Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-04-26
2003-03-04
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S289000, C096S004000, C096S010000, C096S221000
Reexamination Certificate
active
06527827
ABSTRACT:
This invention pertains to a method to separate fluid mixtures consisting of at least two components, the fluid mixture being separated through contact with a selectively adsorbing/permeating membrane in conjunction with a porous transport matrix utilizing basic kinetic concentration effects in combination with thermal desorption of the sorbates.
Furthermore, this invention pertains to a device to separate a fluid mixture consisting of at least two components comprising a carbon membrane system comprising a carbon membrane layer as the active zone adjacent to or on one surface of a porous transport matrix, the membrane being adjacent to a first operating zone and another surface of the transport matrix, which surface is spatially removed from the active membrane layer, being arranged adjacent to a second operating zone as well as devices to heat the surface and/or a part of the transport matrix which is adjacent to the second operating zone.
This invention also pertains to the use of the abovementioned method or of the specified device to clean polluted inflowing or outflowing air, to concentrate a component from process gases, in particular hydrogen and/or to separate air.
The separation of fluid mixtures into their components represents a very significant field of technology from an economic viewpoint. It extends from the extraction of individual components of the mixture in their purest forms to the production of product mixtures containing a least one component in concentrated form, relative to its contents in the initial mixture. Typical operational areas include the recovery of solvent vapours, the extraction of hydrocarbons and/or hydrogen from appropriate mixtures, for example following crack processes (“rectisorption processes”) but also the separation of permanent gases from natural sources or technical processes, such as the separation of oxygen and nitrogen from air or the recovery of hydrogen from ammonia/hydrogen mixtures during the Haber-Bosch process or in other technical process gases.
As well, the cleaning of harmful, polluting or even toxic components from air or outflowing air requires that individual components be separated from complex fluid mixtures. In recent years such cleaning processes have become increasingly significant, such as in the cleaning of the air in industrial production facilities, e.g. in the super-clean rooms of the semi-conductor industry, or also in the feeding of purified ambient air into the interior of motor vehicles.
The separation of gas and gas/vapour mixtures as well as the separation of individual components from such mixtures is usually done technically through adsorption or recently, in conjunction with the development of selective membranes, also using permeation processes.
Adsorption utilizes the capacity of porous solids with large surfaces, such as activated charcoal, silica gel, and aluminium silicates, to concentrate gases and vapours contained in small concentrations in gas mixtures and, therefore, to separate them from the mixture.
Essentially, all known adsorption processes consist of two operating steps: adsorption and desorption. Both processes can be carried out discontinuously with stationary adsorbent layers and continuously in the countercurrent process. Adsorption usually takes place at the lowest temperatures possible because high temperatures reduce the adsorption capacity of the adsorption agent. As a rule, desorption takes place at higher temperatures at which the loaded adsorption agent is freed of the adsorbed adsorbate thermally through the injection of super-heated steam.
Pressure swing adsorption (PSA) was developed to separate permanent gases. In this process, adsorption, once again using activated charcoal in particular, is carried out from the mixture at high pressures of up to almost 10 bar because at such high pressures the gas with the higher boiling point is adsorbed better than the gas with the lower boiling point. A precise separation, for example of hydrogen and carbon monoxide, or of hydrogen and methane, can be achieved. The adsorbent is regenerated, i.e. the more strongly adsorbed gas components are desorbed, by means of a reduction in pressure in parallel flow and subsequent rinsing with low-pressure pure gas. In this discontinuous process, part of the pure gas extracted must be re-used for regeneration. PSA fluid separation is employed on an industrial scale but is too impracticable, as well as not economic enough, for many conceivable applications.
The so-called “BF process” was developed specifically to separate oxygen and nitrogen. This process utilizes the small difference in size between nitrogen and oxygen which takes effect when adsorption is carried out using such finely-pored adsorbents that the pore size is in the range of the critical molecular diameter. At that point the difference in molecular sizes has a great effect on the diffusion velocity in the adsorbent. It has been found, for example, that the velocity determining step in the adsorption of oxygen from an oxygen
itrogen mixture on carbon molecular sieves with pore diameters in the 0.5 nm to 0.7 nm range is the so-called surface diffusion, which depends on the molecular size and, in particular, on the electron density. Despite similar equilibrium loadings for nitrogen and oxygen, oxygen is adsorbed much more quickly. Accordingly, a discontinuous process has been proposed which is similar to the PSA process and in which air is passed through an adsorber charged with activated coke for a short period under light excess pressure. First the oxygen is adsorbed. The relevant oxygen-loaded adsorber is then evacuated in order to draw off the oxygen. Where two adsorbers in parallel connection are used, one of which is always loading and the other unloading, a quasi-continuous process can be carried out that delivers an oxygen-rich gas containing approximately 50 to 55% oxygen. If part of the desorbed rich gas is fed back to rinse the relevant parallel operating adsorber, an oxygen concentration of up to about 80% by volume can be achieved.
German patent DE 36 18 426 as well as EP 606 306 of Bergwerksverband GmbH [Mining Association PLC] describe the production of carbon molecular sieves such as are used in the PSA or BF processes to extract nitrogen from air. As a rule, high-density, especially finely-pored activated charcoal granulates with apparent densities greater than 500 g/l are used in these processes.
The membrane separation process based on the basic operation of permeation is another method to separate gases and gas/vapour mixtures as well as to concentrate individual components or to separate individual components from such mixtures. Partially permeable, selectively operating membranes are used in this method to separate complex gas or gas/vapour mixtures. In addition to made-to-measure synthetic polymers, the materials for the required fixed membranes also include, among others, inorganic materials, like porous glass or glass ceramic, graphite, graphite oxide, and similar materials.
It has also been proposed that oxygen-rich gases be obtained by means of air separation on membranes. A significant problem with this is the unfavourable volume operation in comparison to liquid phase applications.
U.S. Pat. No. 4,349,356 describes a method to concentrate a component from a gas mixture on porous glass membranes in accordance with the principle of Knudsen diffusion. With this method the gas mixture is fed to the membrane in the form of a pulse and the permeated gases are suctioned off at specific time intervals. The enrichment factor is both time and pore size dependent, the motive force once again being the pressure difference. Adsorption processes play no role in the method described in U.S. Pat. No. 4,349,356.
Separation processes using a combination of adsorption and membrane separation mechanisms are also known in the prior art.
A continuously operating gas separation process at membranes is described in EP 428 052 for example. In this method a semi-permeable composite membrane is used which consists of
Spitzer Robert H.
Vidas Arrett & Steinkraus
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