Single Effect evaporater

8/19/2019 Single Effect evaporater

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1. Heat and Material Balances for Evaporators

2. Effects of Processing Variables on Evaporator Operation

3. Boiling-Point Rise of Soltions

!. Ent"alp# - $oncentration $"arts of Soltions

Calculation Methods for Single-Eect Evaporator


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Heat and Material Balances for Evaporators

The basic equation for solving for the capacity of a single-effect evaporator

which can be written as:

where ΔT is the difference in temperature between the condensing steam

and the boiling liquid in the evaporator.


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Sc"e%atic of single-effect evaporator

x


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The feed to the evaporator is F having a solids content of x F mass fraction,temperature T F , and enthalpy h F .

Coming out as a liquid is the concentrated liquid L having a solids content of x L, temperature T 1, and enthalpy h L.

The vapor V is given off as pure solvent having a solids content of xv !,temperature T 1, and enthalpy H v.

"aturated steam entering is S and has a temperature of T " and enthalpy of H S .

The condensed steam leaving off S is assumed usually to be at T ", thesaturation temperature, with an enthalpy of hS .

This means that the steam gives off only its latent heat, #, where: # H S – hS


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"ince the vapor V is in equilibrium with the liquid L, the temperaturesof vapor and liquid are the same.

$ssumption: %o boiling point rise.

&or the material balance, since we are at steady state, the rate of massin rate of mass out. Then, for a total balance,

F = L + V

&or a balance on the solute 'solids( alone:

F x F = L x L


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*owever, the enthalpies of the feed and products are often not

available+ these enthalpy concentration data are available for only a

few substances in solution.

f the heat capacities of the liquid feed 'c pF ) and of the roduct 'c pL(

are nown, they can be used to calculate the enthalpies.

h L = c pL(T p-T 1 )

/here, T& feed temperature T p product temperature

T1 0eference temperature


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Heat-&ransfer 'rea in Single-Effect Evaporator

' continos single-effect evaporator concentrates ()*2 +g," of a

1.) t salt soltion entering at 311.) / to a final concentration of

1.0 t . &"e vapor space of t"e evaporator is at 1)1.320 +Pa and

t"e stea% spplied is satrated at 1!3.3 +Pa. &"e overall coefficient

U 1*)! ,%2./4. $alclate t"e a%onts of vapor and li5id

prodct and t"e "eat-transfer area re5ired. 'ss%e t"at6 since it is

dilte6 t"e soltion "as t"e sa%e boiling point as ater.

7iven8 c pF !.1! +9,+g./ : h L )


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ffects of rocessing 2ariables on vaporator 3peration

Effect of feed te%peratre

f feed enter the evaporator at 311.) / - cold as compared to the boiling

temperature of 454.6 7.

$bout 189 of the steam is used to heat the cold feed to the boiling point.

*ence, only about 489 of the steam is left for vaporiation of the feed.


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Effect of pressre

n ;ample, a pressure of 1!1.46 a abs was used in the vapor space of

the evaporator.This set the boiling point of the solution at 454.6 7 and gaveΔT of 4


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Boiling-Point Rise of Soltions

vaporation produces concentrated liquors with boiling point is higher

than pure solvent at the pressure prevailing in vapor space. Thisdifference in > is called >0.

>0 of the concentrated liquor decreases the effective temperaturedriving force compared to the boiling of pure solvent.

&or strong solutions of dissolved solutes the boiling-point rise due to thesolutes in the solution usually cannot be predicted.

*owever, a useful empirical law nown as Duhring’s rule can be applied.

$ccording to this rule, a straight line is obtained if the boiling point of asolution in ?C or ?& is plotted against the boiling point of pure water atthe same pressure.


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@uhring lines for aqueous solutions of sodium hydro;ide.


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;se of


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Ent"alp# - $oncentration $"arts of Soltions

f the heat of solution of the aqueous solution being concentrated in

the evaporator is large, neglecting it could cause errors in the heat

balances.

"hould consider the heat-of-solution phenomenon.

f pellets of %a3* are dissolved in a given amount of water, it is found

that a considerable temperature rise occurs+ that is, heat is evolved,

called heat of solution.


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nthalpy concentration chart for the system %a3* water


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Evaporation of an >aOH Soltion

'n evaporator is sed to concentrate !03? +g," of a 2) soltion

of >aOH in ater entering at ?) @$ to a prodct of 0) solids.

&"e pressre of t"e satrated stea% sed is 1*2.! +Pa and t"e

pressre in t"e vapor space of t"e evaporator is 11.* +Pa. &"eoverall "eat-transfer coefficient is 10?) ,%2./. $alclate t"e

stea% sed6 t"e stea% econo%# in +g vaporiAed,+g stea% sed6

and t"e "eating srface area in %2.

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Single Effect evaporater