Reflux rate minimum

The reflux rate and flow of nitrogen gas must be kept at a minimum to assure that the amount of dioxane carried over to the traps in liquid nitrogen is kept as small as possible. 

The indirect effects add to those of Fig. 14-42, widening the gap between true and apparent efficiency. The indirect effects exponentially escalate as minimum reflux is approached. Small errors in VLE or reflux ratio measurement (this includes column material balance as well as reflux rate) alter R/Rmin. Near minimum reflux, even small R/Rmin errors induce huge errors in the number of stages, and therefore in tray efficiency. Efficiency data obtained near minimum reflux are therefore meaningless and potentially misleading. 

Here the feed rate is maximised while the reflux ratio is optimised. The bottom product composition imposes an additional constraint to the problem. The results are summarised in Table 11.8 which gives the maximum feed rate, minimum batch time, optimum reflux ratio, and total number of batches for each mixture and total yearly profit. 

Here, the product IPB is much heavier than the reactants, benzene and propylene, making possible easy separation in bottoms. On the other hand, propylene is much lighter than benzene, which should be used in large excess for better selectivity. It is desirable that the propylene reacts completely to avoid a new separation problem. Therefore, the column should keep only two sections, reaction and stripping. Benzene and propylene are fed at the top and at the bottom of the reaction section creating a countercurrent flow of reactants. The reaction takes place in the liquid phase in the presence of a zeolite catalyst. This time, the minimum reflux rate is dictated not by the separation but by the amount of benzene that must be condensed to remove the heat of reaction. From the heat-... 

E.xample problems are included to highlight the need to estimate the entire set of products that can be reached for a given feed when using a particular type of separation unit. We show that readily computed distillation curves and pinch point cur es allow us to identify the entire reachable region for simple and e.xtractive distillation for ternary mixtures. This analysis proves that finite reflux often permits increased separation we can compute exactly how far we can cross so-called distillation boundaries. For extractive distillation, we illustrate how to find minimum. solvent rates, minimum reflux ratios, and, interestingly, ma.xinnim reflux ratios. 

We have designed and implemented a reactive divided wall distillation column for the production of ethyl acetate from acetic acid and ethanol. Important aspects derived from steady state simulation were considered for instance, a side tank was implemented in order to split the liquid to both sides of the wall and a moving wall inside the column that allows to fix the split of the vapor stream. The dynamic simulations indicate that it is possible to control the composition of the top and bottoms products or two temperatures by manipulating the reflux rate and the heat duty supplied to the reboiler, respectively. The implementation of the reactive divided wall distillation columns takes into account important aspects like process intensification, minimum energy consumption and reduction in Carbon Dioxide emission to the atmosphere. 

R = minimum reflux ratio = R ahi/D, and vapor rate = reflux rate -H D. 

In this entire process, only one equation could not be solved analytically, Eq. (6.4). In order to solve the problem, you needed the Ai-values of the chemicals (or vapor pressures), the flow rates of the feed stream, the thermal condition of the feed stream, the desired split for the light and heavy keys, and either the number of stages or the reflux ratio. The equations then And the splits of all the other components, the minimum number of stages, the minimum reflux rate, and the actual number of stages needed to achieve the desired split. These last three items are very useful when using a more rigorous method of calculation, as shown below. 

The same separation would require a minimum reflux ratio of 0.6. Existing pumping facilities can deliver a reflux rate of 275 kmol/h. Determine the required number of theoretical stages. What minimum overall tray efficiency is needed to make the specified separation with the existing column ... 

If the distillation were to be started at twice the minimum reflux ratio, determine the required number of stages. If the initial charge is 100 kmol and the distillate rate is 10 kmol/h, calculate the reflux rate, the amounts of distillate and residue, and the residue composition as a function of time. Irrespective of tray hydraulics and reboiler and condenser capacity constraints, when should the distillation be stopped Assume negligible tray holdups and use shortcut methods. 

For the conditions of Problem 12.7, compute the minimum external reflux rate and the distribution of the nonkey components at minimum reflux by the Underwood equation if the feed is a bubble-point liquid at column pressure. 

Ibmole/hr (30% greater than the exact value of the minimum reflux rate from Bachelor) hy the following schemes. 

Similar to step 6 above, check the sensitivity of the number of stages to errors in reflux rate. Near minimum reflux or a pinched condition, minor changes (equivalent to typical flow meter errors)... 

A simple method (22) which rigorously calculates minimum reflux without convergence problems is extrapolation of the reflux-stages plot (Fig. 3.8). Simulation runs are performed at different numbers of stages, keeping the material balance, product compositions, and Ng/N constant, while letting reflux rate vary. For each run, t e number of stipes is plotted s ainst reflux, and the curve is extrapolated asymptotically to an infinite number of stages. 

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