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Ad s o r p t i o n o f G a s e s a n d Va p o r s o n C a rb o n M o l e c u l a r
S i e v e s
I. P. Okoye, M. Benham , and K. M. Thomas*,
Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building,University of Newcastle upon T yne, Newcastle upon T yne N E1 7R U, U.K., and
Hiden Analytical Ltd ., 420 Europa Boulevard, Warrington, WA5 5UN , U.K.
Received October 14, 1996. In Final Form: Jan uary 21, 1997X
The adsorption phenomena of oxygen and nitrogen on a carbon molecular sieve were studied above thecritical temperature ofthe adsorptives as a function ofpressure in order to understand further t he mechanismof air separ ation. The upta ke of both gases studied was virtually linear a t low equilibrium pr essures, inagreement with Henrys law, but deviation occurred at higher pressur es. The adsorption kinetics werestudied with different a mounts of preadsorbed gas for changes in pressur e of 11 kPa and pa rtial pressur ein helium of10 kPa. The gas adsorption kinetics obey a linear driving force mass tr ansfer model. Therat ios of the r ate constan ts (kO2/kN 2) for each pressur e increment were 35-43 for pure gases an d 21-27for gas/helium mixtures, an d th ese ratios clearly demonstr ate t he m olecular sieving char acteristics. Thepresence of water vapor is detr imental t o the operation of carbon m olecular sieves. The adsorption an ddesorption char acteristics of water vapor with different amounts of preadsorbed wat er were stu died forcomparison with oxygen and nitrogen adsorption over the pressure range 0-1.8 kPa for pressure stepsof 0.1 kPa. The resu lts ar e discussed in ter ms of th e mecha nism of gas separ at ion usin g car bon molecularsieves.
I n t r o d u c t i o n
The use of carbon molecular sieves in the separationand purification of mixtures of gases with very similarmolecular dimensions is of great interest in the chemicaland petrochemical indust ries. Awide ra nge ofcommer cialcarbon molecular sieves (CMS) have been manufacturedbyvaryingthetypeofprecursors,method and temperatureof carbonization, a ctivation procedure, pore blockingmethod, and passivation techn iques. These carbon mo-lecular sieves can be prepared from coal, petroleum ,biomass, a nd polymericpr ecursors 1,2 and a re used widelyfor gas separat ion 3 and storage4 applications. A typicalapplication is the indu str ial separa tion of air int o oxygen
and n itrogen by pressure swing adsorption (PSA). Thecapacities of this class of molecular sieves for oxygen andnitrogen adsorption are very sim ilar, but the rates of adsorption differ considerably. The P SA technique isbased on the difference between the kin etics of adsorpt ionofoxygen and n itrogen with oxygen a dsorption being muchfa s t e r t h a n n i t r og en a d s or p t ion . T h is d iffe r en ce i nadsorption kinetics is thought to be related to molecularsize. The kinetic diam eter of oxygen (0.346 nm ) is slightlysma ller th an th at of nit rogen (0.364 nm ). When a car bonsam ple with m olecular sieving cha ra cteristics comes intocontact with air, an oxygen-enriched adsorbed phase anda corresponding nitrogen-rich gas phase are producedinitially.
The pr esent study involved a n investigation of th e
kinetics of oxygen and nitrogen a dsorption on a carbonmolecular sieve with various amoun ts ofpr eadsorbed gasfor a series of pressu re steps . The effects of th e presenceof helium gas on th e adsorption kinetics and capacity ofthese gases were investigated. The presence of watervapor in the air is very detrimental to the performanceof carbon m olecular sieve m a terials. Therefore the
adsorption of water vapor was also investigated forcomparison and to understand the mechanism by whichwater vapor interferes with the separation process.
E x p e r i m e n t a l S e c t i o n
M a t e r i a l s U s e d . The commercial carbon molecular sieve(CMS) used in the present study was supplied by Air Productsand Chemicals Inc., U.S. The CMS was prepared by carbondeposition on a microporous substr at e. Helium, nitrogen, andoxygen (99.99% purity), supplied by BOC Ltd, were dried bypassage through drying tubes containing activated silica gels.
M e a s u r e m e n t o f A d s o r p t i o n K i n e t i c s . The kinetic mea-surements were carried out using the Intelligent Gravimetric
Analyser (IGA) supplied by Hiden Analytical Lt d. The IGAinstr umen t allows the adsorption-desorption isother ms and th ecorresponding kinetics of adsorption or desorption at eachpressure step t o be determined.5 The system consists of a fullycomputerized microbalance which automatically measures theweight of the carbon sample as a function of time with the gaspressure and sample temperature un der computer control. Thecarbon sample was outgassed to a const ant weight at 383 K and10-8 Pa prior to measurement of the isotherms. The pressurean d t em p erat u re were t h en s et t o t h e d es i red v al u e u n d ercomputer control, and the weight uptake was measured as afunction of time under isothermal conditions until equilibriumwas at tained. The approach to equilibrium was m onitored inreal time, an d a comput er algorithm was u sed to esta blish when99% gas uptake was achieved. These weight versus time datawere used to calculat e the adsorption kinetic param eters . After
equilibrium was achieved, the pressure was increased to thenext desired value and the weight versus time monitored. Theprocess was repeated un til sufficient a dsorption dat a points wereobtained for t he isotherm. In t his technique th e adsorptionkinetics for a given pressure step were measured for differentamounts of preadsorbed gases a dsorbed at equilibrium on thecarbon adsorbent. Asimilar pr ocedure was used for th e nitrogen/helium a nd oxygen/helium mixtures. In th is case the par tialpressure of the gas was increased in increments similar t o thepure gases. The total flow ra te used thr oughout the experimentswas 100 cm 3 m in-1. The adsorption and desorpt ion data for watervapor were obtained in a similar m ann er by first increasing an dt h en d ecreas in g t h e p res su re i n i n crem en t al s t ep s . Kin et i cmeasurements were obtained for oxygen, nitrogen, an d wat er
* Corresponding author. University of Newcastle upon Tyne. Hiden Analytical Ltd.X Abstract published in Advan ce ACS Abstracts, J une 15, 1997.(1) Metcalf, J. E.; Kawahata, M.; Walker, P. L. Fuel 1963, 42 , 233.(2) Moore, S . V.; Trimm, D. L. Carbon 1977, 15 , 177.(3) Sircar, S.; Golden, T. C.; Rao, M. B. Carbon 1996, 34 , 1.(4) Verma, S. K.; Walker, P. L. Carbon 1992 , 30 , 837. (5) Benh a m , M. J .; Ross, D. K. Z. Phys. Chem. 1989, 25 , 163.
4054 La n g mu i r 1997, 13 , 4054-4059
S0743-7463(96)01040-2 CCC: $14.00 1997 American Ch emical Society
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vapor a t 293K only. The isotherms are typically repeatable tob et t er t h a n (1%.
R e s u l t s
The carbon dioxide adsorption isotherm at 273 K forth e car bon molecular sieve was Type 1. The sur face ar eacalculated from the Langmuir isotherm gave a surfacearea of242 m 2 g-1 (based on an area of 1.7 10-19 m 2
per molecule) while the micropore volume obtained byextrapolation of th e Dubinin-Radush kevich equat ion was0.152 cm 3 g-1.
Adsorption isotherms of nitrogen and oxygen at 293 Kdeterm ined using the IGA are shown in F igures 1a and1b, respectively. There is litt le difference in th e isotherm sfor oxygen a nd n itrogen a nd t he gases in t he pr esence ofhelium. The upt akes ofboth gases as a function of pressure
are approximately linear at low pressures, but deviationsfrom linearit y are observed as pressur e increases. Theadsorption temperatu re used in this study was above thecritical temperatures of both nitrogen and oxygen, andtherefore, it is not possible to express the pressures int e r m s o f r e l a t i v e p r e s s u r e s i n c e t h e s a t u r a t e d v a p o rpressure (Po) does not exist under the aforem entionedconditions. The virial plots for nitrogen and oxygenadsorption are shown in Figures 2 and 3, respectively.These graphs ar e linear over a ma jor par t of the pr essureran ge but deviate a t low-pressure where Henr ys law isobeyed. In the low-pressure region of th e a dsorptionisother m, small errors pr oduce large errors in the virialplots. Good agreemen t was obta ined for the values ofA 0a n d A 1 for the adsorption of nitrogen gas and nitrogen in
h e li u m, a n d t h e v alu e s a r e g ive n i n T ab le 1 . T h eagreement for oxygen adsorption virial parameter wasless satisfactory (see Table 1). This is due to a higherdegree of scatter in the data points for oxygen/heliumm i xt u r e s . T h e p a r a m e t e r s o bt a i n ed fr om t h e v ir i a lequation graph for n itrogen a re similar to th ose obtainedpreviously.6 The isotherm s calculated from the virialcoefficients for nitrogen and nitrogen/helium mixtures areshown in Figure 1a while the isotherms calculated foroxygen and oxygen/helium mixtures are shown in Figure1b. It is appa ren t tha t in all cases th ere is good agreementbetween the isoth erms calculat ed from the virial equat ionparameters an d th e experimentally determined isotherms.Figure 4 shows the graph of nitr ogen an d oxygen u ptak eversus time. Compar ison of Figures 1 and 4 shows thatth e adsorption capacities for oxygen an d nitr ogen ar e verysimilar while the rates ofadsorption differ markedly. Thedifferences in the rate constants for the adsorption of pure gases an d corresponding gas/helium mixtures for agiven pr essure in crement are comparat ively small com-pared to the differences in the rat es ofoxygen and nitr ogen.Graphs of ln(1 - w t/w e) against t im e t where w t a n d w e
(6) Cole, J. H.; Everet t, D. H .; Marsha ll, C. T.; Paniego, A. R.; Powl,J. C.; Rodriguez-Reinoso, F . J. Ch em. Soc., Faraday Trans. 1 1974, 70 ,2154.
Figure 1. (a) Adsorpt ion isother ms for nitr ogen on th e car bonmolecular sieve; (9) N 2, (b) N 2 /He; determined at 293 K.Isotherm fitting using virial equation parameters in Table 1;s N 2, - - - N 2 /He. (b) Adsorption isotherms for oxygen on thecarbon molecular sieve; (9) O2, (b) O2 /He; determined at 293K. Isotherm fitting using virial equation parameters in Table1, s O2, - - - O2/He.
F i g u re 2 . Virial plots for nitrogen adsorption on carbonmolecular sieve at 293 K; (9) N 2, (b) N 2/He.
F i g u r e 3 . Virial plots for oxygen adsorption on the carbonmolecular sieve at 293 K; (9) O2, (b) O2/He.
T a b l e 1 . V i ri a l C o n s t a n t s f o r Ad s o r p t i o n o f O x y g e n a n d
N i t r o g e n o n t h e C M S
A 1/g mol-1 exp(A 0) 10 9/mol g-1 P a-1
N 2 -921 ( 25 4.109 ( 0.002N 2/He -928 ( 27 3.909 ( 0.002O2 -736 ( 7 3.756 ( 0.001O2/He -1015 ( 116 3.99 ( 0.01
Ad sorption of Gases an d V apors on CM S L an gm uir, V ol. 13, N o. 15, 1997 4055http://dontstartme.literatumonline.com/action/showImage?doi=10.1021/la961040c&iName=master.img-000.png&w=234&h=321
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refer to the adsorbate weight uptake at t im e t a n d a tequilibrium, respectively, for the uptake of both gases at293 K are shown in Figur e 5. It is appar ent tha t the gasuptakes for both oxygen and nitrogen on this class of molecular sieve follow a linear driving force mass transferkinetic model already discussed elsewhere,7 where w t/w e) 1 - e-kt, with linear graphs of ln(1 - w t/w e) againsttim e. The rate of nitrogen and oxygen upta ke can becompa red in terms ofth e pseudo-first -order ra te consta nt ,k, which is determined from the gradient of the kineticplot as shown in Figure 5. The full kineticdat a presentedin Table 2 indicate t hat th e rat e of upt ake of nitr ogen ism uch slower and involves longer equilibrium tim escompared with th at of oxygen. Compar ison of th e rateconstants for gases and the gas m ixtures in helium iscomplicated by th e slightly different pressu re increment s.However, th e rat e consta nt s for both n itr ogen an d oxygen
adsorption for a given pressure increment increase slightlywith increasing initial pressure both for pure gases andfor mixtures with h elium. The kinetic selectivity for thecarbon molecula r sieves can be obtained from th e rat io ofthe rate constants for nitrogen and oxygen (see Table 2).It is apparent that the ratio decreases with increasingpressure.
Figure 6 sh ows th e adsorption-desorption isotherm ofwater determ ined at 293 K. I t is evident t hat the waterisotherm is Type V an d rema rk ably different from tha t ofnitr ogen or oxygen. This is due to th e different m echan ismofads orption which involves initial adsorpt ion on prima ry
centers followed by the growth of clusters of watermolecules ar ound these cent ers. The adsorption-desorp-tion hysteresis is small in these carbon molecular sieves,which is in contrast to adsorption on activated carbonswhich h ave a wider pore size distribution.8 In addition,the steep increase in water vapor uptake around p/p0 0.3-0.4 is lower than for active carbons (p/p0 0.45-0.65)which
have a wider pore size distr ibution. Figure 7 shows graph sof ln(1 - w t/w e) versus time for water vapor uptake forpressu re st eps of (a) 0-100 Pa , (b) 1617-1718 Pa , and (c)91 1-1012 Pa. It is appar ent that th e graphs are stra ightlines for >9 0% of t h e u p t a k e . T h is con fi r m s t h a t t h eadsorption kin etics follow a linear driving force masstra nsfer model for the pressur es steps used. The resultsalso show that the r ate consta nt s differ ma rkedly for thedifferent pr essure increments. Figure 8 shows a graphofra te constant versus water vapor pressure. This graphshows th ree distin ct regions corresponding to specificprocesses in the water adsorpt ion mecha nism. The initial
(7) Chagger, H. K.; Nda ji, F. E.; Sykes, M. L.; Thomas, K. M. Carbon1995, 33 , 1411.
(8) Foley, N.J .; For shaw,P . L.; Thom as,K.M.; St ant on,D.; Nor m an,P. R. La n g mu i r 1997, 13 , 2083.
F i g u r e 4 . Vari at ion of t h e g as u p t ak e w it h t i me for t h eadsorption of nitrogen and oxygen on the carbon molecularsieve at 293 K. Pressure ran ges: (a) pure gases, 55-66 kPa;(b) gas/helium mixtures, N 2 40.3-50.2 kPa, O2 41.2-48.9 kPa .
F i g u r e 5 . Variation of ln(1 - w t/w e) ag ai n s t t i me fo r t h eadsorption of N 2 an d O2 on the car bon m olecular sieve at 293K. Pressure range: 55-66 kPa.
T a b l e 2 . K i n e t i c D a t a fo r N i t r o g e n a n d O x y g e n G a sA d s o r p t i o n o n C a r b o n M o l e c u l a r S i e v e D e t e r m i n e d
a t 2 9 3 Ka
(a) Pure Gases
pure N 2(k/s 10-4)
pure O2(k/s 10-4) kO2/kN 2
0-11 2.143 ( 0.011 83.5 ( 0.7 39.011-22 2.20 ( 0.01 88.9 ( 0.9 40.422-33 2.325 ( 0.009 99.5 ( 1.0 42.833-...