Enthalpy-composition diagram

The heat effects accompanying a crystallization operation may be determined by making heat balances over the system, although many calculations may be necessary, involving knowledge of specific heat capacities, heats of crystallization, heats of dilution, heats of vaporization, and so on. Much of the calculation burden can be eased, however, by the use of a graphical technique in which enthalpy data, solubilities and phase equilibria are represented on an enthalpy-composition H x) diagram, sometimes known as a Merkel chart. 

The use of the FF x diagram for the analysis of chemical engineering unit operations such as distillation, evaporation and refrigeration processes, is now quite common, and the procedures are well described in textbooks, e.g. Coulson and Richardson (1991), McCabe, Smith and Harriott (1985). These charts are less frequently applied to crystallization processes, however, because not many H x diagrams are available. 

Among the few enthalpy-composition charts for solid-liquid systems published in the open literature (all for aqueous solutions) are  

The construction of an H-x diagram is laborious (McCabe, 1935) and would normally be undertaken only if many calculations were to be performed, e.g. on a system of commercial importance. Nevertheless, once an H-x chart is available its use is simple, and a great deal of information can be obtained rapidly. If the concentration x of one component of a binary mixture is expressed as a mass fraction, the enthalpy is expressed as a number of heat units per unit mass of mixture, e.g. Btulb or Jkg Molar units are less frequently used in crystallizer design practice. 

Similarly, if mixture A were to be removed adiabatically from mixture C, the enthalpy and composition of residue B can be located on the straight line through points A and C by means of the equation 

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The Ponchon-Savarit method, using an enthalpy-composition diagram, may also be used to handle sidestreams and multiple feeds, though only for binary systems. This is dealt with in Section 11.5. 

Figure 11.24. Enthalpy-composition diagram, showing the enthalpies of liquid and vapour...

Figure 11.25. Combination and separation of a mixture on an enthalpy-composition diagram...

Figure 11.28. Determination of the number of plates using the enthalpy-composition diagram...

Figure 11.29. Enthalpy-composition diagram for ammonia-water at 1.0 MN/m2 pressure (Example 11.10)...

Figure 11.30. Enthalpy-composition diagram for a system with one sidestream...

Figure 11.32. Enthalpy-composition diagram for carbon tetrachloride-toluene separation with one side-...

Although the thermal demands of crystallization processes are small compared with those of possibly competitive separation processes such as distillation or adsorption nevertheless, they must be known. For some important systems, enthalpy-composition diagrams have been prepared, like those of Figure 16.3, for instance. Calculations also may be performed with the more widely... 

Figure 16.3. Enthalpy-composition diagrams of some salt solutions. Several other diagrams are in the compilation of Landolt-Bomstein, IV 4b, 1972, pp. 188-224. (a) sodium sulfate/water (b) magnesium sulfate/water (after Chemical Engineers Handbook, 1963 edition, McGraw-Hill, New York) (c) sodium carbonate/water.

Figure 1.2 Enthalpy-composition diagram for the ethanol-water stem, showing liquid and vapor phases in equilibrium at 1 atm.

Savarit (1922). Besides the enthalpy-composition diagram, the method makes use of... 

The enthalpies and compositions of the feed, vapor product, and liquid product are now plotted on an enthalpy-composition diagram. Figure 5.9. The point F represents the feed with coordinates YZ.Hf Y. the point V represents the vapor product with coordinates (F,77) and the point L represents the liquid product with coordinates (X,h. It can be seen from Equations 5.27 and 5.28 that the line segments LF and FV are actually the segments on the same straight line since they... 

The above concepts amount to a graphical representation of the overall material, component material, and enthalpy balances around an equilibrium stage. A column solution may be developed by applying these graphical rules on an enthalpy-composition diagram. 

The enthalpies can be determined from an enthalpy-composition diagram or from the heat capacities and latent heats of vaporization. In the case of a total condenser, the composition of the stream is not changed (not true in a partial condenser ) and a mass balance around the condenser can be written ... 

Figure 3.26 is a composition enthalpy diagram for the n-hexane/n-octane system at constant pressure. An example demonstrating the construction and utility of this enthalpy-composition diagram follows. 

The first phase-equilibrium diagrams discussed are for two-component liquid-vapor systems. Next, three-component diagrams used in extraction, absorption, leaching, and ion exchange are developed. Finally, enthalpy-composition diagrams, which include energy effects, are constructed. 

ISOPROPANOL-TOLUENE ENTHALPY-COMPOSITION DIAGRAM. SHAHVD DONNELLY HG CHEM ENG PROGRSYMPSER... 

Figure 2-4. Enthalpy-composition diagram for ethanol-water at a pressure of 1 kg/cm ...

For binary systems it is often convenient to represent the enthalpy functions graphically on an enthalpy-composition diagram such as Figure 2-4. For ideal mixtures the enthalpies can be calculated from heat capacities and latent heats. Then,... 

For binary flash distillation, the simultaneous procedure can be conveniently carried out on an enthalpy-composition diagram First calculate the feed enthalpy, hp, from Eq. t2-81 or Eq. (2=9b) then plot the feed point as shown on Figure 2-9 (see Problem 2-All. In the flash drum the feed separates into liquid and vapor in equilibrium Thus the isotherm through the feed point, which must be the T nun isotherm, gives the correct values for x and y. The flow rates, L and V, can be determined from the mass balances, Eqs. f2-51 and 2-61. or from a graphical mass balance. 

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