Evaporation

Question 1

Why evaporate?

Answer 1

To concentrate a solution of non volatile solute and volatile solvent.

Unlike drying the residue is a liquid
Unlike distillation where vapour is a single component distillation is multiple components

Question 2

Driving force of Evaporation?

Answer 2

The Temperature difference between steam temperature and product temperature

Question 3

Why do we need evaporation?

Answer 3

Reduces Transportation Costs
Storage costs
Prepare for drying or other process
Better Microbial stability 
Recovery of solvent

Question 4

Batch Pan

Answer 4

Long Residence time
improved agitation
Low capacities
Boils at low temp

Question 5

Natural Circulation

Answer 5

High heating efficiency
Large capacities
short tube bundle with shell and tube heater

Question 6

Forced Circulation

Answer 6

The Liquid is circulated at high rate through HX
Minimises buildup of deposits
Pressure is slightly less

Suitable for liquid susceptible to crystallization

Question 7

Rising Film Tubular

Answer 7

Liquid on the inside of the tube brought to boil
Vapour core with liquid film on tube walls
high vapour velocities with faster liquid film speed
Short Product Residence time
Higher Heat Transfer Coefficient

Question 8

Falling Film Tubular

Answer 8

Going with gravity film
Faster moving film
Even shorter residence time

may run dry
uneven liquid distribution

Question 9

Rising/Falling Film Tubular

Answer 9

better liquid distribution
lower head room requirements
Faster moving liquid
half the height

Question 10

Problems of Evaporation?

Answer 10

High Product Viscosity
High Sensitivities
Crystals Formation and deposition

Question 11

Initial selection of evaporator chosen off which characteristics?

Answer 11

Viscosity of product
Fouling Tendency (of product) 
Highest Heat Transfer Coefficient 
Materials of construction:
- Resistant to corrosion 
- Ease of cleaning

Question 12

Two evaporators are used most commonly and why?
e.g. FF and T

And name one for high viscosity fluid?
FC

Answer 12

Falling Film Evaporator:

• Plate or tubular provides highes HTC
• usually chosen if the product permits as it is most economical

Tubular Evaporators :

• for very large evaporators
• less floor space than plate evaporators

Forced Circulation:
viscosities of 5000CP
Reduce Fouling 
OPEX and CAPEX Large 
Tubular when particle diameter greater than 2mm