Evaporator V2

EvaporatorEvaporator Equipment used in evaporation, the process of Equipment used in evaporation, the process of boiling a liquid in order to reduce its volume boiling a liquid in order to reduce its volume

NeedNeedReduces transportation costReduces transportation costStorage costs Storage costs Prepare for the next Unit operation – drying, Prepare for the next Unit operation – drying, crystallisation etc.crystallisation etc. Reduces deteriorative chemical reactions Reduces deteriorative chemical reactions Better microbiological stability Better microbiological stability Recovery of solventRecovery of solvent

EvaporatorEvaporator Driving force:Driving force:Temperature difference in between steam chest Temperature difference in between steam chest

temperature and product temperature.temperature and product temperature. Result :Result :Volatile solvent is removed from the feed.Volatile solvent is removed from the feed.

Solution (volatile solvent + non volatile solute)Solution (volatile solvent + non volatile solute)

Concentrate (Higher solute Conc.)Concentrate (Higher solute Conc.)

ExamplesExamples Concentration of milk to produce condensed Concentration of milk to produce condensed

milkmilk Concentration of juicesConcentration of juices Concentration of NaOH, NaCl from aqueous Concentration of NaOH, NaCl from aqueous

solutions to produce salt.solutions to produce salt. Ether recovery from fat extractionEther recovery from fat extraction

Basic Parts of an EvaporatorBasic Parts of an Evaporator Heat-exchanger Heat-exchanger Vacuum Vacuum Vapour separator Vapour separator Condenser Condenser

EvaporatorEvaporatorVapor out

Feed inSteam in (Saturated vapor)

Product out

Condensate out (Saturated Liquid)

Vapor Separator

Heat Exchanger

Vaccum for non condensable

Condensor unit

Coolant In

Coolant out

Evaporator Type Evaporator Type

Batch Pan Batch Pan

Rising Film Rising Film

Falling Film Falling Film

Multiple Effect EvaporatorsMultiple Effect Evaporators

Processing FactorsProcessing Factors

Conc of solute in feedConc of solute in feed

Conc of solute in product/concentrateConc of solute in product/concentrate

Pressure and Temp. of the systemPressure and Temp. of the system Depends on temperature sensitivity of material.Depends on temperature sensitivity of material. Boiling point elevationBoiling point elevation

Processing FactorsProcessing Factors Steam pressure and temperatureSteam pressure and temperature

Material of construction Material of construction

FoamingFoaming Low heat transfer rateLow heat transfer rate Entrainment lossEntrainment loss

Heat Transfer Coefficient, hHeat Transfer Coefficient, h A coefficient which indicates the amount of A coefficient which indicates the amount of

heat flow that is exchanged across a unit area heat flow that is exchanged across a unit area of a medium or system in a unit amount of of a medium or system in a unit amount of time with a unit of temperature difference time with a unit of temperature difference between the boundary of the system. between the boundary of the system.

S.I. unit W mS.I. unit W m-2-2 K K-1.-1.

Overall Heat Transfer Coefficient, UOverall Heat Transfer Coefficient, U In cases of combined heat transfer for a heat In cases of combined heat transfer for a heat

exchanger, there are two values for h. exchanger, there are two values for h.

Convective heat transfer coefficient for the fluid film Convective heat transfer coefficient for the fluid film inside the tubes inside the tubes

Convective heat transfer coefficient for the  fluid Convective heat transfer coefficient for the  fluid  film  outside  the  tubes.    film  outside  the  tubes.   

  The  thermal  conductivity  (k)  and thickness (Dx) of The  thermal  conductivity  (k)  and thickness (Dx) of the tube wall must also be accounted for.  the tube wall must also be accounted for.  

So an additional term (Uo), called the overall heat So an additional term (Uo), called the overall heat transfer coefficient, must be used instead.  transfer coefficient, must be used instead.  

Feed water Vaporisedtank feed water

Tank Volume out to0.92 litre/cm atmosphere

Overflowto drain Water

main

Steam in

Constantlevel Sight

device glass

Steamtrap

Condensateout

Condensatetank

Tank Volume1.31 litre/cm

Figure 1. The Evaporator

Steam TrapSteam Trap FunctionFunction

Discharge condensate, air and other incondensable Discharge condensate, air and other incondensable gases from a steam system while not permitting the gases from a steam system while not permitting the escape of live steam escape of live steam

Ball Float Steam TrapBall Float Steam Trap

Reduces the ferocity of

discharge and sound. Sound levels can be

reduced by up to 80%.

Vapor Out mvhv

Steam in mshs

Condensate out

mchc

Water in mwhw

QQEE

QQLL

Lab CalculationLab Calculation Calculate U, overall heat transfer coefficient Calculate U, overall heat transfer coefficient

for the evaporatorfor the evaporator

Perform a mass balance and energy balancePerform a mass balance and energy balance

Calculate discrepancy Calculate discrepancy

Mass BalanceMass Balance Feed Side:Feed Side:m (kg/s)m (kg/s)m (feed in) = m (Vapor)m (feed in) = m (Vapor)

Steam SideSteam Sidem (steam)= m (condensate m (steam)= m (condensate

collected) + m (flash)collected) + m (flash)

Flash Steam LossFlash Steam Loss The amount of flash steam produced during the The amount of flash steam produced during the

pressure reduction can be expressed as:pressure reduction can be expressed as:w = (Hw = (Hilil - H - Hflfl) / H) / Hfefe

wherewhere w = ratio of flash steam generated (kg flash steam w = ratio of flash steam generated (kg flash steam

/ kg condensate)/ kg condensate) HHilil = initial liquid enthalpy (kJ/kg) = initial liquid enthalpy (kJ/kg) HHflfl = final liquid enthalpy (kJ/kg) = final liquid enthalpy (kJ/kg) HHfefe = enthalpy of evaporation (kJ/kg) = enthalpy of evaporation (kJ/kg)

Energy BalanceEnergy Balance H (kJ/kg)H (kJ/kg)

Energy from the steam to the water QEnergy from the steam to the water QEE

The Heat Loss from the vessel to the room, QThe Heat Loss from the vessel to the room, QLL

Amount of heat in condensing steam QAmount of heat in condensing steam QSS

QQS = S = QQE + E + QQLL

MeasurementsMeasurements QQS and S and QQE E

Mass Flow rate- condensate, feed waterMass Flow rate- condensate, feed water

T,P will give H Specific EnthalpyT,P will give H Specific Enthalpy

Q = mHQ = mH

QQLL

Surface areaSurface area Surface temperatureSurface temperature QQLL =hA =hA ∆∆TT

QQEE = UA = UA ∆∆TT

Further ReferenceFurther ReferenceTransport Processes and Separation process Transport Processes and Separation process

principle by:principle by:Christie. J. GeankoplisChristie. J. Geankoplis

Animations on evaporator visit:Animations on evaporator visit:www.rpaulsingh.comwww.rpaulsingh.com and click animations then and click animations then

evaporatorevaporator

Thank YouThank You

Multiple-effect EvaporatorMultiple-effect Evaporator Water is boiled in a sequence of vessels, each Water is boiled in a sequence of vessels, each

held at a lower pressure than the last. held at a lower pressure than the last.

Because the boiling point of water decreases Because the boiling point of water decreases as pressure decreases, the vapor boiled off in as pressure decreases, the vapor boiled off in one vessel can be used to heat the nextone vessel can be used to heat the next

Generally the first vessel (at the highest Generally the first vessel (at the highest pressure) requires an external source of heat pressure) requires an external source of heat