Hengstebeck’s method

Hengstebeck s method is used to find the third trial value for L/V. The calculated values are plotted against the assumed values and the intercept with a line at 45° (calculated = assumed) gives the new trial value, 2.4. 

Where the concentration of the non-keys is small, say less than 10 per cent, they can be lumped in with the key components. For higher concentrations the method proposed by Hengstebeck (1946) can be used to reduce the system to an equivalent binary system. Hengstebeck s method is outlined below and illustrated in Example 11.5. Hengstebeck s book (1976) should be consulted for the derivation of the method and further examples of its application. 

For multicomponent mixtures the method of Hengstebeck can be used. Kister s book Distillation Design, by McGraw-Hill, 1992 explains the method and is easier to find than the original book of Hengstebeck. 

The method proposed is the Hengstebeck Method described in Reference 17. The method is quite simple. First, a heavy key component is selected and the relative volatility (a) of all column components to this heavy key are determined. Once this is done for a colunrn, minor operating changes won t affect a much and the same a can therefore often be used for subsequent sets of data. One can use ocfeed or perhaps more accurately, a = VocToD Bott - Then a plot is made of InD/B versus a. A straight line should result. If a fairly straight line does not result, the analyses are probably faulty. Here, D is the mols/hr of each component in the distillate and B is the mols/hr of each component in the bottoms. Actually, from Fenske s equation (Equation 11 and Reference 18), at total reflux it is shown that In D/B versus In a is a straight... 

This compares well with the value of 1.04 obtained from the Hengstebeck dia gram (Sec. 2.3.S), and with a value of 1.07 obtained either from the Underwood method, corrected for nonconatant molar overflow (Sec. 8.2.4), or from extrapolation of computer simulation results (Fig. 3.8). 

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