College of Engineering
Department of Chemical Engineering
Chemical Engineering Apparatus Design (ChEg4110)
Target group: 4th Year ChEng students
Chapter 7
Design of basic chemical engineering equipment
By
Wondimu M. (PhD)
Academic year: 2024/25
Outcome of Chapter 7
Identify different basic chemical engineering equipment
Revise basic terms on Evaporator (capacity, economy, modes of
feed and basic principles of processing)
Clearly identify different types of evaporators & selection criteria.
Compute problems focusing on design of evaporator (economy &
area) based on feed and operating conditions (p, T).
2
Basic chemical engineering equipment:
Re-boiler
Can be handled as heat exchangers
Condenser
Evaporator
Crystallizer Dissolved solid separating equipment
Membrane
On an industrial scale, evaporation and crystallization are the main
processes used for the recovery of dissolved solids from solutions.
3
Evaporation can produce solids contents of 0% -92%
In this chapter evaporator is covered. 4
7.1. Evaporator
Evaporation, a widely used method for the concentration of
aqueous solutions, involves the removal of water from a solution by
boiling the liquor in a suitable vessel, an evaporator, and withdrawing
the vapors.
If the solution contains dissolved solids, the resulting strong liquor
may become saturated so that crystals are deposited.
Evaporation is the removal of a solvent from solids that are not
volatile.
It is categorized as heat and mass transfer unit operation.
Components and streams of evaporator
6
Needs and application of evaporator:
Needs:
Reduce transportation cost
Reduce storage cost
Prepare for the next unit operation (such as drying, crystallization ...)
Solvent recovery
application
7
Classification of evaporators
(1) Natural circulation evaporators
(a) Long tube vertical falling film evaporator
(b) Long tube vertical climbing up evaporator
(2) Forced circulation evaporators
(a) Forced circulation evaporator with horizontal heating element
(b) Forced circulation evaporator with vertical heating element
(3) Agitated film evaporator
(4) Open kettle or pan
8
1. Open kettle or pan are simple, low capital cost, easy to use and
clean; but it is expensive in running cost as
heat economy is poor and the hole liquid
heating leads to component decomposition.
2. Forced Circulation Evaporators (horizontal or vertical tube)
•uses pump for high velocity circulation (2-6 m/s).
•Is more costly than natural circulation
•Suitable for situations such as:
-viscous solutions
-solution with suspended particles
-for heat sensitive materials
Application for concentrating of:
caustic, brine and corrosive solution.
9
3. Natural Circulation Evaporators
Short tube (horizontal or vertical tube)
Long tube (vertically falling and rising film types)
Liquid flow inside the bundle of vertical tube (at 0.3-1 m/s) &
steam condenses outside the tube.
decreased density due to boiling causes liquid to rise in the tubes
by natural circulation.
Disadvantage:
Not suitable for viscous liquids b/s
heat transfer coefficient is very low.
10
3.2.1. Short-Tube
Vertical Evaporators
Consist of short tube bundle (4 to 10 ft in length).
Are the oldest but still widely used in sugar industry.
These are also known as calandria or Robert evaporators.
11
3.2.2 Long-Tube Vertical Evaporators (tube length is 20 to 65 ft.)
Is the most widely employed natural circulation evaporator because it is often the
cheapest per unit of capacity.
• Liquid flows as a thin film on the walls of long vertical heated tube.
• Both rising & falling film types are used with higher rate of heat transfer advantage.
(ii)
(i) Rising film evaporators
Feed enters at the bottom and the liquid Feed enters at the top of the tubes bundle.
starts boiling at lower part of the tube. Used to concentrate fruit juices & heat
sensitive materials b/s of low holdup time.
commonly used in concentrating black Suitable for scale-forming solutions as
liquors in the paper & pulp industries. boiling occur on the surface of the film.
12
Agitated-film evaporator
Is a modified falling film type with a single jacketed tube containing
internal agitator to reduce resistance to heat transfer on the liquid side.
Advantages:
high heat transfer coefficients even with very
high viscous and heat sensitive liquids such as
gelatin, rubber latex, antibiotics and fruit juices.
Disadvantages:
It is very costly and have smaller capacities.
Maintenance is difficult sue to moving parts.
Other application include Starch Industry
Alcohol Industry Beer and Beverages
Dairy Industry Pharmaceutical Industry Edible Oil Industry
Food Industry Natural Products Specialty Chemical
Pulp and Paper Chlor-Alkali Dyes and Pigments
Textile Industry Petrochemical and Soap and Biofuels
Polymer Industry 13
Selection of evaporator type depend on factors as:
14
Properties of evaporating liquid and evaporator performance
Properties of evaporating liquid which influence evaporation process:
Concentration
Foaming nature
Temperature sensitivity
scale
15
Evaporator performance measurement methods:
• Capacity : number of kilogram of water evaporated per hour.
It is related to heat transfer: Q=UAT.
Evaporator capacity ranges from 27 to 37 kg/hr/m2
• Economy: number of kilogram of water evaporated per kilogram
of steam feed to the evaporator.
Use of multiple effect evaporation and vapor recompression
increase the economy of evaporation.
Steam economy is about 0.8 (<1) for single effect evaporator
and about 0.8n for a n-effect evaporators.
16
17
Single-effect and multiple-effect
Evaporator can be implement: single-effect, double-effect , triple-effect
or more-effect.
Single-effect Evaporators are used when:
throughput is low,
cheap supply of steam is available,
expensive materials of construction is
used (corrosive feedstock's) and
vapor is so contaminated.
Operate in batch, semi-batch or continuous.
Heat requirements is obtained from mass and energy balances by
assuming no work is performed on the system (W = 0) and heat loss is
negligible (Q = 0)).
18
Cont…
The operation is simple, it does not use steam efficiently.
1.1 to 1.3 kg of steam is required to evaporate 1 kg of water.
If enthalpy or heat capacity and heat of solution data are not available:
heat heat needed to raise the heat required to
= + .
requirements feed to product temperature evaporate the water
The latent heat of water is taken at the vapor head pressure instead of
the product temperature in order to compensate heat of solution.
19
Multiple -effect Evaporators
Multiple effect evaporators enable the performance to be improved
by either direct reduction in steam consumption, or by improved
energy efficiency of the whole unit.
Recompression is applied to vapor rising from evaporator and
evaporation at low temperatures using a heat pump cycle.
Consider the following triple effect evaporator to examine heat
transfer and mass balances.
20
Methods of feeding of evaporators in series
Purpose of putting evaporators in series: to increase evaporating water per steam used.
Modes of feeding evaporators in series are forward, backward, mixed and parallel.
Vapor to
Forward Vapor to Condenser
Condenser
Steam
Steam
Backward
Condensate
Condensate
Thick Thick
Liquor Liquor
Requires pump for feeding & removes Requires pump between each pair of effects as
thick liquor from the last effect. the flow is from lower pressure effects to
higher pressure effects.
Liquor transfers between effects without
Useful for cold feed entering need to be heated
pump as the flow is in the direction of
to a lower temperature than forward feed type.
decreasing pressure.
Commonly used when products are viscous and
Helpful when concentrated product may
exposure to higher temperature increases rate of
degenerate if exposed to high temperature.
heat transfer due to reduction in liquid viscosity.
21
Mixed Parallel
Vapor to
Vapor to
Condenser
Condenser
Steam Steam
Condensate Condensate
Thick Thick Thick Thick Thick
Liquor Liquor Liquor Liquor Liquor
Feed enters at an intermediate effect & no transfer of liquid from one effect
flows to the next higher effect till it reaches to another effect.
the last effect in forward feed mode.
used primarily when feed is saturated
Partly concentrated liquor is then pumped
& product is solid containing slurry.
back to the effect before the fresh feed was
introduced for further concentration. most common in crystallizing
It eliminates some of the pumps needed in evaporators.
backward configuration as flow occurs due
to pressure difference.
22
Thermal Design Calculation/ Heat Transfer in Evaporators
The rate of heat transfer is the key in evaporators design.
(7.1)
23
Typical overall heat transfer coefficients in evaporators
Type of evaporator Overall heat transfer coefficient
(W.m-2°C-1)
Long-tube vertical:
Natural circulation 1000-2700
Forced circulation 2000-7500
Short-tube vertical 750-2500
Agitated-film:
Low to medium viscosity (<1 P) 1800-2700
High viscosity (> 1P) 1500
Falling film(viscosity <0.1 P) 500-2500
Rising film 2000-5000
24
Cont’d
25
Cont…
(7.5)
(7.6)
26
Cont…
27
The energy balance equations in all effects are given below:
Feed (mf,Tf)
If the sensible heat of the steam is neglected this equation can be rewritten as:
Therefore, use of steam table is a must to get enthalpy and latent heats!
28
29
Example 7.1: 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. If the overall coefficient of
heat transfer is 3 kW/m2 deg K, estimate the heating surface required
and the amount of steam used if the feed to the evaporator is at 294 K
and the condensate leaves the heating space at 352.7 K. The specific
heats of 10 and 50 % solutions are 3.76 and 3.14 kJ/kg deg K,
respectively.
30
Solution:
31
Solution cont’d…
32
Example 7.2: 4 kg/s of a liquor containing 10% solids is fed at 294 K
to the first effect of a triple-effect unit. Liquor with 50% solids is to
be withdrawn from the third effect, which is at a pressure of 13
kN/m2. The liquor may be assumed to have a specific heat of 4.18
kJ/kg K and to have no boiling point rise. Saturated dry steam at 205
kN/m2 is fed to the heating element of the first effect, and the
condensate is removed at the steam temperature in each effect . If the
three units are to have equal areas, estimate the area, the temperature
differences and the steam consumption. Heat transfer coefficients of
3.1, 2.0 and 1.1 kW/m2 K for the first, second, and third effects
respectively, may be assumed.
33
Here, precise solution is not possible since during operation of the evaporator, variation
of liquor alters the heat transfer coefficients & hence temperature distribution.
2200@394K
2235@381K
2288@361K
2377@325K
34
Cont…
(381-294) 2235D1
2235D1 (381-361) 2288D2
2288D2 (361-325)
35
Cont…
D1=0.98 kg/s D2=1.07 kg/s D3=1.15 kg/s D0=1.66 kg/s
36
