III Component Separation Fundamental - ydhermawan's · PDF file1/3/2017 · III Component Separation Fundamental ... Gravity settler (Decanter) ... The decanter is sized on the basis

Chemical Plant Design – 1210384 Chapter-3

Department of Chemical Engineering - UPN “Veteran” Yogyakarta Page 1 of 32

IIIComponent Separation

Fundamental

Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

OutlineHeterogeneous Separation:

1. Gas-liquid (or vapor–liquid)2. Gas–solid (or vapor–solid)3. Liquid–liquid (immiscible)4. Liquid–solid5. Solid–solid.

Homogeneous Separation1. Creation of another phase2. Addition of a mass separation agent




III.1.HETEROGENEOUS SEPARATION


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:• Gas–liquid (or vapor–liquid)• Gas–solid (or vapor–solid)• Liquid–liquid (immiscible)• Liquid–solid• Solid–solid




The principal methods for the separation ofheterogeneous mixtures are:

1. Settling and sedimentation2. Inertial and centrifugal separation3. Electrostatic precipitation4. Filtration5. Scrubbing6. Flotation7. Drying.


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.




Gravity settler for the separation of gas–liquid and vapor–liquid 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 µm 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 µm diameter.


Gravity settler (Decanter) for the separation ofliquid–liquid 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




Gravity settler for the separation of fluid–solid 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.


A thickener for liquid–solid 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.




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


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 liquid–liquid separations, because of theformation of a stable emulsion.

Inertial 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 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.




Inertial separators increase the efficiency of separation bygiving the particles downward momentum.


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.




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 liquid–solid 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.


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.




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.

corona

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.


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.




Filtration can be arranged in many ways


III.1.5. Srubbing

Scrubbing with liquid (usually water) can enhance thecollection of particles when separating gas–solid mixtures.

Three of the many possible designs for scrubbers:1. Packed-bed Scrubber2. Spray Scrubber3. Venturi Scrubber




Packed-bed Scrubber

a packed tower is similar to anabsorption tower.

Whilst this can be effective, it suffersfrom the problem that the packing canbecome clogged with solid particles.

Towers using perforated plates similarto a distillation or absorption columncan also be used. As with packedcolumns, plate columns can alsoencounter problems of clogging.


Spray Scrubber

Spray Scrubber uses a spraysystem that will be less prone tofouling.

The design of spray scrubberuses a tangential inlet tocreate a swirl to enhance theseparation.




Venturi Scrubber Liquid is injected into the

throat of the venturi,where the velocity of thegas is highest.

The gas accelerates theinjected water to the gasvelocity, and breaks upthe liquid droplets into arelatively fine spray.

The particles are then captured by the fine droplets. Very highcollection efficiencies are possible with venturi scrubbers.

The main problem with venturi scrubbers is the high pressure lossacross the device.


III.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 dropletsto rise to the surface.

This is used to separate mixtures of solid–solid particles afterdispersion in a liquid, or solid particles already dispersed in a liquid orliquid–liquid mixtures of finely divided immiscible droplets.

The liquid used is normally water and the particles of solid orimmiscible liquid will attach themselves to the gas bubbles if they arehydrophobic (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 system2. dissolution in the liquid under pressure and then liberation in the

flotation cell by reducing the pressure3. electrolysis of the liquid.




A typical flotation cell for solid separation

The mixture is fed to a flotation cell, and gas is also fed to the cellwhere gas bubbles become attached to the solid particles, therebyallowing them to float to the surface of the liquid.

The separation of the solidparticles depends on the differentspecies having different surfaceproperties such that one species ispreferentially attached to thebubbles.

The solid particles are collected from the surface by an overflowweir or mechanical scraper.


A number of chemicals can be added to the flotation medium tomeet 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 solidsrenders the solid to be more hydrophobic and increases itstendency to attach to the gas bubbles.

c. Activators are used to “activate” the mineral surface for thecollector.

d. Depressants are used to preferentially attach to one of the solidsto make it less hydrophobic and decrease its tendency to attachto the gas bubbles.

e. Frothers are surface-active agents added to the flotation mediumto create a stable froth and assist the separation.




When separating low-density solid particles oroil droplets from water,the most commonmethod used isdissolved-air flotation.

DAF shows some of the effluent water from the unit beingrecycled, and air being dissolved in the recycle under pressure.

The pressure of the recycle is then reduced, releasing the air fromsolution 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 ofthe air bubbles and is removed.

Dissolved air Flotation (DAF)


III.1.7. Drying

Drying refers to the removal of water from a substance through awhole range of processes, including distillation, evaporation andeven physical separations such as centrifuges.

Here, consideration is restricted to the removal of moisture fromsolids into a gas stream (usually air) by heat, namely, thermaldrying.

Some of the types of equipment for removal of water also can beused for removal of organic liquids from solids.

Four of the more common types of thermal dryers used in theprocess industries are:1. Tunnel Dryer2. Rotary Dryer3. Drum Dryer4. Spray Dryer




Tunnel Dryer

Wet material on trays or a conveyor belt is passed through atunnel, and drying takes place by hot air.

The air-flow can be counter-current, co-current or a mixture ofboth.

This method is usually used when the product is not free flowing


Rotary Dryer

Wet material is fed at the higherend and flows under gravity.

Drying takes place from a flow ofair, which can be counter-currentor co-current.

The heating may be direct to thedryer gas or indirect through thedryer shell.

This method is usually used whenthe material is free flowing.

Rotary dryers are not well suitedto materials that are particularlyheat sensitive because of thelong residence time in the dryer.

A cylindrical shell mounted at a small angle to the horizontal is rotated atlow speed.




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

In spray dryer, a liquid or slurry solutionis sprayed as fine droplets into a hot gasstream.

The feed to the dryer must be pumpableto obtain the high pressures required bythe atomizer.

The product tends to be light, porousparticles.

An important advantage of the spray dryer isthat the product is exposed to the hot gas fora short period. Also, the evaporation of theliquid from the spray keeps the producttemperature low, even in the presence of hotgases.

Spray dryers are thus particularly suited toproducts that are sensitive to thermaldecomposition, such as food products.




Spray Dryer for Milk Powder Manufacture




III.2.HOMOGENEOUS SEPARATION


Homogeneous Separation(Smith, R., 2005)

If the mixture is homogeneous, separation can only beperformed by the creation of another phase within thesystem and the addition of a mass separation agent.

For example, if a vapor mixture is leaving a reactor, anotherphase could be created by partial condensation.

Alternatively, a liquid solvent could be contacted with thevapor mixture to act as a mass separation agent topreferentially dissolve one or more of the components fromthe mixture. Further separation is required to separate thesolvent from the process materials so as to recycle thesolvent, and so on.




Partial Condensation of Reactor Product

Partial condensation forseparating H2 from others


The principal methods for the separation ofhomogeneous mixtures are:

1. Distillation2. Absorption and Stripping3. Liquid-Liquid Extraction4. Adsorption5. Membrane6. Crystallization7. Evaporation




III.2.1. Distillation

The separation of a homogeneous fluid mixture requires thecreation of another phase

If this liquid mixture is partially vaporized, then another phase iscreated, and the vapor becomes richer in the more volatilecomponents (i.e. those with the lower boiling points) than theliquid phase.

The liquid becomes richer in the less volatile components (i.e.those with the higher boiling points).

If the system is allowed to come to equilibrium conditions, thenthe distribution of the components between the vapor and liquidphases is dictated by vapor–liquid equilibrium considerations.

All components can appear in both phases.


A cascade of equilibrium stages with refluxing and reboiling

It is assumed in the cascade that liquid and vaporstreams leaving each stage are in equilibrium.

Using a cascade of stages in this way allows themore volatile components to be transferred to thevapor phase and the less-volatile components tobe transferred to the liquid phase.

In 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 feedthe cascade (by condensing the top product, asreflux). total condenser or partial condenser

At the bottom of the column, vapor is also neededto feed the cascade (by vaporizing vaporizingsome of the liquid leaving the bottom stage)

The feed to the process is introduced at an intermediate stage, andproducts are removed from the condenser and the reboiler.




Distillation Tray and Packing

Distillation Tray Distillation Packing


Distillation Column




Choice of operating conditions of distillation

Specified conditions:1. Feed composition and flowrate2. Product specifications: product purities of recoveries of

certain component The operating parameters to be selected by the designer

include:1. operating pressure2. reflux ratio3. feed condition4. type of condenser


Operating pressure of distillation column

As pressure is raised:• separation becomes more

difficult (relative volatilitydecreases), that is, more stagesor reflux are required;

• latent heat of vaporizationdecreases, that is, reboiler andcondenser duties become lower;

• vapor density increases, giving asmaller column diameter;

• reboiler temperature increaseswith a limit often set by thermaldecomposition of the materialbeing vaporized, causingexcessive fouling;

• condenser temperatureincreases.

As pressure is lowered:• The lower limit is often set by the

desire to avoid:1. vacuum operation2. refrigeration in the condenser

Both vacuum operation and the use ofrefrigeration incur capital andoperating cost penalties and increasethe complexity of the design. Theyshould be avoided if possible.




Reflux ratio

For a stand-alone distillation column(i.e. utility used for both reboiling andcondensing), there is a capital–energy trade-off.

As the reflux ratio is increased fromits minimum, the capital costdecreases initially as the number ofplates reduces from infinity, but theutility costs increase as morereboiling and condensation arerequired

The optimal ratio of actual to minimum reflux is often less than 1.1.However, most designers are reluctant to design columns closer tominimum reflux than 1.1, except in special circumstances, since a smallerror in design data or a small change in operating conditions might leadto 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 ratiotends to increase.

Thus the ratio of heat added to preheat the feed divided by theheat saved in the reboiler depends on the change in q, the relativevolatility between the key components, feed concentration andratio of actual to minimum reflux.




Type of condenser (Either a total or partial condenser can be chosen.)

Total Condenser:

Most designs use a totalcondenser.

A total condenser is necessaryif the top product needs to besent to intermediate or finalproduct storage.

Also, a total condenser is best ifthe top product is to be fed toanother distillation at a higherpressure as the liquid pressurecan readily be increased usinga pump.

Partial Condenser: A partial condenser reduces the

condenser duty, which isimportant if the cooling serviceto the condenser is expensive,such as low-temperaturerefrigeration.

It is often necessary to use apartial condenser whendistilling mixtures with low-boiling components that wouldrequire very low-temperature(and expensive) refrigerationfor a total condenser.


III.2.2. Absorption and Stripping

Absorption processes often require anextraneous material to be introduced into theprocess to act as liquid solvent.

Liquid flowrate, temperature and pressure areimportant variables to be set.

In absorption, a gas mixture iscontacted with a liquid solventthat preferentially dissolves oneor more components of the gas.




PFD of CO2 Removal

Addition ofDEA


PFD of Dehydration Unit

Addition ofTEG




III.2.3. Liquid-Liquid Extraction

Liquid–liquid extraction carries out separationby contacting a liquid feed with anotherimmiscible liquid.

The separation occurs as a result ofcomponents in the feed distributingthemselves differently between the two liquidphases.

The liquid with which the feed is contacted isknown as the solvent. The solvent extractssolute from the feed.

The solvent-rich stream obtained from theseparation is known as the extract and theresidual feed from which the solute has beenextracted is known as the raffinate.


Liquid-Liquid Extraction




III.2.4. Adsorption

Adsorption is a process in which molecules of adsorbate become attachedto 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 hydrogensulfide and ferric oxide:

The ferric oxide adsorbent, once it has been transformed chemically, canbe regenerated in an oxidation step:


adsorbent

Activated Carbon

Ferric Oxide




Physical and Chemical Adsortion

Physical Adsorption Chemical Adsorption

Heat of adsorption Small, same order as heat ofVaporization (condensation)

Large, many times greaterthan the heat of vaporization(condensation)

Rate of adsorption Controlled by resistance tomass transfer; Rapid rate atlow Temperatures

Controlled by resistance tosurface reaction; Low rate atlow temperatures

Specificity Low, entire surfaceavailability for physicaladsorption

High, chemical adsorptionlimited to active sites on thesurface

Surface coverage Complete and extendable toMultiple molecular layers

Incomplete and limited to alayer, one molecule thick

Activation energy Low High, corresponding to achemical reaction

Quantity adsorbed per unitmass

High Low


Types of physical adsorbent:1. Activated carbon:

• a form of carbon that has been processed to develop a solid with highinternal porosity.

• The most commonly used methods are the separation of organicvapors from gases, and a liquid-phase application, e.g. decolorizing ordeodorizing aqueous solutions.

2. Silica gel (SiO2) :• its surface has an affinity for water and organic material.• It 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 ◦Cin air.

• Activated aluminas are mainly used to dry gases and liquids, but canbe used to adsorb gases and liquids other than water.




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 absorbsmall molecules and separate them from larger molecules, as“molecular sieves” .

• Typical applications are the removal of hydrogen sulfide from naturalgas, separation of hydrogen from other gases, removal of carbondioxide from air before cryogenic processing, separation of p-xylenefrom mixed aromatic streams, separation of fructose from sugarmixtures, and so on


III.2.5. Membrane

Membranes act as a semipermeable barrier between two phases to createa separation by controlling the rate of movement of species across themembrane.

The separation can involve two gas (vapor) phases, two liquid phases ora vapor and a liquid phase. The feed mixture is separated into aretentate, which is the part of the feed that does not pass through themembrane, and a permeate, which is that part of the feed that passesthrough the membrane.

The driving force for separation using a membrane is partial pressure inthe case of a gas or vapor and concentration in the case of a liquid.Differences in partial pressure and concentration across the membraneare usually created by the imposition of a pressure differential across themembrane.

However, driving force for liquid separations can be also created by theuse of a solvent on the permeate side of the membrane to create aconcentration difference, or an electrical field when the solute is ionic.




Idealized flow patterns in membrane separation


Schematic of Separation using membrane based on pressure




III.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 thatgoes into solution is the solute.

As dissolution proceeds, the concentration of the solute increases.Given enough time at fixed conditions, the solute will eventuallydissolve up to a maximum solubility where the rate of dissolutionequals the rate of crystallization.

Under these conditions, the solution is saturated with solute and isincapable of dissolving further solute under equilibrium conditions.


III.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, andthis arrangement is thus used when the concentrated product is subject todecomposition at higher temperatures. It also has the advantage that it ispossible to design the system without pumps to transfer the solutions fromone 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 higherheat transfer coefficients. Because the solutions flow against the pressuregradient between stages, pumps must be used to transfer solutions betweenstages.

3. Parallel feed: The vapor from each stage is still used to heat the next stage.This arrangement is used mainly when the feed is almost saturated,particularly when solid crystals are the product




Three possiblearrangements for a three-stage evaporator:

(a) forward feed operation(b) Backward feed operation(c) Parallel feed operation

Documents

III Component Separation Fundamental - ydhermawan's · PDF file1/3/2017 · III Component Separation Fundamental ... Gravity settler (Decanter) ... The decanter is sized on the basis