Separations as unit operations - Principles of Chemical Separations with Environmental Applications - page 30

The selectivity of any separation is improved as the value of the separation factor is
increased.
Separation processes require a property difference and a separating agent.
There are two types of separating agents: mass; and energy.
The extent of separation depends on the size of the property difference and the specific
separating agent(s) used.
For separation processes, countercurrent operation is usually preferred over cocurrent
operation because, in the former, the driving force is maintained at a larger value along
a boundary.

2.13 Questions

2.1 Why is the concept of a unit operation useful?
2.2 Why is a complexing agent useful that can selectively and reversibly react with the
solute of interest?
2.3 Describe why a difference in a property is needed for a separation.
2.4 What questions would you ask to evaluate various separation processes for a given
situation?
2.5 Membranes separate species based upon differences in molecular size, a parameter
which is often comparable to molecular weight. Why do you think it is difficult to
partition air into pure oxygen and nitrogen streams using membranes?
2.6 Henry's Law constant is equal to the ratio of the vapor pressure of a species over
the operating system pressure. If the number of stages required in a separation
decreases with increasing Henry's Law constant, how can the Henry's Law constant
be increased (i.e., of what thermodynamic variable is vapor pressure a function)?
2.7 Laboratory experiments were performed to assess the feasibility of separating ethy-
lene from ethane. It was determined that the equilibrium solubilities of ethylene
and ethane in an acidic copper(I) aqueous solution were similar. The rates of up-
take of the two gases into the aqueous solution were measured independently and
it was found that the rate of ethylene absorption is several times greater than that
of ethane. Is this an example of an equilibrium- or a rate-controlled separation
and why?
2.8 In membranes, the flux, or productivity, increases with increasing membrane pore
size. Selectivity increases when the pore size is much smaller than the molecular
diameter of the larger species in a binary mixture, but slightly larger than that of the
smaller species. Explain the tradeoff between productivity and selectivity in terms
of membrane pore sizes.
2.9 Describe circumstances in which steam can be either an energy-separating agent or
a mass-separating agent.
2.10 List the two possible ways a separating agent obtains a different component distri-
bution between two phases. Which of these applies to distillation, and why?