FR2903766A1 - Producing carbon monoxide and hydrogen from a mixture of methane and nitrogen by cryogenic distillation, comprises washing an initial gas mixture with a liquid methane in a first column to provide gaseous hydrogen and a washing liquid - Google Patents

Abstract

The process of producing carbon monoxide and hydrogen from a mixture of methane and nitrogen by cryogenic distillation, comprises washing an initial gas mixture with a liquid methane in a first column (1) to provide gaseous hydrogen and washing liquid, separating the hydrogen dissolved in the washing liquid in a second column (2), separating the remaining liquid by distillation in a third column (3) to provide a liquid enriched in carbon monoxide and methane, and sending a part of liquid enriched in carbon monoxide to a fourth column (4). The process of producing carbon monoxide and hydrogen from a mixture of methane and nitrogen by cryogenic distillation, comprises washing an initial gas mixture with a liquid methane in a first column (1) to provide gaseous hydrogen and washing liquid, separating the hydrogen dissolved in the washing liquid in a second column (2), separating the remaining liquid by distillation in a third column (3) to provide a liquid enriched in carbon monoxide and methane, sending a part of liquid enriched in carbon monoxide to a fourth column (4), drawing out a flow rich in carbon monoxide from the fourth column, heating the third column by a vapor, and heating the second and third columns using a closed refrigerating cycle. The fourth column has an overhead condenser, which is cooled using the closed refrigerating cycle. The produced carbon monoxide is drawn out from the fourth column in a liquid form. The refrigerating cycle is a cycle of a nontoxic and/or inflammable fluid e.g. a nitrogen cycle. An independent claim is included for an installation for producing carbon monoxide and hydrogen from a mixture of methane and nitrogen.

Description

The present invention applies to the production processes of

  carbon monoxide and / or hydrogen by cryogenic distillation. When a process for producing carbon monoxide is fed from a source of synthesis gas containing nitrogen, the carbon monoxide to be produced must be substantially purified of the nitrogen that it could contain. In a conventional production unit producing carbon monoxide and hydrogen (based on a carbon monoxide cycle), where the carbon monoxide to be produced must be substantially purified of the nitrogen that it could contain, it is It has been found that inventories of toxic and / or flammable products grow in the same proportions as the production rates. On larger plants these inventories of toxic and / or flammable products present a significant source of risk in the event of an accident. EP-A-0532 387 proposes to substitute the carbon monoxide cycle with an independent cycle operating especially with nitrogen. This invention deals with the case of separation of the synthesis gas into three columns in which the carbon monoxide is withdrawn at the top of the third column and in liquid form before being pumped, and in this case makes it possible to reduce to a minimum the inventories of liquid carbon monoxide in the unit.

In the event that it is necessary to substantially purify the carbon monoxide of the nitrogen that it may contain, it is necessary to use an additional column which makes it possible to carry out the carbon monoxide / nitrogen separation. In this case the carbon monoxide is withdrawn in the tank of the additional column in liquid form before being pumped.

As this column operates generally at low pressure in order to facilitate the separation of the nitrogen / carbon monoxide binary, large diameter columns result and therefore contain in the tank and in the top condenser amounts of monoxide. of extremely important liquid carbon.

The refrigeration cycle is an independent cycle that can be in particular a nitrogen cycle. According to one object of the invention, there is provided a process for producing carbon monoxide and hydrogen from a gaseous mixture comprising essentially these two bodies, methane and nitrogen, of the type in which the starting gas mixture is washed with liquid methane in a first column to provide hydrogen gas and a washing liquid; the hydrogen dissolved in the washing liquid is separated from it in a second column, and the remaining liquid is distilled off in a third column to provide a carbon monoxide enriched liquid at the top of this third column and a liquid enriched in methane in the tank of the third column, characterized in that at least a portion of the carbon monoxide-enriched liquid is sent to a fourth column, a flow rate rich in carbon monoxide in the bottom of the fourth column is withdrawn. and heating the tank of the fourth column by means of a top steam of the third column. According to other optional objects: the fourth column has a head condenser, the head condenser is cooled by means of a closed refrigeration cycle, the second and / or third column is heated by means of a closed refrigeration cycle, - the production carbon monoxide is withdrawn from the fourth column in liquid form, then pumped to the operating pressure and heated under this operating pressure, - the refrigeration cycle is a cycle of at least one non-toxic and / or non-flammable fluid, for example a nitrogen cycle. According to another object of the invention, there is provided a plant for producing carbon monoxide and hydrogen from a gaseous mixture essentially comprising these two bodies, methane and nitrogen, of the type comprising: a first column provided in the tank with an inlet of said gaseous mixture and at the top of a liquid methane inlet; a second column provided in the tank with a vaporizer and at at least one point of said column with a liquid inlet connected to the tank of the first column; a third column provided in the tank with a vaporizer, at the head of means for creating reflux and means for withdrawing a fluid enriched in carbon monoxide, and at at least one intermediate point of an inlet connected to the vessel 3 2903766 of the second column; and a refrigerating cycle providing heating and / or cooling of the three columns, characterized in that it comprises a fourth column, means connecting an intermediate point of the fourth column with the means for withdrawing a fluid enriched in carbon monoxide. Carbon, the means for creating reflux being constituted by a vessel vaporizer of the fourth column. According to other optional objects: a refrigerating cycle which is a closed cycle, and in which the means for withdrawing carbon monoxide are liquid withdrawal means connected to the suction of a pump, the installation comprising means (4) for heating the fluid discharged by this pump. the refrigerating cycle is a cycle using at least one non-toxic and / or non-flammable fluid, for example nitrogen. By replacing the carbon monoxide refrigeration cycle with a nitrogen cycle, for example, the inventories of carbon monoxide will be substantially reduced (fluid of the condenser becoming liquid nitrogen), in addition it is proposed according to the invention to superimpose the third column (carbon monoxide / methane separation) and the fourth column (carbon monoxide / nitrogen separation) by thermally linking them via a condenser reboiler (conventional or preferably low liquid amount), thus the inventories of toxic liquid and / or flammable are minimized. The installation essentially comprises a first column, or washing column, 1, a second column, or exhaust column called flash column, 2, a third column, or carbon monoxide / methane separation column, 3, a fourth column, or denitrogenation column 4, a heat exchanger 10 of the indirect type and circulating against the flow of fluids in heat exchange relationship, pumps P1 and P2, and a refrigerating cycle. This cycle comprises a compressor 11, an expansion turbine T and an expansion valve. To ensure the heating of the columns 2 and 3 by means of the refrigerating cycle, the column 2 has in the tank a vaporizer 5 and the column 3 has in the tank a vaporizer 6. To ensure the cooling of the 4 290 3766 columns 1 and 4 by means of the cycle 1, the column 1 has at its head a condenser 8. The initial gaseous or di-phase mixture SG, introduced in the vat of the column 1, via a pipe, in a pressure for example of the order of 15 to 40 bars absolute, is washed in this column by means of liquid methane introduced at the top of column via a pipe 16, an intermediate refrigeration of the column being provided by the condenser 9 by means of the cycle refrigeration. A hydrogen gas stream of production H2, under pressure, is withdrawn at the top of column 1 via a pipe, while the bottom liquid, essentially consisting of carbon monoxide, methane, nitrogen and hydrogen. is fed, via a line 18 equipped with an expansion valve 18A, at the top of the column 2. The latter being heated in the tank by the vaporizer 5, the dissolved hydrogen is separated from the liquid in this second column, under a pressure of the order of 5 to 20 bar, and is discharged at the head via a pipe 19. The bottom liquid of the column 2, consisting essentially of methane, nitrogen and carbon monoxide, is introduced via a pipe 20 equipped with an expansion valve 20A, at an intermediate point of the column 3.

The latter, heated in the tank by the vaporizer 6 and cooled at the top by the condenser 7, separates the two constituents of the liquid by distillation under a pressure of the order of 2 to 10 bars. Liquid methane containing the heavy impurities is withdrawn in the bottom of the column 3 via a pipe 21, and liquid carbon monoxide containing light impurities such that the nitrogen is withdrawn at the top of the same column via a line 22. that the liquid methane is partially purged and partly recycled, via a pipe 21A equipped with a pump P2, in the pipe 16 which feeds the head of the column 1, the liquid carbon monoxide mixed with nitrogen is sent at an intermediate point of column 4 after relaxation. The bottom liquid of column 4 is heated by the overhead vapor of column 3 via vaporizer 7. The overhead gas of column 4 is condensed by means of the top condenser 8 supplied with liquid by the refrigerating cycle leading to condenser liquid 5 and 6.

Purge gases PG1 and PG2 are withdrawn from the condensers 7 and 8 respectively. The bottom liquid of column 3 is pure carbon monoxide which is supplied by the pump P1 to the desired operating pressure, and then reheated at this pressure in passages of the heat exchanger 10, from which it flows. spring in the form of gaseous product via a pipe 24. The refrigeration cycle 6 will now be described, which may be a closed nitrogen cycle, that is to say the cycle fluid of which is nitrogen or another non-toxic gas and / or non-flammable and which is entirely separate from the aforementioned fluid circuits. The cycle nitrogen, compressed by the compressor 11 in a high pressure line 25, is cooled in passages of the exchanger 10 and then condensed in vaporizers 5 and 6. The high pressure liquid nitrogen is expanded in expansion valves 13, 15 and sent to the condenser 8.

The low pressure nitrogen gas from this condenser 8 is returned via a pipe 27 and the exchanger 10 to the low pressure suction of the compressor 11. A fraction of the high pressure nitrogen gas leaving the passages is expanded in the turbine T at an intermediate pressure and then reheated in passages of the exchanger 10 and returned to an intermediate stage of the compressor 11. A fraction of the evaporated tank carbon monoxide 29 is withdrawn at an appropriate temperature of the exchanger 10 and relaxed in a valve 29A before being injected in the lower part of the denitrogenation column 4. This makes it possible to decouple the operation of the third and fourth columns 3, 4 by an additional heating that is adjusted, depending, for example, on the amount of nitrogen in the gas mixture SG. A flow of liquid nitrogen can be produced as the final product (not shown). As an example, on a production unit of 33,000 Nm 3 / hr of carbon monoxide, in the conventional case of a carbon monoxide / nitrogen separation column and carbon monoxide ring process, the total inventory of liquid toxic and / or flammable is around 70 m3.

6 2903766 By replacing the carbon monoxide cycle with a nitrogen cycle for example, and by superimposing the last two columns 3,4 by linking them with a reboiler / condenser 7 with low liquid stock, it is possible to reduce the inventory toxic and / or flammable liquid around 25 m3 5 is a reduction of 65%. It should also be noted that the inventory of toxic products only (carbon monoxide) is in the case of the conventional solution of 53 m3, to be reduced to around 8 m3 in the context of the invention, a reduction of 85 0/0 io 7

Claims (9)

  A process for producing carbon monoxide and hydrogen from a gaseous mixture essentially comprising these two bodies, methane and nitrogen, of the type in which the starting gas mixture is washed with liquid methane in a first column (1) for supplying hydrogen gas and washing liquid, the hydrogen dissolved in the washing liquid is separated therefrom into a second column (2), and the remaining liquid is separated by distillation into a third column (3) for supplying a carbon monoxide enriched liquid at the head of this third column and a methane enriched liquid in the bottom of the third column, characterized in that at least a portion of the enriched liquid is sent to carbon monoxide at a fourth column (4), a high flow of carbon monoxide is withdrawn from the bottom of the fourth column and the tank of the fourth column 15 is heated by means of a head vapor of the third neck onne.   The method of claim 1 wherein the fourth column has a head condenser. 20   3. Method according to claim 2 wherein the condenser overhead is cooled by means of a closed refrigeration cycle.   4. Method according to one of the preceding claims wherein the second and / or third column (2, 3) is heated by means of a closed refrigeration cycle.   5. Method according to one of the preceding claims wherein the production carbon monoxide is withdrawn from the fourth column (4) in liquid form, then brought by pump (P1) to the operating pressure and 30 warms it up under this pressure of use.   6. Method according to one of the preceding claims, characterized in that the refrigerating cycle is a cycle of at least one non-toxic and / or non-flammable fluid, for example a nitrogen cycle. 8 2903766   7. A plant for producing carbon monoxide and hydrogen from a gaseous mixture essentially comprising these two bodies, methane and nitrogen, of the type comprising: a first column (1) provided in a tank with an inlet of said gaseous mixture and at the top of a liquid methane inlet; a second column (2) provided in the tank with a vaporizer (5) and at at least one point of said column of a liquid inlet connected to the tank of the first column (1); a third column (3) provided in the tank with a vaporizer (6), at the head of means (7) for creating reflux and means (22) for withdrawing a carbon monoxide enriched fluid, and at least one intermediate point of an inlet connected to the vessel of the second column (2); and a refrigerating cycle (6) providing heating and / or cooling of the three columns (1 to 3), characterized in that it comprises a fourth column (4), means connecting an intermediate point of the fourth column with the means (22) for withdrawing a carbon monoxide enriched fluid, the means (7) for creating reflux being constituted by a tank vaporizer of the fourth column.   8. Installation according to claim 7 comprising a refrigerating cycle (6) which is a closed cycle, and wherein the carbon monoxide removal means are liquid withdrawal means connected to the suction of a pump (P1). ), the installation comprising means (10) for heating the fluid discharged by this pump (P1). 25   9. Installation according to claim 7 or 8, characterized in that the refrigerating cycle is a cycle using at least one non-toxic and / or non-flammable fluid, for example nitrogen.