PTQ Q2 2022 Issue

Importance of testing for vacuum ejectors in refinery service Efforts should bemade to identify and avoid errors in the specification, engineering, andmanufacturing of vacuumsystemequipment before theymanifest at start-up


V acuum systems are criti- cally important to the per- formance of refinery crude vacuum distillation units. Vacuum tower flash zone pressure is a result of vacuum system suction pres- sure plus pressure drop through the overhead vapour line and col - umn internals. To maximise recov - ery of gasoil from vacuum residue, the flash zone should operate at the lowest possible pressure with- out exceeding tower capacity. Unfortunately, many vacuum tow - ers operate above their design or expected pressure, resulting in lower vacuum gasoil yields and reduced profitability. Many papers and articles in the technical literature discuss the per - formance and troubleshooting of vacuum ejector systems in refinery service. 1,2 . This article focuses on the importance of shop testing vac - uum ejectors to ensure they meet their design parameters of pressure versus capacity and consequently prevent significant economic losses that can result from their under - performance. An additional benefit is the ejector performance curves derived from actual testing are par - amount to any future troubleshoot- ing, optimisation, and revamping of vacuum systems in operating units. Crude unit vacuum systems Multi-stage steam jet vacuum ejec - tor systems are almost universally used to produce vacuum in refin - ery crude distillation units. They are particularly well suited to the large vapour volumetric rates pres - ent and high compression ratios normally required. The systems are arranged in two to four stages, with

30% 33% 32% 31% 34% 35% 36% 37% 39% 38% 40%

Unit 1

Unit 2












Flash zone pressure, mmHg absolute

Figure 1 Flash zone pressure impact on vacuum residue yield

each stage consisting of an ejector and discharge surface condenser. Depending on unit capacity and desired flexibility, each stage may have ejectors or surface condens - ers in parallel. Steam works as the motive fluid providing the energy for compression. Load to each stage consists of non-condensable gas, condensable hydrocarbons vapour, and water vapour. Surface con - densers minimise the quantity of condensable vapours and cool the non-condensable gas mixture flow - ing to the next ejector stage and leaving the system. This reduction in load results in smaller ejector size in the intermediate and final stages, as well as lower overall energy usage. Discussed in further detail under vacuum ejector fundamentals, ejec - tor suction pressure is a function of its load and performance curve, as long as its discharge pressure is below its maximum discharge pressure (MDP). In a multi-stage crude vacuum unit system, over - all suction pressure is a function of

vapour overhead load from the vac - uum tower and first-stage ejector performance curve. This holds true provided the first-stage condenser, as well as ejectors, condensers, and interconnecting piping in subse - quent stages do not cause the first- stage MDP to be exceeded. In some cases, high vacuum sys - tem suction pressure, and conse - quently high tower pressure, is caused by process loads pushing the ejectors out on their curves. However, Process Consulting Services (PCS) has encountered sev - eral instances of ejector design errors leading to tower pressures up to 10 mmHgA above design right from start-up. When vacuum system fails to deliver from day one, system performance can further degrade rapidly with normal exchanger foul - ing and the initial 10 mmHgA miss can increase to 20 mmHgA or more above design suction pressure. The impact of flash zone pressure on refinery profitability is mag - nified in units processing heavy

PTQQ 2 2022 41

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