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A single integrated vacuum system

Failure to design the vacuum unit as an integrated system will invariably result in unsatisfactory yield and poor product quality (high vanadium, nickel, microcarbon, or asphaltenes), and ultimately, an unscheduled shutdown. To avoid these revamp problems the charge pump, fired heater, transfer line, column internals and ejector system must all be evaluated and designed as a unified whole so that critical variables – heater outlet temperature, coil steam injection rate, flash zone temperature and pressure – may be properly balanced. A thorough understanding of requisite theory is necessary and computer modeling is useful but theory and simulations are not enough. Process flow sheet models must be calibrated using actual field data, and appropriate allowances made for less-than-ideal equipment performance. One cannot rely alone on manufacturers’ specification sheets because only rarely will they reflect actual field-tested performance. Only by exercising such caution can under-utilized equipment be exploited and investment in new equipment minimized. An experienced revamp engineer will then be able to optimize critical temperatures and pressures to ensure satisfactory yields, product quality and length of run.

The critical link: heater-ejector system
Today many refiners have set 4-5 year run length and higher HVGO TBP cutpoints as goals. Increasing heater outlet temperature to achieve these goals requires all equipment to work together to keep cracked gas low, for if an ejector system is incapable of handling higher cracked gas production from a 10-15ºF increase in heater outlet temperature, column flash zone pressure will increase and gas oil yield can actually be reduced. Hence, not only must the ejector system be sized properly but the heater must be well designed to minimize both cracked gas and coke make – and the two must be balanced. Contrary to conventional wisdom this means designing upstream: from the ejector system back to the heater (and all the way back to the charge pump to ensure total unit performance.) For the designer to accomplish this, baseline and revamp process models must reflect existing equipment design including all shortcomings. This entails making a thorough test run. Following these guidelines can effect a significant increase in revenue – the bottom line goal of every revamp.

Other Literature

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A single integrated vacuum system

Failure to design the vacuum unit as an integrated system will invariably result in unsatisfactory yield and poor product quality (high vanadium, nickel, microcarbon, or asphaltenes), and ultimately, an unscheduled shutdown. To avoid these revamp problems the charge pump, fired heater, transfer line, ...


Is pinch enough?

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