Continuous distillation of methanol in two columns

In the double-tower process, the distillation of crude methanol is carried out in two stages; first, the light fractions, primarily dimethyl ether, are removed in the pre-tower. Methanol from which the light fractions have been removed is then fed into the heavy fraction column – the main column – to further separate high-boiling-point heavy fraction impurities, thereby yielding highly pure purified methanol. Since extremely small amounts of low-boiling impurities may sometimes be present at the top of the main tower, pure methanol is extracted at 3 to 7 trays above the tower top. The pre-column reboiler uses indirect heating outside the column; to our knowledge, there is no system in which fluid exits the reboiler and enters the main column.

It needs to be analyzed based on the specific circumstances. In the case of a kettle reboiler, drawing liquid from a position on the reboiler that is far from the inlet makes the reboiler function like a theoretical plate; drawing it from the bottom of the tower does not have this effect, so it is certainly better to draw the liquid from the reboiler. In a thermal siphon reboiler equipped with baffles inside the tower, these baffles, together with the reboiler, also serve a separation function (since liquid is involved in the circulation, the effect achieved is not equivalent to that of one theoretical plate in a kettle-type reboiler); the presence of baffles requires that liquid be drawn out of the tower. For a thermal siphon reboiler without baffles inside the tower, drawing liquid from within the tower and drawing it from the bottom of the reboiler is the same for separation purposes; however, drawing liquid from the bottom of the reboiler presents issues such as the reboiler’s mechanical structure, increased resistance at the liquid inlet of the reboiler, and piping problems, so it is generally located at the bottom of the tower.