WO2012122620A1

WO2012122620A1 - Improved refrigeration apparatus and process in a natural gas processing facility

Info

Publication number: WO2012122620A1
Application number: PCT/CA2011/001208
Authority: WO
Inventors: Richard BOWER; Brent J. ALLARDYCE
Original assignee: Allardyce Bower Consulting Inc.
Priority date: 2011-03-16
Filing date: 2011-10-31
Publication date: 2012-09-20

Abstract

An improved method for processing a natural gas stream comprises mechanically chilling the natural gas stream; flowing the chilled natural gas stream through a differential pressure control valve such that the natural gas stream temperature and pressure drop in accordance with the Joule-Thomson effect; separating the natural gas stream into a liquid and a residue gas in a low temperature separator; then compressing the residue gas to a selected sales gas pressure suitable for flow through a sales gas pipeline of a natural gas pipeline network.

Description

Improved Refrigeration Apparatus and Process in a Natural Gas

Processing Facility

Field Of Invention

This invention relates generally to an improved refrigeration apparatus and process in a natural gas processing facility which can improve hydrocarbon liquids recovery in the facility.

Background

In a conventional natural gas processing facility, mechanical refrigeration is utilized to chill an incoming raw natural gas stream to remove hydrocarbon liquids. The recovered liquids are then processed to make specification liquid products. The residue gas is then suitable for distribution in a natural gas pipeline network.

The residue gas is typically of insufficient pressure to flow directly to a sales gas pipeline of the natural gas pipeline network. Therefore, a process compressor is provided to compress the residue gas to a specified sales gas pipeline pressure.

Significant energy is required to chill the incoming natural gas stream, resulting in significant operating costs. Significant capital costs must also be expended to provide the refrigeration equipment necessary to handle these significant refrigeration loads. Reducing the operating load of such equipment can reduce the facility's operating costs, or designing a facility with refrigeration equipment with a lower operating load rating can reduce the facility's capital costs. Also, existing refrigeration processes and equipment leave a significant amount of liquids in the natural gas stream; as hydrocarbon liquids generally have a higher financial value compared to natural gas, increasing liquids recovery during the refrigeration process should increase the value of the facility's operation.

Summary

According to one aspect of the invention, there is provided a method for processing a natural gas stream comprising: mechanically chilling the natural gas stream; flowing the chilled natural gas stream through a differential pressure control valve such that the natural gas stream temperature and pressure drop in accordance with the Joule-Thomson effect; separating the natural gas stream into a liquid and a residue gas in a low temperature separator; and flowing the residue gas from the separator at or above a selected sales gas pressure suitable for flow through a sales gas pipeline. Utilizing the Joule-Thomson effect can improve liquids recovery from the natural gas stream and/or reduce loads caused by the mechanical chilling when compared to methods which do not utilize such effect.

When the residue gas pressure leaving the separator is below the sales gas pipeline pressure, the method can further comprise compressing the residue gas to or above the sales gas pipeline pressure. When the incoming pressure of the natural gas stream prior to mechanical chilling is above the sales gas pipeline pressure, the control valve can be set to reduce the pressure of the natural gas stream such that the residue gas leaving the separator is at or above the sales gas pipeline pressure.

The method can further comprise compressing the natural gas stream by an inlet compressor prior to mechanically chilling the natural gas stream. The method can further comprise flowing the residue gas and natural gas stream through a heat exchanger such that heat is transferred from the natural gas stream to the residue gas. The method can further comprise injecting glycol into the natural gas stream prior to mechanically chilling the natural gas stream.

According to another aspect of the invention, there is provided an apparatus for processing a natural gas stream. This apparatus comprises a natural gas feed line for flow of a natural gas stream therethrough; a chiller fluidly coupled to the feed line and configurable to mechanically chill a natural gas stream flowing through the chiller; a differential pressure control valve fluidly coupled to the feed line downstream of the chiller and configurable to cause the natural gas stream temperature and pressure to drop in accordance with the Joule-Thomson effect when the natural gas flows through the control valve; a low temperature separator vessel fluidly coupled to the feed line downstream of the control valve and in which the natural gas stream is separable into a liquid and a residue gas; and a residue gas line fluidly coupled to the separator vessel for flow of the residue gas therethrough.

The apparatus can further comprise a sales compressor fluidly coupled to the residue gas line downstream of the separator vessel and configurable to compress the residue gas to a selected sales gas pressure suitable for flow through a sales gas pipeline. Additionally or alternatively, the apparatus can further comprise an inlet compressor fluidly coupled to the feed line upstream of the chiller; the inlet compressor and control valve are configurable such that the pressure of the residue gas leaving the separator is at or above a selected sales gas pressure suitable for flow through a sales gas pipeline. The apparatus can further comprise a flow meter coupled to the feed line upstream of the chiller and a controller communicative with the control valve and the flow meter. In addition there may be a temperature controller communicating with the control valve to provide feedback to the other control loops. The controller has a memory having stored thereon steps and instructions executed by the controller to dynamically change the control valve settings in response to changes in the natural gas pressure and temperature measured by the flow meter. The controller can be further communicative with the chiller and the steps and instructions can further include dynamically changing the chiller settings in response to the changes in the natural gas pressure measured by the flow meter. Brief Description of Drawings

Figure 1 is a schematic system diagram of a conventional refrigeration skid in a natural gas processing facility (PRIOR ART).

Figure 2 is a schematic system diagram of an improved refrigeration skid in a natural gas processing facility according to one embodiment of the invention.

Figure 3 is a schematic system diagram of an improved automated refrigeration skid in a natural gas processing facility according to another embodiment of the invention. Detailed Description of Embodiments of the Invention

A conventional mechanical refrigeration skid 10 in a natural gas processing facility is shown in Figure 1 (PRIOR ART). This refrigeration skid 10 carries out a refrigeration process that assists in the separation of liquids and gases from a natural gas stream. The constituents of natural gas are well known to those skilled in the art and include raw wellhead gas as well as gas in solution with oil.

Upstream of the refrigeration skid 10, the natural gas stream is processed in a facility inlet separator vessel (not shown) to separate some hydrocarbon liquids and water from the natural gas stream. Freed hydrocarbon liquids are sent for further processing by liquids processing equipment (not shown) and then to sales in the form of specification liquid products 12. The constituents of these liquids are well known in to art and can include one or more of butane, propane, and pentane (although "liquids" in the plural is used in this description, it is understood that "liquids" can consist of only one type of hydrocarbon liquid). These liquids may either be atmospherically stable or be a pressurized Liquefied Petroleum Gas (LPG) product. Freed water is also removed in this separator vessel and normally sent to storage. The remaining natural gas stream continues to the refrigeration skid 10 for further processing.

Depending on the nature and extent of upstream processing, the pressure of the incoming natural gas stream may be too low for processing by the refrigeration skid 10 and/or too low for separated residue gas for flow through a sales gas pipeline. In such circumstances, the facility typically contains a process compressor. The process compressor can be an inlet compressor (not shown) which boosts the incoming natural gas stream pressure to a suitable refrigeration skid operating pressure. This incoming natural gas stream, depending on its various constituents, can be treated to remove H₂S and CO2 (not shown) and sent to the refrigeration skid 10. In cases where compression of the incoming natural gas stream is not required, the natural gas stream from the inlet separator vessel flows directly to treating and processing in the refrigeration skid 10. The process compressor can alternatively or additionally include a sales gas compressor 15 which compresses residue gas separated from the natural gas stream to a suitable sales gas pipeline pressure. More particularly, the system can feature a three stage compressor, with two stages of compression performed by the process compressor and another stage of compression performed by the sales gas compressor or by another compressor.

The mechanical refrigeration of the incoming natural gas stream and the separation of gas and liquids from this stream by the refrigeration skid 10 will now be described in detail. The incoming natural gas stream ("inlet stream") flows through a natural gas feed line 16 and through a pair of gas/gas heat exchangers 17, 18 serially fluidly coupled to the feed line 16 and which utilize cooled residue gas to cool the inlet stream, thereby reducing the refrigeration load. As will be described below, the cooled residue gas is a product of the inlet stream that has been subjected to a separation and cooling process elsewhere in the refrigeration skid 10. A mixture of ethylene glycol and water is sprayed by a series of glycol injectors 20(a), 20(b), 20(c) into the inlet stream to prevent the inlet stream from freezing or hydrating and to assist in water removal from the inlet stream. A first glycol injector 20(a) is fluidly coupled to the feed line 16 immediately upstream of the first gas/gas heat exchanger 17, and a second glycol injector 20(b) is fluidly coupled to the feed line 16 immediately downstream of the second gas/gas heat exchanger 18.

The partially-cooled inlet stream then exits an outlet of the second gas/gas heat exchanger 18, is sprayed by the second glycol injector 20(b) and then flows through the feed line 16 to a gas/liquids heat exchanger 22 where a heat exchange with cooled liquids further cools the inlet stream. As will be described below, the cooled liquids is a product of the inlet stream that has been subjected to a separation and cooling process elsewhere in the refrigeration skid 10. The further cooled inlet stream then exits an outlet of the gas/liquids exchanger 22 and then flows via the feed line 16 to a chiller 24, where the inlet stream exchanges heat with boiling refrigerant 26 to further chill the inlet stream until the gas in the stream reaches the required cold temperature for processing in a low temperature separator (LTS) 19. At the required cold temperature, hydrocarbon liquids and additional water are condensed from the inlet stream leaving behind the residue gas stream; the liquids and water flow with the residue gas as a gas/liquids mixture through the feed line 16. Depending on the design of the system, this residue gas will be chilled to -10° C or as cold as -40°C. Glycol is also injected into the feed line 16 by another glycol injector 20(c) and/or directly into the chiller 24 to prevent freezing and the formation of gas hydrates.

The cooled gas/liquids mixture from the chiller 24 flows through the feed line 16 to the LTS 19 where the mixture is separated into a cooled outlet gas stream and a cooled outlet liquids stream; the outlet gas stream exits the LTS 19 via an outlet gas line 32 and the outlet liquids stream exits the LTS 19 via an outlet liquids line 34. Rich glycol (which includes water absorbed from the gas) is also typically separated in the LTS 18 and flows via a rich glycol line 36 to a regeneration process 38. The cooled hydrocarbon liquids in the outlet liquids stream 34 absorb heat from the inlet stream at the gas / liquid exchanger 22 and then flows via the outlet liquids line 34 to further processing in the facility for further conditioning of the liquids. This further conditioning may involve making a specification atmospheric product or a pressurized hydrocarbon liquid stream. The cooled residue gas in the outlet gas line 32 flows from the LTS 19 and absorbs heat from the inlet stream in the gas/gas heat exchangers 16, 18 as previously discussed and then flows via the outlet gas line 32 as specification quality sales gas.

In many cases, the residue gas flowing out of the LTS 19 ("sales gas") does not have sufficient pressure to flow directly to the sales gas pipeline and then to the rest of the natural gas pipeline network, and thus is compressed to a suitable sales gas pipeline pressure by the sales gas compressor 15 which is located on the outlet gas line 32 downstream of the LTS 19.

There are a number of different variations and adaptations of the refrigeration process described above, including:

• prior to refrigeration at the skid 10, the inlet stream is already dehydrated (water removed) and therefore, glycol injection is not required at one or more injection sites in the refrigeration skid 0;

• there may be only one gas/gas exchanger or there may be multiples of each exchanger depending on equipment sizing

• the cooled liquids do not absorb heat from the inlet stream and instead either exchanges heat with another process stream or flow directly to liquids processing by the liquids processing equipment;

• the LTS 19 may operate at different temperatures depending on the capacity of refrigeration system;

• the rich glycol and condensate are not separated in the LTS 19 but are left together and separated by the liquids processing equipment; and

• a sales gas compressor is provided instead of or in addition to an inlet compressor in the facility. Referring to Figure 2 and according to an embodiment of the invention, a differential pressure control valve 36 is installed on the feed line 16 between the chiller 24 and the LTS 19 to form an improved refrigeration skid 11 and the skid is operated according to an improved refrigeration process which increases liquids recovery and/or reduces mechanical refrigeration loads compared to the conventional skid 10 shown in Figure 1. Suitable commercially available control valves include Fisher's 657 and 667 ET model control valves. The control valve 36 enables an improved refrigeration process that utilizes the Joule-Thomson effect to chill the inlet stream to a lower temperature without the need for further mechanical refrigeration.

In thermodynamics, the Joule-Thomson effect describes the temperature change of a gas or a liquid when it is forced through a valve while kept insulated so that no heat is exchanged with the environment. This process is called a throttling process or Joule-Thomson process. At room temperature, most gases including natural gas cool upon expansion by the Joule-Thomson process. This principle is based on the conservation of energy; if no external work is extracted and no heat is transferred in the expansion process, the total energy of the gas remains the same. As expansion of the gas increases the potential energy of the gas, conservation of energy dictates a corresponding decrease in the kinetic energy and thus temperature of the gas.

The control valve 36 is set to cause a pressure drop in the gas/liquid inlet stream prior to reaching the LTS 19. This pressure drop causes the Joule-Thomson effect. The temperature drop can be significant and depends on the pressure drop caused by the valve's setting. The colder gas and liquid mixture from the control valve 36 then flows to the LTS 19 where additional hydrocarbon liquids can be removed from the gas. These liquids can then be processed to make specification liquid products.

In other words, flowing the inlet stream through the control valve 36 reduces the temperature of the inlet stream, thereby increasing the amount of liquids that are removed from the inlet stream in the LTS 19 compared to a process utilizing the same equipment and operating parameters but without the control valve 36. While flowing the inlet stream through the control valve 36 causes a corresponding decrease in pressure of the inlet stream and residue gas, the sales compressor 15 can be used to bring the residue gas pressure back up to a suitable sales gas pipeline pressure. Alternatively, the operating parameters can be set so that the incoming pressure of the inlet stream (i.e. at the entrance of the skid 11) is sufficiently high that the pressure of the residue gas downstream of the control valve 36 and LTS 19 is still at or above the required sales gas pipeline pressure. An inlet compressor (not shown) can be provided to increase the inlet stream pressure to such an incoming pressure, if the upstream pipeline pressure is not sufficiently high.

The main advantage of this improved process is that for many cases, liquids recovery can be increased without changing the operating parameters of the skid 11. This is because in many refrigeration skids, spare compression is available which is not being fully utilized by the skid especially when inlet stream flow to a facility declines. For example, in many cases, the sales compressor 15 has available capacity to boost gas flow to higher pressure ratios than originally designed without damage to the equipment. Therefore, the extra power and added compression ratio from the sales gas compressor 15 can be utilized to "make up" the extra pressure drop caused by the control valve 36, thereby increasing liquids recovery without expensive equipment retrofit.

In another instances, the incoming pressure of the inlet stream is sufficiently high that spare compression is available to flow the inlet stream through the control valve 36 and still be above the required sales gas pipeline pressure. In such instances, the sales gas compressor 15 is not required to bring the residue gas up to the sales gas pipeline pressure.

Alternatively or additionally, the skid's operating parameters can be adjusted to reduce mechanical refrigeration loads, with a resultant reduction in operating costs. For example, the chiller 24 can be set to chill the inlet stream to a higher temperature than the temperature required for liquids separation in the LTS 19 (thereby reducing the load on the chiller 24), and the control valve 36 can be set to further cool the inlet stream to the required liquids separation temperature.

In view of the above, the chiller 24 and control valve 36 can be set at different settings depending on the degree of liquids recovery and refrigeration load reduction that is desired. For example, at one extreme, the chiller 24 and control valve 36 can be set to maximize the reduction in refrigeration load by the chiller 24 and minimize the increase in liquids recovery, and at another extreme, the chiller 24 and control valve 36 can be set to maximize the increase in liquids recovery and minimize the reduction in refrigeration load by the chiller 24. Referring to Figure 3 and according to another embodiment, the control valve 36 can be a solenoid valve that is communicative with a controller 50 that is programmed to dynamically adjust the control valve's settings depending on the operating conditions of the skid 11 , and in particular on the properties of the incoming inlet stream. One or more flow meters 52 are installed in the refrigeration skid 11 including a first flow meter 52 installed on the feed line 16 upstream of the control valve 36. The controller 50 is programmed to adjust the control valve settings depending on the pressure measured by this first flow meter 52. Additional flow meters (not shown) can also be installed on the feed line 16, for example, between the control valve 36 and LTS 19 to collect additional inlet gas flow data for the processor 50 to process. The processor 50 can be a programmable logic controller or other suitable automated controller with a memory that can store a program which is executed by the controller 50 to dynamically control the control valve settings based on data including data from the flow meter 52. The controller 50 program can also be further programmed to also dynamically control the settings of the chiller 24 based on the properties of the inlet stream and an operator's desired reduction in refrigeration load and increase in liquids recovery. The controller 50 can be locally located in the refrigeration skid 11 and be programmed to operate mostly or entirely autonomously, or the controller 50 can be located remotely and communicate with the control valve 36 and/or chiller 24 via a network connection (LAN or via the World Wide Web).

The improved process and new equipment described in the aforementioned embodiments can be retrofit to a refrigeration skid of an existing natural gas facility or form part of the design of a new facility or a new refrigeration skid in an existing facility. The improved process in such a new facility should enable the refrigeration process to run colder than a conventional mechanical refrigeration with minimal penalty. A natural gas facility operator should be able to improve liquid recovery as the gas stream declines by running colder temperatures in the LTS 19. Alternatively, it may be advantageous to change out the existing LTS 19 and the coldest part of the gas/gas heat exchanger 22 to take advantage of the improved process. That is, taking fuller advantage of the improved process may require operating existing equipment at below their minimum design temperature, which can be between -29°C and -46°C. If building a new facility or a new refrigeration skid 11 for an existing facility, an LTS 19 and a gas/gas heat exchanger capable of operating at these lower temperatures can be included as part of the original design without significant additional cost. Even in a retrofit, the expected expense in swapping out an existing LTS 19 and gas/ gas exchanger should be relatively minimal. It may also be economical to increase the compressor displacement of the sales gas compressor to maximize the allowable pressure differential, thus increasing liquids recovery.

While particular embodiments have been described in the foregoing, it is to be understood that other embodiments are possible and are intended to be included herein. It will be clear to any person skilled in the art that modifications of and adjustments to the foregoing embodiments, not shown, are possible.

Claims

Claims What is claimed is:

1. A method for processing a natural gas stream comprising: mechanically chilling the natural gas stream; flowing the chilled natural gas stream through a differential pressure control valve such that the natural gas stream temperature and pressure drop in accordance with the Joule-Thomson effect; separating the natural gas stream into a liquid and a residue gas in a low temperature separator; and flowing the residue gas from the separator at or above a selected sales gas pressure suitable for flow through a sales gas pipeline.

2. A method as claimed in claim 1 wherein the residue gas pressure leaving the separator is below the sales gas pipeline pressure, and the method further comprising compressing the residue gas to or above the sales gas pipeline pressure.

3. A method as claimed in claimed in claim 1 wherein the incoming pressure of the natural gas stream prior to mechanical chilling is above the sales gas pipeline pressure, and the control valve is set to reduce the pressure of the natural gas stream such that the residue gas leaving the separator is at or above the sales gas pipeline pressure.

4. A method as claimed in claim 3 further comprising compressing the natural gas stream by an inlet compressor prior to mechanically chilling the natural gas stream.

5. A method as claimed in claim 1 further comprising flowing the residue gas and natural gas stream through a heat exchanger such that heat is transferred from the natural gas stream to the residue gas.

A method as claimed in claim 1 further comprising injecting glycol into the natural gas stream prior to mechanically chilling the natural gas stream.

An apparatus for processing a natural gas stream comprising: a natural gas feed line for flow of a natural gas stream therethrough; a chiller fluidly coupled to the feed line and configurable to mechanically chill a natural gas stream flowing through the chiller; a differential pressure control valve fluidly coupled to the feed line downstream of the chiller and configurable to cause the natural gas stream temperature and pressure to drop in accordance with the Joule- Thomson effect when the natural gas flows through the control valve; a low temperature separator vessel fluidly coupled to the feed line downstream of the control valve and in which the natural gas stream is separable into a liquid and a residue gas; and a residue gas line fluidly coupled to the separator vessel for flow of the residue gas therethrough.

An apparatus as claimed in claim 7 further comprising a sales compressor fluidly coupled to the residue gas line downstream of the separator vessel and configurable to compress the residue gas to a selected sales gas pressure suitable for flow through a sales gas pipeline.

An apparatus as claimed in claim 7 further comprising an inlet compressor fluidly coupled to the feed line upstream of the chiller and wherein the inlet compressor and control valve are configurable such that the pressure of the residue gas leaving the separator is at or above a selected sales gas pressure suitable for flow through a sales gas pipeline.

10. An apparatus as claimed in claim 7 further comprising a flow meter coupled to feed line upstream of the chiller and a controller communicative with the control valve and the flow meter, the controller having a memory having stored thereon steps and instructions executed by the controller to dynamically change the control valve settings in response to changes in the natural gas pressure measured by the flow meter.

11. An apparatus as claimed in claim 10 wherein the controller is further communicative with the chiller and the steps and instructions further include dynamically changing the chiller settings in response to the changes in the natural gas pressure measured by the flow meter.