UNIT OPERATIONS Evaporation

In Section 4.8 we discussed the case of heat transfer to a boiling liquid. An important instance of this type of heat transfer occurs quite often in the process industries and is given the general name evaporation. In evaporation the vapor from a boiling liquid solution is removed and a more concentrated solution remains. In the majority of cases the unit operation evaporation refers to the removal of water from an aqueous solution. 

Typical examples of evaporation are concentration of aqueous solutions of sugar, sodium chloride, sodium hydroxide, glycerol, glue, milk, and orange juice. In these cases the concentrated solution is the desired product and the evaporated water is normally discarded. In a few cases, water, which has a small amount of minerals, is evaporated tp give a solids-free water which is used as boiler feed, for special chemical processes, or for other purposes. Evaporation processes to evaporate seawater 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 cooling, salt crystals will form and be separated. This special evaporation process, termed crystallization, is discussed in Chapter 12. 

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 temperature of the process. Some of these properties which affect the processing methods are discussed next. 

Concentration in the liquid. Usually, the liquid feed to an evaporator is relatively dilute, so its viscosity is low, similar to water, and relatively high heat-transfer coefficients 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 turbulence must be present to keep the coefficient from becoming too low. 

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 losses occur. 

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While the unit operation evaporation, that is, the mass transfer from the liquid phase to the vapor phase, still possesses a direct connection with vacuum techniques, the connection of today s single mass crystallization from solution with vacuum techniques is only indirect. The techniques of vacuum cooling and vacuum evaporation are only the mostly used means for inducing the crystallization process. The reason for the dominant position of vacuum crystallization over classical surface cooling crystallization is the considerably reduced inclination to form incrustations. Vacuum crystallization is used in the low vacuum field down to 1 mbar. There are also applications in the overpressure field, although with increasing pressure the number of applications is reduced. In vacuum crystallization, one can find all the classical process control options used in the more familiar vacuum evaporation processes. However, an important difference to evaporation is the fact that the separation process is not concluded with the crystallization step. The suspension formed still has to be separated into crystal mass and mother liquor. Crystallization is therefore always associated with a mechanical separation process. The better this separation, the greater the purity of the crystallized masses. 

In order to make a multipurpose plant even more versatile than module IV, equipment for unit operations such as soHd materials handling, high temperature/high pressure reaction, fractional distillation (qv), Hquid—Hquid extraction (see Extraction, liquid-liquid), soHd—Hquid separation, thin-film evaporation (qv), dryiag (qv), size reduction (qv) of soHds, and adsorption (qv) and absorption (qv), maybe iastalled. 

Now you can reconsider the material balance equations by adding those additional factors identified in the previous step. If necessary, estimates of unaccountable losses will have to be calculated. Note that, in the case of a relatively simple manufacturing plant, preparation of a preliminary material-balance system and its refinement (Steps 14 and 15) can usefully be combined. For more-complex P2 assessments, however, two separate steps are likely to be more appropriate. An important rule to remember is that the inputs should ideally equal the outputs - but in practice this will rarely be the case. Some judgment will be required to determine what level of accuracy is acceptable, and we should have an idea as to what the unlikely sources of errors are (e.g., evaporative losses from outside holding ponds may be a materials loss we cannot accurately account for). In the case of high concentrations of hazardous wastes, accurate measurements are needed to develop cost-effective waste-reduction options. It is possible that the material balance for a number of unit operations will need to be repeated. Again, continue to review, refine, and, where necessary, expand your database. The compilation of accurate and comprehensive data is essential for a successful P2 audit and subsequent waste-reduction action plan. Remember - you can t reduce what you don t know is therel... 

Unit operations in the mass transfer category that may be found at a plant are evaporation, extraction, adsorption, and absorption. 

Vaporization and diffusion of flammable or toxic liquids or gases is a primary consideration with distillation, evaporation, extraction, and absorption operations. The basic principle of safety for tliese unit operations is contaimnent of the materials witliin the system. These operations should be conducted outdoors whenever possible. In tliis way, any accidental release of flammable or... 

The COLEX units operated between 1955 and 1963. Approximately 24 million pounds of mercury were employed and, unfortunately, a good deal was lost through waste, accidental spills, and evaporation. In fact, about 2 million pounds have not been accounted for. The process discharged approximately a quarter million pounds into the surface waters in the vicinity of the plant, and much of this remains in the bottom sediments of the watershed. 

This complex unit operation involves significant microstructural changes in fact, most of the desirable characteristics of fried foods are derived from the formation of a composite structure a dry, porous, crispy and oily outer layer or crust, and a moist cooked interior or core, whose microstructures form during the process (Bouchon et al., 2001). The high temperatures (around 160 and 180°C) cause water evaporation, which is transferred from the food towards the surrounding oil, whereas oil is absorbed by the food replacing part of the released water. This process results in products with a unique flavor-texture combination (Mellema, 2003). 

Evaporation), and in succeeding years under " Unit Operations )... 

The use of solvent extraction also represents a potentially feasible process. Solvent extraction is an engineering unit operation that is adapted effectively to continuous processing. It has been used with success for the isolation of nonpolar compounds of bp >100 °C (58). Solvent extraction (continuous liquid-liquid extraction) may represent a useful process for routinely concentrating 50-100 L of water. The major problem with solvent extraction is the evaporation and recovery for reuse of large volumes of the organic solvent. Other problem areas that must be considered are purification of sufficient solvent and minimization of artifact formation by heat. 

FIG. 19 Schematic flow diagram for the production and purification of fermentation-derived acetic acid, as modified from Office of Industrial Technologies (2003). Unit operation identification items AF, anaerobic fermentation DI, distillation ED, electrodialysis EV, evaporation MF, microfiltration PV, pervaporation-assisted thermal cracking. 

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