Multieffect systems

With a number of heat sensitive liquids it is necessary to work at low temperatures, and this is effected by boiling under a vacuum, as indeed is the case in the last unit of a multieffect system. Operation under a vacuum increases the temperature difference between the steam and boiling liquid as shown in Table 14.1 and therefore tends to increase the heat flux. At the same time, the reduced boiling point usually results in a more viscous material and a lower film heat transfer coefficient. 

Vapor Reuse, A modification of the multieffect system, shown in Fig. 7-24, has been termed the vapor reuse system (Ref. 12). In it, the feed is introduced into a stripping tower, and the vapor from it is used as the source of heat and the feed of the lower pressure column. The two columns may not be in balance f.e., the minimum heat required for the stripping section may be more or less than that... 

This system involves a lower over-all temperature drop than a two-stage multieffect system and thereby makes the design of the heat-transfer surfaces easier. However, the multieffect system has the... 

Mechanical efficiency may range from 25-75% of the theoretical evaporation rate. Efficiencies may be raised with the application of multieffect or vapor compression evaporators. The more complicated efficient systems can seldom be warranted due to the short service offered. 

In this subsection we describe heat pumps, multieffect distillation of binary mixtures, synthesis of multicomponent distillation systems with heat integration, and multieffect distillation for thermally coupled configurations. 

An evaporator provides one stage of separation based on relative volatility. It is typically used with systems having a large relative volatility, such as salts and solvents. When water is being removed as an overhead vapor, multieffect operation may be used to provide improved energy efficiency. Figure 3.14 shows two alternative direct material balance evaporator control schemes. 

Optimal number of stages in a multieffect evaporation system, for which the capital cost is balanced against the cost of heating steam. 

Case 4. Heuristic 11 can also be used to develop a system with one or more multieffect columns. If we use heuristic 7 to do the ethanol-isopropanol separation by itself, one option is to pressurize the liquid feed to column D in case 2 and operate column D at a higher pressure. This multieffect arrangement is shown below. There are many other options possible for using multieffect columns for this separation. 

Multieffect evaporator [5]. In an evaporation process, the fluid is heated to its saturation temperature, and then additional energy is applied to start the liquid evaporation. Evaporation occurs at constant temperature and requires a large amount of energy so that the molecules in the liquid state pass to the vapor state. Unlike dryers, what is achieved in the evaporator is concentration, where normally a diluted "juice" with 5-10 % solids is concentrated to 30-50 % solids. Indeed, evaporation is a concentration process and not a dehydration process. Tomato juice will be concentrated in a three-stage multieffect evaporator (Fig. 7.46). 1,000 kg/h of tomato juice are fed to the system with an initial concentration of 6 % solids (w/w). The object of the operation is to obtain a commercial concentrate of 31 % solids (w/w). The outlet solid concentration of stages 1-3 are 13, 21, and 31 % (w/w), respectively, (a) How much water was removed in each stage of the system (b) What is the flow rate of the concentrate at the outlet in kg/h ... 

An increased number of effects is now economical for multieffect evaporators. Each evaporator system design will be analyzed more closely to define the nrast economical number of effects. 

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