JPH0627619B2 - Liquefaction method of natural gas - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for liquefying natural gas.

2. Description of the Related Art Conventional technology and its problems Conventionally, as a natural gas liquefaction method, a cascade method and a mixed refrigerant (MCR) method have been used. Since the MCR method is simpler than the cascade method, the latter method has recently been used more frequently. However, the MCR system is not so simple and requires a large heat exchanger, so that the construction cost is high. In addition, since the ratio of the mixed refrigerant components greatly affects the performance, it is necessary to carefully adjust the components, it takes a long time for the plant to reach a steady state after the start of operation, and the mixed refrigerant components may not be released during the open inspection. Special handling is required.

Means for Solving the Problems The present inventor has conducted various studies in view of the current state of the art, and as a result, has found that natural gas having a predetermined temperature and pressure obtained by cooling with a Rankine cycle using propane or the like as a refrigerant. Gas (primary flash gas) obtained by flashing with a separator [first separator] from the components contained in the gas itself [cooling gas] or natural gas [cooling gas] (these are primary cooling in this specification) The Brayton cycle that uses a majority of the gas as refrigerant is cold (as cold gas), and the remaining primary cooling gas is cooled by the secondary flush gas described later (this is called secondary cooling). When introduced into a separator (second separator) together with cold gas to flash / liquefy and separate into liquefied gas and secondary flash gas, the problems of conventional technology are widespread. It was found to be reduced to. That is, the present invention provides the following method: (i) a refrigerant liquefied in a condenser after being compressed by a compressor,
A Rankine cycle cooler in which heat is exchanged in a cooler to be vaporized and then circulated in a compressor. A pressure of 30 kg / cm ² · G or more obtained by primarily cooling natural gas and circulating gas,
And a cooling gas having a temperature of −20 to −70 ° C., or a pressure obtained by flushing the cooling gas obtained by primary cooling the natural gas and the circulating gas with the cooler of the Rankine cycle in the first separator. 30kg / cm ² · G or more, and temperature-
A primary flash gas of 20 to -70 ° C is used as a primary cooling gas, (ii) a majority of the primary cooling gas is expanded by an expander to be a cold gas, and (iii) the rest of the primary cooling gas is heat-exchanged with a refrigerant. A method for separating a liquefied gas and a secondary flash gas by introducing a cold gas and a secondary cooling gas into a second separator, as a secondary cooling gas, and (vi) a primary cooling gas and a heat In a natural gas liquefaction method using a Brayton cycle, in which a secondary flash gas is used as a refrigerant to be exchanged, and the secondary flash gas after heat exchange with a primary cooling gas is compressed by a compressor to form a circulating gas, (vii ) The power recovered by the expander that expands the majority of the primary flash gas is used to compress the secondary flash gas after heat exchange with the primary cooling gas and / or to compress the refrigerant in the Rankine cycle. Method of liquefying a natural gas which is characterized in that.

Hereinafter, the present invention will be described in detail with reference to the example of the flow chart according to the present invention shown in the drawings.

In FIG. 1, about 70 kg / cm ² · G of natural gas enters the acid gas removal device (3) from the line (1), and after the acid gases such as carbon dioxide and hydrogen sulfide are removed, the line (5 ) And water is removed by the adsorption dehumidifier (7). This natural gas is combined with the circulating gas from the line (111) in the line (9), enters the first gas cooler (11) and is cooled to about 0 ° C., and then the second through the line (13). Is cooled to about −30 ° C. by the gas cooler (15), enters the LPG recovery unit (19) via the line (17), and the LPG fraction is recovered, and then the third gas via the line (21). It is cooled to about -60 ° C by the cooler (23), and the line (2
After 5), the high pressure separator [first separator] (27) is entered. When the natural gas contains a large amount of heavy hydrocarbons such as ethane and propane, the high pressure separator [first separator] (27) separates the condensed component.

For cooling the natural gas in the gas coolers (11), (15) and (23), a cascade Rankine cycle using a refrigerant such as propane, a mixture of propane and ethane, or R-13B1 is used. That is, the refrigerant compressed in the multi-stage compressor (31) by the prime mover (29) is transferred to the line (3
3) and then liquefies in the condenser (35), then passes through the line (37), part of which passes through the line (39), enters the first cooler (11) and is vaporized, and the line (41) is passed through. Then, the high pressure stage suction of the compressor (31) is restored. Refrigerant that does not enter the first cooler (11) is decompressed in the line (43), and a part of it is vaporized in the second cooler (15) through the line (45) and then in the line (47). Then, the compressor (31) returns to the intermediate pressure stage suction. The remaining liquefied refrigerant is further depressurized through the line (49), vaporized in the third cooler (23), and returned to the low pressure stage suction of the compressor (31) through the line (51). In this way, by the heat of vaporization of the circulating refrigerant,
The gas is cooled.

A part of the gas [primary flush gas (primary cooling gas)] cooled by the third gas cooler (23) and separated by the high pressure separator [first separator] (27) is part of the line (5
After going through 3), enter the first heat exchanger (55), and at about -80 ° C.
Cooled to about −21 ° C. in the second heat exchanger (59) via the line (57), and the obtained secondary cooling gas is passed through the line (61) to the intermediate pressure separator [second Separator] (63), the pressure is reduced to about 9 kg / cm ² · G, the temperature is lowered to about -125 ° C, and the liquid [liquefied gas] and the gas [secondary flush gas] are separated. In addition, the majority (remaining amount) of the gas [primary flush gas (primary cooling gas)] discharged from the high pressure separator [first separator] (27) described above passes through the line (65) and the expander (67). Enter, pressure about 9kg /
cm ² · G, expanded to temperature -125 ° C (becomes cold gas), partly liquefied, and entered the above-mentioned intermediate pressure separator [second separator] (63) via line (69). , And the gas and liquid are separated together with the above-mentioned secondary cooling gas. When a large amount of liquid components are separated by the high pressure separator [first separator] (27) described above, a part of the liquid component passes through the line (71) and the medium pressure separator [second separator] described above is used. Separator] (63) The pressure is reduced, and the secondary cooling gas and the cold gas are separated into gas and liquid.
The liquid component [liquefied gas] separated by the medium pressure separator [second separator] (63) enters the nitrogen separator (77) through the line (75) to separate and remove nitrogen, and then the line (7
After 9), in the low pressure separator (81), the pressure is almost atmospheric pressure,
The temperature is about -162 ° C, and the gas and liquid are separated, and the liquid component is liquefied natural gas tank (85) through line (83).
Sent to. If it is not necessary to remove nitrogen, the nitrogen separator (77) is unnecessary.

The gas [secondary flush gas] separated in the medium pressure separator [second separator] (63) is heat-exchanged in the second heat exchanger (59) via the line (87), and the temperature is about −. It reaches 85 ° C., joins a part of the liquid component of the high-pressure separator [first separator] (27) from the line (73) in the line (89), and enters the first heat exchanger (55). A part of the liquid component of this high-pressure separator (first separator) (27), which is heat-exchanged (to the above-mentioned primary cooling gas) up to about -65 ° C, is recycled to the Rankine cycle. However, when it is not circulated, the first heat exchanger (55) and the second heat exchanger (59) can be common.

The medium pressure gas (secondary flush gas) exiting the first heat exchanger (55) is passed through a line (91) by an intermediate pressure compressor (93) driven by an expander (67). About 27kg
/ Cm ² · G, compressed to the line (9
5) through suction into the high-pressure circulating gas compressor (105) driven by the prime mover (99).

The gas separated in the low pressure separator (81) is fed into the line (9
Approximately 27 kg / in the low pressure circulating gas compressor (101) via 7)
It is compressed to cm ² · G and is combined with the medium pressure circulating gas from line (95) via line (103) and pressurized to about 70 kg / cm ² · G in the high pressure circulating gas compressor (105). It This gas is cooled in the circulating gas cooler (109) via the line (107), and merged with the natural gas in the line (9) through the line (111).

As described above, since the circulating gas and the natural gas are circulated and used, when the natural gas has a large nitrogen content, the circulating gas mainly contains nitrogen, but there is no problem in terms of performance.

The above description is based on the examples, and the pressure and temperature conditions are for the case where natural gas is 100% methane,
When the composition and pressure of natural gas change, the temperature and pressure of each part also change slightly.

In the case of 100% methane, the inlet pressure of natural gas is 70%.
Assuming kg / cm ² · G and cooling water temperature of 20 ° C, if the liquefied natural gas production capacity is 125 t / h (1 million t / year), the input of the circulating gas compressor is about 28,000 kw, propane refrigerant compression The input of the machine is about 19,000kw, and the required input per ton of liquefied methane is 376kwh.

A flow chart for secondary cooling of the primary cooling gas by utilizing the low temperature of the flush gas discharged from the low pressure separator (81) in FIG. 1 is shown in FIG. This is the third heat exchanger (11
3) and the intercooler (115) of the circulating gas compressor are required, but the required input per ton of liquefied natural gas is slightly less than in the embodiment of FIG.

Effects of the Invention According to the natural gas liquefaction method of the present invention, which performs primary cooling of natural gas and circulating gas by Rankine cycle to bring the temperature to −20 to −70 ° C. and then secondary cooling by Brayton cycle, Such an effect can be obtained.

(1) Comparing the embodiment of the present invention with a cascade cycle performed under the same conditions, if the required inputs are the same, the latter requires a three-fluid eight-stage cascade, and the present invention is simpler in system. Construction costs are low.

Also, compared with the MCR method under the same conditions as the embodiment of the present invention, the present invention requires slightly less input. Also, MCR
In the case of the method, since it is necessary to use a heat exchanger of three fluids or more, the total heat exchange amount is large, and the number of heat exchange elements is large, the construction cost is higher than that of the present invention.

After all, although the present invention uses the Brayton cycle, which is often seen as a disadvantage in the refrigeration cycle, the effective energy (exergy) loss is reduced and the expansion heat is reduced because the adiabatic efficiency of the expander is about 80%. The machine will perform a kind of heat exchange, the total amount of heat exchange can be reduced, and the total efficiency can be improved by reducing the exergy loss of heat exchange. large.

(2) Since no special refrigerant is required, the device is simple and the operation is easy. In particular, since the mixed refrigerant is not used, the operation at the time of starting and stopping is easy, and the intermittent operation is easy.

(3) The number of heat exchangers is small, all of which are two-fluid heat exchangers, the aluminum fin heat exchanger can be easily used, and it is small and inexpensive.

(4) In the embodiment, the power recovered by the expander is used to compress the circulating gas (secondary flush gas), but it may be used to compress the refrigerant for the primary cooling.

(5) As the expander and the compressor directly connected to it, a high speed turbo type is usually used, which is compact and economical.

(6) Boil-off gas generated in the tank of the natural gas liquefaction base can also be collected by the device used.

[Brief description of drawings]

1 and 2 are flow charts showing an embodiment of the present invention. (11) …… First gas cooler, (15) …… Second gas cooler, (19) …… LPG recovery machine, (23) …… Third gas cooler, (27) …… High-pressure separator, (29) …… motor, (31) …… multistage compressor, (35) …… condenser, (55) …… first heat exchanger, (59) …… second heat exchange. Vessel, (63) ... Medium pressure separator, (67) ... Expander, (77) ... Nitrogen separator, (81) ... Low pressure separator, (85) ... Liquefied natural gas tank, (93) …… Medium pressure compressor, (99) …… prime mover, (101) …… low pressure circulating gas compressor, (105) …… high pressure circulating gas compressor, (109) …… circulating gas cooler.

Claims (1)

[Claims] (I) A Rankine cycle cooler in which a refrigerant that has been compressed in a compressor and then liquefied in a condenser is heat-exchanged in a cooler to be vaporized and then circulated in the compressor. Pressure obtained by primary cooling of 30 kg / cm
A cooling gas having a temperature of −20 to −70 ° C. of ² · G or more, or a cooling gas obtained by primary cooling natural gas and circulating gas with the Rankine cycle cooler is flushed with the first separator. A pressure of 30 kg / cm ² · G or more and a temperature of −20 to −70 ° C. is used as the primary cooling gas, and (ii) a majority of the primary cooling gas is expanded with an expander to obtain a cold gas. , (Iii) The rest of the primary cooling gas is heat-exchanged with a refrigerant to form a secondary cooling gas, and (iv) the cold gas and the secondary cooling gas are introduced into a second separator to produce a liquefied gas and a secondary flash gas. (Vi) A secondary flash gas is used as a refrigerant for exchanging heat with the primary cooling gas, and the secondary flash gas after heat exchange with the primary cooling gas is compressed by a compressor to circulate gas. Brayton cycle (Vii) The compression and / or Rankine cycle of the secondary flash gas after heat exchange of the power recovered by the expander for expanding the majority of the primary flash gas with the primary cooling gas, A method for liquefying natural gas, which is used for compressing a refrigerant in.