US3918265A - Compensation of refrigeration losses during the storage of liquefied, low-boiling gaseous mixtures - Google Patents

Abstract

A process for the compensation of the refrigeration losses to the surroundings during the storage of liquefied, low-boiling gaseous mixtures, e.g., natural gas, in a storage vessel or a tanker, comprising withdrawing the liquefied gaseous mixture from the storage means, subcooling said liquefied mixture, and recycling resultant subcooled liquid into the storage means, the refrigeration value of the subcooled liquid being sufficient to compensate for the loss of refrigeration values due to heat from the surroundings. Savings in energy are obtained by virtue of the relatively high temperature of subcooling the liquid, as compared to the temperature required to condense vapor in equilibrium with the stored liquefied gas, and containing a substantial proportion of very low boiling components.

Description

United States Patent Etzbach et al.

COMPENSATION OF REFRIGERATION LOSSES DURING THE STORAGE OF LIQUEFIED, LOW-BOILING GASEOUS MIXTURES Inventors: Volker Etzbach, Munich; Karlheinz Muller, Egling, both of Germany Linde Aktiengesellschaft, Wiesbaden, Germany Filed: Dec. 10, 1973 Appl. No.: 423,395

Assignee:

Foreign Application Priority Data Dec. ll, 1972- Germany 2260516 References Cited UNITED STATES PATENTS 4/1974 Delahunty 62/54 Primary E.ran'zinerWilliam F. ODea Assistant ExaminerRonald C. Capossela Attorney, Agent, or Firm-Milieu, Raptes & White [57] ABSTRACT A process for the compensation of the refrigeration losses to the surroundings during the storage of liquefied, low-boiling gaseous mixtures, e. g., natural gas, in a storage vessel or a tanker, comprising withdrawing the liquefied gaseous mixture from the storage means, subcooling said liquefied mixture, and recycling resultant subcooled liquid into the storage means, the refrigeration value of the subcooled liquid being sufficient to compensate for the loss of refrigeration values due to heat from the surroundings. Savings in energy are obtained by virtue of the relatively high temperature of subcooling the liquid, as compared to the temperature required to condense vapor in equilibrium with the stored liquefied gas, and containing a substantial proportionof very low boiling components.

8 Claims, 1 Drawing Figure M] an 7,

US. Patent Nov. 11, 1975 3,918,265

COMPENSATION OF REFRIGERATION LOSSES DURING THE STORAGE OF LIQUEFIED, LOW-BOILING GASEOUS MIXTURES BACKGROUND OF THE INVENTION This invention relates to a process for the compensation of refrigeration losses during the storage of liquefied, low-boiling gaseous mixtures, and is especially applicable to mixtures having widely spaced apart boiling points of the individual components.

For the compensation of evaporation losses during the transportation of liquefied gases, a process is known for the recondensation of gases evaporating from a plurality of storage tanks containing liquefied gases (DAS[German Published Application1No. 1,086,225). According to this process, the evaporated gas from each of the storage tanks is liquified in a closed cycle by heat exchange with a liquid medium, the temperature of which is below the boiling point but above the solidification point of the liquefied gases. The liquified gas is recycled into the same tank from which it evaporated. To compensate for refrigeration losses of stored liquid gas mixtures, this conventional process requires a considerable expenditure of energy because equilibrium considerations dictate that the vapor phase above the liquid bath in the storage tanks is much more enriched in low-boiling components than the liquid itself.

SUMMARY OF THE INVENTION An object of this invention is to provide an improved energy-saving and stucturally simple process for the compensation of the refrigeration losses during the storage of low-boiling, liquefied gaseous mixtures in at least one storage tank.

Upon further study of the specification and appended claims, further objects andadvantages of the invention will become apparent to those skilled in the art.

To attain these objectives, a system is provided comprising withdrawing liquefied gaseous mixtures from the storage tank, subcooling same in heat exchange with an auxiliary refrigerant, and recycling resultant subcooled mixture into the storage tank.

By virtue of this invention, it is possible to provide makeup refrigeration without condensing evaporated liquefied gaseous mixtures. Thus, it is unnecessary to condense at low temperature levels a vapor fraction highly enriched in the low-boiling components of the mixture. Thus, an important savings in energy is gained because the subcooling of the mixture present in the liquid phase can be conducted at a considerably smaller temperature difference (AT) between the subcooled liquid and the stored liquid than between the condensed vapor and the stored liquid. This concept of subcooling liquid instead of evaporated vapor is generally unorthodox in the storage of liquefied gases because it is well-known that the overall heat transfer coefficient is larger and thus the size of heat exchanger is smaller when condensing vapors. Thus, while the present invention is contrary to conventional engineering, energy is conserved.

The present invention is applicable to the storageof all liquefied gases which would otherwise lose refrigeration value because of warming by the surroundings. The invention is especially valuable in connection with liquefied gases having a boiling point less than 40 C preferably less than 150 C. Also, the invention is particularly important when treating gaseous mixtures having about 95 to 0.1percent, preferably 15 to 0.5percent of a component boiling lower than the boiling point of the mixture.

The process of this invention is especially advantageous for the compensation of the cold losses of stored liquid mixtures wherein the boiling points of the individual components are widely spaced apart, since in such mixtures the temperature difference between the boiling point of the liquid and the boiling point of the vapor phase in equilibrium with the liquid is particularly large. Thus, for example, the boiling point of a liquid natural gas composed of about 99 molar percent of methane and 1 molar percent of nitrogen, stored under a pressure of about 1 atmosphere absolute, is approximately 107 K. The vapor phase in equilibrium with the liquid natural gas consists of about molar percent of methane and about 20 molar percent of nitrogen having a boiling point at approximately 86 K. Accordingly, the present invention is especially advantageous when the boiling point difference between two components of the mixture are at least 10C, preferably at least 20C, and most preferably at least 30C. Stated in another way, the invention is particularly important when the difference between the boiling points of the stored liquefied gases and vapor in equilibrium therewith is at least 1C, preferably at least 5C, and most preferably at least 20C.

A further advantage of the present process resides in that the cooling curve of the liquid gaseous mixture can be adapted more efficiently to the warming curve of the auxiliary refrigerant than is possible in case of the corresponding curves of the vapor mixture and the auxiliary refrigerant. Therefore, the heat exchange itself required to compensate for the refrigeration losses can be effected, according to the process of this invention, with lower temperature differences, which again is particularly advantageous from an energy viewpoint. Thus there are two energy savings one from the temperature level of refrigeration which is lower in the present invention, and the other is the lower temperature difference during the heat exchange with the auxiliary refrigerant.

When a liquefied nitrogen-containing natural gas is being stored, it is advantageous for the auxiliary refrigerant to be nitrogen, which can be cooled in a closed Claude refrigeration cycle, for example, or in any other conventional refrigeration cycle.

In the event that the liquid gaseous mixture is stored in several storage tanks, for example in a tanker having a plurality of storage compartments or storage vessels, a preferred embodiment of this invention comprises placing the vapor spaces of the individual storage means in communication with one another and to withdraw liquid gaseous mixture from only one container. This liquid is then subcooled in heat exchange with the auxiliary refrigerant, and then distributed among the individual storage means so that the amounts of subcooled liquid mixture returned into the storage containers are just sufficient to compensate for the refrigeration losses caused in each instance.

The communication between the individual storage means is preferably conducted via the vapor spaces in each storage means. The reason for this is that a system consisting of several vessels can be considered as a single thermodynamic unit.

Because of the communication between the vapor spaces of the individual storage means, some vapor is necessarily going to be transferred between storage vessels so as to compensate for the loss of liquid from the storage means used to deliver liquid for subcooling purposes. Accordingly, to avoid vapor from accumulating in the just-recited storage means, the distributed subcooled liquid is recycled in larger amounts into this storage tank, thus condensing such vapors.

The advantages of this preferred embodiment reside in that there is required only a single heat exchanger for subcooling the liquid gaseous mixture, and only a single pump to convey this mixture from the storage tank to the heat exchanger and from there to the individual storage tanks. Any enrichment in lower-boiling component in the liquid phase of the storage tank where the pump is arranged, which is possible during full load cooling, is not deleterious in most cases.

Moreover, a still further advantage is realized insofar as after the storage tanks are emptied, a small amount of liquid need be retained in each tank, ordinarily, in order to avoid warming of the tank because of the heat transfer from the surroundings. By virtue of the present invention, only in the one storage tank from which the liquid gaseous mixture is withdrawn for subcooling is it necessary to keep liquid, rather than, as heretofore customary, in each of the storage tanks. Thus, in the pres ent system, all other tanks can be completely emptied. This is of significant economic importance in tanker operations because after the return voyage of the ship from the consumer (where the liquid gaseous mixture is unloaded) to the producer (where the ship is reloaded) a smaller quantity of liquid gaseous mixture can be left in the storage tanks, so that, in total, the useful storage capacity of the ship is better utilized.

In accordance with a further feature of this invention, it is advantageous to conduct the heat exchange of the liquid gaseous mixture with the auxiliary refrigerant in switching heat exchangers, so that any solids which may freeze out during this heat exchange, e.g., during the storage of liquid natural gas, higher-boiling hydrocarbons or solid CO do not result in a stoppage of the process since by conventional switching operations, a partially clogged heat exchanger path can be exchanged by a cleaned heat exchanger path.

DESCRIPTION OF PREFERRED ILLUSTRATED EMBODIMENT The drawing is a schematic view of a system comprising a plurality of individual storage means as found in tankers transporting liquefied nitrogencontaining natural gas.

Referring to the drawing, there is a first storage tank 1 and a second storage tank 2. When the tanker travels from the producer to the consumer, both tanks are filled almost completely with liquid natural gas. A pump 3 is disposed within a lower zone of the first storage tank 1, conveying liquid natural gas from this storage tank 1 through the valve 4a, which is open during this switching step, and the cross section 5a of the heat exchanger 6a. In the heat exchanger 6a, the liquid natural gas is subcooled in indirect heat exchange with lowtemperature nitrogen flowing in cross section 70 and derived from a Claude closed refrigeration cycle (not shown) and then recycled into the storage tanks 2 and 1 via valves 10 and 11. The valve 10 is set so that the amount of subcooled natural gas flowing through this valve is just sufficient to compensate for the refrigeration losses caused by the heat from the surroundings Q In contrast thereto, the setting of valve 11 makes it possible to convey a somewhat larger amount of sub cooled natural gas into the storage tank 1, so that there is the possibility, on the one hand, to compensate for the refrigeration loss due to Q and, on the other hand, to recondense the amount of natural gas flowing via conduit 12 from the second storage tank 2 into the first storage tank 1 and vaporized by Q To remove any solid deposits caused by frozenout CO or solidifying higher-boiling hydrocarbons, a warm flushing step is employed. Thus, there is provided a second heat exchanger 6b having a corresponding cross sections 5b and 7b, to which the process can be switched over during the flushing of the first heat exchanger. Filters 9a and 9b, connected downstream of the heat exchangers 6a and 6b, prevent any entrained solids from cross section 5a or 5b from entering into the storage tanks.

If the tanker is on its way back from the consumer to the producer, rather than as illustrated on its voyage from the producer to the consumer, it is sufficient to compensate for the refrigeration losses merely by leaving a small amount of liquid natural gas in the first storage tank 1, whereas the second storage tank 2 can be entirely empty. The amount of subcooled liquid natural gas conveyed through valve 10 into the second storage tank is metered so that it just compensates for the cold losses Q while completely evaporating. After this evaporation, the quantity of gas flows back into the first storage tank 1 and is there recondensed by heat exchange with subcooled natural gas. This latter heat exchange is conducted by direct heat transfer relationship.

From the foregoing description, one skilled in the art can easily ascertain the exxential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

What is claimed is:

l. A process for the compensation of the refrigeration losses to the surroundings during the storage of liquefied, low boiling gaseous mixtures in a plurality of storage means, each having a vapor space and a lower zone for said liquefied gaseous mixture, the vapor spaces thereof being in communication with each other, said process comprising withdrawing said liquefied mixture from only one storage means, subcooling said liquefied mixture, and recycling subcooled liquefied mixture into all of the plurality of storage means, with the provision that a larger portion of subcooled gaseous mixture is recycled into the storage means from which the liquefied gaseous mixture is withdrawn, then into each of the remaining storage means.

2. A process as defined by claim 1, wherein the boiling point of the stored liquefied gaseous mixture is less than 40C.

3. A process as defined by claim 1, wherein the stored mixture is natural gas containing a minor amount of nitrogen.

4. A process as defined by claim I, wherein said plurality of storage means comprise compartments of a liquefied natural gas tanker.

ference between the boiling point of the stored liquefied gaseous mixture and vapor in equilibrium therewith is at least 1C.

8. A process as defined by claim 1, wherein the subcooling is conducted by indirect heat exchange Contact with an auxiliary refrigerant.

Claims (8)

1. A process for the compensation of the refrigeration losses to the surroundings during the storage of liquefied, low boiling gaseous mixtures in a plurality of storage means, each having a vapor space and a lower zone for said liquefied gaseous mixture, the vapor spaces thereof being in communication with each other, said process comprising withdrawing said liquefied mixture from only one storage means, subcooling said liquefied mixture, and recycling subcooled liquefied mixture into all of the plurality of storage means, with the provision that a larger portion of subcooled gaseous mixture is recycled into the storage means from which the liquefied gaseous mixture is withdrawn, then into each of the remaining storage means.

2. A process as defined by claim 1, wherein the boiling point of the stored liquefied gaseous mixture is less than 40* C.

3. A process as defined by claim 1, wherein the stored mixture is natural gas containing a minor amount of nitrogen.

4. A process as defined by claim 1, wherein said plurality of storage means comprise compartments of a liquefied natural gas tanker.

5. A process as defined by claim 4, wherein the stored mixture is natural gas containing a minor amount of nitrogen.

6. A process as defined by claim 1, wherein the components of the stored gaseous mixture have a difference in boiling point temperatures of at least 10* C.

7. A process as defined by claim 1, wherein the difference between the boiling point of the stored liquefied gaseous mixture and vapor in equilibrium therewith is at least 1* C.

8. A process as defined by claim 1, wherein the subcooling is conducted by indirect heat exchange contact with an auxiliary refrigerant.

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