(9) Heating and cooling requirementsAssume Heat loss is relatively small, column itself is essentially adiabatic(). Heat effects of the entire unit are confined to the condenser and the reboiler.

Water consumption/Cooling water flow rate If condensate is not subcooled, heat removed in condenser:(V=Va)

*[For enthalpy balances in detail, see pp.672-675]()EXAMPLE 21.2 [(c)(d)]

(10) Minimum number of plates/Total refluxWhen total reflux,Effect of reflux ratio on number of plates:

Total reflux flow

(1) Constructing method: constructing steps on an x-y diagram between compositions xD and xB, using 45line as the operating line for both sections of the column.Nmin is the ideal plates needed, not including a reboiler.(2)Analytic method for ideal mixtures (pp.665-666)

(11) Minimum reflux/Infinite number of platesWhen the intersection of the two operating lines falls on the equilibrium line (point d),Fig.21.19 Minimum reflux ratio

When the intersection of the two operating lines falls on the equilibrium line (point d),=minimum reflux ratioAll actual columns operates at a reflux ratio between the minimum and infinity.

For the normal type of equilibrium curve,

Eq.(21.44) cannot be applied to all systems.For example, ethanol-water system, abnormal equilibrium curve.

In such a situation the minimum reflux ratio can be computed from the slope of the operating line ac that is tangent to the equilibrium curve.

Question: Is it the only way that the minimum reflux ratio must be found by drawing the operating line that is tangent to the equilibrium curve for abnormal equilibrium curve?Factors influencing RDm include: (1)Equilibrium relation (curve); (2)xD, xF, xB; (3)q (Thermal condition parameter of feed). Therefore, it is not the only way for determining RDm by drawing the operating line that is tangent to the equilibrium curve for abnormal equilibrium curve. For example,

(12)Invariant zone/Invariant zone: At minimum reflux ratio, near pinch point, where there is no change in either liquid or vapor concentrations from plate to plate,so

(13)Optimum reflux ratio Effect of Reflux ratio on distillation:

The total cost reaches a minimum at a definite reflux ratio, that is , optimum reflux ratio RDopt. The optimum reflux ratio depends on the cost of energy; it will be closer to RDm when energy costs are relatively high, and farther from RDm when distillation equipment is made of expensive alloys.RDopt=(1.1~2.0)RDm[EXAMPLE 21.3][p.670]

(14)Nearly pure productsWhen either the bottom or overhead product is nearly pure, a single diagram covering the entire range of concentrations is impractical as the steps near x=0 and x=1 become small. Method 1: Auxiliary diagrams on a large scale [calculations are usually done by computer].

Method 2: Using Eq.(20.27) or (20.28). [At each end of the equilibrium curve, both the equilibrium and the operating lines are straight so Eq.(20.27) can be used, and no graphical construction is required][Basis: Raoults law applies to the major component and Henrys law to the minor component at each end of the equilibrium curve. ]?[p.671][EXAMPLE 21.4.]

(15) Some special cases of distillationa side-stream draw off ():a) 3 operating lines; 2 q-lines (feed lines); b) Equations of operating lines: Equations of operating lines of top and bottom sections are the same as that of the rectifying and stripping sections discussed before. The equation of operating line in the middle section of the column has to be derived.

Assuming constant molal overflow in this column. By material balance:

Assuming constant molal overflow in this column, equation for the operating line of middle section is The upper operating line is given by Eq.(21.13)The lower operating line is given by Eq.(21.16)

The operating lines for the 3 sections are shown in the following diagram. Dashed lines are for RDm=(L/D)min.

How to determine the number of ideal plates?

(b) Direct steam heating Live steam is injected directly below the bottom plate.Equation of operating line of the rectifying section is the same as that discussed before.The equation of operating line in the stripping section of the column has changed.

Assuming constant molal overflow in this column. By material balance:

which cases will live steam be used for in distillation?Water soln. Bottom product nearly pure water.