With desirability of getting the maximum use out of existing equipment when revamps are planned - not least to minimise investment costs - a rigorous approach to CPD is required to avoid scope growth.
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Faulty equipment specifications sometimes carry unforeseen penalties, with loss of performance. But do not blame the original equipment manufacturer, say the authors- it is the process design engineer's responsibility to meet the objectives.
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Basic heat exchange principles are used to explain gas plant thermosiphon reboiler hydraulic constraints. Field measurements help clarify "off-design" operations that were not apparent and were causing high condensate levels.
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Understanding vacuum ejector performance is the key to maintaining reliability and distillate yields. A low VGO yield may result from a breaking ejector system, but bypassing a fouled inter-condenser can restore lost yield.
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To revamp vacuum units, process modelling and equipment design know-how are needed, and the understanding of connected equipment performance can lead to higher gasoil quality and yields, with fewer unscheduled shutdowns.
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Poor heater design leads to unscheduled Vacuum Unit fired heater shutdowns due to coke formation. Oil film temperature and residence time are the variables that determine the rate of coking for a given crude oil blend. Proper heater design and operation controls oil residence time and oil film temperature, allowing refiners to maximize heater run length.
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Inaccurate feed characterisation and process modelling errors are major contributors to poor performance in a vacuum unit as refiners switch to heavier crude types.
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Good vacuum column pressure control is essential to a successful revamp when vacuum column diameter is the primary unit limit. Poor pressure control can lead to massive entrainment of VTB into HVGO. This is especially significant with heavy crudes with high metals and carbon residue, where entrainment causes a rapid rise in the metals and carbon residue of the HVGO product. This article primarily addresses controlling vacuum column flash zone pressure through the design and operation of the first stage ejector pressure control system.
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