Refrigeration – Processes – Circulating external gas
Patent
1985-08-14
1987-05-19
Capossela, Ronald C.
Refrigeration
Processes
Circulating external gas
62 27, 62 38, 62 42, F25J 302
Patent
active
046664831
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for recovering the heaviest hydrocarbons from a gaseous mixture, and in a particular to the recovery of a mixture of propane, butane and pentane from a petroleum refinery residual gas, of the type in which the gas to be treated is cooled and partly condensed and the condensate is distilled.
The mixtures of propane, butane and pentane, which are usually termed liquified petroleum gases or LPG, are condensed at ambient temperature under a pressure on the order of 10 to 20 bars. Their recovery from petroleum refinery residual gases is a recent development dictated by economic factors.
In the petroleum refinery residual gases, the LPG are diluted with lighter constituents, such as ethane, methane or hydrogen. It is usually necessary to remove these light constituents from these gases to a predetermined extent before sending them to a LPG treating unit which separates the constituents therefrom by distillation. This is why the recovery method itself includes a distillation operation.
In order to achieve a good propane, butane and pentane extraction yield, such a recovery method requires a cooling of the residual gas to a temperature on the order of -80.degree. C. Now, the obtainment of this temperature level by the use of a refrigerating unit involves the use of complex refrigerating units having two stages and employing a vaporization under a vacuum, which presents outlay and working problems.
An object of the present invention is to provide a method whereby it is possible to obtain an extraction yield on the same order in a particularly simple and cheap manner.
The invention therefore provides a method of the aforementioned type which comprises vaporizing in counter-current heat exchange with the gas to be treated a part of the liquid condensed at the top of the distillation column.
In advantageous manners of carrying out the invention:
the liquid condensed at the top of the column is subcooled to the final temperature for cooling the gas to be treated and it is expanded before being vaporized;
there is combined with said expanded liquid the vapour at the top of the column expanded at the same pressure and/or the vapour issuing from the cooled gas to be treated, after having caused the latter to be circulated in counter-current to this gas and having expanded it in a turbine.
Another object of the invention is to provide an installation for carrying out such a method. This installation, which is of the type comprising a thermal exchange line provided with means for separating the condensate from the cooled gas, and a distillation column in which this condensate is introduced, is characterized in that the distillation column comprises means for recovering a part of the condensed liquid at the top of the column and means or circulating the recovered liquid in counter-current heat exchange with the gas in the thermal exchange line.
One example of carrying out the invention will now be described with reference to the accompanying drawing, the single FIGURE of which diagrammatically represents an installation for recovering LPG according to the invention.
The gas to be treated, which is conveyed by a conduit 1, is a petroleum refinery residual gas consisting of a mixture of C1 to C5 hydrocarbons and hydrogen. The purpose of the treatment is to eliminate almost completely the light constituents (C1 and C2 hydrocarbons and hydrogen) from this mixture. All the pressures mentioned hereinunder are absolute pressures.
The gas, which is compressed at 23 bars, 30.degree. C. in a compressor 2, is cooled to an intermediate temperature on the order of -30.degree. C. and partly condensed in a heat exchanger 3, and then divided into a liquid phase and a vapour phase in a phase separator 4. The vapour phase is cooled to about -82.degree. C. in a second heat exchanger 5 and then divided into a liquid phase and a vapour phase in a second phase separator 6.
The vapour leaving the separator 6, which is almost solely formed by ethane, methane and hydrogen, is sent in counter-current to the e
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Capossela Ronald C.
L'Air Liquide Societe Anonyme pour l'Etude et, l'Exploitation de
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