PRACTICAL, AVAILABLE TECHNOLOGY
Research-Cottrell and Standard Oil (Calif.) show that imaginative engineering for meeting today's air pollution regulations can keep within refinery space limitations as well as budget Many companies that installed air quality control equipment before the current regulations were established now face a serious problem-bringing this equipment up to standards. Having made a substantial investment in air control equipment, they now find it obsolete, unable to meet current regulations. They have a "white elephant" on their hands that must somehow be incorporated into current or future air control plans or discarded at a total loss of investment. More than 20 years ago, Standard Oil (El Segundo, Calif.) installed a Research-Cottrell electrostatic precipitator as the final stage of a dust recovery system to capture "fines" from a fluid catalytic-cracking unit. With normal maintenance and parts replacement the unit continued to perform satisfactorily. Since that time, however, more stringent air quality control has become one of the conditions for doing business, so the original unit could no longer meet the regulations. Faced with a seven-figure price tag for replacing the precipitator in its entirety, Standard Oil instead solved the problem by "capping" the original precipitator shell with a 4-ft extension. This allowed Research-Cottrellto design new internals that increased precipitator treatment capacity by more than 13 000 ft2-at far below the cost of new construction. The actual cost to Standard Oil was less than 10% of t i e cost for new precipitators.
Capping. This 4-ff extension cost 10% that of new units
The choices In most cases where precipitator efficiency must be increased, companies usually select one of two basic solutions. One is to add on the required treatment capability by building another precipitator either in series or parallel with the existing precipitator. The other is new construction, either by tearing down and replacing the old precipitator with a larger one or by building in a new location. To Standard Oil engineers, both of the foregoing solutions were impractical because of cost and available space limitations. Also. the existing precipitator shell, though undersized, was still in good condition and they were reluctant to scrap it. Their solution The solution was to increase the capacity of that shell by increasing its height. In the original installation, 31 collection plates in each unit treated flue gas in three separate fields-inlet, middle, outlet-in the Precipitator. Each plate measured 9 ft wide by 20 ft high for a total collection area of 66 960 ft2. By replacingeach 2 0 4 plate with a 24-ft plate for a new collection area total of 80 352 f12.Standard Oil gained 13 392 ftZ of treatment area for roughly the construction cost of a fourfoot high precipitator shell. The steel cap itself, 4 ft high X 16 ft wide X 18 fl ionq, was constructed on the
ground in an area near the old precipitator shell and lowered into position by a crane. The cap was then welded to the old shell and the new internals installed. The precipitator, as designed and built by Research-Cottrell, now has a normal rated gas capacity of 150 000 acfm at 650 OF. It is two chambers wide and three fields long in the direction of the gas flow. Each field has 30 9-in. each wide ducts, formed by 31 Opzel-type collecting plates, each 24 ft high by 9 ft long. These collection electrode plates are cleaned by magnetic impulse, gravity impact rappers. The 2160 copper Bessemer discharge electrode wires are cleaned by electric-type vibrators. Electrical energization to the precipitator is supplied by four 25 kVa, 75 kV vacuum tube rectifiers. Six-stage collection unit The particulate collection system is installed downstream of the fluid catalytic cracking unit to control emissions of silica alumina catalyst fines. The cracking operation generates a large quantity of these fine particles. Under current regulations Standard Oil must collection ail but 30 Iblh, and they do. To collect these highly abrasive fines, Standard Oil has three stages of cyclone separators and the three-field ResearchCottrell electrostatic precipitator. The cyclone separators filter out the larger catalyst fines, which are collected and recycled into the cracking process. Those small fines, which cannot be.collected by the cyclones, pass on to the electrostatic precipitator where they are removed from the gas stream before it is released to the atmosphere. Catalyst particles collected by the precipitator that are too fine for re-use are hauled away. Double savings by remodeling With proven technology and some imaginative engineering design, Standard Oil has avoided a potential air pollution problem on a critical process plant. What's more, by not building a new precipitator housing shell, which would have meant an extended shutdown of the cracking unit, they saved, in addition to construction costs, literally tens of thousands of barrels of lost product. Volume 11. Number 4, April 1977
333
