Process Description
NGL Recovery Unit Process Description – Overview
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 effect 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 Section
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 Section
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 Section
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 Section
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 Section
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.