Neutral Shift: Five Years of Continuous Operation with Adjustable Frequency Drives

IN 1998, A LARGE MIDWESTERN refinery undercook a US$150 million project to update and optimize a 50,000-bbl/d fluidized-bed catalytic cracking unit (FCCU). The FCCU is one of the most important units within a refinery and is the largest producing unit of high-octane gasoline blend stock. The scope of the project included a complete replacement of the FCCU reactor, combustion air compressor, wet gas compressor, and the addition of a flue gas scrubber to reduce particulate emissions. The FCCU portion of the project was completed during a planned refinery maintenance turnaround in November 2000. A companion project to construct a new gas plant for an existing 25,000-bbl/d Coker unit was completed in May 2001. This article will discuss the FCC and Coker wet gas compressors. The project was justified on the basis of energy conservation, improved product yield, reduced maintenance costs, improved reliability, and environmental compliance. Much of the energy savings were expected to be realized through efficiency improvements in the combustion air and wet gas compressors. The existing FCCU was a steam turbine driven centrifugal compressor that operated at approximately 65% efficiency. An electric driver was chosen due to the lower maintenance cost versus a steam turbine. Since the refinery must typically purchase natural gas to supplement internally produced gas to make steam, the cost of steam was higher than the cost of electricity on a per-horsepower basis. As is typical of the refining industry, an interruption of either of these critical units would shut down or severely curtail the entire process and cost millions of dollars in lost production. Hence, total process reliability was of prime importance in making equipment selections.