SEPARATIONS

Limits for Air Separation by Adsorption with LiX Zeolite

Salil U. Rege and Ralph T. Yang*

Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109

In the present work the performance of two zeolite sorbents, namely LiX (Al/Si ) 1 with 100%Li+) and NaX (13X), is compared for air separation by pressure swing adsorption, with particularattention focused on the limits in the adsorption/desorption pressure ratio. The product (O2)recovery was optimized for the two sorbents at different pressure ratios (2-10), keeping theproduct purity and product throughput nearly constant. It was found that in the case of theLiX zeolite it is possible to operate at a pressure ratio as low as 2 with only a slight decrease inproduct recovery, compared to the limit of 4 for NaX. The temperature excursion in the LiXbed was 4 times that in the NaX bed. Substitution of a portion of the LiX sorbent by inert,high-heat-capacity particles (e.g., iron) in the bed decreased the temperature excursionsignificantly. It was shown that an optimal amount of 5-10% (v/v) of inert substitution (at thesame total bed volume) could increase the O2 product recovery by 2%.

Introduction

Nitrogen and oxygen are, respectively, the second andthird largest man-made commodity chemicals today.Since the 1920s, N2 and O2 have been produced bycryogenic distillation of air. Since the 1980s, adsorption(i.e., pressure swing adsorption or PSA) has beenadopted rapidly and increasingly for air separation.Approximately 20% of air separation is presently ac-complished by PSA. This is the combined result ofzeolite sorbent development and adsorption processcycle development. A brief history of the sorbent andprocess developments is given elsewhere (Yang, 1987;Ruthven et al., 1994).Zeolites have the unique ability to adsorb N2 more

strongly than O2 (i.e., by a factor of 2 or more in termsof pure component adsorption amounts). The mainreason for this is the interaction between the quadrupolemoment of N2 and the cation that is attached to thezeolite framework. It has been long known that Li+ isamong the cations that provide the strongest interac-tions with N2 (McKee, 1964), due to its high polarizingpower (i.e., charge/ionic radius). A major advance cameonly recently when it was found that (1) a threshold ofapproximately 70% cations must be Li+ for the strongN2 interactions, beyond which the adsorption amountsteeply increases, and (2) N2 adsorption is significantlyincreased when the Si/Al ratio in type X zeolite ap-proaches unity (Chao, 1989; Chao et al., 1992; Coe etal., 1992).A variety of PSA cycles are known (Yang, 1987). The

dominant factor that determines the energy require-ment (hence the cost of N2 and O2) of a PSA cycle is thepressure ratio of the high adsorption pressure to the lowdesorption pressure (Leavitt, 1991). A pressure ratioof 4 or higher has been used in industry, which hasappeared to be a barrier for PSA air separation. It wasfirst suggested by Leavitt (1991) that this barrier couldpossibly be lowered to 2, when LiX zeolite was used.

However, no specifics were given by Leavitt. Mean-while, in the vast literature on PSA simulation, littleor no attention has been paid to the pressure ratio, withthe exception of work by Knaebel and Hill (1985) andKayser and Knaebel (1986; 1989). In their work ana-lytical solutions were derived with simplifying assump-tions, including isothermality and the PSA cycle involv-ing essentially two steps only (i.e., Skarstrom cycle) withcomplete bed utilization. Although these assumptionsdeviated substantially from industrial air separation,their solutions provided a qualitative insight into theprocess dynamics. At a fixed feed throughput and O2product purity using 5A and 13X zeolites, their solutionscorrectly predicted a steep decline in product recoveryas the pressure ratio was decreased to near 4.In this work, we examine the limits for air separation

using the LiX zeolite (with 100% Li exchange). Par-ticular attention was paid to the limits in the pressureratio on the X zeolite with Si/Al ) 1. The PSA cycleused in this work was the well-developed five-step cyclethat is used in industry (Yang, 1987). The cycleconditions were optimized by numerical simulation. Theproduct throughput, expressed by the bed size factor,was within the range for industrial practice. The resultsof LiX were compared with that of NaX (with Si/Al beingapproximately 1.15), which is the sorbent being usedin industry. Moreover, a means to counter the largeheat effects on LiX (due to the high heat of adsorptionof N2 on LiX) is suggested.

Description of the PSA Cycle

A five-step PSA cycle was used in this study. Thesteps involved in each cycle are as follows: (1) Pres-surization with the feed gas (air); (2) high-pressureadsorption, i.e., feed step; (3) cocurrent depressurization;(4) countercurrent blowdown; (5) countercurrent low-pressure purge with the product (oxygen).All the above steps were of equal duration (30 s).

Thus the time required for the completion of each PSAcycle was 2.5 min. The model assumed only twoadsorbable components, namely O2 and N2. The less-strongly adsorbed species like Ar, etc., were clubbed

* Author to whom correspondence should be addressed.Tel.: (313) 936-0771. Fax: (313) 743-0459. E-mail: yang@umich.edu.

5358 Ind. Eng. Chem. Res. 1997, 36, 5358-5365

S0888-5885(97)00521-6 CCC: $14.00 © 1997 American Chemical Society

along with O2, and it was assumed that all contami-nants in air like CO2 and water vapor were removedcompletely prior to feeding by pretreatment beds.The cocurrent depressurization step has been shown

to improve the recovery of the strongly adsorbed com-ponent by increasing the concentration of the strongadsorptive in both gas and adsorbed phases due tolowering of the pressure in the voids (Yang, 1987). Inorder to approach the cyclic steady state faster, the bedwas pressurized initially with 90 mol % O2. In thesubsequent cycles pressurization was carried out withair consisting of 22% O2 (mixture of O2 and Ar) and 78%N2. The product of each cycle comprised of a volumetricmixture of the output stream of the feed step and thecocurrent depressurization step. This product streamwas partly used to purge the bed countercurrently instep (5).In order to study the performance of the two adsor-

bents under study, the product purity, product recovery,and product throughput were studied at various pres-sure ratios. The pressure ratio is defined as

In this paper, the product recovery and purge to feedratio (P/F) are defined as follows:

Another factor called the “bed size factor” (BSF),which gives an indication of the sorbent productivity,is used in this work:

Mathematical Model

The model used assumes the flow of a gaseousmixture of two components in a fixed bed packed withspherical adsorbent particles of identical size and shape.The bed is considered to be adiabatic and diffusionalresistance is assumed to be negligible since the diffusionof O2 and N2 in the adsorbents considered is known tobe fast. Thus, there is local equilibrium between thegas and the solid phase of each gas component. Axialdispersion for mass and heat transfer is assumed, butdispersion in the radial direction is taken to be negli-gible. Axial pressure drop is neglected and the idealgas law is assumed to hold since pressures involved arenear atmospheric. Also the gas is assumed to haveconstant viscosity and heat capacity.The mass balance equation for component k in the

bed is given by the axially dispersed plug flow equation(Sun et al., 1996):

The overall material balance obtained is

For an adiabatic bed with no heat transfer with thesurroundings, the overall heat balance may be writtenas

Assuming local equilibrium, we have

where qk* is the equilibrium amount adsorbed at thesurface of the pellet.

Initial and Boundary Conditions

The boundary conditions employed correspond to theDankwert’s boundary conditions for the closed-closedvessel case with no dispersion to immediate left of thez ) 0 and to the immediate right of z ) L. Here thenotation z ) 0 and z ) L is used with reference to theentrance and exit points of the bed for the high-pressure

Figure 1. N2 isotherms for LiX (Si/Al ) 1, 100% Li) and NaX(13X, Si/Al ∼ 1.15) zeolites at 298 K.

Figure 2. O2 isotherms for LiX (Si/Al )1, 100% Li) and NaX (13X,Si/Al ∼ 1.15) zeolites at 298 K.

pressure ratio )high pressure during adsorption PH

low pressure during purge PL(1)

product recovery )(O2 from steps 2 and 3) - (O2 used in step 5)

(O2 fed in step 1 and step 2)(2)

purge to feed ratio (P/F) )amount of O2 used in step 5

amount of O2 fed in steps 1 and 2(3)

BSF )weight of adsorbent used in the bed (kg)product throughput (kg of O2/h in product)

(4)

εt∂yk∂t

- εDax

∂2yk∂z2

+ ε∂(uyk)∂z

+FbRTP

∂qk∂t

+εtykP

dPdt

) 0

(5)

ε∂u

∂z) -

FbRT

P∑k)1

2 ∂qk

∂t-εt

P

dP

dt(6)

[εFgCpg + Fb(Cps + ∑k)1

2

qkCpg)]∂T

∂t+ εFgCpgu

∂T

∂z-

ελL∂2T

∂z2) Fb ∑

k)1

2

|∆Hj|∂qk

∂t+ ε

dP

dt(7)

∂qk∂t

)∂qk*∂t

(8)

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5359

feed step. For each of the steps having pressuredynamics, a linear pressure variation is assumed.The boundary conditions for the bed mass and tem-

perature variables are summarized below, wherein thesubscript “k” corresponds to the component index:

Isotherm Data

The equilibrium amount adsorbed on respective ad-sorbent was calculated using the extended mixed Lang-muir isotherm:

The temperature dependence of the Langmuir param-eters are assumed to be as follows (Baksh et al., 1992):

The adsorbents under study were LiX and NaX (13X).It should be noted that although Baksh et al. (1992) alsomeasured the adsorption isotherms of N2 and O2 on acommercial LiX zeolite (Si/Al ∼ 1.15), the LiX sampleused in the present case has a different Si/Al ratio (of1). Hence the isotherms obtained by Baksh et al. (1992)differ from those given in this paper. Isotherm data forNaX and LiX used in the present simulations are shownin Figures 1 and 2 and the isotherm parameters arelisted in Table 1. These isotherms were measured inour laboratory using a Micromeritics ASAP 2010 ap-paratus.

Numerical Method Used

The terms in the above partial differential equations,involving spatial derivatives, were approximated bytheir finite difference equivalents using the Crank-Nicolson scheme. In each of the simulations, 100 spatialgrid points were used for discretization with a relativeconvergence accuracy of 1 × 10- 3. The time derivativewith respect to pressure was known. The time deriva-

Table 1. Values of the Parameters in the Temperature Dependent Langmuir Isotherm of N2 and O2 for LiX and NaX(13X) Adsorbents

sorbent sorbate k1 (mmol/g/atm) k2 (K) k3 (atm-1) k4 (K) -∆H (kcal/mol) Cpg (cal/mol/K)

NaX O2 8.21 × 10-4 1592.53 3.51 × 10-3 1544.43 3.16 8.27NaX N2 6.11 × 10-4 2168.55 1.05 × 10-3 2012.92 4.31 6.50LiX O2 1.11 × 10-4 1593.0 1.03 × 10-4 2061.9 3.16 8.27LiX N2 1.25 × 10-3 2168.6 2.07 × 10-4 2455.5 5.60 6.50

(Step 3) Cocurrent Depressurization Step:

at z ) 0, u ) 0

P ) P(t) ) PH + (PCD - PH)(t/τco)

∂yk∂z |z)0 ) ∂T

∂z |z)0 ) 0

∂yk∂z |z)L ) ∂T

∂z |z)L ) 0 (11)

(Step 4) Countercurrent Depressurization Step:

at z ) L, u ) 0

P ) P(t) ) PCD + (PL - PCD)(t/τcn)

∂yk∂z |z)0 ) ∂T

∂z |z)0 ) 0

∂yk∂z |z)L ) ∂T

∂z |z)L ) 0 (12)

(Step 5) Countercurrent Low-Pressure Purge Step:

at z ) L, yk ) yp,k

at z ) L, u ) uL

P ) PL

-Dax

∂yk∂z |z)L ) uL(yL,k - yk|z)L) (uL < 0)

-λL∂T∂z |z)L ) FgCpguL(T|z)L - TL)

∂yk∂z |z)L ) ∂T

∂z |z)L ) 0 (13)

qk* )KkPk

1 + ∑j)1

2

BjPj

k ) 1,2 (14)

K ) k1 exp(k2/T) and B ) k3 exp(k4/T) (15)

(Step 1) Pressurization Step:

at z ) 0, yk ) yf,i

at z ) L, u ) 0

P ) P(t) ) PL + (PH - PL)(t/τP)

Dax

∂yk∂z |z)0 ) uH(yk|z)0 - yH,k)

-λL∂T∂z |z)0 ) FgCpguk(T|z)0 - TH)

∂yk∂z |z)0 ) ∂T

∂z |z)0 ) 0 (9)

(Step 2) High-Pressure Feed Step:

at z ) 0, yk ) yf,i, u ) uf

P ) PH

Dax

∂yk∂z |z)0 ) uH(yk|z)0 - yH,k)

-λL∂T∂z |z)0 ) FgCpguH(T|z)0 - TH)

∂yk∂z |z)L ) ∂T

∂z |z)L ) 0 (10)

5360 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

tive of the average adsorbed concentration could bewritten as a function of time derivatives of gas phaseconcentrations, temperature, and pressure using theisotherm equation. Thus the partial differential equa-tions were converted into a set of ordinary differentialequations (ODEs) having first-order time derivatives oftemperature and mole fractions of the gaseous compo-nents as the differential vector on the l.h.s. and a sparse-banded matrix of finite difference concentration andtemperature terms on the r.h.s. Since the ODE matrixwas stiff in nature, it was solved using an ODE solver,namely the LSODE (Livermore solver for ordinarydifferential equations) subroutine for the solution offirst-order initial value problems of stiff/nonstiff systems(Hindmarsh, 1980) which is based on the Gear’s method.The PSA simulation code was written in FORTRAN

and executed on a SUN-SPARC workstation. In mostof the cases the cyclic steady state was reached within30 cycles. The CPU time was found to depend upon theinitial conditions and the operating parameters. TheCPU time taken for computing one PSA cycle wasbetween 18 and 25 s.

Results and Discussion

Comparison of LiX and NaX (13X) Zeolites asSorbents. The PSA simulations reveal some interest-ing characteristics of the two zeolite sorbents underconsideration. One objective of the present work wasto study the oxygen product recovery at various pressureratios. A similar study has been done analytically inthe past (Kayser and Knaebel, 1989) using the binarynonlinear isotherm theory for the case of a four-stepisothermal PSA cycle for air separation using zeolite13X as the adsorbent at 0 °C.The PSA cycle specifications used in the simulations

are summarized in Table 2. The dispersion coefficientsand effective thermal conductivity used in the simula-tions were predicted using standard correlations re-ported in literature (Yang, 1987). The Peclet numbersmentioned in Table 2 are defined as follows:

In the present study, by proper manipulation of feedthroughput and the purge to feed ratio (P/F), the productpurity and product throughput were kept constantapproximately at 95.2% and 0.027 kg of O2 product/h/kg of adsorbent, respectively, so as to provide a faircomparison of the two sorbents. In other words, the bed

size factor (BSF) for these sorbents were kept constant.It should be noted that the purge to feed ratios areprobably not the optimal values for each of the simula-tion results in themselves. However, by placing aconstraint that the BSF for each of the results be thesame optimization with respect to the purge to feedratios was done. Thus the product recoveries wereoptimized at fixed product purity and fixed BSF.As can be seen in Figure 3, the product recovery for

the NaX sorbent remained fairly constant between apressure ratio of 5 and 10. Below a pressure ratio of 4,the product recovery started to drop drastically. In factbelow a pressure ratio of 4, it was almost impossible tomaintain a product purity above 95% keeping theproduct throughput at the constant value mentionedbefore. An extrapolation (see dotted line) was thereforedone at pressure ratios of 2 and 3 for the NaX sorbentwith data at slightly higher BSF than the other datapoints. The process conditions for these simulations aresummarized in Table 3. In contrast, the LiX sorbentwas seen to maintain its product recovery at a pressureratio of 3. Even at a pressure ratio of 2, the productrecovery is seen to have dropped to just above 50%,which is quite acceptable. At present, industrial pro-cesses employ a pressure ratio between 4 and 5. Lower-ing the pressure ratio implies an increase in thedesorption pressure, thus providing large savings incapital and operating costs related to vacuum equip-ment (Leavitt, 1989). Hence there is an incentive toreduce the pressure ratio in PSA beds using LiXsorbents, provided the product throughput is acceptable.For the operating conditions in this study, the LiX

sorbent was found to converge to a value of 65% at high-pressure ratios while the NaX (13X) sorbent convergedat about 52%. The higher recovery in the case of LiX isobvious.Effect of Pressure Ratio on Temperature Pro-

files. The temperature profiles in the PSA bed follow-ing the high-pressure adsorption step were studied atvarious pressure ratios. It is seen from Figures 4 and5 that the peak temperatures for the LiX and NaXsorbents are 40 and 29 °C, respectively. The differencein the temperature peaks of LiX and NaX for lowerpressure ratios is considerably lower than that at higherpressure ratios which is because of the higher heat ofadsorption of N2 on LiX than on NaX, as can be seenfrom Table 1.Figures 6 and 7 show the effect of the pressure ratio

on the temperature during the low-pressure purge step.

Table 2. Adsorption Bed Characteristics and OperatingConditions Used in the PSA Simulations

bed length 2.5 mdiameter of adsorber bed 1.0 mbed external porosity 0.40bed density 720 kg/m3

heat capacity of gases 6.87 cal/mol/Kheat capacity of bed 0.28 cal/g/Kwall temperature 298 K (ambient)feed gas composition 78% N2, 22% O2adsorption pressure (PH) 1.0 barrange of desorption pressure (PL) 0.1-0.5 bar

(depending on pressure ratio)range of cocurrent

depressurization pressure (PCD)0.6- 0.8 bar

range of feed interstitial velocities 12-80 cm/sPem 150Peh 2average BSF value

(kg of adsorbent/kg ofO2 product/h)

37.5

Pem ) uL/Dax

Pet ) uL/R where R ) λLFg-1Cp

-1

Figure 3. O2 product recovery (%) at different pressure ratios(PH/PL) for LiX (Al/Si )1, 100% Li) and NaX (13X) sorbents. PH )1.0 atm, average bed size factor (BSF) ) 37.5 kg of adsorbent/kgof O2 product/h, O2 product purity ) 95.2%. Refer to Table 3 foroperating conditions.

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5361

For both the sorbents, there was a fall in the temper-ature with an increase in pressure ratio. The effect,however, was more pronounced in the case of LiX, againbecause of its high heat of adsorption.Effect of P/F Ratio on the Product Purity and

Recovery. A certain amount of the product obtainedin each cycle was purged countercurrently at the lowerdesorption pressure through the bed to clean the bed ofresidual N2 impurity. The purge to feed ratio (P/F),defined earlier in eq 3, is an important parameter indeciding the adsorber performance. The effect of P/F

on product purity and recovery has been studied exten-sively (Yang, 1987; Baksh et al., 1992). In the case ofthe LiX zeolite, the P/F was varied from 0.19 to 0.35 ata pressure ratio of 3, keeping the feed throughputconstant at 0.753 kg of O2/h/kg of adsorbent. As can beseen from Figure 8, the O2 product purity increased withan increase in the P/F ratio up to a certain value, beyondwhich there was no significant gain in purity. Thepurging of the bed with the product allowed for asharper O2 wave front during the feed step by cleaning

Table 3. PSA Simulation Operating Conditions Used in Figures 3-5, 10, and 11

pressure ratioPH/PL PCD (atm)

feed velocityUH (m/s)

purge velocityUL (m/s)

purge to feedratio P/F

O2 productpurity (%)

O2 productrecovery (%)

BSF (kg of adsorbent/kg of O2 produced/h

LiX (Al/Si ) 1, 100% Li+) Adsorbent (PH ) 1.0 atm)1.2 0.95 0.30 0.10 0.85 90.2 12.4 3931.5 0.75 0.70 0.25 0.70 95.6 29.0 76.82 0.8 0.70 0.60 0.44 95.1 53.5 37.43 0.7 0.45 0.13 0.23 96.1 61.9 37.74 0.7 0.37 0.11 0.13 95.3 63.3 38.15 0.7 0.35 0.09 0.09 95.1 64.1 37.26 0.65 0.30 0.08 0.06 95.4 65.3 37.77 0.65 0.28 0.06 0.04 94.5 65.3 37.98 0.65 0.28 0.06 0.03 95.9 64.5 37.610 0.65 0.28 0.04 0.02 95.3 64.7 36.7

NaX (13X) Adsorbent (PH ) 1.0 atm)2 0.85 0.30 0.30 0.80 96.0 20.0 138.93 0.80 0.30 0.33 0.49 93.8 42.3 49.64 0.70 0.28 0.42 0.46 95.2 53.3 38.25 0.70 0.23 0.40 0.35 95.6 53.4 37.46 0.70 0.19 0.37 0.27 95.4 53.2 37.37 0.70 0.17 0.35 0.21 95.2 52.8 37.18 0.67 0.12 0.34 0.18 95.3 52.8 37.610 0.67 0.11 0.34 0.14 95.3 53.1 36.5

Figure 4. Cyclic steady-state temperature profiles at the end ofthe high-pressure feed step for LiX (Al/Si ) 1, 100% Li) at BSF )37.5 kg of adsorbent/kg of O2 product/h. Inset figures indicate thecorresponding pressure ratio (PH/PL).

Figure 5. Cyclic steady-state temperature profiles at the end ofthe high-pressure feed step for NaX (13X) at BSF ) 37.5 kg ofadsorbent/kg of O2 product/h. Inset figures indicate the corre-sponding pressure ratio (PH/PL).

Figure 6. Cyclic steady-state temperature profiles at the end ofthe low-pressure purge step for LiX (Si/Al ) 1, 100% Li+) at BSF) 37.5 kg of adsorbent/kg of O2 product/h. Inset figures indicatethe corresponding pressure ratio (PH/PL).

Figure 7. Cyclic steady-state temperature profiles at the end ofthe low-pressure purge step for NaX (13X) at BSF ) 37.5 kg ofadsorbent/kg of O2 product/h. Inset figures indicate the corre-sponding pressure ratio (PH/PL).

5362 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

the bed of the impurity, thus increasing the productpurity. However, it is evident that, by doing so, thereis a loss of O2 in the product stream, and hence therewas a decline in the product recovery with an increasein the P/F ratio. Figure 9 displays the behavior of theproduct recovery as the product purity is increased,keeping the feed throughput constant. It should berecalled that the “oxygen” component actually wascomposed of a mixture of O2 and Ar. Although thepurity of oxygen depicted in Figures 8 and 9 exceeded98%, it also included about 4% Ar. Hence the actualpurity of O2 that would be obtained is about 95-96%.Adsorption Bed Profiles. The cyclic steady-state

N2 concentration profiles at the end of each step of thePSA cycle, for NaX and LiX adsorbents at a pressureratio of 10, are shown in Figures 10 and 11, respectively.In the case of NaX, it is seen that the bed utilizationwas more since the N2 wave front extended furtherdown the bed than it did in the case of the LiX sorbent.This result was expected since the NaX sorbent had asmaller capacity for N2 than the LiX sorbent. Thetemperature profiles (not shown) in the case of NaXwere more flat whereas those of LiX showed some sharppeaks. The temperature variation in the bed fromdesorptive to adsorptive conditions was much smallerin the case of NaX (from 22 to 28 °C) than that in thecase of LiX (from 15 to 40 °C). This is understandably

due to the higher heat of adsorption of the adsorbateon the LiX sorbent.A comparison of the bed concentration profiles at

times half way through a step (not shown) and those atthe end of the step showed a slight advancement for thefeed and cocurrent depressurization steps. There wasa drastic change in the profile involving the counter-current depressurization step which was natural, con-sidering the large pressure variation occurring duringthis step. Also a comparison of the concentrationprofiles for the two adsorbents revealed that the wavefronts were much sharper in the case of the LiX sorbentthan those for the NaX sorbent. This followed from thefact that the N2 isotherm on LiX was more favorablethan that on NaX, as can be seen from Figures 1 and 2.Countering the Heat Effects. Due to the combined

effect of the high heat of adsorption of N2, a greateramount of adsorption and a shorter bed utilization inthe case of the LiX zeolite, there was a substantialdifferential in the bed temperature during the adsorp-tion and desorption steps. This can be seen fromFigures 4 and 5, wherein the temperature rise in theLiX bed was about 17 °C compared to 4 °C in the NaXbed. The increase in temperature during adsorptionand the consequent decrease during desorption isdetrimental to the separation. Yang and Cen (1986) hadsuggested two methods for countering the adverse heateffects in the adsorption process, namely providing heat

Figure 8. O2 product purity (%) and recovery (%) versus purgeto feed ratio (P/F) for LiX (Si/Al )1, 100% Li) zeolite at pressureratio ) 3; PH ) 1.0 atm, feed throughput fixed at 0.753 kg of O2/h/kg of adsorbent. (Note: “O2” also includes inerts like Ar.)

Figure 9. O2 product recovery (%) versus percentage purity forLiX (Si/Al ) 1, 100% Li) zeolite at pressure ratio ) 3; PH ) 1.0atm, feed throughput fixed at 0.753 kg of O2/h/kg of adsorbent.(Note: “O2” also includes inerts like Ar.)

Figure 10. Concentration profile in NaX (13X) bed at the end ofeach step at pressure ratio ) 10. Refer to Table 3 for operatingconditions. Inset numbers indicate step number: (1) pressurizationwith the feed gas (air); (2) high-pressure feed step; (3) cocurrentdepressurization; (4) countercurrent blowdown; (5) countercurrentpurge.

Figure 11. Concentration profile in LiX (Si/Al)1, 100% Li+) bedat the end of each step at pressure ratio ) 10. Refer to Table 3 foroperating conditions. Inset figures indicate step number: (1)pressurization with the feed gas (air); (2) high-pressure feed step;(3) cocurrent depressurization; (4) countercurrent blowdown; (5)countercurrent purge.

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5363

exchange between adsorbers and the addition of highheat capacity additives. The former method is not veryattractive since it adds to the complexity of the multibedadsorptive process. The second proposal suggests theaddition of inert additives like iron particles (of nearlythe same size as that of the zeolite particles) which willserve to store heat during adsorption and release itduring desorption. Thus the problem of temperatureexcursions may be overcome.A similar approach was employed in the present case

of air separation using the LiX zeolite for a pressureratio of 10. Simulations were carried out using 0, 5,10, 20, and 30% of inert iron particles by volume. Theinternal heat-transfer resistance to the iron particleswas assumed to be negligible due to its high heatconductivity. The specific heat of iron (0.11 cal/g/K) isactually less than that of zeolite (0.28 cal/g/K); howeversince its density (7200 kg/m3) is 10 times that of zeolite(720 kg/m3) the effective heat capacity of the zeolite-iron particle system is enhanced to about 2-3 times ofthe value in the case of pure zeolite with 10-30% inertparticle addition by volume. For the present simula-tions, the bed dimensions were maintained and conse-quently, with the addition of inerts, the adsorbentloading (as well as the bed adsorbent density) wasreduced. The effective adsorbent bed density, specificheat and the bed performance for the various cases ofinert particle loadings are summarized in Table 4. Inall the runs, the O2 product purity and the BSF factorwere kept constant. As the adsorbent loading wasreduced due to inert addition, the feed throughput hadto be decreased to keep the bed utilization at a constantvalue.It was found that the temperature deviation in the

case of inert addition was reduced by almost 60% thanthat in the case of the pure adsorbent. For inert particleloading up to 10%, slight enhancements in productrecovery by 1-2% were observed, as can be seen inTable 4. However with inert addition of more than 10%,it was observed that the product recovery actuallydeclined. Thus there appears to be an optimum for theamount of inert addition at about 5% (vol).Limits of Operability. It has been shown in this

work (Figure 3) that it is possible to operate theadsorption bed packed with LiX zeolite with a pressureratio as low as 2 with only a slight loss in productrecovery. This is possible because of the superioradsorptive properties of this adsorbent. However, animportant factor in deciding the effectiveness of the PSAprocess is the bed size factor (BSF) which was definedpreviously (eq 4). The BSF factor provides an idea aboutthe productiveness of the process and is indicative ofthe extent of bed utilization. Since it is desirable to havea maximum product throughput for a given amount ofadsorbent, as per definition, we would like to minimizethe value of BSF as much as possible.It should be noted that optimization of the PSA

process is highly subjective by virtue of the presence ofa large number of process variables and no well-definedalgorithm exists for the same. There always exists a

trade-off between product purity, recovery, and the BSF.In this case, we attempted to extract the minimumpossible BSF, keeping the product purity and recoverywithin tolerable limits. The BSF value may be arbi-trarily optimized by manipulating the following operat-ing conditions: (1) time duration of each step; (2) feedthroughput (kg of feed/h/kg of sorbent); (3) purge to feedratio (P/F); (4) end pressure of cocurrent depressuriza-tion (PCD).It was decided to operate at low cycle times (125-

150 s/cycle) in order to sustain high-feed flow rates. Thefeed throughput was increased to a value just below thebreakthrough threshold. It was found that for the givenconditions (PH ) 1 bar, PL ) 0.5 bar) cocurrent depres-surization pressure (PCD) was optimally located between0.80 and 0.85 bar. A further decrease in PCD resultedin breakthrough during step (3). Next the purge to feedratio (P/F) was adjusted to provide an minimum BSFvalue.It can be seen from Figure 12 that as the BSF value

falls, the product purity is initially above 90%, but thereis a drastic fall just below the BSF value of 18 kg ofadsorbent/kg of O2/h. Similarly, the product recoveryincreases drastically below this BSF value. Thus, theoptimal BSF value for the given LiX adsorbent bedsystem is approximately 18 kg of adsorbent/kg of O2/hunder the process conditions mentioned above.

Conclusions

PSA simulations were made to compare the perfor-mance of LiX (Al/Si ) 1 with 100% Li+) and NaX (13X)adsorbents. A study of the dependence of O2 productrecovery on the pressure ratio at a fixed product purityand product throughput reveals that the LiX sorbentgives higher recovery for PH:PL from 1 to 10. By properchoice of operating conditions it is possible to operate aPSA system with the LiX adsorbent at a pressure ratioof 2 with high product purity, recovery, and throughput.

Table 4. PSA Bed Characteristics and Process Performance in the Case of Different vol % of Iron Particle Addition (PH) 1.0 atm, PL ) 0.1 atm, PCD ) 0.60-0.65 atm)

vol % ironparticles

effective specificheat Cps (cal/g/K)

adsorbent beddensity Fb (kg/m3)

feed velocityUH (m/s)

purge velocityUL (m/s)

O2 productpurity (%)

O2 productrecovery (%)

BSF (kg of adsorbent/kg of O2 produced/h)

0 0.28 720 0.28 0.04 95.3 64.7 36.75 0.38 684 0.23 0.05 95.1 66.3 36.210 0.40 648 0.22 0.05 95.3 65.5 36.320 0.56 576 0.19 0.04 95.2 62.6 36.630 0.75 504 0.14 0.04 95.2 62.5 36.3

Figure 12. O2 product purity (%) and O2 product recovery (%) vsbed size factor (BSF) for pressure ratio ) 2 with LiX (Si/Al)1,100% Li+) adsorbent.

5364 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

The performance of the LiX sorbent is considerablybetter than that of the NaX sorbent as seen from theproduct recovery versus pressure ratio curve. The effectof the P/F ratio on product purity and recovery wasstudied and found to be as expected. The Bed SizeFactor (BSF) is an important parameter for PSA per-formance evaluation. In this work, the lowest optimalBSF value for the LiX adsorbent at a pressure ratio of2.0 was determined to be about 18 kg of adsorbent/kgof O2/h.The temperature excursions in the LiX beds were

approximately 4 times higher than that in the NaXbeds, due to the combined result of higher N2 heat ofadsorption and a shorter bed utilization in the LiXsystem. One method, namely the addition of high heatcapacity inerts, was used in the case of the LiX zeolitein order to overcome the large temperature effects. Themethod was successful in reducing the temperaturedeviations during adsorption and desorption, with animprovement in product recovery of 2%. It was alsoobserved that there was an optimum for the volume ofinert addition (in the present case 5-10% by volume),beyond which a drop in product recovery was observed.

Acknowledgment

We are grateful to Praxair, Inc. for donating the Xzeolite sample (with Si/Al ) 1) and Nick Hutson forperforming Li+ ion exchange and isotherm measure-ments. This work was supported by the NSF underGrant CTS-9520328.

Nomenclature

B ) Langmuir parameter (atm-1)BSF ) bed size factor (kg of adsorbent/kg of O2 produced/h)

Cp ) specific heat (kcal/g/K)Dax ) axial dispersion coefficient in adsorbent particles (m2/s)

∆H ) heat of adsorption (kcal/mol)K ) Langmuir parameter (mmol/g/atm)k1 ) Langmuir temperature dependence constant (mmol/g/atm)

k2, k4 ) Langmuir temperature dependence constant (K)k3 ) Langmuir temperature dependence constant (atm-1)L ) total length of the adsorption bed (m)P ) total pressure (bar)Pem ) mass Peclet numberPeh ) thermal Peclet numberP/F ) purge to feed ratioqj ) volume-averaged adsorbed amount (mmol/g)R ) gas constant (kcal/mmol/K)t ) time (s)T ) temperature (K)u ) interstitial gas velocity (m/s)y ) mole fraction of the components in the gas phasez ) axial coordinate in the bed (m)

Greek Letters

R ) thermal dispersion coefficient (m2/s)ε ) void fraction of the packingεt ) bed void fraction including macropores in particlesλL ) effective thermal conductivity (W/(m‚K))Fb ) bed density (kg/m3)Fg ) gas phase density (kg/m3)

τ ) time duration of process step (s)

Subscripts

CD ) intermediate pressure corresponding to the cocurrentdepressurization step

co ) cocurrent blowdown stepcn ) countercurrent blowdown stepg ) gas phaseH ) corresponding to the feed stepi ) species ij, k ) gas phase component indexL ) low pressure corresponding to purge stepp ) purge stepP ) pressurization steps ) solid phase

Superscripts

* ) at equilibrium

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Received for review July 14, 1997Revised manuscript received September 15, 1997

Accepted September 15, 1997X

IE9705214

X Abstract published in Advance ACS Abstracts, November1, 1997.

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5365