Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
1998-10-09
2001-01-30
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S053000, C095S055000, C095S096000, C095S098000, C095S102000, C095S116000
Reexamination Certificate
active
06179900
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the separation/recovery of gases. More specifically the present invention relates to a process for the separation/recovery of a desired component from multi-component gaseous streams with the aid of membranes of high permeability and medium selectivity combined with adsorption under pressure swing (also known as Pressure Swing Adsorption or PSA). The process of the present invention is specially directed to gaseous streams in which the desired component(s) to be separated/recovered is present at low molar contents and/or low pressure,which renders the state-of-the-art processes using membranes/PSA combined processes inapplicable for economic reasons. The present process is equally suited to gaseous streams where the desired component is present at very low molar concentration and moderate pressure.
The potential use of the present invention encompasses a wide variety of mixtures of gases of industrial use present at low molar contents and/or low pressure in gaseous streams, the separation/recovery of which is of great economical appeal.
The separation of useful industrial gases using the process of the present invention is rendered possible by the use of the polymeric permeable membrane, of high permeability and medium selectivity, this membrane selectively permeating molecules of small kinetic diameter, for example hydrogen and helium. An additional economic advantage stems from the fact that the relatively high pressure inventory of the non permeate stream or retentate is transferred to the permeate stream, with high savings in energy.
The present process is specially useful in the light of the deep interest in the separation/recovery of hydrogen from refinery streams where hydrogen exists in relatively low molar content, for example between 10 and 30 mol %, and low pressure, for example between 8 and 15 bar. The high purity hydrogen recovered through the present process allows the hydrogen recovered to be used in hydrotreating processes, widely used in Brazil in view of the heavy character of the local petroleum oils.
Other industrially interesting gases can be separated/recovered in high yield and low energy consumption by using the present process for the separation/recovery, provided these gases have a small kinetic diameter. For example, helium is an interesting industrial gas which can be separated/recovered from any gas stream containing helium even in very low concentration by using the process of the present invention.
BACKGROUND INFORMATION
Processes designed for the production of a high purity product stream at high recovery from gaseous feed streams by combining single or multi-stage membrane systems and multi-bed PSA units are well-known. The use of stand-alone membrane units to produce a very high purity stream, i.e., greater than 99%, was found to be inefficient since large membrane areas and power demands were required in order to achieve this high purity at high recovery rates. PSA units, on the other hand, proved to be very efficient in producing high purity components from feed streams containing the desired component at concentrations greater than 50 mole %, but become less efficient for treating relatively low purity, less than 50% streams to yield a high purity product at high recovery rates.
Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and light hydrocarbons.
Membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates.
A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other component of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component or components and a stream which is depleted in the selectively permeating component or components. The stream which is depleted in the selectively permeating component or components is enriched in the relatively non-permeating component or components. A relatively non-permeating component permeates more slowly through the membrane than at least one other component of the mixture. An appropriate membrane material is chosen for the mixture so that some degree of separation can be achieved.
Membranes for hydrogen separation have been fabricated from a wide variety of polymeric materials, including cellulose esters, polyimides, polyaramides and polysulfones. An ideal gas separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high separation factor (selectivity) and high gas permeability. Normally polymers possessing high separation factors have low gas permeabilities, and vice-versa.
In any case it is very uneconomical to purify a permeating desired component by membranes only to high purity levels higher than 99%.
On the other hand, a process using pressure swing adsorption or PSA is an adequate tool for separating and purifying hydrogen gas contained in a gaseous mixture with impurities which are selectively adsorbed by one or more adsorbing beds in a PSA system. Adsorption in these beds occurs at a more elevated pressure, the impurities which are more selectively adsorbed being desorbed through pressure reduction at a lower desorption pressure. It is possible to purge the beds at this lower desorption pressure for desorption and further withdrawal of impurities, before repressurization at higher adsorption pressure for adsorption of impurities of further amounts of the feed gaseous mixture during the working out of the process cycle.
As practised in the technique, the PSA process is commercially very important for the purification of hydrogen gas. The purity of hydrogen can be higher than 99.99%. The PSA process can be used to treat a wide variety of available feed streams and is not limited to a specific stream. There are no pre- or post treatment requirements, except for the withdrawal of impurities in order to avoid unduly adsorbent degradation. Further, there is no practically reduction in pressure from the feed stream and the product gas so that the product gas is available at the pressure level for further use downstream of the PSA system and for repressurization of each bed up to the adsorption pressure.
Pressure of hydrogen-containing streams submitted to separation through PSA reach 40 bar (600 psig) or higher.
The number of adsorbing beds in PSA systems vary widely, from two to 12 beds, on a case-to-case basis.
Thus, U.S. Pat. No. 4,398,926 teaches a process for the selective permeation using membranes combined to PSA where the technique of selective permeation separates a major amount of the impurities of the desired component which is contained in a gaseous feed stream at high pressure and relatively impure, final separation and purification being effected in a PSA system to which the membrane system is integrated. It is alleged that in this way the recovery of the product is improved, without sacrificing the purity, while the overall cost for producing the product is reduced. The process is said to be specially useful to treat high pressure streams, that is, having pressures higher than 40 bar (600 psig). It should be understood that the PSA systems show drawbacks when operating at high pressures of adsorption, that is, above 40 bar. At these high pressures the PSA system becomes very expensive in terms of investment costs. The trend is that a higher amount of product be retained in the bed at these high pressures, so that the desired component, for example hydrogen, will be discarded from the bed together with the impurities during the counter-current depressurization step. Thus U.S. P
Behling Rolf-Dieter
da Silva L{acute over (e)}dia Barreto
Peinemann Klaus-Viktor
GKSS - Forschungszentrum Geesthacht GmbH
Spitzer Robert H.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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