Enthalpy-concentration method

FRACTIONAL DISTILLATION USING ENTHALPY-CONCENTRATION METHOD... 

Sec. 11.6 Fractional Distillation Using Enthalpy-Concentration Method... 

EXAMPLE 11.6-2. Distillation Using Enthalpy-Concentration Method A liquid mixture of benzene-toluene is being distilled using the same conditions as in Example 11.4-1 except that a reflux ratio of 1.5 times the minimum reflux ratio is to be used. The value o R = 1.17 from Example 11.4-2 will be used. Use enthalpy balances to calculate the flow rates of the liquid and vapor at various points in the tower and plot the curved operating lines. Determine the number of theoretical stages needed. 

Figure 11.6-3. Plot of curved operating lines using enthalpy-concentration method for Example 11.6-2. Solid lines are for enthalpy-concentration method and dashed lines for constant molal overflow.

Note that in Example 11.6-2 in the stripping section, the vapor flow increases slightly from 125.0 to 126.5 in going from the reboiler to near the feed tray. These values are lower than the value of 133.0 obtained assuming equimolal overflow. Similar conclusions hold for the enriching section. The enthalpy-concentration method is useful in calculating the internal vapor and liquid flows at any point in the column. These data are then used in sizing the trays. Also, calculations of <7r used... 

Use of Enthalpy-Concentration Method to DistiU an Ethanol-Water Solution. A... 

Distillation of Benzene-Toluene Feed Using Enthalpy-Concentration Method. A... 

Himmelbau (1995) or any of the general texts on material and energy balances listed at the end of Chapter 2. The Ponchon-Savarit graphical method used in the design of distillation columns, described in Volume 2, Chapter 11, is a further example of the application of the lever rule, and the use of enthalpy-concentration diagrams. 

The most convenient method for representation of enthalpy data for binary J solutions is by enthalpy/ concentration (Hx) diagrams. These diagrams are graphs T of the enthalpy plotted as a function of composition (mole fraction or mass fraction of one species) with temperature as parameter. The pressure is a constants and is usually 1 atmosphere. Figure 13.10 shows a partial diagram for the H2SO4/H2O system. 

The lower part of the column is covered by stepping off stages in a fashion similar to that in the upper part of the column, and the final conni of theoretical stages is then determined. The Ponchon-Savarit method may be used for many situations more complex lhan the simple one just described mixed vapor-liquid distillate product, side draw streams, multiple feeds, and so on. Standard unit operations textbooks should be consulted for more dentils on this methnd. As mentioned, it suffers from a need for enthalpy-concentration data, but even a crude approximation based on linear variation of enthalpy with concentration can be better than the McCabe-Thiele approach if there is a very large difference in the latent heats of vaporization of the iwo components being distillnd. 

The isotherms on an enthalpy-concentration diagram for a system with no heat of dilution are straight lines. Enthalpy-concentration diagrams can be constructed, of course, for solutions having negligible heats of dilution, but they are unnecessary in view of the simplicity of the specific-heat method described in the last section. 

The advantage of the enthalpy-concentration diagram method is that both heat and mass effects are taken into account simultaneously. This method has only limited use, however, because of the difficulty in obtaining enthalpy-concentration data that have been published for only a few systems. In using either method, however, it is necessary to make a corresponding mass balance since the heat effects in a crystallization process are related to the quantities of solids produced through the heat of crystallization. 

The McCabe-Thiele constructions described in Chapter 8 embody rather restrictive tenets. The assumptions of constant molal overflow in distillation and of interphase transfer of solute only in extraction seriously curtail the general utility of the method. Continued use of McCabe-Thiele procedures can be ascribed to the fact that (a) they often represent a fairly good engineering approximation and (b) sufficient thermodynamic data to justify a more accurate approach is often lacking. In the case of distillation, enthalpy-concentration data needed for making stage-to-stage enthalpy balances are often unavailable, while, in the Case of absorption or extraction, complete phase equilibrium data may not be at hand. 

Example 10.1. One hundred kilogram-moles per hour of saturated n-hexane-n-octane vapor containing 69 moIe% hexane is separated by distillation at atmospheric pressure into product containing 90 mole% hexane and bottoms containing 5 mole% hexane. The total condenser returns 42.5 mole% of the condensate to the column as saturated liquid. Using the graphical Ponchon method and the enthalpy-concentration data of Fig. 10.8 calculate ... 

Figure 12.15 Top Isothermal Pxy diagram for a binary mixture of an alkane(l) + an aromatic(2) at 330 K, computed from the Redlich-Kwong equation of state using the phi-phi method. This diagram is the same as in Figure 9.5. Bottom The corresponding isothermal (residual) enthalpy-concentration diagram for the same mixture as at top, also computed from the Redlich-Kwong equation using (12.4.1). Note that differences in liquid and vapor values at Xi = 0 and at Xi = 1 estimate the pure-component latent heats of vaporization.

The Ponchon-Savarit method is summarized in Fig. 5.3-8. The method involves an enthalpy-concentration diagram, and the enthalpies of the saturated liquid and vapor are first plotted on the diagram. Next, the equilibrium tie lines are added, based on phase equilibria. Compositions of feed, distillate, and bottoms are then located on the diagram (in the example shown the feed is mixed vapor-liquid and the distillate and bottoms are saturated liquids). A reflux ratio is chosen, and the enthalpy of the reflux is located as the top difference point, Ag- (The reflux ratio is equal numerically to the vertical distance from the difference poim to the value of y/v> divided by the vertical distance from y to Xg.)... 

The method of Ponchon and Savarit [2.61] is a graphical determination of the number of theoretical separation stages in counterflow columns. It is particularly applicable to binary mixtures in rectification processes. Exact results are obtained when the enthalpy-concentration diagram of Pon-... 

A graphical method to determine the absorption enthalpy for physical systems by the aid of the enthalpy-concentration diagram is described in [3.1]. Absorption enthalpies for practical use are given in [3.16]. 

Introduction. In Section 11.4B the McCabe-Thiele method was used to calculate the number of theoretical steps or trays needed for a given separation of a binary mixture of A and B by rectification or fractional distillation. The main assumptions in the method are that the latent heats are equal, sensible heat differences are negligible, and constant molal overflow occurs in each section of the distillation tower. In this section we shall consider fractional distillation using enthalpy-concentration data where the molal overflow rates are not necessarily constant. The analysis will be made using enthalpy as well as material balances. 

Equilibrium diagrams for leaching. The equilibrium data can be plotted on the rectangular diagram as wt fraction for the three components solute A), inert or leached solid (B), and solvent (Q. The two phases are the overflow (liquid) phase and the underflow (slurry) phase. This method is discussed elsewhere (B2). Another convenient method of plotting the equilibrium data will be used, instead, which is similar to the method discussed in the enthalpy-concentration plots in Section 11.6. 

Probably the most satisfactory method of calculating heat effects during a crystallization process is to use the enthalpy-concentration chart for the solution and the various solid phases which are present for the system. However, only a few such charts are available, including the following systems calcium chloride-water.,(HI), magnesium sulfate-water (P2), and ferrous sulfate-water (K2). When such a chart is available, the following procedure is used. The enthalpy if, of the entering solution at the initial temperature is read off the chart, where//, is kJ (btu) for the total feed. The enthalpy i/j of the final mixture of crystals and mother liquor at the final temperature is also read off the chart. If some evaporation occurs, the enthalpy Hy of the water vapor is obtained from the steam tables. Then the total heat absorbed q in kJ is... 

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