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
what does the vertical temperature gradient cause?
some of the vapour (hotter) joining stage n below will condense
some of the liquid (cooler) joining stage n from above evaporate
at stage n where there is liquid evaporating and vapour condensing as it joins the stage what happens if you have equal molar enthalpy changes?
if the molar enthalpy changes associated with these two process are equal then the amount of vapour condensing on the stage will if the equal the amount of liquid evaporating
if the amount of vapour condensing is equal to the liquid evaporating, the what affect does this have on the flowrates?
there is transfer of componentes between the phases but no net change in the flowrates
Vn=Vn+1
Ln=Ln-1
what is constant molar overflow?
because there is no net change in flowrate
Vn=Vn+1
Ln=Ln-1
liquid and vapour flowrates are constant throughout each section of the column and we have constant molar overflow
what causes the flowrate to differ between the rectifying and stripping section
becauseq of the feed, the flow rates may duffer between the rectifying and stripping section
what are the main underlying assumptions that lead to the condition of constant molar overflow?
the main assumptions are:
-the components have equal and constant molar enthalpies of vaporization (Δh,vap, ‘latent heats’)
- other enthalpy changes (Δh,mix , CpΔT) are negligible
- heat loss from the column is negligible
- the pressure is uniform in the column
what leads to the condition of constant molar overflow
the mccabe thiele assumptions and with the stage energy anf mass blance equations leads to the condition of constant molar overflow
what three heat exchangers are typically in a multiple equilibrium separation stage system?
a feed heater
a reboiler at the bottom of the column
a condenser at the top of the column
what is the mccabe thiele construction?
the mccabe thiele construction is a graphical method to fond the number of equilibrium stages necessary to achieve specified compositions for the distillate(xD), bottoms(xB), given the feed composition(zF) and state (q)
what is the mccabe thiele construction based on?
it is based on the assumption of constant molar overflow
what does the mccabe thiele construction show on analysis about the leaving streams
the leaing stream compositions fall on the equilibrium (x,y) curve
what are the design problem specifications?
F,zF, q, P,xD,xB,R
what are the design problem results?
D,B,Vb, N
what is the total material balance?
F=D+B
what is the MVC material balance
zF.F=xxD.D+xB.B
using the total material balance and MVC material balance as simultaneous equations what are the external flow rates D and B, given zF, F, xD and xB
D= (zF-xB)F/(xD-xB)
B= (xD-Zf)F/(xD-xB)
how are the internal liquid and vapour flow rates determinedib the rectifying section?
R=L/D reflux ratio
V=L+D total mass balance in top section
therefore,
L=RD internal liquid flow rate in rectifying section
V=(1+R)D internal vapour flow rate in rectifying section
how are the internal liquid and vapour flow rates determined in the stripping section
Vb= /V / B boilup ratio /L= /V+ B total mass balance on bottoms section
therefore,
/V= Vb. B internal vapour flowrate in stripping section
/L = (1+Vb) B internal liquid flowrate in stripping section
explain how you would obtain the operating line for the rectifying section?
consider a control volume around the top of the columndown to an arbitrary stage n above the feed.
the MVC material balance is
yn+1.V=xD.D + xn.L
hence, passign stream compositions (xn,yn+1) fall on an operating line given by:
y=D/V .xD + L/V .x
this has a slope L/V and passes through the point (xD,xD)
explain how you would obtain an operating line for the stripping section?
consider a control volume around the top of the column down to an arbitrary stgae n below the feed
the MVC material balance is
yn+1./V +zF.F = xD.D + xn./L
also from the MVC material balance on the whole column
zF.F = xD.D +xB.B
hence, the passing stream compostions (xn,yn+1) fall on the operating line given by
y= -xB (B/ /V) + (/L / /V) x
this has a slope of /L / /v and passes through (xB,xB)
what is the intersection of the operating lines?
- rectifying section MVC balance
- stripping section MVC balance
- the difference betweeen 1 and 2
- flow rate of liquid in the feed
- flowrate of the vapour in the feed
substitute 4. and 5. into 3.
hence the point of intersection is on the q-line of the feed mixture
what is the rectifying line in terms of the reflux ratio?
y=(D/V)xD+ (L/V)x operating line equation
L=RD AND V=(1+R)D internal flowrates
y=(xD/R+1) + (R/R+1)x
slope: R/R+1
intercept: xD/R+1
passes through (xD,xD)
explain the mccabe thiele construction
- mark xD,xB and zF on the diagram
- plot the equilibrium curve: y=αx/1+xα-1)
- draw the q-line from (zF,zF) with slope -q/1-q
- draw the rectifying line from (xD,xD) with slope R/R+1
- draw the stripping line from (xB,xB) to the intersection of the q-line and the rectifying line
- draw in the equilibrium stages starting from (xD,xD)
where do the leaving streams plot
on the equilibrium curve
where do the passing streams plot?
passing streams plot on the operating lines (rectifying and stripping)
does the partial reboiler and total condenser both count as equilibrium stages?
no. only the partial reboiler counts as an equilibrium stage
where does the distillate stream plot?
x0=xD plots on the operating line
where does vapour and liquid passing between stages 1+2 plot and between 2+3 plot
on the operating line
where does vapour and liquid leaving stagw 1, stage 2 and stage 3 plot?
on the equilibrium curve
what are the leaving streams related by
equilibrium curve
what are passing streams related by
operating line
what 3 things does a triangle on the x,y diagram represent
each triangle represents a stage with:
- composition of passing stream above( top right)
- composition of leaving streams ( top left)
- composition of passing streams below (bottom left)
how do you find the number of stages N in the column?
the total number of stages on the mccabe thiele construction is N+1, where N is the number of stages in the column
