Oct-2016

Operating vacuum distillation ejector systems

Best practices and opportunities to deliver reliable ejector system performance and reduce performance risk.

JIM LINES
Graham Corporation

Article Summary

Reliable ejector system performance is critical for every refiner. The performance of an ejector system correlates directly to vacuum gas oil yield and refinery profitability. Both charge rate and fractionation are impacted when distillation or fractionation operating pressure is not met. While they have been used widely in distillation service for decades, an understanding of best practices for specifying an ejector system and the important factors that affect ejector system performance are not always well known. This article provides a deeper review of ejector system performance, variables impacting performance, and best practices to specify an ejector system for vacuum distillation service.

Ejector system
An ejector system is a combination of ejectors and condensers arranged in series. The system produces and maintains sub-atmospheric pressure (a vacuum) within the distillation column to permit fractionation of crude oil into its various important components, such as light or heavy vacuum gas oils (LVGO and HVGO, respectively), and reduce the amount of lower valued residuum. The ejector system will continually extract from the distillation column cracked and inert gases along with associated saturated steam and hydrocarbon vapours. Failure to extract the gases and saturated vapours properly will result in an increase in distillation column operating pressure, thereby increasing residuum while lowering LVGO and HVGO yield. The ejector system extracts the gases at sub-atmospheric pressure and compresses them to a pressure typically above atmospheric pressure where they enter another refinery process for treating or repurposing of the gases.
 
An ejector
Ejectors are static equipment with no moving parts. The operating principle follows compressible flow theory. Medium or low pressure steam, typically less than 300 psig (43 kPag), is the energy source that performs the work and creates the vacuum. Steam is expanded isentropically across a converging-diverging nozzle where its pressure is reduced and converted to supersonic velocity. This pressure reduction and expansion to supersonic flow is what creates the vacuum. The low pressure region exiting the converging-diverging nozzle is lower than the distillation column pressure, thereby inducing flow from the column and pulling the cracked gases and inerts plus saturated vapours into the ejector. The vacuum column discharge is referred to as suction load or flow to the first stage ejector. The suction load is entrained by and mixes with the high velocity motive steam, and the combined flow remains supersonic. Again, compressible flow theory is applied where the supersonic mixture of load and motive passes through another converging-diverging conduit, referred to as a diffuser, where high velocity is converted back to pressure. A fundamental principle for compressible flow, which may be counter-intuitive, is that when flow is supersonic and the cross- sectional area of a flow path is progressively reduced, velocity actually decreases. The throat of the converging-diverging diffuser section of the ejector is where cross- sectional area is the smallest and a shock wave is established, which serves to boost pressure. Figure 2 illustrates pressure and velocity profiles across an ejector with a clear step up in pressure at the throat where a shock wave is established.

An ejector, unlike a piston reducing volume to increase pressure, does not create a discharge pressure. Motive steam provides the energy necessary to compress and flow the mixture of motive and load to the operating pressure of a downstream condenser. If the pressure of the condenser is below the discharge capability of the ejector, the ejector will not cause the condenser to operate at a higher pressure. Conversely, if the operating pressure of a condenser downstream of an ejector is above the discharge capability of that ejector, referred to as a maximum discharge pressure (MDP), the performance of the ejector breaks down, the shock wave is lost, and typically suction pressure moves sharply higher. Suction pressure and therefore distillation column pressure may surge or become unstable once the shock wave is no longer present.

An ejector performance curve provides critical information about variables affecting performance. The two most important variables to understand and have correct for proper performance are: motive steam pressure and temperature; and the MDP an ejector is anticipated to operate against. Performance frustration and lost profit for a refiner stem most often from motive steam pressure falling below a minimum pressure or from discharge pressure in operation rising above MDP. In either of these two conditions, there is an abrupt negative change in performance, with distillation column operating pressure rising above its design operating pressure, and also pressure surging may occur. Figure 3 shows a typical ejector performance curve. Notice that, for a given suction load, MDP capability increases with higher motive steam pressure. This particular ejector is designed for 7213 lb/h of water vapour equivalent load at 15 torr, discharging up to 104 torr when motive steam is at 220 psig. If motive steam pressure is 230, 240 or 250 psig, the MDP capability at 7213 lb/h of load is 109, 113 or 117 torr, respectively. Higher pressure motive will increase the motive mass flow rate along with the velocity exiting the converging-diverging nozzle and, therefore, energy from expansion increases, thus with higher motive pressure MDP capability is greater. A dashed line shows an estimated suction pressure if the discharge pressure in operation exceeded MDP. There is essentially a doubling of the vacuum column discharge pressure, from 15 torr to 30 torr, should discharge pressure exceed MDP. That jump in pressure increases vacuum residuum, thereby reducing LVGO and HVGO cuts. The actual broken suction pressure will depend on discharge pressure. The higher the discharge pressure, the higher the broken suction pressure.

A similar break in performance arises when motive steam pressure is below 220 psig for example, while discharge pressure must be 104 torr.

In each case the break in performance is a result of insufficient energy available from the motive steam to perform the required compression. The shock wave breaks down, resulting in loss of compression across the ejector.  Discharge pressure above MDP or motive pressure below design cause the shock wave to move out of the throat and into the converging section where it ultimately breaks down and compression is negatively impacted.

Vacuum system condensers
Condensers within an ejector system are positioned between ejector stages to condense steam and vapours in order to reduce energy requirements for the system. A vacuum condenser may also serve as a pre-condenser positioned between a vacuum column and an ejector system. By condensing steam and vapours it will reduce the loading to a downstream ejector, thereby lowering energy usage in the form of motive steam required by that ejector. A condenser within an ejector system is unlike a typical shell and tube heat exchanger, although it externally appears no different. It has similar construction features that follow Tubular Exchanger Manufacturer Association (TEMA) or American Petroleum Institute API 660 guidelines. However, the internal configuration is different due to operating under a vacuum, condensing vapours with non- condensibles present, handling non-ideally miscible condensates to ensure correct vapour-liquid equilibrium and to permit continual extracting of non-condensibles (see Figure 4). Distinct differences from conventional shell and tube heat exchangers are:
• Open areas above the tube bundle to permit flow distribution and reduce pressure loss
• Lack of conventional flow directing segmental or double segmental baffling in order to reduce pressure loss and appropriately manage vapour-liquid equilibrium
• Extracting non-condensible gases within a tube bundle, in most cases.