Vacuum Ejector Performance Break
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Vacuum ejector performance break Understanding vacuum ejector performance is the key to maintaining reliability and distillate yields. A low VGO yield may result from a breaking ejector system, but bypassing a fouled inter-condenser can restore lost yield Norm Lieberman Process Improvement Engineering Edward Hartman and Daryl Hanson Process Consulting Services Inc oor ejector system per- formance continues to Preduce vacuum gas oil (VGO) product yield due t o design and reliability problems (Figure 1). Since poor performance increases column operating pressure, profit is significantly reduced too, especially during high margin periods. Although there are many potential trouble spots in multi-stage or parallel multi- stage ejector systems, this article focuses on the event called “breaking”. Experi- ence shows that breaking can easily reduce VGO yield by 2–4% on crude charge. N o r m a l l y, changes in the first-stage ejector system’s suction pressure are predictable, with gas loads based on the manufacturer’s certified performance curv e . H o w e v e r, when the first- stage ejector’s operation breaks, it no longer operates Figure 1 Vacuum ejector system on this curve and its suction pressure increases abruptly. Conse- injection, as well as “wasting” time in q u e n t l y, pressure in the vacuum column meetings or performing computer flash zone goes up rapidly, the VGO simulations, in reality it is field product yield drops and the vacuum measurements and the application of tower bottom (VTB) rate increases. The fundamental vacuum ejector system amount of VGO yield loss depends on principles that can improve operations the pressure in the vacuum column, and increase profitability. which in recent cases has shown increases of 15–50 mmHg in the flash How ejectors work zone pressure when an ejector’s The major system components are the performance breaks. ejector and the condenser (Figure 2). Although there are numerous ejector These must operate together to minimise system problem areas, breaking is caused column operating pressure. The ejector by a high first-stage ejector discharge consists of a steam nozzle, steam chest pressure. Since the symptoms of and diffuser. The choice of steam nozzle breaking are distinct, field pressure is based on the gas load it was designed measurements can pinpoint the problem for (rate and composition), steam so that speculation and theories which pressure and temperature, as well as have little to do with the root cause can maximum discharge pressure (MDP). be avoided. While the trend today is to Since the steam nozzle is a critical flow undertake sophisticated tests such as orifice, steam pressure sets the flow rate, Figure 2 Vacuum ejector major system neutron back-scatter or isotope with higher or lower steam pressure components 3 4 P TQ REVAMPS & OPERATIONS controlling the nozzle’s steam flow rate. The pressure rise in the throat from Ejector system fundamentals As the steam flow provides the power for the sonic boost is large. If the mixture The vacuum column’s overhead stream compression, the flow rate it was velocity in the converging section and flow rate and composition set the designed for must be maintained, entrance to the throat exceeds the sonic operating pressure in the first-stage o t h e rwi se the vacuum column operating v e l o c i t y, the entire sonic boost is ejector (Figure 3). Since most refinery pressure will increase. maintained. But if the ejector’s discharge first-stage ejectors are designed for The steam ejector educts process gases pressure exceeds a value (MDP) sufficient critical pressure ratios, their suction into the steam chest and then through a to cause the mixture velocity in the pressure depends only on the gas load as specially designed converging-diverging diffuser throat to fall below Mach 1.0, or long as their discharge pressure is below device that is part of the diffuser. The critical flow, the pressure rise from the their MDP. Thus, the performance of the diffuser dimensions and throat area are sonic boost is lost. The ejector is then second- and third-stage ejectors or the specified to meet the motive steam rate said to break. While the ejector will run hotwell off-gas liquid ring pump have no it was designed for, process gas load and q u i e t e r, it will also have a much lower influence on the first-stage ejector‘s M D P. Each component needs to work overall compression ratio, because the suction pressure as long as the first-stage properly to compress the process gas pressure boost across the shock wave discharge pressure is below its MDP. a n d minimise the column operating suddenly disappears. Understanding this is essential, pressure. o t h e rwise during revamps money may Steam ejectors work by converting the Process gas load – be wasted by adding more capacity to pressure energy of the motive steam into d r y vacuum unit the second- and third-stage ejectors or v e l o c i t y. For a critical flow ejector, the On a dry vacuum unit, the process gas liquid ring pumps. Also, during motive steam enters the steam chest load to the first-stage ejector comes from troubleshooting, time may be wasted through the steam nozzle at velocities cracked gas, condensable hydrocarbons, chasing irrelevant factors. In one typically in the range of Mach 3.0–4.0. air leakage and saturated water in the instance, the authors lowered the third- Localised pressure inside the steam chest feed. Air leakage is generally small. stage ejector’s discharge pressure from drops slightly below suction pressure so Hotwell off-gas should normally have 1100 mmHg absolute to 800 mmHg by that the process gas flows into the steam less than 10 mole % nitrogen. If the online cleaning the after-condenser. In chest from the suction piping. The sample contains 30–40% nitrogen, this another case, hotwell pressure was mixture (motive steam and process gas) indicates that there is a large air leak that reduced from 1200 mmHg absolute to then enters the diffuser. This consists of is unnecessarily raising the gas load and 760 mmHg by lowering the suction the converging (narrowing nozzle) column operating pressure. pressure controller’s set point on the section, the throat (straight piece of Typically, cracked gas generated in liquid ring pump. In each instance, the pipe), and the diverging (widening) the heater is the largest source of first- first-stage ejector’s suction pressure did section. The shape of the diffuser allows stage process gas load. Since most not change. the mixture velocity to exceed Mach 1.0, refiners meter hotwell gas, production During the design of the ejector whereas in a straight pipe it cannot. changes can be monitored. Condensable system, the MDP for the first and second In the converging section of the hydrocarbon load is primarily a stages is determined by optimising d i f f u s e r, process gas is accelerated above function of the upstream crude column overall motive steam, cooling water Mach 1.0 and the motive steam velocity residue stripping section and the consumption and capital costs. Third- drops. Motive energy is transferred to vacuum column top temperature. Poor stage discharge is fixed by the hotwell the process gas, and the fluids begin to stripping increases 300–700°F boiling- operating pressure. This is generally mix. Pressure rises across the converging range hydrocarbons in the vacuum unit between 890 and 1200 mmHg, section. The motive steam and process feed, which raises the ejector’s depending on where it is routed. But gas finally reach the same velocity condensable hydrocarbon load. once installed, each ejector stage has a toward the end of the converging Inadequate stripping is caused by a low certified performance curve and its MDP section. If the ejector’s discharge steam rate, poor tray efficiency or tray cannot be exceeded or its operation will pressure is below its MDP, the mixture damage. The condensable load is also break from its performance curv e . enters the throat above Mach 1.0. Since influenced by the vacuum column’s Breaking the second- and third-stage compressible fluid flow in a straight pipe overhead temperature, but its effect is ejectors only affects the first-stage cannot exceed sonic velocity, there is a small. Since slop oil is metered from suction pressure when the first-stage sonic shock wave inside the throat most ejector hotwells, increased slop oil discharge pressure exceeds its MDP. where velocity drops below Mach 1.0. production indicates a rise in Since an ejector is a thermal Across the sonic shock wave (sonic condensables. c o m p r e s s o r, power comes from steam boost), the mixture pressure rises rapidly. In the diverging section, velocity decreases as the nozzle opening gets w i d e r, and thus kinetic energy is converted to pressure. A large portion of the first-stage ejector’s compression ratio occurs from the sonic shock wave. A single ejector’s operation can be thought of as a multi-stage compressor with no moving parts. In the converging section, pressure rises as energy is transferred from the motive steam to the process gas. In the throat, there is a pressure rise across the sonic shock wave, or sonic boost. In the diverging section, kinetic energy is converted to pressure e n e r g y. Figure 3 First-stage ejector curve 3 5 P TQ REVAMPS & OPERATIONS flow rate and pressure. Ty p i c a l l y, steam Steam tables second-stage ejector, causing a higher temperature is slightly above saturation.