Example 5.2: Design a liquid-liquid gravity separator which can handle a two phase liquid stream of 0.5 m3/ min. The feed contains 45% by volume of light phase and 55% by volume of a heavy phase. Densities of light (ρl) and heavy phase (ρh) are 900 and 1150 kg/m3 respectively. Required settling time of light phase is 5 min while the settling time for heavy phase is 4 min.
Solution :
Feed rate to separator = 0.5 m3/min
Light phase in feed = 0.5 x 0.45 = 0.22m3/min
Heavy phase in feed = 0.5 x 0.55 = 0.28 m3/min
Settling time for light phase = 5 min
Volume of light phase in separator = 0.22 x 5 = 1.1 m3
Design Volume can be taken as 20% more
Design volume of light phase = 1.2 x 1.125 = 1.32 m3
Volume of heavy phase = 0.27 x 4 = 1.08 m3
Design volume of heavy phase = 1.2 x 1.08 = 1.3 m3
Total design volume of separator = 1.32+1.3 = 2.62 m3
Assume free board volume = 0.288 m3
Separator vessel volume = 2.62+0.288 = 2.908 m3
For a horizontal tank with elliptical heads having major to minor axis ratio 2:1