Recovering Natural Gas Liquids

Abstract

Systems and methods for recovering natural gas liquids include a distillation column; a thermosyphon reboiler fluidly connected to the distillation column; a Joule-Thomson valve fluidly connected to the distillation column and operable to control a flow of fluid into the distillation column; and a lift gas control valve operable to control a flow of lift gas into the distillation column through a return line of the thermosyphon reboiler. An interlock system operatively couples the Joule-Thomson valve and the lift gas control valve to operate the lift gas control valve in response to operation of the Joule-Thomson valve.

Claims

1. A system for recovering natural gas liquids, the system comprising: a distillation column; a thermosyphon reboiler fluidly connected to the distillation column; a Joule-Thomson valve fluidly connected to the distillation column and operable to control a flow of fluid into the distillation column; a lift gas control valve operable to control a flow of lift gas into the distillation column through a return line of the thermosyphon reboiler; and an interlock system that operatively couples the Joule-Thomson valve and the lift gas control valve to operate the lift gas control valve in response to operation of the Joule-Thomson valve.

2. The system of claim 1, wherein the interlock system comprises a controller communicatively coupled to the Joule-Thomson valve and the lift gas control valve, the controller configured to perform operations comprising: receiving a signal to start the flow of fluid into the distillation column; operating the Joule-Thomson valve to control the flow of fluid; and operating the lift gas control valve in coordination with operating the Joule-Thomson valve to control the flow of lift gas to reduce thermal stress on the thermosyphon reboiler.

3. The system of claim 2, further comprising a differential pressure sensor to measure a pressure difference across the thermosyphon reboiler.

4. The system of claim 3, wherein the operations further comprise measuring the pressure difference across the thermosyphon reboiler using the differential pressure sensor.

5. The system of claim 4, wherein operating the lift gas control valve is based on the measured pressure difference.

6. The system of claim 2, further comprising a temperature sensor to measure a temperature of the thermosyphon reboiler.

7. The system of claim 6, wherein the operations further comprise: measuring the temperature of the thermosyphon reboiler; and determining a rate of temperature change based on the measured temperature.

8. The system of claim 7, wherein operating the lift gas control valve is based on the determined rate of temperature change.

9. The system of claim 7, wherein the operations further comprise in response to determining that the rate of temperature change exceeds a threshold rate of temperature change, operating the lift gas control valve to increase the flow of lift gas.

10. The system of claim 9, wherein the operations further comprise in response to determining that the rate of temperature change is below the threshold rate of temperature change, operating the lift gas control valve to decrease the flow of lift gas.

11. The system of claim 2, further comprising a turbo-expander fluidly connected to the distillation column in parallel with the Joule-Thomson valve.

12. The system of claim 11, wherein receiving the signal to start the flow of fluid into the distillation column occurs in response to a trip of the turbo-expander.

13. A method for recovering natural gas liquids, the method comprising: receiving a signal to start a flow of fluid into a distillation column; operating a Joule-Thomson valve to control the flow of fluid into the distillation column; and operating a lift gas control valve in coordination with operating the Joule-Thomson valve to control a flow of lift gas to the distillation column through a return line of a thermosyphon reboiler to reduce thermal stress on the thermosyphon reboiler.

14. The method of claim 13, wherein operating the lift gas control valve occurs in response to operating the Joule-Thomson valve.

15. The method of claim 13, further comprising measuring a pressure difference across the thermosyphon reboiler using a differential pressure sensor.

16. The method of claim 15, wherein operating the lift gas control valve is based on the measured pressure difference.

17. The method of claim 13, further comprising: measuring a temperature of the thermosyphon reboiler; and determining a rate of temperature change based on the measured temperature.

18. The method of claim 17, wherein operating the lift gas control valve is based on the determined rate of temperature change.

19. The method of claim 17, further comprising: in response to determining that the rate of temperature change exceeds a threshold rate of temperature change, operating the lift gas control valve to increase the flow of lift gas; and in response to determining that the rate of temperature change is below the threshold rate of temperature change, operating the lift gas control valve to decrease the flow of lift gas.

20. The method of claim 13, wherein receiving the signal to start the flow of fluid into the distillation column occurs in response to a trip of a turbo-expander fluidly coupled to the distillation column in parallel with the Joule-Thomson valve.

Description

DESCRIPTION OF DRAWINGS

[0008] FIG. 1 is a schematic of a portion of a natural gas production system.

[0009] FIG. 2 is a schematic of portion of a distillation system including a Joule-Thomson valve and a lift gas control valve.

[0010] FIG. 3 is a flow chart for a method of recovering natural gas liquids.

[0011] FIG. 4 is a plot of rate of temperature change of a reboiler during startup of Joule-Thomson mode operation.

[0012] FIG. 5 is a plot of rate of temperature change of a reboiler during startup of Joule-Thomson mode operation using lift gas to decrease thermal stress in the reboiler.

[0013] FIG. 6 is a block diagram illustrating an example computer system used to provide computational functionalities associated with described algorithms, methods, functions, processes, flows, and procedures according to some implementations of the present disclosure.

[0014] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0015] NGL recovery facilities can include a reboiler (e.g., a thermosyphon reboiler) fluidly connected to a distillation column. The reboiler can receive NGL from the bottom of the distillation column, partially vaporize the liquid, and return the vapor to the distillation column. When the distillation column is operated in a Joule-Thomson mode, recovery of NGLs can decrease as compared to operation with a turbo-expander. The decreased NGL recovery can result in intermittent flow of liquids to the reboiler. The intermittent flow combined with differences in heat capacity between liquid and vapor phases, the latent heat and bubble point can cause thermal stresses on the reboiler that can lead to failure of the reboiler.

[0016] This disclosure provides an approach for recovering natural gas liquids. The approach can include a distillation column and a thermosyphon reboiler fluidly connected to the distillation column. Flow of feed gas into the distillation column can be controlled by flow control valves such as Joule-Thomson valves. A lift gas control valve can control flow of lift gas into the distillation chamber through a return line of the thermosyphon reboiler where the lift gas is a taken as a small branch from the feed gas. An interlock system can operatively couple the Joule-Thomson valve and the lift gas control valve to operate the lift gas control valve in response to operation of the Joule-Thomson valve to reduce thermal stress on the thermosyphon reboiler.

[0017] FIG. 1 is a schematic of a portion of a natural gas processing system 100 that can be used to separate NGLs from raw natural gas. The system 100 includes a gas-liquid separator 106, a turbo-expander/recompressor system 105, and a distillation column 110 (e.g., a de-methanizer column). Dry natural gas 102 enters the gas-liquid separator 106, which is configured to separate liquid of the natural gas from the gas. The gas stream from the gas-liquid separator is split into two streams: one stream is provided to the turbo-expander 108 of the turbo-expander/recompressor system 105 (e.g., through valve 111), and the other stream is provided to cold boxes 104 for heat exchange purposes before it enters the distillation column 110. The gas stream is expanded by the turbo-expander that lowers the pressure and temperature of the gas stream. The cooled gas stream is then provided to the distillation column 110.

[0018] A reboiler 116 (e.g., a thermosyphon reboiler) is connected to the distillation column 110. The reboiler 116 receives liquid from the distillation column 110, partially vaporizes the liquid, and returns the liquid-vapor mixture to the distillation column 110. A thermosyphon reboiler circulates fluid under natural convection. The higher temperature gases have lower density and rise to the top of the reboiler 116. The rising fluid is replaced at the bottom of the reboiler 116 by cooler fluid (e.g., liquid from the bottom of distillation column 110).

[0019] The distillation column 110 produces residue gas which is used by cold boxes 118 for heat exchange purposes before it is provided to the recompressor 120 of the turbo-expander/recompressor system 105. The recompressor 120 compresses the residue gas product 124 that can be provided to a sales gas distribution header (e.g., a residue gas header).

[0020] The system 100 includes a Joule-Thomson valve 112 that is utilized in a Joule-Thomson mode of operation that bypasses the turbo-expander/recompressor system 105. The system 100 can operate in the Joule-Thomson mode, for example, when the turbo-expander 108 trips.

[0021] The system 100 also includes a lift gas control valve 114. The lift gas control valve 114 controls the flow of lift gas to the distillation column 110 through the return line of the reboiler 116. The lift gas is taken as a slipstream from the gas stream from the gas-liquid separator 106. The lift gas can be warm, dry and mercury-free.

[0022] The Joule-Thomson valve 112 and the lift gas control valve 114 are coupled through an interlocking system 126. The interlocking system 126 includes, for example, a controller or computer system operable to send control signals to the Joule-Thomson valve 112 and the lift gas control valve 114 to control the flow of gas through the respective valves. The interlocking system 126 can operate the lift gas control valve 114 in coordination with the Joule-Thomson valve 112. The interlocking system 126 can also be communicatively coupled to the turbo-expander 108 such that if operation of the turbo-expander stops, the interlocking system 126 can open the Joule-Thomson valve 112 and the lift gas control valve 114.

[0023] When an upset event occurs, such as when the turbo-expander 108 trips, gas flow through the turbo-expander 108 ceases immediately. The system 100 can be operated in Joule-Thomson mode in such circumstances. The interlocking system 126 can open the Joule-Thomson valve 112 to allow gas flow to the distillation column 110. While operating the system 100 in Joule-Thomson mode, the recovery of NGLs decreases as compared to operation in the turbo-expander mode causing intermittent flow of NGLs to the reboiler 116. The intermittent flow can cause thermal stresses on the reboiler 116.

[0024] To mitigate the thermal stresses caused during the Joule-Thomson mode of operation, the interlocking system 126 operates the lift gas control valve 114 to provide lift gas through the return line of the reboiler 116. The lift gas control valve 114 can be operated at any time when the system 100 is operating in the Joule-Thomson mode of operation (e.g., during startup and shutdown procedures, during turbo-expander trips, etc.). The flow of lift gas decreases the pressure at the top of the reboiler 116 and aids in circulation of the fluid through the reboiler 116. Use of lift gas can decrease the rate of temperature change of the reboiler 116 relative to not using the lift gas, thereby enabling a more gradual thermal expansion of components within the reboiler 116. The decreased rate of temperature change reduces thermal shock of the reboiler 116 enabling a longer service life of the reboiler 116.

[0025] In some implementations, the system 100 does not include a turbo-expander, and the system 100 is always operated in Joule-Thomson mode. The interlocking system 126 can control the flow of lift gas to maintain a desired rate of temperature change for the reboiler. This can be advantageous to prolong the operating life of the reboilers when the reboilers have low wall thicknesses, when not enough thermosyphon action is occurring, or when the physical properties of the liquid are far from the design conditions. In some implementations, the interlocking system 126 is used with NGL recovery processes that use J-T mode solely (e.g., without using turbo-expanders) during low integrity or high sensitivity of the reboilers to reduce thermal stresses caused by intermittent liquid flow.

[0026] FIG. 2 is a schematic of an example distillation system 200 for recovering NGLs. The system 200 includes a Joule-Thomson valve 204 interlocked with a lift gas control valve 214 to reduce thermal stress caused by rapid temperature changes of the thermosyphon reboiler 208.

[0027] Feed gas 202 (e.g., raw natural gas containing NGLs) flows through the Joule-Thomson valve 204 to reach the distillation column 206. The Joule-Thomson valve expands the feed gas 202 decreasing the pressure and temperature of the gas. The distillation column 206 separates the species of NGLs in the feed gas 202.

[0028] The distillation column 206 is fluidly coupled to the thermosyphon reboiler 208. Liquid from the bottom of the distillation column 206 enters the thermosyphon reboiler 208 through the inlet 208a located at the bottom of the thermosyphon reboiler 208. The thermosyphon reboiler 208 partially vaporizes the liquid and returns the liquid-vapor mixture to the distillation column 206 through the return line 209.

[0029] Lift gas 210 is taken as a slipstream from the feed gas 202. The lift gas can be warm, dry, mercury-free, and at relatively high pressure compared to the operating pressure of the distillation column 206. The lift gas flows through the return line 209 of the thermosyphon reboiler 208 to the distillation column 206. Flow of the lift gas 210 to the distillation column 206 is controlled by lift gas control valve 214. In an open position (or partially open position), the lift gas control valve 214 allows lift gas to flow to the distillation column 206. In a closed position, the lift gas control valve 214 inhibits the lift gas from flowing to the distillation column 206. Valves 212, 216, 218 are shutdown and isolation valves that can be used as safeguards in case of failures (e.g., due to stuck valves, software malfunction, transmission failures, etc.).

[0030] Interlock system 220 is operatively coupled to the Joule-Thomson valve 204 and the lift gas control valve 214. The interlock system 220 receives signals from the Joule-Thomson valve 204 and operates the lift gas control valve 214 in response to operation of the Joule-Thomson valve 204. For example, the interlock system 220 can send control signals to the lift gas control valve 214 to open the lift gas control valve 214 when the Joule-Thomson valve 204 sends a signal to the interlock system 220 that the Joule-Thomson valve 204 is opening. The interlock system 220 can send signals to the lift gas control valve 214 to partially open or partially close based on the signal from the Joule-Thomson valve 204.

[0031] A differential pressure sensor 222 is coupled between the inlet 208a of the thermosyphon reboiler 208 and the return line 209. The differential pressure sensor 222 is operative to measure a pressure difference across the thermosyphon reboiler 208. The differential pressure sensor 222 is communicatively coupled to the interlock system 220. The differential pressure sensor 222 sends signals to the interlock system 220 indicating the pressure difference across the thermosyphon reboiler 208. The interlock system 220 can operate the lift gas control valve based on the measured pressure difference from the differential pressure sensor 222. For example, the interlock system 220 can send a signal to the lift gas control valve 214 to open the lift gas control valve 214 when the measured pressure difference is less than a lower threshold value. Alternatively, or additionally, the interlock system 220 can send a signal to the lift gas control valve 214 to close the lift gas control valve 214 when the measured pressure difference is greater than an upper threshold value. The interlock system 220 can utilize feedback control techniques to maintain a target pressure difference across the thermosyphon reboiler 208.

[0032] A temperature sensor 224 is coupled to the thermosyphon reboiler 208. The temperature sensor 224 is operative to measure a temperature of the thermosyphon reboiler 208. For example, the temperature sensor 224 can be mounted to a brazed aluminum heat exchanger that forms the thermosyphon reboiler 208 to measure the temperature of the thermosyphon reboiler 208. Alternatively, or additionally, the temperature sensor 224 can be configured to measure the temperature at the outlet of the thermosyphon reboiler 208. The temperature sensor 224 is communicatively coupled to the interlock system 220. The temperature sensor 224 sends signals to the interlock system 220 indicating a temperature of the thermosyphon reboiler 208. The interlock system 220 can determine a rate of temperature change of the thermosyphon reboiler 208 based on signals received from the temperature sensor 224. Alternatively, the temperature sensor 224 can determine the rate of temperature change, and the interlock system 220 can receive the determined rate of temperature change from the temperature sensor 224.

[0033] The interlock system 220 can operate the lift gas control valve 214 based on the measured temperature, the determined rate of temperature change or both. For example, the interlock system 220 can send a signal to the lift gas control valve 214 to open the lift gas control valve 214 when the measured temperature or the determined rate of temperature change exceeds a threshold value.

[0034] In some implementations, the interlock system 220 receives the determined rate of temperature change from the temperature sensor 224. The interlock system 220 opens the lift gas control valve 214 to fully open when the Joule-Thomson mode is operating (e.g., the Joule-Thomson valve is opened). The Joule-Thomson mode can be triggered, for example, when the turbo-expander trips or manually (e.g., in advance of planned activities). The interlock system 220 can determine the amount to open the lift gas control valve 214 to achieve a target rate of temperature change of the thermosyphon reboiler 208. The interlock system 220 can decrease the amount that the lift gas control valve 214 is open until the target rate of temperature change is achieved. The interlock system 220 can minimize the flow rate of the lift gas to the lowest flow rate that results in maintaining the target rate of temperature change by operating the lift gas control valve 214.

[0035] FIG. 3 is flowchart for an example method 300 of recovering NGLs. The method 300 can be implemented on a control system (e.g., interlock system 126, 220 or the computer system of FIG. 6).

[0036] The control system receives a signal to start a flow of fluid into a distillation column (step 302). In some implementations, the signal to start the flow of fluid is generated when a turbo-expander trips (e.g., stops working). The signal to start the flow of fluid can be received in response to the turbo-expander being tripped.

[0037] The control system operates a Joule-Thomson valve in a distillation system to control a flow of fluid into the distillation column (step 304). For example, the control system generates and sends a control signal to the Joule-Thomson valve to open the Joule Thomson valve when the control system receives the signal to start the flow of the fluid.

[0038] The control system operates a lift gas control valve in coordination with operating the Joule-Thomson valve (step 306). Operating the lift gas control valve controls the flow of lift gas to the distillation column through a return line of a thermosyphon reboiler to reduce thermal stress on the thermosyphon reboiler. The control system can operate the lift gas control valve in coordination with operating the Joule-Thomson valve by operating the lift gas valve in response to operation of the Joule-Thomson valve. For example, the control system can generate and send a control signal to the lift gas control valve to open the lift gas control valve in response to the Joule-Thomson valve opening. Alternatively, or additionally, the control system can send a control signal to the lift gas control valve to close the lift gas control valve in response to the Joule-Thomson valve closing. The control system can indicate a partially opened state for the lift gas control valve based on the open status of the Joule-Thomson valve.

[0039] In some implementations, the control system measures a pressure difference across the thermosyphon reboiler using a differential pressure sensor. The control system can operate the lift gas control valve based on the measured pressure difference.

[0040] In some implementations, the control system measures a temperature of the thermosyphon reboiler and determines a rate of temperature change based on the measured temperature. The control system can operate the lift gas control valve based on the determined rate of temperature change. For example, the control system can operate the lift gas control valve to increase the flow of lift gas in response to determining that the rate of temperature change exceeds a threshold rate of temperature change.

[0041] In some implementations, the control system fully opens the lift gas control valve in response to the opening of the Joule-Thomson valve. The control system can decrease the amount that the lift gas control valve is open (e.g., to decrease the flow of lift gas) until the threshold rate of temperature change of the thermosyphon reboiler is achieved.

[0042] FIG. 4 is a composite plot showing rate of temperature change versus time for a thermosyphon reboiler during startup of a Joule-Thomson mode of operation without using lift gas. At many times, the signal saturates the sensor's operating range. The saturated signals exceeded a threshold rate of temperature change of 3.6 degrees Fahrenheit per minute (2 degrees Celsius per minute). Based on detailed analysis of the signals and operating conditions of the thermosyphon reboiler, the extrapolated rate of temperature change of the thermosyphon reboiler reached as high as 28.8 degrees Fahrenheit per minute (16 degrees Celsius per minute).

[0043] FIG. 5 is a composite plot showing rate of temperature change versus time for a thermosyphon reboiler during startup of a Joule-Thomson mode of operation using lift gas. The maximum rate of temperature change experienced by the reboiler was 3.4 degrees Fahrenheit per minute (1.9 degrees Celsius per minute), which is less than the threshold rate of temperature change of 3.6 degrees Fahrenheit per minute (2 degrees Celsius per minute). The Joule-Thomson mode startup using lift gas reduced the thermal shocks and stresses experienced by the thermosyphon reboiler. Reduced thermal stress can increase the service life and/or increase the time between maintenance or repair of the thermosyphon reboiler.

[0044] FIG. 6 is a block diagram of an example computer system 600 used to provide computational functionalities associated with described algorithms, methods, functions, processes, flows, and procedures described in the present disclosure, according to some implementations of the present disclosure. The illustrated computer 602 is intended to encompass any computing device such as a server, a desktop computer, a laptop/notebook computer, a wireless data port, a smart phone, a personal data assistant (PDA), a tablet computing device, or one or more processors within these devices, including physical instances, virtual instances, or both. The computer 602 can include input devices such as keypads, keyboards, and touch screens that can accept user information. Also, the computer 602 can include output devices that can convey information associated with the operation of the computer 602. The information can include digital data, visual data, audio information, or a combination of information. The information can be presented in a graphical user interface (UI) (or GUI).

[0045] The computer 602 can serve in a role as a client, a network component, a server, a database, a persistency, or components of a computer system for performing the subject matter described in the present disclosure. The illustrated computer 602 is communicably coupled with a network 630. In some implementations, one or more components of the computer 602 can be configured to operate within different environments, including cloud-computing-based environments, local environments, global environments, and combinations of environments.

[0046] At a high level, the computer 602 is an electronic computing device operable to receive, transmit, process, store, and manage data and information associated with the described subject matter. According to some implementations, the computer 602 can also include, or be communicably coupled with, an application server, an email server, a web server, a caching server, a streaming data server, or a combination of servers.

[0047] The computer 602 can receive requests over network 630 from a client application (for example, executing on another computer 602). The computer 602 can respond to the received requests by processing the received requests using software applications. Requests can also be sent to the computer 602 from internal users (for example, from a command console), external (or third) parties, automated applications, entities, individuals, systems, and computers.

[0048] Each of the components of the computer 602 can communicate using a system bus 603. In some implementations, any or all of the components of the computer 602, including hardware or software components, can interface with each other or the interface 604 (or a combination of both), over the system bus 603. Interfaces can use an application programming interface (API) 612, a service layer 613, or a combination of the API 612 and service layer 613. The API 612 can include specifications for routines, data structures, and object classes. The API 612 can be either computer-language independent or dependent. The API 612 can refer to a complete interface, a single function, or a set of APIs.

[0049] The service layer 613 can provide software services to the computer 602 and other components (whether illustrated or not) that are communicably coupled to the computer 602. The functionality of the computer 602 can be accessible for all service consumers using this service layer. Software services, such as those provided by the service layer 613, can provide reusable, defined functionalities through a defined interface. For example, the interface can be software written in JAVA, C++, or a language providing data in extensible markup language (XML) format. While illustrated as an integrated component of the computer 602, in alternative implementations, the API 612 or the service layer 613 can be stand-alone components in relation to other components of the computer 602 and other components communicably coupled to the computer 602. Moreover, any or all parts of the API 612 or the service layer 613 can be implemented as child or sub-modules of another software module, enterprise application, or hardware module without departing from the scope of the present disclosure.

[0050] The computer 602 includes an interface 604. Although illustrated as a single interface 604 in FIG. 6, two or more interfaces 604 can be used according to particular needs, desires, or particular implementations of the computer 602 and the described functionality. The interface 604 can be used by the computer 602 for communicating with other systems that are connected to the network 630 (whether illustrated or not) in a distributed environment. Generally, the interface 604 can include, or be implemented using, logic encoded in software or hardware (or a combination of software and hardware) operable to communicate with the network 630. More specifically, the interface 604 can include software supporting one or more communication protocols associated with communications. As such, the network 630 or the interface's hardware can be operable to communicate physical signals within and outside of the illustrated computer 602.

[0051] The computer 602 includes a processor 605. Although illustrated as a single processor 605 in FIG. 6, two or more processors 605 can be used according to particular needs, desires, or particular implementations of the computer 602 and the described functionality. Generally, the processor 605 can execute instructions and can manipulate data to perform the operations of the computer 602, including operations using algorithms, methods, functions, processes, flows, and procedures as described in the present disclosure.

[0052] The computer 602 also includes a database 606 that can hold data for the computer 602 and other components connected to the network 630 (whether illustrated or not). For example, database 606 can hold data 616 (e.g., wall thickness data, ultrasonic emissions data). For example, database 606 can be an in-memory, conventional, or a database storing data consistent with the present disclosure. In some implementations, database 606 can be a combination of two or more different database types (for example, hybrid in-memory and conventional databases) according to particular needs, desires, or particular implementations of the computer 602 and the described functionality. Although illustrated as a single database 606 in FIG. 6, two or more databases (of the same, different, or combination of types) can be used according to particular needs, desires, or particular implementations of the computer 602 and the described functionality. While database 606 is illustrated as an internal component of the computer 602, in alternative implementations, database 606 can be external to the computer 602.

[0053] The computer 602 also includes a memory 607 that can hold data for the computer 602 or a combination of components connected to the network 630 (whether illustrated or not). Memory 607 can store any data consistent with the present disclosure. In some implementations, memory 607 can be a combination of two or more different types of memory (for example, a combination of semiconductor and magnetic storage) according to particular needs, desires, or particular implementations of the computer 602 and the described functionality. Although illustrated as a single memory 607 in FIG. 6, two or more memories 607 (of the same, different, or combination of types) can be used according to particular needs, desires, or particular implementations of the computer 602 and the described functionality. While memory 607 is illustrated as an internal component of the computer 602, in alternative implementations, memory 607 can be external to the computer 602.

[0054] The application 608 can be an algorithmic software engine providing functionality according to particular needs, desires, or particular implementations of the computer 602 and the described functionality. For example, application 608 can serve as one or more components, modules, or applications. Further, although illustrated as a single application 608, the application 608 can be implemented as multiple applications 608 on the computer 602. In addition, although illustrated as internal to the computer 602, in alternative implementations, the application 608 can be external to the computer 602.

[0055] The computer 602 can also include a power supply 614. The power supply 614 can include a rechargeable or non-rechargeable battery that can be configured to be either user-or non-user-replaceable. In some implementations, the power supply 614 can include power-conversion and management circuits, including recharging, standby, and power management functionalities. In some implementations, the power-supply 614 can include a power plug to allow the computer 602 to be plugged into a wall socket or a power source to, for example, power the computer 602 or recharge a rechargeable battery.

[0056] There can be any number of computers 602 associated with, or external to, a computer system containing computer 602, with each computer 602 communicating over network 630. Further, the terms client, user, and other appropriate terminology can be used interchangeably, as appropriate, without departing from the scope of the present disclosure. Moreover, the present disclosure contemplates that many users can use one computer 602 and one user can use multiple computers 602.

[0057] Implementations of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, in tangibly embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Software implementations of the described subject matter can be implemented as one or more computer programs. Each computer program can include one or more modules of computer program instructions encoded on a tangible, non transitory, computer-readable computer-storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively, or additionally, the program instructions can be encoded in/on an artificially generated propagated signal. The example, the signal can be a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. The computer-storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of computer-storage mediums.

[0058] The terms data processing apparatus, computer, and electronic computer device (or equivalent as understood by one of ordinary skill in the art) refer to data processing hardware. For example, a data processing apparatus can encompass all kinds of apparatus, devices, and machines for processing data, including by way of example, a programmable processor, a computer, or multiple processors or computers. The apparatus can also include special purpose logic circuitry including, for example, a central processing unit (CPU), a field programmable gate array (FPGA), or an application specific integrated circuit (ASIC). In some implementations, the data processing apparatus or special purpose logic circuitry (or a combination of the data processing apparatus or special purpose logic circuitry) can be hardware-or software-based (or a combination of both hardware-and software-based). The apparatus can optionally include code that creates an execution environment for computer programs, for example, code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of execution environments. The present disclosure contemplates the use of data processing apparatuses with or without conventional operating systems, for example LINUX, UNIX, WINDOWS, MAC OS, ANDROID, or IOS.

[0059] The methods, processes, or logic flows described in this specification can be performed by one or more programmable computers executing one or more computer programs to perform functions by operating on input data and generating output. The methods, processes, or logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, for example, a CPU, an FPGA, or an ASIC.

[0060] Computer readable media (transitory or non-transitory, as appropriate) suitable for storing computer program instructions and data can include all forms of permanent/non-permanent and volatile/non-volatile memory, media, and memory devices. Computer readable media can include, for example, semiconductor memory devices such as random access memory (RAM), read only memory (ROM), phase change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), and flash memory devices. Computer readable media can also include, for example, magnetic devices such as tape, cartridges, cassettes, and internal/removable disks.

[0061] While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented, in combination, in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

[0062] Particular implementations of the subject matter have been described. Other implementations, alterations, and permutations of the described implementations are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results. In certain circumstances, multitasking or parallel processing (or a combination of multitasking and parallel processing) may be advantageous and performed as deemed appropriate.

[0063] Moreover, the separation or integration of various system modules and components in the previously described implementations should not be understood as requiring such separation or integration in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

[0064] Accordingly, the previously described example implementations do not define or constrain the present disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of the present disclosure.

[0065] Furthermore, any claimed implementation is considered to be applicable to at least a computer-implemented method; a non-transitory, computer-readable medium storing computer-readable instructions to perform the computer-implemented method; and a computer system comprising a computer memory interoperably coupled with a hardware processor configured to perform the computer-implemented method or the instructions stored on the non-transitory, computer-readable medium.

[0066] A number of implementations of these systems and methods have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. Accordingly, other implementations are within the scope of the following claims.

Examples

[0067] In an example implementation, a system for recovering natural gas liquids includes a distillation column; a thermosyphon reboiler fluidly connected to the distillation column; a Joule-Thomson valve fluidly connected to the distillation column and operable to control a flow of fluid into the distillation column; a lift gas control valve operable to control a flow of lift gas into the distillation column through a return line of the thermosyphon reboiler; and an interlock system that operatively couples the Joule-Thomson valve and the lift gas control valve to operate the lift gas control valve in response to operation of the Joule-Thomson valve.

[0068] In an aspect combinable with the example implementation, the interlock system includes a controller communicatively coupled to the Joule-Thomson valve and the lift gas control valve, the controller configured to perform operations including receiving a signal to start the flow of fluid into the distillation column; operating the Joule-Thomson valve to control the flow of fluid; and operating the lift gas control valve in coordination with operating the Joule-Thomson valve to control the flow of lift gas to reduce thermal stress on the thermosyphon reboiler.

[0069] Another aspect combinable with any of the previous aspects includes a differential pressure sensor to measure a pressure difference across the thermosyphon reboiler.

[0070] In another aspect combinable with any of the previous aspects, the operations include measuring the pressure difference across the thermosyphon reboiler using the differential pressure sensor.

[0071] In another aspect combinable with any of the previous aspects, operating the lift gas control valve is based on the measured pressure difference.

[0072] Another aspect combinable with any of the previous aspects includes a temperature sensor to measure a temperature of the thermosyphon reboiler.

[0073] In another aspect combinable with any of the previous aspects, the operations include measuring the temperature of the thermosyphon reboiler; and determining a rate of temperature change based on the measured temperature.

[0074] In another aspect combinable with any of the previous aspects, operating the lift gas control valve is based on the determined rate of temperature change.

[0075] In another aspect combinable with any of the previous aspects, the operations include in response to determining that the rate of temperature change exceeds a threshold rate of temperature change, operating the lift gas control valve to increase the flow of lift gas.

[0076] In another aspect combinable with any of the previous aspects, the operations include in response to determining that the rate of temperature change is below the threshold rate of temperature change, operating the lift gas control valve to decrease the flow of lift gas.

[0077] Another aspect combinable with any of the previous aspects includes a turbo-expander fluidly connected to the distillation column in parallel with the Joule-Thomson valve.

[0078] In another aspect combinable with any of the previous aspects, receiving the signal to start the flow of fluid into the distillation column occurs in response to a trip of the turbo-expander.

[0079] In another example implementation, a method for recovering natural gas liquids includes receiving a signal to start a flow of fluid into a distillation column; operating a Joule-Thomson valve to control the flow of fluid into the distillation column; and operating a lift gas control valve in coordination with operating the Joule-Thomson valve to control a flow of lift gas to the distillation column through a return line of a thermosyphon reboiler to reduce thermal stress on the thermosyphon reboiler.

[0080] In an aspect combinable with the example implementation, operating the lift gas control valve occurs in response to operating the Joule-Thomson valve.

[0081] Another aspect combinable with any of the previous aspects includes measuring a pressure difference across the thermosyphon reboiler using a differential pressure sensor.

[0082] In another aspect combinable with any of the previous aspects, operating the lift gas control valve is based on the measured pressure difference.

[0083] Another aspect combinable with any of the previous aspects includes measuring a temperature of the thermosyphon reboiler; and determining a rate of temperature change based on the measured temperature.

[0084] In another aspect combinable with any of the previous aspects, operating the lift gas control valve is based on the determined rate of temperature change.

[0085] Another aspect combinable with any of the previous aspects includes in response to determining that the rate of temperature change exceeds a threshold rate of temperature change, operating the lift gas control valve to increase the flow of lift gas; and in response to determining that the rate of temperature change is below the threshold rate of temperature change, operating the lift gas control valve to decrease the flow of lift gas.

[0086] In another aspect combinable with any of the previous aspects, receiving the signal to start the flow of fluid into the distillation column occurs in response to a trip of a turbo-expander fluidly coupled to the distillation column in parallel with the Joule-Thomson valve.