• Decisions to be taken

1. General structure

2. Vapor recovery system

3. Liquid recovery system

Best separation system = f (design vars)

•Design guidelines

Level 4 Separation System

Separations

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 propertiesbetween species ( groups of species ) to be separated

Classification of Separation Systems

Mixture of Components

Thermalseparation

Rate-governedseparation

Mechanicalseparation

Absorption Extraction Distillation

Main column Side stripper Feed stream Combined sys.

with without withoutwith

Heat integration Heat integration

Ref) K. Hartmann & K. Kaplick “Analysis and Synthesis of Chemical Process systems.” Elsevier (1990) pp160

SeparatingProcesses

SeparatingOperation

…

…

…

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

productsreactorLiq. Sep.

Sys.feeds

Liq. recycle

liq

• vapor cool down the stream to 100 (cooling water temp.)℉ and use liquid recovery systems (fig 7.1-4)

• Mixed phases ( fig 7.1-3 )

Reactor system

Phasesplit

vap.sep.Sys.

Liq.sep.Sys.

Gas recycle

vap

vap

liq

liq

Liq recovery

liqIf xrec ≫ xpro

purge

products

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

phase split calculation ( flash )

FZi

V, yi

L, xi

0)( ii

iii

iii

xy

xky

LxVyFz

LVF

),,1(

)1,,1(

ni

ni

( approximation calculation, Douglas p166 )

K = K( T,P,x,y )First approx : Raoults law ( ideal mixture, low pressure )

,p

pk

sati

i CT

BAp sat

ln ( Antoine eqn.)

NOT VALID for nonideal solutions ( polar, electrolytic soln )

• FLASH routines are workhorse of CAD packages - PPDS, PROCESS, SpeedUp, FLOWTRAN, ASPEN…..

2. Vapor Recovery System( where? How? )

location of vapor recovery sys.

ab

c

Gasrecycle

purge

Vapor from reactor or phase split

a) On purge stream if significant amounts of valuable materials lostb) 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

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 )

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 ≤ 1.1 → separation difficult fig 7.3-2

A 3.2B 1.7C 1.6D 1.0E 0.4

lump 21 3

Separationtask

B,C,D,E D,E

D

E

A

B,C B

C

Column 1 2 3

A/B,C,D,EA,B,C/D,EA,B,C,D/EA,B,C,D/EA/B,C,D,E

B,C,/D,EA/B,CA,B,C,/DA/B,C,DB,C,D/E

D/ED/EA/B,CB,C/DB,C/D

4 products 5 combinations

Number of alternative schemes is large !

No. of species 2 3 4 5 6 …..

No. of species 1 2 5 14 42 …..( e.g. Table 7.3-2)

a) Simplification using heuristicsb) 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 loopsMany more in Douglas book and references( Table 7.3-5(6), Hartmann )

Separation cost ∝ Feed rateProperty difference

e.g. ..pbT

Alternatives to distillation ( α ＜ 1.1 liquid )• Extraction ( fig 7.3-7 ) • Extractive distillation ( fig 7.3-8 ) • Azeotropic distillation ( fig 7.3-9 ) • Reactive distillation ( fig 7.3-10) • Crystallization ( fig 7.3-11 )

ex. 7.3-1) HDA process : Separation sequence

flash

H2 = 2CH4 = 11Benz = 235.4Tol. = 87.4Diphenyl = 4

Table 7.1-1(p167)

i) If separate light ends and prod. (Benzene)

997.0947.04.235112

4.235＜

x Spec.

ii) If we attempt low pressure flash change ri (ki ) ( table B-4, p531)

specxx ＜9935.0

Moreover, large loss of benzene $105/yr ∴ separate H2, CH4 first

iii) 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

Design of toluene/diphenyl column ( p 532 )

• Vapor pressure data

• Feed comp. From mass balance xF = 0.956

• Recover 99.5% toluene overhead 99.5% diphenyl bottom

From feed

xD = 0.9996 toluenexB = 0.095

• Minimum reflux ( Underwood eqn. )

0347.01

)1(

1

1

F

D

F

Dm x

x

x

xR

• Actual reflux

05.05.1 mRR

P ambient ( can use cooling water )αtop 100

αbot 24.7Conservative average α = 25

• Min. No. of theoritical trays ( Fensk’s eqn. )

31.3ln

]/)1)][(1/(ln[

wwDDm

xxxxN

xxw bottom

• No. of theoritical trays ( Gilliland’s eqn.) NT = 2Nm = 6.2

• Overall plate efficiency ( O’ Connell ) p451

302.0)3.0(

5.025.00

E

F : viscosity of feed

N ( actual no. of trays )

226.21302.0

2.6

• Estimate tray spacing

Calculate height

Vapor load diameter

V = L + D = (R + 1)D =91.73

A eqn ( A. 3-12 )

Guthrie’s correlation

Annual cost $ 26300/yr (B-84)

Case Study:Complete synthesis of conceptual process

Monochlorodecane required for detergentsObjective : devise a few process concepts

1) Common sources of Cl : Cl2 molecule, HCl

Hydrocarbon : nC10H22, decane ( C10H20 ) decanol ( C10H21OH )

2) Alternative Reaction paths

① C10H22 + Cl2 C10H21Cl + HCl C10H21Cl + Cl2 C10H20Cl2 + HCl② C10H20 + HCl C10H21Cl③ C10H21OH + HCl C10H21Cl + H2O

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 )

lightlightcat

Economic potential $/kmol

Decane ( C10H22 ) DEC 10.58Decene ( C10H20 ) 26.46Decanol ( C10H21OH ) 30.86Chlorine ( Cl2 ) Cl2 3.90HCl HCl 2.20Honochloro decane ( C10H21Cl ) MCD 35.00Drchloro decane ( C10H20Cl2 ) DCE 0

① Selectivity

S =Moles of MCD produced

Moles of DEC reacted

DEC Cl2 MCD DCD HCl Basis 1 2-s s 1-s 2-s1 mole MCD 1/s (2/s-1) 1 (1/s-1) (2/s-1)

EP = [35 + 2.20(2/s – 1)] – [10.58×1/s + 3.90 ×(2/s –1) = 36.7 – 13.9/s ( $/kmol MCD )

② EP = [35] – [26.46 + 2.20] = 6.43 ③ EP = [35]-[30.86 = 2.20] = 1.94

22.8②

③

0 0.38 0.458 1

∴ select reaction path 1 if selectivity ≥ 0.46 try and suppress side reaction

4) Species allocation ( reaction path 1 )

①Cl2 r×n

DEC

HCl, Cl2.,DEC wasteMCD productDCD waste

Feed stoichiometricVery large conversion

Discard remaining reactants

②

Cl2 r×n

DEC

HCl wasteMCD productDCD waste

Cl2, DEC

Feed stoichiometricSmaller conversion

Recycle unused reactants

③ AS but recycle Cl2 and DEC as separate streams②

④ same as but complete conversion ( fewer separation )①

HCl wasteMCD p.DCD w.

⑤

Cl2 r×n

DEC

HCl Cl2. MCD productDCD waste

DEC

waste

Large excess of DEC to reduce DCD formation ( Cl2 probably all used )

Cl2 r×n

DEC

HCl MCD productDCD waste

Cl2 waste

⑥

Large excess of Cl2 to completely consume most valuable reactant

Large amount of DCD ?

Feed stoichiometric ( small excess DEC )Large conversion, impurities acceptable in product

Cl2 r×n

DEC

HCl Cl2. DEC MCD productDCD

waste

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 products1 mol Cl2 0.8 mol MCD s = 0.8 1 mol DEC 0.2 mol DCD EP = 19.3

Excess Feed products1 mol Cl2 0.95 mol MCD 5 mol DEC 0.05 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

Scheme ②

r×m separCl2 1DEC 1

DEC 4 Cl2 traceHCl 1.05 W.

MCD 0.95 P.

DCD 0.05 W.

Scheme ⑤

r×n separCl2 1DEC 1

DEC 4

HCl 1.05 W.

MCD 0.95 P.

DCD 0.05 W.

Cl2 trace W.

5) Separations

m.p. ( ) b.p. ( ) solub. In water (kg/m℃ ℃ 3 at 100 )℃

HCl -111 -85 380Cl2 -101 -34 25DEC -30 174 -MCD -50 215 -DCD -40 241 -

Possible separation sequences for scheme 2

a)HCl

Cl2, DEC

MCD

DCD

b)

DCD

MCD

Cl2HCl

c)HCl

Cl2, DEC

MCD

DCD

d)

DCD

MCD

HCl

Cl2, DEC

HCle)

DCD MCD

Cl2, DEC?

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×nPhasesplit

Liq.Sep. sys.

HCl, Cl2

MCDDCD

DEC recycle

To waste ?

Vapor recovery system

Separation HCl / Cl2

Necessary ? Small amount of Cl2 waste possible to recover HCl byproduct ?

a) Distillation large ΔTb butHigh P

Refrigerated condenser

b) Could use absorption with water solvent

Cl2 + H2OH2O

HCl + Cl2 H2O + HCl

Cl2 + H2O cannot be recycled to reactor ( H2O + HCl = highly corrosive )

To remove water drying process

Silica gelH2SO4

Process so far

dryer absorber

reactor Phasesplit

LiquidRecoverysystem

H2SO4

H2O

H2OHCl

H2SO4

H2O

Cl2Cl2

DEC

MCD p.

DCD w.

HClCl2

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

Liquid separation system

4 mole DEC0.95 mole MCD0.05 mole DCD

DEC recycle

MCD productDCD waste

Remove• most plentiful first• direct sequence ( lightest first ) DEC / MCD. DCD• product as distillate

Normalb.p. ( ) ΔT℃ b

DEC 174MCD 215 41DCD 241 26

DECMCDDCD

DEC

MCDDCD

DCD

MCD ( product )

Integration of vapor recovery and liquid separation systems

i) e.g. Use DEC as solvent

r×n Phasesplit

absorb

1

2

Cl2

DEC, Cl2

HClCl2

HCl ( DEC, Cl2 )DEC Fresh

DEC

MCDDCD

MCD

DCD

• large amount of DEC available large L / G in absorber

• simple process scheme• solvent loss ?

ii) Use MCD as solvent

r×n Phasesplit

absorb

1

2

Cl2

DEC, Cl2

HClCl2

HCl ( DEC, Cl2 )

MCDDCD

MCD( Prod )

DCD

FreshDEC

MCDCl2

• 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

iii) use MCD + DCD as solvent

r×n Phasesplit

absorb

1

2

Cl2

DEC, Cl2

HClCl2

HCl

MCDDCD

MCDDCD

DCD

FreshDEC

MCDDCDCl2

MCD ( prod )

DECMCDDCDCl2

7) Material balancesLinear

rigorous

Linear material balances in terms of molar flow rater andfractional recoveries ( selectivity or conversion )

Reactor

sfin,ifout,i

iiniiout ff ,,

)(si

Phase splitter ( flash )

P.Tfin,i

Li

Vi

iinii

iinii

fL

fV

,

,

)1(

).( TP

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

• solve linear material balances in terms of all design variables optimize EP• To obtain the specified fractional recoveries is a design problem

fi

di

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 : 99%• losses in non – key : 0.15~0.3% losses of keys

Example

i mol/h comp

1 4 DEC2 0.95 MCD3 0.05 DCD

fi

di

bi

Light key DEC 99% recovery in distillateHeavy key MCD 99% recovery in bottoms DCD loss = 0.2 loss of MCD to distillate

Can use this model together with other linear models forcomplete material balance

d1 = 0.99 f1 = 3.96 mol / hd2 = 0.01 f2 = 5*10-3 mol / hd3 = 0.2 d2 = 1*10-3 mol / hb1 = 0.01 f1 = 0.04 mol / hb2 = 0.99 f2 = 0.9405 mol / hb3 = f3 – d3 = 0.049 mol / h

Detailed studies – MCD process

Cl2price

0 % conversion of Cl2

100

Small amountof Cl2

Use DEC assolvent to recover Cl2

Use H2O as solvent to recover Cl2

Cl2 to waste

( concentrated sulphuric acid to remove water )

• 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

0 % conversion 100

1

23

profit1) Cl2 recycled ( H2O solvent )2) Cl2 recycled ( DEC solvent )3) Cl2 to waste