Note: Descriptions are shown in the official language in which they were submitted.
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THE SEPARATION OF GAS MIXTURES
The present invention relates to processes in which a product gas forming a
constituent of a feed gas mixture is separated for use i~ an end user's
process. More particularly, the present invention relates to a process in
which a product gas is separated from a feed gas mix~ure but where, to meet
peak demand, a vessel containing the product gas in liquid phase is
available to augment the product gas being produced by the separation
process.
In, for example, a pressure swing adsorption (PSA) plant for the production
of nitrogen from air a bed of adsorbent contained within a pressure vessel
is put through a cycle which includes an adsorption step during which feed
air is initially compressed before being pumped through the bed under
pressure where most of the oxygen and a proportion of the nitrogen and
substantially all of the carbon dioxide and water vapour in the feed air
are adsorbed and a nitrogen rich product gas is supplied from the outlet of
the bed. Frequently, the nitrogen rich product gas is passed first to a
nitrogen receiver vessel from which it is subsequently drawn by the end
user of the product gas. It is common in such processes to have a supply
of liquid nitrogen available to augment the product gas leaving the
nitrogen receiver vessel during periods of peak demand by the ultimate user
of the nitrogen.
It is an aim of the present invention to provide in a process where a feed
gas mixture containing a product gas is first compressed prior to delivery
to a separation unit, steps whereby the output of the compressor and hence
the separation plant is enhanced during periods of high demand.
According to the present invention, in a process where a feed gas mixture
containing a product gas is first compressed prior to delivery to a unit
for the separation of the product gas from the feed gas mixture and in -
which a reservoir of the product gas in li~uid phase is available for
augmenting the supply of product gas from the separation unit during peak
demand; the step of vaporising the product gas in liquid phase by heat
exchange with the feed gas mixture prior to the compression of the feed gas
mixture.
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Preferably, the feed gas mixture is in direct heat exchange with the
product gas in l;quid phase.
Alternatively, a fluid may be used in a closed system which is arranged to
be in heat exchange relation first with the feed gas mixture and
subsequently with the product gas in liquid phase.
Embodiments of the invention will now be described, by way of example,
reference being made to the Figures of the accompanying diagrammatic
drawings in which:-
Figure 1 is a schematic view of a plant for the production of a product gasfrom a feed gas mixture containing said product gas; and
Figure , is a schematic view similar to ~igure 1 but illustrating a
modification thereof.
As shown in Figure 1 a plant for the separation of nitrogen from air
comprises a feed air pipeline 2 which communicates with the inlet of a
compressor 4. The feed air line forms one pass of a heat exchanger 14.
The outlet of compressor 4 communicates in turn with an air separation unit
in the form of a PSA unit 6. From the unit 6 a nitrogen rich product gas
flows either directly to an end user's process or to a reservoir ~not
shown). Forming part of the plant is a pressure vessel 10 containing
liquid nitrogen at substantially the same purity level as the product gas
which emanates from the air separation unit 6. A pipeline 12 extends from
the vessel 10 through the heat exchanger 14 and hence joins the pipeline 8.
It will be apparent, that in use the feed air prior to reaching the
compressor 4 via the pipeline 2 will be in heat exchange relationship with
the fluid in the reservoir 10. This will have the effect of enhancing the
output from the compressor 4 by making more dense the feed air whilst at
the same time vaporising the liquid in the reservoir 10 during peak demand.
By doing this the amount of relatively expensive liquid nitrogen that is
used to support the peak demand will be reduced.
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Turning now to ~igure 2, where like parts have been given the sa~e
reference numerals, the plant for the separat;on of nitrogen from air
includes an air feed pipeline Z which communicates with the inlet of an air
compressor 4. The outlet from the air compressor 4 communicates with a
pipeline 5 which extends through an after cooler 20 and hence to a PSA
nierogen separator 6. A nitrogen rich product gas passes through a
pipeline 8 directly to an end user's process or into a nitrogen receiver
reservoir (not shown). Located in the pipeline 2 is a pre-cooler 22; and a
pipeline 2~s extends between the pre-cooler 22 and the after cooler 20.
Extending from the after cooler 20 is a pipeline 26 which passes initially
through a heat exchanger 28 and hence to the pre-cooler 22. A pipeline 12
extends from a presure vessel 10 containing liquid nitrogen through the
heat exchanger 28 and joints the pipeline 8.
In use air flows through the feed air pipeline 2 through the pre-cooler 22
and hence to the compressor 4. Compressed air from the compressor 4 flows
through the after cooler 20 via pipeline 5 and hence to the PSA separation
unit 6. Product gas rich in nitrogen leaves the unit 6 via ~he pipeline 8
either to go directly to a user's process or to enter a nitrogen reservoir
(not shown). A fluid, for example, water, flows through a closed circuit
system which includes pipeline 26, pre-cooler 22, pipeline 24 and after
cooler 20 as well as the heat exchanger 28. During peak production time
the l;quid phase product gas in the vessel 10 will pass via pipeline 12
through the heat exchanger 28 and will join the pipeline 8. This flow of
gas will chill the water in the closed circuit system and the chilled water
will pre-cool the feed air in the pre-cooler 22. The chilled water
although warmer in the pipeline 24 will also chill the compressed air in
the after cooler 20. The heat picked up by the water in the line 26 will
assist in the vaporisation of the liquid phase product gas in the vessel 10
during its passage through the heat exchanger 8.
Although reference has been made to PSA air separation units, it is
apparent that any other form of feed gas separation unit could be employed.
For example, a temperature swing adsorption (TSA) unit or a cryogenic gas
separation unit.
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