Vertical condenser, vapor in tubes or cleanup system. At startup time total reflux may be achieved by using the reflux valve to control the level in the condensate receiver.

For those columns that must be protected from atmospheric oxygen or moisture, a vent system such as that shown in Figure 3.7 should be used. This is similar to the one recommended later for pressurized or vacuum columns. Note that inerts usually should be added after the condenser, to minimize product losses. Sometimes, however, it is necessary to add inerts ahead of the condenser, for pressure control. Figure 3.7 also shows a more commonly encountered tank arrangement where the reflux drum is common to both the top product system and the reflux system.

A potential and frequent source of trouble with both arrangements is the control of condensate temperature via cooling water. As shown by a study by B. D. Tyreus,11 for a constant subcooled temperature, process gain (°C/pph CW) and dominant time constant both decrease as total heat load increases. This compounds stability problems; we need an increasing controller gain and decreasing reset time as total heat load increases. Further, subcooling heat load must be a reasonable fraction of total heat load—say 5 percent—or the system will lack adequate sensitivity. Finally, many cooling water valves do not have adequate turndown; they are wide open in summer and almost closed in midwinter. A small and a large valve in parallel should often be used.

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