สำนักราชบัณฑิตยสภา

«“√ “√ √“™∫— ≥±‘µ¬ ∂“π ªï ∑’Ë Ú˜ ©∫— ∫∑’Ë Û °.§.-°.¬. ÚıÙı Pramote Chaiyavech ˆˆ˜ was adjusted to the design level on June 3, 1990 sulfur injection into the feed was interrupted on two separate occasions as a result of blockage of the injection line. The unit remained in operation with- out sulfur for some four hours each time during the injection line clearing operation. The reactor pressure drop increase was noted and later soot coke was found in catalyst samples. The pressure drop increase was highest in Reac- tor 3 which finally reached the void blowing zone. The unit was shut down July 12 for inspection and screen cleaning. Sulfur injection is intended to prevent exposure of the metal surface to Olefin mol- ecules at high temperature by pro- viding iron sulfide film on the sur- face. Olefin will react with free or open iron molecules, pulling iron out from the wall. Additional ole- fins continue to add on this coke particle which eventually drops off in rod form. Sulfur injection is therefore extremely important. To ensure uninterrupted sulfur injec- tion a double injection system was installed with a spare. Alternative injection point and low flow alarm were also added. b) Thermal Coke and Coke Precursors. In spite of the tight con- trol of the sulfur injection and the disappearance of soot coke in cata- lyst samples, problems with the re- actor pressure drop increase con- tinued, albeit at a much lower rate. It was noted that the presence of even a small amount of C 4 ’s in the feed also caused a pressure drop increase due to thermal coke and Heavies formation. Olefins also behave the same way as coke pre- cursors. Stricter control of C 4 ’s and Heaviers in feed was thus implemented. Later on the MA- PD converter unit treating the pro- pane recycle stream was switched to complete hydrogenation opera- tion to eliminate any olefins in feed. Positive results were ob- tained. Notwithstanding the above-mentioned fouling of the re- actor screens by coke and Heavies persisting, more measures are re- quired if a long on-stream time is to be achieved. 3. Metallurgical problems a) Carburization and Corro- sion During the screen- cleaning shut down in 1990, met- allurgical inspection of the entire system was carried out. Carburiza- tion and corrosive attack were dis- covered in several areas, particu- larly within the reaction zones. The high temperature environ- ment coupled with the severe coke deposition on the metal surface induced these problems. The cor- rosion was mostly of the under- deposits type. To withstand the re- actor environment better, it was be- lieved that the screen material should be changed from SS 304 to SS 347. This was effected in 1992. b) Thermal Stress and Ther- mal Cycling The temperature pro- file inside the reactor developed thermal stress on metal during op- eration. With the aid of a com- puter program, it was discovered that the reactor basket was not strong enough to withstand the thermal stress developed. More stiffening rods were added to strengthen the basket. In addition, it was discovered that the thermal cycling resulting from the transfer of relatively cold catalyst through the hot reactor contributed signifi- cantly to the stress problem, par- ticularly at high catalyst circulation rate. Catalyst heat-up zones to in- crease catalyst temperature before Fig. 2 Catalyst flow