analysis of mass transfer modeling vapor liquid equilibrium in scrubbing retort of gas fb4adfbc0a

7/28/2019 Analysis of Mass Transfer Modeling Vapor Liquid Equilibrium in Scrubbing Retort of Gas Fb4adfbc0a

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... ~ I T T E L H A U S E R .corporatton- - - - - - - - - - - - - - - - - - -c....:- - .

ANALYSIS OF MASS TRANSFER MODELINGAND VAPOR-LIQUID EQUILIBRIUM IN

SCRUBBING RETORT OFF-GAS

Prepared ByMITTELHAUSER CORPORATION

forU. S. Department of Energy

Laramie Energy Technology CenterP. O. Box 3395, University StationLaramie, Wyoming 82071

December, 1981


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"~ . t i 1 I T T E L H A U S E R ..-.. ' ,. corporatlon_.----..........----..........------------------------------,C, TABLE OF CONTENTS

1.02.03.0

4.0

5.0

6.0

SUMMARYINTRODUCTIONMASS TRANSFER MODELS

Page No.134

3.1 Venturi Scrubber Continuous Contact Model, 43.2 Venturi Scrubber Stage Contact Model 123.3 Sieve Tray Scrubber Stage Contact ~ l o d e l 133.4 Physical Properties 15VAPOR-lIQUID EQUILIBRIUM (VlE) IN NH3-H2S-C02-H20SYSTEM4.1 VlE Data4.2 VlE ~ l o d e l s

4.2.1 Van Krevelen4.2.2 API SWEQ4.2.3 Institute of Gas Technology4.2.4 EMNP4.2.5 Beutier and Renon4.2.6 ECES

4.3 Process Simulation Software4.3.1 ChemShare4.3.2 . SSI Process4.3.3 OlI Systems

SUPPORTING WORK5.1 Preliminary Simulation Work5.2 Determination of Oil Phase Importance5.3 Determination of Organic Acids and BasesCONClUS IONS

202023232324242425262626262929293033

i


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tlt1!TTELHAUSER< - corooration-_.-- _ . - - - ~ - ._- - . - - - . - - - , . , . . - - - . " , ~ ' " " ' ~ , . . . - ~ . , . , ~ - = -.. , . , - " . . . ~ , . . , - ~ " , . , - - . . . . , ~ " . - - - - ~ - - - - - - - - - - - - - - - - - - - - = ' :

~ ~ : : .(....\ 1.0 SUMMARYPrevious experiments at the Laramie Energy Technology Center

.- . - - . - - -indicated that a significant fraction of ammonia (NH3) present in retortoff-gas was removed while operating a venturi scrubbing device for parti- - - - ~ .culate removal. Such results plus the commercial history of using ammonia": __water solutions for removing hydrogen sulfide (H2S) from coal-gas streamssuggests the possibility of using scrubbers for mass transfer as well as

Iparticulate removal. This report addresses mass transfer considerationsrelated to scrubbing retort off-gas. It examines approaches for calculating mass transfer in venturi and sieve tray scrubbers and for determiningvapor-liquid equili&rium.

Two different types of mass transfer models are examined-con-tinuous contact models and stage contact models. Continuous contactmodels are derived from the solutions to d i f f e r e n t i ~ l equations thatmathematically describe material balance, mass transfer rate and equilibrium relationships. Typically such models are appropriate where masstransfer takes place in a continuous manner such as a packed absorber.Stage contact models treat mass transfer as taking place in discretestages such as a tray of a fractionator. Algebrian equations are writtenand solved around the stage to account for material, energy and equilibriumrelationships.-

This work concludes that continuous contact models for masstransfer in a venturi scrubber are not practical. There is l i t t leprevious work on such models for venturis. Further the system is relativelycomplex due to mass transfer of three species - NH3, H2S and C02 - beingconsidered. Previous work, usually on packed towers or spray scrubbers, - - - ~ - - - ~ - -most often is limited to mass transfer of one species. Also the mathematicsof continuous contact models requires simplifying by means of assumptionssuch as linear equilibrium r e l a t i o n ~ h i p s and constant temperature.Unfortunately these assumptions aren't true in scrubbing retort off-gas.As a consequence, the most practical type of modeling is a stage contactmodel. This type modeling can be used for both venturi and sieve trayscrubbers. In addition, this type modeling can be implemented using


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.MiITTELHAUSERo r - O t l o n = . ; ; - . - : . ; . . ; . - = - = - ; ; : ; . ; - . - ; : ; ; ; ; - c . . ; . . ;. ________________ --

commercially available process simulation packages. These packages areC computer programs that have been prey; ous ly used for s imu1at ing processoperations. They incorporate recent d e v e l o p m ~ n t s in the prediction ofvapor-liquid equilibrium (VLE) in NH3-H2S-C02-H20 systems.

Significant advances were made in the 1970's relative toestablishing the thermodynamics of electrolytic solutions and predictingVLE in NH3-H2S-C02-H20 systems. Consequently VLE models are availableand they are described in this report. Several are available ascomputer programs or in conjunction with commercial simulation programs.

A couple potential problems warrant research investigation todetermi ne the ir importance. Scrubb i ng retort off -gas wi 11 produce ahydrocarbon phase in add it on to the gas and aqueous phases. The presenceof this phase could complicate mass transfer predictions; hopefully i tis of minor. importance and can be neglected. The retort off-gas alsocontains polar hydrocarbons that will act as acids and bases in aqueoussolutions. These compounds will tend to fix the soluble NH3 and H2S insolution. Further research can determine the significance of the hydrocarbonphase and polar hydrocarbons.

It is recommended that mass transfer calculations be performedusing one of the available simulation packages. Liquid and vapor rates,compositions and temperatures corresponding to planned field scrubbingtests should be used in simulation work in order to approximate testconditions as closely as possible.


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MITTELHAUSER ________corporation__ _ ........_________________( 2.0 INTRODUCTION

This report addresses mass transfer considerations related toscrubbing oil shale retort off-gases. Experiments during September, 1980,at Laramie Energy Technology Center's 1 5 0 - ~ o n retort, located at the U.S.DOE North Site in Laramie, tested a venturi scrubbing device for particulateremoval. Data from these tests indicated that a significant fraction (50to 75 percent) of ammonia (NH3) present in the retort off-gas was removed inthe venturi scrubber. These results, plus the commercial history of usingammonia-water solutions for selective removal of hydrogen sulfide (HZS) fromcoal-gas streams, suggests the possibility of using scrubbers for mass transfer as well as parliculate removal. Removal of a significant fraction ofthe H2S would reduce the costs associated with downstream acid-gas removalby processes such as Stretford.

Mass transfer in retort off-gas scrubbing systems is complicated.Gas streams are a mixture of gases, hydrocarbons, water and soluble chemicals. Cooling of these streams produces a three phase mixture--gas, aqueousand oil. The calculation of vapor-liquid equilibria under such circumstancesis not straight forward. Several of the techniques for calculating vaporliquid equilibria and mass transfer have been developed only recently.

This report discusses approaches for calculating mass transfer inventuri and sieve tray scrubbers and for determining vapor-liquid equilibrium. Two different mass transfer modeling methods are described. Sourcesof VLE data are summarized and VLE models, several of which are quite recent,are described. Available process simulation software which can be used topredict removal ofNH3 and HZS is summarized. Supporting engineering andlaboratory work that would assist in better understanding absorption ofNH3 and HZS is discussed.

3


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.(MITTELHAUSER- S F - E - R - M - O - D E - L - S - - - - - - - ~ ~ - - - - - - - - - - - - - - - - - - - .

(Mass transfer models may be approached on the basis of stage

contact or continuous contact. In one sense the approach parallels the typeof equipment used to bring about mass transfer. For example, trayedabsorbers or fractionators are designed by calculating heat and materialbalances around stages that are analogous to the trays in the tower.Continuous contact equipment such as packed absorbers are designed usinganalytical or numerical solutions to differential equations that mathematicallydescribe material balance, mass transfer rate and equilibrium relationships.An acceptable approach does not have to parallel the type equipment. Stagecontact calculation procedures, for example, are often used to design packedtowers.

Process conditions and availability of experimental or operatingdata can be an important consideration in the selection of a valid, useablemodel. A continuous contact approach, for example, is usually dependentupon process assumptions such as

Constant temperature Linear vapor-liquid equilibrium relationships.Further it may require data on mass transfer rates and geometric considerationsthat have to be determined experimentally and that are variable with applicationsand conditions.

Rigorous stage contact models are not as sensitive although simpli-fied ones can be. The basic requirements for a good stage contact model areaccurate equilibrium prediction methods and enthalpy data.3.1 VENTURI SCRUBBER CONTINUOUS CONTACT MODEL

Venturi scrubbers are gas-atomized spray devices. The predominantmechanism for mass transfer in such devices is absorption by droplets.(l)

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. l ~ I T T E L H A U S E R___ _:;orporation_____________-_"_--_--_-_____Almost all of the mass transfer takes place where the gas velocity is highest,i .e. , in the venturi throat and immediately downstream.

Mass transfer in venturi scrubbers is not generally discussed inthe literature. The scrubbers are primarily used for particulate removal.The most comprehensive treatment of mass transfer is found in a handbooktitled, Wet Scrubber System Study: Volume I, Scrubber Handbook, preparedfor the Environmental Protection Agency. The modeling approach describedhere parallels the treatment in this handbook.

The general equation for mass transfer of component i between twophases is (see nomenclature pages):

(1)

where mass transfer is characterized by a local gas-phase mass-transfercoefficient kGi' Much of the mass-transfer information in the literature isexpressed in the" form of "overall" coefficients to avoid the problem ofdetermining interfacial properties. Such a coefficient for the gas-phasewould be written KGi, and the flux would be

A A . *Ni = KGi (pl - Pi ) (2)where Pi*, the partial pressure of i at equilibrium with respect to the bulkliquid phase composition of i , is much easier, at least in theory, to determine.For example, if component i obeyed Henry's Law,

* ! \Pi + Hixi (3 )where Xi is the mole fraction of i in the bulk liquid, and is the constantof proportionality, usually called the Henry's Law Constant.

This simple picture is complicated in the retort off-gas system bythe following considerations:

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( Henry's Law in not obeyed . There may be two additional diffusion films to worry aboutbecause of oil condensation.1) gas-oil2) 0 il-water

For the moment, consider the case in which there is no oil film.- - G e n e r o . l l Y - ~ G i is not constant with respect to composition, but if we can - - - - - - -

deterwine that the vapor-liquid equilibrium curve in the range of compositionsof inter"est is essentially straight, we can write:

1(4 )

It is vital to note that Hi in this equation is not the,Henry'sLaw Constant, but the slope of the equilibrium curve. Also i t is essentialto understand that equilibrium curve means the general relation between bulkliquid and bulk vapor compositions at equilibrium. One could plot:

*Yi Pi* fi

versusversusversus

*'1*C 1ai(mole fraction)(partial pressure vs concentration)(fugaCity versus activity)

I t Q ~ ~ t matter so long as one defines the coefficients and constants ofproportionality in such a manner as to make the interfacial fluxes come outin the correct units. The important conclusion to be reached is:

I t is possible in theory to construct for any component i a vaporliquid equilibrium curve, using available models and accurate dataon one of the bulk phases. The slope of this curve in the concentration ranges of interest, when suitably multiplied by appropriateconversion factors, can be used as the constant Hi in equation(4). It is not necessary to assume that Henry's Law holds inorder to use equation (4).To use equation (4), we do need to evaluate kGi, kLi' and Hi' Some

simplification is possible and in the system of interest:

________ ____________ - - - ' - - - - - - . " , . . - - - . . , . . " ,6


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LAITTRHAUSERcorporation For NH3 ana H2S, mass-transfer is gas-film controiledl i i, \Lc; and the dissociation reactions in the liquid are so rapid(lthat we can assume that there is no concentration gradientwithin the liquid droplets. For C02, mass-transfer is liquid-film controlled.(2)First let us consider NH3; gas-film control means that:

(5 )

'"eference (1) gives an expression for kGi of:

( 0 . ) 0.5_Glt (6 )

in which t is the "exposure time" between the gas and 1 quid. This can beapproximated by the ratio of the drop diameter to the relative velocity between the gas and the drop. If F is defined as the ratio between (1), therelative velocity of _the drop to that of the gas and (2) the gas velocityi tse 1f , then

F = ur (7)llG

and t ~ dd (8)FUG -- -- -----Therefore '" ,...., 2 ( DGiUGF) 0.5KGi "' - RT ddEvaluating DGi will be discussed in a later section.

Referring to equation (2), and setting Pi = YiP and Pi*= Yi*P,then l\ /\ . *Ni = KGi P (Yi - Yi ) (10)Now, the molar flux of i that leaves the gas phase can be con

verted into an overall transfer rate by multiplying both sides by the interfacial transfer area, which is assumed to be the total surface area of thedrops, Ad.

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r!l1 ITTELHAUSER__ -.-::.__ == corporotion_____________________ -The droplets in a venturi agglomerate during their passage through

the scrubber . Therefore Ad will change with the distance from the pOint ofatomization. Consider a thin slice of venturi volume at a pOint some distanceaway fl"om the point of atomization. -Within this volume, if the molar gas710wrate is assumed essentially constant, the moles of i transferred fromgas to liquid are

(11 )

where G' is the total gas molar flow rate, gm moles/sec relativeto the flux, G' = GAT, where AT is the total cross-sectional area of theslice of venturi we are considering.

Then, from equations (10) and (11)

This is the general equation of mass transfer in a venturi. Itincorporates no special assumptions other than that of constant -Gi. Any,ralid expressions for KGi' Ad, or Yi*' can be inserted to build a new modelas new data are developed.

For the particular case under consideration, if it is assumed that1 quid d : ~ o p area varies with distance from the atomization point by therelation given in Reference 1:

(13 )

equations (9) and (13) can be substituted into equation (12) to get:,- G dYi =

(Yi-Yi *)

8

(14 )


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MITTELHAUSER. - corporaUon ....... - - - . . : " " " - ~ = - _ - - - - ' ' - - ______ _. _.--_. _-_--_-_____ - -Now assumptions must be made about: The variation of F with z. F will always have a limiting initialvalue less than 1.0, and that i t will decrease with distanceuntil the drops are traveling at the same velocity as the gas,at which point F = O. The variation of AT, the venturi cross-sectional area, with z. How to express or determine the "average" drop size.A useful result is obtained if i t is assumed that, not only is

there zero liquid-phase resistance, but also that the equilibrium partialpressure of i is zero over the bulk liquid. The resulting solution to equation(12) gives the maximum possible degree of mass transfer for a qiven set ofventuri dimensions, rates and initial concentrations.

To obtain this result, assume:(15 )

which comes from Reference 1 AT is constant with z

58,600 0.5 0.45dd = ds = (6pL) + 597 I (I)L(\) 0.5R1.510000G (1

OL

which is an empirical correlation of "average" drop diameter versusliquid properties and gas-liquid ratio.Substituting equation (15) into (14) and integrating both sides

(remember, Yi* = 0 by assumption) between z = 0 (Yi = F = Fi) and z = 1(Yi = F = 0) gives

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MITTELHAUSER., corporatlon___________ --'__ --'______j1I.---- - - - . - - ~ -- .

Yiy11An expression for dd comes from equatlOn (16); The "average"

value of F can be taken from the following curve taken from Reference 1:1 . 5 ~ r - - - - - . - - - - - - - _ r - - - - - - - ,

..o..

1.0 L/G

o . J o h n s t o n ~ e t 'J94G6 . ~ I k r r A.M. ( l ~ O )__ _ fo r . a ~ 5 t r ~ n s f e r.. . (o r part icle col!cct io l lfrol'll FljtUfP 5.3.6""

A A

2.-0 3.0VALUES OF F VS. ( L M / ~ , , ) FOR S02 ABSORPTION BY SODIUM

HYDROXIDE SOLUTION

( 17)

Note that L/G is a molar ratio, not a mass ratio; therefore themolecular weights of both liquid and gas must be known to compute this termfrom run data.

The curve of y? Iy for NH3 and H2S versus, say L/G, ca 1 u 1atedfrom equation (17), should lie below all other curves and below the experimentaldata points for the run.

_ ______ _____________10

However, even higher mass transfer might be accomplished if asignificant liquid film formed on the walls and added to the available areafor mass transfei. If this happens, the equations must be adjusted since kGfor transfer to a film will be different from that for drops.


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MITTELHAUSER ..___corporotJoo___________________

The presence of condensed oi l could affect the efficiency of mass'transfer if the oil formed a film around the outside of the water droplet.If, however, the water formed outside the oil droplets, no significant changein mass transfer would be expected, except for any effect due to changes ininternal liquid circulati?n within the drop and thus to kLi.

When COZ is absorbed, the liquid-film resistance controls the masstransfer because of the relatively slow ionization

COz + HZO ! HC03 + H+which must take place before the combination with dissolved NH3 can occur:

Data on diffusion coefficients in electrolyte solutions are necessary toevaluate kLi' which must be done to evaluate liquid-film resistances. Theproblem is quite complicated especially when mixed electrolyte solutions are

-involved. Ratcliff and Holdcroft (3) reported data on the diffusion of COZinto electrolyte solutions. The diffusion coefficient was found to decreaselinearly with an increase in salt concentration.

For calcualation purposes DCOZ,L could be computed as if it werediffusing in pure water and then an empirical correction factor based uponthe Ratcliff and Holdcroft data could be applied.

AThus in equation (lZ), for COZ the expression for KG; given byequation (4) would be used. In integrating equation (lZ) the variation in Hwith distance along the z -ax i s , would have to be recognized; i .e. , as Yidecrease, xi increases, and therefore Hi, which is dependent on the con-centration not only of i but of all other dissolved species, also changes.However, i t may be possible through construction of a family of Yi vs xicurves at differing concentrations of NH3 and HZS in the liquid, to establish

* *n average slope of the Yi vs xi curve for COZ, and thus to remove Hi fromthe integral.

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MITTELHAUSER-__ ____ corporaUon( 3.2 VENTURI SCRUBBER STAGE CONTACT MODEL

r ~ a s s transfer in a venturi scrubber could be modeled as a stagecontact device wherein the scrubbing liquor and retort off-gas are broughttogether in i nt imate contact. The contact is ana 1ogous to a stage in atray tower.

t1ass transfer in a stage contact dev i ce is determi ned by wri t i ngcomponent material balances and heat balances around each stage. Thematerial balances must account for the equilibrium relationships that existbetween vapor and liquid. In the typical textbook development, a number ofsimplifing assumptions are made in order to reduce the calculations to alevel that can be handled by hand computational methods. The classicalMcCabe-Thiele method for analYSis of fractionators is an example.

The problem in scrubbing retort off-gas is not amiable to simplified hand computational methods. Rigorous heat and material balancesmust be performed. There are multiple compounds that must be included inthe calculations. Further, soluble gases such as ammonia, carbon dioxide,and hydrogen sulfide ionize in the liquid phase and the liquid equilibriummust be accounted for. The equilibrium between the vapor and the liquid isdifficult computationally. Sensible,and latent effects arising from com-bining a liquid and gas stream at different temperatures must be properlyaccounted for. The presence of two liquid phases, oil and aqueous, com-plicates the calculations and requires that the equilibrium between the twoliquid phases be accounted for.

Nonetheless, the recent development of commercially availablesoftware that is applicable to handling problems of this type makes a stagecontact model viable. Section 4.3 of this report describes this software.The capabilities of the software described have been used to calculateabsorption of soluble species into aqueous solutions and the subsequent

. ---stripping of the solutes.A stage contact model is an equilibrium model. I t assumestnat--

everypiace that liquid-vapor contact occurs, the component species in tnevapor and liquid are in equilibrium of one another. In most stage

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MITiELHAUSERcorporation -- --'".'---" .------- -- -- - -- - . --------------------'------contact devices, such as absorbers or tray towers, equilibrium at each stageis not usually achieved because there is not sufficient residence time orcontact between phases. The deviation from equilibrium is accounted for byestablishing an efficiency term.

In a highly efficient contact device, such as a venturi, equili-brium may be approached closely. Absorption of some species such as H2S mayeven exceed equilibrium prediction due to relative absorption rates. Be-cause absorption of NH3 and H2S is fast relative to C02 (due to the slowionization of the C02 in the aqueous phase) H2S removal could exceed projections.

The efficiency term that makes the most sense in the case of aventuri scrubber is one that is analogous to the Murphree Tray Efficiency.The term is defined as follows:

EM y1 - Yi o= 1i *i - YiThis efficiency can be evaluated from scrubber test data. yq and y1 are1 1taken directly from the test data (after correcting for water vapor that mayor may not be reported in the analysis) . . The equilibrium concentration i iis determined by running one of the process simulation programs previouslymentioned. The efficiency is calculated for each component.

Other parameters regarding NH3, H2S and C02 removal can be computedas well and compared to theoretical results predicted by the computer models.For example, the percentage of each compound actually removed in the scrubbertest can be compared to the amount that the computer program predicts willbe removed.3.3 SIEVE TRAY SCRUBBER STAGE CONTACT MODEL

Sieve tray scrubbers are stage contact devices and are most appropriately modeled as such. The discussion in the previous section regarding

, ,I. )


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MITTELHAUSER_'____corporotioo--'-___________________venturi scrubber stage contact model applies here too. The approach isbasically the same but the effort is more complici,l.ted in that the sieve trayscrubber contains multiple stages whereas the venturi is only a single stagecontact.

Again, commercially available software is used to predict scrubberperformance. However, the program used should have the capability to simulatemultiple stage towers whereas a simple flash calculation program will simulatethe venturi scrubber.

The efficiency matter most likely has to be approached differently.The Murphee efficiency described in Section 3.2 is valid, but its determinationrequires gas analysis between stages. The LETC scrubber is not so equippedand if i t were, the analytical requirements would be burdensome.

The common approach on tray towers is to compute an overall efficiency, Eo. The overall efficiency is the ratio of the theoretical stagesrequired to perform a separation to the actual number of trays in thescrubber. Theoretical stages are determined from a process simulation usingavailable software. An overall efficiency is not generally a mathematicallyrigorous approach. However, as a practical design matter, i t is the mostcommonly used efficiency.

An overall efficiency can be mathematically related to a Murpheeefficiency if equilibrium relationships and material balances can be

e x p ~ e s s e d in linear equations and the Murphee efficiency is constant for alltemperatures.

Alog [1 + EGCHG - 1)]' t"0g(HG) .r

It is not likely that the applicable restraints will exist when scrubbingretort off-gas.


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- .MftTELHAUSERcorporation______________________3.4 PHYSICAL PROPERTIES

A continuous contact model approach such as described in Section 3.1requires physical property data. This section gives recommended calculationmethods or data sources for the property data needed. These physical properties include:

'"' gas density, PG or PG gas viscosity, ~ G gas phase diffusivity, DGi liquid density, Pl liquid phase diffusivity, Dli l i q u j ~ _ s u r f a c e tension, etTh-e recommended methods are descr ibed in the book, The Pro pert i es

of Gases and Liquids (4). Table 3.0-1 cites the name of the method and thepage in the book where the method is described. The methods are amenable tohand or computer computation. Alternatively, the commercial process simulation programs described in Section 4.3 can be used to determine severalof these properties. These simulation packages will typically calculatedensity and viscosity. Some will estimate surface tension.

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c PhaseGas

Gas

Gas

LiquidLiquid

LiquidLiquid

TABLE 3.0-1 PHYSICAL PROPERTIES METHODS/SOURCES

Property

DensityViscosity

Diffusivity

Density----

Viscosity

Diffusivity

SurfaceTension

Method/Source

Use Ideal Gas Law

Use Chapman-Enskog theoryapproximation, p.410 ofReference 4Use methods described inChapter 11, p.544 ofReference 4

Use field measurementor .the density for waterUse viscosity of wateror calculate per Table9-13, p.454 of Reference4Use Wilke-Chang method,p.567 of Reference 4Use data from Appendix Bof Reference 5

16

Comment

Deviation from ideal atscrubbing conditions isnegligibleUse Brokaw approximationof parameters 0ijDiffusivity in a multicomponent mixture isdifficult. Recommend calculating diffusivity in Nthe predominant gas phaseconstituent

Use water as the solvent


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MITTELHAUSER

C

(~ "

, corporatioo______________________aAcDdEfFGG'HkKK1LmMNPPQRSrTtuVWwxyz15p]JIjl

AI*i0

NOMENCLATUREactivityarea, cm 2; also interaction parameter in property equationsmass or molar concentration, gm or gm-mole/cm 3diffusivity, cm 2/secdiameter, cmefficiencyfugacity, atmratio of relative velocity of droglet or particle to gas velocitygas mass or molar flux, gm/sec/cm2 or gm-moles/sec/cmZgas mass or molar flow rateHenry's Law Constantlocal mass transfer coefficientsoverall mass transfer coefficientsequilibrium ratio between liquid and vapor mole fractionslength, cmliquid mass or molar flux, gm/sec/cm2 or gm-mols/sec-cm2molality, gm-moles solute/1000 gm solventmolecular weight gm/gm-molemolar flux, gm-mol/sec-cm2partial pressure, atmtotal system pressure, atmvolumetric flow rate, actual cm3/secgas constant, in cm3 - atm = 82.06gm-mole-OKinteraction parameter in viscosity equationradius, cmtemperature, oKtime, secvelocity, cm/secvolumeweight flow, gm/secweight fractionliquid mole fractionvapor mole fractiondistance, cmsurface tension, dynes/cmmass or molar density, gms/cm3 or gm-mol/cm3viscosity, gm/cm-sec. (except where noted)interaction parameter in property equations

Superscriptsmolar quantitiesvalue at phase interfacevalue at equilibrium with respect to bulk property of other phase(in a two-phase system)averageinletoutlet

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MITTELHAUSER. corporation_____________________ -TGLiMaRSdj

totalgas-phase1 quid-phase

Subscripts

pertaining to component "i"Murphree tray efficiencyoverall tray efficiencyrelativeSauter meandropletpertaining to component "j "

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(

IlAltTELHAUSER:,' corporation_____________________

(1)

(2 )

(3 )

(4)

(5 )

REFERENCES FOR SECTION 3Calvert et al, Wet Scrubber System Study: Volume I, ScrubberHandbook, Report No. EPA-R2-72-118a (August 1972).Kohl, A. L. and F. C. Riesenfeld, Gas Purification, 3rd edition,Gulf Publishing Co., Houston (1979).Ratcliff, G. A. and J. G. Holdcroft, Trans. Inst. Chern. Eng.Lond., i,l, 315 (1963).Reid, R. C.; Prausmitz, J. M. and Sherwood, T.K.; The Propertiesof Gases and Liquids, 3rd edition, MCGraw-Hill, New York (1977).Metcalf &Eddy, Inc., Wasterwater Engineering, 2nd edition,McGraw-Hill, New York (1979).

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MITTELHAUSERcorporatioo-=--=--=--=--=-=---=-=-=-________________4.0 VAPOR-LIQUID EQUILIBRIUM (VLE) IN N H 3 - H ~ 2 . : ! : I 2 0 SYSTH14.1 VLE DATA

Experimental measurements of VLEdata are reported in the followingreferences.System ReferenceNH3-COZ-HZS-HZO D. L. Cardon'and G. M. Wilson, "Ammonia-Carbon Dioxide-SOoC t lZOoC Hydrogen Sulfide-Water Vapor-Liquid Study," Publication________ 0____________ of the API, Washington, D.C.NH3-COZ-HZS-HZO E.H.M. Badger and L. Silver, J. Soc. Chem. Ind.,S7,_____c-._l.l 0- 1Z (1938).NH3-HZOOOC to 600CNH3-HZO

970C to 1470CNH3-COZ-HZS-H20200 C

NH3-C02-H20200C to 400C

COZ- H20oOC to 600CH2S- H20800C to lSOoeNH3-HZS -H20800e to l200e

NH3-C02-H20200C to 1000C

NHr H20ODC to 60DC

Breitenbach, Bull. Univ. Wis. Eng. Exp. Sta. Ser. 68(as given in Perry's Chemical Engineers' Handbook,Fourth ed., 1963).1. L. Clifford and E. Hunter, J. Phys. Chern., E, 101(1933) .1. G. C. Dryden, i!. Soc. Chern. Ind., 66, 59 (1947).

S. Ikendo, Kogyo Kagaku Zasshi, 64, 627 (1961).

Lange's Handbook of Chemistry, Eighth'ed., 1952 (originalsource not given).D. H. Miles and G. M. Wilson, "Vapor-Liquid EquilibriumData for Design of Sour Water Strippers," Annual Reportto API for 1974, October 1975.

E. Otsaka, S. Yoshimura, M. Yokabe, S. Inque, KogyoKagaku Zasshi, 63, 1214-1218 (1960).E. P. Perman, 1. Chern. Soc. London, 83,1168(1903).

ZO


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..MITTELHAUSER _______--==_--_---_--_____----System -- -----

NH3-H20OOC to 600 C

NH3-C02-H20200 C to 400 C

- - - - '--'. _ ._- - --(Buffered)80DC to 185C

NH., -CO., -H.,O_._.,__ . ..:.t.. .: L ____________700C to 1200 C

NH3-C02-H20Phase diagrams60 0 C to 1700 CNH3-H2S-H20

200 C to 600 CNH3-C02-H20NH3-H2S-H20NH3-C02-H2S-H20

200 C to 60 0 CNH3-H20

1140 C to 3170 CC02-H20

2700 C to 5500 CH2S- H20

1600 C to 3300 CNH3-H2S-H20

700 C to 900 C

ReferenceSherwood, ~ . ~ . Chern., lZ, 745 (1925). (As given inPerry's Chemi ca 1 Eng i neers I Handbook, Fourth Ed., 1963).S. Pexton and E. H. M. Badger, i!. Soc. Chem. Ind., 2,106 (1938).T. T. C. Shih, B. F. Hrutfiord, K. V. S a r k a n e n ~ - - - a n d L. N.Johansen, TAPPI, Technical Assoc. of the Pulp and PaperIndustry, 50, (No. 12) 630-634 (1967)-.- ---T. Takahashi, Kogyo Kagaku Zasshi, 65, 837-843 (1962).

T.Takahashi, S. Yoshimura, K. Fukii, and E. Otsaka,Kogyo Kagaku Zasshi, ~ , 743-745 (1962).

V. E. Terres, W. Attig, and F. Tscherter, Gas. u.Wasserfach, 98, 512-516 (1957). - -D.-W. Van Krevelen, P. J. Hoftijzer, and F. J. Huntjens,Recuei1 Des Travaux Chimiques Des Pays-Bas, 68, 191-216(1949). - --M. E. Jones, i!. Phys. Chem. 67, 1113-1115 (1963).S. D. Malinin, Geochem. Int., l l , 1060-75 (1975).

T. N. Kozintseva, Geochem. Int., No.4, 750-756 (1964).

V. 1. Oratovskii, A. M. Gamblskii, and N. N. Klimenko,Appl. Chern. USSR, lZ, 2363-2367 (1964).

21.. . - - . -----------_._-----------------.------


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System ReferenceHZS-H20 - - - - - - - - - -H. Gamsjager and P. Schindler, Helv. Chim. A c t a , - 5 2 ~ - - - - - -1395-1402 (1969).5 0CH2S -H20-Sa I t1500 C to 330 0 CH2S- H20H2S-CH4-H20_ 1 0 C to 1400 CNH3-C02-H20

----60oC to 1500 C

T. N. Kozintseva, Geokhemiya, 121-134 (1965).

J. L. Vogel, M. S. Thesis, The University of Tulsa, 1971.

G. J. Frohlich, Ph.D. Thesis, Polytechnic Institute ofBrooklyn University, Microfilms No. 60-3497, 1957

More references are given in I . Wichterle, J. Linek, and E. Hala,Vapor-Liquid Equilibrium Data Bibliography, Elsevier (1973).

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, l ~ l T T F L H A U S E R________ ~ r E S ' r a t i o n ~ _ - - ~ _ - : - -__-:--____::::_=_-:--_- - - - - - -- - -_ -_ - -_-_---_---_______ :

( --"4-.2-- - ~ - - V L : E - - ~ ' O D E l S -\..: --Until recently there was a serious lack of correlations that pre

dicted VLE in aqueous systems containing NH3, H2S and C02' Rigorous developments in the thermodynamics of electrolytic solutions has resulted in improvedapproaches. Great interest in coal gasification and oil shale where thesesystems must be considered has created incentive to develop improved models.

This section describes VLE models that are currently available.4.2.1 Van Kreve 1 n

This model is included for historical reasons.(l) Until the development of other models described here, the Van Krevelin correlation was thebasis for calculating VLE in sour water systems. Its use is no longerrecommended because of the following deficiencies:

1. Onlyd'ata to 600 C were correlated; thus the use of the corre-lation at sour-water stripper temperatures of 100 to l200 Crepresented an extrapolation of existing data.2. The calculation method outlined by Van Krevelen did not allow

for mixtures containing ammonia to hydrogen sulfide ratiosless than 1.5 in the liquid phase.3. The calculation did not take into account reduced volatilitiesof hydrogen sulfide and ammonia at low ppm concentrations duethe ionization constants of the two compounds in water.

4.2.2 API SWEQGrant Wilson and co-workers at Brigham Young University developed

a new sour water equilibrium model (SWEQ) during the mid to late 1970'sunder a contract sponsored by the API Technical Data Committee.(2) The SWEQcorrelation model is very similar to Van Krevelen's except that some of the1 mi tat ions have been removed. It is based on new data over wi ~ ~ r _ T a n g e s inconcentration and temperature as well as Van Krevelen's data. _ t considers

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;1 I T T E L H A U S E R-=-corporation -the chemical equilibrium between ionic species of H2S or C02 and undissociated H2S or C02 in the liquid. Van Krevelen assumed that H2S and C02 only existin aqueous solutions as ionized species which is virtually true where ammoniais in excess .

. Both models use Henry's constants to compute NH3, H2S and C02partial pressures. The SWEQ model improves the correlation of Henry's constants for temperature and composition.4.2.3 Institute of Gas Technology

In connection with the preparation of a Coal Conversion Systems ______ Technical Data Book, _workers at IGT sought to improve the correlation ofpartial pressures with concentrations of NH3, C02 and H2S in water.(3) Their

- approach was to analyze data by modification of Van Krevelen's method and toextend its range by use: of ionization constants, Henry's law correlationsand correlations of activity coefficients as needed. They have extensivelyinvestigated the effect of electrolytes on the solubility or activity of agas dissolved in an aqueous solution, commonly called "salting out."4.2.4 EMNP

Edwards, Maurer, Newman and Prausnitz at the University of Californiaat Berkeley developed a correlation that calculates equilibrium in the NH3-H2S-C02-H20 system as well as systems that include sulfur dioxide and hydrogen

c y ~ n i d e . ( 4 ) This correlation is based on the ionization constants of theelectrolytes, the solubilities of the individual gases in water and saltsolutions, and ionic activity coefficients based on Pitzer's. c o r r e l a t i o n . ~ 5 ) - - The correlation has been incorporated into a computer program called SURFINP.4.2.5 Beutier and Renon

The procedure developed by Beutier and Renon(6) has the same theoreticabasis as the EMNP correlation. However, i t is presently restricted to ternarysystems NH3-C02-H20, NH3-H2S-H20 and NH3-S02-H20. I t may be expected thati t is also useful for the complete multisolute system built up with thesecompounds.


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co! , : " I , ! _ , ' " " " t ' ' ' " ' ' ' ' ~ ( " ~ O l _ ~ ~ ~ f " ' ' t > .

: ; ~ r ~ " ~ j i i t L H ~ ~ ~ t Kcorporation __ .___,------_. _._-_.- --"-- - - _ . _ - , - - - , - - - -4.2.6 ECES

Zemaitis has developed a computer software system called EquilibriumComposition of Electrolyte Systems (ECES) that is applicable to all multicomponent aqueous electrolyte solutions.(7) Whereas previous VLE modelsdescribed here are applicable to. systems containing NH3, C02 and H2S, EeESwas designed to predict vapor-liquid-solid equilibria of multicomponentaqueous solutions of strong and/or weak electrolytes. The system has beendeveloped from a basic thermodynamic framework whereas the other models arebased on a Henry's law approach. The thermodynamic framework incorporatesadvances made in the area of electrolyte thermodynamics during the early1970's.

It is not claimed that EeES predicts better results than the othermodels described. The other models are directed at predicting equilibria inspecific systems and may be more accurate for those systems. The objectiveof EeES is to develop a general purpose system that ' is capable of predictingequilibria in both single and multistage systems over a wide range of conditions.

ECES is available througll software sold byOLI Systems, Inc. Themode 1 details have not been pub 1 shed.

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,;M ITTELHAUSER _____-------------""..

c4.3 PROCESS SIMULATION SOFTWARE

'Process simulation software is available that can predict theoreticalvapor-liquid equilibria and mass transfer in equipment such as venturi scrubbersand sieve tray towers. While a couple of the simulators have been availablefor some time, their capability to handle aqueous electrolytes is recent.They use the VLE models described in Section 4.2 to perform equilibrium andmass balances. The best of the process simulators are described below.4.3.1 ChemShare

ChemShare is a well known computer system used by chemical engineersto simulate process plant systems. Generally these process systems have. been hydrocarbon type processes. In 1980 ChemShare incorporated the APISWEQ model for predicting equilibrium in systems containing NH3, H2S, C02and H20.

ChemShare is a v a i ~ a b l e on a royalty use basis.4.3.2 SST Process

Simulation Sciences Inc. (SST) also provides a process simulationpackage. Their current simulator is named PROCESS. In 1980 SSI also in-corporated the API SWEQ model, giving i t much of the same capabilities asChemShare in the area of weak electrolyte solutions.

SSI's PROCESS is available on a royalty use basis.4.3.3 OLI Systems

OlI Systems Inc. offers a process simulation package that isspecifically directed at multicomponent aqueous solutions of electrolytes.The simulation program is based on the ECES model. Because i t is specificallydesigned to simulate systems of electrolyte solutions, i t has capabilHiesthat other simulators do not have.

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, 1 ~ I T T E L H A U S E R'.___ _ ____ coJPOration_______________"==_""-_--_--______(' OLI's programs are not available on a royalty use basis. The

programs can be purchased or leased. Alternatively, arrangements may bemade with an OLI licensee to have work performed.

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:-1M ITTELHAUSER< corporation --------- - . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ - - - - - - - - - - - ~ - = = - - - = = - - - = - = - - ~ - ----------= ----------

( 1 )

(2 )

(3 )

(4 )

(5 )

(6 )

(7)

REFERENCES FOR SECTION 4D. W. Van Krevelen, P. J. Hoftijzer and F. J. Huntjens, Recueil DesTravaux Chimiques Des Pays-Bas, 68, 191-216 (1949).American Petroleum Inst i tute, "A New Correlation of NH3, C02,and H2S Volatili ty Data From Aqueous Sour Water Systems," Publication955, API, \ ~ a s h i n g t o n , D.C. (March 1978).Mason, D. M. and R. Kao, "Correlation of Vapor-Liquid Equilibriumof Aqueous Condensates from Coal Processin9," Thermodynamics ofAqueous Systems with Industrial Applications, ACS SymposiumSeries 133, American Chemical Society, Washington, D.C. (1980).Edwards, T. J ., G. Maurer, J. Newman, and J. M. Prausnitz, "VaporLiquid Equilibria in Multicomponent Aqueous Solutions of VolatileWeak Electrolytes," A.I.Ch.E. Journal, 24, 966-76 (1978).Pitzer, K. S., "Thermodynamics of Electrolytes, I," J. Phys. Chern.,12, 268-277 (1973).Beutier, D. and H. Renon, "Representation of N H 3 ~ H 2 S - H 2 0 , NH3-C02-H20 and NH3-S02-H20 Vapor-Liquid Equilibria," Ind. Eng. Chern.Process Design Dev., ]2, 220-230_ (1978}.Zemaitis, J. F. Jr. , "Predicting Vapor-Liquid-Solid Equilibria inMulticomponent Aqueous Systems of Electrolytes, II Thermodynamics ofAqueous Systems with Industrial Applications, ACS Symposium Serico133, American Chemical Society, Washington, D.C. (1980).

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~ ~ ~ - h " ~ i ~ L H A U S t R_____ q r p . o r . o t l o n - - - - - - - - - - - - - - - _-_---_---_--_--_-_ _ _ _ _5.0 SUPPORTING WORK

Severaljtems of support work are valuable in terms of thegreater understanding that they wi 11 impart toward mass transfer of NH3,H2Sand C02 in venturi and sieve tray scrubbing systems. This supportingwork is partly engineering in nature and partly laboratory work.5.1 PRELIMINARY SIMULATION WORK

Using commercial process simulation software such as describedin Section 4.3, the performance of the venturi scrubber or sieve trayscrubber can be 'predi cted beforehand. It is recommended that f l ashcalculations or absorber calculations be done using one of the softwaresystems to determine the potential performance with respect to masstransfer._Ltquid_an_d_vapor rates, compositions and temperatures typicalof planned field scrubbing tests should be used in simulation work inorder to approximate test conditions as closely as possible.

This work would not be expensive. It would indicate the amountof NH3, H2S, and C02 removal from the gas stream that could be expected.

~ ~ - " " " - ~ - - - - - - - ~ - -- -__I t would also indicate typical compositions that would have to be analyzedduring field work. Field test results could be compared against thissimulation work to make preliminary determinations of actual mass transferversus predicted. The simulation work should be rerun, however, afterthe field tests at the exact field test conditions to accurately compareactual H2S removal with predicted.5.2 DETERNINATION OF OIL PHASE IMPORTANCE

Scrubbing of retort off-gas produces a liquid hydrocarbonphase through condensation of hydrocarbon vapors and collection of liquidhydrocarqon mist. This hydrocarbon phase presents a complication in thedevelopment of mass transfer models. Theoretically this phase is athird phase that must be in equilibrium with the other two phases--gasC and aqueous liquid. Any mass transfer model is going to be complicatedif it has to consider the third phase.

- "--- "-"-


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'". "MITTELHAUSERcorporotion_________________ --___-( The effect of organic bases in the retort off-gas is to decreaseammonia absorption and increase ammonia stripping. Hydrogen sulfide

absorption is increased and H2S stripping in the sour water stripper isreduced. The hydrogen sulfide and organic base concentration in thestripped process condensate increases.

The combined effect of both organic acids and bases on absorptionand stripping is more difficult to predict. .However, impurity levels inprocess condensates are increased.

There are effects that impact the development of mass transfermodels. The presence of compounds present in the scrubbi.ng solutionsresulting from absorption of acids and bases produces a salting outeffect which impacts vapor liquid equilibrium. Also, most of the processsimulation programs described in Section 4.3 do not have the capabilityto predict VlE in the presence of these types of compounds. Only theOlI Systems program has this capability.

Potential organic acids and bases that might be present inretort off-gas are listed below:Compound Molecular Weight Boiling Point,OC pKPhenol 94 196 9.9Acetic Acid 60 l1S 4.SBenzoic Acid 122 249 4.2m-Cresol lOS 203 10.0Hexanoic Acid 116 205 4.9p-Toluic Acid 136 275Propyl Amine 59 49 3.3Benzyl Amine 107 lS5 4.6Aniline 93 lS4 9.4Acetamide 59 222 12.6Toluidine 107 200 S.9Diethyl Amine 73 56 2.9

A determination of organic acids and bases in retort off-gasis required in order to postulate their impact on mass transfer. It isalso relevant to downstream processing. These compounds tend to fix the

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analysis of mass transfer modeling vapor liquid equilibrium in scrubbing retort of gas fb4adfbc0a

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