Process and apparatus for low temperature refrigeration
United States Patent 3889485
Disclosed is a low temperature refrigeration process employing the working fluid having at least two components, the process comprising the steps of compressing the working fluid; countercurrently contacting the compressed working fluid in a heat exchanger with one or more returning streams to cool the working fluid; substantially isentropically expanding the cooled working fluid to provide a vapor-liquid mixture; separating the vapor and liquid fractions of the working fluid; returning the vapor fraction through the heat exchanger countercurrently with the working fluid; delivering the liquid fraction to an evaporating zone where it is evaporated to deliver refrigeration; returning the evaporated liquid refrigerant to the heat exchanger and countercurrently contacting it with the working fluid; and combining the returned evaporated refrigerant with the returned separated vapor fraction to comprise the working fluid. Apparatus for carrying out this process is also disclosed, particularly for providing low temperature refrigeration for liquefied natural gas.
US Patent References:
Gas liquefaction with work expansion of major feed portion
Knapp et al. - January 1968 - 3360944
Liquefaction of a gaseous mixture employing work expanded gaseous mixture as refrigerant
Swearingen - December 1968 - 3416324
SEPARATION OF GAS MIXTURES SUCH AS METHANE AND NITROGEN MIXTURES
Bernstein - June 1970 - 3516262
Gas liquefaction with work expansion of major feed portion
Knapp et al. - January 1968 - 3360944
Liquefaction of a gaseous mixture employing work expanded gaseous mixture as refrigerant
Swearingen - December 1968 - 3416324
SEPARATION OF GAS MIXTURES SUCH AS METHANE AND NITROGEN MIXTURES
Bernstein - June 1970 - 3516262
Application Number:
05/423089
Publication Date:
06/17/1975
Filing Date:
12/10/1973
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
62/502, 62/114
International Classes:
F25B9/00; F25J1/00; F25J1/02; F25J3/06; F17C13/12
Field of Search:
62/11,23,27,28,29,30,36,38,39,114,502,512,54
Primary Examiner:
Perlin, Meyer
Assistant Examiner:
Capossela, Ronald C.
Attorney, Agent or Firm:
Browning & Bushman
Claims:
What is claimed is
1. A low temperature refrigeration process employing a working fluid having at least two components comprising the steps of
2. The refrigeration process as defined by claim 1, further comprising the step (k) of removing a portion of the working fluid at a point in the refrigeration cycle where the most volatile component of the working fluid is most highly concentrated.
3. The refrigeration process as defined in claim 1, wherein said working fluid comprises predominant portions of nitrogen and methane and a minute portion of helium.
4. The refrigeration process as defined by claim 1, wherein said compression step (a) comprises a plurality of separate compression stages and wherein said expansion step (c) provides the energy to carry out at least one of said compression stages in step (a).
5. The refrigeration process as defined by claim 1, further comprising the steps of (f1) passing the evaporated liquid fraction of step (f) into a separation zone and separating any unevaporated liquid therefrom and (f2) recovering such liquid prior to returning the evaporated liquid fraction according to step (g).
6. The refrigeration process as defined by claim 5, wherein said refrigeration is delivered in step (f) to a second fluid and further comprising the step of (i) introducing a vapor fraction of said second fluid into said working fluid.
7. The refrigeration process as defined by claim 6, further comprising the step (j) of introducing the liquid recovered in step (f2) into said second fluid being refrigerated.
8. The refrigeration process as defined by claim 6, wherein the vapor fraction of said second fluid is a boil-off gas stream from a LNG storage tank and further comprising the step (n) of passing said vapor fraction countercurrently with the working fluid in said heat exchange step (b), as one of said returning streams.
9. The refrigeration process as defined by claim 6, wherein said second fluid being refrigerated is liquefied natural gas (LNG).
10. The refrigeration process as defined by claim 9, wherein the refrigeration is delivered to a boil-off gas stream of a LNG storage tank to condense said gas and further comprising the step (l) of combining a stream of the stored LNG with said boil-off gas stream to be condensed prior to delivery of said refrigeration.
11. The refrigeration process as defined by claim 10, further comprising the step (m) of separating any uncondensed, highly volatile LNG constituents from the combined boil-off gas stream and stored LNG stream subsequent to refrigeration of said combined streams and introducing said separated, uncondensed LNG constituents into the working fluid.
12. The refrigeration process as defined by claim 3, wherein said second fluid is a LNG input stream to be pre-cooled before introduction into a LNG storage vessel.
13. The refrigeration process as defined by claim 12, wherein said second fluid further comprises a recirculated stream of LNG from said storage vessel.
14. Apparatus for low temperature refrigeration comprising a first compressor for compressing a working fluid having at least two components; a first heat exchanger for cooling the compressed working fluid by countercurrent indirect contact with at least one returning stream having a lower temperature; a means for substantially isentropically expanding the cooled working fluid to provide a vapor-liquid mixed working fluid; a first separator for separating the uncondensed vapor fraction from the condensed liquid fraction of the mixed working fluid; conduit means for returning the separated uncondensed vapor fraction through said first heat exchanger countercurrent to said working fluid and for joining said returned vapor fraction with said working fluid at a point before the working fluid enters said first compressor; a second heat exchanger for receiving the separated condensed fraction of the mixed working fluid and for indirectly contacting said working fluid fraction with a second fluid, whereby the working fluid fraction is evaporated and refrigeration is delivered to said second fluid; and conduit means for returning said evaporated working fluid fraction through said first heat exchanger countercurrent to said working fluid and for joining said evaporated working fluid fraction with the working fluid at a point before the working fluid enters said first compressor.
15. The apparatus as defined by claim 14, further comprising means for coupling the mechanical energy from said expanding means with said first compressor.
16. The apparatus as defined by claim 14, further comprising a third compressor for further compressing said working fluid exiting from said first compressor.
17. The apparatus as defined by claim 14, further comprising a second separator for separating any unevaporated working fluid fraction exiting from said second heat exchanger.
18. The apparatus as defined by claim 17, further comprising a third heat exchanger for receiving said separated condensed fraction of the mixed working fluid exiting from said second heat exchanger and for further indirectly contacting said fraction with a stream of said second fluid incoming to said storage vessel; and means intermediate said second and third heat exchangers for introducing into said second fluid (1) unevaporated working fluid fraction separated in said second separator, and (2) a portion of said second fluid contained in said storage vessel.
19. The apparatus as defined by claim 17, further comprising a fourth compressor for compressing the vapor fraction of said second fluid as it exits from the storage vessel; means for conveying a portion of the compressed vapor fraction of said second fluid exiting from said fourth compressor to said second heat exchanger; means for conveying a liquid fraction of said second fluid contained in said storage vessel to said second heat exchanger to be mixed therein with said compressed vapor fraction; a third separator for separating any uncondensed components of said mixed fractions exiting from said second heat exchanger; and means for conveying separated condensed liquid from said third separator back to the storage vessel.
20. The apparatus as defined by claim 17, further comprising a storage vessel for said second fluid and conduit means for conveying a vapor fraction of said second fluid through said first heat exchanger countercurrent to said working fluid and for joining said vapor fraction with the working fluid at a point before the working fluid enters said first compressor.
21. The apparatus as defined by claim 20, wherein said conduit means for the vapor fraction includes a second compressor for compressing the vapor fraction exiting from the first heat exchanger.
1. A low temperature refrigeration process employing a working fluid having at least two components comprising the steps of
2. The refrigeration process as defined by claim 1, further comprising the step (k) of removing a portion of the working fluid at a point in the refrigeration cycle where the most volatile component of the working fluid is most highly concentrated.
3. The refrigeration process as defined in claim 1, wherein said working fluid comprises predominant portions of nitrogen and methane and a minute portion of helium.
4. The refrigeration process as defined by claim 1, wherein said compression step (a) comprises a plurality of separate compression stages and wherein said expansion step (c) provides the energy to carry out at least one of said compression stages in step (a).
5. The refrigeration process as defined by claim 1, further comprising the steps of (f1) passing the evaporated liquid fraction of step (f) into a separation zone and separating any unevaporated liquid therefrom and (f2) recovering such liquid prior to returning the evaporated liquid fraction according to step (g).
6. The refrigeration process as defined by claim 5, wherein said refrigeration is delivered in step (f) to a second fluid and further comprising the step of (i) introducing a vapor fraction of said second fluid into said working fluid.
7. The refrigeration process as defined by claim 6, further comprising the step (j) of introducing the liquid recovered in step (f2) into said second fluid being refrigerated.
8. The refrigeration process as defined by claim 6, wherein the vapor fraction of said second fluid is a boil-off gas stream from a LNG storage tank and further comprising the step (n) of passing said vapor fraction countercurrently with the working fluid in said heat exchange step (b), as one of said returning streams.
9. The refrigeration process as defined by claim 6, wherein said second fluid being refrigerated is liquefied natural gas (LNG).
10. The refrigeration process as defined by claim 9, wherein the refrigeration is delivered to a boil-off gas stream of a LNG storage tank to condense said gas and further comprising the step (l) of combining a stream of the stored LNG with said boil-off gas stream to be condensed prior to delivery of said refrigeration.
11. The refrigeration process as defined by claim 10, further comprising the step (m) of separating any uncondensed, highly volatile LNG constituents from the combined boil-off gas stream and stored LNG stream subsequent to refrigeration of said combined streams and introducing said separated, uncondensed LNG constituents into the working fluid.
12. The refrigeration process as defined by claim 3, wherein said second fluid is a LNG input stream to be pre-cooled before introduction into a LNG storage vessel.
13. The refrigeration process as defined by claim 12, wherein said second fluid further comprises a recirculated stream of LNG from said storage vessel.
14. Apparatus for low temperature refrigeration comprising a first compressor for compressing a working fluid having at least two components; a first heat exchanger for cooling the compressed working fluid by countercurrent indirect contact with at least one returning stream having a lower temperature; a means for substantially isentropically expanding the cooled working fluid to provide a vapor-liquid mixed working fluid; a first separator for separating the uncondensed vapor fraction from the condensed liquid fraction of the mixed working fluid; conduit means for returning the separated uncondensed vapor fraction through said first heat exchanger countercurrent to said working fluid and for joining said returned vapor fraction with said working fluid at a point before the working fluid enters said first compressor; a second heat exchanger for receiving the separated condensed fraction of the mixed working fluid and for indirectly contacting said working fluid fraction with a second fluid, whereby the working fluid fraction is evaporated and refrigeration is delivered to said second fluid; and conduit means for returning said evaporated working fluid fraction through said first heat exchanger countercurrent to said working fluid and for joining said evaporated working fluid fraction with the working fluid at a point before the working fluid enters said first compressor.
15. The apparatus as defined by claim 14, further comprising means for coupling the mechanical energy from said expanding means with said first compressor.
16. The apparatus as defined by claim 14, further comprising a third compressor for further compressing said working fluid exiting from said first compressor.
17. The apparatus as defined by claim 14, further comprising a second separator for separating any unevaporated working fluid fraction exiting from said second heat exchanger.
18. The apparatus as defined by claim 17, further comprising a third heat exchanger for receiving said separated condensed fraction of the mixed working fluid exiting from said second heat exchanger and for further indirectly contacting said fraction with a stream of said second fluid incoming to said storage vessel; and means intermediate said second and third heat exchangers for introducing into said second fluid (1) unevaporated working fluid fraction separated in said second separator, and (2) a portion of said second fluid contained in said storage vessel.
19. The apparatus as defined by claim 17, further comprising a fourth compressor for compressing the vapor fraction of said second fluid as it exits from the storage vessel; means for conveying a portion of the compressed vapor fraction of said second fluid exiting from said fourth compressor to said second heat exchanger; means for conveying a liquid fraction of said second fluid contained in said storage vessel to said second heat exchanger to be mixed therein with said compressed vapor fraction; a third separator for separating any uncondensed components of said mixed fractions exiting from said second heat exchanger; and means for conveying separated condensed liquid from said third separator back to the storage vessel.
20. The apparatus as defined by claim 17, further comprising a storage vessel for said second fluid and conduit means for conveying a vapor fraction of said second fluid through said first heat exchanger countercurrent to said working fluid and for joining said vapor fraction with the working fluid at a point before the working fluid enters said first compressor.
21. The apparatus as defined by claim 20, wherein said conduit means for the vapor fraction includes a second compressor for compressing the vapor fraction exiting from the first heat exchanger.
Description:
BACKGROUND OF THE INVENTION
The present invention relates to low temperature refrigeration, and more especially to the development of refrigeration at a low temperature where the refrigeration is required over a particular range of temperature, for example, in the liquefaction and/or cooling or liquefied natural gas.
In a typical refrigeration system wherein a liquid refrigerant is employed, the refrigerant is usually a pure material which boils at a constant temperature. Therefore, the refrigeration from such a system is developed at a single temperature, and any refrigeration absorbed at a substantially higher temperature results in a waste of power. For example, the development of refrigeration at -300°F. requires 28 percent more power than the development of refrigeration at -270°F. On the other hand, if refrigeration was required over a spread of temperature, for example, from -300° to -270°F., then a weighted average amount of power is the minimum required; however, if all of the refrigeration is developed at -300°F., a substantial waste in power results since the higher power value is required for all of the refrigeration developed.
In many processes, the refrigeration is required over a very wide temperature range, and in such a case, the process may simply consist of an expansion engine, e.g., a turbo-expander, a heat-exchanger and an ambient temperature compressor, all of which operate on a compressible (gaseous) working fluid. The turbo-expander is suitable for refrigerating the gas at low temperatures, but the refrigeration is typically provided over a range of more than about 25 percent of the absolute temperature.
On the other hand, there are many applications where a large quantity of refrigeration is required at a low temperature, but over a rather narrow range, i.e., less than 25 percent of the absolute temperature, and sometimes this requirement is not linearly distributed over the entire required temperature range. An example of such non-linear distribution occurs in connection with the liquefaction of boil-off gas from a liquefied natural gas (LNG) storage tank. Most of this LNG boil-off gas contains nitrogen, methane and sometimes small quantities of other gases. When this gas is reliquefied, the hydrocarbons liquefy first at one or more temperatures, and then the nitrogen liquefies at a substantially lower temperature. In other applications, a pressurized liquefied gas requires further cooling, and in some applications a combination of both the liquefaction and further cooling processes is required. Accordingly, there exists a need to optimize the low temperature refrigeration process which is to be employed to satisfy the foregoing refrigeration requirements, such that the refrigeration is provided over a range of temperature which corresponds as nearly as possible with the temperature range for which the refrigeration requirement exists.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a process and apparatus for the production of low temperature refrigeration over a specific temperature range and in a controlled proportion.
Another object of the present invention is to provide a process and apparatus for the production of refrigeration at different temperature ranges or at a temperature range in which the temperature distribution changes.
It is a further object of the present invention to provide a process and apparatus wherein the boil-off gas of the fluid to be refrigerated is employed as a source of refrigerant working fluid and wherein undesirable constituents are purged from the boil-off gas.
A further object of the present invention resides in the provision of a process and apparatus for producing low temperature refrigeration over a desired temperature range wherein only one main heat exchanger is employed in the refrigeration system and no controls other than simple level controllers are required.
It is also an object of the present invention to provide a process and apparatus for low temperature refrigeration wherein a 2-phase stream is avoided on either side of the main heat exchanger.
It is yet another object of the present invention to provide a low temperature refrigeration process and apparatus to produce refrigeration with a Second Law efficiency (considering 75 percent polytropic compressor efficiency with interstage cooling) of approximately 40 percent.
Another object of the invention is to provide a low temperature refrigeration process and apparatus wherein the refrigeration produced is substantially equal to the power generated by a single low temperature turboexpander.
A further particular object of the present invention is to provide a low temperature refrigeration process and apparatus which is especially well adapted for the further cooling of liquefied natural gas (LNG) and/or the liquefaction of boil-off gas from a LNG storage vessel.
Another particular object of the present invention is to provide a low temperature refrigeration process and apparatus wherein a nitrogen-containing LNG boil-off gas is condensed by cooling at only to a temperature above its bubble point, i.e., by injecting LNG tank liquid into the boil-off gas before the condensing step; wherein the LNG tank liquid is continually circulated; and wherein sub-cooling of the LNG tank liquid is also provided without the necessity of employing an additional heat exchanger.
In accomplishing the foregoing and other objects, there is provided in accordance with the present invention a low temperature refrigeration process employing a working fluid having at least two components, comprising the steps of compressing the working fluid; countercurrently passing the compressed working fluid in indirect contact heat exchange relationship with at least one returning stream having a lower temperature, whereby the working fluid is cooled; substantially isentropically expanding the cooled working fluid to provide a vapor-liquid mixed working fluid; separating the uncondensed vapor fraction from the condensed liquid fraction of the mixed working fluid; returning the separated uncondensed vapor fraction to the heat exchange step and passing the vapor fraction countercurrently with the working fluid, as one of the said returning streams; delivering the separated condensed liquid fraction to an evaporating zone and evaporating the liquid fraction to deliver refrigeration; returning the evaporated liquid fraction to the heat exchange step and passing the evaporated fraction countercurrently with the working fluid, as one of said returning streams; and combining the returned evaporated fraction with the returned separated vapor fraction in order to constitute said working fluid. The refrigeration process of the invention may further include the steps of passing the evaporated liquid fraction into a separation zone and separating any unevaporated liquid therefrom, with subsequent recovery of such liquid prior to returning the evaporated liquid fraction to the heat exchange step. Preferably, the refrigeration is delivered to a second fluid, preferably a liquefied natural gas, and the process then further includes the step of introducing a vapor fraction of this second fluid into the primary working fluid.
Other features of the process according to the present invention include removing a portion of the working fluid at a point in the refrigeration cycle with the most volatile component, i.e., in the case of LNG this would be nitrogen, of the working fluid most highly concentrated. In the case of a LNG refrigeration system, the working fluid comprises predominant portions of nitrogen and methane and a minute portion of helium. In the LNG refrigeration system any liquid collected in the separator following the evaporative refrigeration zone may be introduced directly into the LNG being refrigerated.
In one particular embodiment of the invention, the refrigeration is delivered through a boil-off gas stream of a LNG storage tank to condense this boil-off gas. This process advantageously includes the step of combining a stream of the stored LNG with the boil-off gas stream prior to introduction of the latter into the refrigeration zone. Furthermore, in this embodiment, an additional separation step may also be introduced subsequent to the LNG boil-off gas condensation, wherein the stream exiting the refrigeration zone is conducted to a separator to separate any non-condensible components of the LNG.
In the instance wherein the refrigeration is used to sub-cool an incoming, pressurized LNG stream, the second fluid stream being conducted to the refrigeration zone may comprise further a recirculated stream of LNG from the storage vessel, whereby suitable agitation and cooling for the liquid in the vessel is provided.
Still other features include the provision of a plurality of separate compression stages (if desired) for the initial compression of the working fluid, whereby the energy then produced by the expansion step may be mechanically coupled with one or all of the compression stages in order to minimize the external power requirements for the refrigeration system.
The invention also provides an apparatus for low temperature refrigeration comprising a first compressor for compressing a working fluid having at least two components; a first heat exchanger for cooling the compressed working fluid by countercurrent indirect contact with at least one returning stream having a lower temperature; a means for substantially isentropically expanding the cooled working fluid to provide a vapor-liquid mixed working fluid; a first separator for separating the uncondensed vapor fraction from the condensed liquid fraction of the mixed working fluid; conduit means for returning the separated uncondensed vapor fraction through the first heat exchanger countercurrent to the working fluid and for joining said returned vapor fraction with the working fluid at a point before the working fluid enters the first compressor; a second heat exchanger for receiving the separated condensed fraction of the mixed working fluid and for indirectly contacting the working fluid fraction with a second fluid, whereby the working fluid fraction is evaporated and refrigeration is delivered to the second fluid; and conduit means for returning said evaporated working fluid fraction through the first heat exchanger countercurrent to the working fluid and for joining the evaporated working fluid fraction with the working fluid at a point before the latter enters the first compressor.
The apparatus of the invention may also include a second separator for separating any unevaporated working fluid fraction exiting from the second heat exchanger. Typically, the apparatus includes a storage vessel for the second fluid and conduit means for conveying a vapor fraction of this second fluid through the first heat exchanger countercurrent to the working fluid and for joining this vapor fraction with the working fluid at a point before the working fluid enters the first compressor. Such a conduit for the vapor fraction may include a second compressor for compressing the vapor fraction exiting from the first heat exchanger, i.e., before it is joined with the working fluid. A third compressor may also be incorporated for further compressing the working fluid exiting from the first compressor, and in this case, there may be provided a means for coupling the mechanical energy developed by the turbo-expander with the first compressor.
There is also provided according to the present invention an apparatus designed especially for sub-cooling a stream of LNG entering a storage tank and/or for sub-cooling a stream of LNG being recirculated out of and back into such a storage vessel. In this instance, the apparatus described above further comprises a third heat exchanger for receiving the separated condensed fraction of the mixed working fluid exiting from the second heat exchanger and for further indirectly contacting this fraction with a stream of the second fluid incoming to said storage vessel, and means intermediate the second and third heat exchangers for introducing into the second fluid (1) unevaporated working fluid fractions separated in the second separator, and (2) a portion of the second fluid (LNG) contained in the storage vessel.
In another embodiment of the present invention there is provided an apparatus particularly adapted for recondensing the boil-off gas stream from an LNG storage tank. In this embodiment, the basic apparatus defined above includes also a fourth compressor for compressing the vapor fraction of the second fluid (the LNG boil-off gas) as it exits from the storage vessel, means for conveying a portion of the compressed vapor fraction of second fluid exiting from this fourth compressor to the second heat exchanger, means for conveying a liquid fraction of the second fluid contained in the storage vessel to the second heat exchanger to be mixed therein with the aforesaid compressed vapor fraction, a third separator for separating any uncondensed components of the mixed fractions exiting from the second heat exchanger, and means for conveying separated condensed liquid from the third separator back to the storage vessel.
Other objects, features and advantages of the present invention will become apparent from the following detailed description of the invention which is to be considered together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a flow diagram of one embodiment of the present invention particularly adapted for the sub-cooling of a LNG stream entering and/or being recirculated to a storage vessel;
FIG. 2 is a graph illustrating the temperature distribution in the tube side of the refrigeration heat exchangers plotted against the heat transfer in Btu's per hour (curve A- B- C) and illustrating the evaporation characteristics of a particular working fluid refrigerant (curve D-- E); and
FIG. 3 is a flow diagram of a second embodiment of the invention adapted particularly for the recondensation of a boil-off gas stream from a LNG storage tank.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, in FIG. 1 is illustrated one embodiment of the present invention wherein, for example, there exists a requirement for cooling a pressurized, liquefied gas stream entering the system through line 31, this stream being initially at -260°F. and it being necessary to cool the liquefied gas stream down to a temperature of -290°F. The system is characterized in general by a storage tank 4 wherein a quantity of liquefied gas, in this case liquefied natural gas (LNG), is being held. The LNG in the tank is at a temperature of -290°F., and this body of liquid must have heat removed from it on a continuous basis in order to offset the leakage of heat into the storage tank. In the apparatus illustrated in FIG. 1, the two cooling functions, i.e., cooling the incoming gas and removing heat from the bulk of tank liquid, are accomplished inside the tubes of heat exchangers 1 and 2, respectively.
The pressurized, liquefied natural gas enters heat exchanger 1 through line 31 at a temperature of -260°F. and is cooled within heat exchanger 1 down to a temperature of -290°F., while holding the pressure in its elevated state. Subsequently, as the cooled LNG leaves heat exchanger 1, its pressure is released to the pressure of the storage tank by means of valve 3. The storage tank liquid 4 which is already at -290°F. is recirculated by means of pump 5, mixed with the stream of expanded LNG exiting from heat exchanger 1, passed through heat exchanger 2 and thence back into the storage tank 4 by means of line 6. Thus, the LNG from the tubes of heat exchanger 1 is joined with the recirculating stream of tank LNG at the inlet to the tubes of heat exchanger 2.
The refrigeration which is accomplished in heat exchangers 1 and 2 is provided by introducing a liquid refrigerant mixture through line 7 into the shell side of heat exchanger 2, wherein it countercurrently flows and evaporates as it cools the liquid flowing inside the tubes. The refrigerant mixture exits from heat exchanger 2 into line 8 and then enters the shell side of heat exchanger 1, where it further evaporates and cools the incoming pressurized, liquefied gas stream entering via line 31. The vapor resulting from this evaporation, and any small amounts of liquid which may remain as a result of incomplete vaporization, leave heat exchanger 1 through line 9.
Referring now to FIG. 2 of the drawings, there is illustrated therein by means of the curve A-B-C the temperature distribution in the tube side of both heat exchangers 1 and 2 plotted as a function of the heat transferred in Btu/hour, the curve illustrating the temperature distribution for a total of 1.5 × 10 6 Btu/hour transferred. On the same coordinates is presented a second curve D-E which illustrates the evaporation characteristics of the liquid refrigerant flowing in line 7 of the apparatus when said refrigerant is comprised of 20 percent methane and 80 percent nitrogen and is at a pressure of approximately 40 psia. It is evident from the match of curve A-B-C with curve D-E that the temperature difference between the liquid being cooled on the tube side of the heat exchanger and the refrigerant being evaporated on the shell side of the exchanger is surprisingly uniform in spite of the fact that the cooling requirement for LNG from -260°F. to -290°F. is a non-linear one, and hence, a very difficult cooling requirement, as explained hereinabove. Thus, by employing a specifically tailored refrigerant composition in the apparatus illustrated in FIG. 1, it is possible to carry out this difficult cooling requirement in a very efficient manner. It will be described below how the particular refrigerant composition is tailored to meet this refrigeration requirement, and further, how the apparatus of the invention automatically compensates for any changes in condition and/or composition and thereby assures that the refrigerant compostion is constantly correct for the particular refrigeration requirement. The goal of the process is to seek the highest possible efficiency by providing that the curve D-E, the evaporating pressure of the refrigerant and the composition of the refrigerant are such that the refrigerant is completely evaporated at the shell side discharge point of heat exchanger 1 (the warmest refrigeration requirement).
In the process of the invention, the cold liquid refrigerant is generated in a completely closed system, as illustrated with reference to FIGS. 1 and 3 of the drawings. As a beginning point for this closed system, there is chosen a low pressure ambient temperature point 10. At this point, the refrigerant is rich in the less valuable constituent (nitrogen in the case of LNG compositions), and accordingly, any helium which is also present in LNG compositions can be removed from the system by a small purge to the atmosphere at this point. In this way, the non-condensable helium component can be removed without any appreciable loss of methane or any other more valuable constituent of the refrigerant working fluid.
Any makeup for the refrigerant working fluid may conveniently be provided by taking a small stream of the boil-off gas from the storage tank 4, for example, by means of line 11, and pressurizing it with compressor 12 so that it can be injected into the closed refrigeration system downstream of point 10, as illustrated in FIGS. 1 and 3. This makeup gas is richer in methane than the purge gas leaving through the purge line from point 10 in the system, so that in time, the system will build up a higher concentration of this higher boiling constituent, i.e., methane, from the storage tank. The control of this concentration, if necessary, is controlled automatically by the system, as explained below.
If the working fluid which is initially condensed and delivered through line 7 to heat exchanger 1 fully evaporates before it reaches line 9, no liquid will be contained in the stream of line 9 when it passes to the separator 13, and consequently, no liquid will collect in the separator. However, with the introduction at point 17 of makeup gas which is richer in methane than the gas purged at point 10, the higher boiling constituent in the refrigerant composition will increase until some residual liquid will fail to evaporate in heat exchanger 1. This liquid will then discharge through line 9 along with the evaporated portion of the refrigerant liquid and will accumulate in separator 13. Provision is made to withdraw any accumulated liquid from separator 13 through line 14, and the refrigerant vapor returning to the condensation system through line 15 will be approximately correct for producing the refrigerant required to evaporate as it passes through heat exchangers 1 and 2 such that it will be completely evaporated before leaving exchanger 1 through line 9.
Accordingly, if the refrigeration conditions should change in the heat exchangers 1 and 2, the refrigerant liquid composition will automatically adjust itself in accordance with the foregoing feature of the process in order to accommodate to the new conditions. In other words, the dew point of the refrigerant is automatically adjusted to the warm end temperature of heat exchanger 1 by an adjustment in composition as described above. As has also been pointed out above, the refrigerant must be effective beginning from the cold end temperature of the heat exchangers 1 and 2, and the adjustment of this factor is by means of the evaporation pressure in the refrigerant stream. This pressure is controlled in the illustrated embodiment of the invention by means of adjusting the refrigerant content of the system, which can be accomplished by controlling either the amount of makeup gas injected into the system by compressor 12 and/or the amount of purge gas withdrawn at the vent point 10. This may also be controlled by the amount of liquid which is withdrawn from the separator 13 through drain line 14.
Regeneration of the cold refrigerant liquid takes place as follows: The evaporated refrigerant in line 15 and a vapor of approximately the same temperature from line 16 flow through passes 18a and 18b, respectively, in heat exchanger 18 and then join together at point 17, along with makeup stream 11 which is passed through heat exchanger pass 18d and compressor 12. The resulting stream is partially compressed in compressor 19 and then further compressed in compressor 20, whereupon it is discharged through line 21 and flows back into another pass 18c in heat exchanger 18 wherein it is cooled by absorption of the refrigeration from the aforesaid streams 11, 15 and 16 entering at the cold end of the heat exchanger. Therefore, in FIGS. 1 and 3, stream 18c leaves heat exchanger 18 through line 22 at about -230°F. in the case of LNG, whereby an equivalent amount of heat has been removed from the stream. This cold pressurized gas is introduced into expansion engine 23 wherein it is substantially isentropically expanded, which typically results in a portion of the gas being condensed. The expanded and usually partially condensed stream passes through line 24 into separator 25, wherein any liquefied portion collects and is discharged through line 7 as the refrigerant for use in heat exchangers 1 and 2 to deliver the necessary refrigeration for the incoming and recirculated LNG. As already described,, the refrigerant is evaporated in heat exchanges 1 and 2 and this evaporated mixture then returns through lines 9 and 15 via separator 13 in order to repeat the process.
The uncondensed portion of the exhaust from expansion engine 23, which exhaust stream enters separator 25 through line 24, is disengaged from the condensed portion in separator 25 and is discharged from the separator through line 16, whereupon it joins stream 15 as aforesaid to produce the combined stream 17.
Preferably, the power generated by expansion engine 23 is absorbed by compressor 19 to aid in the recompression of the circulating refrigerant gas. This is most readily accomplished by providing a turboexpander as the expansion engine 23 and simply mechanically connecting the output of this turboexpander as the power supply for compressor 19. While individual compressors 19 and 20 are shown, a single multi-stage compressor or one single-stage compressor of adequate capacity can be used, either being driven by engine 23.
In other applications, the boil-off gas from the storage tank 4 may have a composition of about 25 percent nitrogen and 75 percent methane, and it may be required to recondense this boil-off gas stream, even though the liquid in the storage tank may not require sub-cooling or require that the feed stream to the tank be subjected to a pre-cooling step. Where this boil-off gas stream is to be recondensed, the composition thereof is often about 25 percent nitrogen, as pointed out above, and therefore, the final condensation temperature for the gas stream is about 43°F. below the tank temperature.
If recondensation of the boil-off gas stream is a requirement, either of the embodiments of the invention illustrated in FIGS. 1 and 3 may be employed; however, for this type of application, it is preferred to recirculate some of the storage tank liquid along with the boil-off gas in order to reduce the percentage of nitrogen in the ultimate stream to which the refrigeration is to be delivered. In this way, the bubble point or final condensation temperature is substantially raised.
For example, if approximately 10 mols of LNG from the tank are comingled with every mol of boil-off gas before condensation, the final condensation temperature is approximately 12°F. lower than the tank temperature, instead of 43°F. lower in the case where no comingling is provided. This is still a significant temperature range over which the refrigeration is required, and therefore, the refrigeration process of the present invention is particularly well suited to meet this requirement.
A particular configuration of apparatus for carrying out the recondensation of an LNG boil-off gas stream is illustrated in FIG. 3 of the drawings. It will be observed that the closed refrigeration system shown in FIG. 3 is identical in structure and operation as the closed system described hereinabove with specific reference to FIG. 1 of the drawings. Only the location at which the actual refrigeration is delivered and the configuration of the storage tank together with the recirculation and boil-off stream lines associated therewith are arranged somewhat differently in FIG. 3. Thus, instead of two heat exchangers, there is provided in the embodiment of FIG. 3 only a single heat exchanger 32 into which the refrigerant mixture flows via line 7. On the shell side of the heat exchanger 32 there is combined a recirculated stream of tank liquid entering through line 36 and a stream of LNG boil-off gas entering through line 33 after it has been compressed by compressor 35. This mixture of gas and liquid passes through heat exchanger 32 countercurrently to the refrigerant and is thereby condensed. The condensed stream exits heat exchanger 32 through line 27 and is introduced into a separator 26, wherein the liquid portion is separated and reintroduced into the storage tank through line 28. Any non-condensable components of the composition leaving heat exchanger 32 are removed in the separator and directed through line 29 into the makeup stream for the refrigeration system provided at line 11. Non-condensable components of this type, such as helium, are ultimately vented to the atmosphere at point 10, as described hereinabove.
It will be observed from FIGS. 1 or 3 by an envelope examination of the illustrated refrigeration cycle that the enthalpy content of the compressed refrigerant fluid entering heat exchanger pass 18c is substantially equal to the total enthalpy of the three low pressure returning streams 18a, 18b, and 18d at the warm end of the heat exchanger 18. This is equivalent to stating that the cooling or refrigeration (neglecting heat entry through the insulation) is substantially equal to the power of the turboexpander. Since the turboexpander operates at near the refrigeration temperature, the cycle is operating at near its optimum. Since the expander is condensing within its nozzles and rotor, there is no apparent way of making the cycle more efficient. Accordingly, there has been provided in accordance with the present invention a novel refrigeration process and apparatus wherein a mixture of refrigerants is employed so that by controlling the composition and proportion of the mixture together with the evaporator pressure, the refrigeration can be distributed according to any necessary refrigeration requirement, thereby resulting in a considerable saving in power. The invention also provides a refrigeration system wherein a portion of the boil-off gas from a liquefied gas storage tank may conveniently be employed as a source of refrigerant and wherein it is possible to reject some of the undesirable constituents of the boil-off gas, such as helium or oxygen, which it does not desire to return to the storage tank.
The present invention has been described with reference to several specific embodiments thereof, and accordingly, it will be apparent that many modifications, substitutions and omissions will be readily suggested to a person of ordinary skill in the art without departing from the spirit of the present invention. Therefore, it is to be understood that the scope of the invention is to be determined solely by the claims appended hereto.
The present invention relates to low temperature refrigeration, and more especially to the development of refrigeration at a low temperature where the refrigeration is required over a particular range of temperature, for example, in the liquefaction and/or cooling or liquefied natural gas.
In a typical refrigeration system wherein a liquid refrigerant is employed, the refrigerant is usually a pure material which boils at a constant temperature. Therefore, the refrigeration from such a system is developed at a single temperature, and any refrigeration absorbed at a substantially higher temperature results in a waste of power. For example, the development of refrigeration at -300°F. requires 28 percent more power than the development of refrigeration at -270°F. On the other hand, if refrigeration was required over a spread of temperature, for example, from -300° to -270°F., then a weighted average amount of power is the minimum required; however, if all of the refrigeration is developed at -300°F., a substantial waste in power results since the higher power value is required for all of the refrigeration developed.
In many processes, the refrigeration is required over a very wide temperature range, and in such a case, the process may simply consist of an expansion engine, e.g., a turbo-expander, a heat-exchanger and an ambient temperature compressor, all of which operate on a compressible (gaseous) working fluid. The turbo-expander is suitable for refrigerating the gas at low temperatures, but the refrigeration is typically provided over a range of more than about 25 percent of the absolute temperature.
On the other hand, there are many applications where a large quantity of refrigeration is required at a low temperature, but over a rather narrow range, i.e., less than 25 percent of the absolute temperature, and sometimes this requirement is not linearly distributed over the entire required temperature range. An example of such non-linear distribution occurs in connection with the liquefaction of boil-off gas from a liquefied natural gas (LNG) storage tank. Most of this LNG boil-off gas contains nitrogen, methane and sometimes small quantities of other gases. When this gas is reliquefied, the hydrocarbons liquefy first at one or more temperatures, and then the nitrogen liquefies at a substantially lower temperature. In other applications, a pressurized liquefied gas requires further cooling, and in some applications a combination of both the liquefaction and further cooling processes is required. Accordingly, there exists a need to optimize the low temperature refrigeration process which is to be employed to satisfy the foregoing refrigeration requirements, such that the refrigeration is provided over a range of temperature which corresponds as nearly as possible with the temperature range for which the refrigeration requirement exists.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a process and apparatus for the production of low temperature refrigeration over a specific temperature range and in a controlled proportion.
Another object of the present invention is to provide a process and apparatus for the production of refrigeration at different temperature ranges or at a temperature range in which the temperature distribution changes.
It is a further object of the present invention to provide a process and apparatus wherein the boil-off gas of the fluid to be refrigerated is employed as a source of refrigerant working fluid and wherein undesirable constituents are purged from the boil-off gas.
A further object of the present invention resides in the provision of a process and apparatus for producing low temperature refrigeration over a desired temperature range wherein only one main heat exchanger is employed in the refrigeration system and no controls other than simple level controllers are required.
It is also an object of the present invention to provide a process and apparatus for low temperature refrigeration wherein a 2-phase stream is avoided on either side of the main heat exchanger.
It is yet another object of the present invention to provide a low temperature refrigeration process and apparatus to produce refrigeration with a Second Law efficiency (considering 75 percent polytropic compressor efficiency with interstage cooling) of approximately 40 percent.
Another object of the invention is to provide a low temperature refrigeration process and apparatus wherein the refrigeration produced is substantially equal to the power generated by a single low temperature turboexpander.
A further particular object of the present invention is to provide a low temperature refrigeration process and apparatus which is especially well adapted for the further cooling of liquefied natural gas (LNG) and/or the liquefaction of boil-off gas from a LNG storage vessel.
Another particular object of the present invention is to provide a low temperature refrigeration process and apparatus wherein a nitrogen-containing LNG boil-off gas is condensed by cooling at only to a temperature above its bubble point, i.e., by injecting LNG tank liquid into the boil-off gas before the condensing step; wherein the LNG tank liquid is continually circulated; and wherein sub-cooling of the LNG tank liquid is also provided without the necessity of employing an additional heat exchanger.
In accomplishing the foregoing and other objects, there is provided in accordance with the present invention a low temperature refrigeration process employing a working fluid having at least two components, comprising the steps of compressing the working fluid; countercurrently passing the compressed working fluid in indirect contact heat exchange relationship with at least one returning stream having a lower temperature, whereby the working fluid is cooled; substantially isentropically expanding the cooled working fluid to provide a vapor-liquid mixed working fluid; separating the uncondensed vapor fraction from the condensed liquid fraction of the mixed working fluid; returning the separated uncondensed vapor fraction to the heat exchange step and passing the vapor fraction countercurrently with the working fluid, as one of the said returning streams; delivering the separated condensed liquid fraction to an evaporating zone and evaporating the liquid fraction to deliver refrigeration; returning the evaporated liquid fraction to the heat exchange step and passing the evaporated fraction countercurrently with the working fluid, as one of said returning streams; and combining the returned evaporated fraction with the returned separated vapor fraction in order to constitute said working fluid. The refrigeration process of the invention may further include the steps of passing the evaporated liquid fraction into a separation zone and separating any unevaporated liquid therefrom, with subsequent recovery of such liquid prior to returning the evaporated liquid fraction to the heat exchange step. Preferably, the refrigeration is delivered to a second fluid, preferably a liquefied natural gas, and the process then further includes the step of introducing a vapor fraction of this second fluid into the primary working fluid.
Other features of the process according to the present invention include removing a portion of the working fluid at a point in the refrigeration cycle with the most volatile component, i.e., in the case of LNG this would be nitrogen, of the working fluid most highly concentrated. In the case of a LNG refrigeration system, the working fluid comprises predominant portions of nitrogen and methane and a minute portion of helium. In the LNG refrigeration system any liquid collected in the separator following the evaporative refrigeration zone may be introduced directly into the LNG being refrigerated.
In one particular embodiment of the invention, the refrigeration is delivered through a boil-off gas stream of a LNG storage tank to condense this boil-off gas. This process advantageously includes the step of combining a stream of the stored LNG with the boil-off gas stream prior to introduction of the latter into the refrigeration zone. Furthermore, in this embodiment, an additional separation step may also be introduced subsequent to the LNG boil-off gas condensation, wherein the stream exiting the refrigeration zone is conducted to a separator to separate any non-condensible components of the LNG.
In the instance wherein the refrigeration is used to sub-cool an incoming, pressurized LNG stream, the second fluid stream being conducted to the refrigeration zone may comprise further a recirculated stream of LNG from the storage vessel, whereby suitable agitation and cooling for the liquid in the vessel is provided.
Still other features include the provision of a plurality of separate compression stages (if desired) for the initial compression of the working fluid, whereby the energy then produced by the expansion step may be mechanically coupled with one or all of the compression stages in order to minimize the external power requirements for the refrigeration system.
The invention also provides an apparatus for low temperature refrigeration comprising a first compressor for compressing a working fluid having at least two components; a first heat exchanger for cooling the compressed working fluid by countercurrent indirect contact with at least one returning stream having a lower temperature; a means for substantially isentropically expanding the cooled working fluid to provide a vapor-liquid mixed working fluid; a first separator for separating the uncondensed vapor fraction from the condensed liquid fraction of the mixed working fluid; conduit means for returning the separated uncondensed vapor fraction through the first heat exchanger countercurrent to the working fluid and for joining said returned vapor fraction with the working fluid at a point before the working fluid enters the first compressor; a second heat exchanger for receiving the separated condensed fraction of the mixed working fluid and for indirectly contacting the working fluid fraction with a second fluid, whereby the working fluid fraction is evaporated and refrigeration is delivered to the second fluid; and conduit means for returning said evaporated working fluid fraction through the first heat exchanger countercurrent to the working fluid and for joining the evaporated working fluid fraction with the working fluid at a point before the latter enters the first compressor.
The apparatus of the invention may also include a second separator for separating any unevaporated working fluid fraction exiting from the second heat exchanger. Typically, the apparatus includes a storage vessel for the second fluid and conduit means for conveying a vapor fraction of this second fluid through the first heat exchanger countercurrent to the working fluid and for joining this vapor fraction with the working fluid at a point before the working fluid enters the first compressor. Such a conduit for the vapor fraction may include a second compressor for compressing the vapor fraction exiting from the first heat exchanger, i.e., before it is joined with the working fluid. A third compressor may also be incorporated for further compressing the working fluid exiting from the first compressor, and in this case, there may be provided a means for coupling the mechanical energy developed by the turbo-expander with the first compressor.
There is also provided according to the present invention an apparatus designed especially for sub-cooling a stream of LNG entering a storage tank and/or for sub-cooling a stream of LNG being recirculated out of and back into such a storage vessel. In this instance, the apparatus described above further comprises a third heat exchanger for receiving the separated condensed fraction of the mixed working fluid exiting from the second heat exchanger and for further indirectly contacting this fraction with a stream of the second fluid incoming to said storage vessel, and means intermediate the second and third heat exchangers for introducing into the second fluid (1) unevaporated working fluid fractions separated in the second separator, and (2) a portion of the second fluid (LNG) contained in the storage vessel.
In another embodiment of the present invention there is provided an apparatus particularly adapted for recondensing the boil-off gas stream from an LNG storage tank. In this embodiment, the basic apparatus defined above includes also a fourth compressor for compressing the vapor fraction of the second fluid (the LNG boil-off gas) as it exits from the storage vessel, means for conveying a portion of the compressed vapor fraction of second fluid exiting from this fourth compressor to the second heat exchanger, means for conveying a liquid fraction of the second fluid contained in the storage vessel to the second heat exchanger to be mixed therein with the aforesaid compressed vapor fraction, a third separator for separating any uncondensed components of the mixed fractions exiting from the second heat exchanger, and means for conveying separated condensed liquid from the third separator back to the storage vessel.
Other objects, features and advantages of the present invention will become apparent from the following detailed description of the invention which is to be considered together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a flow diagram of one embodiment of the present invention particularly adapted for the sub-cooling of a LNG stream entering and/or being recirculated to a storage vessel;
FIG. 2 is a graph illustrating the temperature distribution in the tube side of the refrigeration heat exchangers plotted against the heat transfer in Btu's per hour (curve A- B- C) and illustrating the evaporation characteristics of a particular working fluid refrigerant (curve D-- E); and
FIG. 3 is a flow diagram of a second embodiment of the invention adapted particularly for the recondensation of a boil-off gas stream from a LNG storage tank.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, in FIG. 1 is illustrated one embodiment of the present invention wherein, for example, there exists a requirement for cooling a pressurized, liquefied gas stream entering the system through line 31, this stream being initially at -260°F. and it being necessary to cool the liquefied gas stream down to a temperature of -290°F. The system is characterized in general by a storage tank 4 wherein a quantity of liquefied gas, in this case liquefied natural gas (LNG), is being held. The LNG in the tank is at a temperature of -290°F., and this body of liquid must have heat removed from it on a continuous basis in order to offset the leakage of heat into the storage tank. In the apparatus illustrated in FIG. 1, the two cooling functions, i.e., cooling the incoming gas and removing heat from the bulk of tank liquid, are accomplished inside the tubes of heat exchangers 1 and 2, respectively.
The pressurized, liquefied natural gas enters heat exchanger 1 through line 31 at a temperature of -260°F. and is cooled within heat exchanger 1 down to a temperature of -290°F., while holding the pressure in its elevated state. Subsequently, as the cooled LNG leaves heat exchanger 1, its pressure is released to the pressure of the storage tank by means of valve 3. The storage tank liquid 4 which is already at -290°F. is recirculated by means of pump 5, mixed with the stream of expanded LNG exiting from heat exchanger 1, passed through heat exchanger 2 and thence back into the storage tank 4 by means of line 6. Thus, the LNG from the tubes of heat exchanger 1 is joined with the recirculating stream of tank LNG at the inlet to the tubes of heat exchanger 2.
The refrigeration which is accomplished in heat exchangers 1 and 2 is provided by introducing a liquid refrigerant mixture through line 7 into the shell side of heat exchanger 2, wherein it countercurrently flows and evaporates as it cools the liquid flowing inside the tubes. The refrigerant mixture exits from heat exchanger 2 into line 8 and then enters the shell side of heat exchanger 1, where it further evaporates and cools the incoming pressurized, liquefied gas stream entering via line 31. The vapor resulting from this evaporation, and any small amounts of liquid which may remain as a result of incomplete vaporization, leave heat exchanger 1 through line 9.
Referring now to FIG. 2 of the drawings, there is illustrated therein by means of the curve A-B-C the temperature distribution in the tube side of both heat exchangers 1 and 2 plotted as a function of the heat transferred in Btu/hour, the curve illustrating the temperature distribution for a total of 1.5 × 10 6 Btu/hour transferred. On the same coordinates is presented a second curve D-E which illustrates the evaporation characteristics of the liquid refrigerant flowing in line 7 of the apparatus when said refrigerant is comprised of 20 percent methane and 80 percent nitrogen and is at a pressure of approximately 40 psia. It is evident from the match of curve A-B-C with curve D-E that the temperature difference between the liquid being cooled on the tube side of the heat exchanger and the refrigerant being evaporated on the shell side of the exchanger is surprisingly uniform in spite of the fact that the cooling requirement for LNG from -260°F. to -290°F. is a non-linear one, and hence, a very difficult cooling requirement, as explained hereinabove. Thus, by employing a specifically tailored refrigerant composition in the apparatus illustrated in FIG. 1, it is possible to carry out this difficult cooling requirement in a very efficient manner. It will be described below how the particular refrigerant composition is tailored to meet this refrigeration requirement, and further, how the apparatus of the invention automatically compensates for any changes in condition and/or composition and thereby assures that the refrigerant compostion is constantly correct for the particular refrigeration requirement. The goal of the process is to seek the highest possible efficiency by providing that the curve D-E, the evaporating pressure of the refrigerant and the composition of the refrigerant are such that the refrigerant is completely evaporated at the shell side discharge point of heat exchanger 1 (the warmest refrigeration requirement).
In the process of the invention, the cold liquid refrigerant is generated in a completely closed system, as illustrated with reference to FIGS. 1 and 3 of the drawings. As a beginning point for this closed system, there is chosen a low pressure ambient temperature point 10. At this point, the refrigerant is rich in the less valuable constituent (nitrogen in the case of LNG compositions), and accordingly, any helium which is also present in LNG compositions can be removed from the system by a small purge to the atmosphere at this point. In this way, the non-condensable helium component can be removed without any appreciable loss of methane or any other more valuable constituent of the refrigerant working fluid.
Any makeup for the refrigerant working fluid may conveniently be provided by taking a small stream of the boil-off gas from the storage tank 4, for example, by means of line 11, and pressurizing it with compressor 12 so that it can be injected into the closed refrigeration system downstream of point 10, as illustrated in FIGS. 1 and 3. This makeup gas is richer in methane than the purge gas leaving through the purge line from point 10 in the system, so that in time, the system will build up a higher concentration of this higher boiling constituent, i.e., methane, from the storage tank. The control of this concentration, if necessary, is controlled automatically by the system, as explained below.
If the working fluid which is initially condensed and delivered through line 7 to heat exchanger 1 fully evaporates before it reaches line 9, no liquid will be contained in the stream of line 9 when it passes to the separator 13, and consequently, no liquid will collect in the separator. However, with the introduction at point 17 of makeup gas which is richer in methane than the gas purged at point 10, the higher boiling constituent in the refrigerant composition will increase until some residual liquid will fail to evaporate in heat exchanger 1. This liquid will then discharge through line 9 along with the evaporated portion of the refrigerant liquid and will accumulate in separator 13. Provision is made to withdraw any accumulated liquid from separator 13 through line 14, and the refrigerant vapor returning to the condensation system through line 15 will be approximately correct for producing the refrigerant required to evaporate as it passes through heat exchangers 1 and 2 such that it will be completely evaporated before leaving exchanger 1 through line 9.
Accordingly, if the refrigeration conditions should change in the heat exchangers 1 and 2, the refrigerant liquid composition will automatically adjust itself in accordance with the foregoing feature of the process in order to accommodate to the new conditions. In other words, the dew point of the refrigerant is automatically adjusted to the warm end temperature of heat exchanger 1 by an adjustment in composition as described above. As has also been pointed out above, the refrigerant must be effective beginning from the cold end temperature of the heat exchangers 1 and 2, and the adjustment of this factor is by means of the evaporation pressure in the refrigerant stream. This pressure is controlled in the illustrated embodiment of the invention by means of adjusting the refrigerant content of the system, which can be accomplished by controlling either the amount of makeup gas injected into the system by compressor 12 and/or the amount of purge gas withdrawn at the vent point 10. This may also be controlled by the amount of liquid which is withdrawn from the separator 13 through drain line 14.
Regeneration of the cold refrigerant liquid takes place as follows: The evaporated refrigerant in line 15 and a vapor of approximately the same temperature from line 16 flow through passes 18a and 18b, respectively, in heat exchanger 18 and then join together at point 17, along with makeup stream 11 which is passed through heat exchanger pass 18d and compressor 12. The resulting stream is partially compressed in compressor 19 and then further compressed in compressor 20, whereupon it is discharged through line 21 and flows back into another pass 18c in heat exchanger 18 wherein it is cooled by absorption of the refrigeration from the aforesaid streams 11, 15 and 16 entering at the cold end of the heat exchanger. Therefore, in FIGS. 1 and 3, stream 18c leaves heat exchanger 18 through line 22 at about -230°F. in the case of LNG, whereby an equivalent amount of heat has been removed from the stream. This cold pressurized gas is introduced into expansion engine 23 wherein it is substantially isentropically expanded, which typically results in a portion of the gas being condensed. The expanded and usually partially condensed stream passes through line 24 into separator 25, wherein any liquefied portion collects and is discharged through line 7 as the refrigerant for use in heat exchangers 1 and 2 to deliver the necessary refrigeration for the incoming and recirculated LNG. As already described,, the refrigerant is evaporated in heat exchanges 1 and 2 and this evaporated mixture then returns through lines 9 and 15 via separator 13 in order to repeat the process.
The uncondensed portion of the exhaust from expansion engine 23, which exhaust stream enters separator 25 through line 24, is disengaged from the condensed portion in separator 25 and is discharged from the separator through line 16, whereupon it joins stream 15 as aforesaid to produce the combined stream 17.
Preferably, the power generated by expansion engine 23 is absorbed by compressor 19 to aid in the recompression of the circulating refrigerant gas. This is most readily accomplished by providing a turboexpander as the expansion engine 23 and simply mechanically connecting the output of this turboexpander as the power supply for compressor 19. While individual compressors 19 and 20 are shown, a single multi-stage compressor or one single-stage compressor of adequate capacity can be used, either being driven by engine 23.
In other applications, the boil-off gas from the storage tank 4 may have a composition of about 25 percent nitrogen and 75 percent methane, and it may be required to recondense this boil-off gas stream, even though the liquid in the storage tank may not require sub-cooling or require that the feed stream to the tank be subjected to a pre-cooling step. Where this boil-off gas stream is to be recondensed, the composition thereof is often about 25 percent nitrogen, as pointed out above, and therefore, the final condensation temperature for the gas stream is about 43°F. below the tank temperature.
If recondensation of the boil-off gas stream is a requirement, either of the embodiments of the invention illustrated in FIGS. 1 and 3 may be employed; however, for this type of application, it is preferred to recirculate some of the storage tank liquid along with the boil-off gas in order to reduce the percentage of nitrogen in the ultimate stream to which the refrigeration is to be delivered. In this way, the bubble point or final condensation temperature is substantially raised.
For example, if approximately 10 mols of LNG from the tank are comingled with every mol of boil-off gas before condensation, the final condensation temperature is approximately 12°F. lower than the tank temperature, instead of 43°F. lower in the case where no comingling is provided. This is still a significant temperature range over which the refrigeration is required, and therefore, the refrigeration process of the present invention is particularly well suited to meet this requirement.
A particular configuration of apparatus for carrying out the recondensation of an LNG boil-off gas stream is illustrated in FIG. 3 of the drawings. It will be observed that the closed refrigeration system shown in FIG. 3 is identical in structure and operation as the closed system described hereinabove with specific reference to FIG. 1 of the drawings. Only the location at which the actual refrigeration is delivered and the configuration of the storage tank together with the recirculation and boil-off stream lines associated therewith are arranged somewhat differently in FIG. 3. Thus, instead of two heat exchangers, there is provided in the embodiment of FIG. 3 only a single heat exchanger 32 into which the refrigerant mixture flows via line 7. On the shell side of the heat exchanger 32 there is combined a recirculated stream of tank liquid entering through line 36 and a stream of LNG boil-off gas entering through line 33 after it has been compressed by compressor 35. This mixture of gas and liquid passes through heat exchanger 32 countercurrently to the refrigerant and is thereby condensed. The condensed stream exits heat exchanger 32 through line 27 and is introduced into a separator 26, wherein the liquid portion is separated and reintroduced into the storage tank through line 28. Any non-condensable components of the composition leaving heat exchanger 32 are removed in the separator and directed through line 29 into the makeup stream for the refrigeration system provided at line 11. Non-condensable components of this type, such as helium, are ultimately vented to the atmosphere at point 10, as described hereinabove.
It will be observed from FIGS. 1 or 3 by an envelope examination of the illustrated refrigeration cycle that the enthalpy content of the compressed refrigerant fluid entering heat exchanger pass 18c is substantially equal to the total enthalpy of the three low pressure returning streams 18a, 18b, and 18d at the warm end of the heat exchanger 18. This is equivalent to stating that the cooling or refrigeration (neglecting heat entry through the insulation) is substantially equal to the power of the turboexpander. Since the turboexpander operates at near the refrigeration temperature, the cycle is operating at near its optimum. Since the expander is condensing within its nozzles and rotor, there is no apparent way of making the cycle more efficient. Accordingly, there has been provided in accordance with the present invention a novel refrigeration process and apparatus wherein a mixture of refrigerants is employed so that by controlling the composition and proportion of the mixture together with the evaporator pressure, the refrigeration can be distributed according to any necessary refrigeration requirement, thereby resulting in a considerable saving in power. The invention also provides a refrigeration system wherein a portion of the boil-off gas from a liquefied gas storage tank may conveniently be employed as a source of refrigerant and wherein it is possible to reject some of the undesirable constituents of the boil-off gas, such as helium or oxygen, which it does not desire to return to the storage tank.
The present invention has been described with reference to several specific embodiments thereof, and accordingly, it will be apparent that many modifications, substitutions and omissions will be readily suggested to a person of ordinary skill in the art without departing from the spirit of the present invention. Therefore, it is to be understood that the scope of the invention is to be determined solely by the claims appended hereto.