US3446029A - Method for heating low temperature fluids - Google Patents

Description

y 27, 1969 Y D. A. GRGURICH ET AL 3,446,029

METHOD FOR HEATING LOW TEMPERATURE FLUIDS Filed June 28, 1967 No 00 A u g :x/: mm 3 3 r mm A A ow A NF 2 8 3 R Q 1/ 3 8 2 R R 8 2 7 i mm on Q. r Y I L F w $2 Q a 3 8 5 n a, a V J J 4 2 A .!O I NW Q A l n 1 2 Q o H 00 2 a INVENTORS D. A. GRGURICH W. K. JOHNSTO W. EISSMAN PATENT ATTORNEY United States Patent 3,446,029 METHOD FOR HEATING LOW TEMPERATURE FLUIDS Donald A. Grgurich, Morristown, William K. Johnston, Parsippany, and Walter Weissman, Morristown, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed June 28, 1967, Ser. No. 649,706 Int. Cl. F17c 7/02 US. C]. 62-52 Claims ABSTRACT OF THE DISCLOSURE This invention provides a method for heating low temperature fluids by using a recycled equilibrium process fluid as a heat exchange intermediate. By using this process fluid rather than a separate intermediate fluid, advantages are gained, in that, less heat exchange area is required, no separate handling facilities are needed for the intermediate fluid, and intermediate fluid losses are reduced.

BACKGROUND The present invention relates generally to a method for heating low temperature fluids. More particularly, the process of the instant invention provides a method for heating low temperature fluids by using a recycled equilibrium process stream as a heat exchange media. The teachings of the instant invention are particularly applicable in the regasification of liquefied natural gas, hereinafter referred to as LNG.

Natural gas is often available in areas remote to Where it will be ultimately used. Quite often the source of this fuel is separated from the point of utilization by a large body of water and it then may prove necessary to effect bulk transfer of the natural gas by large tankers designed for such transport. Under these circumstances, economics dictate that the natural gas be liquefied so as to greatly reduce its volume and that it be transported at substantially atmospheric pressure. Under these conditions the LNG is at a temperature in the range of 258 F. This temperature represents the boiling point of methane at atmospheric pressure. It is to be noted, however, that the LNG often contains amounts of heavier hydrocarbons such as ethane, propane, butane and the like. These will vary the boiling range of the LNG so that it usually will fall somewhere between 240 F. and -258 F. However, nitrogen which is also typically present may cause the low end of the temperature range to get down as low as 270 F.

When the LNG arrives at the point of utilization, it is, of course, in liquefied form and, consequently, it becomes necessary to regasify it before it is used as a fuel. It is often highly desirable to utilize some of the higher molecular weight constituents as raw materials and as feedstocks for the production of various petrochemicals and for the manufacture of liquefied petroleum gas. In addition, it may prove necessary to adjust the heating value of the natural gas to conform with local requirements prior to its entrance into the actual fuel distribution system. This adjustment is conveniently achieved by the use of a reforming operaion of the type to be more fully discussed hereinafter.

It will be appreciated that the reconversion of the LNG to a gaseous form requires the addition of a substantial amount of heat. When heating a cryogenic stream, such as LNG, many problems, both of an economic and technical nature, are often encountered. For example, when heating such streams, direct heat exchange is only feasible with heating streams which are dry since any 3,446,029 Patented May 27, 1969 moisture present would freeze out and deposit and eventually block the heat exchangers employed. Coupling this with the fact that the least expensive heat sources are usually wet (e.g. cooling Water, air and flue gas), it is not surprising that problems in this area are often encountered. In the past problems of this nature have been solved by using a separate intermediate heat exchange fluid contained in a closed cycle or the like. The function of this type of intermediate fluid system is to absorb heat from an available hot wet heat source and reject it to the cold LNG stream. However, there are numerous economic disadvantages which accompany such a system. For example, heat exchanger area requirements are, in effect, doubled since the number of needed exchangers is twice as high. Furthermore, separate facilities are required to store, handle and possibly even produce the separate intermediate exchange fluid.

In contrast, the teachings of the instant invention provide a method for heating cryogenic fluids such as LNG while avoiding the above-mentioned problem. Thus, a utilization of the method to be herein discussed results in a substantial reduction in heat-exchange area require- SUMMARY OF THE INVENTION According to the instant invention the above highly desirable results may be achieved by using a recycled equilibrium process rfluid in place of an intermediate heat exchange fluid. In a preferred embodiment of the invention, the LNG regasification is accomplished in combination with a reforming operation of the character to be hereinafter discussed. Thus, a cold liquid LNG stream is heated by direct contact with an equilibrium recycle stream which has, itself, been heated in an exchanger carrying the hot products of the reforming operation. According to applicants process, as exemplified in this particular embodiment, the LNG feed is pressurized and partially heated and then flashed in a series of three flash drums. The cold liquid from the second of said drums is heated to a temperature in the range of approximately 25 F. to F. by direct contact with a hot equilibrium recycle stream and the resulting mixture flows into the third drum. The equilibrium LNG recycle is obtained by drawing off a portion of the stream from the bottom of this third drum and heating it by use of one or more external process streams such as one or more of the hot streams available from the reforming operation. Some of the heat of the equilibrium recycle stream is then rejected to the incoming LNG feed in a series of exchangers and the remainder is rejected to the LNG entering the aforementioned third drum. By operating this third drum in the aforementioned temperature range and preferably about 30 F., freezing out is avoided in the exchange operation with the reformate streams. By direct injection of this recycle stream into the process stream entering the third drum, it is thus possible to effeet a considerable amount of LNG heating without additional exchanger area.

Accordingly, an important object of this invention is to provide an eflicient method for heating a cold fluid without the use of a separate intermediate heat exchange media.

Another important object of this invention is to facilitate the regasification of LNG so that it is in a suitable condition for delivery and use as a fuel.

Still another object is to provide a method in which LNG is heated by direct contact with a recycled equilibrium process fluid in place of separate intermediate heat exchange media.

Yet another object is to provide a system which requires less heat exchange area and no separate intermediate heat transfer fluid facilities thereby effecting substantial economies in equipment costs.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawing.

In the figure a flow diagram illustrating a process according to the present invention is given.

Referring to the figure in detail, reference numeral 2 designates a stationary insulated storage tank which rereceives the LNG at atmospheric pressure from a tanker (not shown). The LNG in tank 2 will normally have a temperature of about 240 F. to 260 F. and may have, for example, the following composition range.

Table I Mole percent 65-85 7-17 5-12 The LNG from tank 2 is fed through the line 3 into pump 4 wherein its pressure is increased from about 15 p.s.i.a. to 1,100 p.s.i.a., preferably about 1,000 p.s.i.a. Pump 4 causes the pressurized LNG feed stream to flow through a series of heat exchangers, 8, 12 and 18, to be further discussed hereinafter. Upon exiting from exchanger 18 via the line 20 the LNG stream has been heated to a temperature in the range of 40 F. to F. It is then fed into a first flashing drum 22. Drum 22 may be operated in the range of 1,000 to 1,100 p.s.i.a. with a preferable range of from 1,000 to 1,050 p.s.i.a. The flashed vapors from drum 22 will be mostly methane and will have a composition range such as shown in Table H, depending on the composition of the initial feedstock, and the exact operating temperatures and pressures of the drum.

Table II Mole percent or percent by volume Methane 80-90 Propane 1-5 N +lighter 0-2 Butane 0-2 Ethane -15 From drum 22 these vapors are fed via conduit 24 to distribution pipeline 100. These vapors will represent about 40% to 80% of the original moles entering the LNG feed system. The liquid bottoms from drum 22 pass via line 26, valve 28 and line 30 into a second flashing drum 32. Drum 32 is operated in the pressure range of 200 to 500 p.s.i.a. with a preferable range of about 250 to 350 p.s.i.a. The liquid bottoms from drum 32 pass via the line 36 into a third flashing drum 42. This drum is operated in the pressure range of 200 to 350 p.s.i.a. with a preferable range of about 250 to 300 p.s.i.a. The equilibrium stream leaving the bottom of drum 42 is split into two fractions designated a and b, each having a temperature of about 25 F. to 80 F. Fraction a will normally be in the range of from 3 to 5 times as large as fraction b, and will vary somewhat depending upon the original composition of the entering LNG and the exact operating temperatures and pressures of drums 22, 32 and 42. However, since fraction b is held constant, the amount of fraction a is primarily dependent on the temperature of the heat source in exchanger 50. Upon its exit from drum 42 fraction a, which will have a temperature in the range of 25 to 80 F. and preferably about 30 F., is drawn into pump 46 and conducted via line 48 through the heat exchangers 50, 60 and 18. The temperature of fraction a as it leaves exchanger 18 will be in the range of 50 F. to 100 F. and preferably about 70" F. From exchanger 18 it is then injected into the line 36 at point X where it rejects its heat directly to the main LNG process stream so that the combined stream entering drum 42 will have a temperature in the range of about 25 F. to F. By operating drum 42 in this temperature range and preferably in the range of 30 F. to 40 F., freezing out of moisture in the tubes of the reformate heat exchanger 50 is avoided. The upper operational temperature of drum 42 corresponds to the dew point of the fluid entering therein.

It will be understood that as long as the heat flowing into and out of fraction a recycle stream is in reasonable balance, no restrictions need be placed on the degree of vaporization which takes place in the recycle circuit. That is, when fraction a is injected into the cold LNG at point X, it can be either in the liquid, vapor or mixed phase. Here again, those skilled in the art will recognize a further improvement over conventional separate intermediate heat exchange fluid systems which require, in addition to good heat balances, that the intermediate fluid be only in the liquid phase at the accumulator drum. To insure this, the conventional systems must either use compression devices or vent the portion of the intermediate fluid that has vaporized and then supply additional liquid makeup to the heat exchange system. In either event the result would be a more complicated and more expensive system than is available using the teachings of the instant invention.

. Under certain conditions, e.g. wide changes in feedstock composition, it is deemed advantageous to be able to bypass all or a portion of fraction a around point X. This may be accomplished by closing the valve 57 and opening the valve 59, which allows fraction a to enter the drum 47 through the line 53. By regulating the valve 49, in conjunction with valves 57 and 59, the change in the composition of fraction a, resulting from a change in the initial feedstock, may be made to take place over a longer period of time, thus giving plant operators more time to make adjustments. The recycled intermediate (i.e. fraction a) is eventually brought to equilibrium over this larger time period, at which point valve 59 is closed and the operation proceeds as originally discussed.

The vapors from drum 42 leave via line 40 and are then combined with fraction b. This combined stream, which represents approximately 10% to 30% of the original feed, enters a distillation column 56 via the line 58. The composition of the feed column 56 will range as indicated in Table III, again depending on the initial LNG feed composition and the various operating temperatures and pressures hereinbefore mentioned.

Vapors (mostly C and C leave the top of column 56 via line 70 and pass through heat exchanger 12 where they serve to heat the main LNG feed stream from approximately 140 F. to about F., and in so doing are partially condensed and collected in the separator drum 74. The liquid from drum 74 is refluxed via pump 78 back into the column 56. The vapors from drum 74 are combined with the overhead from drum 32 in the line 92. These vapors serve to add additional heat to the incoming LNG stream in heat exchangers 8 and 60. Upon their exit from the exchanger 8 as liquid, their pressure is increased to approximately 1,000 p.s.i.a. by pump 94. After leaving exchanger 60 as vapor, these vapors are passed to pipeline and are ready for distribution.

The bottoms from column 56, pass through a reboiler 63 and will have the following composition range,

5 c 040 c 40420 c, 20-60 5-30 are split into three fractions. The first fraction is recycled TABLE VI Typical stream composition (mole percent) Tempera- Pressure Location ture F.) (p.s.i.a.) C1 C2 C3 Line 3 -250 66. 7 17. 0 1'1. 1 Line 24. -15 1, 000 84. 0 10. 7 3. 7 Line 30. -s5 250 53. 2 21. 9 16. 9 Line 36 +30 250 21.3 32. 0 31.6 Line as +30 250 21. 3 32 31.6 Line as. +50 425 67. 0 Line 100 +50 1, 000 74. s 11.3 1.6

1 Before point X.

back into the column via the line 64. This fraction repre- It is to be understood that the foregoing arrangement sents about 5 t 7 and normally about /3 of the can be modified in various details and need not necessartotal bottoms product. The second fraction, representing ily be restricted to the processing of natural gas, per so. from about 5% to 10% and normally a ut /20 0 the The flow diagram and description are given by way of total bottom product is fed t0 distribution pipeline 100 example for the purposes of illustrating more clearly how via the line 68. The third fraction, which represents the invention may be performed. Moreover, the operating from 25% to 35% of the bottoms Pr duct of column 56 percentages, temperatures and pressures specified hereinand pr fer ly about /10 0f the number of Original LNG above can be varied considerably for given mixtures. Acfeed moles, passes via the line 66 to a reforming complex cordingly, reference should be made to the following aprepresented by the box 80. pended claims in determining the full scope of the inven- The composition of the material in the line 66 which tion, comprise the heavy ends of the LNG feed will be in the What is claimed is: following range: 1. A process for regassifying a feed of liquefied natural gas comprising in combination the steps of: Table Iv (agepiressuring and heating said liquified natural gas (b) flashing said feed in a series of flashing drums; C Mole 3 3 (c) drawing of a portion of the stream from the bottom 2 n n of the last drum in said series; C 45 65 C 2040 (d) heating said portion by use of an external process 40 stream;

5+ (e) using the heated portion resulting from step (d) In t reform-ing Operation this composltlon 13 in at least one heat exchanger to further heat the verted so that the reformate will have a composition as i i natural gas f d; and h indicated in Table V. (f) directly combining said portion with the material entering said last drum.

Table V 2. The process of claim 1 further characterized in that the heating of step (a) is partially accomplished by utilizing heat from the vapors leaving the top of the next to Mole Percent last drum in said series. CO 3 7 3. The process of claim 1 wherein the external process CO 1 2 stream of step (d) is a hot reformate gas stream obtained H 2040 by reforming the heavy ends of said natural gas feed. CH 6070 4. In a process of the type wherein a liquefied natural gas feed is regasified, fractioned and subjected in part to A great deal of 2 Comprising about 50% 0f the a reforming operation, the improvement which comprises total molar flow will also be present. This water is rethe following steps in combination: moved after the reformate gas steam in line 821s heat ex- (a) pressuring and heating Said li fi d natural gas changed with the equilibrium recycle fraction (1 1n exfeed; changer 50 as hereinbefore discussed. After this heat ex- (b) flashing Said pressurized and heated feed in a series change the reformate gas is ultimately fed v1a the line 86 of flashing drums; to a Compressor 88 Where pressure raised to (0) drawing of a portion of the stream from the bottom 9 l and 18 then fed via hue 90 mm of the last drum in said series, said portion being e dlstrlbuflon plpelme in equilibrium with the liquid in said last drum;

- (d) heating said portion by passing it in indirect heat EXAMPLE exchange with a hot process stream from said reforming operation; (e) passing the heated portion resulting from step (d) Stamng wlth a typlcal LNG feedstock fiz i :f throught at least one heat exchanger to further warm C 66] 70 the incoming natural gas feed; and 170 (f) directly combining said portion with the material t: 11,1 stream entering said last drum whereby said last men- 4.2 tioned stream is further heated.

5 5. The process of claim 4 further characterized in that N2 and other 0,4 the heating in step (a) is accomplished by indirect he exchange with process streams resulting from the frac- 2,907,176 10/1959 Tsunoda 62--52 tionation of said liquefied natural gas feed. 3,091,096 5/ 1963 Rendos 62-52 References Cited LLOYD L. KING, Primary Examiner.

UNITED STATES PATENTS 5 US Cl. X'R 2,037,673 4/1936 Zenner 62-53 62 53 2,362,968 11/1944 Bliss 62-53

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