Question

Problem 2

A sieve tray column is designed with 18” tray spacing to operate at a pressure of 4 atm. The value

of the flow parameter Flv = 0.5 and the flooding velocity is 4 ft/s. Unfortunately, the distillation

temperature is found to be too high, and unacceptable thermal degradation occurs. To reduce the operating temperature, you suggest reducing the column pressure to 1.0 atm. Estimate the new

flooding velocity.

As an engineer, what aspect of the distillation column design you have to check next to ensure that

the pressure can be decreased? Hint: Think in terms of hydraulics and separations.

Assumptions: Ideal gas, surface tension and liquid densities are unchanged. ρL >> ρG so that (ρL –

ρG) ~ ρL. Ignore the effect of temperature change in the calculation of vapor density, and L/G is a

constant.

P

Answer 1

Now when the pressure is decreased we need to check

• weeping point
• Plate pressure drop-There could be two man sources of pressure loss :that is due to static head of liquid on plate and due to vapour flow rate through holes
• A simple additive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hd); the head of clear liquid on the plate (hw + how); and a term to account for other, minor, sources of pressure loss, the so-called residual loss hr. The residual loss is the difference between the observed experimental pressure drop and the simple sum of the dry-plate drop and the clear-liquid height. It accounts for the two effects: the energy to form the vapour bubbles and the fact that on an operating plate the liquid head will not be clear liquid but a head of “aerated” liquid froth, and the froth density and height will be different from that of the clear liquid. It is convenient to express the pressure drops in terms of millimetres of liquid.

Total pressure drop ,ht= hd+(hw+how)+hr