Caustic Wash Process: Aluminum Alkyl Removal

• Liquid streams from the bottoms of D-310A/B, T-330, D-330, and D-344 combine with liquid streams from the bottoms of D-110 and D-120A/B.
• The combined liquids mix with heated circulating caustic in M-340 (Caustic Wash Static Mixer).
• Aluminum alkyls react with water to form n-paraffins and aluminum hydroxide.
• Caustic reacts with aluminum hydroxide to form soluble sodium aluminate.
• The catalyst is destroyed, removed from the hydrocarbon phase, and solubilized in the caustic phase.
• Nickel catalyst reacts with caustic to form a nickel salt, which is removed with the spent caustic.
• Maintain circulating caustic strength at or above 12% free caustic and less than 30% to keep sodium aluminate in solution.
• The three-phase stream from M-340 goes to the bottom of T-340 where the vapor separates.
• Vapor rises through the packed bed in T-340, is washed by fresh process water, and flows to E-360 (Deethenizer Feed Cooler).
• The two liquid phases drop into D-340 (Caustic Wash Settler) and separate into a hydrocarbon stream and a caustic stream.
• The hydrocarbon stream along with water flows to T-350 (Water Wash Vapor Scrubber) for water washing.
• Most of the caustic stream from D-340, plus make-up caustic from P-342A/B mixed in M-341 with process water from P-433A/B, is routed by P-340A/B to E-340 (Caustic Wash Heater) and heated by 100 psig steam before going to M-340.
• Excess caustic is routed to T-344 (Degassing Tower).
• Wastewater is routed to D-353 (Wastewater Degassing Drum).
• D-340 has a continuous skim system.

Continuous Skim System

• D-345 pulls the rag layer from the skim pan in D-340.
• The rag layer impacts NAO product quality and fouls the F-383A/B and D-383A/B.
• A continuous draw-off flows from the rag layer skim pan in D-340 into D-345.
• Lighter components are separated from heavier components and wastewater.
• Lighter components go to T-350 for stripping.
• Oily water goes to T-344 for stripping.
• Wastewater goes to D-353.
• The rag layer is periodically removed manually from D-345.

Equipment Details

• M-340 (Caustic Wash Static Mixer) has 2 fixed carbon steel elements and mixes heated circulating caustic into the process.
• T-340 (Caustic Wash Vapor Scrubber) is 2’D x 26’L and separates vapor from liquid phases.
• The three-phase stream enters the side bottom, and fresh process water enters the side top.
• Overhead vapor exits the top, and the two liquid phases exit the bottom.
• D-340 (Caustic Wash Settler) is 7’D x 28’L and separates the two liquid phases into caustic and hydrocarbon streams.
• A skim system removes the rag layer to prevent interference with separation and caustic entrainment.
• The skim pan spans the drum at 50% of the interface float column range.
• M-341 (Caustic Makeup Static Mixer) has 6 carbon steel fixed elements and blends 50% makeup caustic with process water to form a 25% dilution.
• P-340A/B (Caustic Wash Circulation Pumps) are single-stage centrifugal pumps driven by 15 HP 1750 RPM electric motors and circulate recycle and fresh caustic.
• Normally, one pump is in service, and the other is in standby.
• E-340 (Caustic Wash Heater) is a multi-tube, U-type heat exchanger that heats circulating caustic on the shell side using 100 psig steam on the tube side.
• D-345 (Continuous Skim Drum) is 3’D x 9’L and pulls the rag layer from the skim pan in D-340.
• D-345 has a nuclear level sensor to display hydrocarbon, caustic water, and rag layers.
• The nuclear source must be shuttered by a qualified technician before maintenance to reduce the risk of personnel exposure.