Defined optimum reflux ratio

Optimization As stated previously, optimization studies should include the entire system. Such a study was made by Fair and BoUes [Chem. Eng., 75(9), 156 (1968)], using a hght-hydrocarbon system and with the objective of defining optimum reflux ratio. Coolants used were at —87, —40, and +30°C (—125, —40, and +85°F), corresponding to different pressures of operation and associated different condens-... 

Mayur et al. (1970) formulated a two level dynamic optimisation problem to obtain optimal amount and composition of the off-cut recycle for the quasi-steady state operation which would minimise the overall distillation time for the whole cycle. For a particular choice of the amount of off-cut and its composition (Rl, xRI) (Figure 8.1) they obtained a solution for the two distillation tasks which minimises the distillation time of the individual tasks by selecting an optimal reflux policy. The optimum reflux ratio policy is described by a function rft) during Task 1 when a mixed charge (BC, xBC) is separated into a distillate (Dl, x DI) and a residue (Bl, xBi), followed by a function r2(t) during Task 2, when the residue is separated into an off-cut (Rl, xR2) and a bottom product (B2, x B2)- Both r2(t)and r2(t) are chosen to minimise the time for the respective task. However, these conditions are not sufficient to completely define the operation, because Rl and xRI can take many feasible values. Therefore the authors used a sequential simplex method to obtain the optimal values of Rl and xR which minimise the overall distillation time. The authors showed for one example that the inclusion of a recycled off-cut reduced the batch time by 5% compared to the minimum time for a distillation without recycled off-cut. 

The column configurations and other parameters are defined in Table 11.11. Thirteen cases were considered using different initial charge compositions and different product purity specifications (same specifications for light and heavy product). The results are summarised in Table 11.11. The results clearly show the cases (shown in bold) when an IBD column is superior to a CBD column. The optimum reflux or reboil ratio profiles for 3 cases are shown in Figure 11.10. Refer to the original reference for further details. 

In this example, the reflux ratio is to be minimized by finding the optimum feed plate location for a fixed number of total stages N or plates [k2 = N - 2]. This example was included in order to demonstrate the use of the optimization procedure for solving problems involving existing columns. Again the variables listed as usual specifications are fixed. Of the four remaining variables required to completely define the column, three are fixed, namely, bh/dh)Li and... 

---