UNIT OPERATION EVAPORATION

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Chapter Evaporation Presented by:- Ashish Namdev

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Evaporation: Factors affecting evaporation Evaporators Film evaporators Single effect and multiple effect evaporators

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Evaporation Suppose that we have a dilute solution of a solute (say, sugar) dissolved in a solvent (say, water) We need to remove part of the solvent (water) to produce a concentrated solution This can be achieved by heating the solution so as to evaporate the solvent (water) This process is known as evaporation. The industrial equipment used for this purpose in known as an evaporator.

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Types of evaporators Open kettle or pan evaporator:

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Types of evaporators Open kettle or pan evaporator: Concentrate Condensate Pan Boiler Jacket Pressure gauge Steam

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Types of evaporators Open kettle or pan evaporator: simplest form of evaporators inexpensive simple to operate very poor heat economy in some cases paddles and scrapers for agitation are used

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Types of evaporators Horizontal-tube evaporator: Dilute feed Condensate Concentrated product Vapour Steam inlet

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Types of evaporators Horizontal-tube evaporator: - relatively cheap used for non-viscous liquids having high heat-transfer coefficients and liquids that do not deposit scales poor liquid circulation (and therefore unsuitable for viscous liquids)

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Types of evaporators Vertical-type short-tube evaporator: - Liquid is inside the tubes - Steam condenses outside the tubes - used for non-viscous liquids having high heat-transfer coefficients and liquids that do not deposit scales

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Types of evaporators Vertical-type short-tube evaporator: Dilute feed Condensate Concentrated product Vapour Steam inlet

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Types of evaporators Falling-film-type evaporator:

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Factors effecting evaporation: Concentration in the liquid: Liquid feed to an evaporator is relatively dilute. So its viscosity is low, and heat-transfer coefficient high. As evaporation proceeds, the solution becomes concentrated. So viscosity increases and heat-transfer coefficient drops. Density and the boiling point of solution also increase.

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Factors effecting evaporation: Solubility: As solution is heated, concentration of the solute in the solution increases. In case the solubility limit of the solute in solution is exceeded, then crystals may form. Solubility of the solute therefore determines the maximum concentration of the solute in the product stream. In most cases, the solubility of the solute increases with temperature. This means when a hot concentrated solution from an evaporator is cooled to room temperature, crystallization may occur.

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Factors effecting evaporation: Temperature sensitivity of materials: Pharmaceuticals products, fine chemicals and foods are damaged when heated to moderate temperatures for relatively short times. So special techniques are employed to reduce temperature of the liquid and time of heating during evaporation

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Factors effecting evaporation: Foaming and frothing: Solutions like organic compounds tend to foam and froth during vaporization. The foam is carried away along with vapor leaving the evaporator. Entrainment losses occur.

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Factors effecting evaporation: Pressure and temperature: The boiling point of the solution is related to the pressure of the system. The higher the operating pressure of the evaporator, the higher the temperature at boiling. Also, as the concentration of the dissolved material in solution increases by evaporation, the temperature of boiling may rise (a phenomenon known as boiling point rise / elevation ). To keep the temperatures low in heat-sensitive materials, it is often necessary to operate under atmospheric pressure (that is, under vacuum).

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Factors effecting evaporation: Scale deposition: Some solutions deposit solid materials (called scale ) on the heating surfaces. The result is that the overall heat-transfer coefficient ( U ) may drastically decrease, leading to shut down of the evaporators for cleaning purposes.

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Factors effecting evaporation: Materials of construction: Evaporators are made of some kind of steel. However many solutions attack ferrous metals and are contaminated by them. Copper, nickel, stainless steels can also be used.

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Method of operation of evaporators When a single evaporator is used ,the vapor from the boiling liquid is condensed and discarded. This is called single effect evaporation. It is simple but utilizes steam ineffectively. To evaporate 1 kg of water from the solution we require 1-1.3 kg of steam. Increasing the evaporation per kg of steam by using a series of evaporators between the steam supply and condenser is called multiple effect evaporation Single-effect evaporation: Multiple-effect evaporation:

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Prof. R. Shanthini 28 May 2012 Calculation methods for single-effect evaporators Steam, S P S , T S , H S Feed, F x F , T F , h F Condensate, S P S , T S , h S Vapour, V y V , T 1 , H V Concentrate, L x L , T 1 , h L P T 1 Feed: F – mass flow rate x F – mass fraction of solute in feed T F – temperature of feed h F – enthalpy of feed Vapour leaving the evaporator: V – mass flow rate y V – mass fraction of solute in vapour T 1 – temperature of vapour H V – enthalpy of vapour Concentrate leaving the evaporator: L – mass flow rate x L – mass fraction of solute in concentrate T 1 – temperature of concentrate h L – enthalpy of concentrate Steam: S – mass flow rate P S – steam pressure T S – steam temperature H S – enthalpy of steam h S – enthalpy of condensate P – pressure in the evaporator T 1 – temperature in the evaporator

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Calculation methods for single-effect evaporators Overall material balance: F = L + V Solute balance: F x F = L x L + V y V If the vapour is free of solute: F x F = L x L Heat balance: F h F + S H S = L h L + V H V + S h S Rewriting: F h F + S (H S - h S ) = L h L + V H V F h F + S λ = L h L + V H V where λ = H S - h S Steam, S P S , T S , H S Feed, F x F , T F , h F Condensate, S P S , T S , h S Vapour, V y V , T 1 , H V Concentrate, L x L , T 1 , h L P T 1

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Calculation methods for single-effect evaporators Energy lost by the steam q = S λ = S (H S – h S ) In case of no energy loss to the environment, q amount of energy gets transferred from steam to the solution through the tube wall of area A and overall heat transfer coefficient U. Therefore, q = U A Δ T = U A (T S – T 1 ) Steam, S P S , T S , H S Feed, F x F , T F , h F Condensate, S P S , T S , h S Vapour, V y V , T 1 , H V Concentrate, L x L , T 1 , h L P T 1

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Effects of processing variables on evaporator operation: Effect of feed temperature: - The inlet temperature of the feed has a large effect on the evaporator operation. When feed is not at its boiling point, steam is needed first to heat the feed to its boiling post and then to evaporate it. Preheating the feed can reduce the size of evaporator heat-transfer area.

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Effects of processing variables on evaporator operation: Effect of pressure: Pressure in the evaporator sets the boiling point of the solution (T 1 ). Steam pressure determines the steam temperature (T s ) Since q = U A (T S – T 1 ), larger values of (T S – T 1 ) will help reduce the heat-transfer area needed and hence the cost of evaporator. Vacuum can be maintained in the solution side using a vacuum pump. For example, if the pressure in Example 1 is reduced to 41.4 kPa, boiling point of water reduces to 349.9 K and that would increase the (T S – T 1 ) from 10 K to 33.3 K. A large decrease in heat-transfer area would be obtained.

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Effects of processing variables on evaporator operation: Effect of steam pressure: High pressure provides high T s values, and hence T S – T 1 will increase. High pressure steam is however more costly. Therefore, overall economic balances must be considered to determine the optimum steam pressure.

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Boiling point rise of solutions: In example 1, the solution is assumed to be dilute enough to be considered to have the same thermal properties as water. It is not true always. For concentrated solutions, heat capacity and boiling point are quire different from that of water. Duhring’s rule is an empirical law that relates the boiling point of a solution to the boiling point of the solvent at different pressures for a solution of given concentration.

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Boiling point rise of solutions (an example): Duhring plot for boiling point of sodium chloride solutions

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Prof. R. Shanthini 28 May 2012 Double-effect evaporators

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