Running The Simulation

The blue N button ("next") at the top of the Data Browser window on the far right is clicked to run the simulation. If any information is needed, the program will go to that location on the window and display a red symbol. If everything is ready to calculate, the message shown in Figure 3.33 will appear, and you should click OK. The Control Panel window shown in Figure 3.34 opens and indicates that the column was successfully converged. It took four iterations to converge the column.

Figure 3.32 Valves V11 and V12 specifications.

Now we want to look at the compositions of the product streams leaving the column to see if they satisfy their desired purities. We assume that the specification of the heavy impurity in the distillate (isobutane = iC4) is 2 mol% and that the specification of the light impurity in the bottoms (propane = C3) is 1 mol%. To look at the properties of these streams, we open the C1 block in the Data Browser window and click the item Stream Results, which is at the very bottom of the list.

There are three streams in the table shown in Figure 3.35. Stream 1 is the feed inlet to the column. Stream D1 is the liquid distillate leaving the reflux drum. Stream B1 is the liquid bottoms leaving the base of the column. We can see that there is about 12 mol% iC4 in the distillate and 8 mol% C3 in the bottoms. The purities are too low, so we need to increase the reflux ratio or add more stages to get a better separation.

If we go back to Setup in the C1 block, change the reflux ratio to 3, and click the N button, the simulation converges with the results shown in Figure 3.36. The distillate impurity has decreased to about 2 mol% iC4, and the bottoms now has about 1.5 mol% C3. So we are getting pretty close to the desired purities. We could continue to manually change the reflux ratio and the distillate flowrate to attempt to achieve the desired product purities by trial and error. However, there is a much easier way, as discussed in the next section.

Figure 3.33 Ready-to-run message.

Figure 3.33 Ready-to-run message.

H Control Panel

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:- B Calculation Sequence

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