Systems and Methods for Improving the Efficiency of Combined Cascade and Multicomponent Refrigeration Systems

Abstract

Systems and methods for improving the efficiency of combined cascade and multicomponent refrigeration systems by utilizing one or more ejectors to reduce and/or eliminate compression stages. The systems and methods change the temperature profile, which reduces the energy consumption of both the mixed refrigeration system and the pre-cooling system.

Claims

1. A system for chilling a feed gas, which comprises: a first heat exchanger enclosing a first portion of a feed gas line and a portion of a first chilled refrigerant line; 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; a second heat exchanger enclosing a second portion of the feed gas line and a portion of a second chilled refrigerant line; 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; a third heat exchanger enclosing a third portion of the feed gas line and a portion of a third child refrigerant line; a drum in fluid communication with the third chilled refrigerant line for receiving a vaporized refrigerant from the third heat exchanger, the drum having a drum vapor outlet line; an ejector in fluid communication with the drum vapor outlet line, the first chilled refrigerant line, and a compressed refrigerant line; and a compressor in fluid communication with the first vapor outlet line, the second vapor outlet line, and the compressed refrigerant line connected to a chiller for chilling a compressed refrigerant in the compressed refrigerant line.

2. The system of claim 1, further comprising a first expansion valve positioned between the first heat exchanger and the ejector for producing a chilled refrigerant in the first chilled refrigerant line.

3. The system of claim 2, further comprising a second expansion valve positioned between the second heat exchanger and the first flash drum for producing a chilled refrigerant in the second chilled refrigerant line.

4. The system of claim 3, further comprising a third expansion valve positioned between the third heat exchanger and the second flash drum for producing a chilled refrigerant in the third chilled refrigerant line.

5. The system of claim 1, further comprising a pump positioned between the chiller and the ejector for pumping the chilled compressed refrigerant in the first chilled compressed refrigerant line.

6. The system of claim 1, further comprising a supplemental refrigeration system in fluid communication with the feed gas line and an outlet line for one of a chilled feed gas and a liquified feed gas.

7. The system of claim 6, wherein the supplemental refrigeration system includes a mixed refrigerant.

8. The system of claim 1, wherein the compressor is in fluid communication with the drum vapor outlet line.

9. The system of claim 6, further comprising: another ejector in fluid communication with the outline line and a boil-off gas line connected to a tank for holding boil-off gas.

10. The system of claim 9, further comprising a let-down valve positioned in fluid communication with the outlet line for controlling flow of the chilled feed gas or the liquified feed gas in the outlet line to the another ejector.

11. A method for chilling a feed gas, which comprises: introducing a feed gas stream through a first heat exchanger, a second heat exchanger and a third heat exchanger; 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 using a compressor and a chiller to convert a first vapor refrigerant stream from a first flash drum into a liquid refrigerant stream and an ejector to convert the liquid refrigerant stream into the first chilled refrigerant stream; 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 the first flash drum; chilling the feed gas stream in the third heat exchanger by circulating a third chilled refrigerant stream adjacent the feed gas stream in the third heat exchanger using a second liquid refrigerant stream from the second flash drum; transferring a vapor refrigerant stream from the third heat exchanger to a drum; and returning at least a portion of the vapor refrigerant stream in the drum to the ejector for lowering the temperature of the first chilled refrigerant stream.

12. The method of claim 11, further comprising expanding the first chilled refrigerant stream before circulating the first chilled refrigerant stream in the first heat exchanger.

13. The method of claim 12, further comprising expanding the second chilled refrigerant stream before circulating the second chilled refrigerant stream in the second heat exchanger.

14. The method of claim 13, further comprising expanding the third chilled liquid refrigerant stream before circulating the third chilled refrigerant stream in the third heat exchanger.

15. The method of claim 11, further comprising pumping the liquid refrigerant stream from the compressor to the ejector.

16. The method of claim 11, further comprising chilling the feed gas stream in a supplemental refrigeration system to produce one of a chilled feed gas stream and a liquified feed gas stream.

17. The method of claim 16, wherein the supplemental refrigeration system includes a mixed refrigerant.

18. The method of claim 11, further comprising returning at least a portion of the vapor refrigerant stream in the drum to the compressor.

19. The method of claim 16, further comprising: recompressing a boil-off gas stream from a boil-off gas tank by transferring the one of the chilled feed gas stream and the liquified feed gas stream to another ejector; and directing at least a portion of the boil-off gas stream through the another ejector.

20. The method of claim 19, further comprising controlling a flow of the chilled feed gas stream or the liquified feed gas stream to the ejector with a let down valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The detailed description is described with reference to the accompanying drawings, in which like elements are referenced with like reference numbers, in which:

[0014] FIG. 1 is a schematic diagram illustrating a conventional C3MR system.

[0015] FIG. 2 is a schematic diagram illustrating a conventional IPSMR system.

[0016] FIG. 3 is a schematic diagram illustrating one embodiment of the present disclosure retrofitted in a pre-existing liquefied natural gas process.

[0017] FIG. 4 is a schematic diagram illustrating one embodiment of the present disclosure applied to a C3MR liquefied natural gas process.

[0018] FIG. 5 is a schematic diagram illustrating another embodiment of the present disclosure applied to an IPSMR liquefied natural gas process.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0019] 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.

[0020] 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 third portion of the feed gas line and a portion of a third child refrigerant line; vi) a drum in fluid communication with the third chilled refrigerant line for receiving a vaporized refrigerant from the third heat exchanger, the drum having a drum vapor outlet line; vii) an ejector in fluid communication with the drum vapor outlet line, the first chilled refrigerant line, and a compressed refrigerant line; and vii) a compressor in fluid communication with the first vapor outlet line, the second vapor outlet line, and the compressed refrigerant line connected to a chiller for chilling a compressed refrigerant in the compressed refrigerant line

[0021] In another embodiment, the present disclosure includes a method for chilling a feed gas, which comprises: i) introducing a feed gas stream through a first heat exchanger, a second heat exchanger and a third 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 using a compressor and a chiller to convert a first vapor refrigerant stream from a first flash drum into a liquid refrigerant stream and an ejector to convert the liquid refrigerant stream into the first chilled refrigerant stream; 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 the first flash drum; iv) chilling the feed gas stream in the third heat exchanger by circulating a third chilled refrigerant stream adjacent the feed gas stream in the third heat exchanger using a second liquid refrigerant stream from the second flash drum; v) transferring a vapor refrigerant stream from the third heat exchanger to a drum; and vi) returning at least a portion of the vapor refrigerant stream in the drum to the ejector for lowering the temperature of the first chilled refrigerant stream.

[0022] Referring now to FIG. 3, a schematic diagram illustrates one embodiment of the present disclosure retrofitted in a pre-existing liquefied natural gas process. A vaporized refrigerant from the lowest stage drum 120 is taken through line 302 to an ejector 304 that is preferably a liquid motive ejector. The motive for the ejector 304 is supplied via line 130 and is passed at saturated liquid conditions through a high-efficiency pump 306. The propane chilling compressor 118 can comprise three stages or can employ two stages of compression and instead redirect the total flow of vaporized refrigerant from the lowest stage drum 120 through line 302. This facilitates a significant decrease in mass flow to the compressor 118, as depicted in Table 3 below, based on a HYSYS™ simulation. As a result, capacity in the propane chilling system is increased, facilitating the change in temperature profile. The adjustment of the temperature profile differs by implementation, but generally is a reduction of about 2° F. to about 4° F. in the feed gas stream 102 and about 5° F. to about 100° F. in the supplemental refrigeration system 312. The supplemental refrigeration system 312 produces one of a chilled feed gas stream and a liquified feed gas stream in line 116. The supplemental refrigeration system 312 may be a mixed refrigeration system that includes a mixed refrigerant.

TABLE-US-00003 TABLE 3 Prior Art FIG. 4 Refrigeration Equipment Mass Flow Mass Flow Loop Tag % Difference % Difference Mixed K-6000.I Base −11.66% Refrigerant 138 K-6000.II Base −11.66% Propane 118 K-3001.I Base — K-3001.II Base −52.48% K-3001.III Base −10.86%

[0023] Due to the fact that the chilled feed gas stream or the liquified feed gas stream in line 116 is subcooled, the boil-off gas recompression system can be eliminated in favor of another liquid motive ejector 310 that is controlled by means of the letdown valve 148. Pressure in the form of vapor suction through line 308 to the ejector 310 is monitored to ensure that the LNG tank does not reach vacuum pressure. A small temperature increase of approximately 3-5° F. is noted from HYSYS simulation models, but due to the significant subcooling of the chilled feed gas stream or the liquified feed gas stream at the letdown valve 148, no vapor generation occurs.

[0024] Referring now to FIG. 4, a schematic diagram illustrates one embodiment of the present disclosure applied to a C3MR liquefied natural gas process. In this embodiment, the lowest compression stage from the drum 120 to the compressor 118 is eliminated. The entire vaporized refrigerant from drum 120 is thus, diverted through line 302 to the liquid motive ejector 304. The resultant effect is a reduction in the temperature in line 111 from about 23° F. to about 19° F. The temperature of the mixed refrigerant in line 146 is also reduced from about −30° F. to about −34° F. As a result, the vapor fraction in flash drum 142 is adjusted from about 43% to about 41%. The inter-stage flashes conditions in the supplemental refrigeration system improve from about −162° F. in the conventional C3MR liquefied natural gas process to about −190° F. The temperature remains the same. Table 4 (below) illustrates the impact of this embodiment applied to a natural gas liquefaction process for two cases, modeled using HYSYS™. One case maintains the natural gas feed rate to the natural gas liquefaction terminal. The second case increases the feed rate to maintain the compressor 118 at a capacity like a conventional C3MR liquefied natural gas process. An observed feed rate increase of nearly 17% is depicted when the terminal is revamped with the present embodiment. Additionally, a brake power reduction of nearly 22% is observed.

TABLE-US-00004 TABLE 4 FIG. 4 (w/increased Prior Art FIG. 4 feed rate and revamp) Brake Power hp % Difference Base −21.96% −11.98% Feed Rate MMtpa % Difference Base 0.00% 16.57% Feed Temperature ° F. Value 60.00 60.00 60.00 Product Rate MMBtu/hr % Difference Base 1.82% 18.82% MMtpa % Difference Base 0.33% 18.42% Thermal Efficiency % % 92.75 94.34 94.52 UA Btu/hr-° F. % Difference 0.00% 48.13% 564.08%

[0025] Referring now to FIG. 5, a schematic diagram illustrates another embodiment of the present disclosure applied to an IPSMR liquefied natural gas process. In this embodiment, the lowest compression stage from the drum 220 to the compressor 218 is eliminated. The entire vaporized refrigerant from drum 220 is thus, diverted through line 302 to the liquid motive ejector 304. Because of the relatively small size of the propane chilling system in this embodiment, compared to the embodiment in FIG. 4, the increased capacity in the propane chilling system is used to cool the mixed refrigerant in line 501. The mixed refrigerant passed through heat exchangers 206, 208 and 210 is chilled to a temperature of about −13° F. in line 502. The mixed refrigerant is then re-introduced into the brazed aluminum heat exchanger 212. Table 5 (below) illustrates the impact of this embodiment applied to a natural gas liquefaction process for two cases, modeled using HYSYS™. One case only modifies the propane chilling system. The second case modifies both the propane chilling system and the mixed refrigeration system depicted in FIG. 5. Additionally, a brake power reduction of nearly 12% is observed.

TABLE-US-00005 TABLE 5 FIG. 5 (w/MR Prior Art FIG. 5 integration) Brake Power hp % Difference Base −1.52% −11.54% Feed Rate MMtpa % Difference Base 0.00% 0.00% Feed Temperature ° F. Value 95 95 95 Product Rate MMBtu/hr % Difference Base 0.11% 0.81% MMtpa % Difference Base 0.11% 0.81% Thermal Efficiency % % 93.35 93.45 94.10 UA Btu/hr-° F. % Difference 0.00% 1.68% 221.98%

[0026] The systems and methods disclosed herein thus, improve the efficiency of combined cascade and multicomponent refrigeration systems by utilizing one or more ejectors to reduce and/or eliminate conventional compression stages. The systems and methods change the temperature profile, which reduces the energy consumption of both the mixed refrigeration system and the pre-cooling system.

[0027] 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 the mixed refrigeration systems described herein and other multi-stage refrigeration processes for chilling a feed gas, such as other cascade refrigeration cycles and mixed refrigerant cycles, to achieve similar results. Although propane is used as an exemplary refrigerant for the pre-cooling system, it is not intended to preclude other refrigerants from being used instead of propane. 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.