Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-03-22
2001-03-06
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S106000, C095S115000, C095S130000, C095S143000, C095S902000
Reexamination Certificate
active
06197092
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to separation of nitrogen in admixture with methane by Pressure Swing Adsorption (PSA) utilizing in a first stage certain crystalline zeolites which selectively adsorb nitrogen and passing the purge from said first stage which is rich in nitrogen but contains a significant quantity of methane to a second stage containing an adsorbent which selectively adsorbs methane. The novel process of this invention results in an integrated process characterized by the fact that a high recovery of both nitrogen and methane is obtained without generating any innocuous purge effluent gas streams.
DESCRIPTION OF THE PRIOR ART
First applications of PSA processes were performed to achieve the objective of removing smaller quantities of adsorbable components from essentially non-adsorbable gases. Examples of such processes are the removal of water from air, also called heatless drying, or the removal of smaller quantities of impurities from hydrogen. Later this technology was extended to bulk separations such as the recovery of pure hydrogen from a stream containing 30 to 90 mol percent of hydrogen and other readily adsorbable components like carbon monoxide or dioxide, or, for example, the recovery of oxygen from air by selectively adsorbing nitrogen onto molecular sieves.
The carrying out of the PSA process in multi-bed systems is illustrated by the Wagner patent, U.S. Pat. No. 3,430,418, relating to a system having at least four beds. As is generally known and described in this patent, the PSA process is commonly performed in a cycle of a processing sequence that includes in each bed: (1) higher pressure adsorption with release of product effluent from the product end of the bed; (2) co-current depressurization to intermediate pressure with release of void space gas from the product end thereof; (3) countercurrent depressurization to a lower pressure; (4) purge; and (5) repressurization. The void space gas released during the co-current depressurization step is commonly employed for pressure equalization purposes and to provide purge gas to a bed at its lower desorption pressure.
Similar systems are known which utilize three beds for separations. See, for example, U.S. Pat. No. 3,738,087 to McCombs. The faster the beds perform steps 1 to 5 to complete a cycle, the smaller the beds can be when used to handle a given hourly feed gas flow. If two steps are performed simultaneously, the number of beds can be reduced or the speed of cycling increased; thus, reduced costs are obtainable.
U.S. Pat. No. 4,589,888 to Hiscock et al. discloses that reduced cycle times are achieved by an advantageous combination of specific simultaneous processing steps. The gas released upon co-current depressurization from higher adsorption pressure is employed simultaneously for pressure equalization and purge purposes. Co-current depressurization is also performed at an intermediate pressure level, while countercurrent depressurization is simultaneously performed at the opposite end of the bed being depressurized.
U.S. Pat. No. 4,512,780 to Fuderer discloses a pressure swing adsorption process with intermediate product recovery. Three products are recovered from a pressure swing adsorption process utilizing a displacement step in conjunction with pressure equalization between beds of a multi-bed adsorption system. This process is not cost efficient for the recovery of two products.
PSA processes were first used for gas separations in which only one of the key components was recovered at high purity. For example, from 100 mols feed gas containing 80 mols hydrogen and 20 mols carbon monoxide, the process of the Wagner, U.S. Pat. No. 3,430,418, or of the Hiscock et al, U.S. Pat. No. 4,589,888, could separate 60 mols of hydrogen at 99.999% purity, but no pure carbon monoxide could be recovered; 20 mols of carbon monoxide and 20 mols of hydrogen remained mixed at 50% purity each. Neither of these processes can make a complete separation. Only the less adsorbable, light component is recovered at high purity.
For the recovery of a pure, stronger adsorbed, “heavy” component, an additional step is necessary, namely, rinsing of the bed with a heavy component to displace the light component from the bed prior to depressurization. The rinsing step is described in several earlier patents. The problems with these processes are the following: (a) if the rinsing is complete and the light component is completely displaced from the bed, pure heavy component can be obtained, but the adsorption front of the heavy component breaks through to the light component and the latter cannot be recovered at high purity; (b) if the displacement of the light component is incomplete, the typical concentration profile of the heavy component in the bed is not optimum and such bed is depressurized countercurrently to recover the heavy key component at the feed end, the light component still present in the bed reaches the feed end very rapidly and the purity of the heavy component drops. Therefore it is not practical with the prior art processes to obtain both key components at high purity in a single PSA unit.
Such complete separations can be obtained, for example, by two separate pressure swing adsorption processing units wherein each unit includes several fixed beds. From a feed gas containing, for example, hydrogen and carbon monoxide (CO), the first unit recovers pure hydrogen and a carbon monoxide rich gas containing 70% carbon monoxide. This gas mixture is compressed and passed through a second PSA unit which recovers pure carbon monoxide and a hydrogen rich gas. The hydrogen rich gas can be added as feed gas to the first PSA unit and then the cycle is repeated. The combination of two independent PSA units can make an excellent separation at very high flexibility. For example, from a gas mixture with two components this system can recover more than 99.8% of the adsorbable “light” component such as hydrogen at a purity of 99.999% and also recover essentially 100% of the more readily adsorbed component such as carbon monoxide at a purity higher than 99.5%.
Although pressure swing separation adsorption (PSA) has been used to separate a wide variety of gases, the simple fact remains that there is no commercially practiced PSA process for the separation of nitrogen from methane. This is due to many factors including the lack of a nitrogen specific adsorbent and environmental regulations.
The instant invention provides a process for the separation of nitrogen from methane, particularly natural gas streams containing nitrogen.
As pointed out in U.S. Pat. No. 5,669,958, a significant percentage of U.S. natural gas reserves contain more than 4% nitrogen. The bulk of these reserves cannot be exploited because no economical technology exists for removing nitrogen especially at low flow rates, i.e., less than 25 MMSCFD process feed gas.
Cryogenic distillation is the only process being used to date on any scale to remove nitrogen from methane in natural gas. Cryogenic plants are not used more widely because they are expensive and complicated and exhibit poor scale down economics.
There has been mention of the use of adsorbents for the removal of nitrogen from a natural gas. U.S. Pat. No. 2,843,219 discloses a process for removing nitrogen from natural gas utilizing zeolites broadly and contains specific examples for the use of zeolite 4A. This patent does not disclose a pressure swing adsorption process but rather discloses a process where molecular sieve adsorbent is regenerated by thermal swing. The process disclosed in this patent is not practical and it does not provide a cost efficient method for the separation of nitrogen from natural gas.
Another patent utilizing molecular sieves for the removal of nitrogen from natural gas is U.S. Pat. No. 4,964,889 which discloses the use of natural zeolites such as clinoptilolites in various cationic forms for the removal of nitrogen.
However, this patent is silent as to a further process for the waste gas. There is no disclosure of a high overall system recovery o
Butwell Kenneth F.
Dolan William B.
Kuznicki Steven M.
Engelhard Corporation
Miller Stephen I.
Spitzer Robert H.
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