Evaporation Flashcards
generally refers to the removal of water, by vaporization, from aqueous solutions of non-volatile substances.
Evaporation
It is an operation that converts a liquid into a vapor leaving behind a concentrated solution or a higher boiling liquid
Evaporation
In evaporation, several rate processes occur. Name them.
First, heat is transferred from the heating medium to the solution. Second, mass and heat are simultaneously transferred from the
liquid to the vapor phase.
Two basic processes involved in evaporation
Transfer of Heat
Transfer of Mass
The heat supplied includes
The sensible heat for the solution to reach its boiling point.
Latent heat of vaporization for the water evaporated
controls the overall rate of evaporation and the effect of the other transfer processes is only minor.
The transfer of heat from the heater to the solution
is generally provided by the condensation of a vapor, such as steam, on one side of a metal surface with the evaporating liquid on the other side.
The heat required for evaporation
Why is it necessary to remove the vapor from the evaporator for the continuation of vaporization?
the pressure above the solution soon equals the v.p. of the water and boiling stops.
An evaporator consists of
- Heat Exchanger capable of boiling the solution
- and a device to separate the vapor phase from the boiling liquid.
The solution in the evaporator body is violently mixed and therefore homogeneous in composition, except in the case of
long tube vertical evaporators and turbulent evaporators.
To find ℎ𝐹 𝑎𝑛𝑑 ℎ𝐿:
- Use concentration-enthalpy diagram if available
If concentration-enthalpy diagram is not available, instead specific heat of the feed should
be given, use boiling point of the solution as base temperature
Each unit in such a series is called an
effect
is the principal means in use for economizing in the energy consumption.
multiple effect evaporation
the method of reusing the latent heat is
multiple effect evaporation
In the _______ feed operation, the feed is introduced in the first effect and passed from effect to effect parallel to the stead flow.
forward
Disadvantage of forward feed operation
all the heating of cool feed is done in the first effect, so that less vapor is generated from each unit mass of steam, resulting in lower steam economy.
In the _____ feed operation, feed is introduced in the last effect and the concentrated product is taken out from the first effect.
backward
are required between effects in backward feed operation
Pumps
The backward feed operation is used when
- feed is cold, since less liquid must be heated to the higher temperature existing in the early effects
- also used when the product is viscous that high temperature existing in the early effects
- also used when the product is viscous that high temperature is needed to keep the viscosity low enough to give reasonable heat transfer coefficient.
In the _____ feed operation, the fresh feed is introduced directly into each effect and evaporated to find concentration in each effect
parallel
The parallel type of operation is used when
feed is saturated solution and solid crystal are product, as in the manufacture of common salts.
is used only for special applications, as when liquor at an intermediate concentration and a certain temperature is desired for addition processing.
Mixed feed operation
