RadFrac Model with Explicit Heat-Exchanger Dynamics

5 RADFRAC MODEL WITH EXPLICIT HEAT-EXCHANGER DYNAMICS 

The top tray is now labeled as Stage 1 in this absorber model. So the feed stage and the bottom stage must be decreased by one to make this column equivalent to the basic column. The feed and reflux are fed to column along with a partially vaporized stream from the 

The overhead vapor from the column is condensed in a water-cooled heat exchanger. The design specification for this HeatX model is the exit temperature of the hot stream at 64.2 °C (the bubblepoint temperature of the distillate at 1 bar). Cooling water flows through the condenser (3.7 x lO kg/h), entering at 32.2 °C and exiting at 47.6 °C. With a heat-transfer rate of 60.06 MW and an overall heat-transfer coefficient of 730.9 kcal/(hm K), the required area is 2788 m.  

The volume of the tubes is estimated by assuming tubes 0.05 m in diameter and 5 m in length. The number of tubes required to give the necessary heat-transfer area is calculated and the inside volume of these tubes (35 m ) is used to give dynamics in the HeatX model. Shell volume is set equal to tube volume. In the Dynamics section of the HeatX block, the default is Instantaneous. Use the drop-down arrow to select Dynamics, and enter the inlet and outlet volumes on both hot and cold sides of the heat exchanger. 

An Aspen Flash model is used for the reflux drum with pressme set at 1 bar and design specification of a vapor fraction of 10 , which makes the drum essentially adiabatic. A small vapor flow rate is necessary so that the control valve in this vent line can be sized. In the Aspen Dynamics simulation, the valve is completely closed. The liquid holdup in the drum is set to give 5 min at 50% full (diameter 3 m and length 6 m). 

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