2LEVEL I Decision: Batch vs. Continuous Favor batch operation, if1. Production ratea ) less than 10×106 lb/yr (sometimes)b ) less than 1×106 lb/yr (usually)c ) multi-product plants2. Market forcea ) seasonal productionb) short production lifetime3. Scale-up problemsa ) very long reaction timesb ) handling slurries at low flow ratesc ) rapidly fouling materials.
4Heuristics: Completely recover and recycle all valuable reactants Recover more than 99% of all valuable materials.assumeCompletely recover and recycle all valuable reactants
5DECISIONS FOR THE INPUT/OUTPUT STRUCTURE Flowsheet Alternatives(1)Feed streamsProcessProductsby-productsno reactants(2)PurgeProductsProcessFeed streamsBy-Productsreasons:a. inexpensive reactants, e.g. Air, Water.b. gaseous reactants + (inert gaseous feed impurity or inert gaseousreaction by-product)
6LEVEL 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?Products&By productsPROCESSFeedsORPurgeProducts&By productsPROCESSFeeds
71 ) 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 theimpurity.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 thanthe feed, it is better to process the impurity.7 ) If a feed impurity in a liquid feed stream is also a byproduct or a productcomponent, usually it is better to feed the process through the separationsystem.
93 ) Gas Recycle and Purge “Light” reactant “Light” feed impurity, or “Light” by-product produced by a reactionWhenever a light reactant and either a light feed impurity or a light by-product boil lower than propylene (-55ºF), use a gas recycle and purgestream.Lower boiling components normally cannot be condensed at high pressurewith cooling water.
114 ) 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 excessto try to force some more valuable reactant to completion.
125 ) Number of Product Streams TABLE 5.1-3Destination codes and component classificationsDestination code Component classifications1. Vent Gaseous by-products and feed impurities2. Recycle and purge Gaseous reactants plus inert gases and/or gaseous by-products3. Recycle ReactantsReaction intermediatesAzeotropes with reactants (sometimes)Reversible by-products (sometimes)4.None Reactants-if complete conversion or unstable reaction intermediates5.Excess - vent Gaseous reactant not recovered or recycles6.Excess - vent Liquid reactant not recovered or recycled7.Primary product Primary product8.Fuel By-products to fuel9.Waste By-products to waste treatmentshould be minimizedA ) List all the components that are expected to leave the reactor. This list includes allthe components in feed streams, and all reactants and products that appear in everyreaction.B ) Classify each component in the list according to Table and assign a destinationcode to each.C ) Order the components by their normal boiling points and group them withneighboring destinations.D ) The number of groups of all but the recycle streams is then considered to be thenumber of product streams.
13EXAMPLE EXAMPLE Process b.p. A B C D E F G H I J Waste Recycle Fuel Primary productValuable By-productA + 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 EXAMPLEb.p.-253C-16180111253H2CH4BenzeneTolueneDiphenylRecycle and PurgePrimary ProductRecycleFuelPurge : H2 , CH4H2 , CH4ProcessBenzeneTolueneDiphenyl
145 Purge H2 , CH4 H2 , CH4 1 3 Process Benzene Diphenyl 2 4 Toluene Production rate = 265Design variables: FE and xComponentH FH FECH FM FM + PB/SBenzene PBToluene PB/SDiphenyl PB(1 - S)/(2S) 0TemperaturePressurewhere S = /(1 -x) FH2 = FE + PB(1 + S)/2SFM = (1 - yFH)[FE + PB(1 + S)/S]/ yFH FG = FH2 + FEFIGURE 5.2-1.TolueneStream table
15Alternatives for the HDA Process 1. Purify the H2 feed stream.2. Recycle diphenyl3. Purify H2 recycle stream.
16REACTOR PERFORMANCE Conversion (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
17STOICHIOMETRIC FACTOR (SF) The stoichiometric moles of reactant required per mole of product
18Material Balance of Limiting Reactant in Reactor Tolueneunconverted(1-x) molerecycleBenzeneproducedSx moleToluenefeed(1 mole)Tolueneconvertedx moleDiphenylproduced(1-S)x / 2
19Material Balance of the Limiting Reactant (Toluene) Gas recyclePurgeH2 , CH4TolueneBenzeneDiphenylBenzeneH2 , CH4ReactorsystemSeparationsystemTolueneDipheny1Toluene recycleMaterial Balance of the Limiting Reactant (Toluene)Assumption: completely recover and recycle the limiting reactant.
20POSSIBLE 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.
21PROCEDURES FOR DEVELOPING OVERALL MATERIAL BALANCE 1 ) Start with the specified production rate.2 ) From the stoichiometry (and, for complex reactions, the correlation for productdistribution) find the by-product flows and the reactant requirements (in terms of thedesign variables).3 ) Calculate the impurity inlet and outlet flows for the feed streams where the reactant arecompletely recovered/recycled.4 ) Calculate the outlet flows of reactants in terms of a specific amount of excess for streamswhere 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 designvariables without considering recycle flows.
22( PB/S ) - [( PB/S )( 1 - S )/2] EXAMPLEPurge ; H2 , CH4 , PGFG , H2 , CH4FFT , TolueneBenzene , PBDiphenyl , PDProcessrelationknowndesign variableS( x ) = selectivity = givenPB( mol/hr ) = production rate of Benzene =givenFFT( mol/hr ) = toluene feed to process ( limiting reactant ) = PB/SPR , CH = 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 = yFHFGdisapp. in reactionFG = 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 = PCH4SPBgivenFEdesignvariable( PB/S ) - [( PB/S )( 1 - S )/2]yFHFGpurge rateof H2FH2wheremethane in purge streammethane product in reactionmethane in feed