This distillation column consists of a glass column shell and six bubble cap trays, making seven stages in total. Each tray contains three bubble caps arranged in a triangle. The liquid from the top tray comes through a downcomer pipe to one side of the tray of the tray below. The outlet weir is right in the centre of the tray with half weir pipe at a height of 10 mm. This means the liquid needs to flow around the caps and across the tray to get over the lower weir outlet and into the downcomer. This ensures that the tray holds a pool of clear liquid at a depth of at least 10 mm. The vapour flows into the bubble caps through the riser pipes at the bottom of the tray, reverses its flow direction and flows through the annulus between the riser and cap, and flows out from those 26 slots around each cap. A key design aspect for bubble cap trays is that the top of the slots must be lower than the outlet weir height, so that vapour is forced to bubble out through the clear liquid on the tray, creating vigorous bubbling. These bubbles, or froth, provide a large interfacial area, which is essential for good vapour-liquid contact and mass transfer. Note that the tray spacing for the column is 6 inches, which corresponds to the smallest practical spacing in commercial column designs. It is important to observe whether the tray spacing is sufficient to prevent significant entrainment or flooding during column operation.
The vapour stream leaving the top tray is directed through the large vapour pipe into the condenser. Note that the reflux pipe runs inside the vapour pipe (similar to a double-pipe heat exchanger) to heat the reflux stream back to saturation temperature before entering the top tray; this is why the reflux pipe emerges from the vapour pipe. This is an example of energy integration, a very important concept in energy-intensive processes such as distillation. The total pressure drop across all six trays is monitored by a differential pressure transducer, which enables determination of the operating pressure in the reboiler. For detailed performance evaluation, samples of both liquid and vapour can be taken from the sampling ports on each tray.
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