Gas separation: apparatus – Solid sorbent apparatus
Reexamination Certificate
2000-12-26
2002-07-30
Simmons, David A. (Department: 1724)
Gas separation: apparatus
Solid sorbent apparatus
C502S416000, C502S519000
Reexamination Certificate
active
06425946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of gas purification, and more specifically to the removal of trace impurities from inert gases such as nitrogen, helium, and argon using solid scavenger adsorption materials. More particularly, this invention comprises a method of reducing concentrations of trace impurities, such as hydrocarbons, carbon monoxide, and carbon dioxide, from process gases to parts-per-billion and sub-parts-per-billion levels using an ultra-low emission carbon based scavenger. This invention further relates to reducing concentrations of impurities such as organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide, and carbon dioxide, as well as other impurities such as oxygen and larger quantities of moisture (H
2
O), from process gases by combining an ultra-low emission carbon material of this invention and a second scavenger material capable of removing oxygen and/or moisture from the process gas.
2. Description of the Prior Art
Numerous products and processes require pure gases. One known method of gas purification involves the adsorption of process gas impurities on a bed or column of solid scavenger material. In these solid adsorption methods, impurities are caught by the surface of the scavenger material while the process gas preferably passes unaltered through the bed or column.
Commonly used solid scavenger adsorption materials include alumina, activated carbon, silica, adsorption clays, and secondary scavengers. Activated carbon, for example, is used in PSA (Pressure Swing Adsorption) plants and for solvent recovery from air in painting facilities (See, for example, Wood and Stampfer,
Carbon,
30:593 (1992); Wood and Stampfer,
Carbon,
31:195 (1993); Nelson et al.,
Am. Ind. Hyg. Assoc. J.,
33:797 (1972); and Nelson et al.,
Am. Ind. Hyg. Assoc. J.,
52:235 (1991)). These techniques are known to reduce selected impurities in a gas stream down to single digit percentages, and perhaps even as low as ppm (parts per million) concentrations. However, the use of solid scavenger adsorption materials operating at ambient conditions to reduce parts-per-billion (ppb) levels of impurities, particularly hydrocarbons, to sub-ppb levels without contaminating the gas stream with other impurities such as moisture is not known.
For most applications, reducing impurities in gases down to the ppm level is satisfactory. However, ultra-pure gases having impurity concentrations not exceeding ppt (parts-per-trillion (ppt) levels are required in a growing number of industries. For example, in semiconductor fabrication processes, gases such as nitrogen, helium and argon are often required to not have more than low ppb or sub-ppb impurity levels to ensure that the impurities do not degrade the quality, and hence the performance of the semiconductor chips. Gas purification systems are therefore widely used in the manufacture of semiconductors to remove process gas impurities to very low, trace concentrations.
The desire to develop methods to reduce impurities in process gases down to sub-ppb concentrations is further driven by the present ability to measure impurities at extremely low levels. Modem chemical instrumentation such as Atmospheric Pressure Ion Mass Spectrometry (APIMS) permits the detection of process gas impurities such as carbon monoxide, carbon dioxide, oxygen, and moisture (H
2
O) at sub-ppb concentrations.
The advances in the detection of trace levels of hydrocarbons with APIMS has motivated researchers to further reduce the levels of these impurities in ultra-pure process gases to below the limits of detection of this supersensitive instrumentation. One challenge has been to develop gas purification materials and techniques that remove hydrocarbon impurities from an ultra-pure gas without adding trace amounts of other impurities.
Conventionally activated carbon, for example, is known as a very effective adsorbent for removing hydrocarbon impurities from gases. However, conventionally activated carbon is typically activated at 200° C. to 400° C. in gas streams contaminated with ppm levels of impurities such as moisture and CO
2
. After conventional activation, the carbon material contains trace amounts of water and CO
2
that are either not completely removed during activation or re-adsorbed in the contaminated environment of the treatment process. The carbon material may also produce trace amounts of moisture and CO
2
during thermal activation due to chemical reaction of residual functional groups or adsorbed species, such as by dehydroxylation or decarboxylation reactions. The residual water and CO
2
in the conventionally activated carbon material are then released in small quantities into a gas stream during a gas purification process, thereby causing significant contamination of the gas and rendering the effluent gas useless for high purity applications. In some cases, conventionally activated carbon is characterized as “hydrophobic” (repels or fails to adsorb water), even though traditionally activated carbon has been shown to weakly adsorb moisture upon exposure of a gas containing several hundreds to several thousands of ppm of moisture (see, for example, Barton et al.,
Carbon,
22:22 (1984), which is specifically incorporated herein by reference). However, this adsorbed moisture, is also easily released into a process gas stream during purification of the gas. Thus, reducing hydrocarbon impurities in a process gas to sub-ppb levels while maintaining very low levels of water vapor and CO
2
has proven extremely difficult.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a method for reducing the concentration of hydrocarbon impurities as well as other impurities in a process gas to sub-parts-per-billion (sub-ppb) levels, while at the same time not emitting higher levels of other contaminants, such as water vapor and CO
2
, into the process gas being purified.
Another object of this invention is to provide “ultra-low emission” (ULE) carbon materials for reducing trace impurities such as organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide (CO), carbon dioxide (CO
2
), and small amounts water vapor from process gas streams such as helium (He), nitrogen (N
2
) and argon (Ar) to parts-per-billion (ppb) and sub-parts-per-billion (sub-ppb) levels.
Another object of this invention is to provide a method of producing ultra-low emission (ULE) carbon materials capable of reducing the concentration of organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide (CO), carbon dioxide (CO
2
), and water vapor (H
2
O) from a process gas to ppb and sub-ppb levels.
It is a further object of the present invention to provide a method of purifying gases with ultra-low emission (ULE) carbon materials prepared according to the method of this invention, wherein the method reduces trace amounts of hydrocarbon, carbon monoxide (CO), carbon dioxide (CO
2
), and water vapor (H
2
O) impurities to ppb and sub-ppb levels.
It is a further object of the present invention to provide a one-component gas purifier system comprising a bed of an ultra-low emission (ULE) carbon material of this invention capable of reducing trace levels of organic compounds including, but not limited to, substituted and unsubstituted hydrocarbons, wherein said hydrocarbons include saturated, unsaturated, and aromatic hydrocarbons, carbon monoxide, carbon dioxide, oxygen and water vapor from a process gas to ppb and sub-ppb levels.
Yet another object of this invention is to provide a two-component gas purifier system comprising an ultra-low emission (ULE) carbon material of this invention and a secondary scavenger mate
Fraenkel Dan
Funke Hans H.
Houlding Virginia H.
Hogan & Hartson LLP
Lawrence Frank M.
Matheson Tri-Gas, Inc.
O'Rourke Sarah S.
Petersen Steven C.
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