Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2001-06-18
2003-06-17
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S100000, C095S103000, C095S105000
Reexamination Certificate
active
06579346
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to processes and apparatuses for separation of gaseous components by means of selective and adiabatic adsorption and desorption of usually considered unwanted impurities, using suitable adsorbents. The processes concerned are commonly known under the name pressure swing adsorption. Many different systems have been described in the patent literature, all characterized by the general feature that removal of at least one impurifying component is effected through selective adsorption at high pressure by at least one type of adsorbent, densely packed in a pressure vessel, called adsorber. During the adsorption step, feed gas is introduced at the inlet end of the adsorber, producing what is called primary product at the outlet end thereof. Dependent on the feed gas composition, adsorbers may contain more than one type of adsorbent, packed in different vertical zones on top of one another. Types of adsorbents, commonly used in the art may include zeolitic molecular sieves, activated carbon, silica gel or activated alumina.
The indications cocurrent and countercurrent, used hereafter in the description of the various process steps are related to the direction of feed gas flow inside the adsorbers during the adsorption step.
The adsorbents are regenerated by desorbing components through counter-current depressurization until the lowest pressure, producing what is called dump gas and by countercurrent purging at the lowest pressure with near product quality gas, producing purge offgas.
After adsorption, additional near product quality gas of lower pressure is recovered, hereafter called secondary product gas, by depressurizing the adsorber co-currently with the feed inlet end closed. This gas originates from the total void space in the adsorber and from fractionated desorption from adsorbents therein. Production of said secondary product gas by said cocurrent depressurization is made possible without a significant breakthrough of an adsorption front of an unwanted component, provided the adsorption step is ended early enough.
Final depressurization takes place countercurrently down to the lowest pressure, thereby releasing said dump gas. Such dump gas consists of at least one desorbed component in admixture with some of the product component.
The purging of the adsorbent takes place at the preferably lowest pressure by a countercurrent flow through it of purge gas, being a part of said secondary product gas, which thereafter, enriched with at least one desorbed component is collected as said purge offgas.
The combined streams of dump gas and purge offgas are discarded as offgas. Repressurization of the adsorber is realized with its inlet end closed, by admission through its outlet end of, (1) the remaining part of said secondary product gas for the hereinafter as such indicated low level repressurization and finally (2) a part the high pressure product gas, available as a split-off thereof, for the hereinafter as such indicated high level repressurization. On reaching the highest pressure, the contained regenerated adsorbents are ready to undergo a new adsorption step.
According to U.S. Pat. No. 3,430,418 to J. L. Wagner, a minimum of four adsorbers is required for a continuous operation, without requiring additional gas storage vessels, such that always at least one of them is used for adsorbing impurities from feed gas, while the adsorbents in the remaining adsorber or adsorbers are undergoing the other process steps of cocurrent and/or countercurrent depressurization, purging and repressurization.
For a given set of process conditions of feed gas composition, feed gas pressure and desirable offgas pressure, while aiming for a maximum product recovery efficiency, a certain optimum can be established with respect to the distribution of the secondary product gas over its reuse for purging and for low level repressurization. Maximum product recovery efficiency is consistent with the lowest possible concentration of the product component in the offgas, a condition which is met by using a restricted amount of purge gas. If the restricted amount of purge gas is sufficient for an adequate degree of adsorbent regeneration, any amount of available secondary product gas in excess of said restricted amount of purge gas should be used in a useful manner. It is one of the subjects of this invention to improve the utilization of said excess for low level adsorber repressurizations.
The effect of using more or less purge gas as clarified by the diagram of
FIG. 1
, showing the molar concentration of product component in the offgas each adsorber produces during dumping and purging. The value “C” of said concentration is plotted versus the percentage “Q” of the total of such offgas. The composition of the front end of the dump gas, shown at the left hand side of said diagram, is always identical to the composition of the feed gas; as it originates from the void spaces of the inlet end of an adsorber and its connected piping. While dumping continues towards a continuously dropping pressure, the adsorbent's void space releases gas which due to desorption contains more and more impurities, causing the value C to drop. Upon reaching the purging pressure, dumping is stopped. At this point Qd percent of the total offgas which an adsorber releases during a cycle has been produced. The amount indicated between Qd and 100% represents the purge offgas, the concentration of product component therein gradually rises until the available quantity of purge gas has been spent and the purging is finished. This clarifies, that by using less purge gas, less purge offgas is produced, containing less product component as well, consistently resulting in a higher product recovery efficiency. However, a sufficient quantity of purge gas remains required for an effective regeneration of the adsorbents near the outlet end of the adsorbers and to realize a sufficiently large loading difference after the adsorption step consistent with a commercially acceptable utilization of the adsorbents.
By increasing the internal pressure recovery efficiency, defined as pressure rise realized by secondary product gas relative to the pressure rise with total repressurization, then a smaller complementary part of said secondary product gas can be used for purging. One way of achieving a better control over the recovery and the distribution of secondary product gas over low level repressurization and purging is described in U.S. Pat. No. 3,564,816 granted to L. B. Batta. According to this description for a system consisting of 4 adsorbers, the internal pressure recovery efficiency is increased and the proportion of the total recovered quantity of secondary product gas reused for purging is reduced effectively with respect to realizing a higher product recovery efficiency. Increase of the internal pressure recovery efficiency in this system is realized by using the tail end of the released secondary product gas for the initial low level repressurization of an adsorber instead of for continuation of the purging of said adsorber.
Although this method, hereafter indicated as Batta-method, also when used in systems with more than 4 adsorbers, increases the internal pressure recovery and as a result, the product recovery efficiency, the offgas production is interrupted during said first stage of repressurizing, requiring large surge drums to dampen the irregularities of the offgas flow. Furthermore, an increase of the internal pressure recovery may have a limited effect on the product recovery efficiency in such cases, because for such increase, the following parameters are likewise increased: (1) the start-of-dump pressure, (2) the quantity of dump gas, (3) the content of the product component in the dump gas, and therefore (4) product loss. In addition, any such breakthrough of impurities will be at its maximum at said increased start-of-dump pressure and since the recovered secondary product gas is used for countercurrent low level repressurization of an adsorber, these impurities, due to the
Esselink BV
Hayes & Soloway P.C.
Spitzer Robert H.
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