Vacuum V acuum Unit Revamps Following vacuum unit revamps, it is not uncommon to have operating problems. Case studies show how the root cause can be identified using differential pressure measurements.
Daryl W. Hanson Process Consulting Services Inc., Houston, Texas
Norm P. Lieberman Process Improvement Engineering, Metairie, LA
roubleshooting refinery vacuum unit revamps(Photo 1) begins with accurate differential pressure measurements. Specific pressure measurement ele vations and detail drawings of the column internals are essential to properly interpret the data. Without complete information wrong conclusions can be drawn from the differential pressure measurements. Three case studies will show how accurate field measured differential pressure can be interpreted incorrectly. These cases present only a few of the ways to misinterpret data.
Column internals generate pressure drop if they are functioning properly, properly, but so will liquid level. Because pressure drop generated by packed column internals is very low, two gauges must be used with the readings taken simultaneously. To ensure there is no offset between the two instruments, they should both be connected to the same process point and each should have the same reading. By measuring differential pressure simultaneously the readings are not affected by normal pressure variations. When measuring pressures less than 10
Measuring Low Pressure Being able to accurately measure pressures under deep vacuum requires proper equipment. Acceptable measuring devices are absolute pressure mercury manometers or absolute pressure electronic devices. Moreover, tubing from the instrument to the process connection must not leak or measured pressure will be higher than true process pressure. To test for leaks the process isolation valve should be first opened so that the measuring device reads the process pressure, then the valve must be blocked in. If the measured pressure gradually increases then the tubing is leaking, if it remains constant there is no leak. If the pressure does not fall to a steady value within a few seconds, then the connection is partially plugged. The steady state reading will then read high because a very minor air leak cannot be completely off-set by a relatively larger opening into the connection.
Photo 1. Vacuum unit Reprinted from PTQ Revamps & Operations® , Autumn 2003
mmHg absolute, most digital gauges read a few mmHg low. Special high precision gauges are required if accurate pressure readings are required. When measuring pressure drops, this is not a problem.
Rules of Thumb Column internals generate pressure drop and so does liquid head. Properly functioning stripping section trays will generate between 3.5-5 mmHg per tray. Liquid level also produces P, therefore 10” of 0.75 specific gravity liquid generates approximately 13-14 mmHg. Since packing gener-
Case 1: Black HVGO Product – “Low” Wash Section Pressure Drop Cat feed hydrotreater (CFHT) run lengths were being dramatically reduced by rapid metals deposition on the catalyst. Measured 20 during survey Recent refinery configuration Structured changes allowed heavy Mexican Packing heavily packing coked in middle and Venezuelan crudes containGrid of bed ing very high vanadium to be Slop wax tray processed. While the vacuum unit had a history of coking after a 3 year run, switching to heavy Actual flash crude caused downstream unit zone pressure contamination problems in only 31 + 8mmHg higher Flash zone than inside horn 18 months. CFHT catalyst had reached end-of-run and an Vapor 23 Measured Transfer horn during survey line unscheduled outage was planned. A decision needed to be made to go into the vacuum column and replace the packing or Absolute pressure, mmHgA continue running. A pressure survey was conducted because coke reduces Figure 1. Measured pressures pressures and locations packing open area and generates ates only 0.375 mm Hg pressure drop per higher P as the coke builds up in the foot of packing, pressure must be meapacking. Once pressure drop reaches sured simultaneously to determine differapproximately 1 mmHg per foot of packential pressure. Five feet of packing genering, heavy vacuum gas oil (HVGO) prodates only 2 mmHg when operating properuct will begin to turn black due to vacuum ly and 5 mmHg when coke is being formed. tower bottoms (VTB) entrainment. Small variations in operating pressure can Pressures were measured simultaneously materially change the conclusions about in the flash zone and above the wash secthe condition of a packed bed when a sintion because of convenient platform gle gauge survey is performed. access. Figure 1 shows the measured pres-
Photo 2. Coked grid
sures and the locations. Measured P across the slop wax (dirty gas oil or overflash) collector tray and five feet of wash section packing was only 3 mmHg. Interpreting pressure drop correctly requires complete knowledge of column internals and how they influence pressure. Pressures were taken above the wash bed and at the same location as the flash zone pressure instrument. Detailed review of vendor drawings showed a vapor horn horn had been installed; yet it did not appear on the vessel elevation drawing. The flash zone pressure measurement was taken inside the vapor horn. Pressure should never be measured inside vapor horns because vapor and liquid velocities are very high. Kinetic energy can make the pressure reading lower or momentum from liquid impinging on the pressure point can produce readings higher than the flash zone pressure. Pressure in the flash zone was 8 mmHg higher than inside the horn. Therefore, the kinetic energy component of system pressure was very high causing the measured pressure to be lower than the flash zone. Because the true pressure drop across the packing was very high, it confirmed the bed was badly coked (Photo 2 and 3) and a shutdown was needed to replace the packing. Furthermore, high column pressure drop reduced HVGO product yield. Conversely,, pressures outside the vapor Conversely horn and below the overflash pan can be 3 to 6 mmHg lower than the measurement inside the horn due to liquid impingement
Photo 3. Coked structured packing Reprinted from PTQ Revamps & Operations® , Autumn 2003
16 during survey
Packing clean when inspected during turnaround 10" liquid
Measured across collector
18 No liquid head
Five (5) sieve trays
during survey (in liquid level)
15" liquid Quench flow
21 Actual flash zone
Slop wax tray
With steam 755 No steam 757
pressure + 10mm Hg lower than at slop wax tray
Reduced liquid level
Pressure, mmHgA Temperature, F °
Absolute pressure, mmHgA
Figure 2. Pressure measurement measurement at the upper-le upper-level vel tap on the slop wax collector, below the wash bed and in the vapor space above on the pressure tap. Using this reading alone, will lead the troubleshooter to a conclusion that the wash zone packing and overflash tray have high pressure drop, but this is often not so when the refiner opens the tower. The lesson is clear: a pressure tap immediately below the wash zone packing should be used to measure critical wash zone differential pressure.
Case #2: Black HVGO Product –“High” Wash Section Pressure Drop Black HVGO product contained 1.6 wt % microcarbon residue that increased FCC catalyst consumption by more that 4 tons/day. Black HVGO is a symptom of wash bed coking, yet the refiner had no history of coking. Moreover, flash zone temperature was only 730°F and the wash bed packing depth was only 4 feet. Experience shows it is nearly impossible to form coke in short crude vacuum column (versus visbreaker or hydrocracker) wash zones. Because entrainment from the flash zone and wash liquid from the top keep the packing sufficiently wetted to avoid “dryout” in short packed beds and operating temperature was only 730°F, coking was not likely. Yet in spite of low temperature
Figure 3. Measured pressures pressures above and below the stripping section
and a short bed, black HVGO product pointed to coke. In addition, the pressure survey measured 15 mmHg across the wash bed. High P indicated that coke was likely causing the black HVGO. Figure 2 shows pressure was measured at the upper level tap on the slop wax collector, below the wash bed and in the vapor space above. Furthermore, slop wax tray level measured 60% which indicated the upper instrument tap was in the vapor space. But, slop wax collector trays have no level glass to confirm the instrument reading is a true level and these instruments often do not work. Therefore, pressure was measured in the flash zone as a check of slop wax tray liquid level. Measured flash zone pressure was only 21 mmHg versus the 31 mmHg measured on the slop wax tray. Level on the tray was reduced until the instrument reading was 15%, at the same time the measured pressure decreased from 31 to 18 mmHg. Thus, the initial pressure reading on the tray included 10 inches of liquid, which increased the pressure by 13 mmHg. Further review showed that the top of the collector tray risers were 10 inches higher than the instrument top tap. Consequently, slop wax was overflowing the riser during Reprinted from PTQ Revamps & Operations® , Autumn 2003
normal operation measured at 60% of level. Vapor flowing through the risers entrained slop wax that contained vacuum tower bottoms from flash zone entrainment. Also when tray level was lowered, HVGO product color became a translucent dark green. The packed bed was not coked (Photo 4)!
Case #3: Low HVGO Product Yield“Normal” Stripping Section Pressure Drop HVGO product yield had dropped by 4,000 bpd following start-up even though the same crude rates and blends were being processed. Problems with column boot level instrumentation were common during start-up and throughout normal operation. High liquid level is the most common cause of stripping section tray damage. Yet in the most recent turnaround no tray damage was found. Until the refiner switched to heavier crude, very little emphasis was placed on stripping section performance. But as crudes get heavier, stripping section performance plays an increasing important role in meeting HVGO product yield targets. Properly functioning stripping trays generate 3-5
When troubleshooting vacuum column strippers, the quench flow rate s hould also be monitored. If stripping trays are working properly there will be temperature drop. Thus when steam is blocked in, the temperature of the liquid leaving the stripping trays increases, hence quench flow rate must increase to maintain constant temperature in the boot or the columns bottom temperature will increase. When stripping trays are not working, there is little or no change in quench flow rate or bottoms temperature when steam is blocked in. High liquid level also results in loss of vacuum due to liquid phase cracking. If lowering the pool level restores vacuum, liquid phase cracking is occurring. THE AUTHORS
Photo 4. Grid without coke mmHg pressure drop per tray. Figure 3 shows measured pressures above and below the stripping section. Measured differential pressure was 20 mmHg or approximately 4 mmHg per tray so the stripping section appeared to be working. Stripping trays work by reducing oil partial pressure, which vaporizes a portion of the liquid. Heat for vaporization comes from the liquid stream; therefore, liquid temperature drops as light material is stripped. Yet temperature measurements between the flash zone and the bottom of the stripping section showed no temperature drop. Further testing was needed to clarify the inconsistent data.
Because pressure is generated by stripping steam and stripped hydrocarbon flowing through the sieve holes and liquid level on the trays, if steam is blocked in, then the measured pressure drop will normally decrease to about 10% or less of normal values. However, when the steam was blocked in there was no change in the measured pressure drop. This indicated that measured pressure included liquid level. By decreasing column bottom level the measured pressure drop decreased to zero. When stripping steam was put back in service, measured pressure drop was still zero. Hence the trays had been dislodged by high liquid level (Photo 5).
Daryl W. W. Hanson is a chemical
engineer with Process Consulting Services in Houston, TX. His respon sibilities include process and equipment design. He specializes in all phases of refinery distillation from proces s simulat ion through field inspection. Previously he was lead process specialist for Koch-Glitsch Inc. where he was involved with more than 100 column revamps including heavy oils and light-ends recovery towers. Hanson has authored more than 20 technical papers on revamping and distillation. He holds a BS degree in Chemical Engineering from Texas A&M University.
Norman P. Lieberman Lieberman is an independent refinery troubleshooter for his company Process Improvement Engineering in Metairie, Louisiana. He specializes in revamping distillation equipment and correcting process operating problems. He has authored several books and numerous articles.
Process Consulting Services, Inc. 3400 Bissonnet Suite 130 Houston, Texas 77005 U.S.A. Phone: -(713)-665-(713)-665-7046 7046 Fax: -(713)-665-7246 E-mail: [email protected]
Photo 5. Stripping tray damage Reprinted from PTQ Revamps & Operations® , Autumn 2003