Process and apparatus for the cryogenic separation of a mixture of carbon monoxide, hydrogen and methane for the production of CH4

Abstract

In a process of the separation of a mixture of carbon monoxide, hydrogen and methane, the mixture is sent to a scrubbing column, a bottom liquid withdrawn at the bottom of the scrubbing column is depleted in hydrogen with respect to the mixture and is sent to a stripping column, a bottom liquid from the stripping column is sent to a separation column and a liquid enriched in methane withdrawn from the bottom of the separation column is vaporized in order to form a final product.

Claims

1. A process for separation of a mixture comprising carbon monoxide, hydrogen and methane, the process comprising the steps of: i) sending the mixture or a fluid derived from the mixture, after cooling to a cryogenic temperature in a heat exchanger, to a scrubbing column fed at a top portion with a liquid containing at least 80 mol % carbon monoxide or to at least one phase separator; ii) withdrawing a bottom liquid at a bottom portion of the scrubbing column or of the phase separator or of one of the phase separators that is depleted in hydrogen with respect to the mixture and then sending the bottom liquid to a stripping column; iii) withdrawing a gas at a top portion of the stripping column; iv) sending a bottom liquid from the stripping column to a separation column that is configured to separate the bottom liquid into a top gas enriched in carbon monoxide and a liquid enriched in methane; and v) withdrawing the liquid enriched in methane from a bottom portion of the separation column and then vaporizing the liquid enriched in methane in the heat exchanger in order to form gaseous methane, wherein the gaseous methane is compressed in a compressor and a first part of the gaseous methane following compression is returned at the bottom of the separation column for separation therein, and a second part of the gaseous methane following compression is collected as a compressed methane product stream, wherein the separation column has a top condenser cooled by a closed nitrogen cycle comprising a gaseous nitrogen compressor or by a closed carbon monoxide cycle comprising a gaseous carbon monoxide compressor, wherein the first part of the gaseous methane following compression that is returned to the bottom of the separation column has the same methane concentration as the liquid enriched in methane withdrawn the bottom portion of the separation column.

2. The process according to claim 1, in which the first part of the gaseous methane following compression is at a lower pressure than that of the compressed methane product stream.

3. The process according to claim 1, wherein the liquid fed to the top portion of the scrubbing column comprises condensate formed from at least a part of a gas from the top portion of the scrubbing column.

4. The process according to claim 1, in which the mixture contains nitrogen and in which the separation column produces, at the bottom portion, the liquid enriched in methane and depleted in carbon monoxide and, at the top portion, the top gas enriched in carbon monoxide.

5. The process according to claim 1, in which the nitrogen cycle is used to reboil the stripping column.

6. The process according to claim 1, in which a maximum pressure of the nitrogen or carbon monoxide cycle is less than the critical pressure of the nitrogen or the critical pressure of the carbon monoxide.

7. The process according to claim 1, in which a maximum pressure of the nitrogen or carbon monoxide cycle is chosen so that a condensation temperature of nitrogen or carbon monoxide in the heat exchanger at the maximum pressure is within 10? C. warmer than a vaporization temperature of the liquid methane in the heat exchanger.

8. The process according to claim 1, in which the mixture or the fluid derived from the mixture is used to reboil the stripping column.

9. The process according to claim 1, in which the separation column operates at between 1.5 and 15 bars abs.

10. An apparatus for separation of a mixture comprising carbon monoxide, hydrogen and methane, the apparatus comprising: a scrubbing column having a top portion and/or at least one phase separator; a stripping column having a top portion and a bottom portion; a separation column having a top portion and a bottom portion; a heat exchanger; means for sending the mixture to be cooled to a cryogenic temperature to the heat exchanger; means for sending the cooled mixture or a fluid derived from said cooled mixture to the scrubbing column fed at the top portion with a liquid containing at least 80 mol % carbon monoxide and/or to at least one phase separator; means for withdrawing a bottom liquid from the scrubbing column and/or the at least one phase separator, wherein the bottom liquid is depleted in hydrogen with respect to the mixture; means for sending the withdrawn bottom liquid to the stripping column; means for withdrawing a gas at the top portion of the stripping column; means for sending a bottom liquid from the stripping column to the separation column, wherein the separation column is configured to separate the bottom liquid into a top gas enriched in carbon monoxide and a liquid enriched in methane; means for withdrawing the liquid enriched in methane from the bottom of the separation column, wherein the means for withdrawing the liquid enriched in methane are in fluid communication with the heat exchanger in order to form gaseous methane; a compressor; means for sending the gaseous methane to the compressor; and means for returning a part of the gaseous methane following compression in the compressor to the bottom portion of the separation column for separation therein, wherein the separation column comprises a top condenser that is cooled by a closed refrigeration cycle, wherein the closed refrigeration cycle is selected from the group consisting of a closed nitrogen refrigeration cycle and a closed carbon monoxide refrigeration cycle, wherein the part of the gaseous methane following compression that is returned to the bottom of the separation column has the same methane concentration as the liquid enriched in methane withdrawn the bottom portion of the separation column.

11. The apparatus according to claim 10, wherein the closed refrigeration cycle comprises the closed nitrogen cycle comprising a gaseous nitrogen compressor.

12. The apparatus according to claim 10, wherein the closed refrigeration cycle comprises the closed carbon monoxide cycle comprising a gaseous carbon monoxide compressor.

13. The apparatus according to claim 10, in which the means for returning the part of the gas compressed in the compressor at the bottom portion of the separation column are connected to the heat exchanger.

14. The apparatus according to claim 10, wherein the separation column comprises an absence of a reboiler.

15. The process according to claim 1, wherein the separation column comprises an absence of a reboiler.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features, advantages and possible applications of the invention are apparent from the following description of working and numerical examples and from the drawings. All described and/or depicted features on their own or in any desired combination form the subject matter of the invention, irrespective of the way in which they are combined in the claims the way in which said claims refer back to one another.

(2) FIG. 1 provides a first embodiment of the present invention.

(3) FIG. 2 provides a second embodiment of the present invention.

(4) FIG. 3 represents the exchange of heat between the fluids which are cooled and the fluids which are heated within heat exchanger E1 in accordance with an embodiment of the present invention.

(5) FIG. 4 provides a third embodiment of the present invention.

(6) FIG. 5 provides a fourth embodiment of the present invention.

(7) FIG. 6 provides a fifth embodiment of the present invention.

(8) FIG. 7 provides a sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(9) The invention will be described in more detail with reference to FIGS. 1, 2, 4, 5, 6 and 7, which represent processes according to the invention, and to FIG. 3, which represents the exchange of heat between the fluids which are cooled and the fluids which are heated in the heat exchanger E1.

(10) In FIG. 1, a synthesis gas 1 purified from carbon dioxide and from water is cooled in a heat exchanger E1.

(11) It is cooled down to an intermediate temperature of the exchanger and then feeds the bottom of a CO scrubbing column K1.

(12) A top gas rich in hydrogen 3 is withdrawn at the top of the column K1, cooled in the exchanger E1 and sent, partially condensed, to a phase separator P1. The gas 7 from the separator P1 is reheated in the exchanger E1, while the liquid 5 is returned at the top of the column K1 as reflux. Alternatively or in addition, a liquid originating from the top of the column K3 can be sent to the top of the column K1.

(13) The liquid 5 contains at least 80 mol % of carbon monoxide.

(14) The bottom liquid 9 from the scrubbing column K1 is reduced in pressure and sent to the top of the stripping column K2. The top gas 11 from the column K2 is reheated in the exchanger E1. The bottom liquid 13 is vaporized in the heat exchanger E2 against a part 39 of the cycle nitrogen. The remainder 15 of the bottom liquid is sent to an intermediate point of the CO/CH.sub.4 separation column K3. This separation column K3 does not have a bottom reboiler; on the other hand, it does have a top condenser C1. The top carbon monoxide 17 from the column K3 is at least partially condensed in the condenser C1 by exchange of heat with the cycle nitrogen. The condenser C1 is located inside a bath, the walls 8 of which are shown.

(15) The bottom liquid 21 enriched in methane is reduced in pressure and then vaporized in the heat exchanger E1 to form a gas. All the gas is compressed in the compressor V1 and a part of the gas continues the compression in the compressor V2 in order to form the product gas 25 at at least 25 bars abs.

(16) The remainder 23 of the gas compressed in V1 alone is cooled in the heat exchanger and divided into two. A portion 27 is returned to the bottom of the column K3 in order to reboil the column by direct exchange of heat and in order to participate in the distillation.

(17) Another part 29 is cooled down to an intermediate temperature of the exchanger E1 and then joins up with the liquid 21 to be vaporized in the exchanger after reduction in pressure in a valve.

(18) The cycle nitrogen does not participate in the distillation but is used to reboil the column K2 and to condense the top gas 17 from K3. The liquid nitrogen 35 from the condenser C1 is vaporized and is sent to a nitrogen compressor V4. Gaseous nitrogen 19 vaporized by the condenser C1 and is mixed with the vaporized liquid 35 in the exchanger. Another flow of liquid from the bath of the condenser 33 is reduced in pressure to a relatively low pressure and is subsequently compressed in the compressor V3. The nitrogen compressed in V3 joins up with the nitrogen flows 19, 35 and the combined flow is compressed in V4. This compressed flow 37 is cooled in the exchanger and is divided into two. A part 39 is used to heat the exchanger E2 in order to reboil K2. A part 41 is liquefied after cooling in the heat exchanger and is sent to the bath of condenser C1.

(19) The separation column K3 operates at between 1.5 and 15 bars abs, indeed even between 7 and 10 bars abs.

(20) The separation column K3 does not comprise a bottom reboiler.

(21) The scrubbing column K1 operates at between 15 and 60 bars absolute,

(22) The maximum pressure of the cycle of the nitrogen (outlet pressure of V4) is chosen so that the condensation temperature of nitrogen 37 in the heat exchanger E1 at this pressure is lower by less than approximately 10? C. than the vaporization temperature of the liquid methane 21 in the heat exchanger.

(23) In FIG. 2, an alternative form of FIG. 1 is encountered where the mixture 1 is used to heat the heat exchanger E2 and thus the bottom of the column K2. The mixture is partially condensed therein, is sent to the separator P1 and the gas formed 3 feeds the column K1.

(24) The liquid 5 from the separator P1 joins up with the liquid 9 from the column K1 and feeds the top of the stripping column K2.

(25) In this instance, all the nitrogen from the compressor V4 is sent to the condenser C1.

(26) As illustrated in FIG. 3, the stationary phase for vaporization of the liquid methane 21 occurs opposite the stationary phase of condensation of the nitrogen originating from the compressor V4 (two vertical lines at between ?155? C. and ?150? C.) and the exchange diagram indicates particularly noteworthy performance qualities.

(27) However, the presence of the nitrogen cycle is not essential; it can, for example, be replaced by a carbon monoxide cycle.

(28) In FIG. 4, which is an alternative form of FIG. 1, the gas to be treated 1, after having heated the bottom of the column K2 via E2, is first separated in a phase separator P1. The gas formed 3 is cooled in the heat exchanger E1 and subsequently partially condensed in a second phase separator P2. The gas from the separator P2 exits from the apparatus as gas 4. The liquid 9 is reduced in pressure in order to join up with the liquid 5 originating from the first phase separator P1 and the liquid formed feeds the top of the column K2.

(29) Thus, the phase separator and the column of FIG. 1 are replaced with two phase separators.

(30) FIG. 5 is an alternative form of FIG. 2 where the column K3 is not surmounted by a condenser of the top gas but by a reservoir of liquid rich in carbon monoxide. This liquid participates in a carbon monoxide cycle. The liquid 33 is withdrawn from the reservoir, reduced in pressure, vaporized in the exchanger E1 and sent to a compressor V3. A top gas 19 from the reservoir is reheated in the exchange E1 as flow 22 and sent to the outlet of the compressor V3 in order to be compressed in the compressor V4. A part 37 of gas compressed in V4 is returned as liquid 41 to the reservoir. A second flow 35 of liquid from the reservoir is reduced to a higher pressure than the inlet of the compressor V3 and is sent to the inlet of V4 also. Top gas from the column K3 is also sent to the inlet of the compressor V4. The flow of pressurized carbon monoxide 31, preferably pressurized to between 10 and 15 bars, is used as product.

(31) Thus, it is seen that the carbon monoxide cycle replaces the closed nitrogen cycle.

(32) FIG. 6 comprises the carbon monoxide cycle of FIG. 5 and the two phase separators of FIG. 4.

(33) For the cases with carbon monoxide cycle, for example in FIGS. 5 and 6, it is possible, as illustrated in FIG. 7, to take liquid carbon monoxide 37 at the outlet pressure of the compressor V4, to condense it and to send at least a part 11 to the top of the column K1.

(34) The remainder 41 of the liquid can be returned to the top reservoir of the column K3. The liquid 11 preferably contains at least 80 mol % of carbon monoxide.

(35) In the case of the presence of nitrogen, a CO/N.sub.2 separation column can be added upstream or downstream of the column K3.

(36) For all the figures: the scrubbing column K1 operates at between 15 and 60 bars absolute, the stripping column K2 operates at between 3 and 20 bars absolute, the separation column operates at between 1.5 and 15 bars absolute, the maximum pressure of the nitrogen cycle (outlet pressure of V4) is 35 bars absolute (critical pressure of the nitrogen). The process is possible with a pressure of greater than and up to 70 bars but less effective if above the critical pressure of the nitrogen.

(37) While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

(38) The singular forms a, an and the include plural referents, unless the context clearly dictates otherwise.

(39) Comprising in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of comprising). Comprising as used herein may be replaced by the more limited transitional terms consisting essentially of and consisting of unless otherwise indicated herein.

(40) Providing in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

(41) Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

(42) Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

(43) All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.