Exam 3 Flashcards
Name four criteria for selecting a proper solvent for LLE
- Must be immiscible
- Density difference
- Solute must be soluble in solvent. Does not have to have greater solubility than in feed.
- Solvent must be regenerable. Usually by distillation, so solvent must have finite volatility.
- Solvent should be environmentally acceptable, nontoxic.
Name three types of extractors
Podbielnak
Reciprocating Plate Extractor (Karr)
Mixer-Settler
Hollow Fiber Extractor
Oldshue-Rushston Column
Why is rate-based calculation more accurate than equilibrium stages?
Rate-based modeling takes mass and heat transfer coefficients into account.
What should happen to column performance, pressure drop and profiles as the gas velocity approaches the flooding limit?
Efficiency: goes down
Temperature profile: smaller temperature difference
Visual appearance of froth
Pressure drop increases with more then v^2
What are the three methods used in DSTWU?
Winn method to estimate minimum number of stages
Underwood to calculate minimum reflux ratio
Gilliland to relate actual number of stages and RR
In a feed mixture containing propane, butane, pentane, and hexane what pair of these keys are possible, or not:
ltkey-propane, hvykey- butan
ltkey pentane, hvkey butane
ltke butane, hv key – hexane
ltkey-propane, hvykey- butan: possible
ltkey pentane, hvkey butan: not possible
ltkey butane, hv key – hexane: possible
Give four heuristics for sequencing distillation columns. Why does each make sense?
- Separations where the relative volatility of the keys is close to 1 should be performed in the absence of nonkeys (save the hardest distillation for last because nonkeys make the separation even harder)
- Sequences which remove the component one by one in overheads should be favoured (remove it first so we don’t have to boil it over and over again)
- A product comprising a large fraction of the feed should be removed first. Sequences which give a more nearly equimolal division of the feed should be favoured (remove it first so we don’t have to boil it over and over again)
- Separations requiring very high recovery should be reserved to late in the sequence. (save the hardest distillation for last because monkeys make the separation even harder)
What are the two major functions of a liquid/liquid contactor such as a trayed column or a column full of packing? What properties of the liquids enhance each of these functions?
Provide mass transfer and separation
Mass transfer: viscosity, diffusion coefficient. Separation: density difference, low viscosity
How does the diameter vary with the total volumetric flow rate of the solvent and feed?
More solvent means larger diameter, diameter varies with square root of solvent rate
Why does greater purity require a taller contactor?
Because more mass transfer is needed
Why are other separation operations needed with LLE?
You need to separate solvent and solute to recycle solvent
What are the important characteristics of a good solvent for a given problem? (high or low values?)
Low miscibility with carrier
High density difference between solvent and carrier
High solubility of solute in solvent
Must have finite volatility, so it can be regenerated by distillation
Should be environmentally acceptable, non-toxic
Low cost and high availability
Name and describe 3 types of liquid/liquid contactors that are also used for gas/liquid contacting. Name one type of liquid/liquid contacting that is not used for G/L contacting?
Sprays, trays and packing are used for both.
Define extract and raffinate
Extract: solvent with solute
Raffinate: carrier without solute
By analogy to absorption, what is the limiting condition with a large number of equilibrium stages for extraction?
The solvent rate
In typical liquid/liquid extraction with an infinite number of equilibrium stages, where does the “pinch” occur?
The feed point
Why are triangular diagrams useful for representing LL equilibria?
Because it’s a ternary system
List the lines and points on a Hunter-Nash plot.
Equilibrium curve, tie lines, miscibility curve
P point and operating lines
Feed/solvent line
Extract raffinate line
Mixing point
How is extraction analogous to absorption?
Both use a solvent to remove a component from the feed
There is a solute
There is a minimum solvent rate
What key thermodynamic properties make extraction work?
Activity coefficients
How does each type of extractor get mass transfer? Liquid/liquid separation?
(Mass transfer)
Creating area:
- Spray contactors use nozzles to create drops
- Tray contactors create drops as the liquid passes through holes on the tray
- Packed contactors provide metal surface that is selectively wetted by one phase
- Agitated contactors break the discontinuous phase into smaller drops
(Mass Transfer)
Creating turbulence:
- The sum of the fluid velocities is related to turbulence
- Rotating contactors provide additional velocity without flooding
- Agitated contactors create additional turbulence
(L/L Separation)
- Usually gravity separates the fluids based on the density difference
- Rotating contactors provide additional centrifugal force to facilitate L/L separation
What types of equations are used to solve liquid/liquid extraction for equilibrium stages in EXTRACT?
Material and energy balances
Calorimetry equations
Equilibrium relationships
What curves on a Hunter-Nash Plot correspond to those on a McCabe-Thiele plot? What additional curves are used?
Equilibrium curve and tie lines correspond to equilibrium curve on McCabe-Thiele
P point and operating lines correspond to operating lines on McCabe Thiele
Extract raffinate line and feed solvent line corresponds to q-line on McCabe-Thiele
Name three types of liquid/liquid contactors that are not used for gas/liquid contacting
Mixer/settler
Podbielnak
Hollow fiber extractor
How do contactors get L/L separation?
Usually, gravity separates the fluids based on the density difference. Rotating contactors provide additional centrifugal force to facilitate L/L separation.
Under what conditions is extraction preferred to distillation?
When the components to be separated have low relative volatility or form an azeotrope and usually when the liquid is nonideal such that the solute has a greater solubility in a suitable solvent
