CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 62/885,958, filed August 13, 2019, which is incorporated herein by reference. This application and PCT Application No. PCT/US20/41676, which is incorporated herein by reference, are commonly assigned to Bechtel Oil, Gas and Chemicals, Inc.

FIELD OF THE DISCLOSURE

[0002] The present disclosure generally relates to systems and methods for improving the efficiency of open-cycle cascade-based liquefied natural gas systems. More particularly, the systems and methods improve the efficiency of open-cycle cascade-based liquefied natural gas systems by utilizing one or more ejectors to reduce and/or eliminate compression stages.

BACKGROUND

[0003] The natural gas liquefaction process takes natural gas, primarily comprised of methane at high pressure and passes it through consecutive refrigeration cycles to produce liquified natural gas (LNG). The present disclosure relates to open-cycle cascade-based liquefied natural gas systems. One example of such a system is illustrated in FIG. 1.

[0004] In FIG 1, a schematic diagram illustrates a conventional open-cycle cascade-based liquified natural gas system comprising propane chilling, ethylene chilling and open-loop methane liquefaction stages.

[0005] The propane chilling stage includes a three-stage compressor 118, three drums 120, 122, 124 and a chiller 126 to reject heat. The vaporized refrigerant (propane) is introduced into the first compression stage of compressor 118 and is compressed through three successive stages before being transferred through line 128 to the chiller 126. The chiller 126, typically an air cooler or cooling water exchanger, chills the compressed propane in line 130 to a temperature of about 100°F. The outlet pressure in line 130 is equivalent to the pressure at which the refrigerant is liquid, which for propane is approximately 190 psia. The liquid propane in line 130 is flashed through an expansion valve 132 to a pressure of approximately 90 psia and a temperature of about 48°F. It is then introduced into the first heat exchanger 106 to pre-chill the feed gas 102. The outlet of the first heat exchanger 106 consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the high stage flash drum 124. The vapor is recompressed in the compressor 118. The liquid is flashed to a lower pressure through a second expansion valve 134 to a temperature of approximately 8°F and 45 psia and is introduced into the second heat exchanger 108. The outlet of the second heat exchanger 108 consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the middle stage flash drum 122. The vapor is recompressed in the compressor 118. The liquid is flashed to a lower pressure through a third expansion valve 136 to a temperature of approximately -31°F and 20 psia and is introduced into the third heat exchanger 110. The propane is completely vaporized in the third heat exchanger 110 and is distributed into a low stage suction drum 120.

[0006] The ethylene chilling stage includes a two-stage compressor 138, four flash drums 140, 142, 176, 152 and four brazed aluminum heat exchangers 144, 146, 148, 150. A two phased mixture of refrigerant (ethylene) from the lowest pressure heat exchanger 150 in line 153 is combined with vaporized ethylene from the higher pressure flash drum 140 in line 156 and is used as the chilling medium in the heat exchanger 144. This facilitates conventional metallurgy selection of the compressor 138. The vaporized ethylene at about 285 psia from the compressor 138 is chilled with a chiller 154 at ambient conditions to a temperature of about 100°F. The vaporized ethylene is further chilled by the three heat exchangers 106, 108, 110 to a temperature of about - 24°F in line s. At this stage, the ethylene in line 158 is predominately a liquid. The liquid ethylene in line 158 is further chilled in heat exchanger 144 to a temperature of about -88°F. The liquid ethylene is then expanded across expansion valve 160 to a pressure of about 100 psia and a temperature of about -87°F. The liquid ethylene enters heat exchanger 146 and exits as a two-phase mixture of liquid and vapor ethylene in line 164 before being flashed in flash drum 142. The vaporized ethylene from flash drum 142 passes through heat exchanger 144 and is recompressed in the compressor 138. The liquid ethylene from flash drum 142 is chilled further in the heat exchanger 144 before being expanded across expansion valve 166 to about -111°F and 50 psia. A two-phase mixture of vapor and liquid ethylene from the heat exchanger 148 in line 168 is then flashed in the flash drum 140.

[0007] A feed gas 102 enters the system at about 60°F and 650 psia. After passing through the propane chilling stage, the feed gas 102 in line 172 is about -24°F and 635 psia. The feed gas 102 in line 172 is mixed with a recycle stream comprising a chilled feed gas in line 174 at similar conditions to produce a mixed feed gas that is further chilled in heat exchangers 146 and 148. The liquid ethylene is used to further chill the mixed feed gas in heat exchangers 146 and 148.

[0008] The chilled-mixed feed gas is flashed in flash drum 152 wherein the resulting vapor is further chilled in heat exchanger 150 to produce a two-phase mixture of vapor and liquid mixed feed gas in line 178 and the resulting liquid is further flashed in flash drum 176 at about 180 psia. The two-phase mixture of vapor and liquid mixed feed gas from heat exchanger 150 is approximately -100°F and 605 psia. The liquid from flash drum 176 is withdrawn as a natural gas liquids product and the vapor is sent to the highest compression stage of compressor 197 in the open-loop methane liquefaction stage. The liquid ethylene from flash drum 140 in line 170 is used to chill the vapor from flash drum 152 in heat exchanger 150.

[0009] In the open-loop methane liquefaction stage, the two-phase mixture of mixed feed gas in line 178 is further chilled in a brazed aluminum heat exchanger 180 and then expanded through expansion valve 182 to lower the pressure to about 180 psia and the temperature to about - 187°F before entering flash drum 184. The vapor from flash drum 184 is passed through the heat exchanger 180 and the liquid is passed through a brazed aluminum heat exchanger 186. A two- phase mixture of mixed feed gas from heat exchanger 186 is expanded through expansion valve 188 to approximately 70 psia and -219°F before being flashed in flash drum 190. The vapor from flash drum 190 in line 192 is passed through heat exchangers 186, 180 and the liquid is expanded through expansion valve 194 to approximately 30 psia and -242°F before entering the low stage flash drum 196. The liquefied mixed feed gas from flash drum 196 is sent to storage in an LNG tank and the vaporized mixed feed gas is mixed with the compressed boil-off gas from compressor 199. After the boil-off gas from the LNG tank in line 198 is compressed in compressor 199 and is mixed with the vaporized mixed feed gas from flash drum 190, it is chilled in heat exchangers 186, 180 and then compressed in compressor 197.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] The detailed description is described with reference to the accompanying drawings, in which like elements are referenced with like reference numbers, in which:

[0011] FIG. 1 is a schematic diagram illustrating a conventional open-cycle cascade-based liquefied natural gas system.

[0012] FIG. 2 is a schematic diagram illustrating one embodiment of the present disclosure retrofitted in a pre-existing open-cycle cascade-based liquefied natural gas system.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS [0013] The subject matter of the present disclosure is described with specificity, however, the description itself is not intended to limit the scope of the disclosure. The subject matter thus, might also be embodied in other ways, to include different structures, steps and/or combinations similar to and/or fewer than those described herein, in conjunction with other present or future technologies. Although the term “step” may be used herein to describe different elements of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless otherwise expressly limited by the description to a particular order. Other features and advantages of the disclosed embodiments will be or will become apparent to one of ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features and advantages be included within the scope of the disclosed embodiments. Further, the illustrated figures and dimensions described herein are only exemplary and are not intended to assert or imply any limitation with regard to the environment, architecture, design, or process in which different embodiments may be implemented. The pressures and temperatures described herein thus, illustrate exemplary advantages and/or parameters of the various embodiments.

[0014] In one embodiment, the present disclosure includes a system for chilling a feed gas, which comprises: i) a first heat exchanger enclosing a first portion of a feed gas line and a portion of a first chilled refrigerant line; ii) a first flash drum in fluid communication with the first chilled refrigerant line for receiving a two-phase refrigerant from the first heat exchanger, the first flash drum having a first vapor outlet line and a first liquid outlet line; iii) a second heat exchanger enclosing a second portion of the feed gas line and a portion of a second chilled refrigerant line; iv) a second flash drum in fluid communication with the second chilled refrigerant line for receiving a two-phase refrigerant from the second heat exchanger, the second flash drum having a second vapor outlet line and a second liquid outlet line; v) a third heat exchanger enclosing a portion of the first vapor outlet line, a portion of the second vapor outlet line and a portion of a third chilled refrigerant line; and vi) a first ejector in fluid communication with the second vapor outlet line, the first chilled refrigerant line and the third refrigerant line. [0015] In another embodiment, the present disclosure includes a method for chilling a feed gas, which comprises: i) introducing a feed gas stream though a first heat exchanger and a second heat exchanger; ii) chilling the feed gas stream in the first heat exchanger by circulating a first chilled refrigerant stream adjacent the feed gas stream in the first heat exchanger; iii) chilling the feed gas stream in the second heat exchanger by circulating a second chilled refrigerant stream adjacent the feed gas stream in the second heat exchanger using a first liquid refrigerant stream from a first flash drum; iv) pumping a third chilled refrigerant stream from a third heat exchanger to an ejector for converting the third chilled refrigerant stream to the first chilled refrigerant stream; and v) returning at least a portion of a first vapor refrigerant stream from the first flash drum to the first ejector.

[0016] Referring now to FIG. 2, a schematic diagram illustrates one embodiment of the present disclosure retrofitted in a pre-existing open-cycle cascade-based liquefied natural gas process.

[0017] In the propane chilling stage, a vaporized refrigerant (propane) from the lowest stage suction drum 120 may be taken through line 202 to an ejector 204, which is preferably a liquid motive ejector, at a rate based on the entrainment ratio and efficiency of the selected ejector. The motive for the ejector 204 is supplied via line 130 and is passed at saturated liquid conditions through a high-efficiency pump 206. The compressor 118 can thus, include three stages of compression or can employ two stages of compression when the total flow of vaporized refrigerant from the lowest stage suction drum 120 is redirected through line 202.

[0018] In the ethylene chilling stage, a vaporized refrigerant (ethylene) from the lowest stage flash drum 140 is taken through line 212 to another ejector 208 that is preferably a liquid motive ejector. The motive for the ejector 208 is supplied from the outlet of the brazed aluminum heat exchanger 144 and is passed through another high efficiency pump 210. The compressor 138 employs two stages of compression.

[0019] In the open-loop methane liquefaction stage, the vaporized mixed feed gas (methane) from the lowest stage flash drum 196 is redirected through line 216 to another ejector 220 that is preferably a liquid motive ejector. The motive for the ejector 220 is supplied from flash drum 184 via line 222 and is passed through another high efficiency pump 218. A two-phase mixture of mixed feed gas from the ejector 220 may be directed to flash drum 190 and/or flash drum 196 through line 230 with the use of valves (not shown) where it is flashed to produce a vapor and a liquid. Boil off gas generated from the LNG tank in line 198 is redirected through line 228 to another ejector 224 that is preferably a liquid motive ejector. The motive for the ejector 224 is also supplied from flash drum 184 via line 226 after being passed through the high efficiency pump 218. A two-phase mixture of mixed feed gas from the ejector 224 may be directed to flash drum 190 and/or flash drum 196 through line 230 with the use of valves (not shown) where it is flashed to produce a vapor and a liquid. Ejector 224 serves to recompress the boil-off gas. Since the liquefied feed from flash drum 196 is close to saturation conditions, the other ejector 220 serves to compress the liquified feed from flash drum 196.

[0020] As demonstrated by the HYSYS™ simulation results in Table 1 below, the open- cycle cascade-based liquefied natural gas system illustrated in FIG. 2 could achieve a brake power reduction of as much as 42% while processing the same amount of LNG as a conventional open- cycle cascade-based liquefied natural gas system. If the same system were modified to increase the feed rate and process more LNG, an increase in product rate of 45% is possible, with a reduction in brake power of 22%. Prior Art FIG. 2 FIG. 2 (w/ increased feed rate)

Brake Power hp % Difference Base -42.17% -22.04% Feed Rate MMtpa % Difference Base 0% 39.2% Feed Temperature °F Value 60 60 60 Product Rate MMBtu/hr % Difference Base 3.93% 45%

Thermal Efficiency % % 92.18 95.48% 95.71%

UA Btu/hr-°F % Difference 0.00% 13.31% 63.92%

TABLE 1

[0021] The systems and methods disclosed herein thus, may be used to improve the efficiency of new and preexisting open-cycle cascade-based liquified natural gas systems by utilizing one or more ejectors to reduce and/or eliminate compression stages. The resultant effect of this enhancement is a reduction in the flow rate through each compressor, which reduces the energy consumption of the overall process.

[0022] While the present disclosure has been described in connection with presently preferred embodiments, it will be understood by those skilled in the art that it is not intended to limit the disclosure to those embodiments. For example, the present disclosure may be implemented in other open-cycle cascade-based liquified natural gas systems or other open-cycle refrigeration or chilling systems, to achieve similar results. Although propane, ethylene and methane are used as exemplary refrigerants in the open-cycle cascade-based liquified natural gas system described herein, they are not intended to preclude other refrigerants from being used. It is therefore, contemplated that various alternative embodiments and modifications may be made to the disclosed embodiments without departing from the spirit and scope of the disclosure defined by the appended claims and equivalents thereof.