A.) A reverse osmosis unit is to be used to concentrate the aqueous solution of

Question

A.)  A reverse osmosis unit is to be used to concentrate the aqueous solution of metal complexes left over from a metal finishing operation.  A pressure of 100 bar is exerted on the solution side of the membrane during concentration.  A disgruntled employee pours salt into the solution on the retentate (metal containing solution) side of the membrane.  Is the pressure required to be exerted on the metal-solution side in order to keep the flux constant increased or decreased?

B.) A hollow fiber membrane module is used to separate a CO2/CH4 mixture.  CO2 permeates easier through the membrane, and a high-flowrate sweep gas is passed outside the fibers to remove any permeating species. The current module is to be replaced with a new module with a larger cross-sectional area (ie, a larger number of fibers to pass the feed gas through), while maintaining all other parameters.  How will this affect the mole fraction of CO2 in the gas exiting the fibers? (increased or decreased?)

Please give explanation whenever possible.

Explanation / Answer

A) The pressure required to be exerted in the metal solution side is increased. As the solute concentration increases on the retentate side upon addition of salt, water will now begin to flow on this side of the membrane resulting in an increase in the osmotic pressure.

B) As the cross sectional area of the module is increased i.e the number of fibers is increased, more CO2 will pass through. Since mole fraction of CO2= moles of CO2/ moles of CO2 + moles of CH4, the mole fraction would correspondingly decrease.

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