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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

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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

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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

.

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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.

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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.

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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.

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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

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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

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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)

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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

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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

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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

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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.

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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

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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.

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

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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.

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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

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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)

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

Distillation Tray and Packing

Distillation Tray Distillation Packing

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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

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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

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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.

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

PFD of CO

2

Removal

Addition of DEA

PFD of Dehydration Unit

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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

.

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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

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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.

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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.

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

I dealized flow patterns in membrane separation

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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.

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

Three possible

arrangements for a

three-stage evaporator

:

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