Calculation distillation

Many industrial separations require a series of columns that are connected in specific ways. Some distillation programs can model such a system as a hypothetical single column with arbitrary cross-flows and connections and then carry out the distillation calculations for the modeled hypothetical column. Alternatively, such a system can be modeled as a process flow sheet using a process simulator. 

In using a spreadsheet for process modeling, the engineer usually finds it preferable to use constant physical properties, to express reactor performance as a constant "conversion per pass," and to use constant relative volatiHties for distillation calculations such simplifications do not affect observed trends in parametric studies and permit the user quickly to obtain useful insights into the process being modeled (74,75). 

The widespread availabihty and utihzation of digital computers for distillation calculations have given impetus to the development of analytical expressions for iregression equation and accompanying regression coefficients that represent the DePriester charts of Fig. 13-14. Regression equations and coefficients for various versions of the GPA convergence-pressure charts are available from the GPA. 

The design of a plate tower for gas-absorption or gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical plates needed to achieve a specified composition change (see Sec. 13). Distillation differs from gas absorption in that it involves the separation of components based on the distribution of the various substances between a gas phase and a hquid phase when all the components are present in Doth phases. In distillation, the new phase is generated From the original feed mixture by vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

When chemical equilibrium is achieved qiiickly throughout the liquid phase (or can be assumed to exist), the problem becomes one of properly defining the physical and chemical equilibria for the system. It sometimes is possible to design a plate-type absorber by assuming chemical-equilibrium relationships in conjunction with a stage efficiency factor as is done in distillation calculations. Rivas and Prausnitz [Am. Tn.st. Chem. Eng. J., 25, 975 (1979)] have presented an excellent discussion and example of the correct procedures to be followed for systems involving chemical equihbria. 

Edmister, W. C., Absorption and Stripping-factor Functions for Distillation Calculation by Manual- and Digital-computer Methods, A.I.Ch.E. Journal, June 1957. 

Commercial computer services are available to do rigorous distillation calculations. Perhaps the licensor will provide copies of rigorous computer runs to validate his balances. Alternately, the operating company can make such runs. For highly non-ideal systems, literature data for binary pairs may have to be sought. In some cases, laboratory equilibrium data may have to be obtained in-house or contracted out to one of several organizations or universities that are in this business. 

It is essential to calculate, predict or experimentally determine vapor-liquid equilibrium data in order to adequately perform distillation calculations. These data need to relate composition, temperature, and system pressure. 

The Hausbrand vapor-pressure diagram [127, 128] in Figure 8-40 is a useful approach for the steam distillation calculation. This particular diagram was prepared for six organic compounds and the corresponding water vapor pressure as (it - ps) for three system pressures of 760, 300, and 70 mm Hg versus temperature,... 

Edmister, W. C., Hydrocarbon Absorption and Fractionar tion Process Design Methods, Pet. Engr. May 1947-March 1949 and, Absorption and Stripping—Factor Functions for Distillation Calculations by Manual and Digital—Computer Methods. A.I.ChJE. foumal, V. 3, No. 2 p. 165 (1957). 

Distillation calculations result in a reflux ratio L/D = 0.8, with 4 theoretical trays for rectification and 4 theoretical trays for stripping, or a total of 8 trays. The design heat balance (neglecting heat losses) is as follows ... 

Figure 1.8. Information flow, binary flash distillation calculation (a) Information recycle (b) Information flow...

The complexity of multicomponent distillation calculations can be appreciated by considering a typical problem. The normal procedure is to solve the MESH equations (Section 11.3.1) stage-by-stage, from the top and bottom of the column toward the feed point. For such a calculation to be exact, the compositions obtained from both the bottom-up and top-down calculations must mesh at the feed point and match the feed composition. But the calculated compositions will depend on the compositions assumed for the top and bottom products at the commencement of the calculations. Though it is possible to... 

Haas, J. R. (1992) Rigorous Distillation Calculations, in Distillation Design, Kister, H. Z. (McGraw-Hill). 

A distillation calculation is to be performed on a multicomponent mixture. The vapor-liquid equilibrium for this mixture is likely to exhibit significant departures from ideality, but a complete set of binary interaction parameters is not available. What factors would you consider in assessing whether the missing interaction parameters are likely to have an important effect on the calculations ... 

The errors associated with the Underwood Equations were discussed in Chapter 9, which tend to underpredict the minimum reflux ratio. This introduces uncertainty in the way that the calculations were carried out in Examples 11.2 and 11.3. The differences in the total vapor load between different sequences are small and these differences are smaller than the errors associated with the prediction of minimum reflux ratio and minimum vapor load using the Underwood Equations. However, as long as the errors are consistently low for all of the distillation calculations, the vapor load from the Underwood Equations can still be used to screen between options. Nevertheless, the predictions should be used with caution and options not ruled out because of some small difference in the total vapor load. 

The relationship between the composition of the vapour yA and of the liquid xA in equilibrium may also be expressed in a way, which is particularly useful in distillation calculations. If the ratio of the partial pressure to the mole fraction in the liquid is defined as the volatility, then ... 

Figure 11.47. Vapour pressure curves for steam distillation calculations...

Bruno, J.A., Yanosik, J.L., and Tierney, J.W. Distillation Calculations with Nonideal Mixtures, Extractive and Aziotropic Distillation, Advances in Chemistry Series 115, ACS, Washington, DC... 

For purposes of distillation calculations, a rough diagram of saturated vapor and liquid enthalpy concentration lines can be drawn on the basis of pure component enthalpies. Even with such a rough diagram, the accuracy of distillation calculation can be much superior to those neglecting enthalpy balances entirely. Example 13.8 deals with preparing such a Merkel diagram. 

The iterative calculation procedure is outlined in Figure 14.10. The method is an adaptation to extraction by Tsuboka and Katayama (1976) of the distillation calculation procedure of Wang and Henke [Hydrocarb. Proc. 45(8), 155-163 (1967)]. It is also presented by Henley and Seader (1981, pp. 586-594). 

No distillation calculations of any kind were carried out, other than the simple material balance type. It was assumed that all of the ammonium bicarbonate fed to a still would go to the distillate product, plus sufficient water to form a 13% ammonium bicarbonate solution, and that all of the sodium chloride plus the remaining water would be the still bottoms product. 

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