Leakage total reflux

For vacuum distillation, a preliminary test for leakage is necessary. The apparatus is evacuated when the required pressure has been reached the vacuum pump is stopped and the rate of increase of pressure is observed (section 5.4.1). A further test for tightness is performed when the column is in operation and the thermal load has been established. The still pot heater is switched on only when the desired pressure is attained, and the column jacket heater as soon as vapour evolution starts. The cooling water should be tiumed on before the heaters are put into operation during the initial period its rate of flow should be checked at intervals since it usually tends to vary somewhat at the start with the expansion of tap washers. When the vapour has reached the colunm head the load is adjusted to the required value (section 4.10.7) unless the column is to be flooded (section 4.10.8) to wet the packing. The apparatus is then allowed to run for at least half an hour at total reflux, so as to allow the initial boiling point to be accurately determined, and only then is it adjusted to the required reflux ratio (section 4.10.4). 

Figure 12.7 Events preceding butadiene column explosion during total reflux operation, (a) Fraction of column contents batch-distilled due to leakage, as a function of time (b) corresponding rate of vinylacetylene accumulation (From "Butadiene Explosion at Texas Cify-2 RJI. Freeman and MP. McCre y, in Loss Prevention, vol. 5, p, 66,1971 reproduced by permission of the American In-ttiUde of Chemical Engineers.)...

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