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Rivera, Alyssa A.
5ChE-C
Equipment Design
1. Introduction
Evaporation is basically a separation step which uses heat transfer to separate products
presenting differences at boiling point. In order to concentrate a non-volatile solute, such as
organic compounds, inorganic salts, acids or bases from a solvent, evaporation process is used
where in solvent is removed as vapor from a solution, slurry or suspension of a solid in a liquid.
Evaporation requires the use of a heating medium, usually steam which is in indirect contact
through a steam chest. The most common solvent in most of the evaporation systems is water.
Evaporation differs from the other mass transfer operations such as distillation and drying. In
distillation, the components of a solution are separated depending upon their distribution between
vapor and liquid phases based on the difference of relative volatility of the substances. Removal
of moisture from a substance in presence of a hot gas stream to carry away the moisture leaving a
solid residue as the product is generally called drying. Evaporation is normally stopped before the
solute starts to precipitate in the operation of an evaporator.
1.1 Evaporation
The major requirement in the field of evaporation technology is to maintain the quality of the
liquid during evaporation and to avoid damage to the product. This may require the liquid to be
exposed to the lowest possible boiling temperature for the shortest period of time.
Evaporation process comprises three main steps:
1. Pre-heating of a solution prior to evaporation
2. Removal of water (solvent) as vapor by steam heating
3. Condensing the vapor removed
1.2 Types of evaporation
Because of numerous requirements and limitations for feed, products and steam, it have
resulted in a wide variation of evaporator designs today. In almost all evaporators the heating
medium is steam, which heats a product on the other side of a heat transfer surface. The following
list contains the descriptions of the most common types of evaporators: Falling Film Evaporators,
Rising Film Evaporators, Forced Circulation Evaporators and Plate Evaporators.
Typical evaporator applications: Product concentration Dryer feed pre-concentration
Volume reduction Water / solvent recovery Crystallization
Types of Evaporators
a. Falling Film Evaporators
In a falling film evaporator, the liquid is fed at the top of the tubes in a vertical tube bundle.
The liquid is allowed to flow down through the inner wall of the tubes as a film. As the liquid
travels down the tubes the solvent vaporizes and the concentration gradually increases. Vapor
and liquid are usually separated at the bottom of the tubes and the thick liquor is taken out.
Evaporator liquid is recirculated through the tubes by a pump below the vapor-liquid separator.
b. Rising or Climbing Film Evaporators
The LTV evaporator is frequently called a rising or climbing film evaporator. The liquid
starts boiling at the lower part of the tube and the liquid and vapor flow upward through the
tube. If the heat transfer rate is significantly higher, the ascending flows generated due to higher
specific volume of the vapor-liquid mixture, causes liquid and vapor to flow upwards in
parallel flow. The liquid flows as a thin film along the tube wall. This cocurrent upward
movement against gravity has the advantageous effect of creating a high degree of turbulence
in the liquid. This is useful during evaporation of highly viscous and fouling solutions.
c. Forced circulation Evaporator
This system is used when the product has a strong tendency to foul the heating surfaces;
therefore, it is recirculated at a rather high rate through the tubes. Forced circulation evaporator
is commonly used for concentration of caustic and brine solutions and also in evaporation of
corrosive solution.
d. Gasketed Plate Evaporator
The gasketed-plate evaporator is also called the plate evaporator because the design is
similar to that of a plate heat exchanger. The heat transfer coefficient is greatly enhanced due
to high turbulent flow through narrow passages. This evaporator is suitable for high viscous,
fouling, foaming and heat sensitive solutions. This type of evaporators is mainly used for
concentration of food products, pharmaceuticals, emulsions, glue, etc
Other types of evaporators:
e. Stirrer Evaporator
This type of evaporation is rarely used today. It applies mostly for highly viscous feed and
for very specific products.
f. Circulation Evaporator
The operations are similar to the rising film principle, but the liquid recovered in the
separator is sent back to the bottom of the evaporator creating a closed loop.
g. Fluidized bed Evaporator
This system operates under the same principle as forced circulation evaporators and is used
when the feed solution contains particles.
1.3 Multiple Effect Evaporator
Multiple Effect Evaporation remains one of the popular methods used for the concentration
of aqueous solutions. Water is removed from a solution by boiling the liquor in an evaporator and
withdrawing the vapor. If the solution contains dissolved solids, the resulting strong liquor may
become saturated so that crystals are deposited.
Evaporation is carried out by supplying heat to the solution to vaporize the solvent. The
heat is supplied basically to provide the latent heat of vaporization and by adopting methods for
recovery of heat from the vapor, it has been possible to achieve great economy in heat utilization.
The normal heating medium is generally low pressure steam (1 to 1.5 kg/cm2g).
An industrial evaporator systems generally comprises:
1. A heat exchanger to supply sensible heat and latent heat of evaporation to the feed.
Saturated steam is usually used as the heating medium.
2. A separator in which the vapour is separated from the concentrated liquid phase.
3. A condenser to effect condensation of the vapour and its removal from the system.
There are two main types of ways of improving steam economy in evaporators. One is to
use a multiple effect evaporator, the other is to use mechanical vapor recompression.
The chief factor influencing the economy of an evaporator system is the number of effects. By
increasing the number of effects we can increase the economy of an evaporator system. The first
effect of a multiple effect evaporator is the effect to which the raw steam is fed, vapors obtained
from first effect act as a heating medium for another effect.
1.3.1 Types of Feed Arrangement in Multiple Effect Evaporator
a. Forward feed
The typical feeding method of multi-effect evaporators is forward. Both feed and steam are
introduced in the first effect and the feed passed from effect to effect parallel to the vapor from the
earlier effect. Concentration increases from the first effect to the last. Forward feeding operation
is helpful when the concentrated product may degenerate if exposed to high temperature. The
product is withdrawn from the last effect.
b. Backward feed
In backward feed configuration, the feed enters at the last effect (coldest effect) and is
pumped through the successive effects. The product is withdrawn from the first effect (hottest)
where the steam is introduced. This method of feeding requires a pump between each pair of effects
to transfer liquid from lower pressure effects to higher pressure effects. It is advantageous when
cold feed entering needs to be heated to a lower temperature than in forward feed operation.
Backward feed is commonly used when products are viscous and exposure to higher temperature
increases the rate of heat transfer due to reduction in viscosity of the liquid.
c. Mixed feed
In the mixed feed operation, the dilute feed liquid enters at an intermediate effect and flows
in the next higher effect till it reaches the last effect of the series. In this section, liquid flows in
the forward feed mode. Partly concentrated liquor is then pumped back to the effect before the one
to which the fresh feed was introduced for further concentration. Mixed feed arrangement
eliminates some of the pumps needed in backward configuration as flow occurs due to pressure
difference whenever applicable.
d. Parallel feed
The fresh feed is introduced to each effect and in this configuration the product is
withdrawn of from the same effect in parallel feed operation. In parallel feeding, there is no transfer
of liquid from one effect to another effect. It is used primarily when the feed is saturated and the
product is solid containing slurry. This is most common in crystallizing evaporators.
Method of feeding of evaporator: a: forward feed; b: backward feed; c: mixed feed; d:
parallel feed.
2. Feed Stock Analysis
Sugar (C12H22O11) is an organic compound, colourless, sweet-tasting crystals that
dissolve in water. In addition to providing a sweet taste and flavour, sugar performs a variety
of functions in food products. Sugar is used as a preservative, to prevent large ice crystals from
forming in frozen sweet mixtures, and to support fermentation in products containing yeast.
Making sugar an important and versatile food ingredient. Sugars are found in the tissues of
most plants, but are present in sufficient concentrations for efficient extraction only in
sugarcane and sugar beet. Sugar beets accounted for 20% of the world's sugar production.
Sugar beets, after washed and sliced go through a large tank called a diffuser where raw
sugar juice is extracted. Sugar juice is now purified resulting to a solution called thin juice.
This thin juice having a 10-14% solids is evaporated to 60% to be fed to a crystallizer to
prudence sugar crystals.
2.1 Operating Conditions, Assumptions and Constraint
For the evaporation for thin juice of 14% to 60%, multiple effect evaporator will be used
specifically three effects. Normally, 5 multiple evaporators are used for evaporation of thin
juice but the assumption will be, the feed that will be treated in an evaporator is less than the
feed treated in a 5 multiple effect. In this manner, triple effect evaporator will be designed.
Entering steam temperature is at its saturation with an operating pressure of 233.54kPa at 1st
effect evaporator. The 3rd evaporator will be at 550mmHg vacuum pressure. Entering feed is
at 20C. Boiling point rise will be neglected for it is a organic solution. Sucrose will only have
a significant boiling point rise if it is above 60Brix or 60% solute. For the heat transfer
coeffiecient, assume values from 2500-1600 (W/m2K) for the falling film evaporator ranges
from these values for heat transfer coefficient.
3. Rationale for equipment Selection
Multiply effect evaporator will be used for the process, specifically, triple effect
evaporator. Because the feed is at 20C and the vaporization of water in the solution will make
it viscous, backward feeding will be chosen. There are different types of evaporator today, but
the falling film tubular evaporator will be appropriate for the evaporation of thin juice. The
typical application for falling film evaporators are the concentration of dairy products (such as
whey, milk protein, skim milk, cream and hydrolyzed milk), sugar solutions, urea, phosphoric
acid, concentrated juices and black liquor.
4. Equipment Specification
Some of the problems associated with the equipment can be traced back to improper or
poor specification of the equipment. The following details are designed to assist in this process
and minimize the consequences of poor selection.
4.1 Material of Construction
The two parameters which control the selection of the material of construction are
corrosion and ease of cleaning. Stainless steel will be used as the material of construction
because it does not readily corrode, rust or stain with this kind of feed solution as ordinary steel
does.
4.2 Shell Specifications
a. Material: stainless steel: type (304)
b. Maximum allowable stress of stainless steel = 14884.5psi
c. Length: 7.916 meters
d. Diameter: 1.979 meters
e. Thickness: 6.9236 mm
f. Evaporator heads(thickness): hemispherical(4.958mm) & flat head(71.174mm)
4.3 Pipe Inside the Evaporator Specifications (Assume: Based on Heuristics)
a. Material: 3 1/3 Schedule 40 stainless steel
b. Length: 4meters
c. Inside Pipe Diameter: 3.5 in
d. Outside Pipe Diameter: 3.334 in
e. Number of tubes: 40 tubes
5. Theoretical Calculations and Material Balances
*Assumptions: no heat loss in the environment, no solute evaporated with the solvent, negligible
boiling point rise (BPR), same contact area for the 3 effects of the evaporators.
Step 1: Over-all Material Balance: Feed = Effluent + Liquid
10000 lb/hr = E + L
Solute Balance: FxF = LxL
10000(0.14) = (0.60)L
Solving simultaneously: E = 7.667X103 lb/hr
L= 2.333X103 lb/hr
Step 2: Assume pressure for 3rd effect. (Setting it to 550mmHg Vacuum). Entering
steam is at its saturation temperature.
Assume or setting over-all heat transfer coefficient (W/m2 K): U1=2500, U2= 2000 and
U3=1600
Po=233.54KPa To= 398.15K
T1T
1U1
U2
U1
U3
15.106
∑∆T= 57.591K
Step 3: Temperature and enthalpy profile
(Hs-hc)= Lo
To=398.15K L0=2198.165 kJ/kg
T1=383.044K L1=2240.316 kJ/kg
T2=364.162K L2=2289.5605 kJ/kg
T3=340.559K L3= 2300.8843kJ/kg
Step 4: Energy Balance in 3 Evaporators
1st Effect : VoLo=(F-V3-V2)Cp(T1-T2)+V1L1
2nd Effect : V1L1=(F-V3)Cp(T2-T3)+V2L2
3rd Effect: V2L2=FCp(T3-Tf)+V3L3
Step 5: Solve for stream rates Vo,V1,V2, V3
1st eq: Vo(2198.165)=(10000-V3-V2)*(3.8029)*(109.894-91.012)+ V1*(2240.316)
2nd eq: V1(2240.316)=V2(2289.5605) + (10000-V3)(3.8029)(91.012-67.409)
3rd eq; V2*(2289.5605)=(10000)*(3.8029)*(67.409-20)+(V3)*(2300.8843)
4th eq: 7.667X103=V1+V2+V3
Vo= 3.31283x103 V1= 3.07708x103 V2=2.6932x103 V3=1.89638x103
T2U1
U2T1 18.882
T3U1
U3T1 23.603
Step 6: Check if the three areas are equal
1st Effect : VOLO=U1 A1 ∆T1
2nd Effect : V1L1=U2 A2 ∆T2
3rd Effect : V2L2=U3 A3 ∆T3
A1= 53.564 m2 Amean= 49.875m2
A2= 50.706 m2
A3= 45.356 m2
* Area must be less than 3 % difference with the Amean
%Diff: 𝐴𝑛−𝐴𝑚𝑒𝑎𝑛
𝐴𝑚𝑒𝑎𝑛X100
%Diff1: 7.396%
%Diff2= 1.666%
%Diff3= 9.061%
If error > 3%
∆Tn= (An* ∆Tn)/Amean
∆T1= 16.425
∆T2= 19.436
∆T3=21.731
Repeat Step 3: temperature and enthalpy profile
(Hs-hc)= Lo
To=398.15K L0=2198.165 kJ/kg
T1=381.725K L1=2243.8754 kJ/kg
T2=362.289K L2=2294.2492 kJ/kg
T3=340.558K L3= 2300.8421kJ/kg
Step 4: Energy Balance in 3 Evaporators
1st Effect : VoLo=(F-V3-V2)Cp(T1-T2)+V1L1
2nd Effect : V1L1=(F-V3)Cp(T2-T3)+V2L2
3rd Effect: V2L2=FCp(T3-Tf)+V3L3
Step 5: Solve for stream rates Vo,V1,V2 , V3
1st eq: Vo(2198.165)=(10000-V3-V2)*(3.8029)*(108.575-89.14)+ V1*(2243.8754)
2nd eq: V1(2243.8754)=V2(2294.2) + (10000-V3)(3.8029)(89.14-67.409)
3rd eq; V2*(2294.2492)=(10000)*(3.8029)*(67.409-20)+(V3)*(2300.8421)
4th eq: 7.667X103=V1+V2+V3
Vo= 3.3033x103 V1= 3.05841x103 V2=2.69978x103 V3=1.90847X103
Step 6: Check if the two areas are equal
1st Effect : VOLO=U1 A1 ∆T1
2nd Effect : V1L1=U2 A2 ∆T2
3rd Effect : V2L2=U3 A3 ∆T3
A1= 49.121 m2 Amean= 49.8216 m2
A2= 49.042 m2
A3= 49.484 m2
*Area should be less than 3 % difference with the Amean
%Diff: 𝐴𝑛−𝐴𝑚𝑒𝑎𝑛
𝐴𝑚𝑒𝑎𝑛X100
1st Effect
U1=2500
(W/m2 K)
2nd Effect
U1=2500
(W/m2 K)
3rd Effect
U1=2500
(W/m2 K)
%Diff1: 0.192%
%Diff2= 0.354%
%Diff3= 0.546%
6. Heuristics
Treated as pressure vessel, operating pressure inside is up to 233.54KPa. Design pressure for
these evaporators are equal to operating pressure plus 25psig or 10%. Design temperature is
equal to Operating temperature plus 30Oc or 50F. Shell and head thickness is obtain with design
pressure and design temperature. Gas/Liquid separators are vertical in alignment. Used diameter:
length ratio of 1:4. Maximum allowable stress is dependent on the material of construction and
the design temperature. Internal tubes for the evaporator can be range to 19–63mm (0.75–24.8
in.) in diameter and 3.66–9.14m (12–30 ft) long.
7. Diagram
Diagram of Falling Film (Backward Feeding)
Legends:
Steam Flow
Feed Flow
Pumps
xf:0.14
F:10000
lb/hr
xl:0.60
Po= 233.54 kPa
550 mmHg vacuum
Steam inlet pipe
Front view of Triple effect Falling Film Evaporator
Top View
Bottom view Side views
Entering feed
to the 3rd
evaporator
(showing
internals)
Triple effect Falling Film evaporator with support
8. Conclusion and Recommendation
In designing an evaporator equipment, an engineer must consider a lot of condition. One must
know first what he/she wants to process and produce. Characteristics of feed and product are
important factor in designing an evaporator; their properties before and after they’re fed to the
equipment is strictly observe by the engineer. Because there are a lot of available type of
evaporator, characteristics of feed must be known to be efficient in yielding the product. Also,
knowing its characteristics, material for construction for the equipment will be easily chosen.
Operating temperature and pressure must be specified to allow the designer to make an equipment
having a maximum allowable working temperature and pressure. Having the design parameters,
thickness and allowable stress can be computed.
9. References
http://nptel.ac.in/courses/103107096/module4/lecture2/lecture2.pdf
http://www.spxflow.com/cn/assets/pdf/Evaporator_Handbook_10003_01_08_2008_US.pdf
https://www.bma-worldwide.com/products/sugar-and-sweeteners/tubular-fallingfilm-
evaporator.html
http://multiple-effect-evaporation.webs.com/
Apendix (Calculations)
A= ᴨ*D*L
Diameter and length has a ratio of 1:4
L=4D
49.231= ᴨ*D*(4D)
D=1.9793m
L= 4D= 7.9173m
Maximum Allowable Working Pressure:
233.54kPa*(14.7/101.325)= 33.88145 psia
33.88145+ 25= 58.88psia
Maximum Allowable Temperature:
125C+ 30C= 155C= 311F
Interpolate for Stainless Steel (304):
500 − 311
500 − 300=
12.9 − 𝑥
12.9 − 15
X=14.8845 ksi= 14884.4 psi (Maximum allowable stress)
Thickness of the shell (Cylinder)
P<0.385(SE)
0.385(14884.5)(1)=5730.53
t=𝑃𝑟𝑖
𝑆𝐸−0.6𝑃+ 𝐶𝑐 =
58.88∗989.5
(14884.5)−(0.6)∗(58.88)+ 3 = 6.9236𝑚𝑚
Thickness of heads:
(Hemispherical)
t=𝑃𝑟𝑖
2(𝑆𝐸)−0.2(𝑃)+ 𝐶𝑐 =
58.88∗(989.5)
2∗(14884.5)(1)−(0,2)(58.88)+ 3 = 4.957951
(Flat Head)
t=2𝑟𝑖 ∗ √0.3𝑃
𝑆+ 𝐶𝑐 = 2(989.5) ∗ (√
0.3∗58.88
14884.5) + 3 = 71.1747