Pressure swing adsorption process for co-producing nitrogen...

Details Pressure swing adsorption process for co-producing nitrogen... Pressure swing adsorption process for co-producing nitrogen...

2001-10-03

2003-03-04

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Solid sorption

Including reduction of pressure

C095S100000, C095S105000, C095S130000

Reexamination Certificate

active

06527830

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a pressure swing adsorption process for dual recovery of a more readily adsorbable component, such as nitrogen, and a less readily adsorbable component, such as oxygen, from a feed gas mixture in which the copurge:feed gas ratio of the process is less than about 1.15:1 so that the ratio of the more readily adsorbable component (nitrogen) to the less readily adsorbable component (oxygen) is less than about 3:1.
DESCRIPTION OF THE PRIOR ART
In numerous chemical processing, refinery, metal productions and other industrial applications, high purity oxygen or nitrogen may be needed. Enriched oxygen gas or nitrogen gas is frequently required for metal treating atmospheres and other applications. Nitrogen and oxygen gases can, of course, be obtained by various known techniques for air separation. Pressure swing adsorption (PSA) processing is particularly suited for such air separation in a variety of applications, particularly in relatively small sized operations for which the use of a cryogenic air separation plant may not be economically feasible.
The most common PSA systems produce a single enriched purity gas stream, usually the less readily adsorbable (light) component, from a given feed supply. In these systems, a feed gas mixture is passed through an adsorbent bed capable of selecting the more readily adsorbable (heavy) component at a higher pressure. The light component passes through the bed and is collected as product. The heavy component is then desorbed from the adsorbent at low pressure and exhausted from the system as waste. PSA systems are designed differently when the primary product of interest is the heavy component. In such systems, a co-current displacement purge, consisting of product quality gas, is passed through the bed in the direction of the feed following the feed step. The light component passes through the bed and is exhausted at the discharge end of the bed as waste. High purity heavy product is collected at the feed end of the bed through a series of blowdown, evacuation and purge steps. In either of the prior art systems, only a single component of the feed mixture is captured as product, while the remaining components are exhausted from the system as waste. In such cases, the waste stream is generally not sufficiently enriched in a desired component for use in a chemical or industrial application. In each of these systems, the unit cost of product is determined relative to the single product of interest, either the heavy or the light product.
U.S. Pat. No. 4,599,094 describes an air separation process in which the primary product is high purity nitrogen, but from which a reasonably high purity coproduct oxygen is also produced. This process uses a high-pressure ratio (ratio of maximum adsorption pressure to minimum desorption pressure) of ten or greater in combination with 13× molecular sieve. High nitrogen recovery is a major objective. Product ratios (N
2
:O
2
) much greater than 3:1, and even approaching the theoretical maximum of 4:1, are achievable with this process. Product nitrogen purities ≧99.8% (with recovery ≧98%) and oxygen purities of 90% to 93.6% were demonstrated. A chief reason for such high recoveries and purities is that the oxygen/air mass transfer front, discharged in the adsorption step, is partitioned and completely refluxed to other beds in the process. This complexity and the high-pressure ratio generate high capital and operation costs resulting in expensive products.
U.S. Pat. No. 4,810,265 teaches a co-current displacement process for the production of nitrogen. Feed air is introduced into the inlet of the bed, nitrogen is selectively adsorbed and oxygen enriched gas is withdrawn from the discharge end of the bed. The air feed step is followed by a co-current purge (co-purge) displacement step in which high purity nitrogen is fed into the bottom of the bed while oxygen enriched gas continues to be withdrawn from the top. The nitrogen purge gas displaces the oxygen that had been co-adsorbed with nitrogen, as well as the oxygen remaining in the interparticle void space. The co-purge flow continues until the mass transfer front erupts from the discharge end of the bed and the oxygen purity degrades. The oxygen-rich (light component) gas is discharged from the top of the bed and is either vented as waste or collected for countercurrent purge and/or pressurization. The adsorbent and void spaces are then saturated with high purity heavy product. U.S. Pat. No. 4,810,265 also teaches the use of LiX or 13× adsorbents in the above-described process.
U.S. Pat. No. 4,013,429 relates to pressure swing adsorption process in which ambient air, during an on-stream period, is passed serially through a pretreatment adsorbent bed removing moisture and carbon dioxide therefrom. The dried and purified air is then passed through a main adsorbent bed selective for retention of nitrogen, the oxygen-rich effluent being collected in an expandable receiving vessel. The pretreatment and main adsorbents are contained in separate vessels connected in series. Nitrogen of high purity is desorbed by evacuation from the main bed in a direction opposite to that of the initial air charge. This nitrogen product passes from the main bed into and through the pretreatment bed to a collection vessel. Preceding the vacuum desorption step both the pretreatment bed and the main bed are rinsed with the high purity nitrogen product gas from a previous stage in the operation. Following evacuation, the beds are repressured with a portion of the oxygen-rich gas drawn from the expandable receiving vessel. By operation in the described manner there are recovered for any desired use, nitrogen of high purity (99.7 to 99.9%) and oxygen-enriched (78 to 90%) gas product. Recovery of both nitrogen and oxygen products is about 95%. Such high recovery of both products is indicative of prior art processes seeking to optimize the N
2
:O
2
product ratio as near to 4:1 as possible. The process of U.S. Pat. No. 4,013,429 is both complex and expensive, having sixteen cycle steps, thirteen valves, four compression machines and three expandable gas receivers.
U.S. Pat. No. 4,892,565 relates to a process for recovery of a more selectively adsorbed key component from a gas mixture containing the key component and one or more less selectively adsorbed secondary components using vacuum swing adsorption. The process minimizes capital costs by reducing or eliminating gas storage vessels and reduces power requirements by operating without a feed compressor, whereby feed is introduced at least in part by vacuum conditions achieved by pressure equalization between parallel adsorption beds. The major thrust of this invention is the production of the more selectively adsorbed component at relatively high purity (≧95%). Alternatively, at least a minor amount of less selectively adsorbed secondary component product can be recovered. Clearly for air separation, this three or four-bed process is directed at the recovery of the heavy nitrogen component alone, or alternatively at a high N
2
:O
2
product ratio.
U.S. Pat. No. 4,915,711 relates to an adsorptive separation process set forth for recovery of two gas products in high recovery and high purity using adsorption, depressurization, low pressure purge, evacuation and repressurization. Depressurization and purge effluents are recycled to feed. Optionally, pressure equalizations are performed after the adsorption and after the evacuation steps. This invention is aimed at CO
2
/CH
4
separation. In terms of an air separation option, only processes utilizing an oxygen-selective adsorbent are considered, i.e. the oxygen is the heavy component. The process also differs markedly from the present invention in the use of the co-current depressurization step following the adsorption step and in the use of the low-pressure purge of the heavy component.
The prior art has been primarily driven to produce high purity nitrogen (99.9% or greater) and to maximize the yield of nitrogen from such air

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