Membranes - Principles of Chemical Separations with Environmental Applications - page 272

9.14 Questions

9.1 What are the three important points with respect to the definition of a membrane?
9.2 What are three advantages and disadvantages of membrane processes?
9.3 Name two environmental applications for each membrane process discussed in this
chapter.
9.4 Why is it that Knudsen diffusion typically cannot provide high selectivity for light
gas separations?
9.5 What is the “upper bound” on a Robeson plot?
9.6 Is concentration polarization reversible?

9.15 Problems

=

9.1 Determine the osmotic pressure for a sodium-chloride solution with concentration

Lat25 ◦ C.
9.2 A wastewater stream at 25 ◦ C is to be treated using a reverse osmosis system. The
wastewater has a flowrate of 50 gal

23,400 mg /

L of sodium
chloride. A 75% recovery rate is required. A reverse osmosis vendor has recommended
a membrane with the following characteristics:
permeability coefficient

/ min and a concentration of 5000 mg /

gmol
cm 2

10 − 6 = 1 . 9 ×

· ·

s atm

area for a bundle

300 ft 2

=

50% recovery rate
optimal pressure differential across the membrane

= 500 psi

95% rejection of the salt.
Determine:
(a) The osmotic pressure of the solution;
(b) the water flow through the membrane;
(c) the number of units required for 75% recovery.
9.3 A reverse-osmosis membrane to be used at 25 ◦ C for a NaCl wastewater stream con-
taining 2.5 g NaCl m 3 , m 3 ) has a water permeability

/ L (2.5 kg NaCl / ρ = 999 kg /

10 − 4 m 2

coefficient K = 4.8 × kg / · s · atm and a solute (NaCl) permeability coeffi-

10 − 7 cient = 4.4 × m / s. Calculate the water flux using a P = 28 atm and a solute

m 3 .

rejection R . The permeate concentration is 0.1 kg NaCl /

9.4 An electroplating plant has 2,000 m 3 / day of a nickel-bearing waste. The nickel con-

centration is 15,000 mg / L as NiSO 4 . Assume the following characteristics of the

system:
resistance through unit

= 10.5 ohms

current efficiency

90%
maximum CD/N ratio

=

m 2 ) = 6,000 (A / / (g-equivalent / L)

membrane area

1.0 m 2 .

=