1. Level 4 Separation System
Decisions to be taken
1. General structure
2. Vapor recovery system
3. Liquid recovery system
Best separation system = f (design vars)
Design guidelines

2. Separations Data requirements
Together with choice of reactions and reaction conditions
is the most important process task
Data requirements
Phase behavior
boiling point ( vapor pressure )
melting point
volatility ( relative )
Solubility in various solvents
Density
Size
Adsorptivity on surfaces
Magnetic + electrostatic properties
Chemical reactivity
Must find way of exploiting difference in some properties
between species ( groups of species ) to be separated

3. Classification of Separation Systems
Mixture of Components …
Separating
Processes
Thermal
separation
Rate-governed
separation
Mechanical
separation …
Separating
Operation
Absorption
Extraction
Distillation
Main column
Side stripper
Feed stream
Combined sys. …
with
without
Heat integration
Heat integration
Ref) K. Hartmann & K. Kaplick
“Analysis and Synthesis of Chemical Process systems.”
Elsevier (1990) pp160

4. 1. General structure ( vapor – liquid processes, no solids )
Decision : do we need a vapor or liquid ( or both )
separation system?
depends on the base of ( reactor )
exit stream
liquid
reactor
liq
Liq. Sep.
Sys.
feeds
products
Liq. recycle
vapor
cool down the stream to 100 ℉ (cooling water temp.)
and use liquid recovery systems
(fig 7.1-4)

5. Mixed phases ( fig )
Gas recycle
vap.sep.
Sys.
vap
purge
Phase
split
liq
liq
vap
If xrec ≫ xpro
Reactor
system
liq
Liq.sep.
Sys.
products
Liq recovery
Basis for schemes above
phase splits are cheapest method of separation
if phase split not obtained by cooling water(100℉), try
a) pressurize reactor (for gas feed and recycle)
b) compressor or refrigerator (for gas feed and recycle)
if small amounts of V or L obtained, eliminate phase split

6. phase split calculation ( flash )Zi
V, yi
L, xi
( approximation calculation, Douglas p166 )
K = K( T,P,x,y )
First approx : Raoults law ( ideal mixture, low pressure )
( Antoine eqn.)
NOT VALID for nonideal solutions ( polar, electrolytic soln )
FLASH routines are workhorse of CAD packages
- PPDS, PROCESS, SpeedUp, FLOWTRAN, ASPEN…..

7. 2. Vapor Recovery System ( where? How? )
location of vapor recovery sys. a b c
Gas
recycle
purge
Vapor from reactor or phase split
a) On purge stream if significant amounts of
valuable materials lost
b) On gas recycle stream if recover materials
may shift product distribution or catalyst poisoning
c) On the vapor stream if both reasons for a, b,
d) No recovery system required if neither reasons
for a nor b are valid

8. The cheapest vapor recovery system ?
condensation ( high pressure and/or low T )
good separation when K ≫ 1 ( e.g. 10 )
K ≪ 1 ( e.g. 0.1)
absorption ( liquid solvent )
adsorption
membrane
reaction
Normally required to estimate size and cost of units
to determine the cheapest separation scheme
Design the vapor recovery system first then consider
the liquid separation system
( ∵ the vapor recovery processes usually generates
a liquid stream that must be further purified )

9. 3. Liquid Recovery System
Decisions
applicability of distillation
sequence of columns/separations
method of separation
Distillation is often the least expensive for liquids
if relative volatility ≤ → separation difficult
fig
B,C
Separation
task B A C A B C D E D 1 2 3
lump E
B,C,D,E
D,E
Column
A/B,C,D,E
A,B,C/D,E
A,B,C,D/E
B,C,/D,E
A/B,C
A,B,C,/D
A/B,C,D
B,C,D/E
D/E
B,C/D
4 products
5 combinations

11. Number of alternative schemes is large !
No. of species …..
No. of species …..
( e.g. Table 7.3-2)
Simplification using heuristics
Evaluation of all alternatives using short cut methods
Heuristics for column sequencing
Remove corrosive components
Remove reactive components or monomers
Remove products as distillate
Remove recycle streams as distillate
for simple columns ( i.e., one top, one bottom column)
Remove most plentiful first
Remove lightest first
High – recovery separations last
Difficult separations last
Favor equimolar splits
Next separation should be cheapest
Minimize no. of columns in recycle loops
Many more in Douglas book and references
( Table 7.3-5(6), Hartmann )

12. Separation cost ∝
Feed rate
Property difference
e.g.
Alternatives to distillation ( α＜1.1 liquid )
Extraction ( fig )
Extractive distillation ( fig )
Azeotropic distillation ( fig )
Reactive distillation ( fig )
Crystallization ( fig )
ex ) HDA process : Separation sequence
flash
Table 7.1-1
(p167)
H2 = 2
CH4 = 11
Benz = 235.4
Tol. = 87.4
Diphenyl = 4

13. i) If separate light ends and prod. (Benzene)
Spec.
ii) If we attempt low pressure flash
change ri (ki ) ( table B-4, p531)
Moreover, large loss of benzene $105/yr
∴ separate H2, CH4 first
Benz = 235.4
Tol = 87.4
Dip. = 4
Heuristics
lightest first
most plentiful first
equimolar split
∴ direct sequence
H2, CH4
Benz.
Tol.
Dip
Last column involves the least species design first

14. αtop 100 αbot 24.7 Design of toluene/diphenyl column ( p 532 )
Vapor pressure data
P ambient ( can use cooling water )
αtop
αbot
Conservative average α = 25
Feed comp. From mass balance
xF = 0.956
Recover % toluene overhead
99.5% diphenyl bottom
From feed
xD = toluene
xB = 0.095
Minimum reflux ( Underwood eqn. )
Actual reflux
Min. No. of theoritical trays ( Fensk’s eqn. )
bottom

15. No. of theoritical trays ( Gilliland’s eqn.)
NT = 2Nm = 6.2
Overall plate efficiency ( O’ Connell ) p451
: viscosity of feed
N ( actual no. of trays )
Estimate tray spacing
Calculate height
Vapor load diameter
V = L + D = (R + 1)D =91.73
A eqn ( A )
Guthrie’s correlation
Annual cost $ 26300/yr (B-84)

16. Complete synthesis of conceptual process
Case Study:
Complete synthesis of conceptual process
Monochlorodecane required for detergents
Objective : devise a few process concepts
1) Common sources of
Cl : Cl2 molecule, HCl
Hydrocarbon : nC10H22, decane ( C10H20 )
decanol ( C10H21OH )
2) Alternative Reaction paths
light
C10H22 + Cl C10H21Cl + HCl
C10H21Cl + Cl C10H20Cl2 + HCl
C10H20 + HCl C10H21Cl
C10H21OH + HCl C10H21Cl + H2O
light
cat
3) Reaction path screening
Guideline
Select paths with large economic potential
( value of products – reactants )
Avoid adding species, solvents etc.
Avoid hazardous materials, disposal problems
Avoid excessive high pressures, low temp etc.
( high utility + operating costs )

17. Economic potential $/kmol
Decane ( C10H22 ) DEC
Decene ( C10H20 )
Decanol ( C10H21OH )
Chlorine ( Cl2 ) Cl
HCl HCl
Honochloro decane ( C10H21Cl ) MCD
Drchloro decane ( C10H20Cl2 ) DCE
① Selectivity
S =
Moles of MCD produced
Moles of DEC reacted
DEC Cl MCD DCD HCl
Basis s s s s
1 mole MCD 1/s (2/s-1) (1/s-1) (2/s-1)
EP = [ (2/s – 1)] – [10.58×1/s ×(2/s –1)
= 36.7 – 13.9/s ( $/kmol MCD )
② EP = [35] – [ ] = 6.43
③ EP = [35]-[30.86 = 2.20] = 1.94
22.8 ② ③
0.38
0.458 1
∴ select reaction path 1 if selectivity ≥ 0.46
try and suppress side reaction

18. 4) Species allocation ( reaction path 1 )
HCl, Cl2.,DEC waste
MCD product
DCD waste ①
Cl2
r×n
DEC
Discard remaining
reactants
Feed stoichiometric
Very large conversion ②
Cl2
r×n
DEC
HCl waste
MCD product
DCD waste
Cl2, DEC
Feed stoichiometric
Smaller conversion
Recycle unused
reactants ③
AS but recycle Cl2 and DEC as separate streams ②
④ same as ① but complete conversion ( fewer separation )
HCl waste
MCD p.
DCD w.

19. DEC ⑤
HCl
Cl2.
MCD product
DCD waste
waste
Cl2
r×n
DEC
Large excess of DEC to reduce DCD formation
( Cl2 probably all used )
Cl2
r×n
DEC
HCl
MCD product
DCD waste
waste ⑥
Large excess of Cl2 to completely consume
most valuable reactant
Large amount of DCD ?
Cl2
r×n
DEC
HCl
Cl2.
MCD product
DCD
waste
Feed stoichiometric
( small excess DEC )
Large conversion, impurities acceptable in product

20. Questions high conversion feasible with stoichiometric feed ??
what is selectivity to MCD ?
what is DEC/Cl2 ratio for good selectivity ?
Pilot plant, bench chemist etc.
Lab. data
Stoich. Feed products
1 mol Cl mol MCD s = 0.8
1 mol DEC mol DCD EP = 19.3
Excess Feed products
1 mol Cl mol MCD
5 mol DEC mol DCD s = 0.95
4 mol DEC, HCl EP = 22
traces Cl2
∴ loss of selectivity fairly large with stoich. Feed.
second scheme probably more interesting although
difference in EP not too large.
keep first possibility as alternative schemes ② ⑤

21. 5) Separations Scheme ② r×m separ Cl2 1 DEC 1 DEC 4 Cl2 trace
HCl W.
MCD P.
DCD W.
Scheme ⑤
DEC 4
Cl2 trace W.
Cl2 1
DEC 1
r×n
separ
HCl W.
MCD P.
DCD W.
5) Separations
m.p. ( ℃ ) b.p. ( ℃ ) solub. In water (kg/m3 at 100 ℃)
HCl Cl
DEC
MCD
DCD

22. ? Possible separation sequences for scheme 2 a) HCl Cl2, DEC MCD DCD

23. 6) General structure of separation system
reaction is at high T
reactor product stream is a gas phase
Is phase split possible/desirable ?
Large ΔTboiling pt between HCl, Cl2 / DEC, MCD, DCD
Condensation possible at atmosphere pressure
w / cooling water
r×n
Phase
split
Liq.
Sep. sys.
HCl, Cl2
MCD
DCD
DEC recycle
To waste ?

24. Vapor recovery system
Necessary ? Small amount of Cl waste
possible to recover HCl byproduct ?
Separation HCl / Cl2
a) Distillation large ΔTb but
High P
Refrigerated condenser
b) Could use absorption with water solvent
Cl H2O
H2O
HCl + Cl2
H2O + HCl
Cl2 + H2O cannot be recycled to reactor
( H2O + HCl = highly corrosive )
To remove water
drying process
Silica gel
H2SO4

25. Process so far
dryer
absorber
reactor
Phase
split
Liquid
Recovery
system
H2SO4
H2O
HCl
Cl2
DEC
MCD p.
DCD w.
Fresh DEC
Q. Other solvents possible ?
DEC
absorbs Cl2 from HCl – Cl2 mixture
already in the process
could recycle without separation of DEC / Cl2
DCD
absorbs Cl2 from HCl – Cl2
avoids drier
MCD
could be used before final purification of product

26. Liquid separation system
mole DEC
0.95 mole MCD
0.05 mole DCD
DEC recycle
MCD product
DCD waste
Remove
most plentiful first
direct sequence ( lightest first ) DEC / MCD. DCD
product as distillate
Normal
b.p. (℃) ΔTb
DEC
MCD
DCD
DEC
MCD
DCD
MCD ( product )

27. Integration of vapor recovery and liquid
separation systems
i) e.g. Use DEC as solvent
r×n
Phase
split
absorb 1 2
Cl2
DEC, Cl2
HCl
HCl ( DEC, Cl2 )
DEC
Fresh
MCD
DCD
large amount of DEC available
large L / G in absorber
simple process scheme
solvent loss ?

28. ii) Use MCD as solvent
HCl ( DEC, Cl2 )
HCl
Cl2
absorb
Phase
split
r×n
MCD
( Prod )
Cl2
MCD
Cl2
DEC, Cl2 1
Fresh
DEC 2
MCD
DCD
DCD
Absorbed Cl2 recovered + recycled
Increased load or separations 1, 2
Also possible to feed MCD, Cl2 from absorber to
column 2, and recover Cl2 from a partial condenser
at top of column 2
Alternatively – could use MCD, DCD stream as solvent

29. iii) use MCD + DCD as solvent
r×n
Phase
split
absorb 1 2
Cl2
DEC, Cl2
HCl
MCD
DCD
Fresh
DEC
( prod )

30. 7) Material balances fout,i fin,i fin,i Linear rigorous
Linear material balances in terms of molar flow rater and
fractional recoveries ( selectivity or conversion )
Reactor s
fin,i
fout,i
Phase splitter ( flash ) Vi
P.T
fin,i Li
Same for dividers ( purge ), columns, etc.
∴ i) assemble linear equations for all units
ii) identify degree of freedom of the system
design vars
S.T.P. fractional recoveries in columns feed
ratios, recycle rate, purge compositions

31. fi solve linear material balances in terms of
all design variables optimize EP
To obtain the specified fractional recoveries is
a design problem di fi bi
e.g. find No. of stages
pressure
feed position ……
That yields desired fractional recovery
For fractional recovery model choice ( distillation )
recovery of key component : %
losses in non – key : 0.15~0.3% losses of keys

32. fi Example di i mol/h comp 4 DEC 0.95 MCD 0.05 DCD bi
Light key DEC 99% recovery in distillate
Heavy key MCD 99% recovery in bottoms
DCD loss = 0.2 loss of MCD to distillate
d1 = 0.99 f1 = mol / h
d2 = 0.01 f2 = 5* mol / h
d3 = 0.2 d2 = 1* mol / h
b1 = 0.01 f1 = mol / h
b2 = 0.99 f2 = mol / h
b3 = f3 – d3 = mol / h
Can use this model together with other linear models for
complete material balance

33. Detailed studies – MCD process
Use DEC as
solvent to recover Cl2
Use H2O as
Cl2 to waste
Cl2
price
0 % conversion
of Cl2
100
( concentrated sulphuric
acid to remove water )
Small amount
of Cl2
High conversion + low Cl2 price waste
Intermediate conversions better to recover Cl2
using DEC as solvent
Low conversion amount of DEC to be used
becomes very large
- better to use water as separation agent
% conversion 100 1 2 3
profit
1) Cl2 recycled ( H2O solvent )
2) Cl2 recycled ( DEC solvent )
3) Cl2 to waste