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Hierarchy of Decisions

Hierarchy of Decisions. Purge H 2 , CH 4. Reactor. Separation System. H 2 , CH 4. Benzene. Toluene. Diphenyl. LEVEL 2. LEVEL 3 DECISIONS 1 ) How many reactors are required ? Is there any separation between the reactors ? 2 ) How many recycle streams are required ?

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Hierarchy of Decisions

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  1. Hierarchy of Decisions

  2. Purge H2 , CH4 Reactor Separation System H2 , CH4 Benzene Toluene Diphenyl LEVEL 2

  3. LEVEL 3 DECISIONS 1 ) How many reactors are required ? Is there any separation between the reactors ? 2 ) How many recycle streams are required ? 3 ) Do we want to use an excess of one reactant at the reactor inlet ? Is there a need to separate product partway or recycle byproduct ? 4 ) Should the reactor be operated adiabatically or with direct heating or cooling ? Is a diluent or heat carrier required ? What are the proper operating temperature and pressure ? 5 ) Is a gas compressor required ? costs ? 6 ) Which reactor model should be used ? 7 ) How do the reactor/compressor costs affect the economic potential ?

  4. 1 ) NUMBER OF REACTOR SYSTEMS If sets of reactions take place at different T and P, or if they require different catalysts, then we use different reactor systems for these reaction sets. Acetone  Ketene + CH4 Ketene  CO + 1/2C2H4 700C, 1atm Ketene + Acetic Acid  Acetic Anhydride 80 C, 1atm

  5. Number of Recycle 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 thecomponentsinfeedstreams, and allreactantsandproductsthatappearinevery 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.

  6. 2 ) NUMBER OF RECYCLE STREAMS EXAMPLE HDA Precess ComponentNBP , CDestination H2 -253 Recycle + Purge Gas CH4 -161 Recycle + Purge Recycle Benzene 80 Primary Product Toluene 111 Recycle liq. Recycle Diphenyl 255 By-product Compressor CH4 , H2(Purge) (Gas Recycle) Benezene (PrimaryProduct) Reactor Separator (Feed)H2 , CH4 (Feed) Toluene Diphenyl (By-product) Toluene (liq. recycle)

  7. 2 ) NUMBER OF RECYCLE STREAMS EXAMPLE Acetone Ketene + CH4700C Ketene  CO + 1/2C2H41atm Ketene + Acetic Acid Acetic Anhydride 80 C, 1atm ComponentNBP , CDestination CO -312.6 Fuel By-product CH4 -258.6 “ C2H4 -154.8 “ Ketene -42.1 Unstable Acetone 133.2 Reactant Acetic Acid 244.3 Reactant Acetic Anhydride 281.9 Primary Product CO , CH4 , C2H4 (By-product) Acetic Acid (feed) Acetone (feed) R1 R2 Separation Acetic Anhydride (primary product) Acetic Acid (recycle to R2) Acetone (recycle to R1)

  8. 3. REACTOR CONCENTRATION (3-1) EXCESS REACTANTS  shift product distribution  force another component to be close to complete conversion  shift equilibrium ( molar ratio of reactants entering reactor ) is a design variable

  9. ( 1a ) Single Irreversible Reaction force complete conversion ex. C2H4 + Cl2 C2H4Cl2 excess ex. CO + Cl2 COCl2 excess ( 1b ) Single reversible reaction shift equilibrium conversion ex. Benezene + 3H2 = Cyclohexane excess ( 2 ) Multiple reactions in parallel producing byproducts shift product distribution type (3) if (a2 - a1) › (b2 - b1) then FEED2 excess if (a2 - a1) ‹ (b2 - b1) then FEED1 excess

  10. ( 3 ) Multiple reactions in series producing byproducts type (3) shift product distribution ex. CH3 + H2 + CH4 excess 5:1 2+ H2 ( 4 ) Mixed parallel and series reactions  byproducts shift product distribution ex. CH4 + Cl2  CH3Cl + HCl Primary excess 10:1 CH3Cl + Cl2 CH2Cl2+ HCl CH2Cl2+ Cl2  CHCl3 + HCl Secondary CHCl3 + Cl2  CCl4 + HCl O O O O O

  11. ( 3-2 ) FEED INERTS TO REACTOR ( 1b ) Single reversible reaction FEED PROD1 + PROD2 Cinert   Xfeed  keq = FEED1 + FEED2 PRODUCT Cinert   Xfeed1 or Xfeed2  keq = ( 2 ) Multiple reactions in parallel  byproducts FEED1 + FEED2  PRODUCT FEED1 + FEED2 BYPRODUCT Cinert   Cbyproduct  FEED1 + FEED2  PRODUCT FEED1 BYPROD1 + BYPROD2 Cinert   Cbyprod1-2  Cp1Cp2 CF CP CF1CF2

  12. Some of the decisions involve introducing a new component into the flowsheet, e.g. adding a new component to shift the product distribution, to shift the equilibrium conversion, or to act as a heat carrier. This will require that we also remove the component from the process and this may cause a waste treatment problem. Example Ethylene production C2H6= C2H4 +H2Steam is usually used as the C2H6 + H2 = 2CH4diluent. Example Styrene Production EB = styrene +H2 EB  benzene +C2H4Steam is also used. EB + H2 toluene + CH4

  13. ( 3-3 ) PRODUCT REMOVAL DURING REACTION to shift equilibrium + product distribution ( 1b ) single reversible reaction ex. 2SO2 + O2= 2SO3 H2O H2O SO2 REACT ABSORB REACT ABSORB O2 + N2 H2SO4 H2SO4 ( 3 ) multiple reactions in series  byproduct FEED  PRODUCT remove PRODUCT = BYPRODUCT remove .

  14. ( 3-4 ) RECYCLE BYPRODUCT to shift equilibrium + product distribution CH3 + H2 + CH4 2 = + H2 O O O O O

  15. ( 4-1 ) REACTOR TEMPERATURE T   k   V  Single Reaction : - endothermic AHAP ! - exothermic * irreversible AHAP ! * reversible continuously decreasing as conversion increases.  Multiple Reaction max. selectivity T  400C  Use of stainless steel is severely limited ! T  260C  High pressure steam ( 40~50 bar) provides heat at 250-265 C T  40C  Cooling water Temp 25-30C

  16. ( 4-2 ) REACTOR HEAT EFFECTS Reactor heat load = f ( x, T, P, MR, Ffeed) QR = ( Heat of Reaction )  ( Fresh Feed Rate ) ……..for single reaction. ……..for HDA process ( approximation ) Adiabatic Temp. Change = TR, in - TR, out = QR / FCP  If adiabatic operation is not feasible, then we can try to use indirect heating or cooling. In general, Qt, max  6 ~ 8  106 BTU / hr  Cold shots and hot shots.  The temp. change, ( TR, in- TR, out), can be moderated by - recycle a product or by-product ( preferred ) - add an extraneous component. ( separation system becomes more complex ! )

  17. Figure 2.5 Heat transfer to and from stirred tanks.

  18. Figure 2.5 Heat transfer to and from stirred tanks.

  19. Figure 2.5 Heat transfer to and from stirred tanks.

  20. Figure 2.5 Heat transfer to and from stirred tanks.

  21. Figure 2.6 Four possible arrangements for fixed-bed recators.

  22. Figure 2.6 Four possible arrangements for fixed-bed reactors.

  23. Figure 2.6 Four possible arrangements for fixed-bed recators.

  24. Figure 2.6 Four possible arrangements for fixed-bed reactors.

  25. ( 4-3 ) REACTOR PRESSURE ( usually 1-10 bar )  VAPOR-PHASE REACTION - irreversible as high as possible P     V  r  - reversible single reaction * decrease in the number of moles AHSP * increase in the number of moles continuously decreases as conversion increases - multiple reactions  LIQUID-PHASE REACTION prevent vaporization of products allow vaporization of liquid so that it can be condensed and refluxed as a means of removing heat of reaction. allow vaporization of one of the components in a reversible reaction.

  26. RECYCLE MATERIAL BALANCE ( Quick Estimates !!! ) Example HDA process  Limiting Reactant : Toluene ( first ) yPH RG Purge , PG FG, yFH H2 , CH4 Benzene , PB reactor separator FFT FT ( 1-X ) PD Toluene FT Diphenyl LEVEL 3 FT ( 1-X ) LEVEL 2 always valid for limiting reactant when there is complete recovery and recycle of the limiting reactant

  27. RECYCLE MATERIAL BALANCE ( Quick Estimates !!! ) Example HDA process  other reactant : (Next ) molar ratio extra design variable Note that details of separation system have not been specified at this level. Therefore, we assume that reactants one recovered completely.

  28. 5 ) COMPRESSOR DESIGN AND COST Whenever a gas-recycle stream is present, we will need a gas- recycle compressor. Covered in “Unit Operation (I)”

  29. 6 ) EQUILIBRIUM LIMITATIONS 7 ) REACTOR DESIGN AND COSTS Covered in “Reactor Design and Reaction Kinetics”

  30. ECONOMIC POTENTIAL AT LEVEL 3 Note, $ $ EP3=EP2-annualized costs of reactors -annualized costs of compressors 2  106 1  106 0.2 0.4 0.6 $/year 0 0.1 0.3 0.5 0.7 -1  106 -2  106   does not include any separation or heating and cooling cost

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