Improve Glass-Lined ReactorEfficiency with Safe, Effective Cleaning of the Reactor Jacket

James P. McIntyreBetzDearbornHorsham, PA USA

Technical Paper

ABSTRACT

This paper details a process for cleaning glass-linedreactor jackets that functions at neutral pH and is safefor workers, equipment and the environment. Thisunique chemistry successfully dissolves rust andscale, resulting in clean, passivated reactor jacketsand increased production efficiency. This process pro-vides all of these benefits without the risk of non-repairable spalling (a.k.a. fishscaling) damage to theglass lining.

INTRODUCTION

Glass-lined reactors are used in virtually all of theworld's pharmaceutical manufacturing facilities. Thereare several key reasons for their selection by designengineers.

In a pharmaceutical process, cleanability is critical.Between batches, each reactor and its associatedprocess equipment must be thoroughly cleaned inorder to assure product quality and minimize heattransfer resistance caused by product buildup.Fortunately, glass has a high degree of surfacesmoothness which makes it easy to clean using non-corrosive, low pressure cleaning systems .

Glass is also chemically resistant and as a result, canserve for many years in environments that wouldquickly render most metal vessels unserviceable.Aggressive reaction environments also tend to dis-solve metals from unlined mild steel or alloy reactors.These metals can compromise product quality in anindustry where purity is essential. By comparison, theglass-lining protects the base metal so effectively thatthe relatively benign heat transfer fluids used in thejacket space will generally attack the jacket and reac-tor exterior long before the reaction environment com-promises the glass-lined interior of the vessel.

The technology certainly exists to minimize corrosionin the jacket space , but it is not often implemented.There are many reasons for this but the major factor isthat productivity is usually a higher priority. Reactionschemes are designed to be as brief as possible inorder to maximize production. From the standpoint ofcorrosion and deposition, it would be ideal to have oneproperly inhibited fluid that could serve as both a heat-ing and cooling medium. However, there are fastermethods of heating and cooling. Steam is commonlyused because it heats reactors very quickly and wateris used for cooling because it cools reactors veryquickly. For rapid chilling to lower temperatures, thereare mechanically chilled glycol solutions or salt brines.

In a typical application steam, cooling water and achilled solution might all be used in the same jacket atdifferent points in the reaction cycle. It is very efficientfrom a production standpoint but it is also potentiallydamaging from a mechanical standpoint. In short,good process designs can and do result in poordesigns for corrosion and deposit control.

To see how this can occur, one has only to think aboutthe process. There is an endless variety of reactionschemes in use, so for the sake of brevity, we willexamine just a few of the many possible scenarios.

Let's consider the process of heating a batch.Frequently, steam is injected directly into the jacketspace until the target reaction temperature is reached.If the target temperature is below boiling, the steamwill be condensed to what is essentially, pure water.This condensate is very aggressive to mild steel andwill act to corrode the metal in the jacket space.

Once the heating cycle is complete, our hypotheticalreaction proceeds. If heat must be removed from thereaction mass, cooling fluid is passed through thejacket in order to precisely control the reaction tem-perature. The cooling fluid is often common plant ser-vice water and can contain a high level of dissolvedminerals including calcium, magnesium, silica, ironand alkalinity. These minerals have retrograde solu-bility with temperature. In other words, instead ofbecoming more soluble at high temperatures, (liketable salt), these minerals become less soluble at hightemperatures. Under high heat fluxes at the reactorwall, the skin temperatures will be elevated. That highskin temperature promotes mineral deposition on thereactor wall within the jacket space.

IMPROVE GLASS-LINED REACTOR EFFICIENCY WITH SAFE, EFFECTIVECLEANING OF THE REACTOR JACKET

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ZIRCONIUMHASTELLOY B

TITANIUM-PALLADIUM

TITANIUM

MONEL

HASTELLOY G

ZIRCONIUM

HASTELLOY C

STAINLESS STEEL

GLASTEEL

TANTALUMFLUOROCARBONS

FIBER-REINFORCED PLASTICS

HASTELLOY C

CARPENTER 20 Cb-3

INCONEL

MONEL

REDUCINGOXIDIZING

REDUCINGOXIDIZING

CH

LOR

IDE

SN

O C

HLO

RID

ES

Comparison of Corrosion Resistance

Figure 1: Glass lining has the widest range of corrosionresistance of any material used for equipment.

Deposition of minerals onto steel is a problem for tworeasons: first, the mineral scale impedes heat transferwhich can extend batch times thereby limiting produc-tion; second, soluble ions in the water such as chlo-rides and sulfates tend to concentrate underneath thedeposit. These ions increase the solubility of iron andresult in what is typically called underdeposit corro-sion. Underdeposit corrosion is really just a specialcase of crevice corrosion. Both phenomena are man-ifestations of concentration-cell corrosion. This typeof corrosion can result in highly localized metal loss.

In addition to introducing minerals into the reactor jack-et space, cooling water usually brings in dissolved oxy-gen as well. Oxygen, like most gases, is more solubleat lower temperatures than it is at higher temperatures.When the cool water contacts the hot reactor wall, oxy-gen is liberated from the water. Since corrosion isnothing more than a special type of oxidation, and oxy-gen is an oxidizing agent, it contributes to corrosion.This phenomenon is so common that it has its ownname. It is generally recognized as oxygen pitting oroxygen corrosion. Oxygen pitting is a common causeof metal perforation in heating zones of water systems.

After our hypothetical reaction is complete, there is acooling cycle during which the temperature is reducedto a predetermined target. Once the product is cooland it is transferred out of the vessel, the reactor isprepared for another run. As jacket fouling builds upover the years, both heating and cooling cycle timescan be extended significantly because of the addedresistance to heat transfer.

The significance of this comes down to one thing - pro-duction. Corrosion and fouling directly increase cycletime and reduce the efficiency of the production facili-ty. There are, however, other, more serious problemsthat can result from corrosion. In cases of extrememetal loss, a reaction vessel may need to be removedfrom service. Glass-lined reactors are typically rated

for specific maximum pressure. If the wall thickness istoo severely reduced, the vessel may be required bycode to be de-rated to a lower pressure. If the pres-sure is lower than that required in the process, thereactor may have to be removed from service.

These problems are nothing new to operators of glass-lined reactors. The problems have been occurring fordecades and corrective action has been required fordecades. The most commonly used corrective actionis acidic cleaning In fact, pharmaceutical companieshave been performing them for many years. Acidiccleaning, however, carries with it the inherent risk ofglass damage. Such damage can cause considerabledowntime.

Glass damage occurs as a result of monatomic hydro-gen (H0) permeating the steel from the jacket side.Because monatomic hydrogen is chargeless and verysmall, it is able to travel through the steel. When the

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Figure 2: Damage to glass lining during cleanup(spalling).

Figure 3

hydrogen atoms reach the metal/glass interface, theyreact to form molecular hydrogen(H2). Along with thisreaction comes a roughly five-fold increase in volume.In time, the resulting pressure causes the glass tochip. Because the initial chip is crescent shaped andclosely resembles a small fishscale, the term "fish-scaling" is commonly used . It is important to note thatfishscale damage extends from the substrate metalthrough the glass coating to the surface. As a result,the integrity of the glass lining with respect to corrosionresistance is completely compromised.

Because fishscale-type spallation often extends over aconsiderable area. For that reason it is almost impos-sible to repair on-site. Practically speaking, it can onlybe repaired at a qualified repair facility.

The process involves removing the vessel from ser-vice and shipping it to the repair facility where thejacket is removed and the reactor is grit-blasted toremove the original glass surface and any depositsthat may have built up in the jacket space. It is theninspected to ensure that the reactor can be repaired tothe original code specifications. If corrosion has beensevere and wall thickness has been reduced overlarge areas, it may be cost-prohibitive to repair thevessel. Replacement may be a more economicaloption.

After that determination is made, the necessary metalrepairs are completed and the vessel interior is againgrit blasted in preparation for re-glassing. The correctnumber of glass coatings is applied, with a thoroughintegrity inspection between each coat. After a finalglass inspection, the jacket is welded onto the reactor,the exterior is painted and any required accessoryitems are installed. The vessel is now ready for ship-ment back to the owner. The owner must then reinstallthe reactor along with its associated piping and con-trols. This process is very costly in terms of downtimeand the actual cost of the re-glassing. It is importantto remember that these costs could have been avoid-ed through the correct selection of jacket cleaningmaterials and procedures.

SAFELY RESTORING REACTOR HEATTRANSFER

The risk of fishscaling is proportional to the rate ofhydrogen permeation through the reactor wall and thecontact time of the jacket cleaning media. Hydrogenpermeation has been shown to be significant whetherusing strong mineral acids, or milder, inhibited organicacids . It follows that the safest reactor jacket cleaningwould be neutral or alkaline, rather than acidic. Inpractice though, alkaline cleaning products have little

effect on the mineral scales which need to be removedin order to restore heat transfer.

One neutral pH cleaning program has been identifiedand in fact, is recommended by the majormanufacturers of glass-lined equipment, as the onlysafe and effective way to clean the reactor jacketspace. This neutral pH cleaning service is designed toremove iron or calcium-based fouling whilemaintaining a pH between 6.5 and 7.5. This cleaningsolution has been documented as generating nomeasurable hydrogen flux through mild steel . As aresult, the risk of fishscaling glass damage is almostnon-existent. To date there are no known glassfailures from the use of this material.

CASE STUDY: IRON OXIDE FOULEDPHARMACEUTICAL REACTOR

A leading pharmaceutical manufacturer relied on sev-eral glass-lined steel reactors in its productionprocess. As is common, the plant was operating at fullcapacity within the constraints of current process cycletimes. The production process had very stringentheating and cooling requirements. Variations in heat-ing and cooling times negatively impacted productquality and product yield. The reaction equipment wasserviced by multiple utility fluids including steam andcooling water. Over years of use, the reactors becameconsiderably iron oxide fouled resulting in productionproblems. Because of reactor jacket fouling, the strin-gent heating and cooling requirements were not met.Process batch cycle times became extended andproduct quality suffered. At the same time, batchescould not be produced as quickly, so production suf-fered a substantial decline.

Plant personnel knew that they needed to remove theoffending deposits from the reactor jacket space. Theyknew that, short of replacing the reactors, it was theonly way to restore productivity. Being familiar with thedanger of fishscaling, they were understandably reluc-tant to undertake any type of acidic cleaning. In addi-tion to concern for the integrity of the glass lining, therewas expressed concern about excessive acid corro-sion of the mild steel jacket. This concern was wellfounded because if acid cleanings are not properlyneutralized, flushed and rechecked, acid hideout cancause severe metal loss. Substantial metal loss mightpreclude the use of the vessels at the required pres-sure.

In the past, the plant had attempted caustic-basedcleaning. While it was quite safe with respect to pro-tecting the glass-lining, and it was safe with respect tothe base metal, it was ineffective. The caustic-based

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cleaning agent was unable to remove the mineralscales that were robbing the facility of its productioncapacity.

The plant consulted with several glass-lined reactorexperts including the manufacturer of the plant's reac-tors. They recommended the new, neutral pH clean-ing service as the only safe and effective means ofremoving the iron oxide fouling.

The service was scheduled for a five-day periodincluding set-up, cleaning and post-inspection of theglass lining. Prior to conducting the on-site service,the cleaning service examined a sample of the jacketspace deposits and analyzed it to determine the causeof the fouling and to match the foulant with the appro-priate neutral pH cleaning chemicals.

Prior to initiating the cleaning, the cleaning team tookultrasonic thickness measurements of the vessel walland the jacket itself to determine the extent of thedamage. Vessel entry was performed to inspect theinterior of the reactor for any existing glass damage,wear patterns, chipping or etching. In addition, sparktesting was performed to ensure that there were no"holidays" (pin holes) in the glass lining. Glass thick-ness was also measured in order to confirm that thelining was within specification.

The cleaning team then embarked on the actual clean-ing. They connected a custom-built cleaning skid tothe jacket piping and recirculated the cleaning solutionthrough the jacket over the course of 72 hours. Duringthis time, cleaning solution pH and concentration werecarefully monitored. In addition, the iron level in thecleaning solution was measured to verify that the solu-tion was removing iron oxide. Iron levels will rise whilecorrosion products are being removed from the jacketand reactor walls. Once the system is down to baremetal, iron concentrations in the cleaning fluid nolonger increase.

After the cleaning phase was completed, the cleaningsolution was flushed from the system and disposed todrain. Unlike acidic cleanings, there was no need forhazardous waste disposal, and there was no need fora separate neutralization step. Prior to reconnectingthe system piping, the condition and direction of theagitating nozzles and impingement plates wasassessed. The cleaning team recommended severalways to improve reactor life and water treatment in thejacket system. The reactor was then returned to ser-vice.

As a direct result of the cleaning , reactor operatingefficiency increased more than 30%. Cool-down timesfor each batch decreased by 45%. Heat-up timesdecreased by 27%. When the plant assessed the full

impact of the cleanings on its production process, thebenefits were quite impressive.

The savings realized was calculated to be $430,000annually.

• Production increased by more than 5%. As a resultof shorter batch times, the plant could now producemore batches per month.

• Product quality was improved through better tem-perature control. Process temperature was nowmore responsive because of the newly increasedheat transfer coefficient.

• Equipment life was increased by effective removalof corrosion sites and deposits. Active corrosioncells were halted. With the deposits now removed,corrosion inhibitors could reach the metal surfaceand promote the formation of a passive film.

The plant's production personnel were so impressedwith the results that they have planned to clean allremaining reactors on a regular basis.

CASE STUDY: IRON OXIDE FOULEDCHEMICAL REACTOR

A chemical manufacturer in Tennessee uses glass-lined reactors in their production process. A combina-tion of steam and cooling tower water is used to heatand cool the reactors. The constant switchingbetween steam and cool water accelerates iron corro-sion. Iron oxide deposits built up to the point whereheat transfer was seriously hampered. Batch timeswere increased as a direct result of fouling.

Removal of iron oxide deposits was indicated as theonly way to regain lost reactor efficiency. As in the pre-vious case study, acidic cleaning was discouraged bythe reactor manufacturer because of the significantrisk of glass damage. Upon consulting with glass-lined equipment manufacturers, the plant learned ofthe novel, neutral pH approach and opted to proceed.

Again, careful inspections of reactor jacket conditionswere performed including deposit composition deter-minations. The cleaning then proceeded for 72 hourswith all the critical parameters under close scrutiny. Atthe end of the cleaning, the reactor was again inspect-ed to ensure that the cleaning was successful.Calculations on the composition of the spent cleaningfluid revealed that the cleaning had removed approxi-mately 30 pounds (13.5 kg) of iron corrosion productsfrom the system.

The vessel was then returned to service. Heat trans-fer rates were observed to increase by more than30%. Reaction times were improved by more than 30minutes per batch. This was a significant improve-

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ment over previous operation and was achieved with-out the risk of glass damage

CONCLUSION

Glass-lined reactors are widely used because theywork where lesser materials simply fail. In the questfor optimum production efficiency, jacket space corro-sion and deposit control are frequently neglected orforgotten until problems arise. Corrosion and deposi-tion result from the high temperatures, low flows andvariable operating conditions within the jacket.Resulting fouling robs the production process of effi-ciency. Heating and cooling times are extended andproduct quality can be compromised.

Traditional methods of reactor jacket cleaning are riskyor ineffective. Acidic cleanings put the reactor's glasslining at risk for fishscaling damage while alkalinecleanings do not effectively remove iron and calciumbased deposits.

A novel, neutral pH cleaning program has demon-strated the ability to improve plant reactor efficiencysignificantly. It does so by removing mineral depositswithout risking damage to the glass lining or the reac-tor base metal and is recommended by the major man-ufacturers of glass-lined equipment as the only safeand effective means of glass-lined reactor jacketcleaning.

REFERENCES

The author gratefully acknowledges the assistanceand contributions of Mr. Donald H. De Clerck, consul-tant for Pfaudler, Inc.

1. Donald De Clerck, "The Care and Feeding ofGlass-lined Steel", Chemical Engineering, October1998

2. A. Marshall, W. M. Walker, A. Dito, "SolvingCorrosion Problems In Reaction Vessel JacketCooling Systems", Corrosion 91, Paper 301,Cincinnati, Ohio

3. Betz Handbook of Industrial Water Conditioning,Betz Laboratories, Inc., 1991, 9th edition p. 215

4. Donald De Clerck, "The Care and Feeding ofGlass-lined Steel", Chemical Engineering, October1998

5. A. Dito, A. Marshall, W. M. Walker, "Neutral pHCleaning of Glass Lined Reactor Jackets",Proceedings of the 8th European Federation ofCorrosion, September 18-22, 1995, Ferrara Italy

6. A. Dito, A. Marshall, W. M. Walker, "Neutral pHCleaning of Glass Lined Reactor Jackets",Proceedings of the 8th European Federation ofCorrosion, September 18-22, 1995, Ferrara Italy

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