Batch distillation is time dependent and requires preparation of time cycles on the Gantt chart so that each task can be estimated in minutes and the overall cycle time to distill off the lights and the product heart cut can be calculated.
Typical tasks on the Gantt chart include pumping time to charge the batch distillation unit (still), heating up the batch, distilling off the methanol, cooling down the batch and pumping time to empty the still. Other things that tie into the Gantt chart are: how long does it take to distill off the lights and how long does it take to distill off the product?
As soon as the still is empty another batch of feed material may be in the holding tank that is ready to be charged into the still. The batch still is then charged. The vacuum pump turned on. The vacuum is drawn down to the desired vacuum and the steam on the jacket is turned on.
During the initial boil up phase of lights vapor, all that is condensed by the overhead condenser is returned to the distillation column and will not be drawn often as lights byproduct. This is called total reflux mode. Of course, there must be a liquid level in the reflux drum in order to start the reflux pump. At this point the column should be at steady state conditions. With a specific rate set on the reflux pump the lights should be removed from the reflux drum on an automatic or manual level control system. Some distillation systems require that the reflux rate change over the cycle time of the distillation unit. When the entire light material and the product has been distilled off of the pot charge, the steam is turned off to the still, the vacuum pump is turned off and nitrogen gas is allowed to flow backwards into the still instead of air. The nitrogen gas is strictly a safety precaution and prevents the buildup of a hydrocarbon/air explosive mixture inside the still. At this point the heavies are pumped down from the still pot, through the run down cooler and to final storage.
Heating and Cooling
In the batch distillation process, there is the heat up phase and the cool down phase with the purification by distillation occurring in between. Heating up the batch can be performed by a steam jacket on the distillation pot, an internal coil or a pump around shell and tube heat exchanger. Likewise, cooling of the batch can occur by one of the same three methods or a rundown cooler can be used on the heavies stream. The best heating and cooling methods to use for each individual project are beyond the scope of this article.