Hierarchy of Decisions

1. Hierarchy of Decisions

2. One Level-1 Decision: Batch vs. Continuous

3. 連續或批次程序的選擇 • Douglas, Conceptual Design of Chemical Process, 1988 • 經驗法則: • 設定產率在每年一百萬磅(106 lb/yr ≒ 454 ton/yr) 以上時盡量採用連續式程序 • 在下列狀況時雖產率較高也建議採用批次的方式 • 同一套設備必須彈性生產眾多種類產品 • 市場變化快導致產品生命週期短 • 程序有放大的問題(如結垢較快) • 批次程序主要必須決定操作順序步驟，操作上極富彈性，同一套設備可生產多樣化產品，產率要大時則可由數套批次設備平行運作來達成。

4. Hierarchy of Decisions

5. Heuristics: Recover more than 99% of all valuable materials. Therefore, it is reasonable to assume Completely recover and recycle all valuable reactants

6. The Input-Output Structures Let us consider two flowsheet alternatives: (1) Feed streams Process Products By-products no reactants (2) Purge Products Process Feed streams By-products Reactants Reasons for using 2nd structure: inexpensive reactants, e.g. air and water. gaseous reactants + (inert gaseous feed impurity or inert gaseous reaction by-product)

7. Seven (7) Level-2 Decisions: Should we purify the feed streams before they enter the process? Should we remove or recycle a reversible by-product? Should we use a gas recycle and purge stream? Should we not bother to recover and recycle some reactants? How many product streams will there be? What are the design variables for the input/output structure? What economic trade-offs are associated with these variables? Products & Byproducts PROCESS Feeds       OR Purge Products & Byproducts PROCESS Feeds      

8. Q1: Purification of Feeds (Liquid/Vapor) If a feed impurity is not inert and is present in significant quantities, remove it. If a feed impurity is present in large amount, remove it. If a feed impurity is catalyst poison, remove it. If a feed impurity is present in a gas feed, as a first guess, process the impurity. If a feed impurity is present as an azeotrope with a reactant, often it is better to process the impurity. 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. 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 Compressor H2, CH4 Purge H2 CH4 Heat Reactor Coolant Flash Toluene 500 psia Heat Heat H2, CH4 Benzene Product Toluene Stabilizer Recycle Dipheny1 Toluene

10. Q2: Should we remove or recycle a reversible by-product? (是否程序中要將副產物之出流設計為部份迴流？) 若產生副產物的反應物為主產物且此反應為可逆反應— 則應該將副產物部份迴流回反應段以促進產物的生成。尤其在此副反應的平衡常數小時更應如此設計 A +B C (主產物) C D (副產物)

11. Q3: Gas Recycle and Purge • “Light” reactant + “Light” feed impurity • “Light” reactant + “Light” by-product produced by a reaction • Whenever a light reactant and either a light feed impurity or a light byproduct 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.

12. A HIERARCHICAL APPROACH Toluene + H2  Benzene + CH4 2 Benzene Diphenyl + H2 1150  F ～ 1300  F 500 psia

13. Q4:Do not recover and recycle some reactants which are inexpensive, e.g., air and water. We could try to make them react completely, but often we feed them as an excess to try to force some more valuable reactants to completion.

14. Q4:是否有某類過量反應物不需要設計迴流回收此反應物，而讓其直接出料？Q4:是否有某類過量反應物不需要設計迴流回收此反應物，而讓其直接出料？ • 通常將較便宜的反應物設計使其過量，然後設計此反應物的回收系統,使其迴流回反應段.但若過量反應物為價值甚低的空氣時,則不需要設計迴流而其直接排出。 • 過去對水亦同，但水資源日漸珍貴， 目前先進工廠設計多有水網路設計， 盡量迴流利用。 UOP Phenol Process

15. Q5: Number of Product Streams 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 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.

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

17. 5 Purge H2 , CH4 H2 , CH4 1 3 Process Benzene Diphenyl 2 4 Toluene Production rate = 265 Design variables: FE and x Component 1 2 3 4 5 H2 FH2 0 0 0 FE CH4 FM0 0 0 FM + PB/S Benzene 0 0 PB0 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 15 465 where 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 + FE FIGURE 5.2-1 . Stream table

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

19. Reactor Performance

20. Material Balance of Limiting Reactant in Reactor recycle

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

22. Q6: Design Variables

24. 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 , CH4 = 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

25.  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 derived 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 FH2+FCH4

26. 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 PGyPH FG+(PB/S)(1-S)/2 FCH4+PB/S FH2 PCH4 Q7: 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

27. End of Level 2

28. 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