Minimum solvent flowrate

As the Amin point moves toward point Vo, the ratio Vb/Ojv+i will increase but it is still the minimum that we use. Since the actual Vq/On+i is a multiple of the minimum, we need to make sure that the actual value will not cause a pinch point anywhere in the column. 

Ai and A2 correspond to two pinch points. Follow the same sequence as for Case 1 to find the mixing points corresponding to Ai and A2. 

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This is shown in Figure 10.1 to be a straight line with slope L/V. If it is assumed that the gas flowrate is fixed, the solvent flowrate can be varied to obtain the minimum solvent flowrate, as shown in Figure 10.2. 

In Figure 10.6a, the operating line for minimum solvent flowrate touches the equilibrium line at xR = 0.1576 and xR = 0.05166. Thus ... 

Figure 5.18 Minimum solvent flowrate calculation water-acetic acid-isopropyl ether, Example 5.3.

Find the minimum solvent flowrate for Example 5.2. Solution ... 

The concentration of solute (A) in the outlet extract stream is limited to a relatively low value because the outlet extract stream is a passing stream with feed On+i. So the maximum concentration of A in Vn will occur when the column is operated at the minimum solvent flowrate. As with distillation, this limitation can be overcome by using... 

Minimum reflux corresponds to the overlap of an operating line and a tie-line (infinite stages at a pinch point). This concept is similar to minimum solvent flowrate for an extraction process without reflux. Total reflux corresponds to the minimum number of stages. Remember that total reflux means that no streams are going into or out of the column, so that F, B, and D are zero, and A = A. ... 

For Example 5.3, illustrating minimum solvent flowrate, show that increasing the solvent flowrate will decrease the required number of equilibrium stages. 

The separation in the extractive column depends on the amount of solvent circulating around the system. Figure 12.4 shows that high solvent flowrates reduce the impiuity of chloroform in the distillate acetone product. For each solvent flowrate, there is a nonmonotonic effect of reflux ratio. To achieve the desired distillate purity of 99.5 mol% acetone, the minimum solvent flowrate is 145kmol/h (solvent-to-feed ratio of 1.45). These results are obtained with the impurity of acetone in the bottoms held at 0.1 mol% acetone using the design spec/vary feature of Aspen Plus and manipulating distillate flowrate. 

Although the procedure in Example 4 works well (provided solvent flowrate is above the minimum required), the coordinates of the mixing points Mj, M2 etc. have to be calculated manually. This tedious step can be avoided by observing that all the operating lines (i.e. L0V2, L1V3 etc.) have a common point called the difference point. 

To get the minimum solvent/feed flowrate, we want to find the mixing point M (Figure 5.15) that ... 

Because ab intersects the equilibrium line, no further separation can occur. In other words, the equilibrium concentration of the solute in the solvent cannot be exceeded. This is the minimum liquid flowrate. Just as with the minimum reflux ratio, this situation is hypothetical because an infinite number of equilibrium stages (or an infinitely tall column) would be required. 

The optimum solvent flowrate is found by determining the minimum total energy required in the reboilers of the two columns Qja and Qr2), using four design spec/vary specifications. In the extractive column, the distillate impurity is held at 0.5 mol% chloroform and the bottoms impurity is held at 0.1 mol% acetone by varying distillate flowrate Di and reflux ratio RRi. In the solvent recovery column, the distillate impurity is held at... 

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