This document defines evaporator concepts like steam economy, evaporator capacity, and steam consumption. It describes single effect evaporators, where vapor from boiling liquid is condensed and discarded, resulting in low steam economy. Multiple effect evaporators improve steam economy by using vapor from one effect to heat the next. Forward, backward, mixed, and parallel feeding arrangements are described for transferring liquid between multiple effects. Forward feed provides increasing concentration from first to last effect without pumps between effects.

1. Definitions
Steam economy
 Kilograms of solvent evaporated per
kilogram of steam used
Evaporator capacity
 Number of kilograms of water vaporized
per hour.
 Steam Consumption
 Capacity divided by economy (steam
used/hr)

2. Single Effect Evaporators
 When a single evaporator is used , the
vapor from boiling liquid is condensed
and discarded. This method is single effect
evaporation
 It is simple, but use steam ineffectively i.e.
steam economy is low, so to evaporate
one kg of water from solution 11.3 kg
steam is required

3. Cont…
Thick Liquor
Condensed vapor

4. Where single effect are used
 Single-effect evaporators are used when the
throughput is low
 when a cheap supply of steam is available
 when expensive materials of construction
must be used as is the case with corrosive
feed stocks
 when the vapor is so contaminated so that it
cannot be reused
 For liquids that boil at high temperatures
(high boiling-point elevation liquors)

5. Operation of single effect evaporator
Single effect units may be operated in batch, semi-
batch or continuous batch modes or continuously
Batch Operation
 Batch units require that filling, evaporating and
emptying are consecutive steps
 Such a method of operation is rarely used since it
requires that the vessel is large enough to hold
the entire charge of feed and that the heating
element is low enough to ensure that it is not
uncovered when the volume is reduced to that of
the product

6. Cont…
Semi-Batch Operation
 Semi-batch is the more usual mode of
operation in which feed is added
continuously in order to maintain a
constant level until the entire charge
reaches the required product density
Continuous-Batch Operation
 Have a continuous feed and, over at least
part of the cycle, a continuous discharge

7. Cont…
 often a feed drawn from a storage tank is
returned until the entire contents of the
tank reach the desired concentration
 The final evaporation is then achieved by
batch operation
Continuous Evaporator
 continuous evaporators have a
continuous feed and discharge and
concentrations of both feed and discharge
remain constant

8. Multiple Effect Evaporators
Three methods are available which enable
the performance to be improved, either
by direct reduction in the steam
consumption or by improved energy
efficiency of the whole unit
 Multiple effect operation
 Recompression of the vapor rising from
the evaporator
 Evaporation at low temperatures using a
heat pump cycle

9. Multiple Effect Operation
 If vapor generated from the first effect is
used for heating in the 2nd effect, and then
vapor from the 2nd effect is sent to a
condenser, the operation becomes double
effect. Addition effects can be added
similarly
 Method of increasing the evaporation/kg
of steam by using a series of evaporators
b/w steam supply and condenser is called
multiple-effect evaporation.

10. Cont…

11. Cont…
 First Effect, in which steam is fed and in
which the pressure in the vapor space is
the highest (for liquid transfer b/c no
pump used) and in last effect it is
minimum. Thick liquid out from last effect
but it depends on feeding arrangement
 The boiling point in 2nd effect must be
reduced, b/c by concentrating, solution
B.P increases, also vapor heat content
reduces going from first effect to 2nd. B.P.
can be reduced by reducing pressure or
applying vacuum.

12. Calculations for forward feed system

13. Cont…
 For three evaporators arranged as shown
in Figure, in which the temperatures and
pressures are T1,T2,T3, and P1, P2, P3,
respectively, in each unit, then the heat
transmitted per unit time across each
effect is
Effect 1 Q1 =U1A1∆T1, ∆T1= (T0 − T1)
Effect 2 Q2 = U2A2 ∆ T2, ∆ T2=(T1 − T2)
Effect 3 Q3 = U3A3∆ T3, ∆ T3 = (T2 − T3)
T0 is steam température

14. Cont…
 Neglecting the heat required to heat the
feed from Tf to T1, the heat Q1 transferred
across where A1 appears as latent heat in the
vapor D1 and is used as steam in the second
effect, so
Q1 = Q2 = Q3
U1A1∆T1 = U2A2∆T2 = U3A3∆T3
 If, as is commonly the case, the individual
effects are identical,
A1 = A2 = A3, Then
U1∆T1 = U2∆T2 = U3∆T3

15. Cont…
 The latent heat required to evaporate 1
kg of water in 1, is approximately equal to
the heat obtained in condensing 1 kg of
steam at T0.
 Thus 1 kg of steam fed to 1 evaporates 1
kg of water in 1. Again the 1 kg of steam
from 1 evaporates about 1 kg of steam in
2. Thus, in a system of N effects,1 kg of
steam fed to the first effect will evaporate
in all about N kg of liquid, so economy
increase with number of effects

16. Cont…
 The water evaporated in each effect is
proportional to Q, since the latent heat is
approximately constant. Thus the total
capacity is:
Q = Q1 + Q2 + Q3
= U1A1∆T1 + U1A2∆T2 + U3A3∆T3
 If an average value of the coefficients Uav
is taken, then:
Q = Uav(∆T1 + ∆T2 + ∆T3)A
Q = UavA∆T

17. Heat Balance on Forward Feed
 Effect 1:
D0λ0 = GFCp(T1 − Tf ) + D1λ1
 D0 is steam flow rate to 1st effect, at λ0 latent
heat of vaporization
 Effect 2:
D1λ1 + (GF − D1)Cp(T1 − T2) = D2λ2
 Effect 3:
D2λ2 + (GF − D1 − D2)Cp(T2 − T3)= D3λ3

18. Feeding Arrangement
 Forward Feed
 Conc. of liquid increase from 1st effect to
last effect
 Pump is required to feed dilute solution
to 1st effect & for thick liquid in last effect
 Transfer b/w effects can be done with out
pumps b/c its in decreasing order

20. Cont…
 Backward Feed
 Dilute feed is fed to last effect and
pumped back to 1st effect
 Additional pumps b/w effects are required
b/c flow is from low pressure to high
pressure
 Gives higher capacity for viscous feed
 Gives low economy for cold feed

21. Cont…

22. Cont…
 Mixed Feed
 Dilute liquid enters in intermediate effect
 Flow back to first effect, where final conc.
Is done at highest temp
 It eliminate some of the pumps required
b/w effects

23. Cont…

24. Cont…
 Parallel Feed
 For crystallization operation, where slurry
crystals and mother liquor is drawn,
parallel feed arrangement is used.
 No transfer of liquid b/w effects and no
pumps required

25. Cont…

26. Thank You
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