Properly Employ Overhead Condensers for Vacuum Columns

Vacuum distillation may be used to enhance the separation of hydrocarbons because of the improvement in relative volatility at the lower pressure. Vacuum may also be employed to break azeotropes by using two columns in series that operate at different pressures. And, it may be required to reduce the boiling point of the bottom composition so conventional heat-transfer fluids can be used for heating. The improved performance of vacuum column operation must be weighed against the additional capital and operating costs of the vacuum system. Because of these high costs, vacuum operation is normally reserved for situations where the components are either very difficult to separate at higher pressures or where the products would be degraded at elevated temperatures. Once a decision is made to operate under vacuum, it is generally advantageous to provide a condenser upstream of the vacuum source to reduce the size and cost of the vacuum system and recover process material. A vent condenser after the main condenser, possibly using a colder cooling medium, may be used to further reduce the load on the vacuum system. The main process parameters involved in specifying vacuum systems are the absolute pressure and the amount of noncondensables. Other considerations include the cooling medium and its temperature, the desirability of recovering process vapors, and the need to treat either the gaseous or liquid effluent stream, or both. In a batch system, the time necessary to pull the system down to the desired vacuum level is also relevant, and in fact might control the sizing of the vacuum system.