Modern Separation Processes
UNIT- I
Review of conventional processes, Recent advances in separation techniques based on size,
surface properties, ionic properties and other special characteristics of substances, Process
concept, Theory and equipment used in cross flow filtration, cross flow electrofiltration, dual
functional filter, Surface based solid – liquid separations involving a second liquid, Sirofloc filter.
In Chemical process industries, separation operations are of great importance. Such
operations are employed to:
(i) separate feed mixture into other mixtures and relatively pure components
(ii) recover solvents for recycle
(iii) remove wastes
(iv) purify reactant feeds
(v) recover reactants from reactant effluents for recycle
(vi) recover by-products
(vii) recover and purify products to meet required specifications.
Mechanism of separation:
A schematic diagram of a general separation process is shown:
The feed mixture can be vapor,liquid or solid. It consists of different chemical species
in various compositions. The product streams are two or more differing from each other
in their compositions. The separation is accomplished by forcing the different chemical
species in the feed into different spatial locations by any of the 5 separation technique.
The most common industrial technique (fig.1.7a) involves the creation of a second
phase (V,L or S) that is immiscible with the feed phase. The creation is accomplished by
energy (heat and/or shaft work) to or from the process or by pressure reduction.
A second technique is to introduce the second phase into the system in the form of a
solvent that selectively dissolves some of the species in the feed.
The use of a barrier (fig. 1.7c) is less common but of growing importance. It restricts
and / or enhances the movement of certain chemical species with respect to other species.
The addition of solid particles is another technique that is of growing importance (fig.
1.7d). The solid particles act directly or as inert carriers for other substances so as to
cause separation. Finally, external fields (fig. 1.7e) of various types are sometimes
applied for specialized separations.
For all the techniques of fig. 1.7, separations are achieved by enhancing the rate of
mass transfer by diffusion of certain species relative to mass transfer all species by bulk
movement within a particular phase. Both transport and thermodynamic considerations
are crucial in separation operations.
The difference in properties of the different species is made use of in separation
operations. Some properties of importance are:
1. Molecular properties:
-----------------------------------------------------------------------------------------------------Molecular weight
Polarizability
Van der Waals volume
Dielectric constant
Van der Waals area
Electric charge
Molecular shape (acentric factor)
Radius of gyration
Dipole moment
2. Thermodynamic and transport properties:
------------------------------------------------------------------------------------------------------.
Vapor pressure
Adsorptivity
Solubility
Diffusivity
------------------------------------------------------------------------------------------------------
(a)Separation by Phase Addition or Creation:
Separation
Initial
Created
Separatng
operations
or Feed
or Added Agent(s)
Phase
Phase
Partial
V and / or L and/or Heat
Condensation L
V
Transfer
or
(ESA)
vaporization
Flash
L
V
Pressure
vaporization
reduction
Distillation
V and / or V and L
Heat transfer
L
(ESA)
Extractive
V and / or V and L
Liquid solvent
Distillation
L
(MSA) & Heat
transfer
(ESA)
Reboiled
V and / or V and L
Liquid
Absorption
L
absorbent
(MSA) & Heat
transfer
(ESA)
Absorption
V
L
Liquid
absorbent(MSA)
Stripping
L
V
Stripping vapor
(MSA)
Refluxed
Stripping
V and / or
L
V and L
Reboiled
Stripping
L
V
Stripping vapor
(MSA) & Heat
transfer(ESA)
Heat
transfer(ESA)
Azeotropic
Distillation
V and / or
L
V and L
Liquid entrainer
(MSA) & Heat
transfer(ESA)
Liquid-Liquid
extraction (2
solvent)
L
L
Two liquid
solvents (MSA1
& MSA2)
Industrial Example
Recovery of H2 and N2 from
ammonia by partial
condensation and high
ressure phase separation
Recovery of water from sea
water
Purification of styrene
Separation of acetne and
methanol
Removal of ethane & lower
MW hydrocarbons for LPG
production
Separation of CO2 from
combustion products by
absorbing with an aqueous
solution of ethanolamine
Steam stripping of naphtha,
kerosene & gas oil side cuts
from crude oil & to remove
light oils
Separation of products from
Delayed coking
Recovery of amine
absorbent
Separation of acetic acid
from water using n-butyl
acetate as entrainer to form
an azeotrope with water
Separation of paraffins
from aromatics &
naphthenes using propane
And cresylic acd as solvents
Drying
L & often
S
V
Evaporation
L
V
Gas (MSA)
and/or Heat
transfer(ESA)
Heat
transfer(ESA)
Heat
transfer(ESA)
Crystallization L
S (& V)
Desublimation V
S
Heat
transfer(ESA)
Leaching (L-S
extraction)
Foam
fractionation
S
L
Liquid solvent
L
G
Gas
bubbles(MSA)
Removal of water from
polyvinyl chloride with hot
air in a fluid-bed dryer
Evaporation of water from a
solution of urea & water
Crystallization of p-xylene
from a mixture with mxylene
Recovery of phthalic
anhydride from
noncondensible gas
Extraction of sucrose from
Sugar beets with hot water
Recovery of detergents from
water solutions
(1) Osmosis (2) Reverse osmosis (3) Dialysis (4) Microfiltration (5) Ultrafiltration
(6) Pervaporation (7) Gas Permeation (8) Liquid membranes
Key factors based on which separation occurs.
Fig.11.1 shows the key factors enabling separation of a mixture or solution. The
difference in these properties of the chemical species present in the mixture decides the
separation method. In some cases 2 such properties will be made use of to bring about
separation; eg.,Electrodialysis. Here, diffusivity as well as the electric charge of the
particular species are exploited to separate them from other species differing in these
properties.
Filtration:
Conventional filtration is the most cost effective method for the separation of large
particles and cells from slurries and fermentation broths. The feed is passed through a
filter medium, and a filter cake is formed as aresilt of deposition of solids on the filter
surface. Plate and frame filters, continuous rotary filters and rotary vacuum precoat
filters are the most widely used types.
Rotary Vacuum Filter:
In this filter, the drum is covered with a layer of precoat, usually diatomaceous earth,
prior to filtration. A small amount of coagulating agent or filter aid is added to the feed
generally, before it is pumped into the filter. As the drum rotates under vacuum, a thin
layer of solids adhere to the drum. The thickness of this layer increases in the suction
designed for forming the cake. The layer of solids is washed and dewatered during its
passage to the discharge point, where a knife blade cuts off the cake. A vacuum
maintained in the drum provides the driving the driving force for the liquid and air flow.
Modern Filtration Methods:
Pressure-driven membrane processes for Liquid Separation:
The most important pressure-driven processes (ΔP across the membrane is the driving
force is the driving force for permeation) are: Microfiltration, Ultrafiltration and Reverse
Osmosis (also called hyperfiltration).
Consider an aqueous stream containing (i) suspended particles-bacteria and blood cells,
(ii) large, dissolved molecules-proteins and carbohydrates and (iii) small, dissolved
molecules-salts. These solutes have to be separated from the carrier solution.
Microfiltration and Ultrafiltration are most commonly used for such systems. (Second
option is RO).
Microfiltration:
Separation of fine particles and colloids from a liquid or separation of particulates from a
gas using a porous membrane having pore sizes in the range 100 to 104 nm (0.1 to 10
μm).
MF is an old membrane separation process retaining particles of micron size ; one of the
traditional use is for separation of microorganisms (eg. separation of yeast from
fermentation broth).
Ultrafiltration:
In Ultrafiltration, the membrane retains particles of submicron size by ultramicroporous
membranes. Usually UF retains solutes in the 300-5,00,000 molecular weight range,
including biomolecules, polymers, sugars and colloidal particles.
As with Reverse Osmosis (RO), MF & UF are pressure driven, with the membrane
permselective for the solvent (usually water). MF & UF separate mainly by size
occlusion of solutes.
Membrane
process
Micro
Filtration
Separation
Mechanism
Sieving
Feed
Driving
force
Pressure
Difference
ΔP < 2
bars
Rejected
species
0.1 μm to
20 μm
Permeated
species
The
suspending
medium, L/G
Ultra
filtration
Sieving
Liquid
Pressure
Difference
ΔP=2 to
3.5 bars
Relatively
large
molecules
(1-100 nm)
Solvent, low
molecular
weight solute
Reverse
osmosis
Solution
diffusion
Salt
solutions,
Sea water
ΔP =10 to
100 bars
or more
Dialysis
Sieving,
Solution
diffusion
Solutions
Concentration
difference
Pervapora
tion
Solution
diffusion
Liquid
Activity
difference
Gas
separation
Sorption
diffusion
Gas
mixtures
Difference
of
fugacity,
or ΔP
Low MW
Solvent
solutes,
(generally
hydrated
water)
ions
Large
Micro solutes,
molecules
Solvent
(5 -50 A)
MW 50 to
104 dalton
Molecules that have higher
affinities for themembrane are
preferentially sorbed on the
feed side of the membrane and
then diffuse through it. The
molecules get desorbed on the
permeate side under vacuum
(for pervaporation) or under
the effect of pressure
difference causing enrichment.
Liquid/ Gas
containing
suspended
particles
Applns
Separation of cells,
sterilization of L/G
streams, sep. of
emulsified oil from
water, clarification
of liquids/
beverages in food
industry
Separation of
biomolecules,
proteins,
emulsions,
dispersed droplets,
macro
Molecules, autopaints from
solutions
Desalination of sea
water,, brackish
water, treatment of
waste water
Haemo
Dialysis,
Recovery of
selected solutes
Production of
absolute alcohol,
dehydration of
solvents, removal
of trace organics
from water
Separation of air
(O2/N2), CO2 from
methane, organic
vapors from air or
a carrier gas, H2
from other light
gases.
(c) Reverse Osmosis:
If separation of small molecules from a solution is required, a dense or non porous
membrane is generally used. In the case of separation of salt from brackish water, the
transport of the solvent (water) occurs under the effect of a pressure difference and
potable water with only traces of salts is obtained as the permeate.
Two types of flow and filtration arrangements:
(i) cross-flow filtration and (ii) dead-end filtration.
(i) Cross-flow filtration:
The feed flows parallel to the membrane surface. Most of the particles or solute
retained are swept away with the flowing feed side liquid. A part of the liquid is
recirculated if the desired concentration of the retentate is not obtained in a single pass.
(ii) Dead-end filtration:
The feed flow is normal to the membrane surface. The retained particles or the solute
remain on the membrane forming a cake or a gel layer. Particle accumulation on the
membrane increases the filtration resistance.
7.2 The Present: Current Status and Potential of the Membrane Industry 313
Fig. 7.7 Use of hydrogen-permeable membranes to recover
and reuse hydrogen from hydrocracker purge gas streams.