Hierarchy of Decisions

1. Batch versus continuous

2. Input-output structure of the flowsheet

3. Recycle structure of the flowsheet

4. General structure of the separation system Ch.5

a. Vapor recovery system

b. Liquid recovery system

5. Heat-exchanger network Ch.6, Ch.7, Ch.16

Ch. 4

ReactorSeparation

System

Purge

H2 , CH4

Benzene

Diphenyl

H2 , CH4

Toluene

LEVEL 2

LEVEL 3 DECISIONS

1 ) How many reactors are required ? Is there any separation between the reactors ?

2 ) How many recycle streams are required ?

3 ) Do we want to use an excess of one reactant at the reactor inlet ? Is there a need to separate product partway or recycle byproduct ?

4 ) Should the reactor be operated adiabatically or with direct heating or cooling ? Is a diluent or heat carrier required ? What are the proper operating temperature and pressure ?

5 ) Is a gas compressor required ? costs ?

6 ) Which reactor model should be used ?

7 ) How do the reactor/compressor costs affect the economic potential ?

1 ) NUMBER OF REACTOR SYSTEMS

If sets of reactions take place at different T and P, or if they require different catalysts, then we use different reactor systems for these reaction sets.

Acetone Ketene + CH4

Ketene CO + 1/2C2H4

700C, 1atmKetene + Acetic Acid Acetic Anhydride

80 C, 1atm

Number of Recycle Streams

TABLE 5.1-3Destination codes and component classifications

Destination code Component classifications 1. Vent Gaseous by-products and feed impurities 2. Recycle and purge Gaseous reactants plus inert gases and/or gaseous by-products 3. Recycle Reactants Reaction intermediates Azeotropes with reactants (sometimes) Reversible by-products (sometimes) 4.None Reactants-if complete conversion or unstable reaction intermediates 5.Excess - vent Gaseous reactant not recovered or recycles 6.Excess - vent Liquid reactant not recovered or recycled 7.Primary product Primary product 8.Fuel By-products to fuel 9.Waste By-products to waste treatment should be minimized

A ) List all the components that are expected to leave the reactor. This list includes all the components in feed streams, and all reactants and products that appear in every reaction.

B ) Classify each component in the list according to Table 5.1-3 and assign a destination code to each.

C ) Order the components by their normal boiling points and group them with neighboring destinations.

D ) The number of groups of all but the recycle streams is then considered to be the number of product streams.

2 ) NUMBER OF RECYCLE STREAMS

EXAMPLE HDA Precess

Component NBP , C Destination

H2 -253 Recycle + Purge Gas

CH4 -161 Recycle + Purge Recycle Benzene 80 Primary Product Toluene 111 Recycle liq. Recycle Diphenyl 255 By-product

Reactor

Compressor

Separator

CH4 , H2 (Purge)

Benezene(PrimaryProduct)

Diphenyl(By-product)

(Feed)H2 , CH4

(Feed) Toluene

(Gas Recycle)

Toluene (liq. recycle)

2 ) NUMBER OF RECYCLE STREAMS

EXAMPLE Acetone Ketene + CH4 700C Ketene CO + 1/2C2H4 1atm Ketene + Acetic Acid Acetic Anhydride 80 C, 1atm

Component NBP , C Destination CO -312.6 Fuel By-product CH4 -258.6 “ C2H4 -154.8 “ Ketene -42.1 Unstable Acetone 133.2 Reactant Acetic Acid 244.3 Reactant Acetic Anhydride 281.9 Primary Product

R1 R2 Separation

Acetic Acid (feed)

Acetic Acid (recycle to R2)

Acetone (recycle to R1)

Acetone(feed)

CO , CH4 , C2H4

(By-product)

Acetic Anhydride(primary product)

3. REACTOR CONCENTRATION

(3-1) EXCESS REACTANTS

shift product distribution

force another component to be close to complete

conversion

shift equilibrium

( molar ratio of reactants entering reactor )

is a design variable

( 1a ) Single Irreversible Reaction

force complete conversion

ex. C2H4 + Cl2 C2H4Cl2

excess

ex. CO + Cl2 COCl2

excess

( 1b ) Single reversible reaction

shift equilibrium conversion

ex. Benezene + 3H2 = Cyclohexane excess

( 2 ) Multiple reactions in parallel producing byproducts

shift product distribution type (3)

if (a2 - a1) › (b2 - b1) then FEED2 excess

if (a2 - a1) ‹ (b2 - b1) then FEED1 excess

121221

1

2

1

2 bbFEED

aaFEED CC

k

k

r

r

( 3 ) Multiple reactions in series producing byproducts

type (3) shift product distribution

ex. CH3

+ H2 + CH4

excess 5:1

2 + H2

( 4 ) Mixed parallel and series reactions byproducts

shift product distribution

ex. CH4 + Cl2 CH3Cl + HCl Primary excess 10:1

CH3Cl + Cl2 CH2Cl2+ HCl

CH2Cl2+ Cl2 CHCl3 + HCl Secondary

CHCl3 + Cl2 CCl4 + HCl

O O

O O O

( 3-2 ) FEED INERTS TO REACTOR

( 1b ) Single reversible reaction

FEED PROD1 + PROD2

Cinert Xfeed keq =

FEED1 + FEED2 PRODUCT

Cinert Xfeed1 or Xfeed2 keq =

( 2 ) Multiple reactions in parallel byproducts

FEED1 + FEED2 PRODUCT

FEED1 + FEED2 BYPRODUCT

Cinert Cbyproduct

FEED1 + FEED2 PRODUCT

FEED1 BYPROD1 + BYPROD2

Cinert Cbyprod1-2

Cp1Cp2

CF

CP

CF1CF2

Some of the decisions involve introducing a new component into the flowsheet, e.g. adding a new component to shift the product distribution, to shift the equilibrium conversion, or to act as a heat carrier. This will require that we also remove the component from the process and this may cause a waste treatment problem.

Example Ethylene production

C2H6 = C2H4 +H2 Steam is usually used as the

C2H6 + H2 = 2CH4 diluent.

Example Styrene Production

EB = styrene +H2

EB benzene +C2H4 Steam is also used.

EB + H2 toluene + CH4

( 3-3 ) PRODUCT REMOVAL DURING REACTION

to shift equilibrium + product distribution

( 1b ) single reversible reaction

ex. 2SO2 + O2 = 2SO3

REACT ABSORB REACT ABSORB

H2O

H2SO4

H2O

H2SO4

SO2

O2 + N2

( 3 ) multiple reactions in series byproduct

FEED PRODUCT remove

PRODUCT = BYPRODUCT remove

.

( 3-4 ) RECYCLE BYPRODUCT

to shift equilibrium + product distribution

CH3

+ H2 + CH4

2 = + H2

O O

O O O

( 4-1 ) REACTOR TEMPERATURE

T k V

Single Reaction :

- endothermic AHAP !

- exothermic

* irreversible AHAP ! * reversible continuously decreasing as conversion increases.

Multiple Reaction max. selectivity

T 400C Use of stainless steel is severely

limited !

T 260C High pressure steam ( 40~50 bar) provides heat at 250-265 C

T 40C Cooling water Temp 25-30C

( 4-2 ) REACTOR HEAT EFFECTS

Reactor heat load = f ( x, T, P, MR, Ffeed )

QR = ( Heat of Reaction ) ( Fresh Feed Rate )

……..for single reaction.

……..for HDA process ( approximation )

Adiabatic Temp. Change = TR, in - TR, out = QR / FCP

If adiabatic operation is not feasible, then we can try to use indirect heating or cooling. In general, Qt, max 6 ~ 8 106 BTU / hr

Cold shots and hot shots.

The temp. change, ( TR, in - TR, out ), can be moderated by - recycle a product or by-product ( preferred ) - add an extraneous component. ( separation system becomes more complex ! )

Figure 2.5 Heat transfer to and from stirred tanks.

Figure 2.5 Heat transfer to and from stirred tanks.

Figure 2.5 Heat transfer to and from stirred tanks.

Figure 2.5 Heat transfer to and from stirred tanks.

Figure 2.6 Four possible arrangements for fixed-bed recators.

Figure 2.6 Four possible arrangements for fixed-bed reactors.

Figure 2.6 Four possible arrangements for fixed-bed recators.

Figure 2.6 Four possible arrangements for fixed-bed reactors.

( 4-3 ) REACTOR PRESSURE ( usually 1-10 bar )

VAPOR-PHASE REACTION

- irreversible as high as possible

P V r - reversible single reaction * decrease in the number of moles AHSP * increase in the number of moles continuously decreases as conversion increases - multiple reactions

LIQUID-PHASE REACTION

prevent vaporization of products

allow vaporization of liquid so that it can be condensed and refluxed as a means of removing heat of reaction.

allow vaporization of one of the components in a reversible reaction.

RECYCLE MATERIAL BALANCE ( Quick Estimates !!! )

Example HDA process

Limiting Reactant : Toluene ( first )

reactor separatorFT ( 1-X )

FT ( 1-X )

FTLEVEL 3

LEVEL 2

PDDiphenyl

Benzene , PB

Purge , PGRG

FFT

yPH

Toluene

H2 , CH4

FG , yFH

always valid for limiting reactant when there is complete recovery and

recycle of the limiting reactant

XFF FT

T

RECYCLE MATERIAL BALANCE ( Quick Estimates !!! )

Example HDA process

other reactant : (Next )

X

FMRRyFy FT

GPHGFH )(

molar ratio

extra design variable

GPH

FH

PH

FTG F

y

y

y

MR

X

FR

Note that details of separation system have not been specified at this level.

Therefore, we assume that reactants one recovered completely.

PHGH yRR 2

)1(4 PHGCH yRR

5 ) COMPRESSOR DESIGN AND COST

Whenever a gas-recycle stream is present, we will need a gas-

recycle compressor.

Covered in “Unit Operation (I)”

6 ) EQUILIBRIUM LIMITATIONS

7 ) REACTOR DESIGN AND COSTS

Covered in

“Reactor Design and Reaction Kinetics”

ECONOMIC POTENTIAL AT LEVEL 3

Note,

GFHFT

PHG

PH

FH

PH

FTG

FTT

FyX

FMR

yF

y

y

y

MR

X

FR

XFF

1

,,0 FTFX $ R

,,0 GPH Ry $ C

EP3=EP2-annualized costs of reactors -annualized costs of compressors

0.2 0.4 0.6

PHy

0.1 0.3 0.5 0.7

$/year 0

2 106

1 106

-1 106

-2 106

does not include any separation or heating and cooling cost