Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Patent
1997-07-28
1999-01-19
Capossela, Ronald
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
62 482, F25J 100
Patent
active
058602941
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method for recondensation of gaseous hydrocarbons from mixtures of inert gases and gaseous hydrocarbons, as stated in the introductory of claim 1.
BACKGROUND
Gas present in tanks in LPG/LEG ships at reloading is removed by means of nitrogen or an inert gas. The method which is used presently involves release of the off-gas to atmosphere. This off-gas constitutes a loss of about 250,000 tons of hydrocarbons per year, mainly propane and butane.
NO Patent Application 932417 discloses a method for recovering a portion of this off-gas by condensing the off-gas in heat exchange with liquid nitrogen, whereupon the evaporated nitrogen is used in the removal of gas from the tanks. In relation to the gas removed the consumption of nitrogen with this method is unnecessary high.
U.S. Pat. No. 5,176,002 describes a process for recovery of vapor released during filling of hydrocarbons in storage tanks. Inert gas is supplied to the tank and intermixed with hydrocarbon vapor to form a non-combustible composition. The composition is thereafter subjected to a two-step compression, and a separation process to condense and separate the hydrocarbon vapor from the inert gas. This process involves the use of a closed coolant circuit in the separation/condensation step.
OBJECT
The object of the invention is to provide a method for recondensation of off-gas in connection with gas removal from gas tanks to avoid increased nitrogen consumption during gas removal, and simultaneously recover in liquid form a substantial part of the hydrocarbon content of the off-gas.
THE INVENTION
This object is achieved by a method according to the characterizing section of patent claim 1. Further favorable features appear from the dependent claims.
According to the present method, the condensate and the cargo gas composition are heat exchanged with a part of its condensate and cargo gas/inert gas mixture, which is to be released to the atmosphere or burned, in a separate condensation step.
More particularly, after compression in at least one step the gas phase from the gas tank is partly condensed in the recondensation plant cargo condenser by means of cooling water or another suitable coolant. The mixture comprising condensate and cargo gas/inert gas is then conveyed to a first phase separator, e.g. the ordinary liquid collector in the recondensation plant.
In accordance with the invention the uncondensed part of the cargo gas/inert gas composition is conveyed from the top of the first phase separator to a separate condensation or heat exchange step. It is then cooled by expanding a part of the condensate from the first phase separator, after cooling in the ordinary intermediate cooler in the recondensation plant, into one side of a separate heat exchanger. After evaporation and superheating in the separate heat exchanger, the gaseous cargo gas is led back to the suction end of the compressor in the recondensation plant.
In the separate heat exchange step the cargo gas component in the cargo gas/inert gas composition is subjected to further condensation, whereupon the resulting condensate/gas composition is expanded to a lower pressure and led into a second phase separator, e.g. constructed to serve as a storage tank for collected cargo gas condensate. Inert gas from the second phase separator including some cargo gas is expanded into a separate section in the separate heat exchange step and is heated by heat exchange with the cargo gas/inert gas composition from the first separator and released to atmosphere, optionally after combustion in a flare.
The remaining part of the condensate from the first phase separator or the liquid collector is also conveyed to the second phase separator or storage tank.
In this way it is possible to obtain a high degree of recovery, typically up to 93% of the original content of cargo gas in the tank, by means of a simple and cost effective process. The term `cargo gas` is intended herein to include hydrocarbon residue from gas tanks, such as propane, mixtures of different hydrocarb
REFERENCES:
patent: 3271965 (1966-09-01), Maher et al.
patent: 4249387 (1981-02-01), Crowley
patent: 4645522 (1987-02-01), Dobrotwir
patent: 5036671 (1991-08-01), Nelson et al.
patent: 5176002 (1993-01-01), O'Brien et al.
Capossela Ronald
Sinvent AS
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