1. Rand water research committee consisting of 12 chemical engineers design a re
ID: 702980 • Letter: 1
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
Flow rate
Concentration
Pressure
Feed
150 m3
45000 ppm
140 atm
Permeate
700 ppm
Brine (retentate)
65000 ppm
Explanation / Answer
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
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