Web & Social Media Analytics Previous Year Question Paper.pdf
Evaporators
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 11.3 kg
steam is required
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.
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.
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
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
19.
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
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
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
