Refrigeration system for a natural gas liquefaction process

Abstract: Provided is an improved refrigeration system for pre-cooling natural gas or cooling a mixed refrigerant for natural gas liquefaction in a propane refrigeration process widely used for the liquefaction of natural gas. The system comprises a plurality of plate-fin heat exchangers (31) preferably arranged in a parallel relationship for passing a propane refrigerant as a vertical flow and pre-cooling natural gas or cooling a mixed refrigerant for liquefying natural gas, and a thermo siphon drum (33,35,37,39) for the propane refrigerant consisting of a horizontally disposed, laterally elongated tank. Because the passages of the heat exchanger for the natural gas or the mixed refrigerant extend over their entire length in mutually separate relationship, even when the propane refrigerant, the natural gas or the mixed refrigerant is in both gas and liquid phases, a high efficiency of heat transfer can be attained, and the size of the heat exchanger can be reduced. In particular, from an economic view point, it is preferable if the thermo siphon drum serves also as a flash tank.

Description: TECHNICAL FIELD The present invention relates to a refrigeration system for pre-cooling natural gas or cooling a mixed refrigerant for liquefying natural gas in a refrigeration process using a propane refrigerant which is widely used for a natural gas liquefaction process. BACKGROUND OF THE INVENTION In a normal natural gas liquefaction process, as illustrated in Figure 1, high pressure natural gas from which acid gases such as CO 2 and H 2 S are removed is cooled to approximately 20 °C in a shell and tube heat exchanger 1 through which HHP propane is passed so that a majority of the water content in the natural gas may be removed and separated in a drum 2. Then, the water content is further reduced to the order of 1 wt ppm in a dryer 3, and the natural gas is cooled to 0 °C in a shell and tube heat exchanger 4 through which HP propane is passed. The natural gas is further cooled in a shell and tube heat exchanger 5 through which MP propane is passed, and is cooled in a shell and tube heat exchanger 6 through which LP propane is passed before it is supplied to a scrub column 7 where heavy fractions are removed. Then, as illustrated in Figure 2, the natural gas is cooled to -145 °C and liquefied by exchanging heat with a mixed refrigerant in a main heat exchanger 8. This stream is flashed twice in drums 9 and 10 so as to be removed of its N 2 content, and is fed to a storage facility by a pump 11 as LNG at its boiling point under the atmospheric pressure. Meanwhile, in the mixed refrigerant cycle, as illustrated in Figure 2, after the mixed refrigerant has exchanged heat with the natural gas in the main heat exchanger 8, the mixed refrigerant is fed to a LPMR compressor 12 at 3 bar, -30 °C, and it is pressurized to 13 bar by the compressor 12, and cooled to the ambient temperature in an after-cooler 13. It is then pressurized to 25 bar in a HPMR compressor 14, and again cooled to the ambient temperature in an inter-cooler 15 before it is further pressurized to 40 bar by the HPMR compressor 14. The thus pressurized mixed refrigerant is cooled to the ambient temperature in an after-cooler 16, and is then further cooled to 15 °C by HHP propane in a shell and tube heat exchanger 17, to 0 °C by HP propane in a shell and tube heat exchanger 18, to -10 °C by MP propane in a shell and tube heat exchanger 19, and to -25 °C by LP propane in a shell and tube heat exchanger 20. In this case, the mixed refrigerant starts partial condensation in the shell and tube heat exchanger 17, and is three quarters condensed in the shell and tube heat exchanger 20. It is then introduced into a separation drum 21 where the separated gas and liquid are passed through the main heat exchanger 8 for exchanging heat with the natural gas. Now consider an example of an LNG plant with a capacity of 2.6 million tons per year. The (kettle type) shell and tube heat exchangers 1, 4, 5 and 6 that are to be cooled by propane are each required to be a large kettle type heat exchanger in the order of 1,000 to 2,000 m 2 , and the shell and tube heat exchangers 17, 18, 19 and 20 are each required to be a large kettle type heat exchanger in the order of 2,000 m 2 x 2. Such heat exchangers are so large in size that they are not suitable for land transportation, and the cost for the foundation and other construction work will be substantial. Further, since the natural gas or the mixed refrigerant enters these shell and tube heat exchangers 5, 6, 18, 19 and 20 in mixed phases, the liquid to gas ratio of the stream in each part of the tubes deviates so much from a theoretical value that the performance of the heat exchangers inevitably drops. A refrigeration system according to the preamble of claim 1 is provided by FR-A-2237158. BRIEF SUMMARY OF THE INVENTION In view of such problems of the prior art, a primary object of the present invention is to provide an improved refrigeration system for pre-cooling natural gas or cooling a mixed refrigerant for natural gas liquefaction in a propane refrigeration process widely used for the liquefaction of natural gas. A second object of the present invention is to provide a refrigeration system of the above mentioned type which is economical to construct, and highly efficient in operation. According to the present invention, such objects can be accomplished by providing a refrigeration system for pre-cooling natural gas or cooling a mixed refrigerant for liquefying natural gas by using a propane refrigerant in a natural gas liquefaction process, comprising: a plate-fin heat exchanger including a plurality of passages for the natural gas or the mixed refrigerant which extend in a mutually separated relationship substantially over an entire length thereof, the propane refrigerant being passed vertically in the plate-fin heat exchanger; and a separation drum for the propane refrigerant connected to the plate-fin heat exchanger characterised in that said plate-fin heat exchanger consists of a plurality of segments disposed parallel to each other and each defining at least one of said mutually separated passages, each segment being separated into a plurality of stages along the length thereof, and a plurality of separation drums, each consisting of a laterally elongated, horizontally disposed tank, are arranged such that each of said separation drums extends laterally across said segments with individual conduits connecting corresponding stages of said segments so as to serve as a common header. From an economical view point, the separation drums may each consist of a thermo siphon drum which preferably serves also as a flash tank. By using a plate-fin heat exchanger having a ten times larger heat transfer area for unit volume than a shell and tube heat exchanger, the above mentioned cost can be reduced. By combining a number of heat exchangers into a single plate-fin heat exchanger and thereby reducing the amount of piping between different heat exchangers, a necessary heat transfer area can be obtained without excessively increasing the size of the overall heat exchanger. An example of plate-fin heat exchanger that can be used for such a purpose is disclosed in Japanese patent publication (kokoku) No. 58-55432 and United States Patent No. 4,330,308. The problem with the prior art that a desired heat transfer efficiency cannot be obtained due to the fact that the natural gas or the mixed refrigerant consists of mixed phases can be avoided by keeping the passages within the plate-fin heat exchanger separate from each other throughout the length of the plate-fin heat exchanger. With the view of maintaining the efficiency of the system even in a partial capacity operation, the stream flow may be directed vertically downward or horizontally in the cases of the natural gas and the mixed refrigerant, and vertically upwards in the case of the propane. BRIEF DESCRIPTION OF THE DRAWINGS Now the present invention is described in the following with reference to the appended drawings, in which: Figure 1 is a diagram illustrating a pre-cooling device for natural gas in a natural gas liquefaction process to which a refrigeration system using a propane refrigerant according to the present invention is applied; Figure 2 is a diagram illustrating a liquefying device for natural gas in a natural gas liquefaction process to which a refrigeration system using a propane refrigerant according to the present invention is applied; Figure 3 is a diagram showing an essential part of a first embodiment of the refrigeration system according to the present invention; Figure 4 is a diagram showing an essential part of a second embodiment of the refrigeration system according to the present invention; Figure 5 is a plan view showing the layout of the system illustrated in Figure 4; Figure 6 is a vertical view showing the layout of the system illustrated in Figure 4; Figure 7 is a plan view of a third embodiment of the refrigeration system according to the present invention; and Figure 8 is a vertical view of the third embodiment of the refrigeration system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 3 shows an essential part of a propane refrigeration system according to the present invention employing a plate-fin heat exchanger 31 in place of the heat exchangers 17, 18, 19 and 20 illustrated in Figure 2, and numerals 33, 35, 37 and 39 denote thermo siphon drums while numerals 33', 35', 37', and 39' denote flash drums for preparing low pressure propane refrigerants. In the present embodiment, four thermo siphon drums are provided for each plate-fin heat exchanger 31. The liquefied propane at 15 bar, 43 °C is converted by a regulating valve 32 into HHP propane at 7 bar, 10 °C, and is introduced into the flash tank 33' in mixed phases. It is then separated into a gas fraction and a liquid fraction, and the gas fraction is returned to the compressor and other parts of the propane refrigeration system via a conduit 40 while the liquid fraction is fed to the thermo siphon drum 33 eventually to be circulated in the heat exchanger 31, a part of the liquid fraction being converted into HP propane at 5 bar, - 5 °C by a regulating valve 34 in mixed phases before it is supplied to the flash tank 35' in the next stage. The propane which has circulated the heat exchanger 31 exchanges heat with the mixed refrigerant in the heat exchanger 31, and is partly evaporated before it is returned to the thermo siphon drum 33. The gas fraction which has been separated in the thermo siphon drum 33 is also returned to the propane refrigeration system via the conduit 40. The thermo siphon drums 35, 37 and 39, the flash tanks 35', 37' and 39', and the regulating valves 36 and 38 in the subsequent stages operate in similar fashion, and their operation will be understood without any further description. A plate-fin heat exchanger when used for cooling natural gas in place of the heat exchangers 4, 5 and 6 of Figure 1 also operates in a similar fashion. However, when natural gas is to be cooled, it is preferable not to exchange heat with the HHP propane in the plate-fin heat exchanger because with the view of preventing the generation of hydrates in the shell and tube heat exchangers of Figure 1 it is necessary to rigorously control the temperature of the HHP propane, and it can be most conveniently carried out by using a control valve provided separately from the gas phase line. Such a control can be advantageously carried out by using a shell and tube heat exchanger provided separately from the plate-fin heat exchanger 31. In a base load LNG plant having a capacity of 2.6 million tons per year, in theory, six to eight plate-fin heat exchangers 31 of the largest possible size are necessary, and if separation drums such as thermo siphon drums are installed for each plate-fin heat exchanger an extremely large cost is incurred. Therefore, it is conceivable to provide a large vertical separation drum for the propane at each different level, to distribute the liquid fraction to each of the plate fin heat exchangers via a header, and to return the propane in mixed phases expelled from each of the plate-fin heat exchangers to the separation drums by collecting the various conduits to the header. According to the Inventor's recognition, by taking some measures such as providing horizontal baffles not to cause bubbles to be submerged in the liquid in the inlet end of each of the thermo siphon drums, it is possible to assign the function of a flash tank to the gas and liquid separator of the thermo siphon, and thereby reduce the cost. A flow diagram showing the outline of an embodiment based on such a recognition is given in Figure 4. However, because the refrigerant is in mixed phases, it is difficult to keep the pressure drop between each of the plate-fin heat exchangers and the corresponding separation drum either uniform or small, and this adversely affects the heat transfer in the plate-fin heat exchangers. One of the reasons for not using plate-fin heat exchangers in this field can be attributed to the loss of the efficiency of heat transfer due to the imbalance in the pressure drop. In view of this fact, according to the present invention, separation drums which may consist of a thermo siphon drum are placed horizontally with their length extending in a lateral direction, and the separation drums are provided with the function of a header so that the conduits returning from the plate-fin heat exchanger to the separation drums may be directly connected thereto, one conduit for each segment of the heat exchanger. As a result, the overall pressure drop is reduced, and the heat transfer in each segment of the plate-fin heat exchanger is improved. More specifically, as illustrated in Figures 5 and 6, five segments of a vertical plate-fin heat exchanger 31 are placed on next to the other, and thermo siphon drums 33, 35, 37 and 39 are arranged laterally so that they may each serve as a common header to each stage of the relevant segment of the plate-fin heat exchanger 31. In the present embodiment, the thermo siphon drums are provided one over the other on either side, or four thermo siphon drums for each segment of the plate-fin heat exchanger 31. Since the propane flows vertically, in particular, vertically upwards, and through a plurality of passages which are separated from each other throughout their length, even though the propane is in mixed phased, the pressure drop is not only minimized but also distributed evenly to different passages in the plate-fin heat exchanger. Meanwhile, the natural gas or the mixed refrigerant is passed as a vertical down flow or a horizontal flow, and by taking into account that it is in mixed phases, the passages of the natural gas or the mixed refrigerant in the plate-fin heat exchanger are preferably kept separate from each other over their entire length so that the loss in the efficiency of heat transfer may be avoided. Figures 7 and 8 show a third embodiment of the present invention. The parts corresponding to those of the previous embodiments are denoted with like numerals, and the description of such parts are not repeated here. In this case, a plate-fin heat exchanger 31 is placed horizontally, and natural gas or a mixed refrigerant is passed horizontally while a propane refrigerant is passed vertically upward. The separation drums 33, 35, 37 and 39 serving as thermo siphon drums are placed horizontally in the same manner as in the second embodiment. Similarly, the separation drums are each provided with the function of a header so that the conduits returning from the plate-fin heat exchanger to the separation drums are directly connected thereto with the individual conduit from each segment of the heat exchanger being connected to a corresponding one of the separation drums so that the overall pressure drop may be not only reduced but also evenly distributed among the different passages in the plate-fin heat exchanger, and the heat transfer efficiency of the heat exchanger may be improved. Because natural gas or a mixed refrigerant is passed horizontally in the heat exchanger, and the condensate of the stream tends to be separated in a lower part of the heat exchanger during the cooling process therein, thereby impairing the heat transfer efficiency of the heat exchanger, it is necessary to use straight fins in the plate-fin heat exchanger. Straight fins are relatively lower in the coefficient of heat transfer as compared to perforated fins normally used for condensing upward or downward flow, but may need a less space after all because the passage of the coolant or the propane refrigerant may be increased in size, and the distributor for each level of the propane may be omitted, thereby increasing the effective area for heat transfer. In a refrigeration system for pre-cooling natural gas or a mixed refrigerant for liquefying natural gas, by using plate-fin heat exchangers instead of shell and tube heat exchangers, and keeping the passages for the propane, the natural gas or the mixed refrigerant separate from each other, unevenness in the ratio of the gas content to the liquid content in different passages is reduced, and a high heat transfer efficiency and a substantial reduction in the equipment cost can be achieved. Further, by flowing the propane in the plate-fin heat exchanger as a vertical upward flow, and placing the associated thermo siphon drums horizontally, even when the propane is in mixed phases, the pressure drop can be not only reduced but also evenly distributed to different passages in the heat exchanger.

Claims: A refrigeration system for pre-cooling natural gas or cooling a mixed refrigerant for liquefying natural gas by using a propane refrigerant in a natural gas liquefaction process, comprising: a plate-fin heat exchanger (31) including a plurality of passages for said natural gas or said mixed refrigerant which extend in a mutually separated relationship substantially over an entire length thereof, said propane refrigerant being passed vertically in said plate-fin heat exchanger; and a separation drum (33) for said propane refrigerant connected to said plate-fin heat exchanger, characterised in that said plate-fin heat exchanger consists of a plurality of segments disposed parallel to each other and each defining at least one of said mutually separated passages, each segment being separated into a plurality of stages along the length thereof, and a plurality of separation drums (33, 35, 37, 39), each consisting of a laterally elongated, horizontally disposed tank, are arranged such that each of said separation drums extends laterally across said segments with individual conduits connecting corresponding stages of said segments so as to serve as a common header. A system according to claim 1, wherein said plate-fin heat exchanger (31) is placed vertically such that each of said segments may extend vertically, and said separation drums (33, 35, 37, 39) extend horizontally laterally across said segments at least on one side of said plate-fin heat exchanger (31). A system according to claim 1, wherein said plate-fin heat exchanger (31) is placed horizontally such that each of said segments may extend horizontally, and said separation drums (33, 35, 37, 39) extend horizontally and laterally across said segments of said plate-fin heat exchanger (31). A system according to claim 1, wherein said separation drums also serve as flash tanks. A system according to claim 4, wherein said separation drums (33, 35, 37, 39) each consist of a thermo siphon drum.

Inventors: KIKKAWA YOSHITSUGI, NOZAWA NORIYOSHI, OHMORI KENROU, OHMORI MOTOHIRO, YAMAMOTO OSAMU, KIKKAWA, YOSHITSUGI, NOZAWA, NORIYOSHI, OHMORI, KENROU, OHMORI, MOTOHIRO, YAMAMOTO, OSAMU, KIKKAWA, YOSHITSUGI, CHIYODA CORPORATION, NOZAWA, NORIYOSHI, CHIYODA CORPORATION, OHMORI, KENROU, CHIYODA CORPORATION, OHMORI, MOTOHIRO, CHIYODA CORPORATION, YAMAMOTO, OSAMU, CHIYODA CORPORATION
Applicants: CHIYODA CHEM ENG CONSTRUCT CO, CHIYODA CORPORATION
Application year: 1993
Main classification: F25J3
IPC codes: F25J1, C10L3, F25J3, F25D9, C09K5