Topic 3: Introduction to Distillation in Plate Columns Flashcards
Is multi -stage distillation more effective than single stage flash distillation?
yes
What are the limitations of flash units?
a single flash unit usually does not provide sufficient separation but further units could be added
what happens if you cascade many flash units
greater separation can be achieved but:
only small amounts of product will be obtained
there will be many intermediate product streams
many flash units would require many heaters and coolers
what us the best way to achieve separation?
the solution is to integrate many stages within a single distillation column with internal recycle
describe the liquid and vapour flow paths in a plate column
-vapour rising through the openings in the plate column
contacts the liquid flowing acorss the plate
- close contact between vapour and liquid promotes
rapid mass transfer
- ideally, equilibrium is achieved between the phases
leaving each plate of the column
- plates are also known as trays
name 4 types of trays
sieve tray
movable valve tray
fixed valve tray
bubble cap tray
describe the anatomy of a distillation column
single feed total condenser at the top -reflux liquid returned to the column reboiler at the bottom -vapour returned to the column rectifying section above the feed stripping section below the feed plates are numbered from 1 and the top to N at the bottom we treat each plate as an equilibrium stage where a plate is a stage in the analysis
what is the reflux ratio
the reflux ratio is defined as:
R = L/D
where R= (0, infinity]
what enters the total condenser at the top of the column?
the vapour leaving the top of the column enters the total condenser
what is the purpose of a total condenser
the total condenser condenses but does not subcool all the vapour
how is the condensate split?
part of the condenstae is returned to the column as reflux, flow rate L
part if the condensate is produced as the top product, distillate, flow rate D
why do we need a reflux?
reflux is essential for column operation
R= (0,infinity]
what is the boil up ratio?
the boil up ratio is defined as:
Vb= /V /B
where Vb= (0,infinity]
what is entering the partial reboiler?
the liquid leaving the bottom of the column enters the partial reboiler
what happens to the liquid as it passes through the reboiler
part of the liquid is reboiled(vaporised) and part is not
what are the outflows from the reboiler
the outflows from the reboiler are:
saturated vapour, flow rate /V
saturated liquid, flow rate B
why is reboiling necessary?
reboiling(together with reflux) is essentia to create the counter-current flow necessaryfor effective column operation
what counts as an equilibrium separation stage in a column?
each plate on the column behaves like an equilibrium separation stage
what assumption can be made based on each plate of the column behaving like an equilibrium separation stage?
the vapour and liquid phases leaving a given plate are assumed to be in equilibrium
so x and y fall on the equilibrium curve
what comprises the feed to a given plate?
generally, the feed to a given plate comprises liquid from the stage above below
is the vapour and liquid streams passing between plates in equilibrium
the vapour and liquid streams passing between plates ( e.g. x1 and y2) are not in equilibrium but their compositions can be found from material and possibly energy balances