Gas separation: processes – With control responsive to sensed condition – Concentration sensed
Utility Patent
1998-10-15
2001-01-02
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Concentration sensed
C095S014000, C095S090000, C095S116000, C096S111000, C096S112000, C096S117000, C096S419000, C096S420000
Utility Patent
active
06168645
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to gas purifiers and, more particularly, to a gas purifier having a safety system for cutting off gas flow to and removing impure gas from a gas purifier.
Gas purifiers working on sorption of impurities mainly belong to two categories: getter-based purifiers and catalyst-based purifiers. The difference between the two types is that getter-based purifiers operate using the chemisorption principle, that is, non-reversible sorption, so that once the purifier is exhausted it must be replaced; while catalyst-based purifiers operate using the physisorption principle, so that these purifiers may be regenerated by thermal or chemical treatment once exhausted. Catalyst-based purifiers may be desirable for their regeneration characteristic, but the range of gases which may be purified is narrow compared to the possibilities of getter-based purifiers. Both kinds of purifiers are meant to remove traces of reactive impurities from an inert (to the purifier chemical) gas. Both types of purifiers may also be ruined in case a wrong gas or a high concentration of a particular gas is fed into the purifier, or in case an atmospheric gas gets in due to failure of the gas line.
Getter-based gas purifiers produce extremely pure gases, e.g., argon, helium, hydrogen, and nitrogen, for use in applications such as semiconductor processing. These gas purifiers, which are sometimes referred to as “getter columns,” typically include a bed of getter material which purifies gas flowing therethrough by adsorbing impurities therefrom.
Getter-based gas purifiers are potentially hazardous because the getter material contained therein may be highly reactive with high concentrations of certain impurities. For example, a high concentration of an impurity gas, such as oxygen, may be accidentally introduced into a gas purifier containing a known zirconium-based getter material, and an exothermic reaction results. A “high” concentration of impurity gas (or “reactive gas”), as referred to herein, means a quantity of gas per unit time such that the heat generated by the exothermic reaction with the getter has not time to be dissipated (by conduction, or by the flowing gas itself) and accumulates with a steep rising of temperature. The critical amount of gas depends on the nature of the gas (i.e., the higher the heat of reaction with the getter, the lower the critical amount of gas), on the concentration of the reactive species in the overall gas composition, and on the gas flow velocity. In general, oxygen concentrations of more than 1-2% may be critical, whereas it has been observed that nitrogen concentrations below 5-6% pose no problem to the purifier, due to nitrogen's lower reactivity.
The steep temperature rise resulting from the exothermic reaction may cause melting of the containment wall of the purifier. The containment wall, which is typically formed of stainless steel, may melt at temperatures as low as about 1,000C. because the getter material contacting the containment wall reacts therewith and forms a eutectic composition. If melting of the containment wall results in the formation of a hole therein, then breach of containment of the getter material occurs, which is potentially catastrophic. The destruction of the getter purifier and consequent processing interruptions would result from such a reaction.
Catalyst-based gas purifiers may also be damaged from high concentrations of reactive gases. The most widely known and used catalyst-based purifiers are those based on zeolites-supported nickel for purification of nitrogen. In case of contact with atmospheric gases, the zeolite-nickel bed can heat up to temperature values of about 600-800° C., that may lead to sintering of particles and heavy formation of nickel oxide, with the result that the purifier is no longer regenerable.
In view of the foregoing, there is a need for a safety device for gas purifiers which protects against breach of containment of the purifier material in the event high concentrations of impurity gases are introduced therein. To ensure that the gas purifier is always protected against breach of containment of the purifier material, the safety device must be extremely reliable. In other words, the safety device preferably should not include complex instrumentation which, in addition to being expensive, may either malfunction or generate false alarms which are disruptive and costly to a semiconductor fabrication facility.
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Lorimer D'Arcy H.
Succi Marco
Vergani Giorgio
Hickman Coleman & Hughes LLP
SAES Getters S.p.A.
Spitzer Robert H.
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