Water - RO Unit

Question 1

What interface are we interested in for reverse osmosis membranes?

Answer 1

Solid-liquid interface

Question 2

What happens at the membrane surface? Why does this happen? What are the impacts of these?

Answer 2

• Stagnant liquid film builds up on the retentate and permeate side
• Due to friction force at membrane surface
• creates additional mass transfer resistances

Question 3

How is the velocity of the water affected by the membrane surface? How does this affect mass transfer and why?

Answer 3

• The closer to the membrane the greater the resistance so the slower it moves
• Means there is less mass transfer across the membrane as there is less mechanical mixing

Question 4

What arrest are the three components of the mass transfer co-efficient?

Answer 4

K1 = mass transfer co-efficient due to the stagnant liquid on the retentate side
K2 = mass transfer co-efficient due to the stagnant liquid on the permeate side
Km = mass transfeer co-efficeient due to the resistance of the membrane

Question 5

What is the formula for the overall mass transfer co-efficient?

Answer 5

1/K = 1/K1 + 1/K2 + 1/Km

Question 6

What are the units of the overall mass transfer co-efficient?

Answer 6

Depends on the desired final unit (i.e. they can vary)

Question 7

What is the formula for the water flow rate?

Answer 7

Vw = KA (∆P - ∆π)
Vw = flow rate of water
K = overall mass transfer co-efficient of water
A = area of membrane
∆P = Pressure difference of feed and  permeate (Pf-Pp)
∆π = osmotic pressure diff of feed and permeate (πf-πp)

Question 8

Does temperature have an impact on the membrane area required to filter water? Why?

Answer 8

Yes because as temperature varier the mass transfer rate changes

Question 9

What is the formula to calculate the area needed for a membrane at a specific temperature?

Answer 9

At = Z * A25
At = area total
Z = membrane area correction factor (in reference to 25˚C as too how much larger an area is required for an equivalent flow rate)
A25 = area used in membrane at 25˚C

Question 10

How does the membrane flow rate and effective membrane area change over time? Why

Answer 10

• they decrease over time
• Scaling and fouling from salts and other impurities cause the membrane area to decrease (however this can be reversed by chemical treatment and/or hydraulic force) and slow densification (i.e. pores become ‘worn’) ‘f the membrane structure decreases mobility of water through membrane

Question 11

What is an important design consideration involving the retentate and the salt concentration? Why does this need to be considered?

Answer 11

• If the salt concentration is higher than its saturation point it will precipitate so ensuring retentate does not become too concentrated is important
• salt precipitate will cause damage to the membrane

Question 12

What is the formula for the flow rate of salt through a membrane? What is one assumption that can be made about this equation?

Answer 12

Vs = Ks A (Cf-Cp)
Vs = volume of salt
Ks = overall mass transfer co-effcient of salt
A = are of membrane 
Cf = concentration of salt in feed
Cp = concentration of salt in permeate (can assume this is 0)

Question 13

What is the relationship between pressure and flow rates of water and salt? What impact does this have on the rejection?

Answer 13

• Vw is proportional to pressure while Vs is independent to pressure (e.g. changes in pressure cause changes in water flow rate but not salt flow rate)
• This means that increases in pressure increase the rejection because more water is filtered from the salt

Question 14

What is the relationship between solute concentration and flow rates of water and salt? What impact does this have on the rejection?

Answer 14

• Increasing solute concentration increases osmotic pressure so for same driving force there is a lower water flow rate, there will also be a higher salt flow rate as Vs∝Cf
• With less water going through and more salt the rejection decreases

Question 15

What are the effects of water temperature? Why are they like this?

Answer 15

• As temp increase the flux increases but rejection decreases
• Temperature changes lots of variables (e.g. Ks, Kw, ∆π etc) so the outcomes are more complicated

Question 16

What is one of the main advantages of membrane processes?

Answer 16

They have a small physical size or process footprint than equivalent separation equipment