Fractional Distillation

the common use of the term fractional distillation refers to a destillation operation in which a fractionating column has been inserted between the boiler and the vapor takeoff to the condenser.

the effect of this column is to give in a single distillation a separation equivalent to several successive simple distillations.

Fractionating Columns.

the easiest way to understand the principles by which fractionating columns give their superior separations is to consider first a rather special type of column known as a bubble plate column.

the essential features of a bubble plate column, illustrated in figure consist of (1) a series of horizontal plates, A, which support a layer of distillate; (2) capped risers, B, through which the distilling vapors ascend; and (3) overflow pipes, C, which return any excess distillate to the next lower plate. At the beginning of a distillation, the vapors coming up from the boiler pass through the first riser and are deflected downward by the cap onto the first plate, where they are condensed.

As simple vaporization and condensation continue, the rising vapors are forced to bubble through the liquid on the plate. The liquid level rises to the top of the overflow tube and then flows downward to the boiler. The liquid on. the first plate corresponds to the first fraction in a simple distillation- it is enriched inthe lower-boiling component. It follows that the temperature of the vapor bubbling through the liquid is above the boiling point the liquid on the plate; through heat exchange, the liquid its brought to its boiling point and its vapor rises to the second plate where the same processes are repeated. As the distillation continues, each plate becomes filled with a layer of liquid whose composition is that of the vapor rising from the next lower plate. Under ideal circumstances, each plate achieves an increment of separation equivalent to one simple distillation.

The overflow tubes serve a more important function than just acting as returns for excess condensate. Since the vapor leaving any plate is richer in the lower-boiling component than the vapor entering the plate, the higher-boiling materials tend to accumulate on the plate. The overflow returns this higher-boiling material to the lower plate, so that an equilibrium balance of low-boiling to high-boiling components is maintained. in effect, vapor and condensate are passing in opposite directions through the column, the more votatile component ascends the column in the vapor stream, while the less votaltile components discends in the condensate stream.

The counterflow is essential for effective separation in a fractionating column.

The separation process can be understood more clearly by reference to a liquid-vapor compotition diagram such as that shown in figure 5.9 for carbon tetrachloride (bp 77⁰) and toluene (bp 111⁰). A liquid mixture containing 50 mole-percent carbon teterachloride (point A on the liquid line) is in equilibrium with vapor containing 71-mole-percent carbon tetrachloride (point A’ on the vapor curve). If liquid with composition A is partially vaporized and the vapor with compotition A’ condensed completely on the first bubble plate, the condensed is represented by B (on the liquid line). Repetition of the vaporization condensation process with liquid B yields a new distillate, C, containing 85% carbon tetrachloride, which condenses on the second bubble plate. Each successive bubble plate achieves an additional increment of separation.

Bubble plate coulumns have the drawback of requiring large samples for effective operation, and a substantial portion of material is condensed on the plates (holdup). To overcome these disadvantages, small-scale laboratory fractionations are usually done with cylindrical colums packed with materials having large surface area (glass beads or helices, small section of twisted metal, carborundum chips, and the like). The principles of operation of packed columns are quite similar to those of the bubble plate column. The layers of packing material, like the bubble plates, serve as support for films of condensate; vapor passing through the layers is enriched in the lower-boiling component, and the higher-boiling components drip downward to lower layers. Tha packing material provides the thorough mixing of vapor and condensate that is essential for fractionating efficiency.

Relative Efficiency of Fractionating Columns. Since column packings differ widely in efficiency, it is desirable to have a means of comparing their effectiveness for separating mixtures. It is useful to define the enrichment factor α, for two components as the ratio of their effective volatilities (f1/f2). From equation (5.2) and (5.3), α can be expressed as the quotient of the ratio of the mole fractions in the liquid.

(5.4)

A theoretical plate is defined as the unit of separation corresponding to the composition ratio, α, that exists at equilibrium between a liquid mixture and its vapor. This concept may be illustrated by considering a 50:50 –mole-percent mixture of carbon tetrachloride and toluene. The vapor in equilibrium with the liquid (bp 90⁰) contains 71-mole-percent carbon tetrachloride and 29-mole-percent toluene. This amount of enrichment corresponds to one theoretical plate.

In other words, this means that the first drop of distillate is 2.5 times richer in the lower-boiling component. The length of packed column required to obtain this degree of separation in the mixture is known as the height equivalent to a theoretical plate (usually abbreviated HETP). The smaller the value of the HETP, the more efficient the column. Although the exact HETP of any given packing depends on operating factors (diameter of the column, density of packing, rate of distillation,etc.), it is useful to have rough estimate of relative values. Table 5.2 records representative values of HETP for several packings as measured under normal working conditions using student apparatus to separate a benzene-toluene mixture. Also shown in table 5.2 are representative values of the column holdup per plate. Both the HETP and the holup values will vary with the manner of packing and subsequent treatment of the column.

In addition to packed columns, special columns are available that achieve mixing of the ascending vapor and the descending condensate by their special construction. One of the simplest, least expensive, and most widely used is the Vigreux column illustrated in figure 5.14. it is essentially an empty tube with many finger-like indentations that point downward at a 45⁰ angle. The rising vapors condense on the fingers and any excess liquid drips down to lower parts of the column. The film of condensate on each finger equilibrates with the rising vapor. Under normal conditions working the Vigreux column has a relatively low efficiency (high HETP of 10 cm), but its low resistence to vapor flow permits a large throughput (volume of distillate per unit of time) that makes the column well suited to distillation of bulk solvent. Because of its small surface area the column has a low holdup and is sometimes used for preliminary purification of small samples.

The spiral wire column is also common. It consists of a wire wound spirally on a glass rod that is held concentrically within an outer glass tube. Spiral wire columns are slightly more efficiency than columns packed with glass beads (HETP of 2 cm) and have about half the holdup of a packed column capable of the same throughput. Their limitation is throughput, which is essentially fixed (0,5 mL/min maximum), whereas packed columns can be scaled up as needed. Because of their simple construction, spiral wire columns are generally built in the laboratory rather than purchased.

A unique column deserving special mention is the Auto Annular Still. This unit, resembling a spiral wire column, cantain an annular Teflon helix wrapped around a Teflon rod. A motor spins the Teflon helix at high speeds, so that the ascending vapors are effectively mixed with the descending condensate. It is more than 150 theoretical plates with a holdup of less than 0,5 mL and a throughput of 15-60 mL/hr. this remarkable unit unfortunetely costs several thousand dollars. Smaller (and less effective) spinning band columns costing only a few hundred dollars are available for small-scale work.

Separation Efficiency. The total number of theoretical plates, n, present in a column is equal to the height of the packed portion of the column divided by the HETP of the packing material. The composition of the vapor at the top of the column, Y1/Y2, is related to the composition in boiler, X1/X2, in following way.

5.5

The exponent of α is n + 1, rather than n, because in vaporizing the mixture in the boiler an additional enrichment factor is introduced. Although equation 5.5 has theoretical significance, it is more useful to have an expression for the number of theoretical plates required to separate a given mixture. An approximate expression (equation 5.6) as been derived for fractional distillation of 50:50 mixtures, which gives the number of theoretical plates required to achieve a separation such that the first 40% of the material distilled will have an average purity of 95% in the lower-boiling component. Equation (5.6) shows that as the relative volatility, α, approaches unity, the number of theoretical plates required to achieve 95% purity increases sharply (as α = 1, log 10 α = 0 dan 1/log 10 α = ~).

5.6

This relationship becomes still more useful (but also more approximate) if one substitutes for log 10 α an expression involving the difference in boiling points of the two components. Equation (5.7) gives results for ideal mixtures being fractionally distilled under perfect conditions.

5.7

Under practical conditions, discussed in the next section, the number of plates required can double. It is clear that tall,high-platage fractionating columns are required for clean separation of materials boiling a few degrees apart. When the required number of theoretical plates is unavailable, it is necessary to collect a smaller portion of the low-boiling distillate.

Figure 5.10 shows the distillation curves for a 50:50 (by volume) mixture of methanol and water using columns with several different numbers of theoretical plates. The number associated with each curve give the number of column plates, including the one additional plate coming from the enrichment of the vapor leaving the boiler. Thus, the number of plates provided by each of these columns is one less than the indicated total platage number.

Reflux Ratio and Holdup. Equations (5.5) and (5.6) were derived for an ideal fractional distillation where there is equilibrium between the rising and descending counterflowing streams of materials. For this equilibrium to be attained, it is essential that vapor reaching the top of column be condensed and liquid returned to the column (reflux). If a large portion of the vapor reaching the top of column is removed as distillate (takeoff), the equilibrium is seriously disturbed and much lower separation efficiency results. The extreme modes of operation are known as total reflux and total takeoff. Since the first mode yields no distillate and the second gives distillate of much lower purity than is possible with the column, in practice some intermediate ratio of takeoff to reflux is employed. The best practical compromise seems to be to adjust the reflux ratio so that it equals the number of theoretical plates of the column. Higher rates of collecting distillate (lower reflux ratios) give poorer separations; slower rates are overly time-consuming and do not provide significantly better separations.

Another factor that seriously affects separation efficiency is the total amount of liquid and vapor in the column at any instant (holdup). A great drop in separation efficiency occurs if the holdup is more than about 10% of the amount of sample to be distilled.

Under practical conditions of partial takeoff and some holdup, the number of plates required to achieve a given separation can be twice as many as predicted by equation (5.7). if the number of plates available is insufficient, the distillation rate must be slowed to more nearly approach ideal conditions.