Abstract:
A process for manufacturing carbon membranes having controlled pore sizes comprises the steps of: a) causing a stream of a pyrolysable carbon-containing gas to come into contact with a carbon membrane having a pore size and/or distribution larger than the desired pore size and/or distribution; b) raising the ambient temperature of the said membrane to or above the temperatures at which the adsorbed carbon-containing gas pyrolyzes; and c) substantially purging the atmosphere surrounding the membrane from carbon-containing gas.

Description:
This is a Continuation of application Ser. No. 08/516,722, filed Aug. 21, 1995 now abandoned, which is a continuation of application Ser. No. 08/213,157, filed Mar. 15, 1994, now abandoned. Application Ser. No. 08/213,157 claims priority to Isreal application No. 015142 filed Mar. 23, 1993. Entitlement to this priority date is claimed. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to carbon membranes for gas separation. More particularly, the invention relates to a method of improving the permeability-selectivity combination of carbon membranes toward gas separation by the carbon chemical vapor deposition (CVD). 
     The manufacturing of carbon membranes has been described before, e.g., in U.S. Pat. No. 4,685,940 and in British Patent GB 2,207,666. However, membranes made according to the known art were produced by attempting to tailor the size of the pores by modifying them in chemical-thermo treatments which substantially only increased the size of the pores. 
     It has now been found, and this is an object of the invention, that it is possible to increase the selectivity of carbon membranes by closing and reopening its pores under controlled conditions. 
     It has further been found, and this is another object of the invention, that it is possible to manufacture carbon membranes having controlled pore sizes, by the chemical vapor deposition (CVD) of hydrocarbon gases in the pore system. 
     It has also been found, and this is still another object of the invention, that, by means of CVD, it is possible to produce asymmetric membranes using homogeneous (symmetric) carbon membranes as the starting membrane. 
     Offer objectives and advantages of the invention will become apparent as the description proceeds. 
     BACKGROUND OF THE INVENTION 
     Carbon membranes for gas separation are porous solids which may be developed so as to acquire pores of molecular dimension. If they have no pores of larger dimensions, these membranes act as molecular sieves. 
     As a separation membrane material, its quality is determined mainly by two properties: the permeability of the more permeable gas, and the selectivity which is the ratio between the permeabilities of the more permeable and the less permeable components of the gas mixture separated. However, when a membrane is formed from the raw (membrane) material, its wall thickness becomes as important as its permeability as a membrane material, as will be more fully explained hereinafter. 
     The permeability of a membrane is defined as the amount of fluid (in moles or an equivalent unit such as cc (STP) that flows through a membrane of unit thickness, per unit surface area, per unit time and per unit pressure difference. It expresses the inherent property of the membrane material indifferent of its thickness. A common permeability unit is: cc(STP) cm-sec -1  -cm -2  -cm(Hg) -1 . 
     Another unit is the &#34;Barrer&#34; whereas: 
     
         1Barrer=10.sup.-10  cc(STP)-cm-sec.sup.-1 -cm.sup.-2 -cm(Hg).sup.-1 !. 
    
     The specific flux of a fluid through a membrane is defined as the amount flowing per unit time per unit area per unit partial pressure difference of the permeate across the membrane. It dose not include the wall thickness, therefore it is of more practical value since it expresses the membrane module productivity. 
     The scientific flux unit is cc(STP)-sec -1  -cm -2  -cm(Hg) -1 . 
     The practical unit is: liters-hour -1  -m -2  -bar -1 . 
     The last unit will be used in what follows. 
     With the aid of the permeability P and flux F definitions, the relation between them is given by equation (1) below: ##EQU1## where l is the membrane thickness. 
     Increasing the effectiveness of a membrane separator demands increasing both permeability and selectivity. 
     Unfortunately, there is always a trade-off between permeability and selectivity so that increasing permeability is made at the expense of selectivity, and vice versa. This is a very common phenomenon, and it is valid for polymer and for carbon membranes, as well. 
     It is therefore very important to be able to change the wall thickness (&#34;l&#34; in equations 1) so that it can serve as a third parameter that enables improving the specific flux through a membrane without changing either permeability or selectivity. As will be apparent to the skilled person from eq. 1, the thinner the wall, the greater is the flow through the membrane. 
     There are some practical limits to the membrane wall thickness. These are related mainly to mechanical qualifies such as the pressure difference rating, the handleability of the product in the course of assembling a membrane module, the shock resistance of the finished product, and to pinhole and fracture formation. It is known to fabricate the membrane wall into two layers: a coarsely porous support which provides the mechanical strength and enables practically free gas cross-flow, and a thin selective layer which takes the role of gas separation. The variable &#34;l&#34; in equations 1 and 2 relates therefore to the thin selective layer. 
     SUMMARY OF THE INVENTION 
     The method of manufacturing carbon membranes having controlled pore sizes according to the invention comprises the steps of: 
     a) causing a gas or vapor stream of pyrolysable carbon-containing molecules (referred to hereinafter as &#34;carbon-containing gas&#34;) to come into contact with a carbon membrane having a pore size and/or distribution larger than the desired pore size and/or distribution; 
     b) raising the ambient temperature of the said membrane to or above the temperatures at which the adsorbed carbon-containing gas pyrolyzes; 
     c) substantially purging or evacuating the atmosphere surrounding the membrane from carbon-containing gas; 
     d) cooling the membrane. 
     The above-described process steps can be carried out in any order chosen from: a-b-c-d, b-a-c-d, a-b-d-c, or b-a-d-c. 
     In the context of the present invention the term &#34;carbon-containing gas&#34; is used to indicate any gas which contains the carbon element, and which can be adsorbed on the surface of the carbon membrane, and should be taken to comprise, e.g., pure gases or mixture of gases, including mixtures of a carbon-containing gas with inert, non-carbon containing gases, hydrocarbons as well as non-hydrocarbons, e.g., C 2  H 3  Cl 3 . Furthermore, the word &#34;gas&#34; should be construed in its amplest sense, and includes, e.g., vapors. Alternative terms used to indicate carbon-containing materials herein are: &#34;CVD molecule&#34; or &#34;CVD material&#34;. 
     According to the process of the invention the following additional steps can be carried out, whenever required: 
     e) partially opening the pores by partial burn-off in the presence of oxidant; 
     f) checking the membrane permeability and selectivity at any selected time or temperature; 
     g) repeating the CVD procedure and steps e) and f), as required. 
     According to a preferred embodiment of the invention, the size of the carbon-containing gas molecule (the CVD material) is selected so as to permit or to prevent penetration into the pores, as desired. In one preferred embodiment of the invention a homogeneous membrane is produced by what will be termed &#34;homogeneous CVD&#34; by using a small, penetrating molecule, as will be more fully detailed hereinafter. 
     Illustrative examples of CVD materials are CCl 3  -CH 3  and (CH 3 ) 4  C. Of course, different molecules can be employed, depending on the particular requirements of the starting and final membrane, and the skilled engineer will be able to select appropriate CVD materials which are suitable for a specific process. 
     In another preferred embodiment of the invention, the homogeneous starting membrane is used to produce an asymmetric membrane. This can be done in a variety of ways, and according to one particular embodiment of the invention the asymmetric membrane is produced by using a CVD molecule that is commensurate with the pore size, so that only partial, shallow penetration of the CVD material into the pores will be effected. This is because the carbon atoms deposited on the pore walls will narrow the pore and block the way for deeper penetration of subsequent CVD molecules. 
     In still another preferred embodiment of the invention, the asymmetric membrane is produced by using a hydrocarbon molecule of a size which does not substantially penetrate the pore system, thereby creating an adlayer. 
     A preferred carbon membrane to be used in connection with the invention is a hollow fiber membrane, but it should be understood that the invention is not limited to any particular size, type or shape of membrane. Throughout the following description, hollow fiber membranes will be used, since they are the most convenient shape used for manufacturing carbon membranes. 
     When a hollow fiber membrane is used, and when it is desired to obtain an asymmetric membrane, in one particular embodiment of the invention, the CVD gas is applied from the bore side (inner side) of the hollow fiber, thereby creating pores having smaller sizes at the inside than at the outside of the membrane. 
     As will be apparent to the skilled engineer, it is possible to supply the CVD material either alone, as a pure mixture, or in a mixture with other gases, e.g., a carrier gas. This, in some instances, is advantageous, as will be more fully detailed hereinafter. 
     In order to clarify the following description, a number of basic procedures and considerations are detailed hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a qualitative projection of the expected form of the pore distribution function for various treated and non-treated membranes; 
     FIG. 2 is the integral fore of the pore distribution function; 
     FIG. 3 shows the results obtained in Example 5; 
     FIG. 4 shows the results obtained in Example 10; 
     FIGS. 5A, 5B, 5C and 5D schematically shows different configurations of apparatus for effecting CVD; 
     FIG. 6 is a schematic representation of three different results obtainable by chemical vapor deposition onto a carbon membrane. 
    
    
     BASIC PROCEDURES 
     The basic procedures of adjusting the permeabilities and selectivities are detailed in the aforementioned U.S. Pat. No. 4,685,945 and GB 2,207,666. It was shown therein that the permeability may be increased at the expense of selectivity by activation, namely controlled oxidation or burn-off of the carbon membrane. In another procedure in the same patents, it was shown that the selectivity may be increased and the permeability decreased by heating the carbon membrane in an inert atmosphere to significantly high temperatures. 
     It will be shown hereinafter that the CVD process provides an additional improvement in obtaining the permeability-selectivity combination over the above-mentioned two procedures. 
     Activation 
     It is believed that the activation process increases the permeability by expelling carbon atoms from the pore walls, thus enlarging it, although the inventors do not wish to be bound by any particular theory. It is obviously understood that the permeation through larger pores is faster. The fact that this increase is gained at the expense of selectivity is shown in the two schematic pore distribution functions displayed in FIGS. 1 and 2. In each figure, curve 1 represents the prior-to-activation or as received membrane, and curve 2 the membrane after activation. FIG. 1 shows the differential pore distribution functions. These are curves showing the relative contribution of the various ranges of pore sizes to the pore volume. In FIG. 2 the integrals of the curves of FIG. 1 are presented. The integration functions are obtained starting from very large pore sizes down to the desired value, so that they show, for each pore size value at the abscissa, the total volume of pores larger than that size. As such, they denote the pore volume available for the penetration of a molecule the size of which is denoted at the abscissa. The approximate pore sizes of 3 molecules are denoted in the pore size scale. In order to gain an idea about the pore volume available for the permeation of, say, oxygen on the as-received membrane, one may consider the area under curve 1 of FIG. 1, extending to the right-hand side of the vertical line located at the oxygen molecular size. In order to gain an idea about selectivity between say, oxygen and nitrogen, one may compare the sizes of the areas which correspond to oxygen and nitrogen. These estimates are more readily made in FIG. 2, where comparisons are to be made between the height of the corresponding curves, rather than between the areas under the curves. 
     Keeping these facts into account, the changes in the membrane properties upon activation can be analyzed. 
     Prior to activation, the pore distribution function (curve 1, FIG. 1) is low, indicating low average porosity, thus low permeability to any molecule. Its maximum falls at low size values, indicating low average pore size. Furthermore, the function is sharp and falls abruptly to low values at larger pore sizes, indicating fair selectivities between large and small molecules. 
     After activation (curves 2 in FIGS. 1 and 2), there is both an increase in the pore volume which increases permeability, and in the average pore size, as expressed by the shift of the maximum of the pore distribution function to larger pore size values. The implication of this shift is a decrease in the selectivity, since the ratio between the pore volumes available for the smaller and the larger molecule (FIG. 2) is not as high as it was prior to activation. This illustrates the trade-off made between permeability and selectivity. 
     Sintering 
     The process of sintering is one of a decrease of the pore volume through pore collapse which proceeds preferably on the smaller pores. This preference derives from the fact that the surface energy of high curvature (small pore size) surfaces is higher, so that the contractive surface forces acting to diminish the pore walls surface and to close the pores are greater. Therefore, the smaller pores are the first to be closed off by sintering. Due to the overall pore closure there is significant decrease in the permeability as well so that if a carbon membrane is manipulated through a few steps of sintering and activation, there is a net loss in selectivity or in permeability. These changes are also illustrated in FIGS. 1 and 2, where curve 3 should be compared to curve 2. Sintering was effected by heating in an inert or reducing atmosphere to above 400° C. (typically, 800° C.), as described in Koresh and Softer, J. Electrothem. Society, 124, 1379(1977), and in Koresh and Soffer, Sep. Sci. and Tech., 18, 723(1983). 
     Chemical Vapor Deposition 
     The carbon chemical vapor deposition onto a carbon membrane may bring about three distinct results, as schematically illustrated in FIG. 6: 
     (i) homogeneous deposition of carbon into the molecular sieve pores leading to a homogeneous closure of the pore system throughout the whole depth of the membrane wall (indicated as &#34;H&#34; in FIG. 6); 
     (ii) carbon deposition into the molecular sieve pores but to a limited depth which creates a dense layer at the membrane wall surface which is part of the original membrane thickness. This layer is called an &#34;inlayer&#34;, which should be distinguished from the &#34;adlayer&#34; of case (iii) (indicated as &#34;A&#34; in FIG. 6). 
     (iii) The creation of an adlayer on the membrane surface (indicated as &#34;A&#34; in FIG. 6). 
     In practice, the three modes need not necessarily be completely distinct. Thus, there may be an adlayer on top of the inlayer (mixing modes iii and ii) or some deposition in depth in addition to the inlayer (mixing modes i and ii). 
     Homogeneous Deposition 
     The homogeneous deposit decreases the permeability and increases the selectivity. However, if a properly large molecule serves as the source of carbon in the CVD process, it may not be able to enter the smaller pores so that the closure of larger pores will be preferred. The permselectivity results are thus better than in the sintering process described above, in which closure of the smaller pores is preferred. 
     The effect of homogeneous CVD which follows activation is again illustrated in FIGS. 1 and 2, in terms of the differential and integral pore size distribution functions. In these figures comparison should be made between the activated membrane stance (2) and the subsequent homogeneous CVD state (4). 
     Inlayer Deposition 
     The inlayer effect creates an asymmetric membrane out of an homogeneous membrane. An interesting feature of this mode is its self-curing property: when the pore system is closed at the very thin layer where carbon is deposited, further deposition at that site is stopped and continued in the other more pore-open sites. This provides an autohomogenizing effect during the formation of the very thin inlayer. 
     Adlayer Deposition 
     The adlayer also creates an asymmetric membrane out of an homogeneous membrane. However, in terms of the molecular sieving properties the carbon membrane material produced by this process is much different from that of the original matrix, which is typically formed by polymer carbonization. 
     The experimental conditions at which the CVD is applied enables the preference of any of the three above-mentioned modes, but it is believed that a single mode is not likely to be realized. It is particularly difficult to form an adlayer without initial inlayer formation. 
     In the following, the means of creating the various CVD deposits will be explained. 
     i) In order to create a homogeneous layer, provision has to be made to prevent the formation of a superficial dense layer which may block penetration into the depth of the matrix. The safest way to achieve this end is the choice of a small enough decomposing molecule. Slowing down the chemical process by lower temperatures and pressures are other means which help in this respect. 
     ii) The formation of an inlayer is achieved by selecting a decomposing molecule which partially penetrates the pore system. The very first superficial CVD layer prevents deeper penetration of subsequent decomposing molecules, thus keeping a thin inlayer. The fragments formed during decomposition are small; therefore, they behave as fast penetrants, and may thus precipitate carbon into the depth of the matrix. To minimize this undesirable effect, the fragments should carry as few carbon atoms as possible. This consideration imposes a further constraint on the choice of the chemically decomposing molecule. 
     In order to form an inlayer as thin as possible, it is desirable that the partial pressure of the decomposing components be progressively decreased across the membrane wall. This requires that the CVD gas be admitted from one side of the membrane, while vacuum is kept at the other side. This also minimizes the problem of the in-depth fragments decomposition. 
     In order to prevent the formation of an adlayer, the CVD time should be minimized so that the much greater extent of CVD required for adlayer formation (explained below) will be avoided. 
     iii) Unlike the homogeneous and the inlayer modes where the carbon matrix already exists and only minor filling of pores in the range of molecular size is required from the CVD process, a complete layer of carbon precipitate is necessary in the adlayer case. An adlayer may therefore be significant only if ample time is provided for the process. If the side effects of inlayer formation and deep deposition in the matrix are to be minimized, the decomposing molecule must be considerably larger than the membrane pore size. It is preferred that its smaller and penetrating fragments be low in carbon and, to maintain a vacuum at the other side of the membrane, as explained, for the inlayer CVD mode. 
     All the above and other characteristics and advantages of the invention will be better understood from the following illustrative and non-limitative examples of preferred embodiments. 
     DETAILED DESCRIPTION 
     Activation, as used herein, refers to pore-size opening by partial burnoff in the presence of oxidants from the gaseous phase at elevated temperatures. In the following examples, oxygen in most cases and air in some cases served for this purpose. By this treatment, there remains some oxygen which is bound chemically to the surface of the pores at the carbon membrane. The temperatures applied were 200° C. to 320° C. The treatment with the oxidant is normally followed by treatment in an inert or reducing atmosphere at further higher temperatures, such as hydrogen, argon+5% hydrogen or pure argon or nitrogen. By this last treatment, the carbon loses its chemibound oxygen as the gases carbon dioxide and carbon monoxide so that carbon atoms are also lost from the pore surface, and the pore size is enlarged correspondingly. The temperatures applied for this part of the activation process may range from 300° C. to 1200° C., and are preferably in the range of 500° C. to 800° C. In the following tables, the term &#34;activation&#34; means treatment with an oxidant followed by an inert or reducing gas as mentioned above. 
     The methods of activation have been previously described in U.S. Pat. No. 4,685,940 and British Patent GB 2,207,666, the specifications of which are incorporated herein by reference, and therefore, the details of the activation steps are not repeated here, for the sake of brevity. 
     CVD causes a tremendous increase in the selectivities. Activation increases flux, although a substantial drop in selectivity occurs in parallel. 
     In all examples, the same starting material was employed, which were carbon fibers formed from cellulose hollow fibers (175 μm OD, 150 μm ID) which were carbonized as taught in U.S. Pat. No. 4,685,940. These are denoted D 2 . 
     The improving effect of the CVD on the flux-selectivity combination process is best observed by comparing the selectivity values before and after CVD for similar fluxes or the fluxes for similar selectivities, as will become apparent from the following examples. 
     The gas fluxes through the membrane are given in liters per hour per square meter of membrane area per 1 atmosphere of driving pressure across the membrane wall, namely: liter/(hour-m 2  -bar). 
     The positive effect of CVD in the following examples becomes evident by comparing the membrane performance before and after the CVD step. &#34;Performance&#34; implies the permeability of the more permeable components (O 2  and H 2 ) and the selectivity (of each of O 2  and H 2  vis-a-vis N 2 ). 
     It has been found that it is frequently important to apply the CVD gas at elevated pressures. If the material is a low boiling temperature gas, it is available at a wide range of pressures, and therefore presents no problem in this respect. If, on the other hand the CVD material is liquid or solid at ambient temperature, its pressure of application to the membrane is limited to its vapor pressure. However, as mentioned above, in some cases, e.g., with CH 3  Cl 3 , it is important to apply the CVD material at a high pressure, frequently at a pressure higher than its ambient vapor pressure. One way of achieving this result is to heat up the whole system (storage vessel, connecting tubing, whole membrane module), which is complicated and impractical. 
     According to a preferred embodiment of the invention, this result is obtained by using a non-permeable CVD material and a permeable carrier gas that passes through a trap containing the volatile CVD material. The CVD-gas mixture is fed to one side of fie membrane, while a vacuum is maintained at the other side. The permeable carrier gas crosses the membrane wall so that the CVD material is purified and concentrated up to the limit where its pressure equals that of the original mixture. Since the temperature under CVD conditions is high, the CVD material remains in the vapor phase. 
     CVD Systems 
     FIG. 5 illustrates four different systems useful for carrying out CVD activation. FIG. 5A schematically shows a universal system which can be used both with and without a carrier gas, and in which intermediate permeability measurements (useful to follow membrane development) can be carried out by three different methods. This is the system which was used in the following examples. 
     FIGS. 5B-5D illustrate reduced systems, each suitable for a single permeability measurement. 
     In the figures, the following elements are shown: 
     T--Trap for liquid CVD material (CVD vapor carried by a gas). 
     CV--Calibrated Volume 
     FC--Flow Controller 
     FM--Flow Meter 
     DPT--Differential Pressure Transducer 
     Pg--Pressure gauge 
     PR--Pressure Regulator 
     BPR--Back Pressure Regulator 
     Additionally, arrows 1 and 1&#39; indicate an outlet to ambient, vacuum or gas analyzer; numeral 2 is the hollow fiber; 3 is the oven; 4 is a valve, in a gas manifold; 5 is the valve, for the CVD gas. 
     Example 1 
     A module of carbon hollow fiber membranes containing 40 fibers with active length of 30 cm, and having initially low fluxes for the different gases was pre-treated by three activation steps, each composed of an exposure to a flow of O 2  at 270° C. for 30 minutes, followed by exposure to H 2  at 620° C. for 10 minutes, until its fluxes for the different gases was raised to the values mentioned in line 1 of Table I below. It was then evacuated from both sides of the membrane, then subject, from the bore side only, to a dose of argon saturated at room temperature with 1-1-1 trichloroethane, while the shell side was still connected to the vacuum line. 
     The exposure to the CVD mixture started at room temperature and was heated up to 620° C. within 5 minutes. Immediately after reaching 620° C., the CVD gas mixture was pumped out, pure argon was introduced, and the membrane module was cooled down to room temperature. The operation of this system is schematically shown in FIG. 5, as described above. The CVD material is fed at one end of the membrane module contained in oven 3, and flows through the hollow fiber 2 contained therein. Vacuum is applied at the other end of the tube. Alternatively, the CVD material can be introduced at the shell side, or both at the shell and bore sides. These alternative modes are not shown in the figures, for the sake of brevity, as they are apparent to the skilled engineer. 
     The fluxes measured immediately after the CVD are shown in line 2 of Table I, indicating a tremendous increase in the selectivity and a corresponding drop in O 2  /N 2  and H 2  /N 2  fluxes. Two subsequent activation steps were made with oxygen at 250° C. for thirty minutes, each followed with treatment in a mixture of 5% hydrogen in argon at 620° C. for twenty minutes, yielded higher permeabilities at the expense of selectivities. 
     By comparing the data in line 1 with those of line 3 for hydrogen/nitrogen separation and with line 4 for oxygen/nitrogen separation, the increase in the membrane performance becomes evident. 
     
                       TABLE I______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1             750     480   1380  1.6    2.92     CVD     1       &lt;.5   140   &gt;2     &gt;2803     act.    140     14    1350  10     974     act.    570     140   1750  4.1    12.5______________________________________ 
    
     Example 2 
     A module was prepared for CVD as in Example 1. 2,2-Dimethylpropane was used as the CVD material applied at the bore side at a pressure of 1 bar without a carrier gas. It was applied upon heating the sample from 600° C. up to 700° C. Then the process was stopped by pumping out as described in Example 1. The results are shown in Table II. 
     
                       TABLE II______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act.    226     61    910   3.7    152     CVD     &lt;.5     &lt;.5   201   --     &gt;4003     act.    68      5     1213  13.6   2424     act.    310     45    2100  6.9    475     act.    840     247   2380  3.4    9.6______________________________________ 
    
     Example 3 
     Operating as in Example 1, except that there were two CVD-activation cycles instead of one. The results shown in Table III were obtained. 
     
                       TABLE III______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     --      1006    721   2003  1.4    2.782     CVD     180     18    960   10     533     act.    1320    780   2400  1.7    3.084     CVD     140     10    1570  14     1575     act.    870     180   2420  4.8    13.4______________________________________ 
    
     Example 4 
     Example 1 was repeated, but the CVD step was split into two steps: exposure to the CVD mixture was made only to 450° C. and continued at this temperature for two minutes, then a 5% H 2  in argon mixture was introduced, and the membrane cell was heated up to 700° C. and removed at this temperature for two minutes. In this case the permeabilities are higher immediately after CVD, so that it takes fewer activation steps to increase it; thus, the sacrifice of selectivity is reduced, and the process is made simpler due to the smaller number of treatment steps. The results are shown in Table IV. 
     
                       TABLE IV______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     --      540     330   1120  1.66   3.42     CVD     260     47    980   5.5    20.83     act.    720     390   1400  1.9    3.6______________________________________ 
    
     Example 5 
     Example 4 was repeated, but the CVD was made at 480° C. rather than at 450° C., and heating of the membrane module after the CVD at 480° C. was made up to 800° C. instead of 700° C. As in Example 3, there were two CVD-activation cycles in this case, which improves the permselectivity twice. The results are shown in Table V and FIG. 3. 
     
                       TABLE V______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     --      1070    660   1810  1.64   2.72     CVD     331     40    1190  8.28   29.83     act.    1000    410   1990  2.42   4.84     CVD     30      2     710   15     3555     act.    250     28    1770  8.9    63.26     act.    480     74    1920  6.5    267     act.    970     290   2330  3.38   8.0______________________________________ 
    
     Example 6 
     Example 3 was repeated, but the CVD was introduced at lower initial permeabilities and was applied in one step, starting at room temperature and ending at 620° C., with heat-up time lasting about 5 minutes. The results are reported in Table VI below. 
     
                       TABLE VI______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act.    440     150   1110  2.84   7.42     CVD     3       3     350   1      1183     act.    520     130   1510  3.87   11.64     act.    880     440   2030  2.0    4.65     CVD     5       4     200   1.1    506     act.    280     38    1700  7.5    44.77     act.    550     120   1910  4.7    15.9______________________________________ 
    
     Example 7 
     Example 3 was repeated, but the activation steps were made for a shorter time, namely 10-15 minutes, and at temperatures as high as 320° C. The final selectivities and permeabilities obtained were very high. The results are seen in Table VII below. 
     
                       TABLE VII______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act.    770     270   1860  2.8    6.92     CVD     43      5.9   2130  7.3    36.13     act.    1460    570   4440  2.58   7.74     CVD     140     2     410   70     2055     act.    130     11    1990  12     1816     act.    1410    370   4970  3.8    13.4______________________________________ 
    
     Example 8 
     Example 3 was repeated, but the first CVD step was terminated at 550° C. and the second at 700° C. instead of 620° C. The results are shown in Table VIII. 
     
                       TABLE VIII______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act     387     116   1099  3.3    9.52     CVD     5       .5    327   10     6543     act     65      7     782   9      1124     act.    309     60    1205  5.1    205     act.    760     334   1648  2.3    4.96     CVD     .5      .2    67    2.5    3357     act.    13      1.7   698   7.5    4108     act.    91      8.8   1357  10     159     act.    302     39    1820  7.8    4710    act.    634     120   2156  5.3    18______________________________________ 
    
     Example 9 
     Example 3 was repeated, but the carrier gas in the CVD mixture was hydrogen instead of argon. A very high hydrogen permeability and selectivity towards nitrogen were obtained, as shown in Table IX. 
     
                       TABLE IX______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act.    432     116   1340  3.3    122     CVD     &lt;1      &lt;1    152   --     &gt;1503     act.    57      &lt;1    1120  &gt;57    &gt;11204     act.    238     25    1680  9.4    675     act.    737     146   2380  5.1    16______________________________________ 
    
     Example 10 
     The CVD process with 1-1-1 trichloroethylene converts an homogeneous to an asymmetric membrane. This was demonstrated by cooling the membrane cell to liquid nitrogen temperature (77K) and admitting nitrogen doses from both sides of the membrane. At this temperature, absorption of nitrogen is very substantial, and it occurs into the pore system if the adsorbate molecular size is smaller than the pore size (adsorption is experimentally observed by the uptake of gas from the gaseous phase). 
     Before the CVD process, the adsorption of nitrogen at 77K was substantial and fast if introduced from either side. However, after CVD nitrogen adsorption was still possible, but only if admitted to the shell side of the membrane. This indicates that the pores on that side are large enough to be permeable to the nitrogen molecule. Admission through the bore side, where the CVD carbon film was formed, did not lead to any measurable uptake, indicating that due to the CVD, the pores of that side had become impermeable to nitrogen at this temperature. This is clear evidence that a film of pores narrower than the molecular size of nitrogen was formed at the bore side. 
     The permeabilities and selectivities (at room temperature) of this membrane before and after CVD are shown in Table X below. The two measured isotherms of FIG. 4 were taken after activation, and after CVD respectively, as described in Table X below. 
     
                       TABLE X______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act.    790     250   1650  3.10   6.592     CVD     50      8     350   6.3    43.6______________________________________ 
    
     Example 11 
     The same adsorption tests to examine asymmetry were carried out as in Example 10 on a different module. But the CVD for this module was performed with 2,2-dimethylpropane as in Example 2. The permeabilities and selectivities are shown in Table XI below, and the nitrogen adsorption isotherms at 77K, admitted from the shell side before and after CVD are shown in FIG. 4. The isotherm before CVD was taken after step 6 in the table and the one after CVD was taken after step 7. As in Example 10, there was no adsorption when the gas was admitted from the bore side from which the CVD gas was introduced. The 12% decrease in the isotherm after CVD indicates that the inlayer film thickness in this particular case was 8×12/100=0.96 micrometers, whereas the original membrane wall thickness was 8 micrometers. 
     
                       TABLE XI______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act.    1289    450   2650  2.8    5.92     CVD     100     10    1200  8.9    1203     act.    230     30    1760  7.35   594     act.    470     70    2000  6.4    28.55     act.    770     140   2300  5.4    16.56     act.    1300    340   3150  3.77   9.27     CVD     140     17    1570  8.2    92______________________________________ 
    
     Example 12 
     Example 1 was repeated, but the carrier gas was saturated with the CVD material at 0° C. rather than at room temperature. Therefore, its partial vapor pressure and thus its concentration is lower than the room temperature saturation case. The results obtained are shown in Table XII below. 
     
                       TABLE XII______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act     359     125   970   2.8    82     CVD     &lt;.5     &lt;.5   233   --     &gt;4663     act.    69      5     1181  14     2364     act.    239     38    1876  6      52______________________________________ 
    
     Example 13 
     Example 1 was repeated, but with hydrogen as the carrier for the 1-1-1 trichloroethane CVD material. The very high H 2  /N 2  selectivity combined with high H 2  flux obtained after the first activation step following CVD (line 3 in Table XIII below) is noteworthy. 
     
                       TABLE XIII______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     act.    432     130   1400  3.3    112     CVD     &lt;.5     &lt;.5   152   --     &gt;3003     act.    57      &lt;.5   1120  &gt;144   &gt;22404     act.    238     25    1680  9.4    675     act.    737     146   2380  5.1    16______________________________________ 
    
     Example 14 
     Combined inert heat-up for activation and CVD. 
     It was shown in Example 4 that the CVD may be divided into two steps: exposure to the CVD mixture at a mildly elevated temperature which we may denote by C, followed by heat-up in an inert atmosphere (denoted by H). It is known in the art that the activation steps are similarly divided into a step of exposure to the oxidant, which is denoted by O, followed by heat-up in an inert atmosphere (again denoted H). The activation-CVD-activation sequence of the previous examples assumes the form OHCHOH if detailed into the substeps. In this example the two heat-up steps, that of the CVD and that of the activation are combined so as to lead to the sequence OHOCH, as shown in Table XIV below. 
     The molecular implementation of this mode is that the CVD falls over an oxidized surface, so that during heat-up in the inert atmosphere there is both pore opening by the removal of carbon oxides and pore closure by CVD, with the net result of some improvement in the permselectivity. 
     In order to appreciate the improvement in the permselectivity, data prior to the activation step (line 2) which preceded CVD (line 3) are also provided in line 1 in the table. Comparing the oxygen flux and the O 2  /N 2  selectivity between lines 1 and 3, the improvement is clearly evident. 
     
                       TABLE XIV______________________________________treatment     flux              selectivity1-hour.sup.-1 -m.sup.-2 -barNo.   type    O.sub.2 N.sub.2                       H.sub.2                             O.sub.2 /N.sub.2                                    H.sub.2 /N.sub.2______________________________________1     --      112     19    1797  5.9    952     OH      1334    639   --    2.1    --3     OCH     492     80    2800  6.2    35______________________________________ 
    
     The above descriptions and examples have been provided for fie purpose of illustration and are not intended to limit the invention in any way. Many modifications can be effected in the CVD method, including, e.g., different systems, carrier gases, CVD materials and membranes, all without exceeding the scope of the invention.