Principles - Liquid/liquid Extraction Flashcards
What is liquid liquid extraction?
A method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent.
Useful for mixtures with similar boiling points or heat sensitive materials.
In liquid liquid extraction, what is the feed, solvent, extract and raffinate?
Feed - the solution which is to be treated
Solvent - the liquid the feed is in contact with
Extract - the solvent rich product of the liquid liquid extraction operation
Raffinate - the residual liquid from which the solute has been removed.
What is raffinate and extract?
The raffinate (from French raffiner, to refine) is a product which has had a component or components removed.
The extract is the solvent rich product, being removed.
[If the solvent and carrier have some solubility in each other then the raffinate will have a small amount of solvent in it and the extract will have a small concentration of the carrier]
How are equilibrium relationships demonstrated for liquid-liquid extraction processes?
As triangles - Gibb’s Triangle
The corners represent 100%: extract, raffinate and solvent ( and the middle represents equal amounts of each)
What is the Lever rule?
What does it mean?
If ‘R’ kg of a mixture at point R is added to ‘E’ kg at point E then, (on a Gibb’s triangle) a new pixture is shown as point M, on a straight line between R and E.
R/E = length of ME line / length of RM line
It enables the relative amounts of 2 phases, a and b which are in equilibrium, to be found by a construction in a phase diagram.
What does the Lever rule state?
It enables the relative amounts of 2 phases, a and b which are in equilibrium, to be found by a construction in a phase diagram.
It forms from a similar rule: mala = mblb
which relates the moments of 2 masses, ma and mb, about a point in a lever.
How can the position of M (a mixture e.g. of R and E) on a Gibb’s triangle be determined?
1) From the weight fractions of the solvent at different positions.
R/E = length of line ME / length of line RM
= (xₑ-xₘ)/(xₘ-xᵣ)
2) The position of M can also be found by a mass balance: R + E = M
A solute balance produces:
xᵣR + xₑE = xₘM = xₘ(R + E)
Therefore:
xₘ = (xᵣR + xₑE)/(R + E)
What are the features of an LLE Gibb’s triangle?
For a triangle with the top point as Extract - E and two bottom points as Raffinate - R and Solvent - S, with a dome above the bottom face of the triangle (between R and S).
- The region above the dome (towards E) is the single phase region
- The line joining to parts of the dome is the tie line
- The region below the dome shows the two phase region
- The top of the dome shows the plait point, a critical point at which the liquid and vapor phases are identical. This is near the top of the 2-phase region, at the inflection point
When enough solvent is added so that the mixture composition falls under the dome, the mixture separates (partitions) into 2 phases. The points representing the phase compositions can be joined by the straight tie line, which passes through the overall mixture composition.
For an LLE Gibb’s triangle, what do the tie lines demonstrate?
Tie lines connect compositions of liquid and vapor phases in equilibrium. Any mixture with an overall composition along a tie line gives the same liquid and vapor compositions.
What is ‘miscibility’?
The property of two substances to mix in all proportions, forming a homogeneous solution.
The capability to be mixed
What are the variables for LLE Gibbs triangles?
Composition (e.g. %)
Pressure and temperature are held constant
