Evaporation in single stage mode of operation or in a single evaporator is not efficient at all since heat is not completely used during the evaporation process. For example, the heat taken by the condensate, thickened liquor and solvent vapor streams is just wasted.
One old method of making the evaporation process more efficient is by connecting in series single stage evaporators. This is called: multiple effect evaporation mode. Another way is by a more modern approach called: process intensification (not discussed here).
CONTENTS
1 Two types of multiple effect evaporation
2 What is multiple effect evaporation?
2.1 The mode of operation of multiple effect evaporators
2.1.1 Forward feed evaporator operation mode
2.1.2 Backward feed evaporator operation mode
1 Two types of multiple effect evaporation
Before going further on this post, a distinction of two types of evaporation operation procedures needs to be made: one aimed for power plant process purposes and another one that could be classified as chemical. This distinction is well presented by D. Q. Kern in his book Process heat transfer.
A number of features can be listed to distinguish between them,
- in process plant evaporators there a blowdown or purge of fluid with dirt is used while in chemical evaporation this not used,
- process plant evaporation is used as part of the services of a chemical process plant, for example: for the production of distilled water,
- chemical evaporation is at the heart of the chemical process, for example: for concentration of high value products,
- chemical evaporators require larger heating surfaces in comparison to power plant evaportors,
- in chemical evaporation the properties of the fluid such as strong viscosity changes and foaming formation may be undesirable but are part of the product.
2 What is multiple effect evaporation?
As mentioned early, multiple effect evaporation is for short the connection in series of two or more single stage evaporators. In this way, you may find double-effect evaporators, triple-effect evaporators and so on. Notice that even if double, triple, etcetera evaporators are made of individual evaporators these are treated as single units.
In most cases presented in textbooks only the heat carried by the solvent vapor and thick liquor is taken advantage of. This means that the heat in the condensate is still being lost. This does not mean that in practice or modern equipments the heat in this stream is lost but that most textbooks do not show these cases. Perhaps the energy remaining in the condensate ma be used for warming of the dilute solution prior it enters the evaporator.
A possible reason triggering multiple effect units creation, but also a limitation, comes from the fact that the solvent vapor is made of water. If the solvent vapor is not water then some other strategy rather than multiple effect should be looked for.
2.1 The mode of operation of multiple effect evaporators
Considering that the solvent vapor is made of water, in a multiple effect evaporator the solvent vapor stream becomes attractive as heating medium. When a mass of steam is suplied to an evaporator it is used to evaporate water, from the dilute solution, which leaves the equipment through the top. The solvent vapor is little less than the steam supplied but it still has enough energy to be used as the steam for another evaporator.
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| Fig. 01 A double effect evaporator in which the energy in the solvent vapor and boiling solution is not wasted but used to thicken the liquor. |
The solvent vapor is just high quality clean water vapor. This is what makes it so attractive. The thing is, that multiple effect evaporators are useful when the solution can not be exposed to high temperatures for a long time but concentration of the solution is needed. Therefore, two important observations can be made,
- at every effect the solution is passed, it concentration increases,
- as the solution passes through an effect the extracted solvent vapor also decreases along with the energy available in it and
- as the solution passes through an effect its temperature decreases too.
Figure 01 shows an schematic of how a double effect evaporator works. However, in practice, three different modes of operation can be identified:
- forward feed,
- backward feed and
- mixed feed.
which are independent of the number of connected single stage evaporators.
As you may have already imagined: the aformentioned modes of operation will have a different heat and mass balance, so that attention has to paid when dealing with this kind of equipments.
The greatest cost in the operation of an evaporator comes from supplied steam.
2.1.1 Forward feed evaporator operation mode
In this operation mode the solution travels, continuosly, from the first effect to the second effect and from there to the third effect, after which it leaves as the concentrated liquor. As shown in Fig. 02 the solution si pumped into the first effect and the liquor is pumped from the third effect.
Notice that the solution at the effect 2 is not pumped into third effect. This may be due to the gradient pressure between these two effects so that no pump is needed.
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| Fig. 02 Forward feed load operation mode for a triple-effect evaporator. |
As you can see, both the solution and the solvent vapor travel in parallel across the whole set of effects. The temperature of the solution decreases with every effect just as the energy of the solvent vapor decreases as well. This could be a disadvantage. In this type of operation mode the solution at the third effect shall be exposed to the lowest temperature so that the concentration effect would be mild. However, long time exposition to high temperatures could lead to degradation of the valuable compounds in the liquor so that this arrangement would be recommended for evaporation of solutions with high sensitivity temperatures.
On the other hand, as temperature decreases the viscosity of the solution is increased. This will affect heat transfer in the form of the heat transfer coefficient.
The economy for this mode operation is the lowest of the three options.
2.1.2 Backward feed evaporator operation mode
Despite the simplicity of the idea it is just not only to reverse the forward feed operation mode. As presented in Fig. 03, the solution has to be pumped into every effect and from it. Again, temperature has to with it.
Notice, that in this case, the streams of solution and solvent vapor travel in opposite directions (say counter current). Also, as the solution enters the evaporator the it is subject to the lowest temperature but as the liquor, thickened in the first effect, leaves, it is subject to the highest available temperature.
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| Fig. 03 Backward feed load operation mode for a triple-effect evaporator. |
One may argue that the viscosity should be lower at the first effect but not only the solution is thicken. Also the temperature gradient is adverse for a natural circulation from the inlet to the outlet. On further observation of this mode of operation is that as the solution is fed into the evaporator it goes from low to high temperature so that the energy in the steam is better used than in forward feed operation mode. In other words, this arrangement would create a temperature ramp. The economy in backward feed mode operation is higher than in forward feed. However, again, this mode of operation is not recommended for fluids that tend to decompose with high temperature exposition.
2.1.3 Mixed feed evaporator operation mode
Mixed feed operation mode is just a combination of the forward and backward feed operation mode trying to come up with the best of two worlds. For a triple-effect evaporator as shown in Fig. 04 the arrangement would seem simple but as the number of effects increases the flow of the solution from one effect to the other may become messy. Just think about a quadruple-effect evaporator in which the main feed could be either at the 2nd or 3rd effect.
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| Fig. 04 Mixed load operation mode for a triple-effect evaporator. |
Of course, mixed feed evaporation mode is only possible for triple-effect, an higher, evaporators (if only one main feed exist). Mixed feed operation mode has better economy than backward feed mode.
3 What you know about multiple effect evaporation
Please, follow the link below to answer a small quizz and check what you have learned from this post,
What you know about multiple effect evaporation
This is the end of the post and I hope you will find it useful.
Other stuff of interest
- LE01 - AC and DC voltage measurement and continuity test
- The pros and cons of bulb thermometers
- The pros and cons of bimetallic thermometers
- Some examples of temperature instruments
- Minor losses - Formulas
- What is a process variable?
- What are the most important process variables?
- Time dependence of process variables
- A list of process variables
- PID controllers for temperature
- Valves - Gate design
- Evaporation - General comments
- The lines of Dühring
- Mass and heat balance in a single stage evaporator
- Example: Mass and and heat balance in an evaporator
Ildebrando.
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