Process Of Complete Cryogenic Vaporization Of Liquefied Natural Gas

Abstract

Liquefied natural gas is cryogenically vaporized completely while being utilized as a cryogen for cryogenic heat-exchange process by vaporizing the liquefied natural gas by a first vaporization step at a temperature lower than -110.degree.C until the non-vaporized residue of the liquefied natural gas becomes a predetermined amount which is in a range between a critical percentage at which higher hydrocarbons in the residual liquefied natural gas are frozen out therefrom and a percentage higher by 8% than the critical percentage, separating the residual liquefied natural gas from the vaporized natural gas, conditioning the first vaporized natural gas to a temperature from -110.degree. to -40.degree.C and then, vaporizing the residual liquefied natural gas by a second vaporization step by introducing it into the conditioned first vaporized natural gas.

Description

The present invention relates to a process of cryogenic vaporization of liquefied natural gas, particularly, relates to a process of completely cryogenically vaporizing liquefied natural gas while utilizing it as a cryogen for cryogenic heat-exchange process.

It is well-known that the liquefied natural gas is vaporized at a very low temperature while being utilized as a cryogen for isolating ethylene gas from mixture gas generated in the ethylene producing process or for separating and liquefying oxygen and nitrogen from air. In all of the conventional processes, the liquefied natural gas is always vaporized in a single step of cryogenic vaporization wherein the entire amount of the liquefied natural gas is cryogenically vaporized while heat-exchanging with other fluid. For example, in the process of producing liquefied oxygen and nitrogen from air, it is necessary that the liquefied natural gas be vaporized at a temperature of about -161.degree.C under a normal pressure condition.

When the entire amount of the liquefied natural gas is vaporized by a single vaporization step at the very low temperature as stated above, it can be observed that it is possible to merely carry out the vaporization discontinuously because of the reasons detailed hereinafter. Such discontinuity of the vaporization naturally results in a discontinuity of the entire plant operations.

Generally, the liquefied natural gas is a mixture, the major part of which is methane and the minor part other hydrocarbons. The other hydrocarbons contain various higher molecular hydrocarbons having a relatively high boiling point. For example, Alaskan liquefied natural gas having a high content of methane, contains a small amount of various higher hydrocarbons as detailed in Table I.

Table I ______________________________________ Hydrocarbon Content (mol p.p.m.) ______________________________________ Propane 100 C.sub.4 hydrocarbons 99.2 C.sub.5 do. 41.1 C.sub.6 do. 54.4 C.sub.7 do. 66.7 C.sub.8 do. 11.7 C.sub.9 do. 0.1 ______________________________________

In the case where the cryogenic vaporization of the liquefied natural gas is effected at a temperature of -161.degree.C under normal pressure conditions in order to heat-exchange with notrogen gas to be liquefied during the vaporization for a few days, some portions of the higher hydrocarbons in the liquefied natural gas are frozen out on a heat-transfer wall surface so as to obstruct the heat-exchange between the nitrogen gas and the liquefied natural gas. This obstruction results in a decrease in the efficiency of the cryogenic vaporization of the liquefied natural gas and the cryogenic liquefication of the nitrogen gas. Additionally, since the vaporization of the liquefied natural gas has to be carried out at a very low temperature not exceeding -110.degree.C, for example, of -161.degree.C, the heat-exchanger for the cryogenic vaporization is different from the conventional heat-exchanger for ordinary use. This difference is in the complicated configuration of the heat-transfer wall surface and the excellent high heat-insulation for transfer of ambious heat thereinto. Therefore, the freeze-out of the higher hydrocarbons on the heat-transfer wall surfaces of the heat-exchanger is a fatal defect with regard to the continuous cryogenic vaporization process of the liquefied natural gas.

Accordingly, in order to remove the frozen out higher hydrocarbons from the heat-transfer wall surface, it is necessary to stop the cryogenic vaporization of the liquefied natural gas at intervals of time about 2 to 3 months and then, fuse away the frozen out higher hydrocarbons by heating them.

The higher hydrocarbons frozen out on the heat-transfer wall surface consist of various hydrocarbons having a relatively high freezing point of -100.degree. to 6.5.degree.C.

Further, after the cryogenic vaporization, when the frozen out hydrocarbons are fused and dissolved into the residual liquefied natural gas at a relatively high temperature, and then the resultant liquid is gradually cooled, it can be observed that the higher hydrocarbons firstly freeze out at a temperature from -80.degree. to -100.degree.C, but not at a temperature higher than this. The freeze-out contains, for example, the higher hydrocarbons as detailed in Table 2.

Table 2 ______________________________________ Hydrocarbon Content (% by weight) ______________________________________ Dimethyl cyclohexane 6.4 n-Heptane 5.6 Cyclohexane 5.1 Methyl cyclopentane 4.4 n-Hexane 4.1 Mixture of Methyl Cyclohexane and 2-2-Dimethylhexane 11.1 n-Nonane 2.3 1-tr-2-Dimethyl Cyclopentane 4.5 C.sub.9 Hydrocarbons 12.0 C.sub.10 Hydrocarbons 6.4 ______________________________________

The higher hydrocarbons have a relatively low vaporization property under normal pressure, and, therefore, are concentrated in the residual liquefied natural gas with the lapse of time of the cryogenic vaporization of the liquefied natural gas. When the concentration of the higher hydrocarbons, exceeds the maximum soluble in the liquefied natural gas under the cryogenic vaporizing condition, the higher hydrocarbons are separated from the residual liquefied natural gas and frozen out on the heat-transfer wall surfaces. That is, in the case where the entire amount of the liquefied natural gas is vaporized through a single step of cryogenic vaporization, the higher hydrocarbons are always frozen out on the heat-exchange wall surfaces as viscous and solid matters. This is an inevitable disadvantage of the conventional cryogenic heat-exchange process utilizing the cryogenic vaporization of the liquefied natural gas.

In an attempt to eliminate the disadvantage stated above, an improvement of the process has been provided. That is, prior to the cryogenic vaporization, the higher hydrocarbons are removed by flowing the liquefied natural gas through an adsorption column containing adsorbent for the higher hydrocarbons. However, such adsorption process has a technical disadvantage of difficulty of process control and an economic disadvantage of high cost of the adsorption column and process.

The object of the present invention is to provide a process of complete cryogenic vaporization of liquefied natural gas without freeze-out of higher hydrocarbons therein while utilizing the liquefied natural gas as a cryogen for cryogenic heat-exchange process at a high efficiency.

The above-stated object can be accomplished by the process of the present invention. According to the present invention, liquefied natural gas is vaporized by a first vaporization step at a temperature lower than -110.degree.C until the non-vaporized residue of the liquefied natural gas becomes a predetermined amount higher than a critical percentage with respect to the initial weight of the liquefied natural gas at which critical percentage higher hydrocarbons in the residual liquefied natural gas are frozen out therefrom, but not higher than a percentage of 8 percent above the critical percentage; the residual liquefied natural gas is separated from the first vaporized natural gas, the first vaporized natural gas is conditioned to a temperature of -100.degree. to -40.degree.C, and then, the separated residual liquefied natural gas is vaporized by a second vaporization step by introducing it into a flow of the conditioned first vaporized natural gas.

The object, features and advantages of the process of the present invention will be better understood by reading the following description and referring to the accompanying drawings, in which:

FIG. 1 is an explanation view of a device for determining the critical amount of the residual liquefied natural gas at which higher hydrocarbons being to be frozen out therefrom, and;

FIG. 2 is a flow chart showing an embodiment of the process of the present invention.

In the process of the present invention, the liquefied natural gas is first vaporized at a temperature lower than -110.degree.C until the non-vaporized residue of the liquefied natural gas becomes an amount higher than a critical percentage, with respect to the initial weight of the liquefied natural gas, at which higher hydrocarbons in the residual liquefied natural gas are frozen-out therefrom, but not higher than a percentage of 8 percent, preferably 3 percent, above the critical percentage.

The first vaporization of the process of the present invention is carried out at a temperature lower than -110.degree.C. The vaporization at a temperature higher than -110.degree.C results in a low merit of the liquefied natural gas when used as the cryogen for the cryogenic heat-exchange process.

When the residual liquefied natural gas becomes an amount as specified above, the first cryogenic vaporization of the liquefied natural gas has to be released. If the first cryogenic vaporization is released at an amount of the residual liquefied natural gas higher than a percentage of 8 percentage above the critical percentage, the first cryogenic vaporization results in a very low efficiency of the utilization of the liquefied natural gas as a cryogen for the cryogenic heat-exchange process. This is undesirable from the point of view of economy. Also, if the first cryogenic vaporization of the liquefied natural gas is contained until the residual liquefied natural gas becomes an amount lower than the critical percentage, the higher hydrocarbons are frozen out from the residual liquefied natural gas and deposited on the heat-transfer plate surface of the heat-exchanger. This causes the disadvantages detailed hereinbefore.

The critical percentage of the residual liquefied natural gas can be determined by using a device as shown in FIG. 1. Referring to FIG. 1, a gas tight Dewar vessel 7 for receiving liquefied natural gas is covered with a heat-insulating material 7a and contains therein a coiled tube 8 for flowing a heating medium therethrough. An end of the coiled tube 8 is joined to a tube 8a for feeding the heating medium from a heating medium supply source (not shown in the drawing), and the other end of the coiled tube 8 is joined to a tube 8b for discharging the heating medium. The Dewar vessel 7 is provided with a tube 9 for discharging the vaporized natural gas therethrough and a thermometer 10, a sensitive end 10a of the thermometer 10 being present in the Dewar vessel 7. A transparent window 11, for example, made of glass, is formed on the side wall of the Dewar vessel 7 to allow observation of the inside of the Dewar vessel 7.

The critical percentage of the residual liquefied natural gas is determined by the following procedure. The Dewar vessel 7 is charged with a predetermined weight of the liquefied natural gas and hermetically closed. A heating medium, for example, dry air at room temperature, is flowed through the feed tube 8a, the coiled tube 8 and the discharge tube 8b to vaporize the liquefied natural gas in the Dewar vessel 7, under predetermined temperature and pressure conditions. The vaporized natural gas is discharged through the tube 9. The amount of the liquefied natural gas remaining in the Dewar vessel 7 decreases with the lapse of time of the vaporization. When it is observed, through the transparent window 11, that the residual liquefied natural gas in the Dewar vessel 7 has become muddy and, at the same time, the thermometer 10 indicates an elevation of boiling point of the residual liquefied natural gas, the vaporization is stopped and the weight of the residual liquefied natural gas is measured. The critical percentage is a ratio of the measured weight of the residual liquefied natural gas to the initial weight of the liquefied natural gas, in percent. The critical percentage of the liquefied natural gas depends on the composition of the natural gas and the vaporization temperature and pressure.

FIG. 2 of the accompanying drawings is a flow sheet showing an embodiment of the process of the present invention in which the liquefied natural gas is cryogenically vaporized and at the same time, utilized as a cryogen of a cryogenic heat-exchange process. Referring to FIG. 2, a first heat-exchanger 21 for the cryogenic vaporization of liquefied natural gas is provided with a path 21a for the liquefied natural gas and a path 21b for the substance to be cooled. The entrance end of the path 21a is connected to a supply source of the liquefied natural gas (not shown in the drawing) through a conduit 20. An exit end of the path 21a is connected to a separator 22 through a conduit 23. An entrance end of the path 21b is connected to a conduit 24 for supplying the substance to be cooled, and an exit end of the path 21b is connected to a conduit 25 for discharging the cooled substance therethrough. A second heat-exchanger 26, for preliminarily cooling the substance to be cooled, is provided with a path 26a for the vaporized natural gas and a path 26b for the substance to be cooled. The entrance of the path 26a is connected to an upper portion 22a of the separator 22 through a conduit 27 so as to flow a gas in the upper portion 22a into the path 26a, and the entrance of the path 26b is joined with the conduit 24. A third heat-exchanger 28, for preliminarily cooling the substance to be cooled, is provided with a path 28a for the vaporized natural gas and a path 28b for the substance to be cooled. The entrances of the paths 28a and 28b are connected to the paths 26a, 26b, of the second heat-exchanger 26 through conduits 29 and 30, respectively, the exit of the path 28a is connected to a receiver of the vaporized natural gas (not shown in the drawing) through a conduit 31. The exitof the path 28b is connected to a supply source (not shown in the drawing) of the substance to be cooled through a conduit 32, a flow rate meter 33 and compressor 34. A lower portion 22b of the separator 22 is connected to a middle portion of the conduit 30 through a conduit 35 and a valve 38. The conduit 24 is communicated to the discharge conduit 25 through a conduit 36 and a three-way valve 37.

The separator 22 is provided with a device 40 for controlling a level of liquid contained in the lower portion 22b of the separator 22, which device 40 is electrically connected to the three-way valve 37 through an electric circuit 41 so as to automatically actuate the three-way valve 37. A device 42 for controlling a flow rate of liquid flowing through the conduit 35 is located on the conduit 35. The device 42 is electrically connected to the valve 38 through an electric circuit 43.

According to the process of the present invention, the liquefied natural gas is fed into the first heat-exchanger 21 through the conduit 20 and first cryogenically vaporized in the path 21a until the non-vaporized residue of the liquefied natural gas becomes a predetermined amount which is higher than a critical percentage of the liquefied natural gas but not higher than a percentage of 8 percent above the critical percentage. During the first cryogenic vaporization, the liquefied natural gas absorbs latent heat for vaporization from the substance flowing through the path 21b, through heat-transfer wall disposed between the paths 21a and 21b so as to cool the substance to a desired very low temperature.

A mixture of the first vaporized natural gas and the non-vaporized residue of the liquefied natural gas flows into the separator 22 through the conduit 23, and the two types of natural gases are separated from each other in the separator 22. The separated residual liquefied natural gas is contained in a lower portion 22b of the separator 22, and the separated first vaporized natural gas is received in an upper space 22a. The separated first vaporized natural gas flows into the path 26a of the second heat-exchanger 26. While flowing through the path 26a, the first vaporized natural gas is conditioned to a temperature of -110.degree. to -40.degree.C by heat-exchanging with the substance flowing through the path 26b, which preliminarily cools the substance. The first vaporized natural gas flows from the path 26a towards the path 28a of the third exchanger 28 through the conduit 30. The residual liquefied natural gas in the lower portion 22b of the separator 22 is introduced into the middle portion of the conduit 30 through the conduit 35 and ejected into the flow of the first vaporized natural gas which has been conditioned at a temperature of -110.degree. to -40.degree.C. The flow rate of the residual liquefied natural gas through the conduit 35 is controlled by actuation of the valve 38 by the control device 42. The ejected residual liquefied natural gas is vaporized completely by a second vaporization step in the flow of the first vaporized natural gas while absorbing latent heat for the vaporization from the first vaporized natural gas. This heat absorption causes a lowering of the temperature of the first vaporized natural gas. Therefore, the mixture of the first and second vaporized natural gases has a temperature lower than that of the first vaporized natural gas, which has been conditioned at a temperature of -110.degree. to -40.degree.C. The vaporized natural gas thus prepared is introduced into the path 28a of the third heat-exchanger 28 so as to preliminarily cool the substance flowing through the path 28b. After passing through the third heat-exchanger 28, the vaporized natural gas is discharged into a receiver (not shown in the drawing) through the conduit 31.

The substance to be cooled flows from a source thereof (not shown in the drawing) through the conduit 32 and is compressed to a desired pressure by the compressor 34. It is then fed into the path 28b of the third heat-exchanger 28 at a predetermined flow rate, which is detected by the flow rate meter 33. The substance thusly fed is preliminarily first cooled by heat-changing with the vaporized natural gas flowing through the path 28a. The first cooled substance is introduced into the path 26b of the second heat-exchanger 26 through the conduit 29, and then, preliminarily second cooled by heat-exchanging with the first vaporized natural gas flowing through the path 26a. The substance thusly second cooled is introduced into the path 21b of the first heat-exchanger 21 through the conduit 24, and then finally cooled to the desired temperature by heat-exchanging with the liquefied natural gas flowing through the path 21a.

The substance thusly finally cooled is discharged into a receiver (not shown in the drawing) through the three-way valve 37 and the conduit 25.

According to the process of the present invention, the separated residual liquefied natural gas in the separator 22 is introduced into a flow of the first vaporized natural gas in order to vaporize it by a second vaporization step. In this second vaporization, it is important that the first vaporized natural gas has been conditioned at a temperature of -110.degree. to -40.degree.C, preferably, -100.degree. to -50.degree.C, and more preferably, -100.degree. to -80.degree.C. If the temperature of the conditioned first vaporized natural gas is lower than -110.degree.C, it is difficult to completely vaporize the residual liquefied natural gas in the second vaporization step. Also, if the temperature of the conditioned first vaporized natural gas is higher than -40.degree.C, the resultant mixture of the first and second vaporized natural gases has a low merit when used as a cryogen for the cryogenic heat-exchange due to its relatively high temperature.

Referring to FIG. 2, the level control device 40 is to prevent the freeze-out of the higher hydrocarbons from the residual liquefied natural gas in the first heat-exchanger. In the case where the feed rates of the liquefied natural gas and/or the substance to be cooled are varied, and thus, the level of the residual liquefied natural gas in the separator 22 is varied, the variation of the liquid level is detected by the level control device 40 and the three-way valve 37 is opened or closed in response to the type of variation detected. That is, the flow rate of the substance introduced into the path 21b is controlled by discharging a portion of the substance flowing through the conduit 24, through the conduit 36, the three-way valve 37 and the conduit 25. Particularly, in the case where the liquid level in the separator 22 becomes lower than the predetermined height, that is, the liquefied natural gas is vaporized over the predetermined amount, the three-way valve is actuated by the level control device 40 so as to discharge a portion of the substance flowing through the conduit 24. The cryogenic vaporization of the present invention may be carried out under normal pressure or a pressurized condition, preferably, lower than 20 atmospehres.

Preferable modes of the process of the present invention will be better understood from the following examples.

EXAMPLE 1

The cryogenic vaporization of the present invention was applied to a process of preparing liquefied nitrogen gas.

Referring to FIG. 2, recycled nitrogen gas was introduced into a conduit 32 at a flow rate of 20,000 Nm.sup.3 /hr at room temperature and under a middle pressure, and then compressed to a pressure of 28 atmospheres by the compressor 34. The nitrogen gas thus compressed flowed through the path 28b of the third heat-exchanger 28 and was preliminarily first cooled to a temperature of -50.degree.C by heat-exchanging with the vaporized natural gas flowing through the path 28a. The nitrogen gas thus first cooled flowed through the path 26b of the second heat-exchanger 26 and was preliminarily second cooled to a temperature of -90.degree.C by heat-exchanging with the first vaporized natural gas flowing through the path 26a. Thereafter, the nitrogen gas thus second cooled flowed through the path 21b of the first heat-exchanger 21 and was finally cooled to a temperature of -151.degree.C so as to be liquefied by heat-exchanging with the liquefied natural gas flowing through the path 21a. The liquefied nitrogen gas was discharged through the conduit 25 into the receiver (not shown in the drawing). The liquefied natural gas was fed into the first heat-exchanger 21 at a flow rate of 12,750 Nm.sup.3 /hr under normal pressure. While flowing through the path 21a, the liquefied natural gas was first vaporized to a vapor of a temperature of -161.degree.C in a percentage by weight of 98 percent based on the initial weight thereof. The remaining 2 percent by weight of the liquefied natural gas remained in the liquid state. The critical percentage of the liquefied natural gas used, was 1.5 percent. The mixture of the first vaporized natural gas and the residual liquefied natural gas was introduced into the separator 22 and the two gases were separated from each other. The separated first vaporized natural gas of a temperature of -161.degree.C was introduced into the second heat-exchanger 26. While flowing through the path 26a, the first vaporized natural gas was conditioned to a temperature of -100.degree.C while preliminarily second cooling the nitrogen gas flowing through the path 26b. The first vaporized natural gas of a temperature of -100.degree.C flowed through the conduit 30 at a flow rate of 12,495 Nm.sup.3 /hr. The residual liquefied natural gas was vaporized completely by a second vaporization step by introducing it into the flow rate of the first vaporized natural gas in the conduit 30 at a flow rate of 255 Nm.sup.3 /hr. By this second vaporization, the mixture of the first and second vaporized natural gas was adjusted to a temperature of -107.degree.C.

The vaporized natural gas of a temperature of -107.degree.C flowed through the path 28a of the third heat-exchanger 28 at a flow rate of 12,750 Nm.sup.3 /hr so as to preliminarily first cool the nitrogen gas flowing through the path 28b. After passing through the path 28a, the vaporized natural gas had a temperature of 20.degree.C, and was discharged into the receiver (not shown in the drawing) through the conduit 31.

Throughout the above-stated cryogenic vaporization, no freeze-out of the higher hydrocarbons was observed, and the cryogenic vaporization could be continuously carried out for a period longer than 12 months.

EXAMPLE 2

The procedure of Example 2 was repeated using liquefied natural gas of a temperature of -132.degree.C under a pressure of 7 atmospheres and air containing no moisture or carbon dioxide. The air was compressed to a pressure of 6 atmospheres by the compressor 34, and fed into the third heat-exchanger 28 at a flow rate of 10,000 Nm.sup.3 /hr. After passing through the path 28b, the compressed air was preliminarily first cooled to a temperature of -19.degree.C. The compressed air thus first cooled was preliminarily second cooled to a temperature of -37.degree.C by flowing through the path 26b of the second heat-exchanger 26. Thereafter, the compressed air thusly second cooled was fed into the path 21b of the first heat-exchanger 21 and finally cooled to a temperature of -130.degree.C. The cooled air was discharged through the conduit 25 into the receiver (not shown in the drawing).

The liquefied natural gas having a temperature of -132.degree.C under a pressure of 7 atmospheres was fed into the path 21a of the first heat-exchanger 21 at a flow rate of 4,132 Nm.sup.3 /hr. After passing through the path 21a, 98 percent by weight of the liquefied natural gas was first vaporized, and 2 percent by weight of the liquefied natural gas remained in liquid phase. The critical percentage of the liquefied natural gas was 1.5 percent at a temperature of -132.degree.C under a pressure of 7 atmospheres. The first vaporized natural gas was separated from the residual liquefied natural gas in the separator 22. The separated first vaporized natural gas flowed through the path 26a of the second heat-exchanger 26 in order to second cool the air flowing through the path 26b. While passing through the path 26a, the first vaporized natural gas was conditioned to a temperature of -100.degree.C. The conditioned first vaporized natural gas flowed through the conduit 30 at a flow rate of 4,049 Nm.sup.3 /hr and was mixed with the residual liquefied natural gas ejected through the conduit 35 at a flow rate of 83 Nm.sup.3 /hr. The residual liquefied natural gas thus ejected was vaporized completely by a second vaporization step. By this second vaporization, the mixture of the vaporized natural gases from the first and second vaporization steps was adjusted to a temperature of -105.degree.C. The vaporized natural gas flowed through the path 28a of the third heat-exchanger 28 while preliminarily first cooling the air flowing through the path 28b. After passing through the path 28, the vaporized natural gas having a temperature of 10.degree.C was discharged into the receiver (not shown in the drawing).

The cooling of the compressed air and the cryogenic vaporization of the liquefied natural gas could be continuously carried out without difficulty for a period longer than 12 months. After operating for a period of 12 months, no-freeze-out of the higher hydrocarbons was observed in any of the paths of the natural gas.

Claims

What we claim is:

1. A process of completely vaporizing at a low temperature liquefied natural gas by a first vaporization step at a temperature lower than -110.degree.C until the non-vaporized residue of the liquefied natural gas becomes a predetermined amount higher than a critical percentage with respect to the initial weight of the liquefied natural gas at which critical percentage higher hydrocarbons in the residual liquefied natural gas are frozen-out therefrom, but not higher than a percentage of 8 percent above the critical percentage;

separating the residual liquefied natural gas from the first vaporized natural gas;

heating the first vaporized natural gas to a temperature from -110.degree. to -40.degree.C; and

vaporizing the residual liquefied natural gas by a second vaporization step by introducing it into the conditioned first vaporized natural gas.

2. A process as claimed in claim 1, wherein the cryogenic vaporization is carried out under normal pressure.

3. A process as claimed in claim 1, wherein the cryogenic vaporization is carried out under a pressurized condition.

4. A process as claimed in claim 1, wherein the conditioned temperature of the first vaporized conditioned natural gas is from -100.degree. to -50.degree.C.

5. A process as claimed in claim 4, wherein the conditioned temperature of the first vaporized natural gas is from -100.degree. to -80.degree.C.

6. A process for completely vaporizing at a low temperature liquefied natural gas while utilizing said natural gas, in both its liquid and vaporous phases, as a coolant for a substance without mixing said natural gas and said substance, comprising the steps of:

exchanging heat between natural gas in the liquid phase and said substance to be cooled whereby sufficient heat is removed from the liquefied natural gas such that the natural gas is in a two-phase liquid-vapor mixture;

flowing said two-phase liquid-vapor mixture into a separator;

separating the natural gas vapors from the natural gas liquid;

heating said natural gas vapors by heat exchange with said substance to be cooled;

mixing said liquefied natural gas with said heated natural gas vapors at a controlled rate;

vaporizing said liquefied natural gas by heat exchange with said already vaporized natural gas;

and further heating said vaporized natural gas by heat exchange with said substance to be cooled.

7. In an apparatus for completely vaporizing at a low temperature liquefied natural gas while utilizing said natural gas, in both its liquid and vaporous stages, as a coolant, for a substance without mixing said natural gas with said substance, the improvement comprising:

first heat exchange means for exchanging heat between liquefied natural gas and a substance to be cooled;

heat conduit means for feeding said heated liquefied natural gas into a separator means;

separator means for separating liquefied natural gas from vaporous natural gas;

sensing means for sensing the liquid level of liquefied natural gas in said separator means;

second heat exchange means or exchanging heat between vaporous natural gas and a substance to be cooled;

second conduit means for feeding vaporous natural gas from said separator to said second heat exchange means;

control means for controlling the liquid level in said separator by discharging residual liquefied natural gas into vaporous natural gas leaving said second heat exchange means;

third conduit means in which residual liquefied natural gas fed into said third conduit means is vaporized by vaporous natural gas flowing through said third conduit means such that upon leaving said third conduit means all natural gas is in a vaporous state;

third heat exchange means for exchanging heat between said vaporized natural gas leaving said third conduit means and a substance to be cooled;

and electrical sensing means for sensing the level of residual liquefied natural gas in said separator and for diverting via by-pass valve means said substance to be cooled around said first heat exchange means such that the rate of vaporization of liquefied natural gas in said first heat exchange means is controlled.