Distillate from the condenser or overhead product first comes to an overhead reservoir or overhead drum. The overhead reservoir is important for distillation equipment and process design because of the following unique purposes. First, the overhead drum needs to store a certain level of liquid to keep vapour from escaping into the system. Second, the liquid level is commonly used as a process input to control the overhead flow rate so that the condenser is not getting flooded or dried out. As well, the overhead drum sometimes serves as a decanter for immiscible overhead product mixtures. Note that there is a differential pressure transducer measuring the liquid level in the reservoir for control purpose in this distillation column.
The overhead product flow controlled by the level controller is then split into a reflux stream and a top product stream. The reflux ratio, a flow rate ratio of the reflux stream to the top product stream, is controlled by a flow controller for the reflux stream. Since the overhead flow controlled by the level controller is relatively constant under controlled operation condition, for example, boil-up rate, increasing the reflux flow rate on the flow controller increases the operation reflux ratio. A precise operation reflux ratio needs to be set up through trial and error.
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