Evaporation

8.1 Introduction

8.1A Purpose
In section 4.8 we discussed thechase of heat transfer to a boiling liquid. An important instance of this type of heat transfer occurs quite often in the process indudtries and is given the general name evaporation. In the evaporation the vapor from a boiling liquid solution removed and amore concentrated solution remains. In the majority of cases the unit operation evaporation revers to the removal of water from an aqueos solution.
     Typical examples of evaporation are concentration of aqueous solutions of sugar, sodium cholride, sodium hydroxide, glycerol, glue, milk, and orange juice. In these cases the concentrated solutions is desired product and the evaporated water is normally discarded. In few cases, water, which has a small amount  of  minerals, is evaporated to give a solid-free water which is used as boilers feed, for special chemical process, or for other purposes. Evaporation process to evaporate sweater to provide drinking water have been developed and used. In some cases, the primary purpose of evaporation is to concentrate the solution so that upon colling, salt crystals will from and be separated. This special evaporation process, termed crystallization, is discussed in chapter 12.

8.1 Processing Factors
The physical and chemical properties of the solution being concentrated and of the vapor being removed have a great effect on the type of evaporator used and on the pressure and temperatuture of the process. some of these properties which affect the processing methods are discussed next.

1.  Concentration in the liquid. Usually, the liquid feed to an evaporator is relatively dilute, so its visicocity is low, similar to water, and relatively high heat-transfer coeficients are obtained. As evaporation proceeds, the solution may become very concentrated and quite viscous, causing the heat-transfer coefficient to drop markedly. Adequate circulation and/or turbulance must be present to keep  the coefficient from becoming too low.

2. Solubility. As solutions are heated and concentration of the solute or salt increases, the solubility limit of the material in the solution may be exceeded and crystalls may from. This may limit the maximum concentration in solution which can be obtained by evaporation. Some solubility of typical salts in water are shown as a function of temperature. In most cases the solubility of the salt increases with temperature. This mean that when a hot concentrated solution from an evaporator is cooled to room temperature, crystallization may occur

3. Temperature sensitivity of material. Many products, especially food and other biological materials, may be materials, may be temperature-sensitive and degrade at higher temperatures after prolonged heating. Such product are pharmaceutical products; food product such as milk, orange juice and vegetable extract; and fine organics chemicals. The ammount of degradation is a function of the temperature and the length of time.

4. Foaming or frothing. In some cases materials composed of caustic solutions, food solutions such as skim milk, and some fatty acid solutions form a foam or froth during boiling. This foam accompanies the vapor  coming out of the evaporator and entrainment losser occur.

5. 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 the temperature at boiling. Also, as the concehtration of dissolved material in solution increases by evaporation, the tenperature of boiling may raise. this phenomenon is called Boiling-point rise or elevation and is discussed in section 8.4. to keep the temperatures low in heat-sensitive materials, it is often necessary to operate under 1 atm pressure,i.e., under vacum

6. Scale deposition and materials of construction. Some solutions deposit solid materials called scale on  the heating surfaces. These could be formed by decomposition products or or solubility decreases. The result is that the overal heat-transfer coefficient decreases and the evaporator must eventually be cleaned. the material of construction of the evaporator are important to minimize corrosion.

TAKEN FROM : Transport Processes and Unit Operations

BY : Christie J. Geankoplis