CENTRAL INSTITUTE OF TECHNOLOGYKOKRAJHAR

EVAPORATION

PURPOSE OF EVAPORATION

To concentrate solution by removing the vapor from a boiling liquid solution .

In the majority of cases, evaporation refers to the removal of water from an aqueous solution.

Example: concentration of aqueous solutions of sugar, sodium chloride, sodium hydroxide, glycerol, glue, milk, and orange juice.

In these cases the concentrated solution is the desired product and the evaporated water is normally discarded.

In a few cases, water, which contains a small amount of minerals, is evaporated to give a solids-free water to be used as boiler feed, for special chemical processes.

Evaporation processes to evaporate seawater to provide drinking water have been developed and used.

EVAPORATION VS DISTILLATION VS DRYINGEVAPORATION & DISTILLATION

In the distillation method, vaporization takes place at the boiling point whereas, in evaporation, vaporization takes place below the boiling point.

Evaporation takes only from the surface of the liquid. In contrast, distillation is taking place from the whole liquid mass.

At the boiling point of the distillation process, the liquid forms bubbles and there is no bubble formation in evaporation.

Distillation is a separation or purifying technique, but evaporation is not necessarily so.

In distillation, the vaporization happens rapidly, whereas the evaporation is a slow process.

EVAPORATION AND DRYING

The term drying usually refers the removal of relatively small amounts of water from solid or nearly solid material, whereas Evaporation is usually limited to the removal of relatively large amounts of water from solutions.

  In most cases drying involves the removal of water at temperatures below its boiling point, whereas , Evaporation means the removal of water by boiling a solution.

WHAT IS EVAPORATION ?

Evaporation is a type of phase transition; it is the process by which molecules in a liquid state (e.g water) spontaneously become gaseous (e.g water vapor).

The equipment used for evaporation is known as Evaporator.

APPLICATION OF EVAPORATION

MANUFACTURING OF BULK DRUGS

MANUFACTURING OF BIOLOGICAL PRODUCTS

MANUFACTURING OF FOOD PRODUCTS

MANUFACTURING OF DE MINERALISED WATER

PROCESSING FACTORS

1. Temperature:

-Higher the temperature greater will be the evaporation.

2. Vapor pressure:

- Rate of evaporation is directly proportional to the vapor pressure of the liquid.

- Lower the pressure ,greater will be the evaporation.

3. Surface area:

- Greater the surface area of the liquid, greater will be the evaporation.

4. Time of evaporation:

- Exposure time is longer, more will be the evaporation.

5. Density:

- The higher the density, slower the liquid evaporates.

6. Concentration:

- Low concentration of the substance, faster the evaporation.

7. Film and deposits:

- Some solutions deposit solid materials called scale on the heating surfaces.

- results in the overall heat-transfer coefficient decreases and evaporator must be cleaned.

8. Foaming or frothing:

- food solution such as skim milk and some fatty-acid solution form a foam or froth during boiling.

9. Economic factors:

- The recovery of solvent and utilization of waste heat are also important as they involves considerable reduction of cost.

WHAT IS ECONOMY AND CAPACITY OF THE EVAPORATOR

ECONOMYEconomy of an evaporator is the total mass of water vaporized per unit mass of steam input to the evaporator.

CAPACITYCapacity of an evaporator is the amount of water vaporized in the evaporator per unit time.

TYPES OF EVAPORATION EQUIPMENT

1. Open kettle or pan

2. Horizontal-tube natural circulation evaporator

3. Vertical-type natural circulation evaporator

4. Long-tube vertical-type evaporator

5. Falling-film-type evaporator

6. Forced-circulation-type evaporator

7. Agitated-film evaporator

8. Open-pan solar evaporator

OPEN KETTLE EVAPORATOR

OPEN KETTLE OR STEAM JACKETED EVAPORATOR

Steam is supplied to the jacketed kettle in which the aqueous extract is placed. Heat is transferred to the extract by conduction and convection.

CONSTRUCTIONCopper is the excellent material for the kettle because of its conductivity.

Iron is used for construction of jackets since it has minimum conductivity.

WORKINGAqueous extract to be evaporated is placed in the kettle. Steam is supplied through inlet. Steam gives out its heat to the contents and the condensates leaves through the outlet.

ADVANTAGES1. Used for both small scale & large scale operation.

2. Simple in construction and easy to operate.

3. Low maintenance & installation cost.

4. Wide variety of materials.

DISADVANTAGE1. Heat economy is less.

2. Not suitable for heat sensitive materials.

3. Heat decreases on product concentration.

4. Since, open type so vapor passes to atmosphere.

USESConcentrating aqueous and thermo-stable liquors.

Eg. Cooking pickels, liquorice extract etc.

(a) HORIZONTAL TUBE TYPE / (b) VERTICAL TUBE TYPE EVAPORATOR

HORIZONTAL TUBE EVAPORATOR

Steam is passed through the horizontal tubes, which are immersed in a pool of liquid to be evaporated. Heat transfers through the tubes and the solvent evaporates. Concentrated liquid is collected.

CONSTRUCTIONLarge cylindrical body with doomed shaped at the top and bottom, made of cast iron or plate steel. Stainless steel tubes are used in steam compartments.

WORKINGFeed is introduced into the evaporator until the steam compartment is immersed. The horizontal tubes receives the heat and conduct it to the liquid. The feed absorbes heat and solvent gets evaporated. Concentrated liquid is collected.

USESBest suited for non-viscous solution. E.g. Cascara extract.

VERTICAL TUBE EVAPORATOR

Liquid is passed throgh the vertical tubes and steam is supplied from outside tubes. Heat transfer takes place through the tubes and the liquids inside tube gets heated. The solvent evaporates, vapor escapes from the top and concentrated liquid is collected from bottom.

CONSTRUCTIONConsist of long cylindrical body made up of cast iron with dome shaped top and bottom. Calandria are fitted at the bottom.

Calandria consist of number of vertical tubes with diameter 0.05- 0.075 meters & length of 1-2 meters.

About 100 such tubes are fitted in the body of 2.5 mtr. Inlets are provided for steam and feed. Outlets are provided for vapor, concentrated products, non-condensed gases and condensate.

ADVANTAGES1. Increases the heating surface 10-15 times than steam

jacketed kettle.

2. Vigorous circulation enhances rate of heat transfer.

3. More units can be joined.

DISADVANTAGES4. Liquid to be maintained above calandria.

5. Complicated- increased installation cost.

6. Pressure has to maintain.

7. Cleaning and maintenance is difficult.

USES

Manufacture of cascara extract, sugar, salt, caustic soda etc.

CLIMBING FILM EVAPORATOR ( RISING FILM EVAPORATOR )

Tubes are heated externally by steam. The preheated heat enters from the bottom and flows up through the the heated tubes. Liquid near walls becomes vapour and forms small bubbles. Larger bubbles flow up with slag and strikes deflector. Deflector throws the concentrate, down to the bottom.

CONSTRUCTIONHeating unit consists of steam jacketed tubes. Long and narrow tubes are held between the plates. Deflector is placed at the top of the vapour head.

Inlets are provided for steam and feed. Outlets are provided for vapour, concentrated product, non condensed gases and condensate.

CLIMBING FILM EVAPORATOR

USESInsulin, liver extracts, vitamins, foaming liquids, corrosive solutions can be concentrated

ADVANTAGES

1. Large area for heat transfer

2. Enhanced heat transfer

3. Suitable for heat sensitive materials

4. Used for foam forming liquids

5. Instrument needs less space

DISADVANTAGES

1. Expensive, construction is quite complicated

2. Cleaning and maintenance is quite difficult

3. Large head space required

4. Not for viscous, salting and scaling liquids

FALLING FILM EVAPORATOR

Feed enters from the top and flows down the walls of the tube. The liquid becomes vapour and forms small bubbles. They tend to fuse to form layers of bubbles. Concentration takes place during downward journey. Vapour and liquid are separated at the bottom.

CONSTRUCTIONIt resembles climbing film evaporator, but it is inverted. Feed inlet is from the top of the steam compartment. The outlet of the product is at bottom and is connected to a cyclone separator.

FALLING FILM EVAPORATOR

USES1. Separate volatile and non volatile liquids2. Concentration of yeast extracts3. Manufacture of gelatin4. Extracts of tea and coffee

ADVANTAGES

1. Suitable for high viscous liquids

2. Liquid hold up time is less

3. Liquid is not overheated

4. Highly acidic and corrosive feeds can be concentrated

DISADVANTAGES

1. Not for suspensions, salting and scaling liquids

2. Poor feed distribution in tubes

3. Feed ratio is high

FORCED CIRCULATION EVAPORATOR

Liquid is circulated through the tubes at high pressure by means of pump. Hence boiling does not takes place as boiling point is elevated. Forced circulation creates agitation. When liquid leaves the tube and enters the vapour head, pressure falls suddenly. This leads to flashing of superheated liquor. Thus evaporation is effected.

CONSTRUCTIONHeating unit consists of steam jacketed tubes. Inlets are provided for steam and feed. Outlets are provided for vapour, concentrated products, non condensed gases and condensate. Pump is connected near the inlet.

FORCED CIRCULATION EVAPORATOR

USES1. Insulin and liver extract2. Crystallizing operations

ADVANTAGES

1. Heat transfer coefficient is high

2. Salting, scaling are not possible

3. Suitable for high viscous preparations

DISADVANTAGES

1. Equipment is expensive

2. More power supply is required

METHODS OF OPERATION OF EVAPORATORS

1. Single-effect evaporators

2. Forward-feed multiple-effect evaporators

3. Backward-feed multiple-effect evaporators

4. Parallel-feed multiple-effect evaporators

5. Mixed feed multiple- effect evaporators

SINGLE-EFFECT EVAPORATORS

FIGURE 8.2-2. Simplified diagram of single-effect evaporator

The feed enters at TF

Saturated steam at TS enters the heat- exchange section Condensed steam leaves as condensate or drips The solution in the evaporator is assumed to be completely

mixed Hence, the concentrated product and the solution in the

evaporator have the same composition Temperature T1 is the boiling point of the solution

The temperature of the vapor is also T1, since it is in equilibrium with the boiling solution

The pressure is P1, which is the vapor pressure of the solution at T1

If the solution to be evaporated is assumed to be dilute and like water, then 1 kg of steam condensing will evaporate approximately 1 kg of vapor (if the feed entering has TF near the boiling point)

Single-effect evaporators are often used when the required capacity of operation is relatively small and/or the cost of steam is relatively cheap compared to the evaporator cost

However, for large-capacity operation, using more than one effect will markedly reduce steam costs

FORWARD FEED MULTIPLE-EFFECT EVAPORATORS

A single-effect evaporator as shown in Fig. 8.2-2 is wasteful of energy

The latent heat of the vapor leaving is not used but is discarded

Much of this latent heat, however, can be recovered and reused by employing a multiple - effect evaporator

A simplified diagram of a forward-feed triple-effect evaporation system is shown in Fig. 8.2-3.

FORWARD FEED MULTIPLE-EFFECT EVAPORATORS

FIGURE 8.2-3. Simplified diagram of forward -feed triple-effect evaporator

If the feed to the first effect is near the boiling point at the pressure in the first effect, 1 kg of steam will evaporate almost 1 kg of water

The first effect operates at a temperature that is high enough that the evaporated water serves as the heating medium to the second effect

Here, again, almost another kg of water is evaporated, which can then be used as the heating medium to the third effect

As a very rough approximation, almost 3 kg of water will be evaporated for 1 kg of steam in a three-effect evaporator

Hence, the steam economy, which is kg vapor evaporated/kg steam used, is increased

This also holds approximately for more than three effects

However, the increased steam economy of a multiple-effect evaporator is gained at the expense of the original first cost of these evaporators

In forward-feed operation as shown in Fig. 8.2-3, the fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow

This method of operation is used when the feed is hot or when the final concentrated product might be damaged at high temperatures

The boiling temperatures decrease from effect to effect. This means that if the first effect is at P1 = 1 atm abs pressure, the last effect wilt be under vacuum at a pressure P3

BACKWARD FEED MULTIPLE-EFFECT EVAPORATORS

In the backward-feed operation shown in Fig. 8.2-4 for a triple-effect evaporator, the fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect.

FIGURE 8.2-4. Simplified diagram of backward-feed triple-effect evaporator.

This method of reverse feed is advantageous when the fresh feed is cold, since a smaller amount of liquid must be heated to the higher temperatures in the second and first effects

However, liquid pumps must be used in each effect, since the flow is from low to high pressure

This reverse-feed method is also used when the concentrated product is highly viscous

The high temperatures in the early effects reduce the viscosity and give reasonable heat-transfer coefficients

PARALLEL FEED MULTIPLE-EFFECT EVAPORATORS

Parallel-feed in multiple-effect evaporators involves the adding of fresh feed and withdrawal of concentrated product from each effect

The vapor from each effect is still used to heat the next effect

This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt

PARALLEL FEED MULTIPLE-EFFECT EVAPORATORS

MIXED FEED MULTIPLE-EFFECT EVAPORATORS

MULTIPLE EFFECT EVAPORATOR

ADVANTAGES Suitable for large scale and continuous process

Highly economical

Upto 5 evaporator can be attached

DISADVANTAGES

Monitoring of Evaporators

USES

Concentration of salt solution.

TYPICAL TYPES OF EQUIPMENT USED FOR BIOLOGICAL MATERIALS

1. Long-tube vertical evaporator: condensed milk

2. Falling film evaporator: fruit juices

3. Agitated-film evaporator: rubber latex, gelatin, antibiotics, fruit juices

4. Heat-pump cycle evaporator: fruit juices, milk, pharmaceuticals

thanks

Presented By: Deepanka Saikia

Bhaskar BasumataryJakir HussainJaydeep Das