4. 1.1 Distillation
• Distillation is a separation process requires differences
to be recognized and utilized.
• Separation by distillation implies a difference in boiling
points of two or more materials.
• We separate many things by detecting a difference in a
physical properties.
• color, size, weight, shape
4
5. • The components or compounds making up crude oil or
natural gas are numbered in thousands.
• Many of these components have similar physical
properties including boiling points that may differ by
only a few degrees.
• Therefore, it is difficult to separate some pure
compounds from the complex mixture of components
in crude oil or gases by distillation alone.
6. • There are other methods of separation used in oil and gas industry
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.
• If crude oil were a final product, it would have just been a low-
grade fuel struggling to establish itself against coal.
7. • If we separate many compounds in crude oil into groups
or fractions, we find that these groups have
characteristics that make them considerably more
valuable than the whole crude oil.
• Some of these groups or fractions are products.
• Some may be feedstock to other processing units where
they are chemically changed into more valuable
products.
• These products, in turn, are usually separated or purified
by distillation in towers
8. 1.2 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.
9. • Tow component mixture is
contained in a vessel.
• When heat is added, the more
volatile material (red dotes)
starts to vaporize.
• The vapor contains a higher
proportion of red dots than the
dose of the original liquid.
10. It is important to note that an equilibrium in
composition will be established:
At a given temperature and pressure
By equilibrium we mean there is a given
concentration as “red dots" in the vapor and in the
liquid depending upon the original concentration of
each component in the liquid and their respective
properties in relation to each other.
11. 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.
12. Now, let's develop this simple
distillation concept into a practical
operation as it is used in the
refinery
First, let’s separate and
remove the product.
• Light Material
+ Heavy Material
13. By cooling the overhead vapor, we
condense and remove it from the
original mixture.
Thus to have made a partial
separation, partial because you
will note that there are a few
“blue dotes" in the distillate
product.
• Light Material
+ Heavy Material
14. This has occurred because at the temperature
and pressure we are conducting the distillation,
the heavier component still vaporizes to some
extent.
This is because the components of interest in a
given distillation usually have fairly close
boiling points
15. Therefore, to purify the distillate product, we
may have to conduct a second distillation.
Obviously, we can
continue to cascade
these simple
distillations until we
achieve the desired
purity of product.
16. The distillations depicted so far are those we call patch,
and are normally practical in the refinery, although it is
done frequently in the laboratory.
Let us make our distillation equipment look more like
refinery pieces of equipment and let us make
continuous instead of patch operation.
17. This is called Flash Vaporization.
The liquid is pumped continuously through a heater and into a
drum where the pressure
is lower.
The lighter material flashes instantaneously
(vapor and liquid flow from the drum continuously).
18. The same system is shown diagrammatically in the
following drawing.
19. Suppose we have 50% of the charge taken overhead.
That is, we set the temperature and the pressure of the
system in such a way that half the charge is boiled off.
And further, suppose the resulting overhead product does
not contain the desired concentration of the lighter
product.
As we have seen before, we can increase the purity by
adding a stage of distillation.
20. 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.
21. 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.
Stripping
stages
Rectifying
stages
22. 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.
23. Equilibrium Stage
A stage, or more specifically, an equilibrium stage, is defined as:
Any portion of the distillation column such that the liquid and
vapor leaving it have composition in equilibrium with each
other.
By definition, then, a stage should be designed in such a way as
to provide intimate contact, or mixing, of the rising vapor and
the descending liquid.
25. The concept of an equilibrium stage is converted to an
actual mechanical separation tray by using an
efficiency factor which is less than one and depends on
the tray design.
27. The design of trays has taken many forms.
Some common ones are :
Valve trays,
Bubble cap trays,
Sieve trays,
S-shaped trays
29. Alternate designs include packing instead of
trays.
Various kinds of packing have used, some of
which are :
Pall rings,
Saddles, and
Mesh.
30. The type of column internally used depends on the
application.
The considerations being :
Purity of feed,
Efficiency,
Capacity,
Reliability,
Pressure drop,
Liquid holdup, and
Cost.
31. The column shown is a simple
binary column with trays.
There is only one feed and two
products, the overhead and
bottoms.
More complex columns may have
several feed streams.
32. 32
• Distillation is based on the fact that the vapour of a boiling mixture
will be richer in the components that have lower boiling points.
• Thus, when this vapour is cooled and condensed, the condensate
will contain the more volatile components. At the same time, the
original mixture will contain more of the less volatile components.
• Distillation is the most common separation technique and it
consumes enormous amounts of energy, both in terms of cooling
and heating requirements.
• Distillation can contribute to more than 50% of plant operating
costs.
33. 33
Distillation columns are classified by how they are operated:
1. Batch, in which 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. When the desired task is
achieved, the next batch of feed is introduced.
2. 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.
34. 34
Classified according to:
1. Nature of the feed that they are processing:
• binary column - feed contains only two components;
• multi-component column - feed contains more than two components.
2. Number of product streams they have:
• multi-product column - column has more than two product streams.
3. Where extra feed exits when used to help with the separation:
• extractive distillation - where the extra feed appears in the bottom product
stream;
• azeotropic distillation - where the extra feed appears at the top product stream.
4. Types of column internals:
• Tray column - trays of various designs used to hold up the liquid to provide better
contact between vapor and liquid;
• packed column - packings are used to enhance vapour-liquid contact.
35. Fractionating “Tower”
Is used in referring to a counter-current operation in
which a vapor mixture is repeatedly brought in contact
with a liquid having nearly the same composition as
the respective vapors
Types of towers
36. Atmospheric Distillation "Tower"
Is the first step in any petroleum refinery, in which the
separation of crude oil into various fractions. These
fractions may be products in their own right or maybe
feedstocks for other refining or processing units
Types of towers
38. Vacuum Distillation "Tower"
Is used to reduce the temperature for the distillation of
heat-sensitive materials and where very high
temperatures would otherwise be needed to distill
relatively nonvolatile materials
Types of towers
39. Stabilization "Tower"
It is a fractionation operation conducted for the
purpose of removing high-vapor pressure
components.
Types of towers
41. Splitting "Tower"
It is a simple distillation process, in which the
separation of naphtha into two streams before
further processing can take place.
Types of towers
43. Stripping "Tower"
Is the process where the requirements, to strip a
volatile component or group of similar
components from a relatively non-volatile
solution or product by the action of stripping gas
or steam
.
Types of towers
44. Absorber "Tower"
Is the process where the requirements, are to absorb a
specific component or groups from a mixture of
components.
This absorption may be physically or chemically occurred.
this application commonly occurs for gas processing to
remove H2S or H2O.
Types of towers
45. Extractor "Tower"
Is the process where the requirements, are to extract a
specific component or groups from a mixture of
components. This extraction may be physically or chemically
occurred.
this application commonly occurs for extracting or removing
specific components from liquid or the liquefied mixture by
using another liquid.
Types of towers
46. 1.3 Reflux
The word reflux is defined as:
"flowing back“
Applying it to the distillation tower, reflux
is:
The liquid flows back down the tower from
each successive stage.
47. Kinds of Reflux
cold Reflux
Hot Reflux
Internal Reflux
Circulating Reflux
Side Reflux
48. Cold Reflux
Cold reflux is defined as:
Liquid that is supplied at a 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 the reflux drum
temperature to the temperature at the top of the
tower.
49. A constant quantity of reflux
is recirculated from the reflux
drum into the top of the
tower.
It is vaporized and condensed
and then returns in like
quantity to the reflux drum.
50. 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.
51. Internal Reflux
It is the liquid that
overflows from one
plate to another in the
tower,
52. Circulating Reflux
It can remove only the
sensible heat which is
represented by its change in
temperature as it circulates.
The reflux is withdrawn and is
returned to the tower after
having been cooled.
53. Side Reflux
This type of reflux (circulating
reflux) may conveniently be
used to remove heat at points
below the top of the tower.
If used in this manner, it tends
to decrease the volume of
vapor the tower handles.
54. Reflux Ratio
It is defined as the amount of internal reflux divided
by the amount of top product.
Since internal hot reflux can be determined only by
computation.
Plant operators usually obtain the reflux ratio by
dividing actual reflux by the top product.
It is denoted by R which equals L/D.
55. 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.
56. 1. A minimum number of plates (stages) is required
at total reflux.
2. There is a minimum reflux ratio below which it is
impossible to obtain the desired enrichment
(separation) however many plates are used.
Two points to consider
57. 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.
58. 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
59. Optimum Reflux Ration
The 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
60. 1.4 Reboiling
In all distillation processes
Heat being added by:
Feed, and
Reboiler
61. The reboiler is a heat exchanger through which the
bottom liquids circulate.
Heat is transferred to the bottom materials which
causes vaporization of the lighter components.
This vapor travels up the column to provide:
The stripping action, and
The additional heat necessary to
vaporize the down coming reflux.
63. The purpose of crude oil distillation is primarily to split
the crude into several distillate fractions of a certain
boiling range.
The sharpness of fractionation is of secondary
importance.
A crude distillation tower, producing 6 fractions
has 40 to 50 trays.
64. Distillation is an operation in which vapors rising
through fractionating trays in a tower are in
intimate contact with liquid descending across
the trays.
So that higher boiling components are
condensed, and concentrated at the bottom of
the tower while the lighter ones are
concentrated at the top or pass overhead.
66. If a slug of water has 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 1760 times upon
vaporization at 100ºC at atmospheric pressure.
67. Fractionation
Crude entering the flash zone of the fractionating
column flashes into:
The vapor which rises 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.
68. Flashed vapors rise up the fractionating column
countercurrent to the internal reflux flowing down the
column.
The lightest product, which is generally Light gasoline passes
overhead and is condensed in the overhead receiver.
The temperature at the top of the fractionators is a good
measure of the endpoint of the light and this temperature is
controlled by returning some of the condensed light gasoline
as reflux to the top of the column.
69. Increasing the reflux rate lowers the top
temperature and results in the net overhead
product having a lower endpoint.
The loss in a net overhead product must be
removed on the next lower draw tray.
This will decrease the initial boiling point of
material from this tray.
70. External reflux which is returned to the top of the
fractionators passes downwards against the rising
vapors.
Lighter components of the reflux are revaporized and
returned to the top of the column while the heavier
components in the rising vapors are condensed and
returned down the column.
We have then an internal reflux stream flowing from,
the top of the fractionators all the way back to the
flash zone and becoming progressively heavier as it
descends.
71. The products heaver than the overhead are obtained
by withdrawing portions of the internal reflux stream.
The endpoint of a sidecut will depend on the quantity
withdrawn.
If the sidecut withdrawal rate is increased, the extra
product is material that was formerly flowing down
the fractionators as an internal reflex.
Since the internal reflux below the draw-off is reduced,
heavier vapors can now rise to that point and result in
a heavier product.
72. The temperature of the draw-off 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
Some people use IBP and FBP
But the IBP varies with stripping
74. Fractionation can be improved by increasing the reflux
in the fractionators, which is done by raising the
transfer temperature.
There may be occasions when the internal reflux
necessary to achieve satisfactory fractionation
between the heaver products is so great that if it was
supplied from the top of the fractionators the upper
trays would flood.
An “Intermediate Circulating Reflux” solves this
problem.
75. Sometimes fractionators will be “pulled dry”
The rate at which a product is being withdrawn is
greater than the quantity of internal reflux in the
fractionators.
All the internal reflux then flows to the stripper, the
trays below the draw-off run dry, and therefore no
fractionation takes place, while at the same time, there
is insufficient material to maintain the level in the
stripper, and the product pump will tend to lose
suction.
76. It is necessary then to:
Either lower the product withdrawal rate,
Or to increase 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.
77. 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.
