how is the temperature of the plates in the rectifier column mantained?

[perfection]

Each sieve plate in the rectifier column of a two column (continuous) still is cooler than the plate below it with the spirit plate.  If i understood that correctly, please share with me HOW is the temperature (or temperature difference) of these plates maintained ?

Are the temperature of the bubble plates in the analyser (stripping) column maintained as well?  If so is it done in the same way as those for the plates in the rectifier column?

[SlickFloss]

Columns are ran at energy equilibrium....constant steam column loses energy as it travels away from source of column and refluxes out

[meerkat]

The most important point to understand regarding the control of tray temperatures is that it is not the tray temperature that you really want to control. The tray temperature is just a consequence of the composition on the tray, and it is the composition that you are really trying to control. A liquid with a higher ABV boils at a lower temperature than a liquid with a lower ABV, so the temperature is just an indication of the composition (i.e. ABV).

People will say "Of course I can control the temperature - if I put more steam into the boiler the tray temperatures all increase". That is true, but it helps if you understand why the tray temperatures increase. The boiler is the part of the column where you will find the lowest ABV. If you put more steam into the boiler you force this low ABV material up the column and dilute the ABV on all of the trays. And spirit with a lower ABV boils at a higher temperature and so the higher temperatures observed only mean that the ABV on each of the trays has decreased.

The opposite effect is when you increase the reflux. Now you are taking the highest ABV material in the column (i.e. the top product) and pouring it down the column increasing the ABV on each plate. Increased ABV means it boils at a lower temperature and we see the tray temperatures decrease when we increase reflux. It's tempting to think that the tray temperatures have decreased because we have added cold reflux, but that is not the true cause. It is the higher ABV resulting in lower boiling points on each of the trays.

It is difficult to measure ABV in real time so we use the tray temperature as a proxy for the ABV. Because we are measuring temperatures all the time we come to think of the temperature as the variable that we are controlling, but remember it is actually the ABV that you are controlling and the temperature is just an alias for ABV.

[perfection]

Thanks meerkat - that makes total sense and so lucidly explained...

AS i now understand the mechanics, apart from the steam being injected into the analyser (stripper) column/section there is no other source of heating either overall or the individual plate level?

[Alex_Sor]

Well said, meerkat!

The strength of alcohol is measured indirectly, namely by the temperature of the plate (tray).

There are graphs and tables of correspondence "temperature == strength of alcohol".

https://alcoprof.ru/wp-content/uploads/2020/09/faq-spirt-soderzhanie-spirta-v-parah-i-zhidkosti2.jpg

The horizontal axis is the alcohol content of the liquid, the vertical axis is that of the vapor, expressed as a percentage by mass. A is the azeotrope point, after which it makes no sense to continue rectification, alcohol will not become purer.

Alcohol content of the liquid
Calculating the alcohol content of a liquid during distillation is as difficult as calculating the alcohol content of steam.
Experts suggest using the following formula for calculations:

The strength of alcohol %=17,26-Ti*(18,32-Ti*(7,81-Ti*(1,77-Ti*(4,81-Ti*(2,95+Ti*(1,43-Ti *(0,8+0,05*Ti)))))))

Ti=(t-89):6,49, 

where "t" is the temperature in degrees Celsius. ( temperature anywhere (where alcohol-containing liquid boils) in degrees Celsius. )

(

The strength of alcohol % = 16,75 - 19,05 * Ti + 12,64 * Ti ^ 2 - 3,69 * Ti ^ 3 - 0.38 * Ti ^ 4
Ti = ( t[degrees_C] - 89,03 ) / 6,54

)

 

Code for VisualBasic:

Dim temper As Double
Private Sub Form_Load()
Text1.Text = 84
ENTER1
txtInput_GotFocus1
End Sub
Private Sub Text1_Change()
    If Val(Text1.Text) < 78.15 Or Val(Text1.Text) > 100 Then
        Text3.Text = "Bad..."
    Else
        ENTER1
    End If
End Sub
Private Sub ENTER1()
    temper = Val(Text1.Text)
    Text3.Text = Round((17.26 - 18.32 * ((temper - 89) / 6.49) + 7.81 * ((temper - 89) / 6.49) ^ 2 - 1.77 * ((temper - 89) / 6.49) ^ 3 + 4.81 * ((temper - 89) / 6.49) ^ 4 - 2.95 * ((temper - 89) / 6.49) ^ 5 - 1.43 * ((temper - 89) / 6.49) ^ 6 + 0.8 * ((temper - 89) / 6.49) ^ 7 + 0.05 * ((temper - 89) / 6.49) ^ 8), 1)
End Sub
Private Sub txtInput_GotFocus1()
    Text1.SelStart = 0
    Text1.SelLength = 4
End Sub

 

And further .
With a sharp increase in the extraction of alcohol from the column, when the temperature rises from below the cube, the spread of this heat up the surface of the pipe exceeds the temperature rise inside by about 10 centimeters.
And vice versa, when the extraction of alcohol stops, when the volume and strength of the return (reflux) increase, the decrease in temperature from top to bottom along the surface of the pipe outstrips its decrease inside by about 10 cm.
(experiments were carried out for columns 2-4")
This effect is very useful when controlling the temperature extraction at the bottom of the nozzle (alcohol extraction).
The temperature on the surface at a level of 20 cm from the level of the lid of the tank (cube) jumps before the temperature jumps in the nozzle from which alcohol is removed.
And it goes down earlier.