C 5 Vacuum Distillation

7/27/2019 C 5 Vacuum Distillation

1/20

Oil Refining Technology Chapter 5 Vacuum Distillation

Chapter 5Vacuum Distillation

OGS (Rev.0) Feb. 2008 Page 1 of 20


2/20


Chapter 5 Contents

5.1 Introduction

5.2 Reduced Crude Flashing5.2.1 Vacuum Bottoms Handling5.2.2 Entrainment Control5.2.3 Product Condensation5.2.4 Vacuum Pressure Measurement

5.3 Vacuum Fractionator

5.4 Steam Jet Ejectors5.4.1 Introduction5.4.2 Operating Principle

5.4.2.1 Steam Pressure5.4.2.2 Discharge Pressure5.4.2.3 Load5.4.2.4 Cooling Water Temperature

5.5 Vacuum Tower Control System

5.6 Startup

5.7 Troubleshooting



3/20


Chapter 5

Vacuum Distillation

5.1 INTRODUCTIONIn 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 oilmeans 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 C at 50mm. would not boil until about 500 C at atmospheric pressure, at which temperaturemost hydrocarbons crack.

For distillation to take place, the vapor pressure of the liquid being distilled must be alittle greater than the pressure above it. The molecules that comprise a liquid areheld together by two forcesnatural cohesion and the weight of the atmosphere

pressing down. This pressure is equal to 14.7 psi. at sea level and will support acolumn 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 byremoving 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 fractionatingcolumn of larger diameter with bubble trays farther apart. This is necessary becausemuch 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 onlydesired to vaporize the gas oil, remove the entrained pitch, and condense the liquid

product as efficiently as possible. Bottoms from the crude tower contain materialthat 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 thatwould cause thermal cracking. Thermal cracking is undesirable because it wouldcause a loss of valuable product, degradation of valuable product, and shortened runtime due to coke formation in pipes and vessels. For these reasons we conduct thedistillation of the heavy reduced crude under vacuum in the vacuum tower.



4/20


To achieve a deep vacuum, pressure drop through the column must be kept low.Instead of the type trays weve discussed earlier, we use random packing anddemister pads, to keep the vapor velocities low a large diameter tower is used. Anactual operating vacuum tower is show in Figure 5.1. The side draws from thevacuum tower may be lube oil stocks or charge to the Cat. Cracker. The bottoms,vacuum residual, may be heavy fuel oil or asphalt.



5/20


Figure 5.1 Vacuum Tower


Vertical Baffle

HIGH SULFUR RESIDLOW SULFUR RESIB


6/20


5.2 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 cm, the pressurein a vacuum column is very much lower. The vacuum heater transfer temperature isgenerally used for control, even though the pressure drop along the transfer linemakes the temperature at that point somewhat meaningless. The flash zonetemperature has much greater significance.

The heater transfer and flash zone temperature are generally varied to meet thevacuum bottoms specification, which is probably either gravity or viscosityspecification for fuel oil or a penetration specification for asphalt. The penetrationof an asphalt is the depth in 1/100 cm, which a needle carrying a 100g weight sinksinto a sample at 77F 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 toohigh 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 sometimesindicated by a positive result from the Oliensis Spot Test. The Oliensis Spot Test is asimple 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 isdropped on a filter paper. Some crude always yield positive Oliensis asphalt,regardless of process conditions. If a negative Oliensis is demanded, operation at thehighest vacuum and lowest temperature should be attempted. Since the degree of cracking depends on both the temperature and the time during which the oil isexposed to that temperature. The level of pitch in the bottom of the tower should beheld 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 beobserved 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 pitchspecifications, the quantity of product taken overhead, and the flash zone pressureand temperatures from below 315 to oven 425 C have been used in commercialoperations.



7/20


Some vacuum units are provided with facilities to strip the pitch with steam, and thiswill tend to lower the temperature necessary to meet an asphalt specification, but anexcessive quantity of steam will overload the jets.

5.2.1 Vacuum Bottoms Handling

Pitch must be handled more carefully than most refinery products. The pitch pumpwhich 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 andso reducing the tendency of vapor to form in the suction line. It is also important thatthe 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 equipmentmust either be kept active, or flushed out with gas oil, when it is shut down. Steamtracing 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 efficientin this service. It is often desirable to send pitch to storage at high temperature tofacilitate 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 thewater 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 pitchis sent to storage at over 100 C, care should be taken to insure that the tank isabsolutely free from water. Pitch coolers should always be flushed out with gas oilimmediately once the pitch flow stops, since melting contents of a cooler is a slow

job.

5.2.2 Entrainment Control

The vapor rising above the flash zone will entrain pitch, which cannot be tolerated incranking unit charge. The vapor is generally washed with gas oil product, sprayedinto the slop wax section. The mixture of gas oil and entrained pitch is known asslop wax, and it is often circulated over the decks to improve contact, though thecirculation 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 thefresh gas oil is sprayed.



8/20


Most of the gas oil spray is revaporized by the hot rising vapors and returned up thecolumn. 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 thecharge. If the carbon residue or the metals content of the heavy vacuum gas oil ishigh, 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 theeffectiveness of entrainment control.

Slop wax is a mixture of gas oil and pitch, and it can be recirculated through theheater to the flash zone and reflashed, if the plant has the capacity to do so. If,however, crude contains volatile material compounds, these will be recycled with theslop wax and can finally rise into the gas oil. Where volatile metals are a problem, itis necessary either to yield slop wax as a product, or to make lighter asphalt, whichwill contain the metal compounds returned with the slop wax.

5.2.3 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 raisethe flash zone pressure. The most efficient method is to contact the hot vapors withliquid 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 exchangeagainst crude oil, so we must arrange to have the circulating liquid at a high enoughtemperature 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 pantemperature 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 rateuntil some of the vapors escape uncondensed. The problem of uncondensed vaporsis easily solved by adding a small circulating LVGO section to catch these vapors bycondensation against LVGO from a water cooler.



9/20


The heavy vacuum gas oil circulation rate is chosen to maximize crude heatexchange. The best way of doing this on an operating unit is to observe thetemperature of the crude leaving the crude / HVGO exchanger, then lower theHVGO circulation rate by 10%. If the crude temperature rises, the effect of thehigher HVGO pan temperature has been greater than the effect reduced circulation,and we should try some more of 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, andsome 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 arewasted. The HVGO product is cooled and pumped to storage on HVGO pan levelcontrol.

The LVGO section is a final contact condenser and normally the circulation rateshould be adequate to keep the vapor to the jets within about 5C of cooling water temperature. A high circulation rate will provide a cushion against upsets.

5.2.4 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/cm 2. Another means of measurement is to measurein millimeters of mercury. Atmospheric pressure (sea level) is 760 millimeters of mercury absolute while a perfect vacuum is 0 millimeters absolute. When vacuumare measured we can more conveniently do it by using millimeters of mercuryabsolute.

5.3 VACUUM FRACTIONATOR

The function of a vacuum tower (Figure 5.2) is to fractionate hydrocarbons that boilabove approximately 700F (370C) 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 totalreduction in absolute pressure of perhaps 28.7 psi from the bottom of the crudetower. This large difference in pressure enables a great deal of hydrocarbon to flashoverhead in the vacuum tower while maintaining a bottoms temperature notexceeding, for example, 730780F depending on the crude source. To further aid inremoval 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



10/20


vacuum tower is swaged down to decrease the time that the bottoms liquid spends atthe elevated bottoms temperature. A quench oil inlet line is also provided to protectthe 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 thevacuum column feed to be vapor. This also requires a special distributor called atangential distributor that imparts a swirling direction to the feed and preventsdamage to equipment above the distributor due to the rapid expansion of the feed asit enters the low pressure of the vacuum tower.

The internals in the vacuum column are designed for a minimum amount of pressuredrop, and the slop wax accumulator, the grid end demister pads are the only internalsthat extend completely across the entire column. The grid and demisters providecoalescing mediums to remove entrained liquid particles from the rapidly risingvapors. 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. Hotvapors pass through the cascade to re-vaporize the lower boiling components of theliquid. 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 non-

condensibles 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 materialthrough the top of the column is much less than at the side draws. In fact, too manylight ends in the feed or light ends formed by thermal decomposition of the bottomswould place an undue burden on the vacuum ejectors that have created and aremaintaining the low pressure on the vacuum column. Vacuum columns are generallydesigned to withstand an internal pressure of 50 psi (3.5 Kg/cm2 gauge) and a 14.7

psia (760 mm absolute) external pressure. To strengthen the vessel walls to work between these two pressures, stiffeners are used. These are merely rings weldedaround the column and spaced a few feet apart.



11/20


Figure 5.2 Vacuum Column



12/20


The materials of construction used in the design of a vacuum tower are killed carbonsteel for the vessel with the lower section clad with an 11-13% Cr. S.S. The slopwax accumulator is made of a 12% Cr. S.S. and the wall of the accumulator is linedwith concrete. The grid is constructed of 304 stainless steel. The upper demister padis constructed of Monel. Side-by-side pans are constructed of 12% Cr. S.S. Theremainders are constructed of carbon steel.

Designs for vacuum columns with different corrosion severities may allow theelimination of alloy cladding and some of the alloy in side-by-side pans andaccumulators.

Flash Towers

Flash towers are used in services where good fractionation of the charge is notnecessary. Their main purpose is to obtain the maximum amount of overhead

product for a given transfer-line temperature. The feed material is heated to thedesired temperature and the mixture of vapors and liquids is introduced into the flashspace of a vessel.

The liquid remaining after flashing falls to the bottom, usually through baffles to aidin separation and the vapors go overhead. Normally, an operation of this nature isused to remove asphalts and tar from crude before processing. Tar separators and tar

stripper are the common names used for this type of tower.

5.4 STEAM JET EJECTOR

5.4.1 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 usedin the laboratories. The vacuum system is used to remove vapors from the systemwhich 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 vacuumconditions. Their wide acceptance is based upon their having no moving parts andrequiring very little maintenance.

These vapors consist of non-condensible hydrocarbon vapors and air coming into thesystem 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 motivatingmaterial. A series of jets (normally three) is used to boost the gases from the



13/20


pressure of the vacuum tower to atmospheric pressure. The steam used to pull thegases and is condensed in each stage and removed as water. Figure 5.3 is aschematic 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. Atany height lower than 34 ft. the water must be pulled off with a pump.

Figure 5.3 Schematic Diagram of Vacuum Jet System

Barometric systems are generally controlled by changing the water flow to thecondenser 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 thequantity 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 thequantity of non-condensed gases exceeds the capacity of the jet and causes thevacuum to fail off.

5.4.2 Operating Principle

Figure 5.4 illustrates a typical two-stage ejector system. In an ejector the steam isinjected at high velocity through a specially designed nozzle (Figure 5.5) andtransfers sufficient energy to the gases from the suction header to entrain themthrough the diffuser into the first-stage discharge header. The pressure in the first-



14/20


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 toatmospheric pressure at which they can be vented, the steam is condensed and asecond 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 onlyoperate ever a relatively limited range. A substantial change in suction or dischargeconditions will probably demand a change in the dimensions of the nozzle or diffuser, or both. The effect of changes in operating conditions can be summarized asfollows:

5.4.2.1 Steam Pressure

Must be maintained quite close to that for which the equipment was designed. If thesteam pressure greatly exceeds that for which the nozzle was designed, the quantityof steam discharging into the diffuser will be greater than can pass through thediffuser, and steam will back into the suction header. Too low a steam pressure willmean 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.

5.4.2.2 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 toatmosphere is only possible if the discharge is obstructed. But on multi-stage unitsan increase in interstage pressure due to high condensate temperatures or failure of asecond or third-stage ejector will immediately affect the performance of the first-

stage unit.

5.4.2.3 Load

A decrease in load (kg/hour of vapor to ejector) will result in a somewhat higher vacuum being obtained, but if the load is increased above design, the vacuumobtained will fall off quite suddenly and dramatically.



15/20


Figure 5.4 Typical Arrangement Vacuum Unit 2-Stage Jets



16/20


Figure 5.5 Cut-Away View of Steam Jet Ejector



17/20


5.4.2.4 Cooling Water Temperature

The temperature at which the steam is condensed in the inter and final condenserswill have a relatively minor effect on the vacuum obtained, but will substantiallyreduce the load at which the ejector system breaks down, since an increase incondensate temperature increases the inter-stage pressure.

In order to insure flexibility a refinery ejector system for a vacuum unit willgenerally be constructed using two parallel sets. The minimum combination of equipment which will achieve a satisfactory vacuum is normally used. The vaporsdrawn 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, andany hydrocarbons lighter than gasoline which have not been stripped from thecharge. The steam and light hydrocarbons will condense in the inter-condenser sothat the first-stage ejectors can be heavily loaded under conditions which only lightlyload the second-stage ejectors. Any actual cracking in the furnace will produce lightgases which will very rapidly overload the second-stage ejectors. Both thecondensable 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.

5.5 VACUUM TOWER CONTROL SYSTEM

Steam pressure below a critical value of a jet will cause the ejector operation to beunstable. Therefore, it is recommended to install a pressure controller on the steamto keep it at the optimum pressure required by the ejector.

The recommended control system for vacuum distillation is shown in Figure 5.5. Air or gas is bled into the vacuum line just a head of the ejector. This makes themaximum 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.



18/20


Figure 5.6 Vacuum Column Pressure Control

5.6 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 thatthis 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 closeend 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 theequipment name plate. As soon as a level appears in the inter-condenser, startthe condensate pump and place the level on control.



19/20


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 themost stable operation on a given ejector set. On most two-stage units, this is about260-130 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. hasincreased), an additional second-stage jet should be added. If the interstage pressureis unchanged, but the lower pressure has risen, an additional first-stage jet should beadded 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.

c. Check that the nozzle steam pressure is under good control.

d. Open the ejector suction valve.

5.7 Troubleshooting

Occasionally, ejectors will fail to pull an adequate vacuum or will performerratically. This can be the result of a large number of troubles, a few of which arelisted 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 packingglands. Plug all vents and drain valves and tighten any screwed connection.Check that pump gland flushing is adjusted to maintain a positive pressureon 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 fromthe second stage.



20/20


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 thegland.

l. Faulty Installation: Failure to properly align gaskets and similar detailswhich are normally insignificant can trap condensate pockets or causeturbulence which can affect the performance of vacuum equipment. If anejector set has 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 pressurelower than the vapor pressure of a liquid in the system. If a very littlevacuum 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 hightemperature 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 andcan 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 non-condensable being vented and these vent gases can be analyzed in the laboratory.

Date post:	14-Apr-2018
Category:	Documents
Upload:	ahmed-mohamed-khalil
View:	225 times
Download:	1 times

C 5 Vacuum Distillation

Documents