REFRIGERATION SYSTEMS ASSOCIATED WITH CRYOGENIC PROCESS PLANTS FOR ETHANE OR PROPANE RECOVERY FROM NATURAL GAS

Abstract

A method for an improved integration of refrigeration into conventional natural gas processing plants which use propane or similar hydrocarbon refrigerants either to supplement cooling by turbo-expanders or as the sole source of refrigeration.

Description

TECHNICAL FIELD

This disclosure relates to the processing of natural gas using refrigeration.

BACKGROUND

Modern natural gas processes for the separation of valuable hydrocarbons such as ethane, propane, and less volatile components from methane are commonly accomplished by fractionation at low temperatures using various refrigeration system designs. These valuable hydrocarbons are typically produced as a methane-free liquid stream.

Most commonly, turbo-expanders are used as the primary source of cooling. However, frequently additional refrigeration is required in order to achieve improved recoveries of the desired hydrocarbons. This supplementary refrigeration is typically supplied with a closed loop refrigeration system using propane or some other hydrocarbon mixture.

A number of turbo-expander designs have become standardized all of which use a fractionation column, and a number of heat exchangers, most commonly of the multi-stream brazed aluminum type. These designs must efficiently integrate the refrigeration system with the gas chilling train. Conventionally this has been accomplished using a heat exchange train which partially cools the inlet gas stream using low temperature boiling liquid propane to further cool the inlet gas steam. This further cooling is accomplished with a separate heat exchanger commonly referred to as a “Chiller”. The present disclosure addresses an improvement on the integration of this propane chilling system which reduces the capital cost of the natural gas processing plant while simultaneously reducing the energy required for the propane refrigeration.

SUMMARY

In aspects, the present disclosure provides a method for improving the overall thermodynamic efficiency of a natural gas liquids recovery plant. The method includes a closed loop hydrocarbon refrigeration system (typically utilizing propane) which eliminates the use of a stand-alone chiller exchanger and all of the associated equipment. Instead, the refrigerant is first sub-cooled and then vaporized and superheated in the inlet heat exchangers of the plant, including both the residue gas exchanger and if required, the reboiling exchangers.

The refrigeration system can be usefully integrated into those ethane and propane recovery plants which benefit from the use of supplementary refrigeration. Unlike the existing designs which return the refrigerant to the refrigerant compressor as one or more dew point temperature gas streams, this method and system returns the refrigerant to the refrigerant compressor as one or more super-heated gas streams significantly reducing the energy requirements of the refrigerant compressor. In the existing designs the piping which returns the refrigerant to the compression system may be so cold as to require special low temperature metallurgy and insulation at significant expense. In this new system and method described herein, this piping is warmed to ambient temperatures and requires no special metallurgy nor insulation. In other words, the present new system and methods permit non-specialized piping and lines, by which is meant the piping and lines may be made of relatively less expensive metallurgy suitable for use at ambient temperatures, and/or which do not have to be insulated.

This method and system described herein is not intended to modify other aspects of the processing plant such as the turbo-expansion, fractionation, or fractionation reflux steps. It stands alone as a method of improving the plant efficiency of these processes by improving the integration of the associated refrigeration system.

BRIEF DESCRIPTION OF THE DRAWINGS

For detailed understanding of the present disclosure, references should be made to the following detailed description of the disclosure, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein:

FIG. 1 is a diagram of a conventional gas processing system which utilizes both a turbo-expander and refrigeration system for cooling the processed natural gas stream. This diagram is provided to help illustrate the differences of the disclosed method and system described herein and the conventional design of FIG. 1.

FIG. 2 is a diagram of one non-limiting embodiment of the disclosed new design for an improvement of the conventional gas processing system also utilizing both a turbo-expander and refrigeration system for cooling the processed natural gas stream.

DETAILED DESCRIPTION

The present disclosure provides improved methods and systems for integrating refrigeration cooling into natural gas separation plants. The present disclosure is susceptible to embodiments of different forms which nevertheless fall within the scope of the method and system described herein. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present disclosure with the understanding that the present disclosure is to be considered an exemplification of the principles of the present disclosure and is not intended to limit the disclosure to that illustrated and described herein.

In aspects, the present disclosure provides an improved hydrocarbon refrigeration system for various types of low temperature natural gas liquid recovery plants. These plants which are designed to extract heavier molecular weight liquids from largely methane-containing natural gas streams, most frequently utilize either turbo-expander refrigeration, closed loop hydrocarbon refrigeration or a combination of both in order to cool and partially condense the inlet natural gas stream. Thus, beneficially, a natural gas stream may be processed with a reduced energy requirement when compared with the conventional designs.

DESCRIPTION OF CONVENTIONAL SYSTEM OF FIG. 1

To better illustrate the advantages of the present disclosure it is useful to compare in detail the existing design with the improved design. To this end FIG. 1 shows a typical ethane recovery plant using both a turbo-expander and propane refrigeration. It is also possible to use only propane refrigeration without a turbo-expander in these plants. There are any number of gas liquids recovery plant designs and the illustrated example serves as representative of the method by which a closed loop refrigeration system supplies cooling to the natural gas stream being processed. These designs initially cool and partially condense the inlet natural gas stream in a combination of exchangers, here, 100, the Gas/Gas Heat Exchanger and 105, the Reboiler Heat Exchanger. The inlet gas streams leaving these exchangers is combined and delivered to an exchanger 110, the Inlet Gas Chiller. It should be noted that at this point the gas stream, while considerably cooled in 100 and 105 is still at a warmer temperature than desired for desirable plant operations.

It should be appreciated that the arrangement of exchangers 100 and 105 while typical, is not universal and many other configurations are possible. What is essential however is that the inlet gas stream in any of the configuration covered by this disclosure will require further cooling than can be provided by the cold streams leaving the plant and the cold streams from the Demethanizer, 150.

The partially cooled stream resulting from the combined streams leaving the Gas/Gas Exchanger, 100, and the Reboiler Exchanger, 105, is then further cooled using a refrigerant stream, most typically cold bubble point liquid propane in shell and tube exchanger. This exchanger, 110, the Chiller, uses cold boiling liquid propane to further chill the gas stream. This exchanger completely vaporizes the entering liquid propane refrigerant. The inlet gas is chilled to a close temperature approach to the liquid propane. Typically, this temperature approach is 5° F. but may be as little as 2° F. or as much as 30° F. The propane refrigerant exits the Chiller 110 as a cold dew point vapor which then returns to the Refrigeration Compressor 112. It should be noted that the cold vapors leaving the Chiller 110 still have considerable cooling potential however this cooling potential is unexploited in the typical closed loop refrigeration system and lost as an inherent system inefficiency.

In conventional natural gas processing plants, the refrigeration system most typically consists of two or more stages of compression due to the high pressure ratio between the inlet and discharge of the compressor. In FIG. 1 the multistage compressor is illustrated as a Low Stage, 115 and a High Stage 120, although it is recognized that this could be three or more stages and this example is chosen for illustrative purposes.

The refrigeration system of FIG. 1 is well known to anyone well versed in the existing or conventional designs but is described here for the purpose of better illustrating the improvements of the disclosed new design. Cold, low pressure refrigerant vapor enters the low stage 115 of the refrigeration compressor and is compressed to pressure intermediate between the inlet pressure and the Refrigerant Condenser, 130, pressure. The cold refrigerant vapors are immediately warmed upon contact with the Low Stage compressor 115 thereby losing any potential refrigerant value. The refrigerant vapors after being compressed to feed the High Stage Compressor 120 are comingled with cold vapors at an intermediate pressure which leave vessel 125, the Economizer, also known as an intermediate pressure separator. Although not all refrigeration systems utilize an Economizer 125 this technique significantly improves the overall energy efficiency of most natural gas plant refrigeration systems and is accordingly illustrated here.

The refrigerant vapor stream, 126 of the comingled Low Pressure stage vapors and the Economizer 125 vapors is directed to the High Stage of compression, 120. The High Stage 120 of compression further raises the pressure of the refrigerant vapor to a pressure that will permit these vapors to be condensed by ambient temperature means such as an air-cooled or water-cooled heat exchanger. The high pressure vapors enter the Refrigerant Condenser, 130, and are cooled and ultimately condensed as a warm, liquid refrigerant.

The condensed liquid refrigerant in this illustration is reduced in pressure by means of a pressure reduction device to the intermediate pressure of the Economizer, 125. Upon the reduction of pressure, the refrigerant is partially vaporized and cooled. The Economizer 125 serves to separate the resultant refrigerant vapor and liquid streams. The vapor stream returns to the Refrigerant Compressor between the Low Stage 115 and High Stage 120 as previously described. The liquid refrigerant leaving the Economizer 125 is then further dropped in pressure, partially vaporized, and further cooled. This mixed vapor and liquid stream then feed to the Chiller, 110. This completes the description the closed loop refrigeration system, one found in the preponderance of conventional natural gas processing plants.

The remainder of the existing conventional design in FIG. 1 is described. It should be noted that the parts of the system of FIG. 2 that are identical to that in FIG. 1 will not be further described later in this disclosure. This portion of the processing plant is dedicated to the fractionation of the gas stream in the Demethanizer Column, 150. Many variants of the fractionation system are possible, and it is illustrated here primarily to show how it interfaces with the refrigerated portion of the plant.

In this particular embodiment, cold gas vapors leave the Cold Separator, 135. This vapor stream may be divided into a first stream 139 which feeds the turbo-expander 140, and a second stream 144 which feeds the Reflux Heat Exchanger 145. The cold vapor stream feeding the turbo-expander 140 is work expanded and exits the turbo-expander 140 at a lower pressure and temperature. This is a second source of chilling in the plant. The second cold vapor stream 144 passes through the Reflux Heat Exchanger, 145, in a heat exchange relationship with the cold vapors leaving the Demethanizer 150. In so doing this cold vapor stream 144 is largely or completely condensed at high pressure. It then expands across a pressure reduction device, 180 and feeds the top of the Demethanizer 150 where it serves as a pseudo-reflux stream.

The Demethanizer 150 produces a cold vapor overhead stream 146 which contains almost all of the methane feeding this distillation tower, and a liquid product bottom stream 151 which contains the small amount of methane not leaving in the overhead stream as well as the desired gas liquid components. Side draw 152 and bottom draw 153 from the Demethanizer 150 form the feed streams for the Reboiler Heat Exchanger, 105, previously described.

The demethanizer, 150, is usually provided with a Liquid Product Pump, 160, to deliver the cool ethane rich bottom product to a pipeline. This stream may provide a useful amount of cooling capacity for the plant.

The cold overhead vapors 146 leaving the Demethanizer Overhead are cross exchanged in the Reflux Heat Exchanger, 145, and warmed in a close temperature approach to the Cold Separator vapors. These partially warmed vapors (147) are then directed to the inlet portion of the plant and feed the Gas/Gas Exchanger 100 where they are further warmed before leaving the processing plant for a sales gas pipeline.

DESCRIPTION OF INVENTIVE SYSTEM OF FIG. 2

It can be seen that the process shown in FIG. 2 largely duplicates that shown in FIG. 1 with the readily apparent difference being that the Chiller, 110, and the Economizer, 125 have been removed. The cold separator, 135, Demethanizer, 150, Reflux Heat Exchanger, 145, Turbo-Expander, 140, Reflux Valve, 180 and Liquid Product Pump 160 are duplicated in FIG. 2 as 235, 250, 245, 240, 280 and 260, respectively. The Gas/Gas Exchanger, 100 and Reboiler Exchanger, 105 have been replaced with what is now called a Gas/Gas/Chiller Exchanger, 200 and a Reboiler Exchanger, 205, respectively, which serve the same purpose as 100 and 105 in FIG. 1 but with some additional passes for the refrigerant services.

In FIG. 2, the condensed refrigerant stream leaving the Refrigerant Condenser, 230, is not flashed and fed to an Economizer. Instead, this new system directs the condensed refrigerant stream to the warm side inlet of the Gas/Gas/Chiller Exchanger, 200. This condensed refrigerant steam further cooled while flowing through the Gas/Gas/Chiller Exchanger 200 in a cooling pass and is sub-cooled well below its boiling point temperature to give stream 202. “Sub-cooling” refers to a liquid existing below its normal boiling point. This sub-cooled refrigerant stream will now serve as the source of refrigeration in various ways, in sharp contrast with the conventional design illustrated in FIG. 1.

It will be particularly appreciated that the refrigerant stream while similar to those seem in mixed refrigerant systems used in LNG plants which only partially condense the refrigerant stream before feeding the heat exchange train, is here completely condensed. In one non-limiting embodiment, “completely condensing” means that at least 99.5 vol % of the gas is condensed. Furthermore, while in one non-limiting embodiment, the refrigerant is a single hydrocarbon, e.g., propane, in a different non-limiting embodiment, the refrigerant may be a mixed hydrocarbon refrigerant. In one non-limiting embodiment, at least one of the hydrocarbons in the mixed refrigerant is propane. In another non-limiting embodiment, in addition to propane the mixed refrigerant may also include ethane, butane, pentane and heavier hydrocarbons.

In a first step of pressure reduction the sub-cooled liquid refrigerant is directed to a pressure reduction device, 270 in one non-limiting embodiment a valve, by which it is dropped in pressure to approximately the interstage pressure of the Refrigeration Compressor 212. A portion of this stream, between 30% and 90% is returned directly to exchanger 200 without further pressure reduction. This cold stream vaporizes at this intermediate pressure thereby providing a substantial portion of the plant's overall refrigeration requirements. In this intermediate-pressure refrigerant return pass, 272, the fluid is not only vaporized but also warmed in the vapor state in a close temperature approach to the warm condensed propane entering the exchanger. Typically, the warm end temperature approach is between 4° F. and 20° F. This additional warming, commonly referred to as “super heating” permits recovering an additional increment of cooling from the refrigerant stream that would otherwise be lost. The warm intermediate pressure refrigerant vapors, 208 leave exchanger 200, and are directed to the interstage feed point of the Refrigerant Compressor 212. It will be appreciated that in the Description herein the temperatures and pressures that are part of the implementation of any particular design of the system are considered “predetermined temperatures” and “predetermined pressures” because they are part of the design.

In a second step of pressure reduction, a portion of the sub-cooled refrigerant at intermediate pressure as described above, is further dropped in pressure to permit yielding its refrigeration value at a lower temperature. The final pressure is sufficient to overcome the pressure losses of the heat exchanger circuit and feed the Refrigerant Compressor low pressure stage inlet. As with the intermediate pressure refrigerant stream, this low pressure refrigerant stream is not only completely vaporized but is also super-heated in a close temperature approach to the warm inlet streams of the Gas/Chiller Heat Exchanger, 200. In other words, the multiple refrigeration streams are heated such that they are completely vaporized and superheated to a close temperature approach of at least 2° F. and at most 30° F. with the inlet gas stream in order to supply refrigeration at a predetermined temperature. Just as in the case of the intermediate pressure refrigerant stream, this additional warming permits recovering an additional increment of cooling from the refrigerant stream that would otherwise be lost.

The sub-cooled liquid refrigerant is also used in a third service, that of providing additional cooling to Reboiler/Heat Exchanger, 205. This refrigeration service is normally also provided at the low pressure and temperature corresponding to that of the cold service in the Gas/Chiller/Exchanger 200, described above. A portion of the sub-cooled refrigerant, stream 206, is reduced in pressure to approximately the same conditions as the low temperature circuit in reboiler heat exchanger 200. As in exchanger 200, the final pressure is sufficient to overcome the pressure losses of the heat exchanger circuit and feed the Refrigerant Compressor low pressure stage inlet. Again, this refrigerant stream is not only completely vaporized but also super-heated in a close temperature approach to the warm inlet streams of the Reboiler Exchanger, 205.

The ability to superheat the refrigerant streams as described in the above paragraphs is the largest factor in the improvement of thermodynamic efficiency of this novel refrigeration system. The mixed liquid and vapor streams leaving 200 and 205 are comingled and fed directly to the Cold Separator, 235 in a state of thermodynamic equivalence to the stream leaving the Chiller, 110 in FIG. 1.

The remainder of the process, including the Cold Separator, 235, the Turbo Expander 240, the Reflux Heat Exchanger, 245, the Demethanizer, 250 and the Liquid Product Pump, 260 replicate that shown in FIG. 1.

It is useful to show a comparison of the performance of the previous, conventional design and the disclosed method and system described herein to better illustrate the advantages which accrue. Several cases are illustrated in which the boundary conditions of the designs are kept identical and only the refrigeration systems adjusted for comparison. The degree of improvement of this novel refrigeration cycle is a function of many variables such as the composition of the inlet gas, the pressure of the inlet gas and the degree of ethane or less volatile hydrocarbons which are desired to be recovered. For the sake of illustration two gas composition cases are compared, at two inlet gas pressures, while ethane recovery percentages are kept the same. In all cases it will be shown that the disclosed design has significant energy advantages seen as reductions of the horsepower of compression.

TABLE 1
Material Balance Rich Gas Case FIG. 1
Flow Rate 200 MMSCFD
Inlet Pressure 900 PSIG
Inlet Temperature 120° F.
Stream No.	101	102	103	148	113	131	121
Flow in Moles per hour	21960.0	21960.0	21960.0	15054.2	6905.5	6061.00	1866.00
Pressure (psig)	900.0	895.0	890.0	285.0	1100.0	230.0	80.0
Temperature (° F.)	120.0	6.4	−30.0	110.0	100.0	120.0	51.0
Component
N2	0.02410	0.02410	0.02410	0.03515	0.00000	0.0000	0.00000
CO2	0.00850	0.00850	0.00850	0.00689	0.01200	0.0000	0.00000
C1	0.61590	0.61590	0.61590	0.89469	0.00814	0.0000	0.00000
C2	0.21380	0.21380	0.21380	0.06216	0.54437	0.0050	0.00992
C3	0.09940	0.09940	0.09940	0.00107	0.31376	0.9900	0.98797
iC4	0.00870	0.00870	0.00870	0.00001	0.02764	0.0025	0.00116
nC4	0.02310	0.02310	0.02310	0.00002	0.07343	0.0025	0.00094
iC5	0.00320	0.00320	0.00320	0.00000	0.01018	0.0000	0.00000
nC5	0.00280	0.00280	0.00280	0.00000	0.00890	0.0000	0.00000
C6+	0.00050	0.00050	0.00050	0.00000	0.00159	0.0000	0.00000
Low Stage Compressor Horsepower:	3155.00
High Stage Compressor Horsepower:	2833.00
Total Refrigerant Compressor Horsepower:	5988.00
	Stream No.	122	123	124	126	127
	Flow in Moles per hour	4195.00	4195.00	4195.00	6061.00	6061.00
	Pressure (psig)	80.0	3.5	3.0	80.0	235.0
	Temperature (° F.)	51.0	−35.0	−35.0	80.0	166.0
	Component
	N2	0.00000	0.00000	0.00000	0.0000	0.0000
	CO2	0.00000	0.00000	0.00000	0.0000	0.0000
	C1	0.00000	0.00000	0.00000	0.0000	0.0000
	C2	0.00281	0.00281	0.00281	0.0050	0.0050
	C3	0.99090	0.99090	0.99090	0.9900	0.9900
	iC4	0.00309	0.00309	0.00309	0.0025	0.0025
	nC4	0.00319	0.00319	0.00319	0.0025	0.0025
	iC5	0.00000	0.00000	0.00000	0.0000	0.0000
	nC5	0.00000	0.00000	0.00000	0.0000	0.0000
	C6+	0.00000	0.00000	0.00000	0.0000	0.0000

TABLE 2
Material Balance Rich Gas Case FIG. 2
Flow Rate 200 MMSCFD
Inlet Pressure 900 PSIG
Inlet Temperature 120° F.
Stream No.	201	212	248	213	211	202	203	209
Flow in Moles per hour	21960.0	21960.0	15054.2	6905.5	3813.0	3813.0	1135.0	1135.0
Pressure (psig)	900.0	900.0	285.0	1100.0	230.0	225.0	4.0	3.0
Temperature (° F.)	120.0	−30.0	110.0	107.0	120.0	−35.0	−35.0	110.0
Component
N2	0.02410	0.02410	0.03515	0.00000	0.0000	0.0000	0.00000	0.00000
CO2	0.00850	0.00850	0.00689	0.01200	0.0000	0.0000	0.0000	0.0000
C1	0.61590	0.61590	0.89469	0.00814	0.0000	0.0000	0.0000	0.0000
C2	0.21380	0.21380	0.06216	0.54437	0.0050	0.0050	0.0050	0.0050
C3	0.09940	0.09940	0.00107	0.31376	0.9900	0.9900	0.9900	0.9900
iC4	0.00870	0.00870	0.00001	0.02764	0.0025	0.0025	0.0025	0.0025
nC4	0.02310	0.02310	0.00002	0.07343	0.0025	0.0025	0.0025	0.0025
iC5	0.00320	0.00320	0.00000	0.01018	0.0000	0.0000	0.0000	0.0000
nC5	0.00280	0.00280	0.00000	0.00890	0.0000	0.0000	0.0000	0.00000
C6+	0.00050	0.00050	0.00000	0.00159	0.0000	0.0000	0.0000	0.00000
Low Stage Compressor Horsepower:	2625
High Stage Compressor Horsepower:	2097
Total Refrigerant Compressor Horsepower:	4722
	Stream No.	204	208	206	207	210	226	227
	Flow in Moles per hour	1229.0	1229.0	1449.0	1449.0	2584.0	3813.0	3813.0
	Pressure (psig)	78.0	76.0	4.0	4.0	1.0	78.0	235.0
	Temperature (°F)	−34.0	110.0	−35.0	95.0	101.0	120.0	167.0
	Component
	N2	0.00000	0.00000	0.00000	0.00000	0.0000	0.00000	0.00000
	CO2	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
	C1	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
	C2	0.0050	0.0050	0.0050	0.0050	0.0050	0.0050	0.0050
	C3	0.9900	0.9900	0.9900	0.9900	0.9900	0.9900	0.9900
	iC4	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025
	nC4	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025
	iC5	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
	nC5	0.00000	0.00000	0.00000	0.00000	0.0000	0.00000	0.00000
	C6+	0.00000	0.00000	0.00000	0.00000	0.0000	0.00000	0.00000

TABLE 3
Material Balance Lean Gas Case FIG. 1
Flow Rate 200 MMSCFD
Inlet Pressure 900 PSIG
Inlet Temperature 120° F.
Stream No.	101	102	103	148	113	131	121
Flow in Moles per hour	21960.0	21960.0	21960.0	17040.2	4919.5	3878.81	1194.25
Pressure (psig)	900.0	895.0	895.0	285.0	1100.0	230.4	80.0
Temperature (° F.)	120.0	−2.4	−30.0	110.0	100.0	120.0	51.4
Component
N2	0.02675	0.02675	0.02675	0.03447	0.00000	0.0000	0.00000
CO2	0.00714	0.00714	0.00714	0.00421	0.01728	0.0000	0.00000
C1	0.73891	0.73891	0.73891	0.94988	0.00817	0.0000	0.00000
C2	0.12413	0.12413	0.12413	0.01105	0.51583	0.0050	0.00992
C3	0.07254	0.07254	0.07254	0.00038	0.32249	0.9900	0.98797
iC4	0.00574	0.00574	0.00574	0.00000	0.02562	0.0025	0.00116
nC4	0.01737	0.01737	0.01737	0.00001	0.07751	0.0025	0.00094
iC5	0.00265	0.00265	0.00265	0.00000	0.01182	0.0000	0.00000
nC5	0.00267	0.00267	0.00267	0.00000	0.01194	0.0000	0.00000
C6+	0.00209	0.00209	0.00209	0.00000	0.00934	0.0000	0.00000
Low Stage Compressor Horsepower:	2054.00
High Stage Compressor Horsepower:	1816.00
Total Refrigerant Compressor Horsepower:	3870.00
	Stream No.	122	123	124	126	127
	Flow in Moles per hour	2684.57	2684.57	2684.57	3878.81	3878.81
	Pressure (psig)	80.0	3.5	3.0	80.0	235.0
	Temperature (° F.)	51.4	−35.0	−35.0	80.0	166.0
	Component
	N2	0.00000	0.00000	0.00000	0.0000	0.0000
	CO2	0.00000	0.00000	0.00000	0.0000	0.0000
	C1	0.00000	0.00000	0.00000	0.0000	0.0000
	C2	0.00281	0.00281	0.00281	0.0050	0.0050
	C3	0.99090	0.99090	0.99090	0.9900	0.9900
	iC4	0.00309	0.00309	0.00309	0.0025	0.0025
	nC4	0.00319	0.00319	0.00319	0.0025	0.0025
	iC5	0.00000	0.00000	0.00000	0.0000	0.0000
	nC5	0.00000	0.00000	0.00000	0.0000	0.0000
	C6+	0.00000	0.00000	0.00000	0.0000	0.0000

TABLE 4
Material Balance Lean Gas Case FIG. 2
Flow Rate 200 MMSCFD
Inlet Pressure 900 PSIG
Inlet Temperature 120° F.
Stream No.	201	212	248	213	211	202	203	209
Flow in Moles per hour	21960.0	21960.0	15054.2	4919.6	2408.0	2408.0	1135.0	1135.0
Pressure (psig)	900.0	890.0	285.0	1100.0	230.0	225.0	4.0	3.0
Temperature (° F.)	120.0	−30.0	110.0	107.0	120.0	−35.0	−35.0	110.0
Component
N2	0.02675	0.02675	0.03447	0.00000	0.0000	0.0000	0.00000	0.00000
CO2	0.00714	0.00714	0.00420	0.01729	0.0000	0.0000	0.0000	0.0000
C1	0.73891	0.73891	0.94989	0.00817	0.0000	0.0000	0.0000	0.0000
C2	0.12413	0.12413	0.01105	0.51584	0.0050	0.0050	0.0050	0.0050
C3	0.07254	0.07254	0.00038	0.32248	0.9900	0.9900	0.9900	0.9900
iC4	0.00574	0.00574	0.00000	0.02562	0.0025	0.0025	0.0025	0.0025
nC4	0.01737	0.01737	0.00001	0.07751	0.0025	0.0025	0.0025	0.0025
iC5	0.00265	0.00265	0.00000	0.01182	0.0000	0.0000	0.0000	0.0000
nC5	0.00267	0.00267	0.00000	0.01194	0.0000	0.0000	0.0000	0.00000
C6+	0.00209	0.00209	0.00000	0.00934	0.0000	0.0000	0.0000	0.00000
Low Stage Compressor Horsepower:	2088
High Stage Compressor Horsepower:	1334
Total Refrigerant Compressor Horsepower:	3422
	Stream No.	204	208	206	207	210	226	227
	Flow in Moles per hour	1129.0	1129.0	1449.0	1449.0	2025.4	2408	2408
	Pressure (psig)	78.0	76.0	4.0	3.0	1.0	78	235
	Temperature (°F)	−34.0	110.0	−35.0	110.0	110.0	120	215
	Component
	N2	0.00000	0.00000	0.00000	0.00000	0.00000	0.00000	0.00000
	CO2	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
	C1	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
	C2	0.0050	0.0050	0.0050	0.0050	0.0050	0.0050	0.0050
	C3	0.9900	0.9900	0.9900	0.9900	0.9900	0.9900	0.9900
	iC4	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025
	nC4	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025	0.0025
	iC5	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000	0.0000
	nC5	0.00000	0.00000	0.00000	0.00000	0.00000	0.00000	0.00000
	C6+	0.00000	0.00000	0.00000	0.00000	0.00000	0.00000	0.00000

The above Tables illustrate the disclosed method operating at two conditions. The first of these is entitled “Rich Gas Case” in which a material balance is shown for a gas stream which contains a large quantity of ethane and heavier gas liquids. It is typical of natural gas found in the Bakin Field in North Dakota and elsewhere. The second is entitled “Lean Gas Case” in which a material balance is shown for a gas stream with a lesser quantity of natural gas liquids. This example is from the Permian Basin Fields in Texas and elsewhere.

The material balance covers primarily the refrigeration systems of the plant designs shown in FIGS. 1 and 2. The simulations were performed to match the liquid recoveries between FIG. 1 and FIG. 2. It can be seen that the total required flow rate of propane refrigerant is significantly less in the FIG. 2 designs whether operating in the Rich Gas Case conditions or the Lean Gas Case Conditions. This, and the superheating of the propane combine to produce a reduction of compressor horsepower of 21% in the Rich Gas Case and 11% in the Lean Gas Case.

Similar energy savings are seen in other plant designs.

It will be appreciated that the systems and methods of the present method and system have significant advantages over the convention systems and methods including, but not necessarily limited to, eliminating equipment such as the economizer or intermediate pressure separator, a chiller or chiller exchanger, a head drum, As well as all of their associated piping and control systems. Further, the system and methods here do not require insulated lines feeding the low or high stage compressors, and may use non-specialized metallurgy, in a non-limiting example carbon steel; that is, specialized metallurgy such as stainless steel is not required, but of course may be used.

The present invention may suitably comprise, consist of, or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. For instance, there is provided a closed cycle refrigeration method comprising, consisting essentially of, or consisting of completely condensing a refrigerant stream at the bubble point pressure of said refrigerant at ambient temperatures to give liquid refrigerant, where the refrigerant stream is selected from the group consisting of ethane, propane, butane, and pentane.

There is additionally provided in another non-limiting embodiment a closed cycle hydrocarbon refrigeration system comprising, consisting essentially of, or consisting of a condenser adapted to completely condense a refrigerant stream at the bubble point pressure of said refrigerant at ambient temperatures to give liquid refrigerant, where the refrigerant stream is selected from the group consisting of ethane, propane, butane, and pentane.

Further there may be provided in another non-restrictive version closed cycle hydrocarbon refrigeration system comprising, consisting essentially of, or consisting of a refrigerant condenser comprising, consisting essentially of, or consisting of a refrigerant condenser configured to completely condense a refrigerant vapor stream gas into a condensed refrigerant stream directed to a multi-pass gas/gas/chiller/exchanger, where the refrigerant stream is at the bubble point pressure of said refrigerant at ambient temperatures to give liquid refrigerant, where the refrigerant stream is selected from the group consisting of ethane, propane, butane, and pentane; and a pressure reduction device configured to flash the liquid refrigerant in the absence of a unit selected from the group consisting of a chiller, an economizer, and both.

As used herein, the terms “comprising,” “including,” “containing,” “characterized by,” and grammatical equivalents thereof are inclusive or open-ended terms that do not exclude additional, unrecited elements or method acts, but also include the more restrictive terms “consisting of” and “consisting essentially of” and grammatical equivalents thereof. As used herein, the term “may” with respect to a material, structure, feature, or method act indicates that such is contemplated for use in implementation of an embodiment of the disclosure and such term is used in preference to the more restrictive term “is” to avoid any implication that other, compatible materials, structures, features, and methods usable in combination therewith should or must be, excluded.

As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

As used herein, relational terms, such as “first,” “second,” “top,” “bottom,” “upper,” “lower,” “over,” “under,” etc., are used for clarity and convenience in understanding the disclosure and accompanying drawings and do not connote or depend on any specific preference, orientation, or order, except where the context clearly indicates otherwise.

As used herein, the term “substantially” in reference to a given parameter, property, or condition means and includes to a degree that one of ordinary skill in the art would understand that the given parameter, property, or condition is met with a degree of variance, such as within acceptable manufacturing tolerances. By way of example, depending on the particular parameter, property, or condition that is substantially met, the parameter, property, or condition may be at least 90.0% met, at least 95.0% met, at least 99.0% met, or even at least 99.9% met.

As used herein, the term “about” in reference to a given parameter is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the given parameter).

Claims

1. A closed cycle refrigeration method comprising: completely condensing a refrigerant stream at the bubble point pressure of said refrigerant at ambient temperatures to give liquid refrigerant, where the refrigerant stream is selected from the group consisting of ethane, propane, butane, and pentane.

2. The closed cycle refrigeration method of claim 1 further comprising sub-cooling of the condensed refrigerant in a multi-pass heat exchanger in combined service with the cooling of the inlet gas stream, such that when the pressure drop associated with cooling occurs there is less than 0.5% vaporization.

3. The closed cycle refrigeration method of claim 1 further comprising: dividing the sub-cooled liquid refrigerant into multiple refrigeration streams followed byflashing of the sub-cooled liquid refrigerant to a predetermined pressure selected to provide cooling at predetermined temperatures.

4. The closed cycle refrigeration method of claim 3 further comprising: heating the multiple refrigeration streams such that they are completely vaporized and superheated to a close temperature approach of at least 2° F. and at most 30° F. with the inlet gas stream to supply refrigeration at a predetermined temperature.

5. The closed cycle refrigeration method of any one of claim 2 further comprising integration of the method into a single heat exchanger which also serves to cool the plant inlet gas and to warm various cold gas streams leaving the plant.

6. The closed cycle refrigeration method of any one of claim 2 further comprising heating of at least one of the multiple sub-cooled refrigerant streams in a reboiler associated exchanger such that this stream is vaporized and superheated to a close temperature approach to the inlet gas stream.

7. A closed cycle hydrocarbon refrigeration system comprising: a condenser adapted to completely condense a refrigerant stream at the bubble point pressure of said refrigerant at ambient temperatures to give liquid refrigerant, where the refrigerant stream is selected from the group consisting of ethane, propane, butane, and pentane.

8. The closed cycle hydrocarbon refrigeration system of claim 7 further comprising a cooling pass adapted to sub-cool the condensed refrigerant in a multi-pass heat exchanger in combined service with the cooling of the inlet gas stream, such that when the pressure drop associated with cooling occurs there is less than 0.5% vaporization.

9. The closed cycle hydrocarbon refrigeration system of claim 7 further comprising lines dividing the sub-cooled liquid refrigerant into multiple refrigeration streams followed by a valve adapted to flash the sub-cooled liquid refrigerant to a predetermined pressure.

10. The closed cycle hydrocarbon refrigeration system of claim 9 further comprising heat exchangers adapted to heat the multiple refrigeration streams such that they are completely vaporized and superheated to a close temperature approach of at least 2° F. and at most 30° F. with the inlet gas stream.

11. The closed cycle hydrocarbon refrigeration system of claim 9 further comprising a single heat exchanger adapted to cool the plant inlet gas and to warm various cold gas streams leaving the plant.

12. The closed cycle hydrocarbon refrigeration system of claim 9 further comprising at least one reboiler heat exchanger adapted to heat at least one of the multiple sub-cooled refrigerant streams such that this stream is vaporized and superheated to a close temperature approach to the inlet gas stream.

13. A closed cycle hydrocarbon refrigeration system comprising: a refrigerant condenser comprising: a refrigerant condenser configured to completely condense a refrigerant vapor stream into a condensed refrigerant stream directed to a multi-pass gas/gas/chiller/exchanger, where the refrigerant stream is at the bubble point pressure of said refrigerant at ambient temperatures to give liquid refrigerant, where the refrigerant stream is selected from the group consisting of ethane, propane, butane, and pentane; anda pressure reduction device configured to flash the liquid refrigerant;in the absence of a unit selected from the group consisting of a chiller, an economizer, and both.

14. The closed cycle hydrocarbon refrigeration system of claim 13 where the refrigerant stream consists essentially of propane.

15. The closed cycle hydrocarbon refrigeration system of claim 13 further comprising the pressure reduction device further configured such that when the pressure drop associated with cooling occurs there is less than 0.5% vaporization.

16. The closed cycle hydrocarbon refrigeration system of claim 13 further comprising heat exchangers adapted to heat the multiple refrigeration streams such that they are completely vaporized and superheated to a close temperature approach of at least 2° F. and at most 30° F. with the inlet gas stream.

17. The closed cycle hydrocarbon refrigeration system of claim 16 further comprising a single heat exchanger adapted to cool the plant inlet gas and to warm various cold gas streams leaving the plant.

18. The closed cycle hydrocarbon refrigeration system of claim 16 further comprising at least one reboiler heat exchanger adapted to heat at least one of the multiple sub-cooled refrigerant streams such that this stream is vaporized and superheated to a close temperature approach to the inlet gas stream.