Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-11-27
2003-06-10
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S105000, C095S106000, C095S120000, C095S129000, C095S137000, C095S139000
Reexamination Certificate
active
06576044
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a process for purifying nitric oxide, and more particularly to a process for adsorbing nitrous oxide, nitrogen dioxide, nitrous acid, carbon dioxide, sulfur dioxide, carbonyl sulfide and moisture from a nitric oxide stream by adsorption.
BACKGROUND OF THE INVENTION
Nitric oxide plays an important role in medicine and electronic component manufacture. For example, in the medical field, inhaled nitric oxide helps maintain blood pressure by dilating blood vessels, and kills foreign invaders in the body's immune system. It can be appreciated that it is imperative that the nitric oxide used in such medical applications be of medical grade, i.e., it must not contain more than 5 parts per million by volume (ppm) nitrogen dioxide, and must be substantially free of all other impurities that are harmful to humans, such as slfur dioxide. In electronic applications, nitric oxide is used for nitriding gate oxides in the manufacture of silicon semiconductor devices. The purity requirements for electronic grade nitric oxide are likewise stringent. For example, electronic grade nitric oxide must contain less than about 30 ppm nitrogen dioxide and nitrous oxide. Nitric oxide can be produced by a variety of methods. U.S. Pat. No. 5,670,127, incorporated herein by reference, discloses a particularly desirable nitric oxide manufacturing method which involves the reaction of nitric acid with sulfur dioxide. According to this process aqueous nitric acid is introduced into the top of a trickle bed reactor while sulfur dioxide, introduced into the bottom of the reactor, passes upwardly through the bed. Nitric oxide, produced by reaction of the nitric acid and sulfur dioxide, passes out through the top of the reactor. Water vapor and any sulfur dioxide not consumed in the reaction also pass out of the reactor with the nitric oxide, and thus become impurities in the nitric oxide product gas. Additionally, nitrous oxide and nitrogen dioxide are also impurity byproducts of the process. Many of the above impurities are produced in most other nitric oxide production processes.
Various techniques are employed to remove nitrogen dioxide and sulfur dioxide from the nitric oxide. U.S. Pat. No. 3,489,515 discloses the purification of nitric oxide by washing the nitric oxide with a dilute aqueous solution of nitric acid. The water reacts with the nitrogen dioxide to produce nitric and nitrous acids, which can be washed from the gaseous product stream by washing the stream with water. This method is not satisfactory for producing electronic grade nitric oxide because it does not adequately reduce the concentration of nitrogen dioxide in the product gas stream. Nitrogen dioxide can also be removed from nitric oxide by cryogenic distillation. This method likewise leaves a lot to be desired because of the high capital cost of distillation equipment and because not all of the valuable nitric oxide is recovered during the distillation. Furthermore, liquid nitric oxide is known to be shock-sensitive and has been observed to detonate under certain conditions.
Another nitric oxide purification technique that has been reported is adsorption using various adsorbents. For example, U.S. Pat. No. 5,417,950 discloses the adsorptive removal of nitrogen dioxide and sulfur dioxide from nitric oxide using alumina-deficient type Y zeolite of ZSM5 zeolite as adsorbents; U.S. Pat. No. 5,514,204 discloses the adsorptive separation of nitrogen dioxide and moisture from nitric oxide using metal cation-free silica gel, alumina, or various zeolites, such as types A, X and Y zeolites; and U.S. Pat. No. 5,670,125 discloses the purification of nitric oxide by adsorbing nitrogen dioxide and sulfur dioxide from the nitric oxide using zeolites having a silica to alumina ratio not greater than about 200.
In addition to the above nitric oxide purification methods, adsorption has been used to remove nitrogen oxides (including nitric oxide) and sulfur dioxide from gas streams. U.S. Pat. Nos. 2,568,396 and 4,149,858 disclose the separation of sulfur and nitrogen oxides from use of activated coke or activated charcoal; and U.S. Pat. Nos. 3,674,429 and 4,153,429 disclose the removal of nitrogen oxides from gas streams using zeolites. Oxygen present in or added to the gas streams effects the oxidation of nitric oxide to nitrogen dioxide, and the nitrogen dioxide is adsorbed by the zeolite. The disadvantage of using most of the above adsorbents for the purification of nitric oxide is that they tend to promote the disproportionation of nitric oxide to nitrogen dioxide and nitrogen and/or nitrous oxide, and the oxidation of nitric oxide to nitrogen dioxide.
Because of the importance of producing nitric oxide that is substantially free of nitrogen dioxide, sulfur dioxide and other impurities for medical and electronic applications, highly effective methods for purifying nitric oxide are continuously sought. The present invention provides a simple and efficient method of achieving this objective.
SUMMARY OF THE INVENTION
According to the invention, gaseous impurities are adsorbed from nitric oxide gas using as the adsorbent a porous polymer.
According to a broad embodiment, the invention comprises a method for purifying a nitric oxide gas stream containing one or more gaseous impurities, comprising an adsorption step comprising passing the gas stream through at least one adsorption zone containing a porous, metal-free polymeric adsorbent that is selective for the one or more impurities, thereby adsorbing the one or more impurities from the nitric oxide gas stream and producing purified nitric oxide.
The porous, metal-free polymeric adsorbent that is selective for one or more impurities in the nitric oxide does not promote the disproportionation of nitric oxide to nitrogen dioxide and nitrogen or nitrous oxide, or by promoting the oxidation of nitric oxide to nitrogen dioxide.
The method preferably further comprises an adsorbent regeneration step comprising desorbing the one or more impurities from the adsorbent. More preferably, the adsorption step and the adsorbent regeneration step are steps of cyclic adsorption process. Most preferably, the cyclic adsorption process is pressure swing adsorption, temperature swing adsorption or a combination of these.
The polymeric adsorbent preferably comprises aromatic polymers, heterocyclic polymers, acrylic polymers, acrylic ester polymers, imine polymers, fluorocarbon polymers and combinations thereof.
Generally, the adsorption step of the method is carried out at a temperature in the range of about −200 to about 200° C. and a pressure in the range of about 0.5 to about 50 bara.
According to one preferred embodiment of the invention, the cyclic adsorption process is pressure swing adsorption and the adsorbent regeneration step is carried out at a pressure in the range of about 0.5 to about 5 bar. In this preferred embodiment, the polymeric adsorbent preferably comprises divinylbenzene polymers, styrene polymers, acrylic polymers or combinations thereof. Likewise, in this preferred embodiment, it is preferred that the adsorption step be carried out at a temperature in the range of about −150 to about 100° C. and a pressure in the range of about 1 to about 20 bara. It is also preferred in this preferred embodiment, that the adsorbent regeneration step be carried out at a pressure in the range of about 0.1 to about 2 bara.
According to another preferred embodiment of the invention, the cyclic adsorption process is temperature swing adsorption and the adsorbent is regenerated at a temperature in the range of about −150 to about 300° C. In this preferred embodiment, the polymeric adsorbent preferably comprises divinylbenzene polymers, styrene polymers, acrylic polymers or combinations thereof. Likewise, in this preferred embodiment, it is preferred that the adsorption step be carried out at a temperature in the range of about −150 to about 100° C. and a pressure in the range of about 1 to about 20 bara. It is also preferred in this p
Ho Dustin Wenpin
Tang Deming
Whitlock Walter H.
Cheung Wan Yee
Neida Philip H. Von
Pace Salvatore P.
Spitzer Robert H.
The BOC Group Inc.
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