Vacuum Distillation (Ok)

7/27/2019 Vacuum Distillation (Ok)

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1. I ntroduction


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In order to maximize the production of gas oil

and lighter components from the bottomsmaterial 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


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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 50 mm.wouldnot boil until about 500 C at atmospheric

pressure, at which temperature most

hydrocarbons crack.


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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.


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Now if boiling will begin when the vaporpressure 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 alower temperature.


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The vacuum unit differs from the atmospheric

type in that it has a fractionating column oflarger diameter with bubble trays farther apart.

This is necessary because much larger volumes

of vapors have to be handled because of thelower pressure.

Any sudden increase in vacuum will expand the

volume of the vapor rapidly and possibly result

in puking the tower.


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In the vacuum unit, almost no attempt is made

to fractionate the products.

It is only desired to :

Remove the entrained pitch,

Vaporize the gas oil , and

Condense the liquid product.

as efficiently as possible.


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Bottoms from the crude tower contain material

that can be charged to:

Catalytic cracking unit, or

Be used for lube oil stocks .


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Distilling this material at atmospheric pressurewould 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 formationin pipes and vessels.

For these reasons we conduct the distillation ofthe heavy reduced crude under vacuum in thevacuum 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 use random packing

and demister pads to keep the vapor velocities

low a large diameter tower is used.


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An actual operating

vacuum tower as shown

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.


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2. Reduced Crude

Flashing


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The reduced crude is charged through a heater

into the vacuum column in the same manner aswhole crude is charged to an atmospheric

distillation unit.

However,whereas the flash zone of an atmospheric

column may be at 11.3 kg cm2,

the pressure in a vacuum column is very muchlower


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The heater transfer and flash zone temperatureare generally varied to meet the vacuum

bottoms specification, which is probably either:

Gravity or viscosity for fuel oil or Penetration specification for asphalt.


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The penetration of an asphalt is:

The depth in 1/100 cm, which a needle

carrying a 100gweight sinks into a sample at

77F in 5 seconds.

So that the lower the penetration the heavier

the pitch.

Very heavy pitches are called asphal ts.


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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 th is occurs,

it is necessary to lower the temperature.

And if a heavier bottoms product is stillrequired, an attempt should be made to obtain a

better vacuum instead.


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Slight cracking may occur without breaking the

vacuum.

This is sometimes indicated by a positive result

from the Oliensis Spot Test.

The Oliensis Spo t Test

Is a simple laboratory test which purports to

indicate the presence of cracked componentsby the separation of these components when a

20% solution of asphalt in naphtha is dropped

on a filter paper.


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Some crudes always yield a 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, andThe time during which the oil is exposed

to that temperature.


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The level of pitch in the bottom of the tower

should be held at a minimum, and itstemperature 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 rises the column, vacuum

falls because of cracking due to increased

residence time.


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The flash zone temperature will vary widely

depending upon:

The crude source pitch specifications,

The quantity of product taken overhead,and

The flash zone temperatures from below

315 to even 425 C have been used incommercial operations.


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2.1 Vacuum Bottom s Hand l ing

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.


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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.

The pitch pump glands be sealed in such a

manner so as to prevent the entry of air.


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Since most pitches are sold at atmospheretemperatures.

All pitch handling equipment must either be:

Kept active, or

Flushed out with gas oil when it is shutdown.

Steam tracing alone is sometimes inadequate to

keep the pitch fluid, but where this is done, thehighest pressure steam available should beused.


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It is often desirable to send pitch to storage at

high temperature to facilitate blending.

It is sometimes cooled in open box units, while

shell and tube units are not efficient in this

service.

If it is desired to increases the temperature of

the pitch:

It is betterto do so by lowering the level

of water in the open box, and not by

lowering the water temperature.


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If the water in the box is too cold:

Pitch can solidify on the inside wall of thetube 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 100 C,

insure that the tank is absolutely free from water.


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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.


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2.1 Entrainment Con trol

The vapor rising above the flash zone willentrain pitch, which cannot be tolerated incranking unit charge.

The vapor is generally washed with gas oil

product, sprayed into the slopwax section.

Slop Wax

The mixture of gas oil and entrained pitch.

it is often circulated over the decks to improve

contact.


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slop wax can also be re-circulated through the

heater to the flash zone and re-flashed, if the

plant has the capacity to do so.

If, however, a crude contains volatile material

compounds, these will be recycled with the slop

wax and can finally rise into the gas oil .


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Where volatile metals are a problem, it isnecessary either:

To yield slop wax as a product, or

To make a lighter asphalt.

which will contain the metal compounds

returned with the slop wax.


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Some slop wax must be;

Yielded in order to reject the capturedentrainment..

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 re-vaporized by the

hot rising vapors and returned up the column.

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 .

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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.

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2.3 Produc t Condensat ion

The scrubbed vapor rising above the demister

blanket is the product, and no furtherfractionation is required.

It is only desired to condense these vapors as

efficiently as possibly.

This could be done in a shell and tubecondenser.

But these are inefficient at low pressures, and the

high pressure drop through such a condenser

would raise the flash zone pressure.

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The most efficient method is to contact the hotvapors with liquid product which has been

cooled by pumping through heat exchangers.

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It is further desired to usefully recover the heat

of the rising vapors by heat exchange againstcrude oil,

So we must arrange to have the circulating

liquid at a high enough temperature to permitefficient heat exchange.

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We therefore have to compromise.

If the gas oil circulation is high enough tocondense 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 thecirculation rate until some of the vapors escape

uncondensed.

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The problem of uncondensed vapors is easilysolved by:

Adding a small circulating LVGO section to

catch these vapors by condensation

against LVGO from a water cooler.

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HVGO circu lat ion rate

The HVGO circulation rate is chosen to maximizecrude 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% .

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If the crude temperature rises

The effect of the higher HVGO pan temperaturehas been greater than the effect reducedcirculation, and we should try some more of the

same %age.

If the crude temperature is lower

We should try a 10 percent change in theopposite direction.


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The HVGO product is cooled and pumped to

storage on HVGO pan level control.

The LVGO section is a final contact condenser.

Normally the circulation rate should be adequateto keep the vapor to the jets within about 5C of

cooling water temperature.

A high circulation rate will provide a cushionagainst upsets.

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2.4 Vacuum Pressu re MeasurementConfusion often arises because of the differentscales used to measure vacuum.

Positive pressures are commonly measuredas:

kilograms per square-centimeter gaugeabove atmospheric pressure.

Atmospheric pressure is:

1.035 kg/cm2

760 millimeters of mercury absolute.

While a perfect vacuum is 0 millimeters

absolute.

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3.Vacuum

Fractionator

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The function of a vacuum tower:

Is to fractionate hydrocarbons that boil above700F (370C) in the crude tower.

Vacuum Column Pressu re

The pressure can be reduced to around 1.0 psia,

below the slop wax tray.

This is a total reduction in absolute pressure ofperhaps 28.7 psi from the bottom of the crude

tower.

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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 780F

(388 415C) depending on the crude

source.

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Str ipp ing Steam

Can be injected into the

bottom boot of thecolumn to:

o Further aid in removal ofusable products from the

bottoms material,

o Help produce properpenetration asphalt, and

o Decrease the partialpressure of the bottoms

liquid.


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Quench oi lA quench oil inlet line is

also provided to protect

the bottoms pumps.

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Feed Line

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.

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Tangent ia l d istr ibu tor

Also requires a special distributor called atangential distributorthat:

Imparts a swirling direction to the feed and

Prevents damage to equipment above thedistributor.

Due to the rapid expansion of the feed as it

enters the low pressure of the vacuum tower.

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Internals

Are designed for a

minimum amount ofpressure drop.

The only internals thatextend completelyacross the entire columnare:

The slop wax

accumulator, The grid pad, and

Demister pad.

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The grid and demisterpads provide coalescing

mediums:

To remove entrained

liquid particles from

the rapidly rising

vapors.

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Spray dist r ibutors

Spray distributors

are used to aid the

grid and demisters

in coalescing.

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Side by Side Pans

There are no of trays in

this vacuum tower.

But what appears to be

trays are side-by-side

pans.

Their outer edges

Perforated byholes, and

Stiffened by metal

lattice.

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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 boilingcomponents of the liquid.

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Accumulator trays are designed to providea vapor-free liquid to the suction of side

draw pumps.

Pump vents are returned to the column toallow removal of non-condensable from the

pump during startup.

This helps a great deal in getting the pumpstarted.

After the pump is pumping properly, the vent

should be closed.

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Top sect ion

The top section of the

vacuum column is swaged

down because the traffic ofmaterial through the top of

the column is much less

than at the side draws.

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In fact, too many light ends

in the feed or light endsformed by thermal

decomposition of the

bottoms would place an

undue burden on thevacuum ejectors that have

created and are

maintaining the low

pressure on the vacuum

column

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Vacuum columns are generally designed to

withstand an internal pressure of 50 psi (3.5Kg/cm2 gauge), and an external pressure of14.7 psia (760 mm absolute)

To strengthen the vessel walls to work betweenthese two pressures

st i f feners are used

These are merely rings welded around the

column and spaced a few feet apart.

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Mater ials of construct ion

The materials of construction used in the design

of a vacuum tower are:

The vessel with

killed carbon steel.

The lower sectionclad with an 11-13% Cr S.S.

The slop wax accumulatoris made of a

12% Cr S.S.The wall of the accumulatoris

lined with concrete.

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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 remainderare constructed of

carbon steel.

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4. Steam Jet

Ejector

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4.1 In troduc tion

4.2 Operat ing Princ ip le

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4.1 In troduct ion

Vacuum is maintained by two generalmethods:

Vacuum Jets, and

Vacuum pumps Jets.

Vacuum jetsare used extensively in yardequipment, whereas

Vacuum pumps are widely used in the

laboratories.

The vacuum system is used to remove vapors

from the system which cannot be condensed.

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Steam jet ejectors are commonly used in

distillation units and 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.

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Vacuum jets pull gases

from the tower by using

air, steam, or waterin the

jet.

The jets generally use

steam as the motivatingmaterial.

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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.

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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 outby gravity.

At any height lower

than 34 ft. the water

must be pulled off with a pump.

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Barometric systems are generally controlled by:

Changing the water flow to the condenserfor the first-stage jet.

Varying steam to the jet.

Varying steam 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.

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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.

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4.2 Operat ing Princ ip leFor two-stage ejector system.

The steam is injected

at high velocity through a

specially designed nozzle

(next Figure) and transfers

sufficient energy to the

gases from the suction

header to entrain themthrough the diffuser into

the first-stage discharge

header.

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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.

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If a single ejector is incapable of raising thegases 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.

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The effect of changes in operating conditions

can be summarized as follows:

Steam Pressure,

Discharge Pressure,

Load, and

CoolingWater Temperature.

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Steam Pressure

If the steam pressure greatly exceeds that forwhich the nozzle was designed:

The quantity of steam discharging into thediffuser will be greater than can passthrough the diffuser, and

Steam will back into the suction header.

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NOTES

Too low a steam pressure will mean,a drastic loss in performance of theejector.

Wet steam will cause: Random fluctuations in ejector

performance, and

Will erode the nozzle and diffuser.

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Discharge Pressure

If the discharge pressure rises above design insingle stage ejector, there is an increasing

probability of reverse flow.

But in 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 theperformance of the first-stage unit.

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Load

A decrease in load (kgs/hour of vapor toejector) will result in a somewhat

Higher vacuum being obtained

But if the load is increased above design

The vacuum obtained will fall off quitesuddenly and dramatically.

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Coo l ing Water Temperatu re

The temperature at which the steam iscondensed 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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In order to insure flexibility a refinery ejectorsystem for a vacuum unit will generally beconstructed using two parallel sets.

The minimum combination of equipment which

will achieve a satisfactory vacuum is normallyused.

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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,

Sulfur and nitrogen compounds from

thermal decomposition in the heater, and

Any hydrocarbons lighter than gasolinewhich have not been stripped from the

charge.

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The steam and light hydrocarbons will

condense in the inter-condenser so that:

The first-stage ejectors can be heavilyloaded, andLightly load the second-stage ejectors.

Any actual cracking in the furnace:

will produce light gases

which will very rapidly overload thesecond-stage ejectors.

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Both the condensable and non-condensable

vapors handled by a typical vacuum unitejector set are highly odiferous so that:

The condensate must be stripped, and

The non-condensable vapors incinerated.

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Vacuum5. Tower

Control System

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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.

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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.

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A pressure control valve regulate 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.

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6. Startup

Vacuum DistillationWhen starting a vacuum unit it is common


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gpractice to

S team out the heater and tower and thenPressure test the tower with steam atabout 50 psig

It will be assumed that this has been done andthat all valves around the ejectors are closed.

a. Shut off steam to the tower.

b. Vent steam from top of tower until thepressure is about 0.2 Kg/cm2, then close

end plug the top vent.

Vacuum Distillation

O th i l t d tl t l f f


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c. Open the inlet and outlet valves of one of

the second-stage ejectors.

d.Open the inlet and outlet valves of one ofthe 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 thesteam pressure to that given on the

equipment name plate.

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As soon as a level appears in the inter-

condenser, start the condensate pump andplace the level on control.

h. Set the ejectors run until a constant

vacuum is obtained even though thepresence of water in the tower mayresult in a poor initial vacuum.

i. Charge the vacuum unit and proceedwith normal operations.

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Add ing Addi t ional 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 130mm Hg. absolute vacuum.

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When the tower vacuum:

Either decreases, or

Becomes sensitive to process conditions.

Additional ejector capacity should be added.

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If the interstage pressure has risen


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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 pressure is unchanged

but the lower pressure has risen

An additional first-stage jet should be added andthis may render the addition of another second-

stage jet necessary.

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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.

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7. Troubleshooting

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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:

Vacuum Distillation

1 A ir Leaking In to the Sys tem :


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1. A ir Leaking In to the Sys tem:

Hot bolt all flanges and manways on the vacuum

tower and on the overhead line.

Tighten and oil all valve packing glands.

Plug all vent and drain valves and tighten

any screwed connection.

Check that pump gland flushing is adjusted

to maintain a positive pressure on the

gland.

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2. A ir Leaking In to Interstage:

This will be confirmed by a rise in interstagepressure.

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.

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3. 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.

4. Wrong Steam Pressure:

Check ejector name plate data andChange pressure controller setting

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5. Wet Steam:

Causes erratic performance.

Check performance of steam traps.

6. Worn Nozzles and Diffusers :

The result of using wet steam.

Vacuum Distillation7. Clogged Steam Filters:


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gg

There is generally

A main filter ahead of the steam pressurecontroller, and

A filter in the nozzle of each ejector.

8. High Condensate Temperatu res:

The result of:

Insufficient cooling water flow, or

Fouling of either the tube or the shell sideof the condensers.

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9. Flooded Condensers:

The result of malfunctions of:

The level controller, or

The condensate drain trap, orPump failure.

If the pump will not hold suction:

Check that air is not leaking in at the gland.

Vacuum Distillation

10 Fau lty Ins tallation :


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10. Fau lty Ins tallation:

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 vacuumequipment.

Vacuum DistillationIf an ejector set has been dismantle:


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If an ejector set has been dismantle:

Each nozzle must of course be reinstalled

in the correct diffuser.

11. Back Pressure:

Due to:

Deposits in the condensers,

Plugged flame arrester in the vent,

Condensate pocket or other obstruction.

Vacuum Distillation

12 Impossib le Operat ion :


110/112

110

12. Impossib le Operat ion :

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 lightthat it will be impossible to pull a vacuum on thetower.

Vacuum Distillation

13 High Tower Bottom Level :


111/112

111

13. 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.

14. Steam Entering Sys tem:

Check:

Steam-out connections on the tower,Heater,

Exchangers, etc.

Vacuum Distillation15. Crack ing in Furnace:


112/112

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 non-condensable being vented and

These vent gases can be analyzed in the

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Vacuum Distillation (Ok)

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