Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2002-07-29
2003-12-09
Esquivel, Denise L. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06658894
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
FIELD OF THE INVENTION
The present invention relates to a process and adsorbent for the recovery of krypton and/or xenon from gas or liquid streams, and to an apparatus for use in an adsorption process.
BACKGROUND OF THE INVENTION
The use of the noble gases krypton and xenon is expected to rise in the coming years. Krypton is primarily used in the global lighting industry, for example in long-life light bulbs and automotive lamps. Xenon has applications in the aerospace, electronics and medical fields. In the aerospace industry, xenon is used in ion propulsion technology for satellites. Xenon provides ten times the thrust of current chemical propellants, is chemically inert and can be stored cryogenically. This results in lower “fuel” weight so that satellites can accommodate more useful equipment. Xenon also finds applications in the medical market as an anaesthetic and in X-ray equipment, and in the electronics market for use in plasma display panels.
Krypton and xenon are produced by concentration from air. Since their concentrations in air are so small (krypton 1.14 ppmv and xenon 0.086 ppmv).large volumes of air must be processed to produce reasonable quantities of krypton and xenon. An issue of interest is the recycling of xenon from the air of operating rooms where it has been used as an anaesthetic.
In practice, krypton and xenon are reclaimed from the liquid oxygen portion of a cryogenic air distillation process. Since the volatilities of krypton and xenon are lower than that of oxygen, krypton and xenon concentrate in the liquid oxygen sump in a conventional air separation unit. This concentrated stream of krypton and xenon can be further concentrated by stripping some oxygen in a distillation column to produce “raw” krypton and xenon. However, this “raw” stream contains other air impurities less volatile than oxygen which have to be removed before pure krypton or xenon can be produced. In particular, the “raw” stream contains carbon dioxide and nitrous oxide, both of which have low solubility in liquid oxygen and tend to freeze out during the concentration of krypton and xenon, resulting in operational problems. In addition, various hydrocarbons (C
1
to C
3
) present in the liquid oxygen can concentrate during the stripping of oxygen to produce a liquid oxygen stream with dangerously high levels of hydrocarbons.
These problems may be addressed by the use of a “guard adsorber”, that is, an adsorber capable of adsorbing impurities from the liquid oxygen stream before the oxygen stripping step.
A number of U.S. patents (U.S. Pat. No. 4,568,528, U.S. Pat. No. 4,421,536, U.S. Pat. No. 4,401,448, U.S. Pat. No. 4,647,299, U.S. Pat. No. 5,313,802, U.S. Pat. No. 5,067,976, U.S. Pat. No. 3,191,393, U.S. Pat. No. 5,309,719, U.S. Pat. No. 4,384,876 and U.S. Pat. No. 3,751,934) describe krypton and xenon recovery processes where guard adsorbers are not used. These patents disclose various ways of reducing methane concentration in krypton and xenon by reducing reflux ratios in the raw distillation column.
U.S. Pat. No. 3,779,028 describes an improved method for recovery of krypton and xenon from a reboiler of an air separation unit. The oxygen-rich liquid which leaves the reboiler passes through an adsorber for the removal of acetylene and other hydrocarbons. There is no disclosure of the type of adsorbent used or of the removal of carbon dioxide or nitrous oxide. Oxygen and residual hydrocarbons are removed from the oxygen-rich liquid, for example using a hydrogen blowpipe, and the resulting secondary concentrate of krypton and xenon is vaporised and passed through an adsorbent, for example active charcoal, silica gel or molecular sieve. Separate krypton and xenon fractions may be collected from the adsorbent.
U.S. Pat. No. 3,768,270 describes a process for the production of krypton and xenon. A portion of the liquid oxygen from the reboiler passes through an adsorber for removal of acetylene and carbon dioxide. As in U.S. Pat. No. 3,779,028, the adsorbent used in the adsorber is unspecified and removal of nitrous oxide is not addressed. The oxygen and hydrocarbons that are not removed in the adsorber are subsequently removed by combustion with hydrogen. The resulting concentrate of krypton and xenon is treated as in U.S. Pat. No. 3,779,028.
U.S. Pat. No. 3,609,983 also describes a krypton and xenon recovery system. In this system, a liquid oxygen stream is passed through a pair of alternating guard adsorbers where acetylene and higher hydrocarbons are removed. The stream is then further purified by distillation. The hydrocarbons which are not removed in the guard adsorbers are catalytically combusted, and the resultant carbon dioxide and water are frozen out by heat exchangers. The stream is purified by a final distillation. This document discloses the use of silica gel as a guard bed adsorbent.
U.S. Pat. No. 3,596,471 also describes a krypton and xenon recovery process. The process employs a hydrocarbon adsorber for removal of hydrocarbons from a krypton- and xenon-containing liquid oxygen stream. The stream is then stripped of oxygen by contact with gaseous argon, residual hydrocarbons are burned and the combustion products removed, and the stream is distilled to afford a mixture of krypton and xenon. No disclosure is made of the type of adsorbent used or of carbon dioxide and/or nitrous oxide adsorption.
U.S. Pat. No. 5,122,173 also discloses a process for recovery of krypton and xenon from liquid oxygen streams. The process employs an adsorber for higher hydrocarbons and nitrous oxide, but the adsorbent material is not indicated.
U.S. Pat. No. 4,417,909 describes a process for recovering krypton and xenon from the off-gas stream produced during nuclear fuel reprocessing. Water and carbon dioxide are removed by adsorption at ambient temperature and at −100° F. respectively, using molecular sieves. The water and carbon dioxide free stream is then passed through a bed of silica gel which removes essentially all of the xenon from the stream. The xenon is then recovered from regeneration effluent of the silica gel bed by freezing out in a liquid nitrogen cooled metal container. This art teaches selective xenon adsorption on silica gel.
U.S. Pat. No. 3,971,640 describes a low temperatures adsorptive process for the separation of krypton and xenon from a nitrogen-rich stream. The separation is carried out in an oxygen-lean stream to minimise the potential of explosions between oxygen and hydrocarbons. The krypton- and xenon-containing stream at 90 to 100 K is sent through a first adsorbent bed of silica gel to adsorb xenon, krypton and nitrogen. The effluent from the first bed is then sent to another bed which contains synthetic zeolite. Krypton, nitrogen, oxygen and hydrocarbons are adsorbed on the second adsorbent. Alternatively, the gases are adsorbed on one adsorbent only. The adsorbed gases are then desorbed by stepwise heating from 105 to 280 K, then to 650 K. This document thus teaches the use of silica gel as an adsorbent for xenon. No guard adsorbent is disclosed.
U.S. Pat. No. 4,874,592 also describes an adsorptive process for the production of xenon. As in U.S. Pat. No. 3,971,640, silica gel (or active carbon or zeolite) is used as a selective xenon adsorption agent. The concentrated xenon so obtained is purified by catalytic removal of hydrocarbons.
U.S. Pat. No. 5,833,737 describes an ambient temperature pressure swing adsorption process for the recovery of krypton from air. The key to the process is the use of hydrogen mordenite as the adsorbent selective for krypton.
U.S. Pat. No. 5,039,500 describes an adsorptive xenon recovery process which uses an adsorbent such as silica gel to selectively adsorb xenon and krypton from a liquid oxygen stream. The concentrated krypton and xenon stream is desorbed by heating and evacuation. The desorbed stream is then admitted to a low temperature solid-gas separating column to solidify and capture the xenon. No guard adsorbent is used in this process.
U.S. P
Chiang Robert Ling
Farris Thomas Stephen
Golden Timothy Christopher
Taylor Fred William
Whitley Roger Dean
Air Products and Chemicals Inc.
Drake Malik N.
Esquivel Denise L.
Jones II Willard
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