Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Patent
1997-12-24
1999-07-20
Capossela, Ronald
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
62626, 62909, F25J 100
Patent
active
059243066
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to the separation of hydrocarbon and, more particularly, ethylene-containing gases, using a low-temperature rectification technique and can be used in the petrochemical, oil-processing, and gas industries, largely in the production of ethylene.
PRIOR ART
A method has been proposed (see U.S. Pat. No. 3,119,677, Int. Cl. 62-23, separation, down to temperatures from minus 120 to minus 130.degree. C., the process being exercised in a stepwise manner with the intermediate condensates brought out to a demethanizer. It should be noted that cooling to temperatures between minus 90 and minus 95.degree. C. is effected through an external cascade-type cycle on propylene and ethylene used as the coolants. Then, the mixture is cooled via an "internal" cycle using a partial condensate as follows. Part of the condensate that is formed as the gas mixture is cooled to minus 100.degree. C. or so, is throttled (as this takes place, its temperature decreases), evaporated by taking the heat off the gas mixture that, accordingly, becomes less heated, and recycled to the initial gas mixture. At a hydrogen/methane ratio of 1:2 by volume (gasoline pyrolysis gases) this condensate comprises a methane-ethylene mixture containing as much as 90% of methane, so that the above-mentioned "internal" cooling cycle ensures the gas mixture being cooled in the range between minus 120 and minus 130.degree. C. Used as the cooling the vapors tapped after the rectification step with the ethylene (the coolant). efficiency when applied to gas mixtures in which hydrogen is a major component. Thus, for the ethane pyrolysis gases (a hydrogen/methane ratio of 4:1) the hydrocarbon condensate resulting from cooling this mixture to temperatures between minus 120 and minus 130.degree. C. contains as much as 65 vol. % ethylene and ethane, so that when evaporating such a condensate, the bulk of the cold can be generated at temperatures above the demethanization of ethane pyrolysis gasses affords their cooling prior to being rectified to temperatures between ca. minus 100.degree. C. to minus 105.degree. C. only. The result is a higher loss of the target product, namely, ethylene, because of a decreased amount of the condensate involved in the rectification process and higher ethylene losses with the residual gas.
Another gas mixture demethanization method (see the USSR Author' in this respect. It consists in a stepwise cooling of the gas mixture under pressure via the internal and external cycles with the formation of intermediate partial gas mixture components and their rectification with the production of a liquid demethanized phase and a gaseous methane fraction. As this takes place, the internal cooling cycle is obtained by throttling the coolant in the form of the partial condensate produced from the above methane fraction that is cooled via the external cycle and recycling the said throttled condensate, after it has been used as the coolant, to the initial gas mixture. mixtures in which hydrogen is a relatively abundant component, e.g., such as products resulting from ethane pyrolysis. This occurs due to lower gas mixture cooling temperatures prior to rectification, as compared to those partial condensate (the coolant) containing methane, a low-boiling (70 vol. % and 35 vol. %, respectively). since it also entails a high loss of the target product, i. e., ethylene, escaping with the gas phase which is detained after the partial condensate (the internal cycle coolant) has been obtained from the methane fraction, and, moreover, is characterized by a limited specific refrigeration capacity of the internal cooling cycle, which fact generates a need for additional power for the gas mixture demethanization purpose. These depth in the production of the partial condensate (the internal cycle coolant). This leads to the low-boiling components such as ethylene and methane being insufficiently condensed with the result that the concentration of ethylene in the residual gas is higher than normal (10 to 18 vol. %)
REFERENCES:
patent: 3119677 (1964-01-01), Moon et al.
patent: 4225329 (1980-09-01), Bailey et al.
patent: 4987744 (1991-01-01), Hanoley
patent: 5566555 (1996-10-01), Hewitt
Capossela Ronald
Nath Gary M.
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