METHOD FOR SEPARATING A MIXTURE CONTAINING AT LEAST NITROGEN AND METHANE

Abstract

A method for separating a mixture containing at least nitrogen and methane by cryogenic distillation in a first column operating in a first pressure and a second column operating at a second pressure lower than the first pressure, the mixture being separated in the first column to form a gas enriched in nitrogen and a liquid enriched in methane, at least a portion of the gas enriched in nitrogen being at least partially condensed in a heat exchanger and returned to the first column, the gas enriched in nitrogen is sent into the heat exchanger by the bottom, ascends in a first series of passages of the exchanger and condenses therein at least partially, the liquid formed descending in these passages of the first series and exiting by the bottom of the exchanger.

Claims

1. A method for separating a mixture containing at least nitrogen and methane by cryogenic distillation in a system of columns comprising a first column operating in a first pressure and a second column operating at a second pressure lower than the first pressure, the mixture being separated in the first column to form a gas enriched in nitrogen and a liquid enriched in methane, at least a portion of the gas enriched in nitrogen being at least partially condensed in a heat exchanger and returned to the first column, the liquid enriched in nitrogen being sent from the exchanger or from the first column to the second column, a liquid rich in methane being withdrawn at the bottom of the second column and a gas rich in nitrogen being withdrawn at the top of the second column, the heat exchanger comprising a stack of plates and fins, the plates being disposed with their axis vertically and the space between the plates forming vertical passages, wherein the gas enriched in nitrogen is sent into the heat exchanger by the bottom, ascends in a first series of passages of the exchanger and condenses therein at least partially, the liquid formed descending in these passages of the first series and exiting via the bottom of the exchanger, the liquid descending in the second column being distributed to descend in another series of passages of the exchanger, therein evaporating partially to form a biphasic mixture which is collected at the bottom of the exchanger.

2. The method according to claim 1, wherein non-condensable gases are extracted at least periodically from the first series of passages.

3. The method according to claim 1, wherein the condensed gas enriched in nitrogen is sent to the top of the first column.

4. The method according to claim 3, wherein the condensed gas enriched in nitrogen is sent to the top of the first column via two or more conduits connecting the exchanger to the top of the first column.

5. The method according to claim 1, wherein the liquid descending in the second column arrives in the exchanger at between −160° C. and −164° C.

6. The method according to claim 1, wherein the liquid descending in the second column arrives in the exchanger at a temperature T1 and emerges therefrom at a temperature T2, with T2>T1+2° C.

7. The method according to claim 1, wherein the liquid descending in the second column exits from the other series of passages, therein evaporating partially at between −153° C. and −157° C.

8. The method according to claim 1, wherein the gas ascending from the first column arrives in the exchanger at a temperature T3 and emerges therefrom at a temperature T4, with T3>T4+2° C.

9. The method according to claim 6, wherein T4−T1<1.5° C.

10. The method according to claim 1, wherein the gas ascending from the first column to the first series of passages of the exchanger exchanges material with the liquid formed descending in these passages.

11. The method according to claim 10, wherein a distillation step is carried out in the passages of the first series.

12. The method according to claim 1, wherein at least a portion of the liquid enriched in methane is sent from the first column to the second column.

13. The method according to claim 1, wherein no portion of liquid enriched in methane is sent from the first column to the second column.

14. The method according to claim 1, wherein the percentage of nitrogen in the mixture differs from the percentage of methane in the mixture by at most 20%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] 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:

[0037] FIG. 1 is a schematic representation of system for separating nitrogen from natural gas as is known in the art.

[0038] FIG. 2 is a H-T diagram illustrating an exchange using an exchanger known in the art.

[0039] FIG. 3 illustrates part of a separating apparatus according to one embodiment of the present invention.

[0040] FIG. 4 illustrates the change in enthalpy H with temperature for the evaporator of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] One solution is to condense the gas enriched in nitrogen ascending in the other series of passages, but deliberately to allow the liquid to descend again and to collect it at the bottom of the heat exchanger. The system is a dephlegmator system. The gas ascending from the first column to the first series of passages of the exchanger exchanges material with the liquid formed descending in these passages, for example via a distillation step carried out in the passages of the first series.

[0042] The liquid enriched in methane is, moreover, partially evaporated in direct contact with this hotter gas resulting from the evaporation of the liquid enriched in methane, so giving rise to the beneficial effect of distillation (which enables a reduction in the height of the column K1 for equivalent performance levels). FIG. 3 illustrates the exchanger required, which is a film evaporator-dephlegmator.

[0043] This technology makes it possible to obtain a counter-current heat exchange with a beneficial distillation effect and a very low level of proximity between hot fluid and cold fluid. FIG. 4 illustrates the exchange diagram for the evaporator E3 of FIG. 3, with the enthalpy H on the x axis and the temperature T on the y axis. In this case there is a true exchange between fluids which flow in counter-current. The cold end of the exchanger corresponds to the appearance of the first bubble in the liquid without recirculation—in the example selected: at −162° C. As illustrated, the curve for condensation of gas at the top and the curve for evaporation of liquid descending in the second column are almost parallel and therefore the ΔT remains reasonable, with the temperatures of the liquid descending in the second column, which evaporates, ranging between −162° C. at the cold end and −155° C. at the hot end. Furthermore, with a temperature difference which can be reduced by virtue of the film evaporator (˜1° C. or even lower), the condensation temperature at the cold end in this example may reduce to −161° C.

[0044] More generally, the liquid descending in the second column K2 arrives in the exchanger E3 at a temperature T1 and emerges therefrom at a temperature T2, with T2>T1+2° C., preferably T2>T1+3° C.

[0045] More generally, the gas ascending from the first column K1 arrives in the heat exchanger E3 at a temperature T3 and emerges therefrom at a temperature T4, with T3>T4+2° C., preferably T3>T4+3° C., more generally T4−T1<1.5° C.

[0046] FIG. 3 shows that: [0047] The nitrogen-enriched gas NG for condensation, from the top of the first column K1, is introduced at the bottom of a first series of passages of the exchanger E3 through two conduits 5. [0048] The nitrogen-enriched gas NG for condensation from the top of the first column K1 is not introduced into the second series of passages of the exchanger E3. [0049] The uncondensed, nitrogen-enriched gas NG′ is collected at the top of the passages of the first series (optionally with a zero flow rate in normal operation or extraction of a small flow rate of non-condensables). [0050] The liquid NL resulting from the condensation of the nitrogen-enriched gas is collected at the bottom of the first series of passages of E3. [0051] The liquid L for evaporation is distributed at the top of the second series of passages of the exchanger E3, falling directly from the lowest heat and mass exchange section of the second column K2. [0052] The liquid for evaporation falling directly from the lowest heat and mass exchange section of the second column K2 is not sent to the first series of passages. [0053] A biphasic fluid L+V obtained from the partial evaporation of liquid L is collected at the bottom of the second series of passages. It is then sent partly as liquid 11 to the second column K2 and partly via the conduits 5 to the top of the first column K1 as reflux.

[0054] In the example, the biphasic fluid exits in the bottom of the column K2, which then serves as a phase separator, with the gas of the biphasic fluid ascending in the heat and mass exchange section, and the liquid accumulating in the bottom of the column K2.

[0055] The exchanger E3 may also be located within a chamber arranged in the second column, with the bottom of the chamber serving to collect the biphasic mixture. In this case, the chamber is used as a phase separator for the biphasic fluid.

[0056] The exchanger E3 may be located inside a chamber arranged below the second column, with the bottom of the chamber serving to collect the biphasic mixture. In this case, the chamber serves as a phase separator for the biphasic fluid, and the gas formed is returned to the column K2; the liquid may be withdrawn as product 15.

[0057] Alternatively a reservoir R may be provided at the bottom of the column K2.

[0058] The mixture 1 comprises nitrogen and methane, preferably as the main components. The percentage of nitrogen in the mixture differs from the percentage of methane in the mixture preferably by at most 20%, or even at most 10%. For example, the mixture may contain 30% of nitrogen and 50% of methane (a difference of 20%) or 45% of nitrogen and 50% of methane (a difference of 5%).

[0059] 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.