Hierachy of desicion

8/15/16 LEVEL I Decision: Batch vs Continuous Hierarchy of decisions Favor batch operation, if Batch versus continuous Input-output structure of the flowsheet Production rate Recycle structure of the flowsheet General structure of the separation system a Vapor recovery system b Liquid recovery system Heat-exchanger network Ch Ch.5 a ) less than 10×10 lb/yr (sometimes) b ) less than 1×10 lb/yr (usually) c ) multi-product plants Market force a ) seasonal production Ch.6, Ch.7, Ch.16 b) short production lifetime Scale-up problems a ) very long reaction times b ) handling slurries at low flow rates c ) rapidly fouling materials Hierarchy of decisions Batch versus continuous Input-output structure of the flowsheet Recycle structure of the flowsheet General structure of the separation system a Vapor recovery system b Liquid recovery system Heat-exchanger network Heuristics: Ch Ch.5 Ch.6, Ch.7, Ch.16 Recover more than 99% of all valuable materials assume Completely recover and recycle all valuable reactants LEVEL DECISIONS: DECISIONS FOR THE INPUT/OUTPUT STRUCTURE ) Should we remove or recycle a reversible by-product? • Flowsheet Alternatives ) Should we use a gas recycle and purge stream? (1) Feed streams ) Should we not bother to recover and recycle some reactants? Products by-products no reactants Process (2) Feed streams ) Should we purify the feed streams before they enter the process? Purge Process Products By-Products ) 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 • • • • & PROCESS • • Feeds By products reasons: a inexpensive reactants, e.g Air, Water b gaseous reactants + (inert gaseous feed impurity or inert gaseous reaction by-product) CuuDuongThanCong.com OR Feeds • • • PROCESS https://fb.com/tailieudientucntt • • • Purge Products & By products 8/15/16 ) Purification of Feeds (Liquid/Vapor) Heat Compressor H2, CH4 Purge H2 Heat CH4 Reactor ) If a feed impurity is catalyst poison, remove it ) 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 Toluene Heat Coolant Heat Toluene Flash 500 psia Benzene Recycle ) If a feed impurity is present in a gas feed, as a first guess, process the impurity 95°F 1150° ~ 1300° ) If a feed impurity is present in large amount, remove it H2, CH4 Product ) If a feed impurity is not inert and is present in significant quantities, remove it Dipheny1 ) 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 Toluene A HIERARCHICAL APPROACH ) 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 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 Toluene + H2 ® Benzene + CH4 Benzene Diphenyl + H2 1150 ° F ～ 1300 ° F 500 psia ) Number of Product Streams TABLE 5.1-3 Destination codes and component classifications Destination code Vent Recycle and purge Recycle ) 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 4.None 5.Excess - vent 6.Excess - vent 7.Primary product 8.Fuel 9.Waste Component classifications Gaseous by-products and feed impurities Gaseous reactants plus inert gases and/or gaseous by-products Reactants Reaction intermediates Azeotropes with reactants (sometimes) Reversible by-products (sometimes) Reactants-if complete conversion or unstable reaction intermediates Gaseous reactant not recovered or recycles Liquid reactant not recovered or recycled Primary product By-products to fuel should be By-products to waste treatment 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 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 CuuDuongThanCong.com https://fb.com/tailieudientucntt 8/15/16 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 ① H2 , CH4 D + E to fuel stream # ② F to primary product ③ (storage for sale) H2 CH4 Benzene Toluene Diphenyl Recycle and Purge Recycle and Purge Primary Product Recycle Fuel Process H2 CH Benzene Toluene Diphenyl Temperature Pressure FH2 FM 0 100 550 Benzene Diphenyl 0 P B/S 100 15 0 PB 0 100 15 ① where S = - 0.0036/(1 -x) 1.544 ② ③ FIGURE 5.2-1 Stream table FE F M + P B/S 0 0 P B(1 - S)/(2S) 100 100 15 465 F H2 = F E + P B(1 + S)/2S F M = (1 - y FH)[F E + P B(1 + S)/S]/ y FH ① Purge : H2 , CH4 H2 , CH4 Toluene Process Production rate = 265 Design variables: F E and x Component I to valuable by-product (storage for sale) ④ J to fuel stream # ⑤ EXAMPLE b.p -253°C -161 80 111 253 Toluene Purge H2 , CH4 F G = F H2 + F E ② Benzene ③ Diphenyl REACTOR PERFORMANCE Alternatives for the HDA Process Purify the H2 feed stream Recycle diphenyl Purify H2 recycle stream STOICHIOMETRIC FACTOR (SF) The stoichiometric moles of reactant required per mole of product CuuDuongThanCong.com 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 Material Balance of Limiting Reactant in Reactor Toluene unconverted (1-x) mole Toluene feed (1 mole) Toluene converted x mole https://fb.com/tailieudientucntt recycle Benzene produced Sx mole Diphenyl produced (1-S)x / 8/15/16 Gas recycle Purge H2 , CH4 Toluene x Benzene Sx H2 , CH4 Diphenyl (1 S) x Reactor system Toluene POSSIBLE DESIGN VARIABLES FOR LEVEL Benzene Sx Separation system x Dipheny1 (1 x For complex reactions: Reactor conversion (x), reaction temperature (T) and pressure (P) S)x If excess reactants are used, due to reactant not recovered or gas recycle and purge, then the excess amount is another design variable Toluene recycle Material Balance of the Limiting Reactant (Toluene) Assumption: completely recover and recycle the limiting reactant EXAMPLE PROCEDURES FOR DEVELOPING OVERALL MATERIAL BALANCE ) Start with the specified production rate relation known ) Calculate the impurity inlet and outlet flows for the feed streams where the reactant are completely recovered/recycled ) 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) SS( x ) = selectivity = given PPBB ( mol/hr ) = production rate of Benzene =given ÚFFT ( mol/hr ) = toluene feed to process ( limiting reactant ) = PB /S PR , CH = methane produced in reaction = FFT = PB /S ÚPD design variable = diphenyl produced in reaction = FFT (1 - S /2 ) = (PB /S)(1 - S /2 ) Let FFEE = excess amount of H2 in purge stream= PH FE + Ú ( PB /S ) - [( PB /S )( - S )/2] purge rate of H2 = yFH FG disapp in reaction FH where FG = make-up gas stream flowrate (unknown) yFH = ) Calculate the inlet and outlet flows for the impurities entering with the reactant streams in Step 4) mole fraction of H2 in FG ( known ) Let PCH = purge rate of CH4 methane in purge stream ( - yFH ) FG + PB /S = PCH Ú Normally, it is possible to develop expressions for overall MB in terms of design variables without considering recycle flows Benzene , PB Diphenyl , PD Process design variable given ) 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) Purge ; H2 , CH4 , PG FG , H2 , CH4 FFT , Toluene methane product in reaction methane in feed Ú PG = total purge rate = PH2 + PCH4 = FE + (1 - yFH) FG + PB/S = FG + ( PB/S )[( - S )/2] Known : yFH PB Design Variables : y x, PH Define yPH = purge composition of H2 = PH2/PG = FE/PG S(x) P B/S F FT(1-S)/2 FFT PD PB[1-(1- yPH)(1-S)/2 S( yFH - yPH) design variable It can be derined that FG = Known : FCH4 PB [ 1- (1- yPH)(1-S)/2 ] S (yFH - yPH) F CH4+P B/S S (x) x PB PCH4 FCH4+PB/S FN2+FCH4 CuuDuongThanCong.com FG FG F G+(P B/S)(1-S)/2 ) ECONOMIC POTENTIAL AT LEVEL EP2 = Annual profit if capital costs and utility costs are excluded FFT [(1- yFH)/ yFH]FH2 FE+[PB(1+S)/2S] FCH4 FH2 PD PCH4+FE PG PB/S (PB/S)[(1-S)/2] FE PG PCH4 design variable Design Variable : yFH 1- yPH yPH F H2 F E+P B(1+S)/2S FH2 FE (PH2) P G yPH = Product Value + By-product Value - Raw-Material Costs [EXAMPLE] HDA process ´10^6 ´10^6 $/yr -2 ´10^6 -4 ´10^6 yPH 0.1 0.3 Ỵ 0.5 0.1 https://fb.com/tailieudientucntt 0.1 0.7 0.9 8/15/16 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 byproduct 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 CuuDuongThanCong.com https://fb.com/tailieudientucntt ... 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 CuuDuongThanCong.com https://fb.com/tailieudientucntt... the reactant are completely recovered/recycled ) Calculate the outlet flows of reactants in terms of a specific amount of excess for streams where reactants are not recovered and recycled (recycle... fraction of H2 in FG ( known ) Let PCH = purge rate of CH4 methane in purge stream ( - yFH ) FG + PB /S = PCH Ú Normally, it is possible to develop expressions for overall MB in terms of design