Evaporators

Nimish Shah

INTRODUCTION:

vaporization of a liquid or that of a

solvent from a solution.

Thus, water 'evaporates' from a river,

or cane-sugar juice is 'evaporated' to

concentrate it.

INTRODUCTION:

In chemical engineering terminology,

evaporation means … removal of a

part of the solvent from a solution of

a non-volatile solute by vaporization.

INTRODUCTION:

The objective of vaporization is to

concentrate the solution.

What is difference between

Evaporation and Vaporization?

Evaporator

Vapor out

Feed in

Steam in (Saturated

vapor)

Product out

Condensate out

(Saturated Liquid)

Vapor

Separator

Heat Exchanger

Vaccum for non

condensable

Condensor unit

Coolant In

Coolant out

Classification of Evaporators:

most evaporators are broadly classified as: (1) natural circulation, and (2) forced circulation.

1 Solar evaporator 2 Batch pan evaporator

3 Natural circulation E. 4 Short tube vertical evaporator

5 Horizontal tube evaporator 6 Basket type evaporator

7 Long tube vertical E. 8 Forced circulation evaporator

9 Rising film evaporator 10 Falling film evaporator

11 Agitated thin film evaporator 12 Horizontal spray film E

13 Plate type evaporator 14 Vapour compression E.

Vertical Tube Evaporator

Short tube vertical (STV)/horizontal

evaporator:Advantages:

fouling liquids can behandled

So cleaning of tube is easy.

High heat transfercoefficients can be achievedwith dilute solutions.

Cost of unit is less iffabricated from carbonsteel.

Their main advantages arelow cost, easy cleaning, andlow 'headroom'requirement.

Limitations: Large liquid holdup. It cannot be used for:

– heat sensitive materials.– if the solute has a tendency to crystallize.– for viscous liquids– for solutions in which precipitation or salting out of a

solid may occur.

low heat transfer coefficient need a larger floor space. The natural circulation velocity in the evaporator is

not sufficient to keep the solid particles in suspension. The problem may be overcome by installing a

propeller in the down take pipe to increase thecirculation rate.

Application: used to concentrate a variety of solutions. It is good for non-corrosive, clear and non-crystalline

liquors, e.g. cane sugar solution.

Horizontal Tube Evaporator

Advantages:

high heat transfercoefficient, low cost, lowliquid hold-up and lessfloor space requirement.

large heat transfer surfacearea can be provided incompact surface.

Limitations:

High headroomrequirement,

unsuitable for viscous andscale-forming materials.

Advantages:

Heat transfer coefficient is high.

Offer highest operationalflexibility.

It is useful for crystallizing andconcentrating thermallydegradable product.

It is useful for viscous solutions.

Disadvantages:

Cost is very high, because ofhigh maintenance cost.

Corrosion-erosion can occur inthe evaporator.

If liquid forms scale, tubeplugging can occur.

High headroom requirement,and high floor area requirementif a horizontal heat exchanger isused.

Forced Circulation Evaporator

Forced Circulation Evaporator

A = Product

B = Concentrate

C = Condensate

D = Heating steam

E = Vapour

1 = Flash vessel/separator

2 = Calandria

Rising film evaporator

Rising film evaporator

Rising film evaporator

Falling film evaporator

Falling film evaporator

Falling film evaporator

A = ProductB = ConcentrateC = CondensateD = Heating steamE = Vapour

1 = Head2 = Calandria3 = Separator4 = Separator duct5 = Calandria base

Agitated thin film evaporator

Plate Type Evaporator

A = ProductB = ConcentrateC = CondensateD = Heating steamE = Vapour

1 = Main separator2 = Pre-separator3 = Plate calandria

Plate Type Evaporator

Characteristics of different types of evaporators

Evaporator Typical products

handled

Comments

Calandria Salt; glycerine from

spent soap lye

Suitable for batch or

continuous operation in

single or multiple effects.

Natural

circulation

(Thermos

yphon)

Foaming liquids;

less viscous

materials; black

liquor from the

pulp industry;

spent soap lye;

electroplating

solutions; spin

bath liquid.

External separator provides

some holding time

adjustment;

integral vapour-head type with

downcomer gives

minimum hold-up.

Forced

circulation

Salting or scale-

forming materials

depending on

steam-chest

configuration;

caustic soda

solution, sodium

sulphate, tomato

juice to 30%

concentration, etc.

Available with:

(1)horizontal steam-chest with

external vapour-separator

(less used now);

(2)vertical steam-chest with

external separator;

(3)vertical steam-chest with

integral vapour head.

Operates with either

submerged or partially filled

tubes in single or multiple

effects.

Characteristics of different types of evaporators

Evaporator Typical products

handled

Comments

Rising film Caprolactum;

ammonium nitrate;

fruit juices; for

crystal producing

solutions with

suspended solids.

Allows single-pass operation

with high liquid and

vapour velocities;

minimum liquid hold-up

Falling film Low to medium

viscosity materials,

heat sensitive

products, fruit

juices, and

pharmaceuticals.

Single or multiple effects;

can be operated on single

pass or with partial

recycle of concentrated

products.

Agitated

film

Handles the full

range of feed

viscosities;

gelatin; fruit

puree; glue

Available:

(1) vertical with integral

vapour separator;

(2) vertical with external

separator, cocurrent

flow;

(3) horizontal with

tapered shell,

countercurrent flow;

Plate type Fruit juices, extracts;

gelatin;

condensed and

whole milk.

Liquid and vapour flow

essentially as in rising-

and falling-film

evaporators without

liquid distribution

problems.