systems of columns, the column modular method is generally recommended over the system modular method except for the possible exception of pipestills. Since, however, each sidestripper is small relative to the main column, it is perhaps easier to treat this particular system by the column modular method rather than the system modular method. To illustrate the system modular method, an abbreviated development of the pipestill problem, Example 4-11, is given.

The pipestill problem solved herein was originally solved by Cecchetti et al.8 by use of the original 0 method of convergence which is described in Chap. 3. This problem is based on data from field tests which were made on the pipestill shown in Fig. 4-6. The 6 method for distillation columns may fail to converge for some absorber-type problems, such as the pipestill. The pipestill is classified as an absorber-type problem because the main column has a condenser but no reboiler; the first sidestripper has a reboiler but no condenser; and all of the remaining strippers are of the conventional type.

For the pipestill shown in Fig. 4-6, Cecchetti et al.8 found a theoretical analogue column by trial. The theoretical analogue column is defined as thai column having perfect plates which gives calculated results that are in good agreement with field test results for the pipestill. The theoretical analogue column shown in Fig. 4-7 was proposed by Hess et al.13 This is essentially the same as the analogue proposed by Cecchitti et al.

Figure 4-6 Actual stage numbers in the main column and the side strippers of the pipestill. [Hess et al, Hydrocarbon Process.. 56(5); 241 (1977), by courtesy Hydrocarbon Processing.]

The minor differences between the theoretical analogue shown in Fig. 4-7 and the one used by Cecchetti et al. are a reflection of the different manner in which water was treated. In the present analysis, which follows that of Hess et al.,13 water was regarded as being distributed between the vapor and liquid phases on stages 2 through 37. whereas Cecchetti et al. regarded water to be in the vapor phase alone on these stages. On stage 1 (the accumulator), two immiscible liquid phases (a water and a hydrocarbon phase) are assumed to be in equilibrium with the vapor phase. In Fig. 4-7, the withdrawal rate of the liquid water phase is W0, and the withdrawal rate of the liquid hydrocarbon phase is Wx.

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