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VERSION 1 - Standardization of Structured Packing Efficiency Measurements 14 March 2008

A Report on

STANDARDIZATION OF STRUCTURED PACKINGEFFICIENCY MEASUREMENTS

A Work in ProgressVersion 1

by

arko OlujiZO Consulting

Associate Professor

Delft University of TechnologyProcess & Energy Department

Leeghwaterstraat 442628 CA Delft, The Netherlands

Encouraged and Supported

by

BASF SE

Delft, 14 March 2008

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CONTENTS

FOREWORD 5

SUMMARY OF THE PROJECT 6

1. INTRODUCTION 8

2. PURPOSE AND AVAILABILITY OF DISTILLATION PILOT PLANTS 9

2.1 Packing Manufacturers Facilities 102.1.1 Sulzer-Chemtech2.1.2 Koch-Glitsch

2.2 FRI 11

2.3 University Based Facilities 122.3.1 SRP2.3.2 TU Delft2.3.3 University of Alberta

2.4 User Companies Pilot Plants 142.4.1 Bayer Technology Services2.4.2 Praxair

2.5 Plant Test Columns 142.5.1 Koch-Glitsch2.5.2 Air Products

3. PILOT PLANT DESIGN AND OPERATION CONSIDERATIONS 15

3.1 Column Diameter 15

3.2 Bed Height/Depth 15

3.3 Packing Installation 163.4 Liquid Distribution 163.4.1 Drip point density3.4.2 Drip point layout3.4.3 Distributor positioning

3.5 Vapour Distribution 18

3.6 Test System and Conditions 183.6.1 Test system properties estimation

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3.7 Liquid Sampling 203.7.1 Column profiles

3.8 Measurement of Flow Rates, Pressure, and Temperature 21

3.9 Pressure Drop Measurement 22

3.10 Reproducibility 22

3.11 Mode of Operation 23

3.12 Avoiding Systematic Errors 24

3.13 Process and Mechanical Design Considerations 25

4. TEST DATA EVALUATION, PRESENTATION AND INTERPRETATION 25

4.1 Representative F-factor 28

4.2 Representative Relative Volatility 29

4.3 Representative Efficiency 30

4.4 Representative Pressure Drop 32

4.5 Packing Geometry Effects 33

4.6 Test Documentation 34

5. CONCLUSIONS AND RECOMMENDATIONS 34

NOMENCLATURE 36

REFERENCES 37

APPENDICES

Appendix A Physical Properties of Common Test Systems 42

A1 Orto- / Para-Xylene 42Table A1 FRI data

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FOREWORD

In what follows you will find a document that summarizes the results of aliterature study that was encouraged and supported by BASF SE. The objective wasto collect and evaluate relevant, publicly available information, and to come with aproposal for standardization of structured packing efficiency measurements, which, inturn, should lead toward obtaining more reliable basic performance data to be usedwith more confidence in (re)design of industrial packed columns.

The present study is made publicly available by BASF SE to serve as a basisfor further improvements in all respects as a sort of open source document. The

home will be the website of Process & Energy Laboratory of TU Delft(www.pe.tudelft.nl) and the coordination will be in the hands of the author of thisdocument.

Upon critical evaluation of the present text, the potential contributors areencouraged to come with comments, discussions, and suggestions forimprovements, which should enable us to arrive at best practices. However, only theproductive proposals will be implemented, and this will occur upon approval ofinvolved contributors. This will be posted as new version and exposed to criticalevaluations, in form of an open forum. However, coordinator will try to ensureconvergence within a few iterations, i.e. achieving the best solution, to be acceptedand considered to serve as a general standard in the near future.

It is our hope that competent people from user companies, equipmentmanufacturing companies as well as academic environment will be willing andallowed to participate actively in this undertaking that should finally lead to adoptingthe best practices as a standard for experimental characterization of structuredpackings performance.

This could then serve as basis for establishing basic standards for testing alltypes of gas or vapour/liquid contacting devices used in separation processes.

Delft, 12 March 2008 Z. Olujic

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http://www.pe.tudelft.nl/http://www.pe.tudelft.nl/


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available for installation of packing, 4 m being a good measure. Certainly, a testsection of 7 m, consisting of two flanged sections, would provide a great degree offlexibility. For instance, it would allow installing two beds of 3 m, separated by a liquidredistribution section, which also provides an opportunity for performing continuous,

finite reflux operation tests.The liquid distributor should be a high turndown, high free area (around 50 %)narrow trough distributor with uniformly distributed drip pipes, providing an initialirrigation density of 100 to 150 drip points per meter square. If the operating pressurerange of the column is too wide, than two distributors of the same design should beemployed, one for lower and the other one for higher operating pressures, i.e. liquidloads. Narrow trough distributors are industrial standard and that one or twodesigned specially for a pilot plant should be used in all tests. In other words, adistributor should not be changed, except if one wishes to observe possibledistributor design effect.

Vapour distribution is not critical, however this means not that any solution will

work effectively. Care should be paid and vapour inlet and distribution device shouldbe designed properly, in accordance with the chosen type of the reboiler. The bestprovision in this respect is to have enough space between the gas (vapour) inlet andpacking support to install a chevron type liquid collector/gas redistributor, at adistance in both directions equivalent to one column diameter, which, in addition canbe used to provide liquid sample representing quite well the composition of the liquidleaving the bed.

Continuous (non-stop) operation under total reflux conditions is the bestoperating mode. The time periods required to get to steady state after column start-up, change of vapour load, and for taking three samples at each vapour load dependon the size of pilot plant and need to be established during the start-up and initialoperation period. Then also the decision will be made whether to go from maximumto minimum vapour load or vice-versa. Middle of the bed conditions arerecommended as reference for presenting tests results and making comparisonsbetween performances of different packings and operating conditions.

The effort behind this study was motivated by the need to arrive at a standardfor total reflux distillation installation and procedure for the purposes of comparativetesting of structured packing performances, which would enable obtaining results thatcould be used with confidence for scale-up purposes. Elaborate analysis provided inthis report and thorough evaluations of all relevant factors provide a good indicationof best choices and practices. The experience gained by building and operating a

pilot-plant along the lines outlined in this report would serve as best foundation forreaching the final goal, i.e. a standard test.

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1. INTRODUCTION

There is no doubt that distillation was and still is by far the most widely appliedseparation technique in process industries. Regarding the ever growing needs forbulk-chemicals and the extent of grow of process industries in Far East it seems thatits importance will even increase in the foreseeable future. A major consequence ofthis strong grows is that larger and larger equipment is built to stay competitive,which implies further challenges regarding the scale-up and design practices. Theamounts of capital and energy involved require as tight as possible column designsand this posses challenges on scale-up know-how side, which, in turn relies mainlyon efficiency and hydraulics data obtained from total reflux experiments carried outwith much smaller equipment using one of standard test systems, often at conditionsdifferent from that of intended application.

Although a lot of experimental work has been done in the past and wellestablished large companies have also collected addition practical knowledge fromoperation of their plants, the design of new as well as redesign of existing columnsstill encompasses a wide range of concerns. These are mainly related touncertainties with respect to expected hydraulic and mass transfer performance ofnovel, performance improving gas/vapour-liquid contacting devices, developed andbrought to the market in the meantime by equipment manufacturers.

All three main families of vapour (gas)-liquid (G-L) contactors for distillationand related separations, i.e. trays, random packing, and structured packings haveexperienced significant improvements in this respect. The main stream of suchdevelopments is concerned with capacity increase (revamps-retrofits) or sizereduction (new designs) of distillation columns, accompanied, if feasible, by energysaving per unit of product (reduction of external reflux), but most of thesedevelopments aim at reaching the capacity increase goal without affecting theseparation efficiency adversely. The certainty regarding the hydraulic and masstransfer performance of the G-L contactor considered plays a crucial role for successof such a total cost reduction effort. In order to get this certainty, i.e. to minimize risksassociated with building very expensive industrial plants, user companies tend tohave dedicated pilot plant scale tests performed, which may be used tovalidate/verify the predictive models.

The emphasis of this study is on the performance characteristics of structured

packings, more precisely on clearing the issues associated with usability of totalreflux measurements, carried out with established test systems in pilot to semi-industrial scale test installations, for (re)design purposes. In this respect the reportfollows the line of a presentation given by T. Friese of BASF, at GVC-FachausschussFluidverfahrenstechniek meeting held in February 2005 in Luebeck, Germany [1].This presentation moved a group of distillation user companies to considerestablishing standardized testing procedures as a measure to arrive at moreconfidence in this respect, and this expectation has been considered and handledaccordingly in this report.

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2. PURPOSE ANDAVAILABILITY OF PILOT DISTILLATION TESTS

Packing hydraulics related experiments are usually carried out using air-water

system at ambient conditions, to observe the suitability of the packing with respect tothe liquid hold-up, pressure drop and capacity. Promising configurations are thenexposed to distillation experiments, most commonly under total reflux conditions, toget impression on achievable efficiency. In general, the main objective of pilot testingof a packing under total reflux conditions is to determine:

(i) The mass transfer efficiency of the packing;(ii) The maximum capacity of the packing; and(iii) The pressure drop of the packing as a function of vapour load (boil-up

rate).

Usually the packing performance data are generated using large enough totalreflux distillation columns in conjunction with one of established test systems,described in the booklet Recommended Test Mixtures for Distillation Columns [2,3].Namely, long ago it has been realised that some standardisation is requiredregarding the choice of the test system, and the EFCE Working Party on Distillationcame with a proposal in this respect and produced in 1960s under editorship of F.Zuiderweg the first edition of this booklet [2]. The next, thoroughly revised andupdated edition, written by Onken und Arlt , was published in 1990 [3].

There are several more or less accessible sources of total reflux data. First weshould mention Billets books [4,5]. Until the retirement of Prof. R. Billet the Universityof Bochum was the place where many conventional and novel random, and certainlyall first generation structured packings were tested. Unfortunately, the total refluxcolumn used for these purposes had an insufficient diameter (d = 0.22 m) to beconsidered as source of data to be used directly for scale-up purposes. All the detailsaround this installation, test systems used and many of generated results producedin cooperation with Dr. J. Mackowiak and later on Dr. M. Schultes are welldocumented in Billets book Packed Towers [5]. Additional information on packingstested in Bochum can be found in Mackowiaks book Fluiddynamik von Kolonnen mitmodernen Fllkrpern [6]. Billets book contains also some data on performanceof conventional random packings obtained in a column with an internal diameter of0.5 m. This one as well as a column with an even larger internal diameter (0.8 m),

available in 1960s at BASF in Ludwigshafen, were extensively used to generatescale-up know how, needed for design and/or operation of, at that time, extremelylarge columns for separation of ethylbenzene/styrene mixture. It should be notedthat in these devoted experiments, the ethylbenzene/styrene system was used atappropriate operating pressure. A detailed description of the test installations andobtained results including Sulzer gauze packing BX performance evaluation can befound in Billets book Industrial Destillation [4]. It should however be noted that 0.8m diameter was chosen for the purposes of testing tray performances appropriately,and that a 0.5 m diameter total reflux column was considered as large enoughregarding the scale-up needs for packed columns. A recent BASF based study [7],discussing the needs for even larger, industrial scale testing facilities, suggests that

operating pressure should be considered when choosing the diameter of the test

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column, indicating that for most common packed column applications (0.1 bar toatmospheric pressure operations) the diameters above 0.3 m should be sufficient.

Large scale equipment manufacturers, like presently Koch-Glitsch and SulzerChemtech prefer to relay on their own facilities, and their data are generally not

publicly available. Most prominent source of distillation equipment test data isFractionation Research Inc. (FRI), and their data are available to member companiesonly. FRI as well as some other institutions (Bayer Technology Services, BTS, andSeparations Research Program, SRP) are willing to perform commercial tests forcustomers, which implies that the customer decides whether and to whom to releasethe data, and these remain mainly unavailable to public. The chance that someperformance data will be published is largest with data produced using adequatefacilities available at universities, however these are the least consistent regardingthe nature of experimentation and available equipment and technical support. Finally,the user-companies have often adequate facilities to be used preferably for ownpurposes only. However, it must be realized that during the last two decades of last

century due to increased costs and reduced technical support, most of the bulkchemicals manufacturing companies abandoned this practice. In fact this occurredeverywhere and today there are few facilities left, which can satisfy the growingneeds in this respect. The exceptions in this respect are air distilling companies.Distilling air at cryogenic conditions to produce high purity oxygen, nitrogen andargon, respectively, is a very specific activity and a really large scale worldwidebusiness, and the competition of big players in this field is especially strong.Therefore they all prefer to use own test facilities, which, due to the nature of thesystem and cryogenic test conditions, have been held on smaller size. With the needto design the columns with larger and larger diameters for very sharp separationsthey also got confronted with the need to consider performing experiments at thescale close to their industrial operations and conditions. From these reasons, fewyears ago Air Products decided to design and build a large enough (industrial scale)test column. On the other hand, Koch-Glitsch invested into transformation of suitableformer industrial column into a test unit. However, soon after performing first series ofdedicated tests, the related expenses appeared that high that this discouragedoffering these facilities for commercial purposes and even discouraged similar effortsfor own purposes. Therefore it looks to be that present day, state of the art facilitieshave to be as small as bearable, i.e. technically responsible, to have a chance to beused frequently.

Regarding this fact, it appears that the overall quality and usability of data

produced using adequate as small as good enough - size facilities becomes aconcern. In what follows an overview is given of available total reflux test facilities.

2.1 Packing Manufacturers Facilities

2.1.1 SULZER ChemtechFrom beginning on Sulzer recognised the importance of the scale of

experimentation and most of their test data on performance of various Mellapak andMellapakPlus family packings come from a 1.0 m internal diameter column, which isoperated in conjunction with thermodynamically ideal Chlorobenzene/Ethylbenzene

system at two pressures, i.e. at 100 and 950 mbar. It should be noted that in Sulzer

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laboratory in Winterthur there are also total reflux columns with smaller diameterwhich can be operated at high pressures, as well as a number of hydraulic, air-waterinstallations which provide for collecting all range of data of interest for understandingand properly correlating the performance of new packings. The collected data have

been correlated and can be found for all Sulzer packings in the software packageSulpak, which is available on request from Sulzer Chemtech via their website. Itshould be noted that Sulpak simulates the hydraulics of Sulzer structured packingswell, but there are no provisions made for estimation of the packing efficiency withdifferent systems. The only efficiency related information contained is that onexperimentally determined number of equilibrium stages (theoretical plates)contained per unit bed height for main types and sizes of Sulzer packings asdetermined with CB/EB system using Sulzer test facilities. Occasionally, some ofefficiency data are presented at conferences [8-11], and the results obtained over theyears with Mellapak 250.Y at FRI have been published [12].

2.1.2 Koch-Glitsch (including former Norton)The Koch-Glitsch Company, which relies on know-how developed by two

companies in the time Glitsch and Koch were on their own. Both however used ownfacilities to substantiate development of own packings and perform customer tests,and this is done presently by Koch-Glitsch. Unfortunately, there is practically nopublicly available information on their experimental work. However, first generation ofKoch structured packings (Flexipack, which was in the time of association with Sulzeractually Mellapak with larger element height) have been tested at SRP [13].

Nevertheless, Koch-Glitsch is now in possession of the state of the art pilotscale total reflux distillation installations of former Norton CPP Corporation which wasa very well established manufacturer of random and structured packings anddistillation and absorption columns in general, and until recently a global distillationequipment player. Some of the comprehensive experimental work carried out withNorton packings in a column with internal diameter of 0.381 m is documented in openliterature [14], including a thorough description of pilot plant facilities and testprocedures. Norton know-how and design practices as well as experiences withapplication of Norton packings in industrial practice are discussed in [15] and in widercontext in the book Packed Tower Design and ApplicationsRandom and StructuredPackings [16], written by a pioneer in development of new generation of highperformance packings and designs of large diameter packed columns, Ralph Strigle.

Interestingly, unlike other institutions Norton used the toluene/iso-octane

mixture at atmospheric pressure and 133 mbar (100 mmHg absolute) as standardsystem, which does not belong to recommended tests systems [3], but was usedoccasionally by Billet [5].

2.2 FRIThis well known independent distillation equipment testing institution makes

dedicated performance tests in one of two 1.2 m internal diameter column, covering awide range of operating pressures, from high vacuum (down to 0.1 bar) to highpressure (up to 27 bar) using appropriate test systems. FRI member companies arewell acquinted with all technical details related to FRI installations and testprocedures. Tables A1, A3b, and A6 in Appendix A summarise average properties of

tests systems used at FRI for performance evaluation and interpretation purposes. A



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detailed description of the test facilities and procedures adopted can be found inrecent publications [12,17].

2.3 University Based Facilities2.3.1 SRPDuring the last two decades the Separations Research Program, associated

with the University of Texas at Austin developed into an established pilot-scaletesting institute, where total reflux distillation facilities are extensively used forcommercial testing. Some of these test results are available for the members of theSRP, and very limited amount of data is available publicly. Limited information onperformance characteristics of first generation of American sheet metal structuredpackings can be found in a paper discussing the accuracy of SRP predictive modelfor structured packings [13]. Most complete in this respect are recently publisheddata on performance of some conventional J. Montz structured packings [18] as well

as of some of the first generation Montz high capacity packings [19].These publications contain a detailed description of the installation used. The

heart of this installation is a 0.43 m internal diameter column, allowing bed heightsbetween 1 and 4 m, which, however can be nearly doubled by installation of anotherbed above the common one. The test system is Cyclohexane/n-Heptane, and thecolumn is operated at 0.17, 0.33, 1.03, 1.66, and 4.4 bar absolute, depending on thechoice of the customer. Table A3a in Appendix 3 contains typical bottom of the bedtemperature related physical properties and operating data. The liquid distributorpreferred is a wide range, Montz narrow trough type, with 21 drip tubes, which is anequivalent to 145 drip points per meter square.

In order to be convincing regarding the reliability and usability ofmeasurements performed in a 0.43 m ID column, SRP carried out an internal studycomparing results obtained with some conventional random and structured packingswith that obtained with the same packings at FRI at same conditions [20]. Althoughdifferent types of liquid distributors were used, there were no significant differencesobserved in compared efficiencies, except those imposed by a too large difference inoperating stripping factors. Stronger deviations in results were observed only atlowest F-factors (end effects, due to imperfection of liquid distributor used at FRI),and hydraulic behaviour was similar enough to exclude any significant difference dueto scale of experimentation. Anyhow, it should be noted that this exercise wascompleted using a lot of engineering judgement, because the differences in facilities,

methods and data were too large to provide a basis for an objective evaluation. Afterthis experience, SRP decided to standardise their equipment and procedures asmuch as possible.

It should be noted that SRP facilities, used extensively for proprietary tests,are handled and operated by an experienced and skilled group of permanent staffmembers under supervision of Dr. A. F. Seibert. Since the last complete overhaul ofthe available facilities, the tests are run on day and night basis, starting Mondaymorning and ending on Thursday evening or Friday morning, depending on thenumber of points to be measured. The samples are taken at regular intervals andduring the night this is done by trained external people (mostly UT students) hired forthis purpose.



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2.4 User Companies Pilots2.4.3 Bayer Technology Services

New technical laboratory at Bayer in Leverkusen contains two semi-industrialscale columns, one with an internal diameter of 0.8 m and the other one with 0.6 m

that are used occasionally for small scale production purposes. The smaller one waschosen to be used as packing testing column in conjunction with CB/EB system,operated at 0.1 and 1 bar, respectively. This column was used for testing a numberof prototypes of new generation of high performance Montz packings. The BTSinstallation and some results are described in a most recent publication [26].

2.4.4 PraxairA column with 0.305 m internal diameter operated at total reflux with argon

and nitrogen under cryogenic conditions was used extensively for evaluation of basicperformance of commercially available structured packings suitable for application incryogenic distillations. The packed height was typically 2.4 m. This unit was also

used to evaluate performance of new generation of high capacity structuredpackings, as introduced by researchers from this organisation [27].

It should be noted that newest generation of air distilling columns with feedrates well above 100 t/h, require large diameters (large specific area packings arepreferred to reduce column height from energy conserving reasons) and this poses achallenge regarding the uncertainties related to scale-up based on small columndiameter data. In answer to this Air Products decided to design and operate a smallindustrial scale column, described in following section.

2.5 Plant Test Columns2.5.1 Koch-Glitsch

As described in a paper by Weiland et al. [28], an existing depropanizercolumn, with an internal diameter of 1.52 m, available at Koch HydrocarbonsMedford, OK, plant has been transformed into a research column. However, thereported results concern modern trays only, and there is still no evidence that thiscolumn has been employed for structured packing testing as announced. Since thiscolumn is a part of the production plant and receives the feed from a deethanisercolumn, its application is limited to typical depropaniser feed mixture and operatingconditions. Therefore this large scale installation is not suitable for structured packingtests.

2.5.2 Air ProductsThe heart of the sophisticated plant scale installation of AIR Product at

Carrington in U.K. is a cryogenic column with internal diameter of 0.9 m and enoughheight to install long beds (up to twenty theoretical plates). Argon/Oxygen (Ar/Ox)separation tests indicated that deeper beds experience deterioration in efficiency dueto developed liquid maldistribution [29]. The essential information came from theutilised liquid sampling system; with a cross type sampler with four points locatedbellow the bed periphery and the fifth sampling point below the middle of the bed.Interestingly, a comparison with a similar test carried out in their standard 0.2 m IDcolumn indicated a significant discrepancy in efficiency, i.e. a much lower efficiency



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than that obtained with small column. Hydraulics was also affected but not sopronouncedly as efficiency.

3. PILOT PLANT DESIGN AND OPERATION CONSIDERATIONS

This section addresses factors of importance for test equipment design andoperation as well as operating procedures and measurements.

3.1 Column DiameterAs mentioned before there is a general belief that columns with diameters

above 0.3 m should bi sufficient regarding the usability of efficiency and capacity datafor structured packings. As shown elsewhere [30], early Sulzer studies indicatedabsence of diameter effect on efficiency of gauze BX and sheet metal Mellapak250.Y packings. However, the pressure drop and capacity appeared to be strongly

dependent on diameter and a recent scale-up hydraulics study [31] indicated thatcolumn diameter should be larger than two times the packing element height tominimize the adverse effect of column wall zone, due to cross section reduction bywall wipers and increased number of bends in the vapour flow, with respect to that inthe bulk of packing. This indicates that the column diameter should be about 0.40 mor even larger. For instance, 0.6 m diameter as encountered at BTS is a finemeasure. Namely, Koch-Glitsch and former Norton packings come with elementheights around 0.3 m, and in this case a diameter of 0.6 m meets the two elementheights rule.

However, the larger the equipment the longer the time to reach the steadystate and, importantly, the larger the investment and related operating costs. Thelatter suggests that 0.45 m, as encountered in Delft should be the best compromisebetween 0.3 m on lower and 0.6 m on larger side.

3.2 Bed Height or DepthUsually the bed heights involved in performance tests are lower than those

installed in practice. Regarding the tendency of liquid to maldistribute with increasingbed depth this means that the results obtained with short beds could be on optimisticside. This is certainly so in case of sharp separations. However a pilot scaleexperiment particularly that performed under total reflux conditions can not give fullcertainty in this respect. In any case doubling the common bed height would be a

good provision, particularly if additional length is provided to install a redistributor, toenable a comparison of a short bed and single long bed and a two shorter bed inseries. This kind of experimental exercise is useful and as demonstrated in Ar/Oxcryogenic distillation case may provide information on the extent of deteriorationexpected if the bed height/depth is maximised, which is often the case with retrofittingthe existing tray columns (manhole location imposed).

However doubling the bed depth should not compromise the accuracy ofexperiment. This means that a tests system needs to be chosen, which, inconjunction with maximum bed height, will not result in extreme overheads purity thatcould lead to substantial errors in composition and consequently the packingefficiency. A good measure is to have enough height to ensure 20-25 theoretical

stages in a bed, which will certainly depend on the specific geometric area of the



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packing and, in same cases, F-factor. So if a packing with 250 m2/m3 is taken asreference size and 15 theoretical stages as a basis than 6 m height reserved forpacking installation would be a good choice, in conjunction with a close boilingsystem like chlorobenzene/ethylbenzene, which allows having such a high number of

theoretical stages without compromising the above mentioned overheads purity. Ifsingle bed, packing performance comparison studies are main purpose, then apacked height of 4 m will be sufficient.

Another consideration related to the height of a bed is the mechanical strengthof the packing and the support system. With present very low material thickness(about 0.1 mm) the weight of packing elements fitting into a test column is that lowthat this is not a concern. In any case, the packing should be supported by a highlyopen flat grid and installed in layers with each element rotated 90 degrees toprevious one. This is a normal bed configuration, providing for maximum large scalemixing of both phases. According to our knowledge only Norton deviated from thispractice by rotating the subsequent layers by 60 to 70 degrees, which was most

probably done to keep the pressure drop acceptable. Namely, the surface of NortonIntalox Structured Packing was very rough, thus causing a pronounced pressure dropdue to skin friction, which had to be compensated somehow, and reducing the angleof rotation of packing layers was a practical solution to this.

3.3 Packed Bed InstallationMethod of packing the test column must be established and used every time

the column is packed. This means that first packing element needs to be placedproperly, because following ones will be placed by rotating them accordingly (90degrees is standard) with respect to previous one. In TU Delft column the firstelement is placed so that parallel channels are oriented perpendicularly to theorientation of the bayonet liquid sampler. This is done to ensure a representativesample. Other organizations prefer using cross samplers and take some otherparameter as reference point for placing the first packing element. At SRP in USA, itis common practice to take South-Nord or East-West orientations as a guide.

Another important thing to do while installing the packing is to ensure that wallwipers are turned out to fit tight to the column wall (they should scrap the wall whilepushed down the column). The smaller the column diameter the greater is thecertainty that bypassing of vapour and liquid will damage the efficiency of the packingto some extent.

3.4 Liquid DistributionThe liquid distribution quality is primary concern in this case and a thorough

consideration is needed to arrive at a satisfying distributor design. As proven inindustrial practice the narrow trough distributors appeared to be the bestconfiguration design in conjunction with structured packings. These are generallydesigned with perforations (not bottom, fouling sensible) in trough walls or in thewalls of drip tubes.

The turndown ratio of a test distributor is often maximised to cover a widerange of liquid loads associated with operations at different pressures. Indeed it isvery practical to use the same distributor for different tests. The high turndown can

be ensured only by having a high enough liquid head in combination with several



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3.5 Vapour distributionVapour distribution is not generally a problem in pilot-scale columns if the

packing is placed at least one column diameter above the vapour inlet. Hoverer, insmall diameter columns the distance between gas inlet and packing support is short

and with increasing column load the inlet gas velocity increases and if there is noeffective distributor involved the vapour stream may hit the opposite wall and bedeflected upwardly to enter the packing concentrated in one section leaving someother areas with much less flow. This strongly maldistributed inlet vapour flow will besmoothed fast, within few packing elements, however this condition results inpronounced liquid maldistribution in lowest packing elements, which, in case of shortbeds can affect efficiency adversely. Therefore it is recommended to have a workinginitial vapour distributor design, and for pilot scale columns with a two-phase feed ashroud baffle design was found to be effective in this respect. If enough space isavailable between vapour inlet and bed and only vapour is entering the column thena chimney-tray or chevron (vane)-type liquid collector/gas redistributor can be used. It

should have a free area of 40 % or more and be placed at least one diameter belowthe bed. This option is recommended.

3.6 Test System and ConditionsThis is a crucial point. Certainly the most suitable among recommend test

systems [2, 3] should be chosen. FRI prefers three systems, depending on pressurerange, orto-/para-xylene for high vacuum range (below 0.1 bar), CH/nH for lowvacuum to above atmospheric pressures (0.1 1.7 bar-a) and isobutane/n-butane forhigh pressure applications (6 27 bar). In Europe, the most common tests system isCB/EB, adopted at Sulzer and BTS, in conjunction with two operating pressures, i.e.100 (0.1 bar) and 960/1000 mbar, respectively. The characteristic properties of thesystems mentioned are summarised at a chosen test condition in correspondingtables in Appendix A.

As mentioned under Bed Height or Depth there is a correlation between thetest system and bed height, i.e. the number of equilibrium stages contained in a bed.Practically, this means that separation power of bed should be kept low enough toavoid extreme purities of the overheads, which can lead to large errors incomposition. In general, bottoms containing less than 10 mole %, and overheadscontaining more than 90 % of more volatile component should be avoided. Also, feedcomposition should be balanced to avoid depletion of one component. A 50/50 feedcomposition is natural choice, but to avoid excessive top purities with installed bed

height, some quantity of more volatile component is removed. So an initial feedcontaining around 40 mole % of the light boiling component is a good initial value,however this is something to be manipulated accordingly to ensure desired distillatepurity.

Since the primary purpose of the experimental effort is to compare theperformances of different packings, relaying on only one proven system isrecommended. Taking all factors mentioned in consideration, the most appropriatetest system for structured packings looks to be the chlorobenzene/ethylbenzene. Theonly deficiency may be the limitation with respect to achievable specific liquid load,which is well below 20 m3/m2h at atmospheric pressure. Nevertheless, higher liquidloads can be obtained with this system at atmospheric pressure if operated in

conjunction with low specific surface area, high capacity packings that can operate at



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F-factors well above 3 Pa0.5. Operation with this system at higher pressure is avoidedbecause it imposes impractical (very high) operating temperature. CH/nH system isbetter in this respect, and 4 bar operation, as adopted at SRP [18,19], allowsreaching specific liquid loads up to 50 m3/m2h at an operating temperature which is

50 degrees lower than that corresponding to the pressure required to reach the samespecific liquid load with CB/EB system (around 200 oC).In general, 0.1 bar and 1 bar pressures as applied in conjunction with CB/EB

system are standard operating pressures adopted at BTS and Sulzer, which coversmost of industrial applications of structured packings. These two operating pressuresare well established reference values. Going deeper into vacuum, say to 0.05 bar, isuseful because it approaches closer conditions associated with distillation of finechemicals, which is usually carried out using high specific geometric area, wire gauzepackings like A3 series from Montz or BX series from Sulzer. An intermediate value,say 0.3 (roughly factor 3 to both, 0.1 bar on lower and 1 bar on upper side) could beattempted, but it may prove unnecessary. While going below 0.1 bar is rather easy to

accomplish, provided enough heat transfer area is installed, above-atmosphericpressure operation with CB/EB system may prove demanding. Major concern is, asmentioned above, the rather high operating temperature. In addition, relative volatilityis then expected to drop below 1.1, where sensitivity to measurement errors maybecome substantial.

3.6.1 Test system properties estimationFor the test system chosen, various properties need to be estimated as a

function of pressure, temperature and composition in the operating range of interest.First of all, we deal here mainly with binary mixtures and average mixture propertiesare used, based on molar or weight fractions of pure components, present in top andbottoms mixtures, respectively. In other words, the characteristic mixture propertiesare determined on additive (weight or molar) basis from pure component properties,separately for top and bottom conditions. Usually, an average, either geometric orarithmetic mean is used as representative value, which can be considered asrepresenting the middle of the bed conditions.

For a test system, the pure component properties required for representationof the results of the experimental work are:

(i) Molecular weights,(ii) Vapour pressures,

(iii) Densities of vapour and liquid(iv) Enthalpies of vaporisation(v) Specific heat capacity of the liquid

For data interpretation and performance modelling studies following properties will beneeded:

(vi) Viscosities of liquid and vapour,(vii) Diffusivities of the liquid and vapour, and(viii) Surface tension.



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For instance, for an ideal tests system like CB/EB, the relative volatility isequal to the ratio of vapour pressures of pure components, which can be determinedusing Antoine equation with corresponding constants. Gas density can be estimatedwith confidence using the ideal gas law based expression. However for liquid density

and for other properties it is best to use polynomials with experimentally determinedcoefficients, covering the full range of operating conditions.The related sensitivities and uncertainties in conjunction with possible sources

of errors will be discussed later on where appropriate.

3.7 Liquid SamplingUsually liquid samples are preferred, taken above and below the bed. Reflux

return line is a natural choice for taking the representative sample of the overheadvapour/liquid composition. Certain degree of sub-cooling the reflux is desired to avoidflashing in reflux return line. The problem is how to obtain a representative sample ofthe liquid leaving the bed. Most common practices are using a cross bayonet type of

the sampler (FRI, SRP) or a simple bayonet sampler (TU Delft) placed directly underthe packing support plate. In the latter case, the half-open cut (slotted) tube is placedperpendicularly to the orientation of the packing sheets in the bottom layer ofpacking, to allow collecting the liquid from nearly all channels, which however coversonly limited part of column cross sectional area. This sample is compared with thesample taken from the line leaving the bottom of the column to get an impressionabout the extent of possible mass transfer during the fall of droplets from bed into tothe column sump (negligible in TU Delft case). If a chimney tray or chevron typeliquid collector is placed in between the bottom of the packed bed and the vapourinlet, then a side draw can be used to collect the liquid sample, which will be a goodrepresentative of the composition of liquid leaving the bed. If partial reboiler is usedand the sample is taken from the line connecting column sump with the recirculationpump, than a separation effect equivalent to one stage can be expected and thisneeds to be accounted for appropriately when determining the number of stagescontained in the packed bed.

The main concern with sampling is to get the representative sample at themoment of the measurement without taking a lot of liquid out of the column to get afresh liquid sample. This can be achieved by using sophisticated commerciallyavailable devices as it is done in Delft, which by virtue of their design enables takingthe fresh sample only. The device itself resembles a ball valve placed in a line, anddoes not incorporate any dead volume.

The critical step is transfer of the liquid sample from the samplers bottle tosmall bottles for gas chromatograph processing. Early days the cheapest way toanalyse liquid samples was using a refractometar, which implies exposure of thinliquid layer to atmosphere and the possibility for the some loss of the light componentby evaporation. A precaution was to cool the sample to room temperature or lower toavoid this, but with present gas-chromatograph of other more advanced analysingtechniques the samples do not need to be exposed to atmosphere and by properhandling the chance for errors is reduced to minimum. A practical advantage of gaschromatography is that rather small quantities of fluid sample are needed. To reducechances for analysis errors the liquid sample should be split into three separatesamples and it is always recommended to run reference samples with compositions



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matching the range of change produced during test. This should be done at frequentbasis and duplicate samples should be provided for sample and analysis verification.

3.7.1 Column profiles

As demonstrated in the last paper by Zuiderweg [32], knowing the compositionand temperature profiles of a test column could be very useful. Main reason for thiswas tracing and identification of sources of liquid maldistribution, observed duringtests with first generation structured packings at FRI. Indeed, FRI has practisedtaking the liquid samples along the bed, utilising bayonet type samplers protrudingthrough the bed, perpendicularly to the orientation of flow channels in the packinglayer. This worked satisfactorily however it inflicts damage to the packing and createspossibility for certain extent of lateral transport of liquid, i.e. maldistribution, as well asfor flow disturbances associated with local splitting and acceleration of gas flow. Ifconcentration profile information is desired, then splitting the bed into correspondingnumber of sections, separated by extended packing support structure should be

considered, because it allows practically undisturbed taking of liquid (and if desiredvapour) samples, as well as recording the corresponding temperature, and pressureif desired.

3.8 Measurements of Flow Rates, Pressure, and TemperatureCertainly, the measurements of all relevant operating parameters must be of

adequate quality and continuously recorded and saved. This includes the flow rate,temperature, pressure, and composition of reflux and bottom liquid streams. Toppressure and temperature should be measured continuously as well as thetemperature below the bed. In addition steam pressure at inlet to reboiler andtemperature and flow rate of condensate at the outlet of reboiler need to be recordedas well as the flow rate and inlet and outlet temperatures of cooling water. Theaccuracy of the state of the art devices for measurement of temperature, pressure,pressure difference, and flow rates is rather high, with deviations usually not largerthan 0.1 % of maximum or measured value. Choice, calibration and use ofthermometers, manometers and flow meters suitable for different purposes should bedone in accordance with established company practices, and will not be consideredhere.

With known conditions of external streams overall and component materialbalances and overall heat (enthalpy) balance can be calculated. An extremely usefulaid for analysing the performance of a packed bed is the temperature profile obtained

by measuring the temperature at a number of points along the bed. This could bedone by inserting the thermo-wells on top of each second or third packing element.By placing five equally spaced sensors, one in the middle and four at peripheryforming a cross, reliable average values could be obtained while the differencesbetween individual readings, if pronounced, could be a good indication of liquid andvapour distribution across and along the column. This is basically a simple version ofa well known technique implemented by Stichlmair and co-workers to trace andquantify the extent of liquid maldistribution in packed beds [33,34]. This technique ismuch less intruding than that using bayonet type samplers to collect liquid samplesfrom a bed. However, both techniques are demanding and should be implementedonly if justified by the objective of the tests.



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3.9 Pressure Drop MeasurementAccurate differential pressure meters are required. Sources of possible

measurement errors should be eliminated by design, and the most notoriouswrongdoer in this respect is the inevitable condensation of a part of vapour in tubes

connecting pressure taps with sensors which in case of small diameters leads topressure line plugging, causing excessive fluctuations in pressure. Namely, vapourfilled lines are preferred because this way pressure is sensed directly, and to avoidcondensation related problems large enough diameters have to be taken say 10 or12 mm for pressure lines. They must be free draining; otherwise provisions should bemade to enable inert gas blowing the condensate from the line during the start upand occasionally during operation. Free draining is ensured if pressure sensingdevices are installed above the lower and upper pressure tap. Seal pots arerecommended and should be placed between the pressure tap and pressure taptubes. Pressure taps on column walls must be placed on locations that are notexposed to any lateral flow of vapour as it may be imposed by distributor or vapour

inlet device.The best practice is to have two pressure drop systems installed in parallel,

one to cover the low pressure drop range, say from 1 to 10 mbar, and the other onethe whole range, say 1 to 100 mbar. In this way more certainty is present regardingthe accuracy of the measured data at low F-factors values.

Since the packed bed height will differ from test to test, and the distributorneeds to be placed immediately above the packing, the best solution is to have topside pressure tap fixed at highest position. This implies that the pressure dropmeasured will include the static pressure related to the height of empty part of thecolumn, and if shorter beds are installed, also the pressure drop associated withdistributor itself. The first one will be appreciable at high pressure operation (high gasdensity) and the second one in case of vacuum operation (high gas velocity).Providing additional pressure taps along the column to reduce and possibly eliminateone or both of these sources may prove to be impractical, because it requires acomplex manifold (many valves) to make necessary adjustments. This is complicatedand prone to errors and therefore I would recommend maximum distance option, inconjunction with appropriate corrections for contributions of static pressure and/ordistributor. The first one is easy to quantify, and a useful correlation for pressure dropof a narrow trough type liquid distributor can be found elsewhere [35]. If the bottomside pressure tap is located bellow the liquid collector/gas distributor device, usuallya chimney tray or chevron type device, then there will be an additional pressure drop

involved, which however can be accounted for by using correlations available in theabove mentioned reference.In case that the top pressure is measured via a separate manometer installed

at the cop of the column, a practical double check is possible if provisions are madeto attach a precalibrated manometer to pressure lines connecting the top and bottomside pressure taps to the pressure difference cell. If this is done and the difference oftwo absolute values does not agree well with that from pressure difference cell Iwould trust the latter one.

3.10 ReproducibilityTwo to three points at different F-factors should be repeated at the end of the

measurement series, and in some cases the same packing should be tested again,



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after a while, to get experience and the confidence regarding the data reproducibilityand interpretation. This implies that the method of installing packing needs to be fixedand repeated every time a column is (re)packed, as discussed under 3.3.

One of worrying problems associated with this is that running up and down

with respect to vapour load can lead to some hysteresis effect, which is a difficultsituation and needs to be handled with care, to avoid excessive discrepancies in thesame data points. The time required to establish a steady state when going fromhigher to lower F-factors is longer than that associated with going up in F-factor.Liquid hold-up is the most sensitive parameter in this respect, and higher than normalvales of pressure drop could be expected when going down too fast. Dedicatedexperiments should provide proper answer to this question, i.e. to establish the testprocedure and related time intervals.

3.11 Mode of OperationPerforming efficiency tests by doing them at total reflux is practical and

therefore accepted as a common standard. However, the results may differ fromthose at finite reflux ratio. A dedicated study carried out at Sulzer in late 1980s hasindicated that there is no significant difference in the results obtained at total reflux(L/G = 1) and finite refluxes (L/G 1) [36]. From that moment on Sulzer reports onlythe data obtained in total reflux tests. Most recently, a study performed in Japanincluded continuous operation and it appeared that the structured packing (Montz-pak B1-250) performed slightly better under continuous operation than at total refluxconditions [37]. This indicates that total reflux results may be considered generally tobe on the safe side.

To eliminate uncertainties in this respect, i.e. to gather own experience, it isadvisable to have provisions for simulating experimentally a finite reflux ratiooperation. This could easily be arranged for simulation of a common rectificationsection situation, with L/G < 1, by having a provision to divert a part of the refluxstream to the feed tank, and to simulate (L/G) > 1 a separate pipe could be installedto enable bringing the liquid from feed tank to the liquid distributor. If continuousoperation is considered as a possibility then enough column height needs to bearranged, i.e. at least three meter per section and one meter for collector andredistributor, as well as enough place for placing the top distributor and demister inthe vapour disengagement area.

In general, total reflux operation is the most convenient one and widelyaccepted. In order to get ready as fast as possible operators of total reflux distillation

columns prefer to start with highest reboiler load, i.e. at an F-factor around the floodpoint, to ensure thorough wetting of the packing before start of measurements, whichis necessary. Being at highest load it is convenient to go down with column load inpredetermined steps. However in this case enough time must be provided uponchange to new F-factor for column to reach the steady state. Other possibility, afterflooding the column is to go down to lowest F-factor, say about FG = 0.5, andincrease the load gradually, increasing the density of the points in the loading region,where some stronger deviations in performance can be expected. These two optionsare normal for a continuous (non-stop) test run. For tests performed on daily basis,the start-up procedure repeats and every day one or two neighbouring points aremeasured. Non-stop runs, starting on Monday morning and ending on Friday, with

sampling performed at predetermined times during both day and night are best



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25/50


experiment without insulation. Also, the effect of sub-cooling, which is dealt with inelaborated way later on, needs to be evaluated appropriately.

Anyhow running a distillation pilot plant will bring necessary amount ofheadache to those involved, if striving for absolute accuracy/certainty, which is

difficult if not impossible to achieve in practice.

3.13 Process and Mechanical Design ConsiderationsEarly days it was a common practice to place the column atop of a ketlle

reboiler. To enable high vacuum operation the condenser was usually installed as apart of the top of the column. Modern pilot plants (FRI, SRP, TU Delft, BTS) preferhaving these three main units as separate devices assembled into a functional unit.Condenser and reboiler need to be large and flexible enough to cover the operatingrange. For structured packings this means operating pressures of 100 mbar on lowerside and atmospheric pressure on upper side. If one wish to be able to operate athigh pressures then additional units, connected in parallel may be considered. Water

is convenient as cooling medium and a small after-cooler unit may be useful tocontrol properly the extent of sub-cooling of reflux. Falling film evaporators areconvenient as reboilers, and the accompanying tank should allow feed volumes to besignificantly larger than the holdup of the column, condenser and tubing, to avoidexcessive depletion of the more volatile component. Usually a 10 bar steam supply isenough for heating purposes, but in case a high pressure operation this may beinsufficient, which can be compensated eventually by having extra heat transfer areaavailable. Certainly, the relevant numbers depend on the system chosen.

From construction point of view, all units should be made of stainless steel,SS316 being a good choice. A useful provision is to have observation windows. If theposition of the liquid distributor is fixed then one window should be placed to allowobservation of operation of the distributor. This location is important because itenables observation of the extent of entrainment of liquid in the loading region. Anadditional window placed perpendicularly is recommended to enable light to beprovided to increase visibility inside the distributor section of the column. A windowalong the bed offers the possibility of observing the piece of periphery of packing inoperation. In preloading region the degree of (active) wetting of packing can beobserved, and under high operating loads, approaching flooding a fluctuating action(surging) of the bed can be observed which is a useful experience increasing boththe understanding of packing behaviour and confidence regarding the interpretationof measured data.

Finally, to enable operation at below-atmospheric pressures a vacuum pump isrequired with corresponding manifold and control equipment. If all is put togetherthan we arrive at test installation capable of operating under vacuum and at aboveatmospheric pressure, connected to and operated via a computer in the control room,equipped with appropriate data acquisition and process operation software.

4. TEST DATA EVALUATION, PRESENTATION AND INTERPRETATION

Usually, the HETP and pressure drop of the tested packing are presented inperformance plots as a function of the superficial (based on full cross section area of



26/50


the column) vapour load, i.e. F-factor, FG, or/and capacity factor, cG, which arerelated through the square of the difference between liquid and vapour densities:

GGsG

uF = (1)

and

GL

Gs

GL

GGsG

Fuc

=

= (2)

where uGs(m/s) is the superficial vapour (gas) velocity, G(kg/m3) is the gas density

and L (kg/m3) is the liquid density.

The superficial gas velocity is obtained by dividing the volumetric flow rate, i.e.the ratio of mass flow rate and the density of vapour, and the cross sectional area of

the column.

=

4

2

cG

GGs

d

Mu

(3)

where dc(m) is the internal column diameter and MG(kg/s) is mass flow rate of gas.In case of the total reflux, mass flows of gas and liquid streams along the

column are equal, and the corresponding superficial liquid velocity is:

L

G

G

L

GGsLs Fuu

== (4)

Multiplied by 3600 this equation delivers information on corresponding column liquidload, expressed in m3/m2h, which is a more practical unit.

Figures 1 and 2 show HETP and pressure drop curves as a function of the F-factor, for top, middle of the bed and bottom conditions, as measured at total reflux ina TU Delft experiment with Montz-pak B1-500MN using CH/nH system atatmospheric pressure, and in a Bayer TS experiment with B1-250MN using CB/EBsystem at 0.1 bar, respectively. As it can be seen from Fig. 1, in this particular, wideboiling test mixture case, even for a rather short bed, the effect is significant. Usingthe bottom temperature as representative for data evaluation would lead to lowestpressure drop and highest capacity, as well as the best efficiency (lower HETP). Theefficiencies differ considerably, because the relative volatility in this case issignificantly higher at top than at the bottom of the bed. For the same bed height andcomposition change, lower relative volatility means more equilibrium stages per unitheight, i.e. a lower HETP, and consequently better efficiency at bottom of the bedconditions. According to Fig. 2, similar situation is with CB/EB system at 0.1 bar, butin this case the difference in top and bottom efficiencies is much less pronounced,i.e. practically negligible. The efficiencies hardly differ because the relative volatility in

the top is only slightly larger (third decimal place) than that in the bottom of the bed.



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0.0

0.1

0.2

0.3

0.4

0 0.5 1 1.5 2 2.5

F-factor (Pa0.5)

0

4

8

12

16

HETP - topHETP - averageHETP - bottomdp/dz - topdp/dz - averagedp/dz - bottom P

ressuredrop

(mba

r/m)

B1-500MN, TUD, CH/nH, 1.013 bar , d = 0.45 m, h = 1.58 m

Ideal mixture, top > bottom

)

P

(m

HET

Figure 1 Effect of operating conditions on packing efficiency and pressure drop forCH/nH system at top pressure of 1.02 bar

0

0.1

0.2

0.3

0.4

0.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

F-factor [Pa0.5

]

HETP[m]

0

3

6

9

12

15

P

ressuredrop[mbar/m]

HETP, top

HETP, bottom

HETP, average

dp/dz, top

dp/dz, bottom

dp/dz, average

B1-250MN, BTS, CB/EB, total reflux, 0.1 bar, dc = 0.6 m, hpb = 2 m

F-factor uLs

(Pa0.5) (m3/m2h)

0.54 1.29

1.04 2.50

1.55 3.74

1.92 4.66

2.27 5.54

2.59 6.35

2.94 7.27

3.22 7.98

3.54 8.85

3.67 9.23

3.78 9.56

4.16 10.67

Figure 2 Effect of operating conditions on packing efficiency and pressure drop for

CB/EB system at top pressure of 0.1 bar.



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It should be noted that the F-factors shown in Fig. 1 and Fig. 2 were calculatedtaking into account the effect of bed pressure drop.

The extent of temperature effect is elaborated in more detail for all importantdesign and operating parameters in what follows. Usually the average values based

on top and bottom of the bed conditions are taken as representative for test resultspresentation.

4.1 Representative F-factorApplying Eq. (3) to Eq. (1) yields a practical expression for gas flow F-factor:

=

4

2

c

G

G

Gd

MF

(5)

indicating that for a given column internal diameter the F-factor is proportional tointernal mass flow rate of the vapour and to the inverse of the square root of vapour(gas) density. Due to difference in top and bottom pressure and temperature as wellas vapour mixture composition, vapour density will be different at the top and thebottom of a bed.

The density of the vapour at the top of the bed is defined by the top pressureand temperature and can be determined for an ideal mixture from:

T

TGwT

TGTR

Mp,,

,= (6)

where pT (bar) is pressure of the top of the column, Mw,G,T (kg/kmol) is molecularweight of the gas mixture, TT (K) is absolute top temperature, and R(kJ/kmol K) isuniversal gas constant. In conjunction with pressure expressed in bar: R= 0.08314.

Similarly, for bottom conditions:

( )

B

BGwT

BGTR

Mpp,,

,

+= (7)

where p(bar) is the measured pressure drop of the packed bed, Mw,G,B (kg/kmol) ismolecular weight of the vapour mixture, and TB (K) is absolute temperature at bottomof the bed.

A simple arithmetic average of gas density can be taken as representative ofthe middle of the bed conditions:

)2

,,

,

BGTG

averG

+= (8)

The situation with the representative internal mass flow rate of vapourhowever is not so simple. Namely, as appeared in practice, it is always somewhatlarger than that based on measured reflux flow rate.

Per definition, under total reflux the molar flows of gas and liquid are equal andconstant along the column, however the reflux, obtained by total condensation of thevapour leaving the top of the bed, returns into the column subcooled. According to



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my knowledge, based on evaluation of BTS, FRI, SRP and TU Delft data, the refluxreturns to the top of the column as a liquid at a temperature 10 to 20 degrees Kbelow its boiling point. Upon entering the column this sub-cooled liquid is heated upto reach its boiling temperature, and the heat for this is provided by condensation of a

part of the rising vapour. In other words latent heat or enthalpy of condensation ofvapour provides the sensible enthalpy to heat the subcooled liquid to its boiling point.Fortunately a relatively small amount of total heat is required to get this done.

The liquid created by the condensation of vapour joins the reflux, i.e. liquidintroduced through the distributor, and, therefore, it is reasonable to expect that theflow rate of this internal reflux is greater than the measured flow rate of externalreflux. In order to obtain a reasonable estimate of the internal reflux, Seader andHenley [38] recommend an equation derived from an approximate energy balancearound the top of the bed, which is transformed here into mass units to express themass flow rate of the gas at bottom conditions, assuming that under total reflux themolar/mass flow rates of gas and liquid streams are equal:

+=

Bv

RRLpTTLp

TGBGh

TcTcMM

,

,,,,

,, 1 (9)

where cp,L (kJ/kg K) is the specific heat of the liquid at corresponding temperature:top, TT (K), or reflux, TR (K), and hv,B (kJ/kg) is specific enthalpy of vaporisation orcondensation at bottom temperature.

Then, keeping in mind the fact that the gas flow rate at the top is equal to(measured) reflux flow rate, a reasonable representative F-factor is that obtained as

an arithmetic average of top and bottom values:

+

=

44

2

1

,

,

,

,

,

cBG

BG

cTG

TG

averGd

M

d

MF

(10)

Specific vaporisation enthalpies and specific heat capacities of top and bottombinary mixtures can be determined from pure component properties on additive

basis, using experimentally determined mass fractions of more volatile component.Suitable simple polynomial equations, with experimentally determined coefficients,can be used to describe pure component properties.

In case of larger bed heights the pressure drop effect will be more pronouncedand may cause even larger difference between top and bottom related F-factors.Nevertheless, an average representing the middle of the bed conditions can alwaysbe used with confidence for the purposes of packing performance comparisons.

4.2 Representative Relative VolatilityOnly with an ideal close boiling system, such as CB/EB mixture, it is certain

that the assumption of a constant relative volatility will hold for the whole bed length.With other systems, such as commonly used CH/nH system, there is a significant



30/50


difference between relative volatilities of the top and bottom of the bed, resulting, asdemonstrated in Fig. 1, in a rather strong effect on efficiency.

In the case shown in Fig. 1, the bottom temperature was 10 0C higher than thetop temperature, and corresponding relative volatilities were 1.75 at the top and 1.6

at the bottom, respectively. This, nearly ten percent difference in relative volatilities,results in a nearly twenty percent difference in corresponding HETP values. In otherwords, for the same composition change, at bottom conditions more theoreticalplates are contained in given bed height, resulting in a lower HETP value, i.e. betterefficiency.

This is based on assumption of ideal behaviour of CH/nH mixture, which isgenerally valid for CH/nH system. However, by considering a certain degree of non-ideal behaviour in liquid phase, the situation can turn upside down, i.e. a largerrelative volatility obtained at the bottom than at the top conditions. The extent of thiswill depend on the method used to estimate the characteristic activity coefficient.However it should be noted that in this case the average value remained nearly the

same. These kind of thermodynamics related uncertainties is generally avoided if aclose boiling mixture like CB/EB system is used.

If an average is taken then the geometric average of top and bottom values ispreferred:

BT = [11]

For binary mixtures, the chosen relative volatility in conjunction with therepresentative composition (mole fraction of more volatile component, x) determinesthe slope of the equilibrium line, m, via:

( )[ ] 211 +==

xdx

dym [12]

For total reflux case, (L/G) = 1, the stripping factor, = mG/L = m/(L/G), whichrepresents the ratio of slopes of equilibrium and operating lines, is equal to the slopeof the equilibrium line ( = m). L and G represent molar flow rates of the liquid andvapour (gas), respectively.

4.3 Representative EfficiencyIt is a common practice to take the most conservative HETP value around theloading point as representative, providing this value will be larger or equal to thatcorresponding to the point of the onset of flooding. Generally the HETP, i.e. theinverse of the number of theoretical plates (equilibrium stages) per meter bed heightis taken as the measure for mass transfer efficiency of a packing. In other words, theinstalled packed bed height, hpb (m), divided by the contained number of theoreticalplates (equilibrium stages), N, determines the efficiency of the packing expressed asthe height equivalent to a theoretical stage (HETP):

N

hHETP

pb= [13]



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Another theoretically more appropriate measure is the HTUoG (orHoG), i.e theheight of an overall gas side transfer unit, which however requires the information onthe number of transfer units contained in a bed:

oG

bedoG

NTU

hHTU = [14]

The number of transfer units is not readily available and the most commonpractice is to use HETP, which is based on the number of theoretical plates, which inturn can easily be determined using established graphical or analytical methods inconjunction with accurate equilibrium data.

For ideal systems (CB/EB) and nearly ideal systems (CH/nH), the number oftheoretical plates or stages generated in the installed bed during a total refluxexperiment can be determined with confidence using well known Fenske equation:

ln

1

1ln

= BD

x

x

x

x

N [15]

where x(-) is the mole fraction of more volatile component in the binary mixture and (-) is the relative volatility, i.e. its representative value, most frequently thegeometric mean of top and bottom values (Eq. 11).

The sensitivity of this equation to measurement errors in relative volatility and

compositions on both ends has been thoroughly elaborated in a paper by Deibeleand Brandt [39], indicating that the effect of deviation in relative volatility is strongerand tends to increase with decreasing relative volatility, and that keeping the molefraction of more volatile component in the bottoms in the range between 10 and 40mole % and in the top between 60 and 90 mole % ensures that relative error will bekept below 2 %. This exercise clearly indicates that significant errors can beexpected if dealing with relative volatility below 1.1, which is a value the CB/EBsystem could assume if operated at pressures above 2 bar. Certainly, it is essentialto use accurate equilibrium data for the test system in question.

From time to time doubts arise about the reliability of Fenske equation. Arecent study of B. Kaibel [40] introduced an alternative, in a way mathematically more

rigorous, integral solution based analytical form of McCabe-Thiele plot, whichindicated that Fenske equation may be wrong to some extent, depending ondifference in top and bottom compositions, which however becomes pronounced atrelative volatilities above 1.5, and should definitely be considered if dealing with non-sharp separations. From the evaluation of SRP data on Montz structured packings[17] it appeared that CH/nH system is practically insensitive in this respect and incase of CB/EB system with relative volatilities in the range 1.1 to 1.2 there is noeffect at all. This means, that Fenske equation in conjunction with properthermodynamic model and measured top and bottom compositions can beconsidered to be a reliable tool for determination of mass transfer efficiency ofpackings tested under total reflux conditions.



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Applying the Eq. [15] to Eq. [13] yields the working expression used widely todetermine the efficiency of a packing from measured top and bottom compositions:

=

BD

pb

x

x

x

x

h

HETP 1

1ln

ln

[16]

There are also stage by stage methods available, used in conjunction withcommercial software, but these are useful mainly if it is dealt with multicomponentmixtures. A common practice for wide boiling binary mixtures is to use McCabe-Thiele plot cut into concentration regions where the equilibrium line can beapproximated as a straight line and the representative average HETP obtained inadditive way.

HETPand HTUoG

are related to each other through stripping factor, , i.e. theratio of slopes of equilibrium and operating lines in the McCabe-Thiele plot of thegiven case. If the equilibrium and operating lines are straight and in parallel: HETP =HTUoG, and this is the case with CB/EB system. If the lines are straight but notparallel then following expression holds:

oGHTUHETP1

ln

=

[17]

A practical demonstration of this equation can be found in a most recent

reference discussing the effect of the test system as experienced during a total refluxdistillation study carried out with a high performance, high specific surface areapacking (Mellapak 752.Y) [21].

Also, it should be noted that the additional internal reflux generated bycondensing the vapour along the column wall is not so pronounced in the columnswith diameters as considered here, to have a substantial influence on efficiency.Practically, the same is with sub-cooling effect that effectively creates additionalinternal reflux, which, however, in total reflux situation appeared to be much lessinfluential than generally anticipated. However, one should not forget that apronounced difference in enthalpies of vaporisation of two components forming thetest mixture could do some harm to efficiency and should be recognised and

accounted for appropriately. This is not a concern with CH/nH and CB/EB systems,but it is recommended to perform the necessary calculation exercise to determine theextent of the effect and make appropriate decisions toward standardisation ofcalculation methods. The same is recommended for subcooling and heat loss relatedeffects.

4.4 Representative Pressure DropThe pressure drop is measured directly and the obtained value divided by bed

height gives the unit pressure drop of the packing in question, expressed usually inmbar/m. However this is only the case if the pressure taps are placed immediatelyabove and below the bed, and this may not always be the case. Namely the top and

bottom section pressure taps are usually fixed and if a short bed is measured than



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there is a certain length of empty column included. For such a rather short tube thefriction with the column walls may be considered negligible, however, if the operatingpressure is high, i.e. well above atmospheric, the static pressure of the empty columncould play a significant role. In such a case, the packing pressure is that obtained by

detracting the static pressure of the empty part of the column from the measuredpressure drop. If, in conjunction with a short bed, the liquid distributor is placed belowthe upper pressure tap, then this needs to be considered, particularly if the free areaof used liquid distributor is on lower side, i.e. well below 50%, and high gas velocitiesare involved. As suggested elsewhere [35], for a test column operating at 100 mbarat a F-factor of 3 Pa0.5, a narrow trough distributor with 40 % free area would cause apressure drop of about 0.2 mbar, which is not significant if the measured pressuredrop is above 10 mbar. Also, if a chimney-tray or chevron type liquid collector is usedto distribute gas, and placed above the bottom side pressure tap, then a significantadditional pressure drop can be expected. Reference [35] contains correlations whichenable estimation of related pressure drop of these frequently used devices.

In other words, the pressure drop per meter packed height is:

( )pb

lcldstmeas

pbh

ppppzp

= / (18)

where subscript pb refers to packed bed, and subscripts st, ld, and lc denotepressure drop associated with static pressure of empty column, liquid distributor andliquid collector, respectively.

4.5 Packing Geometry EffectsSince the primary purpose of the total reflux distillation tests is to get properimpression on the performance characteristics of different packings, it is important tobe aware of macro geometry details of each packing tested, because these maydeviate from nominal, which, in some cases could have profound effect particularlyon pressure drop and capacity. This means that packing to be tested needs to becarefully inspected to observe and notice the design characteristics of corrugations.The best way to get certainty regarding the relevant dimensions of the corrugations isto ask the packing manufacturer to deliver separately two-three corrugated sheetsused for manufacture of the test packing. If this is not done then if there are packingelements in excess or upon completion of tests one packing element could be

dismantled and two-three sheets taken out for closer inspection. Corrugation heightand base width need to be measured precisely, as well as the corrugation inclinationangle. The latter exhibits very strong influence on pressure drop, and can deviatefrom the common (nominal) 45 degrees. One should note that novel, highcapacity/performance packings often come with a corrugation inclination angle below45 degrees! For high performance packings also the height of the bend at the topand/or bottom part of corrugations needs to be known. A useful aid for evaluation ofpossible macro geometry effects is the Delft predictive model for structured packings[41, 42], and some of the geometry related effects can be found quantified elsewhere[21, 43].



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4.6 Test DocumentationThe test should be documented properly, which requires establishing a

standard for preparing a summary report. Excel is the best environment for thisbecause it allows easy transfer, handling and use of data and can be used in

conjunction with process simulation software. In Appendix B, two Excell sheets aregiven, one suitable for data collection (B1), containing values of all measuredparameters, and the other one suitable for data presentation (B2), containing mostimportant design and operating parameters. The latter ones are supporting the mainresult, i.e. a plot showing measured HETP and unit pressure drop as a function of F-factor. In this plot all measured points, usually three per vapour load need to begiven, including repeated ones, done additionally to check the reproducibility. Thelatter ones need to be indicated as such. Useful additional information is a tableinserted in the graph (see Fig. 2), showing the specific liquid loads in m3/m2h,corresponding to F-factors of measurement points.

5. CONCLUSIONS AND RECOMMENDATIONS

There is no doubt that for a company manufacturing fine and bulk chemicalsthe best practice is to have at disposal a test unit of a size sufficient to ensureobtaining representative packing performance data, to understand and to minimizetechnical risk associated with (re)design and operation of an industrial scaleinstallation if equipped with a new type of packing.

For structured packings, a column diameter of 0.4 m or larger in conjunctionwith a maximum bed height of 4 m, allowing 5 to 25 theoretical stages to bedeveloped, depending on packing size, looks to be a reasonable choice.

Total reflux operation is a natural choice and provisions can be easily made toensure performing some tests with a liquid to vapour flow rate ratio below or aboveone.

Since structured packings are mainly considered for application under vacuum(100 mbar is a representative value) and nearly atmospheric pressures, thereasonable choice is to have an installation capable of covering this range ofoperating pressures with one test system and using one, high turndown, high freearea narrow trough liquid distributor with drip tubes to ensure constancy in initialliquid distribution pattern.

The ideal CB/EB mixture is a good test system, however, being a low

(constant) volatility system it provides higher efficiencies from those obtained withcommon wide boiling hydrocarbon mixtures, with like CH/nH system. Also, becauseof high operating temperature this system is not suitable for above atmosphericoperation. In other words, it cannot be used for the purposes of evaluating a packingperformance under relatively high liquid load, say above 20 m3/m2h. The exceptionsin this respect are tests carried at atmospheric pressure in conjunction with lowspecific surface area, high capacity packings, which operate at F-factors well above 3Pa0.5. In case of the CH/nH system, operated at SRP at 4.14 bar, at highest F-factorliquid loads up to 50 m3/m2h are generated, without any temperature imposedlimitation.

Test procedure should be established based on own evaluations and accepted

as a standard to ensure consistence of test results. A continuous, i.e. non-stop



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operation is preferred. If safety and/or personal reasons require daily operation thenthe equipment size should be minimised, to reduce stabilisation times required toarrive at steady state conditions after start-up and changes in column loads.

Evaluations of relevant factors, summarised in this report have provided a

good guidance regarding pilot-plant column design and operation, however furtherexperiences, based on pilot-plant building, start-up and dedicated operation areneeded to arrive at desired final goal, i.e. definition and adoption of a standard totalreflux distillation test, to be considered useful for scale-up purposes.



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Nomenclature

cG capacity factor, m/scp,L specific heat capacity of the liquid, kJ/kg K

dc internal column diameter, mF-factor = FG = uGs(G)

0.5, Pa0.5 or (m/s)(kg/m3)0.5

G molar flow rate of gas or vapour, kmol/shbed height of the packed bed, mhv specific vaporisation enthalpy, kJ/kgHETP height equivalent to a theoretical plate, mHTUoG height of an overall gas side transfer unit, mL molar flow rate of the liquid, kmol/sMG mass flow rate of gas (vapour), kg/sMw molecular weight, kg/kmolm slope of the equilibrium line, -N number of equilibrium stages or theoretical plates, -NTUoG number of gas side transfer units, mp pressure, bar or mbarT temperature, oC or KuGs superficial gas (vapour) velocity, m/suLs superficial liquid velocity, m/sy mole fraction of more volatile component in vapour, -x mole fraction of more volatile component in liquid, -

Greek letters

relative volatility, -p pressure drop, bar or mbar stripping factor, -G vapour density, kg/m

3

L liquid density, kg/m3

Subscripts

aver refers to average valueB bottoms, bottom of the packed bed

D distillateG gas or vapourR refluxT top of the packed bedlc refers to liquid collectorld refers to liquid distributorpb refers to packed bedst refers to static pressure


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