Liquefaction of a hydrocarbon-rich fraction

Abstract

A process for liquefying and subcooling a hydrocarbon-rich fraction, particularly natural gas, is described wherein, once cooled down, the fraction is subjected to a partial condensation to remove heavy hydrocarbons, particularly benzene, by the steps of: a) the liquefied hydrocarbon-rich fraction is subcooled in a separate heat exchanger (normal mode), b) the supply of the liquefied hydrocarbon-rich fraction to the heat exchanger is interrupted at the latest when a defined solid deposition value in the heat exchanger is reached (cleaning mode), c) the solid in the heat exchanger is melted with a defrost gas and drawn off from the heat exchanger and d) the liquefied hydrocarbon-rich fraction is subsequently returned to the heat exchanger.

Claims

1. A process for liquefying and subcooling a hydrocarbon-rich fraction comprising: a) providing a flow of a hydrogen-rich fraction; b) cooling the hydrogen-rich fraction in a first heat exchanger to form a partially condensed fraction; c) separating the partially condensed fraction to form a heavy hydrocarbons-containing liquid fraction and a hydrocarbon-rich gas fraction; d) liquefying the hydrocarbon-rich gas fraction in a further heat exchanger to form a liquefied hydrocarbon-rich fraction; wherein the method further comprises operating a separate heat exchanger comprising: e) operating in a normal mode comprising e1) the liquefied hydrocarbon-rich fraction in the separate heat exchanger to produce a subcooled hydrocarbon-rich fraction; e2) monitoring an amount of solid deposition formed in the separate heat exchanger; f) when the amount of solid deposition reaches a defined value, operating in a cleaning mode which comprises; f1) interrupting the supply of the liquefied hydrocarbon-rich fraction to the separate heat exchanger at the latest when the amount of solid deposition value in the separate heat exchanger reaches a defined value, such that the separate heat exchanger switches to the cleaning mode; f2) feeding a defrost gas into the the separate heat exchanger so that the solid deposition is melted to form a melted deposition; f3) drawing off the melted deposition from the separate heat exchanger and g) subsequently returning to operating the separate heat exchanger in the normal mode in which the liquefied hydrocarbon-rich fraction is returned to the separate heat exchanger to be subcooled.

2. The process according to claim 1, wherein the hydrocarbon-rich fraction is natural gas.

3. The process according to claim 1, wherein the heavy hydrocarbon is benzene.

4. The process according to claim 1, wherein in step e) in the normal mode, the liquefied hydrocarbon-rich fraction is subcooled in the heat exchanger against at least one refrigerant stream and/or at least one mixed refrigerant stream, and in step f) in the cleaning mode said refrigerant stream and/or mixed refrigerant stream are also used to cool the hydrocarbon-rich fraction in the first and/or second heat exchangers.

5. The process according to claim 1, wherein in steps b) and d) the hydrocarbon-rich fraction is liquefied and subcooled against at least one refrigeration cycle having a refrigerant, wherein a substream of the refrigerant is the defrost gas.

6. The process according to claim 1 wherein the cleaning mode of step e) further includes after the solid deposition in the heat exchanger has been melted purging passages of the heat exchanger in which the solid deposition occurs.

7. The process according to claim 4, wherein the liquefied and subcooled hydrocarbon-rich fraction is sent to an intermediate storage; and the passages of the heat exchanger are purged by a supply of dry nitrogen and/or a boil-off gas fraction generated during the intermediate storage of the liquefied and subcooled hydrocarbon-rich fraction.

8. The process according to claim 1, wherein cooling-down, liquefaction and subcooling of the hydrocarbon-rich fraction is carried out in helically coiled heat exchangers and/or welded plate exchangers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The process according to the invention for liquefying and subcooling a hydrocarbon-rich fraction and also further advantageous embodiments thereof are more particularly elucidated hereinbelow with reference to the working examples shown in FIGS. 1 and 2.

(2) FIG. 1 shows a regime where the hydrocarbon-rich fraction is liquefied and subcooled against a mixed cycle while the regime shown in FIG. 2 employs a two-stage nitrogen expander cycle.

DETAILED DESCRIPTION OF THE INVENTION

(3) Hydrocarbon-rich feed fraction 1 to be liquefied, for example so-called lean natural gas, is sent, prior to actual liquefaction, to removal unit A in which a chemical scrub and/or an adsorptive process are used to remove water and carbon dioxide which are drawn off via line 2. The thus prepurified feed fraction 3 is sent to first heat exchanger or heat exchanger zone E1 in which it is cooled down and partially condensed. Partially condensed fraction 4 is then sent to separator D1 and separated into heavy hydrocarbons-containing liquid fraction 5 and hydrocarbon-rich gas fraction 6. While the former is drawn off from the bottom of separator D1 via control valve V6, gaseous fraction 6 is liquefied in second heat exchanger or heat exchanger zone E2. According to the invention, liquefied hydrocarbon-rich fraction 7 is subcooled in separate heat exchanger or subcooler E3. Subcooled hydrocarbon-rich fraction 8in the case of natural gas the LNG product fractionis sent for further use and/or intermediate storage via valve V4. Heat exchangers E1 to E3 described above may be helically coiled heat exchangers and/or welded plate exchangers.

(4) In the regime shown in FIG. 1, cooling-down, liquefaction and subcooling of the hydrocarbon-rich fraction are achieved against a mixed cycle comprising two-stage compressor unit C1. The refrigerant vaporized and warmed in heat exchangers E1 to E3 is sent via line 20 to vessel D2 disposed upstream of the first stage of compressor unit C1. Gas fraction 21 accumulating in said vessel is compressed to an intermediate pressure in the first compressor stage of compressor unit C1, cooled down and partially condensed in intermediate cooler E4 and sent via line 22 to second separator D3. Gas fraction 23 accumulating in said second separator is compressed to the desired final cycle pressure in the second compressor stage of compressor unit C1 and sent to third separator D4 via line 27 in which aftercooler E5 is disposed.

(5) Liquid fraction 25 drawn off from the bottom of second separator D3 is cooled down in heat exchanger E1. This fraction is subsequently subjected to refrigerating expansion in valve V1 and passed, countercurrently to hydrocarbon-rich feed fraction 3 to be cooled down, through heat exchanger E1 via line 26. While liquid fraction 28 accumulating in third separator D4 is recycled to a point upstream of second separator D3 via control valve V5, gas fraction 29 accumulating in third separator D4 is likewise cooled down and partially condensed in heat exchanger E1 and then separated into liquid fraction 30 and gas fraction 32 in separator D5.

(6) The latter is condensed and subcooled in heat exchangers E2 and E3, subjected to refrigerating expansion in valve V3 and is passed via line 33 through separate heat exchanger E3 to provide the peak refrigeration required therein. This fraction is subsequently admixed via control valve V7 and line 34 with liquid fraction 30 cooled down in heat exchanger E2. Said liquid fraction is subjected to refrigerating expansion in expansion valve V2 and subsequently passed, countercurrently to hydrocarbon-rich feed fraction 3/6 which is to be cooled down and liquefied, through heat exchangers E2 and E3 via line 31.

(7) According to the invention, heat exchanger or subcooler E3 is a discrete apparatus. Said apparatus is connected to heat exchangers E1 and E2 only via conduits. Now, when a defined solid deposition value in heat exchanger E3 is reached, the process switches from normal mode to cleaning mode. This is achieved by closing valve V4 and opening valve V9, so liquefied hydrocarbon-rich fraction 7 bypasses heat exchanger E3 via line 9. In a simultaneous operation valves V3 and V7 are closed and valve V8 is opened, so gas fraction 32 drawn off from separator D5 is now passed exclusively through heat exchanger E2. Due to this rerouting of refrigerant fraction 32, heat exchanger E2 assumes, at least to an extent, the subcooling of the liquefied hydrocarbon-rich fraction which in normal mode is effected in separate heat exchanger E3.

(8) Simultaneously with the above-described opening and closing of valves V3, V4 and V7 to V9, and with valves V10 and V11 open, a suitable amount of defrost gas at a suitable temperature is passed via line 10 through heat exchanger E3 and drawn off via line 11. Heat exchanger E6 provided in line 10 heats this defrost gas. Now, rather than refrigerant fraction 32 which flows through heat exchanger E3 in normal mode, defrost gas 10 serves as heat-transfer medium and melts the solids deposited in heat exchanger E. Said solids can be drawn off in concentrated form at a suitable point between heat exchangers E2 and E3, for example at the conduit low points, via appropriate shutoff valves which, for clarity, are not shown.

(9) In the regime shown in FIG. 2, cooling-down, liquefaction and subcooling of the hydrocarbon-rich feed fraction are achieved via a two-stage nitrogen expander cycle. Since the regime for the hydrocarbon-rich feed fraction to be liquefied and subcooled here is identical to that of FIG. 1, it will not be discussed further in what follows; hence what follows describes only the nitrogen expander cycle.

(10) Nitrogen-rich refrigerant 40 warmed in heat exchangers E1 to E3 is compressed to an intermediate pressure in the first compressor stage of compressor unit C1, cooled down in intermediate cooler E4 and sent via line 41 to the second compressor stage of compressor unit C1. Refrigerant 42 compressed to the cycle end pressure is cooled down in aftercooler E5 and cooled down in heat exchangers E1 and E2. A first substream 43 of the cooled-down refrigerant is sent to a first expander X1, subjected to refrigerating and work-performing expansion therein and passed, countercurrently to hydrocarbon-rich feed fraction 3 which is to be liquefied, through heat exchangers E2 and E1 via line 44. The second refrigerant substream 45 is sent to second expander X2 to likewise undergo refrigerating and work-performing expansion, passed, countercurrently to the hydrocarbon-rich fraction 7 which is to be subcooled, through separate heat exchanger E3 via line 46 and subsequently admixed via valve V with the above-described refrigerant substream 44.

(11) When the defined solid deposition value in heat exchanger X3 is reached, second expander X2 is taken off stream. In a simultaneous operation valve V7 is closed and valves V8, V10 and V11 are opened. With valve V8 open, second refrigerant substream 45, hitherto sent to second expander X2, is now sent via line 52, shown dashed in the figure, to a point upstream of first expander X1. With valve V10 opensaid valve is used for adjustment of the desired defrost gas pressurea substream of the refrigerant drawn off upstream of the second compressor stage is sent as defrost gas to heat exchanger E3 via line 50 shown with a dotted line in the figure. Heat exchanger E6 is used for any defrost gas heating required. Having passed through heat exchanger E3, and with valve V11 open, the defrost gas is recycled via line 51, shown with a dotted line in the figure, to a point upstream of the first compressor stage of compressor unit C1.

(12) The process according to the invention for liquefying and subcooling a hydrocarbon-rich fraction, particularly of natural gas, achieves reliable and economical removal of heavy hydrocarbons, particularly of benzene, even when a so-called lean gas is used. The implementation of the concept according to the invention is independent of the chosen type of liquefaction and subcooling of the hydrocarbon-rich fraction.