Chapter 9 evaporation

Effects on evaporation rateHeat rate Laten heat of water at certain condition Maximum allowable temperature: higher is better Operating pressure: lower is better Solution properties: vis

CHAPTER

Objective: “To concentrate a dilute solution consisting of non

volatile solute and volatile solvent”

 In this operation, the solvent to be evaporated is generally water and concentrated solution is a product

 The vapour generated usually has no value, it is condensed and discarded

Evaporation

Properties of evaporating liquids that influence

the process of evaporation

1 Concentration

2 Temperature sensitivity

Pharmaceuticals products, fine chemicals and foods

are damaged when heated to moderate temperatures for relatevely short times

3 Foaming and frothing

4 Scale: Solutions deposit scales on the heating

surface

5 Material of construction

Factors effecting evaporation:

Concentration in the liquid:

- Liquid feed to an evaporator is relatively dilute

- So its viscosity is low, and heat-transfer coefficient high

- As evaporation proceeds, the solution becomes concentrated

- So viscosity increases and heat-transfer coefficient drops

- Density and the boiling point of solution also increase

As the concentration increases, the viscosity and densityincreases thereby the boiling point of solution increases

Factors effecting evaporation:

Solubility:

- As solution is heated, concentration of the solute in the solution increases

- In case the solubility limit of the solute in solution is

exceeded, then crystals may form

- Solubility of the solute therefore determines the maximum concentration of the solute in the product stream

- In most cases, the solubility of the solute increases with temperature This means when a hot concentrated solution from an evaporator is cooled to room temperature,

crystallization may occur

Factors effecting evaporation:

Temperature sensitivity of materials:

- Pharmaceuticals products, fine chemicals and foods are

damaged when heated to moderate temperatures for

relatively short times

- So special techniques are employed to reduce temperature

of the liquid and time of heating during evaporation

Factors effecting evaporation:

Foaming and frothing:

- Solutions like organic compounds tend to foam and froth during vaporization

- The foam is carried away along with vapor leaving the evaporator

- Entrainment losses occur

Factors effecting evaporation:

Pressure and temperature:

- The boiling point of the solution is related to the pressure

of the system

- The higher the operating pressure of the evaporator, the higher the temperature at boiling

- Also, as the concentration of the dissolved material in

solution increases by evaporation, the temperature of

boiling may rise (a phenomenon known as boiling point rise /elevation)

- To keep the temperatures low in heat-sensitive materials,

it is often necessary to operate under atmospheric pressure (that is, under vacuum)

Factors effecting evaporation:

Scale deposition:

- Some solutions deposit solid materials (called scale )

on the heating surfaces

- The result is that the overall heat-transfer coefficient ( U ) may drastically decrease, leading to shut down of the evaporators for cleaning purposes.

Factors effecting evaporation:

Materials of construction:

- Evaporators are made of some kind of steel

- However many solutions attack ferrous metals and are contaminated by them

- Copper, nickel, stainless steels can also be

used.

Effects on evaporation rate

Heat rate

Laten heat of water at certain condition

Maximum allowable temperature: higher is better Operating pressure: lower is better

Solution properties: viscosity, foaming, frothy

Once through / Circulation

Natural / Forced / Agitated

Once through is useful for heat sensitive materials and adapted to multiple effect, agitated and falling film

Circulation is not for heat sensitive materials and adapted to single effect, natural / forced and rising film

Classification

Natural circulation with low heat transfer coefficient

= 0.3 ÷ 1 ⁄ , relative cheap, poor circulation, for nonviscous / non deposite scale liquids

Forced circulation with high heat transfer coefficient

= 2 ÷ 6 ⁄ , fouling reduction, high pumping cost

Agitated with high heat transfer coefficient, reduce thermal resistance of liquid, high capital cost and low capacity

Classification

Falling film is for highly heat sensitive materials which requires short residence time

Classification

Vertical tube is for foaming liquids

Horizontal tube is for low viscosity and non deposit scale liquids

Classification

Single effect is simple, big temperature difference between steam and solution, ineffective energy usage

1 ÷ 1.3 kg of steam kg water evaporation ⁄

Multiple effect is more complex and effective energy usage

Classification

Internal heater with short vertical tube = 1 ÷ 2

External heater with long vertical tube = 3 ÷ 10

Classification

Open kettle & pan evaporator

Open kettle & pan evaporator

Low maintenance & installation cost.

Wide variety of materials.

DISADVANTAGE Heat economy is less.

Not suitable for heat sensitive materials Heat decreases on product concentration Since, open type so vapor passes to atmosphere.

Once through evaporator

Falling film

External heater

Rising filmExternal heater

Once through evaporator

Rising filmInternal heater

Natural circulation evaporator

RisingInternal heaterRising

External heater

Natural circulation evaporator

Horizontal tube

Internal heater Internal heaterVertical tube

Forced circulation evaporator

FallingInternal heater Rising

Internal heater

Forced circulation evaporator

RisingExternal heaterFalling

External heater

Forced circulation evaporator

Horizontal tube – External heater

Agitated evaporator

Natural circulation – Internal heater

• Increases the heating surface 10 ÷ 15 times thansteam jacketed kettle

• Vigorous circulation enhances rate of heat transfer

• More units can be joined

Advantages

• Liquid to be maintained above calandria

• Complicated and increased installation cost

• Pressure has to maintain

• Cleaning and maintenance is difficult

Disadvantages

Natural circulation – External heater

• Large area for heat transfer, enhanced heat transfer

• Short residence time, suitable for heat sensitiveliquids

• Rising film is for foaming and frothy liquids

• Falling film is for viscous and corrosive liquids

Advantages

• Quite complicated, high capital cost

• Cleaning and maintenance is difficult

• Space required

• Rising is not for viscous, salting and scaling liquids

• Falling is not for suspension, salting and scaling liquid

Disadvantages

Forced circulation

• High heat transfer coefficient

• Suitable for high viscous liquids

Advantages

• High capital and operation cost

• Not possible for salting and scaling liquids

Disadvantages

Once through evaporator

Rising film

Internal heater

Plate external heater

Once through evaporator

Rising film – External heater

Once through evaporator

Falling film

External heater Separation chamber

Heat exchanger

Forced circulation evaporator

RisingExternal heaterFalling

External heater

Forced circulation evaporator

Rising – External heater

Forced circulation evaporator

Rising – External heater

Forced circulation evaporator

Rising – External heater

Forced circulation evaporator

Horizontal tube – External heater

Single effect with recompression

Mechanical recompression

Single effect with recompression

Mechanical recompression

Single effect with recompression

Mechanical recompression

Single effect with recompression

Mechanical recompression

Single effect with recompression

1 The vapors, which contain latent heat, are generally discarded in an evaporator, thereby wasting energy

2 But thermal energy in the vapor evolved from a boiling solution can be utilized to vaporize more water

1 Multiple effect evaporation

2 Vapor recompression

Methods of improving Evaporator economy

The following techniques are used to utilize the

thermal energy that is available in the vapors coming out from the evaporator

1 Multiple-effect evaporation

Salient features

 The vapors, which contain latent heat, are generally discarded in an evaporator, thereby wasting energy

 But it can be used as steam supply to another unit operating under

lower pressure and temperature

 The vapor from the second unit can be further used as a steam supply

to a third unit operating at a still lower pressure and temperature

 Each unit in such a series is called an effect and the method of re-using the latent heat is called multiple-effect evaporation.

 In the case of multiple effect evaporators the economy increases at the cost of capacity

 Operating cost is same, but the capital cost, repair and maintenance cost increases with increase in number of effects

Multiple effect evaporator

Forward feed operation

SteamSolution

Backward feed operation

SteamSolution

Mixed feed operationParallel feed operation

SteamSolution

Forward: feed flows naturally (without pump), low boiling point for heat sensitive concentrate

Backward: pump required, for cold feed and highly viscous concentrate

Parallel: for feed is almost saturated, solid crystal

Mixed (forward–backward): for very highly viscous concentrate

Multiple effect evaporator

This arrangement is simplest and

no need of any pump to transfer

liquid from effect to effect as the

liquid flows in the direction of

decreasing pressure.

 This method requires a pump between each pair of effects since the flow is from lower pressure to the higher pressure.

 If the liquid is very viscous then we have to adopt this arrangement for better capacity.

This arrangement is

combination of forward and

backward feed adopted for

best overall performance

 The fresh feed is fed to each effect simultaneously and the thick liquor is taken out from the same effect separately

 In this arrangement there is no transfer of liquid from one effect to another effect

Multiple effect evaporator

• Suitable for large scale and continuous process

• Highly economical

Advantages

• Monitoring of evaporators

Disadvantages

Multiple effect evaporator

Backward feed operation

Multiple effect evaporator

Parallel feed operation

Multiple effect evaporator

Parallel feed operation

Multiple effect evaporator

Mixed feed operation

Multiple effect evaporator

Forward feed operation

Multiple effect evaporator

Forward feed operation

Multiple effect evaporator

2 Vapor recompression

In this method, the vapors from the evaporator are

compressed to a saturation pressure of steam to

upgrade the vapors to the conditioning of original steam

to permit the use as heating media

These are two types

(a)Mechanical recompression or

(b)Thermal recompression

Mechanical Recompression:

In this method the vapor evolved from the evaporator is compressed to some what higher pressure by positive displacement (or) centrifugal compressor and fed to a heater as a steam

Thermal Recompression:

In this method vapor is compressed by means of steam jet ejector Here the high pressure steam is used to draw and compress the major part of vapors from the evaporator

Thermal recompression is better suited than

mechanical recompression to vacuum operation

Jets are cheaper and easier to maintain than

compressors

Disadvantages of thermal recompression include low mechanical efficiency of jets

Overall heat transfer coefficient

Type Overall coefficient ⁄ ℃

Vertical tube

Natrural circulation Forced circulation

1000 ÷ 2500

2000 ÷ 5000 Agitated film

1 1 100

2000 1500 600

Raoult’s law

The partial pressure of each component of an ideal mixture is equal to the vapor pressure of the pure component multiplied by its mole fraction.

For nonvolatile solution, there is only solvent evaporized:

Boiling Point Elevation

• The Boiling point elevation, Δtbp, is directly

proportional to the molality of the solute

Δtbp= Kbpmsolute

Kbp is called the molal boiling point elevation constant

by solvent and is (oC/m)

msolute = molal solute concentration

Ex: How many grams of ethylene glycol, HOCH2CH2OH,

do you have to add to 125 g of water to increase the

bp by 1oC? (The KbpWater = +0.5121 oC/m)

Boiling-Point Elevation

• van’t Hoff Factor, i: This factor equals the number of ions

produced from each molecule of a compound upon dissolving For compounds that dissociate on dissolving, use:

∆Tb = i × Kbp m

Raoult’s law

∆Tb = i × Kbp m

: number of ions produced by each molecule of solute

: called the molal boiling point elevation constant by solvent

Problem: We add 475g of sucrose (sugar) to 600g of water What will

be the Boiling points of the solution?

for BP elevation using the constants from above table

Solution:

Sucrose (C12H22O11) has molar mass = 342.30 g/mol

Example 1

How many grams of ethylene glycol do you have to add

to 125 of water to increase the boiling point by 1℃?

Example 2

Problem: We add 475g of sucrose (sugar) to 600g of water What will

be the Boiling points of the solution?

for BP elevation using the constants from above table

Solution:

Sucrose (C12H22O11) has molar mass = 342.30 g/mol

Duhring’s rule

A linear relationship exists between the temperatures at which two solutions exert the same vapor pressure The rule is often used to compare a pure liquid and a solution at a given concentration.

Duhring’s rule

Boiling point of water

Salt (sodium chloride) – Water at atmospheric pressure

Nomograph for boiling point of aqueous solutions

Example 1

An aqueous, NaOH solution is being evaporated at 6

If the solution is 35% NaOH, determine:

a) The boiling temperature of the solution

b) The boiling point elevation

a) The boiling point of the solution from the plot is

207℉

b) The boiling point elevation: ∆ = 207 − 170 = 37℉

Vapor

= + ∆ = ,

− (pressure drop, concentration)

Temperature difference

ConcentrateCondensate

Vapor

Feed

Steam

,

= + ∆ = ,

= + ∆ = ,

− (pressure drop, concentration,

hydrostatic head & friction loss)

Temperature difference

ConcentrateCondensate

Vapor

Feed

Steam

,

Calculation for single effect evaporator

Calculation methods for single-effect evaporators

F – mass flow rate

xF – mass fraction of solute in feed

TF – temperature of feed

hF – enthalpy of feed

Vapour leaving the evaporator:

V – mass flow rate

yV – mass fraction of solute in vapour

T1 – temperature of vapour

HV – enthalpy of vapour

Concentrate leaving the evaporator:

L – mass flow rate

xL – mass fraction of solute in concentrate

P – pressure in the evaporator

T1 – temperature in the evaporator

Calculation methods for single-effect evaporators

Overall material balance:

T1

Calculation methods for single-effect evaporators

Energy lost by the steam

q = S λ = S (HS – hS)

In case of no energy loss to the environment, q amount of energy gets transferred from steam to the solution through the tube wall

of area A and overall heat transfer coefficient U

T1

Example 1:

A continuous single-effect evaporator concentrates 9072 kg/h of a 1.0 wt % salt solution entering at 38ºC to a final concentration of 1.5 wt %

The vapor space of the evaporator is at 101.325 kPa (1.0 atm abs) and the steam supplied is saturated at 150 kPa The overall

coefficient U = 1704 W/m2.K

Calculate the amounts of vapor and liquid products and the transfer area required Assumed that, since it its dilute, the solution has the same boiling point as water