Distillation Temperature

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Temperature control is crucial to the operation of distillation system, correct temperatures are necessary to vaporize the lighter components to the liquid mixture while keeping the heavier component in the vapor to a minimum. If the temperatures in certain point of the system are either too high or too low, acceptable products wouldn’t be produced.

The temperature at the top of the tower should be at or slightly above the boiling temperature of the desired overhead products at the operating pressure of the tower. If the temperature at the top of the tower is higher than it should be, more of the heavier component would vaporize and become part of the overhead product instead of flowing down the tower as a liquid. On the other hand if the temperature at the top of the tower is lower than it should be, less of the lighter component would vaporize. Some of the lighter components would remain as liquid and flow down the tower.

The temperature at the bottom of the tower is also important. The temperature at this section is usually slightly below the boiling point of the heavier component. If the temperature at the bottom of the tower is too high more of the heavier components would vaporize and move up the tower instead of remaining as liquid. If the temperature at the bottom of the tower is too low less of the lighter components would vaporize and move up the tower.

Another place where temperature control is important is at the feed point. Temperature at feed point should be within the boiling range of the mixture. The temperature at this section should be close to the temperature of feed trays. The temperature at feed trays depends on its physical location in the tower. The lower the feed tray is in the tower the higher its temperature would be. If the temperature at feed point is higher than it should be more of the heavier component would vaporize and move up the tower instead of moving down the tower as a liquid. On the other hand if the temperature at feed point is too low, less of the lighter component would vaporize and more lighter components would end up in the bottom of the tower.

The temperature decreases as the material move higher in the tower. The gradual decrease in temperature from the bottom of the distillation tower to the top is called the temperature gradient. The temperature gradient is measured in term of difference between the temperature at the bottom of the tower and the temperature at the top of the tower.

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