The input streams to the process must be specified. In this example, there is only one input stream, the feedstream F1. The flowrate, composition, temperature, and pressure of this stream must be specified. Clicking Streams and F1 and then Input opens the window shown in Figure 3.23.
In distillation calculations, molar flowrates, and compositions are usually employed. Let us assume that the feed flowrate is 1 kmol/s and the feed temperature is 322 K (120°F). These are entered in the middle of the window. The feed composition is 40 mol% propane and 60 mol% isobutane. The composition can be entered in terms of mole or mass fractions, or it can be entered in terms of molar or mass flowrates. In our example, we use the dropdown arrow to change to Mole-Frac and enter the appropriate values (see Fig. 3.24).
Specification of the feed pressure takes a little thought. We will discuss the selection of column pressure in more detail later in this chapter. We know that the distillate product is propane. We will want to use cooling water in the condenser since it is an inexpensive
heat sink compared to refrigeration. Cooling water is typically available at about 305 K. A reasonable temperature difference for heat transfer in the condenser is 20 K. Therefore the reflux drum temperature will be ~325 K. The vapor pressure of propane at 325 K is ~ 14 atm (206 psia). Therefore, the column will have a pressure at the feed tray of something a little higher than 14 atm.
In addition, we need to allow for some pressure drop over the control valve on the feedstream. The subject of selecting control valve pressure drops will be discussed later
in this book. For the moment, let us assume that we need a 5 atm pressure drop. Thus the feed pressure should be set at ~20 atm. Be careful to specify "atm" instead of the default "N/m2" for the pressure units. The final conditions of the feed are shown in Figure 3.24.
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