Chemical Plant Design 1210384 Chapter-3
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IIIComponent Separation
Fundamental
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
OutlineHeterogeneous Separation:
1. Gas-liquid (or vaporliquid)2. Gassolid (or vaporsolid)3. Liquidliquid (immiscible)4. Liquidsolid5. Solidsolid.
Homogeneous Separation1. Creation of another phase2. Addition of a mass separation agent
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
III.1.HETEROGENEOUS SEPARATION
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
Heterogeneous Separation(Smith, R., 2005)
If a heterogeneous (multiphase mixture), separation can bedone physically by exploiting the differences in densitybetween the phases.
Separation of the different phases of a heterogeneousmixture should be carried out before homogeneousseparation
Phase separation tends to be easier and should be donefirst.
The phase separations likely to be carried out are: Gasliquid (or vaporliquid) Gassolid (or vaporsolid) Liquidliquid (immiscible) Liquidsolid Solidsolid
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
The principal methods for the separation ofheterogeneous mixtures are:
1. Settling and sedimentation2. Inertial and centrifugal separation3. Electrostatic precipitation4. Filtration5. Scrubbing6. Flotation7. Drying.
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
III.1.1. Settling and Sedimentation
Particles are separated from a fluid by gravitational forcesacting on the particles.
The particles can be liquid drops or solid particles. The fluid can be a gas, vapor or liquid.
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
Gravity settler for the separation of gasliquid and vaporliquid mixtures
The velocity of the gas or vapor through thevessel must be less than the settling velocityof the liquid drops.
It is normally not practical to separatedroplets less than 100 m diameter in sucha simple device.
Thus, the design basis forsimple settling devise isusually taken to be a vessel inwhich the velocity of the gas(or vapor) is the terminalsettling velocity for droplets of100 m diameter.
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
Gravity settler (Decanter) for the separation ofliquidliquid mixtures
The horizontal velocity must be low enough to allow the low-density droplets to rise from the bottom of the vessel to theinterface and coalesce and for the high density droplets tosettle down to the interface and coalesce.
The decanter is sized on the basis that the velocity of the continuousphase should be less than the terminal settling velocity of the droplets ofthe dispersed phase. The velocity of the continuous phase can beestimated from the area of the interface between the settled phases
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
Gravity settler for the separation of fluidsolid mixtures
A mixture of gas, vapor or liquid and solid particles enters at oneend of a large chamber.
Particles settle toward the base. Again the device is specified onthe basis of the terminal settling velocity of the particles.
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
A thickener for liquidsolid separation. When separating a mixture of water and fine solid
particles in a gravity settling device, it is common insuch operations to add a flocculating agent to themixture to assist the settling process.
This agent has the effect of neutralizing electric charges on the particlesthat cause them to repel each other and remain dispersed.
The effect is to form aggregates or flocs, which, because they are largerin size, settle more rapidly.
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
Simple gravity settling classifier
The larger particles, faster-settlingparticles settle to the bottom close tothe entrance
The smaller particles, the slower-settlingparticles settle to the bottom close tothe exit
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
III.1.2. Inertial and Centrifugal Separation Sometimes gravity separation (discussed earlier) may be too slow
because of the closeness of the densities of the particles and thefluid, because of small particle size leading to low settling velocityor, in the case of liquidliquid separations, because of theformation of a stable emulsion.
Inertial or momentum separators improve the efficiency of gassolid settling devices by giving the particles downward momentum,in addition to the gravitational force.
Centrifugal separators take the idea of an inertial separator a stepfurther and make use of the principle that an object whirled aboutan axis at a constant radial distance from the point is acted on bya force. Use of centrifugal forces increases the force acting on theparticles.
Particles that do not settle readily in gravity settlers often can beseparated from fluids by centrifugal force.
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
Inertial separators increase the efficiency of separation bygiving the particles downward momentum.
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
A cyclone generates centrifugal force by the fluid motion.
The simplest type of centrifugal device is thecyclone that consists of a vertical cylinder witha conical bottom.
Centrifugal force is generated by the motion ofthe fluid.
The mixture enters through a tangential inletnear the top, and the rotating motion socreated develops centrifugal force that throwsthe dense particles radially toward the wall.
The entering fluid flows downward in a spiraladjacent to the wall.
The particles of dense material are throwntoward the wall and fall downward, leaving thebottom of the cone.
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
A centrifuge uses rotating cylindrical bowl to producecentrifugal force.
In centrifuges, a cylindrical bowl isrotated to produce the centrifugalforce.
The cylindrical bowl is shown rotatingwith a feed consisting of a liquidsolid mixture fed at the center.
The feed is thrown outward to thewalls of the container.
The particles settle horizontallyoutward.
Different arrangements are possibleto remove the solids from the bowl.
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
A centrifuge uses rotating cylindrical bowl to producecentrifugal force.
two liquids having differentdensities are separated by thecentrifuge.
The more dense fluid occupiesthe outer periphery, since thecentrifugal force is greater onthe more dense fluid.
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Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
III.1.3. Electrostatic Precipitation
Electrostatic precipitators are generally used to separate particulate matterthat is easily ionized from a gas stream.
Particles collect on the plates and are removed by vibrating the collectionplates mechanically, thereby dislodging particles that drop to the bottomof the device.
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Electrostatic precipitation is mosteffective when separating particleswith a high resistivity.
The operating voltage typicallyvaries between 25 and 45 kV ormore, depending on the designand the operating temperature.
The application of electrostaticprecipitators is normally restrictedto the separation of fine particlesof solid or liquid from a largevolume of gas.
Dr. Eng. Y. D. Hermawan ChemEng - UPNVY
III.1.4. Filtration
Suspended solid particles in a gas, vapor or liquid are removed bypassing the mixture through a porous medium that retains theparticles 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, whichis 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)2. The cake is a product (filtrate as a waste): it is usual to wash
the cake to remove the residual filtrate from the filter cake.
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Filtration can be arranged in many ways
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III.1.5. Srubbing
Scrubbing with liquid (usually water) can enhance thecollection of particles when separating gassolid mixtures.
Three of the many possible designs for scrubbers:1. Packed-bed Scrubber2. Spray Scrubber3. Venturi Scrubber
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