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020438 Properly Employ Overhead Conden

020438 Properly Employ Overhead Conden - Heat Transfer...

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38 www.cepmagazine.org February 2004 CEP Heat Transfer V acuum 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 compo- nents 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 up- stream of the vacuum source to reduce the size and cost of the vacuum system and recover process materi- al. 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 in- clude the cooling medium and its temperature, the desir- ability 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. Pressure Vacuum can be classified as high, medium or low, where “high vacuum” corresponds to the lowest abso- lute pressure: low vacuum 760–100 torr; medium In a vacuum distillation system, the condenser is an important determinant of overall performance. Follow these guidelines for specifying, selecting and designing overhead vacuum condensers. Properly Employ Overhead Condensers for Vacuum Columns David Greene Consultant George J. Vago Consultant Typical overhead condenser installed on a vacuum distillation column.
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CEP February 2004 www.cepmagazine.org 39 Heat Transfer vacuum 100–10 torr; and high vacuum 10–1 torr. The costs of producing vacuum increase exponentially with the level of vacuum. Once the level of vacuum is established by evaluat- ing the process requirements against cost and perfor- mance, the system can be specified. The first decision is how to allocate the pressure drop between the process user and the vacuum system and how to split this pres- sure drop between the piping and condenser. A rule-of- thumb is to allocate about 10% of the absolute pressure to the pressure drop in the piping and condenser. Be- cause condensation increases with pressure, the pres- sure loss between the process equipment and condenser should be minimized. In the case of medium- and high- vacuum systems, the preferred arrangement is to inte- grally connect the process equipment and condenser.
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020438 Properly Employ Overhead Conden - Heat Transfer...

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