I I I
Component Separation
Fundamental
Outline
Heterogeneous Separation:
1. Gas-liquid (or vapor–liquid)
2. Gas–solid (or vapor–solid)
3. Liquid–liquid (immiscible)
4. Liquid–solid
5. Solid–solid.
Homogeneous Separation
1. Creation of another phase
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I I I .1.
HETEROGENEOUS SEPARATI ON
Heterogeneous Separation
(Smith, R., 2005)
I f a heterogeneous (multiphase mixture), separation can be
done physically by exploiting the differences in density
between the phases.
Separation of the different phases of a heterogeneous
mixture
should
be
carried
out
before
homogeneous
separation
Phase separation tends to be easier and should be done
first.
The phase separations likely to be carried out are:
•
Gas–liquid (or vapor–liquid)
•
Gas–solid (or vapor–solid)
•
Liquid–liquid (immiscible)
•
Liquid–solid
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
The principal methods for the separation of
heterogeneous mixtures are:
1. Settling and sedimentation
2. I nertial and centrifugal separation
3. Electrostatic precipitation
4. Filtration
5. Scrubbing
6. Flotation
7. Drying.
I I I .1.1. Settling and Sedimentation
Particles are separated from a fluid
by gravitational forces
acting on the particles.
The particles can be
liquid drops
or
solid particles
.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Gravity settler for the separation of gas–liquid and vapor–
liquid mixtures
The velocity of the gas or vapor through the vessel must be less than the settling velocity of the liquid drops.
I t is normally not practical to separate droplets less than 100 µm diameter in such a simple device.
Thus, the design basis for simple settling devise is usually taken to be a vessel in which the velocity of the gas (or vapor) is the terminal settling velocity for droplets of 100 µm diameter.
Gravity settler ( Decanter) for the separation of
liquid–liquid mixtures
The horizontal velocity must be low enough to allow the low-density droplets to rise from the bottom of the vessel to the interface and coalesce and for the high density droplets to settle down to the interface and coalesce.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Gravity settler for the separation of fluid–solid mixtures
A mixture of gas, vapor or liquid and solid particles enters at one
end of a large chamber.
Particles settle toward the base. Again the device is specified on
the basis of the terminal settling velocity of the particles.
A thickener for liquid–solid separation.
When separating a mixture of water and fine solid particles in a gravity settling device, it is common in such operations to add a flocculating agent to the mixture to assist the settling process.
This agent has the effect of neutralizing electric charges on the particles that cause them to repel each other and remain dispersed.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Simple gravity settling classifier
The larger particles, faster-settling particles settle to the bottom close to the entrance
The smaller particles, the slower-settling particles settle to the bottom close to the exit
I I I .1.2. I nertial and Centrifugal Separation
Sometimes gravity separation (
discussed earlier
) may be too slow
because of the closeness of the densities of the particles and the
fluid, because of small particle size leading to low settling velocity
or, in the case of liquid–liquid separations, because of the
formation of a stable emulsion.
I nertial or momentum separators improve the efficiency
of gas–
solid settling devices by giving the particles downward momentum,
in addition to the gravitational force.
Centrifugal separators take the idea of an inertial separator a step
further and make use of the principle that an object whirled about
an axis at a constant radial distance from the point is acted on by
a force. Use of centrifugal forces increases the force acting on the
particles.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I nertial separators increase the efficiency of separation by
giving the particles dow nw ard momentum.
A cyclone generates centrifugal force by the fluid motion.
The simplest type of centrifugal device is the
cyclone
that consists of a vertical cylinder with
a conical bottom.
Centrifugal force is generated by the motion of
the fluid.
The mixture enters through a tangential inlet
near the top, and the rotating motion so
created develops centrifugal force that throws
the dense particles radially toward the wall.
The entering fluid flows downward in a spiral
adjacent to the wall.
The particles of dense material are thrown
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
A centrifuge uses rotating cylindrical bow l to produce
centrifugal force.
I n
centrifuges, a cylindrical
bowl is
rotated to produce the centrifugal
force.
The cylindrical bowl is shown rotating
with a feed consisting of a liquid–
solid mixture fed at the center.
The feed is thrown outward to the
walls of the container.
The
particles
settle
horizontally
outward.
Different arrangements are possible
to remove the solids from the bowl.
A centrifuge uses rotating cylindrical bow l to produce
centrifugal force.
two liquids having different
densities are separated by the
centrifuge.
The more dense fluid occupies
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I I I .1.3. Electrostatic Precipitation
Electrostatic precipitators are generally used to separate particulate matter that is easily ionized from a gas stream.
Particles collect on the plates and are removed by vibrating the collection plates mechanically, thereby dislodging particles that drop to the bottom of the device.
corona
Electrostatic precipitation is most effective when separating particles with a high resistivity.
The operating voltage typically varies between 25 and 45 kV or more, depending on the design and the operating temperature.
The application of electrostatic precipitators is normally restricted to the separation of fine particles of solid or liquid from a large volume of gas.
I I I .1.4. Filtration
Suspended solid particles in a gas, vapor or liquid are removed by
passing the mixture through a porous medium that retains the
particles and passes the fluid (filtrate).
The solid can be retained on the surface of the filter medium,
which is
cake filtration, or captured
within the filter medium, which
is
depth filtration.
The filter
medium can be arranged in many ways:
1. Plate and Frame Filter (
separation of solid-liquid
)
2. Bag Filter (
separation of solid-gas
)
3. Belt Vacuum Filter (
separation of solid-liquid
)
4. Rotary Vacuum Filter (
separation of solid-liquid
)
When separating solid particles from a liquid filtrate:
1. Filtrate is a product (cake as a waste)
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Filtration can be arranged in many w ays
I I I .1.5. Srubbing
Scrubbing with liquid (usually water) can enhance the
collection of particles when separating gas–solid mixtures.
Three of the many possible designs for scrubbers:
1. Packed-bed Scrubber
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Packed- bed Scrubber
a
packed
tower
is
similar
to
an
absorption tower.
Whilst this can be effective, it suffers
from the problem that the packing can
become clogged with solid particles.
Towers using perforated plates similar
to a distillation or absorption column
can also be used. As with packed
columns,
plate
columns
can
also
encounter problems of clogging.
Spray Scrubber
Spray Scrubber uses a spray
system that will be less prone to
fouling.
The design of spray scrubber
uses a tangential inlet
to
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Venturi Scrubber
Liquid is injected into the
throat
of
the
venturi,
where the velocity of the
gas is highest.
The gas accelerates the
injected water to the gas
velocity, and breaks up
the liquid droplets into a
relatively fine spray.
The particles are then captured by the fine droplets. Very high
collection efficiencies are possible with venturi scrubbers.
The main problem with venturi scrubbers is the high pressure loss
across the device.
I I I .1.6. Flotation
Flotation is a gravity separation process that exploits the differences in the surface properties of particles.
Gas bubbles are generated in a liquid and become attached to solid particles or immiscible liquid droplets, causing the particles or droplets to rise to the surface.
This is used to separate mixtures of solid–solid particles after dispersion in a liquid, or solid particles already dispersed in a liquid or liquid–liquid mixtures of finely divided immiscible droplets.
The liquid used is normally water and the particles of solid or immiscible liquid will attach themselves to the gas bubbles if they are hydrophobic (e.g. oil droplets dispersed in water).
The bubles of gas can be generated by three methods:1. dispersion, in which the bubbles are injected directly by some form of sparging system
2. dissolution in the liquid under pressure and then liberation in the flotation cell by reducing the pressure
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
A typical flotation cell for solid separation
The mixture is fed to a flotation cell, and gas is also fed to the cell
where gas bubbles become attached to the solid particles, thereby
allowing them to float to the surface of the liquid.
The
separation
of
the
solid
particles depends on the different
species having different surface
properties such that one species is
preferentially
attached
to
the
bubbles.
The solid particles are collected from the surface by an overflow
weir or mechanical scraper.
A number of chemicals can be added to the flotation medium to
meet the various requirements of the flotation process:
a. Modifiers are added to control the pH of the separation.
These
could be acids, lime, sodium hydroxide, and so on.
b. Collectors are water-repellent
reagents that
are added
to
preferentially adsorb onto the surface of one of the solids.
Coating or partially coating the surface of one of the solids
renders the solid to be more hydrophobic and increases its
tendency to attach to the gas bubbles.
c. Activators are used to “activate” the mineral surface for
the
collector.
d. Depressants are used to preferentially attach to one of
the solids
to make it less hydrophobic and decrease its tendency to attach
to the gas bubbles.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
When
separating
low-density solid particles or
oil droplets from water,
the
most
common
method
used
is
dissolved-air flotation.
DAF shows some of the effluent water from the unit being
recycled, and air being dissolved in the recycle under pressure.
The pressure of the recycle is then reduced, releasing the air from
solution as a mist of fine bubbles.
This is then mixed with the incoming feed that enters the cell.
Low-density material floats to the surface with the assistance of
the air bubbles and is removed.
Dissolved air Flotation ( DAF)
I I I .1.7. Drying
Drying refers to the removal of water from a substance through a
whole range of processes, including distillation, evaporation and
even physical separations such as centrifuges.
Here, consideration is restricted to the removal of moisture from
solids into a gas stream (usually air) by heat, namely,
thermal
drying.
Some of the types of equipment
for removal of water also can be
used for removal of organic liquids from solids.
Four of the more common types of thermal dryers used in the
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Tunnel Dryer
Wet material
on trays or a conveyor belt is passed through a
tunnel, and drying takes place by hot air.
The air-flow can be counter-current, co-current or a mixture of
both.
This method is usually used when the product is not free flowing
Rotary Dryer
Wet material is fed at the higher end and flows under gravity.
Drying takes place from a flow of air, which can be counter-current or co-current.
The heating may be direct to the dryer gas or indirect through the dryer shell.
This method is usually used when the material is free flowing.
Rotary dryers are not well suited to materials that are particularly heat sensitive because of the long residence time in the dryer.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Drum Dryer
Drum dryer consists
of a heated metal roll. As the roll rotates, a
layer of liquid or slurry is dried.
The final dry solid is scraped off the roll. The product comes off in
flaked form.
Drum dryers are suitable for handling slurries or pastes of solids in
fine suspension and are limited to low and moderate throughput.
Spray Dryer
I n spray dryer, a liquid or slurry solution is sprayed as fine droplets into a hot gas stream.
The feed to the dryer must be pumpable to obtain the high pressures required by the atomizer.
The product tends to be light, porous particles.
An important advantage of the spray dryer is that the product is exposed to the hot gas for a short period. Also, the evaporation of the liquid from the spray keeps the product temperature low, even in the presence of hot gases.Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I I I .2.
HOMOGENEOUS SEPARATI ON
Homogeneous Separation
(Smith, R., 2005)
I f the mixture is homogeneous, separation can only be
performed by the creation of another phase within the
system and the addition of a mass separation agent.
For example, if a vapor mixture is leaving a reactor, another
phase could be created by partial condensation.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Partial Condensation of Reactor Product
Partial condensation for separating H2from others
The principal methods for the separation of
homogeneous mixtures are:
1. Distillation
2. Absorption and Stripping
3. Liquid-Liquid Extraction
4. Adsorption
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I I I .2.1. Distillation
The separation of a homogeneous fluid mixture requires the
creation of another phase
I f this liquid mixture is partially vaporized, then another phase is
created, and the vapor becomes richer in the more volatile
components (i.e. those with the lower boiling points) than the
liquid phase.
The liquid becomes richer in the less volatile components (i.e.
those with the higher boiling points).
I f the system is allowed to come to equilibrium conditions, then
the distribution of the components between the vapor and liquid
phases is dictated by vapor–liquid equilibrium considerations.
All components can appear in both phases.
A cascade of equilibrium stages w ith refluxing and reboiling
I t is assumed in the cascade that liquid and vapor streams leaving each stage are in equilibrium.
Using a cascade of stages in this way allows the more volatile components to be transferred to the vapor phase and the less-volatile components to be transferred to the liquid phase.
I n principle, by creating a large enough cascade, an almost complete separation can be carried out.
At the top of the cascade, liquid is needed to feed the cascade (by condensing the top product, as
reflux). total condenser or partial condenser At the bottom of the column, vapor is also needed
to feed the cascade (by vaporizing vaporizing some of the liquid leaving the bottom stage)
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Distillation Tray and Packing
Distillation Tray Distillation Packing
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Choice of operating conditions of distillation
Specified conditions:
1. Feed composition and flowrate
2. Product specifications: product purities of recoveries of
certain component
The operating parameters to be selected by the designer
include:
1. operating pressure
2. reflux ratio
3. feed condition
4. type of condenser
Operating pressure of distillation column
Aspressure is raised:
• separation becomes more difficult (relative volatility decreases), that is, more stages or reflux are required;
• latent heat of vaporization decreases, that is, reboiler and condenser duties become lower;
• vapor density increases, giving a smaller column diameter;
• reboiler temperature increases with a limit often set by thermal decomposition of the material being vaporized, causing excessive fouling;
• condenser temperature increases.
As pressure is low ered:
• The lower limit is often set by the desire to avoid:
1. vacuum operation
2. refrigeration in the condenser
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Reflux ratio
For a stand-alone distillation column (i.e. utility used for both reboiling and condensing), there is a capital– energy trade-off.
As the reflux ratio is increased from its minimum, the capital cost decreases initially as the number of plates reduces from infinity, but the utility costs increase as more reboiling and condensation are required
The optimal ratio of actual to minimum reflux is often less than 1.1. However, most designers are reluctant to design columns closer to minimum reflux than 1.1, except in special circumstances, since a small error in design data or a small change in operating conditions might lead to an infeasible design.
Feed condition
Heating the feed most often:
•
increases trays in the rectifying section but decreases trays in
the stripping section
•
requires less heat in the reboiler but more cooling in the
condenser.
As the condition of the feed is changed from saturated liquid feed
(q = 1) to saturated vapor feed (q = 0), the minimum reflux ratio
tends to increase.
Thus the ratio of heat added to preheat the feed divided by the
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Type of condenser
(Either a total or partial condenser can be chosen.)Total Condenser:
Most designs use a total condenser.
A total condenser is necessary if the top product needs to be sent to intermediate or final product storage.
Also, a total condenser is best if the top product is to be fed to another distillation at a higher pressure as the liquid pressure can readily be increased using a pump.
Partial Condenser:
A partial condenser reduces the condenser duty, which is important if the cooling service to the condenser is expensive, such as low-temperature refrigeration.
I t is often necessary to use a partial condenser when distilling mixtures with low-boiling components that would require very low-temperature (and expensive) refrigeration for a total condenser.
I I I .2.2. Absorption and Stripping
Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent.
Liquid flowrate, temperature and pressure are important variables to be set.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
PFD of CO
2Removal
Addition of DEA
PFD of Dehydration Unit
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I I I .2.3. Liquid- Liquid Extraction
Liquid–liquid extraction carries out separation
by contacting a liquid feed with another
immiscible liquid
.
The
separation
occurs
as
a
result
of
components
in
the
feed
distributing
themselves differently between the two liquid
phases.
The liquid with which the feed is contacted is
known as the
solvent. The solvent extracts
solute from the feed.
The
solvent-rich stream obtained from the
separation is known as the
extract
and the
residual feed from which the solute
has been
extracted is known as the
raffinate
.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I I I .2.4. Adsorption
Adsorption is a process in which molecules of adsorbate become attached to the surface of a solid adsorbent.
Adsorption processes can be divided into two broad classes:
1. Physical adsorption, in which physical bonds form between the adsorbent and the adsorbate.
2. Chemical adsorption, in which chemical bonds form between the adsorbent and the adsorbate.
An example of chemical adsorption is the reaction between hydrogen sulfide and ferric oxide:
The ferric oxide adsorbent, once it has been transformed chemically, can be regenerated in an oxidation step:
adsorbent
Activated Carbon
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Physical and Chemical Adsortion
Physical Adsorption Chemical Adsorption
Heat of adsorption Small, same order as heat of Vaporization (condensation)
Large, many times greater than the heat of vaporization (condensation)
Rate of adsorption Controlled by resistance to mass transfer; Rapid rate at low Temperatures
Controlled by resistance to surface reaction; Low rate at low temperatures
Specificity Low, entire surface availability for physical adsorption
High, chemical adsorption limited to active sites on the surface
Surface coverage Complete and extendable to Multiple molecular layers
Incomplete and limited to a layer, one molecule thick
Activation energy Low High, corresponding to a
chemical reaction Quantity adsorbed per unit
mass
High Low
Types of physical adsorbent:
1. Activated carbon:
• a form of carbon that has been processed to develop a solid with high internal porosity.
• The most commonly used methods are the separation of organic vapors from gases, and a liquid-phase application, e.g. decolorizing or deodorizing aqueous solutions.
2. Silica gel ( SiO2) :
• its surface has an affinity for water and organic material.
• I t is primarily used to dehydrate gases and liquids
3. Activated aluminas:
• a porous form of aluminum oxide (Al2O3) with high surface area, manufactured by heating hydrated aluminum oxide to around 400 ◦C in air.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Types of physical adsorbent:
4. Molecular sieve zeolites:
• Zeolites are crystalline alumi-nosilicates.
• They differ from the other three major adsorbents in that they are crystalline and the adsorption takes place inside the crystals.
• This results in a pore structure different from other adsorbents in that the pore sizes are more uniform.
• Access to the adsorption sites inside the crystalline structure is limited by the pore size, and hence zeolites can be used to absorb small molecules and separate them from larger molecules, as
“molecular sieves” .
• Typical applications are the removal of hydrogen sulfide from natural gas, separation of hydrogen from other gases, removal of carbon dioxide from air before cryogenic processing, separation of p-xylene from mixed aromatic streams, separation of fructose from sugar mixtures, and so on
I I I .2.5. Membrane
Membranes act as asemipermeable barrier between two phases to create a separation by controlling the rate of movement of species across the membrane.
The separation can involve two gas (vapor) phases, two liquid phases or a vapor and a liquid phase. The feed mixture is separated into a
retentate, which is the part of the feed that does not pass through the membrane, and a permeate, which is that part of the feed that passes through the membrane.
The driving force for separation using a membrane is partial pressure in the case of a gas or vapor and concentration in the case of a liquid. Differences in partial pressure and concentration across the membrane are usually created by the imposition of a pressure differential across the membrane.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I dealized flow patterns in membrane separation
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
I I I .2.6. Crystallization
Crystallization involves formation of a solid product from a homogeneous liquid mixture.
Often, crystallization is required as the product is in solid form.
The reverse process of crystallization is dispersion of a solid in a solvent, termeddissolution. The dispersed solid that goesinto solution is the solute.
As dissolution proceeds, the concentration of the solute increases. Given enough time at fixed conditions, the solute will eventually dissolve up to a maximum solubility where the rate of dissolution equals the rate of crystallization.
Under these conditions, the solution is saturated with solute and is incapable of dissolving further solute under equilibrium conditions.
I I I .2.7. Evaporation
Evaporation separates a volatile solvent from a solid.
Single-stage evaporators tend to be used only when the capacity needed is small.
For larger capacity, it is more usual to employ multistage systems that recover and reuse the latent heat of the vaporized material.
Three different arrangements for a three-stage evaporator are:
1. Forward feed: The boiling temperature decreases from stage to stage, and this arrangement is thus used when the concentrated product is subject to decomposition at higher temperatures. I t also has the advantage that it is possible to design the system without pumps to transfer the solutions from one stage to the next
2. Backward feed: is used when the concentrated product is highly viscous. The high temperatures in the early stages reduce viscosity and give higher heat transfer coefficients. Because the solutions flow against the pressure gradient between stages, pumps must be used to transfer solutions between stages.
Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY
Three possible
arrangements for a
three-stage evaporator
: