Single Stage Evaporation

Question 1

Evaporation almost always refers to the removal of….

Answer 1

….water (valueless component)

Question 2

The process of evaporation is usually stopped before…

Answer 2

…the solute begins to precipitate from the solution

Question 3

What kind of problems can be encountered in evaporation?

Answer 3

Foaming
Scaling
Heat sensitivity
Corrosion
Space limitations in design

Question 4

Liquors that are evaporated can be classified in which ways?

Answer 4

1. By temperature - Liquors that can be heated to high temperatures without decomposition and those that can only be heated to low temperatures
2. By yield - Liquors that yield solids on concentration and those that do not
3. By boiling point - Liquors that at any given pressure boil at the same temperature as water, and those that have a much higher boiling point

Question 5

What are two basic processes that occur in evaporation?

Answer 5

Mass transfer

Heat transfer

Question 6

Heat is added to the solution for what?

Answer 6

To provide the latent heat of vaporization - to vaporize the solvent (the vapor is not the product that is important)

Question 7

The normal heating medium in evaporation is…

Answer 7

…steam or Dowtherm/flue gases

Question 8

List some equipment used in evaporation

Answer 8

• Open pans (large surface area exposed to sun; used primarily for vaporization of sea water to get salt)
• Horizontal-tube evaporator (steam tubes arranged horizontally on inside of evaporator)
• Vertical-tube evaporator (mixture is passed through tubes while steam surrounds the tube)

Question 9

Give the equation for heat transfer

Answer 9

Q = UA(change in T)

Question 10

Define the boiling point rise

Answer 10

The difference between the boiling point of a solution and that of water

Question 11

What does the overall heat coefficient for any type of evaporator depend on?

Answer 11

The value of the film coefficients on the heating side and for the liquid, as well as the allowances for scale deposits and the tube wall

Question 12

What is Duhring’s Rule?

Answer 12

If the boiling point of a solution is plotted against that of a reference material, usually water at the same pressure, then a straight line is obtained

Question 13

Define Raoult’s Law?

Answer 13

The equilibrium partial pressure that is exerted by a component in a mixture, is the same as the mole fraction of the component times the equilibrium partial pressure if the pure component were at the same temperature

Question 14

The rate of heat transfer in a continuous evaporator operating under steady-state conditions is determined by what two factors

Answer 14

1. The amount of sensible heat required for heating the feed from its entering temperature to the boiling temperature
2. The amount of latent heat required for vaporizing the water

Question 15

How is sensible heat calculated?

Answer 15

The difference between the boiling temperature in the evaporator and the entering feed temperature

Question 16

If the Cp for inorganic salts in the feed is not given what is used?

Answer 16

The Cp for H2O but only for the H2O part of the solution

Example: for a 6% mixture inorganic salt: mCp = 0,94 x m solution x Cp water

Question 17

Define evaporation

Answer 17

The removal of water from a solution by boiling the liquor in a suitable vessel, the evaporator, and withdrawing the vapour