GB2069119A - Refrigeration process - Google Patents

Abstract

A fluctuating refrigeration demand load, such as is met in the reliquefaction of vapours from stored LNG, is satisfied from a substantially constant flow of compressed gaseous refrigerant by providing a Joule Thomson refrigeration loop in which a part of the compressed refrigerant is condensed, the uncondensed vapour is recycled and the condensate is stored in a buffer vessel, the condensate being withdrawn from the buffer vessel at the variable rate required to meet the fluctuating refrigeration demand load and the vapours formed therefrom being combined with the recycling uncondensed vapour in the refrigeration loop, the supply of refrigerant vapour in the loop for recompression being maintained substantially constant by venting vapour from the system in periods of high refrigeration demand and drawing fresh gaseous refrigerant into the system from an external supply at periods of low refrigeration demand. <IMAGE>

Description

SPECIFICATION Refrigeration process This invention relates to a refrigeration method and apparatus able to satisfy a fluctuating refrigeration demand load and is particularly concerned with such a method and apparatus suitable for use in the reliquefaction of the vapours formed by the evaporation of liquefied gases, e.g.

liquefied natural gas (LNG) which are in storage.

The storage of LNG in large quantities and its transfer to carrier vessels for overseas transport is subject to significant losses through evaporation of the cold liquid. Although during normal storage periods these losses may appear small when expressed as a percentage of the tank capacity, generally not exceeding 0. 1% per day, the actual quantities can be large in view of the large capacities of the tanks employed. Additional losses occur during the periods in which LNG is transferred to the ship's cargo when the liquid comes into contact with portions of equipment at a higher temperature.

Attempts have therefore been made to avoid these losses of valuable fuel by installing equipment to reliquefy the evaporated LNG, referred to as boil-off gas, and return it to the tanks.

Until recently, major efforts to reliquefy boil-off gas have been restricted to providing installations to perform such operations at sea on board carriers. Thus, British Patent Specification No.

1472533 describes a method of reliquefying a part of the boil-off gas in a ship's cargo and British Patent No. 1471404 describes several modes of operation of a process in which, if desired, the whole of the boil-off gas can be reliquefied.

The extension of this procedure to large landbased storage tanks raises new problems connected with the large size of the required installations and the varying load factor caused by the intermittent transfer of LNG fo the carrier vessels. It is economically undesirable to install reliquefaction plants having a refrigeration capacity large enough to accommodate heavy loads incurred during relatively short periods such as those in which the LNG is being transferred from a land-based storage tank to a carrier, since the plant would be severely underloaded in the longer periods of normal operation.

The present invention provides a refrigeration method and apparatus which makes it possible to cope with peak demands from a plant whose maximum refrigeration capacity is substantially less than that required to satisfy the peak demand and, indeed, need be only slightly above that required to satisfy the level of demand of normal operating conditions.

According to the present invention there is provided a method of providing refrigeration at a variable rate from a substantially constant flow of compressed gaseous refrigerant, the method comprising providing a compressed stream of gaseous refrigerant, cooling the compressed refrigerant and expanding it by Joule Thomson expansion to partially condense it, recovering the uncondensed gas and recycling it, providing said refrigeration by vaporisation of the liquid refrigerant obtained by said partial condensation and recycling the vapour formed by said vaporisation to comprise, with said uncondensed gas, the stream of gaseous refrigerant to be compressed, and in which the liquid refrigerant is stored in a storage vessel and is withdrawn from it for said vaporisation at a rate which can be varied, and the rate of flow of said stream of gaseous refrigerant is maintained within predetermined limits, and preferably substantially constant, by injecting fresh gaseous refrigerant into the system when the rate of flow of said vapour falls below a predetermined level and withdrawing vapour from the system when the rate of flow of said vapour exceeds a predetermined level.

The invention also provides refrigeration apparatus comprising means for compressing gaseous refrigerant, removing the heat of compression, cooling the compressed refrigerant in the first heat exchange means, expanding the cooled compressed refrigerant to partially condense it, storing the liquid refrigerant obtained by said partial condensation, passing uncondensed expanded refrigerant in indirect heat exchange relationship with said compressed refrigerant in said first heat exchange means and recycling it to said compressor means as one part of said gaseous refrigerant, said apparatus further comprising second heat exchange means including means for passing a fluid to be refrigerated in indirect heat exchange relationship with liquid refrigerant supplied at controlled rate from said storage means, means for recovering vapour formed by vaporisation of said liquid refrigerant in said second heat exchange means, passing it in indirect heat exchange relationship with said compressed refrigerant in said first heat exchange means and recycling it to said compressor means as another part of said gaseous refrigerant means responsive to the rate of flow of said fluid through said second heat exchange means for controlling the rate of flow of liquid refrigerant from said storage means to said second heat exchange means conduit means connecting a supply of fresh gaseous refrigerant to said compressor means and including a first valve adapted to control the flow of said fresh gaseous refrigerant from said supply to said compressor means, means for withdrawing the recovered vapour being recycled to said compressor means and including a second valve adapted to control said withdrawal, and flow control means for maintaining the flow of gaseous refrigerant to said compressor means within predetermined limits by controlling said first valve to admit fresh gaseous refrigerant when the rate of flow of said recovered vapour drops below a predetermined level and controlling said second valve to withdraw recovered vapour when the rate of flow of said recovered vapour exceeds a predetermined level.

In accordance with the invention, the liquid refrigerant formed by the expansion passes to a storage tank, and the liquid level in this tank can rise and fall according to the demand for refrigeration and hence for liquid refrigerant.

The refrigeration capacity of the plant, i.e. the maximum rate of flow of gaseous refrigerant that can be compressed, can be designed such that during periods of low refrigeration demand, the plant can provide somewhat more refrigeration than is required to satisfy the demand. Under these conditions the rate at which liquid refrigerant is revaporised is insufficient for the vapour so formed to satisfy the demand for gaseous refrigerant by the compressor means and fresh gaseous refrigerant is drawn into the refrigeration cycle. The liquid level in the liquid refrigerant storage vessel will thus build up.

In a subsequent period of high refrigeration demand when the rate of production of liquid refrigerant is inadequate, the extra demand is satisfied by withdrawing liquid refrigerant from the storage vessel at a greater rate than it is supplied to the vessel and the liquid level in the vessel falls.

Under these conditions, more vapour is being generated by revaporisation of the liquid refrigerant than can be accepted by the compressor means and the excess is withdrawn from the system, e.g. by venting.

Suitably, the supply of fresh gaseous refrigerant to, and the withdrawal of vapour from the system may be controlled automatically e.g. by valves controlled by means responsive to the pressure of the vapour being recycled whereby when the pressure drops below a predetermined level fresh gaseous refrigerant is drawn into the system and when the pressure exceeds a predetermined level, vapour is withdrawn from the system.

In general, it will be preferred to divide the compressed gaseous refrigerant into two streams, one of which is cooled, expanded and partially condensed to provide the liquid refrigerant and the other of which is cooled, expanded with the performance of external work and passed in indirect heat exchange relationship with compressed gaseous refrigerant to cool the latter.

Work expanding a part of the compressed nitrogen in this manner will reduce the work of compression required. Moreover, a part of the power for compression can be provided from said external work.

The method and apparatus of the invention are particularly suitable for the provision of the refrigeration required to reliquefy boil-off gas from a tank of LNG, especially a land-based storage tank of LNG, the reliquefied gas normally being returned to the tank. Nitrogen is the preferred refrigerant for this purpose because of its low boiling point relative to that of LNG, which eliminates the need to compress the boil-off gas, and because LNG tank sites usually carry a supply of nitrogen for use for purifying and inerting the tanks whenever repair or maintenance is to be carried out and as a refrigerant in the initial liquefaction of the gas for storage in the tanks. The nitrogen is generally provided from an air separation plant in gaseous form at a pressure suitable for distribution in pipelines.

The invention is now described in more detail with reference to one embodiment thereof adapted for the reliquefaction of boil-off gas using liquid nitrogen as the refrigerant, and with the aid of the accompanying drawing; however it will be understood that the invention is generally applicable to the provision of refrigeration in any application having a fluctuating demand for refrigeration and using refrigerants other than nitrogen.

Referring to the drawing, which is a flow sheet of a refrigeration plant in accordance with the invention for the reliquefaction of boil-off gas from a land-based LNG storage tank, reference numeral 1 is a large LNG storage tank, 2 is a storage tank for liquid nitrogen, 3 is a condenser/evaporator for condensing boil-off gas and evaporating liquid nitrogen, 4, 5 and 6 are heat exchangers, 7 is a compressor, which can be a multistage compressor and which is driven by an external power source 7a, which may be, for example, an electric motor or a gas or steam turbine, 8 is an expansion turbine and 9 a booster driven by the turbine. 10 and 11 are coolers for removing heat of compression; 12 is a pump and 13 is an installation, such as an air separation plant, producing gaseous nitrogen.

Boil-off natural gas leaves the tank 1 through line 21, is condensed in the condenser/evaporator 3 by indirect heat exchange with evaporating liquid nitrogen and returned to the tank 1 by the pump 12 and pipeline 22.

The liquid nitrogen is provided as follows.

Warm gaseous nitrogen at low pressure enters the suction of compressor 7 through line 23 and after compression is delivered in line 24. After removal of the heat of compression in cooler 10 the compressed stream is further compressed in the booster 9, cooled again in cooler 11 and then passed through heat exchangers 4, 5 and 6 where it is sub-cooled by indirect countercurrent heat exchange with cold low pressure nitrogen vapour returning in line 28. In the embodiment illustrated, after leaving heat exchanger 4 a part of the stream is withdrawn through pipeline 26 and passed to the turbine 8 where it is expanded and leaves through line 27 which empties into line 28 between heat exchangers 6 and 5. The remainder of the compressed nitrogen, after passing through exchangers 5 and 6, is expanded through expansion valve 29, as a result of which a high proportion is liquefied. The liquid and vapour pass through line 30 to the tank 2, whence the vapour returns through line 31 thence to line 28 and through exchangers 6, 5 and 4 in that order. As indicated above, the expanded nitrogen from the turbine exhaust in line 27 joins the low pressure nitrogen in line 28 between exchangers 6 and 5.

Work expanding a part of the compressed nitrogen In expansion turbine 8 reduces the work of compression required in compressor 7 and booster 9 and also provides a part of the energy required for compression, by driving booster 9.

Liquid nitrogen from tank 2 passes through line 32 and condenser/evaporator 3, where it is evaporated in condensing the boil-off gas; the vapour leaving through line 34 and joining the stream leaving tank 2 in line 31 to be returned through heat exchangers 6, 5 and 4 in that order to cool the compressed refrigerant in line 25 by indirect countercurrent heat exchange.

The amount of liquid nitrogen passing to condenser/evaporator 3 is varied in accordance with the refrigeration load, that is the rate of flow of boil-off gas in pipeline 21, in any suitable manner. In the embodiment illustrated, liquid nitrogen is maintained in a sump of the condenser/evaporator and the rate of supply of liquid nitrogen to the sump is controlled by valve 33 which is controlled to maintain the level of liquid nitrogen in the sump substantially constant.

The apparatus is so designed that in periods of low demand, the amount of nitrogen vapour being generated by evaporation of liquid nitrogen in condenser/evaporator 3 is less than that required by the compressor 7 and thus the pressure in line 28 drops. This pressure drop activates pressurecontrolled valve 37 and permits the entry into line 23 of gaseous nitrogen supplied from nitrogen production plant 13. This nitrogen is liquefied in the refrigeration plant and passes to storage tank 2, thus raising the level and generating a reserve of liquid nitrogen.

In periods of peak load, more liquid nitrogen is required in condenser/evaporator 3 than can be supplied to tank 2 and the level of the tanks falls.

The vapour formed by the evaporation of the nitrogen in condenser/evaporator 3 is more than can be accepted by compressor 7, thus leading to an increase in pressure in line 28. This increase in pressure activates pressure-controlled valve 36 in line 35 to open it and permit the excess vapour to be vented.

The invention is now illustrated by the following Example.

EXAMPLE The apparatus and method described above with reference to the drawing are employed in the liquefaction of boil-off gas stored in large insulated tanks 1 at a pressure slightly above atmospheric pressure and a temperature which is approximately160 C but which varies according to the precise composition of the LNG stored in the tanks 1. The boil-off gas is somewhat warmer on leaving the tank as it takes up heat from the walls and roof, that is from those portions of the tank which are not in direct contact with the liquid, and is warmed further in the pipeline 21 which conducts it to the condenser/evaporator 3, and esters heat exchanger 3 at about -1 000C.

Carrier vessels are loaded from the tanks 1 on an average of once every 14 days. When the LNG is just being stored in the tanks and no discharging into tankers is taking place, the average boil-off rate is 60 tonnes per day. During loading of carrier vessels, which takes place over a 48 hour period, the boil-off rate increases to 1 50 tonnes/day for the first 12 hours and then runs at 90 tonnes/day for the following 36 hours. Thus, the average boiloff rate over the full 14 day period is about 66.5 tonnes/day and the plant is therefore designed with a capability on full load of liquefying 70 tonnes/day of boil-off gas in condenser/evaporator 3.

Under the conditions described, approximately 4.3 m3 of liquid nitrogen at 7 atma are required to reliquefy each tonne of boil-off gas in heat exchanger 3 and the required nitrogen gas flow rate in pipeline 23 is 30000 Nm3/hr.

The nitrogen vapour in line 23, at 7 bar pressure, is compressed to 48 bar in compressor 7 and then to 70 bar in booster 9. The compressed gas is cooled to -730C in heat exchanger 4 and about three-fifths of it is then withdrawn through line 26 and expanded to 8 bar and thereby cooled to -1 600 C. The remaining two-fifths is cooled to -1 660C in heat exchangers 5 and 6 and then expanded to 8 bar in valve 29 from which it exits at --1720C.

For a 12 day period in the 14 day cycle, the refrigeration load is below peak capability of the plant and 800 Nmsof nitrogen is drawn in per hour through line 38 and liquefied, enabling a reserve of 300 m3 of liquid nitrogen to be built up in tank 3. In the next 48 hours, during the loading of the LNG from tank 1 on to a carrier vessel, the boil-off rate increases to 1 50 tonnes/day for the first 12 hours and then 90 tonnes/day for the next 36 hours. During this period, while the refrigeration plant continues to deliver liquid nitrogen at the same rate, the reserve of 300~rnLof liquid in tank 3 is consumed in coping with the increased demand for refrigeration and the excess nitrogen vapour so generated is vented through line 35 and valve 36. The 14-day cycle is then repeated.

Claims (14)

1. A method of providing refrigeration at a variable rate from a substantially constant flow of compressed gaseous refrigerant, the method comprising providing a compressed stream of gaseous refrigerant, cooling the compressed refrigerant and expanding it by Joule Thomson expansion to partially condense it, recovering the uncondensed gas and recycling it, providing said refrigeration by vaporisation of the liquid refrigerant obtained by said partial condensation and recycling the vapour formed by said vaporisation to comprise, with said uncondensed gas, the stream of gaseous refrigerant to be compressed, and in which the liquid refrigerant is stored in a storage vessel and is withdrawn from it for said vaporisation at a rate which can be varied, and the rate of flow of said stream of gaseous refrigerant is maintained within predetermined limits, and preferably substantially constant, by injecting fresh gaseous refrigerant into the system when the rate of flow of said vapour falls below a predetermined level and withdrawing vapour from the system when the rate of flow of said vapour exceeds a predetermined level.

2. A method as claimed in claim 1 wherein the vapour and the uncondensed gas are passed in indirect heat exchange relationship with compressed gaseous refrigerant to cool the latter.

3. A method as claimed in claim 1 or claim 2 wherein the compressed gaseous refrigerant is divided into two streams, the first of which is cooled, expanded and partially condensed to provide said liquid refrigerant and the second of which is cooled, expanded with the performance of external work and passed in indirect heat exchange relationship with compressed gaseous refrigerant to cool the latter.

4. A method as claimed in claim 3 in which a part of the power for compressing the gaseous refrigerant is provided from said external work.

5. A method as claimed in any one of the preceding claims in which the liquid refrigerant is vaporised in indirect heat exchange with a boil-off gas from a tank of liquefied natural gas to condense the latter, the condensate being returned to the tank.

6. A method as claimed in claim 5 in which the refrigerant is nitrogen.

7. A method as claimed in claim 1, substantially as hereinbefore described and as illustrated in the accompanying drawing.

8. Refrigeration apparatus comprising means for compressing gaseous refrigerant, cooling the compressed refrigerant in first heat exchange means, expanding the cooled compressed refrigerant to partially condense it, storing the liquid refrigerant obtained by said partial condensation, passing uncondensed expanded refrigerant in indirect heat exchange relationship with said compressed refrigerant in said first heat exchange means and recycling it to said compressor means as one part of said gaseous refrigerant, said apparatus further comprising second heat exchange means including means for passing a fluid to be refrigerated in indirect heat exchange relationship with liquid refrigerant supplied at controlled rate from said storage means, means for recovering vapour formed by vaporisation of said liquid refrigerant in said second heat exchange means, passing it in indirect heat exchange relationship with said compressed refrigerant in said first heat exchange means and recycling it to said compressor means as another part of said gaseous refrigerant means responsive to the rate of flow of fluid through said second heat exchange means for controlling the rate of flow of liquid refrigerant from said storage means to said second heat exchange means conduit means connecting a supply of fresh gaseous refrigerant to said compressor means and including a first valve adapted to control the flow of said fresh gaseous refrigerant from said supply to said compressor means, means for withdrawing the recovered vapour being recycled to said compressor means and including a second valve adapted to control said withdrawal, and flow control means for maintaining the flow of gaseous refrigerant to said compressor means within predetermined limits by controlling said first valve to admit fresh gaseous refrigerant when the rate of flow of said recovered vapour drops below a predetermined level and controlling said second valve to withdraw recovered vapour when the rate of flow of said recovered vapour exceeds a predetermined level.

9. Apparatus as claimed in claim 8 further including an expansion turbine and means for passing a portion of the compressed refrigerant to the expansion turbine, passing the expanded refrigerant from said expansion turbine through said first heat exchange means in indirect heat exchange relationship with compressed refrigerant and recycling it to said compressor means.

10. Apparatus as claimed in claim 9 in which said compressor means comprises at least two compressors at least one of which is coupled to said expansion turbine to be driven by it.

11. Apparatus as claimed in claim 9 or claim 10 in which said first heat exchange means comprises three heat exchange zones in series, the apparatus being constructed and arranged for passing compressed refrigerant through the first, second and third zones in that order, passing said recovered vapour and said uncondensed expanded refrigerant through said third, second and first zones in that order and in countercurrent indirect heat exchange relationship to said compressed refrigerant, withdrawing said portion from compressed refrigerant between said first and second zones and passing the expanded refrigerant from the expansion turbine back through said second and first zones in that order and in countercurrent relationship to said compressed refrigerant.

12. Apparatus as claimed in any one of claims 8 to 11 in which said flow control means comprise means responsive to the pressure of the vapour being recycled and adapted to control said first valve to admit fresh gaseous refrigerant when said pressure drops below a predetermined value and to control said second valve to withdraw said vapour when said pressure exceeds a predetermined value.

13. Apparatus as claimed in any one of claims 8 to 12 in which the refrigerant is nitrogen and the supply of fresh gaseous refrigerant comprises an air separation plant.

14. Apparatus as claimed in claim 8, substantially as hereinbefore described and as illustrated in the accompanying drawing.