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Vacuum Unit Pressure Control: Impact on Refinery Profitability

 

Summary

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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Refinery vacuum unit pressure control is essential to meeting crude unit revamp profitability objectives. While minimum vacuum unit operating pressure always increases the heaviest distillate product yield, low pressure operation is not always the optimum to meet the feedstock quality and rate targets of downstream units. Low operating pressure can cause massive entrainment of vacuum tower bottoms (VTB) into the heaviest distillate product if the column diameter is too small. Often, optimum vacuum unit operation requires higher column operating pressure, which must be off-set by increasing the heater outlet temperature. This maximizes the heaviest distillate product yield while avoiding high VTB entrainment.

Vacuum distillate quality must be monitored and controlled; otherwise, downstream unit performance suffers. The higher the VTB contaminants level, the lower the quantity of entrainment that can be tolerated before the downstream unit is affected. VTB quality is crude dependent; it is not uncommon to have vanadium and nickel levels greater than 500 weight ppm and 26-28 weight percent carbon residue with heavy crude oils. Small amounts of entrained VTB, when processing Maya and Venezuelan crude oils, will dramatically increase the metals and carbon residue in the heaviest distillate product. Some unsuccessful revamps have produced heavy vacuum gas oil (HVGO) products with carbon residue of 1.5 weight percent or higher and 30-40 weight ppm nickel and vanadium.

Pressure control is necessary when the column diameter is the primary unit limit, which often is the case when a unit is revamped. The three main causes of poor pressure control are...

 

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Tags: CDU, VDU