Low Temperature Refrigeration System

Abstract

A refrigeration system in which a pressurized gas provides the energy. The gas is expanded through an expansion engine and then passed through a heat exchanger, wherein the refrigeration effect resulting from its expansion is recovered. The expansion engine operates a conventional closed loop refrigeration system which is, in turn, cascaded through the low temperature gas discharging from the expansion engine. Prior to being cooled by the low temperature gas, the refrigeration cycle may also reject heat at a higher temperature to any suitable cooling medium. The invention is particularly suited for the liquefaction of natural gas or for low temperature separation of gas constituents.

Description

BACKGROUND OF THE INVENTION

This invention relates to a low temperature refrigeration method and apparatus particularly adapted for liquefying natural gas or the like.

There are a number of known processes for the production of very low temperature (cryogenic) refrigeration. Most of such processes rely upon the application of externally applied mechanical work to drive conventional refrigeration equipment which may be cascaded from one refrigeration cycle to the next to provide the very low temperature which may be required to liquefy natural gas or to accomplish other cryogenic tasks.

Several expansion cycles have been utilized for the lique-faction of gases which utilize the potential energy of the feed gas as the source of liquefaction refrigeration. While such expansion cycles have been used successfully for the liquefaction of both atmospheric gases, i.e., Helium, Nitrogen and Oxygen, and natural gas, as well as the low temperature separation of natural gas constituents, they have not fully utilized the potential energy available in the expansion of a high pressure stream of gas.

In the natural gas industry, it is often necessary to expand large volumes of natural gas from a high pressure to a low pressure to enable its use. Several commercial installations have been constructed to utilize the refrigeration available from this expansion for the purpose of liquefying or separating the natural gas constituents. However, such expander-cycle plants have had limited application because of the relatively small amount of refrigeration that they are capable of producing.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an improved method and apparatus which is commercially advantageous for the economic production of large quantities of cryogenic refrigeration.

It is a further object of this invention to provide a highly efficient method of liquefying natural gas, or the like.

It is a further object of this invention to provide a low temperature refrigeration, utilizing a pressurized gas to drive a refrigeration system which contributes to the cooling and lique-faction of gas, and a means for the rejection of heat from the liquefaction cycle.

It is still a further object of this invention to provide a low temperature refrigeration system utilizing the refrigeration effect produced by expanding a pressurized gas through an expansion engine, which, in turn, drives a system producing additional refrigeration, which may be cascaded from the already low temperature produced by the expansion engine.

It is a further object of this invention to provide a system for liquefying natural gas or the like wherein a portion of the gas is expanded through an expansion engine, which, in turn, drives a closed loop refrigeration cycle. Both the refrigeration effect produced by the expanded gas and the refrigeration cycle are employed to cool a liquefaction stream of the natural gas.

Other objects and advantages of this invention will become apparent from the description to follow, particularly when read in conjunction with the accompanying drawing.

BRIEF SUMMARY OF THE INVENTION

In the low temperature refrigeration system of this invention, a source of gas under pressure is expanded through one or more expansion engines, such as expansion turbines, wherein the temperature of the gas is greatly reduced. The mechanical work of the expansion engine is used to drive a closed loop refrigeration cycle. After compression, the refrigerant is cooled initially to a suitable heat sink, such as a cooling tower, and is further cooled by passing it through a heat exchanger wherein the low temperature expanded gas from the expansion engine is the cooling medium. After expansion, the refrigerant is passed through a second heat exchanger wherein it withdraws heat from another medium.

Where the refrigeration system of this invention is employed for the liquefaction of natural gas or the like, the gas to be liquefied is drawn from a pressurized source, and may be tapped from the same supply of energy gas for the expansion engine. In any event, the gas is passed through the two heat exchangers, whereby it is cooled first by the expanded gas and then by the at least partially vaporized refrigerant. In the meantime, the expanded energy gas may be conducted from the system for commercial use, as in a distribution system, or it may be compressed and recycled.

THE DRAWINGS

FIG. 1 is a schematic flow diagram of a gas liquefaction system embodying features of this invention; and

FIG. 2 is a partial schematic flow diagram of another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Embodiment of FIG. 1

In the flow diagram of FIG. 1, a pressurized supply of gas is provided at pipeline 10 from any suitable source (not shown). The gas, for example, may comprise natural gas supplied directly from a gas well or from a gas transmission pipeline. In any event, a stream of the gas is drawn off at 12 as the energy source for the refrigeration system of this invention.

After removal of water and other contaminents if required, for example, with an adsorber 14, the gas is conducted through line 16 along a processing or expander stream E wherein it is employed to provide the mechanical work necessary to operate a closed refrigeration cycle R which is the principal component of this refrigerating system. Specifically, the processing stream P is conducted through conduit 16 to one or more turbines 18, 20 and 22 (T.sub.1, T.sub.2 and T.sub.3), three being shown for purposes of illustration. The expander or processing stream E expands in the expansion turbines 18, 20 and 22 to produce mechanical work, and exhausts therefrom at E.sub.1 at a greatly reduced pressure and at a substantially lower temperature. Because the gas conducted to the turbines 18, 20 and 22 is at the relatively high temperature of the pipeline gas 10, there is a concomitant high drop in enthalpy through the turbine, which results in a correspondingly high output of work.

The low temperature expanded processing stream E.sub.1 from the final turbine stage is fed through conduit 24 to and through a heat exchanger 26 for cooling of the liquid refrigerant stream, and the medium to be refrigerated, as will be hereinafter described. From the heat exchanger 26, the processing gas E.sub.1, which is now at a commercially usable pressure, may be returned through line 28 to pipeline 10 for transmission to a distributing system (not shown). A pressure reducing regulator 30 enables some gas to be fed from the pipeline 10 to augment that fed from the processing system discharge line 28.

The turbines 18, 20 and 22 are mechanically coupled to compressors 32, 34 and 36 (C.sub.1, C.sub.2 and C.sub.3) which compress and circulate a refrigerant through heat exchangers. More particularly, in the refrigerating system R, a refrigerant, such as fluorinated hydrocarbon or ethylene, is pumped from the first compressor 32 and heat exchanger 38a, to the second compressor 34 and the heat exchanger 38b, and then through compressor 36 and heat exchanger 38c. At the heat exchangers 38a, 38b and 38c, the heat resulting from the compression of the refrigerant is dissipated to the surrounding air, water, or other cooling medium, depending upon the type heat exchanger used. From the heat exchanger 38c, the refrigerant flows through a conduit 40 to the heat exchanger 26 where it gives up heat and is condensed. The thus cooled liquid refrigerant emerges from the heat exchanger 26 and is then passed through an expansion or throttle valve 42 whereupon the liquid expands and returns to a mixed liquid-vapor state at a greatly reduced temperature. The cold fluid is then conducted through heat exchangers 46 and 26 where the sensible heat and latent heat of vaporization of the refrigerant are withdrawn from a medium to be cooled. In the heat exchanger 26, the vaporized refrigerant may also help cool the oncoming liquid refrigerant flowing therethrough. From the heat exchanger 26, the vaporized refrigerant is returned to the compressor 32 through a conduit 48 to complete the refrigeration cycle.

Where the refrigeration system of this invention is employed for liquefaction of natural gas or for low temperature separation of its constituents, the natural gas is delivered to the heat exchangers 26 and 46 along a liquefaction path L which includes a conduit 50 connected to the outlet of a conventional carbon dioxide separator 52, if required. With carbon dioxide thus removed, the liquefaction fluid L passes sequentially through the heat exchangers 26 and 46 with negligible pressure loss, emerging from the heat exchanger 46 in a pressurized, but low temperature liquefied condition.

The cold, pressurized liquefaction fluid L from the heat exchanger 46 then flows through temperature controlled expansion valve 54, which results in further cooling of the already cold fluid by reason of the Joule-Kelvin (Joule-Thomson) effect. After expansion through the valve 54, liquefaction is substantially complete, and the fluid is conducted through line 56 to an insulated storage tank 58.

The refrigeration system of this invention may be employed to liquefy a portion of the natural gas in pipeline 10. In such event, a portion of the gas is drawn off at 60 from the line 16 and directed through the liquefaction path just described. Thus, the natural gas from the same source is diverted into two streams, an expander or processing stream E and a liquefaction stream L, one to provide the energy and the other to receive the refrigeration affects.

Reviewing the cooling process, the liquefaction stream L is first cooled at heat exchanger 26 by the second cooling pass of the refrigerant stream R and by the cooled, expanded process stream E.sub.1 from the expansion turbines 18, 20 and 22. The net effect might be, for example, to cool the gas from about 70.degree.F. to, say -110.degree.F. Also cooled at heat exchanger 20 is the refrigerant which, as described, was precooled at the heat exchangers 38a, 38b and 38c.

At the second heat exchanger 46, the gas is further cooled by the first cooling pass of the refrigerant stream, wherein the result could be to reduce the temperature of the liquefaction stream further to about -200.degree.F. which, at its relatively high pressure is sufficient to liquefy it. Then, after expansion through the valve 54, the stream is cooled to about -260.degree.F. and still liquefied. It is within the capability of those skilled in the art to utilize the liquefied stream for further cooling if desired, as by diverting some of the stream to return through the heat exchanger as a flash sub-cooler, or by drawing off vaporized fluid from the tank 58 for the same purpose.

In the refrigeration system, the process gas E may enter the system at about 1000 p.s.i. and at about 70.degree.F. and be cooled by expansion through the turbines 18, 20 and 22 to about -114.degree.F. Then, after passing through the heat exchanger 26, where it absorbs heat from the liquid refrigerant in line 40 and from the liquefaction stream L in line 50, it is reheated to about 80.degree.F. and is at a pressure suitable for a distribution system. In the meantime, the refrigerant is delivered to the compressors 32, 34 and 36 at about atmospheric pressure and at about 80.degree.F. in temperature. The pressure is progressively increased through the compressors to about, say 200 p.s.i. Of course, the temperature is also increased at each compressor, but after each pass through the heat exchanger, it is returned to about 90.degree.F. This greatly reduces the work load imposed on the expanded gas at the heat exchanger 26. There, the refrigerant is further cooled to about -100.degree.F. and liquefied. At the expansion valve, the temperature is further reduced to about -200.degree.F. as it enters the heat exchanger in a liquid-vapor state.

Thus, in this system, a pressurized gas is cooled by expansion to function as a cooling medium and the mechanical work produced during such expansion drives a closed loop refrigeration cycle as the principal cooling source. Optionally, the same gas may be directed in a separate path through heat exchangers to be cooled by the two sources just recited.

THE EMBODIMENT OF FIG. 2

In the embodiment of FIG. 2, the expanded process stream E.sub.1 in line 28 may be passed through a compressor 62 which may be driven by an engine or motor 64. The compressed gas is then reintroduced to input line 12 at the pressure therein to circulate back through the processing stream E to the first turbine 18 and then continuing as in the embodiment of FIG. 1.

While this invention has been described in detail in conjunction with preferred embodiments thereof, it is obvious that various changes and modifications may suggest themselves to those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

What is claimed is:

1. A low temperature refrigeration method comprising the steps of:

drawing off a processing stream of pressurized gas from a supply source thereof,

expanding the stream through an expansion engine, then

passing the expanded stream through a first heat exchanger,

employing said expansion engine to drive a refrigeration system compressor,

circulating a refrigerant in said system from said compressor through said first heat exchanger to cool said refrigerant prior to expansion and vaporization thereof,

conducting said refrigerant from said first heat exchanger,

expanding said refrigerant to at least partial vaporization,

then passing said refrigerant through a second heat exchanger,

drawing off a liquefaction stream of pressurized gas from a supply source thereof, and

passing the pressurized gas successively through said first and second heat exchangers to be cooled first by said expanded stream and further by said expanded refrigerant.

2. The method defined by claim 1 wherein:

said liquefaction stream and said processing stream are drawn from the same supply source.

3. The low temperature refrigeration method defined by claim 1 including the further steps of:

compressing the expanded processing stream to the level of said supply source, and

then conducting said processing stream to said source.

4. The method of providing refrigeration defined by claim 1 wherein:

said processing stream is not cooled prior to expansion thereof through said expansion engine.

5. The method defined by claim 1 wherein:

said refrigerant is precooled between said compressor means and said first heat exchanger.

6. A refrigeration system comprising:

a source of pressurized gas,

a processing stream conduit connected to said source,

expansion engine means to which said processing stream conduit is connected,

a first heat exchanger connected to the outlet of said expansion engine means,

compressor means to which said expansion engine means are coupled for driving said compressor means,

a closed refrigeration system including said compressor means, a refrigerant circulated by said compressor means, and an expansion means,

refrigerant conductor means circulating said refrigerant through said first heat exchanger for cooling the liquid refrigerant prior to said expansion means,

a liquefaction stream conduit connected to said source and successively to said first and second heat exchangers.

7. The system defined by claim 6 including:

auxiliary heat sink means, and

conduit means connecting the outlet of said compressor means to said heat sink means for initial cooling of said refrigerant.