Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-02-01
2001-10-02
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S642000, C062S908000, C062S909000
Reexamination Certificate
active
06295835
ABSTRACT:
The present invention relates to a process and to a plant for air separation by cryogenic distillation.
In particular, it relates to processes for producing oxygen gas containing between 60 and 96 mol. % oxygen.
It is known from EP-A-229803 and US-A-4022030 to use a mixing-column process to produce impure oxygen under pressure. Variants of this process, such as those described in EP-A-531182, use different operating pressures in the medium-pressure and mixing columns, and operate the mixing column at a pressure below or above that of the medium-pressure column. The air feeding the base of the mixing column can hence come from an intermediate stage of the main air compressor; in this case, two drying/decarbonating systems are necessary, one on each of the air streams.
Alternatively, the air may come from the exhaust of an expansion turbine, as described in EP-A-698772: in this case, there is either a minimum oxygen pressure or a minimum liquid production for the assembly to be energetically optimal.
US-A-5802872 describes the use of a brazed-plate exchanger and a reversible exchanger to cool the air intended for the medium-pressure column of a double column.
WO-A-99/42773, published on Aug. 26, 1999, describes a process in which between 50 and 80% of the air intended for an air separation unit is purified with respect to water in regenerators, and the rest of the air is purified by adsorption.
One object of the invention is to reduce the investment costs of impure-oxygen production units.
According to one subject of the invention, a process is provided for air separation in an air separation plant comprising at least two air distillation columns, including a medium-pressure column and a low-pressure column, in which:
a) a first compressed air flow is cooled in one or more first passage or passages of a heat-exchange system
b) a second compressed air flow is cooled in one or more second passage or passages of the heat-exchange system
c) at least one gas flow coming from a column of the plant is warmed in one or more third passage or passages of the heat-exchange system, characterized in that at least some of the gas flow coming from the plant is sent periodically and cyclically to the first passage or passages in order to regenerate the first passage or passages, and the first air flow is then returned to the/at least one of the third passage or passages of the heat-exchange system, which is or are free from the gas coming from the plant and is or are substantially without any impurities.
The first air flow is hence cooled periodically and cyclically in at least one first passage of the system and in at least one third passage of the system. If the first air flow is cooled in at least one first passage of the system, the gas coming from a column of the plant is warmed in at least one of the third passages. If the first air flow is cooled in at least one third passage of the system, at least some of the gas coming from a column of the plant regenerates the first passage or passages and no longer circulates in at least some of the third passages.
However, if only some of the gas is used to regenerate the first air passages, the rest of the gas may be warmed in the third passage or passages which is or are not used by the air.
It will be understood that the heat-exchange system may include a single exchange line or may comprise two separate exchange lines, including a first in which the first compressed air flow is cooled and a second in which the second compressed air flow is cooled. At least one of the third passages (or the third passage) in which the gas flow coming from the plant is warmed is located in the first exchange line. Optionally, at least one of the third passages, in which the gas flow coming from the plant is warmed, may be located in the second exchange line.
In the case in which only some of the gas is used to regenerate the first air passages, only this regenerating gas is sent to the first exchange line, it being possible for the rest of the gas to be warmed in the third passage or passages situated in the second exchange line.
Preferably, the first flow is sent to the heat-exchange system at a pressure below that at which the second flow enters.
The first flow rate may be less than the second flow rate, and preferably constitutes between 3 and 50% of the total air flow rate sent to the plant, in particular between 10 and 40% of the total flow rate.
At least some of the first air flow cooled in the exchanger may feed a column operating at a pressure at least 0.5 bar lower than the medium pressure.
According to a variant, the column operating at a pressure at least 0.5 bar lower than the medium pressure is a mixing column, a column operating at a pressure intermediate to the medium and low pressure or at the pressure of the low pressure column.
Preferably, a nitrogen-enriched residual gas from the low-pressure column and/or an oxygen-enriched gas from the mixing column or the low-pressure column and/or an argon-enriched gas from an argon column is warmed periodically and cyclically in the first passage where the first flow is cooled.
At least some of the first air flow may be withdrawn at an intermediate point of the exchange system, instead of at its cold end.
Either only the second air flow (not the first flow) is purified with respect to water and CO
2
, for example by adsorbent beds, before being cooled in the exchanger, or the second passage or passages is or are also regenerated by a gas coming from the plant substantially without any impurities, for example impure nitrogen from the low-pressure column. In this case, the purification upstream of the heat-exchange system may be dispensed with completely or partially.
In certain cases, the impure oxygen gas containing between 50 and 96 mol. % oxygen could be used to regenerate the first passage or passages and/or the second passage or passages and be used as a product while containing water and CO
2
. For example, a gas having this composition can feed a blast furnace. The impure oxygen is hence not wasted.
According to another subject of the invention, a plant is provided for air separation by cryogenic distillation, comprising:
at least two columns, including a medium-pressure column and a low-pressure column,
a heat-exchange system,
means for sending a first air flow to one/some first passage or passages of the heat-exchange system without purifying it before it enters the heat-exchange system,
means for sending a second air flow to one/some second passage or passages of the heat-exchange system,
means for sending a gas from the plant to one/some third passage or passages of the heat-exchange system where it is warmed,
characterized in that it comprises means for sending at least some of the gas from the plant to the first passage or passages in order to regenerate it cyclically, and means for sending the first air flow to the third passage/to at least one of the third passages.
Preferably, it comprises a mixing column, means for sending oxygen-rich liquid from the low-pressure column to the mixing column, and means for sending air from the heat-exchange system to the mixing column.
At least some of the required cooling power may be produced by a blower turbine fed with air from the exchanger.
According to a variant, there are means for sending impure oxygen to the first passage or passages as a regenerating gas, coming optionally from the mixing column or the low-pressure column.
There are either means for purifying the second air flow before sending it to the exchanger, or means for sending the gas from the plant to the second passage or passages in order to regenerate it, and not comprising means for purifying the second air flow entirely before sending it to the exchanger.
The invention will now be described in more detail with reference to
FIGS. 1
,
2
,
3
and
4
, which are diagrams of plants according to the invention.
In order to simplify the description, it will be assumed here that there is only a single first air passage and a single second air passage. In reality, that will probably be sev
Doerrler William
L'Air Liquide Societe Anonyme pour l'Etude et l'E
Young & Thompson
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