Enthalpy-concentration charts

However, some of the water vaporizes (40% is in the two phase region of the enthalpy concentration chart). 

From the enthalpy concentration chart at 40.3% H2SO4 solution and AH = 159 Btu / lb, we find the solution is in the two phase region at 249°F. The liquid concentration is -46% H2SO4... 

Engineering manhours for projects, 2 Enthalpy residual, 158, 159 Enthalpy-concentration chart, 524 application example, 529 construction example, 390... 

Merkel diagram. See Enthalpy-concentration chart MESH equations, 405 Methanol svnthesis. 580.585... 

Use the heat of solution data in Table B.IO and solution heat capacity data to (a) calculate the enthalpy of a hydrochloric acid, sulfuric acid, or sodium hydroxide solution of a known composition (solute mole fraction) relative to the pure solute and water at 25 C (b) calculate the required rate of heat transfer to or from a process in which an aqueous solution of HCl, H2SO4, or NaOH is formed, diluted, or combined with another solution of the same species and (c) calculate the final temperature if an aqueous solution of HCl, H2SO4, or NaOH is formed, diluted, or combined with another solution of the same species adiabatically. Perform material and energy balance calculations for a process that involves solutions for which enthalpy-concentration charts are available. 

Figure 8.5-1 Enthalpy-concentration chart for H2SO4-H2O. (Redrawn from the data of W, D. Ross, Chem. Eng. Progr., 43 314,1952.)...

Once someone has gone to the trouble of preparing an enthalpy-concentration chart like that of Figure 8.5-1, energy balance calculations become relatively simple, as shown in Example 8.5-2. 

Compare the ease of this computation with that of Example 8.5-i. Having the enthalpy-concentration chart eliminates the need for all of the hypothetical healing, cooling, and isothermal mixing steps that would normally be required to evaluate the total enthalpy change for the process. 

Enthalpy-concentration charts are particularly useful for two-component systems in which vapor and liquid phases are in equilibrium. The Gibbs phase rule (Equation 6.2-1) specifies that such a system has (2 -I- 2 - 2) = 2 degrees of freedom. If as before we fix the system pressure, then specifying only one more intensive variable—the system temperature, or the mass or mole fraction of either component in either phase—fixes the values of all other intensive variables in both phases. An H-x diagram for the ammonia-water system at 1 atm is shown in Figure 8.5-2. 

Suppose the mass fraction of ammonia in a liquid solution of NH3 and H2O al 1 atm is specified to be 0.25. According to the phase rule, the system temperature and the mass fraction of NH3 in the vapor phase are uniquely determined by these specifications. (Verify.) A tie line may therefore be drawn on the enthalpy-concentration chart from x - 0.25 on the liquid-phase curve to the corresponding point on the vapor-phase curve, which is at y = 0.95 and the... 

EXAMPLE Use of the Enthalpy-Concentration Chart for a Two-Phase System... 

If the overall composition of a two-phase two-component system at a given temperature and pressure is known, the fraction of the system that is liquid or vapor may easily be determined from the enthalpy-concentration chart. 

Prepare an enthalpy-concentration chart for a, binary mixture given the enthalpy or heat capacity data for the pure components, the heat capacity data for mixtures at various concentrations, and the necessary heats of mixing at various concentrations. 

Use the enthalpy-concentration chart in solving material and energy balances. 

A convenient way to represent enthalpy data for binary solutions is via an enthalpy-concentration diagram. Enthalpy-concentration diagrams (H-x) are plots of specific enthalpy versus concentration (usually weight or mole fraction) with temperature as a parameter. Figure 4.21 illustrates one such plot. If available, such charts are useful in making combined material and energy balances calculations in distillation, crystallization, and all sorts of mixing and separation problems. You will find a few examples of enthalpy-concentration charts in Appendix I. 

At some time in your career, you may find you have to make numerous repetitive material and energy balance calculations on a given binary system and would like to use an enthalpy-concentration chart for the system, but you cannot find one in the literature or in your files. How do you go about constructing such a chart ... 

Figure 4.21 Enthalpy concentration chart for n-butane-n-heptane at 100 psia. Curve DFHC is the saturated vapor curve BEGA is the saturated liquid. The dashed lines are equilibrium tie lines connecting y and X at the same temperature.

To include the vapor region on the enthalpy concentration chart, you need to know ... 

One hundred pounds of a 73% NaOH solution at 350°F is to be diluted to give a 10% solution at 80°F. How many pounds of water at 80°F and ice at 32°F are required if there is no external source of cooling available See Fig. E4.43. Use the steam tables and the NaOH-HiO enthalpy-concentration chart in Appendix I as your source of data. (The reference conditions for the latter chart are AH = 0 at 32 for liquid water and AH = 0 for an infinitely dilute solution of NaOH, with pure caustic having an enthalpy at 68°F of 455 Btu/lb above this datum.)... 

Use the same NaOH-H20 enthalpy-concentration chart as in Example 4.43 to obtain the enthalpy data. We can write the following material balance ... 

On the enthalpy-concentration chart for NaOH-HaO, for a 26% NaOH solution with an enthalpy of 207 Btu/lb, you would find that only a two-phase mixture of (1) saturated H2O vapor and (2) NaOH-H20 solution at the boiling point could exist. To get the fraction H2O vapor, we have to make an additional energy (enthalpy) balance. By interpolation, draw the tie line through the point x = 0.26, = 207 (make it parallel to the 220 and 250°F tie lines). 

Enthalpy-concentration charts help make vaporization problems easy to solve. According to the phase rule f = C — 9 + 2, and for a binary mixture of two phases without reaction, C = 2, = 2, and F = 2. If the pressure is fixed at some value, only one more intensive... 

Use the NH3-H2O enthalpy-concentration chart in Appendix I to determine for 1 atm pressure what the mass fraction ammonia is at equilibrium in the gas and liquid phases for a liquid temperature of 130°F. 

Estimate the heat of vaporization of an ethanol-water mixture at 1 atm and an ethanol mass fraction of 0.50 from the enthalpy-concentration chart in Appendix I. 

Prepare an enthalpy-concentration chart for ethanol and water using the data in Table 1.1 similar to the one in Appendix I for the acetic acid-water system. 

Draw the saturated-liquid line (1 atm isobar) on an enthalpy concentration chart for caustic soda solutions. Show the 200°F, 250°F, 300 F, 350°F, and 4CX)°F isotherms in the liquid region, and draw tie lines for these temperatures to the vapor region. Using the data from National Bureau of Standards Circular 500, show where molten NaOH would be on this chart. Take the remaining data from the following table and the steam tables. 

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