Plant continuous catalyst regeneration

Figure 6.5. Modem catalytic reforming plant with continuous catalyst regeneration (by permission of UOP).

Dehydrogenation of /i-Butane. Dehydrogenation of / -butane [106-97-8] via the Houdry process is carried out under partial vacuum, 35—75 kPa (5—11 psi), at about 535—650°C with a fixed-bed catalyst. The catalyst consists of aluminum oxide and chromium oxide as the principal components. The reaction is endothermic and the cycle life of the catalyst is about 10 minutes because of coke buildup. Several parallel reactors are needed in the plant to allow for continuous operation with catalyst regeneration. Thermodynamics limits the conversion to about 30—40% and the ultimate yield is 60—65 wt % (233). 

Continuous processes are designed to operate 24 hours a day, 7 days a week, throughout the year. Some down time will be allowed for maintenance and, for some processes, catalyst regeneration. The plant attainment that is, the percentage of the available hours in a year that the plant operates, will usually be 90 to 95%. 

MTO was first scaled up in MRDC s 4 B/D fluid-bed pilot plant in Paulsboro, New Jersey. Following successful completion of the 100 B/D MTG project, the project was extended, and the plant modified to demonstrate MTO (refs. 16, 17). The plant is shown schematically in Fig. 4. Methanol is converted in a turbulent fluid bed reactor with typical conversions exceeding 99.9%. The products are recovered in a simple gas plant. Coked catalyst is continuously withdrawn from the reactor, and the coke is burned in a fluid-bed regenerator. Coke yield and catalyst circulation are an order of magnitude lower than in fluid catalytic cracking. 

A supported cobalt/molybdate catalyst, probably based on the ones developed in the 1930s, was one of the first types to be used in modem ethylene plants. The front-end reactor was located in the compressor train after heavy hydrocarbons were removed but before sulfur removal or gas drying. The catalyst was, therefore, partly sulfided. Careful temperature control was required to limit ethylene loss. About 10% steam was added to cracked gas, which limited the temperature rise and improved selectivity. An unusual feature of operation was that a significant proportion of the acetylene was removed as a polymer. This decreased the potential temperature rise but meant that catalyst regeneration and subsequent reactivation was a routine procedure at intervals of 2-4 weeks and that a spare reactor was needed. To compensate for loss of activity the gas temperature was continuously increased throughout the operating cycle. Acetylene levels were reduced to about 10-20 ppm with 1-3% ethylene loss. Up to 50% of any butadiene present in the gas was also hydrogenated. The catalyst was replaced after 1-2 years. 

Phosgene addition is continued until all the phenoHc groups are converted to carbonate functionahties. Some hydrolysis of phosgene to sodium carbonate occurs incidentally. When the reaction is complete, the methylene chloride solution of polymer is washed first with acid to remove residual base and amine, then with water. To complete the process, the aqueous sodium chloride stream can be reclaimed in a chlor-alkah plant, ultimately regenerating phosgene. Many variations of this polycarbonate process have been patented, including use of many different types of catalysts, continuous or semicontinuous processes, methods which rely on formation of bischloroformate oligomers followed by polycondensation, etc. 

They may require pH adjustment and settling. These effluents should preferably be recycled or reused. Spent catalysts are usually sent for regeneration or disposed of in a secure landfill. Air emissions should be monitored aimually, except for nitrate acid plants, where nitrogen oxides should be monitored continuously. 

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