Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
1999-11-18
2002-03-19
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S117000, C095S128000, C095S139000, C095S143000, C095S902000
Reexamination Certificate
active
06358302
ABSTRACT:
FIELD OF THE INVENTION
The present invention provides for a process for separating components of a gas stream. More particularly, the present invention is directed to multi-composite adsorbents for the removal of contaminants such as H
2
O, CO
2
, oxides of nitrogen, hydrocarbons and other trace impurities from feed gas streams.
BACKGROUND OF THE INVENTION
Conventional air separation units (ASUs) for the production of nitrogen and oxygen by the cryogenic separation of air are basically comprised of two integrated distillation columns which operate at very low temperatures. Due to the extremely low temperatures, it is essential that water vapor and carbon dioxide be removed from the compressed air feed to an ASU. If this is not done, the low temperature sections of the ASU will freeze up making it necessary to halt production and warm the clogged sections to revaporize and remove the offending solid mass of frozen gases. This can be very costly. It is generally recognized that, in order to prevent freeze up of an ASU, the content of water vapor and carbon dioxide in the compressed air feed stream must be less than 0.1 ppm and 1.0 ppm, respectively.
A process and apparatus for the pre-purification of air must have the capacity to constantly meet, and hopefully exceed, the above levels of contamination and must do so in an efficient manner. This is particularly significant since the cost of the pre-purification is added directly to the cost of the product gases of the ASU.
Current commercial methods for the pre-purification of air include reversing heat exchangers, temperature swing adsorption and pressure swing adsorption.
Reversing heat exchangers remove water vapor and carbon dioxide by alternately freezing and evaporating them in their passages. Such systems require a large amount, typically 50% or more, of product gas for the cleaning, i.e. regenerating, of their passages. Therefore, product yield is limited to about 50% of feed. As a result of this significant disadvantage, combined with characteristic mechanical and noise problems, the use of reversing heat exchangers as a means of pre-purification has steadily declined over recent years.
In temperature swing adsorption (TSA) pre-purification, the impurities are removed at low temperature, typically at about 5-15° C., and regeneration is carried out at elevated temperatures, e.g. from about 150°-250° C. The amount of product gas required for regeneration is typically only about 10%-25%, a considerable improvement over reversing heat exchangers. However, TSA processes require evaporative cooling or refrigeration units to chill the feed gas and heating units to heat the regeneration gas. They are, therefore, disadvantageous both in terms of capital costs and energy consumption.
Pressure swing adsorption (PSA) processes are an attractive alternative to TSA, since both adsorption and regeneration are carried out at ambient temperature. PSA processes, in general, do require substantially more regeneration gas than TSA. This can be disadvantageous when high recovery of cryogenically separated products is required. When a PSA air pre-purification unit is coupled to a cryogenic ASU plant, a waste stream from the cryogenic section which is close to ambient pressure is used as purge for regenerating the beds. Feed air is passed under pressure through a layer of activated alumina particles to remove the bulk of water vapor and carbon dioxide and then through a layer of 13X zeolite particles to remove the remaining low concentrations of carbon dioxide and water vapor. Arrangement of the adsorbent layers in this manner is claimed to increase the temperature effects, i.e. temperature drop during desorption, in the PSA beds. In other configurations only activated alumina is used to remove both water vapor and carbon dioxide from feed air. This arrangement is claimed to reduce the temperature effects.
It will be appreciated that, although many pre-purification methodologies based on PSA have been proposed, in the literature, few are actually being used commercially due to high capital costs associated therewith.
In general, known PSA pre-purification processes require a minimum of 25%, typically 40-50%, of the feed as purge gas. As a result of having low sieve specific product, such processes have high capital cost. Reduction in the air pre-purification system capital cost is particularly important when a large plant is contemplated. Therefore, it will be readily appreciated that, for large plants, improvements in pre-purification system operation can result in considerable cost savings.
In addition the current PSA systems fail to remove substantially oxides of nitrogen and some of the hydrocarbons.
SUMMARY OF THE INVENTION
The present invention provides a multi-composite adsorbent for removing gaseous impurities from feed gas streams in either pressure swing adsorption (PSA) or thermal swing adsorption (TSA) processes. The multi-composite adsorbent is a mixture of adsorbents which have different functionalities such that the H
2
O, CO
2
, nitrogen oxides, hydrocarbons and other trace impurities can be selectively adsorbed from the gas stream.
The multi-composite adsorbent comprises one H
2
O vapor removal adsorbent, one CO
2
removal adsorbent, and a third adsorbent which can selectively adsorb hydrocarbons and/or nitrogen oxides. The multi-composite adsorbent offers several advantages over multi-layered or single layer zeolite, or activated alumina beds: uniform distribution of each adsorbent along the bed to achieve maximum adsorbent performance; uniform temperature distribution to avoid “cold/hot spot” problems and maintain stable performance; potential synergies amongst different adsorbents; and lower costs to operate than a multi-layer bed.
DESCRIPTION OF THE RELATED ART
U.S. Pat. No. 5,779,767 teaches a mixed adsorbent of alumina and zeolite which can remove CO
2
and H
2
O from gas streams. The two-component mixture also demonstrates some efficacy at adsorbing acetylene and nitrogen oxides.
EP 0 862 938 A1 teaches the removal of nitrogen oxides, water and carbon dioxide from gas streams by passing the gas stream through an alumina adsorbent and a zeolite adsorbent in a pressure swing adsorption process. The PSA bed uses an initial layer of alumina adsorbent followed by the zeolite adsorbent layer.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides for a process for the adsorption of contaminating gas components from a feed gas stream by contacting the feed gas with a multi-composite adsorbent comprising a mixture of a CO
2
adsorbent and an H
2
adsorbent, the improvement further comprising including a nitrogen oxide and/or hydrocarbon adsorbent in the mixture.
The multi-composite adsorbents are employed in either pressure swing adsorption (PSA) or thermal swing adsorption (TSA) processes. These processes are typically used for removing CO
2
and H
2
O from compressed air prior to its cryogenic distillation in air separation units (ASU).
The multi-composite adsorbent is a mixture of adsorbent materials that are selected for their specialty towards one or more gaseous contaminants. The carbon dioxide adsorbent has a high CO
2
capacity and is selected from the group consisting of zeolite types A, X and Y; ZSM-5, polymeric and organometallic adsorbents.
The water adsorbent is selected from the group consisting of activated alumina, silica gel and non-zeolite desiccants.
The adsorbent for hydrocarbons and nitrogen oxides should have a high adsorption capacity for both of these species and is selected from the group consisting of zeolite type A and Y, activated carbon, organometallic adsorbents and other non-zeolite, non-activated alumina adsorbents.
Additionally, in another embodiment of the present invention, another adsorbent may be added to this mixture to remove trace impurities that are not nitrogen oxides or hydrocarbons such as ammonia and sulfur oxides.
Polymeric adsorbents are highly cross-linked polymer matrix with uniform pore size distribution, large surface area and pore volume, and a high capacity for
Andrecovich Mark J.
Deng Shuguang
Kumar Ravi
Wolf Rudolph J.
Neida Philip H. Von
Pace Salvatore P.
Spitzer Robert H.
The BOC Group Inc.
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