Hierarchy of decisions LEVEL I Decision: Batch vs. Continuous Favor batch operation, if 1. Production rate a ) less than 10×10 6 lb/yr (sometimes) b.

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

LEVEL I Decision: Batch vs. Continuous

Favor batch operation, if

1. Production rate

a ) less than 10×106 lb/yr (sometimes)

b ) less than 1×106 lb/yr (usually)

c ) multi-product plants

2. Market force

a ) seasonal production

b) short production lifetime

3. Scale-up problems

a ) very long reaction times

b ) handling slurries at low flow rates

c ) rapidly fouling materials.

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

Heuristics: Recover more than 99% of all valuable materials.

assume

CompletelyCompletely recover and recycle all valuable reactants

DECISIONS FOR THE INPUT/OUTPUT STRUCTURE

Flowsheet Alternatives

ProcessFeed streams Productsby-productsno reactants

(2)

Process

Purge

ProductsBy-Products

Feed streams

reasons:

a. inexpensive reactants, e.g. Air, Water.

b. gaseous reactants + (inert gaseous feed impurity or inert gaseous reaction by-product)

(1)

LEVEL 2 DECISIONS:

1 ) Should we purify the feed streams before they enter the process?

2 ) Should we remove or recycle a reversible by-product?

3 ) Should we use a gas recycle and purge stream?

4 ) Should we not bother to recover and recycle some reactants?

5 ) How many product streams will there be?

6 ) What are the design variables for the input/output structure? What economic trade-offs are associated with these variables?

PROCESS

Products&By products

Feeds

PROCESS

PurgeProducts&By products

Feeds

OR

1 ) Purification of Feeds (Liquid/Vapor)

1 ) If a feed impurity is not inert and is present in significant quantities, remove it.

2 ) If a feed impurity is present in large amount, remove it.

3 ) If a feed impurity is catalyst poison, remove it.

4 ) If a feed impurity is present in a gas feed, as a first guess, process the impurity.

5 ) If a feed impurity is present as an azeotrope with a reactant, often it is better to process the impurity.

6 ) If a feed impurity is inert, but it is easier to separate from the product than the feed, it is better to process the impurity.

7 ) If a feed impurity in a liquid feed stream is also a byproduct or a product component, usually it is better to feed the process through the separation system.

HeatH2, CH4

HeatH2 CH4

Heat

Toluene

Toluene

Heat

Compressor

Reactor Coolant Flash

Rec

ycle

Dipheny1

Pro

duct

Benzene

Sta

bili

zer

H2, CH4

Purge

95 F1150 ~ 1300

Toluene

500 psia

3 ) Gas Recycle and Purge “Light” reactant

“Light” feed impurity, or

“Light” by-product produced by a reaction

Whenever a light reactant and either a light feed impurity or a light by- product boil lower than propylene (-55ºF), use a gas recycle and purge stream.

Lower boiling components normally cannot be condensed at high pressure with cooling water.

A HIERARCHICAL APPROACH

Toluene + H2 Benzene + CH4

2 Benzene Diphenyl + H21150 F ～ 1300 F

500 psia

4 ) Do not recover and recycle some reactants which are inexpensive, e. g. air and H2O.

We could try to make them reacted completely, but often we feed them as an excess to try to force some more valuable reactant to completion.

5 ) Number of Product 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.

EXAMPLEABCDEFGHIJ

b.p. WasteWasteRecycleFuelFuelPrimary productRecycleRecycleValuable By-productFuel

A + B to waste

D + E to fuel stream # 1

F to primary product (storage for sale)

I to valuable by-product (storage for sale) J to fuel stream # 2

EXAMPLE b.p.-253C-161 80 111 253

H2

CH4BenzeneTolueneDiphenyl

Recycle and PurgeRecycle and PurgePrimary ProductRecycleFuel

ProcessH2 , CH4

Toluene

Purge : H2 , CH4

Benzene

Diphenyl

Process1

2

3

4

5 PurgeH2 , CH4

Benzene

Diphenyl

H2 , CH4

Production rate = 265Design variables: FE and x

Component 1 2 3 4 5

H2 FH2 0 0 0 FE

CH4 FM 0 0 0 FM + PB/S Benzene 0 0 PB 0 0 Toluene 0 PB/S 0 0 0 Diphenyl 0 0 0 PB(1 - S)/(2S) 0 Temperature 100 100 100 100 100 Pressure 550 15 15 15 465

where S = 1 - 0.0036/(1 -x)1.544 FH2 = FE + PB(1 + S)/2S

FM = (1 - yFH)[FE + PB(1 + S)/S]/ yFH FG = FH2 + FE

FIGURE 5.2-1

.

Stream table

Toluene

Alternatives for the HDA Process

1. Purify the H2 feed stream.

2. Recycle diphenyl

3. Purify H2 recycle stream.

REACTOR PERFORMANCE

Conversion (x)

= (reactant consumed in the reactor)/(reactant fed to the reactor)

Selectivity (S)

=[(desired product produced)/(reactant consumed in the reactor)]*SF

Reactor Yield (Y)

=[(desired product produced)/(reactant fed to the reactor)]*SF

STOICHIOMETRIC FACTOR (SF)

The stoichiometric moles of reactant required per mole of product

Material Balance of Limiting Reactant in Reactor

Toluenefeed

(1 mole)

Tolueneunconverted(1-x) mole

Tolueneconverted

x mole

recycle

BenzeneproducedSx mole

Diphenylproduced(1-S)x / 2

Reactorsystem

Separationsystem

Gas recycle PurgeH2 , CH4

Benzene

Dipheny1

H2 , CH4

Toluene

Toluene recycle

Material Balance of the Limiting Reactant (Toluene)

x

x1

TolueneBenzene

Diphenyl

x1Sx

xS)1(2

1

Sx

xS)1(2

1

Assumption: completely recover and recycle the limiting reactant.

POSSIBLE DESIGN VARIABLES FOR LEVEL 2

For complex reactions:Reactor conversion (x), reaction temperature (T) and pressure (P).

If excess reactants are used, due to reactant not recovered or gas recycle and purge, then the excess amount is another design variable.

PROCEDURES FOR DEVELOPING OVERALL MATERIAL BALANCE

1 ) Start with the specified production rate.

2 ) From the stoichiometry (and, for complex reactions, the correlation for product distribution) find the by-product flows and the reactant requirements (in terms of the design variables).

3 ) Calculate the impurity inlet and outlet flows for the feed streams where the reactant are completely recovered/recycled.

4 ) Calculate the outlet flows of reactants in terms of a specific amount of excess for streams where reactants are not recovered and recycled (recycle and purge, or air, or H2O)

5 ) Calculate the inlet and outlet flows for the impurities entering with the reactant streams in Step 4).

Normally, it is possible to develop expressions for overall MB in terms of design variables without considering recycle flows.

EXAMPLE

Process

Purge ; H2 , CH4 , PG

Benzene , PB

Diphenyl , PD

FG , H2 , CH4

FFT , Toluene

S( x ) = selectivity = given PB( mol/hr ) = production rate of Benzene =givenFFT( mol/hr ) = toluene feed to process ( limiting reactant ) = PB/S

PR , CH4 = methane produced in reaction = FFT = PB/S

PD = diphenyl produced in reaction = FFT (1 - S/2) = (PB/S)(1 - S/2)

Let FE = excess amount of H2 in purge stream= PH2

FE + = yFHFG

disapp. in reaction

FG = make-up gas stream flowrate (unknown)

yFH = mole fraction of H2 in FG ( known )

Let PCH4 = purge rate of CH4

( 1 - yFH ) FG + PB/S = PCH4

( PB/S ) - [( PB/S )( 1 - S )/2]

where

yFHFG

FH2

design variable

purge rateof H2

S

PB

relationknown

given

designvariable

FE

methane in purge stream

methane in feed methane product in reaction

PG = total purge rate = PH2 + PCH4

= FE + (1 - yFH) FG + PB/S

= FG + ( PB/S )[( 1 - S )/2]

Define

yPH = purge composition of H2 = PH2/PG = FE/PG

It can be derined that

PB [ 1- (1- yPH)(1-S)/2 ]

S (yFH - yPH)

FG =

designvariable

design variable

Known : Design Variable :

yFH x

PB FE

S (x) FFT

PB/S

PDFH2FCH4PCH4

PG FG

(PB/S)[(1-S)/2]

FE+[PB(1+S)/2S][(1- yFH)/ yFH]FH2FCH4+PB/S

PCH4+FE

FN2+FCH4

Known : yFH

PB

Design Variables :

x, yPH

S(x) PB/S FFT

FFT(1-S)/2 PD

PB[1-(1- yPH)(1-S)/2

S(yFH -

yPH)

FGPG

FG+(PB/S)(1-S)/2PG yPH

FE

(PH2)

FH2

FE+PB(1+S)/2SFCH4

1- yPH

y

PH

FH2

PCH4

FCH4+PB/S

6 ) ECONOMIC POTENTIAL AT LEVEL 2

EP2 = Annual profit if capital costs and utility costs are excluded

= Product Value + By-product Value - Raw-Material Costs

[EXAMPLE] HDA process

4 10^6 2 10^6 $/yr -2 10^6 -4 10^6

yPH

0.1 0.7 0.9

0.1 0.5 0.3 0.1

Douglas, J. M., “Process Synthesis for Waste Minimization.” Ind. Eng. Chem. Res., 1992, 31, 238-243

If we produce waste by-products, then we have negative by-

product values. Solid waste : land fill cost / lb

Contaminated waste water : - sewer charge : $ / 1000 gal. (e.g. $0.2 / 1000 gal) - waste treatment charge : $ / lb BOD lb BOD / lb organic compound (e.g. $0.25 /lb BOD)

Solid or liquid waste to be incinerated :

$ 0.65 / lb

BOD - biological oxygen demand