5 Flashcards
(32 cards)
Shortcut column unit purpose
Uses shortcut methods to solve mass/ energy balances of column solves parameters like number of trays, feed stage location, reflux ratio
What is mass transfer in distillation driven by
Differences in volatility between liquid and vapour phases
What is mass transfer driver by in absorption
Differences in solubility between liquid and vapour phases
Light key component
Mainly in top product ( check tutorial 5)
Heavy key component
Mainly in bottom product
How can separation be made easier
Light and heavy key components should have very different boiling points
What is purity
Looks at the concentration of the component in the product stream
What is recovery
Amount of component in the product stream/ amount of component in the feed stream
MESH equations
Material/ energy balances, equilibrium relationship , summation relationship on mole fractions
Solver in distillation column
Distillation column has its own solver it is isolated
Default specifications of distillation columns
- 100% tray efficiency
- condenser and reboiler pressure = 0
- heat losses in condenser, reboiler and column = 0
User sets column pressure drop
Typically 0.1 psi per tray
Is the pressure at the bottom of the column higher or lower than the top of the column
It is higher as there is a reboiler at the bottom which produces vapour at high pressure at the bottom
De ethaniser / de propaniser are what types of columns
Distillation columns
What does high desired product purity mean?
There are more stages and greater heating and cooling duties and thus capex/ opex
What is reflux ratio
Check this: the ratio of liquid going back to the column: distillate flow rate
What happens if we increase reflux ratio?
This means
-more liquid enters the column
- condenser and reboiler duties increase
Because more liquid is present in the column so more liquid needs to be vaporised by reboiler so now more vapour is in the column and the condenser now needs to do more work and condenser duty increases
What happens to the number of stages if we increase reflux ratio?
Decreases
As more liquid enters, there is more mass transfer between the liquid and the vapour.
What happens to the diameter of the column, if we increase reflux ratio?
Increases, assuming height is kept the same
As it needs to adjust for the extra volume entering the column
Condenser options
Full reflux (common) : top product is vapour
Partial : vapour and liquid top produce
Total : top product is liquid
How many degrees of freedom do we need for a distillation column with a total or full reflux condenser?
2 (we usually use this)
How many degrees of freedom do we need for a distillation column with a partial condenser?
3
Light key and heavy key components difference
The light key component is the more volatile one (smaller molecular weight) out of the two
Name some Shortcut methods
The following are used to calculate:
- Fenske : minimum number of trays
- Underwood : minimum reflux
- Gilland : actual number of trays
- Kirkbride : optimum feed-stage location
