Revamps 2022 Issue

Identifying the root cause of underperforming ejector systems Underperforming ejector systems illustrate the importance of the long air baffle, its proper operation, and how to quickly identify problems when they arise

Scott Golden, Tony Barletta, Steve White and Darrell Campbell Process Consulting Services, Inc.

V acuum column steam ejector systems are typically engineered with very small design margins. The incen- tives to design with low margin are lower installed ini- tial cost and/or lower utility consumption. Systems designed with low margin must operate as designed. Otherwise, loss in performance can lead to millions of dollars in lost oppor- tunity. A common problem which results in breaking of the first-stage ejector operation, and as a consequence signifi - cantly higher operating pressure, is a failure to seal the first- stage inter-condenser ‘long air baffle’. First-stage inter-condenser design Figure 1 is an elevation view of a first-stage inter-condenser using a typical X-shell exchanger with a long air baffle. Vapour from the first-stage ejector is partially condensed in the inter-condenser to reduce the load on downstream ejec- tors. The vapour mixture from the first-stage ejector is non- condensable gas, condensable oil, and water vapour. The non-condensable vapour is cracked hydrocarbon gas created in the heater and air leakage. The condensed oil is primarily unstripped naphtha/kerosene/diesel boiling range material that leaves the bottom of the crude tower. The water vapour is process steam leaving the vacuum tower (stripping and heater coil) and first-stage ejector motive steam. There is a small amount of saturated water leaving with crude tower bottoms product. Figure 2 shows the flow pattern across the tube bundle. The coldest cooling water contacts the vapour from the fluid mixture exiting the bundle area above the long air baffle,

ensuring that the gas leaving is colder than the condensate. The purpose of the long air baffle is to direct the vapour flow through the coldest tube section to allow sub-cooling of the gas compared to the liquid out. This minimises vapour to the second-stage ejector. First-stage ejector outlet vapour enters the top of the exchanger and ideally is distributed uniformly along the length of the exchanger bundle top side. The inlet vapour partially condenses as it moves from the top of the bundle to the bottom. Condensate falls to the bottom of the exchanger shell, and the uncondensed vapour enters the area under the long air baffle for further cooling. The first-stage inter-condenser X-shell design with a long air baffle is conceptually two exchangers in series, as illus - trated in Figure 3 . The long air baffle is a seal plate, either slanted or ‘L’ shaped and typically contains 20-25% of the tubes under the projection of the baffle. This essentially creates two exchangers in series. When modelling perfor- mance of an X-type first-stage inter-condenser, a two series exchanger arrangement is a better representation of true performance. In the example, 90ºF CW has a 3ºF rise across the area below the long air baffle, with the remaining 7ºF rise across the area above the long air baffle. Condensate leaving the first-stage inter-condenser is created from the area outside the long air baffle and the remaining area under the baffle.

Vapour leaving 1st stage ejector

‘Long air bae’

1st stage ejector vapour inlet

Cooling water outlet (typically 10˚F higher than inlet)

Tube bundles

Gas outlet

CW outlet

VAPOUR

CW intlet

Condensate from area outside bae

Condensate from ‘Long air bae’

‘Long air bae’

Vapour outlet

Tube support

Condensate outlet

Cooling water inlet (coldest)

Total condensate

Figure 1 Elevation view of TEMA X-shell

Figure 2 Vapour flow across bundle

Revamps 2022