New Separation Processes: Questions and Answers
Questions 1: GE in general, Phase equilibrium
1. A general process scheme of a SF process is given in the figure. Write down in the
figure typical operating conditions for a process with supercritical fluids (e.g. the
extraction of caffeine fr...
1. A general process scheme of a SF process is given in the figure. Write down in the
figure typical operating conditions for a process with supercritical fluids (e.g. the
extraction of caffeine from green coffee beans) and for the process step of
precipitation/regeneration, if this step is carried out by pressure reduction only.
5 – 8 MPa
290 or 310 K
10 – 30 MPa
320 – 350 K
2. What for do we need the knowledge on phase equilibrium in context with a process
using supercritical fluids as solvent?
The equilibrium state provides information about:
the capacity of a supercritical (gaseous) solvent, which is the amount of a substance
dissolved by the gaseous solvent at thermodynamic equilibrium,
the amount of solvent, which dissolves in the liquid process streams, and the
equilibrium composition of the liquid phase;
one or more compounds, expressed by the separation factor as defined by:
the selectivity of a solvent, which is the ability of a solvent to selectively dissolve
i j,
y /y
3.1
xi / x j
= separation factor,
with
xi, xj = equilibrium concentrations of component i and j in the liquid phase, in mole
or mass fractions,
yi, yj = equilibrium concentrations of component i and j in the gaseous phase, in mole
the dependence of these solvent properties on conditions of state (P, T),
or mass fractions,
the extent of the two-phase area, as limiting condition for a two-phase process like
gas extraction.
1
,If capacity and selectivity are known, a good guess can be made about whether a
separation problem can be solved with the separation process of gas extraction and on
its mode of operation.
3. Draw a rough sketch of a generalized solubility diagram (solubility versus
temperature) of a solid in a supercritical solvent (e.g. like SiO2 in H2O, naphthalene in
ethylene, caffeine in CO2). Include lines of constant density.
Solubility of naphthalene in ethylene.
4. Indicate in the P,x-diagram, shown below, possible operating conditions for a
process with a supercritical solvent and possible conditions for regeneration of the
solvent and precipitation of the extract.
P,x-diagram of oleic acid - ethylene
2
, 5. Explain the characteristic features of the solvent power of a supercritical fluid!
Nonpolar supercritical fluid (like CO2) dissolves preferably nonpolar substances.
A polar supercritical fluid (like NH3) dissolves preferably polar substances.
The solvent power of a nonpolar SF decreases with increasing polarity of the solute.
Polar subcritical fluids added to a SF enhance polarity and increase solvent power for
polar substances.
A supercritical fluid with low critical temperature (like nitrogen) added to a SF will
reduce the solvent power.
Solvent power of a SF increases with density.
Solvent power of a SF is high at supercritical temperatures near the critical
temperature.
Different SF have a different solvent power for the same substance.
6. In the Figure given below you see typical diagrams for binary systems including the
SF region. Indicate the following features in the diagrams:
Vapour pressure (VP), critical points (CP), solubility of low volatile compound in the
gaseous phase, solubility of SF in the liquid phase.
Are the critical points in binary systems with respect to pressure (P,x-diagram), or with
respect to temperature (T,x-diagram) always at maximum or minimum condition?
P,x-diagrams of n-heptane – ethane T,x-diagrams of n-heptane - ethane
7. Separation of a (quasi-) binary system with a SF involves three compounds. Indicate
in the ternary diagram given below on the left, the solubility of low volatile
compounds in the gaseous phase, the solubility of the SF in the liquid phase, and the
probable place of the critical point.
Purification of the di-/triglyceride fraction is limited to the extent of the two phase
region. Referring to the figure below on the right, indicate what change in operating
conditions is necessary in order to being able to purify the di-/triglyceride fraction to
higher values than possible with the equilibrium given in the figure on the left!
3
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