245

PROCESSING

Deodorization principlesStripping efficiency in cross-now and counter-current operations

1I Degassing

II Flashing r

II Stripping rlj Scrubbing Condensing

I I I

New deodorizer designs, basedon counter-current gas-liquidcontact and large surface

areas. have recently been marketed.No operational mathematical modelsare currently available for characteri-zation of the performance of such sys-tems. The aim of this paper is to pre-sent a simple mathematical model,which may be useful in understandingthe principles of counter-currentdesigns. and 10 compare the theoreti-cal performance of counter-currentdesigns with the traditional cross-flowsystems with respect to stripping gasconsumption.

Deodorization is the final processstep in the refining of edible oils. Thetotal deodorization process comprisesa number of separation steps, which isillustrated in Figure I. The separationsteps comprise one or two individualflashing steps. here called deaerationand flashing, and a stripping step. Thelatter flashing operation is often inte-grated into the following strippingstep. A series of operations concernthe establishing of vacuum and han-dling of oil distillate. The separationobtained by the stripping operation istheoretically dependent on the designof the deodorizer. In most deodorizerdesigns, the stripping operation takesplace simultaneously with some ther-

Feed oil

Finishedoil

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mal reactions, but this docs not pre-vent individual analyses of the behav-ior and performance of the strippingoperation. The purpose of the strip-ping operation is to reduce the contentof undesirable volatile components[odorous components and free fattyacids (FFA)] of the oil to satisfactorylevels, defined by the specifications ofthe end product.

The stripping operation. also called"desorption" or "steam distillation," isdefined as a purification of the liquidoil phase by mixing a stripping gaswith the oil, which facilitates masstransfer of the volatile impurity to thegas phase. The gas phase is continuoously removed from the processchamber, thus preventing the volatilesfrom reentering the liquid. Theamount of stripping gas required toreduce the volatiles to satisfactory lev-els is an imponant cost parameter, asit is greatly influenced by the dimen-sions of the deodorizer as well as vue-

uum system, etc. Almost every currentcommercial application uses steam asstripping gas, owing to its ability tocondense under moderate conditions,thus diminishing the costs of the vacu-um system.

Some COnfusion prevails about thedefinition of terms, but there seems tobe almost consensus about the follow-ing definitions. A stripping processcarried out as a batch process usingsteam is normally called a "steam dis-titlaricn," while stripping carried outin a counter-current manner in multi-stage towers is ealled a "desorption ...The special case of batch "stripping,"in which no stripping gas is used, isanalogous to a one-stage "flash distil-lation" or just "flashing." "Stripping"with no use of stripping gas carriedout in a counter-current manner inmultistage towers has similarities tothe well-known "distillation" process.If the mass transfer is not affected bya simultaneously occurring chemical

Non-condensiblematter

Fatty acids Condensate

Figure 1. The separation steps conalitulln\illh<t ct.ocIorIution pt'OC •••

INFORM. Vol. 10.00.3 (Morch 1999)

246

PROCESSING

Mass tnlnsftor drtving foree, Of'. pv...... pv.....

p~p~ p~

os 0 S" v"s 0S ---'>"";;''':'-/

ov

ov "o

Sb) Fully krMsdbIe c) Non4mllKlb1e

stripping gas ~ gas

pv.oa."'PIOI"Po

pv ..... • P'v -(1 -XoJ

OF" P'v' (1·Xg)· (PIG!" Po)

Py p....·po·p, e, -p•• Po' p,pv -p'v·(1·Xo·XsI Pv ·P'y·(1.Xo)

Of .. P'y·(1-Xo-Xa)· (P.-Po..p,) OF· P'v·(1-Xo)· (PIDI"Po' Ps)

Figure 2. Driving loree In !lash dlstlllallon (III)and stripping, using fully Immiscible (b), orusing nonlmmlscible gaB (e)

reaction, the desorption process isnamed pure "physical desorption, "which presumably is the case whenusing steam as stripping gas.

Furthermore, additional termsregarding the stripping perfonnance aredefined in this paper. The "reductionfactor" [RF (dimensionless)] of a spe-cific volatile is defined as the ratiobetween the concentration in and out ofthe process. RF is a means of compar-ing the stripping performance of differ-ent deodorizer systems, if the samevolatile component, for example, oleicacid, is used as reference. Such a spe-cific reduction factor equals the"stripping intensity" (SI), which is aneffective tool for comparing the perfor-mance of different deodorizers. A wayof describing the actual utilization ofthe gas is the tenn "stripping gas carrierrate," which is defined as the moles of aspecific volatile (i.e., FFA) removed permole of stripping gas used. The strip-ping gas carrier rare may be expressedas an instantaneous rate or a rate accu-mulated over the entire process.

The "stripping efficiency" (E), alsocalled the "vaporization efficiency," isdefined as the relative actual perfor-mance of a process compared to adefined theoretical maximum perfor-mance. The efficiency has to be relat-ed to the theoretical (equilibrium)

model in question, and the unit can beread as a percentage or fraction.

The actual deodorization condi-tions mainly depend on the particularoil type, on the oil quality, and onwhich refining route is applied. Tradi-tional deodorization is conducted as aprocess step in chemical, or caustic,refining, in which the FFA previouslyhave been reduced to near the finalspecifications by reaction with alkali,and is usually carried out at tempera-tures in the range 220-260·C, at avacuum of 1-10 mbar, and at a hold-ing time of 30--60 minutes, while dos-ing stripping steam in the range1.5-2.0 wt% of oil. The strippingintensity obtained is determined bythe process parameters temperature,pressure, and stripping dosage, as wellas the deodorizer design.

During the last few years, a shift inrefining technology toward physicalrefining, in which FFA exclusively isremoved in the stripping step, seemsto have been adopted. However, appli-cation of physical refining requiresincreased stripping intensity, as FFAin this case is the critical key-compo-nent. The required increase in strip-ping intensity has been estimated to betwo- to threefold compared to strip-ping of chemically neutralized oil.The increased stripping intensity

required that physical deodorizationbe achieved by altering one or more ofthe stripping parameters or by modify-ing the deodorizer design.

The increased use of physical refin-ing has initiated the development ofmore efficient stripping designs, i.e.,where the contact between liquid andgas is going on in a counter-currentmanner, and where a large contactarea between the phases is established.Counter-current mode reduces thepotential (equilibrium) gas dosage sig-nificantly, while a large coruacr areaincreases mass transfer, with the pur-pose of ensuring that the actual perfor-mance approaches the potential.

The basic principles of strippingStripping theory is briefly summarizedhere. Mass transfer from the liquid tothe gas phase of a certain volatilecomponent is driven by the differencebetween the equilibrium partial pres-sure and the actual partial pressurepresent at the spot of transfer. The"equilibrium" partial pressure, PV,eq'of the volatile, V, is a function of theconcentration of the volatile in the oilphase, usually described by Raoulr'sor Henry's law. The actual partialpressure is determined by the resis-tance to mass transfer occurring in thesystem. The perfect deodorizer designminimizes all resistances in a way thatno single resistance has the main con-trolling role, thus achieving near-equi-librium conditions.

The effect of the stripping gas canbe explained in this way. The totalpressure is the sum of the individualpartial pressures, according to Dal-ton's law. When injecting a strippinggas, S, while maintaining unchangedtotal system pressure, PIOI' the gascontributes to the total pressure, thusreducing the actual partial pressures ofall other components, i.e.. the volatile,PV.OCI, and the oil, PO.OCI, present in thesystem, It is important to note that thiscondition is highly dynamic and ispossible to maintain only by continu-ously removing the gas phase. If equi-librium conditions existed. the totalpressure would increase significantly,

The maximum effect of the strip-ping gas is obtained if the gas injec-tion does not reduce PJ.(eq' whichimplies that the stripping gas must not

INFORM, Vol.10, no. 3 (March 1999)

247

be present as liquid in the same liquidphase as the volatiles. which wouldreduce the actual concentrations andhence affect their partial pressures.That can be accomplished by using astripping component thai is immisci-ble with the oil liquid. or just bychoosing a stripping component that ispresent exclusively as gas at thedeodorization conditions.

Stripping, in contrast 10 distilling,provides a satisfactory residual con-centration of the volatiles in questionwithout applying very high tempera-ture, which would lower the oil quali-ty, or very low vacuum. which is verycostly. Figure 2 illustrates the qualita-tive difference in the equilibriummass-transfer driving force, based onReoun's and Dalton's laws. Where nostripping gas is injected and pure flashdistillation occurs. there is no dilutionof the gas phase. Where the appliedstripping agent is diluting the oil liq-

uid phase, the equilibrium partial pres-sure of the volatiles is affected. Where(proper) stripping with an immisciblegas is applied, the maximum drivingforce is achieved.

Under normal circumstances, whennear-equilibrium conditions areobtained, stripping provides a muchmore complete separation of volatilesthan normal distillation. with the sametemperature and pressure conditions.The better performance of stripping is"paid" for by the increased dimensionof the vacuum system. which has toovercome the extra gas load (of strip-ping gas).

Contact mode between the phasesThe contact mode between the gas andthe liquid phase is a very importantfeature of deodorizer designs. as itdetermines the equilibrium conditionsin the design and thus sets the poten-tial for (maximum) utilization of the

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stripping gas. The contact mode hasthree significant features: the timecourse of the operation type (batch orcontinuous), the flow pattern, and thetype of phase distribution.

The time course. In large-scale pro-cessing. the continuous process isoften preferred to batch because ofbetter energy recovery, simpler opera-tion at steady state conditions, etc.However. plug flow is difficult tomaintain in continuous systems. and acertain amount of back-mixing. inwhich a time distribution of the com-pounds is developed, may reduceproduct quality or at least reduce theperformance of the process. In batchprocessing, full back-mixing is pre-ferred. which eliminates all concentra-tion gradients, resulting in the bestdriving force in reactions and masstransfers. A full back-mixed batchprocess has similar properties to thepure plug-flow continuous operation.

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INFORM. Vol. 10. no. 3 (March 1999)

248

PROCESSING

.) b) 0~ Plot-+-

s, oP.,. .. rt s, .. ')

0 o S. cS

<,

Figure 3. Cross-flow stripping. Batch stripping mode (s) and .. mlcontlnuoua multltraydesign (b)

Flow pattern. In commercialdeodorization, two principles of flowpattern are relevant: cross-flow (alsocalled cross-current flow), andcounter-current flow. Cross-flow (CF)implies that the flow directions of thephases cross each other. The cross-flow process may be carried out as ausual batch process, in which the con-centrations change with time, or as acontinuous steady state process, inwhich the concentrations change withlocation.

Counter-current (steady state) flowimplies inlet of the gas at the outlet ofthe oil, and outlet of the gas at theinlet of the oil. As the gas risesthrough the column, it meets oil withan increasing volatile content, thusachieving the highest possible equilib-rium driving force when leaving thecolumn. Thus, the equilibrium con-centration of volatiles in the gas phasecorresponds to the inlet oil concentra-tion at all times. The actual number ofequilibrium stages in the systemdepends on the design.

All deodorizer designs are charac-terized by a very short contact timebetween the stripping gas and the liq-uid, compared 10 the total strippingtime, resulting In practicallyunchanged composition of the liquidphase during its contact with a specif-ic gas molecule. Hence, cross-flowcontact results in only a single equilib-rium stage, whereas several stages canbe obtained in a counter-current sys-tem. There is no difference in tbeequilibrium conditions of the two nowpatterns when the stripping processbegins. However, the equilibrium con-ditions change dramatically during

cross-flow processing, resulting ingreatly reduced gas-utilization poten-tial at the end. Hence, the partial pres-sure, P'v (in the equations in Figure2c), is constant throughout thecounter-current process but decreasesdramatically during the cross-flowprocess.

The number of equilibrium stageshas a very important effect in separatingmulticomponent mixtures. Let us con-sider a three-component mixture, com-prising triglyceride. fatty acid, andanother volatile component X, whichhas a lower partial pressure than thefatly acid. Stripping is carried out intwo types of equipment, i.e., a cross-flow stripper with approximately oneequilibrium stage, and a counter-currentstripper with several stages. at condi-tions thai result in the same strippingintensity with respect to fatty acid. Thestripping intensity of the heavier com-ponent X is then lower in the multistagestripper. resulting in relatively greaterretention of that component. This maybe positive with respect to preservationof tocopherol in the oil. but may also beconsidered negative with respect toremoval of other low-vapor pressurevolatiles. e.g., some pesticides. Thevariation in the degree of retentiondepends on the number of equilibriumstages and the stripping intensity.

Phase distribution. Phase distribu-tion governs the size of contact areaavailable for mass transfer. In batchdeodorizers, the oil phase is continu-ous, white the gas phase is distributedherein as bubbles (gas-in-liquid). Thecontact area is limited by the tendencyof the gas bubbles to merge into fewerand larger bubbles. Although such

design is widely used, no data measur-ing or calculating the effective contactarea have been published.

A few commercial systems offertwo continuous phases, in which thecontact area is limited by the filmthickness of the oil. The actual con-struction governs the total contactarea, which can be estimated quiteeasily if the film thickness is known.The mass transfer is proportional tothe available contact area. The effec-tive contact area should be largeenough to facilitate sufficient masstransfer, in order to achieve near-equi-librium conditions.

Principles of deodorizer designsThe designs of deodorizers availableon roday's market can be grouped inthree categories: the batch cross-flow,the horizontal cross-flow, and thecounter-current designs. Each designtype will be characterized with respectto factors affecting the stripping effi-ciency.

Batch cross-nowIn batch-wise deodorization, the oil ischarged to a compartment and thestripping gas is injected near the bot-tom of the compartment by a distribu-tion device to bring the gas into closecontact with the oil. The operation canbe carried out in a single tray (Figure3a) or in a series of trays (Figure 3b).In chemical engineering terms, theprocess can be described as "steamdistillation in a continuously stirredtank reactor (CSTR)," in whichintense agitation ensures uniform con-centration of volatiles throughout theliquid. While the steam passes upthrough the oil, volatile molecules aretransferred across the surface area intothe gas bubble, thereby reducing thecontent in the oil. The stripping iscontinued until the residual contentmeets the specifications. At the end ofthe deodorization process, the volatileconcentrations are low, affecting theequilibrium partial pressure, and thusresulting in low utilization of the gas.Stripping and thermal reactions occursimultaneously.

The single-batch designs typicallyare found in small-scale operations,in which running costs constituteonly a small fraction of the total

INFORM. Vol. 10. no. 3 (March 1999)

249

x.r.,=1.0 wt%

0.18 ------"~~::::'::::;:=~:::::'1l:!l:::=:::;::::""'11I Calculationsare basedon the BaileyeqeauonTemp. = 232C (P'",,"25 moor), PtoI- 5 mbarInlet cone. : 1.0 wt% C18 ratty add.. 0.16

L~ a. 0.14

-~~l 0.12

~~•~. 0.10.~~l 0.08

!I0.06

g-~'a 0.04

f 0.02 t"Th"~=~"''''''''''';;''=~EITicIency = 1

0.00 +b-....~'""---+----+----+----;-~~--1

x~.~~~~~mM~CFo~~~"'-------1r "Yl*al performance: EfIicien::y. O.SSteam dosage 2.-4 wt% 18 required to achieve areduction facU of 2O,le.19aChing 0.05 wt% tra

X-o.10-.'-- __ X~.,.05

Stripping Ume [ilrtJltr.II1Y time units)d .. m doslJge ... umed Ulllpe<:Ified. but conabnl

Figure 4. UIHization of stripping gas.t difference stages of the cro .... low stripping.ftl, free fatty acid

COSts. The majority of the commer-cial deodorizer designs available onthe market are semicontinuous, mul-titray designs. The stripping processis in principle exactly the same typeas in the one-tray batch process; herethe lime course has just been split upinto a number of compartments toachieve a pseudoconttnuous flow.The process can be described as a"series of consecutive batch distilla-tions" in CSTR. Stripping is startedon one tray, then interrupted whendownloading the batch to the nexttray. where the stripping is continued.The semi-continuous types sufferfrom the same limitations in strippinggas utilization as the single-batchtype. However, multitray designsoffer other features, for instance, theyenable almost continuous inlet andoutlet. resulting in simple and effi-cient heat recovery.

The efficiency of the cross-flowbatch process is limited by the follow-ing factors:

• Time-dependent driving forceThe liquid concentration of

volatiles is reduced as a function oftime, resulting in decreasing equilibri-um partial pressure of the volatile andthus decreasing driving force of themass transfer during the process.When approaching the end of the

stripping, the driving force is reducedto a small fraction of the initial force,resulting in very poor utilization of thestripping gas, which is illustrated inFigure 4. Starting with 1.0 wt% oleicacid in triglyceride, and processing at23rc and 5 mbar system pressure.the Bailey equation predicts the initialstripping gas carrier rate (dVldS) to be0.16 mole FFA removed per moleconsumed gas. However, it dropsquickly, and ends at below 0.01mol/mol at a residual FFA concentra-lion of 0.05 wt%. The total steam con-sumption is 2.4 and 1.2 wt% at effi-ciencies equaling 0.5 and unity.respectively.

• Equilibrium stageOnly one equilibrium stage is

obtainable in the cross-flow mode.limiting the carrier rate consider-ably.

• Bubble volume and contact areaThe stripping gas is injected near

the bottom of the tray. resulting in apressure gradient as the gas bubblesrise to the surface. Hence the partialpressure of the volatile is higher at thebottom. and "full" mass transfer driv-ing force is therefore available only atthe surface of the oil.

The contact area available formass exchange depends on bubblesizes and the gas volume. When the

gas bubbles rise through the oil, thepressure change causes an expansionof the bubble volume. As long as allthe individual bubbles are intact. thetotal surface area also increases.However, at a certain volume, proba-bly mainly determined by the surfacetension, the bubble becomes unstableand collapses. Theory about the col-lapse of bubbles is not welldescribed, and the situation is furthercomplicated by the very turbulentenvironment. The initial bubble sizeprobably has an effect only on thetotal bubble surface area near the gasdistributor, thus affecting only themass transfer occurring in thisregion. If for instance the oil heightis 50 crn , the bottom pressure isapproximately 40 mbar higher thanthe surface pressure. If the surfacepressure is 4 mbar, both the volumeexpansion and the partial pressure areaffected approximately tenfold dur-ing the passage of the oil.

Present batch or tray designs areequipped with the so-called gas liftpumps (mammoth pumps), whichincrease the contact between the phas-es and reduce the negative effects ofthe oil height. Such devices mayincrease efficiency considerably. Onthe other hand, the gas lift pumpsrequire a certain load of gas to oper-ate, which restricts the minimum gasdosage.

Pilot experiments have shown thatefficiencies of near unity are achiev-able by improving the isothermaloperation and the stripping steam dis-tributor design. However. large-scaledeodorizers seldom show efficiencieshigher than 0.8, implying that severemass transfer resistance occurs underthese circumstances.

Continuous (horizontal) cross-nowAnother design type on the market isthe continuous, horizontal cross-flowdeodorizers. The oil typically flowscontinuously in an almost horizontaldirection, while the gas is injected inthe oil and rises through the liquid in across-flow way.

The equilibrium considerationsvalid for batch designs also apply tothis design, if the oil now is com-pletely plug flow, i.e., if no back-mixof treated and less-treated oil occurs.

INFORM Vol. 10. no. 3 (March 1999)

250

PROCESSING

II is clear that a certain degree ofback-mix is unavoidable, due to thehorizontal flow and the turbulentenvironment caused by stripping gas.Other things being equal. the greatestefficiency is achieved in the case ofno back mix, implying that the equi-librium driving force of thesedesigns is generally lower than thatof cross-flow conducted in well-defined batch designs. Co-currentflow may be present in some parts ofthe equipment. which further reducesthe driving force. as co-current flowalways results in lower equilibriumdriving force than cross-flow. How-ever. greater efficiencies may com-pensate for a low equilibrium drivingforce, which would result in higheractual gas utilization. The designoffers excellent possibilities of fullcontinuous flow, enhancing heatrecovery, etc.

Counter-current systemsThe third deodorizer design type isthe thin-film type, which is relativelynew on the market. The term "thin-film" refers to the way the oil is dis-tributed throughout the column,namely on a large surface area. typi-cally provided by a sort of packingor just by the wall of a pipe. Themost important features, which arevalid for most thin-film strippers, arethe true continuous process and thecounter-current contact between gasand oil. The oil typically flowsdownward by gravity and the gas isinjected at the bottom of the column.Full exploitation of the high equilib-rium driving force requires a largecontact area, which can be obtainedby distributing the oil over a largesurface. e.g .. a structured packing.On the other hand, a large surfacearea increases the pressure drop

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INFORM. VOl. 10. no. 3 (March 1999)

For lnlormolion circle '115 on Ionn on Iosl page

across the packing, thus reducing thedriving force at the bottom of thecolumn. and the designer has to bal-ance those two effects when select-ing the packing. Packings having aspecific surface area in the range of200-600 m2 per cubic meter arebeing used commercially. The ther-mal treatment desired in deodoriza-tion is often accomplished in a sepa-rate compartment.

However. deposits of particles, etc.,tend to break the thin oil film, thusreducing the effective contact area.Funhennore. settling of material mayfoul the packing and thus increase thepressure drop and reduce efficiency.The IWO most important reasons forfouling probably are formation of con-densation-reaction products of phos-pholipids and oxygen-dependent poly-merization of triglycerides. However,these phenomena may be reduced byproper design of the stripper, properup-stream processing, and completedeaeraricn and avoidance of leakageduring operation.

ModelingModels are useful for designing newequipment or for predicting perfor-mance of existing equipment. Hence.models also can be used for easy com-parison of different systems. It isimportant to distinguish between equi-librium-based models and rate-basedmodels.

Equilibrium considerations revealthe potential of a specific design butare not concerned with the actualmass-transfer rates. Considerationsabout mass-transfer rates and resis-tance to mass transfer reveal to whatextent that potential is actually accom-plished. As one design may have amodest potential but excellent masstransfer while anomer has high poten-tiul but low mass transfer, both aspectsneed to be considered when evaluat-ing efficiency and practical perfor-mance.

The stripping potential is affectedby a number of equilibrium-relatedfactors:

• Increasing temperature greatlyenhances the efficiency as the equilib-rium vapor pressure of the volatilecomponent increases and raises thedriving force.

251

2..

210

260

! 250''i>1!.=&i 2"~!1.I-

volatile component:SCF - stripping gas consumption

(moles) in cross-flow operation mode(Sec - counter-current operationmode)

o - triglyceride load (moles)Plot - system pressurep v - vapor pressure of a specif-

ic volatile component, e.g., FFA(moles), which is a function of thetemperature (T as degrees C)

x f - mole- or weight-fractionof the volatile in feed

x r - mole- or weight-fractionof the volatile in residual

£ - efficiency factorThe equation is adjusted for actual

mass transfer resistance by adding anefficiency factor, E, and is mainly acorrection for the gas lacking satura-tion 10 equilibrium partial pressure,but all effects of approximations andnon idealities are included. Typicalvalues of E are in the range of0.40--0.80, and no values above unity(when corrected for the spontaneousFlashing effect) have been reponed. Ithas been suggested that effects fromthe oil height could be accounted forby adjusting the value of the pressureterm of the equation to correspond tothe average pressure existing in thegas phase (being equal to the pres-sure existing halfway down the oilbed).

The inlet concentrations of volatilecomponents are substantially higher inphysical refining than in chemicalrelining. The error of the approxima-tion V/(V + 0) - VlO applied in thereduced Bailey equation increases sig-nificantly with increasing inlet con-centrations. for instance the relativeerror is 3, 5, and 7% at I, 2, and 3wt%- FFA, respectively.

It is therefore recommended thatthe "complete" Bailey equation (Eq.2) be used to predict performancewhen higher concentrations ofvolatiles occur. i.e., FFA > I wt%.

In some cases the equation may becorrected by an activity coefficientthat varies with the concentration ofthe volatile. However, this complica-

~ Bra baed 00 the UI Baley equation IIaWIg ... eIIIdIIncy • 1.Redudion ofolele.eld at """'"" by I1!Idu<:tion 1acIor. Rf.RF. 10 equaIa a raducIion from~ ~ D.51o 0.05"4X'" ~ • ratio ~ ste.n dosage arxI .system pnISIUI8,a.liI. a rIItIo of 0.' equaIa 0.' wt"JVI .mar or I.Dwt1IJ2.mar.

230 ----------- -- --~--S---~--------------220

RF=10 RF=:20 RF-40 RF-"

210 -I---->----+---+---+---;----+--~0.0 0.1 0.2 0.3 0.4 0.5

Steam dosageIPressure[wt%lmbar]

0.6 0.7

Rgu,.. 5. C.lculated lnlluenea of proce •• parameters in cross-now .trlpping in order 10echleve eeruln r1IduetJ.on 'acton.

• Maximizing the volatile compo-nent concentration in the liquid andmaximizing the fraction of gas in con-tact with this liquid increases themass-transfer driving force.

Lowering the total pressure (whilemaintaining the stripping-gas load)decreases the actual partial pressure ofvolatiles and increases the drivingforce.

• Increasing the stripping-gas loadwhile maintaining the total pressurealso reduces the actual partial pres-sure.

Similarly, the stripping efficiencyis limited by a number of mainlymass-transfer-related factors:

• Increasing the specific contactarea or contact time between the phas-es increases mass transfer (until satu-ration of the gas phase).

• Decreasing the thickness of liquidfilm reduces "traveling distance" ofthe volatile components and lowersthe resistance in the film.

• Recondensation phenomena.These models take into considera-

tion a simplified three-componentsystem. composed of one heavycomponent (triglyceride). onevolatile component (typically FFA),and one stripping gas type (usuallysteam).

Cross-flow modelsThe most used model is the Baileyequation, which is derived fromRaoun's and Dalton's laws, Similarequilibrium models can be derived bysubstituting Raoult's law withHenry's, or Lewis and Luke's laws,which probably would be more appro-priate, as the precision of Rnouh's lawis optimal at volatile concentrationsapproaching unity. Other models havebeen presented in the literature, butnone has gained popularity.

The commonly used form of theBailey equation is the reduced formshown in Eq. I, which has proved itsvalue as a practical tool for manyrefineries, although it contains severalapproximations.

[Eq. II

It is, in principle, an equilibriummodel, relating the process parametersand the reduction factor of a certain

PIOI·O

P'v ·E [Eq.21

INFORM. Vol. 10. no. 3 (March 1999)

252

PROCESSING

Olllnl.1 _Fl =O+(O+V) .X,., 1 I

counter-currentsbipper

Figure 6. Mass nows in the C<XJnter-eurrenl stripper

011 outlet .... _F3= 0 + 0 .Xv....

lion of the model is only feasible if therelation between activity coefficientand concentration is known. As suchdata regarding FFA in triglyceridesare nor widely accessible, one is betteroff omitting the activity factor in theequation. and incorporating that effectinto the overall efficiency E.

Figure 5 illustrates calculated val-ues of cross-flow stripping intensitiesof different combinations of processparameters, assuming that Bailey'sequation is valid. It also reveals whichcombinations of process parameterswould result in the necessary increasein stripping intensity in order to dophysical refining. At unit efficiency, atwofold stripping intensity can beachieved by increasing the tempera-ture in the range of 5_8°C, or byincreasing the steam/pressure ratioapproximately 0.1 wt%/mbar. If forinstance the current situation is dosingI wt% steam at 5 mbar, the latter maybe achieved by either increasing steamdosage to 1.5 wt% or reducing thepressure to 3.33 mbar.

An approximate adjustment forefficiencies lower than unity, whichare found in commercial deodorizers,can be made simply by dividing thevalues by the actual efficiency. Forexample, a twofold increase in strip-ping intensity requires an increase intemperature of 7-11.5cC in a deodor-izer having an efficiency (E) of -0.7.

Increasing temperature is generallythe least expensive way of increasingthe stripping intensity in an existingdeodorizer. However, the increase intemperature is limited by occurrenceof undesirable thermal reactions, e.g.,formation of trans isomers and poly-mers, which is further compounded by

the ongoing tendency in consumer-driven tightening of the product speci-fications.

For use in continuous cross-nowsystems, the quantity parameters S, 0,and V in the Bailey models are simplysubstituted by flow parameters (quan-tity per time units).

Counter-current modelModeling of the counter-current pro-cess has been discussed on severaloccasions. but no single model hasgained general popularity, probablydue to the complexity of such models.Some models introduced in the gener-al literature and handbooks use theconcept of equilibrium stages or ofmass transfer units in a similar way as

Rgure 7. Modeling the per10nnance of counter-current operation at different processconditions

T

0.8%

0.7%

i0.6%&:s• •• 0

02 0.5%'a<ll(!:"EI&.Q.~~

0.4%S .0fI g ~rl-. 0.3%"2h 0.2%"',l

0.1%

Vapour phaseF2=S+Vt

in absorption processes. These modelsfocus on mass-transfer rates. whichare very helpful in the design phasebut are quite complex and not suitablefor use in the production environment.

However, the counter-current situa-tion (in the steady state) can be han-dled by exactly the same and simpleway as the cross-flow situation is dealtwith in the Bailey equation.

The counter-current situation isillustrated in Figure 6, in which themass transfer of volatile between thetwo phases is designated Vr. In con-trast to cross-flow, the mass transferof volatile from liquid to gas phase isat all times in equilibrium with thefeed concentration and therefore con-stant in time. Expressing the liquidmass balance in molar concentrationsgives Eq. 3, in which the symbols cor-

Stripping gasF4=S

[Eq.31

respond to the previous definitions(except that now .r exclusivelyexpresses the molar concentration ofthe volatile component). Eq. 3 is validfor the residual content of volatile, V,.being less than approximately 3 mol%(or I wt% when considering FFA intriglyceride oil).

:::T-E~::,:::~--- -------- Temp_232C(P .. IE25mbar)

Equilibrium oonditions: Sbipplng efficiency. E IE 1_J _Pressure

__________________________ .f-!(,"mbo~')-l

......"-<3-5___ - __ - - - - - - - - - - - - - - - - - - - - - - - ~2,5

""*" 1,25~0.625-~---------------------

~ II HI(--0.0% l--"='-+-=--+----+--+---+---+---+--'0.... 0.'" 0.•'" 0.6% 0.8% 1.0%

FFA Inlet cone. [wt%)1.2%

INFORM. \tlI. 10. no. 3 (March 1999)

253

90%

'OO%.----------- ~----_.~~~----~KaP ... IE.p.

p... " S. p•• 25, E. 1.0·"1(. O.2tip.. = 3,P.-25, E -0.' ooKaO.l!5. ------------r------,

=; -- ~------L---~------..JI- ,,

Range for X.'C~:::::' fI'ICIIe% (0.3 wn'o), ,..... • !5 moiIf'1o (1.1 wt'J,)_____________ ~--------- !:-:-,. 10mole'Jlo{3.!5wt'J.),

~~--------~'----------~------~,,~

RIII9t for K0.25 • dotted up\*" line

0.15. tullowei' line

10 100Reduction factor RF [dimensionless]

Figure 8. Superiority 01 .lflpplng gal usage in counter-current compared 10 cross-llowprocess

The gas phase mass balance can betreated after the same principles asused for deriving the Bailey equation.Assuming that the inlet oil and theoutlet gas are in equilibrium, that Dal-ton's law explains the relationbetween S and V, . and that Raouh'slaw describes the partial pressure ofthe volatile component. then substitut-ing the equilibrium pressure by theefficiency factor multiplied by theactual vapor pressure gives Eq. 4,

See = P'OI - P; -XI- EV, P; -XI - E

IEq.41

Eq. 4 reveals that the stripping gasflow corresponds to the differencebetween total pressure and volatilepartial pressure, while the volatile gasflow corresponds to its partial pres-sure. However. it is obvious that Eq. 4is valid only when the actual partialpressure of the volatile. P'v - x= E, isless than the total pressure. An actualpartial pressure being higher than thesystem pressure results in a sponta-neous evaporation of the volatile. cor-responding to the flashing process. ln

'000

this case, Eq. 4. indicates reducedstripping gas consumption. comparedto cases with lower initial volatileconcentrations. which cannot be true.The validity of Eq. 4. is therefore lim-ited to cases where PUll - P'v -XI - £is positive. For negative values it isassumed that maximum stripping gasconsumption calculated for the specif-ic conditions applies.

Combining Eq. 3 and Eq. 4 resultsin the final Eq. 5 for counter-currentoperation, which can be compareddirectly to the Bailey cross-flow equa-tion (Eq. 2).

As in the case of Eq. 4, Eq. 5 isvalid only when the actual partialvolatile pressure is less than the sys-tem pressure. It is important to notethe balanced Slate of the equation,assuming an indefinite number ofequilibrium stages being present in thecolumn, even though a correction effi-ciency factor E has been added. Theequation therefore lacks accuracywhen extreme values of x, or XI occur,as can be seen by the prediction ofstripping gas consumption approach-ing zero volatiles in the outlet.

Eq. 5. may be written in a moregeneral form, in which the relation

between partial pressure and liquidconcentration is predicted by Raoult's.Henry's. or Lewis and Luke's laws.respectively.

The potential of a specific set ofprocess parameters in counter-currentoperation can be evaluated quickly byusing the simple Eq. 6, whichdescribes the maximum stripping gasconsumption required for any value ofinlet concentration.

Sec, max -

[Eq.61

The equation does not directlycontain the residual volatile compo-nent concentration, which in turnaffects the efficiency factor. Anincrease in the reduction factorresults in a lower efficiency (givenan unchanged number of equilibriumstages present in the deodorizer). Eq.6 is particularly useful if the effi-ciency of the deodorizer has beencharacterized.

The theoretical performance of thecounter-current operation, accordingto Eq. 5 when full equilibrium existsbetween inlet oil and outlet gas phaseis shown in Figure 7.

Comparison of stripping efficiencyComparison of Eq. 2 and Eq. 5 revealsthe theoretical efficiency of cross-flowand counter-current flow designs.Rearrangement of the equationsreveals that counter-current strippingis superior to cross-flow whenever Eq.7 is true,

[Eq.11

For all practical purposes the ratioVIN, corresponds 10 the reductionfactor (RF) previously defined as theratio between inlet and outlet concen-tration Xlix,. Eq. 7 is true for all val-ues of RF > I. implying that, giventhat the efficiencies (E) in the twoprocesses are the same, counter-cur-rent consumption of stripping gas isalways less than that of cross-flowconsumption.

INFORM, Vol. 10. no. 3 (March 1999)

254

PROCESSING

2.5"4 T"";:;:;:::;;=;;;;,-------------,cakulllioI'4 ... baMd on:Raoult's and OIoIIon'l ........ obe)oId. Triglyceride, MW-870. Ind ollie KId, MW-2lI2.p... 5~, T.-.p _232C (p•• 25 ~ Sq,png e!IiI;iency.' In cak:uIIIt-.I ~.

the two processes performed underequal conditions. SCdSCF is a func-tion of xf RF and K, where K equalsPcO(IE·P v, and therefore includes theinfluence of the temperature and thephysical properties of the volatilecomponent in question .

Typical operation conditions can bedescribed as reducing the FFA-con-centration from around I wt% in thefeed, requiring a reduction factor inthe range 10-100. K typically lies inthe range 0.15-0.25, i.e.. a systempressure of 3-5 mbar. a partial pres-sure of the volatile component around20-30 mbar (at 230-240°C). and theefficiency E being 0.7-0.9 .

It can be seen from Figure 8 that.assuming the same efficiency factor isachieved in the two processes, thecounter-current operation in generalreduces the stripping medium con-sumption to less than 30% of theamount used in cross-flow operations.The higher the inlet concentration of

(cOfIlinued on page 162)

..' .....' ...'..'.' .• • • Bailey. reduced__ Ful8alleyequllllon

--CounIer'<lU...m

•."'I-------~-----~---~~..'" s.'"''''' ..'",..FFA inlet cone. [wt%)

Figure 9. Comparison of Indu.trial performance of cross·flow and counter-currentdeodorization

Figure 8 illustrates the relative I ticn. SCCISCF, expressed by the ratiosuperiority of counter-current opera- between the stripping consumptions of

HeadquartersBuilding Update

ConstructionThis picture, taken in early February 1999, shows progress atthe new headquarters site. At this point of construction, theoccupancy dale is anticipated in July 1999, barring unforeseen,serious problems.

Fund raisingPersonal contacts with highly targeted companies and individuals continue by the Campaign AOCS committee,and the mailing to AOCS members and others has been launched.

Be sure to look for the Campaign AOeS materials from the committee, and please consider helping AOeSprovide facilities to ensure continued quality programs and services for members and the industries in whichwe work.

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INFORM. Vol. 10. no. 3 (March 1999)

262

PROCESSING(cQminul!'d!rom pug~ 2j4)

volatile components and the higherthe reduction factor required, the high-er the savings that will be achieved. Itcan also be seen that performance isaffected by K. implying that at highertemperatures (which increase Pv) or atbetter vacuums, the counter-currentprocess is relatively more effective.

Both in counter-current and incross-flow operation the carrier effectis related to the stripping gas volume,and the pressure is inversely correlat-ed to the steam consumption. Theeffects of different stripping gas mediaare expected to be equal. when thecarrier rate is compared on a mole-to-mole basis.

However. these calculated equilibri-um models may not reflect the actualperformance achieved on industrialscale. During the past two decades,experience with thin-film stripping hasbeen obtained in palm oil refining in theFar East. Recent installations of theSoftColumn from Alfa Laval furtherprovide comparable information. Figure9 provides a comparison of the industri-al performance of the two process types,"standardized" to the specific processconditions noted. The figure is supple-mented with theoretical curves of equi-librium performances of the processes.

It can be seen that the savingspotential under typical process condi-tions amounts to approximately I wt%steam, which correlates well with thetheoretical relative saving of approxi-mately 70%. The savings are higher ifthe volatile inlet concentration is high,which is also in accordance with theo-ry. II seems possible to achieve effi-

ciency factors of around 0.8 in thecounter-current process, which iscomparable to the efficiency factortypically achieved in cross-flow pro-cesses. However. precise measure-ments of efficiency factors of packedcolumns have not yet been published.

The trends in product quality andtechnology are moving towardreduced residual FFA content andreduced processing pressure. whichfurther enhance the promisingprospective of the counter-current pro-cess. However. even though the strip-ping gas consumption is an importantcost parameter in deodorizing. otherfactors may favor the traditionalcross-flow process.

AcknowledgmentThis work was supported in part bygrants from the Danish Academy ofTechnical Sciences.

BibliographyJournalsFat Science TechnologyE. Deffense, Theorie und Praxis der

Physikalischen Raffination (97:481--484, 1995).

Fette Seifen AnstriciuniuelV.A. Shadiakhy, Kolonnenauslegung

zur physikalischen Raffination vonOlen- Theorie und Praxis(8S, 173-176, 1986).

Food and Bioproducts ProcessingW. Hamm, The processing of edible

oils: challenges and opportunities(7HI-72. 1996).

INFORMO. Stenberg and P. Sjoberg. Thin-film

deodorizing of edible oils(7,1296-1304, 1996).

Journal of the American OilChemists' Society

H. Stage. The physical refining pro-cess (62:299-308, 1985).

lipid TechnologyP. Sjoeberg, Deodorization technology

(3'52-57,1991).

Transactions of the American tnsunueof Chemical Engineers

H.J. Garber and F. Lerman. Prin-ciples of Stripping Operations:Particularly Steam Distillation(39,113-131.1943).

Y.T. Shah and M.M. Sharma, Desorp-tion with or without chemical reac-tion (54: 1--41, 1976).

BooksR. Billet, Packed Towers in Pro-

cessing and Environmental Tech-nology, VCH VerlagsgesellschaftmbH. New York. New York1995.

K. Carlson, Deodorization. inBailey's Industrial Oil & FatProducts, Vol. 4, fifth edition,edited by Y.H. Hui, John Wiley& Sons Inc., New York, NewYork. 1996, pp. 339-391.

K. Sattler and H.J. Feindt, Ther·mal Separation Processes.VCH VerlagsgesellschaftmbH, New York, New York.1995. •

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