Liquid-Liquid Extraction Flashcards
how reliable is LLE design?
LLE is a reasonably mature operation, however not as mature as distillation, absorption, and stripping.
can LLE be used to find number of equilibrium stages?
Procedures to determine no. of equilibrium stages to desired solute recovery is well defined.
what 7 cases is LLE preferred to distillation?
- Dissolved complex inorganic substances in organic- or aqueous solutions
- Removal of contaminant present in small
concentrations - A high boiling contaminant present in relatively small fractions in waste water stream
- Recovery of heat sensitive material (Vacuum
distillation not economical) - Separation of mixtures according to chemical type (rather than relative volatility)
- Separation of close boiling/ melting point liquids, where solubility differences can be exploited.
- Separation of azeotropic mixtures.
what notation is used for LLE?
- solute A
- solvent S
- carrier liquid C
- feed F
- raffinate R
- extract E
- mole fractions xi
what is the raffinate composed of?
mostly C, a little A and ideally no S
what is the extarct composed of ?
A , S and a little C
describe the ternary system?
a ternary system is composed of a feed which contains solute A and carrier liquid C which are miscible. a selvent S passes the feed. C and S are partially soluble wheras A is completelly/ partially soluble in S. Mass transfer of a from the feed to S occurs with less transfer of C to S, or S to the feed.
what promotes efficient extraction?
suitable solvents are required for efficienct extraction.
what 12 factors are required for chossing an ideal solvent S?
- High selectivity for the solute (A) relative to the carrier (C) (minimise recovery of carrier).
- High capacity for dissolving solute (A) (minimise solvent feed (S/F) ratio)
- Minimum solubility in carrier.
- Volatility should be sufficiently different from that of Solute (A)
- low viscosity
- moderate interfacial tension
- large difference in density relative to the carrier
- lack of tendency to form stable rag/scum layer at the phase interface
- desirable wetting characteristics with respect to column internals
- stability to maximise solvent life/ minimise make-up
- intentness to avoid complex process engineering
- nontoxic and nonflammable characteristics
- low cost
why is high selectivity of solute relative to carrier desired?
to minimise recovery of the carrier C
why is high capacity of dissolving solute desired?
to minimise solvent feed ratio S/F
what is the desired volatility
volatility should be sufficiently different from that of solute A. however it shouldnt be too high that it causes high pressure in the extractor and it shouldnt be too low that high temperature is needed in distillation recovery unit.
why is low viscosity required?
low viscosity is required to promote phase separation, minimise pressure drop, and promote high-solute mass transfer rate
why is moderate interfacial tension desired?
to balance ease of dispersion and promotion of phase separation
nwhta properties are priorities in design of LLE?
compromise needs to be made but selectivity and environmental considerations are more important than cost and capacity
define the partition/ distribution coefficient for LLE?
the phase equilibrium ratio is the ratio of mole fractions of species in two phases at equilibrium. the two phases are;
1. raffinate phase
2. extract phase
define relative selectivity for LLE
relative selectivity is defined by forming a ratio of equilibrium ratios
βij=(kD)i / (kD)j
in what ratio is the extract phase to the raffinate phase in KD
the ratio is the extract phase (2) to the raffinate phase (1)
KD=xA2 / xA1
for high selectivity how should βAC behave
βAC should be high for high selectivity because there would be high concentration of A and low concentration of C in the solvent S
for solvent recovery how should (KC)D and (KS)D behave
for solvent recovery (KS)D should be large and (KC)D should be as small as possible to minimise solvent S in raffinate and carrier c in extract
for high solvent capacity, nhow should (KA)D behvae?
(KC)A should be high
what is a ternary diagram
ternary diagrams are a way of graphically representing the composition of a ternary mixture
what is miscible out of A,C,S
- A and C are completely miscible
- A and S are completely miscible
- S and C are partially miscible
why is there only miscibility limits for S and C
there are only miscibility limits for S and C because they are only partially miscible
