Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-11-03
2003-01-07
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S100000, C095S103000, C095S117000, C095S130000, C095S139000, C095S140000, C095S143000
Reexamination Certificate
active
06503299
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a two-bed pressure swing adsorption (PSA) process for purifying impure gas streams containing more than 50 mole % hydrogen, and more particularly to such a process for the production of high purity hydrogen from various hydrogen-containing feed mixtures such as synthesis gas. The process provides higher hydrogen recoveries and requires fewer adsorption beds than previously known PSA processes for hydrogen production.
BACKGROUND OF THE INVENTION
The need for high purity (>99.9%) hydrogen is growing in the chemical process industries, e.g., in steel annealing, silicon manufacturing, hydrogenation of fats and oils, glass making, hydrocracking, methanol production, the production of oxo alcohols, and isomerization processes. This growing demand requires the development of highly efficient separation processes for H
2
production from various feed mixtures. In order to obtain highly efficient PSA separation processes, both the capital and operating costs of the PSA system must be reduced.
One way of reducing PSA system cost is to decrease the adsorbent inventory and number of beds in the PSA process. In addition, further improvements may be possible using advanced cycles and adsorbents in the PSA process. However, H
2
feed gas contains several contaminants, e.g. a feed stream may contain CO
2
(20% to 25%) and minor amounts of H
2
O (<0.5%), CH
4
(<3%), CO(<1%) and N
2
(<1%). Such a combination of adsorbates at such widely varying compositions presents a significant challenge to efficient adsorbent selection, adsorbent configuration in the adsorber, and the choices of individual adsorbent layers and multiple adsorbent bed systems to obtain an efficent H
2
-PSA process.
There are a variety of known processes for producing hydrogen. For example,
FIG. 1
of the accompanying drawing shows the steam reforming of natural gas or naptha wherein a feedstock, e.g., a natural gas stream
11
, is compressed and fed to a purification unit
12
to remove sulfur compounds. The desulfurized feed is then mixed with superheated steam and fed to a reformer
13
to produce primarily H
2
and CO. The effluent stream from the reformer is sent to a heat recovery unit
14
, then to a shift converter
15
to obtain additional H
2
. The effluent from the shift converter goes through a process cooling and recovery unit
16
prior to sending the effluent (e.g., a synthesis gas stream
17
having on a dry basis a composition of about 74.03% H
2
, 22.54% CO
2
, 0.36% CO, 2.16% CH
4
, and 0.91% N
2
) to a PSA purification system
18
to produce a high purity hydrogen product stream
19
.
Representative prior art PSA processes for hydrogen purification include the following: (1) Wagner, U.S. Pat. No. 3,430,418, (2) Batta, U.S. Pat. No. 3,564,816, (3) Sircar et al., U.S. Pat. No. 4,077,779, (4) Fuderer et al., U.S. Pat. No., 4,553,981, (5) Fong et al, U.S. Pat. No. 5,152,975, (6) Kapoor et al., U.S. Pat. No. 5,538,706, (7) Baksh et al., U.S. Pat. No. 5,565,018, and (8) Sircar et al., U.S. Pat. No. 5,753,010.
Wagner, U.S. Pat. No. 3,430,418 describes an eight-step PSA cycle for hydrogen purification. At least four beds are used in the process; following the bed-to-bed equalization step each bed undergoes a co-current depressurization step prior to countercurrent blowdown to recover void space gas for purging of another bed.
Batta, U.S. Pat. No. 3,564,816 describes a twelve-step step PSA cycle using at least four adsorbent beds and two pressure equalization stages for separating hydrogen-containing gas mixtures contaminated with H
2
O, C
2
O, CH
4
and CO produced in steam reforming of natural gas. In the Batta process, after the first bed-to-bed equalization step, a co-current depressurization step is used to recover void space gas for purging of another bed, then a second bed-to-bed equalization step is used prior to the countercurrent blowdown step in the PSA cycle.
Scharpf et al., U.S. Pat. No. 5,294,247 discloses a vacuum PSA process for recovering hydrogen from dilute refinery off gases, preferably containing less than 60% hydrogen. The patent discloses the use of six adsorbent beds. Baksh et al., U.S. Pat. No. 5,565,018 discloses a 12 bed PSA process using external gas storage tanks to allow gases of increasing purity to be used during repressuzation.
Sircar et al., U.S. Pat. No. 5,753,010 discloses a PSA hydrogen recovery system where a portion of the hydrogen is recovered from the PSA depressurization and recycled to the PSA system.
Baksh, U.S. application Ser. No. 09/373,749 (D-20731), for Pressure Swing Adsorption Process for the Production of Hydrogen, filed Aug. 13, 1999 discloses a pressure swing adsorption process for purifying an impure gas stream by passing it through an adsorbent bed containing an alumina layer for adsorption of H
2
O, an activated carbon layer for adsorption of CH
4
, CO
2
, and CO, and a layer containing the zeolite for adsorption of nitrogen from the gas stream. The pressure swing adsorption process provided in the Baksh application is a 4 bed system employing a 12 step process (see inter alia pages 12-14). The invention described in the present application differs in several important respects from the process disclosed in the Baksh application. These differences include, but are not limited to, the fact that the present invention uses a 2 bed system which allows for a reduction in the bed size factor; and in several embodiments, the present invention uses storage tanks (separate from the adsorption beds) which allow for the use of gas of increasing H
2
purity during refluxing.
It is among the objects of the present invention to provide an improved PSA process for the production of hydrogen from an impure gas stream containing more than 50 mole % hydrogen, which provides increased hydrogen recovery and reduced PSA adsorbent requirements with consequent lower capital and operating costs. Other objects and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing.
SUMMARY OF THE INVENTION
This invention provides a two bed pressure swing adsorption process (as distinguished from the four or more bed processes utilized in prior art designs) for recovering a primary component (e.g. hydrogen) at a purity of over 99% from a feed gas, e.g., synthesis gas, comprising the primary component and one or more impurities. The process is capable of producing high purity (>99.99%) hydrogen at high recoveries with a significant reduction in the total cycle time versus prior art PSA processes used in H
2
production.
This invention includes a two bed pressure swing adsorption process for recovering a primary component at a purity of over 99% from a feed gas comprising the primary component and one or more impurities, wherein the process comprises: (a) passing the feed gas through a first adsorption bed to remove one or more impurities; (b) conducting a pressure swing adsorption cycle in the first bed; (c) separately passing effluent gases from the first bed into at least two separate tanks for subsequent purging and pressurization of the beds; (d) storing a gas mixture in the first of the tanks containing the primary component in a concentration higher than the concentration of the primary component in the gas mixture in the second of the tanks; (e) refluxing the mixture of the primary component from the second tank in the first adsorption bed during the regeneration steps therein; (f) refluxing the mixture of the primary component from the first tank in the first adsorption bed during the regeneration steps therein; (g) simultaneously and non-concurrently performing steps (a) to (f) in a second bed; and (h) recovering the product gas stream.
In accordance therewith, decreased adsorbed inventories are required (without decreasing the H
2
product purities and recoveries), greater flexibility in controlling the duration and the pressures and end points of each step are achieved, and significant reductions (>45%) in the amount of the adsorbent (e.g. zeolite)
Baksh Mohamed Safdar Allie
Terbot Charles Edward
Follett Robert J.
Praxair Technology Inc.
Spitzer Robert H.
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