SEPARATIONS

Adsorption of Binary Mixtures of Propane-Propylene in CarbonMolecular Sieve 4A

Carlos A. Grande and Alırio E. Rodrigues*

Laboratory of Separation and Reaction Engineering (LSRE), Department of Chemical Engineering,Faculty of Engineering, University of Porto, rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal

We studied binary adsorption equilibrium and kinetics of propane and propylene in carbonmolecular sieve 4A (Takeda Corp., Tokyo, Japan). Adsorption equilibrium was measured at 373K with a manometric-chromatographic unit at two different total pressures: 47 and 250 kPa.Equilibrium of the binary mixture was well predicted with the multicomponent multisite modelequation based on pure gas data. The integral thermodynamic consistency test was applied tothe set of data. Adsorption kinetics were measured in fixed-bed experiments at 343, 373, and423 K with three different propylene molar fractions: 0.29, 0.55, and 0.80. The total pressurewas also varied from 110 to 320 kPa. A temperature increase in the curves with high propylenemolar fractions can be detrimental to the separation of propane-propylene mixtures by vacuum-pressure-swing adsorption.

1. Introduction

Propane-propylene separation is the most difficultseparation practiced in the petrochemical industryperformed in large distillation columns containing over200 trays with high reflux ratios. As an alternative todistillation, adsorption was already proposed and pres-sure-swing adsorption (PSA) technology is still in theresearch stage. This separation by adsorption requiresa highly selective adsorbent for propylene, which is themost adsorbed gas and is recovered in the low-pressuredesorption step (blowdown step).

Several publications reporting adsorption equilibriumand kinetic data of pure gases in many differentadsorbents to carry out this separation can be found:silica gel,1-3 zeolites,4-11 activated carbon,5,12 π-com-plexation adsorbents,7,13-17 mesoporous materials, andtitanosilicates.17-21 Data of propane and propylene oncarbon molecular sieve are also available over com-mercial and laboratory-made samples.22 Only in a fewworks can propane-propylene binary adsorption equi-librium be found,2,6,23-25 and in some of them, predic-tions with the ideal adsorption solution theory (IAST)can lead to serious errors.2 Even more difficult is to findadsorption kinetics of the binary mixture, particularlyat high hydrocarbon concentration, conditions that aremuch closer to real adsorption operations.5,26-28

In this work, we have measured binary adsorptionequilibrium data by a manometric-chromatographicmethod. Adsorption experiments were performed at 373K, a temperature where the adsorption equilibrium ofpure gases was previously measured.22 The data ob-tained in the manometric-chromatographic unit weremeasured at a constant pressure and different molar

fractions to perform the integral thermodynamic con-sistency test (TCT), validating the data for theoreticalconsistency of new models of multicomponent adsorptionequilibria. The other topic of this work is measurementsof the binary fixed-bed experiments by varying propane-propylene molar ratios and the total pressure at tem-peratures of 343, 373, and 423 K to study the possibilityof using carbon molecular sieve 4A for the separationof this mixture in a PSA unit.

2. Experimental Section

Carbon molecular sieve 4A was kindly provided byTakeda Corp. (Tokyo, Japan). Adsorption equilibria andkinetics of pure propane and propylene measured onthis adsorbent were already reported.22 Adsorptionequilibria of pure propane and propylene at 373 K areshown in Figure 1. The solid lines in Figure 1 representthe multisite Langmuir model,29 and equilibrium andkinetic parameters of both gases are shown in Table 1.

The binary adsorption equilibrium data were mea-sured in a manometric-chromatographic equipment.The pressure transducer used for pressure measure-ments and the total amount adsorbed determination hasan error of (0.04 kPa. The description, scheme, andoperation of this unit for binary adsorption measure-ments were described elsewhere.25 The activation of thesamples was made under vacuum at 523 K for 24 h aftereach equilibrium point. For each measurement, thebinary gas mixture was in contact with the adsorbentfor at least 48 h. The main difficulty with data collectedin manometric-chromatographic equipment is that thefinal pressure is different in almost all of the measure-ments. Even when this is one of the main disadvantagesof the technique,30 under certain conditions, this prob-lem can be circumvented. Assuming that the mixtureis ideal,31 we proceed as follows: first propylene is

* To whom correspondence should be addressed. Tel.: +35122 508 1671. Fax: +351 22 508 1674. E-mail: arodrig@fe.up.pt.

8057Ind. Eng. Chem. Res. 2004, 43, 8057-8065

10.1021/ie049327p CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 11/10/2004

introduced (faster adsorbing gas), and only when puregas equilibrium is reached, propane is inserted in thesystem. By assuming ideal behavior, we predict thenumber of moles adsorbed of each component (IAST),and accounting for the moles that have to remain in thegas phase, we can calculate the amount of each gas thathas to be in the system. If the targeted pressure is notachieved (within experimental errors), we have the firstindication that the mixture is nonideal, but this has tobe confirmed by measuring the molar fractions of thegas phase.

Binary breakthrough curves were measured in alaboratory-scale fixed-bed column with 0.87-m lengthand 0.021-m diameter. The detailed description of theequipment was reported elsewhere.32 Temperature varia-tions due to adsorption are measured at three differentpoints of the column (0.17, 0.40, and 0.65 m) from thefeed inlet. Technical details of the equipment are listedin Table 2 together with some relevant properties of theadsorbent. Activation of the samples was made underflow of nitrogen at 523 K for 24 h only once and keptwith a low flow rate of nitrogen between experiments(one breakthrough curve per day to allow full propanedesorption).

All gases used in this paper were provided by AirLiquide: propane N35 and propylene N24 (puritiesgreater than 99.95 and 99.4%, respectively). HeliumN50 was used for the calibration procedure.

3. Theoretical Section

In a previous paper where adsorption equilibria andkinetics of pure propane and propylene were reportedon carbon molecular sieve 4A, the Toth model was usedfor data analysis.22 A thermodynamically correct equa-tion with a theoretical direct extension to multicompo-nent systems is used in this work to describe pure andbinary adsorption equilibria: the multisite Langmuirmodel.29 This model considers the solid surface ashomogeneous but allows a molecule to adsorb in morethan one adsorption site. When adsorbate-adsorbateinteractions are neglected, the multicomponent multi-site Langmuir model can be expressed as

where qmax,i is the saturation capacity of component i,P is the total gas pressure in equilibrium with theadsorbed phase, and ai is the number of neighboringsites occupied by component i. The equilibrium constantKeq,i is described by an Arrhenius law:

where Keq,i° is the adsorption constant at infinitetemperature for component i, (-∆Hi) is the isostericheat of adsorption of component i on the homogeneoussurface, and Rg is the universal gas constant. Thesaturation capacity of each gas is imposed by thethermodynamic constraint aiqmax,i ) constant33 to satisfya material balance of sites in the adsorbent.

One of the ways to verify the validity of the experi-mental data is to perform the TCT. In this work, wewill limit the analysis of this test to a binary mixture.30

The TCT relies on the use of the nonisothermal Gibbsadsorption equation expressed in terms of the Gibbsiansurface excess variables as34

where φ is the surface potential of the Gibbsian ad-sorbed phase, Sm is the excess entropy, and µi is theequilibrium chemical potential of the gas phase ofcomponent i at constant P, T, and gas molar fraction,yi. This equation may be integrated or differentiated,offering two possibilities of calculating the consistencyof binary data: the integral and differential tests.35

In the case of a binary mixture, when eq 3 isintegrated, the integral test is obtained:

where n1m and n2

m are the excess adsorbed-phase con-centrations of components 1 and 2 in the multicompo-nent system, y1 and y2 are the gas-phase molar fractionsof components 1 and 2, respectively (satisfying theconstraint y1 + y2 ) 1), and φi

/ is the surface potentialof adsorption of pure gas that can be determined frompure gas adsorption isotherms as

Figure 1. Adsorption equilibria of propane and propylene oncarbon molecular sieve 4A (Takeda Corp.) at 373 K.22 Solid linesare the multisite Langmuir model fitting.

Table 1. Adsorption Equilibrium and KineticParameters of Pure Gases

gasqmax,i

[mol/kg]Ki°

[kPa-1] ai

-∆Hi[kJ/mol]

Dc,i°/rc2

[s-1]EA,i

[kJ/mol]

C3H6 2.197 6.10 × 10-7 4.700 93.931 1.0441 14.507C3H8 2.065 3.33 × 10-6 5.000 32.088 0.3087 23.086

Table 2. Fixed-Bed Details and Adsorbent Properties

bed radius [m] 0.0105bed length [m] 0.83bed porosity 0.246bulk density [kg/m3] 678column wall density [kg/m3] 8238wall specific heat [J/kg‚K] 500wall film heat-transfer coefficient [W/m2‚K] 40overall heat-transfer coefficient [W/m2‚K] 20pellet radius (infinite cylinder) [m] 8.0 × 10-3

pellet density [kg/m3] 900pellet porosity 0.315tortuosity 2.0a

specific solid heat [J/kg‚K] 880a Assumed value.

q*i

qmax,i

) Keq,iP(1 - ∑j)1

N q*i

qmax,i)ai

(1)

Keq,i ) Keq,i°e-∆Hi/RgT (2)

dφ ) -Sm dT - ∑i

nim dµi (3)

φ*1(P) - φ*2(P)RT

) ∫0

1n1my2 - n2

my1

y1y2dy1|

T,P(4)

φ*i(P)RT

) -∫0

Pnim*P

dP|T

(5)

8058 Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004

where nim* is the excess adsorbed-phase concentration

of component i calculated from pure-component gasadsorption isotherms. The integral test allows thecalculation of the left-hand side of eq 4 from pure-component adsorption data, while the right-hand sidehas to be calculated from data at constant temperatureand pressure and varying y1 (and consequently y2).

The model used for simulations of binary fixed-bedadsorption relies on the following assumptions:32 the gasbehavior is ideal; mass, heat, and momentum variationsin the radial coordinate were neglected; the pressuredrop in the column is described by the Ergun equation;macropore and micropore diffusion equations are de-scribed by a bilinear driving force (bi-LDF); a film masstransfer in the layer surrounding the extrudates isconsidered. The bi-LDF simplification has a largeimpact on the computational time used for simulations.

With these assumptions, the fixed-bed componentmass balance is

where Ci is the gas-phase concentration, Dax,i is the axialdispersion coefficient, u is the interstitial velocity, εc isthe column porosity, yi is the molar fraction, ki is thefilm mass-transfer resistance, Bii is the Biot number,and ci is the averaged concentration in the macropores,all valid for component i, while CT is the total gasconcentration and a is the extrudate specific area.

In this model the Ergun equation was used to accountfor the pressure drop:

where P is the total pressure, µg is the gas viscosity, dpis the pellet diameter, and Fg is the gas density.

With the LDF approximation for macropore resis-tances, the mass-transfer rate from the gas phase to theextrudate is expressed by

where Dpi is the pore diffusivity, Rp is the extrudateradius (assumed to be an infinite cylinder), Fp is theparticle density, εp is the pellet porosity, wc is theadsorbent weight, and ⟨qi⟩ is the extrudate-averagedadsorbed-phase concentration.

The LDF equation for the micropores averaged overthe entire extrudate is expressed by

where Dci is the crystal diffusivity, rc is the crystalradius, and qi

/ is the gas-phase concentration in theequilibrium state expressed by eq 1.

The energy balance takes into account the threephases present: gas, solid, and column wall. The gas-phase energy balance is

where Cv is the molar constant volumetric specific heatof the gas mixture, Tg is the temperature of the gasphase, λ is the axial heat dispersion, Cp is the molarconstant pressure specific heat of the gas mixture, hf isthe film heat-transfer coefficient between the gas andsolid phases, Ts is the solid (extrudate) temperature),hw is the film heat-transfer coefficient between the gasphase and the column wall, Rw is the column radius,and Tw is the wall temperature.

The solid-phase energy balance is expressed by

where Fb is the bulk density and (-∆Hi) is the isostericheat of adsorption.

Finally, the wall energy balance can be expressed by

where Rw is the ratio of the internal surface area to thevolume of the column wall, Rwl is the ratio of thelogarithmic mean surface area of the column shell tothe volume of the column wall,32 Cpw is the specific heatof the column wall, U is the global external heat-transfercoefficient, and T∞ is the oven constant set-point tem-perature. Several correlations published in the litera-ture were used to calculate the transport parameters:axial dispersion,36 film mass-transfer coefficient,36 andglobal external heat-transfer coefficient.37

Equations 6-12 were solved with the following bound-ary conditions:

The initial condition of the column was considered inall of the cases as without any of the adsorbates and atconstant temperature prior to the start of the experi-ment.

εc

∂Ci

∂t) ∂

∂z (εcDax,iCT

∂yi

∂t ) -∂(uCi)

∂z-

(1 - εc)aki

Bii + 1(Ci - ci) (6)

∂P∂z

) -150µg(1 - εc)

2

εc3dp

2u +

1.75(1 - εc)Fg

εc3dp

|u|u (7)

εp

∂ci

∂t+ Fp

∂⟨qi⟩∂t

) εp

15Dpi

Rp2

Bii

Bii + 1(Ci - ci) (8)

∂⟨qi⟩∂t

)15Dci

rc2

(q*i - ⟨qi⟩) (9)

εcCTCv

∂Tg

∂t) ∂

∂z (λ∂Tg

∂z ) - uCTCp

∂Tg

∂z+ εcRgTg

∂C∂t

-

(1 - εc)ahf(Tg - Ts) -2hw

Rw(Tg - Tw) (10)

(1 - εc)[εp∑i)1

n

ciCvi + Fpwc∑i)1

n

⟨qi⟩Cv,ads,i + FpCps]∂Ts

∂t)

(1 - εc)εpRgTs

∂ci

∂T+ Fbwc∑

i)1

n

(-∆Hi)∂⟨qi⟩

∂t+

(1 - εc)ahf(Tg - Ts) (11)

FwCpw

∂Tw

∂t) Rwhw(Tg - Tw) - RwlU(Tw - T∞) (12)

u(z)0) C(i,z)0)|z+ ) u(z)0) C(i,z)0)|z- (13)

P(z)L) ) Pexit (14)

-εDzm(i)

u(z)0)∂y(i,z)0)

∂z |z+

+ y(i,z)0)|z+ - y(i,z)0)|z- ) 0(15)

∂y(i,z)L)∂z |z-

) 0 (16)

-λ∂Tg(z)0)

∂z |z+

+ uCCpTg(z)0)|z+ -

uCCpTg(z)0)|z- ) 0 (17)

∂Tg(z)L)∂z |

z-) 0 (18)

Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004 8059

Parameters of the multisite Langmuir model forsingle-component isotherms were fitted using MATLAB6.0 (The Mathworks, Natick, MA) optimization functionfmins. The binary data prediction was done in the samesoftware environment. The fixed-bed model describedabove was solved in gPROMS (PSE Enterprise, London,U.K.) using orthogonal collocation method on finiteelements. We used 25 finite elements and 2 interiorcollocation points per element.

4. Results and Discussion4.1. Manometric-Chromatographic Data. The

data obtained by this method were only measured at373 K. The total pressures targeted for the measure-ments were 47 and 250 kPa. The x-y (adsorbed vs gas-phase molar fractions of the most adsorbed compound,propylene) diagram at 47 kPa is shown in Figure 2. Thesolid line in the figure represents the prediction of thebinary behavior based on pure-component data fittedwith the multisite Langmuir model using parameterslisted in Table 1. The comparison of experimental andpredicted data is very good, confirming the validity ofthe multisite Langmuir model to be used in adsorbermodeling when using low pressures (vacuum-swingadsorption, VSA).

The same plot for a different total pressure of 250 kPais shown in Figure 3. The prediction with the multisiteLangmuir model (solid line in the figure) is also verygood. To simplify the visualization of the multisiteLangmuir model prediction, the predicted total amountadsorbed measured at 47 and 250 kPa is plotted againstthe experimental values in Figure 4.

Experimental data plotted in these figures are tabu-lated in Table 3 for data storage and further application

and/or equilibrium model testing. At both pressuresmeasured (47 and 250 kPa), the point around yC3H6 )0.7 is apparently outside the prediction of the model.Note from Table 3 that the total pressure of these twopoints is smaller than 47 and 250 kPa, respectively. Thispoint was measured without calculating the correctnumber of moles that has to be inserted to target adesired final pressure. If the subject is to determinewhether the behavior of the mixture is ideal or not, aconstant value of the pressure is not very important,even though in this case we were interested in collectingbinary points in such a way as to allow us to performthe integral TCT of the data.

The integral TCT was performed following eq 4. Onthe left-hand side, the surface potential of pure gaseswas calculated using eq 5. Then the right-hand side ofeq 4 was calculated only with binary data (see Table3). Both terms have to be equal, but because of experi-mental errors, there is a difference between them. The

Figure 2. Adsorbed-phase concentration vs gas-phase concentra-tion (x-y diagram) at 373 K of C3H6-C3H8 on carbon molecularsieve 4A at 47 kPa. Solid line: multisite Langmuir prediction at47 kPa.

Figure 3. Adsorbed-phase concentration vs gas-phase concentra-tion (x-y diagram) at 373 K of C3H6-C3H8 on carbon molecularsieve 4A at 250 kPa. Solid line: multisite Langmuir prediction at250 kPa.

Figure 4. Total adsorbed-phase concentration at 373 K forC3H6-C3H8 on carbon molecular sieve 4A at 47 kPa (a) and 250kPa (b). Solid line: multisite Langmuir prediction using pure-component data.

Table 3. Binary Adsorption Equilibrium Data ofPropane-Propylene on Carbon Molecular Sieve 4A(Takeda Corp.) at 373 K Measured in theManometric-Chromatographic Unit

P [kPa] y x qt [mol/kg]

47.25 0.882 0.960 1.03746.92 0.117 0.291 0.85847.37 0.047 0.124 0.84046.89 0.217 0.455 0.92047.14 0.377 0.624 0.92946.92 0.555 0.782 0.98242.00 0.698 0.869 0.972

249.43 0.550 0.800 1.288249.86 0.362 0.627 1.258251.02 0.148 0.373 1.195249.68 0.025 0.100 1.127249.27 0.922 0.982 1.341249.66 0.100 0.271 1.160244.30 0.750 0.921 1.273

8060 Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004

numerical values of these terms are shown in Table 4for the pressures of 47 and 250 kPa. The differencebetween both terms is acceptable and is comparablewith that of previous integral TCT results.35 Becausethe binary set of data satisfied the integral TCT, it canbe used for the design of PSA units for propane-propylene separation as well as for calibration of newadsorption equilibrium models. In fact, the initial treat-ment of adsorption equilibrium of pure components wasperformed with the Toth model. To satisfy the integralTCT test, the maximum amount adsorbed and theheterogeneity parameters have to have the same valuefor propane and propylene, which was not the case.22 Afitting with the same values for propane and propylenewas not very good, and the multisite Langmuir modelfit better the entire set of pure-component data.

4.2. Binary Fixed-Bed Experiments. Fixed-bedadsorption equilibrium experiments were already usedin the literature to determine binary adsorption equi-librium data.38,39 In this work we have carried out aset of experiments covering the entire range of tem-peratures used for single-component determinations(343-423 K) and also explore different pressures(110-320 kPa). Measurements were performed usingthree feed mixtures with propylene molar fractions of0.29, 0.55, and 0.80. These molar fractions correspondapproximately to three different compositions that maybe found in industrial streams (downstream of thepropane dehydrogenation reaction, refinery grade pro-pylene, and fluid catalytic cracking for olefin production,respectively).

Experiments started passing a fixed flow rate ofnitrogen in the column (considered inert in the entirerange of temperature) at the corresponding pressureused for measurements. At time zero, the stream isswitched to the binary mixture with almost the sameflow rate to avoid serious variations due to changes inthe gas velocity conditions.

The experiments performed at 343 and 423 K at thedifferent propylene molar fractions are presented in twofigures to show the data collected in a simplified fashion.

Table 4. Integral TCT for Binary AdsorptionEquilibrium of Propane-Propylene Mixtures on CarbonMolecular Sieve 4A (Takeda Corp.)

pressure[kPa]

φ*1(P) - φ*2(P)

RT[mmol/g]

∫0

1(n1my2 - n2

my1)y1y2

dy1|T,P

[mmol/g]|difference|[mmol/g]

47 1.091 1.010 0.081250 1.678 1.580 0.098

Figure 5. Binary propane-propylene breakthrough curves (expressed in molar flow of gases at the exit of the column) in carbon molecularsieve 4A (Takeda Corp.) at 250 kPa. Parts a and b have a molar fraction of propylene of 0.29 (0.71 of propane); parts c and d have a molarfraction of propylene of 0.55 (0.45 of propane); parts e and f have molar fractions of 0.80 for propylene and 0.20 for propane. Parts a, c,and e were measured at 343 K, while parts b, d, and f were measured at 423 K.

Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004 8061

Figure 5 contains the molar flow rates of propane andpropylene at the exit of the column, while Figure 6shows the temperature profiles measured at threedifferent points of the column (0.17, 0.40, and 0.65 m)from the feed inlet. Solid lines in both figures correspondto the prediction of the model described by eqs 6-18. Itcan be seen that the model can describe well the fullset of data based only on pure-component adsorptionequilibria and kinetics previously measured.22

Note that for high propylene molar fractions thetemperature variations due to adsorption are largerthan 30 K in the experiment made at the lower tem-peratures (343 K). Also, in all of the curves, separatetemperature peaks corresponding to the adsorption ofpropane and propylene can be separately observed. Thisallows us to confirm that the isosteric heats of adsorp-tion determined from the multisite Langmuir model canbe correctly used in the energy balance to describemulticomponent adsorption equilibria in fixed-bed ad-sorption. This verification is very important in the de-sign of vacuum-pressure-swing adsorption (VSA-PSA)units.

Another operational variable that was studied in thiswork was the total pressure. In Figure 7, we can seethe curves with 55:45 (propylene-to-propane ratio) at

110, 180, 250, and 320 kPa measured at 373 K. In thiscase also the model represents well the molar flow rateexiting the column as well as the temperature variationsin the column. It can be seen also that the higher thepressure is, the higher the amount of gases adsorbedin the bed (takes more time for breakthrough of propaneand propylene) and also the higher the temperaturevariations.

From the molar flow-rate curves shown in Figure 5,it can be seen that, for molar fractions of 80% propylene,the VSA-PSA separation of both gases with thisadsorbent will be very difficult when compared to otheradsorbents such as zeolite 4A where a purity over 99%can be obtained.32,40 Also, large temperature excursions(increase of the temperature when adsorbing and de-crease of the temperature when desorbing for adsorbentregeneration) are also detrimental to the VSA-PSAperformance.41

As shown in Figures 5-7, the fitting of the modelproposed using a bi-LDF approximation to describediffusion in the extrudate particles was good. Previousreports have shown that diffusion of small molecules(CH4, CO2, N2, and O2) in the carbon molecular sievesamples is controlled by micropore resistance togetherwith a barrier resistance at the mouth of the micro-

Figure 6. Temperature profiles corresponding to the binary propane-propylene breakthrough curves in carbon molecular sieve 4A (TakedaCorp.) at 250 kPa. Molar fractions of the curves are as follows: (a and b) 0.29 C3H6 and 0.71 C3H8; (c and d) 0.55 C3H6 and 0.45 C3H8; (eand f) 0.80 C3H6 and 0.20 C3H8. Parts a, c, and e were measured at 343 K, while parts b, d, and f were measured at 423 K. Temperatureswere measured at 0.17 m (bottom), 0.43 m (middle), and 0.68 m (top) from the feed inlet.

8062 Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004

pore.42-45 In those studies, the LDF approximation todescribe micropore diffusion was a model simplificationthat gave a reasonably good fit of adsorption uptakesand breakthrough curves.43,44 The barrier resistancewas more important for larger molecules controlling thediffusion process in the case of methane and nitrogenin carbon molecular sieve 3A (Takeda Corp.). For thecase of CO2, which is the smaller molecule, diffusion canbe described well only using micropore resistance. Also,all gases mentioned above exhibit concentration-de-pendent diffusivity; however, this effect is less importantfor the smaller molecule.

In our previous paper, when we characterize thecarbon molecular sieve adsorbent used in this study

with carbon dioxide adsorption,22 we mentioned thatthis sample has an average pore diameter of 6 Å, whichresults from a mixture of very fine micropores and otherlarger micropores. When adsorption kinetics of puregases are measured, only micropore resistance wasdetected, and propane breakthrough curves at lowhydrocarbon concentration were not very well fitted,while propylene experiments seem to be controlled onlyby micropore resistance. While using the diffusivityparameters determined from such experiments andusing the LDF model, we could describe well binarybreakthrough curves for more concentrated mixtures.Whether or not there is some effect of the contribu-tion of a surface barrier resistance at the mouth of

Figure 7. Molar flow rate and temperature profiles corresponding to propane-propylene breakthrough curves at 373 K in carbon molecularsieve 4A (Takeda Corp.) at 110 kPa (a and b), 180 kPa (c and d), 250 kPa (e and f), and 320 kPa (g and h). Molar fractions of the curvesare 0.55 C3H6 and 0.45 C3H8. Temperatures were measured at 0.17 m (bottom), 0.43 m (middle), and 0.68 m (top) from the feed inlet.

Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004 8063

the micropore, particularly in propane diffusion, is atask that may require additional experiments. Also,the experiments performed here did not show concen-tration dependence of the diffusion coefficients. Theimportance of these results is focused on the modelingof the VSA-PSA process where diffusion and equilib-rium parameters determined previously can be directlyused to model propane-propylene separation.

5. Conclusions

Binary adsorption equilibria of propane and propylenein carbon molecular sieve 4A (Takeda Corp.) weremeasured in a manometric-chromatographic unit at373 K and pressures of 47 and 250 kPa. The data werewell predicted by the multicomponent extension of themultisite Langmuir model based on pure gas adsorptiondata. The integral TCT was successfully applied to theentire set of data, indicating that these data can alsobe used for calibration of new multicomponent modelsto describe adsorption equilibrium.

Binary adsorption kinetics were measured in fixed-bed experiments at 343, 373, and 423 K for threedifferent propylene-to-propane ratios: 29:71, 55:45, and80:20 at 250 kPa. Also, the total pressure was variedfrom 110 to 320 kPa. The data were well correlated withthe model proposed using only pure-component adsorp-tion equilibrium data. Two separate peaks can beobserved in the temperature profiles inside the columnbecause of adsorption of propane and propylene, respec-tively. The large temperature excursions can be a majorproblem in VSA-PSA operation. Also from the curveswith a propylene molar fraction of 80%, it was observedthat separation of propane and propylene would be verydifficult to perform in a PSA unit using this adsorbent.

Acknowledgment

The authors are thankful for financial support fromFoundation for Science and Technology (FCT) by ProjectPOCTI/1999/EQU/32654, CYTED V.8 project, and thegift of carbon molecular sieve 4A adsorbent by TakedaCorp. C.A.G. acknowledges a FCT grant (SFRH/BD/11398/2002).

Nomenclature

a ) extrudate specific area, m-1

ai ) number of neighboring sites occupied by component iBii ) Biot numberCi ) gas-phase concentration, mol/m3

ci ) averaged concentration in the macropores for compo-nent i, mol/m3

Cp ) molar constant pressure specific heat of the gasmixture, J/mol‚K

Cpw ) specific heat of the column wall, J/kg‚KCT ) total gas concentration, mol/m3

Cv ) molar constant volumetric specific heat of the gasmixture, J/mol‚K

Dax,i ) axial dispersion coefficient, m2/sDci ) crystal diffusivity, m2/sdp ) pellet diameter, mDpi ) pore diffusivity, m2/shf ) film heat-transfer coefficient between the gas and solid

phases, W/m2‚Khw ) film heat-transfer coefficient between the gas phase

and the column wallKeq,i ) equilibrium constant, kPa-1

Keq,i° ) adsorption constant at infinite temperature forcomponent i, kPa-1

ki ) film mass-transfer resistance, m/sni

m ) excess adsorbed-phase concentration of componenti, mol/kg

nim* ) excess adsorbed-phase concentration of componenti, mol/kg

P ) total pressure, kPaqi/ ) gas-phase concentration in the equilibrium state,mol/kg

⟨qi⟩ ) extrudate-averaged adsorbed-phase concentration,mol/kg

qmax,i ) saturation capacity of component i, mol/kgrc ) crystal radius, mRg ) universal gas constant, J/mol‚KRp ) extrudate radius, mRw ) column radius, mSm ) excess entropy, J/kg‚KTg ) temperature of the gas phase, KTs ) solid (extrudate) temperature, KTw ) wall temperature, KT∞ ) oven constant set-point temperature, Ku ) interstitial velocity, m/sU ) global external heat-transfer coefficient, W/m2‚Kxi ) adsorbed-phase molar fraction of component iyi ) gas-phase molar fraction of component iwc ) adsorbent weight, kg

Greek Letters

Rw ) ratio of the internal surface area to the volume ofthe column wall, m-1

Rwl ) ratio of the logarithmic mean surface area of thecolumn shell to the volume of the column wall, m-1

εc ) column porosityεp ) pellet porosityFb ) bulk density, kg/m3

Fg ) gas density, kg/m3

Fp ) particle density, kg/m3

(-∆Hi) ) isosteric heat of adsorption of component i, J/molλ ) axial heat dispersion, W/m2‚Kµg ) gas viscosity, kg/m‚sµi ) equilibrium chemical potential of the gas phase of

component i, J/molφ ) surface potential of the Gibbsian adsorbed phase, J/kgφi/ ) surface potential of adsorption of pure gas, J/kg

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Received for review July 29, 2004Revised manuscript received September 23, 2004

Accepted September 27, 2004

IE049327P

Ind. Eng. Chem. Res., Vol. 43, No. 25, 2004 8065