p50-100

quite often, particularly in the democratic countries. A manager in industrialR&D cannot expect to know all of these regulations just from reading thenewspapers or from personal experience. Lawyers and specialists should becommissioned for the task of collecting up-to-date information that is relevantto the project and organizing it into different files from which project manage-ment can decide what needs to be included into the working program.

3.8.3 Waste disposal

A critical aspect of any chemical project is the definition and quantification ofall the possible waste streams and of the general options for their disposal withinthe framework of the particular region considered. This should be addressedquite early in any development program. This is again a specialized activityincluding technical, commercial, and legal aspects. This should be dealt with incollaboration with suitable consultants to find at least one (but preferably more)acceptable and affordable disposal procedures for each waste stream.

3.9 Support of specific codes relevant to plant design and operation, and product quality

Many products have to conform to a client’s own purchasing specifications.However, certain groups of clients are buying on express condition that theproduct is fulfilling the requirements of specific “official” codes controlledby a suitable governmental regulation, e.g., a food-grade, reagent-grade, orpharmaceutical-grade product (i.e., the FDA in the U.S. sphere of influence).

Such codes are issued, controlled, and maintained generally by publicorganizations (mostly manufacturers, but also consumers), and they regulatenot only the final composition and packing of the product, but also the rawmaterials and additives used, and the conditions prevailing in the executionof most stages of the production.

For instance, the food-grade code specification dictates not only that allthe raw materials and additives introduced in the process should be of food-grade quality, but also that the conditions in every process stage should bedesigned and controlled to prevent contamination, oxidation, microbialactivity, etc. This code also details the routine quality control with verydetailed analyses and formal reporting and recording. External quality con-trol also is often required.

If such specific code can be relevant to the new process and/or to thenew product, it should be studied from the beginning by the marketing andanalytical professionals and well understood by the core team. In addition tothe general principles of the code, its practical implications may relate insome detail to the selection of raw materials, the plant design, or the analyticalcontrol. For example, it took one producer of food-grade phosphoric acidmany years to drop from 2 ppm arsenic in the product to less than l ppm,as required by the food-grade code.

1360_frame_C03 Page 34 Monday, April 29, 2002 3:35 PM

Copyright © 2002 by CRC Press LLC

3.10 EconomicsThe ability to do economic evaluations of investment and of operation costson the whole process or on a defined part of it is needed from the beginningin order to assist in the choice between alternative options. These economicevaluations can start with simple “order of magnitude” calculations, but canbecome increasingly complex as the project’s scope gains substance.

Therefore, one should assure from the onset of the project the contribu-tion of a specialized cost engineer, who will lay down standardized spread-sheets adapted to the particular framework of the project and collect theexact unit costs that should be used by the corporation at the intended site.

As the process and the implementation framework begins to firm up,the standard tools and the experienced staff of an engineering companyshould produce investment budgets and operating cost estimates with anaccuracy (plus/minus) margin starting at 30%. This can be progressivelyreduced to 15% in the final report.

3.11 Development expense budgetLast but not least in the list of essential resources needed is an expense budgetto cover all the development costs (salaries, transfers, suppliers, consultants,materials, patents, special equipment, etc.).

At the beginning, this budget has to come from the promoters’ ownsources. At a later stage, if the project can be incorporated in the form of alimited responsibility shares company, those investing in risk capital fundscan possibly be convinced to buy a certain portion if it looks promising andif the promoters have good personal records.

In certain countries, public funds can be procured as a partial contributionto specific industrial or scientific developments, mostly in the form of loansrepayable in case of economic success, with many conditions attached. This isgenerally a very lengthy procedure as always with public funds.

When an implementing corporation takes over the project, it covers thepast and future costs from its own financial resources, through one of manydifferent financial formulas. However, as every project is different and pastrecords can only be indicative, “the future is no longer what it used to be.” Theability to predict logically a future process development budget has always beena weak point, although this fact of life is not always admitted or even recog-nized. Most promoters naturally tend to be rather optimistic in that regard.

The only practical method, while consulting with all experienced partici-pants around, is to:

• Divide the development program into a number of functional periodswith specific aims.

• Define and estimate separately every possible cost item in each period.• Draw up a detailed list of every possible cost item.• Then add generous safety factors.



3.12 Worth another thought

• The success of the development and implementation of a new chem-ical process depends mostly on the interactions and cooperation be-tween many critically important human factors.

• The core project group is expected to work as a team, so that all itsmembers have access to all the documents and are aware of all thefacts, and each can contribute his opinion freely inside the team.

• There is generally a direct link between the accuracy of the resultsfrom analyses and the unit cost and time delay. The highest level ofaccuracy is not always justified and affordable for all results, partic-ularly in the exploratory stages of the R&D where a fast procedureis preferable.

• The accuracy and significance of the result from any chemical anal-ysis cannot be any better than the sampling procedure used to pro-cure the sample.

• The process development team should understand which features ofthe product are really important to the final users and what the finalusers would be ready to pay for these results if they were given thechoice between different qualities.

• The process developing team should get a clear and early picture ofthe eventual implementation conditions in an existing facility, whichcould impose objective limitations that need to be taken into account.

• In many cases, the design of a novel process section may be criticallylinked to one particular piece of equipment or specific technology,and the process results will depend not only on the process chemistry,but also on a particular combination of equipment design factors andof operating conditions.

References1. Mizrahi, J., People, organization, and process implementation, Chem. Tech.,

459–464, 1972.

1360_frame_C03 6 Monday, April 29, 2002 3:35 PM


chapter 4

Actual case examples

The following cases are given here only to the extentneeded to illustrate the general principles that are dis-cussed in this book. Obviously, the detailed account ofeach of these cases could have filled a book (assumingthat such details were allowed for publication). Theparticular cases chosen here come mostly from the au-thor’s direct experience, but are “old” cases, not anywith relevance to an ongoing operating corporation.

4.1 Nature and man: the Dead Sea

The Dead Sea is one of the world’s natural wonders; the deepest and one ofthe hottest places on Earth. During milleniums, it has accumulated chloridesand bromides of magnesium, calcium, sodium, and potassium. The averagecomposition of the sea brine reached a steady state, as the average yearlyamount of fresh water that was brought by the Jordan River into the DeadSea brine was equaled by the amount of water that had evaporated fromthis brine.

In the first half of the 20th century, processes were investigated to recoverthe potassium chloride from this brine as a vendable product (potash).Chemists at the Hebrew University in Jerusalem (M. Novominski, M. Lan-goski, and others) studied the entire relevant physical–chemical solids/liq-uid saturation system. They found that when the Dead Sea brine evaporatedand gradually concentrated in a

solar pond

, salt (sodium chloride) reachedfirst its saturation point and precipitated. Then carnallite (a hydrated doublesalt of magnesium and potassium chloride) was crystallized together withsome more sodium chloride. The scientists also found that the mixture ofthe carnallite crystals and sodium chloride, obtained from solar ponds, couldbe leached at ambient temperatures with a large amount of water to leavea number of fine potash crystals with a rather low yield. Or the mixturecould be leached alternatively with a limited amount of water at a highertemperature to decompose the carnallite and dissolve all the magnesium



chloride, allowing to separate by filtration the remaining solid sodium andpotassium chlorides. These can be hot-leached and then the hot filtrate brinecan be cooled and concentrated under vacuum in conventional equipmentto crystallize the potash. The remaining brine can be recycled back into thesolar pond to repeat the process.

1

This straightforward process was eventually developed and an indus-trial plant was built to produce potash at the southern end of the Dead Seanear the biblical site of Sodom. The plant included the following successivesections (see the excellent description by J. Epstein, Reference 2): large solarponds for salt, solar ponds for carnallite,

wet harvesting

of the crystals fromthe carnallite, solids–brine separation, decomposition of carnallite in twocountercurrent stages, hot leaching of the solids in a circulating brine, hotfiltration of the salt, vacuum cooling crystallizers, potash washing and dry-ing, and all the adjacent services required for a desert location. In this firstventure, the solar ponds with

wet-harvesting

were, indeed, the critical newelement essential to efficiently handle millions of tons per year of corrosiveslurry. This was done with floating dredges that crisscrossed the ponds,slurrying the crystals from the bottom, and pumping the slurry into a floatingpipeline to the shore.

But as we said at the beginning, this is a changing world and two

human-induced changes

, which were outside the control of the operating Dead SeaPotash Company, occurred later and required new process developments.First, starting from the 1950s,

all the fresh water

from the Jordan River wasdiverted for agriculture development in Israel and Jordan, and practically

no water

was allowed to drain anymore into the Dead Sea. The age-oldsteady-state was ended and the concentration of the Dead Sea brine startedto increase,

slowly but inexorably

with more salt precipitated at the bottom ofthe sea and, consequently, the carnallite

production

of the existing solar ponds

increased

. The trend of these changes could be followed, analyzed, and pre-dicted exactly from the 1960s, and since it was imperative that

all

crystalsproduced in the carnallite solar ponds be removed to avoid clogging thewhole system, it meant that the potash production capacity should be

increased

accordingly. Finally, this additional raw material was availablealmost for free, so why not build more potash production in the 1970s?

But when expansion plans were prepared and approved and their execu-tion was about to start, the so-called

“energy crisis”

of 1973 happened and thecost of thermal energy jumped almost overnight by a factor of four to fivetimes. With the new production costs and the uncertainty concerning thefuture situation in this regard, the additional potash production by the hotleach process could possibly lose money. So the

processes basic concepts

had tobe urgently reconsidered, including the potential use of some elements whichwere known but not considered essential in the previous economic context.

It was known, for example, that in the crystals mixture produced in thesolar ponds, the salt and the carnallite were precipitated practically in sep-arate crystals, and that the size distribution of the carnallite crystals wasrelatively coarser than that of the salt crystals. It was possible to separate



about 25 to 30% of the carnallite, in rather pure form, from the feed to thepotash plant just by coarse wet-sieving of the slurry.

It also was known for a long time that, when a controlled quantity ofwater is added to carnallite crystals at ambient temperature, all the magne-sium chloride would go into a solution with about 20 to 25% of potassiumchloride, leaving the remaining potassium chloride as fine solids. There wereno incentives to make use of this information up to that point, since it wouldhave only complicated the straightforward “hot leach” process. But whenseverly pressed by the energy crisis, its reconsideration allowed a corporatetask force to develop a “

cold crystallization”

process, which required almostno thermal energy.

From a carnallite stream with a relatively small salt content, a

cold crys-tallization

system would produce

reasonably coarse and clean

potash crystalswithout heating and cooling. This process was analyzed in detail and itsimplementation depended on the development of a

novel type of continuousindustrial reactor–crystallizer

, in which the rather pure carnallite solids werefed and dissolved in one part, while the potash was crystallized in anotherpart, solids were decanted in quiet zones and brine was circulated betweenthe different parts.

3

This new design was piloted and demonstrated in an intensive program.The crystal mixture pumped from the carnallite ponds to feed the existinghot leach potash plant was first wet-screened to separate as much as possiblethe coarse carnallite fraction. A first cold crystallization plant was success-fully build for several hundred thousands ton per year of potash. This newdevelopment allowed a few years of breathing time in the race against natureand the oil “lords.”

Then, the gist of the problem was proposed to the physical mineral sep-arations scientists. Given a crystal mixture of carnallite (specific gravity 1.6)with salt (specific gravity 2.1) in a slurry with

end brine

, a residual solar pondby-product from the potash production of Dead Sea brine (in fact, a concen-trated solution of magnesium chloride with a specific gravity 1.35), how canone

separate

a greater part

of the carnallite in a

reasonable pure form

, in millionsof tons per years, and at

very low cost

? This physical separation did not haveto be completed, since the remaining mixture could still be treated by hotleach, but the content of the

pure

carnallite fraction should be above 95%.This challenge was again solved by a novel technology: by

centrifugaljigging

on a tumbler centrifuge equipped with a conical wedge-wire screenwith a rather large aperture. This type of centrifuge was developed earlierin Germany as a large-capacity screening device to produce low moisturecoarse salt cakes. It was found that the pulsations in the expanded fluid bedof crystals, flowing on the inside of the conical wedge-wire screen, causedthe heavier salt crystals to concentrate nearer the screen and, thus have thepriority of passage through, leaving most of the lighter carnallite crystalsbehind. The large-scale application of this technology allowed anotherexpansion of the “cold crystallization” plant and more breathing time in thecontinuing race against the clogging of the solar ponds.



Finally, the separation of the salt from carnallite in the finer size frac-tions was obtained by adaptation of the conventional froth-flotation tech-nology for salt used in other lands, to the particular conditions of the DeadSea chemistry. Today, the multimillion tons per year production of potashfrom the Dead Sea is

using all of these originally developed technologies

in anoptimum combination.

4.2 Magnesium chloride-based industries

In the early 1960s, it was apparent that end brine was a raw material verysuitable for the production of magnesium oxide (MgO = periclase). Thismaterial is widely used for refractory bricks. Up to that time, a part ofsmall deposits of natural magnesium carbonate, all the existing worldproduction of this material was based on precipitation of magnesiumhydroxide from sea bitterns. Such production is done in a

very dilute system

with hydrated calcined lime, which is an energy-intensive raw material.The application of a similar technology to the Dead Sea end brine woulddeny any advantage of its higher concentration, leaving only the disad-vantages of a desert location.

On the other hand, it was known that the thermal decomposition of suchend brine can produce solid magnesium oxide and a vapor phase with awater/hydrochloric acid mixture. The detailed conditions required to conductcontinuously such thermal decomposition processes were studied by Dr. J.Aman from the Hebrew University in Jerusalem, who developed and patentedin the 1950s the

direct-contact

continuous “Aman reactor.”

4

This is a sort ofspray dryer (vertical cylindrical/conical chamber) in which the end brine issprayed at certain locations from the top while hot flue gases at 800 to 900

°

Care introduced tangentially at the middle height, creating a definite

internalflow pattern

. The solid impure MgO particles remaining from the liquid dropsare settled and removed from the lower conical outlet and the gases exitingfrom the top are directed to a direct-contact absorption column, producing a18 to 20% HCl solution (somewhat below the 22% azeothropic concentration).

This novel process was piloted in the 1960s, and its enormous industrial

potential

was then demonstrated. However, its implementation remainedcritically dependent on the economic utilization of the HCl by-product and,thus, it was delayed until a proper combination could be organized. Otherissues were connected to the presence of smaller quantities of magnesiumbromide in the brine, which would produce elementary bromine in the gases,and this needed to be dealt with. This started a solvent extraction processfor separating a stream of pure magnesium bromide from the end brine, butthis is a different story.

Note that the same Aman process technology was also licensed andapplied successfully in other countries by the Ruthner Company for thedecomposition of the

iron chloride

solution resulting from steel picklingplants, where the recovered HCl solution could be recycled and reused onsite in the pickling plant.



4.3 Economic uses for the HCl by-product solutions

4.3.1 Strategic policy

In the 1960s, the managing team of the IMI Institute for R&D, directed byDr. A. Baniel, created a corporate

strategic policy

defining the need of devel-oping

economic uses

for by-product HCl solutions. As part of the Amanprocess for magnesia, a number of promising “acid-salt” double decompo-sition processes were under consideration aimed at upgrading the value ofthe chlorides of potassium, sodium, and magnesium (available in very largequantities at low cost) into the relevant sulfate, phosphate, or nitrate salts.Implementation of any one of these potential processes would also yieldHCl as a by-product solution (as indeed, the IMI process for potassiumnitrate when it was implemented by Haifa Chemicals Co.).

5

Up to that time, HCl was mainly a traditional by-product of organicchlorination reactions and of small-scale chemical industries. In most of thesecases, the HCl was wasted, neutralized with lime and/or limestone, anddisposed of as CaCl

2

in the sea. Only in very large organic chlorinating instal-lations could the HCl by-product be collected as a solution and recycled to anelectrolysis section to regenerate elementary chlorine, or collected and recycledby the Kellog’s Kel-Chlor process.

6

This route was hardly more economical,but it was possibly less problematic than the neutralization route.

4.3.2 Coupling of HCl-producing and consuming plants

Some industrial uses with economic justification were developed within thisstrategy (see discussion below), but the

basic problem

that remained for sev-eral decades was the

critical coupling

in the implementation between the plant

producing

the HCl and the plant

consuming

it, in their geographical location,in quantity, and in timing. It should be remembered that the HCl–watersystem is dominated by an

azeotrope

at 20 to 22% HCl, so that every ton ofHCl generated below the azeothrope is accompanied over the fence by 4 to5 tons of water, and the transportation of such solutions would be impracticalover any significant distance.

“Breaking the azeothrope” (i.e., obtaining more concentrated solutionsor even 100% dry HCl) is possible, but complicated and expensive, both ininvestment and in energy consumption; for example, by using a cycle ofCaCl

2

brine. This was a wide field of creative process design, aiming at abetter use of the energy and expensive heat exchangers, and for possiblesynergetic utilization of sources of low-temperature waste heat.

7

(See alsoChapter 6, Section 4.)

4.3.3 Timing of implementation

As an acid reagent, HCl could be used to replace sulfuric acid in severalmineral industries. Some new processes in hydrometallurgy and mineralrefining were studied and a few of these could have been developed and



used if a reliable HCl source could have been made

available at the right time

;for example, the cleaning of sand for the glass industry, the purification ofdifferent sorts of clays, the reprocessing of nonferrous scraps, etc.

4.3.4 Production of pure phosphoric acid

The main novel process that was actually developed and used on a largescale in several plants was the production of

phosphoric acid

by hydrochloricacid leaching of (calcium) phosphate rock. The conventional process with

sulfuric acid

gives a

solid gypsum

residue, which is separated by filtration fromthe

impure

“wet”

phosphoric acid (WPA) solution. There exists also processesbased on nitric acid.

When using a hydrochloric acid solution to dissolve the phosphate rock,the water-soluble residual CaCl

2

remains in the same aqueous solution withthe phosphoric acid. A new separation process, therefore, was required toisolate the phosphoric acid from the CaCl

2

(and all the other soluble impu-rities). This result was provided in a pioneering breakthrough by A. Banieland R. Blumberg, by way of solvent extraction.

The first IMI “standard” phosphoric acid process was quite complexwith six different multistage, countercurrent batteries. A comprehensivedescription of all the issues related to its development and implementationwas presented at an international scientific conference,

5

as an

IMI staff report

prepared by a dozen senior staff members, each one in his/her specialty.This

unique

approach started in this book, but unfortunately it was not wellunderstood and was not pursued in further scientific publications.

The new process also accomplished a thorough

purification

of the phos-phoric acid product, which could aim at the higher value markets. Suchmarkets were traditionally supplied by “thermal” phosphoric acid, obtainedvia elementary phosphorus (see below). This novel process was actuallylicensed and implemented first in Japan, Brazil, and Spain, where someexisting sources of by-product HCl already existed, before it was used inIsrael in large plants fed with HCl by-product from potassium nitrate andpericlase productions.

4.3.5 Technological difficulties

After the basic chemical research and the bench-scale demonstration of thenew process, the developing team at the IMI Institute for R&D had to facesome difficult

technological

issues on the way to implementation.

4.3.5.1 Materials of construction

HCl is a well-known, “nasty” component to work with, as it attacks practi-cally any metal. Previously, it could be handled in industry only in smallglass equipment (i.e., Pyrex), or small glass-lined steel (i.e., Pfaudler), or insome cases, in rubber-lined steel (limited to the lower temperature range ofless than 60

°

C). As all of these options for materials of construction were



very expensive, sensitive, and quite limiting in large volumes, it was obviousfrom the start that plants for the large-scale production of relatively cheapmaterials could not be built exclusively from expensive materials.

Fortunately, during the same period, the technology for the design anderection of large equipment and piping made from

plastic

was being devel-oped in several advanced industrial countries. This technology used sheetsand tubes of thermoplastic PVC and polyethylene (and, later, polypropylene)with possibly the external reinforcement of layers of glass-fiber/polyestersetting mixtures and of steel members. Later, the fabrication of equipmentmade exclusively from reinforced polyester or epoxy setting mixtures wasestablished. This new technology was the

critical engineering basis

for anyHCl-based industry at that

time. Thus, the process development group hadto take a very active role in locating the best know-how available worldwide,in establishing specialized companies and workshops in Israel, in creatingdesign standards and testing procedures, and in merging all of these into apractical working system. Another limitation was that the use of

plasticizer

materials in the thermoplastic material was strictly prohibited for all vesselscontaining solvents, as these plastisizers would be leached out by the solvent.(Today, this fabrication technology is essentially available widely as a stan-dard engineering choice, but there are continuing new improvements inmaterials and in design techniques that have to be evaluated.)

At the same time, it was obvious that the use of these thermoplasticmaterials would limit the process temperature to below 60

°

C (at most). Thus,any solvent stripping operating at higher temperatures would remain mostlywith conventional glass-lined equipment, although thermo-setting resinscould sometimes be used for limited functions, and should be minimized aspossible. Another prosaic but important limitation was that, for structuralstrength design considerations, all plastic vessels needed to be

round

(verticalcylinders) and this affected both the

internal functional

design and the plant’sgeneral layout considerations.

4.3.5.2 Safe, stable conditions for solvent extraction in large mineral plants

At the beginning of the project, the “explosion-proof” conditions associatedwith the handling of relatively large quantities of organic solvents withrather low ignition points (i.e., butanol, pentanol, and the like) were wellknown in petroleum refineries and petrochemical installations, but ratherunfamiliar in the mineral/chemical industry. The process developmentgroup had to recruit experienced consultants in this area and make a specialeffort to study, assimilate, and adapt the explosion-proof codes to theseparticular projects, even for such simple items as the venting of excess gases.

In addition, the composition of the solvent stock circulating in the plantcould hardly be taken as a constant, as it undertook various chemicaldegradations and additional reactions, mostly with the unavoidable impu-rities flowing through the plant streams. For example, most phosphate orescontain some organic matter soluble in acidic leach solutions, which are



partly extracted and accumulated in the solvent, and require specific clean-ing procedures.. Such reactions can produce contamination in the productor even change some of the solvent’s properties. Thus, a surprising amountof sophisticated R&D in

organic chemistry

was needed for such mineralprocess development.

4.3.5.3 Clean starting solution for solvent extraction

One of the main enemies of industrial solvent extraction is the

crud

consistingof fine solid precipitates, which accumulates at the interface between thetwo liquid phases and may prevent their separation and cause emulsions.This crud may also clog lines and build up in equipment.

When dissolving, for instance, a typical phosphate ore into a hydrochlo-ric acid solution (with minimum acid excess), most of it goes into a solution.The resulting slurry is degassed under vacuum and the solid residue (con-sisting of sand, clays, dirt, etc.) is then flocculated and separated by coun-tercurrent decantation, and the overflow is polished by filtration. This is theeasy conventional part. However, when the

clear filtrate solution

comes incontact with the organic solvent, the solubility conditions change, and it wasoften found that

crud would precipitate

.For example, part of this crud can be

organic colloidal material

originatingfrom the natural phosphate ore, which was maintained completely in solu-tion in the strongly acid solution, but can be precipitated when part of theacid is extracted and flocculated by the organic reagent. Other parts of thiscrud can be mineral or

metallic salts,

which were initially kept in solution bythe strong acidity. As these elements were defined, the process developmentteam had to devise ways to avoid this crud, or at least reduce it to proportionsthat could be handled with periodical cleaning schedules. This aim could beachieved by either changing the properties of the starting phosphate ore, ifthere was an affordable choice (e.g., by using a more expensive calcinedphosphate concentrate with no organic matter), or by adding pretreatments(such as ion exchange) to the solution before its transfer to the solventextraction section.

4.3.5.4 Recovery of the residual solvent from different exit streams

All the effective solvents were partially water-soluble, and their saturationsolubility in the exit aqueous streams was of the order of a few percents,depending on the temperature and on the other solutes presents. In principle,these residual solvents could be stripped down to the allowed and affordablelevel of, say, less than 100 ppm in a distillation column with sufficient numberof stages and reflux. Again, this may seem a trivial question of engineering,but it was rapidly apparent to the process development team that the invest-ment on the glass-lined equipment, the possible attack of fluoride anions onthe glass lining, the possible deposition of calcium fluoride from wastesolutions whenever heated, and the associated thermal energy and coolingwater would be a

critical load

on the economics.



Thus, every possible way to decrease these costs had to be consideredin the process development. The overall technical–economical optimizationcould recommend a different solvent, which possibly may have been lesseffective in the separation, but cheaper to recover. Other practical questionsalso needed to be addressed, such as the possible fouling of the highertemperature stripping equipment with solid incrustations and, in particular,on the heat exchanger surfaces.

4.3.5.5 Large-capacity liquid–liquid contacting equipment

The implementation of the new processes required

large-capacity

liquid–liq-uid contacting equipment

8–23

for multiple-stage countercurrent batteriesmade of suitable materials, i.e., plastic (see above). The concept of the mixers-settlers was already established on bench scale and used in pilots and rela-tively small industrial installations (i.e., for uranium extraction). However,the design of

efficient large-scale equipment

was not established at the timeand some issues had to be solved.

First of all, hydraulic heads for the flow of liquid streams from stage-to-stage in both directions had to be worked out. The simplest solution formaintaining such hydraulic heads is to install two interstage pumps for eachstage (with the associated sumps and level controls, but with very lowheads), which, in addition to the mixer, amounts to three explosion-proofmotors per stage. No problem. One has only to multiply the number of stagesby three, but the result is not a simple number, but more likely a snowball.The cost of an explosion-proof motor is 2 to 3 times that of an ordinary one,but its installation can cost 10 times more, and the level control loop willdouble that total. One alternative can be to design with only one transferpump, plus a difference in height, but this difference would accumulate andcertainly complicate the vertical layout for large multistage batteries. There-fore, to keep all the mixers–settlers on the same level with only one motorper stage, there was a clear and imperative need to use each motor for morethan one task. This prompted from the beginning a

hydraulic

research anddevelopment program as an integral part of the new chemical process imple-mentation on a large scale.

Figure 4.1 illustrates the principle of the patented IMI “pump-mix” con-cept, with a vertical pump (between two static baffles) on the same shaftand above the mixing axial propeller. This pump design has a very steep {Q@ H} curve, so that the level in the mixer is self-regulated without any levelcontrol hardware. A manual weir for each ratio of liquid densities fixes thelevel of the apparent interface in the settler. This design was successfullyinstalled in a large number of industrial installations for those cases wheretwo liquid phases had relatively close densities and low viscosity and couldbe easily dispersed and circulated in the liquid–liquid mixer. A reasonablemass transfer was obtained with the large range of droplet sizes.

But at a later stage, when implementing such processes involving thecontact and equilibration of a

heavy aqueous brine

with a

light organic solvent

,the above design could no longer give the adequate hydraulic heads, mass



transfer, and phase separation rate. So, as an integral part of these newprocess developments, a new liquid–liquid mixer had to be designed andtested, resulting in the IMI “turbine pump mix” design (see Figure 4.2),which produced a

controlled

droplet distribution when operated with a

vari-able speed

drive.Finally, the use of plastic materials of construction also necessitated

the functional design of a relatively

new round settler

(instead of the con-ventional “shoebox” design), with a central inlet of the two liquid-sus-pension from the mixer, and radial flow of all separated phases. Thisdesign required a fundamental study of the basic hydraulics connected tothe settling coalescence process and of quantitative design procedures forsuch a settler. This study resulted in a later stage to the invention of thepatented “compact settler” design, making use of racks of inclined parti-tions to save the largest part of the area and of the internal volume, andreduce the expensive solvent inventory. (See also Chapter 6, Section 6.4.1and Figures 6.5 and 6.6.)

Figure 4.1

Mixer-settler with pump mix.

Figure 4.2

Mixer-settler with turbine pump mix.

apparentinterface

heavy phase

heavyphase

lightphase

vent

lightin

heavyin

mixed phase

weir

apparentinterface

heavy phase

heavyphase

lightphase

vent

light in

heavy in

mixedphase

tangentialconnection

light phase

turbine

stator

weir



4.4 Phosphoric acid diversification processes

4.4.1 Different quality specifications

The different users of phosphoric acid require

different quality specs

, which arelisted below in order of decreasing purity and purchase cost per unit of P

2

O

5

:

1. Chemically pure/pharmaceutical grade (CP or PG)2. Food grade — FGPA3. Technical grade — for different phosphate salts4. Animal feed grade — for cattle and poultry feed supplements5. Detergent grade — mostly for sodium tripolyphosphate (STPP and

similar)6. Liquid fertilizer grade — giving a clear aqueous solution after neu-

tralization7. Solid fertilizer grade — lowest acceptable grade (almost anything goes)

4.4.2 Solvent extraction opening

Up to the introduction of the solvent extraction processes in several coun-tries,

24,25

there were only two grades of phosphoric acid available:

• The wet process acid (WPA) that was adequate only for the solidfertilizer grade, as it contained a few percents of sulfuric acid, someF, Ca, Mg, Fe, Al, etc.

• Expensive “thermal acid,” which should be used for any of theother grades.

The production of thermal acid (and these markets) was limited not onlyby its cost (2 to 3 times more than WPA), but also by the serious ecologicalhazards related to the elementary phosphorus. The

unfulfilled potential

and

the needs

were clear to the whole industry. Various chemical treatments werestarted in various places in connection with specific partial neutralizationprocesses of WPA.

As soon as the solvent extraction technology got established worldwideand the phosphoric acid extraction and purification was demonstrated,

26–27

there was a worldwide rush by R&D units in this industry to establish newprocesses, to patent different related issues, and to build producing plants.28

The solvent extraction technology allowed for producing different qualitygrades of phosphoric acid at varying production costs, starting with the mer-chant qualities of WPA, which could be produced on site or be purchased. Butone should also note that any one of these processes would leave a more impureresidual stream containing between 30 and 70% of the starting phosphatevalues. This should be downgraded and compounded into a solid fertilizer-grade by-product or mixed, if possible, with merchant WPA. This meant thattheir implementation could only be in proximity to a large solid fertilizer plant.



There was a very intensive worldwide struggle in the 1960s and 1970suntil the novelty appeared to be more or less exhausted and the worldwidemarket saturated; this despite a very fast increase in the demand for the prod-ucts, mostly for the detergent and liquid fertilizer uses. (The Tennessee ValleyAuthority [TVA] point of view is summarized in Reference 29.)

Following are some of the processes developed by the IMI team in thisparticular field.

4.4.3 IMI “cleaning” process

The IMI “cleaning” process30 was implemented in 1974 in a large plant thatis still functioning in the port town of Coatzacoalcas in southern Mexico,near a large WPA producer. The solvent extraction process is extremelysimple and flexible and is based on the extraction of phosphoric acid from a53 to 54% P2O5 WPA feed with a diisopropylether (IPE) organic stream at alower temperature and its back extraction at a higher temperature as cleanproduct. This is at a concentration of 50% P2O5, which is used either fordetergent grade or for liquid fertilizer grade.

The novelty of this process, which is quite unique, is that each operationis conducted in a temperature-controlled invariant system, in which three liquidphases with fixed compositions coexist in the zone delimited by the points:

A. Light phase, almost only IPEB. Intermediate zone with a relatively high solubility of phosphoric acidC. Heavy aqueous solution with very little IPE, as shown in Figure 4.3

on a triangular equilibrium diagram for the tertiary system phospho-ric acid-water-IPE

Figure 4.3 Triangular equilibrium diagram H3PO4-water-IPE.

H2O H3PO4

IPE

WPA

A

B

C



Without this particular feature, IPE would be a quite inefficient solvent. But theWPA is mixed with the recycled solvent in a weight ratio such that thetotal mixture composition fell into the three liquids zone, as close as possibleto the line BC. An extract B is separated and most of the impurities remainwith the residual C. Some water is then added to the extract to separate italong the line AC, which will give the Clean Acid(c) and the recycledsolvent (A). All the mass transfer and the final results are obtained in asingle equilibrium stage for each operation (for a fixed number of components,more phases at equilibrium = less degrees of freedom = simpler process, as Gibbswould have said), although a second mixer-settler was provided in the plantas backup and energy optimization. However, small amounts of the cat-ionic impurities and sulfuric acid entering with the feed WPA (more com-ponents) are co-extracted and can be reduced to the extent needed by acountercurrent, backwash reflux battery.

About 60% of the phosphoric acid is recovered as “clean acid,” whilethe “residual acid” containing 40% P2O5 (with most of the impurities) isreturned and back-mixed into the fertilizer plant. The traces of the volatilesolvent are removed from the two exit streams in two steam-stripping dis-tillation columns.

4.4.4 “Close-cycle” purification process

The IMI “close-cycle” purification process31,32 to produce a quite pure phos-phoric acid from WPA was a modification of the “standard” process in whichthe CaCl2-rejected solution was concentrated, roughly cleaned, and recycledto be mixed with the WPA feed. The rest of the process was similar. Thisallowed by-passing the situation where HCl was not available. The processworked well and the product was very pure, but the process was quitecomplicated. Several studies by large corporations showed that it could bejustified economically only if it was implemented on a very large scale. Suchscale exceeded the demand for such pure product in most geographical areas.

4.4.5 Mixed process

A mixed process is practiced in Israel by mixing a certain portion of WPA(at a 28 to 30% P2O5 solution before its final concentration) into the HClleach operation in which the phosphate concentrate is dissolved. Suchaddition increase the average concentration of P2O5 in the leach solutionand makes use of the acidity of the sulfuric acid in the WPA. The sulfateprecipitates as gypsum with other impurities before the filtration of thesolid residue. Straight sulfuric acid can also be used, but this wouldincreases the load of gypsum on the filter, which needs to be HCl-resistantin such cases. The “mixed” process is then operated in the same way asthe “standard” process, but in a more concentrated and cost-efficient way.It also avoids the restrictions caused by the limited supply of HCl and thelow concentration of its solution.



4.4.6 New proposals

New processes for phosphoric acid published since the 1990s included oenfor obtaining phosphoric acid from phosphate rock and hydrochloric acid viaferric phosphate, which was patented in 1997 and published at the Interna-tional Solvent Extraction Conference (ISEC) in 1999.33 It aimed at cutting dras-tically the volumes and number of contact stages involved in the standard IMIprocess, at the cost of a couple of solid–liquid separations by using the verylow solubility of ferric phosphate (see also Chapter 5, Figure 5.1).

4.5 Citric acid by fermentation and solvent extraction4.5.1 Conventional lime sulfuric acid process for citric acid

Citric acid is an expensive but widely used food additive, giving acidity andlemon flavor to industrial food and soft drink products. Sodium citrate isalso much used as a detergent component for domestic laundry and in manypharmaceutic and fine chemical products. Citric acid is produced by aerobicfermentation in deep tanks, starting with a carbohydrate solution containingdifferent additives and seeded microorganisms. After the practical comple-tion of the fermentation and the filtration of the suspended material, thecitric acid contained in the fermentation “broth” needs to be separated fromany residual carbohydrates and from all the various impurities and by-products, in a form suitable to produce pure citric acid crystals.

The chemical separation route used by most producers consisted, in theaddition of hydrated lime (“liming”), the precipitation of calcium citrate,the filtration and washing of the solids, then the decomposition of the filtercake in a sulfuric acid solution in a strictly controlled ratio to liberate thecitric acid and precipitate the calcium as gypsum. After filtering and wash-ing the gypsum, the solution is concentrated and the citric acid crystallized.The product crystals are washed and the wash solution returned to theconcentrator. The remaining mother liquor is bled and recycled back to theliming. While all producers are keeping confidential the details of theiroperating procedure, it is probable that they are also using additionalpurification steps on different streams, such as active carbon, adsorbingfilter aids, ion exchange, etc. This chemical separation route was used formany years, but is rather delicate to operate, as it had three solid–liquidseparations with washing, resulting in a relatively low citric recovery yield,a high consumption of many reagents, a costly waste disposal, and theoccasional dumping of contaminated batches. This was an obvious placefor a better separation process.

4.5.2 IMI-Miles solvent extraction process for citric acid

With the increasing understanding of the temperature effect on the revers-ible extraction/separation of mildly strong acids with tertiary amine



extracting agents, the IMI team, under Dr. A. Baniel, proposed in 197034 anew process to replace the chemical route described above. This was rap-idly developed, demonstrated, patented, and licensed to one of the majorproducers in those days, Miles Laboratories, Inc. The new process was thenpiloted and implemented in close cooperation with the Miles technicalteam, under Dr. Toby Wegrich, in an existing large plant in the U.S., affect-ing only those sections that were to be replaced. The satisfactory operationresulted in a much increased citric recovery (that relates to the plant pro-duction capacity) and in lower production costs. This process was thenused in other plants of the company, giving them a strong advantage overany competition worldwide.

4.5.3 Newer solvent extraction process for citric acid

About 20 years later, another large American corporation (Cargill, Inc.)decided to get into the citric acid business from the start. The company haddeveloped its own fermentation technology and knew, of course, about theIMI patent licensed to Miles (which was still in force), and were looking fora similar solution. Dr. A. Baniel and David Gonen provided this result in avery creative but simple way35 (which should be very instructive for futuresimilar cases).

The first process intended to use the fermentation broth as it wasproduced then in the existing Miles plants, so the attention of the devel-opers and the original patent claims were referred to this particular rangeof citric concentration. But the citric concentration can be increased 2 to 3times by evaporation, if needed, before the separation process. Such changeallowed not only to avoid the formal wording of the claims in the originalpatent, but also to take advantage of the higher concentrations to get amore compact and efficient separation process with relatively smallerequipment and less solvent inventory. This novel concept was rapidlydeveloped and demonstrated in close cooperation with the designatedcorporate task force. A large plant was built and operated very successfullyon this basis.

Why wasn’t this increase in concentration thought of and introduced inthe IMI-Miles process from the beginning? Only because the exact frameworkof allowed changes (in the existing and producing plant) were defined fromthe start as a precondition for the novel process design, to limit the risks thatthe implementing corporation would be ready to accept. Why wasn’t thisincrease in broth concentration studied and patented later by the operatingcompany after they had a working plant and complete control of the tech-nology? Because then, the common rule in industry was applied: “If it worksand makes a nice profit, don’t touch it.”

Twenty years later, starting with a confident new team and a blank sheet,this preconcentration was a perfectly normal option for consideration. Thislesson can be applied to many other processes.



4.6 Preparation of paper filler by ultra-fine wet grinding of white carbonate

White paper, made with “neutral/alkaline sizing,” contains between 20 and35% by weight of white filler powder, which is mostly precipitated calciumcarbonate in the form of crystalline needles. This filler gives to the finishedpaper its whiteness, opacity, and weight. Such precipitated calcium carbon-ate is not difficult to make, but it is energy-intensive and has to be producedon a relatively large scale. Thus it is relatively expensive, particularly if ithas to be dried, packaged, and transported for long distances. So, the eco-nomic need prompted the question: Why can’t it be replaced by finelyground, white calcium carbonate?

A new process technology was developed and implemented on a mod-erate scale near a large paper mill. This dedicated exclusive user wasreceiving the slurry in accordance with his own specification, ready to mixinto the feed going to the paper machine, naturally flocculated with aconsistent size distribution, characterized by an average size of 1 micron,with most smaller than 2 microns, and with a minimum content of minushalf a micron.36

The novelty and the particular features of the process technology, whichwas needed to obtain such final particles, were in the operating conditionof a regular iron ball mill, such as the pulp density, the residence time, thetemperature and certain chemical additives. However, since the final sizespecification cannot be obtained in a single pass, an extensive external circuitwas needed for fine-size classification, separating the product’s fine particlesfrom the recycled coarser particles. This circuit represented the main processchallenge, considering the requirement that the product particles should retaintheir natural flocculation. Generally, in the technology of “fine particles,”dispersing agents would be used to achieve such size classification, but theywere not allowed in this case.

The process solution was derived from the previous research work doneby one of the developers in the mechanism of hydrocyclones,37–40 whichallowed the design of batteries of microhydrocyclones in a countercurrentarrangement, handling large flows of diluted slurry (Figure 4.4). In addition,an original automatic control scheme was designed to handle the naturalfluctuations in the raw material and mechanical system, based on the con-tinuous measurement of the size distribution in the product slurry, which oper-ated a number of flow splitters affecting the recycle cycles. This lesson canbe applied to other similar microparticle systems.

These filler particles were more or less round, whereas the usual precip-itated calcium carbonate generally consists of elongated needles and thisaffected somewhat the “usual appearance” of the finished paper, althoughmost users were unable to perceive the difference. This ultra-fine grindingplant was happily operated by the Polichrom Company for about 12 years,but then the trading conditions in the area were changed, forcing the papermill to modify its line of products and its operating procedure.



4.7 Worth another thought• The use of the by-product from one process in one plant as a raw material

for another process in another plant creates a critical coupling betweenthe two plants, in geographical location, in quantities, and in timing.

• The implementation of a new process can also require new solutionsas regards materials of construction, design standards, and new func-tional equipment.

• The introduction of a new process to replace part of an existing plantis generally preconditioned into the existing conditions of the remain-ing sections. However, once it is well integrated into the production,an overall optimization should be studied for further improvementor future plants.

References1. Kenat, J., The production of potash from the Dead Sea, Second Symposium on

Salt, Cleveland, OH, 1965.2. Epstein, J.A., The recovery of potash from the Dead Sea, Chem. Ind., 572–576,

July 1977.

Figure 4.4 Principle of an ultra-fine wet grinding and classification process.

Ball Mill

solidfeed

centrifugefine

productslurry

waterrecycle

split

split

water



3. Tzisner, T., The Maklef plant for cold crystallization of potash, Comm. IsraelSoc. Chem. Eng., personal communication, October 1989.

4. Aman, J., British Patent 793.700, 1950; Israel Patent 8722, 1956.5. IMI Corporation, Development and Implementation of Solvent Extraction

Processes in the Chemical Industries, staff report at the Int. Solvent ExtractionConference, The Hague, 1386–1408, 1971.

6. Van Dijk, C.P. and Schreiner, W.C., Hydrogen chloride to chlorine via the Kel-Chlor process, Chem. Eng. Prog., 69, 57–61, 1973.

7. Mizrahi, J., Barnea, E., and Gottesman, E., Production of Concentrated HClfrom Aqueous Solutions Thereof, Israel Patent 36,304, 1972.

8. Mizrahi, J. and Barnea, E., A Gravitational Settler Vessel, Israel Patent 30,304, 1968.9. Mizrahi, J. and Barnea, E., A Liquid–Liquid Mixer, Israel Patent, 43,692, 1973.

10. Mizrahi, J. and Barnea, E., A Gravitational Settler, Israel Patent, 43,692, 1973.11. Barnea, E. and Mizrahi, J., Compact settler gives efficient separation of liq-

uid–liquid dispersions, Proc. Eng., 60–63, 1973.12. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics of

particulate systems, I: General correlation for fluidisation and sedimentationin solid multiparticle systems, J. Chem. Eng., 5, 171–189, 1973.

13. Mizrahi, J., Barnea, E., and Meyer, D., The Development of Efficient IndustrialMixer-Settlers, paper presented at the Int. Solvent Extraction Conference,Lyon, France, 1, 14l-168, l974,

14. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics ofparticulate systems, II: Sedimentation and fluidisation of clouds of sphericalliquid drops, Can. J. Chem. Eng., 53, 461–468, 1975.

15. Barnea, E. and Mizrahi, J., Separation mechanism of liquid-liquid dispersionsin a deep-layer gravity settler (four-parts series), I: The structure of the dis-persion band, II: Flow patterns of the dispersed and continuous phases withinthe dispersion band, III: Hindered settling and drop-to-drop coalescence inthe dispersion band, IV: Continuous settler characteristics, Trans. Inst. Chem.Eng., 53, 61–69, 70–74, 75–80, 83–93, 1975.

16. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics ofparticulate systems, II: Sedimentation and fluidisation of clouds of sphericalliquid drops, Can. J. Chem. Eng., 53, 461–468, 1975.

17. Glasser, D., Arnold, D.R., Bryson, A.W., and Vieler, A., Aspects of mixerssettlers design, Min. Sci. Eng., 8, 23–31, 1976.

18. Barnea, E. and Mizrahi, J., On the “effective” viscosity of liquid-liquid dis-persions, I&EC Fundam., 120, 1976.

19. Barnea, E. and Mizrahi, J., The Effects of a Packed-Bed Diffuser Precoalesceron the Capacity of Simple Gravity Settlers and on Compact Settlers, paperpresented at the Int. Solvent Extraction Conference, Toronto, 374–384, 1977.

20. Barnea, E., The Application of Basic Principles and Models for Liquid Mixingand Separation to Some Special and Complex Mixer-Settler Design, paperpresented at the Int. Solvent Extraction Conference, Toronto, 347–355, 1977.

21. Barnea, E., Meyer, D., and Wahrman, D., Logical Design of Mixers, paperpresented at the Int. Solvent Extraction Conference, Liege, France, 6–12, 1980.

22. Harel, G., Kogan, M., Meyer, D., and Semiat, R., Mass Transfer Characteristicsof the IMI Turbine Pump-Mix, paper presented at the Int. Solvent ExtractionConference, Denver, 26–27, 1983.

23. Cusack, R. and Karr, A., Extractor Design and Specification, Chem. Eng.,113–118, 1991.



24. Toyo Soda Manufacturing Co., Japanese Patent 7,753, 1964.25. Albright and Wilson Ltd., German Patent Application, 2,320,877, 1973.26. Baniel, A. and Blumberg, R., in Phosphoric Acid, Slack, A.F., Ed., Vol.1, Part II,

Marcel Dekker, New York, 1968.27. Blumberg, R., Industrial extraction of phosphoric acid, Solv. Extrac. Rev., 1,

93–104, 1971.28. Blumberg, R., Meyer, D., and Mizrahi, J., Development and implementation

of solvent extraction processes in the chemical industries, paper presented atthe Int. Solvent Extraction Conference, The Hague, 1386–1408, 1971.

29. McCullough, J.F., Phosphoric acid purification: comparing the process choic-es, Chem. Eng., 101–103, 1976.

30. Mizrahi, J., IMI Technology for Cleaning Wet Process Phosphoric Acid bySolvent Extraction, paper presented at the Symp. Am. Chem. Soc., 1973. (Thedata in Figure 4.1 was included by Blumberg, R. in a communication to Isr.Chem. Eng. J., September 1973.)

31. Blumberg, R., Miscellaneous Inorganic Processes, in Handbook of Solvent Ex-traction, Lo, T.C., Baird, M.H.I., and Hanson, C., Eds., John Wiley & Sons, 827,1983.

32. Slack, A.V., Phosphoric Acid, Part 2, Marcel Dekker, New York, 721, 1968.33. Mizrahi, J., New Process for Phosphoric Acid from Phosphate Rock and

Hydrochloric Acid Via Ferric Phosphate, paper presented at the Int. SolventExtraction Conference, Barcelona, 1999; also Israel Patent Application,120,963, 1997.

34. Baniel, A., Bkumberg, R., Haidu, K., U.S. Patent 4,275,234, 1971.35. Baniel, A. and Gonen, D., European Patent 91304805, 28.5.91.36. Hirsch, M., Hirsch, I., and Mizrahi, J., Production of white carbonate paper-

fillers by a new ultra-fine wet grinding technology, Ind. Miner., 67–69, 1985.37. Mizrahi, J., Separation mechanisms in hydro-cyclone classifiers, Brit. Chem.

Eng., 10, 686–692, 1965.38. Cohen, E., Beaven, C.H.J., and Mizrahi, J., The residence time of mineral

particles in hydro-cyclones, Trans. Inst. Miner. Met. (London), 129–138, 1966.39. Mizrahi, J. and Cohen, J., Studies of factors influencing the action of hydro-

cyclones, Trans. Inst. Miner. Met. (London), 318–330, 1966.40. Mizrahi, J. and Goldberg, M., Computer simulation of unflocculated hindered

settling, Isr. J. Tech., 318–392, 1969.



chapter 5

Process definition and feasibility tests

The first review of the proposed idea was done inside the R&D group (seeChapter 2, Section 2.2.1). It was shown in that review that the process can“make sense,” did correspond to a real need, and, on the face of it, was notscientifically incorrect. As a result of these early consultations, a “green light”was given to the promoters’ group for the commissioning of the literaturesurvey, for the preparation of this preliminary process working definitionand for their formal presentation for a second review in a larger forum.

5.1 Translation of the idea into a process definition

5.1.1 Scope of the preliminary process definition

An essential

starting point

for any development program is a preliminaryprocess definition, which will allow:

• Bringing everybody concerned to an

explicit common reference basis

• Illustrating a

concrete venture

in order to develop the interest of thehard-nosed decision-makers in the continuation of the work

• Outlining a proper experimental program and starting its detaileddesign

This field of

synthesis and design of chemical processes

has been the subjectof a number of excellent

theoretical textbooks

.

1–4

These manuals can be usefulmostly for the analysis and understanding of the fundamental principles,and for the definition of

the data which would be needed

for the use of thesophisticated models available. Unfortunately, at the beginning of a newdevelopment program,

most of such data would have to be assumed

.Therefore, this preliminary process definition should be assembled and

presented by an

experienced process engineer

who, in addition to the generalknowledge published, would use:



• Personal interaction with the inventors and promoters• Past experience in similar cases• Some reasonable assumptions (which will always be presented as such)• More specific considerations, which are detailed below

The written preliminary process definition document also will include:

• Results from a comprehensive literature survey describing what isgenerally known in this particular field

• Division of the process into defined sections and interconnectingstreams, as shown on a block diagram

• Calculation of the first process material and heat balances (the so-called “revision 0”)

• Definition of at least one feasible implementation scheme• Projection of an industrial implementation framework and timetable• A detailed list of critical feasibility tests

5.1.2 Comprehensive literature survey

The inventors and the promoters have probably already done the best liter-ature survey they could with the means available to them on the core aspectsof their proposal. Now that the field of interest has been both enlarged andmore focused with the participation of additional experienced professionals,a renewed literature survey can be commissioned, in parallel to the otherwork described below (see also Chapter 3, Section 3.6.1).

This publication’s search can be subcontracted nowadays to specialistsor to academic libraries where it is done by computer screening of largedatabases, according to agreed “key-words.” The first result of such screen-ing is generally a very long list of items, including titles, authors, journal,date of issue, language, and possibly a couple of lines of abstract. A firstmanual selection has to be made from the computer’s output, according tosome criteria to be agreed upon. However, the ordering and collection ofworkable copies of the selected publications (and their translation, if needed)could be sometimes lengthy and expensive.

The senior process team should devote a

continuous

effort to supervisingsuch screening and to the study of these copies/references as they arrive,looking in particular for any factual information, and for any possible numer-ical correlation of the included relevant experimental data. In addition, suchanalysis may give some interesting hints about the reasons for any previousresearch work on this subject and about their potential projection on theindustrial scale up to now.

The continuous recording and distribution of the results and the analysisfrom this survey to the core team and to the relevant consultants had oftenprovoked important

practical responses and proposals

concerning the work athand. In the end, all such findings have to be summarized and included inthe material submitted to the second review.



5.1.3 Block diagram

The overall process will be separated, as far as possible, into

different sections

and represented in a

block diagram

with

numbered interconnecting process streams

.This division is very important for all the following work and, therefore, itneeds to be carefully devised so that each block ( = section of the whole process)would contain, as far as possible,

only one well-defined operation

.In this context, a

section

is a definite part of the process in which the flowrates and compositions of the exiting streams are determined uniquely by:

• Flow rates and composition of the entering streams• Operating conditions that can be controlled by the operator, such

as temperature, pressure, residence time, velocities, reflux ratio, andthe like

The presence of recycle (reflux) streams between certain sections and theexact location of their return point are very important aspects in manyprocesses. The two typical examples given below have been chosen in ordernot to trespass into any actual process or new technology handled by anoperating company.

First Example

— A typical illustration of a block-diagram is given inFigure 5.1 as an example describing a new process, which was not completelydeveloped, for producing diammonium phosphate (DAP) from phosphaterock, HCl solution, phosphoric acid, and ammonia following a patentedsolvent extraction process.

21

However, this proposed process also incorporates a

new process concept

,which is hereby offered to the consideration of the readers, as it may haveapplications in many other fields. Instead of an organic solvent cycle circu-lating inside the plant, there is an

internal cycle of ferric ions

(in various forms)kept inside. In short, the incoming HCl solution (stream 1) encounters aferric hydroxide cake (stream 2, with some solid impurities originating fromthe phosphate ore) and dissolves it, giving a FeCl

3

solution. The solid impu-rities are taken out and the hot FeCl

3

with some HCl excess (stream 3) isused to dissolve phosphate ore (4). FePO

4

is precipitated and separated (6)from the resulting CaCl

2

solution (5). The washed FePO

4

cake is dissolvedin WPA (7) — “wet” phosphoric acid — as a mixture of soluble mono- anddiferric phosphate (8). Ammonia (9) is added to that mixture and to a motherliquor DAP recycle (10); more DAP is formed and ferric hydroxide is pre-cipitated. The later is filtered and recycled, the DAP solution (11) is cooledand crystallized, and the DAP crystals are separated and dried (12).

In total, half of the phosphoric acid in the final product originated fromthe reaction of HCl with phosphate rock. The ferric ions are acting as aseparation tool between phosphoric acid and the resulting CaCl

2

and otherimpurities,

including most of the impurities from the WPA,

which precipitatedin the higher pH section. This proposal is also illustrated as an example ofa “black box” in Chapter 10, Section 10.4.3 and Figure 10.3.



Second Example —

This example of a process block diagram (Figure 5.2)is the Gorin-Mizrahi patented process

8

for the recovery of zirconium fromthe natural zircon mineral. (See Chapter 1, Section 1.4. A more detaileddiscussion of the process choices and issues in the high-temperature sectionscan be found in Chapter 6, Section 6.4.4 and in Figure 6.9.)

A commercial grade of zircon heavy sand is finely ground and mixedwith a concentrated solution of CaCl

2

, granulated and dried at 180 to 300

°

C.The process flow sheet of that section is illustrated in Figure 7.5, Chapter7. In these free-flowing aggregates, the CaCl

2

, with up to 6% water, isdistributed in intimate contact with the solid surfaces. The granules areheated and calcined successively in two rotating kilns in series, at differenttemperatures, in direct contact with combustion gases. In the first kiln, thecalcium chloride melts at 782

°

C, reacts with the zircon solids and with thewater vapor, and decomposes, giving very active CaO while releasing HClinto the exit gas stream.

This HCl is then absorbed adiabatically to give an HCl aqueoussolution while scrubbing the exit gases. The overall reaction is completedin the second kiln at 1400

°

C. The reacted clinker is quenched in waterand ground, then partially attacked with the recycled HCl solution. Thecalcium silicate and other impurities are dissolved in the waste solutionand only zirconium oxide remains in the solid phase, which is filtered,

Figure 5.1

Process block diagram for a DAP process.

FePO4 separation

FePO4dissolution

centrifuge

cooling-crystallyzer

solid /liquid

separation

DAPReactor

Phosphateleaching

wastesolids

separation

Hydroxidedissolution

HCl solutioninsol. waste

phosphate

CaCl2 brine

FePO4 cake

Phosphoric acid

Ammonia

ferrichydroxide

cake

DAP filtrate

1

2

3

4

5

6

7

8

9

10

11

12

DAP crystals

mother-liquor

filtrate slurry



completely washed, and dried. The basic technology of all these opera-tions is more or less conventional, and the novelty of the process residesin the exact conditions in the calcination, which give a

liquid–solid reactingfront

and an

intermediate double salt

of calcium silicate and calcium zir-conate. More details on the high-temperature chemistry are discussed inChapter 6.

5.1.4 Quantitative definitions of the different sections

For

each section

, the quantitative definition should consist of two parts,which have to be detailed in the textual description and presented togetherwith the block diagram, in addition to the available data or the “agreedassumptions.”

• Formal

characterization

of the prevailing mechanisms in the generallyaccepted terminology (and in more detail if there is any doubt)

•

Quantification

of the aims to be achieved

This

physical-chemical

mechanism can be, for example:

• A chemical reaction

11–16

• A heat/mass transfer operation

17,19

• A separation operation (see Chapter 6, References 1 to 9)• It also can be a conventional material handling or a storage operation

Figure 5.2

Process block diagram for a zirconium process.

slurry mixing andgranulation

granulesdrying

second kiln

leaching andfiltration

adiabaticabsorption

ground zircon CaCl2 solution

water

fluegases

wash water

HClmakeup

productdrying

waste stream

first kiln

hotgases

hotgases

HCl solution

quenching andgrinding



For example, such mechanisms can be defined as:

• Homogeneous (one-phase) mixing and equilibration of certainstreams

• Neutralization reaction resulting from mixing and reacting differentphases

• Multiple-stages, countercurrent, liquid–liquid extraction battery,which involves

in every stage

the mixing of liquid streams in order toachieve mass transfer of specific components and approach to equi-librium, followed by the separation of the resulting liquids

• Similar batteries for different process functions, such as “extractwash” or “back extraction” in solvent extraction

• Concentration of a solution by evaporation and the subsequent cool-ing of the resulting solution and of the evaporated condensate

• Filtration and washing of crystals on a wedge-wire screen centrifuge• Drying of the solids from a wet filter or centrifuge cake• Separation of a solids stream into (different, defined) size frac-

tions, etc.

The

aims to be achieved in each separate section

also should be given

quan-titative indexes

, such as the minimum concentration, the specification in theexiting stream, the acceptable upper energy consumption, the minimumrecovery of a valuable component, an acceptable waste composition fordisposal, and so forth.

5.1.5 Process calculations for the preliminary process definition

The chemical engineering calculations can follow well-established proce-dures, which are listed and detailed in some of the basic reference books.

6–12

As far as possible, these calculations will be done

in parallel

and will include:The

correlation and analysis

of all the relevant data already available,either from the previous work of the inventors/promoters, of the processengineer, or from previous publications on related subjects, which may havebeen obtained from an extensive literature search (see Section 5.1.2). Thesecorrelation formulas can be

carefully extended

by extrapolation, if needed, aslong as this is clearly recorded as a provisional mean.

The formulation of the

quantitative relations

(known or assumed) that canaffect the process mechanism for each separate section (e.g., the yield ofreaction, the solubility). These relations sometimes can be based on thetheoretical thermodynamics or the physical chemistry knowledge, as givenin basic reference books.

18,19

But it is seldom that in the first stages of adevelopment effort, these sources could be useful or justifiable. Therefore,quite often at this stage, some of these assumptions have to be based on

known analogies with other processes

from the process engineer’s own back-ground (although he may have to keep some of these references secret, andpersonal trust will be essential).



The

evaluation

of the effect that each of the different operating variablesmay have (for instance) on distribution, solubility, recovery, heat effects, andtheir combinations … for each separate section, on the basis of the abovequantitative relations.

The preparation of computer spreadsheets for

each separate section

with the preliminary design material balance and with the heat balance(if the heat effects are important in such sections, which is not alwaysthe case), on a reasonable arbitrary basis, e.g., 1000 units of the main rawmaterial. These balances are prepared by conventional chemical engineer-ing calculations, and this task should give the process engineer a

goodinsight

into the importance of each of the different factors in the

play-of-forces

inside such process (

leverage

). Of course, there are interactionsbetween the different spreadsheets, since each section starts whereanother ends. These spreadsheets can be easily converted at a later stageto any other basis needed.

A list is also prepared of the

critical feasibility tests

that should be doneto define or confirm

some

of the

high-leverage assumptions

taken in thesecalculations before these are presented for the review (see Section 5.3).

5.1.6 Presentation of one feasible implementation formula

This description is, in fact, a

concrete, possibly optimistic

,

illustration

of theimplementation of the concept,

if it could be made to work as intended

. Thestarting point is the

integration

of the above

quantitative assumptions

into one

possible

implementation case, describing an operating plant with the speci-fications of the raw materials and the products, the material and heat bal-ances, the process control, the recoveries, the disposal or treatment of theresulting waste streams, the relevant safety aspects, the choice of materialsof construction for the contact equipment, and so on. Typically, if one of theraw materials have to be transported in large lots, the material handlingfacilities and storage can be significant.

For example, it may be projected that the new process would need largeheat exchangers made from expensive materials (graphite, glass-lined, tan-talum), or that the rapid scaling of such heat exchangers should be expected,considering the composition of the solutions being heated. In such case, onedesign option could be to resort to the technology of organic “heat carrier”(stable liquid or vapor hydrocarbons) and, if this is considered a practicaldesign possibility, it should be studied, defined, and included in the exper-imental program (see Chapter 6, Section 6.2.1)

5.1.7 Possible industrial implementation framework

A projection of one possible industrial implementation framework (

known

or

assumed

) is presented for the proposed novel process with its specificfeatures, such as the possible site, the scale of production, the equipmentsize and function, the different raw materials available, existing connections



to critical services, any possible synergetic coproduction. Such projectionshould help

specify the technological factors which will have to be solved

.For example, the maximum

supply temperature of the cooling water

thatcould be reliably procured or produced in this site depends on the recordedclimatic conditions. It often has been found in warm countries that suchtemperature can have a

critical

importance for the design of a new processbased on evaporation or distillation

under vacuum

, or involving materialswith

low

boiling points

. If the normal cooling water at this site isn’t coldenough, the cost of supplying

artificially chilled

water would have to beincluded or the process scheme radically changed.

5.1.8 Timetable

A reasonable projected timetable for the whole development and implemen-tation project prior to the plant’s start-up and to the market penetration isalso essential to the decision-making forum, in order to:

• Evaluate the availability of the required resources• Coordinate the assistance of the many different support groups and

experts• Connect with the market projection studies

5.1.9 Important note

Obviously, in the beginning, the above

preliminary

“process working defini-tion” would have to be based, in a large part, on weighted and explicit

assumptions

and on

previous professional experience

.Its purpose is to focus the team’s attention as well as to plan any future

work on the

limited scope of application that is of interest in real life

, and to allowfor a more effective allocation of the available industrial R&D resources.

It is agreed that this process working definition will be progressivelychanged, enlarged, and refined in

future numbered revisions

, as more informationwill be gathered and analyzed, and more promising avenues will be defined.

However, such a

working method

has not always been accepted by all. Inmany cases, it has been resisted and even ridiculed by senior scientists, whowere used to the open-ended academic research approach, claiming, “Whatdo we really know for sure? Let’s collect some data first and then we willsee what kind of process will result.”

Bluntly speaking, their

noncommittal

approach represents the

more seriousdanger

to the development of

any

novel chemical process (like gamblingwithout knowing the odds). The least damage can be that a large part of theexperimental data collected would be

outside the scope that is relevant

forimplementation, resulting in a loss of time, resources, and good will. Moreserious damage could be caused if wasting of time and repeated abortivetrials would erode the corporate management’s interest and promising ideaswould be lost.



5.2 Critical and systematic review of the process definition

5.2.1 Review forum

This

second critical review

aimed at reaching operative decisions with regardto the next steps (“no or maybe”) is generally called in by the decision-makerwho can be, for instance, the managing director of the R&D organization,the corporate vice president for new business, or the director of the fundingcommittee, and comparable in function or in title.

This discussion is conducted in a larger forum with ranking colleaguesof the inventors/promoters (“peers”) and with outside experts who areinvited, if and as needed. It should cover all the essential elements, such asthe technology and patents, the corporate strategy and markets, the profitpotential, and the capability to handle the proposed development programwith the resources available.

In a larger organization, there can often be a competition for the priorityin allocations between different projects. The raising and discussion of themore “difficult” subjects in such a review can sometimes be unpleasant, ifit is seen as a personal criticism among working colleagues with complexhuman relations. Therefore, it can be useful to appoint a merciless “devil’sadvocate” to present the pros and cons of the problematic aspects, in advanceand in writing. His contribution would then avoid wasting time in rhetoricand personal maneuvers.

As a result of the first part of this review, a number of specific activitiesshould be approved for immediate execution, and an additional meeting ofthe critical review forum generally would be reconvened by the “decision-maker” a few months later. This additional meeting will then study andreview the different reports prepared on the

feasibility tests

, the

patent discus-sions

, and the clarification of the

critical economic factors

.

5.2.2 Fundamental process issues

In this review, all the fundamental process issues should be raised andfocused on, and a list prepared all information requested to arrive at thefinal clarification of these issues in the future.

• Any apparent reason why “this cannot work?” Have we forgottenanything?

• Could any “wishful thinking” bias be included in the conceptualreasoning?

• Does any quantitative factor have a leverage large enough that mightturn the balance critically away in the wrong direction?

• How to deal with any relevant existing patent claim (see below).• Is the process flexibility sufficient to accommodate some changes that

could be needed to bypass such typical claims, should they appearin the future?

• Is there a sufficient profitability potential (see below)?



As a result of this review, a list of

critical tests for process feasibility

shouldbe defined and agreed upon (see Section 5.3).

5.2.3 Patent situation

Relevant

patent claims granted to another party, which may have been foundin the first survey, will be discussed in this review. In many cases, suchprevious claims may still be avoided quite fairly, on the basis of their exactformal definition, but such constrain can dictate some changes in the devel-opment program.

Unfortunately, patent applications are not made public for 2 to 3 yearsafter their filing and many inventors are using perfectly legal tactics todelay their publication, so one also may have to look for

hints

fromprofessional circuits and to prepare for eventual “surprises” from thisdirection.

The promoters will also draw up and present to this forum a verydetailed list of

every possible patentable claim

for the new process. After thisreview and the approval to proceed farther, this list will be used for addi-tional discussions with the patent attorneys and the specialists, and after anelimination and selection procedure, for the drafting of a comprehensivepatent application (see Chapter 8).

Note that all the formal procedures of patenting are now quite rigid andvery few procedural decisions are really needed, apart, of course, from the

delicate issue of the names listed as the inventors. In particular, since theestablishment of the PCT (Patent Cooperation Treaty), all international appli-cations can be based on the examination done in one patent office (i.e., inWashington, D.C.).

Therefore, the main deliberations with the patent attorneys on any newpatent and the resulting decisions will be related to the exact formulationand wording of the claims in the application. Even among patent attorneys,there is a certain degree of specialization, since only a professional with areal knowledge of the particular scientific technological field can contributeeffectively to such formulation.

However, in some corporations, any talk about a patent applicationwould immediately involve their top lawyers (who usually are very busy)and, therefore, the patenting procedure could become very slow and veryexpensive without any real additional contribution. This frustrating situationis often a pitfall.

As a curiosity, in the U.S., a corporation is legally bound to pay theinventors cash in return for their assignment of the patent rights, even if theinventors are their own employees or their regular consultants under con-tract. This cash payment is often done in the form of a brand new $1 bill,which is handed over to each inventor with his signature, and which is oftenframed together with the invention certificate.



5.2.4 Profit potential

It also should be shown and agreed in this review that the profit potential ofthe proposed process could be attractive enough to justify the estimated costsof the next stages of a development program.

There can be many different definitions of the profit potential, which areused by different corporations and industrial sectors in various countriesand tax situations. In general, this profit potential should quantify the expectedincrease in ROI (Return On Investment) above what would be the ROI foran available, safe, no-risk investment, over a 10-year period.

The profit potential, as an absolute number for a particular proposal(i.e., in dollars per year) is obviously getting higher as the contemplatedscale of production or the sales turnover are higher, while the costs of theprocess development (the risk?) are much less affected by the size, if at all.Thus, a new process could be brilliant and sophisticated, but if its finalproduct has only a small market potential, the straight prospects of approv-ing funds for its development will be dim. Such a proposal then will begenerally presented as a strategic investment, opening the way to … (asthe popular joke says, “One can save more money by running behind a taxithan by running behind a bus).

At this early stage, the standard economic calculations can only berudimental and based on reasonable assumptions. The bottom-line resultswill generally not be clear-cut either way, but they will indicate mostlyorders of magnitude (so-called “back of an envelope” calculation type). Thedebatable issues should focus on the degree of confidence that can beattributed to some high-leverage factors, e.g., sale prices, cost of certain rawmaterials, possibly transportation, taxes and customs duties ofexport/import, commissions, royalties, etc. As a result of this review, afact-finding program will be defined to confirm or correct the requiredquantitative assumptions for such critical economic factors, in addition tothe publicly available information.

According to their specific strategic considerations, most corporationswould be ready to “gamble” a certain percentage (say between 1 and 20%)of the yearly profit potential on the net costs of a comprehensive develop-ment program. Obviously, such a budget would only be released progres-sively, in installments, as certain objectives are achieved with positive results.

5.3 Design and execution of the feasibility tests5.3.1 Purposes of the feasibility tests

Feasibility tests should convince the decision-makers and demonstrate thatthe results from the new aspects can be achieved more or less as expectedin each stage of the process in order to justify a more extensive experi-mental program.



As for the “more or less” qualification in this context, it is generallyagreed that the better results cannot be obtained in the first attempts, butshould be achieved more likely in the more favorable conditions that will bedefined later, after an extensive process optimization.

It is also realized that such demonstrations can generally be attemptedonly with severe limiting conditions, such as:

• Small, bench-scale, batch tests• Standard or improvised laboratory equipment and analytical facilities• While starting with synthetic “clean” mixtures and reactants “from

the bottle”

The results from each stage can either be shown by the direct analysisof the phases obtained after the test or calculated indirectly on the basis ofthose analyses by accepted chemical engineering methods.

For example, a wet filter cake can contain some impurities, which aredissolved into the layer of liquid that is retained on the solids. This layercan be washed out almost completely on a conventional industrial filter, buta similar washing operation cannot be done conveniently on a small-scalelaboratory batch filter with the same results. In this case, the level of theseimpurities related to the retained filtrate can be calculated on the basis ofthe retained water (or of another soluble component) and then deduced fromthe level found in the unwashed filter cake.

5.3.2 Equilibrium conditions

In the limiting experimental conditions mentioned above, a more convenientfeasibility demonstration can be achieved for those process operations that arebased on equilibrium conditions.

For instance, a particular vapor–liquid equilibrium system can be gov-erning some distillation, rectification, or stripping operations in the proposednovel process. A reliable calculation of the results from these operationscan be obtained on the basis of the correlation of the composition of thevapor phase with respect to that of the liquid phase at equilibrium. Thetheoretical background for the calculations is well established and severalcorrelation formulas were published on the subject. A limited number ofexperimental points on the particular system under consideration can beinterpolated quite safely and used for such process calculations. All such“points” connect the two compositions of the phases at equilibrium inspecific external conditions of temperature, pressure, and the partial pres-sure of inert gases present.

Similarly, a liquid–liquid equilibrium system can be relevant to a proposedsolvent extraction process. The composition of the two liquid phases, foundin a specific equilibrium test in certain defined conditions, can be translatedinto a distribution coefficient for each of the components of interest. Thecorrelation of this distribution coefficient with the operating conditions can



be used for calculation of a multiple-stage countercurrent process, in whichthe two exit liquid phases from every stage are assumed to be at equilibrium.

Solids–liquid equilibrium data can regulate certain dissolution and precip-itation processes, which are widely used in the inorganic salts industriesand mineral treatments, and also in the production of organic crystals. Inall of these processes, the determination of the quantitative relations forthe solubility at equilibrium, in several conditions in the projected range,can be sufficient, at least for the preliminary process design and feasibilitydemonstration.

5.3.3 Scale up of reactors

The scale up of batch reactions in mixed vessels is well established in chemicalengineering using the reaction kinetic curves and the definition of the mixingregimes. Continuous mixed reactors can also be designed reasonably wellfrom small-scale batch tests, or upscaled from small continuous mixed reac-tors, using correction factors. Thus, the feasibility demonstration of suchreactions can be based on straightforward batch reaction tests.

Similarly, the mixing and the reaction between gases flowing in a pipereactor are also relatively easy conditions for the process feasibility demon-stration on a quite small scale, and for reliable scale up based on hydrody-namics conditions and residence time.

5.3.4 Physical separation operations

The scale up of solid–liquid separation equipment has been well estab-lished.21 Many continuous separation operations (solids from liquid or liquidfrom liquid) can be demonstrated, sized, and upscaled quite well from batchtests made in standardized conditions. In this context, the term “standard-ized conditions" involves the definition of a particular set of conditions whichare recommended for a batch test in order to obtain applicable results.

For example, the separation obtained in an industrial decanter or thick-ener — from a “feed” suspension into a more concentrated “underflow”slurry and a clear liquid “overflow” — can be demonstrated and quantifiedfrom a standardized settling test in a 1-liter glass cylinder starting from awell-homogenous slurry (in a thermostatic bath, if necessary). The settlingcurve of the upper limit of the concentrated slurry obtained from each test(see typical example in Figure 5.3) depends on the initial solid concentration,on the differences in density between the solids and the liquid, on the liquidviscosity, and on the degree of flocculation.

The plotting of an empirical settling curve allows the calculation of themaximum solid concentration in the underflow, the level of entrained fines inthe overflow (if any), and the horizontal area of the settler needed, per ton-hour of solids, by the well-established Kinch method following Coe-Clavenger(see p. 4.121 in Chapter 6, Reference 1). This experimental procedure is usedalso for studying the effects of the addition and dozing of flocculating agents.



Similarly, the separation of a cake of solids (including the washing ofthe cake) from a particular slurry in a filter or in a centrifuge can be dem-onstrated and quantified on a small scale with the same type of results.Note, however, that a continuous filter is, in fact, a batch filter that happensto be moving on a belt during the filtration cycle. The difference obtainedin the rate of filtration derives from the operating variables: the pressuredifferential on the filter or the G-forces in the centrifuge, and the hydraulicresistance of the formed cake. Flocculation, however, does not affect sig-nificantly these operations.

5.3.5 Scale-dependant and dynamic flow operations

In contrast with the operations discussed in the above three sections, a feasi-bility demonstration cannot be readily performed for such operations in whichthe results are scale-dependant, such as, for instance, the crystal size distributionobtained in a continuous crystallization (see Chapter 6, Section 6.4).

The feasibility demonstration also cannot be readily performed if themechanisms are based on dynamic flow conditions concerning mostly separa-tions between phases (see Chapter 6, Section 6.3) In such cases, any feasibilitydemonstration has to be connected to a particular equipment choice, and somespecific form of piloting is necessary to determine the dimensions and results.

Figure 5.3 Slurry settling curve — Kinch procedure.

time

final

initial

height

settling curve ofsolids front

point of mostrapid change of

slope

Tx



A shortcut sometimes can be found if a logical analogy can be establishedto another known process, which is in actual operation and accessible to theR&D team or their consultants. Then, if it can be established that the newproposed process and the operating process behave more or less similarlyin simple bench-scale tests, this analogy could justify further piloting.

For example, the feasibility demonstration of many proposed wastestream treatments, based on dynamic flow conditions, is very problematic,but these treatments generally fall in a small number of categories.

5.3.6 Extreme conditions

It is not a simple proposition to improvise on laboratory bench-scale afeasibility demonstration for a process operation which has to be done inextreme conditions of temperature, pressure, electrical fields, etc. If such anextreme operation is an essential element in the new process, small testingequipment could probably be specially designed and operated, but thiswould be expensive, require a long time and expertise, and would divertthe team’s attention.

In certain cases, small-scale testing equipment with associated services(and valuable advice) can be rented from one of many suppliers of furnaces,kilns, autoclaves, electrostatic and magnetic separators, plasma torches, etc.These suppliers can also provide experienced engineers to perform thesetests, since they are interested in promoting good will towards their know-how. The main concerns against such services are the unavoidable secrecyleaks and possibly the geographical distances.

If the extreme operation is a self-contained side element in the newprocess, it should be well defined, isolated, and subcontracted to one of thesespecialized suppliers.

5.3.7 Actual raw materials

In certain processes, it can be very important to perform such feasibility testswith the actual raw materials, since a synthetic mixture cannot duplicateexactly the complex phases structure and/or the compositions of these mate-rials in which a large number of impurities can sometimes be involved.

In many actual projects, the use of certain raw materials in later testsdid result in serious nonexpected problems; for example, the precipitation ofsolids causing incrustation on the walls of the equipment and pipes, or atendency to emulsify, or the precipitation of colloid suspensions, or a coloringphenomenon, etc.

In other cases, the reaction kinetics with the actual solid raw materialswas much slower (by orders of magnitude) than the reaction kinetics withthe synthetic mixtures.

If a practical solution to such troubles cannot be provided in time andincluded in the proposed industrial process design, the project will be“killed” sooner or later, at least in its initial form. Therefore, it is important



to discover these problems as early as possible by using representative samplesof the actual raw materials in the feasibility demonstration tests.

Unfortunately, such representative samples cannot always be procuredat the start. This difficult situation was typically encountered when the newprocess involved the treatment of mineral concentrates from new depositsthat, as of yet, have not been fully explored, or the down-stream treatmentof some material that was expected from certain “future” operations.

5.3.8 Analytical difficulties

In some situations, the available analytical laboratory personnel may nothave previous experience with the exact type of analyses required for thesefeasibility tests and they will have to learn, introduce, and calibrate newmethods. This can be a lengthy procedure, and the time needed can possiblybe reduced with outside help. The allocation of priority in this area, or theneed to compromise on a “second-best” method, had often caused delaysand personal tension.

5.4 Analysis of the results from feasibility testsWhen these results become available, it is advisable that the promoting teamprepares and presents two separate reports, which will be studied and dis-cussed in different contexts and review meetings (sometimes again manyyears later).

1. A report on the results of the feasibility tests, which should bepresented in the normal format for R&D experimental reports witha complete description of all the laboratory procedures and equip-ment, all data collected, and, in particular, any observation aboutunusual features.

2. Another report discussing the significance of the experimental resultsand observations from these tests, as regards the process feasibilitydemonstration and the design issues of the proposed novel process.This report may also include further chemical engineering calcula-tions or economic evaluations.

The discussion in this report should also define exactly a limited rangefor each of the variables to be covered in any future experimental programas basis for the process design.

5.5 Second review of the process definitionThe forum of the second critical review is generally reconvened by thedecision-maker along with participating colleagues (“peers”) and the called-in experts to study and review the different reports that were distributed inadvance and in writing. The reports include:



• Feasibility demonstration tests• Patent discussions• Clarification of the critical economic factors

This second review can end up in one of the following ways:

A. In most cases, the analysis and discussion of these reports wouldgive the “green light” for proceeding with the experimental program(see Chapter 6), the preliminary process design (see Chapter 7), and theeconomic analysis (see Chapter 8). If this program is agreed upon, thiswould be the appropriate time to formalize a contract transmittingthe implementation rights from the promoters to the corporation. Thecorporation would then confirm the appointment of a project managerto carry on the responsibility of the future program. This managerhas most probably already been a member of the process evaluationteam up to this point.

B. In other cases, the results and calculations could indicate a need tocorrect or readjust some of the initial process working definitions. Thefile would be given back to the promoters for the repeat of certaintests, additional reports, and back for another similar review.

C. In certain cases, the continuation of the project could not be logicallyjustified in the present framework, and it would be terminated at thispoint, at least until the promoters’ prayers for some surprising de-velopment are granted.

5.6 Worth another thought

• An essential starting point is a preliminary process definition to bringeverybody concerned to an explicit common reference basis, illustratea concrete venture, outline a proper experimental program and startits detailed design while focusing on the limited scope of applicationthat is of interest in real life, and allowing for a more effective allo-cation of the available industrial R&D resources. This process work-ing definition will be based, in a large part at the beginning, onweighted and explicit assumptions and on previous professional ex-perience, but it will be progressively changed, enlarged, and refined.However, such a working method has often been resisted and evenridiculed by senior scientists who were used to the open-ended ac-ademic research approach.

• A novel process could be brilliant and sophisticated, but if its finalproduct would have only a small market potential, the straight pros-pects of approving funds for its development will be dim. Such aproposal will then be generally presented as a strategic investment,opening the way to new potential products.



• The feasibility tests should convince the decision-makers and dem-onstrate that the results from the novel aspects in each stage of theprocess can be achieved more or less as expected to justify a moreextensive experimental program.

References1. Biegler, L.T., Grossman, I.E., and Westerberg, A.W., Systematic Methods of

Chemical Process Design, Prentice Hall, New York, 1997.2. Douglas, J.M., Conceptual Design of Chemical Processes, McGraw-Hill, New

York, 1988.3. Duncan, T.M. and Reimer, J.A., Chemical Engineering Design and Analysis:

Introduction, Cambridge Press, London, 1999.4. Seider, W.D., Lewin, D.R., and Seader, J.D., Process Design Principles: Synthesis,

Analysis, and Evaluation, John Wiley & Sons, New York, 1999.5. McCabe, W.L., Smith, J.C., and Harriot, P., Unit Operations in Chemical Engi-

neering, 5th ed., McGraw-Hill, New York, 1993.6. Hicks, T.G., Ed., Standard Handbook of Engineering Calculations, Section 6.

Davidson, R.L., Chemical Engineering, McGraw-Hill, New York, 1972.7. Clarke, L. and Davidson, R.L., Manual for Process Engineering Calculations,

McGraw-Hill, New York, 1975.8. Branan, C.R., Rules of Thumb for Chemical Engineers, 2nd ed., Gulf Publishing

Co., 1998.9. Meyers, R.A., Handbook of Petroleum Refining Processes, 2nd ed., McGraw-Hill,

New York, 1996.10. Perry, R.H. et al., Chemical Engineers’ Handbook, various editions, McGraw-

Hill, New York, 1999.11. Froment, G.F. and Bishoff, K.B., Chemical Reactor Analysis and Design, 2nd ed.,

John Wiley & Sons, New York, 1990.12. Smith, J., Chemical Engineering Kinetics, 3rd ed., McGraw-Hill, New York, 1990.13. Schmidt, L.D., The Engineering of Chemical Reactions, Oxford University Press,

Oxford, 1998.14. Fogler, H.S., Elements of Chemical Reaction Engineering, 3rd ed., Prentice Hall,

New York, 1998.15. Levenspiel, O., Chemical Reaction Engineering, 3rd ed., John Wiley & Sons, New

York, 1998.16. Butt, J.B., Reaction Kinetics and Reactor Design, 2nd ed., Marcel Dekker, New

York, 1999.17. Honig, J.M., Thermodynamics Principles Characterizing Physical and Chemical

Processes, 2nd ed., Academic Press, New York, 1990.18. Klotz, I.M. and Rosenberg, R.M., Chemical Thermodynamics: Basic Theory and

Methods, 6th ed., John Wiley & Sons, New York, 2000.19. Coulson, J.M. and Richardson, J.F., Chemical Engineering Fluid Flow, Heat Trans-

fer, and Mass Transfer, different editions, last 6th eds., Butterworth-Heineman,Oxford, 1999.

20. Rohsenow, W.M. et al., Handbook of Heat Transfer, 3rd ed., McGraw-Hill, NewYork, 1997.

21. Purchas, D.B., Ed., Solid/Liquid Separation Equipment Scale Up, Upland Press,Croydon, U.K., 1977.



22. Mizrahi, J., New process producing phosphoric acid from phosphate rockand hydrochloric acid – via ferric phosphate, paper presented at the Int.Solvent Extraction Conference, Barcelona, July 1999. Also Israel Patent120,963, June 1997.



chapter 6

Experimental program

6.1 Basis

6.1.1 Experimental program purposes

Main Purpose —

At this stage of the process development, the main purposeof the experimental program is the collection, the correlation, and the pre-sentation of the

design data

that is specifically needed for the design andoptimization of the new process, as defined and in the

limited range of variablesof practical interest

.It is important to note that the scope of the investigation can depend

also on the

variability

of the particular function under consideration. It canbe found, after the first series of tests, that the already mentioned range ofspecific interest happens to be located in one part of the function wheresharp changes can be seen from a first plotting of all available information.In such case, it is advisable to enlarge the scope of investigation in order toassure a reasonable reliability when interpolating between experimentalpoints. The experimental program, therefore, is formulated in relation to theperceived needs in one particular situation.

Unexpected Problems —

Another important purpose includes the

obser-vations

of possible, but unexpected problems, that can occur and that shouldbe dealt with. These can be, for example, difficulties in the separationbetween phases, slow rates of reaction/mass transfer, colloidal precipitates,unwanted color, etc. (see below). The experimental staff should be instructedto observe carefully and to call their supervisor whenever anything seemsunusual. The “top” R&D managers are often seen circulating between thebenches when such tests are done to obtain a personal “appreciation” of thebehavior of the reacting or separating mixtures.

Preparation —

In addition, the preparation of relatively large represen-tative

samples

of certain products or of certain intermediate phases are oftenneeded for further specialized tests, for market surveys, or just to “showaround” in the promotion contacts for the project. Therefore, generally allthe materials resulting from these tests should be well packaged, labeled,and stored for future use.



6.1.2 Different sections

The separation of the process into the different sections was already rep-resented in the

process “map”

(

block diagram

), with the formal

definition

ofthe prevailing chemical mechanism, of the separation between phases, andof the process results expected in each separate section (see Chapter 5,Section 5.1).

Before starting the experimental program, these definitions should bediscussed systematically,

well understood, and agreed

upon among the inven-tors, the process engineers, and the senior experimental staff. The calcula-tion methods that will be requested for

the process design of each operation

will be formulated and agreed upon by the whole process engineeringteam. For this purpose, use the manuals and textbooks listed as referencesin Chapter 5, as well as for those concerned with the separation processeslisted in this chapter.

7–10

Note that many of the operations in any chemical plant can possibly bedesigned on the basis of

conventional

know-how, with the specific input ofonly a few specific physical properties; for instance, all the sections con-cerned with material handling, liquid flows, blending, packaging, spraying,gas compressing, steamboiler, cooling tower, etc. Thus, the experimentalprogram will not be concerned with such operations at this early stage ofprocess development.

6.1.3 Quantitative data needed for process design

Guided by the definitions, the process engineers also should prepare a listof

all the quantitative data

that will be requested for the process design of eachoperation. Some parts of this design data may be already available in thefiles from the previous analysis of the results of the feasibility tests, from thepromoters’ own sources, or from earlier publications in related fields. Thesystematic organization of what is available will allow the delimitation ofthe missing data that should be generated in the experimental programpresently considered.

Discontinuities are often found between the sets of data obtained fromdifferent sources, as it may be seen when these are plotted on a commongraph, due to the differences in experimental or analytical techniques. There-fore, in this analysis and determination, it is preferable to allow for significantoverlapping to arrive at a reasonably reliable common function.

(

In the author’s considered opinion, there is not much point in designing andstarting any significant experimental program without performing first this processengineering analysis

.)

6.1.4 Format

At the same occasion, it would be useful if the process engineering teamcan specify exactly the preferred format for the results on these data to beused in the experimental reports to allow their direct application in the



process calculations. As there can be many parameters in each stage, the

primary variables

should be indicated in the order used in the calculations(see Section 6.2.1).

This

early

specification

of the format can avoid or reduce the communi-cation problems and the waste of time devoted to clarifications and recal-culations, which often happens between the process engineering team andthe experimental group. In many cases, these two units can also be situatedin geographical locations far apart and may not be able to meet frequentlyface-to-face.

It would also be useful to decide as far in advance as possible thepreferred

order

for the generation and transmittal of

partial data

to theprocess engineering team in

separate, successive, numbered reports

. Thisdemand can appear to be trivial, but this has been, in fact, a sore point inmany projects. If certain parts of the process design work can be startedand advanced before all the data is transmitted in one big bound report,some of the pressure can be relieved from a

serious bottleneck

in processdesign. It generally happens that as soon as the final experimental reportis issued, everybody wants to know all its implications on the new process,the plant under consideration, the economic parameters, etc. On the otherhand, the preparation of serious answers takes time and experienced pro-cess engineers are scarce.

6.1.5 Representative raw materials

As far as possible, these R&D tests should use

representative samples

of theactual raw materials, fuel, water, reagents and additives, filter aid, activecarbon, and IX resins that would be expected to be used in the final plant.As discussed previously, a synthetic mixture of pure laboratory chemicalsfrom the bottles on the shelf cannot duplicate in many cases the exactcomplex physical structure and/or the large number of impurities in theseraw materials. Similarly, whenever it is intended to use combustion gasesin direct contact with the process streams (e.g., in a calciner or a dryer),the exact composition of the fuel can be significant. Such combustion gasescan contain

ash particles

or

gaseous impurities

that can contaminate theproducts or would need to be treated in the waste streams or can accumu-late in the plant.

It has often been found in real cases that the use of

certain raw materials

in such tests did result in

serious, nonexpected problems

. For example, insolvent extraction processes, some impurities can precipitate as fine solidscausing the liquid–liquid mixture to emulsify and, thus, preventing thenormal operation of the process. In one particular case, the raw materialcontained an impurity with oxidizing power, which attacked anddestroyed the organic extracting reagent used. In hydrometallurgy andsalt processes, the precipitation of solids that stick or build on the wallsof the equipment and pipes can stop a plant. Some natural streams can,when heated, release some dissolved noncondensable gases, which may



disturb the vapor–liquid equilibrium, and/or (at least) need to be collectedand vented properly. Other problems can be the precipitation of colloidsuspensions, coloring phenomena, etc. Such troubles may be seriousenough to “kill” sooner or later the proposed industrial process at leastin its original form, so it is very important to discover and diagnose themas early as possible. Possible solutions can include pretreatments or evenchanging the source of the problematic raw material.

Unfortunately, it often happens that, despite all reasonable efforts, rep-resentative samples of all the actual raw materials cannot always be readilyprocured from the beginning of the experimental program.

First Situation —

This difficult situation has been typically found, forinstance, when a new mineral deposit was being explored and small samplesof the expected raw materials can only be separated and reconstituted fromsmall drilling rig cores in quantities hardly enough for the analytical andpreliminary bench tests.

Second Situation —

Another typical case has been the development anddemonstration of a proposed process, which was intended to handle aneffluent stream expected from a “future” operation that was still at the designor construction stage.

Third Situation —

This situation has been also encountered now andagain in the development of large-scale biotechnology processes, which typ-ically consist of two separate parts:

1. The

fermentation

section, which is producing a broth containing avaluable product (e.g., a carboxylic acid)

2. The

recovery

section, intended for the separation of such valuableproduct from the broth into a pure, concentrated, marketable form

These two sections are generally developed and even designed sepa-rately by two specialized groups, and they are often built in different plots“across the road.” Obviously, all the characteristics of the new recoveryprocess are derived from the exact composition of the

expected fermentationbroth

, as defined at the time of the project justification. But it often happenedthat while the “recovery” group was developing, designing, and buildingtheir processing unit, the “fermentation” group was continuing their effortsto improve their part. They would aim at a better

productivity

(which is theaverage production

rate per unit volume

of fermentor) and/or a better yieldon the raw materials. This can be quite natural from their point of view,but the resulting changes in the broth composition (mostly with regard tothe associated impurities) can have serious (negative) effects on the recov-ery process, as developed.

A mutual

understanding and coordination

between these two groupsshould be obvious, but often can be delicate in real life and may have to beimposed from above. To be fair, the fermentation group is not alwaysinformed of the downstream development (“What do these biologists knowabout our separation processes?”). But, at least in one case known to the



author, when the fermentation group was informed that one of the impuritiesgenerated by the microorganism constituted a very serious separation prob-lem, they found a genetic “trick” to prevent this particular impurity andreplace it with a less problematic one.

In such cases, where

representative samples

of the actual raw materialscannot always be readily procured from the beginning, an experimental pro-gram on “synthetic” mixtures can only be done as an

exploratory work

aimedat the

preliminary

process design. The results need to be clearly marked assuch, and this situation reflected in the safety factors included in the economicanalysis.

Repeated tests

should be scheduled for later, in the exact selectedconditions, as soon as the actual representative samples can be obtained.

6.1.6 Classification of missing data

The data missing at the beginning of the experimental program can bedivided into three main categories according to the testing techniques thatwill be required in obtaining the results, as discussed below, and the designmethods for:

• Operations based on

chemical equilibrium

data, (as reviewed in thecomprehensive book by Henley and Seader

2

• Operations based on

dynamic flow conditions

• Operations that are

scale-dependant

, i.e., the results depend on the

absolute size

of such equipment

6.2 Chemical equilibrium data

(See basic reference books on separation processes and, in particular, oneson the equilibrium stages, which are the most useful tools in new processdevelopment.

1–7

)

6.2.1 Vapor–liquid equilibrium system

Vapor–liquid

(reversible) equilibrium systems are used in unit operations,such as distillation, rectification and stripping, evaporation, and condensa-tion. (Note that

gas–liquid

equilibrium systems, which are relevant in unitoperations dealing with scrubbing, cooling, etc. of a gaseous stream, havesome similarity. However, these will be discussed separately, in the nextsection.) Data needed for process design are obtained by correlating the

compositions of both phases

at equilibrium in certain conditions of temperature,absolute total pressure, and the partial pressure of noncondensable gases(inert, nonreactive) that may be present (assuming that such partial pressuredoes not exceed 70 to 80% of the total).

The pair of

compositions for both phases

can be obtained from a

total reflux

test, where the vapors from a boiling liquid phase are totally condensed atthe same absolute pressure and all the condensed liquid is returned to the



boiling liquid. As equilibrium is established, samples are withdrawn fromboth the boiling liquid and the refluxed condensed liquid, and completelyanalyzed for all components. If there are

only two

components, the plottingof the results is straightforward.

But since, in most cases, there are

more than two components

present ineach phase, one has to decide from the start

which two components are thevariables under study

and which other components are to be considered asparameters for the purpose of the present process design, together withthe obvious physical

parameters

, such as the temperature, absolute pres-sure, and partial pressure of noncondensable gases. All the parametershave to be kept constant in each series of tests, to obtain a

cross section fortwo variables

.One may see that the experimental program for a typical system with

four to six components can become very complex unless one limits from thebeginning the

ranges of practical interest

(see Chapter 5, Section 5.1). Once thischosen range is covered with a limited number of experimental “points,”the numerical results can be interpolated quite safely into a mathematicalfunction by using one of the published theoretical correlation formulas. Thecorrelated function then can be used for the process calculations of the

multiple stages

equilibrium system in one of its forms: countercurrent, cocur-rent, or crosscurrent.

The process design can be done using the “theoretical stages” concept,and then translated into an equipment design, by relying on the correlationlinking the “height of theoretical unit” with the operating conditions andthe details of the chosen equipment. Such correlation has been published forseveral basic designs or may be obtained from the suppliers of more spe-cialized equipment.

When there are many condensable components from the beginning (asin petroleum processing, as an extreme case), one may have to “cut” themixed feed by a coarse separation into two or three ranges (“heavies” or“lights”) and to treat

each range as a separate problem

with recycles at thestarting point.

A special case is the concentration of a solution containing nonvolatilesolutes by evaporation of water (or another solvent), leaving the nonvolatilesolutes in the concentrated solution. The vapor phase contains only onecomponent, but the concentrations of the solutes into the liquid phaseincrease gradually, decreasing the partial pressure of the water. In such case,the important data are the quantitative link between the absolute pressureand the boiling temperature of the solution and the concentrations of thesolutes below their saturation limit. These data are essential, for example,for starting the design of energy-efficient,

multiple-effect

evaporators, whichare a critical element in many processes (e.g., salts and sugars).

An equally important result of such tests can be any

observation

aboutthe precipitation of certain solids from the liquid, and the form and behav-ior of such solids, in particular as to their incrustation inside equipmentand pipes, or on heat exchangers surfaces (for their composition, see



Section 6.2.3); as well as the conditions of the release of any noncondens-able gases dissolved in the feed solution.

Another important field of process development is concerned with theseparation between two volatile components that cannot be obtained directlydue to the presence of an azeotrope or another particular feature of theequilibrium curve. (As a reminder, at the azeotropic point, the compositionsof the liquid and of the vapors are identical.)

A well-known case is the system HCl-water (already mentioned inChapter 4, Section 4.3.2) which is dominated by an azeotrope at 20 to22% HCl (the exact number depends on the absolute pressure). Every tonof HCl generated “below the azeotrope” is accompanied by at least 4 to5 tons of water at its maximum practical concentration and this featuremay prevent or limit its use in other processes. “Breaking the azeotrope”means obtaining a more concentrated solution that can be handled atambient temperature (say about 30 to 40% HCl) or even a 100% dry HClgas, if needed.

Such a result is possible by using a cycle of CaCl

2

brine in a close cycle,as the brine absorbs the water and releases the HCl, but this is a compli-cated process with many reflux streams. It is also an expensive process,both in the investment in the HCl-resistant equipment and in the energyconsumption. Another commonly found complication can be the presenceof nonvolatile components in the starting HCl solution, which can accu-mulate in the circulating brine. In such case, the starting solution shouldbe completely evaporated upstream and the heat loads should be redis-tributed. This problem was at the time an open field for creative processdesign, aiming at a better use of the energy and the expensive heat exchang-ers, and of any possible synergetic utilization of sources of low-temperaturewaste heat.

11–14

Different aqueous solutions were used, including MgCl

2

and LiCl. It was also proposed to replace the expensive heat exchangersby direct contact heating with organic “heat carriers.” (See below and alsoChapter 5, Section 5.1.5.)

Direct contact heating technology, with organic “heat carriers” (stable hydro-carbons, liquid, or vapors)

— Certain processes need large heat exchangersmade from expensive materials (resistant to corrosion, such as graphite,glass-lined, tantalum) to introduce heat into the process streams and evap-orate certain components, and similarly for removing heat in condensers. Inother cases, heating of such solutions in a regular heat exchanger wouldprecipitate solids and cause the rapid scaling of such heat exchangers.

One can resort to introducing very hot organic liquid or vapor “heat-carrier” in direct contact with the process stream to be heated. After heattransfer and equilibration, the organic liquid is separated, removed, washed,and reheated in a separate boiler made of cheaper materials. Although theheat carrier material would have a boiling point much higher than any ofthe components present, it can have a definite vapor pressure in the hotterparts of the equipment, which should be taken into account and includedin the experimental program.



6.2.2 Gas–liquid equilibrium system

Gas–liquid (reversible) equilibrium systems are relevant in unit operationsdealing with cleaning or cooling of a gaseous stream in contact with a liquidphase. In such operations, the concentrations of certain components thatexist in both the liquid and the gas phase are related by a definite reversibleequilibrium function, for a definite set of parameters.

For example, water and ammonia can be both in a gas stream and in anaqueous solution (or water and HCl, or water and methanol, and the like).The ammonia can be recovered from the gas stream into an aqueous solutionin a packed column where the gas stream (say with a few percents of ammo-nia) will be introduced from the bottom and will flow upwards (exiting with,say, 0.02% ammonia), while water is introduced at the top and flows down-wards, countercurrently. The liquid also may contain nonvolatile solute com-ponents, and the greater part of the gas stream would consist of “inert”noncondensable gases.

In principle, such a system can be studied as a vapor–liquid equilib-rium system, and a certain number of theoretical equilibrium stages canbe defined to obtain a certain result. But there is a

quantitative difference

compared to a regular vapor–liquid equilibrium system. The

kinetics ofreaching such an equilibrium

are much slower, as they depend on the dif-fusion of the relatively few condensable molecules in the bulk of the gasphase until they reach the liquid interface, and possibly also on the “resis-tance” to mass transfer of the layer adjacent to such interface. So, the

contact conditions

are often

more important

than the theoretical equilibriumand the height of a theoretical equilibrium stage must be

determined exper-imentally

for each of the exact sets of operating conditions and for theexact geometry of the packing in the column. It becomes a completelyempirical design and the position of the equilibrium curve has, in fact,little practical importance.

6.2.3 Liquid–liquid equilibrium system

Similarly, a liquid–liquid reversible equilibrium system can govern a solventextraction process, which is intended to separate, concentrate, or purify aparticular component from a mixed solution, such as a fermentation broth,a leaching solution from a mineral acid reaction, or a waste stream fromother operations.

One can refer to the basic reference books

15–18

and note that the “official”denomination is “liquid–liquid extraction,” but most people in the field keepcalling them “solvent extraction processes.”

Generally, one is considering two liquid phases, but there also existsinvariant systems with

three liquid phases

at equilibrium, according to Gibbs’“phases law.” At least one of these systems was used in the IMI “cleaning”process for separation of clean phosphoric acid from wet phosphoric acid(see Chapter 4, Section 4.4.3, Reference 26).



Again, in almost all processes of

practical

importance, there are manycomponents in each phase. One has to define all these components and theirranges of concentration and to choose, on one hand, the

variable of majorinterest

and on the other hand, the ranges of parameters (such as the con-centrations or ratios of the other constituents and the physical conditions)that can be covered in a

reasonable

experimental program. If one is not carefulin his choice, the number of tests required can easily shoot up exponentially.

The “distribution coefficient” of this major variable can be correlatedand used for multiple stage process calculations in the defined ranges ofparameters. The major difference from the vapor–liquid

physical

equilibriumsystems is that in most liquid–liquid extraction processes, the major variableof interest in any particular process can be either an

ionic

or a

molecular

entity,according to the

chemical

extraction mechanism.Once this procedure is well understood, the bench-scale experimental

program for the development of a separation process based on solvent extrac-tion can be relatively straightforward. The technique of the so-called “separa-tion funnels” tests is based on equilibration in defined conditions, samplingand analyses, and it can be carried out routinely by laboratory technicians.This fact of life was probably one of the main reasons for the successfuldevelopment of many dozens of new solvent extraction processes in the years1960 through 1980 in various countries. Very promising R&D programs in thisfield are continuing nowadays inside some of the large industrial corporations,although not much is published about that at international conferences.

In this connection, it is important to stress the experimental techniquecalled “limiting conditions,” which makes it easier to study the effect of onevariable at a time. If, for example, 50 ml of a starting aqueous solution aremixed with 50 ml of a solvent phase, the concentrations of the componentof interest after equilibration will probably change significantly in bothphases. If, for example, a series of tests are done at different temperatures,the quantitative results can be “all over the place” and a lot of tests will berequired to find a working hypothesis to explain the results. But if 100 mlof the starting aqueous solution are mixed with 1 ml of solvent, the chosencomposition of the aqueous phase will change very slightly, while that ofthe small organic phase can change very significantly. Thus, three to fourtests at different temperatures should give a clear indication of the effect ofthat variable for the particular chosen aqueous composition.

The experimental procedure is also simpler for processes in which thesolvent added is composed of a single component, such as butanol, pentanol,methyl iso butyl ketone (MIBK), and so on. But, for other processes, thecomposition of the solvent phase added can be quite complex by itself andmay present a large number of additional components and parameters, suchas the nature of the extractant (i.e., one particular tertiary amine from thedozens of tertiary amines commercially available), of the modifiers (i.e., oneof many long-chained alcohols available for such duty), and of the diluent(i.e., a light, saturated hydrocarbon), and the relative weight ratios of thesethree classes of components.



In the typical practice of solvent extraction process development, onewould generally start with a screening procedure. (

Even my granddaughterknows by now that, in real life, the Princess would have to kiss many Frogs beforeshe would find, maybe, her Prince

.) This “screening” procedure is generallystarted with one effective composition “formula” found in previous publi-cations, or in the suppliers’ recommendations. This formula may not neces-sarily be the optimal composition for the specific case studied, so that some“exploration tests” with moderate changes outside this range should be donepreferably before any specific commitment. But the prevailing attitude hasoften been: “Let’s start with the composition that works, and we will opti-mize later.” But as often happens, everybody is too busy “later” to look backat this exact composition. This is a well-known pitfall.

It has also been observed that the chemical behavior of some extractants(in particular tertiary amines) does change as the “new” reagent (straightfrom the bottle) is “aged” after a few dozens cycles of loading/regeneration.This change, which may include a significant shift in the equilibrium curve,can be due most probably to the elimination of some traces of impuritieswhich remained in the “new” reagent from its synthesis, and possibly alsoto the oxidation of unsaturated bonds in the experimental manipulations.Since the plant will be working eventually with an aged extractant, thetesting conditions and results should reflect that change from the beginning.

An additional form of aging occurs in functioning plants due to accu-mulation of certain impurities in the solvent cycle. Although a continuouspurification procedure is generally used on a side stream, there is an eco-nomic limit to such purification and any plant has to live with a certain levelof impurities in the solvent. This effect is difficult to reproduce from theseearly tests, but has to be accounted for in the design safety factors.

6.2.4 Solid–liquid equilibrium system

Solid–liquid

reversible

equilibrium data are regulating many processes. Allthe metallurgical transformations relate, in fact, to this field, but the hightemperatures of more than 1000ºC are considered as a “far away situation”by most chemists and chemical engineers. Some chemical industries aretouching these high temperatures, but most remain below 200ºC and, in thisrange, these systems relate to solid dissolution, precipitation, and crystalli-zation, which are widely used in the various inorganic industries, mineraltreatments, and also in the natural sugar and sweetener industries.

In all these processes, the

saturation concentration of one variable component

depends also on the concentration of the other components present, in additionto the physical parameters of temperature, absolute pressure, and non-con-densable gas. The experimental determinations of such saturation concentra-tion can be rather simple, in principle, if only one component is precipitatingor dissolving, while the other components are remaining as parameters eachin its respective phase. (Note that the “limiting conditions” technique men-tioned above should also be used in such experimental determinations).



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