Flash distallation by khalid

Distillation
• Distillation is a method of separating mixtures based on differences in
their volatilities in a boiling liquid mixture. Distillation is a physical
separation process, and not a chemical reaction.
• Uses:-
1. Crude oil
2. Water is distilled to remove impurities
3. Air is distilled to separate its components
4. Distillation of fermented solutions to produce distilled beverages
with a higher alcohol content.
The premises where distillation is carried out, especially distillation of
alcohol, are known as a distillery.

VARIOUS TYPES OF
DISTILLATION
Simple Distillation
Molecular Distillation
Vacuum Distillation
Batch Distillation
Continuous Distillation
Flash Distillation
Fractional Distillation
Azeotropic Distillation

Flash Distillation
⑴ The flash distillation process
consists of three parts.
a. Vaporizing a definite fraction
of the liquid ;
b. Separating the vapor from the
liquid;
c. Condensing the vapor.
Heater
Separator
Condenser
Overhead
Product
Bottom
Product
Throttle

Features
a. Continuous steady state
Operating conditions (T & P)
are constant.
b. Compositions & flow rates of
feed , products of overhead
and bottom are constant。
c. There is equilibrium always
between phases of vapor and liquid .

Flash Distillation
 Flash evaporation is the partial vaporization that occurs when a
saturated liquid stream undergoes a reduction in pressure by passing
through a valve or other device. If the valve or device is located at the
entry into a pressure vessel so that the flash evaporation occurs within
the vessel, then the vessel is often referred to as a flash drum.
 If the saturated liquid is a single-component liquid (for example, liquid
propane or liquid ammonia), a part of the liquid immediately "flashes"
into vapor. Both the vapor and the residual liquid are cooled to the
saturation temperature of the liquid at the reduced pressure. This is often
referred to as "auto-refrigeration" and is the basis of most conventional
vapor compression refrigeration systems.
 If the saturated liquid is a multi-component liquid (for example, a
mixture of propane, isobutane and normal butane), the flashed vapor is
richer in the more volatile components than is the remaining liquid.

Flash Distillation:
Flash distillation (sometimes called "equilibrium distillation") is a single
stage separation technique. A liquid mixture feed is pumped through a
heater to raise the temperature and enthalpy of the mixture. It then flows
through a valve and the pressure is reduced, causing the liquid to
partially vaporize. Once the mixture enters a big enough volume (the
"flash drum"), the liquid and vapor separate. Because the vapor and
liquid are in such close contact up until the "flash" occurs, the product
liquid and vapor phases approach equilibrium.
Simple flash separations are very common in industry, particularly
petroleum refining. Even when some other method of separation is to be
used, it is not uncommon to use a "pre-flash" to reduce the load on the
separation itself.

When is flash distillation used?
flash distillation = a single equilibrium stage
1. when very crude separation is needed
e.g., oil/water separation in crude oil refining
2. when volatilities of components in the mixture
are very different
e.g., water desalination (4000 plants worldwide, producing 3.4
billion gallons potable H2O daily)

Batch Distillation
Production of vapor by boiling the
liquid mixture to be separated and
condensing the vapors without
allowing any liquid to return to the
still.

Continuous Distillation
Based on the return of part of the
condensate to the still under such
conditions that this returning liquid
is brought into intimate contact
with the vapors on their way to the
condenser.

PLATE CONTACTORS:
Cross flow plate are the most commonly used plate
contactor in distillation. In which liquid flows
downward and vapours flow upward. The liquid move
from plate to plate via down comer. A certain level of
liquid is maintained on the plates by weir.

I prefer Sieve Plate because:
• Pressure drop is low as compared to bubble cap trays
• Their fundamentals are well established, entailing low risk.
• The trays are low in cost relative to many other types of
trays.
• They can easily handle wide variations in flow rates.
• They are lighter in weight. It is easier and cheaper to install.
• Maintenance cost is reduced due to the ease of cleaning.

FACTORS AFFECTING DISTILLATION COLUMN
OPERATION
Adverse vapour flow conditions can cause:
• Blowing
• Coning
• Dumping
• Raining
• Weeping
• Flooding

BASIC DISTILLATION EQUIPMENT
AND OPERATION
Main Components of Distillation Columns
Distillation columns are made up of several components, each of
which is used either to tranfer heat energy or enhance materail
transfer. A typical distillation contains several major components:
• a vertical shell where the separation of liquid components is carried
out
• column internals such as trays/plates and/or packings which are used
to enhance component separations
• a reboiler to provide the necessary vaporisation for the distillation
process
• a condenser to cool and condense the vapour leaving the top of the
column
• a reflux drum to hold the condensed vapour from the top of the
column so that liquid (reflux) can be recycled back to the column

AND OPERATION
The vertical shell
houses the column
internals and together
with the condenser
and reboiler,
constitute a
distillation column. A
schematic of a typical
distillation unit with a
single feed and two
product streams is
shown below:

AND OPERATION
Basic Operation and Terminology
The liquid mixture that is to be processed is
known as the feed and this is introduced usually
somewhere near the middle of the column to a
tray known as the feed tray. The feed tray
divides the column into a top (enriching or
rectification) section and a bottom (stripping)
section. The feed flows down the column where
it is collected at the bottom in the reboiler.

BASIC DISTILLATION
EQUIPMENT AND OPERATION
Heat is supplied to the reboiler to
generate vapour. The source of
heat input can be any suitable fluid,
although in most chemical plants
this is normally steam. In refineries,
the heating source may be the
output streams of other
columns. The vapour raised in the
reboiler is re-introduced into the
unit at the bottom of the column.
The liquid removed from the
reboiler is known as the bottoms
product or simply, bottoms.

AND OPERATION
The vapour moves up the column,
and as it exits the top of the unit,
it is cooled by a condenser. The
condensed liquid is stored in a
holding vessel known as the reflux
drum. Some of this liquid is
recycled back to the top of the
column and this is called the
reflux. The condensed liquid that
is removed from the system is
known as the distillate or top
product.
Thus, there are internal flows of
vapour and liquid within the column
as well as external flows of feeds
and product streams, into and out
of the column.

COLUMN INTERNALS
Trays and Plates
The terms "trays" and "plates" are used interchangeably.
There are many types of tray designs, but the most
common ones are :
Bubble cap trays
A bubble cap tray has riser or
chimney fitted over each hole, and
a cap that covers the riser. The
cap is mounted so that there is a
space between riser and cap to
allow the passage of vapour.
Vapour rises through the chimney
and is directed downward by the
cap, finally discharging through
slots in the cap, and finally
bubbling through the liquid on the
tray.

• Because of its high cost and complexity, most
modern column designs favour the use of sieve or
valve trays over bubble-cap trays. Bubble-caps
should only be used where very low vapour rates
have to be handled, or adequate residence time is
necessary for separation and/or chemical reaction,
or in applications where a positive liquid seal is
essential at all flow rates.

COLUMN INTERNALS
Sieve trays
Sieve trays are simply metal
plates with holes in them.
Vapour passes straight
upward through the liquid on
the plate. The arrangement,
number and size of the holes
are design parameters.
Because of their efficiency, wide
operating range, ease of
maintenance and cost factors,
sieve and valve trays have
replaced the once highly thought
of bubble cap trays in many
applications.

• he sieve tray was developed. Sieve tray has almost
the opposite characteristics of the bubble-cap tray.
It is inexpensive to male. With proper design, it has
low pressure drop, fairly good capacity and
efficiency. However, its turn down ratio often does
not meet flexibility demanded by the operating
facility.

COLUMN INTERNALS
Liquid and Vapour Flows in a Tray Column
The next few figures show the direction of vapour and
liquid flow across a tray, and across a column.

COLUMN INTERNALS
Each tray has 2 conduits, one on each side, called
‘downcomers’. Liquid falls through the downcomers by gravity
from one tray to the one below it. The flow across each plate
is shown in the above diagram on the right.
A weir on the tray ensures that
there is always some liquid
(holdup) on the tray and is
designed such that the the holdup
is at a suitable height, e.g. such
that the bubble caps are covered
by liquid.
Being lighter, vapour flows up the
column and is forced to pass
through the liquid, via the
openings on each tray. The area
allowed for the passage of vapour
on each tray is called the active
tray area.

COLUMN INTERNALS
As the hotter vapour passes through the liquid on the tray
above, it transfers heat to the liquid. In doing so, some of
the vapour condenses adding to the liquid on the tray.
The condensate, however, is richer in the less volatile
components than is in the vapour. Additionally, because of
the heat input from the vapour, the liquid on the tray boils,
generating more vapour. This vapour, which moves up to
the next tray in the column, is richer in the more volatile
components. This continuous contacting between vapour
and liquid occurs on each tray in the column and brings
about the separation between low boiling point
components and those with higher boiling

COLUMN INTERNALS
Tray Designs
A tray essentially acts as a mini-column, each accomplishing a
fraction of the separation task. From this we can deduce that
the more trays there are, the better the degree of separation
and that overall separation efficiency will depend significantly on
the design of the tray. Trays are designed to maximise vapour-
liquid contact by considering
• the liquid distribution and
• vapour distribution
on the tray. This is because better vapour-liquid contact means
better separation at each tray, translating to better column
performance. Less trays will be required to achieve the same
degree of separation. Attendant benefits include less energy
usage and lower construction costs.
There is a clear trend to improve separations by supplementing
the use of trays by additions of packings.

COLUMN INTERNALS
Packings
Packings are passive devices that are designed to
increase the interfacial area for vapour-liquid contact.
The following pictures show 3 different types of packings.
These strangely shaped pieces are supposed to impart good
vapour-liquid contact when a particular type is placed together
in numbers, without causing excessive pressure-drop across a
packed section. This is important because a high pressure drop
would mean that more energy is required to drive the vapour up
the distillation column.

COLUMN INTERNALS
Packings versus Trays
A tray column that is facing throughput problems may be
de-bottlenecked by replacing a section of trays with
packings. This is because:
• packings provide extra inter-facial area for liquid-vapour
contact
• efficiency of separation is increased for the same column
height
• packed columns are shorter than trayed columns
Packed columns are called continuous-contact columns
while trayed columns are called staged-contact columns
because of the manner in which vapour and liquid are
contacted.

COLUMN REBOILERS
There are a number of designs of reboilers. It is beyond the scope
of this set of introductory notes to delve into their design principles.
However, they can be regarded as heat-exchangers that are
required to transfer enough energy to bring the liquid at the bottom
of the column to boiling boint. The following are examples of typical
reboiler types.

DISTILLATION PRINCIPLES
Separation of components from a liquid mixture via
distillation depends on the differences in boiling points of
the individual components. Also, depending on the
concentrations of the components present, the liquid
mixture will have different boiling point characteristics.
Therefore, distillation processes depends on the vapour
pressure characteristics of liquid mixtures.

DISTILLATION PRINCIPLES
Vapour Pressure and Boiling
The vapour pressure of a liquid at a particular temperature is the
equilibrium pressure exerted by molecules leaving and entering the
liquid surface. Here are some important points regarding vapour
pressure:
• energy input raises vapour pressure
• vapour pressure is related to boiling
• a liquid is said to ‘boil’ when its vapour pressure equals the
surrounding pressure
• the ease with which a liquid boils depends on its volatility
• liquids with high vapour pressures (volatile liquids) will boil at
lower temperatures
• the vapour pressure and hence the boiling point of a liquid mixture
depends on the relative amounts of the components in the mixture
• distillation occurs because of the differences in the volatility of the
components in the liquid mixture

Flash distallation by khalid

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