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Separation may be defined as a process that transforms a mixture
of substances into two or more products that differ from each
other in composition.
The surface of a solid represents a discontinuity of its structure.
The forces acting at the surface are unsaturated. Hence, when
the solid is exposed to the gas, the gas molecules will form
bonds with it and become attached. This phenomenon is termed
as adsorption. Adsorption, the binding of molecules or particles
to a surface, must be distinguished from Absorption, the filling
of pores in a solid. The binding to the solid is usually weak
and reversible. The adsorption process involves nothing more
than the preferential partitioning of the substances from the
gaseous or liquid phase onto the surface of a solid substrate.
Adsorption, also known as adsorptive separation, can be simply
defined as the concentration of a solute, which may be molecules
in a gas stream or a dissolved or suspended substance in a
liquid stream, on the surface of a solid.
The adsorptive separation is achieved by one of three mechanisms:
steric, kinetic or equilibrium effects. The steric effect derives
from the molecular sieving property of zeolites. In this case
only small and properly shaped molecules can diffuse into the
adsorbent, whereas other molecules are totally excluded. Kinetic
separation is achieved by the virtue of the difference in diffusion
rates of different molecules. A large majority of processes
separate through the equilibrium adsorption of the mixture
and hence are called Equilibrium separation processes.
Physical Adsorption vs. Chemisorption
The phenomenon of adsorption is essentially an attraction of adsorbate
molecules to an adsorbent surface. The preferential concentration of
molecules in the proximity of a surface arises because the surface forces
of an adsorbent solid are unsaturated. Both repulsive and attractive
forces become balanced when adsorption occurs. Adsorption is nearly always
an exothermic process.
We can distinguish between two types of adsorption process depending
on which of these two force types plays the bigger role in the process.
Adsorption processes can be classified as either physical adsorption
(van der Waals adsorption) or chemisorption (activated adsorption) depending
on the type of forces between the adsorbate and the adsorbent.
Physical adsorption is caused mainly by van der Waals forces ad electrostatic
forces between adsorbate molecules and the atoms, which compose the adsorbent
surface.
Thus adsorbents are characterized by surface properties such as surface
area and polarity. A large specific surface area is preferable for providing
large adsorption capacity, but the creation of a large internal surface
area in a limited volume inevitably gives rise to large numbers of small
sized pores between adsorption surfaces. The size of micro pores determines
the accessibility of adsorbate molecules to the internal adsorption surface,
so the pore size distribution of micro pores is another important property
of characterizing adsorptivity of adsorbents.
Chemisorption involves electron transfer and is essentially two-dimensional
chemical reaction. In this type of adsorption, the chemistry of adsorbate
is of central importance. In a particular system both types of adsorption
may occur as well as intermediate types. The solids best suited to adsorption
are very porous, and have very large effective surface areas.
MOLECULAR SIEVES
The discovery of molecular sieves can be traced back to 1756 when the
word zeolite was first used. It came from the Greek meaning "boil" and "rock" after
the observation was made that minerals lost their content of water
when heated. Molecular sieves are a class of adsorbents, which can
trap molecules by adsorption into their pores.
The term "molecular sieve" was originated by J.W. Mcbain to
define porous solid materials, which have the property of acting as sieves
on a molecular scale. Zeolites are example of naturally occurring molecular
sieves.
Molecular sieve adsorbents are crystalline alumino-silicates. Their
unique structure allows the water of crystallization to be removed leaving
a porous crystalline structure. These pores or "cages" want
to readsorb water or other molecules. Aided by strong ionic forces caused
by the presence of cations such as sodium, calcium and potassium, the
molecular sieve will adsorb a considerable amount of water or other fluids.
If the fluid to be adsorbed is a polar compound, it can be adsorbed with
high loading even at very low concentrations of the fluid.
The strong adsorptive force allows molecular sieves to remove many gas
or liquid impurities at very low levels. Another feature of molecular
sieve adsorbents is its ability to separate gases or liquids by molecular
size. The pore or "cage" openings are of the same size as many
molecules. In the case of hydrocarbon paraffins, the normal, straight-chained
molecules can fit into the pores and be adsorbed while the branched chain
molecules cannot enter the pores and pass by the molecular sieve adsorbents.
Molecular Sieve Zeolites
Molecular-sieve zeolites are crystalline aluminosilicates of group IA
and Group IIA elements such as sodium, magnesium, potassium and calcium.
Chemically, they are represented by the empirical formula:
M2/nO * Al2O3 * YSiO2 * wH2O
Where Y is 2 or greater, n is the cation valence and w represents the
water contained in the voids of the zeolite. Structurally, zeolites are
complex, crystalline inorganic polymers based on an infinitely extending
framework of AlO4 and SiO4 tetrahedra linked to each other by sharing
of oxygen ions. The fundamental unit is a tetrahedral complex consisting
of a small cation, such as Si4+, in tetrahedral coordination with four
oxygens (Pauling's first rule). The Al3+ ion commonly coordinates tetrahedrally
as well as Octahedrally with oxygen in silicates.
This framework structure contains channels or interconnected voids that
are occupied by the cations or water molecules. The cations are mobile
and ordinarily undergo ion exchange. The water may be removed reversibly,
generally by the application of heat, which leaves intact a crystalline
host structure permeated by micropores, which may amount to 50% of the
crystals, by volume.
The structural formula of zeolite is based on the crystal unit cell,
the smallest unit of structure represented by:
Mx/n [(AlO2)x (SiO2)y] * wH2O
Where n is the valence of the cation M, w is the number of water molecules
per unit cell, x and y are the total number of tetrahedral per unit cell,
and y/x usually has values of 1-5. The cations are necessary to balance
the electrical charge of the aluminum atoms, each having a net charge
of -1. The water molecules can easily be removed with ease upon heat
and evacuation, leaving an almost unaltered aluminosilicate skeleton.
The molecular-sieve zeolites are distinct from the other three major
adsorbents in that they are crystalline and that adsorption takes place
inside the crystals, the access to which is limited by the pore size.
Zeolite molecular sieves have pores of uniform size (3 to 10A).
Zeolites selectively adsorb or reject molecules based on differences
in molecular size, shape and other properties such as polarity. During
the adsorption of various molecules, the micorpores fill and empty reversibly.
Adsorption in zeolites is a matter of pore filling, and the usual surface-area
concepts are not applicable.
The channels of zeolites are only a few molecular diameters in size,
and overlapping potential fields form opposite walls result in a flat
adsorption isotherm, which is characterized by a long horizontal section
as the relative partial pressure approaches unity. The adsorption isotherms
do not exhibit hysteresis.
Zeolites adsorb molecules, in particular those with permanent dipole
moments, which show other interaction effects, with selectivity that,
is not found in other solid adsorbents.
Separation may be based on molecular-sieve effect or may involve the
preferential or selective adsorption of one molecule species over another.
These separations are governed by several factors:
- The basic framework structure, or topology, of the zeolite determines
the pore size and void volume.
- The exchange cations, in terms of their
specific location in the structure, their population or density, their
charge, and size affect the molecular-sieve behavior and adsorption
selectivity of the zeolite. By changing the cation types and number,
one can tailor or modify with in certain limits the selectivity of
the zeolite in a given separation.
- The cations, depending on their
locations, contribute electric field effects that interact with the
adsorbate molecules.
- The effect of the temperature of the adsorbent
is pronounced in cases involving activated diffusion.
The framework contains channels and interconnected voids, which are
occupied by cation and water molecules. The cations are quite mobile
and may be exchanged to varying degree by other cations. The structure
can be changed depending on the cation exchange.
In crystal form, zeolites are distinct from other adsorbents in that,
for each type, there is no distribution of pore size. The lattice into
which the adsorbate molecules can or cannot enter is precisely uniform.
For this reason zeolites are capable of separating effectively on the
basis of size and they are often known as molecular sieves.
In addition to changes to cationic structure, the Si/Al ration can be
varied, thus zeolites with widely different adsorptive properties may
be tailored by the appropriate choice of framework structure, cationic
form, and silica –to-alumina ration in order to achieve the selectivity
required for a given separation.
The ionic nature of most zeolites means that they have a high affinity
for water and other polar molecules such as carbon dioxide and hydrogen
sulfide. However, as the silica-to-alumina ration is increased the material
can become hydrophobic. Such zeolites can be used in the removal of volatile
organic compounds from air.
More than 150 synthetic zeolite types are known, the most important
commercially being the synthetic types A and X, synthetic mordenite and
their ion-exchange varieties. Of the 40 or so mineral zeolites the most
important commercially are chabazite, faujasite and mordenite.
Applications of Zeolites
- Adsorption: Can adsorb a variety of materials in such processes
as drying, purification and separation.
- Catalysis: Zeolites can shape
selective catalysts on the basis of molecular diameter and are used
in petroleum refining and synthetic fuels production.
- Ion Exchange:
- The largest use where they have replaced phosphates as water softening agents
- The cleaning of radioactive contaminated water with the removal of Caesium and Strontium
- Removal of ammonia and ammonium ions from waste water
Apart from the applications described above, they are also used to remove
hydrogen sulfide and other sulfur compounds from natural gas and LPG
liquids to meet extracting specifications. The removal of carbon dioxide
from air and natural gas before cryogenic processing is routine. Molecular
sieves are the premium desiccants for insulated windows. Special varieties
are used for refrigerant drying.
Selection of adsorbent
The selection of proper sorbent for a given separation is a complex
problem. The predominant scientific basis for sorbent selection is the
equilibrium isotherm. The equilibrium isotherms of all constituents in
the gas mixture, in the temperature and pressure range of operation,
must be considered. As a first and possibly over simplified approximation,
the pure gas isotherms may be considered additive to yield the adsorption
from a mixture. Based on the isotherms, the following factors that are
important to the design of the separation can be estimated:
- Capacity of sorbent, in operating temperature and pressure range
- The
method of sorbent regeneration
- The length of unused bed
- The product purities
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