Evaporation
Learning objectives
At the end of this lecture student will be able to:
- Outline the industrial applications of evaporation
- Explain the differences between evaporation and distillation
- Describe the basic concepts of phase equilibrium
- Discuss the factors affecting evaporation process
- Explain the theory of evaporation process
- Describe the parts of an evaporator
- Discuss the construction and working process of evaporating still
- Explain the process of horizontal and vertical tube evaporator
- Discuss the construction and working process of multiple effect evaporator
- Recommend suitable evaporator for the evaporation process
EVAPORATION
- Evaporation is nothing but a thermal separation process.
- Theoretically, evaporation means simply vaporization from the surface of a liquid.
- Vaporization of a liquid below its boiling point is called evaporation.
Thus, no boiling occurs and the rate of vaporization depends on the diffusion of vapour through the boundary layers above the liquid.
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 vapour).
Evaporation is a unit operation of vaporizing large quantities of volatile liquid to get a concentrated product
Evaporation v/s Distillation
Evaporation
Vaporization takes place below the boiling point
Takes only from the surface of the liquid
There is no bubble formation in evaporation
Not necessarily a separation or purification technique. Vaporisation occurs rapidly
Distillation
Vaporization takes place at the boiling point
Takes place from whole of the liquid
There is bubble formation
It is a separation or purifying technique. It is a slow process
Drying v/s Evaporation
Drying
It refers to the removal of relatively small amounts of water from solid or nearly solid material In most cases drying involves the removal of water at temperatures below its boiling point
Evaporation
It refers to the removal of relatively large amounts of water from solutions
Removal of water by boiling a solution- (wherever removing water is necessary)
Basic concept of phase equilibrium
Phase
- In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform.
- A simple description is that a phase is a region of material that is chemically uniform, physically distinct, and (often) mechanically separable.
- The term phase is sometimes used as a synonym for state of matter
Phase equilibrium
- Many compositions will form a uniform single phase, but depending on the temperature and pressure even a single substance may separate into two or more distinct phases. Within each phase, the properties are uniform but between the two phase properties differ.
- At equilibrium, evaporation and condensation processes exactly balance and there is no net change in the volume of either phase
- For a given composition, only certain phases are possible at a given temperature and pressure. The number and type of phases that will form is hard to predict and is usually determined by experiment.
- The results of such experiments can be plotted in phase diagrams
Triple point
Another interesting feature of the phase diagram is the point where the solid-liquid phase line meets the liquid-gas phase line. The intersection is referred to as the at the triple point, all three phases can coexist
If you think about the three lines which meet at that point, they represent conditions of:
- Solid-liquid equilibrium
- Liquid-vapour equilibrium
- Solid-vapour equilibrium
If you controlled the conditions of temperature and pressure in order to land on this point, you would see an equilibrium which involved the solid melting and subliming, and the liquid in contact with it boiling to produce a vapour – and all the reverse changes happening as well.
Mechanism
When heat applied in solution, the motion of molecules increase and molecules present in the surface overcome the surface tension of the liquid and it evaporates because surface molecules have less cohesive force than others
Distinguishing factors of evaporation
- Residue is a concentrated liquid
- Evaporating liquid is the only one component
- No attempt is made to separate the mixture of vapour
- Purpose is to get concentrated liquid only
Applications
- Manufacturing of bulk drugs
- Manufacturing of biological products
- Miscellaneous – demineralized water
Factors Influencing Evaporation
Rate of evaporation depends on several factors
M=KS (b-b’) / P
M = Mass of vapour formed per unit time (m3/s)
S = Surface area of the liquid exposed (m2)
P = Atmospheric pressure (Kpa)
B = Maximum vapour pressure at the temp of air (Kpa)
b’ = Pressure due to vapour of the liquid (Kpa)
K = Constant (m/s)
Factors Affecting Evaporation
Temperature and time
Surface area
Agitation
Atmospheric aqueous vapour pressure
Atmospheric pressure on the liquid under evaporation Type of product required
Moisture content and concentration of solute Economic factors
1. Temperature and time
The rate of evaporation is directly proportional to the temperature. Higher the temperature, greater will be the evaporation Ex – Alkaloids, Harmones, Enzymes, antibiotics – heat sensitive
If time of exposure is longer, greater will be the evaporation, provided the constituents are thermostable. Exposure of a drug to a relatively high temp. for a short period of time may be less destructive of active principle than a lower temp. with long exposure time.
2. Surface area
- The rate of evaporation is directly proportional to the surface area of the vessel exposed to evaporation.
- Greater the surface area of the liquid, greater will be the evaporation
3. Agitation
When vegetable extracts are concentrated in steam pan, a film may be formed on the surface and/or precipitate matter may deposit on the heating surface. Film reduces the heating surface and precipitated matter hinders the transfer of heat.
4. Atmospheric aqueous vapour pressure
Rate of evaporation is indirectly proportional to the vapour pressure of the liquid Lower the pressure, greater will be the evaporation
5. Atmospheric pressure on the liquid under evaporation
The rate of evaporation is inversely proportional to the atmospheric pressure on the liquid under evaporation.
6. Type of product required
The selection of the method and apparatus to be used for evaporation depends upon type of product required.
7. Moisture Content of Feed
Some drug constituents undergoes hydrolysis readily in presence of moisture at high temperature. To prevent the decomposition, the material is exposed to low temp. initially, then exposed to higher temp.
8. Economic factors
Economies of labour, fuel, floor space & materials are primary considerations. The recovery of solvent & utilization of waste heat are also important as they involves considerable reduction of cost.
Theory of Evaporation
- For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy
- Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. This phenomenon is also called evaporative cooling.
- Let F kg be the feed/h to the evaporator, whose solid content is XF (weight fraction) & enthalpy is hF J/kg
- Let L kg be the product collected/h from the evaporator, whose solute composition is XL (weight fraction) & enthalpy is hL
- Let V kg be the vapor liberated/hr from the evaporator, whose solute composition is y (weight fraction) & enthalpy is hV J/kg
- In most evaporators, the vapor is pure water as there is no entrainment & therefore y is zero
Classification of Evaporators
- Evaporators with heating medium in jacket
e.g. Steam jacketed kettle (Evaporating pan)
- Vapour heated evaporators with tubular heating surfaces
- Evaporators with tubes laced horizontally – Horizontal tube evaporators
- Evaporators with tubes placed vertically
- Evaporators with short tubes
(a) Single effect evaporators – Short tube vertical evaporator – Basket type evaporator
(b) Multiple effect evaporator – Triple effect evaporator
- Evaporators with long tubes
(a) Evaporators with natural circulation
- Climbing film evaporate
- Falling film evaporator
(b) Evaporators with forced circulation
- Forced circulationn evaporator
Material balancing
- The material balance can be obtained from total material entering & total material leaving.
Input = Output
Total material entering = Total material leaving
- It can be represented by an equation for the evaporation process.
- Feed (kg) = product collected (kg) + vapor liberated (kg)
F = L + V
F X F = L X L + Vy
- Written for material balancing in terms of mass x weight fraction for the solute as:
Energy balancing
- Energy balancing Steam is supplied for evaporation
- Let S kg be the steam supplied / h with an enthalpy of hS J / kg
- Let C kg be the condensate removed having an enthalpy of hC J / kg
- The heat balance can be obtained from total heat entering & total heat leaving
It can be represented by
Heat entering (J) =heat leaving (J)
- Also written as
Heat in feed + heat in steam = heat in thick liquor (product) + heat in vapor +heat in condensate + heat lost by radiation
- Loss of heat by radiation is less and can be neglected
- So the equation written in terms of enthalpy as
FhF + ShS = LhL + VhV + ChC
- Energy balancing Temperature difference is the difference between the saturation temperature of the steam & the boiling point of liquid
- Generally, a temperature difference of 20-300 C is sufficient for rapid evaporation of solution. But in practice, the feed may have the temperature less than boiling point of the liquid
- The steam may be superheated & the condensate may get cooled
- All these factors influence evaporator calculation with respect to mass balance
Improving Heat Transfer Coefficient
Improving heat transfer coefficient Evaporator is considered as a heat exchanger
Like any exchanger, heat is transferred from steam to the product
The general equation for heat transfer can be expressed as:
Q = UA Δt
Where Q = rate of heat transfer, W
U = overall heat transfer coefficient, W/m2K
A = heating surface area, m2
Δt = temperature difference, K
Evaporator Economy and Evaporator Capacity
Evaporator Economy
Economy of an evaporator is the total mass of water vaporized per unit mass of steam input to the evaporator
Evaporator capacity
Capacity of an evaporator is the amount of water vapourised in the evaporator per unit time
Evaporator
Equipment used in evaporation, the process of boiling a liquid in order to reduce its volume
Need
- Reduces transportation cost
- Storage costs
- Prepare for the next unit operation – drying, crystallisation etc.
- Reduces deteriorative chemical reactions
- Better microbiological stability
- Recovery of solvent
Driving force
Temperature difference in between steam chest temperature and product temperature
Result
– Volatile solvent is removed from the feed
– Solution (volatile solvent + nonvolatile solute) –> Concentrate (Higher solute Concentation)
Examples
- Concentration of milk to produce condensed milk
- Concentration of juices
- Concentration of NaOH, NaCl from aqueous solutions to produce salt • Ether recovery from fat extraction
Basic Parts of an Evaporator
- Heat source
- Still
- Condensor
- Vapour Separator
Selection criteria
Operating capacity
Degree of concentration required
Capital and operating costs
Heat sensitivity of product
Requirement for facilities
Ease of cleaning
Reliability and simplicity of operation
Size of evaporator
Evaporating Still
Construction
- It consists of a jacketed-evaporating pan with a cylindrical cover that connects it to a condenser. The overall assembly is called still
- The cover is clamped with the evaporating pan
Working
- The dilute liquid is fed into the still, the cover is clamped. Steam is introduced into the jacket • The liquid is evaporated and condensed in the condenser and collected
- The product (i.e. concentrated liquid) is collected through the product outlet
Advantages
- Easy to clean and maintain
- Allow the equipment to be used for solvents other than water. e.g. Ethanol
Disadvantages
- All the liquor is heated all the time
- The heating surface is limited
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.
Construction
Large 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.
Working
Feed 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 absorbs heat and solvent gets evaporated. Concentrated liquid is collected.
USES
- Best suited for non-viscous liquids having high heat-transfer coefficients and liquids that do not deposit scales. E.g. Cascara extract.
- Relatively cheap
- Poor liquid circulation (unsuitable for viscous liquids)
Vertical Tube Evaporator
Liquid is passed through 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.
CONSTRUCTION
- Consist 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 diameter0.05- 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.
Advantages
- Increases the heating surface 10-15 times than steam jacketed kettle
- Vigorous circulation enhances rate of heat transfer
- More units can be joined
Disadvantages
- Liquid to be maintained above calandria
- Complicated – increased installation cost
- Pressure has to maintain
- Cleaning and maintenance is difficult
Uses
Manufacture of cascara extract, sugar, salt, caustic soda etc.
Horizontal and Vertical Tube Evaporators
Horizontal Tube Evaporators
Vertical Tube Evaporators
Multiple Effect Evaporator
A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water.
- Water is boiled in a sequence of vessels, each held at a lower pressure than the last.
- As the boiling point of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the next.
- Generally the first vessel (at the highest pressure) requires an external source of heat.
Construction
Three vertical tube evaporators – Triple effect evaporator (All 3 evaporators are connected in series.
- Vapour from first evaporator serves as heating medium for second evaporator
- Vapour from second evaporator serves as heating medium for third evaporator
- The last evaporator will be connected to a vacuum pump Inlets are provided for steam & feed
- Outlets are provided for vapour, Concentrated product, non-condensed gases & condensate
Other feeding mechanisms of triple effect evaporator
- Forward feed method
- Backward feed method
- Mixed feed method
- Parallel feed method
Forward feeding
- The pressure in the second effect must be reduced below that in the first
- 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 increasingly lower pressures. • Often many of the later effects are under vacuum. Under these conditions, the liquid feed progress is simplest if it passes from effect one to effect two, to effect three, and so on, as in these circumstances the feed will flow without pumping.
Backward feed
- Alternatively, feed may pass in the reverse direction, starting in the last effect and proceeding to the first, but in this case the liquid has to be pumped from one effect to the next against the pressure drops. This is called backward feed • The concentrated viscous liquids can be handled at the highest temperatures in the first effects it usually offers larger evaporation capacity than forward feed systems, but it may be disadvantageous from the viewpoint of product quality.
Parallel Feed
Working
- 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
Advantages
- Suitable for large scale and continuous process
- Highly economical
- Upto 5 evaporators can be attached
- Multiple effects, or stages, are now used to minimize the energy input required to evaporate or boil off undesirable water content
- The total evaporation achieved in these systems is approximately the number of effects times the energy input to the first effect.
Disadvantages
Monitoring of evaporators
Uses
Concentration of salt solutions
Summary
- Evaporation refers to vaporization of liquid below its boiling point.
- Phase is a region of material that is chemically uniform, physically distinct, and (often) mechanically separable.
- Evaporation process is affected by the factors like nature of feed, time of evaporation, temperature, agitation, atmospheric vapour pressure etc..
- Evaporator consists of still, heat source, vapour separator and condenser.
- Evaporator selection depends on the batch size, nature of the feed, product requirement, opearating conditions etc.
- Economy of an evaporator is the total mass of water vaporized per unit mass of steam input to the evaporator.
- Evaporator capacity is the amount of water vapourised in the evaporator per unit time.
- Horizontal tube evaporator is best suited for non- viscous liquids that do not deposit scales on evaporation
- Vertical tube evaporator is best suited for scaling liquid
- In horizontal tube evaporator, the steam is passed through the tubes with the feed outside whereas in vertical tube evaporator the process in vice-versa
- Multiple effect evaporator is used for high heat transfer co-efficient efficiency and for continous process
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