PTQ Q1 2023 Issue

Fouling in VDU ejector systems

Review of critical ejector-condenser interplay, with an overview of fouling in VDU ejector systems, including a case study where fouling impacted a refiner's bottom line

Jim Lines Graham Corporation

E jector system fouling can cost refiners millions of dol - lars annually. It leads to lost yield or lower through - put, either of which affects a refiner’s bottom line. An ejector system is a combination of ejectors and con - densers configured in series, typically e jector-1st intercon- denser-ejector-2nd intercondenser-ejector-aftercondenser . Declining thermal efficiency or heat rejection is seen with progressive fouling in heat exchangers and intercondensers. However, the fouling also increases intercondenser pressure. If intercondenser operating pressure rises beyond the discharge pressure capability of its preceding ejector, over - all system performance breaks and vacuum distillation unit (VDU) pressure rises sharply, an undesirable outcome for any refiner. In VDU service, a breakdown to ejector system performance can mean column overhead pressure may be 25-35 mm Hg rather than, for example, at design of 15 mm Hg abs. When overhead pressure rises in such a manner, vacuum tower bottoms (VTB) increases while vacuum gas oil (VGO) cut is reduced. Condensers within an ejector system serve to a) con - dense vapours discharging from a preceding ejector at a pressure within the discharge capability of that ejector, b) minimise the mass flow rate of vapours exiting the con - denser that must be handled by a downstream ejector to align with the performance capability of that ejector (mass flow rate versus pressure performance curve), and c) in effectively accomplishing a) and b) will keep ejector system energy consumption efficient and vacuum distillation unit pressure within specification. It is the ejector-condenser interplay that is so critical to a refiner meeting vacuum distillation operating objectives. When fouling surpasses the design basis, the consequence for a refiner can be significant, frustrating, and difficult to troubleshoot. VDU ejector system First-stage ejectors maintain vacuum column overhead pressure by evacuating non-condensible gases, hydro - carbon vapours, and steam present in the overhead of the distillation process. The overhead is compressed by the first-stage ejectors and discharged into the first intercondensers, where steam and hydrocarbon vapour are condensed. Non-condensible gases saturated with steam and hydrocarbons flow to the second-stage ejec - tor, where again compression occurs with discharge to the second intercondenser. Condensation of vapours then

occurs and saturated non-condensible gases flow to the third-stage ejector, where they are compressed to a pres - sure above atmospheric pressure and discharged into an aftercondenser. Due to the large scale of a modern refinery, a VDU ejector system, such as one recent case at a 200,000 bbl/day Asian refinery, may have multiple ejectors and condensers in par - allel at each stage. Medium-pressure steam is the energy source for the ejector compression of gases. Water typically is used to effect condensation, such as cooling tower water, river water, or seawater. Fouling can cause condenser operating pressure to rise above the discharge capability of an ejector that precedes it. When that happens, a performance break occurs where the VDU column rises sharply well above desired operat - ing pressure. An elevated VDU column pressure increases VTB, thereby reducing VGO yield. If one considers a $10/ bbl discount for resid versus VGOs, a 1% loss of yield on a 200,000 bbl/day refinery is approximately $7 million per annum. Heat transfer basics Most commonly, condensation takes place on the shellside with water flowing tubeside. Several variables influence the shellside heat transfer rate, including: • Mass flow rate and MW of non-condensibles. The greater the mole fraction of non-condensibles, the lower the shell - side heat transfer rate • Mass flow rate, composition (MW, boiling point, mole fraction) of hydrocarbons. The higher the mole fraction or lighter the hydrocarbon composition, the lower the shell - side heat transfer rate • Proportion of flow that is steam. Higher mole fraction of steam will generally yield a higher shellside heat transfer rate • Operating pressure correlating to velocity • Condensate mass flux (pph/ft2) or condensate film thickness. The complication that arises in establishing the shellside heat transfer rate is immiscible condensate formation, as water and hydrocarbons are immiscible. The mole fraction of non-condensible gases progressively increases as heat is rejected along the release curve, as does condensate film thickness. The following condenser heat release curve example dis - cusses the amount of water and hydrocarbon condensates formed. The amount of non-condensible gas is constant.


PTQ Q1 2023

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