Mixed

Alcohols

FIGURE 8.3.4B CASE STUDY NO.4 (SftSED ON REF 10)

MODIFIED CONTROL SySTEM

POSTMORTEM The improvement resulted from two factors. I

1 The analyzer provided a direct measurement of the water profile, and took action before water contaminated the bottom product, or trays were dried out.

2 The reboil rate was greatly reduced and water concentration was maintained at all times at a minimum level sufficient to form an azeotrope. Following the modification, it was found that sufficient water was always present, and that live stream was never required during normal operation. J

CASE STUDY No. 5 Temperature Control by Throttling Cooling Water (Reference 11).

INSTALLATION An atmospheric column separating chemicals at temperatures greater than 250°F, with a condensate temperature control system shown in Figure 8.3.5. Note that the control system is essentially the same as that shown in Figure 17.8A in Reference 12 adapted to atmospheric pressure. The column was running at very low feed rate.

PROBLEM At one instance, condensate temperature became too low, which caused the control valve to close. The water in the shell began to boil. The valve could not pass the required volume of steam. The cooling water pump stalled, and product vapor was released in large quantities from the vent. Fortunately, the column was quickly shut down before any damage occurred.

SOLUTION Provide an override control from the cooling water exit temperature which will avoid operation in excess of the maximum allowable temperature.

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