Method and apparatus for cryogenic separation of a mixture containing at least carbon monoxide, hydrogen and nitrogen

Abstract

The invention relates to an apparatus for cryogenic separation of a mixture of carbon monoxide, hydrogen and nitrogen, including a stripping column and a denitrogenation column, a pipe for sending the mixture in liquid form to the head of the stripping column, a pipe for removing a liquid depleted of hydrogen connected to the stripping column, a pipe for removing a gas enriched with hydrogen from the stripping column, means for sending the liquid depleted of hydrogen or a fluid derived from said liquid to the denitrogenation column, a pipe for drawing a liquid enriched with carbon monoxide from the denitrogenation column, a pipe for drawing a gas enriched with nitrogen from the head of the denitrogenation column and means for sending at least one portion of the gas enriched with hydrogen to the denitrogenation column.

Claims

1. A method for the cryogenic separation of a mixture containing at least carbon monoxide, hydrogen, nitrogen, and methane, the method comprising the steps of: introducing the mixture, in liquid form, to the top of a stripping column under conditions effective for separation therein to form a hydrogen-lean liquid and a hydrogen-enriched gas; introducing the hydrogen-lean liquid to a separation column under conditions effective for separation therein to form a methane-enriched liquid at a bottom portion and a carbon monoxide-enriched gas at a top portion, the separation column having a top condenser; introducing the carbon monoxide-enriched gas from the top of the separation column to a nitrogen-removal column under conditions effective for separation therein to form a carbon monoxide-enriched liquid and a nitrogen-enriched gas containing the majority of the nitrogen present in the mixture; determining if the nitrogen content of an identified stream(s) selected from the group consisting of the mixture, the nitrogen-enriched gas, the carbon monoxide-enriched gas removed from the top of a separation column, and combinations thereof is/are below a respective threshold(s); selectively introducing at least part of the hydrogen-enriched gas from the stripping column to an intermediate point of the separation column based on the determined nitrogen content of the identified stream, and selectively introducing an uncondensed gas from the top condenser of the separation column to the nitrogen-removal column based on the nitrogen content of the identified stream.

2. The method as claimed in claim 1, wherein the step of determining if the nitrogen content of the identified streams(s) is/are below the respective threshold(s) is based on a signal received from a detector configured to measure the flow rate of the nitrogen-enriched gas.

3. The method as claimed in claim 1, wherein a pressure detector (PIC) is configured to control a flow rate of the carbon monoxide-enriched gas coming from the top portion of the separation column.

4. The method as claimed in claim 1, wherein the uncondensed gas from the top condenser is mixed with the carbon monoxide-enriched gas prior to introduction to the nitrogen-removal column.

5. The method as claimed in claim 1, wherein, if, and only if, the nitrogen content of the identified stream(s) is below the respective threshold(s) is at least part of the hydrogen-enriched gas sent to the separation column at an intermediate point of the separation column and is the uncondensed gas from the top condenser sent to the nitrogen-removal column.

6. The method as claimed in claim 1, wherein the mixture is a bottom liquid from a methane scrubbing column, wherein a scrubbing liquid used in the methane scrubbing column is comprised of the methane-enriched liquid received from the bottom portion of the separation column.

7. A method for the cryogenic separation of a mixture containing at least carbon monoxide, hydrogen and nitrogen, the method comprising the steps of: introducing the mixture, in liquid form, to the top of a stripping column under conditions effective for separation therein to form a hydrogen-lean liquid and a hydrogen-enriched gas; introducing the hydrogen-lean liquid or a fluid derived from the hydrogen-lean liquid to a nitrogen-removal column under conditions effective for separation therein to form a carbon monoxide-enriched liquid and a nitrogen-enriched gas containing the majority of the nitrogen present in the mixture; and determining if the nitrogen content of an identified stream(s) selected from the group consisting of the mixture, the nitrogen-enriched gas, and combinations thereof is/are below a respective threshold(s), selectively introducing at least part of the hydrogen-enriched gas coming from the stripping column to an intermediate point of the nitrogen-removal column based on the nitrogen content of the identified stream.

8. The method as claimed in claim 7, wherein the fluid derived from the hydrogen-lean liquid is produced by expanding the hydrogen-lean liquid through a valve.

9. A method for the cryogenic separation of a mixture comprising carbon monoxide, hydrogen and nitrogen, the method comprising the steps of: introducing the mixture, in liquid form, to the top of a stripping column under conditions effective for separation therein to form a hydrogen-lean liquid and a hydrogen-enriched gas; and introducing the hydrogen-lean liquid or a fluid derived from the hydrogen-lean liquid to a nitrogen-removal column under conditions effective for separation therein to form a carbon monoxide-enriched liquid and a nitrogen; selectively increasing a flow rate of incondensables introduced to the nitrogen-removal column based on the nitrogen content within the nitrogen removal column, wherein the incondensables comprise hydrogen from the hydrogen-enriched gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

(2) FIG. 1 represents a process flow diagram in accordance with an embodiment of the present invention.

(3) FIG. 2 represents a process flow diagram in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION

(4) In FIG. 1, a mixture 10 of carbon monoxide, nitrogen and hydrogen is cooled in an exchanger 9 and sent to a distillation column 1 (or separator). A hydrogen-enriched head gas 12 leaves the top of the column 1 and is heated up in the exchanger 9. The bottom liquor 13 is expanded and then sent to the top of a stripping column 2 which is heated at the bottom by a reboiler 8. The hydrogen-enriched head gas 14 is heated up in the exchanger 9 and then burnt. The bottom liquor 15 containing mainly nitrogen and carbon monoxide is expanded and sent to the middle of a nitrogen-removal column 4. The carbon monoxide-enriched bottom liquor 22 is sent to the exchanger 18 where it is used to condense the head gas from the nitrogen-removal column. In the nitrogen-removal column 4, a nitrogen-enriched gas 21 leaves the top of the column.

(5) If the flow rate of nitrogen 21 drops below a given threshold, the valve of the pipe 26 opens and hydrogen-enriched gas is sent from the top of the stripping column to the nitrogen-removal column 4, preferably mixing it with the gas formed by expanding the liquid from the pipe 16 through a valve.

(6) According to FIG. 2, the mixture that is to be separated contains nitrogen, hydrogen, methane and carbon monoxide.

(7) It is known practice to separate such a mixture using cryogenic distillation, using a CO/CH.sub.4 separation column followed by a CO/N.sub.2 separation column.

(8) The incondensables (H.sub.2N.sub.2) from the CO/CH.sub.4 column are not put to use: they are sent directly to the flare or to a fuel network.

(9) The CO/CH.sub.4 and CO/N.sub.2 columns are operated at equal pressures.

(10) The innovative feature consists in compensating for the reduction in nitrogen by importing head gas (which contains hydrogen) coming from the stripping column upstream of the CO/CH.sub.4 column, to the CO/CH.sub.4 column and then by importing incondensables coming from the CO/CH.sub.4 column to the CO/N.sub.2 column.

(11) All or part of the head gas from the stripping column (which contains hydrogen) is collected so as to be routed toward the feed of the CO/CH.sub.4 column.

(12) Thanks to this new line, the flow rate of incondensables heading toward the CO/CH.sub.4 column can be increased.

(13) All or some of the incondensables are then recovered at the top of the CO/CH.sub.4 column (which contains hydrogen and does not contain methane) so as to be directed toward the CO/N.sub.2 column via the impure-CO line that feeds the CO/N.sub.2 column. Thanks to this other new line, the pressure in the CO/N.sub.2 column can also be maintained if there is a nitrogen deficit in the CO: the presence of hydrogen compensates for the lack of nitrogen.

(14) The flow rates of each of the new lines are controlled by an FIC.

(15) The pressure in the CO/CH.sub.4 column needs to be raised slightly as a result in order to be sure that the flow of incondensables will indeed be directed toward the CO/N.sub.2 column.

(16) The flow rate of impure CO is controlled by a PIC added to the CO/CH.sub.4 column so that the two operating pressures of the columns can be decoupled from one another.

(17) The incondensables at the top of the CO/N.sub.2 column are then directed toward the fuel network via the N.sub.2 purge line which passes via the exchange line.

(18) A mixture 10 of carbon monoxide, nitrogen, hydrogen and methane is cooled in an exchanger 9 and sent to a methane scrubbing column 1 fed at the top with liquid methane 11 or to a separator. A hydrogen-enriched head gas 12 leaves the top of the column 1 and is heated up in the exchanger 9. The bottom liquor 13 is expanded and then sent to the top of a stripping column 2 reheated at its bottom by a reboiler 8. The hydrogen-enriched head gas 14 is heated up in the exchanger 9 and then burnt. The bottom liquor 15 containing mainly methane and carbon monoxide is expanded and sent to the middle of a distillation column 3 having a bottom reboiler 7 and a head condenser 6. A flow of incondensable gases 22 leaves this condenser 6. A carbon monoxide-enriched head gas 16 leaves the column 3 and is sent to the nitrogen-removal column 4. The methane-enriched bottom liquor is pumped by the pump 5, and split into two. One part 20 is vaporized in the exchanger 9 and the rest is fed to the scrubbing column 1 as flow 11.

(19) In the nitrogen-removal column 4, a nitrogen-enriched gas 21 leaves the top of the column and a liquid 22 leaves the bottom enriched in carbon monoxide. The carbon monoxide vaporizes in the head condenser 18 which is a water bath vaporizer.

(20) There is a pipe 26 connecting the head gas outlet 14 of the column 2 to the liquid pipe 15 entering the column 3. There is also a pipe 24 that allows incondensable gas to be sent from the condenser 6 to the nitrogen-removal column, particularly mixing it with the gas 16.

(21) These two pipes operate only if there is a reduction in nitrogen in the flow 10 or 16. When a reduction in nitrogen is detected, all or part of the head gas from the top of the stripping column 2 is sent toward the feed of the CO/CH.sub.4 column 3. By virtue of this new pipe, the flow rate of incondensables heading toward the CO/CH.sub.4 column can be increased.

(22) All or part of the incondensables from the head condenser 6 at the top of the CO/CH.sub.4 column 3 are then recovered to be routed toward the CO/N.sub.2 column via the impure CO line 16 that feeds the CO/N.sub.2 column. By virtue of this other new pipe 26, the pressure in the CO/N.sub.2 column 4 can thus be maintained if there is a deficit of nitrogen in the CO: the presence of hydrogen compensates for the lack of nitrogen.

(23) The flow rates in each of the new pipes 24, 26 are controlled by a detector detecting minimal purity of nitrogen in the nitrogen purge 21 from the head of the CO/N.sub.2 column 4, generally after passing through the exchange lines 9.

(24) The pressure of the CO/CH.sub.4 column 3 needs to be raised slightly as a result in order to be sure that the flow of incondensables will indeed be directed toward the CO/N.sub.2 column 4.

(25) The flow rate of impure CO 16 is controlled by a pressure controller added to the CO/CH.sub.4 column so that the two operating pressures of the columns can be decoupled from one another.

(26) The incondensables, including the hydrogen, at the top of the CO/N.sub.2 column, are then directed toward a fuel network via the nitrogen purge pipe 21 which passes via the exchange line.

(27) 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.

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

(29) 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.

(30) 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.

(31) 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.

(32) 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.

(33) 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.

LIST OF ELEMENTS IN THE DRAWINGS

(34) 1 scrubbing column or separator

(35) 2 stripping column

(36) 3 methane/carbon monoxide separation column

(37) 4 carbon monoxide/nitrogen separation column

(38) 5 methane pumps

(39) 6 methane/carbon monoxide separation column condenser

(40) 7 methane/carbon monoxide separation column reboiler

(41) 8 stripping column reboiler

(42) 9 heat exchanger

(43) 10 synthesis gas feed

(44) 11 scrubbing methane feed

(45) 12 hydrogen-rich gas

(46) 13 stripping column feed

(47) 14 stripping column head gas

(48) 15 methane/carbon monoxide separation column feed

(49) 16 carbon monoxide/nitrogen separation column feed

(50) 17 liquid carbon monoxide reflux

(51) 18 carbon monoxide/nitrogen separation column cooler

(52) 19 pumped liquid methane

(53) 20 liquid methane purge

(54) 21 nitrogen purge

(55) 22 low-pressure carbon monoxide

(56) 23 incondensables

(57) 24 pipe carrying hydrogen-rich gas to the column 3

(58) 26 pipe carrying incondensables to the column 4

(59) 30 cold box