In the Classroom
Applications and Analogies
An Analogy between Fractional Distillation and Separating Physically Fit and Physically Less Fit Persons Doble Mukesh† ICI India R&T Centre, PO Box 155, Thane Belapur Rd., Thane 400601, India;
[email protected] Distillation plays a crucial role in chemical and petrochemical industries, and frequently the entire economy of a process is governed by the efficiency of this unit operation. Distillation is carried out routinely in chemistry laboratories in both schools and colleges. The design of a distillation column is based on (i) several thermodynamic principles, (ii) empirical relationships, and (iii) experimental data (1). In this paper some of the concepts and principles of the distillation process are described in terms of an analogy with physically fit and physically less fit persons. Sorel and Husbrand (2) developed the basic equations upon which all modern distillation calculations are based. Several papers published in this Journal have dealt with the history and applications of fractional distillation and can be referred to for classroom teaching (3–7 ).
A mixture of two liquids at a given temperature vaporizes and reaches an equilibrium with the vapor; the vapor will also contain both the components. The vapor will be richer in the lower-boiling component and the liquid will be richer in the higher-boiling one. For example, if we take a mixture of water and alcohol in a dish and leave it exposed to air, after a few hours a major portion of the alcohol and a small amount of water will have evaporated—leaving behind a liquid mixture that is predominantly water, because alcohol has a higher vapor pressure (lower boiling point) than water. This observation remains unchanged irrespective of the ambient temperature.
Separation of Two Liquids—Separation of Physically Fit and Physically Less Fit Persons
The necessary height of a packed column or the number of trays in a distillation column depends on the difference in boiling points of the two liquids. Only a short column or a relatively small number of trays is necessary if the difference in boiling points is large, but a tall column or a large number of trays is required if the difference in boiling points is small. For example, 30 to 40 trays may be required if the difference in boiling points is of the order of 3 °C, whereas 5 trays may be sufficient if the difference in boiling points is more than 30 °C. Increasing the number of plates or the column height increases capital cost. If the difference in physical fitness between the persons in the group is very large, then the physically less fit persons will be hardly able to climb one or two floors. A short building with a few floors is sufficient for separation; the less fit stay down and the fit go up. On the contrary, if the difference in fitness between the persons in the group is small, then the relatively less fit persons will be able to climb several floors before they feel tired and stop further ascent. Therefore, in such a situation, a building with more floors is required to separate the two groups.
In a fractional distillation, separation of two or more liquids is achieved by passing the vapor mixture upward through a column packed with inert packing material, or a column consisting of a series of trays or plates. The former is called a packed column and the latter a tray or plate distillation column. The vapor collected at the top of the column is condensed; part is removed as liquid product and the other part flows down the column. The vapor moving upward comes in contact with the liquid flowing downward and gets enriched with the low-boiling component, while the liquid flowing downward gets enriched with the higher-boiling component. The vapor reaching the top of the column will be rich in the low-boiling component, whereas the liquid at the bottom of the column will be rich in the higher-boiling component. The packing material or trays provided inside the distillation column improve the contact between the liquid and the vapor. Imagine a group of people consisting of physically fit and physically less fit persons. The physically fit will be able to climb stairs and go to the first or second floor with more ease than the physically less fit. If we ask the group to climb the stairs, we would find the less physically fit on the ground floor and the physically fit on the first or second floor. Therefore difference in fitness is the factor that determines the separation of the group, analogous to the distillation process, in which the separation of two liquids is achieved because of the difference in their boiling points. † Present address: GE India Technology Centre, Units 1&2, Innovator Building, International Technology Park, Whitefield Rd., Bangalore 560066, India.
Number of Stages in a Distillation Column—Height of the Building
Reflux Ratio—Meeting of the Two Groups on the Stairs Part of the vapor that is collected and condensed at the top of the distillation column is made to flow downward so that it comes in contact with the vapor rising up. The lowerboiling component in the liquid flowing downward will vaporize and join the rising vapor while the higher-boiling component will continue flowing downward, thereby leading to better separation. The ratio of the quantity of liquid returned to the column to the quantity of liquid removed as product is called the reflux ratio. Increasing the reflux ratio improves the separation of the two liquids, but increases the
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education
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In the Classroom
distillation time and hence the energy and operating costs. If the difference in physical fitness between persons in the group is small, then the best way to differentiate various fitness levels is to send the group that has reached the top of the column back to the ground and ask them to climb the stairs again, thereby achieving a separation between “physically fit boys” and “physically fit men”. Separation into fitness levels could be achieved by repeating this process several times. This is analogous to refluxing in the fractional distillation column. Flooding in the Distillation Column— Overcrowding at the Stairs The vapor velocity at which the flow of liquid down the distillation column is prevented is known as the flooding velocity. During flooding a liquid layer builds up at the top of the column and the pressure inside the column also increases. Flooding can be overcome either during the operation stage by decreasing the vapor distillation rate, or during the design stage by increasing the column diameter. In our analogy, if the rising group is large, it may stop the falling group, thereby causing a standstill at the stairs similar to flooding in distillation. Such a situation can be prevented by either reducing the size of the rising group or increasing the width of the stairs.
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Conclusion This short paper describes several aspects of fractional distillation—column height, reflux ratio, and flooding—with the help of an analogy using the physical fitness of persons. It should be noted that the boiling point of a liquid depends on several factors including molecular weight, amount of branching, polarity, and ability to form hydrogen bonds. A group of persons will have a distribution of fitness levels; hence it may not be possible to exactly divide it into two “physically fit” and “physically less fit” groups. Literature Cited 1. McCabe, W. L.; Smith, J. C. Unit Operations of Chemical Engineering, 2nd ed.; McGraw-Hill: New York, 1967; pp 483–608 (international science edition). 2. Winkle, M. V. Distillation; McGraw-Hill: New York, 1967. 3. Goedhart, M. J.; van Keulen, H.; Mulder, T. M.; Verdonk, A. H.; de Vos, W. J. Chem. Educ. 1998, 75, 378–381. 4. Mashava, P. M.; Alonge, I. E.; Mlenga, F. J. Chem. Educ. 1989, 66, 1042. 5. Liebmann, A. J. J. Chem. Educ. 1956, 33, 166. 6. Buck, A. C. J. Chem. Educ. 1944, 21, 475. 7. McAdams, W. H. J. Chem. Educ. 1926, 3, 157.
Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
