ECOfreeze Vacuum System

Freeze Condensing Vacuum Systems for Deodorization

Reprinted with permission from inform, vol. 12, no. 3 (March 2001). Copyright © 2001 AOCS Press.

A deodorizer vacuum system utilizingfreeze-condensation technology offersadvantages not available with conven-tional ejector systems.

When revamping existing deodorizersystems, a freeze-condensing vacuumsystem allows deodorizer pressure to bereduced without incurring an excessiveincrease in utility consumption. The keyconcept at work is to remove strippingsteam and free fatty acids before theyenter an ejector system by freezing themonto a cold heat-transfer surface. Theejector system handles essentially aironly.

This paper reviews freeze-condensa-tion and offers a comparison to a con-ventional ejector system for a 75,000pounds per hour (pph) deodorizer.

Freeze-condensation—what is it?The term “freeze-condensation” is amisnomer for what thermodynamicallyhappens, but it is descriptive to theextent it offers a visual image of whatoccurs. The triple point for water is32˚F and 4.6 torr—at that pressure andtemperature, it is possible for ice, water,and steam to coexist. If the pressure andtemperature are below the triple point,

Volume 12 • March 2001 • inform

Freeze-condensing

vacuum systems for

deodorization

This article is by J.R. Lines, vice presi-dent for marketing, Graham

Corporation, 20 Florence Ave., P.O. Box 719, Batavia, NY (phone: 716-

343-2216; fax: 716-343-1097; e-mail:jlines@graham-mfg.com).

Artwork provided by J. Lines

Schematic of a deodorizer vacuum system utilizing freeze-condensation

then it is possible for steam to go direct-ly to the ice phase without passingthrough the liquid phase. The appropri-ate thermodynamic term for this phe-nomenon is “deposition”. Deposition isthe opposite of “sublimation”, the morefamiliar term for a solid passing direct-ly to the vapor phase without passingthrough the liquid phase. Freeze-con-densation has been referred to as a mis-nomer because condensation is associat-ed with vapor going to the liquid state;however, condensation in the classicsense of the formation of a liquid phasedoes not occur at the operating pressureand temperature of a freeze-condenser.

The operating conditions for deodor-izer overhead effluent make depositionpossible if a sufficiently cold coolingfluid is used. Pressure leaving a fattyacid scrubber may range from 1 to 4torr, which is below the 4.6 torr triplepoint pressure. The temperature of thecooling fluid is a function of the operat-ing pressure of the deodorizer. Thelower the desired operating pressure,the colder the cooling fluid must be.Figure 1 depicts recommended coolingfluid temperature for differing deodor-izer pressures. For example, if thedeodorizer pressure is 1.5 torr, then therecommended temperature of the cool-ing fluid is −15 to −30˚F., whereas at0.75 torr, the recommended coolingfluid temperature is −27 to −42˚F.Figure 1 offers a guide to recommendedcooling fluid temperature; however,warmer or colder temperatures may beconsidered.

Condenser designThe heart of a freeze-condensing vacu-um system is the condenser itself.Actually, two condensers are used for atypical application. One condenser ison-line in the freezing or ice-buildingmode. The other is off-line defrosting

and being readied to be brought on-line.Process fluids to a freeze-condenser

are steam, free fatty acid, and air, andthey are on the shell side of the con-denser. Steam is the stripping steam putinto the deodorizer. Free fatty acid iswhat is carried out of the fatty acidscrubber. Air is from leakage into thesystem due to the subatmospheric oper-ating pressure. The thermal design ofthe condenser is sophisticated; design

software is not available. The designmust take into consideration cooling ofgases, deposition heat transfer, and ice-thickness growth. By no means is thisan ordinary heat-transfer problem. Thisis further compounded by the low oper-ating pressure and minimal availablepressure drop. Designs with 0.1 to 0.25torr pressure drop are typical.

The design of the tube bundle is thekey. Tube pitch and layout are tailored

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Figure 2. Measured ice thickness vs. time (−30°F cooling fluid temperature)

Figure 1. Recommended cooling fluid temperature vs. deodorizer pressure

for each particular application to pro-vide the longest run time economicallypossible. The design must factor veloci-ty and heat transfer at the start of oper-ation as well as at the end when there isconsiderable ice buildup. Figure 2shows how ice thickness increases withrun time. In this particular instance, aproperly designed tube field allowed theunit to perform at or below the designoperating pressure for longer than twohours, by which time the ice thicknesson the tubes was approximately three-eighths of an inch.

As ice builds up on the surface of thetubes, two negative effects occur.

First, the ice layer is an insulator thatdiminishes heat-transfer effectiveness.As thickness increases, the temperatureat the ice layer surface becomes warmer,thus reducing the available logarithmicmean temperature difference (LMTD).In a case where boiling ammonia is thecooling fluid, LMTD is approximatelythe difference between the ice surfacetemperature and the boiling ammoniatemperature.

Second, the cross-sectional flow areadecreases as ice layer thickness increas-es. For example, at the start of opera-

tion when tubesare bare and icehas not formed,the gap betweentubes at the inletsection of thecondenser is1.25". As Figure2 shows, aftertwo hours ofoperation the icethickness is0.35", therefore,the gap betweentubes is 0.55".The reduction inthe gap betweentubes results in higher velocity and, con-sequently, greater pressure drop.

Figure 3 shows a comparison of thetop tube row at the start of the cycle(Fig. 3a), when there is no ice and thetubes are bare, and the end of the cyclewhen there is substantial ice buildup(Figs. 3b, 3c).

The tube bundle layout for a well-designed freeze-condenser will have avariable tube pitch. The spacingbetween tubes will vary. This permitsthe entry of high volumetric flow into

the tube bundle at velocities conduciveto low-pressure drop throughout theentire operating cycle. A 75,000 pphdeodorizer will have approximately4,000 ft3/s flow entering a freeze-con-denser. The tube layout is open wherethe high volumetric flow enters and istighter at the back end of the tube bun-dle. Leaving the freeze-condenser, thevolumetric flow rate is approximately50 ft3/s. The open spacing at the frontnot only permits reasonable velocities atthe entrance to the tube field but also

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Figure 3a. Start of cycle (when there is no ice and the tubesare bare)

Figure 3b. End of cycle (when there is substantial ice buildup)

Figure 3c. End of cycle (when there is substantial ice buildup)

allows maximum ice growth becausethe spacing between tubes is wide.Figure 4 illustrates the tube field layoutfor a freeze-condenser that supports a75,000 pph deodorizer.

Freeze-condenser performancecharacteristicsPerformance of a freeze-condenser isaffected by cooling fluid temperature,the amount of noncondensable gas (air),and steam loading.

A sensitivity analysis for coolingfluid temperature was done for a75,000 pph system (Table 1). Thedesign operating pressure was 1.5 torror below. Cooling fluid temperaturewas varied and condenser operationmonitored over time. Cooling fluid tem-perature was varied from 0 to –50˚F in10˚F increments. Run time until 1.25torr operating pressure was reached wasmeasured along with reclamation effi-

ciency. The steam load to the condenserwas metered through a fixed orifice.After defrosting, the weight of conden-sate was measured. Reclamation effi-ciency is pounds of condensate collecteddivided by pounds of steam put in overthe run time.

A 100% reclamation indicates thatall the stripping steam was converted toice. No stripping steam was entering theejector system. The condenser operatedas an effective cold trap.

Operating pressure as a function oftime is shown by Figure 4.

A similar analysis was done for non-condensible air load (Table 2). A highdesired vacuum level is affected by theamount of air inleakage once a set vac-uum system is installed. It is importantto specify a vacuum system that sup-ports the freeze-condenser but does notset the operating pressure. The analysismeasured performance with 0, 100,

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Table 1Cooling fluid temperature efficiencyanalysis for 75,000 pph system

Cooling fluid Run time temperature to reach Reclamation

(°F) 1.5 torr (%)

0 Not possible 0−10 58 minutes 96−20 102 minutes 99.5−30 105 minutes 100−40 125 minutes 100−50 128 minutes 100

Table 2Air load efficiency analysis for 75,000pph system

Air Run time load to reach Reclamation(%) 1.5 torr (%)

0 220 minutes 100100 140 minutes 100200 110 minutes 100300 110 minutes 100400 80 minutes 99

Figure 4.Tube field layout for freeze-condenser that supports 75,000 pph deodorizer

200, 300, and 400% noncondensibleload. Cooling fluid inlet temperaturewas fixed at −30˚F for the analysis. Inthis case the run time to reach 2.0 torroperating pressure was measured.

The data indicate that a well-designed freeze-condenser with a prop-erly matched ejector system yields excel-lent performance across a wide range ofoperating conditions.

Another assessment pertained toincreasing the stripping steam load tothe condenser. The condenser handledthe additional stripping steam withoutproblems. At 200% loading the con-denser behave favorably the change inrun time was reduced because ice depo-sition was greater. Again, reclamationwas essentially 100%.

Comparison of freeze-condensationvs. conventional ejector systemTable 3 shows a comparison of costs fora freeze-condenser vacuum system vs. aconventional ejector system. The com-parison is for a 75,000 pph edible oildeodorizer operating at 1.5 torr. The

load exiting the fatty acid scrubber is1,000 pph stripping steam, 20 pph air, 7pph free fatty acids, at 1.25 torr and160˚F to the vacuum system.

A freeze-condensing vacuum systemhas a greater capital cost when com-pared with a conventional ejector sys-tem. The advantages, however, provide

a reasonable payback for that addedcapital cost. Those advantages include:

• Substantially lower consumptionof high-pressure motive steam, 1,100pph vs. 10,000 pph;

• The caustic flush system used witha conventional ejector system is elimi-nated. The 15 gallons per minute (gpm)

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Figure 5. Freeze-condenser performance vs. cooling fluid temperature

Table 3Comparison of freeze-condensation vs. conventional ejector

Freeze-condenser Conventionalvacuum vacuumsystem system

Capital cost a $500,000Capital costs for ejector system $150,000Utilities

Motive steam (200 psig D&S) 1,100 pph 10,000 pphWater (87˚F) 125 gpm 2,000 gpmCooling fluid (−25°F liquid ammonia) 2,200 pphWaste steam for defrost mode

(25 psig or greater) 2,600 pphCaustic flush solution 15 gpm

Additional costs not shown Cooling tower Cooling towerRefrigeration system Caustic systemInstallation Installation

a For twin freeze-condensers, isolation valves, ejector system, and melt vessel

NaOH solution is eliminated and so ischemical treatment with sodiumhydroxide;

• Cooling water is dramaticallyreduced, 125 gpm vs. 2,000 gpm;

• The ejector system is much smallerand easier to maintain, with the largestejector being 10 to 12 ft long vs. 40 ftlong. A conventional ejector system’sfirst two ejector stages are mounted ver-tically, resulting in accessibility andmaintenance difficulties. The smallerejectors for the freeze-condensingoption are mounted horizontally withinthe structure, making accessibility andmaintenance less difficult;

• Capability to isolate deodorizerfrom vacuum system;

• Environmental effects are lessbecause far less waste water is pro-duced. Only 1,100 pph of motive steamcontacts the process effluent rather than10,000 pph;

• Capability to run deodorizer atlower pressure to improve tocopherolrecovery without a substantial increasein utility usage; and

• Flexible operation makes futureexpansion possible.

SummaryAlthough freeze-condensation is rela-

tively new in the edible oil market, manyrefiners are evaluating the applicabilityof the technology. It does provide sub-stantial benefits, but a properly designedfreeze-condenser that is matched with awell-designed ejector system is vital forreliable operation. Operating cost andenvironmental effects are lower whenfreeze-condensation is used. These fac-tors make it worthwhile—when consid-ering a new deodorizer to evaluaterevamp options, or determine options tolower operating pressure for greatertocopherol recovery—to evaluate freeze-condensation along with conventionaltechnology.❏

Volume 12 • March 2001 • inform