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Hierarchy of decisions

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Presentation on theme: "Hierarchy of decisions"— Presentation transcript:

1 Hierarchy of decisions

2 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.

3 Hierarchy of decisions

4 Heuristics: Completely recover and recycle all valuable reactants
Recover more than 99% of all valuable materials. assume Completely recover and recycle all valuable reactants

 Flowsheet Alternatives (1) Feed streams Process Products by-products no reactants (2) Purge Products Process Feed streams By-Products reasons: a. inexpensive reactants, e.g. Air, Water. b. gaseous reactants + (inert gaseous feed impurity or inert gaseous reaction by-product)

6 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? Products & By products PROCESS Feeds OR Purge Products & By products PROCESS Feeds

7 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.

8 Heat Compressor H2, CH4 Purge H2 CH4 Heat Reactor Coolant Flash Toluene 500 psia Heat Heat H2, CH4 Benzene Product Toluene Recycle Stabilizer Dipheny1 Toluene

9 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.

Toluene + H2  Benzene + CH4 2 Benzene Diphenyl + H2 1150  F ~ 1300  F 500 psia

11 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.

12 5 ) Number of Product Streams
TABLE 5.1-3 Destination 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 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.

13 EXAMPLE EXAMPLE Process b.p. A B C D E F G H I J Waste Recycle Fuel
Primary product Valuable By-product 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. -253C -161 80 111 253 H2 CH4 Benzene Toluene Diphenyl Recycle and Purge Primary Product Recycle Fuel Purge : H2 , CH4 H2 , CH4 Process Benzene Toluene Diphenyl

14 5 Purge H2 , CH4 H2 , CH4 1 3 Process Benzene Diphenyl 2 4 Toluene
Production rate = 265 Design variables: FE and x Component H FH FE CH FM FM + PB/S Benzene PB Toluene PB/S Diphenyl PB(1 - S)/(2S) 0 Temperature Pressure where S = /(1 -x) FH2 = FE + PB(1 + S)/2S FM = (1 - yFH)[FE + PB(1 + S)/S]/ yFH FG = FH2 + FE FIGURE 5.2-1 . Toluene Stream table

15 Alternatives for the HDA Process
1. Purify the H2 feed stream. 2. Recycle diphenyl 3. Purify H2 recycle stream.

= (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

The stoichiometric moles of reactant required per mole of product

18 Material Balance of Limiting Reactant in Reactor
Toluene unconverted (1-x) mole recycle Benzene produced Sx mole Toluene feed (1 mole) Toluene converted x mole Diphenyl produced (1-S)x / 2

19 Material Balance of the Limiting Reactant (Toluene)
Gas recycle Purge H2 , CH4 Toluene Benzene Diphenyl Benzene H2 , CH4 Reactor system Separation system Toluene Dipheny1 Toluene recycle Material Balance of the Limiting Reactant (Toluene) Assumption: completely recover and recycle the limiting reactant.

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.

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.

22 ( PB/S ) - [( PB/S )( 1 - S )/2]
EXAMPLE Purge ; H2 , CH4 , PG FG , H2 , CH4 FFT , Toluene Benzene , PB Diphenyl , PD Process relation known design variable S( x ) = selectivity = given PB( mol/hr ) = production rate of Benzene =given FFT( mol/hr ) = toluene feed to process ( limiting reactant ) = PB/S PR , CH = 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 S PB given FE design variable ( PB/S ) - [( PB/S )( 1 - S )/2] yFHFG purge rate of H2 FH2 where methane in purge stream methane product in reaction methane in feed

23 Known : Design Variable :
 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) design variable FG = design variable Known : Design Variable : yFH x PB FE PB/S S (x) FFT (PB/S)[(1-S)/2] FCH4+PB/S [(1- yFH)/ yFH]FH2 FE+[PB(1+S)/2S] PCH4 FCH4 FH2 PD PCH4+FE PG FG FN2+FCH4

Known : yFH PB Design Variables : x, yPH PB/S FFT(1-S)/2 S(x) FFT PD PB[1-(1- yPH)(1-S)/2 S(yFH - yPH) FE+PB(1+S)/2S FCH4 FH2 FE (PH2) PG FG 1- yPH yPH PG yPH FG+(PB/S)(1-S)/2 FCH4+PB/S FH2 PCH4 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 0.3 0.5 0.1

25 BOD - biological oxygen demand
Douglas, J. M., “Process Synthesis for Waste Minimization.” Ind. Eng. Chem. Res., 1992, 31, 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

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