Lecture 3 Flashcards
(15 cards)
What happens in ternary liquid systems during liquid-liquid extraction?
In liquid–liquid extraction, nonideal ternary mixtures undergo phase splitting to form two liquid phases of different compositions.
What are the two ternary liquid systems we will consider?
- Liquid-liquid extraction in single equilibrium stage
- Liquid-liquid extraction where the carrier (the feed) and solvent are partially soluble in each other
what happens in a Liquid-liquid extraction in a single equilibrium stage?
The simplest case is the single equilibrium stage shown in digital notes, where only component B, called the solute, has any appreciable solubility in either component
A, the carrier, or component C, the solvent.
The solute enters the equilibrium stage in the feed, F, with the carrier, but is not present in the fresh solvent, S. In the stage, B is extracted by the solvent C to produce the extract, E The unextracted B leaves the stage with carrier A in the raffinate, R. Neither A nor B are assumed to be soluble in each other. Therefore, the flow rate, FA, of carrier A is the same in the feed and in the raffinate, R; and the flow rate, S, of fresh solvent C is the same in the solvent and in the extract. By convention, the extract is shown leaving from the top of the stage even though it may not have the lower density.
Important notes before derivation pf Fraction of B unextracted
FA is the MASS (just want you to make sure its mass) flow rate of carrier A in the feed equal to the mass flow rate of carrier A in the raffinate;
S is the mass flow rate of solvent C in the entering solvent equal to the mass flow rate of solvent C in the extract
The calculations are facilitated if compositions are in mass ratios, Xi, instead of mass fractions:
Accordingly, let XB be the ratio of the mass of solute B to the mass of carrier A in the feed (or raffinate) and YB be the ratio of the mass of solute B to the mass of solvent C in the extract
What is the derivation for the fraction of B unextracted in a Liquid-liquid extraction of a single equilibrium stage
Look at written notes
what is a Liquid-liquid extraction where the carrier (the feed) and solvent are partially soluble in each other
In the ternary liquid-liquid system shown in digital notes, components A and C are partially soluble in each other, and component B distributes between the extract and raffinate phases. This is the most commonly encountered case.
Different types of phase diagrams, constructed from experimental liquid-liquid phase equilibrium data, have been devised for using material balances to calculate equilibrium compositions and phase amounts (coming soon).
Before diving deeper into ‘Liquid-liquid extraction where the carrier (the feed) and solvent are partially soluble in each other’ A comment needs to be made about ternary diagrams
Look at written notes
How many phases are represented in the ternary diagram of the liquid-liquid extraction with the carrier (the feed) and solvent being partially soluble?
There is either one liquid phase where all of the solute, solvent and carrier are soluble in each other, and then their a 2 liquid phase where enough amounts of the solvent/ carrier have been added to the system to result in phase separation, forming two liquid phases: 1. carrier and solute 2. solvent and solute
How do we determine if a point in the ternary diagram has 1 or 2 liquid phases?
By doing the cloud point titration where you start either with a specified amount of solute to carrier or solute to solvent (in this FC I will use solute to carrier).
When you start with a certain amount of solute to carrier, start adding slowly some amount of solvent, and keep adding until you spot a point in the data that suggests phase separation has occurred (onset of cloudiness). Repeat the same step but this time starting with a different amount of solute to carrier and again note where phase separation occurs when the solvent is added.
Once you have enough points connect them to form your miscibility boundary curve, also called the binodal curve. Above this curve, only a single liquid phase exists; below the curve, two liquid phases exist.
All of this is visualized in digital notes( more than one diagram)
What are tie lines?
Tie lines are lines that connect points on
miscibility boundary curves that represent compositions of equilibrium phases. An illustration is shown in digital notes
How are tie lines obtained
they were obtained by measuring compositions of different extract and raffinate phases at equilibrium
How many degrees of freedom to we have in a ternary system and what does that tell us?
By Gibbs’ phase rule, there are three degrees of freedom for a three-component, two-liquid-phase system, With T and P specified*, the concentration of one component in either phase suffices to define the equilibrium system. As
shown in digital notes (FC 10), one value for wt% ethylene glycol on the miscibility boundary curve fixes that liquid-phase composition and, by means of the corresponding tie line, the composition of the other phase is fixed.
*Important to note that all ternary diagrams are drawn at a specific temperature and pressure so truly there is one only more degree of freedom.
How can we use Tie line to calculate the equilibrium composition of the Raffinate and the extract
Look at written notes. and used FC 10 as a reference.
What is the plait point (p)?
The tie lines converge to point P, called the plait point, where the two phases become one. Shown in the digital notes
one final note
In the way that Micheal represents a ternary diagram, the extract would have (in most cases) a larger amount of the solute than the raffinate. This causes it to be high up on the miscibility boundary curve
