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Apr-2022

A simple ejector modification

A vacuum column’s economics can be greatly improved with a low cost modification to the ejector system

JIM LINES
Graham Corporation

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Article Summary

Refiners can benefit from improved yield and lower vacuum residuum, thus improving refinery economics with minor modifications to the ejector system. Refiners continually optimise their crude slate, push the vacuum column for greater throughput and, for a variety of other reasons, operate the vacuum distillation unit under conditions differing from the design basis. This can lead to dramatic increases in vacuum column pressure, especially during summer months when cooling water is warmest. Refiners have benefitted from modification to the first stage ejector motive steam nozzle (see Figure 1) to overcome losing millions of dollars during summer months when column pressure abruptly rises, yield declines and vacuum residuum increases. In the parlance of ejector systems, during the summer months ejector performance breaks, shockwave is lost and vacuum column pressure increases dramatically.

Ejector systems
Ejector systems are a combination of ejectors and condensers that evacuate and maintain sub-atmospheric pressure in a vacuum distillation column. Column overhead vapours consisting of non-condensable gases, hydrocarbon vapours, and steam are evacuated continually from the column and compressed above local barometric pressure, typically to 2-5 psig.

It is helpful to understand the operating principle of an ejector where the compression ratio is nominally two or greater and discharge pressure is greater than two times the suction pressure. Ejectors are static equipment with no moving parts. The operating principle follows compressible flow theory. Medium or low pressure steam, typically less than 250 psig, 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 non-condensable gases plus saturated vapours, both steam and hydrocarbons, into the ejector. The vacuum column discharge is referred to as suction load or overhead loading 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 overhead load and motive steam 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 shockwave 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 shockwave 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 overhead 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 shockwave is lost, and typically suction pressure moves sharply higher. This breakdown happens because there is insufficient energy provided by the motive steam to compress to a pressure above the MDP of the ejector. Suction pressure and therefore distillation column pressure may surge or become unstable once the shockwave is no longer present.

This broken ejector operation is what can be remedied inexpensively and will provide enormous payback when ejector break is caused by:
• Cooling water temperature above design
• Excessive condensable hydrocarbon loading
• Fouling above the design basis

These three are rather common causes for performance shortfall by the vacuum distillation ejector systems.

Consider Figure 1 where the design basis for the vacuum column overhead and first stage ejector suction pressure is 7.5 torr and the first intercondenser when supplied with 85ᵒF cooling water will operate at 75 torr. It is common in June-August to have refiners frustrated because the column overhead pressure is not 7.5 torr in this case, but much higher such as 20-25 torr. This results in tremendous lost profit due to substantial reduction in yield with commensurate increase in vacuum column bottoms or residuum.

During the hottest days of summer a refiner’s cooling tower may be strained, and supply temperature to the first intercondenser exceeds 85ᵒF. This can cause the first stage to break performance. As cooling water temperature rises above the 85ᵒF design basis, the first intercondenser pressure rises. When it rises above the discharge capability of the first stage ejector, performance breaks, the shockwave is lost, and vacuum column pressure rises dramatically.

This particular refiner provided output from the data historian for actual cooling water supply temperature. It was evident that summer months would present a performance risk for the ejector system as more than 27 days had periods where the water temperature was above the design basis of 85ᵒF (see Figure 3).

The impact of a warmer cooling water inlet temperature is that condenser pressure must rise. The heat load at the ejector exhaust will be condensed when an intercondenser is present. The critical variable becomes at what pressure must the condenser operate to condense the ejector exhaust. The standard thermal duty equation follows:

Thermal Duty = Area * Heat Transfer Rate * LMTD

Where:
• Thermal duty is the condensation and cooling load from the ejector exhaust in Btu/hr, and this is fixed or unchanged for all intent and purposes
• Area is the heat exchange area of the intercondenser in ft2, which is fixed for the installed exchanger
• Heat transfer rate is the overall heat transfer rate in Btu/hr ft2 ᵒF, which is essentially constant provided overhead load composition is unchanged
• LMTD is the logarithmic temperature difference between the hot side and cold side fluids in ᵒF

If duty, area and transfer rate are fixed, the only variable to affect is LMTD. As cooling water temperature rises above the design basis of 85oF, condenser pressure rises which increases the initial dewpoint and condensing profile such that LMTD is increased, permitting the load to be rejected.


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