Patent Application
20180142949
This disclosure relates generally to a partial open-loop nitrogen refrigeration process and system for an oil or gas production operation.
Oil and gas are often available for production in areas that are not sufficiently served by infrastructure to capture at least the gaseous portion of that production. This is most often encountered in initial well production or during field development. For production operations such as flow testing or fracture treatment clean-up, the liquid portion of the production, water, condensate or oil, is readily captured by simple phase separation from the gaseous stream and subsequently sold or disposed. Should a pipeline or suitable gas processing facility not be accessible, the separated gaseous stream cannot be captured and by necessity is vented to atmosphere or flared. Alternatively, the gaseous production composition may not meet a pipeline or facility inlet specification due to composition or contamination and again vented to atmosphere or flared. Failure of the gaseous production to meet a specification may result from:
This gaseous production cannot be captured unless a compatible pipeline is available or localized suitable processing provided. By necessity, the gaseous production that does not meet pipeline specification is vented or flared with potential negative economic and environmental consequence.
There are methods available to process gases to a desired specification or to otherwise capture those gases. One known method that addresses many of the needs is based upon refrigeration of the gaseous production stream where selected components within the gas stream can be condensed or liquefied and thereby separated. Alternatively, there are known “closed loop” methods to potentially remove contaminants by cooling components of the gas stream to a solid phase and then separating the components. In this manner, a gas stream may have contaminants removed, selected components removed and captured, or the entire gaseous stream suitably processed and liquefied for capture, thereby avoiding venting and/or flaring.
However, existing closed loop methods to process gas streams using refrigeration, though highly efficient, tend to be large and complex such that they are not suitable for interim flow processing and capture. For large scale systems, multiple levels of refrigeration, cascaded systems, and mixed refrigerants are used. The large variation in potential structures, coupled with the integration of heat exchangers, and the implications of heat rejection from refrigeration, results in an extremely complex system. The use of cryogenic refrigerants in closed loop systems is complex mainly due to the requirement to capture and re-use the entire refrigerant load. Typically, application of these systems results in complete vaporization of the refrigerant. These vapors then require compression, with associated unwanted heating, to elevate to the pressures needed for subsequent liquefaction. The vapors are then pre-cooled to at least ambient temperature and adiabatically expanded to cause at least partial re-liquefaction. This compression and cooling of the refrigerant vapor to form some liquid, and the subsequent separation and recycle of the remaining vapor results in a large equipment capacity requirement and a complex process not easily applied to, or suitable for, temporary gas stream processing operations.
To permit processing and capture of remote or contaminated gas streams, a simple, low cost refrigeration process is desired. Such a process will permit ready mobilization to a well site, production collection point or existing facility, present a comparatively small equipment footprint and provide ease of start-up and operation.
According to one aspect of the invention, there is provided a method for cooling a production stream from an oil or gas production operation. The method comprises the following steps: flowing a refrigerant feed stream comprising a non-greenhouse gas (GHG) refrigerant and a production stream comprising a hydrocarbon fluid through a production stream heat exchanger such that the production stream is cooled and the refrigerant feed stream is heated; flowing a refrigerant return stream comprising the non-GHG refrigerant out of the first production stream heat exchanger and into a pre-cooling heat exchanger wherein the refrigerant return stream is cooled; reducing pressure of the refrigerant return stream to further cool the refrigerant return stream and producing a liquid stream and a vapor stream; and recovering at least some of the liquid stream and venting at least some of the vapor stream. The non-GHG refrigerant can be selected from a group consisting of nitrogen, ammonia, helium, neon, oxygen, air, argon and krypton.
In the step of flowing the refrigerant return stream, the refrigerant return stream can be cooled by flowing the vapor stream into the pre-cooling heat exchanger such that the vapor stream is heated. Alternatively, the refrigerant return stream can be cooled by flowing the refrigerant feed stream into the pre-cooling heat exchanger before flowing into the production stream heat exchanger. Also, the step of reducing pressure of the refrigerant return stream can comprise flowing the refrigerant return stream into an expander, such as a throttling valve or a turbo-expander, and the step of producing a liquid stream and a vapor stream can comprise flowing the refrigerant return stream into a phase separator.
The refrigerant return stream can be cooled in the pre-cooling heat exchanger such that condensation occurs within the refrigerant return stream. After cooling in the pre-cooling heat exchanger, the refrigerant return stream can be flowed into the expander wherein the refrigerant return stream is expanded and further cooled such that the refrigerant return stream leaving the expander is a saturated liquid.
Optionally, the vapor stream can be flowed into the production stream heat exchanger such that the vapor stream is heated and the production stream is cooled. Also, the liquid stream can be flowed into a liquid storage tank fluidly coupled to the refrigerant feed stream, such that the refrigerant feed stream comprises at least some of the liquid stream. The refrigerant feed stream can be adiabatically pressured from a selected storage pressure to a selected system pressure.
Furthermore, the production stream and the vapor stream can be flowed from the pre-cooling heat exchanger into a chiller heat exchanger such that the production stream is cooled and the vapor stream is heated. Also furthermore, the production stream can be flowed through a throttling valve such that the production stream is expanded and cooled.
The production stream can comprise natural gas which is liquefied when the production stream is cooled in the production stream heat exchanger.
The method can further comprise flowing the production stream out of the production stream heat exchanger and into a phase separator to produce a lean production stream for flowing into a pipeline or downstream process, and a condensed liquid phase production stream for storage in a production liquids storage tank. The production stream upstream of the production stream heat exchanger can comprise gaseous phase natural gas with a methane composition below a pipeline or process inlet specification, in which case the method further comprises cooling the production stream in the production stream heat exchanger to a temperature which condenses hydrocarbon heavy ends from the production stream to produce the lean production stream comprising gaseous phase natural gas with a methane composition at or above the pipeline or process inlet specification, and the liquid phase production stream comprising the condensed hydrocarbon heavy ends.
Embodiments of the invention described herein relate to a cooling system and a method for cooling a hydrocarbon production stream such as natural gas using a non-greenhouse gas (GHG) such as nitrogen as a cooling medium (“refrigerant”) wherein only a portion of a non-GHG refrigerant stream is recovered, with a vapor portion of the non-GHG refrigeration stream being vented from the system. In other words, a partial sacrificial process is used to form a partial open-loop refrigeration process. Unlike a conventional sacrificial nitrogen refrigeration process which vents all the nitrogen refrigerant after cooling a production stream, the described embodiments comprise means for recovering some of the non-GHG refrigerant thereby improving the operating efficiency of the process compared to conventional sacrificial nitrogen refrigeration processes. Also unlike conventional closed loop nitrogen refrigeration processes which recover all of the nitrogen refrigerant after cooling a production stream, the described embodiments can recover nitrogen or another non-GHG refrigerant without the complex and costly equipment used in closed loop systems to compress vapor. Therefore, the described embodiments provide a method and system that is relatively simple and cost effective to provide refrigeration to oil and/or gas operations.
In addition to cooling a production stream in an oil or gas production operation, the described embodiments can be used to remove contaminants from a hydrocarbon production stream, effect separation and capture of individual or group types of hydrocarbon components from a production stream, liquefy selected hydrocarbon components from a production stream, or liquefy a production stream.
Nitrogen is a suitable refrigerant for the process and will be referenced in the described embodiments; however other non-GHG refrigerants can be used such as ammonia, helium, neon, oxygen, air, argon or krypton, provided that their application provides sufficient cooling to meet the refrigeration need, and that they are not greenhouse gases. Nitrogen exhibits the desired temperature and thermodynamic properties to effectively cool all components of a natural gas stream to meet the desired purposes. Nitrogen in liquid form is typically available at a temperature of −196° C. and at a pressure near atmospheric. This low temperature is sufficient to liquefy or freeze all typical components of a natural gas stream including the lowest boiling point hydrocarbon, methane, which liquefies at −162° C. and freezes at −182° C. Notably, liquid nitrogen is readily available and commonly used within the oil and gas industry with good availability at quantity and supporting logistics and often within proximity of many oil and gas production operations. In many applications nitrogen is typically provided from an offsite nitrogen plant and transported to site using cryogenic tanks Importantly, nitrogen is a primary component of air and thus not harmful to the environment when vented.
First Embodiment of a System for Cooling a Natural Gas Production Stream
According to a first embodiment and referring to
The production stream refrigeration system 10 generally comprises a liquid nitrogen storage tank 100 that is fluidly coupled to an external nitrogen source (not shown) via a supply conduit 170. Nitrogen is stored in the storage tank 100 as cryogenic liquid nitrogen between atmospheric and 5,000 kPa pressure. Excess vapor within the liquid nitrogen storage tank 100 is controllably vented to atmosphere via a venting conduit 160 with vent control provided by a valve 159.
A first refrigerant stream feed conduit 101 fluidly couples the liquid nitrogen storage tank 100 to a high pressure nitrogen pump 110, which is a specialized liquefied nitrogen pump capable of pressuring liquid nitrogen to the desired refrigeration system pressure. This pump 110 may be a positive displacement pump or a centrifugal pump and may be external to the nitrogen storage tank 100 or internal to the nitrogen storage tank 100 as a submerged pump (not shown). The output from the high pressure liquid nitrogen pump 110 is in a liquid or super-cooled liquid state and at or below the critical temperature.
A second refrigerant feed conduit 111 fluidly couples the high pressure nitrogen pump 110 to a refrigerant conduit inlet in a production stream heat exchanger 20. The production stream heat exchanger 20 is part of a natural gas processing system that may utilize a single refrigeration step or multiple refrigeration steps as needed to achieve the desired processing result (not shown). Further, within the natural gas processing system, condensed components from the natural gas production stream may be separated and diverted from the process (not shown). A first refrigerant return stream conduit 125 is fluidly coupled to a refrigerant conduit outlet in the production stream heat exchanger 20, as well as to a refrigerant conduit inlet in a pre-cooler heat exchanger 130. A second refrigerant return stream conduit 135 is fluidly coupled to a refrigerant conduit outlet in the pre-cooler heat exchanger 130, as well as to an inlet of an expander 140. A third refrigerant return stream conduit 145 is fluidly coupled to an outlet of the expander 140 as well as to an inlet of a phase separator 150. A nitrogen liquid conduit 153 is fluidly coupled to a liquid outlet of the phase separator 150, as well as to an inlet of the nitrogen storage tank 100.
A first nitrogen vapor conduit 157 is fluidly coupled to a vapor outlet of the phase separator 150, as well as to a vapor conduit inlet in the pre-cooling heat exchanger 130. A second nitrogen vapor conduit 158 is coupled to a vapor conduit outlet in the pre-cooler heat exchanger 130, as well as to a vapor conduit inlet in the production stream heat exchanger 20. A first nitrogen vapor venting conduit 190 is fluidly coupled to a vapor conduit outlet in the production stream heat exchanger 20 and is open to the atmosphere. A second nitrogen vapor venting conduit 189 is controllably open to the atmosphere by a vent control valve 188 and is fluidly coupled to the vapor conduit outlet in the pre-cooling heat exchanger 130.
A first production stream conduit 195 is fluidly coupled to a production stream source (not shown) and to a production stream inlet in the production stream heat exchanger 20. A second production stream conduit 196 is coupled to a production stream outlet in the production stream heat exchanger 20, as well as to a production stream destination (not shown).
In operation, the high pressure nitrogen pump 110 is operated to flow a cryogenic liquid nitrogen refrigerant feed stream under a selected pressure from the liquid nitrogen storage tank 100 and through the refrigerant feed conduits 101, 111 into a refrigerant stream conduit in the production stream heat exchanger 20. At the same time, a natural gas production stream is flowed through the first production stream conduit 195 and into a production stream conduit in the production stream heat exchanger 20. The nitrogen refrigerant feed stream is in thermal but not physical communication with the natural gas production stream in the production stream heat exchanger 20, such that the nitrogen refrigerant feed stream is heated and the natural gas production stream is cooled therein. The cooled natural gas production stream leaves the production stream heat exchanger 20 through the second production stream conduit 196, and the heated nitrogen refrigerant stream leaves the production stream heat exchanger 20 as a nitrogen refrigerant return stream through the first refrigerant return stream conduit 125.
In the production stream heat exchanger 20, the cryogenic liquid nitrogen refrigerant stream is at least partially vaporized while the natural gas production stream achieves its desired refrigeration. The nitrogen refrigerant return stream flowing through the first refrigerant return stream conduit 125 enters the pre-cooler heat exchanger 130. The pre-cooler heat exchanger 130 is a concurrent heat exchanger wherein the warmed nitrogen refrigerant return stream is in thermal communication with a cooler waste nitrogen vapor stream flowing into the pre-cooler heat exchanger 130 via the first nitrogen vapor conduit 157, thereby cooling the nitrogen refrigerant return stream to a desired temperature.
Upon exiting the pre-cooler heat exchanger 130, the nitrogen refrigerant return stream is directed via the second refrigerant return stream conduit 135 to the expander 140 to cause further cooling and promote liquefaction of at least some vapor within the nitrogen refrigerant return stream. The expander 140 cools the nitrogen refrigerant return stream by pressure reduction such as that provided by a simple throttling valve. Alternatively, cooling may be enhanced by utilizing a turbo-expander where further energy is removed as work. The de-pressured nitrogen refrigerant return stream exits the expander 140 via the third refrigerant return stream conduit 145 and is directed to the phase separator 150. Within the phase separator 150, vapor and liquid nitrogen are separated, with nitrogen vapor exiting through the first nitrogen vapor conduit 157 and liquid nitrogen through the nitrogen liquid conduit 153. The exiting nitrogen vapor within the nitrogen liquid conduit 157 is a waste stream and is directed to the pre-cooler heat exchanger 130 where the waste vapor stream is used to cool the entering nitrogen refrigerant return stream 125 and then optionally vented to atmosphere through the vent control valve 188 and the second nitrogen vapor venting conduit 189. Alternately, upon exiting the pre-cooling heat exchanger 130, the waste vapor stream is directed via the second nitrogen vapor conduit 158 to the production stream heat exchanger 20 for further cooling duty and then vented via the first nitrogen vapor venting conduit 190 as desired.
The recovered liquid nitrogen stream flows through nitrogen liquid conduit 153 from the phase separator 150, and is directed to the liquid nitrogen storage tank 100 wherein nitrogen losses to waste 160, 189, 190, are replaced from an external nitrogen source (not shown) via the supply conduit 170.
Second Embodiment of a System for Cooling a Natural Gas Production Stream
According to a second embodiment and referring to
The components of the second embodiment of the production stream cooling system 10 are the same as the first embodiment, except that the refrigerant feed conduit 111 coupling the high pressure nitrogen pump 110 to the production stream heat exchanger 20 is replaced by a first refrigerant feed conduit 211 that fluidly couples the high pressure nitrogen pump 110 to a refrigerant feed inlet of the pre-cooler heat exchanger 130, and a second refrigerant feed conduit 212 that fluidly couples a refrigerant feed outlet of the pre-cooler heat exchanger 130 to a refrigerant feed inlet of the production stream heat exchanger 20. Additionally, the nitrogen vapor conduit 157 in the first embodiment coupling the phase separator 150 to the pre-cooler heat exchanger 130, is replaced with another nitrogen vapor conduit 257 that fluidly couples the phase separator 150 to the vent control valve 188 and/or to the second nitrogen vapor conduit 158.
With these modifications, the second embodiment utilizes liquid nitrogen in a partially open loop system to provide cooling to a natural gas production stream requiring refrigeration wherein the nitrogen refrigerant return stream is pre-cooled using the nitrogen refrigeration feed stream as the cooling medium prior to it entering the production stream heat exchanger 20. In operation, nitrogen refrigerant stored in the liquid nitrogen storage tank 100 is pressurized and supplied by the high pressure nitrogen pump 110 and flows through conduits 111, 211 into the pre-cooler heat exchanger 130 as the nitrogen refrigerant feed stream, wherein it serves to cool the nitrogen refrigerant return stream flowing through the pre-cooler heat exchanger 130. The nitrogen refrigerant feed stream then exits the pre-cooler heat exchanger 130 via conduit 212 and enters the production stream heat exchanger 20, wherein it serves to cool the production stream flowing through the production stream heat exchanger 20.
As in the first embodiment, the entering natural gas production stream 195 is placed in thermal communication, via heat exchange with the cryogenic liquid nitrogen refrigerant feed stream from conduit 212, causing the liquid nitrogen refrigerant feed stream to at least partially vaporize while achieving the desired natural gas processing refrigeration. The nitrogen exits the natural gas processing system 20 through conduit 125 as the nitrogen refrigerant return stream, and enters the pre-cooler heat exchanger 130 where the warmed nitrogen refrigerant return stream is cooled to a desired temperature. Upon exiting the pre-cooler heat exchanger 130 the nitrogen refrigerant return stream is directed via conduit 135 to the expansion device 140 to cause further cooling to promote liquefaction of at least some vapor within the nitrogen refrigerant return stream. The expansion device 140 cools the nitrogen refrigerant return stream by pressure reduction such as that provided by a simple throttling valve. Alternatively, cooling may be enhanced by utilizing a turbo-expander where further energy is removed as work. The de-pressured nitrogen stream exits the expander 140 and is directed to the phase separator 150 via the conduit 145. Within the phase separator 150, vapor and liquid are separated with vapor exiting through the nitrogen vapor conduit 257 and liquid through the nitrogen liquid conduit 153. The exiting vapor stream within the nitrogen vapor conduit 257 is directed via second nitrogen vapor conduit 158 to the production stream heat exchanger 20 for further cooling duty and then vented through the first nitrogen vapor venting conduit 190 as desired. Optionally, the nitrogen vapor waste stream may be vented to atmosphere through the vent control valve 188 and the second nitrogen vapor venting conduit 189.
The recovered liquid nitrogen stream 153 flows from the phase separator 150 into the liquid nitrogen storage tank 100 wherein nitrogen losses to waste 160, 189 or 190 are replaced from an external nitrogen source (not shown) via the supply conduit 170. Excess vapor within liquid nitrogen storage tank 100 is managed by venting to atmosphere via venting conduit 160 with vent control provided by the valve 159. The liquid storage tank 100 serves as the liquid nitrogen source for the refrigeration process to feed the high pressure nitrogen pump 110 to provide nitrogen for the refrigeration process.
Method for Cooling a Production Stream
According to another embodiment and referring to
The method first comprises providing a liquid nitrogen refrigerant feed stream at a selected pressure and temperature that is sufficient to meet a specified refrigeration load for cooling a natural gas production stream and recover at least a specified (target) amount of liquid nitrogen from a liquid nitrogen refrigerant return stream (step 305). The method can be performed on either of the first and second embodiments of the production stream cooling system 10.
The selected pressure and temperature of the liquid nitrogen refrigerant feed stream should result in at least some of the liquid nitrogen refrigerant return stream to be vaporized in the process of cooling the production stream, as the latent heat of vaporization of nitrogen provides excellent cooling. More particularly, the selected temperature and pressure of the nitrogen refrigerant feed stream should achieve a pressure drop that promotes sufficient cooling across an adiabatic expander to cause re-liquefaction of at least the target amount of the nitrogen vapor in the nitrogen refrigerant return stream. In the case of an expander comprising a throttling valve for isenthalpic nitrogen expansion cooling, target temperatures approaching or below −130° C. and pressures in excess of 2 MPa are desired to gain a suitable liquid. A suitable yield depends upon the economics surrounding the value of the recovered hydrocarbon stream balanced against the cost of nitrogen losses, the cost of capital for equipment and operating costs to achieve the recovery. These economics will vary with each application. In addition, environmental restrictions to venting or flaring, such as a greenhouse gas emissions levy (carbon tax) or legislative restrictions to venting or flaring may further impact the economics. Under these restrictions, the need to vent or flare a portion of the hydrocarbon stream may preclude all production unless the hydrocarbon stream is processed to minimize or eliminate venting or flaring. In the case of an expander comprising a turbo-expander for an isoentropic expansion cooling, a higher expander inlet pressure is a more determining factor for liquid yields than temperature; however reduced temperatures will improve liquid yield. The first and second embodiments of the system 10 have the capability to provide high expander inlet pressures to support efficient turbo-expander type liquefaction where the system pressure is easily and efficiently controlled by the cryogenic liquid pump 110.
To achieve the required temperature in the expander to obtain the target amount of recovered liquid nitrogen, the method comprises “pre-cooling” the nitrogen refrigerant return stream prior to entry into the expander (step 310). This pre-cooling may be accomplished using the sacrificial nitrogen vapor stream according to the first embodiment or by using the nitrogen refrigerant feed stream prior according to the second embodiment.
The nitrogen refrigerant return stream is cooled for a final time via adiabatic expansion in the expander, wherein the outlet pressure of the expander is selected to achieve the desired temperature, pressure, liquid yield and hence the condition of the sacrificial nitrogen stream that is partially recovered (step 315). Notably, a balance is struck between the outlet pressure of the expanded nitrogen stream and the liquid yield for the nitrogen stream. Lower outlet pressures provide better cooling; however a lower liquid yield is achieved at lower pressures. Further, the outlet pressure condition of the liquid nitrogen stream should be compatible with the makeup nitrogen stream and the operating conditions of the process' liquid nitrogen inlet.
After expansion in the expander, the nitrogen refrigerant return stream enters a phase separator wherein the liquid nitrogen is recovered. Nitrogen vapor that is not recovered is considered a waste stream that is directed to the production stream heat exchanger 20 to further cool the production stream (step 320). In the first embodiment, prior to cooling the natural gas production stream, the waste nitrogen vapor stream is used to pre-cool the nitrogen refrigerant return stream as previously discussed in step 310. After cooling the natural gas production stream, the waste nitrogen vapor stream is vented to atmosphere and lost to the refrigeration process.
The recovered liquid nitrogen is returned to the nitrogen refrigerant feed stream for re-use in the refrigeration method (step 325). Because some nitrogen was lost in the waste nitrogen vapor stream, makeup nitrogen is added to the nitrogen refrigerant feed stream; however, the amount of makeup nitrogen required is considerably less compared to prior art cooling systems that have no nitrogen recovery means.
The optimum configuration and conditions to minimize the vented nitrogen stream using this sacrificial refrigeration method, while meeting the desired refrigeration load, can be calculated using thermodynamic correlations and calculators such as those available from the National Institute of Standards as “NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP, National Institute of Standards and Technology, Standard Reference Data Program, Gaithersburg, 2013.” Alternatively, the calculations can be completed using a process simulation package such as the Aspen Technology, Inc., HYSYS Process Simulator or the DWSIM Open Process Simulator.
In this instance, the incoming natural gas production stream is pre-treated such that contaminants or other undesirables have already been removed from the gas stream. The natural gas production stream is comprised of 95% methane, 2% ethane, 1.5% propane, 0.5% iso-butane, 1% n-butane. Only trace amounts of heavier hydrocarbons, water vapor, nitrogen and carbon dioxide are found in the natural gas production stream. The natural gas production stream 495 enters the process at a flow rate of 2,000 kg/h. As specified for the processing conditions, the chiller 491 serves to reduce the gas stream temperature from 10° C. to −90° C. removing approximately 130 kW of energy from the natural gas production stream. The production stream is then expanded across the throttling valve 493, reducing the pressure from 1,480 kPa to 275 kPa with a predicted temperature drop of 15 C°. The natural gas production stream is now at 275 kPa pressure and −105° C. temperature, and enters the liquefier heat exchanger 496. Again, as per the process specification, the natural gas production stream is then cooled to −162° C. to completely liquefy the natural gas production stream. The energy removed from the natural gas production stream to achieve the process target temperature is approximately 352 kW. As per process specification, the created LNG is sub-cooled at −162° C. and 275 kPa such that the entire natural gas production stream is liquefied with no requirement to vent natural gas vapor from the process.
The load on the refrigeration system 30 supporting this natural gas liquefaction process is 130 kW for the chiller and 352 kW for the liquefier heat exchanger. The refrigeration subsystem 30 components include a liquid nitrogen storage tank 400, a cryogenic liquid pump 410, a nitrogen pre-cooler heat exchanger 430, an expander 440 and a nitrogen phase separator 450 which returns liquid back to the liquid nitrogen storage tank 400. Nitrogen is provided to the liquid nitrogen storage tank 400 through conduit 470 and is drawn from the storage tank 400. Pressuring to the refrigeration system operating pressure is provided by the cryogenic liquid pump 410. The pressured liquid nitrogen enters the liquefier heat exchanger 496 through conduit 411 as a nitrogen refrigerant feed stream to cool the natural gas production stream. The nitrogen refrigerant return stream exits the liquefier heat exchanger 496 and flows to the nitrogen pre-cooler heat exchange 430 via conduit 425 for cooling by the sacrificial nitrogen vapor stream 457. Upon exiting the nitrogen pre-cooler heat exchanger 430, the nitrogen refrigerant return stream enters the expander 440, in this instance a throttling valve, for further cooling. The nitrogen refrigerant return stream then enters a phase separator 450 where the liquid portion of the stream is returned to the liquid nitrogen storage tank 400 for re-use. The vapor portion of the stream is directed to the nitrogen pre-cooler heat exchanger 430 for refrigeration duty on the returning stream, then to the natural gas stream production chiller 491 to further cool the natural gas production stream. Upon exiting the natural gas stream production chiller 491 the nitrogen vapor is vented to atmosphere and lost to the process. The refrigeration process conditions for Example 1 are presented in Table 2.
With reference to
Upon exiting the liquefier heat exchanger 496, the nitrogen refrigeration return stream 425 enters the nitrogen pre-cooler heat exchanger 430. Nitrogen vapor stream 457 from the phase separator 450 is at a temperature of −186° C. and serves to cool the nitrogen refrigeration return stream 425 by heat exchange causing condensation within the nitrogen refrigerant return stream. This heat exchange causes condensation to improve the liquid mass fraction to 0.93. The energy change within the nitrogen refrigerant return stream is seen to be 44 kW though the temperatures of the nitrogen refrigerant return stream entering and exiting the nitrogen pre-cooler heat exchanger 430 remain unchanged at −158° C. The LMTD within this nitrogen pre-cooler heat exchanger 430 is 13 C.° and is expected to require a comparatively large heat exchange area. The energy transfer within the nitrogen pre-cooler heat exchanger 430 is important to the recycle yield of the process such that special attention to this heat exchanger is warranted to maximize the cooling or condensation of the nitrogen refrigerant return stream 425. The exiting nitrogen refrigerant return stream 435 enters the expander 440, which in this embodiment is a simple throttling valve, reducing pressure to 275 kPa with an accompanying temperature reduction to −186° C. The flow from the expander 440 is that of a saturated liquid where the pressure expansion has reduced the liquid phase mass fraction from 0.93 to 0.60, with an accompanying temperature reduction of 28 C°. This pressure drop also aligns the liquid pressure such that it is suitable for entry into the liquid nitrogen storage tank 400 for re-use within the system without an additional pressure drop step and further vapor generation. The outlet saturated nitrogen stream 445 then enters the phase separator 450 where vapor and liquid are directed to conduits 457 and 453 respectively.
The nitrogen vapor stream 457 exhibits a low temperature at −186° C. and a very low energy flow at −356 kW. Efficient use of this low energy stream 457 should achieve best efficiency within the refrigeration system as this is the sacrificial vapor stream. The mass rate of this sacrificial vapor stream is 5,651 kg/h from a total nitrogen inlet and rate of 13,951 kg/h representing a loss of approximately 40% of the nitrogen refrigerant return stream. The sacrificial waste stream 457 is first used to pre-cool the nitrogen refrigerant return stream in the pre-cooler heat exchanger 430 as described earlier. The 44 kW heat transfer accomplished within pre-cooler heat exchanger 430 directly reduces the cooling load for the nitrogen refrigerant return stream. The exit temperature of the nitrogen vapor stream 458 from the pre-cooler heat exchanger 430 is −162° C. with an energy flow of −312 kW; still exhibiting significant cooling capacity. The nitrogen vapor stream 458 is then directed to the inlet of the chiller 491 within the natural gas process subsystem 20 where this cooling capacity is used to cool the natural gas inlet stream to −90° C. with an energy exchange of 131 kW. Again, efficient use of this nitrogen vapor stream directly impacts the refrigeration load. In this instance the exiting vapor stream 490 temperature is −85° C. with an energy flow of −181 kW. LMTD within this chiller 491 is 83 C.° where additional cooling capacity exists should the natural gas process require it.
The liquid nitrogen stream 453 from the phase separator 450 exhibits the same temperature as the nitrogen vapor stream 457 at −186° C. As previously mentioned, this liquid nitrogen stream 453 is provided at a pressure of 275 kPa which is compatible with the presumed pressure capacity of the storage tank 400 and the nitrogen makeup supply 470 within this embodiment. Should excessive vapor result from the liquid nitrogen stream 453, venting will be necessary to manage pressure within the storage tank 400 and the cooling capacity of that vented nitrogen will be lost and erode the efficiency of the refrigeration subsystem. Recovery of the process is determined at 8,300 kg/h for a 60% recovery of the nitrogen refrigeration stream. Notably, the process improves efficiency to that of a sacrificial process with little added processing equipment; the required equipment is limited to: a liquid pump 410, a nitrogen pre-cooler 430, an expander throttling valve 440 and a phase separator 450. For simplicity, the nitrogen vapors created within the system are not processed for capture, however the low energy of the vapors are utilized to provide refrigeration to improve process efficiency. Vapor compression and following cooling is not required within the process.
The natural gas process subsystem 20 includes a natural gas production stream chiller 591 as heat exchanger, a condensing heat exchanger 596, a phase separator 585, a lean natural gas outlet 598 connected to a pipeline or process 586, a condensed liquid phase outlet 599, and a natural gas liquids storage tank 587. A natural gas production stream 595 enters the chiller 591 for pre-cooling, is further cooled across an expansion valve 593 and then flows to the condensing heat exchanger 596 wherein the natural gas production stream is refrigerated to condense and form a liquid state of the contained natural gas heavier components. The natural gas processing conditions including energy flow are presented in Table 3. The inlet and separated outlet gas stream compositions are presented in Table 4.
In this instance, the incoming natural gas production stream 595 is pre-treated such that water, carbon dioxide and other undesirables have been sufficiently removed from the gas stream. The composition of natural gas production stream 595 is presented in Table 4 illustrating a content of only 82.9% methane, below the minimum target composition of 95%. The remainder of the natural gas production stream 595 is seen to be comprised of decreasing amounts of ethane, propane, butanes, pentanes, hexanes and heptanes. Only trace amounts of heavier hydrocarbons (C7+), water vapor, nitrogen and carbon dioxide are found in the natural gas production stream 595. The natural gas production stream 595 enters the process at a flow rate of 2,000 kg/h and enters the chiller 591 to reduce the gas stream temperature from 10° C. to −39° C. removing approximately 81 kW of energy from the natural gas stream. The natural gas production stream leaving the chiller 592 is then expanded across the expansion valve 593 reducing the pressure from 1,480 kPa to 700 kPa with a predicted temperature drop of 10 C°. The natural gas leaving the expansion valve 594, now at 700 kPa pressure and −49° C. then enters condensing heat exchanger 596. Then, to meet the process specification of 95% or better methane, the natural gas production stream is then cooled to −100° C. to condense the hydrocarbon heavy ends from the gas stream to result in the gaseous stream 598 having a composition at 96% methane as shown on Table 4. The energy removed from the natural gas production stream 594 to achieve the process target composition is approximately 117 kW. This lean natural gas stream is then directed via the lean natural gas outlet 598 to the pipeline/process 586 as desired. The liquids stream, comprised of condensed natural gas liquids, is directed via liquid phase outlet 599 to the storage tank (587).
The load on the refrigeration system 30 supporting this lean natural gas process is 81 kW for the chiller 591 and 117 kW for the condensing heat exchanger 596. The refrigeration system 30 components include a liquid nitrogen storage tank 500, a cryogenic liquid pump 510, a nitrogen pre-cooler heat exchanger 530, an expander 540 and a nitrogen phase separator 550 which returns liquid back to the liquid nitrogen storage tank 500. Nitrogen is provided to the liquid nitrogen storage tank 500 through conduit 570 and is drawn from the tank and pressured to the refrigeration system operating pressure by the cryogenic liquid pump 510. The pressured liquid nitrogen refrigerant feed stream enters the pre-cooler heat exchanger 530 through conduit 511 to cool the nitrogen refrigerant return stream. The cooled nitrogen refrigerant feed stream then enters the condensing heat exchanger 596 through conduit 512 to cool the natural gas production stream. The nitrogen refrigerant return stream then exits the condensing heat exchanger 596 and flows to the pre-cooler heat exchanger 530 via the conduit 525 to complete the cooling by the incoming liquid nitrogen refrigerant feed stream 511. Upon exiting the pre-cooler heat exchanger 530, the nitrogen refrigerant return stream enters the expander 540, in this instance a throttling valve, and a pressure drop therein causes further cooling or condensation. The nitrogen refrigerant return stream then enters the phase separator 550 where the liquid portion of the stream is returned to the liquid nitrogen storage tank 500 for re-use. The vapor portion of the stream from the phase separator 550 is directed to the natural gas production stream chiller 591 for refrigeration duty on the incoming natural gas stream 595. Upon exiting the natural gas production stream chiller 591, the nitrogen vapor is vented to atmosphere and lost to the process. The refrigeration process conditions for Example 2 are presented in Table 5.
With reference to
Upon exiting the condensing heat exchanger 596, the nitrogen refrigerant return stream 525 enters the nitrogen pre-cooler heat exchanger 530 at a temperature of −158° C. Again, the nitrogen refrigerant feed stream 511, at a temperature of −189° C., cools the returning nitrogen refrigerant return stream 525 by heat exchange causing condensation within the stream. This improves the liquid mass fraction to 0.99. The energy change within the stream is seen to be 90 kW though the temperatures of the nitrogen refrigerant return stream at the entrance and exit through the pre-cooler heat exchanger 530 remain unchanged at −158° C. The LMTD within this pre-cooler heat exchanger 530 is 18° C. The energy transfer within the pre-cooler heat exchanger 530 is important to the recycle yield of the process such that special attention to this pre-cooler heat exchanger 530 is warranted to maximize the pre-cooling of the nitrogen refrigerant return stream 525. The exiting nitrogen refrigerant return stream 535 enters the expander 540, which in this embodiment is a simple throttling valve, reducing pressure to 275 kPa with an accompanying temperature reduction to −186° C. The flow from the expander 540 is that of a saturated liquid where the pressure expansion has reduced the liquid phase mass fraction from 0.99 to 0.63, with an accompanying temperature reduction of 29 C°. This pressure drop also aligns the liquid pressure such that it is suitable for entry into the liquid nitrogen storage tank 500 for re-use within the system without an additional pressure drop step and further vapor generation. The outlet saturated nitrogen refrigerant return stream 545 then enters the phase separator 550 where nitrogen vapor and liquid nitrogen are directed to liquid and vapor conduits 558 and 553 respectively.
The vapor stream 558 is seen to exhibit a low temperature at −186° C. and a low energy flow at −93 kW. Efficient use of this low energy vapor stream 558 is important to achieve best efficiency within the refrigeration system as this is the sacrificial stream and is otherwise a waste stream. The mass rate of this stream is 1,472 kg/h with a total nitrogen inlet rate of 4,000 kg/h representing a loss of approximately 37% of the applied nitrogen refrigeration stream. The vapor stream 558 is directed to an inlet of the natural gas production stream chiller 591 within the natural gas process subsystem 20 where this cooling capacity is used to cool the natural gas inlet stream to −39° C. with an energy exchange of 81 kW. Again, efficient use of this vapor stream directly impacts the refrigeration load. In this instance the exiting vapor stream 590 has a temperature of −2° C. that has a comparatively minimal energy flow of −12 kW. LMTD within this natural gas production stream chiller 591 is 54 C°.
The liquid nitrogen stream 553 from the separator 550 exhibits the same temperature as the vapor stream 457 at −186° C. As previously mentioned, this liquid nitrogen stream 553 is provided at a pressure of 275 kPa which is compatible to the presumed pressure capacity of the storage tank 500 and the nitrogen makeup supply 570 within this embodiment. Should excessive vapor result from the liquid nitrogen stream 553, venting will be necessary to manage pressure within the storage tank 500 and cooling capacity of that vented nitrogen will be lost and erode the efficiency of the refrigeration system. Recovery of the process is determined at 2,528 kg/h for a 63% recovery of the nitrogen refrigerant return stream. Importantly, the process improves efficiency compared to that of a sacrificial process with little added processing equipment; a liquid pump 510, a nitrogen pre-cooler heat exchanger 530, an expander throttling valve 540 and a phase separator 550. For simplicity, created vapors are not processed, however the low energy of the vapors are utilized to provide refrigeration to improve process efficiency. Vapor compression and following cooling is not required within the process.
This example illustrates the impact of pressure and temperature inlet conditions to an expansion device as described within step 310 of
Referring to Table 6, a range of nitrogen inlet temperatures from −90° C. through to −195° C. is simulated. General parameters for the example and relevant key behaviors of nitrogen are provided in the table header; inlet and outlet pressures, nitrogen critical temperature and pressure and the saturation temperature of nitrogen at the outlet pressure. The enthalpy at the inlet pressure of 3.5 MPa and the noted temperature is calculated and presented in column two as “Inlet Enthalpy”. The temperature at the nitrogen outlet pressure of 0.2 MPa to maintain constant enthalpy is determined and presented in the third column as “Outlet Temp”. For reference, the nitrogen saturation pressure is calculated for each resulting outlet temperature and provided in the fourth column as “Saturation Pressure”. The phase state of the outlet nitrogen stream is provided at each outlet temperature for clarity as seen in column five as “Outlet State”. The resulting quality of the outlet stream, quality being the fraction of vapor within the stream, is determined and presented within the sixth column as “Quality”.
Referring to the various outlet conditions determined over the range of inlet temperatures within Table 6, the impact of the inlet temperatures on liquid recovery is illustrated. At the inlet temperatures above −120° C., the outlet temperatures are above the critical temperature of −147° C., i.e. superheated and without any liquid created for recovery. Under these conditions, all applied nitrogen would be vented with no liquid nitrogen returned to the liquid nitrogen storage tank 100 and the application of this process is invalidated. Again, for the inlet temperature range from −120° C. through to −140° C., the outlet temperatures are below the critical temperature; however the outlet pressure of 0.2 MPa is below the saturation pressure resulting in the outlet stream being comprised of only vapor. All applied nitrogen would again, by necessity, be vented under these conditions. A saturated state is seen to result from inlet temperatures below −142° C. through to −190° C. Notably, the quality decreases with decreasing temperatures, minimizing the vapor within the outlet stream, and improving the liquid recovery. At an inlet temperature of −142° C., the quality is 0.99 with only 1% of the stream in liquid phase and able to be recovered. Conversely, at −190° C. at the inlet, the quality is 0.01 with 99% of the returning stream in liquid phase and able to be recovered. It is notable that the recovery limit in any application is set by the operating parameters of the nitrogen pre-cooler heat exchanger 130. The nitrogen vapor stream 157, following phase separation through the phase separator 150, will have as a minimum temperature the temperature of the nitrogen refrigerant return stream 145 at the expander outlet, in this case at −189.5° C. The relatively cold nitrogen vapor stream 157 must meet the cooling load to set the inlet temperature of the nitrogen refrigerant return stream 135 entering the expander 140. Dependent upon the pre-cooling load within the pre-cooler heat exchanger 130, a sufficient cooling capacity may not be available from the cold nitrogen vapor stream 157 to achieve a desired temperature for the nitrogen refrigerant return stream 135. As seen on Table 6, a higher inlet temperature of the nitrogen refrigerant return stream 135 will promote a larger proportion of the outlet flow to vapor, thereby providing more cooling capacity to the pre-cooler heat exchanger 130 and ultimately reach an equilibrium condition. Further, as a simple, low cost and portable refrigeration system, the heat exchanger size needed to achieve a desired nitrogen recovery may not be economically or operationally viable. Achieving a reasonably compact and cost-effective pre-cooler heat exchanger 130 with a cold nitrogen vapor stream 157 inlet temperature of approximately −185° C. is expected to preclude hot nitrogen refrigerant return stream 135 outlet temperatures and thereby expander 140 inlet temperatures much below −170° C. to −175° C. In this instance, a best recovery at ˜80% of the stream would be anticipated.
This example illustrates the impact of pressure and temperature inlet conditions to an expansion device as described within step 310 and deployed within the various embodiments illustrated for isoentropic expansion cooling. With reference to
Table 7, constructed like that of Table 6 in the previous example, is provided to illustrate the effect of inlet temperature on the recovery performance of this theoretical expansion device. In this instance, the nitrogen inlet entropy is calculated from the inlet pressure of 2.0 MPa at the range of inlet temperatures. As a constant entropy process, the outlet temperatures are determined at the outlet pressure for the inlet entropy. Again the saturation pressure, state and quality are then determined for the resulting outlet condition.
Referring to the various outlet conditions determined over the range of inlet temperatures within Table 7, the impact of the inlet temperatures on recovered liquid is illustrated. At inlet temperatures above −110° C., the outlet pressure of 0.2 MPa is below the saturation pressure resulting in the nitrogen refrigerant return stream at the outlet being comprised of only vapor. All applied nitrogen would be vented under these conditions without any recovery. At inlet temperatures of −100° C. and lower, a saturated or sub-cooled state is seen to result. Unlike the previous example, the embodiment of
Further to the above embodiments illustrating application towards liquefaction of a natural gas stream and removal of hydrocarbon heavy ends from the gas stream, undesirable components, as is well understood in the art, may also be removed using the partial open loop refrigeration process. Such undesirable components include nitrogen, water vapor, carbon dioxide and hydrogen sulfide. For example, nitrogen removal from a production stream is accomplished by cooling to liquid state and separating, in single or multiple stages the production stream hydrocarbon components, including methane. Nitrogen within the stream, exhibiting a normal boiling point of −196° C. will remain in a gaseous state and is readily separated from the otherwise liquefied production stream. For example, a vent (not shown) can be coupled to the conduit 498 in the system shown in
While the illustrative embodiments of the present invention are described in detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the scope of the appended claims will readily be apparent to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general concept.
| Number | Date | Country | |
|---|---|---|---|
| 62424284 | Nov 2016 | US |
