Plant 12 – LTS and Deethanizer

Plant 13 – Sale Gas Compression

Introduction

The main process objective of Plant 12 is to recover propane and heavier hydrocarbons from the sweet gas before it is sent to Plant 13 to be compressed into the sales gas pipeline.  The recovered hydrocarbon liquid is fed to the fractionation unit, Plant 16, for separation into three liquid products: propane, butane, and NGL.

Plant 12 Low Temperature Separation (LTS)/Deethanizer

The LTS unit chills the water-saturated sweet gas from the Acid Gas Removal Plants to -30°F to recover heavy hydrocarbons.  The LTS unit consists of heat exchangers, E-1201, E-1202, E-1203, and E-1211, and the HC/EG separator, V-1210.

Sweet gas from the Acid Gas Removal Section enters the LTS unit at about 640 psig and 110°F.  The gas is water-saturated from contacting with the aqueous amine solution.  The gas is first chilled to about 30°F in E-1201 by heat exchange with chilled sales gas.  The gas leaving E-1201 is then cooled to about 20°F in E-1203 by heat exchange with chilled condensate leaving the HC/EG separator, V-1210.  The gas is further chilled to -30°F in the sweet gas chiller, E-1202.  Refrigeration is supplied to E-1202 by vaporizing propane refrigerant on the shellside of the exchanger.

An aqueous solution of 75 wt% EG is sprayed onto the tubesheet at the gas inlets of E-1201, E-1202, and E-1203.  The glycol absorbs the water, which condenses out as the gas is cooled.  Otherwise, the condensed water would combine with the hydrocarbons to form hydrates at the low temperatures and high pressure in the exchangers.  Hydrates will plug exchanger tubes and reduce heat transfer.

The steam leaving E-1202 contains hydrocarbon gas, condensed hydrocarbon liquids, and glycol.  These are separated in the hydrocarbon/glycol separator, V-1210.  The hydrocarbon gas from V-1210 combines with the deethanizer overhead gas from booster compressor, K-1230A and B.  The combined stream is heated to 100^o F in sweet gas/sales gas exchanger, E-1201, and sent to Plant 13 for compression.  Liquid hydrocarbon is removed from the bottom of V-1210, heated to 77^o F by exchange with sweet gas in E-1203 and with propane refrigerant in E-1211, and fed to the deethanizer, C-1220.  Glycol is drawn off the V-1210 glycol boot and sent to the glycol regeneration unit.  The glycol boot is heated by an internal glycol coil to promote hydrocarbon/glycol phase separation.

The purpose of the deethanizer, C-1220, is to remove light gases such as methane and ethane from the LTS liquids.  These components would otherwise end up in the propane product causing the vapor pressure to exceed specification.  Deethanizer overhead gas combines with the sales gas from V-1210.  The bottoms product goes to the debutanizer, C-1600.

The liquid hydrocarbon from V-1210 is routed to the deethanizer, C-1220 via E-1203, E-1211, and V-1210 level control valve.  The liquid is vaporized in E-1203 and E-1211.  The V-1210 level control valve lets the pressure down from about 620 psig to about 452 psig resulting in further vaporization.

Feed enters the column at Tray 32.  Feed to the column is about 30 wt% vapor.  C-1220 contains 40 valve trays for gas/liquid contacting.  As liquid flows down and across the trays in the column, hot gas rising up through the valves in the trays strips out the methane and ethane.  The overhead from C-1220 enters the deethanizer overhead condenser, E-1221.

The overhead gases are chilled to –26^oF in E-1221.  Refrigeration is supplied by vaporizing propane on the shellside of the condenser.  If required, glycol can be injected in the tubeside of E-1221 to prevent hydrate formation.

The outlet stream from E-1221 will contain hydrocarbon vapor, condensed hydrocarbon liquids, and glycol (if injected).  These are separated in the deethanizer reflux drum, V-1221.  The hydrocarbon gas from V-1221 is fed to the deethanizer overhead booster compressors, K-1230A and B.  The compressed vapors from K-1230A and B combine with the sales gas leaving V-1210.  The hydrocarbon liquids from V-1221 are pumped to C-1220 as liquid reflux.  No net overhead liquid product is drawn.  Any glycol in the system is drawn off the boot of the reflux drum and returned to the glycol regeneration system.  The glycol boot is heated by an internal glycol coil to promote hydrocarbon/glycol phase separation.

The bottom of C-1220 operates at 455 psig and 218^oF.  The column bottom is divided into two compartments, with a separation baffle so that all the liquid from the bottom tray enters the deethanizer reboiler, C-1220.  This minimizes light components in the product condensate.  The reboiler inlet compartment runs full to provide a constant static head for reboiler circulation.

E-1220 partially vaporizes the liquid to remove light ends and to provide stripping vapor.  The gases enter C-1220 below the bottom tray.  Liquids from the reboiler compartment overflow into the column net bottoms compartment.  The bottoms product, consisting of propane and heavier hydrocarbon components flow to the debutanizer, C-1600, in Plant 16.

Heat is supplied to E-1220 by condensing 80 psig steam on the tubeside of the exchanger.  Steam condensate is collected in the condensate pot, V-1220, located directly beneath E-1220, and pumped to the condensate collection drum, V-1620, for reuse in steam generation.

Propane Refrigeration System

Plant 12 uses a propane refrigeration system to provide cooling in the sweet gas chiller, E-1202, and the deethanizer overhead condenser, E-1221.  E-1202 and E-1221 are kettle-type exchangers with a U-tube bundle inserted into an enlarged shell.  The tube bundle is submerged in a pool of boiling liquid propane.  Cooling is provided as propane vaporizes on the shellside.  The propane vapor separates from the liquid in the top section of the exchanger shells and enters the first-stage compressor knockout drum, V-1240, at about 0.5 psig and –31^oF.

V-1240 prevents liquid propane from entering the propane compressors, K-1240A and B.  K-1240A and B share two 5000 BHP drivers with the sales gas compressors, K-1300A and B.  Any liquid propane carried over into V-1240 is vaporized by injecting hot gas from the compressor discharge into the bottom of V-1240.

The propane vapor from V-1240 is compressed to 49 psig in the first stage of K-1240A and B.  The compressed vapor is combined with the overhead from the economizer flash drum, V-1355, and fed to the second stage of K-1240A and B.

The second-stage compressor discharge is fed to the propane condenser, E-1245, at about 180^oF and 210 psig.  E-1245 air cools the propane to 110^oF condensing all of the propane.

The liquid propane enters the propane surge drum, V-1250.  V-1250 handles any changes in propane volume in the refrigeration system.  All other vessels are on level control.  Make-up propane can be added to the system at V-1250.  Liquid propane from V-1250 is cooled in the economizer exchanger, E-1211, from   110^oF to 72^oF.  The propane is then flashed to 49 psig.  This causes some of the propane to vaporize, lowering the temperature to about 27^oF.

The propane vapor separates from the liquid in the economizer flash drum, V-1255.  The vapor is sent to the second stage of the propane compressor.  The propane liquid from V-1255 is flashed to about 1.8 psig and –40^oF in the sweet gas chiller, E-1202, and the deethanizer overhead condenser, E-1221.  This completes the refrigerant flow loop.

Note that the refrigeration system could have been simplified by feeding liquid propane directly fro V-1250 to E-1202 and E-1221 without the intermediate flash.  In that case, all of the propane vapor generated from the pressure letdown would have to be compressed from 1.8 psig to 210 psig.  In the design case, the propane is let down in two stages.  The total amount of propane vapor generated in the two-stage flash is about the same as in the single-flash case.  However, only a portion of the vapor needs to be compressed from 1.8 psig to 210 psig; the rest is compressed from 49 psig to 210 psig.  Therefore, addition of the economizer flash stage reduces total compression costs.

Ethylene Glycol (EG) Regeneration System

The rich glycol (72 wt% EG; 28 wt% water) is removed from the glycol boots of V-1210 and V-1221.  It is heated from 0^oF to 125^oF by exchange with lean glycol in exchanger, E-1215.  The glycol contains hydrocarbon gases dissolved in the glycol solution.  These gases must be removed to prevent hydrocarbon flashing in the glycol regenerator and to avoid hydrocarbon emissions.  Hydrocarbon flashing will carry glycol out of the regenerator still column, causing excessive glycol losses.  So, the heated glycol from E-1215 is let down in pressure from 630 psig to 70 psig.  This flashes off most of the dissolved hydrocarbon.  The gases separate from the glycol in the glycol flash drum, V-1260, and to first-stage knockout drum, V-1010A, in the Intake Gas Compression System in Plant 10.

The rich glycol from V-1260 is filtered to remove suspended solids in F-1266.  It is then sent to carbon filter, F-1265, to remove hydrocarbons and other contaminants.  Glycol contamination can cause corrosion, erosion, heat exchanger fouling, and glycol losses due to foaming.

Glycol from F-1265 is let down to near atmospheric pressure and heated to   165^oF in the preheat coil located in the top of the glycol still, C-1260.  Preheated glycol is then fed to the shellside of the glycol regenerator, E-1260.

The glycol regeneration system separates excess water from the glycol solution by distillation.  E-1260 is kettle-type reboiler consisting of a U-tube bundle inserted into an enlarged shell.  An overflow weir keeps the liquid level about the top of the tube bundle.  Net liquid overflows the weir into a drawoff compartment.  The glycol solution is boiled by condensing 80 psig steam in the exchanger tubes.  The vapor generated from boiling the glycol solution contains both glycol and water vapor.

The vapor from the glycol solution in E-1260 rises up to the glycol still, C-1260, located on top of the regenerator.  C-1260 is packed with 10 ft of 1 in. stainless steel packing to provide thorough contacting between the vapor and liquid flowing through the column.  Reflux water condensed by the rich glycol in the preheat coil at the top of the column wets the surface of the packing as it flows downward.  The regenerator vapors contact the liquid as they rise up through void spaces in the packing.  As the vapor and liquid are contacted, glycol will tend to concentrate in the liquid phase and water will concentrate in the vapor phase.  The glycol concentration increases as the liquid flows down in the column.  The vapor leaving the top of the column is essentially pure water vapor and is vented to the atmosphere. 

Lean glycol (75 wt% glycol; 25 wt% water) from the regenerator is cooled from 255^oF to 100^oF in E-1215 by exchange with rich glycol.  The glycol is passed through a second solids filter, F-1270, and enters the glycol surge drum, V-1270.

V-1270 handles any changes in the glycol system volume.  All other vessels in the system are on level control.  A net loss in glycol will be indicated by a drop in level in V-1270.  Make-up glycol can be added to the system at V-1270.  Lean glycol from V-1270, is pumped by P-1270 to the LTS exchangers, E-1201, E-1202, and E-1203.

Plant 13 – Sales Gas Compression

Introduction

The sales gas compression system in Plant 13 compresses the treated gas from the LTS in Plant 12 to the sales gas pipeline pressure. 

Sales Gas Compression and Metering

The heated gas from E-1201 combines with the sales gas compressor spillback and flows to the sales gas compressor knockout drum, V-1300.  This drum removes any entrained liquids from the gas.  Any liquid collected in V-1300 is removed periodically and sent to the second-stage knockout drum, V-1010B, in Plant 10.

Gas from V-1300 enters the sales gas compressors, K-1300A and B.  K-1300 compresses the gas from 625 psig to 1005 psig (maximum).  K-1300A and B are two 100% reciprocating machines.  Each can compress up to 52.5 MMscfd of gas.  K-1300A and B share two 5000 BHP drivers with the propane compressors, K-1240A and B.

The gas leaving K-1300A and B flows to the sales gas cooler, E-1310, which cools the gas from 170^oF to 100^oF.  A part of the cooled gas is spilled back to the inlet of V-1300 to control K-1300A and B suction pressure.  When the sour gas rate to the gas plant is low, a part of the sales gas is also recycled to the front end of Plants 11 and 12.  This recycle maintains a minimum velocity in the tubeside of E-1201, E-1202, and E-1203 for good glycol distribution.  The rest of the sales gas leaving E-1310 is metered and sent to the sales gas pipeline.

Fuel Gas System

A portion of the sales gas from E-1201 goes to the fuel gas knockout drum, V-1305.  This drum removes any entrained liquids from the gas stream.  The gas then enters the fuel gas distribution system.  This system supplies fuel gas to both the Gas and Oil Plants.  Any liquid in V-1305 is removed periodically and sent to the second-stage knockout drum, V-1010B, in Plant 10.

Feed Rates and Composition

Plant 10, 12, 13, and 16 – Gas Processing Section

Gaviota Gas Plant

Product Conditions and Specifications

Plant 10, 12, 13, and 16 – Gas Processing Section

Gaviota Gas Plant

¹ NGL to meet either the H₂S and mercaptan limits or show negative on the Doctor Test.