III Component Separation Fundamental

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}

2. Addition of a mass separation agent

I I I .1. HETEROGENEOUS SEPARATI ON

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

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}

Solid–solid

The principal methods for the separation of heterogeneous mixtures are:

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

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

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 Particles are separated from a fluid by gravitational forces acting 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 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.

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

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.

 The decanter is sized on the basis that the velocity of the continuous phase should be less than the terminal settling velocity of the droplets of the dispersed phase. The velocity of the continuous phase can be estimated from the area of the interface between the settled phases

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

Gravity settler for the separation of fluid–solid mixtures

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



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.

Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY 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.  The effect is to form aggregates or flocs, which, because they are larger in size, settle more rapidly.

Simple gravity settling classifier

The larger particles, faster-settling The smaller particles, the slower-settling

particles settle to the bottom close to particles settle to the bottom close to

the entrance the exit

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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.

Particles that do not settle readily in gravity settlers often can be

 separated from fluids by centrifugal force. _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY}

I nertial separators increase the efficiency of separation by giving the particles dow nw ard momentum

Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY 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  toward the wall and fall downward, leaving the bottom of the cone. _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY}

A centrifuge uses rotating cylindrical bow l to produce centrifugal force

centrifuges, a cylindrical bowl is I n

 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  _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY} 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  the outer periphery, since the centrifugal force is greater on the more dense ﬂuid.

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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.  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. _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY} corona

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)

2. The cake is a product (filtrate as a waste): it is usual to wash _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY}

the cake to remove the residual ﬁltrate from the ﬁlter cake.

Filtration can be arranged in many w ays

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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

2. Spray Scrubber

3. Venturi Scrubber Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

Packed- bed Scrubber

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 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

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 Spray Scrubber uses a spray system that will be less prone to fouling.

 The design of spray scrubber uses a tangential inlet to create a swirl to enhance the separation.

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.

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I I I .1.6. Flotation

differences in

 Flotation is a gravity separation process that exploits the 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 in water).

hydrophobic (e.g. oil droplets dispersed

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 liberation in the

2. dissolution in the liquid under pressure and then

ﬂotation cell by reducing the pressure _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY} 3. electrolysis of the liquid.

A typical flotation cell for solid separation

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



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.

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

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

Dissolved air Flotation ( DAF)

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.

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.



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

process industries are:

1. Tunnel Dryer

2. Rotary Dryer

3. Drum Dryer

4. Spray Dryer

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



Here, consideration is restricted to the removal of moisture from

solids into a gas stream (usually air) by heat, namely, thermal

drying.

Tunnel Dryer

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



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

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

 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.  A cylindrical shell mounted at a small angle to the horizontal is rotated at low 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.

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

Spray Dryer

or slurry solution

 In spray dryer, a liquid

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. Spray dryers are thus particularly suited to



products that are sensitive to thermal decomposition, such as food products. _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY}

Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Spray Dryer for Milk Pow der Manufacture

I I I .2. HOMOGENEOUS SEPARATI ON

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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.

Alternatively, a liquid solvent could be contacted with the



vapor mixture to act as a mass separation agent to

preferentially dissolve one or more of the components from

the mixture. Further separation is required to separate the

solvent from the process materials so as to recycle the

_{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY} solvent, and so on.

Partial Condensation of Reactor Product Partial condensation for

_{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY} separating H from others ₂ The principal methods for the separation of homogeneous mixtures are:

1. Distillation

2. Absorption and Stripping

3. Liquid-Liquid Extraction

4. Adsorption

5. Membrane

6. Crystallization

7. Evaporation 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.

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

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)

 The feed to the process is introduced at an intermediate stage, and products are removed from the condenser and the reboiler. _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY}

Distillation Tray and Packing

_{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY}

Distillation Tray Distillation Packing

Distillation Column

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Choice of operating conditions of distillation

Specified conditions: 

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

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Operating pressure of distillation column As pressure is low ered:

As pressure is raised:

separation becomes more • The lower limit is often set by the difﬁcult (relative volatility desire to avoid: decreases), that is, more stages 1. vacuum operation or reﬂux are required; 2. refrigeration in the condenser
latent heat of vaporization decreases, that is, reboiler and Both vacuum operation and the use of condenser duties become lower; refrigeration incur capital and
vapor density increases, giving a operating cost penalties and increase

They smaller column diameter; the complexity of the design.

reboiler temperature increases should be avoided if possible.

with a limit often set by thermal decomposition of the material being vaporized, causing excessive fouling;

condenser temperature increases. _{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. _{Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY}

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



heat saved in the reboiler depends on the change in q, the relative

volatility between the key components, feed concentration and

ratio of actual to minimum reﬂux.

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

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

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 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.

 I n absorption, a gas mixture is contacted with a liquid solvent that preferentially dissolves one or more components of the gas.

PFD of CO Removal

2 Addition of DEA Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY

PFD of Dehydration Unit Addition of TEG 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  solvent. The solvent extracts known as the solute from the feed.

The solvent-rich stream obtained from the  separation is known as the and the

extract

residual feed from which the solute has been extracted is known as the . raffinate

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

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

I I I .2.4. Adsorption

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

 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:

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

Activated Carbon Ferric Oxide

Physical and Chemical Adsortion Physical Adsorption Chemical Adsorption

High Low

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

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

Types of physical adsorbent:

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.
I t is primarily used to dehydrate gases and liquids
Activated aluminas are mainly used to dry gases and liquids, but can be used to adsorb gases and liquids other than water.

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

1. Activated carbon:

2. Silica gel ( SiO

2 ) : • its surface has an afﬁnity for water and organic material.

3. Activated aluminas:

a porous form of aluminum oxide (Al

2 O

) with high surface area, manufactured by heating hydrated aluminum oxide to around 400 ◦C in air.

Types of physical adsorbent:

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

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

I I I .2.5. Membrane

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

 Membranes act as a semipermeable 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.  However, driving force for liquid separations can be also created by the use of a solvent on the permeate side of the membrane to create a concentration difference, or an electrical ﬁeld when the solute is ionic.

I dealized flow patterns in membrane separation

Dr. Eng. Y. D. Hermawan – ChemEng - UPNVY Schematic of Separation using membrane based on pressure

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

I I I .2.6. Crystallization

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

 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 goes into 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.

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

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 .

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 possible arrangements for a three-

stage evaporator

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

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

III Component Separation Fundamental