LNG liquefaction system and method

Abstract

A system for cooling an LNG refrigeration exchanger, the system comprising: a refrigeration circuit in heat transfer communication with the refrigeration exchanger; an inflow of treated gas; a start-up exchanger arranged to receive at least a portion of the treated gas and cool said treated gas.

Description

[0001] LNG LIQUEFACTION SYSTEM AND METHOD

[0002] Field of the Invention

[0003] The invention relates to the control of liquefaction process of natural gas. In particular, the invention is directed to an initializing process to bring the central heat exchanger to the required liquefaction temperature.

[0004] Background

[0005] A typical LNG cool down process takes up to twelve hours and requires manual handling by the operator during this period. Manual handling, by an operator, of the cool down process may lack consistency requiring intervention and correction throughout the cool down process period.

[0006] Ideally, a cool down process should be done gradually based upon a recommended temperature gradient. The inconsistency of the cool down process may lead to equipment breakdown with capital-intensive equipment being taken offline for repair.

[0007] Summary of Invention

[0008] In a first aspect, the invention provides a system for cooling an LNG refrigeration exchanger, the system comprising: a refrigeration circuit in heat transfer communication with the refrigeration exchanger; an inflow of treated gas; a start-up exchanger arranged to receive at least a portion of the treated gas and cool the said treated gas.

[0009] In a second aspect, the invention provides a method for cooling an LNG refrigeration exchanger, the method comprising the steps: providing cooling to the refrigeration exchanger from a refrigeration circuit; an inflow of treated gas; directing at least a portion of the treated gas and cool said treated gas through a start-up exchanger.

[0010] Brief Description of Drawings

[0011] It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

[0012] Figure 1 is a schematic view of a liquefaction plant according to one embodiment of the present invention;

[0013] Figure 2 is a detailed view of a central refrigerant exchanger coupled to a start-up exchanger and scrub column start-up condenser according to a further embodiment of the present invention, and; Figure 3 is a detailed view of a central refrigerant exchanger, start up heat exchanger, and scrub column start-up condenser coupled to a scrub column according to a further embodiment of the present invention.

[0014] Detailed Description

[0015] Figure 1 shows an LNG liquefaction system 5 according to one embodiment of the present invention.

[0016] The system 5 includes a central refrigerant exchanger 65 arranged to receive treated gas 60.

[0017] The system 5 includes a refrigeration circuit 10 used to maintain the cryogenic temperatures in the central refrigeration exchanger 65. The present invention is not limited to any particular arrangement of the refrigeration circuit, with the refrigeration circuit 10 to be described below being exemplary.

[0018] The central refrigeration exchanger includes a plurality of Passes, respectively arranged to: i) Receive the treated gas inflow and deliver to a Scrub Column 90 (Pass A); ii) Receive a gas inflow from the Scrub Column and deliver to a Flash Drum 100

[0019] (Pass B), and; iii) Receive the treated gas inflow and deliver to a Flash Drum 100 (Pass E); iv) In one embodiment, a further Pass F is used to provide heat transfer for gas from the Scrub Column 90, including a by-pass to a scrub column start-up condenser 75. v) Receive an inflow from the refrigeration circuit (Pass C) and return the cooled refrigerant to the refrigeration circuit (Pass D);

[0020] During normal operation, Passes C and D within the central refrigerant exchanger 65 impart cold energy to the incoming treated gas stream 60. The Pass C receives an inflow of refrigerant, which is then directed through the Pass D, via a Joule-Thomson valve 20. The heated refrigerant is directed back into the refrigerant circuit 10 that, in one embodiment, may include passing through a suction drum 25 and a refrigerant compressor 30. A portion may be pass directly to a discharge condenser 40. The refrigerant then passes back to the Pass C through a discharge drum 50. To compliment the efficiency of the refrigerant, a portion of the discharge from the refrigerant compressor passes through an interstage condenser 45 and interstage drum 27, with a gas line from the interstage drum 27 being compressed at 2ndstage of compressor and combined with the liquid phase from the interstage drum 27 through interstage pump 35. The combined stream enters the discharge condenser 40 and separates into vapour and liquid in discharge drum 50. The overhead vapour from discharge drum 50 return to the central refrigerant exchanger 65 through inlet of Pass C. This is in combination with liquid from pump 55 discharge. The treated gas 60 is directed to the Pass A, chilling the gas and directing it to scrub column 90 in order to remove the heavier hydrocarbons from the gas. In general, the temperature range to cool down is subject to the content of heavy hydrocarbons in the feed gas The scrub column 90 includes a reboiler 95, which provides heat to remove heavy hydrocarbons from the gas. The scrubbed gas is directed back to the refrigerant exchanger 65 through the Pass B before being sent to an end flash drum 100, with flash gas stream 105 directed to a boil off gas compressor and LNG stream 110 directed to storage.

[0021] Before normal operation can commence, the refrigerant exchanger must be cooled to the correct temperature. To do so in an uncontrolled manner may lead to damage and extra maintenance, and so a means of control needs to be identified.

[0022] In a first mode shown in Figure 2, a start-up exchanger 70 and a scrub column start-up condenser 75 are employed. Controlling the flow rate of the gas feed 60 using the temperature monitoring, the feed gas is directed in varying amounts to a start-up exchanger. Thus rather than increasing or decreasing the flow rate of the treated gas 60 to the refrigerant exchanger 65, the flow rate is maintained, with a portion directed 155 to the start-up exchanger 70 which acts to bypass the refrigerant exchanger 65 for at least a portion of the inflow 60. The ability to cool the feed gas 60 during the start-up period can be provided by both the refrigerant exchanger 65 and the start-up heat exchanger 70. An operational process according to this embodiment may include the steps: i) Open Joule-Thomson valve 20 gradually to begin flowing refrigerant vapour through central Refrigerant Exchanger 65 (Pass C and D); ii) Start cooling of the Start-up Exchanger 70 and Scrub Column Start-up Condenser 75 when the Refrigerant Exchanger 65 reaches a pre -determined minimum temperature; iii) Once the Start-up Exchanger 70 and Scrub Column Start-up Condenser 75 have cooled down, begin forward flow of feed gas 155 iv) Start Refrigerant Liquid flow to Pass C of the Refrigerant Exchanger 65; v) Adjusting Joule Thompson valve 20 opening to meet the cooling down rate; vi) Flare feed gas at the midpoint until Scrub Column overhead reaches required quality specification. vii) Forward flow of treated gas to Pass B of the Refrigerant Exchanger 65 to produce LNG; viii) Isolate the Start-up Exchanger 70 and Scrub Column Start-up Condenser 75 and preserve with nitrogen.

[0023] This has the advantage of:

[0024] • Better managing the ramp rate of the heat exchanger, and so avoiding either too rapid or too slow an increase; Reducing the time required for start-up. By diverting to the start-up exchangers to create reflux flow in the scrub column, whilst the feed gas is being cooled, .

[0025] In the first mode of operation, the start-up exchanger 70 also assists in chilling the natural gas by receiving refrigerant 150, a portion of the Pass C flow 120. In so doing, it bypasses the Pass D and directs the refrigerant 180 back into the refrigerant circuit 10.

[0026] In a further embodiment, a start-up condenser 75 may be included, which allows the first mode to be able to remove heavy components in 135, and direct refrigerant 130 during start-up 170 back to suction drum 25.

[0027] In a second mode, forming a further embodiment of the present invention, the treated gas 60 may be directed wholly to the refrigerant exchanger 65 during cooldown. The control of the cooldown process of the refrigerant exchanger 65 may be to manually vary the mass flow rate of the treated gas 60 as a function of the rate of change in refrigerant flow, pressure and composition through the refrigerant exchanger 65. This is distinct from prior art processes whereby the flow rate of the treated gas 60 is varied automatically in order to meet a desired rate of change of temperature of the refrigerant exchanger 65. It will be appreciated that there are three embodiments that form the present invention including: i) the use of start-up exchangers at the beginning of cooldown process, by diverting a portion of flow from refrigerant exchanger 65 ii) To manually vary the mass flow rate of the treated gas 60 as a function of the rate of change in refrigerant flow, pressure and composition going through the refrigerant exchanger 65, and; iii) A combination of both, whereby the flow rate of the treated gas 60 into the refrigerant exchanger 65 may vary according to the rate of change of refrigerant, but also to divert flow to the start-up exchangers 70 & 75. Once the cooldown process is completed, the start up exchangers will be isolated.

[0028] With reference to the second embodiment (ii) the system may include a hybrid control system. The hybrid control system may be arranged to cool the refrigeration exchanger using refrigerant from a refrigeration circuit using a combination of manual and automatic control.

[0029] In doing so, the manual and automatic control may use different control parameters. In a further embodiment, the manual and automatic control may also use the same control parameters. The parameters that either or both the manual and automatic control may use, may include: i) Periodic variation of the cooling rate, and so time based, or; ii) Total volume of the gas passing through the refrigerant exchanger.

[0030] For instance, it may be known that the particular system achieves cooling after a known period or after a known volume of gas has passed the exchanger.

[0031] Alternatively, temperature indicators at various points along the refrigerant exchanger may be used as a means of indicating a need to vary the rate of cooling.

Claims

Claims1. A system for cooling an LNG refrigeration exchanger, the system comprising: a refrigeration circuit in heat transfer communication with the refrigeration exchanger; an inflow of treated gas; a hybrid control system arranged to control the cooling of the LNG refrigeration exchanger.

2. The system according to claim 1, wherein the hybrid control system includes a combination of manual and automatic control.

3. The system according to claim 2, wherein the manual and automatic control use different control parameters cooling of the LNG refrigeration exchanger.

4. The system according to any one of claims 1 to 3, further including a start-up exchanger arranged to receive at least a portion of the treated gas and cool the treated gas.

5. A system for cooling an LNG refrigeration exchanger, the system comprising:a refrigeration circuit in heat transfer communication with the refrigeration exchanger; an inflow of treated gas; a start-up exchanger arranged to receive at least a portion of the treated gas and cool the treated gas.

6. The system according to claim 3 or 5, wherein the start-up exchanger directs the portion of cooled treated gas to a scrub column.

7. The system according to claim 3 or 5, wherein the start-up exchanger receives a portion of refrigerant from the refrigerant exchanger, said start-up exchanger arranged to cool the portion of treated gas and return the heated refrigerant back to the refrigerant circuit.

8. The system according to claim 6 or 7, further including a scrub column start-up condenser arranged to receive a second portion of refrigerant from the refrigerant exchanger, said scrub column start-up condenser arranged to cool down the gas from scrub column overhead and establish sufficient reflux flow for the scrub column during start-up.

9. The system according to claim 8, wherein the scrub column start-up condenser is further arranged to receive gas from the scrub column, cools down the stream and returns to the scrub column to achieve reflux flow faster for the scrub column.

10. A method for cooling an LNG refrigeration exchanger, the method comprising the steps: cooling the refrigeration exchanger using refrigerant from a refrigeration circuit; controlling the cooling using a hybrid control system; said controlling using a combination of manual and automatic control.

11. The system according to claim 10, wherein the manual and automatic control use different control parameters.

12. The system according to claim 10, wherein the manual and automatic control use the same control parameters.

13. A method for cooling an LNG refrigeration exchanger, the method comprising the steps:providing an inflow of treated gas; providing cooling to the refrigeration exchanger from a refrigeration circuit; directing at least a portion of the treated gas, and cooling said treated gas, through a start-up exchanger.

14. The method according to claim 13, further including the step of controlling the cooling using a hybrid control system; said controlling using a combination of manual and automatic control.

15. The method according to claim 13 or 14, further including the step of the startup exchanger directing the portion of cooled treated gas to a scrub column.

16. The method according to claim 14 or 15, further including the steps of: opening a valve and so flowing refrigerant vapour through refrigerant exchanger; cooling the start-up exchanger when the refrigerant exchanger temperature reaches a pre-determined temperature, and then; forward flowing of the treated gas.

17. The method according to any one of claims 13 to 16, further including the steps, after forward flowing, of: start refrigerant liquid flow to the refrigerant exchanger; controlling the feed gas flow to meet the cooling down rate; flaring feed gas at the midpoint until scrub column reaches the required scrub column quality specification, and then; forward flowing of treated gas to the refrigerant exchanger to produce LNG;18. The method according to claim 16 or 17, wherein the cooling step includes cooling a scrub column start-up exchanger.

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