Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-12-17
2001-07-10
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S645000
Reexamination Certificate
active
06257020
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to processes and plants for the cryogenic separation of gases from air.
DESCRIPTION OF THE RELATED ART
The pressures referred to below are absolute pressures. Moreover, the terms “condensation” or “vaporization” should be understood to mean either condensation or vaporization proper, or pseudocondensation or pseudovaporization, depending on whether the pressures involved are subcritical or supercritical.
In recent years, the use of “pump-based” processes for the production of pressurized oxygen has become widespread. These processes consist in extracting an oxygen-enriched liquid fraction from the lower part of the low-pressure column, typically at the bottom, in pumping this liquid to the required pressure, in vaporizing it and warming it to a temperature close to ambient temperature by heat exchange with the incoming air and/or a fluid enriched with pressurized nitrogen. This process therefore allows an oxygen compressor to be saved and is therefore more economical. Likewise, production may be carried out by pumping pressurized nitrogen or argon.
This widespread use of pumping processes has partly been made possible by the use of adsorption for removing, preferably, water and CO
2
at the reversible exchangers.
Moreover, in order to be able to vaporize the high-pressure oxygen, it is necessary to use a high-pressure heat-generating fluid (air or nitrogen-enriched fluid) which will condense by indirect exchange with oxygen, as in U.S. Pat. No. 4,303,428, and/or by isentropic expansion in a turboexpander (see U.S. Pat. No. 5,329,776), so as to balance the heat budget of the distillation part. High pressure should be understood to mean pressure greater than the pressure of the medium-pressure column of a double-column system or greater than the pressure on the condenser side of the vaporizer of a single column. The presence of a high-pressure fluid has, furthermore, favoured the use of more complex cycles with multiple turboexpanders for the production of liquid.
Examples of pumping cycles with two turboexpanders are given in documents U.S. Pat. No. 5,329,776, GB 2,251,931, U.S. Pat. No. 5,564,290 or U.S. Pat. No. 5,108,476. Unfortunately, for all the known processes, the quantity of liquid that can be produced is limited if it is desired not to increase the size of the air compressor (i.e. the flowrate at the first stage).
U.S. Pat. No. 5,758,515 discloses a process for the production of pressurized oxygen using a first turboexpander which feeds the medium-pressure column of a double column and a turboexpander fed by a supercharger, all the expanded air of which is recycled to the primary compressor of the apparatus.
SUMMARY OF THE INVENTION
One object of the present invention is to increase the production of liquid in an apparatus with a pump and two turboexpanders, without increasing the size of the air compressor, while improving the cycle performance. Another object of the present invention is to achieve better optimization of the exchange diagram for an air separation apparatus having two turboexpanders.
According to one subject of the invention, a process is provided for the cryogenic separation of gas from air in a system of columns comprising at least one air distillation column comprising the steps of:
compressing all of the air to a medium pressure and at least one part of the air to an intermediate pressure between the medium pressure and a high pressure,
compressing air from the intermediate pressure to the high pressure,
dividing the compressed air at the high pressure into a first and a second fraction,
cooling the first fraction in a heat exchanger and at least partly expanding it in a first turboexpander,
cooling the second fraction in the heat exchanger and at least partly expanding it to the intermediate pressure in a second turboexpander,
warming the expanded part of the second fraction (or the expanded second fraction) in the heat exchanger and recycling at least one part thereof into the air at the intermediate pressure,
sending air from the first turboexpander to a first column, where it becomes enriched with nitrogen at the top of the column and enriched with oxygen at the bottom, and
withdrawing a liquid coming at least partially from one column of the system and vaporizing it, optionally after pressurization, in the heat exchanger,
characterized in that the feed pressure of the first turboexpander is not less than the feed pressure of the second turboexpander.
According to other optional characteristics of the invention, a process is provided in which:
the inlet pressures of the first and second turboexpanders are identical or the inlet pressure of the first turboexpander is greater than the inlet pressure of the second turboexpander, preferably greater than the inlet pressure of the second turboexpander by at least 1 bar or even at least 2 bar;
the first column forms part of a double column or a triple column;
an oxygen-enriched stream and a nitrogen enriched stream are sent from the first column to a second column of the double column, the first column operating at a higher pressure than the low-pressure column;
a liquid stream is withdrawn from the low-pressure column or the medium-pressure column (or the intermediate column in the case of a triple column) and vaporized by heat exchange with air;
all of the air is compressed to the intermediate pressure;
the intake temperature of the second turboexpander is greater than that of the first turboexpander;
an unexpanded portion of the first fraction condenses by heat exchange with a fluid withdrawn from the column;
the portion which condenses exchanges heat with the liquid, which vaporizes;
an unexpanded portion of the second fraction condenses by heat exchange with a fluid withdrawn from the column;
the portion which condenses exchanges heat with the liquid, which vaporizes;
the liquid stream is enriched with oxygen, with nitrogen or with argon;
several liquid streams vaporize in the heat exchanger;
a fraction of the air is cooled in a refrigerating unit;
at least one part of the second fraction is cooled in a refrigerating unit;
the outlet temperature of the refrigerating unit is the inlet temperature of the turboexpander;
the energy of at least one of the turboexpanders serves to drive one or more compressors;
one stream from the low-pressure column feeds an argon column;
an air stream is sent to the first column without having been expanded in one of the turboexpanders.
According to other aspects of the invention, a plant is provided for the cryogenic separation of gases from air by cryogenic distillation, comprising:
at least one first air distillation column,
an exchange line,
means for compressing all of the air to a medium pressure,
means for compressing at least one part of the air to an intermediate pressure between the medium pressure and a high pressure,
means for compressing air from the intermediate pressure to the high pressure,
means for sending a first and a second air fraction at the high pressure to the exchange line,
a first turboexpander for expanding at least one part of the first fraction, optionally to the medium pressure,
a second turboexpander for expanding at least one part of the second fraction to the intermediate pressure,
means for warming at least one portion of the expanded part of the second fraction,
means for recycling at least one part of this portion into the air at the intermediate pressure and means for withdrawing at least one liquid from one column of the plant and means for sending it to the exchange line, characterized in that it does not include means for increasing the feed pressure of the second turboexpander with respect to the feed pressure of the first turboexpander.
According to other optional characteristics, the plant may comprise means for increasing the feed pressure of the first turboexpander with respect to the feed pressure of the second turboexpander.
By recycling the stream from the warm turboexpander at a pressure greater than the pressure of the medium-pressure column, it is possible
Doerrler William
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Young & Thompson
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