between the column base temperatures and the condensing steam temperature in the reboiler and a 4 atm pressure drop over the steam valve, give supply steam pressures of 6.8 and 17 atm, respectively, for the two processes. The difference in the cost of these two steam supplies would reduce the energy savings.

Total capital investment is also reduced. This is somewhat counterintuitive because one column with two heat exchangers would be expected to be less expensive than two columns with three exchangers. However, the column diameters and the heat exchanger areas are smaller in the heat-integrated design.

An additional aspect of this heat integration simulation is the calculation of the heat transfer rate in the condenser/reboiler heat exchanger. In this steady-state simulation we have specified that the condenser heat removal in the first column is equal to the reboiler heat input in the second column. This satisfies the first law of thermodynamics. The area of the condenser/reboiler is then calculated on the basis of the heat duty, the differential temperature driving force (the temperature in the reflux drum of the first column minus the temperature in the base of the second column), and an overall heat transfer coefficient. In a dynamic simulation this area is fixed. The heat transfer rate will change dynamically as the two temperatures change. So, in the dynamic simulation the heat transfer rates in the two column must be calculated from Q = UA(TD1 — TB2) so that the second law of thermodynamics is satisfied. This can be achieved by using Flowsheet Equations in Aspen Dynamics, which will be discussed in later chapters.

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