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Separation Processes 1 Resit > Topic 3: Introduction to Distillation in Plate Columns > Flashcards

Topic 3: Introduction to Distillation in Plate Columns Flashcards

1
Q

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

A

yes

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2
Q

What are the limitations of flash units?

A

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

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3
Q

what happens if you cascade many flash units

A

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

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4
Q

what us the best way to achieve separation?

A

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

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5
Q

describe the liquid and vapour flow paths in a plate column

A

-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

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6
Q

name 4 types of trays

A

sieve tray
movable valve tray
fixed valve tray
bubble cap tray

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7
Q

describe the anatomy of a distillation column

A 
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
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8
Q

what is the reflux ratio

A

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

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9
Q

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

A

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

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10
Q

what is the purpose of a total condenser

A

the total condenser condenses but does not subcool all the vapour

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11
Q

how is the condensate split?

A

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

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12
Q

why do we need a reflux?

A

reflux is essential for column operation

R= (0,infinity]

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13
Q

what is the boil up ratio?

A

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

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14
Q

what is entering the partial reboiler?

A

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

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15
Q

what happens to the liquid as it passes through the reboiler

A

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

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16
Q

what are the outflows from the reboiler

A

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

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17
Q

why is reboiling necessary?

A

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

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18
Q

what counts as an equilibrium separation stage in a column?

A

each plate on the column behaves like an equilibrium separation stage

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19
Q

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

A

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

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20
Q

what comprises the feed to a given plate?

A

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

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21
Q

is the vapour and liquid streams passing between plates in equilibrium

A

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

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22
Q

what does the vertical temperature gradient cause?

A

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

23
Q

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?

A

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

24
Q

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

A

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

Vn=Vn+1
Ln=Ln-1

25
Q

what is constant molar overflow?

A

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

26
Q

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

A

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

27
Q

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

A

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
28
Q

what leads to the condition of constant molar overflow

A

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

29
Q

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

A

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

30
Q

what is the mccabe thiele construction?

A

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)

31
Q

what is the mccabe thiele construction based on?

A

it is based on the assumption of constant molar overflow

32
Q

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

A

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

33
Q

what are the design problem specifications?

A

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

34
Q

what are the design problem results?

A

D,B,Vb, N

35
Q

what is the total material balance?

A

F=D+B

36
Q

what is the MVC material balance

A

zF.F=xxD.D+xB.B

37
Q

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

A

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

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

38
Q

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

A

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

39
Q

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

A 
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

40
Q

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

A

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)

41
Q

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

A

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)

42
Q

what is the intersection of the operating lines?

A
  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
43
Q

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

A

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)

44
Q

explain the mccabe thiele construction

A
  • 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)
45
Q

where do the leaving streams plot

A

on the equilibrium curve

46
Q

where do the passing streams plot?

A

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

47
Q

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

A

no. only the partial reboiler counts as an equilibrium stage

48
Q

where does the distillate stream plot?

A

x0=xD plots on the operating line

49
Q

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

A

on the operating line

50
Q

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

A

on the equilibrium curve

51
Q

what are the leaving streams related by

A

equilibrium curve

52
Q

what are passing streams related by

A

operating line

53
Q

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

A

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)
54
Q

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

A

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

Separation Processes 1 Resit (7 decks)

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