The response of the system to inadequate cooling will be for the column pressure to rise until there is adequate temperature driving force for complete condensation. The outlet temperature will also rise and increase the pressure in the distillate storage cylinders. The pressure rating of these cylinders is more than adequate to handle any variation resulting from fluctuations in cooling. However, an increase in distillate pressure might endanger the instrumentation. Therefore, the column feed rate should always be reduced until pressure control is regained.
A second double pipe exchanger was installed to cool the bottoms product from an estimated 160°C to approximately 40°C to permit safe handling and easy storage. This equates to a duty of approximately 320 W. The laboratory chilled water was again utilised to provide the necessary cooling. The configuration and dimensions of the bottoms cooler are indicated in Figure 11.6 and are similar to the overhead condenser. The bottoms cooler is considerably shorter than the condenser since a much lower duty is required. A smaller diameter inner pipe was used to accommodate the lower flow rates (a 12 mm pipe would have resulted in sluggish flow and a poor heat transfer coefficient).
The heat transfer area was calculated to be 0.030 m2, the heat transfer coefficient for forced convection was estimated at 198 W/m2/°C (at a chilled water flow rate of 25 L/min and a bottoms flow rate of 0.1 L/min) with an LMTD at normal operating conditions of 74°C. This combination provides a duty of at least 400 W. The surplus duty is desirable for startup and non-optimal operation where the bottoms rate might be significantly higher than the expected design rate of approximately 0.1 L/min.
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