How to use Fractionation still
First, use the barrel of coal-tar with the Fractionation still.
Then, click the right side of the valve to the right twice; the tar should be flowing at maximum. One should see the 'pressure' indicator on the left rise. When the indicator reaches the green region, click the right side of the valve on the left once to let out pressure. This should stall the indicator on the left. Now you can start adding coal (you need 6-8 per barrel). This will raise the indicator on the right. If the heat gauge is below the green region, add more coal. If it's above, don't add coal until it gets lower. The bar on the bottom will start turning green, when it's full you can stop adding coal and close the window on the top right to get your barrel of naphtha.