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Course Manual

Crude Distillation (Vacuum Distillation)

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Chapter 4 Vacuum Distillation

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Chapter 4 Contents 4.1 Introduction 4.2 Reduced Crude Flashing 4.2.1 Vacuum Bottoms Handling 4.2.2 Entrainment Control 4.2.3 Product Condensation 4.2.4 Vacuum Pressure Measurement

4.3 Vacuum Fractionator 4.4 Steam Jet Ejectors 4.4.1 Introduction 4.4.2 Operating Principle 4.4.2.1 Steam Pressure 4.4.2.2 Discharge Pressure 4.4.2.3 Load 4.4.2.4 Cooling Water Temperature

4.5 Vacuum Tower Control System 4.6 Startup 4.7 Troubleshooting

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Chapter 4

Vacuum Distillation 4.1 INTRODUCTION In order to maximize the production of gas oil and lighter components from the bottoms material of an atmospheric distillation unit, these bottoms (reduced crude) can be further distilled in a vacuum distillation unit. Vacuum distillation of an oil means that the pressure on the oil being distilled is lower than the atmospheric pressure. It does not mean that there is a perfect vacuum above the liquid. The distillation of heavy oils is conducted at a low pressure in order to avoid thermal decomposition or cracking at high temperature. A stock which boils at 400 ºC at 50 mm. would not boil until about 500 ºC at atmospheric pressure, at which temperature most hydrocarbons crack. For distillation to take place, the vapor pressure of the liquid being distilled must be a little greater than the pressure above it. The molecules that comprise a liquid are held together by two forces natural cohesion and the weight of the atmosphere pressing down. This pressure is equal to 14.7 psi at sea level and will support a column of water 34 ft. high. Now if boiling will begin when the vapor pressure of the liquid has become a little greater than the pressure holding down, it is clear that by removing some of the holding down force the liquid will start boiling at a lower temperature. The vacuum unit differs from the atmospheric type in that it has a fractionating column of larger diameter with bubble trays farther apart. This is necessary because much larger volumes of vapors have to be handled because of the lower pressure. Any sudden increase in vacuum will expand the volume of the vapor rapidly and possibly result in puking the tower. In the vacuum unit, almost no attempt is made to fractionate the products. It is only desired to vaporize the gas oil, remove the entrained pitch, and condense the liquid product as efficiently as possible. Bottoms from the crude tower contain material that can be charged to the catalytic cracking unit or be used for lube oil stocks. Distilling this material at atmospheric pressure would require high temperatures that would cause thermal cracking. Thermal cracking is undesirable because it would cause a loss of valuable product, degradation of valuable product, and shortened run time due to coke formation in pipes and vessels. For these reasons we conduct the distillation of the heavy reduced crude under vacuum in the vacuum tower.

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To achieve a deep vacuum, pressure drop through the column must be kept low. Instead of the type trays we’ve discussed earlier, we use random packing and demister pads, to keep the vapor velocities low a large diameter tower is used. An actual operating vacuum tower is show in Figure 4.1. The side draws from the vacuum tower may be lube oil stocks or charge to the Cat. Cracker. The bottoms, vacuum residual, may be heavy fuel oil or asphalt.

Figure 4.1 Vacuum Tower

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4.2 REDUCED CRUDE FLASHING The reduced crude is charged through a heater into the vacuum column in the same manner as whole crude is charged to an atmospheric distillation unit. However, whereas the flash zone of an atmospheric column may be at 1-1.3 kg/cm², the pressure in a vacuum column is very much lower. The vacuum heater transfer temperature is generally used for control, even though the pressure drop along the transfer line makes the temperature at that point somewhat meaningless. The flash zone temperature has much greater significance. The heater transfer and flash zone temperature are generally varied to meet the vacuum bottoms specification, which is probably either gravity or viscosity specification for fuel oil or a “penetration” specification for asphalt. The penetration of an asphalt is the depth in 1/100 cm, which a needle carrying a 100gm weight sinks into a sample at 77ºF in 5 seconds, so that the lower the penetration the heavier the pitch. Very heavy pitches are called asphalts. If the flash zone temperature is too high the crude can start to crack and produce gases which overload the ejectors and break the vacuum. When this occurs, it is necessary to lower the temperature; and if a heavier bottoms product is still required, an attempt should be made to obtain a better vacuum instead. Slight cracking may occur without breaking the vacuum, and this is sometimes indicated by a positive result from the Oliensis Spot Test. The Oliensis Spot Test is a simple laboratory test which purports to indicate the presence of cracked components by the separation of these components when a 20% solution of asphalt in naphtha is dropped on a filter paper. Some crude always yield positive Oliensis asphalt, regardless of process conditions. If a negative Oliensis is demanded, operation at the highest vacuum and lowest temperature should be attempted. Since the degree of cracking depends on both the temperature and the time during which the oil is exposed to that temperature, the level of pitch in the bottom of the tower should be held at a minimum, and its temperature reduced by recalculating some pitch from the outlet of the pitch crude exchanger to the bottom of the column. It will often be observed that when the pitch level raises the column, vacuum falls because of cracking due to increased residence time. The flash zone temperature will vary widely depending upon the crude source pitch specifications, the quantity of product taken overhead, and the flash zone pressure and temperatures from below 315˚C to oven 425ºC have been used in commercial operations.

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Some vacuum units are provided with facilities to strip the pitch with steam, and this will tend to lower the temperature necessary to meet an asphalt specification, but an excessive quantity of steam will overload the jets. 4.2.1 Vacuum Bottoms Handling Pitch must be handled more carefully than most refinery products. The pitch pumps which handle very hot, heavy material have a tendency to lose suction. This problem can be minimized by recycling some cooled pitch to the column bottom and so reducing the tendency of vapor to form in the suction line. It is also important that the pitch pump glands be sealed in such a manner so as to prevent the entry of air. Since most pitches are sold at atmosphere temperatures, all pitch handling equipment must either be kept active, or flushed out with gas oil, when it is shut down. Steam tracing alone is sometimes inadequate to keep the pitch fluid, but where this is done, the highest pressure steam available should be used. Pitch is sometimes cooled in open box units, as shell and tube units are not efficient in this service. It is often desirable to send pitch to storage at high temperature to facilitate blending. If it is desired to increases the temperature of the pitch, it is better to do so by lowering the level of water in the open box, and not by lowering the water temperature. If the water in the box is too cold, pitch can solidify on the inside wall of the tube and insulate the hot pitch in the central core from the cooling water. Lowering the water temperature can actually result in a hotter product. When pitch is sent to storage at over 100ºC, care should be taken to insure that the tank is absolutely free from water. Pitch coolers should always be flushed out with gas oil immediately once the pitch flow stops, since melting contents of a cooler is a slow job. 4.2.2 Entrainment Control The vapor rising above the flash zone will entrain pitch, which cannot be tolerated in cranking unit charge. The vapor is generally washed with gas oil product, sprayed into the “slop wax” section. The mixture of gas oil and entrained pitch is known as slop wax, and it is often circulated over the decks to improve contact, though the circulation rate is not critical. The final stage of entrainment removal is obtained by passing the rising vapors through a metallic mesh demister blanket through which the fresh gas oil is sprayed.

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Most of the gas oil spray is revaporized by the hot rising vapors and returned up the column. Some slop wax must be yielded in order to reject the captured entrainment. The amount of spray to the demister blanket is generally varied so that the yield of slop wax necessary to maintain the level in the slop wax pan is about 5% of the charge. If the carbon residue or the metals content of the heavy vacuum gas oil is high, a greater percentage of slop wax must be withdrawn or circulated. Variation in the color of the gas oil product is a valuable indication of the effectiveness of entrainment control. Slop wax is a mixture of gas oil and pitch, and it can be recirculated through the heater to the flash zone and reflashed, if the plant has the capacity to do so. If, however, crude contains volatile metal compounds, these will be recycled with the slop wax and can finally rise into the gas oil. Where volatile metals are a problem, it is necessary either to yield slop wax as a product, or to make lighter asphalt, which will contain the metal compounds returned with the slop wax. 4.2.3 Product Condensation The scrubbed vapor rising above the demister blanket is the product, and no further fractionation is required. It is only desired to condense these vapors as efficiently as possibly. This could be done in a shell and tube condenser, but these are inefficient at low pressures, and the high pressure drop through such a condenser would raise the flash zone pressure. The most efficient method is to contact the hot vapors with liquid product which has been cooled by pumping through heat exchangers. It is further desired to usefully recover the heat of the rising vapors by heat exchange against crude oil, so we must arrange to have the circulating liquid at a high enough temperature to permit efficient heat exchange. We therefore have to compromise. If the gas oil circulation is high enough to condense all the vapors, the gas oil pan temperature will be so low that we will have inefficient heat exchange. In order to obtain a suitable high pan temperature we are forced to reduce the circulation rate until some of the vapors escape uncondensed. The problem of uncondensed vapors is easily solved by adding a small circulating LVGO section to catch these vapors by condensation against LVGO from a water cooler. The heavy vacuum gas oil circulation rate is chosen to maximize crude heat exchange. The best way of doing this on an operating unit is to observe the temperature of the crude leaving the crude/HVGO exchanger, then lower the HVGO circulation rate by 10%. If the crude temperature rises, the effect of the higher HVGO pan temperature has been greater than the effect reduced circulation, and we should try some more of _________________________________________________________________________________ MIDOM

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the same % age. If the crude temperature is lower, we should try a 10 percent change in the opposite direction. Sometimes it is impossible to remove enough heat with crude exchange alone, and some HVGO from the outlet of the HVGO cooler is returned to the circulating line. This should only be done when necessary, since both heat and cooling water are wasted. The HVGO product is cooled and pumped to storage on HVGO pan level control. The LVGO section is a final contact condenser and normally the circulation rate should be adequate to keep the vapor to the jets within about 5ºC of cooling water temperature. A high circulation rate will provide a cushion against upsets. 4.2.4 Vacuum Pressure Measurement Confusion often arises because of the different scales used to measure vacuum. Positive pressures are commonly measured as kilograms per square-centimeter gauge, which are kilograms per square centimeter above atmospheric pressure. Atmospheric pressure is 1.035 kg/cm². Another means of measurement is to measure in millimeters of mercury. Atmospheric pressure (sea level) is 760 millimeters of mercury absolute while a perfect vacuum is 0 millimeters absolute. When vacuum are measured we can more conveniently do it by using millimeters of mercury absolute.

4.3 VACUUM FRACTIONATOR The function of a vacuum tower (Figure 4.2) is to fractionate hydrocarbons that boil above approximately 700ºF (370°C) in the crude tower. In the vacuum column, the pressure can be reduced to around 1.0 psia, below the slop wax tray. This is a total reduction in absolute pressure of perhaps 28.7 psi from the bottom of the crude tower. This large difference in pressure enables a great deal of hydrocarbon to flash overhead in the vacuum tower while maintaining a bottoms temperature not exceeding, for example, 730-780°F depending on the crude source. To further aid in removal of usable products from the bottoms material and to help produce proper penetration asphalt, stripping steam can be injected into the bottom boot of the column to decrease the partial pressure of the bottoms liquid. The bottom of the vacuum tower is swaged down to decrease the time that the bottoms liquid spends at the elevated bottoms temperature. A quench oil inlet line is also provided to protect the bottoms pumps. The feed line to a vacuum column is very large in comparison to the feed lines of most fractionators. This is because of the low pressure which causes almost all the vacuum column feed to be vapor. This also requires a special distributor called a tangential distributor that imparts a swirling direction to the feed and prevents damage to _________________________________________________________________________________ MIDOM

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equipment above the distributor due to the rapid expansion of the feed as it enters the low pressure of the vacuum tower. The internals in the vacuum column are designed for a minimum amount of pressure drop, and the slop wax accumulator, the grid end demister pads are the only internals that extend completely across the entire column. The grid and demisters provide coalescing mediums to remove entrained liquid particles from the rapidly rising vapors. Spray distributors are used to aid the grid and demisters in coalescing. There are no trays in this vacuum tower, but what appear to be trays are side-by-side pans. Their outer edges perforated by holes and stiffened by metal lattice, the side-by-side pans overlap and provide a cascading effect to the condensed liquid. Hot vapors pass through the cascade to re-vaporize the lower boiling components of the liquid. Accumulator trays are designed to provide a vapor-free liquid to the suction of side draw pumps. Pump vents are returned to the column to allow removal of noncondensable from the pump during startup. This helps a great deal in getting the pump started. After the pump is pumping properly, the vent should be closed. The top section of the vacuum column is swaged down because the traffic of material through the top of the column is much less than at the side draws. In fact, too many light ends in the feed or light ends formed by thermal decomposition of the bottoms would place an undue burden on the vacuum ejectors that have created and are maintaining the low pressure on the vacuum column. Vacuum columns are generally designed to withstand an internal pressure of 50 psi (3.5 Kg/cm² gauge) and a 14.7 psia (760 mm Hg absolute) external pressure. To strengthen the vessel walls to work between these two pressures, stiffeners are used. These are merely rings welded around the column and spaced a few feet apart. The materials of construction used in the design of a vacuum tower are killed carbon steel for the vessel with the lower section clad with an 11-13% Cr. S.S. The slop wax accumulator is made of a 12% Cr. S.S. and the wall of the accumulator is lined with concrete. The grid is constructed of 304 stainless steel. The upper demister pad is constructed of Monel. Side-by-side pans are constructed of 12% Cr. S.S. The remainder is constructed of carbon steel. Designs for vacuum columns with different corrosion severities may allow the elimination of alloy cladding and some of the alloy in side-by-side pans and accumulators.

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Figure 4.2 Vacuum Column

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Flash Towers Flash towers are used in services where good fractionation of the charge is not necessary. Their main purpose is to obtain the maximum amount of overhead product for a given transfer-line temperature. The feed material is heated to the desired temperature and the mixture of vapors and liquids is introduced into the flash space of a vessel. The liquid remaining after flashing falls to the bottom, usually through baffles to aid in separation and the vapors go overhead. Normally, an operation of this nature is used to remove asphalts and tar from crude before processing. Tar separators and tar stripper are the common names used for this type of tower.

4.4 STEAM JET EJECTOR 4.4.1 Introduction Vacuum is maintained by two general methods, vacuum pumps and jets. Vacuum jets are used extensively in yard equipment, whereas vacuum pumps are widely used in the laboratories. The vacuum system is used to remove vapors from the system which cannot be condensed. The common means for creating a vacuum in distillation is to use steam jet ejection. They can be employed singly or in stages to create a wide range of vacuum conditions. Their wide acceptance is based upon their having no moving parts and requiring very little maintenance. These vapors consist of non-condensable hydrocarbon vapors and air coming into the system with the feed and from leaks. Vacuum jets pull gases from the tower by using air, steam, or water in the jet. The jets generally use steam as the motivating material. A series of jets (normally three) is used to boost the gases from the pressure of the vacuum tower to atmospheric pressure. The steam used to pull the gases and is condensed in each stage and removed as water. Figure 4.3 is a schematic diagram of a vacuum jet system. Water is removed from the jet stages by a pump or gravity flow from a water column. If the jets are 34 ft. above ground level the water flows out by gravity. At any height lower than 34 ft. the water must be pulled off with a pump.

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Figure 4.3 Schematic Diagram of Vacuum Jet System Barometric systems are generally controlled by changing the water flow to the condenser for the first-stage jet. Control can be attained by varying steam to the jet. This type of control is normally not as effective as varying the condenser water. Every vacuum system has a definite capacity. This capacity is measured as to the quantity of non-condensable removed at a definite vacuum. As the quantity of non-condensable exceeds the capacity of the jets, the vacuum begins to fail off. Therefore as the cooling water to the first stage jets is reduced the quantity of non-condensed gases exceeds the capacity of the jet and causes the vacuum to fail off. 4.4.2 Operating Principle Figure 4.4 illustrates a typical two-stage ejector system. In an ejector the steam is injected at high velocity through a specially designed nozzle (Figure 4.5) and transfers sufficient energy to the gases from the suction header to entrain them through the diffuser into the first-stage discharge header. The pressure in the first stage discharge header is, of course, higher than the pressure in the suction header, but if the velocity of the steam through the diffuser throat is high enough, gas cannot back into the suction header. If a single ejector is incapable of raising the gases to atmospheric pressure at which they can be vented, the steam is condensed and a second ejector taking suction of the non-condensable gases raises them to a higher pressure. The dimensions of an ejector are quite critical so that any given ejector will only operate ever a relatively limited range. A substantial change in suction or discharge conditions will probably demand a change in the dimensions of the nozzle or diffuser, _________________________________________________________________________________ MIDOM

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or both. The effect of changes in operating conditions can be summarized as follows: 4.4.2.1 Steam Pressure Must be maintained quite close to that for which the equipment was designed. If the steam pressure greatly exceeds that for which the nozzle was designed, the quantity of steam discharging into the diffuser will be greater than can pass through the diffuser, and steam will back into the suction header. Too low a steam pressure will mean a drastic loss in performance of the ejector. It should be noted that wet steam will cause random fluctuations in ejector performance and in addition will erode the nozzle and diffuser. 4.4.2.2 Discharge Pressure If the discharge pressure rises above design, there is an increasing probability of reverse flow. An increase in discharge pressure on an ejector discharging to atmosphere is only possible if the discharge is obstructed. But on multi-stage units an increase in interstage pressure due to high condensate temperatures or failure of a second or third-stage ejector will immediately affect the performance of the first-stage unit. 4.4.2.3 Load A decrease in load (kgs/hour of vapor to ejector) will result in a somewhat higher vacuum being obtained, but if the load is increased above design, the vacuum obtained will fall off quite suddenly and dramatically. 4.4.2.4 Cooling Water Temperature The temperature at which the steam is condensed in the inter and final condensers will have a relatively minor effect on the vacuum obtained, but will substantially reduce the load at which the ejector system breaks down, since an increase in condensate temperature increases the inter-stage pressure.

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Figure 4.4 Typical Arrangement Vacuum Unit 2-Stage Jets

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Figure 4.5 Cut-Away View of Steam Jet Ejector

In order to insure flexibility a refinery ejector system for a vacuum unit will generally be constructed using two parallel sets. The minimum combination of equipment which will achieve a satisfactory vacuum is normally used. The vapors drawn from the top of a typical vacuum unit to the jets consists of air from leaks, steam entrained from the bottom of the crude distillation tower, light hydrocarbons, and sulfur and nitrogen compounds from thermal decomposition in the heater, and any hydrocarbons lighter than gasoline which have not been stripped from the charge. The steam and light hydrocarbons will condense in the inter-condenser so that the first-stage ejectors can be heavily loaded under conditions which only lightly load the second-stage ejectors. Any actual cracking in the furnace will produce light gases which will very rapidly overload the second-stage ejectors. Both the condensable and non-condensable vapors handled by a typical vacuum unit ejector set are highly odiferous so that the condensate must be stripped and the non-condensable vapors incinerated.

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4.5 VACUUM TOWER CONTROL SYSTEM Steam pressure below a critical value of a jet will cause the ejector operation to be unstable. Therefore, it is recommended to install a pressure controller on the steam to keep it at the optimum pressure required by the ejector. The recommended control system for vacuum distillation is shown in Figure 6.6. Air or gas is bled into the vacuum line just a head of the ejector. This makes the maximum capacity of the ejector available to handle any surges or upsets. A pressure control valve regulates the amount of bleed air used to maintain the pressure on the reflux Drum. The liquid overhead product shall always be sub-cooled to avoid excessive loss of product vapor to the evacuating equipment.

Figure 4.6 Vacuum Column Pressure Control

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4.6 STARTUP When starting a vacuum unit it is common practice to steam out the heater and tower and then pressure test the tower with steam at about 50 psig. It will be assumed that this has been done and that all valves around the ejectors are closed. a. Shut off steam to the tower. b. Vent steam from top of tower until the pressure is about 0.2 Kg/cm², then close end plug the top vent. c. Open the inlet and outlet valves of one of the second-stage ejectors. d. Open the inlet and outlet valves of one of the first-stage ejectors. e. Open water through both condensers. f. Open steam to both first and second-stage ejectors. g. Check that the steam is dry and adjust the steam pressure to that given on the equipment name plate. As soon as a level appears in the inter-condenser, start the condensate pump and place the level on control. h. Let the ejectors run until a constant vacuum is obtained even though the presence of water in the tower may result in a poor initial vacuum. i. Charge the vacuum unit and proceed with normal operations. Adding Additional Ejectors The operators should observe and get to know the interstage pressure which gives the most stable operation on a given ejector set. On most two-stage units, this is about 260-130 mm Hg. absolute vacuum. When the tower vacuum either decreases or becomes sensitive to process conditions, additional ejector capacity should be added. If the interstage pressure has risen (the vacuum as measured in mm Hg. has increased), an additional second-stage jet should be added. If the interstage pressure is unchanged, but the lower pressure has risen, an additional first-stage jet should be added and this may render the addition of another second-stage jet necessary. a. Open the ejector discharge valve. b. Open the steam to the nozzle. _________________________________________________________________________________ MIDOM

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c. Check that the nozzle steam pressure is under good control. d. Open the ejector suction valve.

4.7 TROUBLESHOOTING Occasionally, ejectors will fail to pull an adequate vacuum or will perform erratically. This can be the result of a large number of troubles, a few of which are listed below for checking. a. Air leaking Into the System: Hot bolt all flanges and manways on the vacuum tower and on the overhead line. Tighten and oil all valve packing glands. Plug all vents and drain valves and tighten any screwed connection. Check that pump gland flushing is adjusted to maintain a positive pressure on the gland. b. Air Leaking Into Interstage: This will be confirmed by a rise in interstage pressure. Tighten all flanges, pump glands and screwed connections. Check that the condensate drain trap is not stuck and bleeding air back from the second stage. c. Leaking Behind Nozzle: Certain styles of ejectors can readily leak air or steam through a leak in the point where the nozzle is attached to the body. d. Wrong Steam Pressure: Check ejector name plate data and change pressure controller setting. e. Wet Steam: Causes erratic performance. Check performance of steam traps. f. Worn Nozzles and Diffusers: The result of using wet steam. g. Clogged Steam Filters: There is generally a main filter ahead of the steam pressure controller and a filter in the nozzle of each ejector. h. High Condensate Temperatures: The result of insufficient cooling water flow or fouling of either the tube or the shell side of the condensers. i. Flooded Condensers: The result of malfunctions of the level controller or the condensate drain trap, or of pump failure. If the pump will not hold suction, check that air is not leaking in at the gland. l. Faulty Installation: Failure to properly align gaskets and similar details which are normally insignificant can trap condensate pockets or cause turbulence which can affect the performance of vacuum equipment. If an ejector set has _________________________________________________________________________________ MIDOM

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been dismantled, each nozzle must of course be reinstalled in the correct diffuser. j. Back Pressure: Due to deposits in the condensers, a plugged flame arrester in the vent, a condensate pocket or other obstruction. k. Impossible Operation: Such as attempting to obtain an absolute pressure lower than the vapor pressure of a liquid in the system. If a very little vacuum gas oil is produced it may be so light that it will be impossible to pull a vacuum on the tower. m. High-Tower Bottom Level: If the level in the tower is permitted to rise, some cracking will occur because the pitch is being held at a high temperature for too long. n. Steam Entering System: Check steam-out connections on the tower, heater, exchangers, etc. o. Cracking in Furnace: Experienced at very high transfer temperatures and can be checked by running an Oliensis on the pitch, or a bromine number on the light vacuum gas oil. Where a leak is elusive, a special meter can be installed to measure the noncondensable being vented and these vent gases can be analyzed in the laboratory.

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Appendix

Vacuum Towers Anyone who has ever seen crude oil distilled in the lab under atmospheric pressure will appreciate the importance of a vacuum tower. At about 680ºF-700ºF, the residual _________________________________________________________________________________ MIDOM

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liquid will start evolving yellowish vapor. This is an indication of the thermal cracking that degrades the quality of virgin distillates and gas oils. To distill most of the gas oil out of the crude while still avoiding excessive temperatures, a vacuum tower is used. The crude unit's primary tower is intended to fractionate between naphtha, kerosene, and furnace oil. The vacuum tower only has one function: to produce clean, high-boiling gas oil suitable for cracking-plant or lubeoils refining feed.

Figure 1 A Typical Crude Unit Vacuum Tower

A vacuum tower's flash zone typically operates at 1-2 psia and 720ºF to 780ºF. The tower is designed to tolerate a small degree of thermal cracking. A sketch of a typical vacuum tower is shown in Figure 1. Some of the more common troubles associated with operating vacuum towers are: 

High flash-zone pressure

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

Black gas oil

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Excessive production of trim gas oil

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High-gravity resid

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Ejector deficiencies

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Bottoms-pump NPSH problems

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Low gas draw temperatures

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Transfer line failures

1. LOSS OF BOTTOMS-PUMP SUCTION PRESSURE Providing net positive suction head (NPSH) for any centrifugal pump can be a tricky business. For vacuum tower bottoms pumps, the difficulties are magnified. A few pounds of vapor that another pump might pass completely gas-up a pump in vacuum service. This is because the low absolute suction pressure expands a small amount of vapor into a very large volume. Moreover, the possibilities of introducing vapor into the suction of a vacuum tower bottom's pump are more numerous than in other services. Especially on start-up, initiating and maintaining good suction conditions for the bottoms pump is one of the most difficult aspects of vacuum tower operation. A few of the more noteworthy problems are discussed below and summarized in Figure 2. 1.1 Insufficient Quench Most vacuum towers are provided with a means to reduce the bottom boot temperature 20ºF-50ºF. This is done with a circulating quench as shown in Figure 2. The purpose of the quench is to reduce thermal cracking of the bottoms product and to suppress vaporization at the suction of the bottom's pump. If the bottom's pump is truly losing suction because of insufficient NPSH, increasing the circulating quench rate or reducing the quench return temperature will help.

1.2 TGO Pan Overflows Trim gas oil (TGO) is a black oil stream withdrawn immediately above the vacuumtower flash zone. It consists of 20%-50% resid and 50%-80% gas oil. This relatively _________________________________________________________________________________ MIDOM

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light material may cause the bottoms pump to cavitate when it overflows its strap-out pan at a non-uniform rate. Reduce the TGO pan level to see if this helps the NPSH problem on the bottoms pump.

Figure 2 Providing Suction Head for a Vacuum Tower Bottoms Pump is Difficult 1.3 Gland Oil The purpose of gland oil is to keep black oil away from the pump seals. A gland oil pressure of 10 psig is usually sufficient to keep black oil out of the seals. Naturally, a small amount of gland oil will leak through the seals into the vacuum bottoms stream. This is of no consequence. If gland oil leakage becomes excessive, either because of a defective seal or excessively high gland oil pressure, the vacuum bottoms pump will lose suction. The gland oil, which consists of a relatively light hydrocarbon, flashes on contact with the hot resid product. The evolved vapor gases-up the pump. Try pinching back on the gland oil pressure to reduce cavitation. If this helps, but only at a very low pressure, the pump's seal is bad. One refiner substituted heavy vacuum gas oil for a lighter gland oil to eliminate this problem.

1.4 Suction Screen Thermal cracking will eventually produce coke in the vacuum tower. The coke will wash down into the pump suction and plug the suction screen. A simple pressure _________________________________________________________________________________ MIDOM

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survey will identify this problem. Measure the pressure at the pump's suction and at a point in the boot above the liquid level. The difference in pressure, expressed in feet of liquid (assume a 0.75-0.80 specific gravity) should equal the height of liquid in the boot above the pump suction. If this pressure difference is quite a bit less than this delta height, the pump's suction screen is plugged and should be cleaned. 1.5 Air Leak A rather small leak in a flange on the pump's suction piping will cause loss of NPSH. Any air sucked in will reduce the average density of the resid in the boot and suction line. The reduction in density cuts the head of liquid and usually precipitates cavitation. On one unit, the level float in the boot would jump an inch or two when the bottoms pump started to lose NPSH. An air leak in a suction line flange was later found.

2. HIGH FLASH ZONE PRESSURE The reduction of resid in a vacuum tower is a function of the flash zone temperature and pressure. A rise in this pressure Increases production of resid at the expense of the more valuable gas oil product. The key tool in troubleshooting flash-zone pressure problems is a vacuum-tower pressure survey. The time to initiate this survey is just after start-up when the trays, demister, and ejector system are clean and in good condition. Pressures are best measured with a portable mercury-filled vacuum manometer. Using a vacuum pressure gauge will reduce the accuracy of observed pressure drops. Relying on permanently installed gauges for pressure drop data will not give reliable results. Figure 3 summarizes two vacuum-tower pressure surveys: one just after unit start-up and the other a year into the run. The data clearly shows that the demister is partially plugged with coke. Pressure drop data should be normalized by correcting for flow rates and pressure as follows:

Where:

M1 = Mass flow through the tower V1 = Superficial velocity through the tower MB

= Mass flow of the base data to which ΔP1 is to be compared

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ΔP1 = Measured pressure drop ΔPN= Normalized pressure drop

Figure 3 Pressure Survey is a key to Troubleshooting High Flash-zone Pressure

Any restriction to vapor flow above the flash zone must increase the flash-zone pressure. An Increase in AP across the wash trays below the demister or across the demister itself is almost certainly due to coke buildup. A generous wash oil flow (Figure 1) will inhibit coke formation, but of course, this increases the production of undesirable trim gas oil. Once the coke is formed, only a shutdown will correct the situation. _________________________________________________________________________________ MIDOM

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Large increases in ΔP across the top or bottom pump around trays are an indication of flooding. A reduction in liquid pump around rate might alleviate the problem. A high vacuum tower top pressure is a result of air leaks, excessive production of hydrocarbon gases due to thermal cracking, or a host of ejector deficiencies. 2.1 Thermal Cracking Increasing the flash-zone temperature will reduce the gas oil left in the resid. Unfortunately, thermal cracking rates double for every increase of 20ºF-25ºF. As shown in Figure 4, the production of non condensable gas (a product of thermal cracking) rises with higher transfer line temperature. This can overload the ejector system. Also, the rate of coke build-up on the demister pad is a function of flash zone temperature. For these reasons, simply raising the furnace outlet temperature to overcome the effect of high flash zone pressure is not always a good idea.

Figure 4 Raising Flash-zone Temperature can Overload Vacuum Ejector

3. EJECTOR PROBLEMS Reduced vacuum at the top of a tower may be due to a number of problems. 

Non condensible hydrocarbons



Air leaks

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

Worn ejector internals



Degradation of ejector steam pressure or temperature



Fouled or overloaded condensers



Plugged seal legs

3.1 Air Leaks A rather sudden loss in vacuum or failure to reestablish normal tower top pressure after a unit turnaround is most likely a consequence of an air leak. Air or oxygen compounds (CO, CO2) in the ejector tall gas are the usual indication. If CO or CO2 is present, the air leak is probably in the hot part of the vacuum tower or the transfer line. Look for a leak in the ejector or vacuum tower overhead piping if oxygen is present in the tail gas. On several units, leaks in the seal leg piping have proven troublesome. A sketch of a single-stage ejector system (Figure 5) shows the location of the seal leg piping. Use aerosol shave cream to test for vacuum leaks. When all else fails, pressure the tower to 3-5 psig. The leak can then be located by the noise it will emit. 3.2 Motive Steam Quality The ejector steam undergoes an isentropic expansion which converts much of its pressure into velocity. It is this high velocity (up to 6,000 mph) that pulls a vacuum. A reduction in steam temperature and pressure or an increase in the steam's moisture content reduces the motive steam's ability to pull a vacuum. 3.3 Condensers The precondenser shown in Figure 5 removes the bulk of the mass of steam and hydrocarbons evolved from the vacuum tower. The purpose of the downstream intermediate condensers is to condense the motive steam used in the ejectors. Fouling of the condenser tubes and high cooling water temperature will increase the pounds per hour that the ejectors must handle.

Regardless of ejector capacity, if the vacuum tower uses stripping steam, the pressure at the top of the tower cannot be lower than the vapor pressure of water at the precondenser outlet temperature. 3.4 Plugged Seal Legs As shown in Figure 5, the seal legs function to drain oil and water from the shell side of the condenser. The inability to maintain condenser drainage will severely reduce _________________________________________________________________________________ MIDOM

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ejector efficiency by backing liquid up in the condenser shell. With experience, one can find the liquid level inside the condenser shell by feeling for the temperature gradient by hand. Condenser tubes submerged in liquid are effectively out of service.

Figure 5 A typical Vacuum Tower Overhead System A sudden rise in vacuum tower top temperature, which occurs simultaneously with the onset of cold weather, can be due to freeze-ups of the seal legs. Plugging with waxy deposits is also possible. 3.5 Ejector Internals The very high velocities that the ejector is exposed to, subject the nozzle and diffuser throat to excessive wear. Low-quality steam will accelerate this erosion. A gradual loss in vacuum may be due to enlargement of the ejector clearances. It is a good practice to caliper these clearances when the system is out of service.

4. BLACK GAS OIL The heavy vacuum gas oil (HVGO) produced in most refineries is the principal component charged to a fluid catalytic cracking unit (FCCU). The catalyst at the FCCU is adversely affected by the nickel, vanadium and sodium which become concentrated in the vacuum-tower bottoms resid. When resid is entrained into the HVGO, the metals in FCCU charge will dramatically increase. _________________________________________________________________________________ MIDOM

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The operating engineer should check the color of the HVGO periodically. A darkish yellow color is normal; black gas oil indicates resids (and metals) are being entrained into the gas oil. Some of the more common causes of resid entrainment are: 







A section, of the demister pad has become dislodged. A pressure drop survey showing a substantial decrease in AP across the demister is evidence of such a failure. The demister pad is partially coked. Quite often, the demister spray header (see Figure 11.1) is designed for too large a wash oil flow. At low flow, the wash oil does not distribute to the ends of the spray header, and the peripheral area of the demister dries up and cokes. The sections of the demister still open are exposed to velocities high enough to promote resid entrainment. A high AP across the demister shows that coke plugging is the problem. An examination of the demister before it is removed from the tower will indicate which areas are not being wetted with wash oil from the spray header. This information should then be used to revamp the spray header. A decrease in flash zone pressure increases velocity and promotes entrainment. Throttle the steam to the ejector system (to temporarily raise flashzone pressure) and see if the HVGO color clears up. Inadequate/low of wash oil will allow entrained resid to reach the HVGO draw-off tray. Naturally, increasing the wash oil flow puts HVGO into black trim gas oil.

5. EXCESSIVE PRODUCTION OF TRIM GAS OIL Trim gas oil (TGO) or slop gas oil is an intermediate cut made in a vacuum tower between heavy vacuum gas oil (HVGO) and resid. The TGO consists of 20%-50% entrained resid and 50%-80% wash oil (HVGO). In some refineries, TGO is dumped directly into resid; in others the TGO is recycled to extinction through the vacuum furnace. Either way, TGO is an undesirable byproduct of vacuum-tower operations.

Excessive production of TGO is often a consequence of efforts to clean up dirty HVGO by over refluxing with wash oil. If TGO production has increased after a unit turnaround, the operating engineer may be sure that the HVGO trap-out pan is leaking. The evidence for this is a lower than normal TGO draw-off temperature and increased TGO gravity (ºAPI). A low HVGO draw temperature is further proof that the HVGO is leaking into the TGO section. Leaks from the HVGO pan may be initiated by cleaning the tower trays during a turnaround or by bumping the vacuum tower during start-up typically with a shot of water. Sometimes the leaks will coke up. Once _________________________________________________________________________________ MIDOM

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workmen left pieces of a trap-out pan out of the tower after a turnaround. The resulting leak did not coke up.

6. LOW PUMP AROUND DRAW TEMPERATURES Why do most vacuum towers have two pumps around (PAR) streams and produce both a light vacuum gas oil (LVGO) and HVGO streams? After all the LVGO and HVGO are usually combined after they are drawn off the vacuum tower (refer to Figure 1.) Try letting the LVGO draw-off pan overflow for 15 minutes, and the answer will become apparent. Both the HVGO and LVGO draw off temperatures will fall as both products become lighter. A leak in the LVGO pan will have the same effect as letting the pan overflow, i.e., draw temperatures and hence crude preheat will decline. Tight trap-out pans in vacuum towers almost always save energy.

7. LIGHT RESID Gas oil left in resid will often have a value equivalent to fuel oil ($20/bbl). Gas oil recovered for FCCU feed will have a value equivalent to crude ($35/bbl). It follows, then, that every barrel possible ought to be boiled out of the vacuum tower bottoms. The best troubleshooting tool for judging overall vacuum-tower performance is to vacuum distill off in the lab the 1,000ºF¯gas oil in the resid. Based on plant data, the following rules of thumb may then be applied: 

5%-10% l, 000ºF¯ gas oil on resid Indicates excellent performance.



30%-35% l, 000ºF¯ gas oil on resid Indicates poor performance.

 Some of the more common reasons for increased losses of gas oil to resid are:  Leaking TGO draw-off' pan. Declining TGO production highlights this failure.  Low flash zone temperature. Have the instrument mechanic check the furnace outlet thermocouple.  High flash-zone pressure. Perform a ΔP survey.  Inadequate velocity steam. Steam is injected into the vacuum furnace coils to promote vaporization by reducing the hydrocarbon partial pressure. Too much steam will Increase residual entrainment into the gas oil and will ruin the FCCU feed.  Stripping deficiencies. Stripping the resid with exhaust steam is a cost effective method to recover gas oil. A properly operated stripper may easily remove half _________________________________________________________________________________ MIDOM

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of the gas oil that is left in the resid after it drops out of the flash zone. Using 0.2 Ibs of steam per gallon of bottoms is a typical steam rate. Effective steam stripping is indicated by a 30°F-50°F temperature drop between the flash zone and the tower bottoms. A lower AT means that the steam is not effectively contacting the resid. This can be due to upset or corroded trays or tray flooding. Often, poor level control in the bottom of the tower will permit resid to back up and flood the stripping trays.

8. PROJECTS TO IMPROVE GAS OIL RECOVERY In a crude-short world, projects that recover gas oil (which is readily converted to gasoline) from resid will almost always have a good payout. Residual fuel prices must, in the long run, compete against coal, while gasoline prices are a direct function of crude oil costs. A few projects have seen successful field application. 











Improve light gas oil recovery in the crude unit primary tower. This will unload the vacuum tower and furnace. Reduce the number of wash trays in the vacuum tower. In one Installation, two bubble-cap trays replaced four tunnel-cap trays. The flash-zone pressure was therefore reduced. The net effect was beneficial in that the lower pressure drop was more useful than the loss in fractionation represented by the two trays removed. Replace pump around trays with packing. Two-inch pall rings are a good bet for this application. The payoff is reduced AP and flash zone pressure. Improve ejector performance. Providing a source of colder cooling water for the interstage condensers is one method of achieving this objective. Lower tower top pressure will translate into a lower flash-zone pressure. Increase the number or efficiency of resid stripping trays. Increasing the super heat of the exhaust stripping steam will also multiply the amount of gas oil stripped out of the vacuum-tower bottoms resid product. Seal weld the HVGO and TGO draw-off trays to eliminate leakage into the resid. Welding a dam around the periphery of the tray between the outermost rows of caps and the tray ring is an effective method to cut tray-ring leakage. The height of the dam must be greater than the height of the outlet wire.

9. TRANSFER LINE FAILURES It seems as if the vacuum tower transfer line is a weak point in many crude units. It is possible, because of an incorrectly sized transfer line, to approach sonic velocity in these lines. Such super-high velocities have lead to rapid erosion and failure of the _________________________________________________________________________________ MIDOM

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transfer line. If a unit's transfer line is experiencing an accelerated rate of failures, the operating engineer should consider several questions. Has the flash zone pressure been substantially reduced? Has the furnace charge rate (including velocity steam) been increased? Is the vacuum tower feed lighter than it used to be? A positive response to these questions means a higher transfer line velocity and enhanced rates of erosion. Operating data indicates such erosion can become a problem at velocities exceeding several hundred ft/sec. An increase in the naphthenic acid content of crude will also accelerate transfer line corrosion. 9.1 Furnace Tube Failures The furnace outlet (or transfer line) temperature, is not usually the highest temperature which the vacuum tower charge reaches. The high pressure drop in the transfer line (caused by high velocities) suppresses vaporization in the furnaces tubes. In turn, this raises the oil temperature in the tubes so that the required duty may be provided even at the reduced percent vaporized. This is called temperature peaking. The high peak temperatures cause furnace-tube failures. The process operating engineer, faced with a series of transfer line and furnace tube failures in vacuum service, should consider enlarging the transfer line diameter and the size of the last few tubes in the furnace.

10. TROUBLESHOOTING CHECKLIST FOR VACUUM TOWERS 10.1 Bottoms Pump NPSH  Insufficient quench  Overflow of TGO pan  Too much gland oil pressure  Leaking seal _________________________________________________________________________________ MIDOM

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 Suction screen plugged  Air leak in suction piping 10.2 High Flash-Zone Pressure  Tower pressure survey  Normalized pressure survey  Coke buildup on demister  Flooding of PAR trays  Ejector deficient  Excess thermal cracking 10.3 Ejector Problems  Air leaks (CO, CO2, or O2)  Non condensible hydrocarbons  Worn ejector internals  Low motive steam pressure or temperature  Wet motive steam  Fouled condense

10.4 Transfer Line Failures  Approach to sonic velocity  Erosion due to high velocity  Reduced flash zone pressure  Lighter feed  More velocity steam _________________________________________________________________________________ MIDOM

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 Increased charge rate  Furnace tube peaking temperature  Naphthenic acid in crude 10.5 Black Gas Oil  Indicates metals in FCCU feed  Demister section upset  Demister pad partially coked  Too low a flash-zone pressure  Wash oil flow too low 10.6 Excessive Production of Trim Gas Oil (TGO)  Symptom of wash oil section problems  Leaking HVGO draw-off tray 10.7 Low Pump around Draw Temperatures  Low HVGO temperature (HVGO draw off tray leaking)  Low HVGO and LVGO (LVGO draw off tray leaking)

10.8 Light Resid  Inadequate velocity steam  High flash-zone pressure  Low flash-zone temperature  Leaking TGO draw-off pan  Stripping steam too low _________________________________________________________________________________ MIDOM

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 Stripping trays upset 10.9 Projects to Improve Gas Oil Recovery  Reduce light gas oil in vacuum tower feed  Reduce number of wash trays  Replace PAR trays with packing  Colder cooling water for ejector condensers  Seal weld HVGO and TGO draw-off trays  Increase the number of bottom stripping trays 

More superheat of exhaust stripping steam

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