Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-08-13
2002-01-22
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S117000, C095S130000, C095S139000, C095S140000, C095S143000
Reexamination Certificate
active
06340382
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a 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 improved process provides higher hydrogen recovery and lower adsorbent inventory as compared with 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. 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 variety of adsorbates at widely varying compositions, combined with the high purity (>99.9%) requirement for H
2
, presents a significant challenge to efficient selection, configuration and amount of adsorbents in each layer of the bed to achieve an efficient H
2
-PSA process.
There are a variety of known processes for producing hydrogen. For example,
FIG. 1
of the accompanying drawings shows the steam reforming of natural gas or naphtha 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 is cooled and recovered in unit
16
. The effluent, 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
is then routed 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 and (6) Kapoor et al., U.S. Pat. No. 5,538,706.
The adsorbers in hydrogen PSA processes have been conceptually divided into multiple zones, depending upon the particular contaminants to be removed in the successive zones. For example, in Wagner (U.S. Pat. No. 3,430,418) a combination of two types of adsorbents is used, i.e. activated carbon for the removal of H
2
O and CO
2
, and calcium zeolite A for removal of CO and CH
4
(see Example 1). The Wagner patent 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 PSA cycle using at least four adsorbent beds and two pressure equalization stages for separating hydrogen-containing gas mixtures contaminated with H
2
O, CO
2
, CH
4
and CO produced in the steam reforming of natural gas. In the Batta process, a co-current depressurization step follows the first bed-to-bed equalization step to recover void space gas for purging of another bed. A second bed-to-bed equalization step is used prior to the countercurrent blowdown step in the PSA cycle.
Sircar, (U.S. Pat. No. 4,171,206), discloses a PSA process in which a crude hydrogen stream (such as the gaseous effluent from a shift converter of a hydrocarbon reforming plant) flows through a first bed of activated carbon (effective for the removal of CO
2
), and then through a second bed of 5A zeolite (effective for the removal of dilute impurities such as CH
4
and/or CO) to produce high purity (>99.9%) hydrogen.
Golden et al, (U.S. Pat. No. 4,957,514), discloses the purification of hydrogen using a barium-exchanged Type X zeolite to remove CO, CH
4
and N
2
contaminants. According to Golden, the preferred BaX zeolite is one in which 60 to 100% of the sodium cations of a NaX zeolite have been replaced by barium cations. Golden compares the adsorbent requirements using BaX (96% Ba, 4% Na), CaX (98% Ca, 2% Na), Ca/SrX (50% Ca, 50% Sr), and commercial 5A zeolites in hydrogen purification processes. For a given feed flow rate and H
2
purity, the quantity of zeolite required in the hydrogen purification process is lowest when BaX is used. Also, Golden ranks the adsorbents in the order BaX>Ba/SrX>5A>SrX>Ca/SrX>CaX for CO or CH
4
adsorption. In particular, CaX is ranked the lowest with respect to the removal of CO and CH
4
impurities.
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, each of which contains a layer of activated carbon, a layer of 13X zeolite, a layer of 5A zeolite and a layer of CaA zeolite or calcium exchanged X zeolite. This four-layer arrangement is described as useful for the removal of large feed concentrations (>1%) of CO and CO
2
.
More recently, Bomard et al in International Patent Application W097/45363, disclosed a method for separating hydrogen from a gas mixture that contains CO and other impurities such as CO
2
and hydrocarbons. In the Bomard application, the feed mixture is passed into a first selective adsorbent (e.g., activated carbon) to remove CO
2
and hydrocarbons. It is then contacted by a second adsorbent, a faujasite-type zeolite with at least 80% lithium exchange, to remove primarily CO impurity and to produce high purity hydrogen. In addition, a third adsorbent (5A zeolite) may be placed between the first and second adsorbents to remove nitrogen if N
2
is also present in the feed mixture.
It is also known to utilize PSA processes for the selective adsorption of N
2
from air to obtain O
2
-enriched gases. Berlin, (U.S. Pat. No. 3,313,091), describes the use of strontium-substituted type X zeolites in such a process and stipulates that Ca
2+
, Sr
2+
, and Ag
+
are the preferred exchanged cations with Sr
2+
being most desirable. In the case of Type A zeolite, Ca
2+
, Mg
2+
and Ag
+
are preferred.
Coe et al, (U.S. Pat. No. 4,481,018), also discloses PSA air separation processes involving N
2
and O
2
separations using calcium-exchanged forms of zeolite X, and showed enhanced PSA process performance with increasing calcium content. However, Chao (U.S. Pat. Nos. 5,698,013 and 5,454,857) discloses for calcium-exchanged forms of zeolite X a peak performance in air separation below maximum calcium content. In particular, the peak performance occurs when the degree of calcium exchange is in the range of 60 to 89 equivalent percent, and the SiO
2
/AlO
2
O
3
molar ratio is in the range of 2.0 to 2.4.
The operating conditions used in H
2
PSA processes are distinct from those used in PSA processes for O
2
production from air. The adsorption pressure is typically less than 2.5 bars in VPSA air separation, while the adsorption pressure is 5 to 20 bars in H
2
PSA.
The amount of N
2
in the feed streams of these two processes is significantly different, e.g. the N
2
fraction in air is about 78 m
Ackley Mark William
Baksh Mohamed Safdar Allie
Follett Robert J.
Spitzer Robert H.
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