The T he D is istilla tillatio tion n Gro Grou up, I nc nc..
Troubleshooting Refinery Vacuum Towers Presented at the AIChE Spring National Meeting 22-26 April 2001 Copyright 2001 Andrew W. Sloley All rights reserved. Not to be uploaded to any other site without written permission from the copyright holder.
Reprinted/distributed Reprinted/distributed with permission.
Distributed by
The Distillation Group, Inc. P.O. Box 10105 College Station, TX 77842-0105 USA [1]-(979)-764-3975 [1]-(979)-764-1449 [1]-(979)-764-1449 fax
[email protected] www.distillationgroup.com
P.O . Box 10105, C ollege Station, T X 77842-0105 U SA
[email protected]
Phone 979-764-3975 Fax 979-764-1449
Troubleshooting Refinery Vacuum Towers Andrew W. Sloley The Distillation Group, Inc. PO Box 10105 College Station, TX 77842-0105 USA Presented at the AIChE Spring National Meeting Session: New Frontiers in Refinery Fractionator Operation 23-27 April 2001 Houston Copyright © Andrew W. Sloley February 2001
Introduction
W ith it h crude price pri ce fluctuation fl uctuations s often compres compressing sing refi refinery nery margins, margins, stable stable and eff effiicient vacuum vacuum tower tower operati operation on is i s more more cri criti tica call than t han eve everr to re r efinery fi nery profits. profi ts. M any refi refineries neries run up to five fi ve years years with with good vacuum vacuum tower yie yi elds. lds. Others O thers have have consiste consistent nt proble probl ems ge gettin tting g past past an eightee eighteen month mont h run. r un. M ajor sources of lost profits include coking, high pressure drops, internal leaks, and loss of vacuum. Simple tools, costing less than $200 each, used correctly can identify and track many common vacuum tower problems. Systematic problem analysis coupled with standard stream analysis methods can identify many others. K nowing nowi ng proble problems before a shutdown cuts mai mainte ntenance costs. costs. U nschedul nscheduled ed procurement procurement and work may cost cost as much much as ten ten times t imes (or more) more) than schedul schedule ed work[1]. work[ 1]. K nowing nowin g what what when when and how a problem starts starts is is key key to solving solvi ng it. Reliab Reli able le operation operation incre in creas ase es ove overall plant profi profits. ts. Ineffec I neffecti tive ve troubleshooti bleshooting ng leads leads to faile fail ed fixes f ixes and continui conti nuing ng losse losses. Four case stud studiies are are shown. T he first fi rst looks l ooks at causes of a coking coki ng wash wash bed. T he sec second ond exam exam-ine in es internal internal le l eaks and and their affect affect on he h eat remova removal. l. T he third thi rd brie bri efly fl y shows an an example xample of an external xternal leak and its it s impact on heat-t heat-transfer ransfer and and yields. yi elds. The T he fourth fourth,, a coked coked wash wash bed in a visbre visbreaker aker vacuum vacuum tower, tower, illu il lus strates that the t he problems are not limit li mite ed to to crude vacuum vacuum towers. towers. They T hey also also occur in visbreakers, hydrocrackers, and other units. Tro T roub uble les shoot hooting ing requ require ires s unde unders rsta tand nding ing how how the the proc proce ess and the the differe different nt equ equipm ipme ent intera interac ct. Tro T roub uble les shoot hoote ers must ust know mor more e tha than n just just how equ equipm ipme ent function functions s in isola isolation tion.. T hey hey mus mustt understand how entire systems work and how different types of equipment interact. Simple, common problems should always be checked before attempting to use expensive, difficult to interpret, and time consuming uming high technol technolog ogy y troubles tr oubleshooti hooting ng tools. M ost ost unit uni t proble problems are are simpl simple e in cause cause and can can be identified with effective use of field technique. Rapid problem identification cuts costs and increases profits.
Troubleshooting Refinery Vacuum Towers Andrew W. Sloley The Distillation Group, Inc. PO Box 10105 College Station, TX 77842-0105 USA Presented at the AIChE Spring National Meeting Session: New Frontiers in Refinery Fractionator Operation 23-27 April 2001 Houston Copyright © Andrew W. Sloley February 2001
Introduction
W ith it h crude price pri ce fluctuation fl uctuations s often compres compressing sing refi refinery nery margins, margins, stable stable and eff effiicient vacuum vacuum tower tower operati operation on is i s more more cri criti tica call than t han eve everr to re r efinery fi nery profits. profi ts. M any refi refineries neries run up to five fi ve years years with with good vacuum vacuum tower yie yi elds. lds. Others O thers have have consiste consistent nt proble probl ems ge gettin tting g past past an eightee eighteen month mont h run. r un. M ajor sources of lost profits include coking, high pressure drops, internal leaks, and loss of vacuum. Simple tools, costing less than $200 each, used correctly can identify and track many common vacuum tower problems. Systematic problem analysis coupled with standard stream analysis methods can identify many others. K nowing nowi ng proble problems before a shutdown cuts mai mainte ntenance costs. costs. U nschedul nscheduled ed procurement procurement and work may cost cost as much much as ten ten times t imes (or more) more) than schedul schedule ed work[1]. work[ 1]. K nowing nowin g what what when when and how a problem starts starts is is key key to solving solvi ng it. Reliab Reli able le operation operation incre in creas ase es ove overall plant profi profits. ts. Ineffec I neffecti tive ve troubleshooti bleshooting ng leads leads to faile fail ed fixes f ixes and continui conti nuing ng losse losses. Four case stud studiies are are shown. T he first fi rst looks l ooks at causes of a coking coki ng wash wash bed. T he sec second ond exam exam-ine in es internal internal le l eaks and and their affect affect on he h eat remova removal. l. T he third thi rd brie bri efly fl y shows an an example xample of an external xternal leak and its it s impact on heat-t heat-transfer ransfer and and yields. yi elds. The T he fourth fourth,, a coked coked wash wash bed in a visbre visbreaker aker vacuum vacuum tower, tower, illu il lus strates that the t he problems are not limit li mite ed to to crude vacuum vacuum towers. towers. They T hey also also occur in visbreakers, hydrocrackers, and other units. Tro T roub uble les shoot hooting ing requ require ires s unde unders rsta tand nding ing how how the the proc proce ess and the the differe different nt equ equipm ipme ent intera interac ct. Tro T roub uble les shoot hoote ers must ust know mor more e tha than n just just how equ equipm ipme ent function functions s in isola isolation tion.. T hey hey mus mustt understand how entire systems work and how different types of equipment interact. Simple, common problems should always be checked before attempting to use expensive, difficult to interpret, and time consuming uming high technol technolog ogy y troubles tr oubleshooti hooting ng tools. M ost ost unit uni t proble problems are are simpl simple e in cause cause and can can be identified with effective use of field technique. Rapid problem identification cuts costs and increases profits.
Coked Wash Beds: A Continuing Problem
C oking oki ng wash wash zones zones have been the t he source source of many vacuum unit uni t shut downs. Some Some packed packed vacvacuum towers have five-year runs reliably. Others shut down for wash bed replacement every 18 months. For one refiner (the first case study), the flash zone pressure increased from 27 mm to 36 mm over a two year run. A simple, low-cost manometer allows for wash zone monitoring. An absolute mercury manometer is accurate to within one-half mm of mercury pressure when used correctly[2]. A pressure survey vey imme i mmedi diately ately after startup startu p sets sets a base base-l -liine perf performance ormance for the the vacuum vacuum towe towerr. T he unit uni t engine engineer should houl d monitor moni tor the operati operation on of the vacuum vacuum tower tower with wi th peri periodi odic c pressure pressure surveys. urveys. If I f the pressure pressure drop, for the same distillate yield, increases by two mm of mercury or more across a packed bed, the has coked. W ith it h the the low liquid li quid rates rates in the vacuum vacuum column’s column’s wash wash zone, press pressure drop incre i ncreas ase es in the wash zone are nearly always caused by coking. A coked wash bed increases flash zone pressure, drops distil distillate late yield, and eve eventu ntually ally le l eads to blac bl ack k H V G O product. I ncreas ncrease ed pre pressure ssure drop across across the wash wash section cti on increas i ncreases es resi residue ent entrain rainme ment nt.. Increa I ncrease sed d carbon carbon and metals from blac black k oil oi l loads l oads the the FC C catacatalyst, produces more FCC cracked gas, and drops FCC product quality. Coked Wash Bed Shuts Unit Down Probl em Hist Hist ory
The T he refine refinerr repla replac ced the the existing isting wash oil oil spr spra ay he header der with with a new new one one des designe igned d for for lowe lower flow flow rate to increase gas oil yields. At startup, the gasoil yield rose and the pressure drop was the same as before before modification modif ications s. U nit ni t operation operation for the first fi rst year year see seemed med troubl trouble e-free. -free. A pressure pressure survey survey imm i mme ediately after startup set a base-line performance for the vacuum tower (Figure 1). Over the second year, the flash zone pressure rose from 18 mm of mercury to 27 mm of mercury (Figure 2). Cut point dropped from 1052°F (566°C) to 1038°F (559°C ). D istill isti llate ate yield losse losses we were cos costi ting ng approximate approximately ly $1,600,000 per year. After two years the unit was shut down for cleaning. W hen the unit shut down, no replace replacement ment grid ri d had bee been ordered. ordered. U pon entry, entry, the the gri grid d was found coked. coked. T he was wash h grid deli delive very ry ti time was was goin going g to be ‘too long lon g to wai wait’ to t o replace replace it. Rather ather than than replacing the grid, the wash zone grid was cleaned in place with a high-pressure water lance from the top. Grid is manufactured in shallow layers approximately 2-5/8 inches (67 mm) thick (Figure 3). Each grid layer is rotated from the layer below. A water lance cannot reach more than the top one, or perhaps two, layer(s). Other equipment inspected during the turnaround included the overflash collector tray and the wash wash oil distri di stributo butor. r. N o obvious obvious dama damag ge was seen on eith eithe er. After restarting the unit with the water-lanced grid, the pressure drop was even worse than before! fore! T he pressure pressure drop across the the was wash h bed had increa i ncrease sed from from te ten mm of mercur mercury y to 19 mm of mercury (Figure 4). Once exposed to oxygen, the coke had hardened and the water-lance debris blocked a large part of the open area that had been available in the grid before the shutdown. Cutpoint dropped again, to 1025°F (552°C). Replacement grid was immediately ordered. As soon as the new grid was received, another shutdown took plac place, e, the old grid grid removed removed and the the new gri grid d plac pl ace ed in in the the tower. tower. I f the the pressure pressure survey survey results had been understood or believed, replacement grid could have been ordered well in advance. A second shutdown would have been avoided. Avoiding the extra shutdown would have easily paid for many $200 manometers and pressure surveys.
First Fix
After the unit restarted with the clean grid, the wash rate was returned to the previous rate. Fully wet packing helps prevent coking. M any literature reports emphasize the importance of having the correct wash oil rate to keep the packing wet. W hile lacking many important details and grossly simplifying the process analysis, the conclusion that many units have insufficient wash oil is correct. Computer models can predict wash oil rates if done correctly. N evertheless, troubleshooting starts in the field. Correct data must be gathered and interpreted first. Successful unit revamps and effective troubleshooting start with field data, not theoretical calculations. The unit seemed to work correctly for the first fourteen months, then pressure drop surveys started showing that the flash zone pressure was rising. Over another fourteen months, the pressure drop across the wash zone increased by six mm of mercury. T he wash bed had coked again, with only a small improvement in run length. Equipment and the process do not exist separately. Equipment details count as much as process details. Process limits come from equipment limits. N ot only must the wash rate be correct, it must also be distributed properly. Figure 5 shows a sketch of the type of spray header used. Spray nozzles on a pipe header distribute the liquid over the packed bed. To develop a proper spray cone, a five psi (34 kPa) to 20 psi (138 kPa) pressure drop across the spray nozzle is required. M uch below five psi (34 kPa), the spray cone does not develop. Above 20 psi (138 kPa) the spray starts to form smaller droplets that entrain more easily. A simple pressure gauge can check the operation of a spray header. Figure 6 shows the pressure reading obtained by putting a gauge downstream of the wash oil control valve. T he pressure gauge read 11 psi (76 kPa) at five feet above grade. This must be adjusted to the spray header elevation by:
Static Head =
feet of elevationdifference × specific gravity of fluid at conditions
2.31
After adjusting for height, the spray header pressure drop is only one psi (seven kPa). W hen checking the nozzle details, for the new flow rate the pressure drop should have been 35 psi (240 kPa). Something was wrong with the spray header. At the second unit shutdown, the spray header was inspected and two things found. First, the wrong size spray nozzles were on the header. T he nozzles had approximately three times the capacity they should have had. Second, many of the flange gaskets had been left out (Figure 7). Instead of forming spray cones, the liquid was just pouring onto the packed bed in a series of solid, small diameter jets (Figure 8). J ust enough liquid was getting onto the wash bed from condensing on the underside of the H VGO collector, by entrainment from the flash zone, and spraying out of the flanges without gaskets to coke up the bed. The spray header nozzles were replaced with ones the correct size and gaskets were installed on all flanges. After this shutdown, the unit has been working without coking the wash bed. Operation has proved successful at the lower wash oil rate. Revamp limited by vacuum tower heat removal Probl em Hist ory
A refiner completed a major revamp to increase capacity and run new, heavier crudes. After the revamp, projected crude unit yields could not be obtained. T he crude unit was limited by heater duty. The oil could not get to the needed temperature. Theoretical analysis of the problem showed that the crude preheat temperature was 40°F (22°C) colder than expected. As crude heat integration exchangers
often suffer from dramatically higher fouling than many engineering standards assume, the initial conclusion was that the crude preheat exchangers were fouling more than expected. Often, newer, heavier crudes have high fouling factors. H eavier crudes also contribute to asphaltene precipitation when mixed with lighter crudes. T his can dramatically increase exchanger fouling. Rather than accepting a preliminary conclusion, management insisted that a plant test and data analysis be done to verify this before further detail engineering began on crude train modifications. A plant test was run and data gathered. Reduced preheat to the atmospheric column has two major affects. First the amount of AGO recovered drops. This decreases high-level AGO heat available for preheat. Second, the lighter material gets into the LV GO, decreasing the LV GO temperature. N ormally, LVGO rejects heat to cooling water and air, so this has minimal direct impact, as long as the LVGO can handle the increased duty. Decreased preheat duty from AGO rundown, reduces the crude tower operating temperature even more. T he decreased crude tower operating temperature then drops more AGO into the vacuum tower feed (Figure 9). Detailed test run data review and modeling showed that the HVGO was cooler than expected. Reduced H VGO draw temperature has an even bigger impact than AGO rundown duty losses. The H VGO draw temperature was 50°F (28°C) lower than expected. Figure 10 shows the basic data around the vacuum unit. D rawing material balance envelopes around the vacuum tower and checking the heat balance shows that 2,400 bpd (380 m3/day) of LVGO is being condensed but is not being drawn as LVGO product (Figure 11). I nstead, it comes out with the HVGO product. T he reduced LV GO temperature was the major reason feed preheat to the crude unit could not be attained. This was costing the refiner three million dollars a year. Fixi ng t he Vacuum Tower
Equipment and the process do not exist separately. Equipment details count as much as process details. Process problems come from equipment limits. M any reasons cause LVGO to leak into H VGO: the collector can leak, the draw nozzle can be too small[3], the collector can be damaged. In one notable case, an H VGO collector was damaged by using a jackhammer to remove coke from it. Originally, the vacuum tower had bubble cap trays. Approximately 15 years ago most of the bubble caps had been replaced with structured packing and grid. H owever, new collectors were not installed at that time. T hree bubble cap trays were modified to act as collectors. M odifications were done in the field to change the bubble cap trays to total draw trays. Figure 12 shows the modifications to the draw tray sumps. A comparison with a regular collector tray sump is shown. Rapid inspection shows a major problem. The sump is not fully sealed. At low pumparound rates this may not matter, the height of liquid above the nozzle is enough to get the pumparound plus product out the draw even though the sump is not sealed. As pumparound rates increase, the liquid level in the sump rises until the sump overflows, spilling LVGO into the HVGO . As a quick check, the HVGO pumparound rate increased to its maximum, and the LVGO pumparound rate decreased. A dramatic increase was seen in H VGO draw temperature when LV GO pumparound rate dropped slightly. For a temporary fix, H VGO rate was set to its maximum (it had the same problem with liquid overflow as the LV GO collector) and LVGO return temperature minimized to attempt to keep the LV GO pumparound rate as low as possible. This helped the unit most of the time, but the plant still suffered from weekly upsets when the LVGO tray overflowed. Additionally, the collector trays had very high pressure drops compared to properly designed collectors. The modified bubble cap trays imposed an extra three mm of mercury pressure drop. For a packed vacuum column this has a significant yield affect.
The solution to this problem was putting in correctly designed LV GO and H VGO collector trays at the next turnaround. T he new collectors solved most of the problems with preheat. O nly minor preheat train changes were needed. Preheat changes are very expensive because of the cost of piping and plot plan problems. Tower internal solutions have no plot plan problems. N o complex computer models or high tech methods were required. Simple mass-balance and heat-balance calculations were all that was needed to identify the vacuum tower problems. Review of the drawings identified the exact cause. Quick field verification showed that the problem identified was the real cause of the preheat loss. Sudden Vacuum Loss Drops Yields Pr obl em Background
A refiner experienced a sudden loss of vacuum in a dry vacuum tower operating with an overhead pressure of 10 mm H g. Overhead pressure had risen to 20 mm H g. Substantial yield losses were being incurred every day. Figure 13 shows the overall unit with a heat and material balance boundary. Sour ces of Vacuum Syst em Load
Figure 14 shows the major sources of unexpected vacuum system load. Vacuum system load changes occur from either different (or additional) material entering the system or by hydrocarbon cracking inside the system. T his includes both intentional sources and environmental sources. Intentional sources are material added to the system with the full knowledge that the material will go to the vacuum system. The major intentional sources are usually stripping steam added to the vacuum tower boot and velocity steam added to the heater coils. Environmental sources are streams that the plant attempts to minimize because they serve no useful purpose, but only consume vacuum system capacity: increasing capital requirements and operating costs. Environmental sources include: Vacuum unit feed changes caused by upstream (atmospheric tower) damage, often in the • stripping section. • Increased heater coking. Improperly metered increases in velocity steam or stripping steam. • H eat exchanger leaks from heat integration circuits. • Vacuum tower coking. • Air leaks. • All of these areas can create problems rapidly from apparently small changes in equipment performance. Vacuum Unit Feed Changes
Atmospheric stripping section damage is one of the most common sources of added vacuum system load. Even small amounts of damage to the stripping section can add large amounts of light material to the vacuum tower feed. T his loads the vacuum ejectors, causing higher vacuum system pressures. Higher vacuum system pressures reduce H VGO yield and increases residue yield. Profits are lost. Increased Heater Coking
H ydrocarbon cracking in the heater breaks forms coke plus cracked gas. Cracked gas may contain hydrogen sulfide, methane, ethane, ethylene, and other light compounds. Heater cracking rates depends upon mass flux, heater temperature profile, heat flux profile, and heater history. H eater design
dramatically affects coking rates for the same conditions. A well designed, operated, and maintained heater may have low cracking rates at the same conditions a poorly designed heater is inoperable. H eater design and operation is critical to profitable vacuum unit operation. Once coke is formed in heater tubes, additional cracking and coke formation is even easier. Coke makes more coke. Coke formation makes cracked gas as well. Improperly Metered Steam
Flow meter drift can add much additional steam to the vacuum heater or vacuum tower stripping section. Additional steam adds load to the ejectors. The higher ejector load drops the system pressure. Heat Exchanger Leaks from Heat Integration Circuits
H eat exchange in the heavy vacuum gas oil (H VGO) and light vacuum gas oil (LVGO) circuits condenses the vacuum tower product. M ost units integrate much of the heat removal with crude preheat. A few integrate with steam generation or other units. In either case, leaks in the heat removal exchangers leak from the heat removal utility into the vacuum process side. Crude contains light material that loads up the vacuum system. Boiler feed water preheat leaks or steam generator leaks adds water, and hence steam load, to the system. Both make the ejectors work harder. Vacuum Tower Coking
Vacuum tower cracking most often occurs in the boot, collector trays, or in the vacuum wash zone. Rarer, but still possible, is cracking in the vacuum tower stripping section. Cracking in the collector tray liquid or in the wash zone results in a coked vacuum tower. I n addition to the cracked gas load increase, coked vacuum towers can produce black products and dramatically lower yields. Cracking in the boot can coke up the tower draw and shut the unit down as well. Cracking of liquid in the boot is the most common source of cracked gas in the vacuum tower. Quench addition to the boot can control this rate and unload the vacuum system. N oticeable changes in cracked gas rate from collector tray, wash zone, or stripping section coking indicate critical problems in the vacuum tower. Coke formed by cracking in these areas builds up inside the tower rapidly. Coked wash sections and collectors increase tower pressure drop, reduce yields, and make black products. Cracking on the stripping section trays plugs the bottom of the tower. Liquid entrainment from the stripping section increases. Proper equipment design and installation can prevent problems. Air Leaks
Loose flanges increase leaks into the system. Increased leakage increases the vacuum system load. System pressure rises and yields drop. Finding t he Probl em
Systematic approaches help troubleshooting. The root causes of most unit problems are simple, even if they are difficult to find. Often, too much attention is paid to rare and difficult to find problems. Simple sources of problems are not checked thoroughly enough before they are eliminated from future consideration. T he attraction of working on something new, unique, or rare (i.e. exciting) lures most engineers past the basics without sufficient consideration given to simple problems.
Good field technique, understanding of the process, and application of engineering fundamentals identifies the vast majority of problems. Troubleshooting should check the easiest, cheapest, most likely, and quickest to find problems first. Load on the first ejector in a steam jet vacuum system sets the suction pressure. M ost refinery and chemical plant vacuum systems are critical ejector systems: the ejector discharge pressure is more than twice the ejector inlet pressure. Figure 15 shows a typical vacuum system ejector curve. In troubleshooting any chemical process system, one of the first steps should be to draw a heat and material balance boundary around the system and check how the entering and leaving streams have changed. Figure 16 shows a three-stage ejector system, its heat and material balance boundary and identifies the entering and exiting streams. Drawing the heat and material balance around the vacuum tower plus the vacuum system (Figure 17) clearly shows that if the total load going to the system has increased, the load change must also show up in the streams leaving the system. T he slop oil rate must change, the sour water rate must change, or the vent gas rate must change. H owever, vacuum systems may be sensitive enough to load changes that rate changes too small to easily see may still cause operating problems. Di r e ct I d en t i f i c a t i o n o f t h e Le a k
Checking the exiting streams showed a slop oil rate 75% higher than normal operation. D istillation tests showed that a large amount of naphtha, kerosene, and diesel was in the stream. If an upstream upset had damaged the atmospheric tower stripping section most of the additional light material would be diesel and a small amount of kerosene. T he large amount of naphtha and kerosene clearly indicated that crude was leaking into the vacuum tower. The unit was dropped to a lower capacity and crude preheat exchangers isolated in groups, then individually. After several days of testing, the leaking exchanger was isolated and the bundle pulled for repair. After the bundle was pulled, the unit was restored to approximately 90% of capacity at desired yields while repairs took place. Coking in Refinery Main Fractionators Thermal ly unst able oil s and coking
M any refinery main fractionators process thermally unstable oils. Common services include: 1. Atmospheric crude columns 2. FCC main fractionators 3. Gas oil crude columns 4. Vacuum preflash columns 5. Vacuum crude columns 6. Delayed coker main fractionators 7. Fluid coker main fractionators 8. Visbreaker atmospheric columns 9. Visbreaker vacuum columns 10. Residue hydrocracker atmospheric columns 11. Residue hydrocracker vacuum columns W hile services differ between units and plants, the list has been sorted into a generally least severe to generally most severe order. Reliable operation with thermally unstable oils requires a great care with mechanical details. Coking is a product of time, temperature, and thermal instability. M echanical details that create small
liquid pockets or films with long residence times initiate coke formation. Once started, coking may continue until major problems develop. Gri d v ersus Packing i n Wash Serv ices
Any type of packing can coke in wash zone service. N o clear evidence exists on the superiority of either grid or structured packing in this service. Vapor and liquid distributor design, fabrication, and installation are so much more important that minor differences between grid and structured packing can be ignored. In general, grid will require a deeper bed than structured packing for the same de-entrainment effectiveness. For a given degree of wash bed effectiveness, pressure drop across grid or structured packing will be approximately equal. Grid has been used more often in this service because it has been available for a longer time than structured packing. Random packing should not be used in refinery main fractionator wash service. Random packing inevitably has parts of the packing that hold liquid for long periods of time. Long residence times increase the risk of coked beds. Figure 18 and Figure 19 show drawings of typical, modern random packings. Random packing fills a vessel randomly. Some packing will always lie with spots where liquid can have long residence times. T his is especially true in low liquid rate services. Wash zones are low liquid rate services. Long residence times, high temperatures, and thermal instability of the oil lead to coking. In contrast, Figure 20 shows a drawing with elements of typical, modern structured packing. The surface can drain freely. Coking tendency is reduced. Wash Zone Liquid Rat es
To keep terminology clear, we will use the following terms as shown in Figure 21: Wash oil: the oil sent to the top of the wash bed to clean entrainment from the wash bed. • • Overflash: the oil that comes from the bottom of the wash bed that is the residue of the wash oil used. Slop wax: the oil from the collector tray immediately below the wash bed. (This term • comes from old-time lubricant column operation and is not strictly applicable to fuels vacuum column operation. H owever, industry standard usage accepts its application to fuels vacuum columns.) Condensate: liquid condensed on the underside of the collector tray that falls (usually) • back into the flash zone. The obvious question looking at Figure 21 is why do we make a distinction between overflash and slop wax? Figure 21 is not complete. We have not considered entrainment. The purpose of the wash zone is to remove entrainment from the feed. Entrained oil makes the products black. A very small amount of entrainment can make a product D 8 color. We normally consider entrainment a very small quantity. T his is not always true. At very low wash rates and high vapor velocities in the column, entrainment can reach a significant fraction of the liquid on the collector tray. I n some units, entrainment may be nearly 100% of the slop wax. Figure 22 expands the definitions from Figure 21 to include entrainment. Obviously, with a functioning wash zone the entrainment never reaches the product above the wash bed. T herefore, the slop wax liquid is:
Slop wax = Overflash + Entrainment M easured slop wax is never equal to overflash.
Care must be taken with data interpretation to make sure that the overflash rate is determined when deciding if a wash bed has liquid coming out the bottom of it. Common methods of determining overflash rates include metals and asphaltene balances around the feed and slop wax. Other techniques can be used as well. It is critical to avoid confusing slop wax rates with overflash. Visbreaker Operation Limited by Coking
Visbreaker products are thermally unstable oils. At the high temperatures in wash zones they readily coke. Extreme attention to mechanical design details must be included to eliminate dead spots where coking can occur. A refinery visbreaker had problems with high pressure drops and coked wash beds over a period of years. After several turnarounds, a thorough attempt to understand and solve the problem started. Cu r r e n t Op e r a t i o n
The main causes of poor quality H VGO products from refinery main fractionators are coked wash beds, poor liquid distribution to the wash bed, and poor vapor distribution to the wash bed. These causes are related. Poor distribution to a wash bed, of either vapor or liquid, leads to coking. Coking partially blocks the wash bed. Vapor velocity increases due to the lower cross section area open. T he higher vapor velocities increase entrainment. Entrainment carries black oil from the feed entry (flash zone) up to the HVGO product. Black H VGO results. T his severely affects downstream operation and product quality. Your D 8 color is typical for a coked wash zone. Data showed a pressure drop of 28 mm H g across the tower before shutting down (Figure 23). This is a very high pressure drop for a normally operating vacuum tower. Unless collectors have exceptionally high pressure drops and the grid beds are very deep, a typical pressure drop in this service would be 12-18 mm H g across the tower. H igh pressure drops occur across coked beds. The process operation was also checked. A commonly used number for grid or structured packing wash zone liquid rates is 0.15 gpm/ft2 (0.367 m3/hr-m2) of tower cross-section area at the minimum liquid wetting point. For wash zones, the minimum liquid wetting rate is found on the bottom of the bed. T he rate the data shows is 0.097 gpm/ft2 (0.237 m3/hr-m2) of slop wax. The observed rate was low and is a probable contributor to coking. W hile incomplete, the available evidence supported the conclusion that the wash bed was badly coked. Coked wash beds lead to poor product qualities. Work Recommendat ions
The wash bed had to be replaced. W hatever caused the bed to coke in the first place had to be identified and fixed. T his is often a complex activity. T he root cause of coking problems may not be apparent. M any units have been ‘fixed’ only to have coking occur again. U nit reliability and product quality depends upon identifying and fixing the real problem. Both grid and packing can coke. N o clear evidence exists on the superiority of either grid or structured packing in this service. Vapor and liquid distributor design, fabrication, and installation are so much more important that any minor differences between grid and structured packing can be ignored. The mechanical support structure and collector design must also be reviewed to eliminate dead spots where liquid can sit with a long residence time. Long residence times increase the risk of coked towers. The process was checked and modifications designed for the wash oil system.
Shut dow n Observat ions and Repai r
The unit was shut down. The wash bed and the collector tray were coked. The entire wash bed was coked solid and large quantities of coke had accumulated on different parts of the collector. M odifications were made to the wash oil system, collector, and mechanical support structure. The unit was restarted and has operated normally. N o evidence of coki ng has yet been seen. Conclusions
M any plants have very good experience and routinely run for five years without wash bed coking. Others coke as often as every 18 months. Understanding the process and correct mechanical design, fabrication, and installation is required for reliable wash zone operation. T he hotter the operation and the less stable the oil, the more important every detail becomes. Standard design approaches based on old-time, low-severity services fail to perform acceptably in severe services. Simple instruments, correctly used, identify many refinery vacuum column problems rapidly and at minimum cost. Absolute pressure manometers and heat and material balances are valuable, inexpensive tools. Accurate data and good field technique identifies problems better than back-office calculations and engineering standards books. Systematic troubleshooting using basic engineering concepts with heat and material balance envelopes identify and locate many other unit problems. Simple, common problems should always be checked before attempting to use expensive, difficult to interpret, and time consuming high technology troubleshooting tools. M ost unit problems are simple in cause and can be identified with effective use of field technique. Rapid problem identification cuts costs and increases profits. Disclaimer
N o performance, suitability for use, or lack of suitability for use for any given process service is implied to any particular model or brand of packing by these comments. Figures used have been used as illustrative of generic classes of equipment.
t o Vacuum Syst em
11 LVGO PA 12
l vgo 13
HVGO PA 14 hvgo 15 was h 16 f eed 18
pr essur e, mmHg vacuum r esi due Figure 1 Pressure survey of tower at start of run
t o Vacuum Syst em
11 LVGO PA 12
l vgo 13
HVGO PA 14
hvgo
15 was h 25 f eed 27
pr es s ur e, vacuum r esi due Figure 2 Pressure survey of coked tower
mmHg
Figure 3 Typical ‘grid’ used in many vacuum tower wash zones
t o Vacuum Syst em
11 LVGO PA 12
l vgo 13
HVGO PA 14 hvgo 15 was h 34 f eed 36
pr essur e, mmHg vacuum r esi due Figure 4 Pressure survey of coked tower: after water lance cleaning
Figure 5 Typical wash zone spray distributor
t o Vacuum Syst em
LVGO PA l vgo
HVGO PA hvgo 15 was h
pr es s ur e, mmHg el evat i on 32f t ( 9. 7m) f eed
vacuum r esi due 11 pr es s ur e, psi a e l ev at i on 5f t ( 1. 5m) Figure 6 Spray header pressure survey
mi ssi ng gaskets and/ or l o os e f l ange s
mi ssi ng gaskets and/ or l o os e f l anges
mi ssi ng gasket s and/ or l o os e f l anges
Figure 7 Spray header with missing gaskets and loose flanges
s pr a y wi t h i ns uf f i c i ent pressure drop t o devel op cone
spray wi t h a pr operl y devel oped spr ay cone
Figure 8 Spray nozzle comparison: improper versus proper cone development
pr obl em
decr eased AGO pr eheat
l ower heat er f eed t emper at ur e
decr eased AGO yi el d
l ower heat er out l et t emper at ur e Figure 9 AGO yield spiral
t o Vacuum Syst em
LVGO PA 54900 bpd 36. 5 M bt u/ hr
411 F l vgo 9100 bpd
HVGO PA 43700 bpd 49. 1 M bt u/ hr 612 F hvgo 14800 bpd was h
f eed
Figure 10 Vacuum tower yield and temperature data
t o Vacuum Syst em
LVGO PA 54900 bpd 36. 5 M bt u/ hr
411 F l vgo 9100 bpd ( l vgo 11500 bpd)
HVGO PA 43700 bpd 49. 1 M bt u/ hr 612 F
hvgo 14800 bpd ( hvgo 12400 bpd)
was h
( c al c ul at ed val ues ) f eed
Figure 11 Vacuum tower heat and material balance check
or i gi nal
t r ays wi t h s ump
modi f i ed t r ay wi t h sump
sump not seal ed\ can over f l ow at hi gh l i qui d rates
c ol l ec t or t r ay s ump
( vapor r i ser s and bubbl e caps not shown f or cl ar i t y )
Figure 12 M odified sumps when converting from a tray to a packed tower
Figure 13 Entire system heat-balance and material-balance envelope
Figure 14 Sources of vacuum system load
Suction pressure, mmHg 28 26
M a x i m u m w o r k i n g d i s c h a r g e p r e s s u r e : 12 5 m m H g M i n i m u m m o t i v e s t e a m p r e s s u r e : 1 50 p s i g
24 22 20 18 16 14 12 10 8 Water vapor equivalent load, mass/hr
Figure 15 Typical vacuum ejector operating curve, first stage of an ejector system
Vacuum Syst em HMB Boundar y mot i ve st eam vent gas
l oad f r om Vacuum Tower
cool i ng wat er cool i ng wat er
ai r l eaks
s l op oi l sour wat er
Figure 16 Vacuum system with heat and material balance boundary
Figure 17 H igh slop oil rate indicating a leaking crude preheat exchanger
Figure 18 [4] Random packing with locations for liquid to have a long residence time
Figure 19 [5] Random packing with locations for liquid to have a long residence time
Figure 20 [6] Structured packing with few locations for liquid to have a long residence time
was h oi l
Was h Bed over f l as h Sl op Wax Col l ec t or
s l op wax
condensat e f eed Fl ash Zone
Figure 21 Flash zone terminology
was h oi l
Was h Bed over f l as h Sl op Wax Col l ec t or
s l op wax
condensat e f eed
ent r ai nment Fl ash Zone
Figure 22 Flash zone terminology with entrainment included
t o Vacuum Sys t em
10- 14 mm Hg
l vgo PA
LVGO Bed l vgo dr aw
3200 mm di ame t er Fracti onat i on Bed
hvgo PA HVGO Bed
hvgo dr aw ( bl a ck)
11600 mm di ame t er
wash oi l ( 100- 130 m3/ h)
Was h Bed
sl op wax ( 25 m3/ h) f eed
38- 42 mm Hg 4300 mm di ame t er
v i sbr eak er bot t oms
Figure 23 Visbreaker operation before shutdown
