Natural Gas Liquefaction Process

Abstract

A method of liquefying a hydrocarbon-rich gas, wherein the gas flows through a cascade of three refrigeration stages, each stage comprising a refrigerant circuit and a compressor, wherein at least part of the flow of refrigerant from the second circuit is used for the pre-cooling of the hydrocarbon rich gas in the first refrigeration stage. This balances the load on each of the compressors. By standardizing the drive units and compressors of the three coolant circuits, it is possible to maximize the attainable liquefaction capacity of the liquefaction process using tried-and-trusted drive units and compressors respectively. This method can be applied to mixed refrigerant cascades and circuits with a carbon dioxide pre-cooling circuit. This latter option has benefits for offshore use where large amounts of hydrocarbons are undesirable.

Description

Preferred embodiments of the present invention shall now be described, by way of example only, with reference to the following drawings, in which:

FIG. 1 shows a load balanced liquefaction process in accordance with a preferred embodiment of the invention;

FIG. 2 show an alternative embodiment of a load balanced process;

FIG. 3 shows a graph of overall power demand as a function of a reference temperature;

FIG. 4 shows a load balanced liquefaction process containing a carbon dioxide pre-cooling circuit;

FIG. 5 shows hot/cold composite curves for the processes shown in FIGS. 2 and 4; and

FIG. 6 shows a comparison of refrigerant inventories of the processes shown in FIGS. 2 and 4.

In FIG. 1 the cooling and liquefaction of the hydrocarbon-rich flow, which is conducted via line 1, are effected against a mixed refrigerant circuit cascade, consisting of three mixed refrigerant circuits. These as a rule have different compositions, such as are described, for example, in the aforementioned German published application 197 16 415.

The hydrocarbon-rich flow which is to be liquefied is cooled in the heat exchanger E1 against the two evaporating mixed refrigerant flows 4b and 4d of the first mixture circuit 4a to 4e, then cooled by the evaporating mixed refrigerant flow 3d, and then conducted via line 1a to a heavy hydrocarbon separation unit S, represented simply as a box.

In this separation unit S the C₃₊ separation described heretofore takes place, whereby the components separated out of the hydrocarbon-rich flow are drawn off from the heavy hydrocarbon separation unit S via line 1b.

According to one advantageous embodiment of the method according to the invention, not shown in the drawing, at least one part flow of one of the two part flows 3b and 3d of the second cooling agent mixture circuit 3a to 3e, which will be discussed in greater detail hereinafter, is used for the provision of cooling in the separation unit S. In this situation, the choice of which of the two part flows 3b and/or 3d is drawn from for this provision of cooling is determined by the temperature level(s) required in the heavy hydrocarbon separation unit S.

The hydrocarbon-rich flow to be liquefied is then conducted via line 1c to a second heat exchanger E2, and is liquefied in this against the evaporating mixed refrigerant flow 3b of the second cooling circuit 3a to 3e.

Once liquefaction has taken place, the hydrocarbon-rich flow is conducted via line 1d to a third heat exchanger E3, and is subcooled here against the mixed refrigerant flow 2b of the third cooling circuit 2a to 2c. The subcooled liquid product is then conducted via line le to its further use.

As can be seen from the drawing, each of the three cooling circuits 2a to 2c, 3a to 3e, and 4a to 4e, has a compressor, V2, V3, and V4 respectively. Not shown in the drawing are the corresponding drives for these compressors V2, V3, and V4. In addition, the coolers or heat exchangers which are located downstream of the compressors V2, V3, and V4 respectively are not shown in the drawing, in which the refrigerant mixture is cooled against a cooling medium, such as water.

The refrigerant mixture of the first refrigerant circuit, compressed in the compressor V4, is conducted via line 4a to the heat exchanger E1, and is divided here into two part flows 4b and 4d after cooling has taken place. The refrigerant mixture in these part flows 4b and 4d, after throttling has been effected in the valves d and e or expansion devices, is evaporated to different pressure levels in the heat exchanger E1 and then conducted via line 4c or 4e to the compressor V4 before the first stage (part flow 4c) or to an intermediate pressure level (part flow 4e).

The refrigerant mixture of the second cooling circuit 3a to 3e, compressed in the compressor V3, is conducted via line 3a through heat exchangers E1 and E2, and is cooled in these. That part flow 3b of this refrigerant mixture flow, which is conducted through heat exchanger E2, after expansion in valve b, is evaporated in heat exchanger E2 against cooling process flows, and is then conducted via line 3c to the intake stage of compressor V3.

According to the invention, a part flow 3d of the refrigerant mixture of the second refrigerant mixture circuit 3a to 3e is drawn off after the heat exchanger E1, expanded in valve c, and then evaporated in heat exchanger E1 against cooling process flows, before being conducted via line 3e, at an intermediate pressure level, to the circuit compressor V3. Accordingly, the refrigerant mixture part flow 3d, according to the invention, makes a contribution to the pre-cooling of the hydrocarbon-rich flow in heat exchanger E1.

In order for this to be achieved, the part flow 3d of the refrigerant mixture of the second mixed refrigerant circuit 3a to 3e, used for the pre-cooling of the hydrocarbon-rich flow, must be evaporated at a pressure which is higher than the evaporation pressure of the mixed refrigerant part flow 3b of the second mixed refrigerant circuit 3a to 3e.

By selecting the intermediate pressure at which the mixed refrigerant part flow 3e is evaporated and conducted to the compressor V3, and by regulating the volume distribution of the two mixed refrigerant part flows 3b and 3d, the distribution of the cooling capacity of the second refrigerant circuit onto the heat exchangers E1 and E2, and therefore to the pre-cooling and liquefaction of the hydrocarbon-rich flow which is to be liquefied, can be adjusted almost at will.

If, for example, 40% of the total drive capacity is required for the pre-cooling and 60% for the liquefaction and subcooling of the hydrocarbon-rich flow, then, with the concept and method according to the invention, one compressor is used in each case with a third of the total drive capacity in the first and third refrigerant mixture circuit, i.e. for the pre-cooling as well as for the subcooling of the hydrocarbon-rich flow which is to be liquefied. The compressor of the second refrigerant mixture circuit is operated according to the invention in such a way that it uses 20% of its capacity, and consequently 6.66% of the total capacity, for pre-cooling, while the remaining 80%, i.e. 26.66% of the total capacity, is used for liquefaction.

The method according to the invention accordingly makes possible the economical exploitation of the available compressors and drive units, because the (circuit) compressors of the three refrigerant circuits obtain approximately the same drive capacity, i.e. a third of total capacity in each case.

Accordingly, large liquefaction plants in particular, with a liquefaction capacity greater than 5 million tonnes LNG per year, can be operated substantially more economically, since, by standardizing the drives and compressors of the three cooling circuits, the achievable liquefaction capacity of the liquefaction process can be maximised with the use of tried-and-trusted drive units and compressors.

FIG. 2 shows an alternative version of the load balanced process. As with FIG. 1 the pre-cooling cycle C10 comprises a first circuit driven by a first compressor V10 and one part 22 of the refrigerant stream 21 from the second cycle C20. Three General Electric MS 7121 EA (Frame 7) gas turbines are used to drive the compressors V10, V20, V30. If highest availability is of the essence, the three refrigeration cycles can be designed with two times 50% gas turbine/compressor trains. In this case six GE MS 6581 B (Frame 6) gas turbines would replace the three Frame 7s.

All LNG plants require the extraction of at least of those hydrocarbons, which would freeze in the LNG under storage conditions (e.g. aromatics and C₅+). In an LNG plant precooling is usually considered as first cooling step between ambient temperature and extraction of the mentioned hydrocarbons.

It should be emphasised that the method according to the invention can be combined with all known separation methods considered to be prior art for relatively high-boiling hydrocarbons.

The precooling portion of the overall power demand of all refrigeration compressors for the two gases defined in Table 1 is shown in FIG. 3 as a function of a reference temperature. This is the temperature, under which all main process streams (natural gas, refrigerant fluids) enter into the cryogenic heat exchangers.

	TABLE 1
		Lean	Rich
	mol %	Gas	Gas
	N2	5.00	5.00
	CH4	88.93	84.07
	C2H6	3.96	5.58
	C3H8	1.37	2.73
	C4H10	0.48	1.34
	C5H12	0.17	0.65
	C6H14	0.06	0.32
	C7H16	0.02	0.16
	C8H18	0.01	0.08
	Benzene	0.01	0.08
		100.00	100.00

The lower the reference temperature and the richer the gas the smaller the required compressor power for precooling becomes. This situation can be addressed reasonably well by designers of dual flow liquefaction processes, if the power mismatch between precooling and liquefaction plus subcooling is compensated by helpers for the gas turbines.

A process with three refrigeration cycles offers a much wider field for even load distribution between the cycles. If part of the refrigerant of the liquefaction cycle C20 is vaporized under elevated pressure in the precooling section C10 and is fed to the LC compressor V20 as side stream 22, a perfect load balancing between all three refrigeration cycles can be achieved. This feature is a major aspect of a cost effective design for large production capacities. As all three (3) cycles are symmetrically driven this arrangement is referred to as MFC*s3.

Unlike the embodiment of FIG. 1, the final compressor V30 of FIG. 2 is split into two casings V31, V32. The second casing V32 is designed to deal with high pressures at which the multistage compressor operates.

In order to provide actual figures for a realistic process design a large LNG train has been studied. On the basis of the lean gas composition with a pressure of 62 bar and a temperature of 35 deg C. at the inlet to precooling a conceptual process design was made. The refrigeration compressors are driven by Frame 7's with additional 20 MW on each shaft, which have been recruited from the starter/helpers. The resulting LNG rundown amounts to 8.5 mtpa at 333 stream days, which is accompanied by an additional quantity of 0.4 mtpa NGL (C₃+ hydrocarbons). The specific energy consumption of the refrigeration compressors is 259 kWh/t_LNG.

In FIG. 4 the precooling circuit C10 of FIG. 2 has been replaced with a pre-cooling circuit C100 which comprises a carbon dioxide stream 101. After compression and condensation/subcooling the stream 101 is split into three separate streams, 102, 103, 104 which are then expanded to different pressures. This compensates for the constant temperature evaporation of CO₂. Unlike hydrocarbon streams 201, 301 only part of the carbon dioxide stream 101 is sub-cooled by the pre-cooling heat exchanger E100 prior to expansion, in order to reduce the internal heat load of this exchanger.

Owing to the higher operating pressure, the CO₂ precooling compressor V100 is split into two casings, V110, V120 with a barrel type casing V120 for the high-pressure stage. After compression the carbon dioxide is cooled by a water cooled condenser C20 and an additional subcooling heat exchanger C22, using seawater to subcool the liquid refrigerant after the condenser C20, in order to improve process efficiency. In addition a desuperheater can also be provided after the compressor, as in many conventional systems.

As with the previous embodiments, “load balancing” is achieved by allowing the liquefaction compressor V200 to take over some of the precooling cycle load, leading to a “symmetrical” process.

Process simulations of the above embodiment as shown in FIG. 4 and FIG. 2 gave power requirement data as shown in Table 2, using the design data as shown in Table 3. As a result of the load-balanced process, the power input to the CO₂-precooled case was only 4.4% higher than the baseline. For a given maximum available power as defined by the hydrocarbon process case, this would correspond to a LNG capacity of 95.6% with CO₂ precooling unless more driver capacity is installed.

	TABLE 2
		C2/C3
	CO2 precooling	precooling
	Total shaft power	162.7	155.8	MW
		(104%)	(100%)
	Precooling	49.6	47.6	MW
	compressor
	Liquefaction	50.5	47.7	MW
	compressor
	Subcooling	50.5	48.5	MW
	compressor
	Other power	12.1	12.0	MW
	consumers

	TABLE 3
	LNG production capacity	5.8	mtpa
	Gross calorific value	40	MJ/Sm3
	of LNG
	Feed gas pressure	69	bar
	(liquefaction inlet)
	Sea cooling water	5	° C.
	temperature

Temperature profiles in the form of hot/cold composite curves for the two cases are shown in FIG. 5. The three CO₂ precooling temperature levels are easily observed in the left diagram. The highest pressure level to the liquefaction compressor is also considered part of precooling. Changes in the subcooling process are minimal between the two cases.

Layout, size and weight of an offshore LNG liquefaction module with CO₂ precooling were compared to the baseline hydrocarbon case (that shown in FIGS. 1 and 2). Among the factors that contributed to reduce the equipment footprints and give smaller dimensions with CO₂ were reduced precooling compressor suction drum sizes and smaller precooling piping dimensions. Additional equipment caused by the third precooling pressure level/drum and the installation of a refrigerant subcooler made the net reduction in footprint area marginal, however. The plate-fin heat exchangers were reduced in size due to larger LMTD (Logarithmic Mean Temperature Difference) and less internal duty. While plate fin heat exchangers were used in this instance it is of course also possible to use other types of heat exchangers, which could also be reduced in size. Some of the major pipe sizes in the liquefaction and subcooling circuit did not change much, and it is these pipes that to a large degree set out the deck heights, so no changes were envisaged relative to deck elevations. In total, it was concluded that the liquefaction module size would be no greater when using a CO₂ precooling circuit, and indeed a reduction of a few square meters is possible. In addition, the weight of the module dropped by 100 tons.

A major safety concern of the LNG process with hydrocarbon precooling, especially when applied offshore, is the possible formation of a flammable and explosive hydrocarbon/air mixture in case of a major leakage in one of the refrigerant circuits. Thus, the minimization of hydrocarbon refrigerant inventory is very important in terms of safety.

As may be observed from the FIG. 6, the HC refrigerant inventory is reduced by about 70% in the CO₂-precooled process. The reduced hydrocarbon charge is positive in relation to loss prevention and to the availability of the three main safety functions of the LNG barge, which are (i) main structural strength, (ii) main escape routes, and (iii) means of evacuation.

If the molecular weight of the hydrocarbon refrigerant is higher than that of air, a flammable cloud can accumulate inside or between the modules, and on the deck surfaces. Thus, in addition to minimizing the total hydrocarbon inventory it is of special importance to eliminate the heavier components, especially propane (52% heavier than air), but also ethane (4% heavier than air). By replacing the hydrocarbon precooling with CO₂, all propane is eliminated from the liquefaction module, and even though ethane is present in the liquefaction and subcooling refrigerants, both these mixtures have a molar mass that is lower than air.

From the above results it has been found that the introduction of CO₂ precooling in a load-balanced MFC*s3 process does not give a significant increase in specific power requirement, or equipment size/weight/cost, while the safety of the process can be improved.

Claims

1. A method for the liquefaction of a hydrocarbon-rich flow, whereby the liquefaction of the hydrocarbon-rich flow is effected against a refrigerant circuit cascade consisting of three refrigeration circuits, whereby the first of the three refrigeration circuits serves to provide preliminary cooling, the second refrigeration circuit serves to provide the actual liquefaction, and the third refrigeration circuit serves the sub-cooling of the liquefied hydrocarbon-rich flow, and whereby each refrigeration circuit comprises at least one single-stage or multi-stage compressor, characterised in that at least one part flow of the refrigerant of the second refrigeration circuit is used for the preliminary cooling of the hydrocarbon-rich flow.

2. A method as claimed in claim 1 wherein the part flow of the refrigerant of the second refrigeration circuit used for the pre-cooling of the hydrocarbon-rich flow is evaporated at a pressure which is higher than the evaporation pressure of the remaining part flow of the refrigerant of the second cooling circuit, and is conducted to the compressor of the second cooling circuit at an intermediate pressure level.

3. A method as claimed in claim 1 wherein the separation of unwanted components and/or components of the hydrocarbon-rich flow which freeze out during the liquefaction of the hydrocarbon-right flow takes place before the actual liquefaction of the hydrocarbon-rich flow.

4. A method as claimed in claim 3, wherein at least one part flow of one of the two part flows of the second cooling circuit is used for the provision of cooling in the separation unit.

5. A method as claimed in claim 1 wherein the volumes and/or evaporation pressures of the two part flows of the second refrigeration circuit are changeable.

6. A method as claimed in claim 1 wherein the hydrocarbon rich stream is a natural gas flow.

7. A method as claimed in claim 1 wherein each compressor has a substantially equal share of the load.

8. A method as claimed in claim 1 wherein the first refrigeration circuit comprises carbon dioxide.

9. A method as claimed in claim 1 wherein all the refrigeration circuits comprise mixed refrigerants.

10. A method of liquefying a hydrocarbon-rich gas, wherein the gas flows through a cascade of three refrigeration stages, each stage comprising a refrigerant circuit and a compressor, wherein at least part of the flow of refrigerant from the second circuit is used for the preliminary cooling of the hydrocarbon rich gas in the first refrigeration stage.

11. A method of liquefaction comprising a plurality of cooling circuits arranged in a cascade formation, each circuit comprising a compressor, wherein each compressor has a substantially equal share of the total load.

12. A method as claimed in claim 11 wherein the cascade comprises at least first and second cooling circuits, the second cooling circuit being used at least partially for pre-cooling the substance to be liquefied.

13. A method as claimed in claim 11 wherein the method is a method of liquefaction of a hydrocarbon rich flow.

14. A method as claimed in claim 13 wherein the first cooling circuit comprises carbon dioxide.

15. A substantially load balanced mixed refrigerant cascade process comprising a carbon dioxide pre-cooling circuit.

16. A substantially load balanced mixed refrigerant cascade process as claimed in claim 15 wherein the carbon dioxide is cooled after condensation to a temperature of 2020 C. or less.

17. A substantially load balanced process as claimed in claim 16 wherein the carbon dioxide is cooled to a temperature of 15° C. or less.

18. A substantially load balanced process as claimed in claim 16 wherein cold cooling water is used to cool the carbon dioxide.

19. A substantially load balanced process as claimed in claim 18 wherein the cold cooling water is sea water.

20. A substantially load balanced process as claimed in claim 15, wherein the carbon dioxide pre-cooling circuit includes a sub-cooling heat exchanger installed after the condenser.

21. A substantially load balanced process as claimed in claim 15, wherein the carbon dioxide cooling circuit comprises three pressure levels.

22. A substantially load balanced process as claimed in claim 15, wherein the carbon dioxide is not subcooled in the pre-cooling circuit.

23. A substantially load balanced process as claimed in claim 15, wherein a high pressure casing is used with the carbon dioxide compressor.

24. A substantially load balanced process as claimed in claim 23 wherein the compressor is split into two casings and a barrel type casing used for the high pressure stage.

25. An LNG liquefaction process comprising three cascade cycles each driven by a compressor, wherein the compressors are substantially equally loaded and one of the cascade cycles is a carbon dioxide cycle.

26. A method for the liquefaction of a hydrocarbon-rich flow, whereby the liquefaction of the hydrocarbon-rich flow is effected against a refrigerant circuit cascade consisting of three mixed refrigeration circuits, whereby the first of the three refrigeration circuits serves to provide preliminary cooling, the second refrigeration circuit serves to provide the actual liquefaction, and the third refrigeration circuit serves the sub-cooling of the liquefied hydrocarbon-rich flow, and whereby each refrigeration circuit comprises at least one single-stage or multi-stage compressor, characterized in that at least one part flow of the refrigerant of the second refrigeration circuit is used for the preliminary cooling of the hydrocarbon-rich flow.