Plant 17  Liquids Treating Section

Design Basis

Design Feed and Product Specifications

The design concentrations of COS, H₂S, and mercaptan in the feed streams to the Liquids Treating Section are calculated from the maximum values expected from upstream processing conditions.  A 50% contingency is then added due to uncertainties in the composition of the sour gas feed to the Gaviota Gas Plant.  The actual values may be less than the design values.

The COS concentration in the C₃/C₄ stream is based on:

1.                  Seventy five percent removal of COS from the raw feed gas in the Acid Gas Removal Plants 11 and 21.

2.                  Seventy five percent of the COS in the Amine Plant product gas condensing into the C₃+ product from the low temperature separator in the Gas Processing Section, Plant 12.

3.                  All the COS in the C₃+ ending up in the C₃/C₄.

The mercaptan sulfur concentration of the C₃/C₄ and the C₅+ NGL is based on:

1.                  No removal of the mercaptan sulfur from C₃/C₄ and C₅+ NGL in upstream processing.

2.                  The C₃/C₄ containing all the methyl mercaptan in the C₃+ LTS liquid and the C₅+ NGL containing all the heavier molecular weight mercaptans from the C₃+.

3.                  All mercaptans from the intake separator bottoms ends up in the C₅+ NGL.

The C₃/C₄ product specifications are set so that the fractionated propane and butane meet National Gas Processing Association (NGPA) specifications.  The C₅+ NGL specifications are set to meet requirements for gasoline blending.

Feed:

Treated Product Specifications

Processing

  C₃/C₄ Treating

    COS/H₂S Removal                                               DEA and Caustic Contacting

    Mercaptan Removal                                             Caustic Extraction

    Caustic Removal                                                   Water Washing

  C₅+ NGL Treating

    Sweetening                                                            Liquid Merox Process

¹C₃/C₄ Liquids Treating Section Product is split into propane and butane in Plant 16 Depropanizer.

²Or Doctor Test Negative.

Process Description

Introduction

This section describes the process objectives and the major process steps in the Liquids Treating Section of the Gaviota Gas Plant.

Process Objectives

The process objective of the Liquids Treating Section of the plant are outlined as follows:

C₃/C₄ Treating

Diethanolamine (DEA) contacting to remove COS and the majority of the H₂S.

Dilute caustic contacting to remove residual H₂S.

Extraction of mercaptans with regenerated caustic.

Removal of entrained caustic by water washing.

Removal of entrained water by coalescing.

C₅+ NGL Sweetening and Caustic Regeneration

Oxidation of mercaptans in the C₅+ NGL to disulfides.

Regeneration of mercaptan extraction caustic.

Separation of vent gas, C₅+ NGL, and regenerated caustic.

Maintenance of catalyst activity.

Vent Gas Separation and Disposal

The C₅+ NGL, caustic, and excess air mixture from the Merox reactor flows into the dome section of the caustic settler, V-1760.  Here the air and any hydrocarbon vapor are vented to the incinerators in Plant 15 and 25.  The liquid C₅+ NGL and caustic flow to the lower horizontal section or settler section of V-1760 where they are coalesced and separated.  All separations in V-1760 are based on physical characteristics only; no chemical reactions are involved.  A back-pressure controller on the vent gas maintains a constant pressure on V-1760 and C-1750.

The dome section of the caustic settler is designed with sufficient residence time to handle upsets and to break down foam.  A demister section at the top of the dome minimizes carry-over of liquid droplets in the vent gas.

The vent gas leaving the dome is diluted with fuel gas to ensure that the vent gas mixture will always be in the nonexplosive range.  The volume of fuel gas added is equal to 120 % of the process air injection rate to the air diffuser, M-1751.  The minimum amount of oxygen required to support combustion in this type of mixture is about 11 vol%.  The 1.2:1 ratio of fuel gas to air reduces the oxygen content to at least 9.55 mole%, even if no oxygen is consumed in the Merox reactor, C-1750.

The vent gas is sent to the vent gas knockout drum, V-1500, in Plant 15 to separate out any liquid carry-over.  The vent gas is then incinerated in the Tail Gas Plant incinerators, FH-1500 or FH-2500 in Plants 15 and 25, respectively.  Flame arresters in Plants 15 and 25 will prevent any flashback from the furnace firebox.  An emergency bypass to the relief system handles the vent gas if the incinerators are out of service.

Separation of C₅+ NGL and Regenerated Caustic

The caustic/C₅+ NGL/disulfide mixture leading the dome section of V-1760 is separated into a regenerated caustic phase and a C₅+ NGL phase in the horizontal section of V-1760.  The regenerated caustic is recirculated to the mercaptan absorber, C-1740, and to the Merox Reactor, C-1750.  The C₅+ NGL, containing the dissolved disulfides, flows to storage on level control.

A 4-ft long bed of 4-8 mesh crushed and screened anthracite coal is used as a coalescing medium in the caustic settler.  This medium helps to coalesce small droplets of disulfide/hydrocarbon in the caustic into droplets large enough to separate rapidly from the caustic.  Caustic droplets in the disulfide/hydrocarbon phase are also coalesced.  In addition, the bed helps to coalesce and separate gas bubbles in the liquid.  A segmental baffle at the top of the separator prevents the gas from flowing out with the disulfide/hydrocarbon phase.  A vent line is connected to the dome to dispose of any vapor accumulating downstream of the coalescing bed.

Replacement of the coal charge will be necessary only when coalescing efficiency deteriorates.  Loss of the coalescing efficiency is usually due to plugging of the bed with scale or sufficient shrinkage to allow significant bypassing.  A bed life of three to five years can be expected if the bed is properly packed.

The caustic settler also provides surge volume for caustic volume changes in the mercaptan absorber.  Caustic is the continuous phase in the absorber, and the caustic interface level is maintained near the top of the absorber by interface level controller.  The hydrocarbon feed to the absorber passes through the caustic as large drops.  When there is a change in hydrocarbon feed rate, the volume of hydrocarbon in the caustic also changes.  This causes the caustic/hydrocarbon interface level to change.  The interface level controller acts to maintain the level.  Thus an increase in feed rate will reduce caustic volume in the absorber and increase caustic inventory in the settler.

Caustic must be withdrawn from the regenerated caustic stream and replaced with fresh caustic to remove Na₂SO₄, Na₂S₂O₃, and Na₂CO₃ from the system.  The Na₂SO₄ concentration should be maintained below 0.2 lb/gal of caustic.  (Measurement of Na₂S₂O₃ and Na₂CO₃ is unnecessary since they will only be present in trace quantities.)  The Na₂SO₄ concentration should never exceed 0.4 lb/gal, otherwise mercaptan removal efficiency in the absorber will be severely reduced.  Caustic blowdown and fresh caustic make-up occurs only when needed.  Waste caustic is sent to the caustic degasser, V-1790.

Merox Reagent Addition

Fresh Merox WS Reagent must be added to the caustic stream to make up for losses in the caustic blowdown, and to compensate for normal catalyst deterioration.

The reagent is a cobalt phthalocyanine disulfonate water slurry which resembles a blue dye.  A 1-gal jug contains 2.2 lb of active reagent.  When mixed with caustic, the reagent forms a micelle dispersion which settles with time.  However, mild mixing will resuspend the reagent.

A catalyst addition drum, V-1745, is used to inject the Merox reagent into the circulating caustic upstream of the Merox reactor.

The drum has an air sparger to mix the reagent with steam condensate and caustic.  After mixing, the reagent mixture is pressured from the drum using process air.

The drum has a level gauge to simplify measuring the correct proportions of condensate and caustic.

The active reagent concentration in the circulating caustic should be about 100-150 ppm_w as determined by calculation.  The initial charge required is 6.6 lb of active reagent, based on the estimated caustic volume in the plant.  One hundred ppm_w is equivalent to about 1 lb of the active reagent in 1000 gal of 25^oBé caustic.

The amount of reagent make-up needed is a function of (1) feed rate, (2) type of mercaptans in the feed, (3) concentration of mercaptans in the spent caustic, and (4) operating conditions in the caustic regeneration system.  About 1 lb of active Merox reagent can treat 15,000 bbl of C₃/C₄ plus C₅+ NGL feed.

Dirt and scale tend to adsorb Merox reagent and remove it from the circulating caustic.  Therefore, considerable additional Merox reagent may have to be added during the first one to two weeks of operation to replace adsorbed reagent until all adsorption surfaces are saturated.

Regenerated caustic with the required Merox reagent concentration has a deep blue or blue-green appearance.  This indicates the reagent has sufficient catalytic activity and that enough air is being injected into the Merox reactor.  If the color is dark green, insufficient air is being used.

The presence of iron rust or iron sulfide particles will color the caustic a muddy olive-drab.  At higher concentrations of iron rust and iron sulfide, the caustic will be dark brown or black in color and activity of the reagent may be reduced.  The caustic will also tend to emulsify with hydrocarbon.  When this occurs, the circulating caustic should be replaced with fresh caustic and new Merox reagent.

Process Air Supply

Process air for mercaptan oxidation is supplied from the instrument air header.  A knockout drum, V-1746, is provided to remove any water in the instrument air and to prevent backflow of caustic or hydrocarbon into the instrument air supply.

Caustic and Wash Water Degassing

The caustic degasser, V-1790, receives spent caustic from several sources on an intermittent basis.  Four hundred gallons of spent caustic is batched to V-1790 from the caustic treater, V-1730, approximately every 12 days.  The degasser can also receive spent mercaptan extraction caustic from V-1760, waste liquid from the vent gas knockout drum, V-1500, in Plant 15 and caustic from the caustic sump, V-1791.

The spent caustic sent to the degasser will contain dissolved and entrained liquid hydrocarbons that have a high vapor pressure.  Most of these hydrocarbons are vaporized as the caustic is heated by the degasser steam coil, E-1790.  The vapors are vented to the vent gas knockout drum, V-1500, in Plant 15.  The degassed caustic is pumped to a tank truck for disposal.  Any liquid hydrocarbon remaining after degassing is pumped out through the hydrocarbon skim outlet to liquid blowdown.  The operator can locate the hydrocarbon/caustic interface using the overlapping gauge glasses on the side of V-1790.

The maximum allowable temperature in the degasser is 140^oF.  Operation above this temperature could result in corrosion and stress cracking of the carbon steel vessel.  Normally a temperature of 120^oF is adequate for degassing.

Wash water from the water washer, V-1770, and coalescer, V-1775, is sent to the wash water degasser, V-1780.  The wash water degasser also intermittently receives condensed water from the depropanizer reflux drum, V-1611, in Plant 16.  Any C₃/C₄ hydrocarbon dissolved or entrained in the wash water is vaporized and vented to V-1500 in Plant 15.  Degassed wash water from V-1780 is pumped to the wash water storage tank, T-1570, for use as SO₂ scrubber make-up water.