Abstract
Evaporation is an operation we use to separate solid from liquid by heating a solution
and create more concentrated solution. Evaporation is very common in a lot of fields in
the industry such as food, chemical, pharmaceutical and more. For creating this effect
we use appliances call evaporators, there are many types of evaporators that differ in
the structure, qualities and the products they design to produce, but they all made of
two section- heating section(by the use of steam) and a liquid\vapor section. The
properties of the solution have a high effect on the process, so they need to be
considered when one design such a process. There are ways to improve the efficiency of
the process using the latent heat of the vapours we separated from the liquid in two
main ways- multiple effect evaporators and recompression by thermo or mechanical
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compressors. A common way to measure the efficiency of evaporator called “steam
economy”, and it is the ratio between kg of water evaporated/ kg of steam we use. You
can perform a mass balance and an energy(mainly enthalpy) on every evaporator, and
discovered the heat transfer in the process. In this paper I will introduce different types
of evaporators, show the properties of solution that need to be consider, demonstrate
how to write a mass and energy balances on an evaporator and hoe calculate the heat
transfer in the process.
EVAPORTION
Evaporation is the process when a substance or a compound change is phase from liquid
to gas. in the industry, we use the evaporation process to remove vapours from a
solution by heating the solution, usually to create more concentrated solution. The
purpose of evaporation is to concentrate a solution of non-volatile solute and a solvent
that is usually water. In most of the cases the desired product is the more concentrated
solution, but in some cases, we want the vapor as well. Evaporation regularly used in a
food industry, chemical industry, pharmaceutical industry and a lot more. Evaporation
can also be the initial step in producing a dry product if the concentrated liquid later
undergoes some drying process.
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Process Factors: the properties of the solution we about to evaporate have a large
effect on the process and on the kind of evaporator we will choose. The main properties
that effect our choices are: 1. Concentration - usually the liquid in the feed will be
relatively dilute so like water he will have low viscosity and high heat transfer coefficient
(HTC). After the process, the viscosity may rise, and the HTC will drop.
Circulation/turbulence must be present to keep the HTC from becoming too low.
2. Solubility- with the rise in the concentration of the solution, the solubility of the
solute in the solvent may reach the maximum, and after this point crystals will form.
3. Sensitivity to temperature- some materials, especially food and other organic
materials can be degraded in high temperature or after prolonged heating.
4. Foaming- some materials can form a foam during boiling. The foam can leave with the
vapor and cause a lost.
5. Pressure and temperature- the increase of the concentration of solution may cause a
boiling point elevation. Some materials are sensitive to high temperature, and therefore
some evaporators work under vacuum to decrease the B.P.
6. Boiling point elevation- important property of solutions that need to consider is the
B.P.E. as more concentrated the solution is, the more it differs from water in her
thermal properties- the B.P is increase over the B.P of water with the increase in
concentration. A useful empirical law that help us find the new B.P of solution call
Duhring’s rule. The rule state that there is a linear relationship between the B.P of pure
liquid (commonly water) and the solution at the same pressure. To find the new B.P we
plot the B.P of water as the x-axis and the B.P of the solution as the y-axis (figure 1).
Figure 1 – “Duhring’s law”
for different fractions of
NaOH.
some other factors that need to be taking under consideration are the materials the
evaporator made of and scale deposit on the heating surface.
Different types of evaporators: all evaporators made of two section- a heating section
(steam chest) and a vapor-liquid separation section. The two section can be in the same
vessel, or in some cases the heating section can be external to the vessel. An evaporator
can be composed by one effect or more, when effect is defined as one vessel(body) or
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more that operate to separate the liquid from vapor in the same temperature. Let us
elaborate about some (not all) of the common evaporators:
1. Horizontal tube natural circulation- steam goes into horizontal tubes, condensed and
get out at the other end of the tubes. The steam tubes run inside the vessel where the
solution is. The solution covers the tube, heat transfers to the solution and vapours are
leaving the surface of the liquid. Usually inside the vessel there is baffles that prevent
from liquid droplet from escaping with the vapours. Due to poor circulation, it is use for
low viscosity liquids.
2. Vertical tube natural circulation- instead of horizontal tubes there is a vertical tube
where the liquid is inside the tube and the steam condense outside the tube. In higher
temperature the densities decrease, this effect causes the rise of the boiling liquid inside
the tube in a natural circulation, and then the liquid is go down through a downcomer or
a large central open. The natural circulation increases the heat transfer coefficient. Use
for low viscosity liquids (figure 2).
3. Falling film- a long horizontal tubes. The feed is on the top of the vessel and the liquid
is fed into the tubes and falling by the force of gravity as thin layer film. Distributers are
sometimes used to obtain uniform distribution of the liquid when he falls. These kinds
of evaporators can handle more viscous liquid because of the use of gravity. Because of
short resident time (time expose to heat) (around 20 sec) this kind of evaporators
mainly used in food industry for more sensitive product like orange juice.
4. Forced circulation- use of pumps to increase the HTC by forcing the circulation of the
liquid inside the tubes. This kind of evaporators usually used a vertical tube. This
method very useful for viscosity liquids1 (figure 3).
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Figure 2: vertical tube type evaporator Figure 3: forced circulation type
evaporator
Single effect evaporator: if we want to know how to evaluate the performance of
evaporator, we need to do a mass and enthalpy balances on evaporator.
Mass balance- the mass balance on a single effect evaporator will include the flowrate
for the feed (Mf), product (Mp), vapor (Mv), steam (Ms), and condensate (Mc), X will
represent the mass fraction of the solute.
(1) Mf = Mp + Mv (2) XfMf = XpMp (3) Ms = Mc
Energy balance- for energy balance we first assume that no work is performed on the
system(w=0), and that the heat loss is negligible (Q=0), typically the heat loss in the
industry is estimate as around 2%. H will represent the enthalpy of the streams.
(4) MfHf + MsHs = MPHP + MVHV + MSHC
the latent heat of the steam λ is represent as(5) λ = Hs-Hc
by combine the equations we get (6) MFHF + MS(HS-HC) = MPHP + MVHV
and al the heat transfer in the evaporator is the latent heat from the steam we insert to
the system so (7) q = Ms∙λ
The basic equation that describe a singe effect evaporator is (8) q = UA∆T
where q is the heat transfer rate(W), U is the overall heat transfer coefficient(W/m 2k) , A
is the surface area of the heat transfer , and ∆T is the different between the condensate
steam and the boiling liquid. U is composed by a few factors- the resistance of the metal
wall and the scale, the liquid film coefficient and the steam side coefficient.
Multiple effect evaporator- a single effect evaporator is very wasteful in a manner of
energy. After we are boiling the liquid, we usually release the vapours(mainly with a
condenser) and not use their latent heat. In a multiple effect evaporator, we reuse the
latent heat of the vapours from every effect(body) to heat the liquid in the next vessel.
Every effect operating under a lower pressure than the one before, so the B.P of the
solution will be lower. Multiple effect evaporator is much more economic than a single
effect evaporator because the reuse of the vapour saves a lot of money we used to
spend on the steam (figure 4). For a multiple effect evaporator system, you can write
overall mass and energy balance, and independed mass/ energy balance for every
effect, where the vapour from every effect use as the steam for the next one.
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Figure 4 – a simple
flowsheet of multiple
effect evaporator
Recompression: another way to reuse the latent hit of the vapor we separated is by
compress it and increase is condensing temperature. The increases of the vapor’s
condensing pressure and temperature create a temperature gradient for latent heat
reuse in the evaporator’s system. There are two main ways to compress the vapours-
thermal vapor recompression (TVR), and mechanical vapor recompression (MVR).
The TVR method use a steam ejector (also called thermo compressor) to compress the
vapours that leaving the vessel. For that purpose, the compressor use steam at very
high pressure (0.8 – 2.1 MPa). another benefit we get from the thermo compressor is
that it is creating a vacuum pressure inside the vessel.
In the MVR method, we use a mechanical compressor or a centrifugal fan to increase
the pressure and temperature of the vapor we reusing to heat the liquid. For driving the
fans, we usually use a steam turbine or electricity motors, depend on the terms of the
system (costs, pressure of steam we need). MVR system can eliminate the need of
condenser for the vapours in the last effect, and after the start up it requires very small
amount of steam at steady state, and only the work of the compressor added energy to
the system(figure 5).
Figure 5 – a flowsheet of a
single effect evaporator with a
MVR.
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Steam economy- steam economy (SE) is a common way to measure and compare the
performances of evaporators. SE is expressed as the ratio of water evaporated (kg) to
steam consumed (kg). in a single effect evaporator due to decrease of the latent heat as
the pressure increase, the heat we use to raise the initial temperature of the solution to
the b.p, and the heat loses the system to the surrounding(which we neglected at the
energy balance) the SE will always be less than 1. The typically value is around 0.75-0.95,
depend on the initial temperature of the feed and the system loses. For improving the
SE, we are reusing the vapor by manipulating the temperature we need via pressure
changing. In a multiple effect evaporator, we can evaluate the SE as SE = e∙N where e
represents the SE of a single effect evaporator, and N is the number of effects. Adding a
TVR can approximately double the SE of a system, and adding a MVR can provide even
much higher value of SE (table 1). also, we need to remember that the SE is affected by
the initial temperature of the feed and the properties of the liquid 2-7.
Table 1- comparison between SE of
different kinds of evaporation
systems.
example of real-life evaporator-
“SPXFLOW” is an American company that design, manufacture and delivered worldwide
process industrial products such as evaporators, heat exchangers, driers and more.
One of the products the company manufacture is rising\falling film plate evaporator.
This evaporator is engineered to handle small to medium production runs of heat-
sensitive product especially for the food industry, such as fruit juice, Gelatine, Milk corn
syrup etc… for a standard size evaporator, the water removal rate range is from 454 to
15,876 Kg/hr, and that Either plate can be arranged in a multiple effect system to
maximize thermal efficiency8.
Let us examinee the case of orange juice’s evaporation. Fresh orang juice consists of
10%-15% wt. of solids in water. Before shipping, in order to reduce the cost of the
shipping, the juice is concentrated by evaporation at a falling film type evaporator.by
order to improve the aroma and flavour of the product, some of the orange juice bypass
the evaporator and mixed with the product.
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Let us say that the mass flow rate into the feed of a new single effect- falling film
evaporator made by SPXFLOW is 9000 kg/hr of 14% wt. fresh orange juice(F 1). The
evaporator rate for orange juice is 6500 Kg of water/hr, and the cut back is 12% wt. of
the feed. What is the concentrate of the final product?
Basis- the easiest basis we can choose to solve the problem is 1 hour.
Mass balance on the feed will give us (1) F1 = F2 + F3; we know that F3 = 0.12F1
so F3= 1080 kg and F2 = 9000-1080 = 7920 kg.
Evaporator- we know that the evaporator rate is 6500 kg of water/ hour, and that
overall balance is (2) F2=F4+F5 so F5 = 7920-6500=1420 kg.
Solid balance: (3) 0.14∙F2=X4∙F4 + X5∙F5 → 0.14∙7920 = 0∙6500 + X5∙1420 →
→X5=0.7808 this is the solid fraction at stream out of the evaporator.
Mixer – overall balance on the mixer is (4) F3 + F5 = F6 → F6 = 1420+1080=2500kg
Solid balance on mixture will give us (5) X5∙F5 + X3∙F3 = X6∙F6 →
→0.7808∙1420 + 0.12∙1080 = X6∙2500 → X6 = 0.4953
What means the concentrate of the solids in the orange juice in the end of the
evaporation process with the falling film evaporator of SPXFLOW will be 49.53%.
Energy balance- because of the sensitivity for heat of products in the food industry, it is
common to use vacuum pressure inside the evaporator for lowering the boiling point of
water. Let us say that in this evaporator we use vacuum pressure of 40 kPa so the
boiling point of the water is 75.88° C . the steam supplied is saturated at 120 kPa(104° C)
The energy balance for evaporator is (1) F2H2 + Sλ = F4H4 + F5H5
we say that stream F2 enter the room at room temperature(25°) and leave at 75.88°.
we can calculate the heat capacity of the orange juice by equation Turell and Perry 9
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where T is the temperature
in Fahrenheit scale between 32°-212° and Xs is the solid fraction.
After unit conversion we get that the Cp of orange juice @steam F2 is 4.21445 and @ F5
is 4.20003 kJ/kg∙C°. the Cp water @ steam F4 is 4.2 kJ/kg∙C° we use 0° as a reference so
(2) H2 = 4.21445∙25=105.3625 kJ/kg;
(3) H5=4.20003∙75.88=318.698 kJ/kg
(4)H4= 4.2∙75.88=318.696 kJ/kg(can also obtained from steam table) →
S∙λ=1689604.16 kJ. The latent heat of steam(λ) @given pressure is 2243.4kJ/kg
(5) S =( F4H4 + F5H5 - F2H2)/λ → S = 753.144 kg which means we use 753.144 kg of steam
per hour to concentrate the orange juice!
Steam economy- the steam economy is 6500 kg water/753.144kg steam = 8.63 a very
high value for a regular single effect evaporator but because of the use of vacuum
pressure is a reasonable value.
*all the numbers in the above example (except of the data about the SPXFLOW’s
evaporator and the orang juice’s concentration) are fictional and not represent a real
factory.
** in today developments of the juice’s industry it is common to use 4-6 effects
evaporators, and use also TVR and MVR to reduce the money they spend on the steam,
because of the high price of fossil fuel who usually use for producing the steam 10.
Conclusions
Design of evaporation system is complicated process that need to consider in many
factors such as the amount of product, his purpose and the properties of the solution. A
lot of calculations must be done in order to choose the right evaporators and other
auxiliary devices such a compressor or pump that will fit most to the process. Efficient
design of the right system that suit the best to the liquid we about to evaporate is highly
important in order to make the process economical and friendly to the environment.
Today, we can see that because of the low SE of single effect evaporators, almost all
evaporation systems compose of multiple effect systems and usually also use MVR and
TVR to improve the efficiency of the process.
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References
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nderdruck.pdf
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