Chapter 6 CO combustion promoters: past and present

Abstract

It has been over 30 years since the use of Pt-metal containing CO combustion promoters were introduced for commercial use in catalytic cracking processes. Their use has revolutionized fluid catalytic cracking (FCC) operations, allowing yield optimization through use of both partial and complete CO combustion operating modes. It is likely that most, if not all, cracking units throughout the world are using CO promoters. In the 1940s and 1950s, afterburning in regenerators was a major problem. Catalytic solutions involving the use of transition metals were proposed, and Cr TCC catalysts were eventually adopted for commercial use, with moderate success. In the early 1970s, a small program was initiated to develop a catalyst to promote CO combustion at Mobil's Paulsboro, N. J. laboratory. The desirable characteristics of such a promoter were as follows: (1) sufficient activity to provide complete CO combustion in the catalyst dense bed; (2) no (or little) production of contaminant products usually associated with metals (excessive coke and hydrogen); (3) sufficient stability to withstand both the reducing conditions of the riser and reactor, and the hydrothermal conditions in the regenerator; and (4) acceptable incremental cost. In late 1972, the concept that economically small amounts of Pt, perhaps 1–10ppm, would not increase the cost of the catalyst unacceptably, and might still have sufficient activity for complete CO combustion, was proposed and tested in laboratory zeolite catalysts. The results were excellent: the regeneration gas contained only CO 2 , no CO; and cracking yields were acceptable. Testing of other noble metals showed that all had activity, but Pt, Pd and Ir seemed the best. The first actual commercial demonstration was done in a TCC unit in 1974; the catalyst contained 5 ppm Pt added by exchange. Results were excellent with complete CO combustion attained at a kiln temperature of 1160°F. Commercial use in FCCs began in 1975 in units with proper metallurgy. By the end ofthat year, at least 19 FCC units were using promoted catalyst routinely. A test in a Mobil refinery demonstrated both partial and complete combustion and the increased yields that result from the lower catalyst circulation rates and higher temperatures. By 1979, several different modes of Pt addition had been tried and the preferred method was the use of additives containing concentrates (100–1000 ppm) of Pt on a support, usually alumina. Subsequent developments, such as the higher NO x levels associated with Pt, will also be discussed.