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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.

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Presentation on theme: "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."— Presentation transcript:

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2 Hierarchy of decisions

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

4 Hierarchy of decisions

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

6 DECISIONS FOR THE INPUT/OUTPUT STRUCTURE  Flowsheet Alternatives Process Feed streams Products by-products no reactants (2) Process Purge Products By-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)

7 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   Purge Products & By products Feeds OR

8 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.

9 Heat H2, CH4 Heat H2CH4 Heat Toluene Heat Compressor ReactorCoolantFlash Recycle Dipheny1 Product Benzene Stabilizer H2, CH4 Purge Toluene 500 psia

10 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.

11 A HIERARCHICAL APPROACH Toluene + H2  Benzene + CH4 2 Benzene Diphenyl + H  F ~ 1300  F 500 psia

12 4 ) Do not recover and recycle some reactants which are inexpensive, e. g. air and H 2 O. 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.

13 5 ) Number of Product Streams TABLE 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.

14 EXAMPLE ABCDEFGHIJABCDEFGHIJ b.p. Waste Recycle Fuel Primary product Recycle Valuable By-product Fuel 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  C H 2 CH 4 Benzene Toluene Diphenyl Recycle and Purge Primary Product Recycle Fuel  Process H 2, CH 4 Toluene    Purge : H 2, CH 4 Benzene Diphenyl

15 Process Purge H 2, CH 4 Benzene Diphenyl H 2, CH 4 Production rate = 265 Design variables: F E and x Component H 2 F H F E CH 4 F M F M + P B /S Benzene 0 0 P B 0 0 Toluene 0 P B /S Diphenyl P B (1 - S)/(2S) 0 Temperature Pressure where S = /(1 -x) F H 2 = F E + P B (1 + S)/2S F M = (1 - y FH )[F E + P B (1 + S)/S]/ y FH F G = F H 2 + F E FIGURE Stream table Toluene

16 Alternatives for the HDA Process 1. Purify the H 2 feed stream. 2. Recycle diphenyl 3. Purify H 2 recycle stream.

17 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

18 STOICHIOMETRIC FACTOR (SF) The stoichiometric moles of reactant required per mole of product

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

20 Reactor system Separation system Gas recycle Purge H 2, CH 4 Benzene Dipheny1 H 2, CH 4 Toluene Toluene recycle Material Balance of the Limiting Reactant (Toluene) Toluene Benzene Diphenyl Assumption: completely recover and recycle the limiting reactant.

21 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.

22 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 H 2 O) 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.

23 EXAMPLE Process Purge ; H 2, CH 4, P G Benzene, P B Diphenyl, P D F G, H 2, CH 4 F FT, Toluene S( x ) = selectivity = given P B ( mol/hr ) = production rate of Benzene =given  F FT ( mol/hr ) = toluene feed to process ( limiting reactant ) = P B /S P R, CH 4 = methane produced in reaction = F FT = P B /S  P D = diphenyl produced in reaction = F FT ( 1 - S / 2 ) = (P B /S)( 1 - S / 2 ) Let F E = excess amount of H 2 in purge stream= P H 2  F E + = y FH F G disapp. in reaction F G = make-up gas stream flowrate (unknown) y FH = mole fraction of H 2 in F G ( known ) Let P CH 4 = purge rate of CH 4  ( 1 - y FH ) F G + P B /S = P CH 4 ( P B /S ) - [( P B /S )( 1 - S )/2] where y FH F G FH2FH2 design variable purge rate of H 2 S PBPB relation known given design variable FEFE methane in purge stream methane in feed methane product in reaction

24  P G = total purge rate = P H 2 + P CH 4 = F E + (1 - y FH ) F G + P B /S = F G + ( P B /S )[( 1 - S )/2] Define y PH = purge composition of H 2 = P H 2 /P G = F E /P G It can be derined that PB [ 1- (1- y PH )(1-S)/2 ] S ( y FH - y PH ) F G = design variable design variable Known : Design Variable : y FH x P B F E S (x) F FT P B /S PDPD FH 2 F CH 4 P CH 4 PGPG FGFG (P B /S)[(1-S)/2] F E +[P B (1+S)/2S][(1- y FH )/ y FH ]FH 2 FCH 4 +P B /S PCH 4 +F E F N 2 +F CH 4

25 Known : y FH P B Design Variables : x, y PH S(x) P B /S F FT F FT (1-S)/2 PDPD PB[1-(1- y PH )(1-S)/2 S( y FH - y PH ) FGFG PGPG F G +(P B /S)(1-S)/2P G y PH F E (P H 2 ) FH2FH2 F E +P B (1+S)/2S F CH 4 1- y PH y PH FH2FH2 P CH 4 F CH 4 +P B /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 y PH  0.1

26 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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