Evaporation

Chapter 5

EVAPORATION 

Introduction

• Evaporation is a surface phenomenon; wherein mass transfer takes place from the surface. It is the process of vaporization of a liquid. In this process, liquid state of a substance is changing to a gaseous state due to an increase in temperature and/or pressure. It is a fundamental part of the water cycle and is constantly occurring throughout nature. During evaporation water changes from a liquid to a gas or vapor. Water boils at 100 °C but in reality, it begins to evaporate at 0 °C but extremely slowly. As the temperature increases, the rate of evaporation also increases. The amount of evaporation depends on the temperature, and it also depends on the amount of water to evaporate.

• Pharmaceutical industries use evaporator for the vaporization of a solvent from a solution. The evaporation is so important operation that it is considered an individual operation. If we continue the evaporation process, the residual mater will be solid, which is known as drying. However, the aim of evaporation is not to dry but to concentrate the solution. In addition, it is not the crystallization, in which the evaporation leads to formation of crystal in the solution. It is expected that students should learn the difference between evaporation, drying and crystallization. The driving force in evaporation is temperature difference between steam chest temperature and product temperature that result in to removal of solvent from the feed that solution is concentrated.

• To understand the evaporation process, few facts about the solution properties must be known. Knowledge of solution properties is important for the design of the equipment for evaporation. Some of the important properties of the solution are given below,

• Concentration: Initially, the solution may be quite dilute, and the properties of the solution may be taken as the properties of solvent. As the concentration increases, the solution becomes viscous and heat transfer resistance increases. The crystal may grow on the heating coil or on the heating surface. The boiling points of the solution also rise considerably. Solid or solute contact increases and the boiling temperature of the concentrated solution became higher than that of the solvent at the same pressure (i.e. elevation in boiling point).  

• Foaming: Many of the materials like organic substance may foam during vaporization. If the foam is stable, it may come out along the vapor known as entrainment. Heat transfer coefficient changes abruptly during foaming for such systems

• Degradation: Some of the products in pharmaceutical industries are very temperature sensitive and may get degraded during evaporation. Thus, special case or technique is required for concentrating such solution. 

• Scaling: Many solutions have tendency to deposit the scale on the heating surface, which may increase the heat transfer resistance. These scales produce extra thermal resistance of significant value. Therefore, scaling in the equipment should not be ignored thus de-scaling becomes an important and routine matter

• Equipment material: The material of the equipment must be chosen considering the solution properties so that the solution should neither be contaminated nor react with the equipment material. 

Advantages: 

• Evaporation reduces transportation and storage cost. 
• It prepares material for the next unit operation, for example, drying, crystallization etc. 
• It reduces rates of deteriorative chemical reactions. 
• It gives better microbiological stability. 
• It helps in recovery of solvent. 

Objectives 

• The primary objective of evaporation is to concentrate a solution consisting of a non-volatile solute and a solvent. In the majority of evaporations, the evaporating solvent is water. When the liquid phase is agitated, mass-transfer in the liquid phase is sufficiently rapid that the rate of evaporation of solvent can be determined by the rate of heat transfer from the heating medium, usually condensing steam, to the solution. Mineral-bearing water often is evaporated to give a solid-free product for boiler feed, for special process requirements, or for human consumption. This technique is often called water distillation, but technically it is evaporation.

• In summarizing, objectives of the evaporation are to generate a sense of the neverending equilibrium condition of water vapour and to know the speed at which humidify is transferred from one system to the other.  

Applications

• Evaporation is used in concentration and recovery of dissolved solutes like sodium chloride from aqueous solutions to produce salt.

• It is used in ether recovery from fat extraction. 

• It is also used in concentration of solutions. For example, concentration of milk to produce condensed milk and to obtain concentrated juices. 

• It is used in concentrating pharmaceutical herbal extracts in herbal industry.

• It is used in pharmaceutical industries to eliminate excess moisture, providing an ease of handling products and improving product stability. 

• It is used in preservation of long-term activity or stabilization of enzymes in laboratories. 

• Evaporation process is used in the manufacture of bulk drugs. 

• Evaporation is used in the manufacture of biological products. For example, insulin, enzymes, hormones etc. 

• It is used in demineralization of water.

• It is also used in concentration of chromatographic fractions, glucose and fructose syrups.  

• It has applications in concentration of effluents. 

Factors Influencing Evaporation 

The main factors that have an effect on evaporation are as follows: 

• Surface area: If the surface area of evaporating solution is increased, then the amount of liquid that is exposed to air is larger. More molecules can escape with a wider surface area. For example, if we spread out clothes to dry it speeds up the process of evaporation. The larger the exposed surface area, the more molecules can escape from the liquid. Thus, larger the surface area faster is the evaporation.

• Temperature: The water molecules move rapidly when the water is heated. This makes the molecules escape faster. Higher temperatures lead to increase in evaporation as more molecules get kinetic energy to convert into vapors. For example, boiling water evaporates faster than normal water. 

• Humidity: Humidity means the number of vapors present in the air. The air around can only hold a certain number of vapors at a certain time and certain temperature. If the temperature increases and the air speed and humidity stay constant, then the rate of evaporation will increase since warmer air can hold more water vapors than colder air. Thus, the rate of evaporation decreases with increasing humidity and increases with vice versa. 

• Moving air: Particles of vapors move away when the speed of air increases. This leads to a decrease in the amount of water vapor in the atmosphere. For example, we use hand dryers to dry our hands. Here the air is expelled from the hand dryer which dries our hand.  

• Boiling point of liquid: Liquids with a lower boiling point evaporate faster. Mercury hardly evaporates at room temperature as it has a boiling point of 357 °C. 

• Density of liquid: As the density increases, the rate of evaporation decreases. Liquids with a higher density have a lower rate of evaporation. For example, honey has a lower rate of evaporation as compared to water, which has a lower rate of evaporation as compared to alcohol. This is because honey is denser than water, and water is denser than alcohol.

Equipment's Used for Evaporation

• In evaporation, heat is added to a solution to vaporize the solvent, which is usually water. The heat is generally provided by steam on one side of a metal surface, with the evaporating liquid on the other side. The equipment used for evaporation is called as evaporator. The type of evaporator used depends primarily on the configuration of the heat-transfer surface and on the means employed to provide agitation or circulation of the liquid. Evaporator equipment may be classified, in general, as horizontal-tube, calandria type vertical, baskettype vertical, forced-circulation, and long-tube or film type vertical evaporators. The general types of equipment are classified either based upon movement of heating medium or type of heating surfaces

Classification based on heating medium movement:

• Simple evaporators: Example: Evaporating pan (Steam jacketed kettle).
• Natural circulation evaporators. Example: Climbing film evaporator.
• Forced circulation evaporators. Example: Forced circulation evaporator.
• Film evaporators. Example: Horizontal tube evaporator

Classification based on type of heating surfaces:

• Evaporators with heating medium in jacket. Example: Evaporating pan.
• Evaporators with horizontal tubes. Example: Horizontal tube evaporator.
• Evaporator with long tubes and natural circulation. Example: Climbing film evaporator.
• Evaporator with long tubes and forced circulation. Example: Forced circulation evaporator.
• Multiple effect evaporators.

Evaporating Pan

• The evaporating pan is also called as steam jacketed kettle. In this type of evaporator the movement of the liquid to be evaporated is due to the convection currents set-up by the heating process. It is a type of natural circulation evaporator. It consists of a hemispherical pan surrounded by a steam jacket. The hemispherical shape provides a large surface area for the evaporation to takes place. The evaporation pan may be fixed and the contents are discharged from the outlet provided at the bottom of pan. In some cases the evaporators are mounted in such a way that they can be tilted on either side to remove the concentrated product. The evaporating pans are heated by the steam which enters the jacket through inlet and leaves it from outlet. The heat from the steam in jacket is utilized for evaporating liquid in pan.

Principle:

• The mechanism involved in this evaporation process is conduction and convection so that the heat is transferred by this mechanism to the extract. Evaporating pan containing aqueous extract is provided with the steam which gives out heat to a jacketed kettle. The temperature raises and the escaping tendency of the solvent molecules into the vapors increases and enhances the vaporization of the solvent molecules

Construction:

• Steam evaporating pan consists of hemispherical structure with an inner pan called kettle which is enveloped with an outer pan called jacket, Fig. 5.1. These two pans are joined to enclose a space through which steam is passed. Several metals have been used for the construction of the kettle. Copper is an excellent material for the kettle due to its superior conductivity. If acidic preparation evaporated in copper kettle, some of the copper gets dissolved in preparation. In order to avoid this, for acidic preparation is tinned copper kettle is used. Iron is also used for the construction of the jacket because it has low conductivity. Rusting of iron with use is major problem and to prevent this iron jacket is tinned or enameled on the inner surface. An inlet for the steam and non-condensed gases are provided near the top of the jacket. Condensate leaves the jacket through the outlet provided at the bottom. The kettle is provided with the outlet for the product discharge at its bottom

Working:

•  The solution to be evaporated is placed in the kettle and steam which gives out heat to the content is supplied through the inlet and condensate leaves through the outlet. For smaller volumes the contents must be stirred manually and mechanically for larger volumes. The rate of evaporation is fast in the initial stages. The room where evaporation is carried must have good ventilation to remove the vapour to avoid fog formation of condensed vapour. To prevent condensation in the room and also to accelerate the rate of evaporation fans are fitted. The kettle may be fixed or made to tilt. A kettle of capacity up to about 90 liters may be made to tilt. The bottom outlet is used to collect the concentrated products.

Applications:

• Evaporating pan is suitable for concentrating aqueous liquids. (ii) 
• It is suitable for concentrating thermostable liquors, for example, liquorices extracts

Advantages:

• Evaporating pan is constructed both for small scale and large-scale batch operations. 

• It is a simple in construction and easy to operate, clean and maintain. 

• Its cost of installation and maintenance is low. 

• Wide variety of materials such as copper, stainless steel and aluminium etc can be used for the construction of evaporating pan. 

• Stirring the contents in pan and removal of the products is easy.

Disadvantages:

• Natural circulation of the product makes poor heat transfer.

• Deposition of solid may cause decomposition of the product. 

• Heating surface is limited and decreases proportionally to increase in size of the pan. 

• It is not suitable for the concentration of thermolabile materials.

• It has no provision to operate under a reduced pressure. 

• No provision to recollect the costly organic solvents. 

• Being the evaporating pan open vapours directly pass into the atmosphere. This may cause discomfort to the worker.

• Saturation of surrounding environment with vapors' may slow down the process.

Horizontal Tube Evaporator

• Horizontal tube evaporator frequently found to be the most adaptable choice for simple evaporation wherein liquids are not viscous and do not deposit scale or salt on surface.

Principle: 

• The principle mechanism involved in this type of evaporator is that steam is passed through tubes arranged horizontally. Heating causes evaporation of the feed outside the tubes discharging concentrate at the bottom and vapours passed out from the outlet at top. The vapour is removed from the top of the chamber and the product circulation take place by natural circulation over the heating coil.

Construction:

• Horizontal-tube evaporators are designed with either rectangular or circular cross-sections, with tubing of stainless steel, aluminum, nickel, carbon, speller zed iron pipe, lead covered copper, or special bronze.

• The tubes are extended between two steam chests and arc is fastened to tube. Four-hole packing plate’s force down conical gaskets around the tube ends into counter-sunk holes in the tube sheets. Secure sealing is obtained, with facility for quick and easy renewal. Horizontal tubes of 2 to 3 cm diameter are extended across the bottom of a cylindrical chamber with 1 to 3 meters diameter and 2.5-to-4.5-meter height. In case of vertical tube evaporators, the tubes are arranged vertically in calendria.

• A calendria is a heating part in evaporator consisting of large number of smaller diameter tubes wherein liquid is concentrated while rising or falling.

Working: 

• In the horizontal-tube evaporator, steam is fed into steam chest and is directed through the horizontal tubes to heat the liquid surrounding the tube in the bottom of the evaporator body. The definite path followed by the steam assures that all non-condensed gases and condensate are swept to the opposite steam chest, where they are withdrawn. The velocity and paths of circulation of the liquid depend upon the distribution, size, anti shape of the heating surface in the liquid compartment.

Applications:   

1. It is used in the manufacture of the cascara extract.
2. It is used in the manufacture of caustic soda. 
3. It is used in the manufacture of salts.

Advantages: 

• A number of units can be joined to obtain more efficient effect. 

• It has low cost per unit of heating surfaces. 

• It has extreme simplicity. 

• Easy renewal of heating surfaces.

• Sectional construction with low maintenance cost. 

• Ease of operation. 

• Ability to carry large volume of liquor in the body. 

• It requires low headspace. 

• Small cargo space required for shipment.

Disadvantages: 

• Cleaning and maintenance is difficult when compared with steam jacketed kettle. 

• During operation the pressure inside the evaporator increases that reduces the effective temperature gradients and may affects heat sensitive materials. 

• It may be used only when rigorous boiling can be obtained with natural circulation. 

• It is not suitable for viscous liquids. 

• Since the boiling liquid is outside of the tubular heating surface, it is not easily cleaned by mechanical means.

• It is not suitable when scaling or salting liquids are involved.

Climbing Film Evaporator

• A climbing film vertical long tube evaporator is a type of evaporator that is an essentially shell and tube heat exchanger. Thus, it is also known as rising film evaporator. This evaporator is superior to falling film evaporator as the upstream film movement causes some particles to remain in the feed stream. The absence of any kind of distributor makes it suitable for such variety of applications.

Applications: 

• A climbing film is used for effluent treatment. 

• It can be used in production of polymers and for juice concentration and food processing. 

• It is used in thermal desalination of sea water. 

• It has many applications in pharmaceuticals especially for solvent recovery. 

• It is used as reboilers for distillation columns. 

• It is used as pre-concentrators or flash evaporators or pre-heaters to remove volatile components prior to stripping.

Principle:  

• The theory of climbing film evaporator is that the ascending force of the steam produced during the boiling causes liquid and vapours to flow upwards in parallel flow. At the same time the production of vapour increases and the product is pressed as a thin film on the walls of the tubes, and the liquid rises upwards. This co-current upward movement against gravity has the beneficial effect of creating a high degree of turbulence in the liquid.

Construction: 

• A Rising Film Evaporator (RFE) is a vertical shell and tube heat exchanger with a vapourliquid separator mounted at the top. Tubes carrying the steam internally are placed vertically in the bottom of the cylindrical evaporator chamber. The length of the boiling tubes is typically not more than 23 ft (7m). This type of unit is known as the Roberts evaporator in Europe and is the calandria evaporator in the United States.

Working: 

• The liquid to be concentrated is fed at the bottom of the heated tube bundle. The feed is heated with steam condensing on the outside of the tube from the shell side. This produces steam and vapour within the tube bringing the liquid inside to boil. The product circulation is by natural convection. The vapour so produced pushes the liquid against the walls of the tubes and causes the upward force that feed moves up. As more vapours are formed, the centre of the tube will have a higher velocity which forces the remaining liquid against the tube wall forming a thin film which moves to the top above the calandria. The velocities generated by the vapour lift are quite high, giving good thermal performance. This is useful while evaporating highly viscous products and products that have a tendency to foul the heating surface. Usually there must be a high temperature difference between the heating and boiling sides of evaporator and is required to convey the liquid and to produce the rising film. The vapour and balance liquid are separated in the vapour-liquid separator.

Advantages: 

• The main advantage of the rising film evaporator is the low residence time of the liquid feed in the evaporator compared to other evaporators. 

• It has relatively high heat transfer coefficient that reduces the overall heat transfer area requirement which in turn will lower the initial capital cost of the evaporator. 

• It is easier to clean the tubes. 

• Thermosiphon action eliminates the need for circulation pump. 

• Trace quantities of suspended particles in the feed are tolerated.

• Can operate under reasonable vacuum. 

• Multiple effect arrangement provides steam economy. 

• It is relatively inexpensive evaporator.

Disadvantages:

• It requires large floor space and is heavy. 

• It has poor heat transfer at low temperature differences.

• Not suitable for thermolabile materials. 

• It evaporates products of low viscosity and have minimal fouling tendencies.

Applications:

• The thermal desalination of sea water. 

• Concentration of dilute solutions such as plant extracts. 

• It is used as a reboiler to distillation column.

• It is an economical alternative to falling film evaporator for moderate vacuum. 

Forced Circulation Evaporator

• Forced circulation evaporator uses force to drive the liquid through the evaporator tubes thus produces high tube velocities. A high efficiency circulating pump, designed for large volume and sufficient head, is used to supply the force. Proper design results in controlled temperature rise, controlled temperature difference and tube velocities that give optimum heat transfer. Forced circulation evaporator conducts evaporation under reduced pressure which is used to evaporate thermolabile substances. 

Principle:

• In forced circulation evaporator liquid is circulated through the tubes at high pressure by means of a pump. Hence boiling does not take places because boiling point is elevated. Forced circulation of the liquids also creates some form of agitation. When the liquid leaves the tubes and enters the vapor head, pressure falls suddenly. This leads to the flashing of super heated liquor leading to evaporation of feed.

Construction:

• In these types of evaporators pumps are fitted to circulate the contents heating in it. The forced circulation evaporator consists of steam jacketed tubes held between two tube sheets, Fig. 5.4. The tube measures 0.1 m inside diameter and 2.5 m long. The parts of the tubes projects in to the vapor head which consists of a deflector. The vapour head is connected to a return pipe which runs downwards and enters in to the inlet of a pump. The liquid is circulated by means of a pump. As it is under pressure in the tubes the boiling point is raised (but no boling take places) and it enters into the body of the evaporator.

Working: 

• Steam is introduced in to the calendria. Pump sends the liquid to the tube with a positive velocity. As the liquid move up through the tube it gets heated and begins to boil. As result the vapour and liquid mixture rushes out of the tubes at a high velocity. This mixture strikes the deflector in a manner that effective separation of liquid and vapour takes place. The vapour enters the cyclone separator and leaves the equipment. The concentrated liquid is circulated through the pump for further evaporation. Finally, the concentrated product is collected at the bottom from discharge outlet.

Advantages:

• There is a rapid movement of liquid due to high heat transfer coefficient. 

• The liquid entering the circulation evaporator boils in the separator and not on a heating surface hence minimizes fouling. 

• Salting, scaling and fouling are not possible due to forced circulation. 

• These evaporators are suitable for thermolabile substances because of rapid evaporation.

• It is suitable for the viscous preparation because pumping mechanism is used. 

• These evaporators are also used for liquids with high solids content. 

• Circulation evaporators are fairly compact and are easy to clean and operate. 

• Disadvantages

1. The holdup time of liquids is high. 
2. The equipment is expensive as well as power requirement is high. 
3. It has high maintenance costs. 
4. The increased velocity can cause the equipment to corrode at a faster rate, which increases the overall maintenance cost.

Applications:

• Forced circulation evaporator is used to concentrate thermolabile substance solutions. 

• It is used for concentration of insulin and liver extracts. 

• It is well suited for crystallizing operation where crystals are to be suspended at all times. 

• These evaporators are used in production of salt, corn steep water and calcium carbonate.

• It is used for effluent treatment in pharmaceutical industries. 

• Forced circulation chambers can be superimposed on each other to form a multi effect tower which can be utilized for increasing concentration of refined sugar and syrups. 

Multiple Effect Evaporators

• As we know, evaporator is a heat exchanger wherein liquid is boiled to produce vapour and simultaneously it generates a low-pressure steam. This steam is further used as heating medium for another following evaporator. Thus, the second evaporator is a low-pressure boiler. Utilization of steam in first evaporator is called as first effect. The vapours generated in first evaporators are used for following second evaporator as heating source, so it is called as second effect. All individual evaporators are single effect evaporators. To have used them as multiple they need to be connected within series. For example, consider two evaporators are connected in such a way that vapors from first evaporator are supplied to the steam chest of the following evaporator as shown in, making-up a two-effect evaporator. 

Steam feeding in multiple effect evaporators:   

• In case of two effect evaporators, the temperature in the steam chest of first evaporator is higher than in the second evaporator. The vapours generated in the first evaporator will the effect boiling of liquid in second evaporator; the boiling temperature in the second evaporator must be lower so that effect must be under lower pressure. The pressure in the second effect must be reduced below that in the first effect.

Forward feed multiple effect evaporators:

• In some cases, the first effect may be at a pressure above atmospheric; or the first effect may be at atmospheric pressure and the second and subsequent effects have therefore to be under further lower pressures. Under lower pressures, the liquid feed progress is simplest if it passes from first effect to second effect, to third effect, and so on. In these situations, the feed flows without pumping. This is called forward feed The most concentrated liquid products occur in the last effect of the process. In backward feed evaporator the feed may pass the last effect and proceeds to the first. This is in reverse of forward feed multiple effect evaporators. In backward feed the liquid is pumped from one effect to the next against the pressure drops, Fig. 5.7. The concentrated viscous liquids are at the highest temperatures in the first effect and thus have larger evaporation capacity than forward feed systems. 

Parallel feed:

• In this method a hot saturated solution of the feed is directly fed to each of the three effects in parallel without transferring the materials from the one effect to the other. The parallel feed arrangement is commonly used in the concentration of the salts solution where the solute crystallizes on concentration without increasing the viscosity. 

Construction:

• A multiple effect evaporator system for concentrating a process liquid consists of more than one evaporator effects arranged in series, each effect includes a process liquid inlet and a process liquid outlet; a heating fluid inlet and heating fluid outlet and an evaporative condenser provided with liquid inlet. Two effect evaporators are connected together with the piping arrangement so that the vapors from the calandria of the first effect are used to heat the calandria of the second effect. The calandria of the second effect is used as a condenser for the first effect. The latent heat of vaporization is used to evaporate more quantity of the liquid. The vapour from the second effect then taken to a condenser and converted into the liquids. In general, not more than two or three effects are combined together to have economical and efficient evaporation of liquids. The construction of the multiple effect evaporators uses three evaporators, so it is called as triple effect evaporators. The vapour from the first evaporator serves as heating medium for the second evaporator. Similarly, vapour from the second evaporator serves as a heating medium for the third evaporator. Last evaporator is connected to a vacuum pump.

Working:

• Multiple effect evaporators (3 - stages) is a long tube forced circulation type evaporators where in the first effect high pressure steam is used to evaporate solvent from the feed. The evaporated solvent in the form of vapor is used for evaporating the feed in the second effect at atmospheric pressure. Evaporated solvent from the second stage is used for evaporating concentrating feed in the third effect under vacuum. Finally evaporated solvent from the third effect is condensed in the steam condenser using cooling water on other side. Condensate from all the three effects is collected in condensate receiving tanks, which is pure solvent and hence reused in the process.

Advantages:

• It is suitable for large scale and continuous operation. 
• It is highly economical when compared with single effect.
• About five evaporators can be attached in series. 
• Minimizes the energy input required to evaporate undesirable solvent.

Applications:

• Used in the manufacture of the cascara extract. 

• Used in the manufacture of salts and caustic soda. 

• Used in the manufacture of salts. 

• Used in order to recover expensive solvents such as hexane which would otherwise be wasted.

• Recovery of sodium hydroxide in Kraft pulping.

• Cutting down waste handling cost is another major application 

• To get a concentrated product and to improve the stability of the products. 

• Used in the concentration of the sodium salts that is obtained as a by-product from the production of p-cresol. 

• In the food and drink industry, for example, coffee, need to go through an evaporation step during processing.   

Economy of Multiple Effect Evaporators

• The performance of a multiple effect evaporators is measured in terms of its capacity and economy. Capacity of evaporator is defined as the number of kilogram of water vaporized per hour. Its economy is the number of kilograms of water vaporized from all the effects per kilogram of steam used. For single effect evaporator, the steam economy is approximately about 0.8 (<1). The capacity is about n-times that of a single effect evaporator and the economy is about 0.8 n for an n-effect evaporators. However, pumps, interconnecting pipes and valves required for transfer of liquid from one effect to another effect increases both the equipment and the operating costs. The economy of multiple effect evaporators is greater than single effect because vapors generated in first effect are used as a heating medium for second effect and so on. Thus, efficiency increases as steam consumption decreases and evaporative capacity increases. At first sight, it may seem that the multiple effect evaporator has all the advantages, the heat is used over and over again, and it appears to be getting the evaporation in the second and subsequent effects for nothing in terms of energy costs. However, fact is that there is a price to be paid for the heat economy. For example.

• Comparative costs of the auxiliary equipment do not altogether follow the same pattern. Condenser requirements are less for multiple effect evaporators. The condensation duty is distributed between the steam chests of the effects, except for the first one, and so condenser and cooling water requirements will be less. The optimum design of evaporation plant is based on a balance between operating costs which are lower for multiple effects because of their reduced steam consumption, and capital charges which will be lower for fewer evaporators. The comparative operating costs are illustrated by the figures in Table 5.5. If the capital costs are known they would reduce the advantages of the multiple effects, but certainly not remove them.