Evaporation
McCabe Ch. 16, Seader Ch.17
Dr. Hatem Alsyouri
IMPORTANT NOTES
Review the use of steam tables
(always bring them,, to the class)
Review the concepts of enthalpy, latent and sensible
heats, and heat transfer coefficient
Pay attention to units
Familiarize your self with INTERPOLATION
Always bring the steam tables to class (in different units).
Saturated Steam Table
Superheated or Sub-cooled Steam
Latent heat
sensible heat
Sub-cooled
T2 SC
sensible heat
saturated
TSat
superheated
T1 SH
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Evaporation
Evaporation is a unit operations used to increase
concentrations of process solutions.
Vapor
Examples: beverages, solvents, (in)organic salts
This is accomplished by evaporation of the
solvent in an evaporators.
Evaporator design involves determination of:
flow rates of products (vapor and thick
solution)
Amount and conditions of heating steam
area of heat transfer needed
Type of evaporator
(dilute solution)
Concentrated
solution
Evaporation normally accompanies other
operations like crystallization.
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Continuous-flow, steady state Evaporator
Model
Assumptions:
1. One volatile component in feed
2. Steam is saturated (latent heat
vaporizes the solution)
3. Boiling creates mixing
Tv= Te
Tp = Te
4. Tv = Tp = Te corresponding to
evaporator pressure P
5. Overall driving force for heat
transfer is T
= Ts Tp
6. No heat loss
Mass and Energy Balance
Total and Solute mass balance
m f m p mv
w f m f wp m p wv mv
Energy balance on Solution
Q m f H f m p H p mv H v
Q ms H
vap
Latent heat for
steam ()
Q U A (Ts Tp )
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Enthalpy of feed/product solutions
1) From chart
m f H f Qs m p H p mv H v
2) Heat capacity values are provided.
A reference temperature (datum) is needed. You can assume 0C, the reference
temp of steam table, or any suitable reference.
m f Cp f (T f Tref ) Qs m pCp p (Tp Tref ) mv H v
3) Assume water
Use steam tables
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Enthalpy of (NaOH-Water)
Solution
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Boiling Point Elevation (BPE) or
Boiling Point Rise (BPR)
Solutions have higher boiling points than pure water.
The increase of boiling point over the pure water is
called Boiling Point Elevation (BPE).
BPE is high for concentrated solutions.
BPE is calculated from empirical (experimental)
relations like Duhring rule (e.g., charts are available
for NaOH aqueous solution)
Large liquid head also causes BPE.
Neglecting the impact of BPE can yield wrong
design of evaporator.
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Calculation of
Boiling Point
Elevation (BPE)
Example:
35 wt% NaOH solution at 6 psia
207 F
From Steam Table at 6 psia
Boiling point of pure water
(Tw) = 170 F
Duhring chart
Tw and 35%
Boiling point of solution
(Tsol) = 210 F
BPE = 210 170 = 40 F
170 F
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14
Exercise (1)
S&H 17-39
15
Heat Input Utilization
Economy
mvapor
. . . (1)
msteam
Qsteam Q feed Qvaporization
. . . (2)
ms s m f Cp f (Tv T f ) mv v
. . . (3)
Equation (3) is equivalent to the general energy balance equation
on the model evaporator:
m f Cp f (T f Tref ) Qs m pCp p (Tp Tref ) mv H v
. . . (4)
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Exercise (2)
McCabe Problem 16.1
A solution of organic colloids in water is to be concentrated from 8 to 45% solids in a
single-effect evaporator. Steam is available at 1.03 atm gauge (120.5C). A pressure of
102 mmHg absolute is to be maintained in the vapor space. The feed rate to the
evaporator is 20,000 kg/h. Overall heat transfer coefficient (U) is 2800 W/m2.C. The
solution has a negligible BPE and negligible heat of dilution.
Calculate:
(1) the steam consumption (ms= 17782 kg/h)
(2) the economy (0.925)
(3) heating surface area required (56.4 m2)
Conditions: The temperature of the feed is: a) 51.7 C, b) 21.1 C, and c) 93.3 C.
Properties:
Specific heat of the feed solution is 3.77 J/g .C
Latent heat of vaporization of the solution can ne taken as that of water.
Answers: slight variations from the ideal answers are ok
(b)
(a)
(c)
21.1 C:
51.7 C:
93.3 C:
ms= 18,831 kg/h
ms= 17,782 kg/h
ms= 16,356 kg/h
Economy= 0.873
Economy= 0.925
Economy= 1.005
A= 59.7 m2
A= 56.4 m2
A= 51.9 m2
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Brain Storming Points
1. Factors leading to boiling point elevation and impact of
ignoring BPE in calculations
2. The physical meaning of Economy in evaporation and
ways to improve economy
3. Main resistances to heat transfer in evaporation and steps
to calculate overall heat transfer coefficient
4. Heat of dilution effect
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Heat of Dilution
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Exercise (3)
Coulson & Rischardson 14.1
A single effect evaporator is used to concentrate 7 kg/s of a solution from 10 to 50%
solids. Steam is available at 205 kN/m2 and evaporation takes place at 13.5 kN/m2. The
overall heat transfer coefficient of heat transfer is 3 kW/m2.K. The feed enters to
evaporator at 294 K and the condensate leaves the heating space at 352.7 K. The
specific heats of the 10% and 50% solutions are 3.76 and 3.14 kJ/kg. K respectively.
Estimate:
(a) Heating surface area required
(b) Amount of steam used
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Classification of Evaporators
Once through
evaporators
One pass mainly used in multiple effect
evaporators
Evaporation capacity is limited
Useful for heat-sensitive materials by operation
under vacuum
Examples : falling film, agitated evaporators
Circulation
evaporators
Multiple passes by circulation
A pool of liquid is maintained. Feed mixes with
the pool, evaporation in tubes, remaining liquid
returns to pool.
Not good for heat sensitive materials
Wide range of concentrations
Natural and forced circulation
Examples: climbing film
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Types of Evaporators
Low viscosity
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Product viscosity
Heat sensitivity
Scale formation and deposition
(a) Horizontal tube evaporator
Horizontal tubes
Solution in shell, steam in tube.
No agitation
Agitation occurs due to bubbling of vapor
Suitable for low-viscosity solutions that do not deposit scales on heating
surfaces
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(b) Short vertical tube evaporators
Short vertical tubes
Solution in tubes, and steam in shell
Boiling solution in tubes provides agitation and higher U
Not suitable for viscous liquids
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(e) Falling film evaporator
Widely used
Feed at top flow as film down the tube
(Downward flow ) as a film
Feed distribution in tubes as film by perforated
plates (a problem)
Heat sensitive solutions (food /juice) by
reducing the residence time (once-through
Vacuum operations
Vapor/liq
separator
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(e) Climbing film evaporator
separator
Upward flow
Long vertical-tube exchanger with steam
in shell and solution in tube
Space or separator for vapor
Good for solutions that tend to foam
which break at the impenging baffle in the
separator
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(d) Forced circulation evaporators
A pump is used to force solution flow
Upward flow
Very viscous solutions
Heat transfer coefficient is increased
by forced circulation.
Liquid velocity is 2-5.5 m/s
compared to 1.5 m/s for natural flow
Tubes under static pressure, liquid
becomes superheated in the vapor
space-> flashes
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(c) Long Vertical tube evaporator
Separate chamber of product disengagement to vapor
and liquid
Longer tubes so feed entering velocity can be higher
thus higher U
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Evaporator Selection
Coulson & Richardson Vol. 2
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Example on Calculating U
McCabe Solved Example 16.1
Condensed milk is produced by evapration of milk in a falling film evaporator
containing stainless steel tubes 32 mm in diameter and 6 m long. Evaporation takes
place at 60C, which is the boiling point of milk at 2.7 Ibf/in2 absolute, using steam at
70C. The feed rate is 40 kg/h per tube at 60 C.
(a) Estimate the internal coefficient hi and the over all coefficient U
(a) What is the evaporation rate per tube?
(b) If the raw milk has 13.5 % fat plus solids, what is the concentration of the condensed
milk?
(c) Calculate the average residence time in the evaporator. The properties of milk at 60
C are:
(cP)
(kg/m3)
k (W/m.K)
(J/g)
Raw milk
0.94
1010
0.62
2357
25% solids
1.6
1030
0.55
2357
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Multiple Effect Evaporators
1.
During evaporation, steam generates vapor. Single effect evaporator
can be wasteful of energy if the vapors heat content is not used.
2.
The latent heat can be recovered and re-used by employing a
multiple effect evaporators (MEE).
3.
Types of MEE: a) forward, b) backward, and c) parallel feed.
4.
How much will 1 kg steam evaporate from solution? (depends on the
feeds temp)
5.
What is the driving force for evaporation? (Tsteam-Te)
6.
Why pressure should successively reduce across evaporators?
7.
What is the economy for a 3-effect evaporator if 1 kg steam is used
and approximately 1 kg vapor is produced from each evaporator?
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Forward feed
Uses:
Feed is hot
Product is heat
sensitive
P and T
Backward feed
Uses:
Higher capacity (mv)
Feed is cold
Lower economy than
forward if feed is cold
Product is viscous
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Parallel feed
Mixed feed
Permits final evaporation
to be done at the highest
temperature
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Cost savings by multiple effect
Coulson & Richardson Vol. 2
34
Overall Heat Transfer Coefficient (U)
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Multiple Effect Evaporator Example
Geankoplis p.544
A triple-effect forward-feed evaporator is being used to evaporate a sugar
solution containing 10 wt% solids to a concentrated solution of 50 wt%. The
boiling point rise of the solution (independent of pressure) can be estimated
from BPR C = 1.78x + 6.22x2 (BPR F = 3.2x + 11.2x2) where x is the weight
fraction of sugar in solution. Saturated steam at 205.5 kPa (29.8 psi) [121.1C
(250 F) saturation temperature] is used. The pressure in the vapor space of the
3rd effect is 13.4 kPa (1.94 psi). The feed rate is 22,680 kg/h (50,000 Ibm/h) at
26.7C (80F). The heat capacity of the liquid solutions is cp = 4.19 2.35x
kJ/kg.K (1.0 0.56x btu/Ibm.F). The heat of solution is considered to be
negligible. The coefficients of heat transfer have been estimated as U1 = 3123,
U2= 1987, U3 = 1136 W/m2.K or 550, 350, and 200 btu/h.ft2.F. If each effect
has the same surface area, calculate the area, the steam rate used, and the steam
economy.
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