Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead,

EVAPORATION

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid.

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.[1][2]

A liquid in a vacuum environment has a lower boiling point than when the liquid is at atmospheric pressure. A liquid in a high pressure environment has a higher boiling point than when the liquid is at atmospheric pressure. In other words, the boiling point of a liquid varies dependent upon the surrounding environmental pressure (which tends to vary with elevation). Different liquids (at a given pressure) boil at different temperatures.

DEFINITION

Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid evaporates from its surface, at the boiling point vapor bubbles come from the bulk of the liquid. For this to be possible, the vapor pressure must be sufficiently high to win the atmospheric pressure, so that the bubbles can be "inflated". Thus, the difference between evaporation and boiling is "mechanical", rather than thermodynamical. The boiling point is lowered when the pressure of the surrounding atmosphere is reduced, for example by the use of a vacuum pump or at high altitudes

The normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, 1 atmosphere. At that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapor to form inside the bulk of the liquid. The standard boiling point is now defined as the temperature at which boiling occurs under a pressure of 1 bar.

The heat of vaporization is the amount of energy required to convert or vaporize a saturated liquid (i.e., a liquid at its boiling point) into a vapor and overcome the surface tension.

Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation. Evaporation is a surface phenomenon in which molecules located near the liquid's edge, not contained by enough liquid pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, resulting in the formation of vapor bubbles within the liquid.

Natural evaporation: Evaporation of the solution below the boiling point

temperature and evaporation rate is low. Evaporation: The solution heated to boiling point, so that

evaporation in the boiling state.

Definitions

It is the removal of relatively large amounts of liquid (solvent) to obtain concentrated solution.

Evaporation is a thermal separation process, widely used for concentration of liquids in the form of solutions, suspensions, and emulsions. Concentration is accomplished by boiling out a solvent, normally water, from the liquid. In most cases, concentrate resulting from the evaporation process is the final product. Sometimes, however, the evaporated, volatile component is also a main product, as, for example, during solvent recovery.

EVAPORATION AS A UNIT PROCESS

Evaporation is a type of vaporization of a liquid that only occurs on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs within the entire mass of the liquid.

Heating will be necessary to provide the latent heat of vaporization, and in general, the rate of evaporation is controlled by the rate of heat transfer.

Vacuum Evaporation: Vacuum evaporation is the process of causing the

pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal. Although the process can be applied to any type of liquid at any vapor pressure, it is generally used to describe the boiling of water by lowering the container's internal pressure below standard atmospheric pressure and causing the water to boil at room temperature

Evaporation in Open System: Evaporation is the process of liquid turning to gas by the

following mechanism: liquid is made up of many molecules that are in constant, random

motion. These molecules collide with each other to gain or lose kinetic energy from or to other molecules. The molecules at the surface of the liquid which have high enough kinetic energy to break the attractive bonds with other molecules will be able to change from liquid to gaseous state and to escape the surface of the liquid. Since the molecules with higher kinetic energy have moved out of the liquid, the average kinetic energy of the liquid drops which in turns means that the temperature of water will drop as well (evaporative cooling).

Theory of Evaporation

Since at higher temperatures the molecules have more kinetic energy, more of them are likely to escape, and so evaporation occurs more quickly at higher temperatures.

In general, evaporation occurs because systems seek equilibrium (there is a low concentration of molecules in the air, and a high concentration in the liquid).

: In a closed system, liquid evaporation continues until the air in the

container is saturated with water vapor. At that point, the vapor in the system is considered saturated because it cannot absorb any more molecules from the liquid. Saturation pressure measures the pressure of the vapor at that point that evaporation cannot increase the number of molecules in the vapor. Saturation pressure increases as temperature increases since more molecules escape from the liquid. Boiling occurs when the saturation pressure is equal to or greater than the atmospheric pressure. When the air is saturated with water vapor, the molecules in the vapor condense to a liquid as fast as the liquid evaporates, and the two processes (evaporation and condensation) continue at equal rates. This is called an equilibrium. The evaporation and condensation are proceeding at the same rate, so there is no net change.

Evaporation in Closed System

Rate of Evaporation is affected proportionally by:

1- Temperature (indicator to kinetic energy)2- Pressure 3- Surface Area

and affected inversely by:  

1- Intermolecular Forces (which affected by matter concentration " dissolved substances as salts and dissolved gases as N2,O2,CO2 2- Humidity3- Wind

Factors affecting the rate of evaporation

1- Temperature :

The rate of evaporation of liquids varies directly with temperature. With the increase in the temperature, fraction of molecules having sufficient kinetic energy to escape out from the surface also increases. Thus with the increase in temperature rate of evaporation also increases

2-Surface Area : Molecules that escape the surface of the liquids constitute

the evaporation. Therefore larger surface area contributes accelerating evaporation.

The exposed surface area: – The rate of evaporation is directly proportional to the

exposed surface area. The greater the exposed surface, the faster the evaporation. This is because water molecules need to be at the surface of the liquid to evaporate. With a large surface area more liquid molecules can be at the surface and therefore more liquid molecules can evaporate.

3-Vapor pressure: If pressure is applied on the surface of a liquid,

evaporation is hindered; 4- Air-pressure: _ Evaporation is also affected by the pressure exerted

on the evaporating surface. Lower pressure on open surface of the liquid results in the higher rate of evaporation

5- Flow rate of air: The rate of evaporation of liquids depends upon the

flow of air currents above the surface of the liquid. Air current flowing over the surface of the liquid took away the molecules of the substance in vapor state there by preventing condensation. In addition it gives the space for new molecules of liquid in the form of water vapor to accommodate.

6-Inter-molecular forces: _ The stronger the forces keeping the molecules together

in the liquid or solid state the more energy that must be input in order to evaporate. The magnitude of inter-molecular forces of attraction in liquid determine the speed of evaporation. Weaker the inter-molecular forces of attraction larger is the extent of evaporation. In diethyl ether rate of evaporation is greater than that of ethyl alcohol.

.

7-Humidity of the air: Humidity is the amount of water vapor found in the air, and is

referred to as relative humidity. It is expressed in a percentage. For example, if an area is at 100 percent humidity, the air is saturated with water. This causes some of it to condense into water.

The relative humidity of the air has a direct effect on the rate of evaporation of liquids. If the air has high humidity, the rate of evaporation will be slower, because if the air is already filled with water vapor, it will not have any place to hold excess vapor and therefore, evaporation will occur at an extremely slow rate

8- Constituents of the solution: - If the fluid is made up of large charged molecules, the

molecules will escape at a slower rate because more energy is required to lift their mass and overcome their electromagnetic interactions with each other to allow the molecules to escape. Also, a mixed fluid can evaporate faster or slower depending on how the different molecules interact with each other.

9- Viscosity of the solution: Viscosity is the resistance of a liquid to flow. Viscosity

increases with increasing strength of intermolecular forces and decreases with increasing temperature. The rate of vaporization increases with increasing temperature, and decreasing strength of intermolecular forces. The viscosity of the solution increased as the process of evaporation proceeds, due to concentration of the solute in the solution. As a result, increases the boiling point. The increased viscosity reduces the heat transfer coefficient thus slow down the rate of evaporation. Since the rate of evaporation depends on the heat transfer equation: q= UA∆T.

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.

When a nonvolatile solute is added to a liquid to form a solution, the vapor pressure above that solution decreases. Why?

Liquid molecules at the surface of a liquid can escape to the gas phase when they have a sufficient amount of energy to overcome the liquid's intermolecular forces. That vaporization process is reversible. Gaseous molecules coming into contact with the surface of a liquid can be trapped by intermolecular forces in the liquid. Eventually the rate of escape will equal the rate of capture to establish a constant, equilibrium vapor pressure above the pure liquid.

Boiling point elevation

If we add a nonvolatile solute to that liquid, the amount of surface area available for the escaping solvent molecules is reduced because some of that area is occupied by solute particles. Therefore, the solvent molecules will have a lower probability to escape the solution than the pure solvent. That fact is reflected in the lower vapor pressure for a solution relative to the pure solvent. That statement is only true if the solvent is nonvolatile. If the solute has its own vapor pressure (volatile), then the vapor pressure of the solution may be greater than the vapor pressure of the solvent.

On the surface of the pure solvent (shown on the left) there are more solvent molecules at the surface than in the right-hand solution flask. Therefore, it is more likely that solvent molecules escape into the gas phase on the left than on the right. Therefore, the solution should have a lower vapor pressure than the pure solvent

The Vapor Pressure of a Solution is Lower than that of the Pure Solvent

 Raoult's law states that the vapor pressure of a solution, P, equals the mole fraction of the solvent, C solvent, multiplied by the vapor pressure of the pure solvent, Po

One consequence of Raoult's law is that the boiling point of a solution made of a liquid solvent with a nonvolatile solute is greater than the boiling point of the pure solvent. The boiling point of a liquid or is defined as the temperature at which the vapor pressure of that liquid equals the atmospheric pressure. For a solution, the vapor pressure of the solvent is lower at any given temperature. Therefore, a higher temperature is required to boil the solution than the pure solvent.

Boiling Point Elevation

The following figure is a phase diagram for both a pure solvent and a solution of that solvent and a nonvolatile solute that explains that point graphically.

As you can see in the the vapor pressure of the solution is lower than that of the pure solvent. Because both pure solvent and solution need to reach the same pressure to boil, the solution requires a higher temperature to boil.

Phase Diagram for a Solvent and its Solution with a Nonvolatile Solute

The equipment used in evaporation may be classified according to the form of the movement, as this is very important in heat transfer, and can be divided into three main groups:

I- Natural circulation evaporators II- Forced circulation evaporators III- Film evaporators

Evaporators

Evaporators in this category are those in which the movement of the liquid results from convection currents set up by the heating process.

Evaporating pan: simplest form of evaporators inexpensive simple to operate very poor heat economy in some cases paddles and scrapers for agitation are

used

I- Natural circulation evaporators

Concentrate

Condensate

Pan

Boiler

Jacket

Pressure gauge

Steam

What are the disadvantages of evaporating pan?

Why the pan shape is hemispherical?

They are called stills because it is essentially a vessel similar to the evaporating pan, with a cover that connects it to a condenser, so that the liquid is distilled off.Often a quick release system of clamps which allows the cover to be removed easily for access to the interior of the vessel for cleansing or removal of the product may be used.Advantages:(a) Simple construction and easy to clean and maintain.(b) The vapor is removed by condensation which(i) speeds evaporation(ii) reduces inconvenience and(iii) allows the equipment to be used for solvents other than water e.g. ethanol.(c) A receiver and vacuum pump can be fitted to the condenser, permitting operation under reduced pressure and, hence, at lower temperature.

Evaporating still

Disadvantages:(a) Natural convection only

b) All the liquor is heated all the time(c) The heating surface is limited.

Uses:(i) Aqueous and other solvents may be evaporated(ii) Thermolabile materials can be evaporated under reduced pressure.(iii) Removing the still head it is convenient for evaporating extracts to dryness.

- Horizontal Tube Evaporators.: The evaporator tube is called horizontal because the tubes are arranged

horizontally. The heating chamber is formed by the horizontal tubes, which are

supported by two tube sheets. The steam enters the tubes and condenses to give up its heat of condensation.

The condensed steam is removed through the condensate oulet. The evaporation chamber comprises a vertical cylindrical body, closed

bases, the solvent evaporated with an outlet at the top and another outlet for the concentrated solution at the bottom. . These evaporators are usually of iron or steel plate with a diameter of approximately 2 meters and 3 meters high. The diameter of the tubes is usually 2 to 3 inches. The following evaporator enters the steam inside the tubes, and fluid to be heated is flowing over the tubes.

Tube Evaporators

Horizontal tube evaporator

Advantage: - relatively cheap - used for batch and continuous operation Disadvantages: poor liquid circulation (and therefore unsuitable for

viscous liquids)

Uses: used for non-viscous liquids having high heat-transfer

coefficients and liquids that do not deposit scales

I- SHORT TUBE EVAPORATORS: 1- Standard type 2- Basket type 3- Forced circulation evaporator II- Long tube evaporators (Film evaporators): 1- falling tube evaporators 2-Rising tube evaporators

Vertical Tube Evaporators

Construction and PrincipleCalendriaThe lower portion of the evaporator consist of a bundle of tubes with the liquor inside and steam outside.Tube length: 1 – 2 mTube diameter: 40 – 80 mmDiameter of evaporator: 2.5 mNumber of tubes: 1000This part of the evaporator is called the calendria.

· The liquor is maintained at a level slightly above the top of the tubes, the space above this being left for the disengagement of vapor from the boiling liquor.· The liquor in the tubes is heated by the steam and begins to boil, when the mixture of liquid and vapor will shoot up the tubes (in a similar manner to that of a liquid that is allowed to boil to vigorously in a test-tube).· This sets up a circulation, with boiling liquor rising up the smaller tubes of the calendria and returning down the larger central downtake.

Advantages1. Use of tubular calendria increases the heating area, possibly by a factor of 10 to 15 compared to that of an external jacket.2. The vigorous circulation reduces boundary layers and keeps solids in suspension, so increasing the rate of heat transfer.3. Condenser and receiver can be attached to run the evaporation under vacuum with nonaqueous solvents.

Disadvantages1. Since the evaporator is filled to a point above the level of the calendria, a considerable amount of liquid is heated for a long time. The effect of this continual heating can be reduced to some extent by removing concentrated liquor slowly from the outlet at the bottom of the vessel.2. Complicated design, difficult for cleaning and maintenance.3. The head (pressure) of the liquor increases pressure at the bottom of the vessel and, in large evaporators where the liquor depth may be of the order of 2 m; this may give rise to a pressure of about 0.25 bar, leading to elevation of the boiling point by 5 to 60C.

The down take is annular A buffle in mounted on top of the calandria to reduce

entrainment The calandria are present in a basket that can be

removed from the apparatus for efficient cleaning.

Basket type evaporator

Forced circulation evaporator

Working Principle· The liquor is circulated by means of the pump and as it is under pressure in the tubes the boiling point is elevated and no boiling takes place.· As the liquor leaves the tubes and enters the body of the evaporator there is a drop in pressure and vapor flashes off from the superheated liquor.

Advantages· Rapid liquid movement improves heat transfer, especially with viscous liquids or materials that deposit solids or foam readily.· The forced circulation overcomes the effect of greater viscosity of liquids when evaporated under reduced pressure.· Rapid evaporation rate makes this method suitable for thermolabile materials, e.g. it is used in practice for the concentration of insulin and liver extracts.

Film evaporators spread the material as a film over the heated surface, and the vapor escapes the film

Long tube evaporators 1- Climbing film or rising film evaporators

Construction and working principle

The heating unit consists of steam-jacketed tubes, having a length to diameter ratio of about 140 to 1, so that a large evaporator may have tubes 50 mm in diameter and about 7 m in length.The liquor to be evaporated is introduced into the bottom of the tube, a film of liquid forms on the walls and rises up the tubes, hence it is called climbing film evaporator.At the upper end, the mixture of vapor and concentrated liquor enters a separator, the vapor passing on to a condenser, and the concentrate to a receiver.

FILM EVAPORATORS

(i). Cold or pre heated liquor is introduced into the tube. (ii). Heat is transferred to the liquor from the walls and boiling begins,

increasing in vigor. (iii). Ultimately sufficient vapor has been formed for the smaller

bubbles to unite to a large bubble, filling the width of the tube and trapping a ‘slug’ of liquid above the bubble

As more vapor is formed, the slug of liquid is blown up the tube, the tube is filled with vapor, while the liquid continues to vaporize rapidly, the vapor escaping up the tube and, because of friction between the vapor and liquid, the film also is dragged up the tube up to a distance of 5 to 6 metres.

Long tube evaporators (Falling film evaporators)

Construction and working principleConstruction is same as climbing film evaporator but is inverted as shown in the figure.The liquor to be evaporated is introduced at the top of the evaporator tubes and the liquor comes down due to gravity.The concentrate and vapor leaves the bottom. They are separated in a chamber where the concentrate is taken out through product outlet and vapor from vapor outlet.

Advantages of long tube evaporator

Since the movement of the film is assisted by gravity, more viscous liquid can be handled by falling film evaporator.(i) Very high film velocity reduces boundary layers to a minimum giving improved heat transfer.(ii) The use of long narrow tubes provides large surface area for heat transfer.(iii) Because of increased heat transfer efficiency, a small temperature gradient is necessary with less risk of damage to thermolabile materials.(iv) Although the tubes are long, they are not submerged, as in the short-tube evaporator; so that there is no elevation of boiling point due to hydrostatic head.

Disadvantages(i) Expense to manufacture and install the instrument is high.(ii) Difficult to clean and maintain.(iii) From the operational point of view the feed rate is critical. if too high, the liquor may be concentrated insufficiently, where as if the feed rate is to low, the film cannot be maintained and dry patches may form on the tube wall.

ENTRAINMENT When a bubble of vapor rises to the surface of liquid and bursts, the liquid film that forms the top of the bubble is usually sprayed as very fine droplets along with the stream of vapor.This droplets greatly vary in size. Some of them drop back quickly into the liquid from which they came; some settle more slowly; and some will not settle at all, at any vapor velocity (that is practicable to maintain). Such finely divided liquid carried along with the stream of vapor is called entrainment.Entrainment may cause :(a) serious losses from the liquid being evaporated,(b) and contamination of the condensate, if desired for other purpose.

Problems encountered during evaporation process

1. A certain amount of separation is made first by making the diameter of the vapor head of the evaporator such that the rising velocity of the vapor is kept down to a reasonable value.

2. The vapor space is made higher. More of the medium-sized droplets can settle back into the liquid in the time that is available before the vapor leaves the evaporator.

3. Baffles are placed over the liquid in the vapor space in the evaporators. The liquid droplets will impinge on the surface of the baffle, they coalesce into a sheet and are not easily picked up again by even extremely high vapor velocities.

Methods for reducing entrainment

4. If the mixture of vapor and entrained liquid were given a rotary motion, centrifugal force would tend to throw the droplets out against the side of the vessel, where they would coalesce and run down as sheet of liquid. The figure describes such an entrainment separator.

The vapor is fed into the entrainment separator by a tangential tube so that it starts a whirling motion at once (hence called cyclone separator). The presence of a spiral vane ensures this whirling motion so that most of the entrained liquid is thrown out against the wall of the separator, where it runs down, to be returned to the evaporator.In the lower part of the separator the vapors turn through 1800 to rise through the central vapor-off take pipe, and this again projects some particles of liquid down into the bottom of the separator.

FOAMINGFoam is the formation of a stable blanket of bubbles that lies on the surface of the boiling liquid. The cause of foaming is not known but it depends on:(a) the formation at the surface of the liquid of a layer whose surface tension is different from that of the bulk of the liquid and,(b) the presence of finely divided solids or colloidal material that stabilizes the surface layer.

(i) The liquid may be carried at a level below the top of the heating surface, so that the bubbles of foam come in contact with a hot surface and thereby burst.(ii) Steam jets are sometimes directed against the layer of foam to break it.(iii) The liquid carrying foam at high velocity may be ejected against baffles, where the bubbles of the foam are broken mechanically. This happens in forced-circulated evaporators.(iv) The best method is the addition of small amount of sulphonated castor oil, cottonseed oil, or other vegetable oils or some of the silicone oils. They often control or completely eliminate foam.

Methods to tackle foaming

Scales are the solid deposits, which accumulate on the heat transfer surface during evaporation. The scale has very low thermal conductivity. As evaporation proceeds, there is gradual increase resistance to heat transfer due to deposition of scales.Inorganic substances such as calcium sulphate, calcium hydroxide, sodium carbonate, sodium sulphate and calcium salts of organic acids have scaling tendency.

SCALE FORMATION

The scale formation can be minimized by:(a) Maintaining high circulation velocities in the tubes e.g. in forced circulation.(b) Arranging the evaporators in forward feed arrangement so that highly concentrated solution is subjected to low temperature (effect - I).(c) Periodic removal of scales should be done.

These are devices those are used for removing condensate from evaporators. The function of a steam trap is to allow condensate to drain but to prevent steam from blowing out of the space drained

STEAM TRAPS

This trap consists of a closed metal cartridge. To one end of this metal cartridge is connected a collapsible corrugated tube. The left hand end of the tube is sealed, and to it is attached the stem of a valve. The space between the cartridge and the corrugated tube filled with oil.

The collection of condensate against the valve, losing heat by radiation, cools the cartridge, the oil contracts, the valve opens, and the condensate is blown out. When the condensate is all discharged and steam enters the trap again, the cartridge expands and the trap closes.Advantage: This device is very simple and has no moving parts.

Uses: It is best suited for small capacities.

Expansion traps