Sunday, 24 May 2015

Objective 2

The Effect of Reflux Ratios on the Performance of the Distillation Column

The performance of a distillation column is determined by many factors, for example:
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feed conditions
  • state of feed
  • composition of feed
  • trace elements that can severely affect the VLE of liquid mixtures
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internal liquid and fluid flow conditions
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state of trays (packings)
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weather conditions

Feed Conditions:
The state of the feed mixture and feed composition affects the operating lines and hence the number of stages required for separation. It also affects the location of feed tray. During operation, if the deviations from design specifications are excessive, then the column may no longer be able handle the separation task. To overcome the problems associated with the feed, some column are designed to have multiple feed points when the feed is expected to containing varying amounts of components.

Reflux Conditions:
As the reflux ratio is increased, the gradient of operating line for the rectification section moves towards a maximum value of 1. Physically, what this means is that more and more liquid that is rich in the more volatile components are being recycled back into the column. Separation then becomes better and thus less trays are needed to achieve the same degree of separation. Minimum trays are required under total reflux conditions, i.e. there is no withdrawal of distillate.

On the other hand, as reflux is decreased, the operating line for the rectification section moves towards the equilibrium line. The ‘pinch’ between operating and equilibrium lines becomes more pronounced and more and more trays are required. This is easy to verify using the McCabe-Thiele method.

The limiting condition occurs at minimum reflux ration, when an infinite number of trays will be required to effect separation. Most columns are designed to operate between 1.2 to 1.5 times the minimum reflux ratio because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty).

Weather Conditions:
Most distillation columns are open to the atmosphere. Although many of the columns are insulated, changing weather conditions can still affect column operation.

Tham, M. (n.d.). Introduction to Distillation. Retrieved May 25, 2015, from  

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