Jul-2001

Vacuum pressure control: impact on profitability

Optimisation of flash zone pressure and temperature can improve feedstock quality to downstream units

Gary R Martin and Jason M Nigg, Process Consulting Services

Article Summary

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 offset by increasing the heater outlet temperature. This maximises 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 wt/ppm and 26–28 wt% 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 revamps have produced heavy vacuum gasoil (HVGO) products with carbon residue of 1.5 wt% or higher and 30–40 wt/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:
- No means to control pressure
- Fundamental errors in the pressure control system design
- Poor ejector spillback piping design and installation.

Numerous equipment problems can cause variations in distillate yield and quality [Golden S W, Troubleshooting vacuum unit revamps; Petroleum Technology Quarterly, Summer 1998]. However, the focus of this article is controlling column flash zone pressure through the design and operation of the first stage ejector pressure control system.

Product yield and quality
Vacuum unit distillate must meet both yield and contaminants specification targets. Refinery vacuum units produce feedstocks for further processing in an FCC, hydrocracker, or lube oil facility (Figures 1 and 2). Maximum, on-specification lube distillate or HVGO product yield occurs when the vacuum column is operated at an optimum flash zone temperature and pressure.

The fired heater, column diameter, heat removal, and/or vacuum ejector system determine the minimum flash zone pressure and optimum temperature. The specific unit equipment limit will determine the optimum combination of temperature and pressure to meet distillate yield and quality targets. If the column diameter is the major limit, then inadequate pressure control often results in high metals, microcarbon residue (MCR), and/or asphaltenes in the distillate products from VTB entrainment.

Vacuum column flash zone pressure and temperature management is the key to maximising profitability. Flash zone temperature and pressure determine the vacuum distillate yield and quality. Whether temperature, pressure, or both, are adjusted depends on specific equipment constraints. The interdependencies of the major equipment complicate this optimisation. For instance, the vacuum unit heater outlet temperature sets the flash zone temperature and it largely determines the cracked gas load on the vacuum ejectors. The maximum heater outlet temperature, assuming no ejector system limit, is set by the coke laydown rate in the radiant section coils.

High coke formation rates will reduce run length and require heater decoking, as well as increase the cracked gas production. The impact of cracked gas on the column operating pressure depends on whether the unit uses coil or stripping steam. Typically, the operating pressure of a dry vacuum column is more dependent on cracked gas production than a unit using steam, because the cracked gas rate is the primary ejector load on a dry unit. If the column diameter is the limit, then operating pressure will set the VTB entrainment level.

Vacuum distillate yield is affected by pressure and temperature changes. Increasing the column flash zone temperature at a fixed pressure will increase the yield of the heaviest lube cut, or the HVGO. When the temperature is increased at a fixed pressure, the vacuum column capacity factor increases by the ratio of the distillate rate increase. For instance, increasing total vacuum distillate production by 5 per cent will increase the column capacity factor by about 5 per cent.

The capacity factor is derived from Stoke’s Law and is a function of the superficial vapour velocity and vapour density. Flash zone operating temperature has very little effect on the vapour density. Conversely, lower operating pressure can significantly increase the capacity factor. Lower pressure reduces the vapour density, which in turn, increases the superficial vapour velocity. The net effect is always an increase in the column capacity factor.

Due to the effect of pressure on capacity factor, it is possible to reduce the column operating pressure to the point where massive VTB entrainment occurs. High distillate metals and carbon residue will require operating changes to reduce entrainment. Flash zone temperature and/or pressure must be optimised.

Many vacuum units have no pressure control; therefore, the vacuum ejectors set the flash zone operating pressure. Higher ejector gas load increases operating pressure, while lower gas load decreases operating pressure. Once the column exceeds its capacity limit, operating changes must be made. Heater outlet temperature is the only operating variable that can be used when no pressure control exists.

Reducing the operating temperature may reduce the capacity factor. However, the lower heater outlet temperature will also make less cracked gas, which will reduce the ejector load and lower the column operating pressure further.