mto-introduction-part 2

 The membrane operate in different ways, depending
upon the nature of the separation to be made.
 The membranes serve to prevent intermingling of two
miscible phases, hydrodynamics flow, and the
movement of substance through them is by diffusion.
 The membranes permit a component separation by
selectively controlling passage of the components from
one side to the other.

 In gaseous diffusion or effusion, the
membrane is micro porous. If a gas mixture
whose components are of different
molecular weights are brought into contact
with such a diaphragm, the various
components of the gas pass through the
pores at rates dependent upon the
molecular weights e.g. separation of
uranium from gaseous uranium
hexafluoride.

 In permeation, the gas transmitted
through the non porous membrane
first dissolves according to the
solubility in it and then diffuses
through. e.g. separation of helium (He)
from natural gas through fluorocarbon
polymer membranes.

 e.g. a liquid solution of alcohol and
water brought into contact with
suitable non porous membrane, in
which alcohol preferentially dissolve
after passage through the membrane,
the alcohol is vaporised on the far side.

 The separation of a crystalline substance from colloid, by
contact of their solution with a liquid solvent with an
intervening membrane permeable only to the solvent and
dissolved crystalline substance is known as dialysis. e.g.
separation of undesired colloidal material from beet-sugar
solution.
 Fractional dialysis for separating two crystalline
substance in solution makes use of the difference in the
membrane permeability for the substances.
 If an electromotive force is applied across the membrane to
assist in the diffusion of charged particles, the operation is
Electrodialysis.

 If a solution is separated from the pure
solvent, the solvent diffuses into the
solution, an operation known as osmosis.
This is not a separation, but by super
imposing pressure to opposite the osmotic
pressure the flow of solvent is reversed, and
the solute and solvent of a solution can be
separated by Reverse osmosis. e.g.
desalination of water.

 This involves the formation of a
concentration difference within single
liquid or gaseous phase by imposition
of a temperature gradient upon the
fluid, thus making a separation of the
components of the solution possible.
E.g. separation of 3He from 4He and
mixture.

 If steam is allowed to diffuse through a
gas mixture, it will preferentially carry
one of the component along with it,
thus making a separation by the
operation known as sweep diffusion.

 If the two zones within the gas phase
where the concentration are different
are separated by a screen containing
relatively large openings, the operation
is called atmolysis.

 If a gas mixture is subjected to a very
rapid centrifugation, the components
will be separated because of the slightly
different forces acting on the various
molecules owing to their different
masses. The heavier molecules thus
tend to accumulate at the periphery of
the centrifuge. e.g. separation of
uranium isotopes.

 By forming a foam of large surface, as
by bubbling air through the solution
and collecting the foam, the solute can
be concentrated. This operation is
known as foam separation. e.g.
separation if detergent from water.

DIRECT MTO INDIRECT MTO
1) In this type of operation the two immiscible
phases are generated from a single phase
solution by addition or removal of heat.
1) In this type of operation the separation
involves the addition of a foreign substance.
2) e.g. fractional distillation, fractional
crystallization and CST type fractional liquid
extraction.
2) e.g. gas absorption, stripping, adsorption,
drying, leaching, liquid extraction and certain
types of crystallization (adductive).
3) Products obtained are free of added substance
and most frequently (if almost pure) doesn’t
require any further separation.
3) The removed substance is obtained as a
solution with foreign substance and requires
further separation of pure substance and added
substance and lead to expense and less quality
of products.
4) Any type of corrosion or losses are not
involved.
4) Addition of foreign substance creates
corrosion problem and cost of inevitable losses.
5) They are rarely used because of high cost
(most if the cost is due to heat supplied or
removed).
5) They are frequently used because of net less
cost.
6) Applications:-
These methods are more frequently used to
obtain extra pure substances.
6) indirect methods used for such as:
i) drying of clothes by air (in summer)!!!
ii) production of HCl aqueous solution by
absorption of HCl containing gas into the water,
with no further separation requirement.

 Choice of separating the component of a
solution is usually limited by the peculiar
physical characteristics of the materials
to be handled.
 Choice of method exists between mass
transfer operation and a purely
mechanical separation method. e.g. in
the separation of desired mineral from its
ore, it maybe possible to use either the mass
transfer operation of leaching with a solvent
or purely mechanical methods of flotation.

 Sometimes both mechanical and mass
transfer operations are used especially
where the former is incomplete, as in
processes for recovering vegetables oils
wherein mechanical expression is followed
by leaching. It is characteristic that at the
end of the operation the substance removed
by mechanical methods is pure, while if
removed by diffusional methods it is
associated with another substance.

 One can also frequently choose between a
purely mass transfer operation and a
chemical reaction or a combination of
both. e.g. H2S can be separated from other
gases either by absorption in a liquid solvent
with or without simultaneous chemical
reaction or by chemical reaction with ferric
oxide. Chemical methods ordinarily destroy
the substance removed, while mass transfer
methods usually permit its eventual
recovery in unaltered form without great
difficulty.

 There are also choices to be made within
the mass transfer operations. e.g. a liquid
solution of acetic acid maybe separated by
distillation, by liquid extraction with a
suitable solvent, or by adsorption with a
suitable solid adsorbent.
 The principle basis for choice in any case is
cost; that method which costs the least is
usually the one to be used.

 Ease of operation:- occasionally other factors
also influence the decision, however, the
simplest operation, while it may not be the
least costly, is sometimes desired because it will
be trouble free.
 Sometimes a method will be discarded because
of imperfect knowledge of design methods
or unavailability of data for design, so that
results cannot be guaranteed.
 Favorable previous experience with one
method maybe given strong considerations.

 Solute recovery and fractionation
 Unsteady state operation
 Steady state operation
 Stage wise operation
 Continuous contact operation (or
Differential contact operation)

 If the components of a solution fall into two distinct groups
of quite different properties, so that one can imagine in
that one group of components constitutes the solvent and
other group the solute, separation according to this groups
is usually relatively easy and amounts to a solute recovery
or solute removal operation. e.g. separation of methane
(solvent) from methane + pentane (solute) + hexane
(solute) mixture by absorption with oil due to the property
(vapour pressure) difference.
 While the component properties differ, the difference are
small and to separate them into relatively pure components
requires a different technique. Such separation are termed
fractionation. e.g. separation of pentane + hexane
mixture by fractional distillation.

 It is characteristic of unsteady state operation that
concentrations at any point in the apparatus
change with time.
 This may result from changes in concentration of
feed material, flow rates or conditions of
temperature or pressure.
 In any case, batch operations are always of the
unsteady state type. e.g. laboratory extraction
procedure of shaking a solution with an
immiscible solvent.
 In semi batch operations, one phase is stationary
while the other flows continuously in and out of
the apparatus. e.g. drying of clothes through air.

 It is characteristic of steady state operations
that concentrations at any positions in
the apparatus remains constant with
passage of time.
 This requires continuous, irreversible flow
of all phases into and out of the apparatus, a
persistence of the flow regime within the
apparatus, constant concentration of the
feed streams and unchanging conditions of
temperature and pressure.

 If two insoluble phases are first allowed to come into
contact so that the various diffusing substances can
distribute themselves between the phases, and if the
phases are then mechanically separated, the entire
operation and the equipment required to carry out it
are said to constitute one stage. e.g. laboratory batch
extraction in a separatory funnel. This operation can
be carried out in continuous fashion (steady state) or
batch wise fashion.
 However, for separation requiring greater
concentration changes, a series of stages can be
arranged so that the phases flow through the
assembled stages from one to another. Such an
assemblage is called cascade.

 In this case the phases flow through the
equipment in continuous, intimate contact
throughout without repeated physical separation
and recontacting.
 The nature of the method requires the operation
to be either semi-batch or steady state.
 Equilibrium between two phases at any position in
equipment is never established; indeed, should
equilibrium occur anywhere in the system, the
result would be equivalent to the effect of an
infinite number of stages.

 There are four major factors to be
established in the design of any plant
involving the diffusional operations.
1. Number of equilibrium stages
2. Time requirement
3. Permissible flow rate
4. Energy requirement

 In order to determine the number of
equilibrium stages required in a
cascade to bring about a specified
degree of separation, or the equivalent
quantity for a continuous contact
device, the equilibrium characteristics
of the system and material balance
calculations are required.

 In stage wise operations the time of contact is intimately
connected with stage efficiency, whereas for continuous contact
equipment the time leads ultimately to the volume or length of
the required device.
 Material balance permit calculation of the relative quantities
require for the various phases.
 The equilibrium characteristics of the system establish the
ultimate concentration possible, and the rate of transfer of
material between phases depends on the departure from
equilibrium which is maintained.
 The rate of transfer additionally depends upon the physical
properties of the phases as well as the flow regime within the
equipment.
 For a given degree of intimacy of contact of the phases, the time
of contact required is independent of the total quantity of the
phases to be processed.

 This factor enters into consideration of
semi batch and steady state operations,
where it leads to the determination of
the cross sectional area of the
equipment. Considerations of fluid
dynamics establish the permissible
flow rate, and material balances
determine the absolute quantity of
each of the streams required.

 Heat and mechanical energies are ordinarily
required to carry out the diffusional operations.
Heat is necessary for the production of any
temperature changes, for the creation of new
phases and for overcoming heat of solution effects.
Mechanical energies required for fluid and solid
transport, for dispersing liquids and gases, and for
operating moving parts of machinery.
 The ultimate design, consequently, requires us to
deal with the equilibrium characteristics of the
system, material balances, diffusional rates, fluid
dynamics and energy requirements.

mto-introduction-part 2

More Related Content

What's hot (19)

Similar to mto-introduction-part 2 (20)

More from Vivek Faldu (11)

Recently uploaded (20)

mto-introduction-part 2