Method for purifying a gas rich in hydrocarbons

Abstract

A process for the purification of a gas rich in hydrocarbons and comprising at least 10 ppm by volume of hydrocarbons having at least six carbon atoms nitrogen.

Claims

1. A process for the purification of a gas comprising hydrocarbons and comprising at least 10 ppm by volume of hydrocarbons having at least six carbon atoms, the process comprising the following stages: Stage a) cooling the gas to a temperature of between −20° C. and −60° C. by heat exchange with at least one coolant in a heat exchanger; Stage b) purifying from compounds containing at least six carbon atoms of the gas partially liquefied in stage a) in a washing column containing a column top at a highest end and a column vessel at the lowest end, thus forming at the column top, a gas stream containing less than 5 ppm by volume of compounds containing at least six carbon atoms and, at the column vessel, a liquid stream enriched in compounds containing at least six carbon atoms; Stage c) condensing the gas stream resulting from stage b) in a heat exchanger, thus forming a two-phase stream; Stage d) separating the two-phase stream resulting from stage c) in a phase-separating pot at a temperature of between −60° C. and −80° C., thus forming a gas stream at the pot top and a liquid stream at the pot vessel; Stage e) introducing the liquid stream resulting from stage d) into the washing column as washing column top reflux; Stage f) condensing the gas stream resulting from stage d) by heat exchange in a heat exchanger at a temperature of less than −100° C., thus forming a liquefied gas containing less than 5 ppm by volume of compounds containing at least six carbon atoms.

2. The process of claim 1, wherein the gas rich in hydrocarbons is natural gas.

3. The process of claim 1, wherein the hydrocarbons having at least six carbon atoms comprise a predominance of benzene.

4. The process of claim 1, wherein the coolant is a mixed coolant comprising nitrogen, methane, ethane and butane.

5. The process of claim 1, wherein at least a portion of the liquid stream formed during stage f) is tapped at a temperature of less than −100° C. and then recycled in the phase-separating pot employed in stage d).

6. The process of claim 1, wherein the liquid stream formed at the vessel of the phase-separating pot in stage d) is pumped using at least one pump in order to feed the top of the washing column employed in stage b).

7. The process of claim 1, wherein the at least one coolant is used to cool the gas in stage a) and to condense the gas in stage f).

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) 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:

(2) FIG. 1 is a schematic representation of one embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(3) The process which is a subject-matter of the present invention is illustrated in FIG. 1.

(4) In FIG. 1, a gas stream 1 rich in hydrocarbons, such as a stream of natural gas, is introduced into a heat exchanger 2.

(5) The pressure of this gas stream is, for example, between 25 and 60 bar abs. Typically, the gas stream 1 contains between 30 ppm by volume and 500 ppm by volume of benzene, usually less than 100 ppm by volume. The gas stream 1 is cooled by heat exchange in the heat exchanger 2 on contact with a coolant. The heat exchanger is fed with at least one coolant stream. For example, this stream can be composed of a mixed coolant stream. The composition and the operating conditions of the mixed coolant are adjusted to the hydrocarbon to be liquefied.

(6) The stream of natural gas cooled to a temperature of between −20° C. and −70° C., typically of between −35° C. and −40° C., at the outlet 4 of the exchanger 2 is introduced into a washing column in which the heavy products are separated from the natural gas. The term “heavy products” is understood to mean the hydrocarbons having more than four carbon atoms and the aromatic compounds, including in particular benzene.

(7) A liquid stream 6 containing all (to within about one ppm by volume) the benzene from the initial gas stream 1 is discharged at the vessel 7 of the column 5.

(8) At the top 8 of the column 5, a gas stream 9 comprising less than 1 ppm by volume of benzene is recovered in order to be introduced into a second heat exchanger 10 which can preferably be a second section of the heat exchanger 2.

(9) The mixed coolant stream 11 recovered at the outlet of the heat exchanger 2 is introduced into a phase-separating pot 12, producing a gas stream 13 containing the light components of the coolant at the pot 12 top and a liquid stream 14 containing the heavy components of the coolant at the pot 12 vessel. These two streams 13 and 14 feed the second heat exchanger 10 (or second stage of the exchanger 2).

(10) The gas stream 9 containing less than 1 ppm by volume of benzene introduced into the second heat exchanger 10 (or second section of the exchanger 2) is at least partially condensed. The two-phase stream 15 at the outlet of the second heat exchanger 10 (or second section of the exchanger 2) is introduced into a phase-separating pot 16 in order to produce a gas stream 17 at the pot 16 top and a liquid stream 18 at the pot 16 vessel. The temperature is then typically between −70° C. and −75° C.

(11) The liquid stream 18 feeds the top 8 of the washing column 5. Depending on the installation of the pot 16 with respect to the column 5, a pair 19 of lift pumps may be present in order to suck the liquid stream 18 in order to carry out the column 5 top 8 reflux.

(12) It should be noted that the liquid reflux (stream 18) may not be sufficient and that, in this case, it is possible to cool the pot 16 by injecting liquid natural gas at the inlet of the two-phase pot (line 22). This line 22 is important as it makes it possible to control the flow of liquid reflux into the column 5 and thus the benzene content of the product to be liquefied. As the composition and the operating conditions of the natural gas can change during the lifetime of the unit, the reflux flow necessary can thus be optimized, and also the liquefaction energy.

(13) There exist at least two alternative forms for this solution, which is a subject-matter of the present invention: Alternative form No. 1: In order to simplify the main exchanger, it is possible to directly “shower” the column 5 top with liquid natural gas (stream 22) but the flow of liquid natural gas 22 is then greater and this option can be costly in liquefaction energy. Alternative form No. 2: In order to reduce the flow of liquid natural gas necessary, it is also possible to shower the top of the separating pot 16 and thus to purify the natural gas by two successive refluxes.

(14) The gas stream 17 is introduced into a third heat exchanger 20, which can preferably be a third section of the exchanger 2, in order to be cooled to a temperature of less than −110° C., for example between −110° C. and −115° C. The stream 21 thus cooled can be partially tapped and form a stream 22 which will be recycled by being introduced with the stream 15 into the phase-separating pot 16.

(15) This is because, according to a specific form of the process for the liquefaction of natural gas 1 which is a subject-matter of the invention, the liquid coolants 14 are drawn off and then subsequently reduced in pressure, for example using valves 23, before being reintroduced and revaporized in the exchange line 24 opposite the natural gas 17, which gas liquefies. Thus, a stream of liquid natural gas can be tapped 22 at this level (cooler than the phase-separating pot 16 forming the reflux of the column 5) and recycled in the pot 16 in order to increase the reflux liquid 18 in the event of lack of the latter, while limiting the loss in effectiveness by instantaneous vaporization.

(16) Finally, the gas stream 21 is introduced into a heat exchanger 26 in order to produce a stream of liquefied natural gas 27, a product purified from heavy compounds and from aromatics, including typically benzene, resulting from the liquefaction and purification process which is a subject-matter of the present invention.

(17) 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.