Carbon molecular sieves used in gas separation membranes

CARBONMOLECULARSIEVESUSEDIN GASSEPARATIONMEMBRANES A. J. BIRD Department of Chemical Engineering and Chemical Technology, Imperial College, London SW7. England and

D. L. ?ZUMM School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, Australia (Receioed 28 February 1981) Abstract-The preparation of unsupported and supported carbon molecular sieve membrances has been studied. It proved impossible to prepare a continuous membrane, but measurements of the mass iransfer of gases showed that diffusion across the membranes occurred both in the gas phase within large interstices and on the surface of the carbons. As a result it was possible to obtain some separation of gases.

1.LNTRODUCTION

Gas adso~tion on various forms of carbon is of particular interest. both as a result of the high adso~tive capacity of the sofids and as a result of the wide range of properties such as surface area, pore volume or pore diameter which can be obtained by careful preparation or pre-treatmentll-31. Carbon molecular sieves, for exampie, can be prepared with microporous diameters ranging from 3 to 10 A and with apparent surface areas ranging from 50 to 1000m2g-‘[4]. In addition, they may be prepared from liquid precursors [2,4,5], thereby allowing the possibility of casting the carbons in various geometries. In view of the molecular sieve properties of the solids. these would seem to offer many advantages for the production of memb~nes that are shape selective for adsorption or d~usion of gas. As a result, the present studies were undertaken to investigate the possibility of preparing such membranes. The carbons chosen for study were prepared from polyfurfuryl alcohol (PFA), previous studies having been completed to characterise the general properties of these solids [4-6). Mass transfer of gas through a porous plug can involve several processes, depending on the nature of the pore structure and the solid[7]. In larger pores, bulk diffusion or Knudsen diffusion may be important. In micropores, surface diffusion involving some kind of mobilie adsorption may also be si~~cant~8~~ These effects may all be related by Fick’s law of diffusion, but the d~usion coefficient, and its dependence on temperature, depends on the processes involved [7]. In addition to these differences, experimental measurements of difIusivity may present some difhculties. Comparisons of diffusivities measured using the Wicke-Kallenbach steady state cell and pulse spreading techniques reveal that the former gives no measure of diffusion in “dead end” pores[9]. Similar arguments apply to the “time lag” method developed by Banner et nl.[lO, 111. Nonetheless, for mass transfer through a membrane, these methods give a more accurate value, in

that deadend pores cannot be penetrated by the gas. As a result, the present studies were based on steady state and time lag measurements in a concentration cell.

The experimental apparatus consisted of a carbon based membrane sealed into a glass cell using Versilic tubing. This seal was stable up to C(I. 200°C although some leakage could be detected after prolonged temperature cycling. For steady state measurements, the pressure across the cell was equalised and a mixture of gas 1 and gas 2 passed across one face of the membrane at the same flow rate as gas 2 alone passed over the other face of the membrane. Both streams of emergent gas were analysed using an on-line gas ~~omato~aph. ~it~gen and hydrogen were separated on a 1.5 m cotumn of silica gel (0°C) while nitrogen and propylene were separated on a I m column of activated alumina (110°C). For time lag experiments, both sides of the cell were evacuated (lo-’ mm Hg) and a gas admitted to one side of the membrane. The rate of pressure rise on the other side of the membrane was then monitored manometrically. Membranes were prepared using the method developed previously [4-61. Unsupported membranes of carbon molecular sieve (CMS) and of cobalt and nickel imprecated CMS were prepared: these had a diameter ~nomin~) of 15 mm and a thickness of 1.5-2.5 mm. CMS membranes were also supported on silica f&s, copper and iron gauzes, sintered bronze, and carbon felts and cloths. Of these, satisfactory membranes were only obtained on silica frits. In all eases, the heating rate during carbonisation had to be carefully controlled to less than 7°C min-‘, followed by carbonisation at 700°C for 4 hr and slow cooling to room temperature. 3.REstlLTs

Steady state diBusivities were measured as a function of temperature for combinations of nitrogen and 177

A. J. Brru,andD. L.TRIMM

'78

hydrogen or nitrogen and propylene. Typical results are shown in Figs. 1 and 2, and the range of results are summarised in Table 1. Attempts were then made to fit the results to a T”* dependence (bulk ditIusion[7]), to a T”’ dependence (Knudsen difIusion[7]) and to an exponential dependence on temperature (activated diffusion[81). Apparent activation energies were measured in relevant cases and are recorded in Table 2. Tie lag experiments were carried out with a variety of gases at 350K. Membranes mounted on silica frits mesh 3 and mesh 4 were too macroporous to show a measurable transient, Results for all the gases are shown in Table 3. Comparisons were also drawn between two similar samples which were prepared separately. These results are also given in Table 3.

370

330

290

410

LM

1 0

T(K)

Fig. 2. Steady state dtiusivities as a function of temperature: Gas N2 in N&H, mixture, SCZ-CMS;C,H, in N&H, mixture,SC'Z-CMS.

4. DlSCUSSIi3N The

2.51

320

340 1 :I0

major problem with the manufacture of CMS membranes was found to be the considerable shrinkage that occurred on carbonisation. This led to cracking and deformation of the membrane. With the exception of silica frits, the use of supports did not alleviate the problem. Even when a continuous membrane was obtained, the factors determining the properties of the carbon were not completely open to control. Thus, for example, comparison of the diffusivity of gases through similar samples prepared separately (Table 2) showed marked differences in some cases.

I 310

360

F’ii 1. Steady state dtiivitics as a function of temperature:0, Gas C&I6 in N&H, mixture, Ni-CMS; I, N2 in N,,/CJi6 mixture, Ni-CMS.

Table 1. Steady state measurementsof di!Tusivitv T = 350K Membrane

Gas Mixture

Oiffusivity

CMS

N2'H2

Co-CMS

N2'"2

Ni-CMS

N2'"2

CO-CMS

N2'C3H6

Si-2-CMS

N2'C3H6

Si-3-W

N2H2

Si-3-CMS

N2'C3H6

(cm&c-')

1

('12)

b

o.9xlo-4

1.2xlo-3

3.6~'0-~

6.4~'0-~

3x10-3

3.1xlo-3

6.4xlo-4

*.5x10-*

1.0x10-*

6.2~10-~

'.8x10-'

1.6x10-*

5xlo-3

a-of gas in the mixture b-apparatus malfunction

Nanenclature : CMS

2

carbon molecular sieve

M-CMS

metal impregnated CMS

Si-X-CMS

silica frit. porosity X, impregnated with CMS

a

Carbon moiecu~ar sieves used in gas separation membranes

179

Table 2. ,4pparent activation energies measured using the steady state cell over the range 290-450 K Gas

Sample

Gas Pair

EA

Kilojoule. Kilo mole-'

Ni-CMS

N*/H*

Co-CMS

6.25x103 6.25x103

N2/H2

CMS

14.9x103

N2'H2

Si-2-CMS

6~10~

N2'C3H6

Si-3-CMS

lOxlO

N2'c3H6

Si-3-CMS

bulk flow

N2'H2

Si-3-CMS

bulk flow

N2'H2

C&MS Si-3-CMS

N2/C3H6

9.8x103

N2/C3"6

23.5x?D3 (1>364K)

Si-2-CMS

C3H6 l

activated trend*

N2'C3H6

Diffusivity appeared to increase exponentially with temperature but it was impossible to measure an apparent. activation energy.

Table 3. Time lag measurements on CMS membranes sample a

GliS

t

D

sample b D

t

I-butene

17.5

4.8x1o-4

trans-2-butene

35

2.54x1o-4

cis-2-butene

30

2.66x1a-4

iso butene

25.0

2.g2x1o-4

12.0

8.52~10-~

12

2,2 dimethylpropane

25.0

3.6X1o-4

16

9

25 45

2.ox1o-4

31

Ar

25

3.62~10-~

15

He

10

9.o3x1o-4

11

1.20x10-3

8

'3"6

N2

7.5

H2

25.0

02 t - time lag (set) D = Diffusivity

cm2s-'

180

A. J. BIRDand D. L. TR~MM

Nonetheless, the membranes showed some evidence of activated diffusion, although mass transfer was not related to molecular size alone. In some cases mass transfer appeared to occur only in the gas phase. Thus, for example, Hwang and Kammermeyer have analysed the surface diffusion of gases through microporous glasses[ 121and have developed general equations for the surface flow in terms of the hindered translational motion of an activated molecule evaluated by quantumstatistical mechanics. They were able to estimate the importance of surface diffusion by comparing gas separation factors based on the surface flow equations with those predicted on the basis of bulk or Kundsen diffusion. Similar comparisons have been made for the present system (Fig. 3) and these show that gas migration in the gas phase above is sufficient to account for mass transfer across the membrane in some cases. Trans-2butene is seen to diffuse almost entirely in this way, despite the fact that the activated process is much more important with the other butene isomers. Such effects are due, almost certainly, to large pores or cracks in the membrane, and it proved impossible to remove these completely. Even with the greatest care during preparation, these major cavities could not be eiiminated and a continuous microporous membrane could not be manufactured. Mass transfer through the membranes was found to show significant contribution from diffusion on the surface in most cases, and the results summarised in Fig. 3 reflect this. Permanent gases such as argon, oxygen, nitrogen and carbon dioxide diffuse mainly in the gas phase and this is not unexpected in view of their low boiling points. Hydrocarbons, which would be expected to bond more easily with the carbon surface to form a mobile adsorbed layer are found, in general, to diffuse across the surface, at least in part. Surface diffusion is also reflected by the activated effects observed with steady state measurements as a function of temperature (Table 2). The apparent energies of activation reported in this table must reflect a combination of surface and bulk diffusion and, in agreement with this, values are considerably higher than would be expected on the basis of surface diffusion alone[4,6]. The results do show, however, that mass transfer across the membrane is an activated effect, probably involving

Fig. 3. Plot of DA(&)“’ vs boiling point/critical point of gases[l2]: ----, Plot against boiling point temperature;-, Plot against critical tempreature; -, Knudsen diflusion assumed;DA= dil&ion coefhcientof A; MA= molecularweight of A; 22dmp= 2.2 dimethylpropane;C2b= cti-2-butene; t2b = truns-2-butene; I = critical temperature; 0 = boiiii temperature.

migration of an adsorbed layer of gas as well as diffusion in the gas phase in large pores or cracks in the membrane. As a result, some separation of gas mixtures may be achieved. Acknowledgement-The authors acknowledge, with gratitude, financialhelp with the purchase of equipmentby the Australian ResearchGrants Committee. REFZRENCES

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