Competitive Absorption−Desorption of Acid Gas into Water−DEA Solutions

SEPARATIONS

Competitive Absorption-Desorption of Acid Gas into Water-DEA Solutions

Renaud Cadours*

IFP-Lyon, BP3-69390 Vernaison, France

Damien Roquet

TOTAL, Tour Coupole-La Defense 6, 92078 Paris La De´fense, France

Gauthier Perdu

PROSERNAT, Tour AREVA-La Defense 6, 92084 Paris La De´fense, France

Measurements of the CO2 absorption rate into a diethanolamine (DEA) aqueous solution were made using aLewis cell to determine the CO2-DEA kinetic parameters. The absorption rate was controlled by a gas-sidemass-transfer phenomenon. This was achieved by monitoring the total pressure inside the Lewis cell. Thegas-side mass-transfer coefficient was deduced from H2S absorption measurements in an aqueous solution ofDEA. The kinetic rate constant of the reaction of formation of the zwitterion determined in this work is ingood agreement with existing literature values. Measurements of the H2S absorption rate into a DEA aqueoussolution previously loaded with CO2 showed a competition between H2S absorption and CO2 desorption. Amass-transfer rate model that takes the effects of chemical reactions into consideration enabled these oppositemass transfers to be modeled.

Introduction

Natural gas treatment involves several steps, resulting inincreased costs and operating complexity. This treatment usuallyinvolves water and hydrocarbon dew-pointing and gas deacidi-fication. Condensables are removed from natural gas, to preventthe formation of liquid phases and hydrates during gas transport.Acid gas removal is necessary to meet current pipelinespecifications for hydrogen sulfide (H2S) and carbon dioxide(CO2) contents. Many processes are available to remove acidgases from sour gas mixtures. The most common solventprocesses used for this operation are based on aqueous solutionsof alkanolamines. Primary and secondary alkanolamines suchas diethanolamine (DEA) are used for total deacidification.Tertiary amines, such as methyldiethanolamine (MDEA), canbe used for selective H2S removal and to maximize CO2slippage, to produce rich H2S gas for Claus treatment.

Several parallel reversible reactions occur during the absorp-tion of CO2 and H2S into an aqueous alkanolamine solution.The case of a reversible reaction is very complex, because ofthe fact that reaction rate expressions are not linear. VanKrevelen and Hoftijzer1 originally proposed approximate ana-lytical solutions. Nevertheless, numerical solutions showed thatthese approximations generally were not valid.2 In the case ofseveral reversible reactions, Onda et al.3-5 suggested someapproximate solutions for specific cases. However, most of therecent work that was dedicated to the absorption of acid gasesin aqueous alkanolamine solutions used a numerical techniqueto solve the equations; this numerical technique describes the

phenomenon of multicomponent mass transfer, coupled withparallel complex reversible chemical reactions. Cornelisse etal.6 studied the simultaneous absorption of CO2 and H2S in anaqueous solution of secondary alkanolamine, by means of thepenetration theory. The resulting model was restricted to a fewstoichiometric schemes. Bosch et al.7 described the simultaneousabsorption of H2S and CO2 in aqueous solutions of alkanola-mines, assuming reversibility, a generalized reaction rate, anda generalized stoichiometry for all reactions. The model waslimited only by the type of reactions. Littel et al.8 used the sameapproach to describe the simultaneous absorption of H2S andCO2 in aqueous solutions of primary, secondary, or tertiaryamines. They generalized the models of Bosch et al.8 byconsidering finite-rate reactions and instantaneous reactions, withrespect to mass transfer. They modeled the latter as finite-ratereactions with very high reaction rate constants, as previouslydescribed by Glasscock and Rochelle.9

In this work, CO2 absorption rates into DEA aqueoussolutions were measured in a thermoregulated reactor with aconstant gas/liquid interfacial area. Mass-transfer resistance wascontrolled by monitoring the total pressure in the cell withnitrogen. Mass-transfer coefficients, for both gas and liquidphases, were deduced from specific chemical systems. Theliquid-side mass-transfer resistance was obtained from N2Ophysical absorption into water-MDEA solvents. H2S absorptiondata into DEA aqueous solutions were used to determine thegas-side mass-transfer resistance. The absorption of CO2 or H2Sinto acid-gas-loaded solutions led to opposite mass transfers ofH2S and CO2. A numerical model based on the film theory wasused to explain the opposite mass transfers of H2S and CO2

observed in the Lewis cell.* To whom correspondence should be addressed.

233Ind. Eng. Chem. Res.2007,46, 233-241

10.1021/ie060019w CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 11/30/2006

Experimental Section

The reaction between CO2 and DEA has been studied severaltimes.10-15 The zwitterion mechanism that has been proposedby Caplow10 is the most widely accepted mechanism forprimary and secondary alkanolamines. It involves the formationof an intermediate zwitterion that is deprotonated by the basiccomponent to produce a carbamate and a protonated base.

The contribution of each base is dependent on its concentrationand its basicity. Depending on the mechanism-limiting step,which could be zwitterion formation or deprotonation, thereaction order in amine concentrations is observed to be between1 and 2. Studies based on short gas-liquid contact time (forexample, wetted-wall column12 or wetted-wall sphere13 orlaminar jet apparatus15) led to a reaction order of 1 in amineconcentrations. Reaction orders between 1 and 2 in DEAconcentrations are usually obtained with studies using stirred-cell reactors. The influence of the zwitterion-deprotonationstep can be significant if long gas-liquid contact timemeasurements are made, especially at low alkanolamine con-centrations. For example, Blauwhoff et al.16 and Littel et al.14

used the same methodology: a batch reactor, with respect toboth gas and liquid phases. The absorption rate was deducedfrom the pressure decrease resulting from the CO2 injection.The overall reaction rate constant was defined when pseudo-first-order conditions were met. This procedure means thatthe CO2 absorption rate into a partially loaded solution ismeasured because of the absorption of a significant amount ofthe injected acid gas, according to the example given byBlauwhoff et al.16 These authors observed a significant influenceof hydroxide ions on the overall reaction rate. Another difficultyin measuring the CO2 absorption rate into DEA aqueoussolutions is the possible influence of a gas-side mass-transferresistance. This resistance may appear, especially at low CO2

partial pressure.In this work, we propose an experimental approach using a

Lewis cell to measure the CO2 absorption rate into an aqueousDEA solution, for temperatures ranging from 313 K to 333 K,and DEA concentrations ranging from 10 wt % to 30 wt %.Monitoring of the total pressure in the cell ensured the stabilityof gas-side mass-transfer resistance, which was characterized

by H2S absorption rate measurements into aqueous DEAsolutions. The injection of very small amounts of CO2 in thecell made it possible to consider a pseudo-first-order approxima-tion for determination of the apparent rate constant. Thisinjection prevented acid gas from accumulating in the solventand solvent composition from being modified, especially forhydroxide ion concentration. The overall apparent constant wascompared to literature data.

Experimental Setup. The experimental apparatus was de-veloped to measure acid-gas absorption rates into water-alkanolamine solutions. The main equipment is the reactor (seeFigure 1), which is composed of a (6.00( 0.02) × 10-2 minternal diameter quartz cylinder that has been closed at bothends by two stainless steel (SS304L) metallic flanges. The totalvolume available for gas and liquids is (346.5( 0.1) × 10-6

m3. The reactor is provided with a six-bladed Rushton turbine,(3.10( 0.02)× 10-2 m in diameter, in the liquid phase and afour-bladed impeller, (2.00( 0.02)× 10-2 m in diameter, inthe gas phase. They are both magnetically driven by adjustable-speed motors. This technique prevents leaking, friction, and heatgeneration, which appear when using stems that pass throughthe top and bottom of the cell. Stirring speeds are checkedperiodically: they remain constant within 1 rpm during the tests.Four vertical baffles that are in place prevent vortices in theliquid. A horizontal plate and a ring, placed halfway up thecell, set both the level and the area of the interface availablefor gas/liquid transfer and ensure its stability during stirring.The gas-liquid interface area, (13.0( 0.1) × 10-4 m2, wasgeometrically estimated.

The temperature in the reactor is known within(0.2 Kthrough a 100Ω platinum probe, calibrated between 273 K and403 K, against a Herau¨s TLH600 100Ω platinum probe thatwas calibrated by the Laboratoire National d’Essais. Theabsorption cell temperature is regulated by immersion of thecell in a thermoregulated bath. The absorption rate is measuredby recording the absolute pressure drop using a Kulite pressuretransducer that was operating in the pressure range of 0-10 ×105 Pa. This pressure transducer is calibrated in the operatingrange within 400 Pa against a Dru¨ck DPI605 reference pressuretransducer calibrated by the Laboratoire National d’Essais. TheKulite pressure transducer is kept at a temperature slightly higherthan the experimental temperature to avoid liquid condensationin its measuring chamber. A computer that was fitted with anacquisition card is used to record temperatures and pressuresas a function of time.

Figure 1. Experimental equipment for kinetic measurements.

CO2 + R2NH T R2NH+COO-

R2NH+COO- + baseT (base)H+ + R2NCOO-

234 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

The reactor is connected to a solvent volumetric pump andan acid gas reservoir (see Figure 1), in which the pressure andtemperature are recorded. The volume of solvent introduced intothe reactor is known within 0.1× 10-6 m3. The amount of acidgas transferred into the reactor is calculated from the pressuredrop inside the acid gas reservoir of a known volume ((302.7( 0.1) × 10-6 m3).

Mode of Operation.The solvent was prepared under vacuumfrom water and alkanolamine that had been degassed indepen-dently. The solvent composition was known by means ofdifferential weighing to within 0.01 g of each component.Degassed solvent was introduced into the reactor using thevolumetric pump. Stirring and heating were started. After theexperimental temperature was attained, the liquid-phase volumein the reactor was calculated from the volume introduced, usingthe density correlation of Amararene et al.,17 and taking intoaccount the temperature difference between the temperatureinside the volumetric pump and the reactor.

The absorbed gas partial pressure was obtained from themeasured pressures:

wherePI is the inert gas pressure in the reactor.PI was measuredbefore the acid gas injection and took solvent vapor pressureand nitrogen partial pressure into consideration.

A controlled amount of acid gas from the thermostated high-pressure gas reservoir was then fed into the vapor phase of thereactor through thermostated tubing. The pressure decreaseversus time, as a result of acid gas absorption through thehorizontal gas/liquid interface, was recorded. This experimentaloperation mode was followed for the different absorbing systemsinvolved in this study. An example of the rough experimentalresults obtained for a typical absorption experiment is displayedin Figure 2.

Liquid-Side Mass-Transfer Characterization

The liquid-phase mass-transfer coefficient was determinedfrom the physical absorption of pure N2O into alkanolamineaqueous solutions. Measurements were conducted in the

303-333 K temperature range, for MDEA compositions inthe 20-40 wt % range, and for various liquid-phase stirringspeeds.

The N2O physical absorption rate was calculated from therelation

We assumed that the gas-phase resistance is negligible duringpure N2O absorption. The interfacial concentration of N2O wasobtained from Henry’s law:

The N2O concentration in the liquid bulk was determined fromthe accumulated N2O absorbed into the alkanolamine aqueoussolution:

The N2O absorption rate was calculated by the mass balance inthe Lewis cell gas phase:

A pressure-time relation was obtained from eqs 2-5:18

The liquid-side mass-transfer coefficient was determined fromthe slope of the curve corresponding to the logarithmictransformation of the pressure-time absorption profile. The N2O

Figure 2. N2O absorption into a 40 wt % MDEA aqueous solution.

Pabsorbed gas) P - PI (1)

æN2O) kL(CN2O,int - CN2O,liquid bulk) (2)

CN2O,int )PN2O

HN2O(3)

CN2O,liquid bulk ) -VG

RTVL(PN2O

- PN2O,ini) (4)

æN2O) -

VG

ART

dPN2O

dt(5)

ln[PN2O+

VG HN2O

RTVL(PN2O

- PN2O,ini)

PN2O,ini] )

-AkL( RTVGHN2O

+ 1VL)t (6)

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007235

Henry’s constant was calculated from a correlation, representing,to within 3%, the more-consistent data available in the literature.The model was similar to that of Wang et al.;19 the newcorrelation’s parameters were fitted on the data selected by Liand Mather.20 We used a well-known dimensionless equationto correlate the data:21 where

The physicochemical parameters used in these equations wereobtained from the Amararene et al. correlations17 for solventdensity; from the Hsu and Li correlations22 for solvent viscosity;and from the Versteeg and van Swaaij correlations23 for N2Odiffusivity.

Gas-Side Mass-Transfer Characterization

The gas-side mass-transfer coefficient was determined fromthe absorption rates of H2S into DEA aqueous solutions, inwhich an instantaneous reaction occurred between the absorbedacid gas and the alkanolamine. The total pressure inside theabsorption cell was monitored by nitrogen injection. Critchfield24

observed that the gas-side mass-transfer coefficient was driving-force-dependent, assuming no contribution from the liquid-phaseresistance. This means that the liquid-phase resistance cannotbe neglected. Taking these observations into consideration, themeasurements were made with low driving forces and DEAconcentrations of>1 kmol/m3, to minimize the effect of theliquid-phase gradients. For each experiment, the followinginequality was checked and fulfilled:

where

where CDEA,bulk is expressed in terms of mol/m3, HH2S isexpressed in terms of Pa m3/mol, and PH2S is given in Pascal.The gas-side mass-transfer coefficient (kG) was obtained directlyfrom the partial pressure of H2S in the reactor and thecorresponding measured absorption rate:

The gas-side mass transfer coefficient was measured for differentoperating conditions. The values reported in Table 1 are themean values of five measurements;σ is the standard deviationobserved in the five experimental values.

Versteeg et al.21 reported the dependence of the gas-side mass-transfer coefficient on gas diffusivity. The H2S and CO2

diffusion coefficients reported in Table 2 were estimated usingthe predictive methods of Wilke and Lee or Fuller andGiddings,25 and they are shown to differ by<4% for totalpressures of 2× 105 and 5× 105 Pa. ThekG values measuredwith N2-H2S/water-DEA systems were to be used to determinethe overall kinetic rate constant of the reaction between CO2

and DEA that occurs during the absorption of N2-diluted CO2

into water-DEA solutions.

CO2 Absorption into Aqueous DEA Solutions

The influence on absorption of all chemical reactions, whichis occurring between dissolved CO2 and reactants in the solution,is usually expressed by an enhancement factor,E. The chemicalreaction contributions and both gas- and liquid-side mass-transfer resistances to the overall mass transfer were estimatedfrom

Small quantities of CO2 were injected. The total CO2 concentra-tion in the liquid phase, including molecular and ionic formsof the molecule, did not change significantly during theabsorption rate measurement and was assumed to be negligible.Because the maximum CO2 partial pressure in the reactor waskept at <104 Pa, and because we were considering initialabsorption rates into unloaded solvents, we assumed that thelimiting step in the mechanism was the zwitterion formationby reaction of CO2 with DEA. Considering a reaction order of1 in CO2 and in DEA, the reaction rate was expressed by

where k is the kinetic constant of the zwitterion formationreaction.

Table 1. Gas-Side Mass-Transfer Coefficient Measured from H2S Absorption into DEA Aqueous Solvents

temperature(K)

DEA(wt %)

total pressure(× 105 Pa)

PH2S

(× 105 Pa)kG

(× 10-5 mol m-2 s-1 Pa-1)

323.15 30 4.510 0.036 ((0.004) 2.81× 10-2 (σ ) 0.24× 10-2)333.15 20 4.656 0.033 ((0.004) 2.71× 10-2 (σ ) 0.07× 10-2)313.15 10 4.687 0.038 ((0.004) 2.82× 10-2 (σ ) 0.05× 10-2)313.15 10 2.417 0.028 ((0.004) 3.49× 10-2 (σ ) 0.10× 10-2)

Table 2. CO2 and H2S Diffusion Coefficients at Infinite Dilution in Nitrogen

Diffusion Coefficient at 313 K (× 10-4 m2/s) Diffusion Coefficient at 333 K (× 10-4 m2/s)

gas Wilke and Lee Fuller and Giddings Wilke and Lee Fuller and Giddings

Ptotal ) 2 × 105 PaCO2 0.092 0.090 0.103 0.100H2S 0.095 0.094 0.106 0.104

Ptotal ) 5 × 105 PaCO2 0.037 0.036 0.041 0.040H2S 0.038 0.037 0.042 0.042

Sh) 0.230Re0.661Sc0.430 (7)

Sh)kLdcell

Dgas(8)

Re)FNdRushton

2

µ(9)

Sc) µFDgas

(10)

kG <kLEi,H2S

HH2S(11)

Ei,H2S) 1 +

DDEA

DH2S(CDEA,bulk)(HH2S

PH2S) (12)

æH2S) kGPH2S

(13)

æCO2) 1

(1/kG) + [HCO2/(kLE)]

(PCO2- HCO2

CCO2,bulk) (14)

r ) kCCO2CDEA (15)


The CO2 quantities introduced were calculated to satisfy thecondition for a pseudo-first-order reaction, as given by Danck-werts and Sharma:26

where Ha is Hatta’s number andEi is the limiting case ofabsorption, with a single irreversible reaction, for the film theory.

whereCDEA,bulk has units of mol/m3, HCO2 has units of Pa m3

mol-1, and PCO2 has units of Pa. As the pseudo-first-orderconditions (eq 16) were satisfied, the enhancement factorE wasequal to Hatta’s number,Ha.

Enhancement factors were obtained from absorption rates.These calculations required the physicochemical parameters ofthe system, especially Henry’s constant and the diffusivity ofCO2, which were calculated using Blanc and Demarais correla-tions.12 Each value reported in Table 3 is the average valuecalculated from five experiments under similar operatingconditions. Standard deviations forE are also reported.

Kinetic constants (k) were then calculated according to eq17 (see Table 3). The uncertainties were estimated usinguncertainty propagation, taking into account the standarddeviation on both the gas- and liquid-side mass-transfer coef-ficients, as well as on CO2 absorption rates. The measuredreaction rates can be directly compared to those of Blanc andDemarais,13 because we used the same assumptions and the samephysicochemical parameters, especially the diffusivity and theHenry’s constant of CO2 in a DEA aqueous solution. Rinker etal.15 measured the overall reaction rate with a short gas-liquidcontact-time apparatus. The overall constant obtained by Rinkeret al.15 can be associated with minimal uncertainty to the reactionrate of the zwitterion formation.

The reaction rates of the zwitterion formation from CO2 andDEA measured in this work with a Lewis cell are consistentwith the results of Blanc and Demarais12 and Rinker et al.,15

who used a wetted-wall or laminar jet apparatus, within theexperimental uncertainties resulting from the use of differentequipment (Figure 3).

Table 3 also reports kinetic rates measured during CO2

absorption into H2S-loaded DEA aqueous solutions. Very lowloadings were obtained through small injections of H2S. Takinginto account the experimental uncertainties, we did not observeany influence of very low H2S loading on the CO2 measuredabsorption rate. In fact, quite the reverse was observed: the

H2S absorption rate into DEA aqueous solutions was signifi-cantly influenced by the initial low loading of CO2 in thesolution. Table 4 reports the significant difference between gas-side mass-transfer resistance coefficients directly deduced fromH2S absorption into an unloaded solution or into a low CO2-loaded solvent.

The dissolved gas desorption is linked to the evolution ofabsorption of the opposite gas. The absorption of a highlyreactive gas such as H2S enhances the desorption of CO2, whichreacts with DEA, with a relative low reaction rate. Thephenomenon was experimentally visible by pressure recording,as the quantity of CO2 desorbed by the H2S absorption wassufficient to change the pressure in the reactor. The use ofanalytical techniques would be useful in providing betterunderstanding of this phenomenon. Real-time measurements ofthe quantity of CO2 and H2S present in the gas phase wouldenable experimental observation of the opposite mass transferof the acid gases in the different cases: H2S absorption intoCO2-loaded DEA aqueous solutions, and CO2 absorption intoH2S-loaded DEA aqueous solutions. Gas chromatography (GC)analysis of the gas-phase samples or in situ analysis usingspectroscopy techniques of the gas phase in the vicinity of theinterface seem to be the most-convenient solutions. Becausewe did not use these techniques, the modeling of competitiveabsorption-desorption enabled us to confirm the experimentalobservations.

Opposite Mass-Transfer Representation

The modeling of acid-gas absorption-desorption involvesparallel reversible reactions that correspond to the reaction of

Table 3. CO2 Absorption into Water -DEA Solutions

solutiontemp,T (K)

DEA(wt %)

total pressure(× 105 Pa)

kG(× 10-5 mol m-2 s-1 Pa-1)

PCO2

(× 105 Pa) E Ei

k(m3 mol-1 s-1)

unloaded 323.15 30 4.482 2.81× 10-2 (σ ) 0.24× 10-2) 0.031 ((0.004) 309 (σ ) 43) 1747 2.7 ((50%)unloaded 333.15 20 4.667 2.71× 10-2 (σ ) 0.07× 10-2) 0.035 ((0.004) 228 (σ ) 51) 1434 3.6 ((50%)unloaded 313.15 10 4.675 2.82× 10-2 (σ ) 0.05× 10-2) 0.040 ((0.004) 123 (σ ) 5) 417 2.0 ((50%)unloaded 313.15 10 2.451 3.49× 10-2 (σ ) 0.10× 10-2) 0.027 ((0.004) 117 (σ ) 8) 663 1.8 ((25%)RH2S ) 0.004 molH2S/molDEA 323.15 30 4.488 2.81× 10-2 (σ ) 0.24× 10-2) 0.033 ((0.004) 327 (σ ) 58) 1859 3.0 ((50%)RH2S ) 0.007 molH2S/molDEA 333.15 20 4.622-4.650 2.71× 10-2 (σ ) 0.07× 10-2) 0.032 ((0.004) 236 (σ ) 59) 1858 3.8 ((50%)RH2S ) 0.015 molH2S/molDEA 313.15 10 4.666-4.683 2.82× 10-2 (σ ) 0.05× 10-2) 0.037 ((0.004) 123 (σ ) 14) 498 2.0 ((50%)RH2S ) 0.015 molH2S/molDEA 313.15 10 2.417 3.49× 10-2 (σ ) 0.10× 10-2) 0.034 ((0.004) 118 (σ ) 8) 623 1.8 ((25%)RH2S ) 0.015 molH2S/molDEA 313.15 10 2.338 3.49× 10-2 (σ ) 0.10× 10-2) 0.030 ((0.004) 122 (σ ) 5) 599 1.9 ((25%)

3 < Ha <Ei,CO2

2(16)

Ha ) ( 1kL

)xkDCO2CDEA (17)

Ei,CO2) 1 +

DDEA

DCO2

(CDEA,bulk)(HCO2

)

(PCO2)

(18)

Figure 3. Kinetic constant comparison for the zwitterion formation reactionin the CO2 + DEA system.


H2S and CO2 with DEA. In this way, a numerical model wasdeveloped. The mass-transfer phenomenon, coupled with chemi-cal reactions, was modeled according to the film theory. Foreach component, a partial differential equation describes thediffusion-reaction processes in each point of the liquid film:

whereφ is the electrical potential gradient and is expressed asa function of ion concentration and ion diffusivity:

Ri is the production term:

For one of the ionic components, the partial differential equationis substituted by the static electroneutrality condition, to maintainelectroneutrality throughout the mass-transfer zone:

The set of coupled differential and algebraic equations wassolved with the appropriate initial and boundary conditions. Theinitial concentration profiles of the transferred species are linear,taking into account the boundary conditions at the gas/liquidinterface and the bulk concentrations. For nonvolatile species,the initial concentrations in the mass-transfer zone are equal tothe bulk concentrations.

Boundary conditions at the bulk side assume chemical equilib-rium for all species.

At the gas/liquid interface, fluxes of nonvolatile species are equalto zero. For the volatile species, we assume continuity of themass-transfer rate in the gas and liquid near the interface:

Equation 19 is put in dimensionless form, using the reducedvariables described by Cadours and Bouallou:27

where D1 is the diffusion coefficient of the first volatilecomponent.

Equations 19 and 22 become

The boundary conditions become the following:At the gas/liquid interface,x ) 0:

At the bulk side,x ) 1:

The set of partial differential equations was solved by thefinite difference iterative method. The numerical treatment wasidentical to that used by Cadours and Bouallou.27

This model was used to represent the opposite mass transferwhen H2S is absorbed into a CO2-loaded DEA aqueous solution

Table 4. Gas-Side Mass-Transfer Coefficient Measured from H2S Absorption into DEA Aqueous Solvents

temp,T (K)

DEA(wt %)

total pressure(× 105 Pa) solution

PH2S

(× 105 Pa)kG

(× 10-5 mol m-2 s-1 Pa-1)

313.15 10 2.417 unloaded 0.028 ((0.004) 3.49× 10-2 (σ ) 0.10× 10-2)313.15 10 2.444 RCO2 ) 0.015 molCO2/molDEA 0.028 ((0.004) 2.89× 10-2 (σ ) 0.15× 10-2)313.15 10 2.324 RCO2 ) 0.015 molCO2/molDEA 0.030 ((0.004) 3.02× 10-2 (σ ) 0.17× 10-2)

∂Ci(x,t)

∂t) Di

∂Ci2(x,t)

∂x2-

ziDi( FRT)[∂(φ(x,t)Ci(x,t))

∂x ] + Ri(x,t) (19)

φ(x,t) ) (RT

F )∑q)1

NC

zqDq

∂Cq(x,t)

∂x

∑q)1

NC

zq2DqCq(x,t)

(20)

Ri(x,t) ) ∑j)1

NR

λi,jki∏q)1

NC

Cqâq,j (21)

∑i)1

NC

ziCi(x,t) ) 0 (22)

For volatile species: Ci(x,0) ) (1 - xδ)Pi

Hi+ Ci,bulk(xδ)

(for 0 ex eδ) (23a)

For nonvolatile species:Ci(x,0) ) Ci,bulk

(for 0 ex eδ) (23b)

Ci(x,δ) ) Ci,bulk (24)

For volatile species: kGi(Pi - HiCi(0,t)) ) -Di

∂Ci

∂x|x)0

∀ t > 0 (25a)

For nonvolatile species:∂Ci

∂x|x)0 ) 0 ∀ t > 0 (25b)

x ) xδ

, t )tD1

δ2, Di )

Di

D1, φ ) φFδ

RT(26)

For volatile species: Ci ) (Hi

Pi)Ci, Ri ) Ri(δ2Hi

DiPi)

(27a)

For nonvolatile species:Ci )Ci

Ci,zm,

Ri ) Ri( δ2

DiCi,bulk) (27b)

∂Ci(x,t)

∂ t) Di

∂2Ci(x,t)

∂x2-

ziDiφ(∂Ci(x,t)

∂x ) - ziDiCi(x,t)∂φ

∂x+ Ri (28)

∑i)1

NC

ziCi(x,t) ) 0 (29)

For volatile species:

Ci(0,t) ) ( Di

kGiHiδ)∂Ci(0,t)

∂x|x)0 + 1 ∀ t > 0 (30a)

For nonvolatile species:∂Ci(0,t)

∂x|x)0 ) 0 ∀ t > 0 (30b)

For volatile species: Ci(1,t) )Ci,bulkHi

Pi∀ t > 0 (31a)

For nonvolatile species:Ci(1,t) ) 1 ∀ t > 0 (31b)


or when CO2 is absorbed into a H2S-loaded DEA aqueoussolution. A simplified mechanism reduced to main reactionswas selected:

We first considered the overall finite rate reaction between CO2

and DEA, where only the alkanolamine was involved in thedeprotonation step of the zwitterion mechanism that wasproposed by Caplow.10 Because absorption rate measurementshave been achieved with very low acid-gas loading, and underspecific operating conditions to set the zwitterion formation asthe kinetic rate-limiting step, the other reactions proposed byRinker et al.15 were not taken into account. We considered areversible reaction, with reaction order of 1 in CO2 and DEAconcentrations. The reaction order value of the reverse reactionwas assumed to be 2, with reaction order 1, with respect toeach ionic compound. The experimentally measured reactionrate was directly introduced into the numerical model for the

forward reaction between CO2 and DEA. The backward rateconstant was then deduced from the reaction equilibriumconstant. Reaction between H2S and DEA was assumed to beinstantaneous, with respect to mass transfer, because it involvesonly a proton transfer. In the numerical treatment, this instan-taneous equilibrium was modeled as a finite-rate reaction witha very high reaction rate constant, as proposed by Glasscockand Rochelle.9 A reaction order of 1, with respect to eachcomponent, was assumed. The reaction rate between H2S andDEA was assumed to be 100 times greater than the reactionrate between CO2 and DEA.

As an initial approximation, the equilibrium constants werecalculated from the Kent and Eisenberg correlations.28 TheHenry’s law constant of CO2 in DEA aqueous solutions wasobtained from Blanc and Demarais.12 The Henry’s law constantof H2S in aqueous DEA solutions was directly obtained fromH2S solubility in pure water. The CO2 diffusion coefficient inthe liquid phase at 298 K was determined from the Blanc andDemarais correlations.12 The H2S diffusion coefficient in thesolvent was estimated from the diffusivity in pure water. The

Figure 4. Absorption of CO2 into H2S-loaded DEA aqueous solution. Conditions: 10 wt % DEA, 313.15 K,RH2S ) 0.015 molH2S/molDEA, andPCO2 ) 3400Pa.

Figure 5. Absorption of H2S into CO2-loaded DEA aqueous solution. Conditions: 10 wt % DEA, 313.15 K,RCO2 ) 0.015 molCO2/molDEA, andPH2S ) 2800 Pa.

CO2 + 2R2NH T R2NH2+ + R2NCOO-

H2S + R2NH T R2NH2+ + HS-


DEA diffusion coefficient was also obtained from the correla-tions reported by Hikita et al.29 The influence of the temperaturewas taken into consideration, according to the relation proposedby Hikita et al.29 for aqueous alkanolamine solutions. Thediffusion coefficients of the different ionic species were assumedto be equal to the DEA diffusion coefficient.

The model was used to highlight the opposite mass transferin the case of the absorption of an acid gas into a solventpreviously loaded with another acid gas. First, we consideredCO2 absorption into a solvent with a loading of 0.015 molH2S/molDEA. The measured reaction rate was not significantlyinfluenced by H2S in the solvent, taking into account theexperimental uncertainties reported in this work. The concentra-tion profiles in the liquid phase, which have been calculatedwith the numerical model previously presented, show that theCO2 absorption in the solvent had a direct influence on the H2S-DEA equilibrium (see Figure 4). The model calculated asignificant molecular H2S formation in the mass-transfer film.CO2 absorption in the solvent was coupled to an H2S desorptionphenomenon. The relative quantities of acid gas transferred werenot observable with the equipment used in this work, and thereaction rate does not seem to be influenced by H2S, under theuncertainties associated with the experimental procedure.

On the other hand, we observed experimentally that H2Sabsorption was significantly influenced by dissolved CO2 insolvent. The phenomenon modeling predicted CO2 desorptionsimultaneously with the H2S absorption (see Figure 5). The highreactivity of H2S with DEA led to a high H2S absorption rateand, as a result, to an important modification of the interfaceconditions. The equilibrium between CO2 and DEA wassignificantly displaced by the high H2S absorption rate and ledto CO2 desorption at the interface.

These experimental results and their representation highlightthe importance of the competitive mass transfer, when theabsorption of two gases is considered. The importance of thereverse reactions must also be taken into consideration torepresent the phenomenon. Integration of this numerical toolinto absorption simulation models will lead to improvement inthe performance prediction of natural gas treatment units.

Conclusion

In this work, the kinetics of the zwitterion formation reactionbetween CO2 and diethanolamine (DEA) were determined fromthe CO2 absorption rate obtained in a Lewis cell. The totalpressure control of the measuring cell with nitrogen made itpossible to study the reaction under pseudo-first-order condi-tions, with unloaded solvent. Experiments performed with verylow acid-gas loadingslower than 0.015 mole of acid gas permole of DEAsshowed the influence of the first acid gasdissolved in the solution when the absorption rates of a secondacid gas are measured.

The experimental observations were represented with ageneral model coupling mass transfer and chemical reactions.This procedure involved a numerical approach to solve the setof coupled differential and algebraic equations. This model wasused to represent the opposite mass transfer when CO2 or H2Sis absorbed in a water-DEA solution partially loaded with theother acid gas. Real-time measurements of the quantity of CO2

and H2S present in the gas phase would be useful to arrive ata better understanding of this phenomenon.

Nomenclature

A ) gas/liquid interfacial area (m2)Ci ) concentration of componenti (mol/m3)

Di ) diffusivity of componenti (m2/s)dcell ) internal diameter of the reactor (m)dRushton) diameter of the Rushton turbine in the liquid phase

of the reactor (m)E ) enhancement factorEi ) enhancement factor in instantaneous reaction regionF ) Faraday’s constant;F ) 96 489 C/molH ) Henry’s law constant in the concentration scale (Pa m-3

mol-1)Ha ) Hatta’s numberk ) kinetic rate coefficientkG ) gas-side resistance mass transfer (mol m-2 s-1 Pa-1)kL ) liquid-side resistance mass transfer (m/s)NC ) total number of componentsNR ) total number of reactionsP ) pressure (Pa)R ) ideal gas constant; R) 8.314 J K-1 mol-1

Ri ) production term for componenti (mol m-3 s-1)Re) dimensionless Reynolds numberSc) dimensionless Schmidt numberSh) dimensionless Sherwood numbert ) time (s)T ) temperature (K)V ) volume (m3)x ) distance from interface (m)zi ) ion charge for componenti

Greek Symbols

R ) acid gas loading (molacid gas/molalkanolamine)â ) reaction orderδ ) film thickness (m)λ ) stoichiometric coefficientµ ) viscosity (Pa s)F ) density (kg/m3)æ ) absorption rate (mol m-2 s-1)φ ) electrostatic potential (V/m)

Subscripts and Superscripts

bulk ) bulkG ) gasI ) Inert (solvent vapor pressure+ inert gas partial pressure)ini ) initial time, corresponding to the absorbed gas injectionint ) interfaceL ) liquid∼ ) dimensionless notation

Component AbbreViations

CO2 ) carbon dioxideDEA ) diethanolamineH2S ) hydrogen sulfideHS- ) hydrosulfide ionN2O ) nitrous oxideOH- ) hydroxide ionR2NH ) primary or secondary alkanolamineR2NH2

+ ) protonated primary or secondary alkanolamineR2NCOO- ) primary or secondary alkanolamine carbamate

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ReceiVed for reView January 5, 2006ReVised manuscript receiVed September 27, 2006

AcceptedSeptember 29, 2006

IE060019W


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Competitive Absorption−Desorption of Acid Gas into Water−DEA Solutions