The adsorption of gas mixtures by solids

Surface Technology, 7 (1978) 165 - 186 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

165

Review Paper

THE ADSORPTION OF GAS MIXTURES BY SOLIDS

P. G. HALL and S. A. MIJLLER Department of Chemistry, University of Exeter, Devon (Gt. Britain) (Received November 24, 1977)

Summary The physical adsorption of gas and/or vapour mixtures by solids is reviewed; the survey covers the period after 1959 and presents an overall picture, although the main emphasis is on the experimental aspects. The experimental work, which tends to be far more complex and time consuming than its single gas counterpart, is classified into two distinct groups: static and flow methods. The various systems which have been investigated are tabulated. Examples of results obtained are discussed, together with such theoretical interpretations as the application of ideal adsorbed solution theory.

1. Introduction The physical adsorption of pure gases and vapours by solids has been studied extensively [1, 2] ;however, data on adsorption from gas mixtures are comparatively rare. The first experiments involving gas mixtures were cartied out in 1863 [3]. A comprehensive review of the period up to 1959 has been published by Young and Crowell [4]. Since then reviews in the literature have been few and far between and deal primarily with the various theoretical aspects of mixed gas adsorption [ 5 - 8]. For example, Brown and Everett [9] have discussed the relationship between adsorption from liquid mixtures and adsorption from gas mixtures (the former can be regarded as the limiting case of mixed gas adsorption when the total pressure in the vapour approaches the saturated vapour pressure of the equilibrium vapour phase), and Pierotti and Thomas [10] have discussed the virial approach to mixed gas adsorption. A text by Ponec et al., published in English translation in 1974 [11], includes sections on some of the experimental and theoretical aspects, but little work other than that previously reviewed by Young and Crowell. No recent compilation of experimental data apparently

166

exists. The present survey therefore covers the period after 1959 and is intended to give an overall picture of the work in this field up to the present day, with emphasis on the experimental rather than the theoretical side. Interest in mixed gas adsorption has increased rapidly as industrial and environmental needs and applications have either been recognised or become more pressing. Such studies are of importance for example in pollution control [ 12 ], air purification [ 13, 14], and catalysis [ 15]. However, the experimental work required tends to be far more complex and time consuming than its single gas counterpart. The number of variables involved increases by two on going from single (where the variables are temperature, pressure, a m o u n t adsorbed) to binary adsorptive systems. The additional requirement for full characterisation of the binary systems is a knowledge of both the equilibrium gas and adsorbed phase compositions. The number of variables naturally increases with the number of gaseous components and it is not surprising that only a few m u l t i c o m p o n e n t systems have been studied. Systems which have been studied since 1959 are listed in Table 1.

2. Experimental methods Results published in the literature are frequently of only limited value because of imprecise definitions of the parameters characterising experimental conditions [70, 53]. Where the initial mixture composition has been kept constant, in static techniques, it is often unclear from the data given how gas phase compositions were affected by adsorption [54, 55]. Because of the increased number of variables, a wide range of experimental approaches is possible and additional difficulties arise from the variety of graphical methods employed to represent experimental data. Comparison of systems thus becomes complicated and complete understanding is hindered. For binary mixed gas adsorption systems, three of the five variables may be depicted unambiguously on a three-dimensional plot, the data being represented as a surface, in contrast to the curve obtained in two dimensions for single gas adsorption [4]. One of the remaining two variables is held constant; any variation in the fifth cannot be depicted. Experimentally, temperature is most frequently held constant. However, the variation of adsorption with temperature has also been investigated other than by the determination of a series of isotherms for the systems concerned [77, 86]. In some, but not all experimental techniques, a second variable is held constant on isotherm determination, e.g. total equilibrium pressure [73] or equilibrium gas phase composition [22, 40]. In practice, results are presented in terms of a series of two-dimensional plots for which the other variables are either defined or not considered. Thus there is a wide choice of graphical representations. The main techniques which have been used for the determination of adsorption isotherms for mixed adsorptive systems may be classified into two distinct groups: static and flow. Both may be further subdivided into

167 TABLE 1 The physical adsorption o f gas mixtures Adsorptive

Adsorbent

Temperature

Ref.

0, 25, 50 °C 20 °C 25 °C Room temp. 20 °C 0, 25, 50 °C 20 °C 0, 10, 20, 30, 40 °C 25 °C 30 °C 0, 25, 50 °C 25 °C 40, 60, 70 °C. 100 °F 100 °F 100 °F 25 °C

16 17 18 a'b 19 17 16 20 21 22 23 16 22 23 24, 25 24, 25 24, 25 22

BINARY ADSORPTIVE SYSTEMS

Hydrocarbonhydrocarbon CH 4 + C2H 4

+ C2H6 + C3H6 + C3H8

+ n-C4H10

+ n-C5H12 + n-C6 H14

Activated alumina Active carbon Activated charcoal Silica gel Active carbon Activated alumina Silica gel Silica gel Activated Charcoal Activated Activated Charcoal Silica gel Silica gel Silica gel Activated

carbon alumina carbon

carbon

C2H 2 + C2H 4 + CH3C~CH + HC'C.C'H + H2C:CH.C~CH

Activated charcoal Zeolite Zeolite Zeolite

25 °C 20~ 40 °C 20, 40 °C 20, 40 °C

18 a'b 26 26 26

C2H4 + C2H6

Graphitised carbon black Zeolite Active carbon Activated charcoal Active carbon Activated alumina Zeolite Activated alumina Zeolite

25 °C

27

25 °C 20, 60 °C 25 °C 20 °C 0, 25, 50 °C 25 °C 0, 25, 50 °C Unspecified (abstr.)

18 a 17 18 a'b 17 16 18 a 16 28

30, 40, 50 °C 25 °C

29 27

20, 60 °C 0 °C 0 °C 35 °C

17 30 30 31

+ C3H6

+ C3H8

C2H6 + C3H6 + C3H8

+ n-C4HI0

Charcoal Graphitised carbon black Active carbon Zeolite (5A) Zeolite (5A) Linde molecular sieve (5A)

(continued overleaf)

168 T A B L E 1 (continued) Adsorptive

Adsorbent

Temperature

Ref.

C3H 6 + C3H 8

Graphitised carbon black Active c a r b o n Activated charcoal CaX zeolite

25 °C

2"1

20 °C 25 °C 25 °C

17 18 a'b 32

C3H 8 + n-C4H10

Active c a r b o n Activated charcoal Zeolite ( 5 A ) C a A zeolite

20 °C 25 °C 0 °C 2 0 , 1 1 0 °C

17 18 a'b 30 33

n-C5H12 + n-C6H14

CaA zeolite Porapak Q C h r o m o s o r b W NAW Silica Linde molecular sieve 5 A

110 °C 100 °C 100 °C 6 0 , 1 0 0 °C 300 °C

33 34 34 34 35

Cab-O-Sil Cab-O~Sil Graphon C h r o m o s o r b W NAW Silica N a X zeolite Silica gel Activated carbon

30 0C 20, 30, 40 °C 20, 30, 40 °C 100 °C 100 °C 85 °C 70 - 130 °C Unspecified (abstr.)

36 37 38 34 34 39 4 0 , 41 42

Silica gel Active c a r b o n

U n s p e c i f i e d (abstr.) U n s p e c i f i e d (ref.)

43 44

N a X zeolite C a A zeolite Zeolite C a A zeolite Zeolite CaA, C a X a n d N a X zeolites

120 °C 85 - 140 °C 200 °C 110 °C 200 °C 100 °C

45 33 46 33 46 47

Zeolite Zeolite Zeolite N a C a A zeolites

200 250 250 250

46 46 46 48

N a X zeolite

30 °C

+ n-CTH16

C6H 6 + C6H12 (cyclohexane) + n-C6H16

+ CTH 6 (toluene) + n-C7H16

+ C12H21 (1,3,5 Triethyl-benzene) n-C6H12 + n-C6H14 n-C6H14 + n-C7H14 + n-C7HI6 n-C7HI6 + n-CsH18 + CH3(CH2)5:CH 2

n-CsHI8 + n-CgH20 n-C9H20 + n-CIoH22 n-CloH22 + n-CIIH24 n-CI2H26 + n-C14H30

°C °C °C - 360 °C

Hydrocarbon + nonhydrocarbon

C3H8 + C2H5CI

7, 49

(continued on facing page)

169 TABLE 1 (continued) Adsorptive

Adsorbent

Temperature

Ref.

C6H 6 + CH3OH + C2H50H

Vycor glass Cab-O-Sil Cab-O-Sil Graphon Activated carbons Porapak Q Glass beads Chromosorb W NAW Silica Activated carbons Activated carbons

15, 25, 30 °C 20, 30, 20, 30, 10, 30, 100 °C 100 °C 100 °C 100 °C 10, 20, 30 °C

50 36 37 38 51 c 34 34 34 34 51 c 51 c

Cab-O-Sil

20, 30, 40 °C

37

Graphon

20, 30, 40 °C

38

n-C6H14 + (CH3)2CO

Chromosorb W NAW Silica

100 °C 100 °C

34 34

C7H 9 + C2H5OH (Toluene) + iso-C3H7OH + n-C4H9OH

Activated carbons

30 °C

51

Activated carbons Activated carbons

10, 20, 30 °C 30 °C

51 c 51

C8H12 + C2H5OH (p-Xylene) + iso-C 3 HTOH + n-C4H9OH

Activated carbons

30 °C

51

Activated carbons Activated carbons

30 °C 30 °C

51 c 51

Activated charcoal Graphitised carbon black Graphitised carbon black Glass beads Chromosorb W NAW

Unspecified (abstr.) 298 K

52 53, 54

298 K

55

120 °C 120 °C

34 34

Activated charcoal Graphitised carbon black Graphitised carbon black Graphitised soot Silica gel

Unspecified (abstr.) 30, 50 °C

52 56

30, 50 °C

57

30, 50 °C 56 °C

58 59

298 K

55

+ (CH3)2CO

+ iso-C3H7OH + n-C4H9OH C6H12 + C2H5OH (Cyclohexane) + C2H5OH

35 °C 40 °C 40 °C 50 °C

30 °C

Non-hydrocarbon + non-hydrocarbon CH3OH + CHCl 3 + CH3CN + C2HsNH 2 + (CH3)2CO

CHCI 3 + C2H5OH + (CH3)2CO

+ CC14 C2H5OH + CH3CN

Graphitised carbon black

(continued overleaf)

170 T A B L E 1 (continued) Adsorptive

Adsorbent

Temperature

Ref.

Graphitised carbon black CaA zeolite (5A)

298 K

55

85, 110, 140 °C

33

C2HsCI + (C2H5)20

Activated charcoal

50, 71 °C

60

n-C3H7OH + CH3CN

Graphitised carbon black Graphitised c a r b o n black

298 K

55

298 K

55

Activated Activated Activated Molecular

212 - 310 K U n s p e c i f i e d (ref.) U n s p e c i f i e d (ref.) 123 - 295 K

61 19 19 d'e 62

+ C2H5NH 2 + n-C3HTOH

+ C2HsNH2

Organic-inorganic mix tures CH 4 + CO 2 + N2

carbon carbon carbon sieve

CH3OH + H 2 0

C h r o m o s o r b W NAW

120 °C

34

C2H 4 + CO + CO 2

Zeolite Active c a r b o n Zeolite A c t i v a t e d alumina Graphitised carbon black

25 °C 20 °C 25 °C 0, 20, 50 °C 25 °C

18 a 17 18 a 16 63

C2H 6 + CO 2

Linde molecular sieve (5A) Silica gel

35 °C

31

U n s p e c i f i e d (ref.)

19d, e

C2H5OH + H 2 0

Glass b e a d s C h r o m o s o r b W NAW

120 °C 120 °C

34 34

C2H5Cl + H 2 0 C3H 6 + CO 2 C3H 6 + 0 2 C3H 8 + CO 2

Active charcoal Activated alumina Stannic o x i d e Activated alumina NaY zeolite

75 °C 0, 25, 50 °C 74 - 500 °C 0, 25, 50 °C U n s p e c i f i e d (abstr.)

64, 65 16 66 h 16 67

C3H 8 + H2S C3H 8 + N 2 HCON(CH3) 2 + H 2 0

NaX zeolite A c t i v a t e d charcoal Active charcoal

25, 40 °C Unspec. (ref.) 40, 60, 80 °C

68 69 60

n-C4H10 + H2S

CaA zeolite

0 - 75 °C

70

C6H 6 + H 2 0 n-C6H12 + H 2 0

C h r o m o s o r b W NAW Silica

100 °C 100 °C

34 34

+N 2

(continued on facing page)

171 T A B L E 1 (continued) Adsorptive

Adsorbent

Temperature

Ref.

n-CTH16 + CCI 4 n-CTH16 + H 2 0

Tripolite Silica gel

25, 45 °C 75 °C

71 72

Ar + N 2

Silica gel NaX zeolite

--78.5 °C 140,160 K

73 74, 75

CO + H 2

Zinc o x i d e Gallium arsenide Zinc selenide Silica gel P y r e x glass Linde m o l e c u l a r sieves 5A and 10A Copper(I) bromide film Gallium arsenide Zinc selenide H y d r o u s ferric oxide P y r e x glass Linde m o l e c u l a r sieves 5A and 10A

24.6 °C - - 1 8 6 - 400 °C - - 1 8 6 - 300 °C 120 - 180 K 77.4 K - - 2 0 0 °F

76 77!, 78 i 771 ' 78 i 79d, f 8O 81, 82

- - 1 9 6 to 200 °C

83

- - 1 8 0 - 300 °C - - 1 8 0 - 300 °C 100 °C

84 i 78 i 85

77.4 K - - 2 0 0 °F

8O 81, 82

Zinc selenide films Silica gel Linde m o l e c u l a r sieve 5A Silica gel Carbolac c a r b o n Silica gel Silica gel Activated c a r b o n Aluminium oxide (two types)

- - 1 9 6 - 300 °C

86 i

120 - 180 K 70 °F

79 87

140 - 160 K 273 K 100 - 180 K 140 - 160 K Room temp. 173 K

89 9O 88 19d, e

CS 2 + H2S

Carbon (carbosorbid)

Unspecified (abstr.)

93

D2 + H2

Two charcoals Silica gel Linde m o l e c u l a r sieves 4A, 5 A and 13X

75, 90 K 75, 90 K

94 94

75, 90 K

94

Gallium arsenide

--183 - 375 °C

95 i

Inorganic--inorganic mix tu res

+ Kr + N2 + 02

CO 2 + H 2

+ H2S + He + Kr + N2

+ Xe

H2 + 02

88d, f

91, 92

(continued overleaf)

172 TABLE 1

(continued)

Adsorptive

Adsorbent

Temperature

Ref.

Iron catalyst Silica gel

50 °C 100 - 180 K

15 i 96d, f

He + N 2 + Xe

Activated c a r b o n Silica gel

100 - 150 K 100 - 180 K

97 96d, f

Kr + N 2 + Xe

Carbolac c a r b o n P y r e x glass

273 K 77.4 K

89 8O

N2 + 0 2

Molecular sieves Linde m o l e c u l a r sieve 5A Linde m o l e c u l a r sieves 5A a n d 10X NaX zeolite

25 °C 10 - 50 °C

98 99

- - 2 0 0 °F

81,82

30 °C

100 j

- - 1 2 0 - 0 °C --117 - 20 °C

101 102d, g

+ N2 + Xe

02 + 03

Silica gel Silica gel

M U L T I C O M P O N E N T A D S O R P T I V E SYSTEMS CH 4 + n-C4H10 + n-C5H12 CH 4 + n-CsH12 + n-C6H14 C3H 6 + CH2:CHCHO + 0 2

Silica gel Silica gel Stannic o x i d e

C6H 6 + n-C6H14 + H 2 0 CO + H 2 + NH 3

Zeolite Chromium oxide

D 2 + HD + H 2

Zeolites Zeolites Synthetic mordenite

100°F

34

100°F

25,34

100-500°C Unspecified(abstr.)

66 103 k

75°C 62.0,77.6,90.4 K 35-112 K 48-62 K

104 1051 1061 1071

CH 4 + n-C4Hlo + n-C5H12 + + n-C6H14

Silica gel

100°F

34

Unspecified M u l t i c o m p o n e n t (abstr.) containing mixtures of CH4, C2H6, C3H8, isoC4H10, n-C4H10

Charcoal

30-120°C

108

Activated c a r b o n

75 - 300 °F, e x a c t temps, unspecified

109

Carbon Zeolites

20 °C 77.5 K

110 1111

M u l t i c o m p o n e n t (abstr.) containing mixtures of light h y d r o c a r b o n s

CO 2 f r o m d r y and m o i s t air C o m p o n e n t s o f air

Please see o p p o s i t e page for Table f o o t n o t e s .

173

Systems containing three gaseous c o m p o n e n t s , the adsorption of one o f which was considered to be negligible (e.g. the carrier gas in flow techniques), have been r e p o r t e d as binary mixtures. T e m p e r a t u r e s have been q u o t e d in the units given in the original reference. aSeparation factors only given. bNot studied in this reference but reported as studied in ref. 32. However, this was found not to be so. CAlthough results are presented in terms of a binary vapour mixture in this reference, (binary vapour}-air mixtures were apparently used in all systems e x c e p t the C6H 6 +

dC2H 5 O H system at 10 °C and 50 °C. Only the adsorption of the more strongly adsorbed component was considered. e Could be considered as a single component isotherm determination technique. f Adsorption of H2 or He disregarded. gAdsorption of 02 disregarded.

h • R e f e r e n c e deals mainly with adsorption from a ternary m i x t u r e with acrolein. 1.Mainly c o n c e r n e d with chemisorption. 3 A t e c h n i q u e is described for the study of this m i x t u r e ; no experimental results are given. kExact p r o c e d u r e uncertain f r o m abstract. l Only selectivity determined.

several different experimental methods, the merits of which depend on the particular information required. There are three static methods: (1) gravimetric, involving the use of McBain springs [50] or electrobalances [27] ; (2) volumetric [23, 34, 36, 54, 66] ; and (3) combined gravimetric-volumetric. Bering and Serpinskii [64, 112] first suggested a combined gravimetric-volumetric technique and have been its prime users. With such a m e t h o d t h e y have studied the adsorption of binary gas mixtures of ethyl chloride and water on two types of activated charcoal at 75 °C [64, 65], hydrogen sulphide and propane on NaX zeolite at 25 and 40 °C [68], ethyl chloride and diethyl ether on activated charcoal at 60 °C [60] and water and n-heptane on silica gel at 75 °C [72]. The combined m e t h o d has also been used by Gamble [113] to study the adsorption of methanol-carbon tetrachloride binary vapour mixtures on futile at 25 °C, by Glessner and Myers [31] to study the adsorption of n-butane-ethane and carbon dioxide-ethane binary gas mixtures on Linde molecular sieve type 5A at 35 °C and by Telipko and Vlasenko [104] to study the adsorption of hydrogen-carbon m o n o x i d e - a m m o n i a ternary gas mixtures on chromium oxide at 75 °C. The technique requires no external analysis for binary adsorptive systems; adsorbate and adsorptive equilibrium compositions may be calculated from the measured total a m o u n t and weight adsorbed. It is in fact the only purely experimental technique that requires no analysis; Friederich and Mullins [27] have reported results for the adsorption of hydrocarbon binary mixtures on carbon black (Sterling FTG-D5) at 25 °C, determined by means of a purely gravimetric m e t h o d for which adsorbed phase compositions were obtained by the iterative solution of a series of equations based on the Gibbs adsorption isotherm. Hall and Mfiller [114] have recently described a combined volumetricgravimetric m e t h o d similar in principle to that of Bering and Serpinskii, but

174

GAS - C IRCULA'T~NG PUMP

i

i II I11 i DB

OA

3

]1

',

II

Fig. 1. Combined gravimetrie-volumetric adsorption apparatus (after Hall and Miiller [114l). in which the mixed adsorption points constituting an isotherm can be determined at the same total equilibrium pressure; the cumulative volumetric errors arising in the Bering and Serpinskii technique are avoided. The main section o f the apparatus is shown in Fig. 1. The essential features are: (a) the gravimetric part, i.e. two helical fused quartz springs suspended one u n d e r n e a t h the o th er inside the adsorption loop and housed in a thermostated water condenser; (b) U-tube m e r cur y m a n o m e t e r (1 - 3) for pressure measurement in the dosing and adsorption sections of the apparatus; (e) the gas dosing and mixing section consisting of two calibrated gas burettes DA and DB; (d) the loop, of calibrated volume, containing the spring and adsorbent housings, two traps containing gold foil (one either side of the spring and adsorbent housings), a gas circulating pump, and a m ercury m a n o m e t e r (3). The volumetric-gravimetric sections of the apparatus were enclosed in an air t h e r m o s t a t maintained at 303.1 ± 0.1 K. The volumes of sections of the apparatus were calibrated to an accuracy of bet t er than ±0.6%, in many eases ±0.1%. Hall and Miiller concluded that the com bi ned vol um et ri egravimetric m e t h o d was suitable for accurate measurements of the adsorption o f gas mixtures at constant total equilibrium pressure, provided the appropriate corrections are made in respect o f non-ideal gas behaviour. Thus, the m e t h o d is limited by the availability of accurate virial coefficient data. This contrasts with single gas adsorption at these pressures for which the non-ideality correction is usually minor or negligible.

175 Use of the ideal, rather than the truncated virial equation of state for b u o y a n c y correction estimation on calculation of the single component adsorption data, led to no noticeable change in the results obtained for any of the systems studied at the pressures used. However, the assumption of an ideal gas phase caused a significant change for all the mixed adsorption systems. In some cases this gave absurd results, i.e. the estimated equilibrium gas phase a m o u n t of one component turned out to be greater than that calculated as initially dosed to the adsorption loop. The adsorbents have not always been exposed directly to a gas mixture in these static methods, but first one gas then the other has been admitted [23, 3 3 , 1 1 5 ] . Results have usually, but not always [23], been checked by reversing the order of gas admission, and also in some cases by use of a preprepared mixture [115]. Practically always, no effect has been observed [33, 68], but cases where such has occurred are known, e.g. the system hydrogen-nitrogen-activated charcoal at 77 K investigated by Stern [ 115]. In all experiments the criteria for equilibrium were constancy of pressure and, where applicable, constancy of adsorbent sample mass; constant adsorptive composition has also been used [62, 94]. In recent work the adsorptive was in most cases circulated over the adsorbent during the experiment [66, 91] ; ref. 50 is an exception to this. A variety of methods exists for the analysis of both gas and desorbed phases; which m e t h o d is selected depends greatly on the particular system under study. For example, refractive index measurement has proved to be a convenient m e t h o d for systems where the gases are in the liquid state under normal conditions [50]. Gas chromatography [24, 30, 66] and to a lesser extent thermal conductivity [29, 75] measurements have often been chosen for the adsorptive analysis necessary in volumetric systems. Thermal conductivity techniques have the advantage that no sampling is required, so that approach to equilibrium may easily be monitored. Other techniques involve volumetric analysis of a gas sample after the chemical reaction of one c o m p o n e n t [115], gas density measurements [91], infrared spectroscopy [54], mass spectroscopy [62], or for binary mixtures containing one paramagnetic component, magneto-magnetic analysis [100]. Isolation of the adsorbent, desorption and analysis of the desorbed phase acts only as a check on internal consistency for volumetric systems [62]. Such a technique has, however, been used to determine the composition of the adsorbed phase in gravimetric methods, e.g. by Reeds and Kammermeyer in the study of the system b e n z e n e - m e t h a n o l - V y c o r glass at 15, 25 and 35 °C [50]. Basmadjian studied the adsorption of hydrogen--deuterium gas mixtures on several adsorbents at 75 and 90 K [94] using a static technique in which the equilibrium gas phase compositions were determined by a thermal conductivity m e t h o d and the total pressure was measured rather than by measuring the a m o u n t adsorbed volumetrically. The a m o u n t of gas desorbed was then assessed volumetrically and an analysis made. This approach is more usually associated with flow techniques, e.g. that used by Markham and Benton [116].

176 With all the static methods, except the gravimetric one where a liquid is used as the adsorptive source, pressure decreases in the system as adsorption occurs. Lasof~ and Nodzehski [91] have reported a modified volumetric technique in which this was avoided; pressure was maintained at the initial dose pressure t h r o u g h o u t a measurement by further addition of the least adsorbed component. They investigated the system xenon-carbon dioxide-alumina (two samples) at 173 K [91, 92]. The total a m o u n t of CO 2 in the apparatus was constant for each point on an isotherm, and the pressure of Xe was varied to give a range of overall compositions and total equilibrium pressures. For one sample, both the partial pressure and the adsorbed a m o u n t of CO2 remained constant throughout, only the adsorption of Xe being affected by the presence of a second component [91]. The adsorption of CO 2 was influenced by the presence of Xe for the other sample [92]. It is probable that this technique could also be usefully employed to study the adsorption of mixtures over a range of compositions at constant total equilibrium pressure, provided that the adsorptive-adsorbent system was insensitive to the order of gas admission. Static methods may be used to investigate the effect of a constant total a m o u n t of one component in the system as noted above (also e.g., in refs. 60, 68, 91). Alternatively a constant composition gas mixture may be added to different total pressures [ 54]. Another approach is to study a range of mixture compositions and let total equilibrium pressure vary [36, 50]. The latter may, in fact, be roughly constant for the system and its variation will produce no noticeable effect [27, 36]. However, such a variation, depending on its magnitude, will lead to increased scatter on plots which are not expressed entirely in terms of mole fractions. Flow techniques may be divided into two classes: those requiring the application of external analysis methods, which for convenience will be termed non-chromatographic, and those based entirely on chromatographic techniques. They may be used to investigate the effect of changing the equilibrium gas phase composition while keeping the total equilibrium pressure constant [81] and vice versa [20]. The non-chromatographic techniques overlap with the chromatographic technique of frontal analysis. In each of these two cases a stream of gas is passed through an adsorbent bed until equilibrium is reached; this is assumed to correspond to equalisation of entrant and effluent gas compositions. Analysis of the effluent gas is generally achieved by thermal conductivity measurement [22, 35, 81, 97]. Both the a m o u n t and composition of the adsorbed phase must then be determined. These may be calculated, in the case of the chromatographic technique, from a knowledge of the entrant gas flow rate and measurements of the change in effluent gas composition with time until equilibrium is attained, i.e. breakthrough volume determination [34, 73]. A carrier gas has sometimes been used in this technique [34, 117]. This was not so, however, in the investigations made by Camp and Canjar [73] into the adsorption of nitrogen and argon on silica gel. A modified frontal analysis method, based on continuous measurement and comparison of the rates at which the gas

177 was fed into the adsorption column and emerged from it, has been well outlined by Schay et al. [117] ;these workers investigated the system carbon dioxide-acetylene-charcoal (Nuxit AL) at 0 and 20 °C and at a total pressure of 1 atm. Breakthrough volumes have also been determined by a non-chromatographic m e t h o d [22]. This was used by Grant and Manes [22] to study the adsorption of binary mixtures of methane with propane, n-butane and nhexane on activated carbon at 25 °C. Effluent gas compositions were monitored by the thermal conductivity, and the breakthrough volume of the heavier component was determined with a wet-test meter. The adsorption of the heavier c o m p o n e n t could therefore be assessed; the adsorption of methane was determined from the weight increase in the adsorbent tube. More conventional non-chromatographic methods necessitate isolation of the adsorbent, desorption and determination of the a m o u n t and composition of the desorbed phase [51]. They have been used to study the adsorption of both substances which are (i) liquids at room temperature [35, 118] and (ii) those which are not [81]. In general, a carrier gas has been employed in both cases. For (i), the a m o u n t adsorbed may easily be determined by weighing [35] and for (ii), for example, by expansion of the desorbed gas into a known volume, accompanied by temperature and pressure measurements [81, 97]. Refractive index measurements [35, 118] ancl chromatography [40] have been used as analysis methods. A modified technique in which the gas mixture was only vented through the adsorbent for a limited time and then the system closed, gas admission stopped, and the resulting mixture recycled through the adsorbent until equilibrium was reached, has been outlined by Danner and Wenzel [81]. The possibility of a significant constant pressure gradient developing along the adsorbent bed, was thus avoided. This is a factor that should be considered in all flow techniques. Care should also be taken to ensure that the gas flow has attained the correct temperature before contact with the adsorbent. This has been ensured in some cases by making the gas path in the thermostat sufficiently long [73]. Gas chromatographic techniques other than those of frontal analysis have, however, been used for the determination of mixed gas adsorption isotherms. The use of development chromatography has been investigated. In this technique, as outlined by Sazonov et al. [69], retention volumes of the peaks arising on the application of a perturbation to a stream of gas mixture passing continuously through an adsorbent bed are determined, and may be related to the adsorption isotherms. Both adsorption systems in which one c o m p o n e n t is strongly adsorbed and the other weakly adsorbed, e.g. propane-nitrogen-activated charcoal (unspecified temperature) [69], and those for which both components are strongly adsorbed, e.g. m e t h a n e ethane-MSM silica gel at room temperature [19], have been investigated with this technique. A similar technique was employed by van der Vlist and van der Meijden [99] to study the adsorption of oxygen-nitrogen mixtures on Linde molecular sieve 5A at temperatures from 10 to 50 °C and at a total pressure of 1 atm.

178 ).0

/

/J

5u

0.S //

// o.6 u_ ×~ / ~,)/

o

-oA~--o7 o~

5"7

O.L,

g

03 D <

0.2

I 02

I 0. t

I 0.6

ADSORBATE

MOLE FRACTION

I 0.8

10

OF C(CH3);

Fig. 2. Adsorption of 2,2-dimethylpropane-n-butane gas mixtures on Vycor glass at 303.1 K and at a total equilibrium pressure of 18.386 +-0.017 kPa.

Adsorption isotherms and equilibrium distributions have been determined chromatographically for the system methane-propane-silica gel at 20 °C [20] by use of radiotracer pulses with the elution technique detailed by Gilmer and Kobayashi [ 119 ]. In the latter technique the retention volume is determined for a small pulse of one c o m p o n e n t added to a column through which a stream of the second is being passed; the pulse component is therefore essentially present at infinite dilution. Although most of the experimental work has concentrated on isotherm determination, various kinetic studies have also been carried out [39, 44, 71, 73, 7 6 , 1 2 0 ] . In particular, K~rger et al. [39] have investigated the adsorption kinetics of the system benzene-hexane-NaX zeolite at 85 °C, and Aharoni and Tompkins [76] the system hydrogen-carbon dioxidezinc oxide at 24.6 °C. The overall mass transfer from gas to adsorbed phase has been studied for the system nitrogen-argon-activated silica gel at --78.5 °C by Camp and Canjar [73] ; movement of molecules from the outside external surface of adsorbent particles to their interstices or pores was found to be rate determining. A chromatographic apparatus for studying the adsorption dynamics of m u l t i c o m p o n e n t mixtures has been described by Dubinin et aL [44]. Apparently, since 1959 differential heats of adsorption have only been directly determined experimentally for the adsorption of methanol and acetonitrile vapour mixtures on graphitised carbon black [ 53] ; details of the microcalorimeter used can be found in ref. 53.

179

3. Results and theoretical interpretation Mixed adsorption isotherms have been determined for a wide range of binary adsorptives, hydrocarbons being perhaps the most investigated. Results [114] for the adsorption of mixtures of 2,2-dimethylpropane and nbutane on Vycor glass (Fig. 2) show that there is a preference for C(CH3) 4 over the whole range of mixture compositions. With this system the adsorption of both mixture components was depressed by the presence of the second component. A compilation of the data published in the literature since 1959 is given in Table 1. Mutual interaction effects have generally been observed, the main exception being where one c o m p o n e n t shows an adsorption far exceeding that of the other [ 9 1 , 1 0 1 ] . Enhanced adsorption, compared with that from the corresponding pure adsorptive, has also been observed [38, 56]. For a binary mixture this may be so for only one [38] or both components [37, 56]. It has been postulated [121] that for systems where adsorbate-adsorbent interactions are weak such effects are due to adsorbate-adsorbate attractive forces between the two substances, e.g. for the adsorption of chloroform and acetone vapours on carbon at 30 and 50 °C [ 56]. Mutual depression of adsorption is, however, a far more c o m m o n occurrence [19, 20, 75], and has been attributed to competition of the components for surface sites [122]. Adsorbent selectivity for a particular mixture component has often been assessed by calculation of the separation factor a = y2xl/ylx2 where Yl, Yz and xl, x2 are the mole fractions of components 1 and 2 in the gas and adsorbed phases respectively. This is generally termed the selectivity coefficient. Systems in which dispersion forces are d o m i n a n t usually show little variation of this factor with changes in temperature and pressure [23]. Considerable variation of the value of a has, however, been observed where the adsorption of binary mixtures containing one polar c o m p o n e n t on zeolites has been investigated [74, 124]. Indeed, for the system n-butane-hydrogen sulphide-CaA zeolite selectivity inversion occurs within the temperature range 0 to 75 °C [70]. This has been attributed to temperature effects on c o m p o n e n t interactions with the zeolite surface, the specific interaction of hydrogen sulphide being more sensitive to temperature change than the non-specific interaction of n-butane. Another indication of the degree of selectivity shown by an adsorbent for a particular adsorptive may be obtained for systems studied at constant temperature and total pressure by plotting the mole fraction of one component in the gas phase against that in the adsorbed phase. This approach has been detailed by Young and Crowell [4]. Systems showing a change in selectivity on such plots (for which there will therefore be a point where a = 1) have been said to exhibit an adsorption azeotrope [ 50]. Such azeotropes have been found for the adsorption of benzene-ethanol, cyclohexane-ethanol and benzenecyclohexane mixtures on Graphon as reported by Perfetti and Wightman [38] An adsorption isobar is characterised by a series of experimental measurements determined at constant partial pressure of one c o m p o n e n t in the

180

/

10

iI

i/j7//l/Ill

.11j.,,f

02

04

j/z"

06

08

10

YE XC(CH314

Fig. 3. B i n a r y v a p o u r a d s o r p t i o n i s o t h e r m for e t h a n o l - c y c l o h e x a n e o n Cab-O-Sil (after Perfetti a n d W i g h t m a n [ 37 ] ). Fig. 4. C o m p a r i s o n b e t w e e n t h e o r e t i c a l a n d e x p e r i m e n t a l results for t h e a d s o r p t i o n o f 2 , 2 - d i m e t h y l p r o p a n e - n - b u t a n e gas m i x t u r e s o n graphite at 303.2 K a n d at a t o t a l pressure o f 1 1 . 8 3 kPa.

gas phase. For some systems isobars have been measured directly [34, 77, 86] ; for others, however, they have been determined from a series of mixed isotherms measured by varying gas phase composition and total equilibrium pressure. They may be represented as plots of the a m o u n t of component 1 adsorbed against its partial pressure, at constant partial pressure of component 2. Various other constructions may be made from a series of mixed adsorption isotherms. These have been detailed by Bering e t al. [68] and enable differential heats and entropies of adsorption to be calculated for the two components, if the system has been studied at more than one temperature. For the system hydrogen sulphide-propane NaX zeolite, Bering e t al. found that the isobars of each component also corresponded to constant differential heats and differential entropies of adsorption [68]. Perfetti and Wightman [37] have used a graphical method of presentation of binary vapour adsorption isotherms (shown schematically in Fig. 3) which illustrates some interesting features. The isotherms are plotted as the a m o u n t of each component adsorbed per unit area of adsorbent v e r s u s vapour composition; the corresponding pure component isotherms are included as broken lines for comparison. In the example chosen {Fig. 3), ethanol-cyclohexane mixtures on Cab-O-Sil, ethanol is the dominant component in the adsorbed phase. Adsorption of cyclohexane is appreciable only at low vapour mole fractions YE of ethanol. The amounts of cyclohexane adsorbed from the mixtures are much less than from the pure state. An anomalous enhancement of adsorption of ethanol over the pure state is observed at high ethanol mole fractions.

181 Much of the theory relating to mixed adsorption is centred on the prediction of binary mixed adsorption parameters from single c o m p o n e n t data. Considerable industrial advantages would be obtained if such were possible, e.g. in gas separation plants. The second All Union Conference on Theoretical Problems (1970) concentrated entirely on mixed adsorption systems. A summary of the theoretical developments in this field has since been published [7]. Billow e t al. [6] have made a fairly extensive review of the more important approaches. These they classified into three groups: (i) those based on isotherm models; (ii) thermodynamic approaches; (iii) independent methods for the determination of adsorption separation factors. The same terminology will be adopted here, and only (i) and (ii) will be considered further. All the isotherm models used are primarily extensions of already existing single-component models. Attempts have been made to describe mixed adsorption on homogeneous [116] and heterogeneous [125] (including microporous [60, 126] ) adsorbents; a summary of the main approaches can be found in Bfilow's review [6]. Recent work includes the extension of a modified form of the Langmuir equation proposed by Jovanovi6 [127] to describe mixed adsorption [ 1 2 8 ] . The fit with experimental data was found to be better than that obtained by application of the Langmuir equations for the test systems [128]. The theory was later developed further to describe adsorption on heterogeneous surfaces [ 1 2 9 ] , the mixed isotherm being used to represent individual homotattic sites. Another approach, based on the T6th isotherm, has also been applied to adsorption on heterogeneous adsorbents [130] ; predicted results agreed well with the data of Szepesy and Ill~s [17] for the adsorption of hydrocarbon mixtures. Jaroniec and Rudzifiski [129, 131] have discussed the overall adsorption isotherm for gas mixtures on heterogeneous solid surfaces in the form of multiple integrals. More recently Jaroniec [132] has derived, on the basis of statistical thermodynamics, the generalised integral adsorption isotherm and main thermodynamic functions for localised monolayer adsorption of gas mixtures on heterogeneous solid surfaces. It was concluded that the fundamental function in mixed gas adsorption on heterogeneous surfaces is an ndimensional distribution of adsorption energies. From the known analytical distribution functions the Gaussian distribution seemed to be the most convenient. Lee has proposed a correlation method for the prediction of mixed adsorption data based on the lattice theory of solutions and the volume filling of micropores [ 1 3 3 ] . As well as pure c o m p o n e n t data, this method requires a mixing parameter which may be obtained from a single mixed adsorption point. Agreement between predicted and experimental data was better than that obtained by various other methods applied to the same systems. This, however, was to be expected because of the additional parameter used. The thermodynamically based theory of Myers and Prausnitz has perhaps aroused the most interest. A review by Sircar and Myers [8], and also that by Biilow [ 6 ] , cover this and related theories founded on the

182 a s s u m p t i o n of an ideal a d s o r b e d phase. T h e ideal a d s o r b e d s o l u t i o n t h e o r y o f Myers a n d Prausnitz has b e e n t e s t e d fairly e x t e n s i v e l y b y its p r o p o s e r s a n d o t h e r s [27, 1 3 4 , 1 3 5 ] a n d f o u n d to c o m p a r e r e a s o n a b l y well w i t h s y s t e m s for w h i c h t h e a s s u m e d ideality w o u l d be e x p e c t e d t o hold. R e c e n t l y , a d a p t a t i o n o f t h e t h e o r y t o include n o n - i d e a l i t y has b e e n attempted [ 136]. Hall and Mfiller [ 1 3 7 ] have e x a m i n e d ideal a d s o r b e d s o l u t i o n t h e o r y f o r its ability t o p r e d i c t t h e e x p e r i m e n t a l results f o r t h e s y s t e m C(CH3)4/ n-C4H10/ g r a p h i t e w h e n used in c o n j u n c t i o n w i t h t h e B r u n a u e r - E m m e t t Teller e q u a t i o n describing t h e simple c o m p o n e n t a d s o r p t i o n . T h e results are s u m m a r i s e d (in Fig. 4) in t e r m s of a m o l e f r a c t i o n p l o t in w h i c h x a n d y r e p r e s e n t t h e a d s o r b a t e and a d s o r p t i v e m o l e f r a c t i o n s ( o f C(CH3)4) r e s p e c t i v e l y ; t h e b r o k e n line r e p r e s e n t s t h e t h e o r e t i c a l d a t a and t h e solid line r e p r e s e n t s t h e b e s t i n t e r p r e t a t i o n o f t h e e x p e r i m e n t a l d a t a o b t a i n e d . T h e c o r r e c t a d s o r b e n t p r e f e r e n c e was p r e d i c t e d b u t t o a m o r e m a r k e d degree. A g r e e m e n t b e t w e e n t h e o r y a n d e x p e r i m e n t was b e s t at a d s o r p t i v e m o l e f r a c t i o n s o f C(CHa) 4 a b o v e 0.5. T h e t h e o r e t i c a l l y p r e d i c t e d a d s o r b e d a m o u n t s w e r e o f t h e c o r r e c t o r d e r of m a g n i t u d e .

Acknowledgment T h e a u t h o r s a c k n o w l e d g e t h e a w a r d o f an S. R. C. R e s e a r c h S t u d e n t ship to S.A.M.

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184

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