FR3118144A3 - METHOD AND APPARATUS FOR THE CRYOGENIC SEPARATION OF A MIXTURE OF HYDROGEN, METHANE, NITROGEN AND CARBON MONOXIDE - Google Patents

Abstract

Title: PROCESS AND APPARATUS FOR THE CRYOGENIC SEPARATION OF A MIXTURE OF HYDROGEN, METHANE, NITROGEN AND CARBON MONOXIDE Process for the cryogenic separation of a mixture of hydrogen, methane, nitrogen and carbon monoxide wherein the mixture (1) is separated by partial condensation and/or stripping to reduce its hydrogen content producing a hydrogen-depleted gas (13), the hydrogen-depleted gas is separated in a first distillation column (K1) to produce a nitrogen enriched gas and a carbon monoxide enriched and nitrogen depleted liquid (21), at least a part of the carbon monoxide enriched and nitrogen depleted liquid is sent to an overhead condenser (R1) of the first distillation column where it partially vaporizes producing a first gas enriched in carbon monoxide containing methane (44) and a liquid containing carbon monoxide and methane (27), the first gas serving as cycle gas for ten ir the cold process. Abstract Figure: Single Figure

Description

METHOD AND APPARATUS FOR THE CRYOGENIC SEPARATION OF A MIXTURE OF HYDROGEN, METHANE, NITROGEN AND CARBON MONOXIDE

The present invention relates to a process and an apparatus for the cryogenic separation of a mixture of hydrogen, nitrogen, methane and carbon monoxide.

The carbon monoxide and hydrogen production units can be separated into two parts:

Generation of synthesis gas (mixture containing H ₂ , CO, CH ₄ , CO ₂ , Ar and N ₂ essentially). Among the various industrial routes for the production of syngas, that based on coal gasification seems to be developing more and more, particularly in countries rich in coal deposits such as China. The design of this unit, which includes a coal gasification reactor with oxygen and steam, is based on the CO and hydrogen production required.

Syngas purification . We find :

• a liquid solvent scrubbing unit to remove most of the acid gases contained in the synthesis gas
• an adsorbent bed purification unit.
• a cryogenic separation unit called a cold box for the production of CO.

Generally, the synthesis gas comprises a mixture at high pressure (around 60 bar) and is very rich in CO (at least 40% mol, for example approximately 50% mol). Another advantage of the carbon gasification process is the low content of impurities (CH ₄ , Argon and Nitrogen) present in the synthesis gas at the inlet of the cold box for the production of pure CO.

When the nitrogen or methane contents are too high to be acceptable in the downstream process, they should be separated from the gas using appropriate columns.

The hydrogen separated from the CO is required at high pressure to be able to recover it, either in a PSA or in a methanol synthesis unit (MeOH).

Part of the separation energy of said cold box is ensured by free expansion between the synthesis gas and the pure CO produced at low pressure, but in most cases, this free expansion is not sufficient to complete the balance. unit refrigerator. A supply of liquid nitrogen is necessary to keep the cold box cold and complete the refrigeration balance.

The synthesis gas at a pressure of around 60 bar coming from a pre-treatment unit (CO ₂ and MeOH separation) is cooled in the main exchange line, partially condensed before feeding a one-stage partial condensation separator pot . The hydrogen-rich vapor is sent to a MeOH unit or to a PSA after heating in the exchange line. The bottom liquid is sent to a stripping column at approximately 16 bar after expansion. The overhead vapor from the stripping column leaves the cold box after warming up and is sent as fuel or recycled to the system via a compressor.

The bottom liquid of the stripping column which contains the CO to be purified is then sent to the other columns for final purification.

Depending on the purities required, it is possible to first send this liquid to the CO/N ₂ separation column or to the CO/CH ₄ column.

Starting with the CO/N ₂ column makes it possible to

• Produce CO directly at production pressure from the CO/CH ₄ column without the need for product compression.
• Reduce the pressure of the CO-CH ₄ column and therefore thereby:
  • Either reduce the pressure of the fluid which will carry out the reboiling of the CO-CH ₄ column and therefore optimize the energy.
  • Either reduce the quantity of components other than methane in the methane purge in the CO-CH ₄ column bottom, which has the effect of reducing the temperature of the CO-CH ₄ bottom but also has the effect of reducing the yield of CO removal.

If we start with the CO/N ₂ separation, three configurations are possible:

• The first in which the reboiling of the column is carried out by means of a reboiler. In this case, the reboiling fluid does not mix with the column vessel.
• The second in which the reboiling is carried out directly by means of a CO fluid warmer than the temperature of the tank.
• The third intermediate solution where both a reboiler and direct reboiling are considered (therefore combining the advantages and disadvantages of each solution in proportion to the reboiling distribution)

It is interesting to note that the lower the nitrogen content required in the product, or in other words, the higher the quantity of nitrogen to be removed, the higher the reboiling rate must be. The need for condensation at the top of the column is also higher. The CO/N ₂ column head is preferentially condensed with the bottom liquid of the CO/N ₂ column which will have been expanded beforehand.

In the first configuration, whatever the reboiling rate, the flow to be processed in the following columns does not increase. However, since the reboiler requires a fluid hotter than the vessel to operate (since there is a minimum approach ∆T), it introduces irreversibility into the system and therefore increases the energy to operate. In addition, given that the flow rate in the column tank remains constant, it is sometimes necessary to use a supplement in order to provide enough cooling capacity. The condensation of the CO/N ₂ column head then becomes more complex.

In the second configuration, the direct reboil increases the amount of tank flow. It then becomes possible again to condense the head of the CO-N ₂ column with the column bottom without having to resort to additional flow. However, since the flow has increased, the next column will therefore have to deal with a greater flow.

According to one object of the invention, there is provided a process for the cryogenic separation of a mixture of hydrogen, methane, nitrogen and carbon monoxide in which:

1. the mixture is separated by partial condensation and/or by exhaustion to reduce its hydrogen content by producing a hydrogen-depleted gas,
2. the gas depleted in hydrogen is separated in a first distillation column to produce a gas enriched in nitrogen and a liquid enriched in carbon monoxide and depleted in nitrogen,
3. at least a portion of the carbon monoxide-enriched, nitrogen-depleted liquid is sent to an overhead condenser of the first distillation column where it partially vaporizes producing a first carbon monoxide-enriched gas containing methane and a liquid containing carbon monoxide and methane,
4. at least one fluid containing carbon monoxide and methane is sent from the condenser of the first column to a second distillation column to separate therein producing a liquid enriched in methane and a second gas enriched in carbon monoxide richer in carbon monoxide than the first gas enriched in carbon monoxide, the second gas enriched in carbon monoxide containing at least 98% mol of carbon monoxide and less than 300ppm of methane,
5. cold for the process is produced by a carbon monoxide-enriched gas refrigeration circuit

characterized in that the carbon monoxide enriched gas consists at least partially of a portion of the first carbon monoxide enriched gas containing methane and the cycle gas contains at least 98 mol% carbon monoxide and at least 300 ppm of methane, or even at least 0.5% mol of methane.

According to other optional aspects:

• part of the bottom liquid of the first column is sent to a top condenser of the first column where it partially vaporizes.
• the liquid not vaporized in the top condenser of the first column is sent to the second column after expansion.
• the part of the bottom liquid of the first column sent to a top condenser of the first column is the only liquid feeding this condenser.
• part of the bottom liquid of the first column is sent to a top condenser of the second column.
• part of the first gas enriched in carbon monoxide containing methane is sent to separate in the second column.
• cycle gas is sent to the bottom of the first column to separate there.
• the liquefied cycle gas is sent to the top condenser of the second column.

According to another aspect of the invention, there is provided an apparatus for the cryogenic separation of a mixture of hydrogen, methane, nitrogen and carbon monoxide comprising means for separating the mixture by partial condensation and/or by stripping to reduce its hydrogen content by producing a hydrogen-depleted gas, a first distillation column, having an overhead condenser, for separating the hydrogen-depleted gas to produce a nitrogen-enriched gas and a carbon monoxide-enriched liquid and depleted in nitrogen, means for sending at least part of the liquid enriched in carbon monoxide and depleted in nitrogen to the top condenser of the first distillation column where it partially vaporizes by producing a first gas enriched in carbon monoxide containing methane and a liquid containing carbon monoxide and methane, a second distillation column, means for sending at least one fluid containing carbon monoxide carbon and methane from the condenser of the first column to the second distillation column to separate therein producing a liquid enriched in methane and a second gas enriched in carbon monoxide richer in carbon monoxide than the first gas enriched in monoxide of carbon, the second gas enriched in carbon monoxide containing at least 98% mol of carbon monoxide and less than 300ppm of methane, a refrigeration circuit for producing cold for the process using a gas enriched in carbon monoxide as cycle

characterized in that the gas enriched in carbon monoxide comes at least in part from the top condenser of the first column and contains at least 98% mol of carbon monoxide and at least 300 ppm of methane, even at least 0.5% mol of methane.

The apparatus may include means for directing gas from the overhead condenser and/or liquid from the overhead condenser to separate into the second column.

The second column may include a bottom reboiler.

The first column may not include a bottom reboiler.

The apparatus may comprise means for sending second gas enriched in carbon monoxide to the cycle.

By choosing to dissociate the purity of the cycle from the purity of the production, it is possible to choose to treat in the following columns only the flow rate necessary for the production.

Thus, we benefit from the advantages of direct reboiling, namely:

• the absence of a necessary approach ∆T,
• the absence of additional fluid to condense,
• a more compact condenser (due to a two-fluid configuration) without oversizing the downstream part.

Thus, the CO cycle will be an impure cycle (because the methane will not be separated).

In the CO product, we can reach < 300ppm or even < 10 ppm depending on the CO application, whereas in the CO cycle, we will have CH ₄ > 300ppm up to 0.5% typically.

For example the composition of the impure CO cycle is:

• CO> 98% mol
• N ₂ < 2% mol
• 10ppm - 300ppm < CH ₄ < 0.5 %mol

This is to be compared with the composition of the produced CO:

• CO > 98%
• CH ₄ < 300ppmv

What changes in the quality of the CO cycle and of the CO product is above all the CH ₄ content.

The advantages of this configuration are:

• Improved energy consumption (no approach necessary, no extra fluid to condense).
• Optimization of sizing (next columns not oversized, more compact condenser).

The invention will be described in more detail with reference to the figure.

illustrates a method according to the invention.

A feed stream 1 containing at least 40% mol of carbon monoxide, methane, nitrogen and hydrogen is cooled in a heat exchanger 3 at a pressure of 60 bars. Other pressures and compositions are possible and the source of the flow can be coal gasification among others. The flow cools in a second heat exchanger 5 where it partially condenses. It is separated in a phase separator 7 to produce a hydrogen-enriched gas 9 sent to heat up in the exchangers.

The liquid formed depleted in hydrogen 11 is sent to a stripping column F to remove a flow enriched in hydrogen and produces a liquid 13 enriched in carbon monoxide.

The liquid 13 is expanded in a valve and the partially condensed flow thus formed is separated in a phase separator 15 whose gas 17 and liquid 19 are sent to supply a first distillation column K1. Column K1 produces a bottom liquid 21 enriched in carbon monoxide and depleted in nitrogen still containing methane. The top gas is enriched in nitrogen and condenses in the top condenser R1 of the first column K1 from which a nitrogen purge 63 is drawn off.

Bottom liquid 21 is split in two, part 23 being expanded and sent to head condenser R1 as the only feed liquid. The head condenser R1 is therefore of simple construction because only two fluids exchange heat there. The liquid 23 partially condenses there, the remaining liquid 27 around the condenser R1 being expanded and sent to a second column K2 at an intermediate level.

The gas formed 44 in condenser R1 is split in two, part 47 being sent to a second column K2 at an intermediate level. The other part 45 is expanded from 7.6 bars to 6.4 bars and constitutes the cycle gas. It contains more than 98% mol CO, less than 2% mol nitrogen and between 10ppm - 300ppm and 0.5% mol methane.

The second column K2 produces a liquid methane purge 29, having an external liquid-fed reboiler R3 31. The carbon monoxide 73 produced overhead is the product of the process and in this case is not used as cycle gas. It contains more than 98% mol of carbon monoxide and less than 300ppmv of methane.

It is nevertheless conceivable to mix part of the gas 73 with the gas 45 to form a cycle gas that is purer in terms of carbon monoxide and less impure because it contains less methane.

The overhead condenser R2 of the second column K2 condenses the overhead carbon monoxide against liquefied cycle gas 49 mixed with part of the bottom liquid 25 of the first column K1. Liquid 33 from condenser R2 is split in two. Part 35 vaporizes in heat exchanger 5 and is then mixed with the gas formed by mixing gas 45 with gas 55 from condenser R2. The formed mixture 57 heats up in the heat exchanger 3.

Part 37 of the liquid from condenser R2 is expanded to 2 bars, partially condensed and sent to a phase separator 39. The liquid formed 43 vaporizes in the heat exchanger 5 and is mixed with the gas 41 to form a gas 59. Once heated in exchanger 3, this gas 53 supplies the first stages C1, C2 of the cycle compressor and is then mixed with gas 57 at 5.8 bars. The mixture formed is compressed in the last stage C3 and the cycle gas 61 at its highest pressure is cooled in the exchanger 3. Then it is divided into two, a part 49 supplying the reboiler R2 and the rest 51 being sent to tank of the first column K1 to heat the column by direct heat exchange.

A flow of liquid nitrogen 71 is used to provide cold to the process by vaporizing in the exchangers 5.3 to form the LAN gas.

For better operating stability, it will be chosen to send with a slight excess of CO to the CO/CH ₄ K2 column towards the CO produced. Thus, part of the CO 73 product will be sent continuously to the compressor. In the event of fluctuations, this forwarding rate will be impacted but the rate 73 produced to the customer may remain stable.

Part of the CO 73 product can be sent to the compressor to improve the stability of the system between the CO which is sent to the CO/CH ₄ column versus the CO flow produced, the CO product flow to the compressor is for the excess of CO produced by the CO/CH ₄ column. It is preferable to operate with a slight excess of CO towards the CO/CH ₄ K2 column versus the CO produced; as this improves the stability of the flow of CO to the product, it will be accepted that the excess CO produced to the compressor C1, C2; C3 varies to maintain constant the CO flow produced 73 towards the customer.

Claims (10)

Process for the cryogenic separation of a mixture of hydrogen, methane, nitrogen and carbon monoxide in which:

1. the mixture (1) is separated by partial condensation and/or by exhaustion to reduce its hydrogen content by producing a hydrogen-depleted gas (13),
2. the gas depleted in hydrogen is separated in a first distillation column (K1) to produce a gas enriched in nitrogen and a liquid enriched in carbon monoxide and depleted in nitrogen (21),
3. at least part of the liquid enriched in carbon monoxide and depleted in nitrogen is sent to a top condenser (R1) of the first distillation column where it partially vaporizes producing a first gas enriched in carbon monoxide containing methane ( 44) and a liquid containing carbon monoxide and methane (27),
4. at least one fluid (27, 47) containing carbon monoxide and methane is sent from the condenser of the first column to a second distillation column (K2) to separate therein producing a liquid enriched in methane (29) and a second gas (73) enriched in carbon monoxide richer in carbon monoxide than the first gas enriched in carbon monoxide, the second gas enriched in carbon monoxide containing at least 98% mol of carbon monoxide and less than 300ppm of methane,
5. cold for the process is produced by a refrigeration circuit of a gas enriched in carbon monoxide

characterized in that the carbon monoxide enriched gas consists at least partially of a portion (45) of the first carbon monoxide enriched gas containing methane and the cycle gas (57, 59, 61) contains at least 98 % mol of carbon monoxide and at least 300 ppm of methane, or even at least 0.5% mol of methane. Process according to Claim 1, in which part (23) of the bottom liquid of the first column (K1) is sent to the top condenser (R1) of the first column (K1) where it partially vaporizes. Process according to Claim 2, in which the liquid not vaporized in the top condenser (R1) of the first column (K1) is sent to the second column (K2) after expansion. Process according to Claim 2 or 3, in which the part (23) of the bottom liquid of the first column sent to the top condenser (R1) of the first column (K1) is the only liquid feeding this condenser. Process according to one of the preceding claims, in which part (25) of the bottom liquid of the first column is sent to a top condenser (R2) of the second column (K2). Process according to one of the preceding claims, in which a part (47) of the first gas enriched in carbon monoxide containing methane is sent to separate in the second column (K2). Process according to one of the preceding claims, in which cycle gas (51) is sent to the bottom of the first column (K1) to separate there. Process according to one of the preceding claims, in which the liquefied cycle gas (49) is sent to the top condenser (R2) of the second column (K2). Apparatus for the cryogenic separation of a mixture of hydrogen, methane, nitrogen and carbon monoxide comprising means for separating the mixture (1) by partial condensation and/or by exhaustion in order to reduce its hydrogen content by producing a hydrogen-depleted gas (13), a first distillation column (K1), having an overhead condenser (R1), for separating the hydrogen-depleted gas to produce a nitrogen-enriched gas and a carbon monoxide-enriched and depleted liquid nitrogen (21), means for sending at least a portion (23) of the liquid enriched in carbon monoxide and depleted in nitrogen to the top condenser of the first distillation column where it partially vaporizes producing a first gas (44 ) enriched with carbon monoxide containing methane and a liquid (27) containing carbon monoxide and methane, a second distillation column (K2), means for sending at least one fluid (27, 47) containing vs arbone and methane from the condenser of the first column to the second distillation column to separate therein producing a liquid enriched in methane (29) and a second gas (73) enriched in carbon monoxide richer in carbon monoxide than the first gas enriched in carbon monoxide, the second gas enriched in carbon monoxide containing at least 98% mol of carbon monoxide and less than 300ppm of methane, a refrigeration circuit (C1, C2, C3, 53, 57, 59 , 61) to produce cold for the process using a gas enriched in carbon monoxide as cycle gas
characterized in that the gas enriched in carbon monoxide of the cycle gas comes at least in part from the top condenser of the first column and contains at least 98% mol of carbon monoxide and at least 300 ppm of methane, or even at least 0.5% mol of methane. Apparatus according to claim 9 comprising means for passing gas (47) from the overhead condenser (R1) and/or liquid (27) from the overhead condenser to separate in the second column (K2).