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. ContinuousFavor batch operation, if

1. Production rate a ) less than 10106 lb/yr (sometimes) b ) less than 1106 lb/yr (usually) c ) multi-product plants2. Market force a ) seasonal production b) short production lifetime3. 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.assumeCompletely recover and recycle all valuable reactants

DECISIONS FOR THE INPUT/OUTPUT STRUCTURE Flowsheet Alternatives ProcessFeed streamsProductsby-productsno reactants(2)ProcessPurgeProductsBy-ProductsFeed streamsreasons: 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?PROCESSProducts&By productsFeedsPROCESSPurgeProducts&By productsFeedsOR

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, CH4HeatH2CH4HeatTolueneTolueneHeatCompressorReactorCoolantFlashRecycleDipheny1ProductBenzeneStabilizerH2, CH4PurgeToluene500 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 (-55F), use a gas recycle and purge stream. Lower boiling components normally cannot be condensed at high pressure with cooling water.

A HIERARCHICAL APPROACHToluene + H2 Benzene + CH42 Benzene Diphenyl + H21150 F 1300 F500 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 StreamsTABLE 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 treatmentshould be minimizedA ) 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.

EXAMPLEABCDEFGHIJb.p.WasteWasteRecycleFuelFuelPrimary productRecycleRecycleValuable By-productFuelA + 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 253H2CH4BenzeneTolueneDiphenylRecycle and PurgeRecycle and PurgePrimary ProductRecycleFuelProcessH2 , CH4ToluenePurge : H2 , CH4BenzeneDiphenyl

Process12345PurgeH2 , CH4BenzeneDiphenylH2 , CH4

Production rate = 265Design variables: FE and xComponent 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 465where 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 + FEFIGURE 5.2-1 .Stream tableToluene

Alternatives for the HDA Process1. Purify the H2 feed stream.2. Recycle diphenyl3. Purify H2 recycle stream.

REACTOR PERFORMANCEConversion (x)= (reactant consumed in the reactor)/(reactant fed to the reactor)Selectivity (S)=[(desired product produced)/(reactant consumed in the reactor)]*SFReactor 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 ReactorToluenefeed (1 mole)Tolueneunconverted(1-x) moleTolueneconvertedx molerecycleBenzeneproducedSx moleDiphenylproduced(1-S)x / 2

ReactorsystemSeparationsystemGas recyclePurgeH2 , CH4BenzeneDipheny1H2 , CH4TolueneToluene recycleMaterial Balance of the Limiting Reactant (Toluene)TolueneBenzeneDiphenyl 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.

EXAMPLEProcessPurge ; H2 , CH4 , PGBenzene , PBDiphenyl , PDFG , H2 , CH4FFT , 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/SPD = 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]whereyFHFGFH2design variablepurge rateof H2SPBrelationknowngivendesignvariableFEmethane in purge streammethane in feedmethane 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 =designvariabledesign variableKnown : Design Variable :

yFH x PB FES (x)FFTPB/SPDFH2FCH4PCH4PGFG(PB/S)[(1-S)/2]FE+[PB(1+S)/2S][(1- yFH)/ yFH]FH2FCH4+PB/SPCH4+FEFN2+FCH4

Known : yFH PBDesign Variables :x, yPHS(x)PB/SFFTFFT(1-S)/2PDPB[1-(1- yPH)(1-S)/2 S(yFH - yPH)FGPGFG+(PB/S)(1-S)/2PG yPHFE (PH2)FH2FE+PB(1+S)/2SFCH41- yPH yPHFH2PCH4FCH4+PB/S6 ) 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 10^6yPH 0.1 0.7 0.90.1 0.5 0.30.1

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