(15h) On Instruction of McCabe and Thiele Method for Binary Distillation

I instruct CHEG 3043, Equilibrium Staged Separations at Prairie View A & M Univeristy, Prairie View, TX.  Since Treybal (1980) a number of developments in the field have come about. Desktop computers with spreadsheets and software such as Aspenplus and RADFRAC simulation package has changed the emphasis from a set of procedures to methods that are more versatile. The design specifications in Treybal (1980) is not accurate. For example when product split, P_split, W/D ratio and distillate compositions are given teh residue composition can be calculated. W/D ratio can be shown from mass balance to be given in terms of the feed, distillate and residue compositions. So once the residue, distillate compositions and W/D are known the feed composition can be calcualted. It cannot be specified as indicated in Treybal (1980).  I go over ⁶C₃ = 20 different problem types.  The emphasis on calculated of number of ideal stages, N_p needed to achieve a desired level of separation is in excess. Detailied methods are provided for the 20 different problem types. For example;

Given Product Split, P_split (x_DD/x_wW), Reboil Ratio, B and Reflux Ratio R   calculate Residue  composition, x_W, N_p, W/D

The 4 unknowns are the coordinates of the intersection point, x_int, y_int and the product compositions, i.e., the distillate composition, x_D and residue composition, x_w.  4 equations can be generated from the operating line for rectification, operating line for stripping, feed line and the given product split, P_split information. Thus,

                                                                                                          (2.77)

Eq. (2.77) along with Eqs. (2.70-2.72) form the 4 simultaneous equations that can be used to solve for the 4 unknowns.   Once the unknowns are solved for the operating lines of rectification, stripping can be constructed in addition to the feed line that is specified.  The W/D can also be calculated from the now available product compositions

The 4 unknowns are the coordinates of the intersection point, x_int, y_int and the product compositions, i.e., the distillate composition, x_D and residue composition, x_w.  4 equations can be generated from the operating line for rectification, operating line for stripping, feed line and the given product split, P_split information. Thus,

                                                                                                          (2.77)

Eq. (2.77) along with Eqs. (2.70-2.72) form the 4 simultaneous equations that can be used to solve for the 4 unknowns.   Once the unknowns are solved for the operating lines of rectification, stripping can be constructed in addition to the feed line that is specified.  The W/D can also be calculated from the now available product compositions.

        During steam distillation, the number of stages N_p can be more than the minimum number of stages, N_m.  This is because there in no vapor return. Useof N_m is not recommended  when steam is used.