Minimum number of trays

The simulation can also be run to find the minimum number of trays by decreasing the number of stages until the required reflux ratio becomes very large. Product purities are held constant. It is assumed that the feed stage is a fixed ratio of the total number of stages. Results are given in Table 3.3 and show that the minimum number of stages is 15. 

Total Stages Feed Stage Reflux Ratio 

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Because the feed tray is essentially non-effective it is suggested that an additional theoretical tray be added to allow for this. This can be conveniently solved by the nomographs [21] of Figures 8-16 and 17. If the minimum number of trays in the rectifying section are needed, the)t can be calculated by the Fenske equation substituting the limits of xpi for x jj and x i, and the stripping section can be calculated by difference. 

Example 8-4 Determine Minimum Number of Trays by Winn s Method (used by permission [99])... 

The minimum number of trays necessary to debutanize the effluent from an alkylation reactor will be calculated. The feed, products, and vapor-liquid equilibrium costants of the key components at conditions of temperature and pressure corresponding to the top tray and reboiler are shown in Table 8-1. 

Example 8-25 Scheibel-Montross Minimum Reflux, 80 Minimum Number of Trays Total Reflux — Constant Volatility, 80 Chou and Yaws Method, 81 Example 8-26 Distillation with Two Sidestream Feeds, 82 Theoretical Trays at Operating Reflux, 83 Example 8-27 Operating Reflux Ratio, 84 Estimating Multicomponent Recoveries,... 

The construction for the minimum number of trays is independent of the heat balance. The minimum reflux corresponds to a minimum condenser load Q and hence to a minimum value of Q = hD + Qc /D. It can be found by trial location of point P until an operating curve is found that touches the equilibrium curve. 

A convenient approximation is that the distributions of nonkeys require the minimum number of trays as given by Eq. (13.116). Designating the nonkey by subscript nk, that equation becomes... 

The minimum number of trays is found with the Fenske-Underwood equation... 

Calculate the minimum number of trays using the Fenske Equation. 

Christensen and Jorgensen (1987) proposed a measure, q, for the degree of difficulty of separation for batch distillation (as presented in Chapter 3). Mujtaba (1997) used the ratio of minimum number of trays required to achieve a separation task to the actual number of trays available for the task to calculate, qc, for... 

For each column, the recoveries of the light and heavy key components have been specified. Then, the minimum number of trays Nmill and the minimum reflux ratio Rmin have been calculated using the shortcut distillation model DSTWU with Winn-Underwood-Gilliland method in Aspen Plus. The reflux ratio was set to... 

Separation constraints The separation in a column can be expressed as the impurity levels of the key components in the two products xg.LK in the bottoms and xD Hx in the distillate. Separation is limited by the minimum reflux ratio and the minimum number of trays. We must always have more trays than the minimum and a higher reflux ratio than the minimum. If the number of trays in the column is not large enough for the desired separation, no amount of reflux will be able to attain it and no control system will work. In extractive distillation columns, there is also a maximum reflux ratio limitation, above which the overhead stream becomes less pure as the reflux increases. 

Figure 13.6 illustrates various aspects of the graphical method. A minimum number of trays is needed at total reflux, that is, with no product takeoff. Minimum reflux corresponds to a separation requiring an infinite number of stages, which is the case when the... 

Assume xi values of bottoms compositions of light key for approximate equal increments from final bottoms to initial feed charge. Calculate L/V values corresponding to the assumed xj values by inserting the various xj values in the Fenske equation for minimum reflux ratio of l-(d). The xj" values replace the xjb of this relation as the various assumptions are calculated. The actual (L/D) are calculated as in l-(d) keeping the minimum number of trays constant. Complete the table values. 

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