CA2546426C - A system and method for vaporizing cryogenic liquids using a naturally circulating intermediate refrigerant - Google Patents

Abstract

A system and a method for vaporizing a cryogenic liquid using a naturally circulating intermediate refrigerant.

Description

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A SYSTEM AND >id$THOD FOR VAPORIZIING CRYOGFNiC
LIOUmS USING A NATURALLY CIILC[]LATIIVG
IIV'PERMBDIA'PE REFRIGF,RANT
FIB,LD OF THE INVENTION
100011 The present invention relates to a system and method for vaporizing cryogenic fluids using a naturaliy circulating intermediate refrigerant in a thonnal siphon type arrangement wherein a tirst heat exchanger is positioned above a second heat exchanger so that the intdmediaoe refrigerant is vaporiaod in the second heat exchanger with the vapor passing upwardly into the first exchanger for heat exchange with a cryogenic liquid with a eondensed intenuediate reftiigeramt liquid being recovered and passed back to the second heat exchanger by gravity flow.

BACICGROUND OF THE Di VENTION
[00021 In many areas of the workl, large natural gas deposits are found, which are remote from any commercial market for the volumes of natural gas available.
Accordingly, methods have been considered for moving natural gas to eommerciel markets by pipeline and by liquefaction of the natural gas followed by transport by ship and the like. When the natnral gas is liquefied and transported by ship or the liloe to a destination it is necessary to revaporize the liqucfied natural gas (LNG) for use as a natural gas.
[0003) Many approaches have been used for such vaporization or revaporization.
For instance, seawater is freqaently used as a hoating medium to vaporize the LNG since seawater is nonnally present at the unloading ama. A continuing problem, however, is the large surface area required in the heat exchangers for revaporization of the LNG by seawater as a heating medium. Further, the use of seawater resutts in contamination of the heat exchanger surfa<xs in maay instaaces so that frequent cleaning is reqnired.
Further, when lower flow rates of seawater or excessively high rates of cryogenic liquid are used, the seawater can fi=eeae in the soawater side of the heat exchange system used.
This can result in damage to the system as well as Interrapting production of vaporized cryogenic matcrial. Accordingly, an improved method has been sought to accomplish the desired heat exchange efficiently and in a smaller area, which is of tremendous benefit when the regasification is accomplished offshore and the like.

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10004] Accordiag to the pt+esent invention, it has been found that cryogenic liquids are madily revaporized by a method for vaporiziag a cryogenic fluid using a naturally circulatnsg iatonaediate ra5igerant, tha method comprisiag: passing the cryogenic liquid in heat exchange contact with a vaporous intermediate re&igeraat in a first heat exchanger having a vaporous intermediate refrigerant inlet and a liquid intermediate refrigerant outlet to heat the cryogenic fluid to produce a gaseous cryogenic fluid and a liquid intermediate reffigerant; passing the liquid intermediate refrigerant in heat exchange contact with a heating fluid in a second heat exchanger having a liquid intermediate refrigerant inlet and a vaporous Intermediate refrigerant outlet to heat the intermediate refrigerant to produce the vaporous intermediate refrigerant, the first heat exchanger being above the sooond heat exchanger allowlag the vaporous intertnediate refrigerant to rise into the first heat exchanger, and, allowing the liquid intecmediate refrigerant to flow downwardly into the aecond heat exchanger 100051 The inveation fittther coalprises a system fbr vaporizing a cryogcnic liquid using a naturaily circttlatiag inbenaediate refrigerant, the system comprising: a first heat exchanger having a liquid cryogenic fluid inlet, a vaporized cryogenic fluid outlet, a vaporized intennediate refrigeraat inlet and a liquid intermediate refrigerant outlet; and, a second heat exchanger having a liquid refrigernnt intet, a vaporized refrigerant outlet, a heating fluid inlet and a heating fluid ouNet, the first heat exchanger being positioned above the second beat exchanger with the vaporized intermediate refrigerant inlet tD the first heat exchanger being in fluid communication with the vaporized intermediate refrigerant outlet from the seeond heat exchanger and with the liquid intennediatc refrigerant outlet from the first heat exchanger being in fluid communication with the liquid intetmediate refrigetattt inlet to the second heat exchanger.
BRILF DESCRIPT[ON OF THE DRAWINGS
[0006] FIG 1. is a schenuitic embodiment of the ptesent invention and cwmprises a heat exchanger comprising two tube bundles in a vertical position inside a low pressure vessel with a welded plate type heat exchanger as an intermediate refrigerant reboiler, [4007] FIG 2. is a schematic diagram of a further embodiment of the present invention using a plate type heat exchanger for both the vaporization of the cryogenic liquid and for vaporizing ofthe intermedimte refrigerant;

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(0008] FIG 3 is a schematic diagram of a futther embodiment of the present inventlon wherein a superheater ls provided to supaireat the re#'rigerant vapcx;

(00091 In FIG 4. a superheater is used to superheat the vaporized natural gas;
and, [0010] In FI(3 5 an intermediate separation vessel is used between the intermediate refrigenmt vapor outlet from the -intemnediate refrigeram heater and the 1 iquid Intermediate refrigerant outlet from the cryogenic liquid vaporizing section.

DESCRIPTION OF PRB FE +' EMBODIIVIENTS
[0011] In the description of the FIGs, the same nurabers will be used tln+oughoart to refer to the same or similar cotnponents.
[0012] In FIG 1 an embodiment of the procoss is shown which uses a typical shell and tube heat exchanger in a vertical configuration. Vaporization system comprises a refrigerant condenser and cryogenic liquid vaporizer having a top 11. The refrigerant condenser (first heat exchanger) 12 includes a cryogenic liquid inlet 14 and a vaporized cryogenic fluid outlet 16. As ahown, line 14 passes liquid cryogenic material into a zone established by a divider 21 and a header 20 and into an inlet of tube bundles 18. The vapor'rced cryogenic material is recovered tluough an outlet from tubes 18 through a header 22 and passed to line 16. Two headers for the two tube bundles 18 are shown. The second bundle of tubes is shown as raoeiving dw cryogenic liquid through a header 24 into the tube bundles 18 and recovering the vaporized cryogenic material through a header 26 from an outlet from tube bundles 18 in header 26. A
variety of arrangements can be used to pass the cryogenic material through headers into heat exchange tubes as well known tD those skilled in the ait. The embodiment shown is illustmtive only.
[00131 The first heat exclumger 12 also includes a refrigerant vapor inlet 38 through which refrigerant vapor is introduced and passes upward through a riser 30 and outwardly into a space for refrigerant vapor 28, as shown by arrows 50. The vapor then exchanges heat with the cryogenic fluid in tube bundles 18 and condenses into a liquid intermediate refrigerant material. A representative liquid level 32 is shown in a bottom 34 of heat exchanger 12. The liquid intermediate refrigerant is passed through an outlet 36 through a line 45 and into an inlet 44 into a second lxat exchanger 40. In second heat exchanger 40, the intermediate refrigerant is heated by heat exchange contact with a heat . . w. .. ,........~ , ti., ~,.-.... .

exchange fluid passed Into heat exchanger 40 via an inlet 46 and a line 47.
T'he intermediate refrigerant is vaporized in second heat exchanga 40 by heat exchango with the heat exxchange fluid which is then discharged through an outlet 48 and a line 49.
Refrigeraat vapor Is discharged througb a line 42 and passes upwardly ftough a line 43 into a vaporous rofrigerant inlet 38 into Srst heat exeltsngor 12. In the operation of the heat exchanger system, the liquid head in bottom 34 of first heat exchanger 12 supplies the motive force to flow liquid intermediate reErigerant back through line 45 into inlet 44 into second heat exchanger 40. Tbe vaporized heaf exchange fluid is then passed upwardly into fusR heat exchangor 12 as a vapor. This cycle is a rapoating cycle and provides a supply of heat to fust heat exchaager 12 from second heat exchanger without the need for mcchanical pumps or the Iiice.
[0014] It will be understood ahat a wide variety of types of heat exchangers could be used. For Instance, either or both of the first and second heat exchangers could be a shelt and tube heat exchanger of a variety of configurations, a core-in-kettle heat exchanger, a piate fin beat eau:hangar, a plate type heat exchanger, multiple tube bundles in a shell heat exchanger and the like as lmown to those skilled in the att.
Any such heat exchanger is considered to be suitable although it is preferred that plate type heat exchangers be used.
[0015) Plate typa heat exchaagera are marketed by many suitable suppliers.
Printed circuit heat exchanges are a type of plate heat exchanges and are markeM~d by HEATRIC. Printed circuit heat exchangers are extremely compact, high cftu;ioncy heat exchangers which readily tolerate high pressure and have extreme temperature capabilities. The printed circuit heat exchangers basically are prepared by etching a flow path into a plate with a matching flow path being etched into another plaw with the two plates then being joined, as kaowa to those skilled in the art. 'Ihe plates are stached and may be diffusion bonded and also welded if desired. These plates caa have a wide variety of heat exchange paths and are considered to be well known to those skilled in the art. Because of their high effici,cncy and compact configuration, plate heat exchangers ara preferred for use in the process of the ptesctt invention. They are also configured to provide certain advantages with respect to their cleaning and use generally when seawater is used as a heat exchange tnaterial.
[0016] It should be noted that not only seawater but any other suitable liquid or vapor which is warm relative to the cryogenic liquid and at a temperature sufficient to vaporize the intientiatiate refrigerant can be used as a heat exchange fluid in the setond . . ,,. ...,..n...+
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heat exchanger. Some such materials are freshwater, seawater, light hydrocarbons, steam, air, quench water and refinery waste heat streams and the like.
[00171 Desirably the intersnediate refrigaant is a material such as propane, mixed refrigerants, fluorocarbon rebrigerants, chlorofluorocarbon refrigerants, such as the family of FREON re&igerants produced by DuPont and the lilae, which do aot fraZe at cryogenic temperatures, ie., below -100 F. A prinmy criterion in the intarmodiate refrigeraat is that it be readily vaporized by the heat exchange fluid available and that it be effective to convey heat to the first heat exchanger and condenso in beat exchango contact with the cryogenie liquid. Since most intermediate refrigerants will readily condense at the temperature of the cryogenic liquid, a primary consideration is the ready vaporization of tho intermediate re&igerant by the heat source available. It is fiwthar desirable that the intermediate refrigerant rentain liquid in contaot with the heat exchange surfaccs in contact with the liquid cryogenic fluid in the first heat exchanger. The refrigerattts named are considered to meet these criteria. Ccrtain of the refrigerants may be more desirable than others for certain applications.
[00181 In ahernate embodiments of the present invention, shown for instance in FIG 2, a plate type heat exchanger may be used for both the first heat exchanger 12 and the second heat exchanger 40. The flow through these plate type heat exchangers is as discussed previously. For instanec, the intq'mediate refrigerant vapor is produced from an intermediate refrigetant vapor outlet 42 from the second heat exchanger and passed through a line 43 to an inlet 38 to first exchaoger 12 where a cryogenic fluid is introduced through a line 14 and recovered through line 16 in a revaporized or partially revaporized form. The condensed interntediate refcigorant is recovered through an outlet 36 from flrst heat exchanger 12 and passed via a line 45 to second heat exchanger 40.
[00191 As shown in FIG 2, a headng fluid is introduaod through an inlet 46 and a line 47 and discharged via an outlet 48 through a line 49. The operation of tha first and second heat exchanger in combination is as discussed previously with the condensed liquid intermediate re&igerant discharged through outlet 36 supplying the necessary fluid head for movement of the intermediate refrigerant through line 45 to inlet 44 to second heat exchanger 40. The vapor is discharged through line 42 and line 43 to inlet 38 with the operation of the refrigerant flow being completely by gravity by a thermal siphon type process. This type process presents significant advantages in that no pumps are necessary for the circulation of the intermediate refrigerant, although a pump could be used if desired. Since the m-frigerant is a material not eeadily frozen in centact with the . ~. .... ......a.. ~.~.~.~.~_..~ ,.~ .

heat exchange surfaces containing the cryogenic liquid and is readily vaporized in the second heat exchanger, an efficient heat transfor is accomplished without exposing the heating fluid in line 47 to direct contact with heat exchange surfaces containing the cryogenic liquid. This is a signiticant advantage with respect to the fn,ezing of the heat exchange fluid during periods of slow heat exchange fluid flow or high flows of cryogenic fluid.
(0020] Desirably outlet 36 is placed suffieiently above inlet 44 to provide the necessary heat for the desired flow. The height is typieally at least about two feet and is preferably from about two to about tat feet. More preferably, the height is at least about six feet and desirably fr' ocn about six to about ten foat.
[0021) In FIG 3, another embodiment ls shown. In this embodiment, a superheater 54 is used with a second heatiog nudium suppliod through a line 56 and reeovered through a line 58 to suporhmt the intermediate refiigerant vapor, which is then passed through an outlet 42 fim supedmtor 54 through a iine 43 into an inlet 38 to first heater 12. 'Ilu liquid refrigerant is retumed as discussed previously through line 44 to second heater 40.
(0022] In FIG 4, a fiuthar embodiment is shown whemin a superheater 60, heated by heating material supplied through lino 56 and recoverod through line 58, is used to superheat the recovered cryogenic materiat which has been liquefied in heat exchanger 12 (downstream). In other aspecb the flow of mamrud is as described previously.
[0023] In FIG 5, a fuRher embodiment is shown where a separator 62, having a liquid level 64, is used to ensura the sepamtion of liquid and vapor from the streams in lines 43 and 45. The liquid refrigerant passed from outlet 36 through line 45 to separator 62 is desirably all liquid. Accordingly, this stream Is introduced into seperaWr 62 below the liquid level 64 in separator 62. Similarly, the stream recovered via outlet 42 and passed through line 43 to separator 62 is desirably all vapor. This stream is introduced into separator 62 at a level above liquid level 64 and a vapor stream is then passed onward through line 43' to inlet 38 to vessel 12. Similarly, a liquid stream is recovered from separator 62 and passed through line 45' to inlet 44 to heat exchanger 40. This embodiment ensures that the vaporous intermediate refrigerant is passed as a vapor to first exchanger 12 and that the liquid intermediate refrigerant is passed to second heat exchanger 40 as a liquid.
100241 It should be understood that liquid may be entn3ined with the vapor passed to fust heat exchanger 12 and that vapor may be absorbod or contauned in the . . . .,.. _,_ ......:w ,.... ..rx..~,...:,,.,i_ . . . . . .

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intermediate refrigerae don stream passed back to second heat exchanger 40.
Such inclusion of liquid or vapor does not aEfect the operation of either vessel significantly since the vessels each act substantially as a sepan+tion vessel in their own right, as well as htat exchengers.
[0025) Accordingly to the present invention, an intermediate refrigenint is used which is not prone to freeze on heat exchange surfaces in contact with the cryogenic liquids. Clearly when a maOrxial attch as seawater is used as a heat exchange material, there is always a risk that seawater in contact with heat exahange surfa<xs contaating cryogenic materials may freexe, thus obstructing the passage of additional heat exchange material and resulting In even greater freezing of the vassel. Since the freezing may be relatively sudden considering the radical diiference in tempensture betwecn the seawater and the cryogenie materials, this can result in substatttial damage Lo heat exchange surfaces in a very short period of time. These problems are avoided by the present invention wherein an intermediate refrigerant resistant to freezing in contact with surfaces contacting cryogenic liquids is used.
(0026] Further the plate type heat exchangers used in the present invention are very readily cleaned in the event that pollution of the heat exchange surfaces occurs as a result of the passage of the seawater. The occurrence of pollution is minimized because the differential temperature across the hmt exchange surfaces is much less.
Further the environmental problems resulting from discharging aeawater at a very low temperatm+e into the sea are avoided. By the present invention, the heat exchange can be a lower temperature since the heat of vaporization is supplied by seawater which can be used readily in larger volumes since the plabe type heat exchangers are very efficient and occupy a relatively small area. Since the heat of vaporization is transferred to the cryogenic fluid, a greater heat transfer can be achieved than if only sensibte heat were available for transfer to the cryogenic liquid. Further, the present invention reduces the need for pumping an inLemiediate refrigerant, then:by making the process more energy efficient.
[0027] By positioning the heat exchangers so that the second heat exchanger is below the first heat exchanger and so that the superheaters are above the first heatr and the second heater respectively, much less area is required for the installation of a revaporizing system having sufficient capaGity to handie large quantities of cryogenic liquid. Further, systems of these types could readily be placed side by side so that the appropriate number of systems could be used to vaporize a desired cryogenic fluid at a , . . ,... .N. ..,.i..Jl..il.iõ. u .~i ii..u..~y u..i 11i 1 . . .

desired rate. It is clear that the inlet and outkt from the seeond heat exchanger could be positioned to draw seawater from a substaatiai distapce from the platform and dischacge it a substaiitial distance from a platform or other facility.
[00281 In summary, the prqscet invention has provided a highly efficient and high effective method and system for navaporiziag a eryogenic liquid by the use of an intermediate refi'igerant, which is not pmne to tlm pmblans associated with the use of most commonly used heat exchaage materials used to ravaporixe cryogenic liquids.
[0029] While the present invention has been described by referenee to certain of its preferred embodiments, it is pointed out that the embodiments described aro illustrative rather than 1'uniting in nature and that many variations and modifications are possible within the scope of the prosent invention. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments.

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Claims (15)

1. A method for vaporizing a cryogenic fluid using a naturally circulating intermediate refrigerant, the method comprising:
a) passing the cryogenic liquid in heat exchange contact with a vaporous intermediate refrigerant in a first heat exchanger having a vaporous intermediate refrigerant inlet and a liquid intermediate refrigerant outlet to heat the cryogenic fluid to produce a gaseous cryogenic fluid and a liquid intermediate refrigerant;
b) passing the liquid intermediate refrigerant in heat exchange contact with a heating fluid in a second heat exchanger having a liquid intermediate refrigerant inlet and a vaporous intermediate refrigerant outlet to heat the intermediate refrigerant to produce the vaporous intermediate refrigerant, the first heat exchanger being above the second heat exchanger;
c) allowing the vaporous intermediate refrigerant to rise into the first heat exchanger; and, d) allowing the liquid intermediate refrigerant to flow downwardly into the second heat exchanger;
wherein the first and second heat exchangers are plate type heat exchangers.

2. The method of claim 1 wherein the cryogenic fluid is liquefied natural gas.

3. The method of claim 1 wherein the first heat exchanger is a printed circuit heat exchanger.

4. The method of claim 1 wherein the heating fluid is seawater.

5. The method of claim 1 wherein the refrigerant comprises at least one of propane, a mixed refrigerant, a fluorocarbon refrigerant and a chlorofluorocarbon refrigerant.

6. The method of claim 1 wherein the cryogenic fluid is further heated in a third heat exchanger downstream from the first heat exchanger.

7. The method of claim 1 wherein the intermediate refrigerant is further heated in a fourth heat exchanger between the second heat exchanger and the first heat exchanger.

8. The method of claim 1 wherein the liquid intermediate refrigerant outlet from the first heat exchanger is placed sufficiently above the liquid intermediate refrigerant inlet to the second heat exchanger to assure natural circulation of the refrigerant.

9. The method of claim 1 wherein the liquid intermediate refrigerant outlet from the first heat exchanger is at least about two feet above the liquid intermediate refrigerant inlet to the second heat exchanger.

10. A system for vaporizing a cryogenic liquid using a naturally circulating intermediate refrigerant, the system comprising:
a) a first heat exchanger having a liquid cryogenic fluid inlet, a vaporized cryogenic fluid outlet, a vaporized intermediate refrigerant inlet and a liquid refrigerant outlet;
b) a second heat exchanger having a liquid refrigerant inlet, a vaporized refrigerant outlet, a heating fluid inlet and a heating fluid outlet, the first heat exchanger being positioned above the second heat exchanger with the vaporized intermediate refrigerant inlet to the first heat exchanger being in fluid communication with the vaporized intermediate refrigerant outlet from the second heat exchanger and with the liquid intermediate refrigerant outlet from the first heat exchanger being in fluid communication with the liquid intermediate refrigerant inlet to the second heat exchanger;
wherein the first and second heat exchangers are plate type heat exchangers.

11. The system of claim 10 wherein the liquid intermediate refrigerant outlet from the first heat exchanger is placed sufficiently above the liquid intermediate refrigerant inlet to the second heat exchanger to assure natural circulation of the refrigerant.

12. The system of claim 10 wherein the liquid refrigerant outlet of the first heat exchanger is at least about two feet above the liquid refrigerant inlet into the second heat exchanger.

13. The system of claim 10 wherein a third heat exchanger is positioned in fluid communication with the vaporized cryogenic fluid outlet to heat the vaporized cryogenic fluid.

14. The system of claim 10 wherein a fourth heat exchanger is positioned in fluid communication with the vaporized intermediate refrigerant outlet to heat the vaporized intermediate refrigerant.

15. The system of claim 10 wherein a vessel is positioned in fluid communication with the liquid intermediate refrigerant outlet, the vaporized intermediate refrigerant outlet, the liquid intermediate refrigerant inlet, and the vaporized intermediate refrigerant inlet to separate vaporized and liquid intermediate refrigerant for passage to the vaporized intermediate refrigerant inlet and to the liquid intermediate refrigerant inlet respectively.