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The intersection of the operating lines It is seen from the example shown in Figure 11.15 in which the feed enters as liquid at its boiling point that the two operating lines intersect at a point having an X-coordinate of x . The locus of the point of intersection of the operating lines is of considerable importance since, as will be seen, it is dependent on the temperature and physical condition of the feed. If the two operating lines intersect at a point with coordinates (xq,yq), then from...

Binary Systems

In previous sections, the case of constant molar latent heat has been considered with no heat of mixing, and hence a constant molar rate of reflux in the column. These simplifying assumptions are extremely useful in that they enable a simple geometrical method to be used for finding the change in concentration on the plates and, whilst they are rarely entirely true in industrial conditions, they often provide a convenient start for design purposes. For a non-ideal system, where the molar latent...

Ye yn1

Simple Distillation Mole Fraction

Where ye is the composition of the vapour that would be in equilibrium with the liquid of composition xn actually leaving the plate. This equation gives the efficiency in vapour terms, although if the concentrations in the liquid streams are used then the plate efficiency Emi is given by Figure 11.55. Compositions of liquid and vapour streams from plates Figure 11.55. Compositions of liquid and vapour streams from plates where xe is the composition of the liquid that would be in equilibrium...

JtkJ

Toluene Octane Mixture

L 1_. - - -t-T T I I_I I i i Bottom 2 4 6 8 10 12 14 16 18 20 22 24 Top Liquid on equilibrium-stage number from bottom Figure 11.45. Composition profile for the azeotropic distillation of ethanol and water, with benzene as entrainer Extractive distillation is a method of rectification similar in purpose to azeotropic distillation. To a binary mixture which is difficult or impossible to separate by ordinary means, a third component, termed a solvent, is added which alters the relative volatility...

Packed Columns For Distillation

Packed Column Distillation

In bubble cap and perforated plate columns, a large interfacial area between the rising vapour and the reflux is obtained by causing the vapour to bubble through the liquid. An alternative arrangement, which also provides the necessary large interfacial area for diffusion, is the packed column, in which the cylindrical shell of the column is filled with some form of packing. A common arrangement for distillation is as indicated in Figure 11.58, where the packing may consist of rings, saddles,...

Plate Columns

Turbogrid Tray

Distillation may be carried out in plate columns in which each plate constitutes a single stage, or in packed columns where mass transfer is between a vapour and liquid in continuous countercurrent flow. Plate columns are now considered, and packed columns are discussed in Section 11.11. The number of theoretical stages required to effect a required separation, and the corresponding rates for the liquid and vapour phases, may be determined by the procedures described previously. in order to...

Steam Distillation

Steam Distillation

Where a material to be distilled has a high boiling point, and particularly where decomposition might occur if direct distillation is employed, the process of steam distillation may be used. Steam is passed directly into the liquid in the still and the solubility of the steam in the liquid must be very low. Steam distillation is perhaps the most common example of differential distillation. Two cases may be considered. The steam may be superheated and so provide sufficient heat to vaporise the...

Azeotropic And Extractive Distillation

Atmospheric Distillation

In the systems considered previously, the vapour becomes steadily richer in the more volatile component on successive plates. There are two types of mixture where this steady increase in the concentration of the more volatile component, either does not take place, or it takes place so slowly that an uneconomic number of plates is required. If, for example, a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 per cent by mass, when...

Y A and yB

Margules Equation Activity Coefficient

The liquid phase activity coefficients y1 and y2 depend upon temperature, pressure and concentration. Typical values taken from Perry's Chemical Engineers' Handbook 14 are shown in Figure 11.8 for the systems n-propanol-water and acetone-chloroform. In the former, the activity coefficients are considered positive, that is greater than unity, whilst in the latter, they are fractional so that the logarithms of the values are negative. In both cases, y approaches unity as the liquid concentration...

P

The vapour pressures of -heptane and toluene at 373 K are 106 and 73.7 kN m2 respectively. What are the mole fractions of -heptane in the vapour and in the liquid phase at 373 K if the total pressure is 101.3 kN m2 At 373 K, PA 106 kN m2 and P 73.7 kN m2 xA P - P B I P P B --- 0.856 106 x 0.856 Ja P BxB P lQl 3 0.896 Equilibrium data usually have to be determined by tedious laboratory methods. Proposals have been made which enable the complete diagram to be deduced with reasonable accuracy from...

Vapourliquid Equilibrium

Atmospheric Distillation Column

The composition of the vapour in equilibrium with a liquid of given composition is determined experimentally using an equilibrium still. The results are conveniently shown on a temperature-composition diagram as shown in Figure 11.3. In the normal case shown in Figure 11.3a, the curve ABC shows the composition of the liquid which boils at any Figure 11.1. Separation of a binary mixture Figure 11.1. Separation of a binary mixture Figure 11.2. Multicomponent separation Figure 11.2. Multicomponent...

Enthalpy Composition Diagram

Ponchon Savarit Method Examples

Equation 11.92 represents any operating line below the feed plate, and it shows that all such lines pass through a common pole M of coordinates xw and H'w. As with the rectifying section, a stream M may be defined by mass Lm 1 Vm, composition xw and enthalpy H'w. Thus It therefore follows that phases F, M, and N are on a straight line on the H x chart, as shown in Figures 11.27 and 11.28. 11.5.2. Determination of the number of plates on the H x diagram The determination of the number of plates...

Optimum Reflux Ratio

Reflux Trays Reboiler Cost

Using the graphical construction shown in Figure 11.18, with yy 0.61, the value of Rm is The cost of a distillation unit includes the capital cost of the column, determined largely by the number and diameter of the plates, and the operating costs, determined by the steam and cooling water requirements. The depreciation charges may be taken as a percentage of the capital cost, and the two together taken as the overall charges. The steam required will be proportional to Vm, which may be taken as...

Batch Distillation

Batch Distillation Reflux

In the previous sections conditions have been considered in which there has been a continuous feed to the still and a continuous withdrawal of products from the top and bottom. In many instances processes are carried out in batches, and it is more convenient to distil each batch separately. In these cases the whole of a batch is run into the boiler of the still and, on heating, the vapour is passed into a fractionation column, as shown in Figure 11.33. As with continuous distillation, the...

Importance Of Reflux Ratio

Line Distillation Column

Heat to vaporise 1 mole of feed molar latent heat of the feed A material balance of the more volatile component over the whole column gives This equation is commonly known as the equation of the q-line. If xq x , then yq x . Thus, the point of intersection of the two operating lines lies on the straight line of slope q q 1 passing through the point x , x . When yq 0, xq x q. The line may thus be drawn through two easily determined points. From the definition of q, it follows that the slope of...

Benzene Toluene Bottom Product With No More

Benzene Toluene Equilibrium Line

The values of x and y required must satisfy, not only the equation, but also the appropriate equilibrium data. Thus these values may be determined graphically using an x y diagram as shown in Figure 11.9. Figure 11.9. Equilibrium data for benzene-toluene for Example 11.6 Figure 11.9. Equilibrium data for benzene-toluene for Example 11.6 In practice, the quantity vaporised is not fixed directly but it depends upon the enthalpy of the hot incoming feed and the...