GB2207666A

GB2207666A - Carbon membranes

Info

Publication number: GB2207666A
Application number: GB08718664A
Authority: GB
Inventors: Abraham Soffer; David Rosen; Shlomo Saguee; Jacob Koresh
Original assignee: Israel Atomic Energy Commission
Current assignee: Israel Atomic Energy Commission
Priority date: 1987-08-06
Filing date: 1987-08-06
Publication date: 1989-02-08
Anticipated expiration: 2007-08-06
Also published as: SE8703107D0; GB8718664D0; GB2207666B; SE8703107L; DE3726730A1; FR2619376B1; FR2619376A1; SE467291B

Description

C.

C P W'hSS FOR 1-jl'-,'(,)DljCING AND R( 2L0 666 The Invontion relateu to the production of carbon me.:Llrar.es which can be used for the separation of gases. The Membranus have a very nharp cutoff of tho upper pore size. which Is for a given muml.rane ot the order of 10 per cient, and preferably 5 per cent of the effective pore vize-Thus, such membranov are suitable for the separation of a variety of gaveous upecles which differ In size from each other by about 10 per cent of molecule size.

The process is one of pretreationt of the mo-ybrane naterlal: pyr62yL;ls at a predetermined rate and subsequent activation or further treatment to change pore size.

The use of menibranes for separation processes has been known fo - r some tens of years. Such separation processes were based on the selective -,eri,.it:ation of different moieties through thin membranes.

In many cil the systems the separation was due to the selectivity of permeation due. to size differences of the various species in the mixture. The first processes were used for the separation of col- loidal species and for the separatign of other suspended srnall size prticles from liquids. Amongst the'first types of membranes there may be iijuittioiii.i tilt,4!ibr-anes with rathel. 1,31..JC and I'JI'(! di%r.eter ( -"-In which were satisfactory for the separation of mixtures with very large size differences of the species to be sep- arated. A later develo;Ai.c.r.t relates to reverse 05;TIOSiS r.,L:-,.brar. es used for the sweetening of brackish water. In parallel, ion exchanqe miembranes were used for separation processes based on electrodialysis.

Anongst the important developments there'may be mentioned the provision of hol low fiber membranes resulting in a very large overall 30;:,,t::n.ibi-arie surf ace area in a smal 1 modul e, providing a] so the poss i bi 1 i ty U of Z2c15Y 504Ing If the edges Of such r,.odulor,.

A fur-t)ier relates to the invention of the ass.i.:etric IIACI[Iibtane which comprises a coarse pore, thick support to which there is applied a very thin, dense and highly selective maribrane, which th.in membrane is used for the actual separation process, while the coarse support provides the required mechanical strenqth. Conventional processes for the separation of a certain gas or gases from gaseotis mixturet. are based on differences in chemical or physical properties. of the various constituents.

Chemical processes involve cyclic processes where certain chemicals are used for the removal of constituents. An example is the removal of hydrogen sulfide from natural gas. This may be effected by the iron sponge method wl.t.re a reactor c(,;nprising a bed of finely divided iron is used in a single run, leaving iron sulfide'i-astes. Another process is the cyclic amine method where an org2.;c aiixitonium salt is obtained during natural gas purification. which [....is to he thermally decomposed so as to recycle the amine.,,r,jngst the most common physical separation methods are those based distillations. The most cw,non is the separation of iiitrog.--ii dt,,d oxygen from air. On a large scale (ahout 1000 ton/day) the multistage cryogenic process is economical from an energy point of view. On a smaller scale other processes are more econo,-.iical. There are also used adsorption processes based on the different adsorbability of various cG-,iponents of the gas mixture. Such process is advarita,Eously effected as a multistage cyclic process in configuration. After use, the adsorbent has to be regenerated for recycling, which can be effected at low pressure or by heating. Various separation processes are based on thp use of specific zeo;ies or of carbon molecular sieves.

cl j The ii)ve,.liori reldtes to novel Cdrbun munibranes for use in the !,epat.dli(,ii of species from each uti.er. 1he invention fur71jer relates to a separation process of gaseous species from each other whenever effected by such process.

The Cdrbon njewbranes re pioduced from suitable precursor materials which undergo pyrolysis and which will not have pore-holes or cracks after the pjirc!vsis process.

Tne permeability is due to the foniation of an open pore system of molecular dimensions, which pores have a sharp cutoff at the predetermined desired upper size. In view of this, molecules which are of smaller size will be able to pass through the carbon membrane, whereas any gaseous molecule having a s.ze litrger than the pore size will not be able to pass through the membrane. The selectivity of the membranes is thus determit,ed by the provision of essentially uniform pores which size is between that of the molecular gaseous species which ought to pass and that wiiicii is to lie reject.t.d.

The terin "pore" is not to be construed as defining a definite hole of given si,-e but rather a pore-systein which has bottfe-necks along the pathway of the permeating gas, which deterntine the permeability and selectiviry. When the effective size of such constrictions are essentially unifor,n a high degree of st.lectivity will be attained. The term "pore si.,e" serves s working theory indicating Ge cutoff value of n.olecul.,r size of cas.-.fs which i.-ith.--r pass through or are prevented from passing tjrcigh such i:,,..,.i)t-ane.

4 1 0 p Tne material of the membrane is carbon, which is a solid black material which does not melt up to about 3500 0 C and which is insoluble in organic solvents; it contains at least 70 weight-" mf carbon (the element) and seems to be a high molecular weight polymer of condensed aromatic rings.

The term "iiieiiibrane" designates a thin layer or sheet of the material. It can be also in tubular form, where the diameter and the wall-thickness definc. tne important dimensions. Membranes of the present invention will be of a thickness of from about 1 to 100 p, the prefered range being about 5 p to about 30 p. Tubular 1(-.ri.embraries have a diameter of from about 5 to 1000 y, with a wall thickness of f rciin 1 p to about 300)J. As starting material there can be used a wide variety of carbon-containing non-s;;ielting materials such as regenerated cellulose, Cuprophan cellulose, cellulose, thermosetting polymers arid also acrylics.

The tf,ii,pe-i-ature of pyroiysis varies in the wide range of from 250 0 to above 1500 0 C, the prefered range being from about 400 0 C to Obout 900 0 C. There:s used a predetermined rate of heating, such as from about lo-1r-iinute to about 10 0 C. At higher heaiing rates there m:ty be formed pinholes and even i,,,icroscoilic cracks, v,' ich reduce the selectivity of the iijciitbt-ane, arid which in extreii,e cases can rend.ir the miembrane useless for the intended purposeE.

Pyrolysis results in the gradual degradation of the material used as starting, niaterial, the morphology and diviensiuns of the ii;atf--rial esSentially utichaiig,-d du-itig pyrolysis.

( 1 W 1 As the gaseous species which are to be separated have an effective size of the order of a few Angstronts, it is essential to produce membrales having a sharp upper cutoff of "pore size" at a predetermined value, generally in the 2 R to 10 R range, and more particularly in the 2.5 to 6 R range. In the process of the invention, for the production of carbon mt:mbr;,nes of des'i.ed pore size, there are used sheets or tubular membranes of organi precursors which preset the membrane geometry, the process of pyrolysis maintains the gien geometry.

The precursors materials ought not to undergo melting or softening during the p.,ttil.Isis; the rate of pyrolysis (carbonization)'is chosen to be at such rdte as to prevent formation of cracks or pinholes.

The atii.osphei-e under which the pyrolysis is effected is controlled so as to prevent un,j'esired burnoff of the men,brai:e material, resulting in pinholes or pores greater than the permitted size. The pyrolysis is advontageously carried out under an inert atmosphere. Suitable media for such pyrolysis are inert gases such as arcon, carbon dioxide, etc. The rate of heating can be varied within wide limits, depending also on the 2Onature of the starting membrane material. It will generally be in the 1 0 C/:..ii,ute to about 100C/minute rate. There may be used slowcr or higher rates of heating. The ne,-,nbrane material can be advantageously subjected to a pretreatment with certain cl.ieiiiicals to provide enhancer.' uniformity of the pore system formed during pyrolysis. Amongst these there may be mentioned aniTionium salts, such as apli.ionium chloride, chlorine, bromine, fluorine,fire retardant material. hydrogen chloride, hydrogen bromide, oxygen, 9 air.

1 G1 Perincability was medsured by conventional means, the results being given by the Ba rer unit. W,en a mefr..')ra,e has a 9-&,.-en pore size after pyrolysis, it is possible to enlarge the effective size of the pores and to retain their uniformity by certain subsequent treatments, such as heating at elevated temperature. Such enlargement csn be effected, for example, by subjecting the membrane to an air-flow at 4000C during about 15 minutes, resulting in an increase of pore size from about 3 R to 6 Tne permeability of the resulting mei;.brane is weasured for a given pair of gases, and the ratio of permeabilities provides the selectivity of the mewbrane for such gases. Following the pyrolysis it is advantageous to subject the membrane to an atmosphere of air, of hydrogen, etc, at elevated temperature. It is also advantageous to subject the membrane afterwards to heating under vacuum 15for a given period of time. experiments vicre effected after sprin kling tile membrane material with ar,.noniu:n chloride, effecting the pyrolysis under a carbon dioxide atmosphere at ambient pressure, exposure to hydro gen at elevted temperature under ambient pressure, cooling under hydrogen and exposure to air.

Ttie process can also be effected in the sain.---manner, with an oxidation step at elevated temperature under oxygen being effected after the pyrolysis step, before the subsequent steps.

1 0 w Pyrolysis starts already at about 200 C, and uncer cer-ain circumstances it is possible to effect the pyrolysis step of the invention already at 150 0 C even lower temperatures. After the pyrolysis it is advantageous to expose the menibrane to air for a number of hours, generally at.a temperature between ambient temperature and about 200 0 C. It is generally advantageous to resort after this to a further step: exposing the membrane to vacuum at an elevated temperature generally in the 400 to 800 0 C range. Instead of such vacuum treatment it is possible to purge the ifien,,bi.ane with an inert 9.3s at an elevated temperature. When a membrane is exposed to an oxidizing atmosphere after the pyrolysis step, this brings about an increase of pore size. The resulting membranes have highly uniform pores, the upper limit of which does not differ by more thin about 5 to 10 percent fi,om each other. Membranes with an upper limit of..ore size make possible to separate gase.ous species having similar diiii:.tisions, with the uniform upper range of the pore size being of course between the size of the gas molecules which are to be separated. Heating to;,!:jve 7000C decreases pore size due to sintering. The iOtial heating step - the pyrolysis - is effected in an inert atmos20phere, sucil as an inert gas, carbon dioxide or the like. It is also feasible A to effect,'.art of the pyrolysis tn an atmosphere of air or oxygen, and to terminate the pyrolysis under an inert atmosphere or in vacuum.

a c i G The following Examples are suminarized in Table 1. Example 1 A cellulose membrane was pyrolysed in presence of pure argon. in a t-emperature programmed oven at a constant heating rate of 1 degree Centigrade per minute from ambient temperature up to 8000C. It was then exposed v air for at least 16 hours and then evacuated to 800 0 C for 0.5 hour. There was obtained a carbon molecular sieving membrane which had a hydrogen permeability of 300 Barrers. and a methane permeability which was too low to be measured by means of the existing apparatus, i.e. its permeability to methani was less than 0.25 Barrers. This implies that the selectivity of this membrane towards hydrogen-methane mixture separation is greater tli,!i; 300/0.25=1200. Example 2 A run was carried out as in example 1, but the final pyrolysis temperature 13 and evacuation was to 600 0 C. The permeability of the resulting carbon meiribrane for oxygen and nitrogen was 98 and 8.2 Barrers respectively, so that its selectivity for separating an oxygen/nitro,_,en mxture was 98/8.2=12. Example 3 A run was carried out as in example 1 but the final pyrolysis and evacuation 0 temperatutes were to 400 C. The permeability of tht: resulting carbon rrieni brane for oxygen and nitrogen was 56 and 5.1 Barrers respectively,o thit its selectilifty for separating an oxygenl'nitrogen mixture was 56/5.1=11.

Examp e 4 A cellulose ii-,eilil!-ant. was subjected to the following treat,.i,ents:

1.

2.

Sprinkled with ammonium ch;oride powder; p., rolysed in- the presence of carbon dioxide at amblent pressure, in temperature programmed oven at a constant beating rate of 1 OC per winute from ambient temperature up to 620 0 C; C) 3. exptised to hydrogen at ambient pressure at 620 0 C for one hou'r and cooled in presence of hydrogen to room temperature The penneability of the resulting membrane to oxygen and nitrogen was 12.6 and 0.85 Barrers respectively, so that its selectivity for separating an 5 oxygerij. iitrogen mixture was 14.8. Exam 5 The niii:ibrane of Example 4 w,.s sujec'tli-d to the following additional treatments:

4. lx,Posed to air at room temperature for at least 16 hours; C;.posed again to hydrojen a!- 620 0 C for 30 minutes. The permeability of the esulting carbon membrane for oxygen and nitrogen was 59 and 4.8 Bai.es re,pective.ly, so that its selectivity for this gas couple beis 11. 1--ExairipLL 6 A cellulose nembrane WdS subjected to the!Following treatments:

5.

1. Sprinkled with ammonium chloride powder; 2. pyrolysed in presence or carbon dioxide at ambient pressure, in a 0 tewperature programined oven at a constant heating rate of 1 C per minute from ambient temperature up to 620 0 c; 3. oxidized in oxygen at 20COC for one hour; 4. expoed to hydrogen at ambient pressure at 6200 C for one hour and cooled in presence of hydrogen to room temperature; 5. expG.sed to air at room temperature for at least 16 hours.--- The pprm.-ability of the resulting carbon n.aiiibrane for oxygen and- nitrogen was 49 and 4.9 Barrers respectively. so that its selectivity for this gas couple was 10.

9 1 C C 0 Example 7

The membrane of Example 6 was further treated by:

6. Evacuation at 2000C for one hour. The resulting 02 and N2 permeabilities were 115 and 16.7 Barrers respe tivjAy and the corresponding selectivity was 6.8.

Example 8

A cellulose memurane was subjected to the following treatm.--.nts.

1. Sprinkled with ammonium chloride powder: 2. pyrolysed in presence of carbon dioxide at ambient pressure, in a te.,,-iperature progranuned oven at a constant heating rate of 10C per minute fr'cm ambient temperature up to 6200c; 3. oxidized in'oxygen at 1800C for 3 hours; _ exposed to hydreigen.3t ambient pressure at 6200C for one hour and cioled in presence of hydrogen to room teinperature; exposed to air at room temperature for at least 16 hours. The resulting 02 and N2 permeabilities were 61 and 5.5 Barrers respectively and the corresponding selectivity was 11. Example The membrane of Example 8 was further treatee by: 20 6. Evacuat:on at 6000C. The resulting 02 and N2 permeabilities were 117 diii 21 2arrers respectively and the corresponding selectivity was 5.6. Example 10 A cellulose men,brane was subjected to the following treatments:

4.

5.

1. Sprinkled with aminonium chloride powder, 25 2. pyroly,.ed in presence oF carbon dioxide at ambient pressure, in a temp;-rature programmed oven at a constant heating rate of 10 C per minute from ambient temperature-up to 620 OC; C 1 M 1 CII 3. 4.

5.

0 oxidized in oxygen at 1800C for one hour; e.%posed to hydrogcn at ambient pressure at 6200 C for 15 minutes and cooled in presence of hydrogen to room temperature, then hydrogen was pumped out; steps 3 and.-4-.were repeated twice, (for the first time step 3 was extended for 16 hours instead of one hour); 6. the membrane was exposed to air for at least 16 hours.

The r2sulting 0 2 and N. permeabilities were 68 and 7 Barrers respectively and the corresponding selectivity was 9.6. 10 Example 11 The membrane of Example 10 was further treated by:

7. Ev:cuation at 200 C for one hour.

The resuning 02 and N, permeabilities were 300 and 50 Barrers respectively and the Lorresponding selectivity was 6. 15 Example 12 A cellulose niembrane was subjected to the following treatments:

1. Sprinkled with ammonium chloride powder; pyrolyst.d in presence of carbon dioxide at ambient pressure, in a teinperature progranmned o.wen at a constant beating rate of 1 0 C per 0 minute from amb.ent tempeFature up to 620 C; 3. oxidized in oxygen at 1800C for 16 hours; exposed to hydrogen at ambient pressure at 620 OC for 10 minutes and cooled in presence of hydrogen to room temperature, then hydrogen ias puirped out; steps 3 and 4 were repeated twice; the membrane was exposed to ambient air for at least 16 hours. The resulting 0 2 and N. permeabilities were 106. and 13.7 Barrers respectively and the corresponding selectivity was 7.7.

1 25 5.

C C Example 13

The njewbrane of Exa.;ile 12 was further treated by: 7. [v.icuatiori at 2000C for one hour. The re-sulting 0 2 and N 2 permeabilities we re 3611 and 51.7 Barrers respectively and the corr-sponding selectivity-was 7. Example 14 A cellulose ii,-.iiibrane was subjected to the following process steps: 1.

1 5 2.

3.

6 Sprinkled t..ith ammonium chloride powder; pyrolysed in presence.of carbon dioxide at ambient pret.surc.-, in a temperature progranhued oven at a constant heating rate of 10C per minute from ambient temperature up to 620 0 C; at 6200 C the C02 gaseous phase was replaced by pure argon and tbe temperature was raised up to 900 0 C at the same constant rate of 1 0 c per minute. The membrane was then cooled to room temperature and it had no permeability to oxygen.

Example 15

The mewibrane of Example 14 was further treated by:

4. Ox,dation with oxygen at 300 0 C for 7 hours.

The resulting 02 and N 2 penneabilities were 850 and 195 Barrers reGpectively 20 and the cori-espond-ing selectivity was 4.4.

Example 36

The membrane of Example 15 was fu-ther tr.---ated by:

5. Further oxydation with oxygen at 3000C for 7 hours The resulting 0 and N crme.--.bilities were 1913 and 786 Urrers respectively 2 2 " !5 and the corresponding selectivity was 2.4.

X -p 1 17 A cellulose membrane received the following treatments.

2.

3.

Sprinkled with aniTionium chloride powder, pyi-oly.,,d in presence of carbon dic,xide at ambient pressure, ill j temperature progranwned oven at a constant heating r;,te oF 10C per minute from ambient temperature up to 620 0 C; oxidized in oxygen at 180 0 C for 16 hours; exposed to hydrogen at ambient pressure at 620 0 C for 15 minutes and cooled in peesence of hydrogen to room temperature; then hydrogen was pumped out; 5. steps 3---nd 4 were repeated twice. The last hydrogenation step was at 8000 C for one hour. The result! ng 0 2 and N 2 permeabilities were 700 and 212 Barrt..rs respectively and the corresponding selectivity was 3.3. This example was made in order to serve as a reference to the next exatnple where a chemical vapour deposition was employed. Eampl_18 The rrieri ane of Example 17 was further treated by chmical vaprur deposition of met -ne at 7200C for 20 minutes.,The resulting 0 2 and N 2 permeabilities were Iz,7 and 19.1 Barrers respectively and the corresponding selectivity was J.7. Co.:.paring this result with that (if Example 12, it is evident that the M had caused a reduction of the oxygen perf.,e..bility and an increase in the electivily vs. nitrogen.

Example 19

A cellulose m..:.ibrane was subjected to the following treatments:

1. Sprinkled with ammonium chloride powder; % 2. pyrolysed in presence of carbon dioxide at atilbient pressure, in a temperature progr;inuoed oven at a constant heatirg rale of IOC per A 9 c 1 C - C minute from ambient temr.erature up to 700 0 C; 3. degassed at 2-000C for one hour.

The resulting H 2 and Ar perineabilities were 520 and 0.5 Barrers respectiiely and the zorresponding selectivity was 1040.

Example 20

A cellulose membrane was subjected to the following treatments:

1.

2.

Sprinkled with aminonium chloride powder; pyrolyfed in presence of carbon dioxide at ambient pressure, in a temperature prograirmned oven at a constant heating rate of 1 0 C per minute from ambient temperature up to 650 0 C; activated in a mixture of 1:1 hydrogen carbon dioxide at 500 0 c for 2 hours.

The resulting 0. and Ar pertneabilities were 122 and 13 Barrers respectively and the corresponding selectivity was 9.4. The resulting H2 and Ar pet.T., ,eabi lities were 2340 and 23 Barrers respectively and the corresponding selectivity was 180. The resulting He and Ar perineabilities were 724 and 13 Barrers res pectively and the corresponding selectivily was 55.7.

Example

The memibi-ane of E:cample 20 was further treated by:

4. Further hydrojenation with hydrogen at 6000C for 112 an 1 iour.

The resulting 0 2 and Ar permeabilities were 285 and 40 Barrers respectively and the corresponding selectivity was 7.1. Example 22 A cellulose nieinli-ant., received the following treatments: 1. Sprinkled with ammonium chloride powder; 2. pyrolysed in-presence of carbon dioxide at ambient pressure, in a temperature programmed oven at a constant heating rate of IOC per minute fom ambient temperature up to 750 0 C; 1 1 C' 3. dctivated in a mixture of 1:1 hydrogen carbon dioxide at 500 0 C for 2 hours. The resulting 0,, and Ar permeabilities were 23 and 3.3 Barrers respectively and '.!c correpscnding sele(Aivity was 7.0.

a 4 1 CJ TABLE 1. Sutwary of ExamAes of MSCM Preparations, Treatments and Perfonl; arices.

Examp] number 3 4 6 7 8 9 11 12 13 14 11-3 16 7 21 21 122 mixture ratio'of 1:1 H21'02 SF 6 is not penucating in all the above mentioned membrane examples.

Treatment sequences given from left to right.

NH451 1 arb?n-loxidation 1. gass J.1zationlactivation dE =L- MWALawanaw C-400 N C-620 membrane 4 N C-620 membrane 6 N C-b20 membrane 8 N C-620 m2mbrane 10 N C-620 membrane 12+ N C-C.0 chem.,,t,a,, Perm "ir, iv depos gases Barrer selec.

G1 10 Ox-20-16 Ox-20-16 Ox-20-16 Ox-20-16 Ox-200-1 Ox-180-3 Ox-20-16 OX-180-1 OX-180-16 Lx-180-16 x- 180-16 Ox-20-16 D-800-0. 5 D-600-0.5 D-400-0.5 H-620-0.5 D-620-0.5 H-620-0.5 D-620-1 H-620-1 D-200-1 D-620-1 H-620-1 D-200-1 D-620-0.17 H-620-0.17 D-200-1 D-610-0.17 H-620-0.17 D-620-0.17 H-620-0.17 D-620-0.17 H-6110-0.17 D-200-1 D-900 H 2 /CH 4 02/N2 02: N 2 021N2 021N2 02/N 2 02/N2 0 2 1 N 2 61 0 2/N 2 117 0 2 / N 2 68 OZ/N2 300 300 '71200 98 12 56 11 13 15 12 49 10 7 5.6 '9. 6 6 02/N2 106 7.7 0 2 M 2 361 0 2 /N 2 0 7 114+ Ox-300-7 02 /N 2 850 4.4 rnembrane 15+ (;x-300-7 0 2 /N 2 1913 2.4 -'0 Ox-180-16 D-620-0.17 H-620-0.17 Ox-180-16 D-620-0.17 11-620-0.17 Ox-180-16 D-620-0.1? fl-300-1 0 2 /N 2 698 3.3 membrane- 17+ CH 4-720-20 02IN 2 147 7.7 14 C-700 D-200-1 H 2 1Ar 520 3000 N C-1550 C02:H2-500-2 G2/Ar 122 9.4 H /Ar 2340 180 H9/Ar 724 5.5 membrane 20+ H-600-112 0 2 /Ar 285 7.1 N C-750 CO:H,-500-2 0 7.0 2 1 2/Ar 23 1 a 0 1 C Cf I 4

Claims

CLAIMS: 1. A process for the production or carbon membranes from membrane

precursor carbon-containing non-melting material of dimensions a(cording to the required product, said 'wembranes having an effective predetermined pore-Eize adapted to preferably permit passage of a desired given gaseous species, while being substantially less pervious to another gaseous species to be separated from the first one, which comprises the optional step of pretreatment uf the membrane precursor with an ogent adapted to increase carbon yield and for preserving geometry of the precursor; effecting pyrolysis of the precursor at a predetermined rate of heating in an inert atmosphere up to a given temperature; if desired, oxidiziiig the resulting membrane in an oxygen or in an oxygen-containing atmosphere at elevated temperature; exposing the resulting membrane, if desired, to an air, hydrogen or carbon dio..,cide atmosphere for a predeterinined period of time;and if desired, subjecting the resulting membrane to vacuum treatment.
2. A process according to claim 1, where the membrane precursor is., non- melting carbonaceous material having a thickness of from 2 to 500 V, and if in tubular form, a svall thickness of from 1 to :,ko 300 p and a diameter from 5 to 1000
3. A process according to claim 1 or 2, where the beating is effected at a rate of from 1OCImin. and tu 10OCImin.

9 f X-
4. A process i.s cliinid in any of claims 1 to 3, where the pyrolyzed membrane is Activated by air. oxygen, carbon dioxide, water.or vapour.
5. A process according tt, any of claims 1 to 3, where the pore size of the' pyrolized ine.mbrani. is ircreased by heating in the range of from 5U 0 0' to 1200 C for from 10 minutes to about 16 hours in an atmosphere of nitrogen, ar93n, hydrogen or in vacuum.
6. A process according to any of claims 1 to 3, where pore size is decre?.sed by sintering at a te:;;perature in the range of fro,n 70(3 0 C to abou- 1500 0 C for about 15 minutes to 12 houls in an inert atimosphere su( as a noble gas, nitrogen; or in hydrogen. or in a vacuum.
7. A process according to any of claims 1 to 3, where the riernbrane is su!-.-;,.-ted to a degassing step at a temnperature in the range about 0 to about 1000 C in a vacuum.
8. A pi-o:ess according to any of claims 1 to 3, wherc. the r.ei-.brane is passil..al,eL. by treatrient with hydrogen at a temperature in the range of fro;n about 5000C to about 10000C at a pressure of from 1 Torr to 1000 Torr for j.jut;-2 hours.
9; A prczess according to;tny of claims 1 to 3, where the is su'u.ier-'.ed to exposure to vapor of an crgaiiic subslarice at a te,"-. perature in the 4500C to 12000C rance fo about 15 rninutes to about 4 hours, resAting in a deposit of carbon in the pores, modifying the peri..eability and the selectivity of the me,-,,brane.
10. Carb.n membranes for separating Gaseous species fr-2-ri eah othdr, whenever obtained by a process cldi.-.ed in any of claims 1 to 9.

Published 1988 at The Patent Office, State House, 66171 High Holborn, London WC1R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Rent. Coa 1187.

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