The predicted bottoms temperature at 950 kPag is around 160°C. This precludes the use of steam as a heating medium since a steam pressure of 1500-2000 kPa would be required to provide an adequate temperature difference. A lower operating pressure would allow steam to be used with a reduced pressure but the attainable performance would be compromised and additional catalyst would be required to compensate for the lower temperatures and, therefore, lower reaction rates.
An internal, direct contact, electric heating element was specified to provide the necessary boilup. This simplifies the construction of the column by eliminating the need for the separate reboiler vessel that would be required if a thermosiphon reboiler was used. Although the element will reach high temperatures (approximately 220°C), auto-ignition is unlikely in the oxygen deficient environment inside the column The design duty for the reboiler could only be met with stainless steel elements and a three phase power supply, requiring three separate elements to be manufactured. Each element was fixed to the base of the column and sealed to prevent leakage. A variable frequency drive (voltage regulator) and ammeter were connected in series with the elements to regulate the reboiler duty and measure the current draw (and, therefore, power supplied). A digital controller on the variable frequency drive allows the voltage to be specified exactly. The maximum rated duty of the installed elements is 7.5 kW.
Inside the column, a baffle separates the liquid being heated from the bottoms product to be removed. Weirs in the baffle ensure that the heating element is covered by liquid at all times and a splash plate connected to the top of the baffle ensures that all liquid coming from the packed bed enters the reboiler sump before leaving the column. This configuration is depicted in Figure 11.1.
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