Antifouling programmes for Caustic scrubber treatment.
Pyrolysis of liquid and gaseous feedstocks for ethylene production is achieved in steam cracking units. The cracked gases contain carbon dioxide and hydrogen sulfide that must be removed from the cracked gas. Hydrogen sulfide is a catalyst poison for hydrogenation reactors. Carbon dioxide can freeze at low temperatures in heat exchangers and fractionation equipment. It can also be absorbed into ethylene, effecting the product quality and further processing. These acid gases are scrubbed with caustic solution (NaOH) in caustic wash towers. The caustic tower (caustic scrubber) is typically integrated upstream of the last compressor stage.
Caustic scrubber systems are frequently subject to polymer fouling. Fouling of the caustic scrubber internals and wet caustic oxidiser are known problems. They are recognised by the operators as “Red-tide fouling” or “Red oil”. Sodium carryover to the next compressor stage is not unusual and leads to problems with the downstream units. Aldol condensation products and high concentrations of C4 and C5 diolefins are formed. Aldol condensation polymerisation is a base catalysed reaction. The cracked gas contains carbonyls like aldehydes and ketones. The presence of acetaldehyde in cracked gas streams is quite common.
The caustic base removes a proton from the aldehyde molecule by forming a carbanion. This carbanion will react with another aldehyde molecule to form the aldol group. It still contains a reactive aldehyde that may continue to react. The polymers create longer chain lengths in the caustic scrubber and remain suspended in the caustic solution. Aldol condensation products are often called “Red Oil” because of the orange to red or brown-red colour. The polymers can absorb other organic materials from cracked gas. This will increase the pressure drop and fouling formation. Additionally, unsaturated compounds such as 1,3 butadiene can be easily dissolved in caustic solution. Together with metal oxides and oxygenated compounds, more polymers are formed to increase the red oil production.
Kurita has developed high-performing antifouling concepts which inhibit the aldol condensation. Formation of red oil polymer materials will be avoided. Antifoulants with dispersant properties keep the polymer particles small enough to avoid agglomeration of the polymers. Antifoulants with radical catcher properties will stop the free radical polymerisation mechanism. The treatment programme can be monitored by analysing the spent caustic. Successful treatment will lead to the elimination of expensive gasoline wash. It will reduce the load on the spent caustic oxidation unit. This will reduce COD load on the wastewater plant. A sodium contamination in the DSG system through recycling of the spent gasoline will be avoided.
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