Topic 3: Introduction to Distillation in Plate Columns

Question 1

Is multi -stage distillation more effective than single stage flash distillation?

Answer 1

yes

Question 2

What are the limitations of flash units?

Answer 2

a single flash unit usually does not provide sufficient separation but further units could be added

Question 3

what happens if you cascade many flash units

Answer 3

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

Question 4

what us the best way to achieve separation?

Answer 4

the solution is to integrate many stages within a single distillation column with internal recycle

Question 5

describe the liquid and vapour flow paths in a plate column

Answer 5

-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

Question 6

name 4 types of trays

Answer 6

sieve tray
movable valve tray
fixed valve tray
bubble cap tray

Question 7

describe the anatomy of a distillation column

Answer 7

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

Question 8

what is the reflux ratio

Answer 8

the reflux ratio is defined as:
R = L/D
where R= (0, infinity]

Question 9

what enters the total condenser at the top of the column?

Answer 9

the vapour leaving the top of the column enters the total condenser

Question 10

what is the purpose of a total condenser

Answer 10

the total condenser condenses but does not subcool all the vapour

Question 11

how is the condensate split?

Answer 11

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

Question 12

why do we need a reflux?

Answer 12

reflux is essential for column operation

R= (0,infinity]

Question 13

what is the boil up ratio?

Answer 13

the boil up ratio is defined as:
Vb= /V /B
where Vb= (0,infinity]

Question 14

what is entering the partial reboiler?

Answer 14

the liquid leaving the bottom of the column enters the partial reboiler

Question 15

what happens to the liquid as it passes through the reboiler

Answer 15

part of the liquid is reboiled(vaporised) and part is not

Question 16

what are the outflows from the reboiler

Answer 16

the outflows from the reboiler are:
saturated vapour, flow rate /V
saturated liquid, flow rate B

Question 17

why is reboiling necessary?

Answer 17

reboiling(together with reflux) is essentia to create the counter-current flow necessaryfor effective column operation

Question 18

what counts as an equilibrium separation stage in a column?

Answer 18

each plate on the column behaves like an equilibrium separation stage

Question 19

what assumption can be made based on each plate of the column behaving like an equilibrium separation stage?

Answer 19

the vapour and liquid phases leaving a given plate are assumed to be in equilibrium
so x and y fall on the equilibrium curve

Question 20

what comprises the feed to a given plate?

Answer 20

generally, the feed to a given plate comprises liquid from the stage above below

Question 21

is the vapour and liquid streams passing between plates in equilibrium

Answer 21

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

Question 22

what does the vertical temperature gradient cause?

Answer 22

some of the vapour (hotter) joining stage n below will condense
some of the liquid (cooler) joining stage n from above evaporate

Question 23

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?

Answer 23

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

Question 24

if the amount of vapour condensing is equal to the liquid evaporating, the what affect does this have on the flowrates?

Answer 24

there is transfer of componentes between the phases but no net change in the flowrates

Vn=Vn+1
Ln=Ln-1

Question 25

what is constant molar overflow?

Answer 25

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

Question 26

what causes the flowrate to differ between the rectifying and stripping section

Answer 26

becauseq of the feed, the flow rates may duffer between the rectifying and stripping section

Question 27

what are the main underlying assumptions that lead to the condition of constant molar overflow?

Answer 27

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

Question 28

what leads to the condition of constant molar overflow

Answer 28

the mccabe thiele assumptions and with the stage energy anf mass blance equations leads to the condition of constant molar overflow

Question 29

what three heat exchangers are typically in a multiple equilibrium separation stage system?

Answer 29

a feed heater a reboiler at the bottom of the column a condenser at the top of the column

Question 30

what is the mccabe thiele construction?

Answer 30

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)

Question 31

what is the mccabe thiele construction based on?

Answer 31

it is based on the assumption of constant molar overflow

Question 32

what does the mccabe thiele construction show on analysis about the leaving streams

Answer 32

the leaing stream compositions fall on the equilibrium (x,y) curve

Question 33

what are the design problem specifications?

Answer 33

F,zF, q, P,xD,xB,R

Question 34

what are the design problem results?

Answer 34

D,B,Vb, N

Question 35

what is the total material balance?

Answer 35

F=D+B

Question 36

what is the MVC material balance

Answer 36

zF.F=xxD.D+xB.B

Question 37

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

Answer 37

D= (zF-xB)F/(xD-xB) B= (xD-Zf)F/(xD-xB)

Question 38

how are the internal liquid and vapour flow rates determinedib the rectifying section?

Answer 38

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

Question 39

how are the internal liquid and vapour flow rates determined in the stripping section

Answer 39

``` 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

Question 40

explain how you would obtain the operating line for the rectifying section?

Answer 40

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)

Question 41

explain how you would obtain an operating line for the stripping section?

Answer 41

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)

Question 42

what is the intersection of the operating lines?

Answer 42

1. rectifying section MVC balance 2. stripping section MVC balance 3. the difference betweeen 1 and 2 4. flow rate of liquid in the feed 5. 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

Question 43

what is the rectifying line in terms of the reflux ratio?

Answer 43

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)

Question 44

explain the mccabe thiele construction

Answer 44

- 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)

Question 45

where do the leaving streams plot

Answer 45

on the equilibrium curve

Question 46

where do the passing streams plot?

Answer 46

passing streams plot on the operating lines (rectifying and stripping)

Question 47

does the partial reboiler and total condenser both count as equilibrium stages?

Answer 47

no. only the partial reboiler counts as an equilibrium stage

Question 48

where does the distillate stream plot?

Answer 48

x0=xD plots on the operating line

Question 49

where does vapour and liquid passing between stages 1+2 plot and between 2+3 plot

Answer 49

on the operating line

Question 50

where does vapour and liquid leaving stagw 1, stage 2 and stage 3 plot?

Answer 50

on the equilibrium curve

Question 51

what are the leaving streams related by

Answer 51

equilibrium curve

Question 52

what are passing streams related by

Answer 52

operating line

Question 53

what 3 things does a triangle on the x,y diagram represent

Answer 53

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)

Question 54

how do you find the number of stages N in the column?

Answer 54

the total number of stages on the mccabe thiele construction is N+1, where N is the number of stages in the column