Multi-stage process for the separation/recovery of gases

Details Multi-stage process for the separation/recovery of gases Multi-stage process for the separation/recovery of gases

1998-10-09

2001-04-24

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

C095S053000, C095S055000, C095S096000

Reexamination Certificate

active

06221131

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a process for the separation and/or recovery of gases from gas and/or gas vapour mixtures by means of a membrane separating device to which the gas and/or gas vapour mixture is supplied, the latter being separated in the membrane separating device into permeates enriched with gas/vapours and retentates which are depleted of gas/vapours. The membrane separating device may comprise the use of different membrane types. Typically one said membrane type used is made of glassy polymers and is highly permeable to gases of small kinetic diameter, such as H
2
or He, whilst another said membrane type used is made of rubbery polymers and is highly permeable to condensable gases/organic vapours, such as C
2
and higher hydrocarbons. If further purification of the desired component(s) is sought, a PSA or cryogenic system may be incorporated to the membrane separating device.
BACKGROUND INFORMATION
Up to now, the economic treatment of gas mixtures, such as occur for example in the petroleum industry, but also in petrochemicals, represents a problem which has not yet been satisfactorily solved. Processes using the membrane separating technique for the separation or recovery of gases of high purity, greater than 99%, are fundamentally known, particularly in the combination of membrane separating devices with pressure swing adsorption devices. Where high product purities are demanded, exclusively membrane separating processes are classified as being uneconomic because the multi-stage recirculation of the permeate streams demand an excessively high expenditure of energy and excessively high costs for recompression. The combination of membrane separating devices and pressure swing adsorption devices can only be economically operated where the initial content of desired gases to be separated from the gas/vapour mixture, for example H
2
, is above 40 mole % and where a high feed pressure is available. Because the glassy polymer membranes previously employed for this purpose in the membrane separating device must, as a precondition, exhibit selectivity which is as high as possible despite low fluxes, it is necessary to set the highest possible pressure to the inlet of the separating device, for example between 40 and 130 bar, in order to be able to utilise the selectivity of the membranes. The downstream pressure swing adsorption device was likewise operated in the known arrangements at a relatively high adsorption pressure, preferably 40 bar and higher. The compression costs are very high so that the total separating process can only be operated economically when the mixture to be separated is already available at high pressure.
Processes of the type previously described are known, for example, from U.S. Pat. No. 4,398,926 and U.S. Pat. No. 4,690,695. In EP-A-0684066, the recovery of H
2
or He is known from high pressure supply flow in which the inlet concentration of H
2
or He can be below 30% by volume, where the impurities are caused by higher hydrocarbons. Under these conditions, the hydrogen can be raised in a membrane unit to 40% in the permeate before the permeate is supplied, after recompression, to a pressure swing adsorption device in which the light component, for example H
2
, is enriched to over 98%. The energy consumption for the recompression is controlled, at the expense of the yield, by extracting only a small permeate flow.
It is, therefore, an object of the present invention to create a process by means of which, at low energy requirements, simultaneous separation or recovery of high purity gases having a small kinetic diameter and of condensable gases/vapours in highly concentrated form can be achieved by applying moderate pressure only, it being possible for the gas and/or gas vapour mixture which has to be separated to have a low concentration of the components to be enriched:
The object is achieved, according to the invention, by at least one membrane separating unit having an organophilic membrane which is selective for C
2
and higher hydrocarbons combined with at least one membrane separating unit having a glassy membrane which is selective for gases of small kinetic diameter to which is supplied at the inlet end the retentate from the organophilic membrane separating unit.
The advantage of the process according to the invention lies essentially in the fact that, as desired, simultaneous economical separation and/or recovery of gases with a small kinetic diameter such as hydrogen or helium and condensable gases/vapours, such as C
2
and higher hydrocarbons from gas and/or gas vapour mixtures is possible. From the point of view of ecology also, the process according to the invention is advantageously characterised by substantially reduced energy requirements as compared with known processes because the process can be operated at relatively moderate pressures. The process is also advantageously applicable where the gases/vapours to be separated are only available in small or even in only very small molar concentrations in the mixture.
PREFERRED MODES
In carrying out the present process, it is advantageous that the organophilic membrane(s) used in the organophilic membrane unit(s) are made from elastomer or rubbery polymers which are for example manufactured from silicone rubber, the silicone rubber advantageously consisting of POMS or PDMS. These membranes of the first membrane separating device (b) are therefore organophilic membranes through which gases or vapours, for example C
2
and higher hydrocarbons, permeate selectively.
In said advantageous embodiment of the process, the membranes of the glassy membrane unit(s) consist of a so-called glass-type membrane which is for example manufactured either from polyamide-imide or from aromatic polyimides. These membranes are selective with respect to gases of small kinetic diameter, for example hydrogen or helium.
In order to have sufficient pressure energy (pressure inventory) available for the separation of the gas and/or gas vapour mixture to be separated and supplied to the organophilic membrane unit(s) and sufficient pressure energy for the permeate to be generated there, the primary gas and/or gas vapour mixture to be separated has a pressure between 1 and 30 bar, advantageously between 2 and 15 bar at inlet to the organophilic membrane unit(s).
In another embodiment of the invention, the primary gas and/or gas vapour mixture to be separated preferably has its temperature moderated before inlet to the organophilic membrane unit(s) in order to optimize the separation result, the temperature being then advantageously in the range between 0 and 25 C.
Fundamentally, the moderation of the temperature of the gas and/or gas vapour mixture to be separated can take place in any given suitable manner and will essentially depend on whether its temperature is located higher or lower within the range of the temperature which is optimum for the separation procedure of the gas and/or gas vapour mixture. In a further advantageous embodiment of the process, the moderation in the temperature then takes place by means of a heat exchanger/cooler upstream of the organophilic membrane unit(s), through which heat exchanger/cooler is fed the gas and/or gas vapour mixture to be separated before inlet to the organophilic membrane unit(s).
In order to remove impurities from the gas and/or gas vapour mixture to be separated, which impurities could easily, should they enter unhindered, lead to destruction of the actual membrane or also to blockage and therefore likewise to making the membrane ineffective, it is advantageous to pretreat/filter the primary gas and/or gas vapour mixture to be separated before inlet to the first membrane separating device; a separate filter device can be provided for this purpose. To optimise the separating procedure of the gas and/or gas vapour mixture in the organophilic membrane unit(s), a reduction in pressure is preferably effected on the permeate side of the organophilic membrane unit(s), the pressure on the permeate side being advantageously in the region of 1-2

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