Process for the production of carbon monoxide

Details Process for the production of carbon monoxide Process for the production of carbon monoxide

1999-06-28

2001-01-16

Capossela, Ronald (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Separation of gas mixture

C062S920000

Reexamination Certificate

active

06173585

ABSTRACT:

FILED OF THE INVENTION
The present invention relates to a process and to a plant for the separation of a gas mixture containing hydrogen, carbon monoxide and small amounts of impurities (methane and inert gases) in order to produce hydrogen and carbon monoxide as pure products.
BACKGROUND OF THE INVENTION
In the conventional process called “partial condensation”, the hydrogen separated from the gas mixture, after being cooled to a low temperature, is expanded to a pressure of about 5 to 10 bar so as to provide the refrigeration for keeping the plant cold and to cool the gas mixture to below −200° C. A plant of this type is described in U.S. Pat. No. 4,217,759 and uses at least four columns to produce carbon monoxide with a yield of 95% and a purity of approximately 99%. The hydrogen yield is about 99.9%, with a purity of approximately 95%.
The object of the invention is to simplify the process and the plant by reducing the number of columns used for the separation steps, but still maintaining these high purities and yields. Thus, the scrubbing columns usually placed at the head of the process and the distillation column for carbon monoxide/methane separation are eliminated.
The object of the invention is also to ensure that the coldest temperature in the plant is above that of the conventional process, with comparable hydrogen purities and carbon monoxide yields.
DE-A-2,460,515 describes a methane-contaminated hydrogen separation apparatus comprising a first separator, a first stripping column receiving the gas from the first separator, means for sending the head gas into a second separator and means for sending the liquid from the second separator into a second column fed with the liquid from the first separator.
EP-A-0,017,174 describes an apparatus for separating a mixture of carbon monoxide, hydrogen and methane, in which the mixture is partially condensed and the condensed portion is sent into the head of a first stripping column. The head gas from the stripping column is cooled and sent into a second phase separator. The liquid portion is sent into a second column. The base liquid from the second column is fed into the head of a third column and the uncondensed portion from the first phase separator is fed into this base of the third column.
The object of the present invention is also to provide a plant having a lower investment cost than in the prior art, by dispensing with any means of refrigeration.
The invention makes it possible in some cases to achieve complete hydrogen recovery and to obtain the hydrogen at a pressure greater than that obtained in the conventional process.
SUMMARY OF THE INVENTION
For this purpose, the subject of the invention is a process for the production of carbon monoxide and hydrogen from a gas mixture which essentially comprises these two substances and methane, the process comprising the following steps:
cooling the gas mixture to be treated so as to partially condense it;
separating a condensed portion of the mixture from an uncondensed portion containing mainly hydrogen in a first phase separator;
sending at least one fraction of the condensed portion into a first stripping column in order to produce a gas fraction containing hydrogen at the head of the column and a liquid fraction containing carbon monoxide and methane at the base of the column;
sending at least some of the head gas from the first stripping column into a second heat exchanger in order to partially condense it and to form a two-phase fluid;
separating the two-phase fluid into a condensed portion and an uncondensed portion containing mainly hydrogen in a second phase separator;
sending at least one fraction of the condensed portion into a second stripping column in order to produce a hydrogen-rich gaseous portion at the head and a carbon-monoxide-rich liquid fraction at the base, characterized in that the uncondensed portion is warmed and recycled into the mixture to be treated, or withdrawn.
At least some of the liquid fraction from the first stripping column may be sent into a purification column in order to produce substantially pure carbon monoxide at the head of the column and a methane-rich liquid at the base of the column.
Optionally, some of the head gas from the first stripping column (B) and/or at least some of the hydrogen-rich gas coming from the first phase separator is sent into a permeation unit or an adsorption unit and a carbon-monoxide-enriched gas is sent from the permeation unit or adsorption unit into the gas mixture to be treated.
Alternatively, some of the head gas from the first stripping column is recycled into the gas mixture to be treated without passing through a permeation unit or adsorption unit.
If the pressure of the mixture to be treated is high, the energy of separation is entirely provided by Joule-Thomson expansion of the starting mixture. Thus, the apparatus does not include expansion turbines, even for expanding a cycle gas.
If this is not the case, some of the energy of separation is provided by heat exchange between the gas mixture to be treated and a cryogenic liquid, or by injecting a cryogenic liquid into one of the columns or by means of a refrigeration cycle using an autonomous refrigerant.
Otherwise, the uncondensed portion from the second phase separator may be expanded in a turbine.
At most 15%, and preferably at most 10% of the mixture to be treated is sent into the second phase separator.
Preferably, the mixture to be treated contains less than 1% methane.
The head gas of the first stripping column may contain between 60 and 80% hydrogen and preferably between 71 and 74% hydrogen.
The base liquid of the first stripping column may contain between 95 and 99.9% carbon monxide and preferably between 98.8 and 99.1% carbon monoxide.
The subject of the invention is also a plant for the production of carbon monoxide and hydrogen from a gas mixture which essentially comprises these two substances and small amounts of methane and impurities, the plant comprising:
a first heat exchanger, a first phase separator, a first stripping column, means for cooling the gas mixture to be treated in the first heat exchanger, means for introducing at least some of the cooled gas mixture into the first phase separator and for withdrawing therefrom a gaseous portion, essentially comprising hydrogen, and a liquid portion, means for sending at least one fraction of the liquid portion into the stripping column for withdrawing therefrom a liquid containing mainly carbon monoxide and methane;
a second heat exchanger, a second phase separator, a second stripping column, means for sending at least some of the head gas from the first stripping column into the second heat exchanger and then into the second phase separator, means for sending at least some of the condensed portion from the second phase separator into the second stripping column and means for withdrawing a hydrogen-rich gas and a carbon-monoxide-rich liquid from the second stripping column, characterized in that it comprises means for heating the gaseous portion from the first phase separator. The plant may comprise a purification column and means for sending at least some of the base liquid from the first stripping column into the head of the purification column and means for withdrawing a carbon-monoxide-enriched head gas and a methane-enriched base liquid.
Preferably, the liquid from the second phase separator is sent into the head of the second stripping column.
The plant may comprise means for separating a gas by permeation or adsorption, means for sending some of the head gas from the first stripping column and/or at least some of the uncondensed portion from the first phase separator into permeation or adsorption separation means, this gas being mixed in order to produce a carbon-monoxide-enriched gas fraction, and means for sending this fraction back into the mixture to be treated.
The plant may comprise a refrigeration cycle using an autonomous refrigerant and/or a turbine and means for sending an uncondensed portion from the second phase separator into the turbine.
In some cas

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