EPE_course_presentation_002.pptx

Eng.Said Elsayed
Elementary process engineering

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Elementary Process Engineering
Eng.Said Elsayed
Course Overview
1. Introduction
3. Basics of Oil Refining process
2. Fundamentals of gas processing
4. Towers internals
7. Tanks & Valves
8. Pumps
9. Heat Exchangers & Fired
Heaters
5. Conversion & Rearrangement
processes
6. Safety in refinery
10. Compressors

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Eng.Said Elsayed
Course Overview
1. Introduction
3. Basics of Oil Refining process
2. Fundamentals of gas processing
4. Towers internals
7. Tanks & Valves
8. Pumps
9. Heat Exchangers & Fired
Heaters
5. Conversion & Rearrangement
processes
6. Safety in refining
10. Compressors

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Session 5 : Distillation & Tower Internals
Distillation
Separation by distillation implies a
difference in boiling points of two or
more materials.
The components or compounds
making up crude oil or natural gas are
numbered in thousands
It is difficult to separate some pure
compounds from the complex mixture
of components in crude oil by
distillation alone.

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There are other methods of
separation used in a refinery for
example:
 Extraction with a solvent,
 Crystallization, and
 Absorption.
Fortunately, rarely need pure
compounds and it is often enough to
separate groups of compounds from
each other by boiling range.

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Eng.Said Elsayed
If we separate many compounds in
crude oil into groups we find that these
groups have characteristics that make
them considerably more valuable than
the whole crude oil.
Some of these groups are products.
Some may be feedstock to other
processing units where they are
chemically changed into more valuable
products.
These products, are usually separated or
purified by distillation.

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Principles of Distillation
The basic principle of distillation is
simple:
1. When a solution of two or
more components is boiled,
2. The lighter component (the
one most volatile or the one
with the greatest tendency to
vaporize) vaporizes
preferentially.

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Two component mixture is contained in a vessel.
When heat is add, the more volatile material
( red dotes ) start to vaporize.
The vapor contains
A higher proportion
of red dots than
dose the original
Liquid.

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This results in:
The vapor above the liquid being
relatively rich in the lighter (more
volatile material).
And the liquid is left with
proportionately more of the less
volatile (heavier liquid).
Thus a separation, to some degree,
has taken place.

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• Light Material
+ Heavy Material

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Suppose we add two more stages of
distillation.
Although this is accomplishing our
goal of increasing the purity of the
light friction, we are also making large
amounts of the intermediate product,
each of which contains the same light
friction.

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Tower Sections
We have described staging for the
purpose of concentrating the lighter
component in the overhead.
The same principles apply to
concentrating the heavier component
in the bottom product.
The upper two stages are called
rectifying stages.
These below the feed are called
stripping stages.

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The upper rectifying section
increases the purity of the overhead
product.
The lower stripping section
increases the recovery of the
overhead product.
In many cases, the bottom product
is the one of primary interest.
For the bottom, or heavy, product
the rectifying section improves
recovery.

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Rectification and Stripping
The section of the distillation column
below the feed plate is known as the
stripping section. In this section the
more volatile component is removed
(stripped) from the liquid (on each
plate) by the vapor rising from the
boiler.
The section above the feed plate is
known as the rectification section. In
this section the purity of the vapor
gradually improves as the higher boiling
point fractions condense out and join
the liquid on each plate.

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TYPES OF DISTILLATION COLUMNS
One way of classifying distillation column
type is to look at how they are operated.
Thus we have: batch and continuous
columns.
• Batch Columns In batch operation, the
feed to the column is introduced
batch‐wise. That is, the column is charged
with a 'batch' and then the distillation
process is carried out.

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Batch distillation has always been an
important part of the production of
seasonal, or low capacity and high-
purity chemicals. It is a very
frequent separation process in
the pharmaceutical industry and
in wastewater treatment units

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Continuous Columns In
contrast, continuous columns
process a continuous feed stream.
No interruptions occur unless
there is a problem with the column
or surrounding process units.
They are capable of handling high
throughputs and are the most
common of the two types. We shall
concentrate only on this class of
columns.

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Types of Continuous Columns:‐
Continuous columns can be further
classified according to: ‐
The nature of the feed that they are
processing:‐
– Binary column ‐ feed contains only two
components
– Multi‐component column ‐ feed
contains more than two
components
The second classification is based on
the number of product
streams they have:‐
Multi‐product column ‐ column has
more than two product streams

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Classified based on the
type of column internals:
• Tray column ‐ where trays of
various designs are used to hold
up the liquid to provide better
contact between vapor and liquid,
hence better separation packed
column.
• Packing instead of trays are used
to enhance contact between vapor
and liquid.

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Packed column are called
continuous contact column
while tray column are called
staged-contact column because
o the manner in which vapor
and liquid are contacted

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Main Components
A typical distillation contains several
major components:
1. A vertical shell where the
separation of liquid components is
carried out.
2. Column internals such as
trays/plates and/or packing , which
are used to enhance component
separations.

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3. A re‐boiler to provide the
necessary vaporization for the
distillation process a condenser to
cool and condense the vapor leaving
the top of the column.
4. A reflux drum to hold the
condensed vapor from the top of
the column

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The type of column internal used
depends on the application.
The considerations being :
 Purity of feed,
 Efficiency,
 Capacity,
 Reliability,
 Pressure drop,
 Liquid holdup, and
 Cost.

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Reflux
The word reflux is defined as: "flowing back“
Applying it to distillation tower, reflux is:
The liquid flowing back down the tower from each
successive stage.

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Kinds of Reflux
 Cold Reflux
 Hot Reflux
 Internal Reflux
 Circulating Reflux

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Cold Reflux
Cold reflux is defined as:
Liquid that is supplied at temperature a
little below that at the top of the tower.
Each pound of this reflux removes a
quantity of heat equal to the sum of its:
latent and sensible heat
required to raise its temperature from
reflux drum temperature to the
temperature at the top of the tower.

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Hot Reflux
It is the reflux that is admitted to the
tower at the same temperature as that
maintained at the top of
the tower.
It is capable of removing the latent heat
because no difference in temperature is
involved.

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Internal Reflux
It is the liquid that overflow from one plate to another in the
tower,
and may be called hot
reflux because it is
always substantially
at its boiling point.
It also capable of
removing the latent
heat only because no
difference in temperature
is involved.

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Circulating Reflux
It is able to remove only the sensible
heat which is represented by its
change in temperature as it circulates.

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Reflux Ratio
It is defined as the amount of reflux
divided by the amount of top
product.
Plant operators usually obtain the
reflux ratio by dividing actual reflux
by the top product.
It is denoted by R which equals L/D.

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The Importance of Reflux
Ratio
In general, increasing the reflux:
Improves overhead purity, and
Increases recovery of the
bottom product.
The number of stages required for a
given separation will be dependent
upon the reflux ratio used.

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Eng.Said Elsayed
1. A minimum number of plates (stages)
required at total reflux
2. There is a minimum reflux ratio below
which it is impossible to obtain the
desired enrichment however many
plates are used.
3. TURNDOWN ratio of a column is an
indication of the operating flexibility. If
a column, for example, has a turndown
ratio of 3, it means that the column can
be operated efficiently at 33% of the
maximum design throughput.
points to consider

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Total Reflux
Total reflux is the conclusion when all the
condensate (distillate) is returned to the
tower as reflux, no product is taken off
and there is no feed.
At total reflux, the number of stages
required for a given separation is the
minimum at which it is theoretically
possible to achieve the separation.
Total reflux is carried out at:
1. Towers start-up.
2. Testing of the tower.

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Minimum Reflux
At minimum reflux, the
separation can only be achieved
with an infinite number of stages.
This sets the minimum possible
reflux ratio for the specified
separation.

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Optimum Reflux Ration
Practical reflux ratio will lie between:
The minimum for the specified
separation and Total reflux
The optimum value will be the one at
which the specified separation is
achieved at the lowest annual cost.
For many systems, the optimum value of
reflux ratio will lie between:
1.2 to 1.5 times the minimum reflux
ratio

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Reboiling
In all distillations processes
Heat being added by:
 Feed, and Steam
 Reboiler.

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The Reboiler is a heat exchanger through
which the bottom liquids circulate.
Heat is transferred to the bottom
materials which cause vaporization of the
lighter components.
This vapor travels up the column to
provide:
The stripping action, and
The additional heat necessary
vaporize the down coming reflux.

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Fractionation
in Gas Processing

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Process Introduction
The Fractionation Unit consists of four fractionation
columns in series:
 a De-methaniser
 a De-ethaniser
 a De-propaniser
 a De-butaniser
The unit is closely integrated with the Scrub Section in
order to achieve optimum cold economy and a minimum
processing/recycle of 'excess' components.

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Eng.Said Elsayed
De-methaniser - Methane from the top of the column is sent to
H.P Fuel Gas. Column bottom feeds the De-ethaniser.
De-ethaniser - Ethane from the top of the column is returned to
the column as refux, Column bottom feeds the De-propaniser.
De-propaniser - Propane from the top of the column is returned
to the column as refux, sent to Refrigerant Storage, or sent for
re-injection. Column bottom feeds the De-butaniser.
De-butaniser - Butane from the top of the column is returned to
the column as refux, or sent for re-injection. Column bottom is
sent to Condensate Storage.

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De-methaniser

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Demethanizer
A distinguishing feature of gas
plants with high ethane-recovery
rates is the columns in the
following ways:
• It has an increased diameter at
the top to accommodate the
predominately vapor feed to the
top tray.
• It is typically primarily a stripping
column, with no traditional
condenser–reflux stream.

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• It may have several liquid feed
inlets further down the column
that come from low-temperature
separators.
• It has a large temperature
gradient; over 170°F (75°C) is
common.
The column serves two main
functions: it acts as a flash drum
for the top feed, which comes in as
a cold, two-phase stream, and it
removes methane

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To obtain high ethane recovery, the
column must operate near the low
end of the temperature range,
which translates into a higher
compression load.
The demethanizer usually has 18 to
26 trays, which operate at a tray
efficiency of 45 to 60%

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Sales Gas
FEED
DEA
Chilling &
Turbo-expander
Compression
Drying & Sweetening
Boosting
Fractionation
De-Ethenizer
De-Butanizer
Mol.
Sieves
LPG
Cond.
Gas Plant

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stabilizer

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A petroleum refinery is a collection of unit
operations, such as fractionation towers,
pumps, and heat exchangers. Analysis and
design of these units require knowledge of the
thermodynamic and physical properties of the
petroleum fluids.
The Nature of Petroleum
Crude oil is a mixture of hydrocarbons with
between 100,000 to 1,000,000 different
molecules contributing to a boiling range well
over 1,000o F.
At the lower end are the gaseous products,
methane and ethane and various inorganic
gases such as hydrogen sulfide and carbon
dioxide.

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over the years chemical and
petroleum refining engineers have
developed special methods or
correlations to estimate the
properties of petroleum fraction
from easily measured properties
like normal boiling point (NBP) and
specific gravity.

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The characterization of petroleum fractions
requires several measurable laboratory
properties:
• Specific gravity (SG)
• Boiling point curve (ASTM or true boiling
point (TBP distillation)
• Kinematic viscosity at 37.8 C (100 F) and
98.9 C (210 F) (n100, n210)
•Molecular weight (M)

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Specific Gravity:
Specific gravity for liquid oils is defined as:
where roil is the oil density and water is the
water density.
Both densities of oil and water are at the
same standard temperature and pressure
conditions, which are 1 atm (14.7 psia) and
15.6 C (60 F).
Since under the same conditions, most
petroleum fractions are lighter than water

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Initial Boiling Point (IBP)
Is the temperature at which the first
drop of condensate is collected during a
laboratory distillation test.
In a mixture of hydrocarbons, the first
molecules to vaporize are the light ones.
So, the IBP test is used to check for light
hydrocarbons that are present in a
product.

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End Boiling Point (EP)
Is the temperature at which the last drop
of liquid vaporizes during the test.
In a mixture of hydrocarbons, the last
molecules to vaporize are the heavy ones.
So, the EBP (EP) test is used to check for
heavy hydrocarbons that are present in a
product.

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ASTM Distillation
ASTM distillation is carried out in a
relatively simple apparatus
consisting of a flask holding the
sample connected to an inclined
condenser, which condensed the
rising vapours. The fractions
distilled are collected in a
graduated cylinder.
The temperature of the rising
vapours is recorded at specific
interval of the collected distillates.

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The high degree of fractionation in this
test gives an accurate component
distribution.
Because the degree of separation for a
TBP distillation test is much higher than
that of the ASTM distillation test, its IBP is
lower and its EP is higher than those of
the ASTM test.
The TBP curve (a plot of the TBP versus
the percent volume of sample distilled)
True Boiling Point Distillation

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where M is the molecular weight of the petroleum fraction, Tb
is the mean average boiling point of the, and SG is the specific
gravity, 60 F/60 F.
Molecular Weight

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Viscosity is a measure of a fluid's
resistance to flow
Dynamic (absolute) Viscosity
Absolute viscosity or the coefficient of
absolute viscosity is a measure of the
internal resistance
For practical use the Poise is to large and it's
usual divided by 100 into the smaller unit
called the centiPoise (cP) where
1 p = 100 cP
1 cP = 0.01 poise = 0.01 gram per cm second
= 0.001 Pascal second = 0.001 N.s/m2

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Kinematic Viscosity
is the ratio of absolute or dynamic viscosity to
density .Kinematic viscosity can be obtained by
dividing the absolute viscosity of a fluid with it's
mass density
ν = μ / ρ
In the SI-system the theoretical unit is m2/s or
commonly used Stoke (St) where
1 St = 10-4 m2/s
Since the Stoke is an unpractical large unit, it is
usual divided by 100 to give the unit
called Centistokes (cSt) where
1 St = 100 cSt
1 cSt = 10-6 m2/s

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Saybolt Universal Seconds
(or SUS, SSU)
Saybolt Universal Seconds (or SUS) is
used to measure viscosity. The efflux
time is Saybolt Universal Seconds (SUS)
required for 60 milliliters of a
petroleum product to flow through the
calibrated orifice of a Saybolt Universal
viscometer, under carefully controlled
temperature and as prescribed by test
method ASTM D 88

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Crude distillation unit (CDU)
is at the front-end of the refinery, also
known as topping unit, or atmospheric
distillation unit. It receives high flow
rates hence its size and operating cost
are the largest in the refinery.
Many crude distillation units are
designed to handle a variety of crude oil
types.
The design of the unit is based on a light
crude scenario and a heavy crude
scenario.

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By distillation at atmospheric
pressure, crude oil can be
separated into:
 Fuel Gases
 LPG,
 Gasoline,
 Jet Fuel (Optional),
 Kerosene,
 Gas oil,
 Diesel oil, and
 Residue (Fuel Oil)

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Typical products from the unit are:
Gases
Light straight run naphtha (also called light gasoline or light
naphtha)
Heavy gasoline (also called military jet fuel)
Kerosene (also called light distillate or jet fuel)
Middle distillates called diesel or light gas oil (LGO)
 Heavy distillates called atmospheric gas oil (AGO) or heavy gas
oil (HGO)
Crude column bottoms called atmospheric residue or topped
crude.

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Crude is generally pumped to the
unit directly from a storage tank,
and it is important that charge tanks
be drained completely free from
water before charging to the unit.
If water is entrained in the charge
It will vaporize in the exchangers
and in the heater, and cause a high
pressure drop through that
equipment.

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If a slug of water been charged to the
unit, the quantity of steam generated
by its vaporization is so much greater
than the quantity of vapor obtained
from the same volume of oil, that the
trays in the fractionating column
could be damaged.
Water expands in volume 1600 times
upon vaporization at 100ºC at
atmospheric pressure.

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Process Description
Heat Exchange
In order to reduce the cost
of operating a crude unit.
As much heat as possible is
recovered from the hot
streams by heat exchanging
them with the cold crude
charge.

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A record should be kept of heat exchanger outlet temperatures
so that fouling can be detected and possibly corrected before
the capacity of the unit is affected.

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The heater transfer temperature
is merely a convenient control,
and the actual temperature,
which has no great significance,
will vary from 325ºC to as high as
430ºC, depending on:
 The type of crude, and
 The pressure at the
bottom of the fractionating
tower.

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Since the atmospheric tower does not
have a reboiler, the heat content of the
furnace supports the total vapor rate
to the column plus additional duty
called overflash.
Heat removal in the tower is
accomplished by condensation of
vapor with liquid cooled in
pumparounds.
Depending on the crude slate,
perhaps half or more of the crude is
flashed.

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Heater Outlet and Transfer Line
Heater outlet temperature is limited to
approximately 750°F by thermal cracking of the
feedstock, which impairs distillate product
smoke points and color.
Depending on the crude, this temperature may
range from 700°F to 800°F. But cracking of
paraffinic and naphthenic crudes occurs at
approximately 650°F - 700°F.
The outlet from the furnace is directed to the
flash zone in the fractionation tower via a
transfer line. The

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Overflash
The furnace is normally operated to produce
overflash.
Overflash is defined as vaporization in
excess of requirements for lifting all of the
products taken overhead and withdrawn as
sidestreams.
The purpose of overflash is to generate
internal reflux in the wash trays between
the flash zone and the bottom sidestream
draw tray.
Overflash vapors are condensed and wash
the trays to prevent carryover and coking.

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Improving fractionation on the trays
above the flash zone, thereby
improving the quality of the HGO
and reducing the overlap with the
bottom products below the flash
zone.
The overflash provides heat input to
the column in excess to that needed
to distill the overhead products.
It also prevents coke deposition on
the trays in the wash zone.

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Fractionation
Crude entering the flash zone of the
fractionating column flashes into:
 The vapor which rises up the
column, and The liquid residue
which drops downwards
This flash is a very rough separation.
The vapors contain appreciable quantities
of heavy ends, which must be rejected
downwards into reduced crude, while the
liquid contains lighter products, which
must be stripped out.

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Eng.Said Elsayed
Increasing the reflux rate lowers the
top temperature and results in the
net overhead product having a lower
endpoint.
The loss in net overhead product
must be removed on the next lower
draw tray.
This will decrease the initial boiling
point of material from this tray.

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Cut point
Temperature of the drawoff decks is
a fair indication of the endpoint of
the product drawn at that point.
The degree of fractionation between
cuts is generally judged by
measuring the number of degrees
centigrade between the 95% point
of the lighter product and the 5%
point of the heaving product.

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Flash Zone and Stripping Section
The flash zone pressure is set as low as possible to
maximize vaporization, minimize flash zone
temperature, and reduce furnace duty while
optimizing compression on the tower overhead
vapor stream.
Flash zone pressure is determined by overhead
condensation pressure plus pressure drop in the
tower. If the reflux drum operates at 5 psig, the
pressure drop across the overhead cooling system is
5 psid, and the pressure drop through
the tower is 5 psid, the flash zone will operate at 15
psig.

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It is necessary then to:
 Either lower the product
withdrawal rate,
 Or to increases the internal
reflux in the tower by raising the
transfer temperature,
 Or by reducing the rate at
which the next lightest product is
being withdrawn.

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Product Stripping
The flashed residue in the bottom of the
fractionators and the sidecut products
have been in contact with lighter boiling
vapors.
These vapors must be removed to meet
flash point specifications and to drive
the light ends into lighter and more
valuable products.
Steam (usually superheated steam) is
used to strip these light ends.

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Eng.Said Elsayed
All the stripping steam is condensed in
the overhead receiver and must be
drained off.
Refluxing water will upset the
fractionators
If the endpoint of the overhead product
is very low, water may not pass
overhead, and will accumulate on the
upper trays and cause the tower to
flood.

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Eng.Said Elsayed
The effect of steam
Steam is frequently used in fractionating
columns, strippers and sometimes in
furnaces.
If the quantity and temperature of the
steam are known, its effect can be
determined by calculating the partial
pressure exerted by the steam.
This partial pressure is then subtracted from
the total system pressure resulting lower
pressure.
In other words steam has the same effect as
lowering the pressure.

99
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Eng.Said Elsayed
Reflux Rate Changing
Reflux as a "coolant" that removes
heavy fractions by condensing them.
This extra reflux flowing down the
tower causes the temperature on each
tray to decrease.

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Eng.Said Elsayed
Some of the heavier hydrocarbons in
the upward flowing vapors will now
condense and fall back down the tower.
The extra reflux flowing down the tower
reduces the temperature of the liquid at
the bottom of the column.
When the bottom temperature
decreases, the amount of light material
vaporized out of the liquid at the
bottom of the tower is decreased.

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Eng.Said Elsayed
Because fewer vapors are now going
overhead, the amount of top product
formed is decreased, or less.
By increasing the reflux rate:
 Lighter overhead,
 Lighter bottom, and
 Lighter side draw products are
produced.

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Eng.Said Elsayed
If we decrease the reflux rate
the cut point changes are
reversed.
The temperature on each of the trays
increases, and a higher tower
temperature mean heavier products.
So overhead, bottom, and side draw
products become heavier.
The amount of overhead product
produced increases and the amount
of bottom product formed decreases.

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Eng.Said Elsayed

104
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Eng.Said Elsayed
Feed Temperature Changing
Suppose we raise the temperature of the
feed and hold the reflux rate and other
tower variables constant.
As the crude enters the column more of
the feed is vaporized because of the
higher temperature.
Some of the heavy material that
previously fell to the bottom of the tower
is contained in these vapors.
So the products formed above the feed
tray become heavier.

105
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Eng.Said Elsayed
Side Product (Draw off) Rate Changing
Another way to change the cut point in a
crude column is to vary the amount of
liquid that is drawn to the stripper
columns.
Suppose we increase the kerosene draw by
100 barrels.
When we open a stripper draw on the side
of a crude unit less reflux flows to the trays
below the draw-off tray.

106
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Eng.Said Elsayed
Reducing the amount of reflux going
to the trays below the kerosene draw
causes these trays to heat up.
As the temperature on these trays
increases more heavy material begins
rising up the tower.
Because the temperature of the
vapors rising to the kerosene draw is
higher, the draw-off tray temperature
will also be higher.

107
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Eng.Said Elsayed
High temperatures produce heavy
products, so increasing the kerosene
draw makes this product heavier.
The products formed below the
kerosene draw also become heavier.

108
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Eng.Said Elsayed
Let’s reverse the situation and look at
what happens when the kerosene draw
is decreased.
Now the amount of reflux flowing to
trays below the kerosene draw
increases.
An increase in reflux causes more heavy
components to condense out of the
rising vapors because the temperature
on the trays falls.

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Eng.Said Elsayed
Low temperatures produce light
products, so reducing the kerosene draw
results in a lighter kerosene product.
The products formed below the
kerosene draw also become lighter.
Opening a stripper draw makes this
product and products formed below
this point heavier.
Closing a stripper draw makes this
product and products below this tray
lighter.

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Eng.Said Elsayed
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.

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Eng.Said Elsayed
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.

113
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Eng.Said Elsayed
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.

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Eng.Said Elsayed
Thermal cracking is undesirable
because It would cause:
 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.

115
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Eng.Said Elsayed
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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Eng.Said Elsayed
Process description
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.
the pressure in a vacuum
column is very much lower.

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Eng.Said Elsayed
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.

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Eng.Said Elsayed
If Cracking occurred :
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.

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Eng.Said Elsayed
Flash Zone
The hot feed is flashed into the
flash zone of the vacuum tower
through a system of injection
nozzles.
The flash zone pressure is set as
low as possible to maximize
vaporization, minimize flash zone
temperature, and reduce furnace
duty while optimizing the vacuum
system and tower pressure drop,
and obtaining a deep cut without
thermal cracking.

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Eng.Said Elsayed
The Steam Stripping Section
Steam is injected into the tubes and the bottom
of the tower to increase vaporization and
reduce hydrocarbon partial pressure at the flash
zone. Steam is injected at the bottom of the
tower below stripping trays.
There are 3 to 4 trays just above the injection
point to enhance stripping of the bottoms.
Steam is also injected in the feed in the furnace
before the feed starts to vaporize to increase
velocities and prevent coking.
The steam is superheated in the furnace
convection section.

121
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Eng.Said Elsayed
Bottoms Recycle and Product
Cooling
A boot for collecting tower bottoms is
located below the flash zone.
The complete bottoms vacuum resid
stream is cooled to about 700°F. It is
typical to make steam in this exchanger.
A slipstream from the cooled vacuum
resid is recycled back to the vacuum
tower bottoms to remove heat and
alleviate coking.
The vacuum resid is further cooled via
crude train heat exchange.

122
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Eng.Said Elsayed

123
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Eng.Said Elsayed

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Eng.Said Elsayed
Top section
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.

125
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Eng.Said Elsayed
Steam Ejectors and vacuum
Pumps
Vacuum on the tower is
maintained with a vacuum system
on the tower overhead.
Two types of vacuum systems are
used: steam ejectors and vacuum
pumps. Steam systems are
considered more reliable but the
waste steam is sour and must be
treated.

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Eng.Said Elsayed
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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Eng.Said Elsayed
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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Eng.Said Elsayed
Theory of Operation
The converging-diverging steam jet
is rather like a two-stage
compressor, but with no moving
parts.
A simplified drawing of such a
steam jet is shown in Fig High-
pressure motive steam enters
through a steam nozzle.
As the steam flows through this
nozzle, its velocity greatly
increases. But why? Where is the
steam going to in such a hurry?
Well, it is going to a condenser.

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Eng.Said Elsayed

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Eng.Said Elsayed
condense the steam at a low
temperature and low pressure. It
will condense the steam quickly.
The steam accelerates toward the
cold surface of the tubes in the
condenser, where its large volume
will disappear as the steam turns
to water.
Similarly, as the high-velocity
steam enters the mixing chamber
it produces an extremely low
pressure.
The gas

131
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Eng.Said Elsayed
flows from the jet suction nozzle
and into the low-pressure mixing
chamber. It is not correct to say
that the gas is entrained by the
steam.
The gas just flows into the
mixing chamber because there is
a very low pressure in the mixing
chamber.
The rest of the jet is used to
boost the gas from the mixing
chamber up to the higher
pressure in the condenser.

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Eng.Said Elsayed

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Eng.Said Elsayed

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Eng.Said Elsayed

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Eng.Said Elsayed

138
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Eng.Said Elsayed

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Eng.Said Elsayed

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Eng.Said Elsayed
 ACCIDENT Death in oil field
 Watch how source of ignition cause disaster

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Eng.Said Elsayed
To Reboiler
From
Reboiler
Vapor feed
liquid feed
Reflux
To OVHD
Condenser
liquid
vapor
Down comer
Active tray
area
Outlet weir

142
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Eng.Said Elsayed
Why Internals Are Employed?
To bring most intimate contact between
ascending vapor and descending liquid
without reduction in the through put or
capacity of a fractionating column

143
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Eng.Said Elsayed
The selection process for tower
internals can be straightforward for
some designs, but very difficult for
others.
Internals design considerations can
be quite complex, especially when
revamping an existing unit.
but when in doubt about how to
proceed, an experienced in-house
designer should be consulted.

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Eng.Said Elsayed
There are numerous types
of mass transfer devices
available on the market
today.
The main question faced by
the designer and operator is
to decide which types of
hardware to use in which
application.

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Eng.Said Elsayed
Trays are the lowest cost devices
as they introduce the least amount
of fabricated
material into the distillation tower.
Thus compared to packing the costs
are usually lower.
Trays function by developing a
FROTH regime on the tray deck in
which vapor and liquid phases are
brought into intimate contact.

146
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Eng.Said Elsayed
Structured Packing was
developed and optimized
to reduce pressure drop
and thus reduce energy
consumption in vacuum
and medium pressure
applications.

147
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Eng.Said Elsayed
Types of Internals
 Fixed Internals:
• Tray Supports
• Downcomer supports
• Distributors Supports
• Vortex Breakers
 Removable Internals:
• Trays
• Chimney Trays
• Packing (structured, random)
• Distributors
• Bed limiters, Supporting grid
• Demisters

148
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Eng.Said Elsayed

149
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Eng.Said Elsayed

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Eng.Said Elsayed

151
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Eng.Said Elsayed

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Eng.Said Elsayed
Downcomers Purpose
To collect the two phase mixture from
the tray
To separate the vapor from the liquid
To distribute the clarified liquid
uniformly to the tray below

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Eng.Said Elsayed

154
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Eng.Said Elsayed

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Eng.Said Elsayed
Inlet Weirs
These contribute to the uniform distribution of liquid as
it enters the tray from the down comer.
It is not recommended for
fluids that are dirty or tend
to foul surfaces.
If a more positive seal is
required at the downcomer at the outlet, an inlet weir
can be fitted or a recessed seal pan
used.

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Eng.Said Elsayed
Outlet Weirs
These are necessary to
maintain seal on the tray,
thus insuring bubbling of
vapors through liquid.

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Eng.Said Elsayed

158
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Eng.Said Elsayed

159
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Eng.Said Elsayed

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Eng.Said Elsayed
Weep Holes
Holes for drainage must be adequate to drain the tower in a
reasonable time, yet not too large to interfere with tray action.
Draining of the tower
through the trays is
necessary before any
internal maintenance
can be started.
The majority of the holes are placed adjacent to the outlet or
downcomer weir.

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Eng.Said Elsayed
Trays Classification
Type
Flow
pattern

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Eng.Said Elsayed
 Type of Trays
 Sieve Tray

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Eng.Said Elsayed
 Sieve Tray
Sieve trays are simply metal plates with holes
in them. Vapor passes straight upward
through the liquid on the plate.
The arrangement, number and size of the
holes are design parameters.
They are a good choice when vapor and
liquid loads vary little within a tower section,
and turndown requirements are not
stringent. They can be designed for 2:1
turndown typically, unless liquid rates are
low.
.

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Eng.Said Elsayed
Sieve trays have three major
advantages
(1) they are less expensive
(2) have no moving parts
(3) are easier to clean

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Eng.Said Elsayed
 Type of Trays
Valve Tray
• Floating
• Fixed

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Eng.Said Elsayed
Valve Trays
Valve trays are presently most often
specified for distillation towers.
They provide a wider hydraulic operating
range than sieve trays with comparable
efficiency.
They can provide better efficiency and
capacity with lower pressure drop
They can be designed for turndowns in the
range of 2:1 to 8:1, depending on process
conditions and tray design parameters
selected.

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Eng.Said Elsayed

168
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Eng.Said Elsayed

169
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Eng.Said Elsayed

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Eng.Said Elsayed
The capacity of cross flow trays can be
improved by using one or more of the
following design improvements:
• Minimise the active area wasted by
mechanical features such as
clamping,bolting, beams, joints etc..
• Use mini valves.
• Optimise Downcomer design to increase
bubbling area
• Using flow directional vanes on the tray
inlet to minimize misdistribution.
• Optimizing valve layout on the tray deck.
• Using different valve types on the tray deck
to optimize the formation of froth layer.

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Eng.Said Elsayed
The valve units typically
consist of orifices (circular or
rectangular) covered by caps
that open and close with
variation in vapor flow rates.
Liquid flows across the active
areas where it intimately
contacts vapor, flows over
weirs, and falls into
downcomers. The
downcomers are designed to
allow vapor to disengage from
the liquid which flows by
gravity to trays below.

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Eng.Said Elsayed

173
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Eng.Said Elsayed
 Type of Trays
 Bubble cap tray

174
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Eng.Said Elsayed
Bubble cap trays
were used extensively in the past, but
presently have limited practical
applications.
They might be an appropriate choice
for towers in which
1) extreme turndown is required,
2) liquid flow rates are extremely
low,
3) large liquid residence (e.g., in
reactive systems) is required on
the active areas.

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Eng.Said Elsayed
The trays can be designed for minimal liquid leakage even with
high liquid levels on the active area or at low vapor rates.

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Eng.Said Elsayed
 Type of Trays
 Dual Tray

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Eng.Said Elsayed
Dual-flow Trays
Dual-flow trays are similar to sieve
trays, except downcomers are not
used, and both vapor and liquid pass
through the orifices in the tray deck.
Efficiencies generally aren't high, and
change substantially with operating
rates.

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Eng.Said Elsayed
High Efficiency Trays
Valvetrays have 10:1 and
higher turndown

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Eng.Said Elsayed

180
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Eng.Said Elsayed

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Eng.Said Elsayed
 Type of Trays
 Cartridge tray

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Eng.Said Elsayed

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Eng.Said Elsayed
Slit Trays
The Slit Tray is a high efficiency device
with circular symmetry, featuring rows
of concentric slits for vapor passage.
It is mainly used for the distillation of
aqueous systems and specialty
chemicals.
Main characteristics are:
• Low tray spacing: 150 – 250 mm (6” –
10”)
• High fractionation stages per given
column height
• Self supporting structure
• Large operating range up to 1:4

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Eng.Said Elsayed

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Eng.Said Elsayed
Shell HiFi Plus Trays
The Shell HiFi is a fractionation tray equipped
with multiple envelope downcomers, oriented
offset to the tray‘s center line.
it allows for:
• Large downcomer area
• High weir length
• High number of passes
• Low tray spacing per given vapor & liquid
loadings
• High hydraulic capacity
• High number of separation stages per given
column height, and vapor & liquid loadings
• Low pressure drop per given vapor & liquid
loadings

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Eng.Said Elsayed

187
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Eng.Said Elsayed
 Type of Trays
 Chimney Tray

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Eng.Said Elsayed

189
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Eng.Said Elsayed
 Trays Flow Pattern
 Two Passes

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Eng.Said Elsayed

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Eng.Said Elsayed
 Trays Flow Pattern
 Four Passes

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Eng.Said Elsayed

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Eng.Said Elsayed

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Eng.Said Elsayed
Whatever the cause of the tray damage, it is often hard to
prove tray damage without:
 Column shutdown, and
 Inspection.
Effects of damaged trays
Poorer fractionation,
A decrease in temperature difference because of the
poorer fractionation.
An increase in pressure difference.
The damage is often to downcomers or other liquid
handling parts.

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Eng.Said Elsayed

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Eng.Said Elsayed

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Eng.Said Elsayed
Selection of Tray Type
The principal factors to consider
when comparing the performance
of bubble-cap, sieve and valve trays
are:
Cost,
Capacity,
Operating range,
Efficiency, and
Pressure drop.

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Eng.Said Elsayed
Cost:
Bubble-cap trays are appreciably
more expensive than sieve or valve
trays.
The relative cost will depend on
the material of construction used;
For mild steel the ratios,
bubble-cap: valve: sieve, are
approximately
3.0 : 1.5 : 1.0

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Eng.Said Elsayed
Capacity:
There is little difference in the
capacity rating for the three types
(the diameter of the column
required for a given flow-rate).
The ranking is:
sieve, valve, and bubble-cap

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Eng.Said Elsayed
Turn-down Ratio
The ratio of the highest to the
lowest flow rates.
Bubble-cap trays have a
positive liquid seal and can
therefore operate efficiently at
very low vapour rates.

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Eng.Said Elsayed
Pressure Drop:
The pressure drop over the trays can
be an important design consideration,
particularly for vacuum columns.
The trays pressure drop will depend on
the detailed design of the tray but.
In general,
sieve plates give the lowest pressure
drop, followed by valves, with bubble-
caps giving the highest.

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