Process Optimization TABLE 1. Typical density values Strong caustic (10%) Medium caustic (5%) Weak caustic (2%) Water Wedge baffle Hood baffle to divert liquid to pump compartment Yellow oil draw pot Pump compartment Spent compartment Spent caustic Density: 1,112 kg/m3 Density: 1,058 kg/m3 Density: 1,014 kg/m3 Density: 1,000 kg/m3 instead of caustic (FIG. 10). * All temperature sensors should be long enough to accurately read the liquid temperature. * Antifoaming and emulsion breaker facilities, if required. Flow fluctuation in compressor. Possible causes include: * Dislodging of trays and packing due to abnormal compressor condition, such as surge/ choke during startup/trips. * Entrainment of caustic to the downstream line to the knockout drum of the compressor. Pump * Sticking of tray valves can prematurely flood the tower. FIG. 9. Caustic tower bottom portion for phase separation, showing the pump circulation compartment (left) and the spent caustic compartment (right). differences. These types of level transmitters (differential pressure) cannot detect the exact level of liquid because hydrocarbon density is 30% lower than the caustic solution. * The mixture of aqueous caustic and hydrocarbon has a greater tendency to form an emulsion/ foam (fouling is the root cause of the foaming) that can lead to inaccurate level detection. * All LTs calibrated with the same density across the tower in most of the plants. Solutions and recommendations include: * Provision of an LG in the pump circulation compartment to continuously check the level (FIG. 10). * Instead of a typical DP type, one interface-level transmitter can be considered in the spent caustic compartment (FIG. 10). * Skimming facility to remove yellow oil (FIG. 10). 66 JULY 2022 | HydrocarbonProcessing.com * Accurate densities should be considered for calibration across the tower for LTs. Typical values are mentioned in TABLE 1. * A permanent water wash facility can be considered for all level transmitters and pressure transmitters. * All pressure transmitter tapping should be in the vapor phase (along with an inclined slope toward the column) to prevent liquid accumulation. A pressure indicator should be above the tapping altitude. * Two-level gauges can be installed at different elevations2 to detect foaming. * Level transmitter tapping should be in the spent caustic compartment rather than the pump circulation compartment. * Hydrocarbon density should be used for the draw-off pot of yellow oil skimming level transmitter calibration Solutions and recommendations include: * All caustic tower trays and packing should be mechanically designed and support should be strengthened to withstand abnormal compressor conditions. * The caustic tower should have provision for a wedge baffle or impingement plate in front of the feed nozzle to counter gas momentum. This also avoids flow hitting the level transmitter and hood baffle. * The overhead line and successive knock-out drum should be designed for caustic. * Tower metallurgy should be stress released to reduce corrosion. Any welding work after manufacturing should be avoided.3 General problems. Possible causes include: * Salt out (precipitation of salts on tray) of caustic * Liquid holdup or flooding due to blockage of packing with support * Foaming and emulsion at the bottom section * Corrosion * Pyrophoric FeS formation (H2 S + Fe = FeS + H2 ) * Fouling (aldol condensation)4http://www.HydrocarbonProcessing.com