Membranes With Preselected Pore Types for Separation Processes

The invention relates to membranes having preselected pore types which are made from shaped bodies composed of finely divided carbonaceous materials and binders and wherein the formed bodies are subjected to subsequent heat treatment.

Separating processes employing membranes have obtained great importance in recent processing techniques and have resulted in intensive research activity. Of interest are above all areas where the classic separation methods such as distillation, deep-freeze dissociation and similar processes have failed or are too harsh or too expensive. Thus, membranes have been used for almost 50 years in separating isotopes and during the last few years for obtaining helium from mineral oil and natural gas. Membranes are also used for separation of liquid phases, such as in reverse osmosis, in ultra-filtration and micro-filtration. They are furthermore in use in the nuclear reactor and petroleum industry for the concentration of gaseous materials.

Cellulose-acetate membranes have been used also widely in reverse osmosis processes. They permit molecules with hydrogen bridges, particularly H.sub.2 O to pass by their carbonyl groups while other types of molecules are subject to good retention.

From the German published application No. 1,719,557, it has also become known to employ two layer-dry cellulose acetate membranes for gas separation which have a porous and a non-porous layer. With these membranes the gas separation is effected by active diffusion into the non-porous layer while the porous layer only has protective purposes.

The German allowed application No. 2,321,459 also shows acrylonitrile polymer membranes which have a microporous surface to which are added one or several porous layers with increasing pore dimensions. These membranes are particularly intended for ultrafiltration.

From the German allowed application No. 1,925,582, synthetic membranes are also known of polytetrafluoroethylene which likewise are microporous and can be used for gas separation. A separation is, however, possible only if one face of the membrane is contacted with steam.

There are, furthermore, known from the German published application No. 2,055,559, molecular sieves made of thermally degraded polyphenylene oxides. These materials can also be used in the form of foils and have slot-shaped pores.

The problem of the membrane separation processes is that selectivity and permeability are in competition with each other: a high selectivity lowers the permeability and thus requires an increase of the required membrane surface. Besides, it is desirable to use membranes which are as thin as possible since this reduces the resistance towards movement of the process medium. However, this in turn results in a substantial reduction of the mechanical strength of the membrane. Thus, it is continuously necessary to compromise or to resort to expensive auxiliary materials and steps. Besides, the performance of many membranes is still unsatisfactory.

It is therefore the object of the present invention to provide membranes with preselected pore types which have higher permeabilities than the prior art membranes at the same selectivities and may be made to measure for any specific separation problem as it may arise.

This is important for a use of the membranes at reasonable cost where the problem is the separation of gaseous and liquid mixture and where components have to be concentrated which have a great penetration power. Besides, it is an object to provide membranes of high strength which makes it unnecessary to employ special supporting materials.

Particularly it is of interest to have porous membranes of different pore sizes for use by the coke producing and mineral oil industries.

These problems are met by a membrane having a preselected pore type which is suited for separating processes and is formed by heating a shaped body composed of a mixture of finely ground coal material with a binder composed preferably of (a) between about 5 and 20% by weight of natural or synthetic elastomer or a mixture of both elastomers and (b) about 1 and 20% by weight of thermoplastic material to about 400.degree. to 1400.degree. C. in an inert atmosphere at a rate of heat increase between about 0.1.degree. and 10.degree. C./min. The membranes are useful for a wide range of chemical and physical separation operations between different materials. The invention also comprises a process wherein the said heat treatment is effected in an inert atmosphere at a rate of temperature increase between 0.1.degree. and 10.degree. C./min.

The shaped bodies of the invention may be in the form of tubings, foils or discs with a wall thickness between 0.1 and 5 mm.

The preferred carbonaceous material used as the starting material is for instance mineral coal or lignite or a special coke, including charcoal, peat coke and lignite low temperature coke.

It has been surprising to find that porous membranes of different pore sizes could be formed by many different material combinations composed of mixtures of finely ground coal or coke, elastomers and thermoplastic materials if the compositions had the appropriate amounts of each components for the specific size and if the membranes were subjected to the corresponding proper thermal treatment. Some of these membranes are comparatively expensive. This, however, is no obstacle for their use for special purposes.

The membranes of the invention have a distribution of pore radii of a specific amplitude, a specific total pore volume and a specific size of the mean pore radii, all of which is determined by the type of mixture and heat treatment. Thus, it is possible for each specific separation problem to have a suitable pore type which also provides a high permeability.

In order to vary the pore size of the products of the invention the mean macropore radii can be increased among others by increasing proportions of thermoplastic materials and can be decreased by increasingly fine grinding of the carbonaceous starting product. The permeability and pore size is furthermore affected by the proportion of components particularly the amount of binder, and the degree of grinding and also by the type of heat treatment. Among others, increasing rates of temperature rise increase the permeability and pore size.

On the other hand, the smaller the grain size of the coal, the smaller are the macropores. Still more effectively is the pore size furthermore determined by the ratio of coal and binder and different types of binder.

The membranes of the invention, particularly when made in the form of tubes or tubings, have a high strength to an extent that even at low wall thicknesses, even below 1 mm, they can withstand differential pressures of more than 1500 psi without supporting materials.

A further advantage of these membranes is the high degree of homogeneity and facility of reproduction of the pore distribution. This could be determined by measurement of the permeation and pore structure of membranes produced in identical manner. It is all of these points where the prior art membranes usually had shortcomings.

The permeability is usually given as volumes (under standard conditions) per unit of time, surface and pressure difference times the thickness of the membrane, and is expressed in the following manner:

Since different amounts of materials penetrate a membrane in case of different gases or liquids under identical conditions and since this results in a higher concentration of the more easily penetrating components, it is customary to designate the ratio of permeabilities of the corresponding components as the optimum separation factor. A measure for the selectivity thus is expressed as follows:

P.sub.A /P.sub.B =a (A is the component which penetrates more easily).

The permeabilities for gaseous hydrogen which, in most cases, is the component of easiest penetration through a membrane, are for the more recently described synthetic membranes between

This is for instance described in the German published application No. 17 19 557.

The optimum separation factors as against other gases are between 1.5 and 70, it being borne in mind that usually high separation factors coincide with the lowest permeabilities.

As distinguished, the lowest hydrogen permeability which was found in the membranes of the invention was

The optimum separation factor as against methane is 3.0 and against carbon dioxide 4.8.

A similar situation exists for the membranes when used for ultrafiltration of liquids (of particle sizes between 10.sup.-2 and 10.sup.-1 microns). In this case the H.sub.2 O permeabilities of the prior art membranes are between

This must be compared with the membranes of the invention in which the mean macropore radius is

2.multidot.10.sup.-1 micron=2.multidot.10.sup.3 angstrom units and of which the hydrogen gas permeability if

while the water permeability was established as

The following examples will further illustrate the invention:

80 parts by weight of finely ground mineral coal of which 98% of the particles had a particle size of <10.mu. and 50% 3.5.mu., were mixed with 1.5 parts by weight of a butadiene acrylonitrile mixed polymerizate in latex form and 5 parts by weight of polyethylene in pulverulent form and the mixture shaped on an extruder to a tubing having an 0.5 mm wall thickness. The tubing was then heated in a shaft furnace in a nitrogen atmosphere up to a temperature of 750.degree. C. at a rate of temperature increase of 2.degree. C./min.

The mean macropore diameter of the final membrane was 5.times.10.sup.2 angstrom units. The permeability for hydrogen at 20.degree. C. and 1 bar was

The optimum separation factor against methane was 3.

The following table summarizes the Examples 2 to 8. In all these cases, the tubings were made as described in Example 1. In the table there are set out also the mean macropore diameters and the permeability.

The coal grain size in the examples 2 to 7 was more or less uniform as between the different Examples. The size distribution was about as follows:

80%: 63.mu.

50%: 23.mu.

30%: 10.mu.

and

The coal grain size of Example 8 was as in Example 1.

The examples show that with the invention it is possible to select highly varied pore types with comperatively high permeabilities for different membrane separation purposes.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint or prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.