Vacuum unit pressure control

Operational issues such as higher heater duty and column pressure must be weighed against avoiding residue entrainment in HVGO

Tony Barletta, Process Consulting Services Inc

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Article Summary

A crude vacuum unit at a refinery processing a variety of crudes, including some heavy Canadian sour, was revamped in autumn 2005. However, problems maintaining control of the vacuum column pressure persisted. Since this unit produces varying grades of asphalt, including relatively soft roofing flux, and processes various crudes, the column operating pressure has to be adjusted over a wide range. Following startup, however, the spillback system did not allow sufficient recycle flow to meet the targeted operating pressure. This was because the pressure control system was not designed with enough flexibility to prevent the column pressure from “floating” on the first-stage ejector load — a common mistake. Since spillback systems normally consist of some nominal piping and a control valve, they often get too little attention during process design.

Why control pressure?
In principle, the column operating pressure should be kept at a minimum, since this maximises the heavy vacuum gas oil (HVGO) product yield and minimises the heater duty. But vacuum columns are part of an integrated process, with each crude unit having its own constraints. Increasing the column operating pressure is often necessary to maximise revenue, even though it increases the heater duty. Decreasing the pressure raises the column vapour velocity and reduces temperatures throughout the system. Since vacuum unit products and pumparounds are typically the largest sources of crude unit preheat, reducing draw temperatures often lowers the crude preheat temperature and increases the atmospheric crude heater duty.

Column pressure may need to be tightly controlled or increased to prevent massive vacuum residue entrainment into the HVGO product. Entrainment is caused by vapour rates that exceed the column wash section internals capacity. In some instances, improperly designed pressure control systems can cause massive residue entrainment, resulting in an HVGO product with more than 2 wt% MCR and 10–40 wt ppm nickel and vanadium. Some vacuum residue produced from heavy crudes contains more than 500 wt ppm vanadium and 28 wt% MCR. With these crudes, no entrainment can be tolerated. HVGO product contaminants must be controlled to some maximum level; otherwise, hydrotreater, FCC or hydrocracker performance will suffer.

Vacuum units that produce asphalt need to maintain a stable pressure to ensure asphalt penetration, thin film oven and loss-on-heat specifications are met. Furthermore, those units processing heavy feeds while producing soft asphalts may need to significantly increase their operating pressure to avoid a very low vacuum heater outlet temperature. This reduces the heat available to the preheat crude, thereby lowering the crude heater inlet temperature. When the crude charge rate is constrained by atmospheric heater firing, the low vacuum heater outlet temperature reduces the crude charge rate. Therefore, it is sometimes necessary to raise the column operating pressure above minimum to maintain an adequate crude preheat and charge rate.

Ejector performance curves
Ejectors are a form of thermal compressor, with the first-stage load controlling its inlet pressure. Motive steam provides the energy to compress the process gas load. Ejector inlet pressure varies with load, as long as the ejector is not “breaking” or there are no mechanical problems. Maintaining a constant operating pressure requires a constant load even if the process gas load leaving the column is not constant. Figure 1 shows a typical first-stage ejector performance curve. As long as its discharge pressure is below maximum discharge pressure (MDP), the suction pressure depends only on the suction load. If the discharge pressure exceeds the MDP, performance will break, causing the suction pressure to increase dramatically.

Increasing the vacuum column operating pressure requires a higher first-stage ejector gas load. In some instances, it is possible to increase the ejector load by raising the heater coil or residue stripping steam rate, but these methods typically do not permit continuous stable pressure control. What ejector systems need is an operable spillback to control the vacuum column pressure at its desired pressure. 

Pressure control
Many vacuum units have no pressure control. Thus, the first-stage ejector load sets the flash zone operating pressure. A higher ejector gas load increases the operating pressure, while a lower gas load decreases the operating pressure. However, pressure control is necessary in some cases to intentionally avoid low pressure or to raise the column operating pressure to reduce the load on other crude unit equipment. Good pressure control allows the first-stage ejector gas load to be maintained constant in spite of variations in the process load from the top of the vacuum column. The first-stage gas load can be held constant or adjusted by varying the recycle flow rate from its discharge to its suction. In theory, the system shown in Figure 2 will provide good pressure control. 

The pressure measurement used to control the recycle needs to be accurate when vacuum columns operate at maximum capacity factors (Cf). The pressure controller should use an absolute pressure transmitter with a relatively small range. In some instances, 0–500 mmHg absolute pressure transmitters with an accuracy of +/-0.25% of range have been used when trying to control the column flash zone pressure at 8.0 mmHg absolute. In this situation, a 0–50 mmHg absolute range is better suited and should be used instead.

Since some vacuum units need to operate over wide pressure ranges, recycle flow variability needs to be extensive. During design, the process engineer must ensure that the inlet and outlet piping and control valve are sized properly to meet the operating range. The driving force for recycle flow depends on the pressure differential from the ejector discharge to its suction. An ejector develops a maximum discharge pressure before it stops performing. However, the actual discharge pressure also depends on many other variables. Ejector discharge pressure will be at its highest during the summer months, because the cooling water temperature and second-stage ejector loads are highest. In the winter months, when the cooling water temperature is coldest, the ejector discharge pressure will be lowest. The recycle flow rate will be highest when trying to maintain maximum suction pressure. If maximum pressure operations occur during winter, the pressure control system will have a minimum pressure differential. This should be the sizing case for the control station and piping.

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