Water Wash Column Design

1. Purpose

This procedure provides design guidelines for a Water Wash Column used to remove basic nitrogen from LPG.

2. General

This treatment is normally done to the LPG feed of a Catalytic Condensation Unit. Do not confuse this design with a Jet Deck design that is used for Merox Extractors, Sulfolane Raffinate Water Wash Columns, LPG Amine Treaters and Ethermax Raffinate Water Wash Columns which is covered in Procedure ”.

3. Design Criteria

Design a Water Wash Column for treating LPG per the following design criteria:

• Circulating Water Rate, (BPD) = (0.5)(LPG Charge Rate, (BPD))

• Spent Water Rate, (BPD) = (0.025)(LPG Charge Rate,(BPD))

• Column Diameter, FT =

Design the trays in the Water Wash Column per the following design criteria:

• Sieve Hole Size, inch - ¼ (6 mm)

• Hole Velocity, Ft/Sec - 1.8 (LPG phase)

• Downcomer Velocity, Ft/Sec - 0.2 (Circulating Water Rate)

• Number of Trays - 20

• Tray Spacing, inch - 12 (300 mm)

• Number of Passes - 1

• Space above top tray, Ft - 10 (3000 mm)

• Space below bottom tray, Ft - 5 (1500 mm)

is a sample 307 specification for this column.

4. Process Flow

shows the flow scheme for a Water Wash Column. This drawing provides two column entry locations for the circulating water stream. The normal flow scheme allows the circulating water to enter onto Tray 5, while the phosphate and condensate injection go to Tray 1. An alternate flow scheme allows the circulating water to go to Tray 1 along with the phosphate and condensate injection. The following comments apply to the alternate flow scheme:

• It yields a more rapid response of circulating water pH when the phosphate rate changes.

• The top five trays are less likely to have corrosion caused by the higher acid concentration.

• Feeding circulating water to Tray 1 may cause turbulence that results in water carry over.

Distributors

The distributor for the feed to the top tray is normally a simple tee per Standard Drawing 3-144.

The distributor for the feed between the upper and lower wash sections is normally a simply tee per Standard Drawing 3-146.

The distributor for the feed below the bottom tray is normally a single pipe per drawing 3-314. The distributor is to be parallel to the downcomer. There are two slots on the distributor oriented 180 degrees apart in the horizontal plane. The slots have a typical width of 3/8 inch (10 mm) and a minimum width of ¼ inch (6 mm). The design exit velocity should be between 1.5 to 3 ft/s. The nozzle location is normally 1.5 ft (450 mm) below the bottom tray.

General Discussion

The design of a Inflection Point Engineering Catalytic Condensation Unit includes a water wash column to remove basic nitrogen compounds from the LPG feedstock. Basic nitrogen compounds primarily consist of ammonia, nitrites, and amine (soluble and entrained), which will react and deactivate the phosphoric acid based SPA catalyst.

Inflection Point Engineering Catalytic Condensation Units with an LPG feedstock that has been through an Amine Treating Unit, or any LPG feedstock that originates from FCC units or cokers, usually requires a water wash column.

The injection of a sodium dihydrogen phosphate (NaH2PO4) solution maintains the circulating water at a pH of 5.5 to 6.5. The NaH2PO4 solution (1 normal) is prepared in a small break tank and injected into the water wash column on a continuous basis. Typically, the wash water is deaerated condensate before chemical addition and it is continuously added to the water wash column through a water break tank.

The spent water is analyzed and the wash water addition rate is adjusted to keep the total dissolved solids below 2000 ppm. The acid injection rate is adjusted to keep the pH between 5.5 and 6.5.

Project