week 4 Flashcards
Distillation method used for low volume high value products
batch distillation
Two types of batch distillation
- Differential distillation
- Batch rectification
Batch rectification has two modes of operation
- constant product composition
- constant reflux ratio
As material is withdrawn, the MVC composition becomes smaller
Constant Product Composition
To maintain a constant product composition, the reflux ratio must be ____ as the composition of the MVC decreases
Increased
To maintain a constant product composition, the _____ must be increased as the composition of the MVC decreases
Reflux ratio
Material balance of constant product composition
𝑆1𝑥𝑠1 − 𝑆2𝑥𝑠2 = 𝐷𝑏𝑥𝑑
S1 = mols in still initially
S2 = mols I’m still after
xs1 = mol frac of MVC in still initially
xs2 = mol frac of MVC in still after
Equation for amount of product obtained in constant product composition
𝑆1 − 𝑆2 = 𝐷𝑏
S1 = mols in still initially
S2 = mols I’m still after
MVC
Most volatile component
If a column is operated at _____, the composition of MVC will continuously fall
Constant reflux
If a column is operated at constant reflux, the composition of MVC will continuously ___
Fall
Constant product composition reflux ratio equation
dLb/dDb
Most expensive type of tray arrangement
Bubble cap trays
Tray arrangements from highest relative cost to lowest
- Bubble-cap
- Valve
- Sieve (lowest)
Tray arrangements from highest pressure drop to lowest pressure drop
- Bubble cap (highest)
- Valve
- Sieve (lowest)
Which tray arrangement has the lowest efficiency?
Sieve tray
Which tray arrangement has the lowest vapour capacity?
bubble cap trays
3 Tray arrangements
- Sieve trays
- Valve trays
- Bubble cap trays
Typical turndown ratio of a sieve tray
2
Typical turndown ratio of a valve tray
4
Typical turndown ratio of a bubble cap tray
5
indicates the range of flow that is able to be measured with acceptable accuracy
Turndown ratio
https://en.wikipedia.org/wiki/Turndown_ratio
3 types of reboilers
- Kettle type reboiler
- Vertical thermosyphon reboiler (reboiler liquid withdrawn from bottom sump)
- Vertical thermosyphon-type reboiler (reboiler liquid withdrawn from bottom tray downcomer)
Possible vapour liquid flow regimes for a contacting tray
- Spray
- Froth
- Emulsion
- Bubble
- Cellular foam
Overall Column Efficiency equation
E = number of ideal stages/actual number of stages x 100
Values of E tend to vary between 30-100%
Factors affecting column efficiency
- Kinetics slow as you near equilibrium
- Factors related to plate design
measures performance of a single tray by considering the composition of the vapour phase as it enters and leaves a given tray
Murphree Efficiency
https://neutrium.net/equipment/distillation-tray-efficiency/
Murphree efficiency equation
(yn - yn-1)/(ye - yn)
yn = vapour mol fraction on plate n
yn-1 = vapour mol fraction on plate n-1
ye = vapour mol fraction that is in equilibrium with the liquid mol fraction
For a single plate, the vapour leaving is in equilibrium with the liquid leaving, and the ratio of that actual composition achieved to that which could occur if equilibrium between yn and xn were achieved is known as…
Murphree plate efficiency
Shown by the lowest point on the total cost line
min total cost
McCable Thiele Plots with a single side stream have
2 top operating lines with slope Ls/Vs and Ln/Vn
McCable Thiele Plot for a distillation column with two feeds has…
An operating line per section (3)
A q line per feed (2)
Overall column efficiency
E = (number of ideal plates/ actual number of stages) * 100
Factors related this are
- Kinetics slow as you near equilibrium
- Factors related to plate design
