Boiling rate effect on the composition of distillate fractions
Posted by     08/10/2013 00:00:00    0 Comments

This article describes the effect that the boiling rate in a batch boiler (i.e. not a continuous still) has on the composition of fractions within the distilling run. The effect can be summarised as follows:

  • When boiling very, very slowly, you would be able to predict (i.e. know) what comes out of the still over time and be able to select fractions that you prefer to have.
  • When boiling fast, you will not know what comes out of the still and have to be happy with what you get.

We can illustrate, and explain, these summarised results as follows:

In the diagrams below, the following is applicable:

  • The height of the bar, indicates a relative "distillate flow rate"  i.e. ml of distillate per minute or liter of distillate per hour.
  • The width of the bar, indicates the relative time for the distillation. (i.e. minutes or hours)

To simplify matters, we make an assumtion that the flow rate remains the same with the same heat inout (i.e. kW energy) over time. This is strickly not true as the flowrate will gradually decrease over time because more energy is required to evaporate low alcohol mashes, than for high alcohol mashes (because the heat of vaporisation for water is much higher than the heat of evaporisation of alcohol).

In the figures below, we also work on a fixed amount of mash to be distilled for all the examples.

For a very, very slow boiling distillation, the following figure will be applicable:

First the heads (most volatile components such as methanol and acetone) will come through, then the hearts (ethanol) and then the tails (heavier alcohols/ aromatic components). These main fractions are indicated with the colour bands.

1. SLOW BOILING:

As indicated above: With a very, very slow rate of boiling, little vapour is formed and as a result, little vapour can be condensed to become distillate and this gives a relative small distillation flow rate. The result thereof is that the distillation runs over a relative long period of time. 

However, we can see that the heads, hearts and tails are fairly easy to distinguish, as well as various fractions/components within the tails.(indicated by the pink, red, orange and yellow "fractions" within the tails)

An intuitive conclusion might be that we would be able to reduce the distillation time in half by doubling the heat input. This intuitive conclusion is hundred percent correct. Double the heat input WILL half the distillation time (if we ignore slightly higher heat losses for the moment).

However, something else also changes which is not necessarily intuitive.

In fig. 2 hereunder, note that the distinction between the fractions (colour bands) become more difficult and that the fractions at a specific point, changes in composistion.

As fig 2,3,4 and 5 hereunder show, the vertical lines between heads, hearts and tails (and the various fractions within the tails) changes from perfectly vertical, to slightly slanted, to very slanted and eventually to horisontal in figure 5.

During very, very slow boiling (figure above) a fraction will be fairly constant in composistion. In fact, if we look at a very small fraction, it will only contain one very specific component ("type of alcohol") of the distillation. In fig 2, 3, 4 and 5 hereafter, the composition of even a small fraction, progressively becomes more and more complex and in fig 5, even the smallest fraction will contain all the components found in a mash.

2. A LITTLE FASTER BOILING:

 

 

3. EVEN FASTER BOILING:

 

4. FAST BOILING:

5. VERY, VERY FAST BOILING:

The figures above illustrate the effects of the boiing rate (i.e. heat input) on the composition of fractions from the resulting distillate.

1. The more heat input, the faster the boil

2. The faster the boil, the higher the rate of condensation (assuming off-course the condensor is capable of cooling all the vapour)

3. The higer the rate of distillate flow (i.e. production rate), the shorter amount of time required to distil a given amount of mash/wsh.

4. The more heat input/ the faster mash boils, the less distinct fractions become.

The implication of all this is that we can make clearer cuts while doing a batch distillation and decide which fractions we would like to retain. On the other hand, if we boil fast, we cannot differentiate between the various cractions well and "just have to be happy with what comes out".

Note: From the start of a run, to the end of the distillation run, the temperature will gradually increase over time as the composition of the mash changes over time. Initially, with a relative high alcohol percentage, the bilig point will be lower. As the alcohol content becomes less, the boilig temerature rises.

If you correlate the temperatures with the various fractions, you would be able to work on temperatures alone during consecutive distilling runs (for that particular kind of mash at the same ambient airpressure/altitude above sealevel)

Once temperatures have been correlated with the fractions it is possible to compile a distilling heat input profile to boil SLOW when necessary and faster to save time without gettng the fractions "mixed up".

A simple distilling profile might look like this:

Start with a slow boil until the heads have been cleanly separated. Then double up on the boiling rate while ethanol (the heart) comes off. After the hearts we have to discard the "tail fraction 1" but can maintain the higher boiling rate. However, to clearly capture the "Tail fraction 2", we again have to lower the boiling rate to clearly capture it. After the "Tails fraction 2" has been captured, we can actually stop distilling because the rest of the tails may not be wanted (in this particular example) and it is needless to continue.

Following the above reasoning, it is clear that we can cut out all congeners, with almost no ethanol left in it. This also implies that it is needless to take the feints (unused heads and tails) and add it to future washes to extract more ethanol from it IF it boiled very slow.

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