Equipments Distillation Terminology

5/23/2018 Equipments Distillation Terminology

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dist i l la t ionterminologyTo provide a better understanding of the distillation process, the foll ow ing brieflyexplains the terminology most often encountered.SOLVENT RECOVERYThe te rm "solvent recovery" often has been a somewhat vague label applied tothe many and very different ways i n wh ic h solvents can be reclaimed by industry.

One approach employed i n the pri nti ng and coatings industries i s merely totake impure solvents containing both soluble and insoluble particles and evapo-rate the solvent from the solids. For a du ty of this type, APV offers the Paraflashevaporator, a compact uni t wh ic h combines a Paraflow plate heat exchanger anda small separator. As th e solvent ladened liquid is recirculated through the heatexchanger, it i s evaporated and th e vapor and l iquid separated. This w il l recover asolvent but i t will not separate solvents if tw o or more are present.

Af ur th er technique i s available to handle an air stream tha t carries solvents.By chill i ng the air by means of vent condensers or refrigeration equipment, thesolvents can be removed from the condenser.

Solvents also can be recovered by using extraction, adsorption, absorption,and distillation methods.S0 LV E NT EXT R ACTI0 NEssentially a liquid/liquid process where one liquid i s used to extract anotherfrom a secondary stream, solvent extraction generally i s performed in a columnsomewhat similar to a normal disti l lation column . The primary difference is tha tthe process involves two liquids instead of liquid and vapor.

During th e process, the lighter (i.e., less dense) liquid is charged to the baseof the column and rises through packing or trays while the more dense liquiddescends. Mass transfer occurs and a component is extracted fr om one streamand passed to the other.

Liquid/liquid extraction sometimes is used whe n the breaking of an azeotropeis difficult or impossible by distillation techniques.

CAR EON ADSO R PTIO NThe carbon adsorption techn ique is used primarily to recover solvents fro m dil uteair or gas streams.In principle, a solvent ladened air stream is passed over activated carbon andthe solvent is adsorbed in to the carbon bed. When the bed becomes saturated,steam i s used t o desorb the solvent and carry it to a condenser. In such cases astoluene, for example, recovery of the solvent can be achieved simply by decantingthe water/solvent tw o phase mixture whi ch forms in the condensate. Carbon

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t h e APVposi t ionWith nearly 50 years of experience i n thedesign of distillation equipment, the APVGroup of companies ranks among theleaders in this field. As early as 1932, APVengineers developed the West plate whichprovided a good turndown ratio and highlyefficient operation wi th low tray spacings.While this device has been superseded bymodern valve trays that can offer similarturndown characteristics at lower cost,the many thousands of West plates sti ll inoperation attest to their durability andre ab y.

Today, APV CREPACO, Inc. offers con-siderable expertise in the design andfabrication of all types of distillationequipment and systems. As a member ofthe worldwide APV organization and ofFractionation Research Incorporated (FRI)of California, APV has access to specialistsknowledgeable in both the theoretical andpractical aspects of distillation, solventrecovery and pollution abatement. Further-more, well equipped laboratory facilitiesare available for extensive test work andthe company can utilize the Chem Share process simulation programs to simplifytray to tray calculations for binary and multicomponent mixtures and to performenergy and material balance calculations in great detail.

In addition to providing a design and engineering service, APV also canfabricate equipment from various stainless steels, titanium, Monel, or nickel in sizesranging from pilot plant uni ts to column diameters of 12 feet or more. lnternals caninclude valve, sieve or bubble cap trays as well as many types of metal or ceramicpacking.

In effect, a complete distillation service from the preliminary design throughstartup is available from a single, responsible source.

Fig 1Pic tured above are six la rge APV sod ium carbonateregeneration columns being install ed as key elementsin an electric power generation pollut ion controlprogram. Fabricated of stainless steel, the columnsrise 68 to 70 feet h igh by 3 to 4 feet i n diameter.

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FI G 2Portion o f f lue gas desulfurization systemadsorption beds normally are used in pairs so that th e air f low can be diverted tothe secondary bed wh en required.On occasions, the condensate is i n the fo rm of a moderately dilute misciblemixture. The solvent then has to be recovered by distillation. This wou ld applyespecislly to ethyl alcohol, acetone type solvents.A B S O R P T I O NWh en carbon adsorption cannot be used because certain solvents either poisonthe activated carbon bed or create so much heat that the bed can ignite,absorption i s an alternate technique. Solvent i s recovered by pump ing he solventladened air stream through a colu mn countercurrently to a wa ter stream whi chabsorbs the solvent. The air f rom the top of t he col umn essentially is solvent freewhi le the dilute water/solvent stream discharged from the co lumn bottomusually i s concentrated in a distil lation col umn . Absorption also can be applied incases whe re a n oil rather than water is used to absorb certain organic solventsfrom a n air stream.A 2 E O T R P E SDuring distillation, some components for m an azeotrope at a certain stage of thefractionation a nd requ ire a thi rd component to break the azeotrope and achieve ahigher percentage of concentration. In the case of ethyl alcohol and water, forexample, a boiling mixture containing less than 96% by weight ethyl alcoholproduces a vapor richer in alcohol than in water and i s readily disti l led. A t the96% by wei gh t point, however, the e thyl alcohol composition in the vapor remainsconstant, i.e., the same composition as the boiling liqu id. This is know n as theazeotrope composition and further concentration requires use of a process kn ow nas azeotropic distillation. Other common fluid mixtures wh ich form azeotropesare formic acid/water, is0 propyl alcohol/water, and is0 butanol/water.

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FIG 3 System for recovering butanol fro m butano l/water mlxture

AZEOTROPIC DISTILLATIONIn a typical azeotropic disti llation procedure, a thi rd component such as benzene,is0 propyl ether o r cyclohexane is added to a n azeotropic mixture such as ethylalcohol/water to for m a ternary azeotrope. Since the ternary azeotrope is richer inwater than the binary ethy l alcohol/water azeotrope, water is carried over the topof the co lumn. The azeotrope, when condensed, forms t wo phases. The organicphase is refluxed to the co lumn whi le t he aqueous phase is discharged to a thir dcolumn for recoveryof the entraining agent.

Certain azeotropes such as the n-butanol/wate r mixture can be separated ina tw o column system with out t he use of a third component. Wh en condensed anddecanted, this type of azeotrope forms t w o phases. The organ ic phase is fed backto the primary column and the butanol recovered from the bottom of the stil l. Theaqueous phase, meanwhile, is charged to the second column with the waterbeing taken from the c olumn bottom. The vapor from the top of both columns iscondensed and the condensate ru n to a c ommon decanter.EXTR ACTIV E DISTIL LATI0 NThis technique is somewh at similar to azeotropic dist il lation i n that it is designedto perform the same type of task. In azeotropic distillation, th e azeotrope is brokenby carrying over a terna ry azeotrope at the top of the column. In extractive distil-lation, a very h igh bo iling compound i s added and the solvent removed at the baseof the column.

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S T R I P P I N GIn distillation terminology, stripping refers to the recovery of a volatile componentfrom a less volatile substance. Again, referring to the ethyl alcohol/wa ter system,stripping is done i n the f irst column below the feed point where th e alcohol entersat about 10% by weigh t and the result ing l iquid from the column base containsless than .02 alcohol by wei ght . This is kn ow n as the stripping section of thecolumn. This technique does not increase the concentration of the more volatilecomponent but rather, decreases its concentration in the less volatile component.

A stripping col umn also can be used wh en a l iquid such as water contam-inated by toluene cannot be discharged to sewer. For this pure stripping duty, thetoluene is removed wit hin the c olumn wh ile vapor f rom the top is decanted forresidual toluene recovery and refluxing of the aqueous phase.R EC T IF IC AT1 NFor rectification or concentration of the more volatile component, a top section ofcolumn above the feed point is required B y means of a series of trays and ref luxback to the top of the column, a solvent such a s ethyl alcohol can be concentratedto over 95% by weightBATC H D ISTILLAT I0 NWhen particularly complex or small operations require recovery of the morevolatile component, APV can offer batch distillation systems of various capacities.Essentially a rectification type process, batch distillation involves pumping a batchof liquid feed in to a tank whe re boiling occurs. Vapor rising through a columnabove the tank combines wi th reflux coming down the column to effect concen-tration. This approach is not too effective for puri fying the less volatile component.

For many applications, batch distillation requires considerable operatorintervention or alternatively, a significant amount of control instrumentation.While it is more energy intensive than a continuous system, steam costsgenerally are less significant on a small operation. Furthermore, it is highlyflexible and a single batch co lumn can be used to recover many different solvents.C O N T l NU O U S D l STl LLATIONThe most common form of distillation used by the chemical, petroleum andpetrochemical industries is the continuous mode system.In continuous distil lation, feed constantly is charged to t he colum n at a pointbetween the top and bottom trays. The section above the feed po int rectifies themore volatile component while the column section bel ow the feed point strips outthe more volatile component from the less volatile liquid. In order to separate Ncomponents wi th continuous disti l lation, a min imu m of N - l disti llation col umnsis required.T U R N D O W NThe tur ndown ratio of a col umn is an indication of the operating f lexibil i ty If acolumn, for example, has a turn down ratio of 3, i t means that the colum n can beoperated efficiently at 33% of the design maximum throughput

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system com ponentsCOLUMN S, INTERNALS, INSTRUMENTATIONAND AUXILIARY EQUIPMENT

The following briefly defines the many components required for a distil lat ionsystem and th e many variations in components that are available to meet diff erentprocess conditions.C O L U M N S H E L L SA distil lation col umn shell can be designed for use as a free-s tanding module orfor instal lation wi th in a suppo rting steel structure. Generally speaking, unlessa column i s of very small diameter, a self-supporting co lumn s more economic. Thisholds true even under extreme seismic-3 conditions.

APV has bui lt distillation columns in carbon steel, 304stainless steel, 316stain-lesssteel, Monel, itanium, and lncoloy 825. Usually, it is more economic to fabricatecolumns in a single piece withou t shell flanges. This technique not only simplifiesinsta llation but also eliminates danger of leakage during operation. Columns over80 fe et in length have been shipped by road without transit problems.

While columns of over three feet diameter normally have been transportedwit hou t trays to prevent dislodgment and possible damage, recent and more eco-nomic techniques have been devised for factory installation of trays w i t h the traymanways omitted. After th e co lum n has been erected, manways are added and, atthe same time, the fi tter inspects each tray.

Wi th packed colum ns of 20-inch diameter or less wh ic h use high efficiencymetal mesh packing, the packing can be installed prior to shipment. Job site packing,however, is the nor m for larger columns. This prevents packing from bedding do wnduring trans it and leaving voids tha t wou ld reduce operating efficiency. Randompacking always is ins talled after delivery except for those rare occasions whe n acolumn can be shipped in a vertical position.

Access platforms and interconnecting ladders designed to OSHA standardsalso are supplied for on site attachment to free-standing columns.

Installation usually s quite simple since columns are fitted wi th lifti ng lugs and,at the fabrication stage, a temp late is drill ed to match support holes in the columnbase ring . Wit h these exact template dimensions, supporting bolts can be preset forquick and accurate coupling as the column is lowered in to place.C O L U M N I N T E R N A L SDuring recent years, the development of sophisticated computer programs andne w materials has led to many innovations in th e design of trays and packings formore efficient operation of distil lation columns. I n designing systemsfor chemical,petro leum and petrochemical use, APV specialists take fu ll advantage of availableinternals to assure opt imum distillatioh performance.t r a y devicesWhile there are perhapsfi ve basic dist illat ion rays suitable for industr ial use, thereare many design variations of diff ering degrees of importance and a confus ing arrayof tradenames applied to their products by tray manufacturers. The most modernand commonly used devices inc lude sieve, valve, bubble cap, dual flow, and baffletrays each wi th its advantages and preferred usage. Of these, the sieve and valvetype trays curren tly are most often specified.

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FIG 4

FIG 4Tray component ter minologyFIG 5Typical tray of integral truss designFIG 6 Cartridge tr ay assemblyPhoto courtesy of Koch Enqineer i w Co , Inc

FIG. 5

For a better understanding of tray design, Figuie 4d ef in es and locates typicaltray components. The material of construction usually is 14 gauge with moderntrays adopting the integral truss design whi ch simplfies fabrication. Atypica l t russtray is sho wn in Figure 5.For co lumns less than thr ee feet in diameter, it is notpossible to assemble the truss trays in the column. Trays therefore m us t be preas-sembledon rods in to a cartridgesection or loading nt otheco lumn . Figure Gshowsthis arrangement i n scale model size.

The hydraulic design of a tray is a most important factor. The upper operatinglim it generally is governed by th e flood point although, i n some cases, entrainmentalso can restrict performance. By forcing some liq uid to f lo w back up th e column,entra inment reduces concentration gradients and lowers efficiency. A column alsocan flood by downcomer backup whe n tray design provides insuf ficien t downcomerarea or w hen the pressure drop across the tray is high. When th e downcomer isunable o hand le all the liquid involved, the trays start to fill and pressures ncrease.This also can occur wh en a highly foaming l iqu id is involved. Flooding associatedwit h h igh tray pressure drops and small tray spacing takes place wh en the requ iredliquid seal is higher than the tray spacing. Downcomer design also is particula rlyimportant at high operating pressures due to a reduction in th e difference betweenvapor and liquid densities.

The lower limi t of tray operation, meanwhile, is influenced by the amount ofliquid weeping from one tray to th e next. Unlike the upwa rd force of entrainment,weeping l iquid flows in the norma l direction and considerable amounts can be

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tolerated before column efficiency is significantly affected. As the vapor ratedecreases, however, a point eventual ly i s reached wh en all the l iquid is weepingand there is no liquid seal on the tray. This is known as the du mp point below whichthere is a severe drop i n efficiency.S I E VE T R A YThe sieve tray is a l ow cost device wh ic h consists of a perforated plate that usuallyhas holes of 3/16 inch to one inch diameter, a downcomer, and an outlet weir.Although inexpensive, a correctly designed sieve tray can be comparable to otherstyles in vapor and liquid capacities, pressure drop, an d efficiency. For flexibility,however, it is inferior t o valve a nd bubble cap trays an d sometimes is unacceptablefor low liquid oads wh en weeping has to be minimized.

Depending upon process conditions and allowable pressure drop, the tu rndownratio of a sieve tray can vary from 1.5 to 3 and occasionally higher. Ratios of 5 assometimes claimed can be achieved only whe n t he tray spacing is large, availablepressure drop is very high, li quid loadings are high, and th e system i s non-foaming.For many applications, a tu rn do wn of 1.5 s qui te acceptable.

It also is possible to increase the flexibility of a sieve tray for occasional lo wthroughput operation by maintaining a h igh reboil and increasing the reflux ratio.This may be economically desirable when the l ow throughput occurs for a smallfraction of the operating time. Flexibility likewise can be increased by the use ofblanking plates to reduce the hole area. This is particularly useful for initial opera-t ion when it is proposed o increase th e plant capacity after a fe w years. There is noevidence to suggest that blanked-off plates have inferior performance to unblankedplates of similar hole area.D U A L F L O W TR A YThe dual flo w tray is a h igh hole area sieve tray with out a downcomer The liquidpasses do wn through th e same holes throug h wh ic h the vapor rises Since nodowncomer is used, the cost of th e tray is lo wer than that of a conventional sievetray In recent years, use of t he dual fl ow tray has declined somewhat because ofdifficulties experienced w i th partial Iiquid/vapor bypassing of the tw o phases, par-ticularly in larger diameter columns The dual f l ow colu mn also has a veryrestrictedoperating range and a reduced efficiency because there is no crossflow of liqu idV A L V E T R A YWhi le the valve tray dates back to the rivet type first used in 1922, many designimprovements and innumerable valve types have been introduced in recent yearsA selection of modern valves as illustrated provide the following advantages1 Throughputs and efficiencres can be as h igh as sieve or bubble cap

trays2 Very h igh flexibility can be achieved and turn down ratios of 4 to 1 areeasily obtained wi th out having o resort to large pressure drops at thehigh end of the operating range3 Special valve designs w i t h venturi shaped orifices are available forduties involving l ow pressure drops4 Alt hough slightly more expensive th an sieve trays, the valve tray isvery economical in view of its numerous advantages

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FIG 7( lef t) Special two-stage valve w it hl ightweight or i f ice cover for completeclosing(below ) Two typical general purposevalves whic h may be used in all typesof services

5. Since an opera ting valve is continuously in movement, the valve traycan be used for light to moderate fouli ng duties. APV has successfullyused valve trays on brewery effluent containing was te beer, yeast,and other materials wi th foul ing tendencies.

B U B B L E C A P T R A YAlthough many bubble cap columns still are in operation, bubble cap trays rarely arespecified today because of hig h cost factors and th e excellent performance of t hemodern valve type tray. The bubble cap, however, does have a good tu rn dow n ratioand is good for lo w liquid loads.B A F FL E T R A YBaffle trays are arranged in a tower in such a manner that the l iquid flows do wn thecolum n by splashing fro m one baf fle to t he next lower baffle. The ascending gas orvapor, meanwhile, passes through th is 'curtain' of liquid spray.

Although the baffle type tray has a lowefficiency, it can be usefu l in applicationswh en the l iquid contains a h igh fraction of solids.packingsFor many types of duties, particularly those involving smal l diameter columns,packing is the most economical tower internal. One advantage is that most packingcan be purchased rom stockon a cubic foot basis. In addition, the mechanical designandfabricationof a packed column isqui tesimple. Disadvantagesof packing includeits unsui tabil ity or fou ling duties, breakage of ceramic packing, and in APV experi-ence, less predictive performance, parti cular ly at lo w liquid loads or high columndiameters.

The most widely used packing is the random packing, usua lly Rashig rings, Pallrings and ceramic saddles. These are available in various plastics, a nu mber of di f-

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ferent metals and, with the exception of Pall rings, inceramic materials. While packings in plastic have theadvantage of corrosion resistance, the self -wett ing ab ilityof some plastic packing such as fluorocarbon polymerssometimes is poor, particularly in aqueous systems. Thisconsiderably increases the HETP wh en compared w it hequivalent ceramic rings .High efficiency metal mesh packingas shown nFig. 8have fou nd increasing favor in industry during recentyears. One type uses a woven w ir e mesh wh ic h becomesself -wett ing because of capillary orces. This he lps estab-lish good liquid distribution as the liquid f lows throughthe packing in a zig-zag pattern .

FIG 8 Segment of highe f f i c iency meta l meshpacking

If properly used, hig h efficiency packings can provide HETPvalues n he range6t o1 2 in.Thi scan reducecolumn heights,especial lywhena large numb eroft raysis required. Such packings, however, are veryexpensive and each application mustbe studied in great detail.Wi th both random and, in particular, hi gh efficiency packing, considerableattention must begiven tocorrec t liquid distribution. Certain types of high efficiencypacking are extremely sensitive to liquid distribution and should not be used incol umn s over t w o feet i n diameter. Positioning of these devices and the design ofliquid distribution and redistribution are important factors that should be deter-mined only by experts.IN S T R U M E NTAT10 NOne oft he most importantaspectsof anydistil lation system isth e abil i tyto maintainthe correct compositionsfrom the col umn s by means of proper controls and instru-mentation. Wh il e man ual controls can be supplied, thi s approach rarely is usedtoday i n he United States. Man ua l control involves the extensive use of rotametersand thermometers which, in turn, involves high labor costs, possible energywastage and, at times, poor quality control . Far better control i s obtained t hroug hthe use of pneumat ic or electronic control systems.P N E U M A T I C C O N T R O L S Y S T E M SThe most co mm on for m of distil lation colu mn instrumentation is the pneumatictype analog cont rol system Pneumatic instruments have the advantage of beingless expensive tha n other types and, since there are no electrical signals required,there is no risk of an electrical spark One disadvantage is the need to ensure thatthe air supply has a very l ow de w point (usually -40F) to prevent condensation inthe loopsE L E C T R O N I C C O N T R O L S Y S T E M SEssentially, there are th ree types of electronic control systems.

a) conventional electronic instrumentsb) electronic systems w i th all f ield devices explosion proofc) intrinsically safe electronic systems

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Most disti l lation duties involve at least one flammable l iquid whic h is beingprocessed i n both the vapor and liquid phases. Since there always is the possibilityof a leak of liquid or vapor, pa rt icular ly from pu mp seals, i t is essential for completesafety that there be no source of ignit ion in the vicinity of the equipment. Whilemany ins truments such as controllers and ala rms can be located in a control roomremoved rom the process, all local electronic inst ruments mus t be either explosionproof or in trins ically safe.

Wi th explosion proof equipment, elec trical devices and wi ring are protected byboxes or conduit wh ic h will contain any explosion that may occur.

Wit h intrinsically safe equipment, barriers limi t the transmission of electricalenergy to such a l ow level that it i s not possible to generate a spark. Since explosionproof boxes and condui ts are not required, wir ing costs are reduced.

For any int rinsicall y safe system to be accepted for insurance purposes, F M(Factory Mutual )or CSA(CanadianStandards Association) approval usually must beobtained. This approval applies to a combinat ion of barriers and field devices. There-fore, w he n a loop incorporates such inst rum entsfr om different manufacturers, it i sessential to ensure that approval has been obtained for the combination ofinstruments.A U X I L I A R Y E Q U I P M E N TIn any distillation system, the design of aux iliary equipment such as the reboiler,condenser, preheaters and product coolers is as important as the design of thecolumn itselfR E B O I L E RAlthough there are man y types ofrebo i lers , the she l l and tubetherm osyph on reboiler is usedmost frequently Boiling wi th in thevertical tubes of the exchangerproduces liquid circulation andeliminates the need for a pump Atypical arrangement is shown inFig 9

For certa in duties, particularlywhen the bottoms liquid has atendency to fou l heat transfer sur-faces, it is desirable to pump theliquid around the heat exchangerSince boiling can be suppressedby use of an orifice pla te at the ou t-let of the unit, fouling is reducedThe liquid being pumped is heatedunder pressure and then is flashedinto the base of t he co lumn wh erevapor is generated

A n alternate approach is theuse of a p late heat exchanger as aforced circulation reboiler

FIG 9 Typica l shell and t u b ethermosyphon reboiler arrangement

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Wi th this technique, the very high l iquid urbule nt fl ow which i s induced withinthe heat exchanger through the use of multiple corrugated plates holds fouling toa min imum. Meanwhi le, th e superior rates of heat transfer that are achievedreduce the surface area required for t he reboiler.CONDENSERSSince most distillation co lumn condensers are of shell and tube design, the proces-sor has the option of condensing on either the shell or tube side. From the processpoint of view, condensation on the shell side i s preferred since there is less subcool-ing of condensate and a lower pressure drop is required.These are important factorsin vacuum duties. Furthermore, w i th cooling water on the tube side, any fouling canbe removed more easily.

Tube side condensation, on the other hand, can be more advantageouswhenever process flu id characteristics dictate the use of more expensive, exoticmater ials. Capital cost of the unit then may be cut by using a carbon steel shell.PR EHEATER S / C00 LERSThe degree to whi ch flu ids are aggressive to metals and gasketing materials gener-ally determines the selection of plate or shell and tube preheaters and productcoolers.

If flu ids are no t overly aggressive toward gasket materials, a plate heat ex-changer is an extremelyefficient preheater since a veryc lose temperature approachmay be achieved. Added economy s realized by using heat from the top and bottomsproduct for all necessary preheating.

While plate type units can be supplied w i th compressed asbestos fiber gaskets,very aggressive duties normally are handled in a number of tubular exchangersarranged in series to generate a good mean temperature difference. The use ofmultiple tubular units obviously is more expensive than a single plate heatexchanger but is unavoidable for certain solutions such as aromatic compounds.VENT CONDENSERIt is norm al practice on distillation systems t o use a vent condenser after the ma incondenser to minimize he amount of volatiles being driven off in to he atmosphere.Usuallyof the shell and tube type, the vent condenser wi ll have about one-tenth thearea of the mai n unit and wi l l uti lize a chil led water s upplyt ocool he non-conden-sible gases to about 45 -5OoF .PUMPSSince most distil lation duties involve fluids that are highly flammable a nd have alo w flash point, i t is essential that explosion-proof (Class 1, Group D, Division 1)pu mp motors be supplied. Centrifugal pumps generally are specified since they arereliable and can provide the necessary head and volumetric capacity at moderatecosts.

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FIG 10 Packaged disti l lati on system during fabrication

packageddist i I Iat ions y s t e m sFor distillation systems of moderate size, it of ten can be economical to fabricate an dsupply columns, heat exchangers, tanks, pumps and other elements as a fu ll y pre-assembled package.This techn ique was used or th e distillation unit p ictured aboveduring fabrication as a complete, ready to transport system. For th is particularproject whi ch wa s designed to separate both ethyl alcohol and is0 propyl alcoholfrom water, t hree co lum ns of relatively small diameter were positioned w it h as-sociated components and instrumentation on a prefabricated structure. Whi le in-sulation normally w ou ld not be supplied in order to preclude the possibility ofdamage during shipment, major elements on this project were insulated at thecustomer's request and were t rucked 600 miles to the Massachusetts job sitewithout problems. Final size of the package was 65' x 12' x 8'.

The APV packaged approach proved to be beneficial in a number of ways.Factory fabrication and installation of piping was far more economical th an fie ldfinishing wh il e erection time for the system wa s reduced considerablywith signifi-cant savings in local labor costs.

A n alternate approach for IaFger systems is the fabrication of equ ipmentmodules. For one large batch distillation unit, APV supplied he co lumn tself as onemodule, the batch tank and reboiler as t he second, and th e heat exchangers,decanter, pumps an d auxiliary items as th e third . This modular const ruction prior toshipment w as successful despite the fact th at th e batch tank w as over 8 feet indiameter.

For systems involving colu mns in excess of six feet diameter, prepackaginggenerally i s not feasible. Components of such sys tems normally have to be installedand piped at the job site.

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typ ic al in s tal lat io n sfuel a l co h o lTo provide the 199 proof ethyl alcohol required for blending with gasoline duringthe production of gasohol, a compl icated distillation system is needed. A minimumof a three column system is required since ethyl alcohol forms an azeotrope withwater. This azeotrope has to be broken in a separate co lumn through the use of anentraining agent which subsequent ly is recovered for reuse.

The simplified flow diagram shown below represents the APV power alcoholtechnique. This system uses double effect distillat ion so that heat can be cascadedto the reboiler on the dehydrat ion still. Due to this double effect approach, steamconsumption is as low as 16 Ibs/gallon of alcohol product with a feed beer streamcontaining 10% by volume alcohol. The design of the beer still is particularlyimportant and, since the feed has a tendency to foul the metal surfaces, APV offerstray columns.

For certain duties, APV can provide a forced circulation Paraflow plate heatexchanger for the reboiler. This arrangement minimizes fouling and eliminates theneed for direct steam inject ion which dilutes the stillage and causes more effluent.This is an important feature of the design.

APV has operating experience with a number of different entraining agentsincluding benzene, isopropyl ether and cyclohexane.

1 D t G A S S E R 2 DEGASSER JREBOILER 4 F EE D S E E R 6 D E H YD R A T l OH 7 C O N U E N S E W 8 F I N A L P P R E HE A T IN G 1 OC O ND E NS ERCONDENSER PREHEATER STILL COLUM N REBOILER CONDENSER CONDENSER

11DECANTER 12 V E N T l 3 E N T A A t N E R 14 REBOILER lSALEOHOLCONDENSER RECOVERY COOLER

COLUMN

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20 million gallons per yearethanol distillat ion systemConsists of dehydration andentrainer columns designedto take 190 proof ethanolfrom surplus wine to a fuelgrade gasoline additive

As shown by the schematic, a typical fuel ethanol distillation system takes afermented corn beer at 10% by volume alcohol and concentrates the alcohol to99.7 by volume. The system consists of a beer still, dehydration column andentrainer recovery column. Wit h this particular application, the dehydration columnIS operated at approximately 70 psig and the heat from the vapors at the top of thecolumn is used to provide 60% of the heat required for the beer still. This system,incidentally, is designed so that either the beer st ill or the dehydration column canbe operated individually.

Due to the disparity between wine production and consumption over the pastyears, a large surplus has accumulated and has been stored at a concentration of95% by volume. To exploit the recent increase in demand for fuel alcohol, a numberof companies have been established specifically to dehydrate wet ethanol to a fuelgrade product. Pictured is a typical system that was ordered when the demand forfuel grade ethanol was extremely high. Working under a tight fabrication schedule,all of the components were produced wi th in ten weeks from the date of the order.The system subsequently was shipped to the Caribbean, insta lled and put on streamto design capacity wi th in a further eight weeks.

This is but one of six large fuel alcohol distillation systems supplied withinrecent months which have a total combined production capacity in excess o twohundred million gallons per year. Numerous other small systems also have beenprovided for the recovery of ethyl alcohol.

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meth no l recycledDesigned to recover methanol from solvent laden air, this absorber and dist illationsystem uses wate r to continuously assimilate up to 95 %of t he MeOH contained inan airstream of up to 9 00 0c fm w hi ch originates in a drying operation. After absorp-tion is carried out in a 52 high by 6 diameter c olum n equipped wi th valve trays,26,000 Ibs/hr of enriched water is passed through a regenerative Paraflow platepreheater to t he adjacent 75b y 36 diameter disti l lation column. Here, alcohol inthe feed i s str ipped down to .02 w/w. While bottoms are recirculated to t he ab-sorber, enri ched alcohol above th e feed poin t is rect ified in the upper section of th edistillation column.

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antibiotic solvents

Engineering personnel give final he k to IPA system while xylene column in the backgroundawaits shipment

For a major project involv ing the production of a ful l range of antibiotics, APV hascontracted to engineer and furni sh nearly a score of disti llation columns w it h allthe auxiliary equipment needed to recover eight basic solvents from 2 8 differentsources. Included are bu tyl acetate, butanol, xylene, acetone, triethylamine, iso-propyl alcohol, toluene, and me thyl isobutyl ketone (MIBK), many with two to f ivepoints of origin and the latter from eight separate sources. Each wi ll be processedindividually to eliminate any possibility of cross-contamination of the antibiotics.

After a year of laboratory tests, more tha n 16,000 hours of engineering workand 14 volumes of specifications, the fabrication of 19 columns, nine large solventreboil tanks and nearly 100 plate or tubular heat exchangers was undertaken. Tothis wa s added scores of pumps, miles of piping, and system cont rol modules in-volving close to 1500 valves, indicators and controllers. Several pieces of 78-footlength and three to f our feet in diameter we re shipped as integral units whi le t helargest column measuring 106 feet by 3 feet in diameter was transported in twosections. Components are be ing installed in 22 bays wit hin a 400 by 33 foot struc-ture wi th 15 feet hig h firewa lls between solvent modules.

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flue gas desulfurizationAs an integral part of an experimental pollu tion control program, APV is providingSIX sodium carbonate regeneration co lumnsfor he Rockwell International AqueousCarbonate system to be installed at a large utility power station. This demonstrat ionplant isdesigned to reduce the risko f 'acid rain'caused by sulfu r dioxide emissionsproduced wh en coal is burned for electric power generation.Under the experimental process, flue gas from th e po wer plant is diverted to aspray dryer scrubber wh er e the sul fur dioxide reacts with an atomized sodiumcarbonate solution. This produces a fine powder essentially consisting of sod iumsulfite. The gas clean subsystem is designed to remove 90% or more of the SOfrom approximately 300,000 scfm of flue gases and is expected to be effective wit hgases containing from 200 to more than 2000 ppm of sulfu r dioxide.

For the adjacent regeneration subsystem, APV engineered and fabricated sixcolumns which range from 3 to 4 feet in diameter by 68 to 70 feet high. Theseinclude a precarbonator, tw o crystallizers and carbonators (one each on- line, onespare), and a decomposer.In th is phase of the process, th e dry spent absorbent collected i n the gascleaning subsystem IS reducedwi th coke and then i swa ter quenched. The resultantprocess 'green liquor' is charged to th e APV precarbonator where contact with agas stream induces a series of exothermic reactions. After fil tra tion and cooling,the partially precarbonated iquor is pumped to the crystallizer where contact w i thCOz-rich gas from the carbonator produces a sodium bicarbonate slurry. Withsolids content reduced by filtration, t his solution is further crystallized in the car-bonator before being fed to th e fi nal column, the decomposer. Here, t he sodiumbicarbonate is thermallydecomposed o a sodium carbonate solution wh ic h is storedfor eventual reuse in scrubbing more flue gas. Meanwh ile, the hydrogen sulfidegas whi ch evolves during the carbonation cycle is converted to elemental sulfur ina conventional Claus plant.

The regeneration subsystem is sized to produce 12 tons per day of elementalsulfur and regenerate 5000 Ibs/hr of sodium carbonate for reuse in SOz removal.

Trays are positionedprior to closing the column 21


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MEK/heptane/waterAs engineers redesign the space shuttle and eliminate problem areas, there's oneproven item that won't cause concern. During launching, a large throwaway fueltank mounted near the engines has been protected by insulating tiles againstextremely high temperatures. To recover the MEK (methyl ethyl ketone) used to affixthe tiles, APV supplied a preassembled batch and/or continuous mode systemwhich included a 2' diameter by 45' high distillation column and a mechanicallyagitated extraction column. After extracting heptane from a mixture containing MEKand water, the remaining feed is subjected to double disti llation. The MEK azeotropefirst is stripped and the organic phase from the decanter then is ru n back throughthe column. This strips the MEK water azeotrope and provides pure MEK for reuse.acetone/water/PETNBuilt to recover 99% minimum by weight acetone from a water mixture which alsoconrains traces of explosive penta erythr ito l tetranitrate (PETN), th is preassembleddistillation system was designed around a 2' diameter by 40' high rectifying/strippingcolumn of 304 stainless steel. The acetone is used in the manufacture of explosivesand during the recovery phase, some of the PETN precipitates out and contaminatesthe column. To counteract this buildup, the system design incorporates a provisionfor the CIP (cleaning in place) of the entire column wi th 99% acetone as required.

Preassembled to simplify installation, the packaged system included thecolumn, reboiler, condensers, 500 gallon feed tank, control panel and normalpumps, piping and wir ing mounted wi th in a steel frame. System capacity is 4235Ib/hr of feed.tri solvent recoveryArranged for batch operation, a distillation system was supplied to processapproximately 7000 gallons per day of solvents and recover methyl alcohol,isopropyl alcohol, and methyl cellosolve.

The system consisted of a 4000 gallon batch boiling tank combined wi th a 3'diameter by 40' high distillat ion co lumn and all required associated equipment. Toprotect against corrosion from certain chlorine impuri ties, monel was used in thefabrication of the batch tank, the lower part of the distillation tower, the reboiler,and all components around the batch tank. The system was supplied in the form ofthree fully preassembled modules, complete wit h a number of product tanks.cyclohexanone from waterTo recover a solvent while protect ing against possible environmental pol lut ion, APVdeveloped a two column system for use by a manufacturer of video tape. Themixture of cyclohexanone and water originates from carbon adsorbers which areused to clean the air within the tape manufacturing area. Since the solvent andwater form an azeotrope, a two column arrangement was required to purify themixture. The first column strips the cyclohexanone/water azeotrope and allowspure water to be discharged from the column base. The second column, meanwhile,takes an enriched cyclohexanone and strips out the remaining water to provide avirtually pure solvent. Designed to process approximately 3000 gallons per day of astream that contains about 80% cyclohexanone and 20% water, the solvent recoveryfactor is 99.9% while the discharged water carries less than 2 ppm of cyclo-hexanone. The entire dis tillat ion system including all necessary components, pipingand wir ing was shop preassembled and shipped as a unit ready for immediate WestCoast installation.

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acetone from waterFor a pharmaceutical process, APVhas provided a complete distillationsystem designed to recover up to99 6 of acetone from water wh ichalso contains traces of ethanol andMlBK System components include a55' h igh by 3'3" diameter colum nequipped with valve trays, ma in andvent condensers, a forced circulationSeries R57 Paraflow plate reboiler,accumulator vessel, fu ll ins trumenta-tion, and a three section 'Junior'Paraflow to preheat feed regenera-tively with top product and columnbottoms The design also includesmultiple take-off points to preventM l B K and/or ethanol buildup Feedrate is 38 0 gallons/hr of solvent andwater mixturedeactivating n itroIn a somewhat unusual application,an APV sieve tray scrubber is in-corporated into the process for manu-facturing small fuel pelletsfor missilepropulsion. Th is 20' high by 4' di-ameter unit was designed to scrub

acetoneand ethanol w i th causticsodai n th e presence of traces of n itro-glycerin. The acetone and ethanolare removed from an air f lo w of up to550 0cf m originating in the propellantdrying room wh ile the caustic sodaabsorbs and denitrates the tracequantities of nitro.

Fr eon /MCF dist i l lat ionUtilizing an APV batch distil lationsystem, a producer of printed circuitscurrently is separating a two com-ponen t mi xtu re of Freon TF andmethyl chloroform with recoveriesrespectively of better than 98 % and90% by volume. The 28 ' high by 12"diameter distillation column equippedwi th stainless steel mesh packing isfitted to a 1450 gallon batch tank, asolvent holding tank, reboiler, andsystem controls on a platform only12' long and 8' wide. Feed rates areapproximately 12 00 gallons per day,double the processing capability of asimilar packaged system whic h wa ssupp ed previous y.

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a c o m p a r is o n b e t w e e n b e n zen ea n d i s p r o p y l e t h e r f o r use nt h e d eh y d r at io n ofis p rop y l alcohol

In the pharmaceutical industry, it often i s necessary to recover solvents by disti llationboth to save costs and to reduce the volume of effluents. Since one company may use anumber of dif ferent solvents, frequently in small quantities, it i s essential that the disti l-lation system installed or recovery operations be very flexible.

A typical case may be seen at Scientific Protein Laboratories (SPL) of Waunakee,Wisconsin. This small manufacturer of specialty pharmaceuticals currently i s recoveringacetone, methanol and i s 0 propyl alcohol ( (PA ) from water mixtures. The distillationsystem, designed and fabricated by APV, easily handles the various duties involved, i seconomical to operate, and representsa moderate capital investment.IPA dehydrationIt i s a relatively straightforward engineering task to design either a batch or continuousdisti llation system to separate both acetone and methanol from water or to concentrateIPA t o the azeotrope composition. The more volatile components can be collected a t thetop of the column with the reflux ratio being controlled by monitoring the temperaturea t an adjacent point. This controls the product take-of f rate. I f there are sufficienttraysin the column, the required separation wi ll be achieved.

Dehydrat ion of the IPA/water azeotrope, however, i s more difficult since it i s neces-sary to break the azeotrope. This was done a t SPL in a continuous operation that used anentrainer t o form a ternary azeotrope. The duty was to dehydrate 4.5 GPM of IPA/waterazeotrope containing 12.6%w /w water. The product was to contain less than 0.1 w/wwater and less than 0.1 w/w of the entrainer.early experimental workAlthough data were abundantly available on the dehydration of ethanol using benzene asentrainer I ) , there was very l i t t l e information to be had some years ago on the dehydra-tion of IPA. I t therefore was decided to carry ou t t e s t work on a small laboratory columnto determine the optimum design having minimum energy requirements. Rough calcula-tions indicated that almost 30 theoretical trays would be required so the t e s t unit, a 1 'A''diameter column with metal mesh packing, was se t upw it h the equivalent of 30 theoret-ical trays. The overall packed height was 60".

The t e s t column was operated in continuous mode wi th sufficient benzene a t the topto hold the temperature close to that o the ternary azeotrope boiling point. The opti-mum feed point was found to be three theoretical trays from the column top . Higher feedpoints reduced the efficiency of the formation of the t e r n q azeotrope and caused areduction n the water removal rates for a constant reboil rate.

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wENTRAINERB I N A R Y

T E R N A R Y

ENZENEcomposition B.P. O F

Benzene/water 8.83% water 156.6BenzenellPA 33.3% IPA 161.5IPA/water 12.6% water 176.5Benzene 73.8%IPA 18.7% 151.7Water 7.5%

S 0 PROPYL E TH ER~composition B.P. O F

IPE/water 4.5% water 144IPE/IPA 16.3% IPA 151IPA/water 12.6% water 176.5IPE 89.0%IPA 6.0% 143Water 5.0%

Feed was continuously pumped into the column where it came intocontact with thebenzene. The ternary azeotrope (Table 1 ) was formed a t the top of the column and wascarried over as vapor to the condenser. The condensate was cooled and the resultanttwo-phase liquid was decanted in to two layers. The theoretical composition of the tw olayers i s charted in Table 2.All of the organic phase was returned to the column as refluxand the aqueous layer was discharged.

The lower stripping portion of the column can be considered as two sections: thebot tom involved wi th the stripping o f benzene from IPA and the upper wi th strippingwater from IPA and benzene. The experimental work confirmed that about 27 theoret-ical trays were required in the lower section in order t o achieve the composition that i srequired fo r the product.

Since all the condensed vapor a t the t op o f the column was decanted and only theorganic phase returned to the column, there was no clearly defined reflux ratio as i s thecase with most distillation operations. Additionally, since the reflux was no t in equilib-rium wi th the vapor a t that point, any calculation of the conditions a t the t op of thecolumn was difficult and unreliable. Therefore, for a given feed rate the reboil rate wasvaried in order to study the effect on product quali ty. As the reboil rate was reduced, twophenomena would occur. When insufficient azeotrope was being produced, the removalrate of the water was less than required and the water content of the bo ttom productincreased. A t low reboil rates, there also was a tendency for higher benzene contents tooccur a t the column bottom.

Theoretically, the minimum heat required to operate the tes t column was 600 Btu/lbof feed. This was based on no heat losses, 100%decanter efficiency, and both the feedand reflux a t their respective boi ling points. In practice, it was found that 840 Btu/lb offeed was needed to give the proper product composition.

Although the approach t o this problem was somewhat empirical, the work did provideaccurate data on which t o base the design of a full scale continuousdistillation column.The first such column was installed and commissioned in the United Kingdom in 1966and always has operated satisfactorily.the SPL columnWhile benzene in itially was designated as the entrainer in the original column design fo rSPL as a result of the experimental work, subsequent FDA restrictions against carcino-gens in the pharmaceutical industry prevented i t s use for full scale production. Theentrainer therefore was changed t o i s 0 propyl ether (IPE). Fortunately, the process fo reither entrainer i s very similar so no major plant revisions were necessary.

Reference t o Table 2shows that the ternary azeotrope of IPA/I PE/water entrains only3.9 Ibs of water per 100 Ibs of azeotrope compared wit h 6.2 Ibs fo r the correspondingbenzene ternary. However, Table 3 indicates that the latent heats of both IPE and i t s

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FIG.1VAPOR LlOUlO EQUILIBRIUM DATA

MOLE FRACTIONBENZENEOR IPEIN VAPOR

T A B L E 2

IPA 18.7 18.8ne I 7;:; 179.8.4 ti8 9

MOLE FRA CTION: BENZENEOR IPE I N L l O U l D

I 286 EtnzeneIPA/benzene/water258 ternaryazeotropeBenzene 175 WaterI IWaterIPE 74 IPA/IPE/waterternar y azeotropeWaterternary are far less than those of benzene. The entraining efficiency of the IPE per Btu s,therefore, only about 7% lower than benzene. However, since the vapor mass flowrate upthe column i s increased and the condensing temperature i s lower, there i s an extra load onthe column and condenser. This will slightly reduce the capacity of the column.

The separation of the entrainer fr om IP Aa t the base of the column i s accomplished aseasily with IPE as with benzene since the vapor liquid equilibrium characteristics arealmost identical (Fig. 1). The switch, therefore, from benzene to IPE wasachievedwithonly a minor loss in operating capacity and without plant design changes apart frominstrument adjustments.

To dehydrate 4.5 GPM of azeotrope as called for by SPL required an approximate heatload of 1.7-million Btu/hr. A two foot diameter packed column was needed to handle theequivalent f low of vapor. To minimize the height factor, the column was filled with metalmesh packing. Although this packing gave an HETP of 12" on the duty, the overallpacking performance failed to meet the claims of the manufacturer. Despite this, how-ever, a t a 12" HETP the column was no t as high as it would have been had6 actual traysbeen used.

The final design which was used is shown schematically in Fig. 2with a photograph ofthe actual installation shown in Fig. 3.As illustrated, the column was designed for bothbatch and continuousoperation.

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batch operationWhen operated in batch mode, 2500 gallons of either IPA/water, methanol/water oracetone/water were pumped into the batch tank. A t startup, the column wasallowed tocome t o equil ibrium on total reflux. When the temperature a t 'A'attained a value close tothat of either the pure product or the azeotrope boiling point, the column was switchedto automatic control. The product was drawn of f through valve 'V- l 'w ith the valve beingset according to the temperature a t the 'A' point. As this temperature increased, theautomatic controls started to reduce the product removal rate. This increased the refluxratio which maintains t h e required temperature a t 'A'. As the distillation proceeded, theconcentration of the volatile component in the tank decreased and eventually, the tem-perature a t 'A' no longer could be maintained even on total reflux. A t this stage, thedistil lat ion was complete. Residual solvent in the tower was boiled off a t zero reflux ratioto a t a i l s tank and the contents of the batch tank were dumped to the drain. Typical of abatch distillation operation, this procedure combined both f lexibi li ty and simplicity.continuous operation with benzeneas entrainerThe alternate operation of this column for the dehydration of the IPA/water azeotropewas more complex. This operation was carried ou t continuously. At the start of a run,about 500 gallons of IPA/benzene mixture were charged to the batch tank. The columnthen was started up on to tal reflux and when the IPA/benzene azeotrope temperaturewas achieved a t the to pof the column,feed gradually was pumped to the column a t a lowrate of 2 GPM. The temperature a t 'A'gradually fell to the ternary azeotrope value ofabout 152F and a t this point, the condensed top product was diverted through the twostage cooler into the decanter. The organic phase from the decanter was pumped throughthe regenerator section of the heat exchanger back to the top of the column as reflux. Theaqueous phase was pumped to storage for subsequent processing back to the azeotrope ina batch operation.

As soon as the aqueous phase was separating in the decanter, the feed was increased to

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4.5 GPM. The level of the anhydrous IPA in the batch tank was maintained low underlevel control and the product was pumped through a cooler to storage.

I t was necessary t o maintain a small flow of benzene to the column t o compensate forlosses. This makeup of the benzene was controlled fr om a measurement of the tempera-ture a t ' A . I t was found that i f the temperature a t 'A ' increased above 157"F, the liquidcondensate failed t o separate into two phases. This was corrected by adding benzene tothe column and bringing the temperature down to about 152" F. This temperature, there-fore, was used to control the addit ion of benzene to the column. As long as the steamflow was a t 840 Btu fo r every pound of feed, contro l of the column was relatively simple.

A t the end of the run, the benzene was recovered from the top of the column in theform of benzene/lPA azeotrope. This was stored fo r use a t the next startup.continuousoperation with IPE as entrainerSimilar operation was achieved using IPE as entrainer. The only significant differencewasthat the azeotrope temperature was 143F.metal mesh packing performanceThe metal mesh packing used as the mass transfer device did enable the column to beshorter in height than would have been required with trays. However, the performance ofthis packing did fall considerably below expectations. While claims of 4" t o 8" for theheight equivalent to a theoretical plate (HETP) had been made, the column was givingover 12" HETP. The 2 f t diameter column a t SPL was larger in diameter than othercolumns that APV had tested with mesh packing. In addition, the column wasarrangedwith 12' intervals between liquid redistribution points.

I t appears, therefore, that on an industrial s ize column with conventional liquiddistr ibution and redistribution devices, an HETP of less than 12" probably i s difficult toachieve. With small columns of I O diameter or less having good and frequent liquiddistribution, it may be possible o achieve such low HETPvalues.

At a 12" HETP, the cost of using such packing i s more expensive than using trays whentrays are rated as 50% effic ient. Therefore, unless height i s of prime importance, APVnormally would not use metal mesh packings for columns of 12"diameter and larger.practical conclusionsDuring the startup ot the column, a number of very important conclusions were reachedmainly as a result of practical experience.

Good management of the solvent tanks was essential. For example, small amounts ofmethanol or acetone in the IPA reduced the effic iency of the azeotrope dist illat ion. I twas therefore essential to remove al l high volatiles before starting the azeotropicdistilla-tion. It was even more important to ensure that high volatile impurities d id not becomemixed wi th the IPAduringstorage.

Since the decantation of the two phase azeotrope was best achieved a t IOO"F, it wasnecessary to cool the condensate and then reheat the reflux to i t s boiling point. In manydistillation operations, it i s not necessary to reheat the reflux but in this applicationwhere the reflux is of a dif ferent composition than the distillate, cold reflux reduces thedehydration capacity of the column.

I t also was a useful feature t o have sight glasses on both the decanter and on the feedline to the decanter. Those visual aids were essential during startup in order to set up thedecanter to g ive maximum efficiency.Ref erences1. Guinot, H -Clark, F.W.

Trans. Inst. Chem. Engrs. (London) 1938,16,187 31


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t h e

b reW r y effIn recent years there has been a considerable tighte ng of restrictions governingof such regulations coupled wit h technical advancements in the efficiencyof wastetreatment plants has improved many waterways, more stringent restrictions n o ware being imposed to further restore river wate r quality. One industry feeling theeffects of this continuing program is the br ewing industry, where a significantamount of liquid effluent has to be treated for B O D reduction before it can bedischarged to sewer.

Brewery effluent conta ins a mixture of suspended solids, soluble solids, ethylalcohol, water and small quanti tiesof other volatiles. Since the ethyl alcohol contrib-utes significantly to the B O D of the effluent, and since ethyl alcohol is a valuableproduct in concentrated form, it is advantageous to remove th is effluent, purify itand produce a salable l iqu idwh ich w il l help to offset the costs of effluent treatment.Most brewery effluent is a mixture of waste beer, washings and yeast. Normally hisl iquid conta insfro m 2%v /v to 4%v /ve thy l alcohol together with minor amountsofother volatiles such as aldehydes.In conjunction w i th some breweries, APV has developed a number of tech-niques for the trea tment of these waste products and also for the recovery of thealcohol. This paper discusses alcohol recovery and points out someof the difficultiesencountered and techniques used for overcoming these problem areas.

the discharge of liquid effluents in North American in1 try. Whil e the enforcement

ETHYL ALCOHOLEthyl alcohol is a colorless l iquid SG 0.79 )w ith very little aroma. It is otally misciblewi th water and can be partially separated from water by distillation. It does, however,fo rm a n azeotrope wi th water. This azeotrope, or constant boiling mixtu re as it issomet imes called, boils at 78.15"C and consists of 89.43% molar alcohol. It is pos-sible to concentrate the alcohol to 100%purity but this is difficult since it involvesbreaking the azeotrope.

Alcohol, both as i ts azeotropic composition and at 100%purity, is a valuab leproduct. It can be used as a chemical r aw material, in the pharmaceutical ndustry,for food products, and as a fuel.

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I

POSSIBLE RECOVERYTECHN QUESthe c o l u m n / r e b o i l e r s y s t e mAs i l lustrated b yt he Figure 1schematic,one method of alcohol stripping centersaround the util ization of a single freestanding distil lation column equippedw it h the proper number of trays. Com-pleting the system i sa simpledegassingvessel, plate reboiler, condenser, pumpsand piping.In a typical installation, waste beerreceived at t he alcohol recovery area isinitially heated and flashed into the de-gassing vessel. This serves t o purge thealcoholic st ream of carbon dioxide wh ic hwould choke thedis til lat ioncolumn wi t hfoam and completelydisrupt the distilla-tion and heat transfer efficiencies.The degassed stream, whi ch con -tains water and alcohol plus suspendedsludge and yeast solids, is fed in to thedistillation column. A s the mixture fil lseach tray in i ts f l ow dow n the column,vapor generated by the Paraflow plateheat exchanger reboiler at the base ofthe stil l, rises and bubbles through th eliquid on each tray. Since the alcohol inthe feed is more volatile than the waterand most other components, it r iseswithand progressively enriches the vapor ateach successive tray. This action pro-duces a top distillate conta ining 70% o95% v/v alcohol depending upon thenumber of trays ir the col umn and the FIG 2 Two column systemreflux ratio being used. This alcohol is draw n off wh ile the stil l bottomsof wate r andsolids are discharged to the by-product evaporator. The stil l bottoms contain lessthan 0.02%v/v alcohol.In those ins tances wh ere building or local codes impose height limitations, asystem incorporating wo shorter distil lation colu mns can be used wi th comparableresults. This variation, as sh ow n in Figure 2, produces topso f 70%v/v alcohol in thestripping column w it h the discharge being fed to an adjacent rectifying column wherethe alcohol concentrat ion is increased o 95%v/v. At t his level, it is suitable for saleas an industrial solvent. Distillate at 70%v/v , incidentally,will sustain combustion,and if alcohol recovery is not required, may be mixed w i th fuel for easy disposal by

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FIG 3 Multiple patterned plates are positioned and then compressed wit hi n typical Paraflow preheatersburning in a grains dryer. The bottoms containing water, solids and no m ore than.02%v/valcohol are combined w it h non-alcoholic effluent, concentrated and eithersold separately as a syrup or m ixed wit h spent grains for subsequent drying anddisposal as a feedstu ff.A numb er of problems exist in the processing of these effluent streams. Onemajor difficulty is that the effluent l iquidsfoul th e heat transfer surfaces. In order toprevent foaming in the disti l lation column, the waste eed has to be heatedto 220For higher an d then flashed into a degassing vessel whi ch drives off th e carbondioxide and reduces the foaming tendencies. Unfortunately, it is diff icult to heat theprocess fluid above 18OoFbecause of problems of fouling, particularly wi t h certainof the effluent streams. Fouling can, however, be minimized by carefully mixing theeffluent streams prior to th e preheater to ensure tha t none of th e high foulingmaterials are present in a large concentration. In addition, t he preheaters used areParaflow Plate Heat Exchangers similar to that sh ow n in Figure 3.These Paraflowplate heat exchangers are less susceptible to fouling th an many other types of heatexchangers, since the plate corrugations enhance the liquid turbulence whi chassists i n shearing the f oulin g depositsoff thesur face .Theyareeasilycleaned-in-place and can be read ilyopened f the fouling becomes too bad. Generally, even wi thcareful operation, for most applications, the heat exchangers w il l have o be in-placecleaned at least every t w o weeks.

Surprisingly, field operations show th at these very hi gh fouling fluids have notaffected the efficient operation of the c olu mn trays. Every f e w mo nth s he trays arecleaned-in-place and at th e same time are visually inspected. Fouling has no t beena problem, probably because of the choice of valve trays for t he mass transferdevice. The smal l valves in the holes on the trays are continually moving up an d

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4%

d o w n a n d t h e r e is a lso somerotation. This helps prevent thebuildup of foulant in the areasaround the holes.u t i l iz in g e v a p o r a t o r s t e a mAs an alternate to the column/reboiler system APVc an offer extrasteam economy with an arranqe-

A L C O H O L I C

STEAM-

- - - - - - - - - - - - -90% ~ 95%ALCOHOLA L C O H O L SOLUTIONDILUTE CLEAR

H O OUT

STREAM

FIG 4 St e a m ec o n o m y d i s t i l l a t i o n s y s t e m

ment whi ch uses a stripping column wi th evaporator steam as the heating medium,and a packed secondary still . By taking advantage of a bui lt -in source of steam fromthe evaporator and operating th e distil lation column be twe en two evaporatoreffects as s how n i n Figure 4, i t is possible to strip off alcohol as a part of theevaporator process. Vapor generated in one evaporator effect travels up thedisti l lation column and strips the alcohol enroute. The vapor t hen is ed to the nextevaporator effect as th e heating medium. After being condensed, it emerges as aclear, dilu te alcohol solution. Since the stream carries no sludge or suspendedsolids, it n o w can be charged to a secondary column, w hic h for small duties cancontain packing rather than the more expensive trays. The clean condensateranges from 3% to 6% alcohol depending upon characteristics of the alcoholiceffluent. Disti l late from packed colum n can be in th e range of 70% to 95% v/valcohol. This system lowers operating costs and reduces potential fouling problemsi n he distillation column.

The tw o processes discussed are designed primar ily for the recovery of 190"proof indus trial alcohol. This alcohol contains some impurities s uch as fusel oils,aldehydes and other high volatile compounds. Mos t of the fusel oils and some of thehigh volatile compounds (heads) can be removed in these processes. Usually fuseloils can be extracted by taking off a sidestream f rom one of the trays at a point wher eit is kno wn that the fusel oils predominately accumulate. Generally, this is con-sidered to occur at the c olu mn tray w he n the alcohol composition i s about 130"proof. By ensuring an adequate take off at this point, it is possible to keep the fuseloils in the product to a very l o w evel.

The removal of the heads s more difficul t since they are more volatile than thealcohol. The head quantity i n he final product can be minimized by taking the fina lalcohol product off the column at about 4 o r 5 trays belo wthe op and maintaining apurge of heads from th e top. This can, of course, never prove completelysatisfactorysince the heads are contained in a lo w concentration i n the vapor that is passingthroug h the l iquid on the tray where the product is being removed.

To recover a verypurealcohol; it is necessary, therefore, to have another smal lcolumn to strip the volatile from the alcohol product.A B S O L U T E A L C O H O LReference toFigure 5shows th e vapor li quid equilibrium curve for ethyl alcohol/water. The plot is of the mole fraction of ethyl alcohol in the vapor phase aga ins tthemole fraction of ethyl alcohol in the liquid phase. A t a mole fraction of .894%, thecurve oins th e 4 5 degree line. This shows that separation by boiling at this point is

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ETHYL ALCOHOL WATERno longer possible since a constantboiling mixture is obtained. At this point,i t is no longer possible to further can-centrate the alcohol by conventionaldistil lation.Absolute (100%) thyl alcohol canbe obtained by using a benzene entrainerto produce a ternary azeotrope wi th sub-sequent distillation to produce absolutealcohol and to recover the benzene. Thisis a somewhat complex process requiringtwo disti l lation columns and it is, there-fore, considered unlikely that manybreweries will wi sh to produce absolute 0 .1 2 .3 4 .5 e I .8 9

9

8

7

.S

z g ~

2?

MOLE FRACTIONalcohol. E T H YL A L C O H O L I N L I Q UI OFIG Vapor l iquid e q u i l i b r i u m c u r v e ,E C O N O M I C S O F A L C O H O L RECOVERY

The recovery of alcohol by distillation is an expensive process. The costs of theequipment are quite high, as are operational costs. The alternates, however, ofpaying large sewage costs are probably more expensive and there is, of course, nosalable by-product .

The equipment cost for alcoho l recovery is naturally dependent o n the size ofbrewery and the type of effluent t o be produced. All the equipment in contact wi ththe process flu id is usually supplied in 304 stainless steel to minimize corrosion. Toprocess a 190' proof ethyl alcohol from a stream of about 50 U.S. GPM containing3 alcohol would require equipment costing in the order of $350,000 U.S.).hiscost only covers the distillation equipment and does not include tankage, erect ionor any civil engineering. The final installed cost would be in the order of 1% or 2 timesthat figure.

The operating costs are mainly inf luenced by the cost of steam used for runningthe distillation column. For normal dilute alcohol streams, the st ripping ratio atthe base of the column can be about 5 or 6 to 1. This usually means that about.2 pounds of steam are required per pound of effluent processed. The use ofcombined distillation/evaporation scheme as shown in Fig. 5can however reducethe steam consumption .F U T U R E P O T E N T IA LAlthough certain automobile manufacturers ini tially were somewhat conservative intheir attitude toward adding anhydrous ethanol to gasoline because of the possibilityof corrosion, most now approve the use of gasoline containing ethanol and conse-quently, a considerable market has emerged for this product.

Many ethanol recovery plants produce a relatively small amount of ethanol peryear at 190 proof grade. Since it is no t always economic to run separate ethanoldehydration plants, numerous companies ship ethanol to central sources havingsizeable dehydration capabilities. One such location is in the Caribbean where threelarge APV azeotropic distillation plants currently are used to process surplusEuropean 190 proof ethanol to 200 proof.

Although the price of petro leum varies considerably, it now would appear thatthere always will be a demand for ethanol due to its excellent properties in relationto enhancing octane values.

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t h e p r o d u c t i o n o f

n ear andlaw alcohol b eerAlthough near beer wi th i ts very low or even zero alcohol content has been availablefor a number of years, it is only recently that there has been a substantial increasein the demand for this product.

Retaining all the organoleptic qualities of more conventional brews, near beerlooks, tastes and smells like beer but does not have its disruptive side effects. Thesepositive attributes have helped the product gain favor among certain Moslemnations in wh ich alcohol is banned while in many other countries that impose verysevere penalties for alcohol related driving violations, the sale of near beer hasshown dramatic growth. Moreover, wi th the ever increasing concern in the UnitedStates over highway fatalities and wi th the passage and strict enforcement of muchmore stringent driving sobriety legislation, the annual rate of consumption for bothnear and low alcohol beers has risen here as well .To meet the demand for equipment to produce these beers, APV has workedwith a number of breweries in developing several de-alcoholization techniques.Such systems n ow are in use in San Antonio, Texas, Belleville, Illinois, St. Paul,Minnesota and Perth, Australia.

Typical preassembled modular distill ation system designed to reduce alcohol content of beer to less than5 percent

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.OOLIN6CONOENSER

Sl A l PP l NG

O E ESTERIZEAAECOMBINEA

R E O I L E R

TEAM

P A E H E A lHOT WUlEA HOT WATER

P R0 0uE1 I

F EEO BALANCtT ANK

L-O E L LOTSTEAM

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With process plant becoming increasingly larger, there has been a tendency for thechemical and petroleum industries to focus attention on the use of continuous distil la-tion since this approach becomes progressively more economic as the scale of operationgrows. As a result, batch distil lat ion has become a somewhat neglected uni t operation.There are, however, many areas where batch disti llat ion can be used more economicallythan continuous. APV has realized

B TCH DlST LL Tthis large potential and hasdeveloped the technologynecessary to quote and supplyeconomic units for batch sizesextending from about 300 toover 5000 gallons. An APV 300 ONa c l o s e r l o o k atgallon un it i s shown in Fig. 1.advantages o f batch distillationThe main advantage of batch distil lat ion is i t s flexib ili ty. A single un it can, bychanging reflux ratios and boil up rates, be used for many different systems.I t also i s possible to separate more than two components in the samecolumn whereas with continuous distillation, a t least N-1 separatecolumns are required to separate N components.

A batch distillation process i s very simple to control andthere i s no need to balance the feed and draw off as i s the casewith the continuous distillat ion approach.

Batch distillation i s particular ly useful when applied t o feedscontaining residues which have a tendency to foul surfaces sincethese residues remain in the s t i l l and cannot contaminatethe rectif ication column internals. For aqueous systems handlingmaterials which leave very heavy fouling, it i s possible t o heatwi th direct steam injection into the s t i l l and thus al lev iateproblems of buildup on the heat transfer surfaces.column designBefore designing a batch column, it obviously i s desirable tohave as much detailed information on the system as i s possible.Vapor liquid equilibria (VLE) data, vapor and liquid densities,liquid viscosity and the boil ing temperatures of the components areessential if a column i s to be properly designed. Failure to haveaccurate VLE data means that it i s necessary to run a small scaleexperiment in order to characterize the system.In addition, the customer must ident ify theproduct feed composition, the requiredcomposition of the residue and distjllate, thebatch size, and the batch time. Having acquiredthis data, the vendor finally can commencedesign work.

To illustrate the design principles, iti s useful to study the case of binarymixtures. A typical system i s shown in Fig. 2.

40FIG. 1


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F I G , 2 MOLE FRACTION ' A IN LIQUID

Once the V L E data have been established, it becomes a straightforward task to calcu-l a te the number of theoretical stages and reflux ratios.

There are two main techniques for operating a batch column. One i s to work withconstant reflux ratio during the complete run. The effect of this method i s charted inFig. 3. As the composition of the more volatile component ( M V C ) in the s t i l l xw de-creases, so the fraction of M V C in the top product decreases. To obtain a s e t composi-tion of say 90% i n the total amount of top product collected, it will always be neces-sary to collect initially a t a higher composition of about 95% to compensate for acomposition below specification a t the end of the run. The advantage of constantreflux is that control and operation are very simple.

The second method i s to increase the reflux ratio during the run in order t o main-tain a steady to p product composition. This i s shown in Fig. 4where the increase inthe slope of the operating line i s obtained by increasing the reflux ratio. The gradient

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of the operating line ($) i s obtained from the enrichment equation given below.

This equation assumes constant molal overflowY,x,+~ i s the mole fraction of the MVC in the liquid arriving a t the nth stagexdDVL

is the mole fraction of the MVC in the vapor leaving the nth stage

i s the mole fraction of the MVC in the top productis the molal flow rate of top productis the molal vapor flow rate in the columnis the molal liquid f low rate in the column

The reflux ratio R is given byLIV

1 LIV)R =With the VLE data and the top and bottom compositions, it then i s possible to

calculate graphically by the McCabe & Thiele procedure the minimum reflux ratio,minimum number of theoretical stages, and other such parameters. In batch distilla-tion in particular, these procedures are tedious and time consuming since, of course,the composition of the liquid in the s t i l l changes with time and it i s necessary to repeatthe calculations many times. Obviously, the procedures become even more time con-suming wi th multi -component systems.

For this type of design work, APV uses computer programs that enable the engineerto produce an efficient design very quickly. One program i s extremely flexible and

1 Top product and st il l comp osi t ion.

produ ct and stil l com posit ion.Num ber of theor etical stages.

Average top produ ct com posit ion .Init i al and final sti l l co mpo sit ion.St il l chargeBatch t imeNumber of theoretical stagesFIG. 542

Min im um nu mber of theoretical stages.Minim um ref lux rat io.

Ref lux rat io I

Reflux ratio required constantthroughout b atch.Bo il up rateProduct qual i ty


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operates in a different series of modes as is charted in the Fig. 5table. Naturally,the VLE data have to be specified for all modes. The program further assists in thedetermination of the number of theoretica l stages. Other modes also are availableto provide different permutations of operation.

To help determine the sizing of the column diameter, APV uses variousprograms which incorporate different proprietary methods including those ofFractionation Research Inc. (FRI) of Cal iforn ia.c o l u m n i n t er n a lsThe choice of internals for the column depends mainly on the product beingprocessed and the size of the column to be used. To meet virtually every parameter,packed columns as well as sieve, bubble, and ballast trays are available. As ageneral rule, sieve trays are not used frequently in batch columns since the turndown ratio of most trays of this type is only about 1.5 to 1. This reduces one of themain advantages of batch columns, mainly flexibility. Usually, small batch co lumnsare packed since the efficiency of trays of less than 2 diameter often decreasesrapidly. Ballast trays, although expensive, are often used for larger columns sincethey are efficient and have turn down ratios of up to 9 to 1.co n t r o lThe control of batch columns is very simple and therefore, required instrumentationusually is quite inexpensive. Constant reflux operation can be handled by a ratiocontroller o r by using a timed deflection of condensate. For operation with variablereflux, the control of the reflux ratio must be tied to some property of the top productthat undergoes a sufficiently large change in value for change in composition.Manual cont rol of this type of reflux operation is not feasible.

Numerous batch distillation systems have been supplied to customers whorequire the separation of many different components. To simplify the operation ofthese systems, APV CREPACO has developed a range of proprietary computersspecifically designed for process control. Use of an ACCOS microprocessor enablesthe operator to switch from one type of separation to another without having tomake many manual adjustments on the control panel. In addition, the computer canbe programmed to automatically take care of all necessary tails cuts as well as toclean the system during interims between the various separations. In one batchdistillation system supplied to a processor in Puerto Rico, an APV ACCOS 2scomputer is used to control distillation whi le an IBM XT handles the logging ofall data.b a t ch sys t e ms / co n t i n u o u s d i s t i l la t io nIn a number of duties, it is essential that the base product contain only very smallamounts of one component. For example, in the recovery of solvents from water, itis mandated that the water may retain only trace quantities of solvents before beingpumped to waste. Since one disadvantage of a batch system is that there is nostripping section, it is necessary to boil the batch pot for relatively long periods oftime in order to reduce the residual solvent to trace quantities.

To resolve this prob lem, a number of batch distillation systems with thecapability of being operated in the continuous mode have been supplied. Byincorporating one, two or three feed points on the column, the feed can be pumpeddirectly to the column instead of to the batch tank. Furthermore, a separate smallholdup/reboiler is furnished in certain cases so that the batch tank and itsassociated reboiler need not be utilized for the continuous operation.

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APV Crepaco IncExecu t i ve and In te rna tiona l Headqu ar te rs9525 West Bryn Mawr Ave., Rosemo nt, IL60018En g in e er i n g a n d Ma n u f a c t u r i n g395 FillmoreAve.,Tonawanda, NY 14150(708)678-4300 FAX: (708)678-4407

(716) 692-3000 FAX: (716) 692-1715Sales Of f ices:Charlotte, NC/(704) 376-0209 Chicago (Rosemont), L/(708) 678-4300Columbus, OH/(614) 846-6415 Dallas (Irving),Texas/(214) 257-3455Los Angeles (Cerritos),CA/(213) 926-9700 Minneapolis,MN/(612)544-8731Mt. Laurel, NJ/(609) 235-6800 Nashville,TN/(615)255-0342MetropolitanNew York (Montvale,NJ)/(201)930-0001Salt Lake City, UT/(801)262-8494 Seattle, WA/(206) 575-8900APV Canada, Inc.- Toronto/(416) 850-1852

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