CONTENTS
For short evaporation is a unit operation involving heat tranfer and phase change. Also, this operation can be seen from another point of view,
- concentration or purification ,or
- drying
However, this is a confusion to be avoided since drying is a solid-liquid operation and evaporation can be restricted as a liquid-liquid operation. Evaporation is at the heart of increasing the concentration of a substance or purification since a component in a solution is to be split from the whole.
Evaporation refers to the separation of a solvent in a solution by evaporation. This operation is not o be confused with distillation since in this case what matters is the concentrated solution rather than the evaporated phase.
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| Fig. 01 A sketch of the working principle for an evaporator |
Evaporation mau also be confused in some cases with distillation but in evaportation the vapor phase is made of a single component (in distillation the vapor is in fact a mixture).
1 How to evaporate something
There are two possibilities:
- using steam or
- applying electrical heating
Each of these mechanisms have their own restrictions and benefits. Steam is preferred for heating of large volumes of liquids as in a jacketed reactor while electrical heating is used for smaller volumes. Another feature to pay attention is the availability of fuel and electrical energy since there are cases in which very large electrical resistances can be used if fuel is not available for steam generation. One further case would be that of electrical steam generators (if you are steam guy).
1.1 Boiling point
Once heating the solution you will need to know how much energy you need to supply so that over heating does not occur. Over heating would lead to,
- waste of energy,
- thermal degradation of valuable compounds,
- evaporation of something that should not or
- risk of explosion.
Since, you need to separate a solvent from a solution therefore this solvent would need to have a known boling point so that engineering estimation can be performed. However, further implications arise,
- since the liquid is in fact a solution it will have a different boiling temperature to that of the solvent we wish to evaporate,
- the boiling temperature will change according with the pressure in the evaporation chamber,
- the vapor pressure of the solution is usually smaller than that of the pure solvent (we are looking to evaporate),
- (therfore) the boiling temperature of the solution will be greater than that of the pure solvent.
One further term: the boiling temperature difference between that for the solution and that for the pure solvent is called: boiling temperature elevation.
At this point it should obvious that some background on thermodynamics is required since temperature and vapor pressure are to be related later to enthalpy since one common question is on the required energy to evaporate the solvent.
1.2 Boiling mechanism
Much of the knowledge on boiling liquids comes from studies in the past century in 1931 (the reader is advised to read the book Heat Transmission by WH McAdams for an extended presentation of the topic). The mechanism for pool boiling from submerged surfaces is now outlined.
Consider a submerged metallic surface (to be heated), with certain geometry, such as a plate or cylinder for example, in a pool of liquid to a certain depth. Some precisions need to be made,
- the fluid may be stagnant or not,
- the pool of liquid may be open or not, to the atmosphere so that the system may be pressurized. In this way, there is an amount of liquid over the surface and a hydrostatic pressure on it as well,
- if the liquid is at its boiling temperature the term saturated pool boiling is employed and
- if the liquid is at a temperature below its boiling point the term subcooled pool boiling is used.
Once the surface is heated, thermal energy is transferred from the metal to the liquid by conduction. If temperature in the metal surface continue increasing more thermal energy is transferred to the fluid so that if becomes lighter with lower density. Since the fluid on top, near the liquid-gas interface is colder and with larger density than that of the fluid near the hot surface, a second mechanism enters: convection. The unstable arrangement of heavy liquid over lighter liquid drives fluid motion just because of the temperature gradient. Two heat transfer mechanisms are now involved.
Later, if temperature continues to rising the two mechanisms, conduction and convection, could not be enough for the heat transport so that a phase change of the liquid begins: it evaporates. On the metal surface small bubbles (evaporated liquid) appear in random places of it due to non-uniformities of the surface. These bubbles are first, well attached all over the surface but increments on temperature dettach them while convective motions transport them to the top to the liquid-gas surface. At first the thermal energy is released on the way to the top and the bubbles disappear, but eventually the whole pool of liquid is so hot that the bubbles reach the liquid-gas interface (some bubbles may coalesce) so that liquid vapor splits from the original pool.
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| Fig. 02The evaporation mechanism presented from a boiling pool |
The bubbles are minute at the beginning and grow with time. Later, these break lose once the forces of surface tension holding them to the metal surface are smaller than the buoyancy forces (due to density changes).
More can be said about the boiling mechanism. For example, that rough surfaces may be preferred for faster bubble formation than smooth surfaces. Also, there is a maximum for the supplied heat for evaporation on which the best results are obtained. This maximum is discussed in next section.
1.3 The boiling curve
This is a plot, of experimental data, of the rate of heat transfer $q$ per unit area $A$ from the metallic surface to the liquid versus the temperature difference $\Delta T=t_S-t_L$, where $t_S$ is the metallic surface temperature and $t_L$ is that for the liquid.
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| Fig. 03 A sketch of the boiling curve |
Experimental data on boiling are key for control of process purposes. The boiling curve depicted by A through F' is barely the same as A through F with the difference that for A-F' the heat transfer coefficient $h$ is used rather then $q/A$. However, the phisycs is the same.
2 What you know about generalities on evaporation
Follow the link below to check what you have learned about generalities on evaporation
What you know about evaporation generalities
This is the end of the post. 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
Ildebrando.
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