Specifying Equipment Parameters

The parameters for all the equipment must be specified. Clicking on Blocks on the left side of the Data Browser window produces a list of all the blocks that must be handled. Any block with a red color is not completely specified. The column is the most complex and has the most parameters to fix. So we will start with the column C1.

3.4.1 Column C1

Clicking on the block labeled C1 opens a window with a long list of items. The top subitem is labeled Setup. Clicking it opens the window shown in Figure 3.25. There are several page tabs. The first is Configuration, on which the number of total stages, the type of condenser, the type of reboiler, the numerical convergence method, and two other variables are specified. We consider each of these below.

1. Number of Stages. The rigorous way to select the number of stages is to perform an economic optimization. We discuss this in detail in Chapter 4. For the moment let us select a column with 32 stages. Aspen uses the tray numbering convention of defining the reflux drum as stage 1. The top tray is stage 2 and so forth on down the column. The base of the column in this example is stage 32. Therefore this column has 30 trays.

Figure 3.25 Column C1.

2. Condenser. Use the dropdown menu to select Total. If the distillate were removed as a vapor, Partial-Vapor should be selected.

3. Reboiler. Both the kettle and the thermosyphon reboilers are partial reboilers (the vapor from the reboiler is in equilibrium with the liquid bottoms product withdrawn), so it does not matter which you select.

4. Convergence. The standard method works well in hydrocarbon systems. Alternative methods must be use in highly nonideal systems. Examples in latter chapters will illustrate this.

5. Operating Specifications. As discussed in Chapter 2, a distillation column has 2 degrees of freedom once the feed, pressure, number of trays, and feed tray location have been fixed. There are several alternative ways to select these 2 degrees of freedom, as shown in Figure 3.26. At this stage in our simulation, the usual approach is to fix the distillate flowrate and the reflux ratio. Later, once we obtain a converged solution, we will change the specified variable so that the product specifications are met. For now let us fix the distillate flowrate at 0.4 kmol/s since we know that this is the molar flowrate of propane in the feed. In addition, let us select a reflux ratio of 2 since the propane/isobutane separation is neither very difficult nor very easy. Figure 3.27 shows the Configuration page with all these data inserted. Note that the red dot on the C1 block becomes a blue checkmark when all the required input data have been provided.

Now click the Streams page tab. A window opens, on which the location of the feed tray must be given. For the moment we set this in the middle of the column on stage 16 (see Fig. 3.28). Later we will return to this question and determine the "optimum" feed tray location by finding the tray that minimizes reboiler heat input.

Operating specifications

Distillate rate ▼ Mole ▼

kmol/sec ▼

Bottoms rate Reflux rate Boilup rate Boilup ratio Distillate to feed ratio Bottoms to feed ratio Condenser duty Reboiler duty

Feed basis 1

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