Process Description
The NGL Recovery Unit program represents a typical Gas Subcooled Process (GSP) with a turbo-expander. The feed to the unit is gas gathered from wells that has been treated for removal of hydrogen sulfide, water and other detrimental impurities. The feed mainly contains a range of alkane hydrocarbons: methane through octane. Nitrogen and carbon dioxide are also present in small concentrations. The purpose of the NGL recovery unit is to separate out most of the ethane and nearly all of the propane and heavier hydrocarbon compounds from the methane. Nearly all the methane in the feed ends up in the product gas from the unit and is sent to a pipeline along with all the nitrogen in the feed and a fraction of the ethane in the feed. The product gas is termed sales gas. The separated components from the feed are recovered as a liquid product called NGL (natural gas liquids). These are sent offsite to a fractionation unit for separation into ethane, propane, butane and light gasoline.
To effect separation, the feedgas is cooled down using:
- Cold sales gas from the top of the Demethanizer
- Propane refrigerant from battery limit
- Auto-chilling by gas expansion and work extraction through an expander (turbine)
- Auto-chilling by liquid expansion through valves (auto-refrigeration and Joule- Thomson effects)
As the feed cools down, the heaviest components (propane, butane and light gasoline) condense first. These are separated (warm separation) and sent to the lower part of the Demethanizer. The feed gas is chilled further and some lighter components condense (the balance of the feed propane and most of the ethane). These are separated (cold separation) and sent to the lower part of the Demethanizer along with the warm liquid. A portion of the cold gas is further chilled and condensed using cold Demethanizer overhead gas. The condensed feed gas stream is sent to the top of the Demethanizer to serve as reflux. The balance of the cold gas is further chilled in an expander and the resultant cold stream is sent to the middle-upper part of the Demethanizer.
The feed chilling section operates close to the feedgas supply pressure while the Demethanizer overhead operates at a much lower pressure.
The Demethanizer column performs two functions: it strips out most of the dissolved methane in the liquid feeds to the Demethanizer while washing down as much ethane to the bottom. Because the cold temperatures at the top of the column do not allow a sharp separation of these two components, distillation is used to affect their separation, the heavier hydrocarbons will naturally end up in the bottom of the column. Some propane and butane will also tend to rise up the column if there is not enough liquid produced from the Expander and from auto-chilling of liquid feeds to keep the top of the column chilled.
Heat is added at the base of the Demethanizer column with a reboiler that uses hot oil as its heat source. This ensures removal of most of the dissolved methane reaching the bottom of the Demethanizer. The NGL collected in the bottom of the Demethanizer is pumped to the NGL Distillation Unit at battery limit.
Feed Chilling Warm
Treated feedgas at high pressure from battery limits is first cooled in Feed/Sales Gas Exchanger No. 1 E-801 using cool Sales Gas from E-803. The feedgas then passes on to Feed Chiller E-802 which uses propane refrigerant from battery limits. The pressure of vaporized refrigerant on the shell side is controlled to adjust the temperature of the boiling refrigerant which, in turn, controls the outlet temperature of the feedgas from E-802. The vaporized refrigerant is returned back to battery limits.
A portion of the feedgas (approximately 32% of the plant feed rate by weight) is condensed in E-802 and the liquid and vapor are separated in Warm Separator D-801. The collected warm liquid is routed to tray 13 of the Demethanizer T-801. This liquid contains a significant amount of methane which will be stripped out in the lower section of T-801.
Feed Chilling Cold
The vapor stream from D-801 flows through Feed/Sales Gas Exchanger No. 2 E-803 to cool the feedgas further using cold Sales Gas from E-804. Additional liquid is produced from E-803 (approximately 8% of the plant feed rate by weight) and is separated out in Cold Separator D-802.
The cold liquid from D-802 is combined with warm liquid from D-801 and fed to tray 13 of Demethanizer T-801.
A portion of the vapor from D-801 (approximately 24% of the plant feed rate by weight) is further chilled and condensed against Sales Gas directly from the top of Demethanizer T-801 in Feed/Sales Gas Exchanger No. 3 E-804. This stream is depressured to T-801’s pressure across FV-808. The flashing of liquid and the Joule-Thompson effect from depressurization causes the stream temperature to drop. Approximately 33% of the stream, by weight, flashes to vapor. This cold stream, which is rich in methane, is routed to the top of T-801 (tray 40) to provide reflux for washing ethane and propane down the Demethanizer.
Feed Chilling Expander
The balance of vapor from Cold Separator D-802 (about 36% of the plant feed rate by weight) is routed through Feed Gas Expander KT-801. Guide vanes on the inlet of the Expander allow control of the flow rate through the unit. The expander drives the Sales Gas Recompressor K-801 which is an integrated unit. The Expander/Recompressor normally operates at 10,356 RPM but will dynamically change according to changes in expander- and recompressor-side conditions. The outlet flow from KT-801 is routed to tray 27 of Demethanizer T-801. Because of both gas expansion and work performed by the expander in turning the Recompressor, the temperature drops across KT-801. Only a small amount of liquid is produced in the outlet of KT-801.
A bypass valve is provided around KT-801 for startup and shutdown operations. Without the Feed Gas Expander in operation, it is difficult to maintain plant capacity and NGL separation. In case of a trip of KT-801, the control system will automatically open the bypass valve on a one-shot basis to a predefined opening that depends on the current feed flow rate.
Demethanizer
The Demethanizer T-801 is a distillation tower consisting of 40 trays. Its main functions are to strip out any methane from the liquid feeds to the tower and to wash down most of the ethane and all the propane and higher hydrocarbons to the bottom of the column. Cold liquid from E-804 is sent to the top tray (tray 40) to wash down ethane and propane rising with the vapors from the lower sections of the tower. Expander KT-801 outlet is fed to tray 27. Warm liquid from D-801 and cold liquid from D-802 are combined and fed to tray 13.
The top section of T-801 handles most of the uncondensed gas feeding the tower via KT-801. Therefore, the top section of the Demethanizer has a larger diameter than the sections below in order to keep gas velocities through the trays such that flooding (excessive retention of liquid on the trays) does not occur.
Below the bottommost tray of Demethanizer T-801 (tray 1) is a full trap-out pan that routes liquid from tray 1 to the shell side of Demethanizer Reboiler E-805. Hot oil from battery limits is used to heat the liquid to drive off most of the methane that reaches the bottom tray. The vapor from E-805 is returned to the bottom of the Demethanizer and flows through chimney vents in the trap-out pan and then flows up into tray 1. Warm liquid on the shell side of E-805 spills over a weir and flows by gravity into the base of T-801. This liquid is product NGL.
Product NGL in the base of T-801 is pumped by NGL Pumps P-801A/B. These are centrifugal, electric motor-driven pumps. Only one pump is normally in operation. Product NGL is routed to the Deethanizer tower at battery limits.
Sales Gas Compression
Cold Sales Gas is produced at the top of the Demethanizer and is mainly methane with unrecovered ethane. It also contains almost all the nitrogen contained in feedgas as well as a portion of the carbon dioxide contained in the feedgas. Sales Gas is heated up in the Feed/Sales Gas Heat Exchangers E-804, E-803 and E-801 prior to compression back to pipeline pressure.
The Sales Gas from E-801 is first compressed by the Sales Gas Recompressor K-801 which is driven by the Feed Gas Expander KT-801. K-801 is a centrifugal compressor. A check valve is installed in the bypass line around K-801 to allow Sales Gas flow to continue in case of a trip of KT-801. Because the pressure ratio of K-801 is fairly low, the temperature of the gas leaving K-801 is low enough where it does not have to be cooled prior to final compression in Sales Gas Compressor K-802.
Sales Gas Compressor K-802 is a motor-driven centrifugal compressor and takes suction from the discharge of K-801. The flow through K-802 is adjustable by changing the position of the inlet guide vanes. K-802 discharges into the sales gas pipeline at battery limits. A check valve on the discharge of K-802 prevents back-flow from the pipeline.
At startup and shutdown, the discharge of K-801 can be vented to the flare system at battery limits.
Instrumentation
Feed Chilling Warm Section Controls
The temperature of feed gas from battery limits is indicated on TI-801 and its pressure is indicated on PI-801. The ethane content of the feed gas is indicated on AI-801. The feed gas flow FIC-801 is normally controlled by adjustment of the Expander inlet guide vane position. Refer to the Advanced Controls section below for more details of the control strategy. HIC-801 is used to adjust the position of feed gas valve HV-801. Normally, HIC-801 is fully open.
TI-802 indicates the outlet temperature of the feed gas from Feed Gas/Sales Gas Exchanger No. 1 E-801. TI-822 indicates the Sales Gas inlet temperature to E-801 and TI-823 indicates the outlet temperature. The pressure of Sales Gas leaving E-801 is indicated on PI-823.
TIC-803 controls the feed gas outlet temperature from Feed Chiller E-802 by adjusting the setpoint of PIC-802 which controls the pressure of the vapor space of the refrigerant side of E-802. In turn, PIC-802 adjusts the position of control valve PV-802 which will affect the flow rate of vaporized refrigerant returned to the Propane (C3) Refrigeration System at battery limits. Changing the pressure of the refrigerant side of E-802 will directly change the temperature at which the propane refrigerant boils in E-802.
LIC-802 controls the refrigerant level in E-802 by adjusting the position of the refrigerant supply valve LV-802. The flow rate of refrigerant through LV-802 is indicated by FI-802.
The two-phase mixture of feedgas leaving E-802 is separated in Warm Separator D-801. The pressure of D-801 is indicated on PI-803. The temperature of feed gas leaving D-801 is indicated on TI-804. The level of liquid in D-801 is controlled by LIC-803 which adjusts the opening of control valve LV-803. The flow of liquid flowing through LV-803 is indicated on FI-803.
The temperature of warm feedgas entering Feed/Sales Gas Exchanger No. 2 E-803 is indicated on TI-804. The temperature of cold feedgas leaving E-803 is indicated on TI-805. The temperature of Sales Gas entering E-803 is indicated on TI-821 and the temperature leaving E-803 is indicated on TI-822.
The two-phase mixture of feedgas leaving E-803 is separated in D-802 Cold Separator. The pressure of D-802 can be controlled by PIC-805 which can be selected to adjust the guide vane position of Sales Gas Compressor K-802. Refer to the Advanced Controls section below for more details of the control strategy.
The flow of feed gas from D-802 to E-804 Feed/Sales Gas Exchanger No. 3 is controlled by FIC-808 which adjusts the position of control valve FV-808 which adjusts the flow of reflux produced by E-804 to the top of T-801. The temperature of reflux leaving E-804 is indicated on TI-806.
The flow of feed gas to KT-801 Expander is indicated on FI-805.
The level of liquid in D-802 is controlled by LIC-806 which adjusts the opening of control valve LV-806. The flow of liquid flowing through LV-806 is indicated on FI-806.
Expander & Recompressor Controls
Cold gas from Cold Separator D-802 enters Feed Gas Expander KT-801 by passing through expander trip valve XV-801. XV-801 is controlled by Expander Trip Interlock I-801 (see section Interlock I-801 below for details).
The flow of gas to KT-801 is controlled by the position of the inlet guide vane FV-801A. HIC-801A is a hand controller that is normally in cascade mode and receives its setpoint from selector FY-801.
HIC-801B is a hand controller that adjusts the expander bypass control valve FV-801B. HIC-801B is normally used at startup and shutdown and is automatically adjusted in the event of a trip of KT-801 by I-801 (see section Interlock I-801 below for details). Normally HIC-801B is in manual mode with an output of 0% to close FV-801B.
FI-805 indicates the total flow of gas to KT-801 and its bypass.
Switch HS-803 with selector FY-801 form an "A/B switch" used to select the control signal destined for HIC-801A and HIC-801B. Normally HS-803 is in the "A" position to select the output of feed flow controller FIC-801. When it is in the "B" position, it selects the output of Demethanizer overhead pressure controller PIC-814B. The unselected controller of FY-801 is forced into manual mode and its output tracks the selected controller's output. This ensures bumpless control when switching the position of HS-803.
The speed of the shaft connecting Sales Gas Recompressor K-801 and KT-801 is indicated on SI-801. A second speed indication, SAH-801, is used as a trip sensor for interlock I-801. SI-801 will alarm before SAH-801 to give the operator warning of an impending overspeed trip (refer to section Interlock I-801 below for details).
The outlet temperature from KT-801 is indicated on TI-807. The mixed gas from KT-801 and its bypass is indicated on TI-812.
XI-801A indicates the status of the machinery monitoring system of the KT-801/K-801 unit. In case of trouble (e.g. excessive vibration, high bearing temperature, low lube oil pressure) XI-801A will produce an alarm. If the trouble is severe, XA-801A will alarm and cause a trip of I-801 to protect the machines.
HS-801 is a switch used to trip and reset interlock I-801. XA-801 generates an alarm that the Expander/Recompressor train has been tripped.
Pi-803 indicates the pressure of Sales Gas entering K-801. TI-824 indicates the discharge temperature of K-801.
PIC-824 controls the discharge pressure of K-801 in the event of a stop of downstream Sales Gas Compressor K-802. PIC-824 controls the position of control valve PV-824 to route gas to the flare system at battery limits.
Demethanizer Controls
The flashed reflux feed to the top of Demethanizer T-801 is indicated on TI-811. The cold vapor feed from KT-801 is indicated on TI-812. The flashed combined liquid feeds to T-801 is indicated on TI-813.
The temperature of the overhead vapor leaving the top of T-801 is indicated on TI-814. The temperature of tray 33 in the upper section of T-801 is indicated on TI-815. The temperature of tray 20 in the center section of T-801 is indicated on TI-816. The temperature of tray 7 in the bottom section of T-801 is indicated on TI-817.
The differential pressure across the trays in the upper section of T-801 is indicated on PDI-811. The differential pressure across the trays in the middle section of T-801 is indicated on PDI-812. The differential pressure across the trays in the lower section of T-801 is indicated on PDI-813.
The overhead pressure of T-801 is normally controlled by PIC-814A which adjusts the position of the guide vane on K-802 Sales Gas Compressor. An alternate mode of pressure control is to use PIC-814B to control the inlet guide vane of KT-801 Feed Gas Expander. To avoid unstable control, only one of these two controllers is permitted to be in automatic mode at any time. The advanced control system will reset the second controller that is placed in automatic mode back to manual mode.
The outlet temperature of Reboiler E-805 is controlled by TIC-818 which adjusts the setpoint of the hot oil flow controller FIC-818. The pressure of the hot oil supply is indicated on PI-001 and its temperature is indicated on TI-001. The temperature of the bottoms liquid leaving T-801 is indicated on TI-819.
The level of NGL in the bottom of T-801 is controlled by LIC-819 which adjusts the position of LV-819 to regulate the flow of NGL from NGL Pumps P-801A/B to the Deethanizer at battery limits. The flow of NGL through LV-819 is indicated on FI-819. The motors of P-801A/B are operated by switches HS-801A/B, respectively. The methane content in the product NGL is indicated on AI-819.
Sales Gas Compressor Controls
TI-824 indicates the Sales Gas temperature from Recompressor K-801. PIC-824 controls the discharge pressure of K-801/suction pressure of Sales Gas Compressor K-802 in the event of a stop of K-802. PIC-824 controls the position of control valve PV-824 to route gas to the flare system at battery limits.
PI-825 indicates the suction pressure of K-802. PI-826 indicates the discharge pressure. The ratio of the two pressures is indicated on PY-826. PYH-826 generates a trip signal for I-802 Sales Gas Compressor interlock based on the PV of PY-826. PY-826 will alarm before the trip point to warn the operator of an impending trip condition (refer to section Interlock I-802 below for details).
The discharge temperature of K-802 is indicated on TI-826. A second, independent temperature is indicated on TAH-826 which serves as a trip sensor to I-802. TI-826 will alarm before the trip point of TAH-826 to warn the operator of an impending trip condition (refer to section Interlock I-802 below for details).
The flow of Sales Gas from K-802 to the pipeline is indicated on FI-826. The ethane content of the Sales Gas is indicated on AI-826.
HIC-827 is used to isolate K-802 from the pipeline by adjusting the position of control valve HV-827. The pressure of the pipeline is indicated in PI-827.
XI-802A indicates the status of the machinery monitoring system of K-802. In case of trouble (e.g. excessive vibration, high bearing temperature, low lube oil pressure) XI-802A will produce an alarm. If the trouble is severe, XA-802A will alarm and cause a trip of I-802 to protect the machine.
HS-802A is a switch that operates the electric motor of K-802. HS-802B is a switch used to trip and reset interlock I-802. XA-802 generates an alarm that the Sales Gas Compressor has been tripped. When I-802 is in the trip state, HS-802A is locked in the STOP state (refer to section Interlock I-802 below for details).
Switch HS-814 with selector PY-814 form an "A/B switch" used to select the control signal for the inlet guide vane PV-814 of K-802. Normally HS-814 is in the "A" position to select the output of Demethanizer overhead pressure controller PIC-814A. When it is in the "B" position, it selects the output of Cold Separator pressure controller PIC-805. The unselected controller of PY-814 is forced into manual mode and its output tracks the selected controller's output. This ensures bumpless control when switching the position of HS-814.
Advanced Controls
The output signal from feedgas flow controller FIC-801 is connected to the "A" input signal of switch FY-801 which, in turn, is connected to the external setpoint signal of both HIC-801A and HIC-801B which are configured with a ratio of 1.0. Neither HIC will back-initialize any controller when the HIC is not in cascade mode.
An alternate mode of expander guide vane control uses the Demethanizer overhead pressure controller PIC-814B output to the "B" input of FY-801. HS-803 is used to switch between the A and B inputs. To ensure bumpless transfer when switching HS-803, the unselected controller to FY-801 is locked in manual mode and its output tracks the selected controller.
Normally, HS-803 is in the "A" position and feedgas flow controller FIC-801 will control the inlet guide vane position of the Expander while HIC-801A is in cascade mode. The Expander bypass valve will normally be closed with HIC-801B in manual with an output of 0.0.
In case the Expander is taken out of service, the bypass valve can be regulated from FIC-801 (or PIC-814B) by placing HIC-801B into cascade mode.
In case of an Expander trip, special logic is required to do a one-shot initialization of controllers FIC-801 (or PIC-814B), HIC-801A and HIC-801B in order to minimize an upset to the unit. This one-shot occurs when:
- FIC-801 (or PIC-814B) is in automatic mode, and
- HIC-801A is in cascade mode, and
- HIC-801B is not in cascade mode, and
- I-801 has just transitioned from OK to TRIP (one-shot)
When this happens, the following takes place:
- FIC-801 (or PIC-814B) is placed in manual for two seconds
- FIC-801 (or PIC-814B) output is initialized to a value calculated from a lookup table that is a function of the feed flow (FIC-801’s PV before the trip) (one-shot)
- HIC-801B is placed into cascade mode (one-shot)
- HIC-801A is locked by interlock I-801 (see above)
- After a delay of three seconds, FIC-801 (or PIC-814B) is placed into automatic mode (one-shot)
- After a delay of four seconds, the setpoint of FIC-801 (or PIC-814B) is restored to the value just before the trip (one-shot)
Sales Gas Compressor Guide Vane Control
The flow of Sales Gas to the pipeline is controlled by adjusting the position of the guide vane on Sales Gas Compressor K-802. The control signal to the guide vane position is selected by switch PY-814 which has Demethanizer overhead pressure controller PIC-814A's output as its "A" input and Cold Separator pressure controller PIC-805's output as its "B" input. The selection of the guide vane control signal is made using hand switch HS-814. To ensure bumpless transfer when switching HS-814, the unselected controller to PY-814 is locked in manual mode and its output tracks the selected controller.
Normally HS-814 is in the "A" position so that PIC-814A controls the guide vane position of the Sales Gas Compressor.
Demethanizer Overhead Pressure Control
There are two controllers, PIC-814A and PIC-814B, for controlling the Demethanizer overhead pressure. Both controllers share the same pressure transmitter. PIC-814A can be used to control the inlet guide vane of the Sales Gas Compressor while PIC-814B can be used to control the inlet guide vane of the Feedgas Expander. Logic prevents both controllers from being in automatic mode at the same time. The first controller put in automatic mode will remain in automatic mode while the second controller will be set back to manual mode. To switch control, both controllers must first be placed into manual mode.