Question

1. Rand water research committee consisting of 12 chemical engineers design a reverse-osmosis plant to recover 30 % of the seawater feed water as portable water. The permeate pressure used is five time less than the feed side pressure. The total spiral wound membrane surface area is 464000 m² for a single stage crossflow. Calculate the salt passage and water permeance given the characteristic of the feed water, permeate and final brine as in table below.

	Flow rate	Concentration	Pressure
Feed	150 m3	45000 ppm	140 atm
Permeate		700 ppm
Brine (retentate)		65000 ppm

Answer 1

Feed flowrate = 150 m3

Permeate flowrate = Feed flowrate*recovery = 150*30%=45m3

feed flowrate=Permeate flowrate + Brine flowrate

=> Brine flowrate =feed flowrate-Permeate flowrate =150-45=105m3

Salt Permeance = Salt Concentration in Permeate*Permeate flowrate/(Salt Concentration in feed*feed flowrate)=700*45/(45000*150)=0.46%

Salt flowrate in Permeate = Permeate Salt concentration*Permeate flowrate*1000*10^-6 kg=700*45*10^-6=0.0315 kg

Given, Permeate pressure = 1/5 of Feed Pressure = 1/5*140=28 atm

Salt flowrate in Permeate=Permeance*R.O Area*(Feed Pressure-Permeate Pressure)

=>Permeance=.0315/464000/(140-28)=6.06*10^-6 kg/atm/m2