Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-01-25
2001-09-18
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06289693
ABSTRACT:
BACKGROUND OF THE INVENTION
Synthetic natural gas or syngas comprises a gaseous mixture derived from carbon sources and contains chiefly hydrogen (H
2
) and carbon monoxide (CO) together with low amounts of other gases.
Synthetic gases of varying and controlled H
2
/CO molar ratios are employed in various petrochemical processes, such as the production of methanol and oxo alcohols, aldehydes, acids, and other chemical compounds. One type of synthetic gas is produced by the reaction of steam and methane in a reformer, which reaction provides a synthesis gas, after carbon dioxide removal, of about a H
2
/CO molar ratio of three (3) and a residual methane content of up to about five percent (5%).
It is desirable to produce a synthesis gas of a selected and lower H
2
/CO molar ratio for use in particular petrochemical processes, such as a molar ratio of about 0.9 to 1.0 and also with a lower methane content.
U.S. Pat. No. 5,832,747, issued Nov. 10, 1998, hereby incorporated by reference, discloses a wholly cryogenic process for the cryogenic adjustment of the hydrogen and carbon monoxide molar ratio of a syngas, typically of even molar ratio. The process comprises partially condensing, in a cold box, at least a portion of the gaseous feed mixture and then separating the partially condensed feed mixture in a phase separator, to provide a gaseous product stream of a different H
2
/CO molar ratio than the gaseous feed. The process includes warming the resultant product stream without any further cryogenic separation.
It is desirable to provide a new and improved process and system for providing from a syngas, a product gas of a defined selected H
2
/CO molar ratio and a low methane content.
SUMMARY OF THE INVENTION
The invention comprises a system and process for the production of a synthesis gas having a specific H
2
/CO molar ratio, such as in the range of 0.7 to 1.2 and a low residual methane content. The system and process comprises employing a combination of membrane separations to remove excess hydrogen from the syngas feed, and the subsequent cryogenic separation of excess methane to provide a gaseous product stream with a selected H
2
/CO molar ratio; for example, of about 0.7 to 1.2 or other selected H
2
/CO ratio, and where applicable, a methane content of less than about 1.5 percent (1.5%).
In one embodiment, syngas from a steam-methane reformer system with a high molar ratio of H
2
/CO, and with residual methane, is treated to remove carbon dioxide. The syngas is then heated, typically with steam, and introduced into a semipermeable membrane unit or system. Excess hydrogen is removed as a permeate gas and a retentate or treated gas stream of the desired H
2
/CO molar ratio withdrawn; such as, but not limited to: 0.7 to 1.2, or such other ratio as required in the particular petrochemical process in which the syngas is to be employed. The excess permeate hydrogen gas may be recompressed and employed for and in subsequent cryogenic cold boxes or employed for other purposes. The retentate gas contains undesirable high amounts of methane, which is removed by cryogenic fractionation or as a bottoms stream, while the syngas (H
2
/CO) of desired H
2
/CO molar ratio is withdrawn as an overhead stream.
The membrane removal of excess hydrogen or other excess gases from the syngas feed stream is accomplished by one or more semipermeable membrane systems, or units in series or in parallel with the particular semipermeable membrane material selected for the particular gas to be removed as an excess gas; such as, but not limited to: polymeric and metal membranes.
There are a wide variety of membrane materials and designs employed to remove hydrogen gas as a permeate gas. The membrane may comprise hollow fibers or tubes, flat sheets, or spiral wound membranes. Generally, the excess hydrogen removed as a permeate is at a pressure of about 100 psig and may be recompressed for other uses in the process or in other processes.
The system and process shall be described in a particular embodiment directed to the treatment of a syngas feed stream from a steam-methane transformer, with the object to produce a H
2
/CO molar ratio of about 0.9 and with a low residual methane content of 1.5 percent (1.5%) or lower. However, it is recognized that other syngas treatment streams may be used and other molar ratios obtained by employing a combination of a membrane separations to remove hydrogen from or adjust the feed gas stream composition, followed by cryogenic-fractionating of the retentate gas stream, to remove high boiling point materials like alkanes, such as methane, and to recover an adjusted H
2
/CO molar ratio product stream.
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Calfee Halter & Griswold LLP
Chart Industries, Inc.
Doerrler William
Drake Malik N.
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