U.S. patent application number 16/772357 was filed with the patent office on 2021-03-18 for method and apparatus for the cryogenic separation of a synthesis gas containing a nitrogen separation step.
The applicant listed for this patent is L'Air Liquide, Societe Anonyme pour I'Etude et I'Exploitation des Procedes Georges Claude. Invention is credited to Antoine HERNANDEZ.
Application Number | 20210080175 16/772357 |
Document ID | / |
Family ID | 1000005249163 |
Filed Date | 2021-03-18 |
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United States Patent
Application |
20210080175 |
Kind Code |
A1 |
HERNANDEZ; Antoine |
March 18, 2021 |
METHOD AND APPARATUS FOR THE CRYOGENIC SEPARATION OF A SYNTHESIS
GAS CONTAINING A NITROGEN SEPARATION STEP
Abstract
A method for separating a gas mixture comprising carbon
monoxide, nitrogen and hydrogen involves sending a
hydrogen-depleted fluid to a denitrification column (K2) having a
top condenser (C1) and a bottom reboiler (R2) in order to produce a
nitrogen-enriched gas at the top of the column and a
nitrogen-depleted liquid at the bottom of the column, cooling the
condenser of the denitrification column by means of a nitrogen
cycle using a nitrogen compressor (V1, V2, V3), vaporising, in the
heat exchanger of the condenser, the liquid nitrogen (53) from the
nitrogen cycle, and returning the nitrogen (55) vaporised in the
heat exchanger to the nitrogen compressor.
Inventors: |
HERNANDEZ; Antoine;
(Pontault Combault, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
L'Air Liquide, Societe Anonyme pour I'Etude et I'Exploitation des
Procedes Georges Claude |
Paris |
|
FR |
|
|
Family ID: |
1000005249163 |
Appl. No.: |
16/772357 |
Filed: |
December 13, 2018 |
PCT Filed: |
December 13, 2018 |
PCT NO: |
PCT/FR2018/053280 |
371 Date: |
June 12, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25J 2210/42 20130101;
F25J 3/0257 20130101; F25J 3/0261 20130101; F25J 2200/40 20130101;
F25J 2250/20 20130101; F25J 3/0252 20130101; F25J 3/0223 20130101;
F25J 2205/04 20130101; F25J 2270/904 20130101; F25J 2270/902
20130101; F25J 2250/02 20130101 |
International
Class: |
F25J 3/02 20060101
F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2017 |
FR |
1762148 |
Claims
1.-13. (canceled)
14. A process for separating a gas mixture comprising carbon
monoxide, nitrogen, hydrogen, and methane, the method comprising:
i) cooling the gas mixture in a heat exchanger, ii) separating the
gas mixture cooled in the heat exchanger by at least one scrubbing
and/or distillation and/or partial condensation step, to form a
hydrogen-depleted fluid containing carbon monoxide and nitrogen,
iii) introducing the hydrogen-depleted fluid into a denitrification
column comprising a column top, a top condenser, a column bottom,
and a bottom reboiler, thereby producing a nitrogen-enriched gas at
the column top and a nitrogen-depleted liquid at the column bottom,
iv) cooling the top condenser by means of a nitrogen cycle using a
nitrogen compressor comprising at least a first stage and a second
stage, the first stage comprising a first entry pressure and the
second stage comprising a second entry pressure, the first entry
pressure being lower than that of the second entry pressure, v)
expanding the nitrogen-depleted liquid and sending the expanded
nitrogen-depleted liquid to the top condenser wherein it is at
least partially vaporized by heat exchange in the condenser with
the nitrogen-enriched gas, thereby condensing the expanded
nitrogen-depleted liquid, vi) vaporizing a liquid nitrogen from the
nitrogen cycle in the condenser and returning the vaporized
nitrogen to the heat exchanger at an inlet of the second stage of
the nitrogen compressor, and a) sending the bottom liquid from the
denitrification column to a methane and carbon monoxide separation
column comprising a top condenser which is a bath vaporizer placed
in a bath of liquid, or b) the separation in step ii) comprises a
distillation step in a methane and carbon monoxide separation
column for separating a methane-depleted flow from a
methane-enriched flow, and at least a portion of the
methane-depleted flow constitutes the hydrogen-depleted fluid
supplying the denitrification column, the methane and carbon
monoxide separation column comprising a top condenser which is a
bath vaporizer placed in a bath of liquid, the bath of liquid of a)
or b) being supplied with liquid nitrogen from the nitrogen
cycle.
15. The process of claim 14, wherein liquid nitrogen from the top
condenser of the methane and carbon monoxide separation column is
sent to vaporize in the top condenser of the denitrification
column.
16. The process of claim 14, wherein the gas mixture cooled in the
heat exchanger is separated by at least one partial condensation
step so as to form a hydrogen-depleted gas, the hydrogen-depleted
gas is sent to an intermediate level of a stripping column
comprising a bottom reboiler, and bottom liquid of the stripping
column is sent to a denitrification column in case a) or to the
methane and carbon monoxide separation column in case b).
17. The process of claim 14, wherein the reboiler of the stripping
column and/or the reboiler of the methane and carbon monoxide
separation column is reheated with at least a portion of the gas
mixture.
18. The process of claim 14, wherein the operating pressure of the
denitrification column is at least 7 bar abs and/or the operating
pressure of the methane and carbon monoxide separation column is at
least 5 bar abs.
19. The process of claim 14, wherein the top condenser of the
CO/CH.sub.4 column is cooled solely by cycle nitrogen.
20. The process of claim 14, wherein the reboiler of the
denitrification column is reheated by means of the cycle
nitrogen.
21. The process of claim 20, wherein the nitrogen used for
reheating the reboiler of the denitrification column is at the
maximum pressure of the nitrogen cycle.
22. The process of claim 14, wherein the nitrogen sent to the bath
of the condenser of the column CO/CH.sub.4 is condensed at the
maximum pressure of the nitrogen cycle.
23. The process of claim 14, wherein the operating pressure of the
methane and carbon monoxide separation column is at least 5
bar.
24. An apparatus for separating a gas mixture comprising carbon
monoxide, nitrogen, hydrogen and methane, comprising a heat
exchanger for cooling the gas mixture, means for separating the gas
mixture cooled in the heat exchanger by at least one scrubbing
and/or distillation and/or partial condensation step, to form a
hydrogen-depleted fluid containing carbon monoxide and nitrogen, a
denitrification column comprising a top condenser and a bottom
reboiler, a pipe for sending the hydrogen-depleted fluid to the
denitrification column, to produce a nitrogen-enriched gas at the
column top and a nitrogen-depleted liquid at the column bottom, a
nitrogen cycle using a nitrogen compressor comprising at least a
first stage and a second stage, the entry pressure of the first
stage being lower than that of the second stage, means for sending
liquid of the nitrogen cycle to the condenser of the
denitrification column, means for expanding bottom liquid from the
denitrification column, means for sending the expanded liquid to
the top condenser of the denitrification column for at least
partial vaporization by heat exchanger in a heat exchanger of the
condenser with the nitrogen-enriched gas, which is thereby
condensed, means for sending nitrogen vaporized in the heat
exchanger of the condenser to the inlet of the second stage of the
nitrogen compressor, a methane and carbon monoxide separation
column comprising a top condenser which is a bath vaporizer placed
in a bath of liquid, a) means for sending the bottom liquid from
the denitrification column to the methane and carbon monoxide
separation column, or b) the methane and carbon monoxide separation
column, means forming part of the means for separating the gas
mixture cooled in the heat exchanger by at least one distillation
step, the apparatus further comprising means for sending liquid
nitrogen from the nitrogen cycle to the top condenser of the
methane and carbon monoxide separation column.
25. The apparatus of claim 24, comprising means for sending liquid
nitrogen from the top condenser of the methane and carbon monoxide
separation column to the top condenser of the denitrification
column.
26. The apparatus of claim 24, further comprising at least one
phase separator for separating the gas mixture cooled in the heat
exchanger by a partial condensation step, to form a
hydrogen-depleted gas, a stripping column, and means for sending
the hydrogen-depleted gas to an intermediate level of the stripping
column.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a 371 of International Application No.
PCT/FR2018/053280, filed Dec. 13, 2018, which claims priority to
French Patent Application No, 1762148, filed Dec. 14, 2017, the
entire contents of which are incorporated herein by reference.
BACKGROUND
[0002] The present invention relates to a process and to an
apparatus for cryogenic separation of a synthesis gas containing
nitrogen. This gas commonly contains carbon monoxide, hydrogen,
methane, and nitrogen. It preferably contains less than 50 mol % of
methane. It preferably contains more than 10 mol % of carbon
monoxide.
[0003] Units for producing carbon monoxide and hydrogen can be
separated into two parts: [0004] Generation of Synthesis Gas
(mixture containing H.sub.2, CO, CH.sub.4, CO.sub.2, Ar, and
N.sub.2, essentially). Among the various industrial routes for the
production of synthesis gas; that based on coal gasification
appears to be increasingly expanding, in particular in countries
rich in deposits of coal, such as China. The process of partial
oxidation of natural gas may also be advantageous for production of
CO, alone or with low H.sub.2/CO production ratios. Another route
is steam reforming. [0005] Purification of Synthesis Gas. The
following are found: [0006] a unit for scrubbing with a liquid
solvent in order to remove most of the acid gases present in the
synthesis gas; [0007] a unit for purification on a bed of
adsorbents; [0008] a unit for cryogenic separation, referred to as
a cold box, for producing CO.
[0009] In the case where the synthesis gas is produced from an
entrained bed or fluidized bed coal gasification, the cold box
process is partial condensation. In the case where the synthesis
gas is contaminated with methane, for MEG, TDI/MDI or PC
applications; for example, it is necessary for the cold box to
include a CH.sub.4 separation column. In the case where the
synthesis gas is contaminated with nitrogen, if the nitrogen is
used for transporting coal, for example, it is necessary for the
cold box to include a nitrogen separation column.
[0010] In DE19541339, a CO/N.sub.2 column is sited upstream of a
CO/CH.sub.4 column. Reboiling of the CO/CH.sub.4 column is provided
by condensation of cycle nitrogen. The condensation at the
CO/N.sub.2 column top is provided by vaporization of N2 liquid from
the cycle at low pressure.
[0011] The nitrogen vaporized in the condensers of the CO/N.sub.2
and CO/CH.sub.4 columns returns to the intake of the compressor of
the nitrogen cycle.
[0012] The CO/N.sub.2 column operates at a relatively low pressure
(2.6 bar).
[0013] The pressure of the CO/CH.sub.4 column is relatively
low.
[0014] The top condensation of the CO/N.sub.2 column is provided by
vaporization of the bottoms from the CO/CH.sub.4 column and, in
addition, by reheating of the hydrogen-rich fraction from the
vessel for partial condensation of the synthesis gas.
[0015] The CO product at the outlet of the CO/N.sub.2 column goes
back to the intake of the CO compressor for compression to the
required pressure.
[0016] The top condenser of the CO/N.sub.2 column has a substantial
volume, since the complement is supplied by reheating of the
hydrogen, and hence a high gas flow: since this exchanger must be
positioned in height relative to the top of the column CO/N.sub.2
column, its substantial volume would make it difficult to transport
the packet containing the CO/N.sub.2 column.
[0017] The configuration leads to high energy consumption at the
level of the CO cycle compressor, because the CO produced has to be
compressed.
[0018] This necessitates the procurement of a CO compressor, which
is more expensive than an N.sub.2 compressor.
[0019] The coupling of the CO/N.sub.2 condenser and CO/CH.sub.4
reboiler causes the unit operating difficulties when the amounts of
CH.sub.4 and N.sub.2 in the incoming synthesis gas are varied.
[0020] DE2814660 describes a separation process utilizing a column
for removal of methane, followed by a double column in which the
top of the CO/argon separation column heats the bottom of the
denitrification column.
[0021] The top denitrification condenser vaporizes the liquid from
the bottom of the denitrification column after expansion and
vaporization of a nitrogen cycle liquid. Conversely, the nitrogen
cycle is not used as a refrigerant fluid for condensing the
separation top from a CH.sub.4 separation column, either in the
column 26 or in the column 13. The top of the CH.sub.4 separation
column 13 is cooled with hydrogen.
[0022] This results in a larger exchanger at the top of the column
13, which takes up more space in the cold box packet and is
therefore more difficult to transport. Moreover, the supply of cold
is insufficient, and CH4 remains in the fluid sent to the second
column, whereas, according to the invention, a single column
removes all of the CH4.
[0023] DE2814660 has an N2 circuit to the reboiler of the bottoms
from the argon/CO column, which is also at a pressure greater than
that of the present invention, where the reboiling of the CO/CH4
column is performed by the synthesis gas.
[0024] According to this prior art, the reboiling of the CO/N2
separation is provided by the N2 cycle via the bottom reboiler of
the argon/CO column, thus requiring a higher pressure than that
according to the invention, where there is a need for reboiling
nitrogen solely at the pressure of the CO/N2 column.
SUMMARY
[0025] According to one subject of the invention, a process is
provided for separating a gas mixture comprising carbon monoxide,
nitrogen, hydrogen, and optionally methane, where: [0026] i) the
mixture is cooled in a heat exchanger, [0027] ii) the mixture
cooled in the heat exchanger is separated by at least one scrubbing
and/or distillation and/or partial condensation step, to form a
hydrogen-depleted fluid containing carbon monoxide and nitrogen,
[0028] iii) the hydrogen-depleted fluid is sent to a
denitrification column having a top condenser and a bottom
reboiler, to produce a nitrogen-enriched gas at the column top and
a nitrogen-depleted liquid at the column bottom, [0029] iv) the
condenser of the denitrification column is cooled by means of a
nitrogen cycle using a nitrogen compressor having at least a first
stage and a second stage, the entry pressure of the first stage
being lower than that of the second stage, [0030] v) bottom liquid
from the denitrification column is expanded and sent to the top
condenser of the denitrification column for at least partial
vaporization by heat exchange in a heat exchanger of the condenser
with the nitrogen-enriched gas, which is thereby condensed, [0031]
vi) the liquid nitrogen from the nitrogen cycle is also vaporized
in the heat exchanger of the condenser and the vaporized nitrogen
is returned to the heat exchanger at the inlet of the second stage
of the nitrogen compressor, and [0032] a) bottom liquid from the
denitrification column is sent to a methane and carbon monoxide
separation column comprising a top condenser which is a bath
vaporizer placed in a bath of liquid, or [0033] b) the separation
in step ii) comprises a distillation step in a methane and carbon
monoxide separation column for separating a methane-depleted flow
from a methane-enriched flow, and at least a portion of the
methane-depleted flow constitutes the hydrogen-depleted fluid
supplying the denitrification column, the methane and carbon
monoxide separation column comprising a top condenser which is a
bath vaporizer placed in a bath of liquid, the bath of liquid of a)
or b) being supplied with liquid nitrogen from the nitrogen
cycle.
[0034] According to other, optional aspects of the invention:
[0035] the mixture contains methane. [0036] the separation in step
ii) comprises a step of distillation in a methane and carbon
monoxide separation column, to separate a methane-depleted flow
from a methane-enriched flow, and at least a portion of the
methane-depleted flow constitutes the hydrogen-depleted fluid
supplying the denitrification column. [0037] bottom liquid from the
denitrification column is sent to a methane and carbon monoxide
separation column. [0038] the methane and carbon monoxide
separation column comprises a top condenser which is a bath
vaporizer placed in a bath of liquid. [0039] the top condenser of
the methane and carbon monoxide separation column is supplied with
liquid nitrogen from the nitrogen cycle. [0040] liquid nitrogen
from the top condenser of the methane and carbon monoxide
separation column is sent to vaporize in the top condenser of the
denitrification column. [0041] the mixture cooled in the heat
exchanger is separated by at least one partial condensation step,
to form a hydrogen-depleted gas, the hydrogen-depleted gas is sent
to an intermediate level of a stripping column having a bottom
reboiler, and bottom liquid from the stripping column is sent to a
denitrification column in case a) or to the methane and carbon
monoxide separation column in case b). [0042] the reboiler of the
stripping column and/or the reboiler of the methane and carbon
monoxide separation column are reheated with at least a portion of
the gas mixture. [0043] the operating pressure of the
denitrification column is at least 7 bar abs or even 8 bar abs.
[0044] the operating pressure of the methane and carbon monoxide
separation column is at least 5 bar abs or even 6 bar abs. [0045]
the top condenser of the CO/CH4 column is cooled solely by cycle
nitrogen. [0046] the reboiler of the denitrification column is
reheated by means of cycle nitrogen. [0047] the nitrogen used for
reheating the reboiler of the denitrification column is at the
maximum pressure of the nitrogen cycle. [0048] the nitrogen sent to
the bath of the condenser of the CO/CH4 column is condensed at the
maximum pressure of the nitrogen cycle.
[0049] According to another subject of the invention, an apparatus
is provided for separating a gas mixture comprising carbon
monoxide, nitrogen, hydrogen, and optionally methane, comprising a
heat exchanger for cooling the mixture, means for separating the
mixture cooled in the heat exchanger by at least one scrubbing
and/or distillation and/or partial condensation step, to form a
hydrogen-depleted fluid containing carbon monoxide and nitrogen, a
denitrification column having a top condenser and optionally a
bottom reboiler, a pipe for sending the hydrogen-depleted fluid to
the denitrification column, to produce a nitrogen-enriched gas at
the column top and a nitrogen-depleted liquid at the column bottom,
a nitrogen cycle using a nitrogen compressor having at least a
first stage and a second stage, the entry pressure of the first
stage being lower than that of the second stage, means for sending
liquid of the nitrogen cycle to the condenser of the
denitrification column, means for expanding bottom liquid from the
denitrification column, means for sending the expanded liquid to
the top condenser of the denitrification column for at least
partial vaporization by heat exchange in a heat exchanger of the
condenser with the nitrogen-enriched gas, which is thereby
condensed, means for sending nitrogen vaporized in the heat
exchanger to the inlet of the second stage of the nitrogen
compressor, a methane and carbon monoxide separation column
comprising a top condenser which is a bath vaporizer placed in a
bath of liquid, [0050] a) means for sending the bottom liquid from
the denitrification column to the methane and carbon monoxide
separation column, or [0051] b) the methane and carbon monoxide
separation column means forming part of the means for separating
the mixture cooled in the heat exchanger by at least one
distillation step, the apparatus further comprising means for
sending liquid nitrogen from the nitrogen cycle to the top
condenser of the methane and carbon monoxide separation column.
[0052] The apparatus may comprise means for sending liquid nitrogen
from the top condenser of the methane and carbon monoxide
separation column to the top condenser of the denitrification
column.
[0053] The apparatus may comprise at least one phase separator for
separating the mixture cooled in the heat exchanger by a partial
condensation step, to form a hydrogen-depleted gas, a stripping
column, and means for sending the hydrogen-depleted gas to an
intermediate level of the stripping column.
[0054] According to the invention, the reboiling of the CO/CH.sub.4
column is by cooling of the synthesis gas, whereas in the DEXX it
is by condensation of the cycle: the advantage of this in our
scheme is that it allows the pressure of the CO/CH.sub.4 column to
be increased without increasing the exit pressure of the N.sub.2
compressor.
[0055] The condenser of the CO/N.sub.2 column top is cooled by
vaporization of at least a portion of the bottom liquid from the
CO/N.sub.2 column after expansion and also by vaporization of
N.sub.2 liquid at medium pressure. In the prior art, the cooling is
performed by vaporization of N.sub.2 liquid from the cycle at low
pressure. The vaporization of the bottom liquid after expansion
enables a considerable reduction in the N.sub.2 cycle flow to be
vaporized in the condenser, thereby reducing the N.sub.2 cycle flow
and hence the power of the N.sub.2 cycle compressor. In the prior
art, the nitrogen flow is relatively high relative to the flow of
CO produced.
[0056] According to the invention, the nitrogen vaporized in the
condensers of the CO/N.sub.2 and CO/CH.sub.4 columns returns to an
inter-stage of the N.sub.2 compressor, whereas in the prior art it
returns to the intake of the compressor. The prior art therefore
results in an increase in the compression energy of the cycle. In
the prior art, the flow 54 returns at the same pressure (2.4 bar)
as the N.sub.2 flow required for the cooling of the synthesis gas
in E2, whereas in our scheme the N.sub.2 flow vaporized in the
condensers returns to the N.sub.2 compressor at a higher pressure
than the N.sub.2 flow needed for the cooling of the synthesis gas.
This is made possible in this case because the CO/N.sub.2 column is
operated at a higher pressure (at least 7 bar, for example at 8.5
bar) relative to the prior art (2.6 bar). [0057] FIG. 3 of
DE102012020469 includes a pump for increasing the pressure of the
CO/CH.sub.4 column, but this pressure remains low (3.6 bar)
relative to the schemes according to the invention (at least 5 bar,
or even at least 6 bar), and, including in FIG. 3, the N.sub.2
liquid from the condenser of the CO/CH.sub.4 column is at low
pressure and returns to the intake of the compressor, whereas in
our scheme the nitrogen from the CO/CH.sub.4 column top condenser
returns to an inter-stage of the N.sub.2 compressor (at a higher
pressure than the nitrogen used for cooling the synthesis gas in
the main exchanger).
[0058] As according to the invention the CO/N.sub.2 column is at a
higher pressure, it is possible to produce the CO directly without
recompression.
[0059] The energy for condensing the top of the CO/N.sub.2 column
is provided by vaporization of the bottom liquid, after expansion,
and additionally by vaporization of low-pressure nitrogen from the
N.sub.2 cycle. This reduces the size of the top condenser and
allows the packet containing the CO/N.sub.2 column and its top
condenser to be transported.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] For a further understanding of the nature and objects for
the present invention, reference should be made to the following
detailed description, taken in conjunction with the accompanying
drawings, in which like elements are given the same or analogous
reference numbers and wherein:
[0061] FIG. 1 illustrates a schematic representation a separation
process in accordance with one embodiment of the present
invention.
[0062] FIG. 2 illustrates another schematic representation a
separation process in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0063] In FIG. 1, a gas mixture 1, resulting for example from coal
gasification, contains carbon monoxide, hydrogen, methane, water,
and nitrogen. The gas 1 is purified in adsorbent beds 3A, 3B and
cooled in a cooler 4, Then it is sent to a first heat exchanger E1
to be cooled. Partial flows of the synthesis gas are used to reheat
the reheaters R1, R2, which are drawn twice at different places in
the drawing for reasons of clarity. Following expansion in a valve,
separation takes place in a phase separator S1, forming a gas 5 and
a liquid 7. The gas 5 is cooled in a heat exchanger E2, expanded,
and sent to a phase separator S4, The gas 9 from this phase
separator S4, which is rich in hydrogen, is reheated in the heat
exchangers E2, E1, and a portion of the gas is used to regenerate
the adsorbent beds 3A, 3B, A portion 11 of the liquid from the
phase separator S4 is expanded and sent to the top of a stripping
column K1 operating at 17.6 bar. The column K1 has no top
condenser, but has a bottom reboiler R1. The remainder 13 of the
liquid from the phase separator S4 is expanded and sent to a phase
separator S3. The top gas 17 from the column K1 is reheated in the
exchangers E1, E2.
[0064] The liquid 7 from the phase separator S1 mixes with other
fluids (top gas from separator S3, derived from the liquid 13 from
the separator S4) to form the flow 8, which is sent to a phase
separator S2 and then to an intermediate level of the stripping
column K1.
[0065] The gas from the phase separator S3 and the liquid from the
phase separator 53, after vaporization in the exchanger E2, are
mixed with the fluid 7 to supply the column K1.
[0066] Bottom liquid 19 from the column K1 is taken at -154.degree.
C., expanded at 8.3 bar, and sent to the phase separator S5, and
the gas and the liquid from the phase separator are sent to an
intermediate level of the CO/N.sub.2 column K2 operating at 8.3
bar. The column K2 has a top condenser C1, consisting of a
plate-type heat exchanger, and a bottom reboiler R2.
[0067] The top gas 27 from the column K2 is partially condensed in
the condenser C1, and the resulting liquid L, 29, is returned to
the top of the column K2 and in part, and the remaining gas V,
enriched in nitrogen, is reheated in the exchangers E2, E1 as gas
31.
[0068] A liquid 53 from the top condenser C2 of the column K3 is
vaporized by heat exchange with the gas 27 in the condenser C1,
forming the gas 55, which is sent to the entry of the compressor
V3.
[0069] The bottom liquid 33 enriched in carbon monoxide and
depleted in nitrogen is divided into two, 21, 35, and expanded. An
expanded portion 21 at 6.5 bar is sent to a phase separator, the
liquid from which is used in part to cool the condenser C1,
Accordingly, the top condenser C1 of the CO/N.sub.2 column K2 is
cooled by vaporization of at least a portion of the bottom liquid
33 from the CO/N.sub.2 column K2 after expansion and vaporization
of the liquid nitrogen 53 at medium pressure. Vaporizing the bottom
liquid 33 after expansion enables a considerable reduction in the
nitrogen cycle flow to be vaporized in the condenser C1 thereby
reducing the nitrogen cycle flow and hence the power of the
nitrogen cycle compressor V1, V2, V3.
[0070] The remainder of the liquid from the separator S8 and the
fraction 35 supply the CO/CH.sub.4 column K3 after passage through
a phase separator S6, from which the gas and the liquid are sent to
different intermediate levels of the column K3.
[0071] The column K3 has a top condenser C2, consisting of a
plate-type heat exchanger disposed in a bath of liquid for
vaporization, and a bottom reboiler R3, The carbon
monoxide-enriched top gas is condensed in the condenser C2, and the
methane-rich bottom liquid 39 is expanded and reheated in the
exchanger E1. Column K3 functions at 6.6 bar.
[0072] The plate-type exchanger is surrounded by an annular barrier
forming an overflow wall P. Accordingly, the liquid surrounding the
exchanger is able to pass over the barrier P to be withdrawn as
liquids 43, 53.
[0073] The top condenser C2 of the column K3 is cooled by
compressed and expanded nitrogen 59 from the nitrogen cycle
compressor V1, V2, V3 after cooling in the exchangers E1, E2. The
vaporized nitrogen is returned upstream of the last stage V3 of the
nitrogen cycle compressor. The nitrogen at the exit pressure of the
stage V3 is also used to reboil the reboiler R2 of the column
K2.
[0074] The reboilers R1 and R3 of the columns K1 and K3 are
reheated by partial flows of the feed 1 downstream of the exchanger
E1 and upstream of the phase separator S1, This reboiling of the
CO/CH.sub.4 column K3 by cooling of the synthesis gas has the
advantage of enabling an increase in the pressure of the column
CO/CH.sub.4 without an increase in the exit pressure of the
nitrogen cycle compressor. The partial flows sent to the reboilers
R1, R3 are at the same temperature and at the same pressure.
[0075] Liquid nitrogen 53 from the bottom of the condenser C2 of
the column K3 is sent for vaporization in the condenser C1 of the
column K3 and is subsequently returned downstream of the stage V2
and upstream of the stage V3. Accordingly, the nitrogen vaporized
in the condensers C1, C3 of the CO/N.sub.2 column K2 and
CO/CH.sub.4 column K3 returns at an inter-stage of the nitrogen
compressor V1, V2; the N.sub.2 flow 57 vaporized in the condensers
C1, C2 returns to the N.sub.2 compressor at a higher pressure than
the N.sub.2 flow required for the cooling of the synthesis gas. In
this case, this is made possible by operating the CO/N.sub.2 column
K2 at a higher pressure (8.5 bar) relative to the prior art (2.6
bar).
[0076] A carbon monoxide-rich gas 41 leaves the column K3 at 6.6
bar at -170.4.degree. C. and is reheated in the heat exchangers E1,
E2. Preferably no carbon monoxide compressor is used. It
constitutes a product of the process and has not been
compressed.
[0077] A supply of liquid nitrogen 69 enables compensation of the
leaks from the nitrogen cycle. Sent to a phase separator S7, the
liquid formed is vaporized in the exchanger E2, and mixes with the
gas from the separator S7, and is sent to the entry of the
compressor V1.
[0078] A portion 47 of the liquid nitrogen in the condenser C2 is
expanded and sent to the separator SI, and the gas 49 formed enters
at the entry of the compressor V1.
[0079] Another portion 45 of the same liquid is expanded at a lower
pressure and is sent to the exit of the compressor V1 and the entry
of the compressor V2.
[0080] The operating pressure of the denitrification column K2 is
at least 7 bar abs or even 8 bar abs; the operating pressure of the
methane and carbon monoxide separation column K3 is at least 5 bar
abs or even 6 bar abs.
[0081] In FIG. 2, the order of the nitrification and methane and
carbon monoxide separation columns is reversed.
[0082] Accordingly, the liquid 19 from the bottom of the stripping
column is sent not to the denitrification column but instead to an
intermediate point of the CO/CH.sub.4 separation column K3, after
separation by a phase separator S5.
[0083] The CO/CH.sub.4 column K3 has a bottom reboiler R3 which is
heated by the feed, and a top condenser C2, which is used to
condense the top gas 51, which is returned to the column K3 in
condensed form. The condenser is cooled with condensed nitrogen 61,
63 produced by condensing the cycle nitrogen 59 from the compressor
V3 in the exchangers E1, E2 and in the reboiler R2. The liquid is
partially vaporized, producing a gas 55 which is returned to the
entry of the compressor V3, and a liquid which passes over the
barrier P. A portion 31 of the liquid is vaporized in the exchanger
E2 and returns to the entry of the compressor V3. The other
portion, 53, is used to cool the top condenser C1 of the column K2,
as before.
[0084] The bottom methane 39 of the column K3 is reheated in the
exchanger E1, to leave the apparatus as a product. The top gas 26,
enriched in carbon monoxide and containing nitrogen, leaves toward
the middle of the denitrification column K2.
[0085] The column K2 has a top condenser C1, consisting of a
plate-type heat exchanger, and a bottom reboiler R2, which is
heated by cycle nitrogen. The top gas 27 from the column K2 is
partially condensed in the condenser C1, and the liquid L formed,
29, is returned to the top of the column K2 and in part, and the
remaining gas V, enriched in nitrogen, is reheated in the
exchangers E2, E1 as gas 31.
[0086] A liquid 53 from the top condenser C2 of the column K3 is
vaporized by heat exchange with the gas 27 in the condenser C1,
forming the gas 55 which is sent to the entry of the compressor
V3.
[0087] The carbon monoxide-enriched and nitrogen-depleted bottom
liquid 21 is expanded. This liquid, at 6.5 bar, is sent to a phase
separator, the liquid from which is used in part to cool the
condenser C1. Accordingly, the top condenser C1 of the CO/N.sub.2
column K2 is cooled by vaporization of at least part of the bottom
liquid 33 from the CO/N.sub.2 column K2, after expansion and
vaporization of the liquid nitrogen 53 at medium pressure. The
vaporization of the bottom liquid 33 after expansion enables a
considerable reduction in the nitrogen cycle flow to be vaporized
in the condenser C1, thereby reducing the nitrogen cycle flow and
hence the power of the nitrogen cycle compressor V1, V2, V3.
[0088] The gas 31 is a carbon monoxide-rich product of the
process.
[0089] The operating pressure of the denitrification column K2 is
at least 7 bar abs or even 8 bar abs; the operating pressure of the
methane and carbon monoxide separation column K3 is at least 5 bar
abs or even 6 bar abs.
[0090] It will be understood that many additional changes in the
details, materials, steps and arrangement of parts, which have been
herein described in order to explain the nature of the invention,
may be made by those skilled in the art within the principle and
scope of the invention as expressed in the appended claims. Thus,
the present invention is not intended to be limited to the specific
embodiments in the examples given above.
* * * * *