F 1 Yo

This equation shows that when the concentration of the volatile component in the boiler is very low, producing vapor with a very small Yo, F needs to be very close to 1 in order to achieve our aim (which means that you'd get hardly any product D). The higher the concentration in the boiler, the smaller F can be, meaning you need less reflux.

This is the reason you need reflux in a column in order to get a pure product. A reflux column on its own will provide some of the total reflux required, but not enough to achieve complete separation. In practice, a reflux ratios in the range of 8/10 to 9/10 give good results with a column containing around 80cm of packing.

The simple equation for reflux ratio also shows why it makes sense to do a quick pot distillation before embarking on rectification (fractional distillation). This increases the concentration of volatiles in the boiler, greatly reducing the time needed for rectification. However, it also shows that towards the end of rectification, when the volatile concentration in the boiler is low and you're trying to squeeze the last traces of volatiles out of the mix, you must increase the reflux ratio to maintain quality.

Some important lessons can be learned from these calculations. Remember that they assume that the pressure is constant throughout the column, and that there are constant molal overflow conditions. One of the conditions for constant molal overflow is that heat losses are negligible. So, if you want your still to be as efficient as possible, the column must be insulated very well.

Using packing rather than physical trays or plates, the spacing of the theoretical plates varies with the position in the column, and as the distillation proceeds. You can check this yourself by measuring the temperature at several points in column when it's in equilibrium. Figure 8-10 is a plot of the temperatures we found when distilling a mixture of ethanol and water. It shows that the temperature falls as you go up the column, but that near the top it levels out at a constant value.

The obvious conclusion is that the top section of the column is filled with ethanol vapor at its highest molar concentration, because the temperature is very close to the boiling point of 95% ethanol. You could argue that the whole section is acting as one "plate", but this is probably too simple. This plateau quickly disappeared when we reduced the reflux ratio, and the product started to smell like moonshine.

The calculations we've done using theoretical plates are valid and well proven, and the overall conclusions apply equally well to packing, as long as the packing has an adequate surface/volume ratio. Good packing material is the key to successful separation.

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