Defined stripping section

The mass balance relationships for the feed plate, the plates in the stripping section, of the column and for the reboiler must, however, be modified, owing to the continuous feed to the column and the continuous withdrawal of bottom product from the reboiler. The feed is defined by its mass flow rate, F, its composition xp and the thermal quality or q-factor, q. The column bottom product is defined by its mass flow rate, W, and composition, xw and is controlled to maintain constant liquid level in the reboiler. 

The rectifying or stripping section of a column must operate somewhere between total reflux and minimum reflux conditions. The range of feasible operation of a column section can thus be defined for a given product composition. It can be seen in Figure 12.19 that these section profiles are bounded for a stage column by the distillation line and the pinch point curve. As noted previously, the pinch point curve provides a minimum reflux boundary for both staged and packed columns,... 

Formulas for the numbers of trays in the enriching and stripping sections at operating reflux also are due to Underwood (Trans. Inst. Chem. Eng. 10, 112-152, 1932). For above the feed, these groups of terms are defined ... 

In each case the point defined is the intersection of the line with the 45° diagonal line. The slopes and intersection points of each of these lines are summarized in Table 2.3, In addition, it has been shown that the rectifying section component balance line and the stripping section component balance line meet on the q-line,... 

K s = stripping section partition ratio, defined as equilibrium concentration of product solute in extraction solvent divided by that in wash solvent (Bancroft coordinates)... 

Columns with a condenser and an external stripping stream include the usual rectifying section between the condenser and the main feed as well as the stripping section between the two feeds. A fixed-configuration, fixed-pressure column of this kind has two independent operating parameters or two degrees of freedom the condenser duty and the stripping stream flow rate. If the two parameters are variable, two process specifications would be required to define the column performance, such as the recovery of one component and the rejection of another in one of the products. 

All five of the feed types can be characterized by the use of a single factor, denoted by q and defined as the moles of liquid flow in the stripping section that result from the introduction of each mole of feed. Then q has the following numerical limits for the various conditions ... 

Equation 3.21 is in fact equivalent to the standard definition of a reflux ration used throughout distillation literature. It is not usually common practice to refer to a reflux ratio for a stripping section, but with the general definition it can be defined by incorporating Equation 3.12... 

The liquid and vapor flow rates in the stripping section (Ls and Vs) can be calculated if the thermal condition of the feed is known. Since the temperature, pressure, and composition of the feed are given, the fraction of the feed that is liquid can be calculated from an isothermal flash calculation. This fraction is defined as the variable q" Knowing q, the liquid and vapor flow rates in the stripping section can be calculated. If the feed is saturated liquid, is 1. If the feed is saturated vapor, q is 0. 

Next, we need to define the feed tray since from that point, we need to change the operation line from that of the rectifying section to that of the stripping section. We call it inter. We compute the transition point defining a function as the cut between the two lines cut function. Alternatively, we can directly provide the solution algebraically. For an example like this, we present the next case of study. 

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