Transcript Document
TASNEE Ethylene Cracker Plant “CGC Inter Stage Cooler Fouling and Its Remedial Approach to Avoid Unplanned Shutdown” By Nafees Ahmad Area Process Engineer Contents • Problem Statement • Operation Overview • Fouling Phenomena • Trouble shootings and Mitigations • Conclusion Problem Statement TASNEE Ethylene Cracker plant started facing high Differential Pressure (DP) across CGC 4th Stage cooler (3E-3114) from November 2013 and it was increasing significantly, forcing to an unplanned shut down for cooler cleaning. Operation Overview • In TASNEE Ethylene Cracker plant Ethane / Propane feedstock is cracked in pyrolysis furnaces to produce Ethylene and Propylene. • The cracked gas is cooled down to ambient temperature in water quench tower after which it is compressed in five stage CG compressor from 0.3 barg to 34 barg. • The compressed gas stream from CG compressor is fed to the separation unit which consists of several distillation columns where the process stream is separated into individual desired products. Operation Overview • Cracked gas compressor consist of 5 stages and every stage have one inter-stage cooler and separator. There is a caustic scrubber in between the 4th and 5th stages of the compressor to remove acidic gases from the cracked gas. • After each compression step, cracked gas is cooled in a cracked gas intercooler and during compression the formed condensate is separated in inter-stage separators. • CGC fouling control is being done by: Continuous injection of boiler feed water into each compressor stages to keep discharge temperatures below 90°C. Intermittent injection of Wash Oil (Solvesso# 200) into each compressor suction to disperse formed polymer from casing and clean the compressor stages. CGC System Diagram 0.25 barg 1.3 barg 3.5 barg & wash oil 17.3 barg 8 barg 3E-3114 34.6 barg Fouling Phenomena • Gas crackers usually crack ethane or propane have high probability of fouling within a compressor as more fouling precursors/active monomers and lesser aromatic compounds are generated during cracking compared to liquid crackers. • Fouling of the CG compressor is most often due the result of polymerization and condensation reactions. The chemical reactions in the fouling are mainly due to the active monomers having double bonds such as butadiene, styrene, isoprene, vinyl acetylene etc. that polymerize and deposit on the internal surfaces of the compressor. • These fouling reactions are sensitive to temperature, pressure, concentrations of active monomers and the presence of precursors and catalysts. Fouling Phenomena • There are a number of factors that promote polymerization: More gas stream → more fouling More temperature → more fouling More cracking severity → more fouling More recycle streams → more fouling Once polymer starts to collect at the inlet of inter stage coolers, it starts to act like a filter. Polymer deposition creates a pressure drop (High ∆P across cooler) that lowers the overall performance of the compressor. Trouble Shooting and Mitigations • Immediate actions BFW injection at 4th stage has been optimized at 85 0C as it was around 85-90 0C. Cracking of Propane feed reduced to design value as plant was operated at higher side due to ethane feed shortage. Propane conversion reduced to design value( Case-2 design) as it was at higher side. Online compressor monitoring by operation & reliability team. Trouble Shooting and Mitigations Trouble Shooting and Mitigations Conclusion Ethylene plant cracked gas compressor system are prone to fouling. In many cases, fouling of the CGC inter stage coolers are one of the prime reason for unplanned shut down of the plant. Fouling can be controlled by wash oil (300-400 kg/hr.) and antifoulant chemical (5-20 ppm) injection directly to fouled cooler through retractable quill can avoid unplanned shut down with sustainable process conditions. THANKS