Eq. (6.74)

•Center-to-side flow. tSide-to-center flow.

•Center-to-side flow. tSide-to-center flow.

Comment All the actual flow parameters exceed the flow parameters at the froth-to-emulsion transition. Therefore, all these checks indicate emulsion regime operation.

Entrapment. If entrainment is excessive, column diameter or tray spacing are usually increased. It has been recommended (2,67) that entrainment from the tray should not exceed about 0.10 lb liquid entrained per pound of liquid flow. At higher values, significant efficiency reduction occurs (34). Depending on the service, a lower or higher value can be set (4). For instance, if the column overhead stream is compressed and no knock-out drum is present, the entrainment that can be tolerated is smaller. Also, for trays operating at a high liquid-to-vapor ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantity of entrained liquid, and a lower limit is set.

Section 6.2.11 recommends the use of Fair's entrainment correlation in the froth (and emulsion) regime. This section also states that entrainment in the emulsion regime is unlikely to be a problem. Fair's correlation (Fig. 6.16) predicts fractional entrainment (pound of entrained liquid per pound of liquid) of 0.011 and 0.0075 at 80 percent of flood for the top and bottom trays, respectively. Even at 90 percent of flood the entrainment is likely to be less than 0.02 lb entrained liquid per pound of liquid at the flow parameters calculated in Sec. 6.5.6. It may therefore be concluded that entrainment is unlikely to be troublesome in the depropanizer.

Downcomer residence time. This check calculates the apparent residence time in the downcomer and ensures it is within the recommended guidelines in Sec. 6.2.9.

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