De-mystifying vacuum ejector systems First-stage inter-condenser design and performance are critical for reliable vacuum system performance
Scott Golden, Tony Barletta and Steve White Process Consulting Services, Inc.
V acuum systems are critically important to maintain column operating pressure and maximise distillate product yield and economic potential. Yet systems continue to operate poorly even upon initial installation or after turnaround modifications. Often, performance break increases column operating pressure by 20-40 mmHg, dra- matically reducing distillate yield. Performance break occurs because the first-stage ejector operating discharge pressure is at or above its maximum discharge pressure (MDP). There are many potential causes of poor vacuum ejector system performance, yet an increasingly common one is failure of the first-stage inter-condenser to operate as intended. This article will highlight the importance of the first-stage inter-condenser and demonstrate how failure to perform as intended, specifically the sub-cooling zone, frequently results in performance break and unstable, high column operating pressure. Fundamental concepts including inter- condenser design, ejector performance curves, MDP, and interaction between system components are reviewed. The critical importance of ejector system design-phase considerations, including cracked gas production, first- stage ejector MDP margin, and system pressure drop, are reviewed. System components interact; first-stage ejector suction pressure is directly tied to the second-stage ejector suction pressure (see Figure 1 ). There are many potential causes of first-stage inter-condenser poor performance; an increasingly common one and the focus of this article is fail- ure of the first-stage inter-condenser to sub-cool. Ejector system design optimisation Most refinery vacuum unit ejector systems are three- or four-stage units. A stage is defined as an ejector and
1st stage intercondenser long air ba e bypass
1st stage intercondenser vapour outlet temperature is high
2nd stage ejector suction load increases
2nd stage ejector suction pressure increases
1st stage ejector discharge pressure exceeds MDP
1st stage ejector performance break
High vacuum column operating pressure
Figure 1 Cause and effect first-stage ejector break
condenser in series. Vacuum ejectors use motive steam energy to raise the process load from the column top pres- sure to the outlet gas discharge pressure. The condenser for each ejector stage condenses steam and oil to reduce the load to the downstream ejector. The cooling water (CW) rate is set by the first-stage inter-condenser require- ments. The design of the vacuum system is typically opti- mised to minimise motive steam and CW consumption. The first-stage ejector consumes up to 70% of the total motive steam. Consequently, the first-stage ejector and inter-condenser are much bigger in size than subsequent stages. They typically represent more than 50% of the total installed cost (TIC) of a typical vacuum system. Multistage
Motive nozzle
Di user
Di user throat
Discharge
Motive steam
P d
P , m m m
m=m + m s m
Steam chest
P = = 5.2 d P 98 19
Compression ratio =
s
P , m s s
m m
42,500 23,690
= m
Entertainment ratio =
= 1.8
P = Absolute pressure m = Mass ow rate
s
Suction load
P P
10,060 19
= m
Expansion ratio =
= 529
s
Figure 2 First-stage ejector design
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