Wednesday, 27 May 2015
Monday, 25 May 2015
Sunday, 24 May 2015
How is petrol for automobiles and kerosene for aircraft engines manufactured? How do we get pure oxygen for use in hospitals? How do you increase alcohol content in fermented beverages*? Distillation is the industrial method of choice for all these products.
When 2 or more molecules in a liquid mixture need separating, their different boiling points are conveniently used as a means of separating the molecules to get pure products. The liquid mixture is vaporized using heat and/or lowering pressure, and subsequently condensed. Separation is achieved due to the difference in vapour pressures of the components at a given temperature. A bi-directional** mass transfer process occurs simultaneously, with the more volatile*** component vaporizing and passing from the liquid to the vapor phase and the less volatile component condensing. The resulting vapour, called the distillate product, is enriched in one component while depleted in the other. The liquid found at the bottom of a distillation column has the highest boiling point, is called the bottom product.
*Fermented Beverages: Examples of fermented beverages consists of wine and beer.
**Bi-directional: Functioning in two different directions.
***Volatile: Easily exaporated at normal temperature.
Figure 1 above shows a typical distillation of crude oil into various grades of fuels for different transportation means. Lighter fractions such petrol is used for automobiles; then comes kerosene for aircraft engines, followed by diesels for automobiles and trains. Heavier fractions are used to make lubricating oils and polishing waxes. This leaves the even more heavier fraction for heating, power generation and ship engines. Lastly, the bitumen residue (bottom product) from crude oil refining is used for road making and roofing.
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:
internal liquid and fluid flow conditions
state of trays (packings)
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.
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).
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 http://lorien.ncl.ac.uk/ming/distil/distilop.htm