Upload
ahmet-baykan
View
225
Download
0
Embed Size (px)
Citation preview
8/11/2019 10 Introduction to Complex Distillation Methods
1/40
1
Chapter 10
Introduction to Complex
Distillation Methods
In the preceding chapters concerning distilla-
tion (chapters 2-7), we have learned how to sepa-
rate binaryand multi-componentmixtures by the
rather simpledistillation systems (either continu-
ousor batch)
Unfortunately, however, the knowledge of such
simple distillation systemsonly is normally not
enoughfor the industrial-scaledistillation, as
the simpledistillation systems are notcapableof,
e.g.,
8/11/2019 10 Introduction to Complex Distillation Methods
2/40
2
completelyseparating azeotropic mixtures
separatingthe mixtureswhen the relative
volatilityis close to unity (1)[in other
words, when the boiling temperaturesof
the componentsin the mixturesare
relatively closeto each other/one another];
note that if we try to do so, the distillation
costcould be very expensive
To overcomesuch problems/complications,
the complexdistillation systems/techniques must
be employed
8/11/2019 10 Introduction to Complex Distillation Methods
3/40
3
Such complexdistillation methods include
azeotropicdistillation
pressure-swing or two-pressuredistillation
extractivedistillation
reactivedistillation
The details of these complexdistillation tech-
niques are as follows
10.1Breaking Azeotropes with Other Separators
When the mixture becomes azeotropic(i.e.
the mixture behaves as it is the pure substance),
it limitsthe separation by distillation(i.e.it
cannot be separated from each other/one
another by distillation)
8/11/2019 10 Introduction to Complex Distillation Methods
4/40
4
For example, when the mixture of ethanol
(EtOH) and waterreaches the concentrationof
0.8943mole fraction of EtOH, it can no longer
be separatedby the simple/ordinarydistillation
To breakthe azeotrope, or to getthe mixture
with the mole fractionof EtOHhigher than
0.8943, how would we do?
Lets consider the simplest, but less likelyto
be used, technique, illustrated in Figure 10.1
8/11/2019 10 Introduction to Complex Distillation Methods
5/40
5
By employing this technique, the distillate(or
the mixture of A and B) whose concentrationis
close/nearto the azeotropic point from thefirst
distillation columnis sent to another separation
device, which is able to yield pureA
Figure 10.1: The distillation column coupled
with another separating device for braking the
azeotrope
(from Separation Process Engineering by Wankat, 2007)
8/11/2019 10 Introduction to Complex Distillation Methods
6/40
6
Note that the waste streamin Figure 10.1 is
the streamthat contains the mixtureof the sepa-
ratingcomponents (e.g., the mixture of A + B)
However, some questions arise:
If the other techniqueor the other separa-
tion deviceis more effectivethan the dis-
tillationin separating the mixture, why do
we use the distillation in the first place?
or why dont we use the other technique
in the first place?
What/how would we do with the wastestream (i.e.the mixture of separating
components)?
8/11/2019 10 Introduction to Complex Distillation Methods
7/40
7
Lets consider another technique, which is
more likelyto be employed, shown in Figure 10.2
Figure 10.2: The distillation column coupled
with another separating device in which the
bottom product of the other separating deviceis recycled back to the distillation column
(from Separation Process Engineering by Wankat, 2007)
8/11/2019 10 Introduction to Complex Distillation Methods
8/40
8
In this separating scheme, the distillateat or
near the azeotropicconcentration is sent to the
other separation device(s), in which the waste
stream from the other separation device(s) is sent
back to the distillation column
By employing this technique, thefirstdistilla-
tion column operates as a two-feedcolumn (i.e.
thefeedand the recycle stream)
This arrangementis commonly usedin indus-
try
A more complexsystem used to breakan
azeotropeor azeotropesis as illustrated in Figure
10.3
8/11/2019 10 Introduction to Complex Distillation Methods
9/40
9
Figure 10.3: The complex separation system, com-
prising the distillation column and another sepa-
rating device, used for breaking azeotrope(s)(from Separation Process Engineering by Wankat, 2007)
The product from the additional separating
device(s) is returned to the distillation column as
a reflux, in which1o
x y>
8/11/2019 10 Introduction to Complex Distillation Methods
10/40
10
When the refluxhas higher concentration( )ox
thanthe azeotropic distillate( )1y , as depicted in
the McCabe-Thiele diagram in Figure 10.3, the
distillationcan continueuntil the desired concen-
trationof A(or even pureA) is obtained
10.2Binary Heterogeneous Azeotropic
Distillation Processes
Surprisingly, the presenceof anotherazeotro-
picmixture can be used to separate the existing
azeotropicsystem, especially when the two azeo-
tropic systems are heterogeneousto each other
Such technique is called the azeotropicdis-
tillation
8/11/2019 10 Introduction to Complex Distillation Methods
11/40
11
Azeotropicdistillation is performed by adding
a solventor an entrainerto the system; this
entrainerformsan azeotropic mixturewith one
or both of the components
An example of the rather simple(although
not common) azeotropicdistillation is the distil-
lation of n-butanoland water
When the vapour that comes out of the top
of the distillation column with the concentration
of azy (this is an azeotropicconcentration) is con-
densed by a condenser, it is separatedinto two
separate liquidphases:
8/11/2019 10 Introduction to Complex Distillation Methods
12/40
12
one is the water-richphase (the aphase)
another one is the organic(i.e.n-butanol)
phase (the bphase)
as shown in Figure 10.4; note that each phase has
the samevapour phaseconcentration of n-buta-
nol,B
y , but different liquid phaseconcentrations
( )0.01 0.02 and 0.41 0.42B Bx xa b
- -
The distillation system used for this binaryheterogeneous azeotropic system is as depicted in
Figure 10.5
In this distillation system, the column 1is
the strippingcolumn receiving the a(water-rich)
phase with the concentration ofB
xa
from the
separator
8/11/2019 10 Introduction to Complex Distillation Methods
13/40
13
Figure 10.4: The heterogeneous azeotropic system
of n-butanol and water
(from Separation Process Engineering by Wankat, 2007)
The McCabe-Thiele diagram for the 1stcolumn
is on the left hand side(LHS) of Figure 10.6; note
that, for the 1stcolumn, n-butanol is the more
volatilethan water; thus, the bottom productcan
be pure water(i.e., bot
0B
x )
8/11/2019 10 Introduction to Complex Distillation Methods
14/40
14
Figure 10.5: The distillation system for the
heterogeneous binary azeotrope
(from Separation Process Engineering by Wankat, 2007)
Another liquid phase, or theb
(organic) phaseis sent to the column 2 (the 2ndcolumn), which is
also a strippingcolumn
8/11/2019 10 Introduction to Complex Distillation Methods
15/40
15
Figure 10.6: The McCabe-Thile diagram for the
two-column distillation system for the heteroge-
neous binary azeotrope
(from Separation Process Engineering by Wankat, 2007)
On the contrary to the 1stcolumn, in the 2nd
column, n-butanol is the less volatilecomponent,
as it is evident that the equilibriumline is under
the operatingline
8/11/2019 10 Introduction to Complex Distillation Methods
16/40
16
This phenomenoncan occursince a solvent
or an entrainerwith a highboiling temperature
that can dissolveonly n-butanol is addedto the
distillationsystem
As a result, the bottomstream of the 2ndco-
lumn can have a very high concentration(or even
pure) of n-butanol
Another commercialexample of breaking aze-
otrope using the entraineris the addition of
benzene(i.e.the entrainer) into the mixture ofethanol(EtOH) and water
8/11/2019 10 Introduction to Complex Distillation Methods
17/40
17
10.3Pressure-swing or Two-pressure Distillation
As we have learned previously, the changes in
both temperature and pressureaffect the vapour-
liquid equilibrium(VLE)
The changes in temperature and pressurecan
also alter() the compositionof the azeo-
trope
For example, at 1 atm (760 mm Hg), the aze-
otropicconcentration of the ethanol-watermix-ture is at 0.8943mole fraction of ethanol, as
mentioned previously
8/11/2019 10 Introduction to Complex Distillation Methods
18/40
18
However, at the pressure below70 mm Hg,
the azeotropedisappearscompletely
Note that, since, the distillationis commonly
operatedat a constant pressure, it is more con-
venientto alterthe systems pressurethan the
temperature
To appreciate () how the pressure-swing
or the two-pressuredistillation works, lets con-
sider the pressure-swingdistillation system, which
is used to separate the mixture of methyl ethylketone (MEK) and water, shown in Figure 10.7
8/11/2019 10 Introduction to Complex Distillation Methods
19/40
19
Figure 10.7: The pressure-swing distillation
system for breaking the azeotrope
(from Separation Process Engineering by Wankat, 2007)
The column 1 (or the 1
st
column) operates at1 atm, and the azeotropic concentrationof the
mixture is 35 wt% waterand 65% MEK; note
that, at 1 atm, MEKis more volatilethan water
8/11/2019 10 Introduction to Complex Distillation Methods
20/40
20
If thefeedto the 1stcolumnhas a concentra-
tion of MEK lower than65%, which is lower than
the azeotropicconcentration,
the distillatecan have as high concentra-
tionof MEKas 65%(i.e.at the azeotropic
concentration)
the bottomcan be the mixturewith a very
high concentrationof water(or even pure
water)
The relative volatilityof MEKand water
changesafter the pressurereaches ~6.8 atm(or
~100 psia)
8/11/2019 10 Introduction to Complex Distillation Methods
21/40
21
At the pressurehigher than~6.8 atm, water
becomes more volatilethan MEK
Additionally, the azeotropicconcentration at
the pressureof ~6.8 atmchanges from 35% water
(at 1 atm) to 50% water
To obtain pureMEK, the distillate from the
1stcolumn, which contains the mixture of 35%
water and 65% MEK, is sent to the column 2(or
the 2ndcolumn) operating at ~6.8 atm
Since, at this pressure (~6.8 atm), wateris
more volatilethan MEK, and the azeotropic con-
centration is at 50% water and 50% MEK,
8/11/2019 10 Introduction to Complex Distillation Methods
22/40
22
the distillateof the 2ndcolumn can be the
mixture with the concentrationof waterof
as high as 50%
the bottomof the 2ndcolumn can be the
mixture with a very high concentrationof
MEK(or even pureMEK)
10.4Extractive Distillation
Extractive distillationis another complexdis-
tillation system used for separatingthe azeotropic
mixture(s)
8/11/2019 10 Introduction to Complex Distillation Methods
23/40
23
In the extractivedistillation, a solventis added
to the distillation column such that only one com-
ponent (e.g., B from the mixture of A + B) is
attracted to it
If the solventhas a highboiling point, the
volatilityof the mixtureof the solventand species
Bis reduced, until it is lowerthan that of species
A
Accordingly, species Abecomes the more vo-
latilecomponent (than the mixture of species B+ solvent); thus, it can easily be removed from
the distillation system as the distillate
8/11/2019 10 Introduction to Complex Distillation Methods
24/40
24
An exampleof extractive distillation is as illu-
strated in Figure 10.8
Figure 10.8: An extractive distillation system
(from Separation Process Engineering by Wankat, 2007)
In this system, the solventwith a high boiling
pointis added to the 1stcolumn several stages
abovethefeed stageand a few stages belowthe
topof the column
8/11/2019 10 Introduction to Complex Distillation Methods
25/40
25
In the topsection (i.e.the section abovethe
solvent feeding point), species Ais removedfrom
the column as the distillate; this is because the
solvent, which is a high boiling temperaturesub-
stance, is mixedwith species B, and the resulting
mixturehas a very high boiling temperaturerela-
tive to that of species A
In the middle section(i.e.the section from
thefeeding pointof the solventto thefeed stage),
the solventseparatesspecies Bfromthe mixture
of A + B
8/11/2019 10 Introduction to Complex Distillation Methods
26/40
26
It is important to note that the amountof
the solventmust be large enoughto completely
separatespecies Bfromspecies A, otherwise, the
distillatewill notbe pureA, as species Bcan
forman azeotropic mixture withspecies A
In the bottom section(i.e.the section below
thefeed stage), since the mixtureof the solvent
and species Bis less volatilethan species A, the
mixtureof species Band the solventis removed
from the 1stcolumn as the bottom
The solventcan then be separatedfrom spe-
cies Bin the 2nddistillation column
8/11/2019 10 Introduction to Complex Distillation Methods
27/40
27
If the solventis chosenproperly, it should
easilybe removedfrom species Bby the simple
distillation column with onlyafewequilibrium
stages
The solventobtained from the 2ndcolumn can
be recycledto the 1stcolumn to be usedas the
solventagain
Note that, generally, the extractivedistillation
is notapplicablefor separating the mixtureof
isomers
8/11/2019 10 Introduction to Complex Distillation Methods
28/40
28
10.5Azeotropic Distillation with Added Solvent
When a homogeneous azeotropeis formed,
the proceduresused for heterogeneousazeotropic
distillation cannotbe employed
In order to solvethis problem, the solvent(or
an entrainer) thatformsa binaryor ternary
azeotropeis addedto the systemto enablethe
separationof the mixture
An example of the azeotropic distillationwithadded solventis as illustrated in Figure 10.9
8/11/2019 10 Introduction to Complex Distillation Methods
29/40
29
Figure 10.9: The separation of butadiene from
butylenes using ammonia as an entrainer
(from Separation Process Engineering by Wankat, 2007)
At the temperature of 40 oC, the azeotropeof
the mixtureof butadieneand butyleneexiting the
1stcolumn as the top product is homogeneous,
and, as a result, very difficultto be separated
from each other; note that, as butadieneis the
less volatilecomponent, it leavesthe 1stcolumn
purely(or nearly purely) as the bottom
8/11/2019 10 Introduction to Complex Distillation Methods
30/40
30
Ammoniais addedto the distillation column
either asa refluxor asafeed
The mixtureof butadiene + butylenes + am-
monia is condensedat the condenser
At a temperature below20 oC(note that, the
colderthe temperatureof the settler, the purer
the twoliquid phases), twoliquidphasesare
formed atthe liquid-liquid settler:
the upperphase: contains mainlybutylene
and ammonia(with a small amount of
butadiene)
the lowerphase: contains mainlyammonia,
with small amounts of butadiene and buty-
lene
8/11/2019 10 Introduction to Complex Distillation Methods
31/40
31
The lowerphase is returnedto the distillation
column (to be usedas the recycle entrainer),
while the upperphase is sent to the subsequent
strippingcolumn
In the strippingcolumn, the mixtureof buty-
leneand ammoniacan be distilled and thus sepa-
rated into
the topproduct (the distillate), which isthe azeotropicmixture of butyleneand
ammonia
the bottomproduct (the bottom), which
can be purebutylene
The distillateof the stripping column is recy-
cledto the liquid-liquid settler
8/11/2019 10 Introduction to Complex Distillation Methods
32/40
32
Another example of the azeotropic distillation
with the added solventis as shown in Figure 10.10
In Figure 10.10, species Aand the solvent(or
the entrainer) form the azeotropewith a low
boiling point; thus leavingthe distillationcolumn
as the distillate
Species B, whose boiling temperatureis higher
than the mixture of species A and the solvent (in
other words, species B is a less volatilecompo-
nent) can be removedfrom the column as pureBas the bottom
8/11/2019 10 Introduction to Complex Distillation Methods
33/40
33
Figure 10.10: An azeotropic distillation with a
low boiling temperature azeotrope
(from Separation Process Engineering by Wankat, 2007)
Species Acan be removedfrom the solventby a water-wash extraction
8/11/2019 10 Introduction to Complex Distillation Methods
34/40
34
The additionaldistillation column(on the
RHS of Figure 10.10) is used to separatethe sol-
ventfrom water
The thirdexample is commonly used for sepa-
rating ethanol(EtOH) from water, using a hydro-
carbonas an entrainer, as depicted in Figure
10.11
Previously, benzeneis used as the solvent
(entrainer); however, because of its toxicity (as
it is a carcinogenicagent), it has been replacedby some other hydrocarbons, e.g., diethyl ether,
n-pentane, or n-hexane
8/11/2019 10 Introduction to Complex Distillation Methods
35/40
35
Figure 10.11: A ternary azeotropic distillation
for separating ethanol from water
(from Separation Process Engineering by Wankat, 2007)
The mixture of EtOH and water at nearly
azeotropic concentration(~70-90% EtOH) is fed
into the azeotropicdistillation column, in which
the hydrocarbon solvent/entraineris added to
the column as a reflux
8/11/2019 10 Introduction to Complex Distillation Methods
36/40
36
The vapour distillateis condensedand then
separatedinto twoliquid phases:
the upperlayer or the solventlayer
the waterlayer
The compositionof each layerdepends onthesolventused
For example, if n-hexaneis used,
the upperlayer contains 96.9 wt% hexane,
2.9% EtOH, and 0.5% water
the loweror waterlayer contains 6.2%
hexane, 73.7% EtOH, and 20.1% water
The upperlayer is returnedto the azeotropic
columnas a refluxto be used as the recycle en-
trainer
8/11/2019 10 Introduction to Complex Distillation Methods
37/40
37
The lowerlayer (with a small amount of
n-hexane, which is used to break the azeotrope)
is sentto the stripping column, to obtain pure
wateras the bottom
The additionof solvent(e.g., n-hexane) into
the azeotropiccolumn (the LHScolumn) makes
the azeotropicmix-ture of EtOH + water and
the solvent becomemore volatilethan EtOH
Thus, EtOH(which is now less volatile) can
be removed purelyas the bottom
10.6Reactive Distillation
The advantageof having the reactivedistilla-
tion system or the distillation witha chemical
8/11/2019 10 Introduction to Complex Distillation Methods
38/40
38
reactionis that the product(s)of the reactioncan
be removedcontinuously bythe distillation, which
drives the reversiblereaction to completion
The arrangement of the distillation system for
the reversiblereactions:
A C
or A C + D
is as depicted in Figure 10.12
If the reactantis less volatilethan the pro-
duct(s), the arrangementon the LHS(A) is ap-propriate; note that the bottom bleedis required
to preventthe build-upof non-volatileimpurities
8/11/2019 10 Introduction to Complex Distillation Methods
39/40
39
Figure 10.12: The reactive distillation system for
a single-reactant reaction
(from Separation Process Engineering by Wankat, 2007)
If the reactantis more volatilethan the pro-
duct(s), the arrangementon the RHS(B) is ap-
propriate; note that the top bleedis required to
re-move volatileor gaseouscomponents
The arrangement for the reaction:
A + B C + D
8/11/2019 10 Introduction to Complex Distillation Methods
40/40
in which the reactants are of intermediate volati-
lity between the two products (i.e.C and D), is
as shown in Figure 10.13
Figure 10.13: The reactive distillation system
with two reactants
(from Separation Process Engineering by Wankat, 2007)
Note also that the arrangements Cand Dare
widely usedfor the esterificationreaction:
Fatty acid + Alcohol Ester+ Water