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Chapter 6 Multiple Reactions

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Overview Multiple reactions types Define “Selectivity”. How it can be used in the design? Solve engineering problems with multiple reactions

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6.1 Definitions 6.1.1 Types of reactions There are four types of multiple reactions 1.Parallel reactions (competing reactions) where the reactant is consumed by two different reaction pathways to form different products e.g oxidation of ethylene to ethylene oxide 2.Series reactions (consecutive reactions) where the reactant forms an intermediate product, which reacts further to form another product e.g EO with NH3 to form mono, di and tri ethanolamine

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3.Complex reactions are reactions that involve a combination of both series and parallel reactions e.g formation of butadiene from ethanol 4.Independent reactions are reactions that occur at the same time independently e.g cracking of crude oil to form gasoline

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We want to minimize the formation of undesired products and maximize the formation of desired products to reduce the cost of separating undesired from desired products

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Selectivity quantifies the formation of desired wrt undesired products Instantaneous selectivity Overall selectivity For a batch reactor For CSTR Selectivity

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Yield Yield is defined as the ratio of the reaction rate of a given product to that of the key reactant Instantaneous yield Overall yield For a batch reactor For CSTR

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6.2Parallel Reactions We examine ways to maximize S D/U for parallel reactions

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Case 1: α 1 >α 2 Keep the conc of reactant A as high as possible – If gas phase no inerts and high pressure are used – If liquid phase use of diluents is minimized – Batch or PFR is used because conc drops progressively while in CSTR the conc is always at the lowest exit value Case 2: α 1 <α 2 Keep the conc of reactant A as low as possible – Diluting the feed with inerts – Recyle reactor product – CSTR is used

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The sensitivity of the selectivity to the temperature can be determined from the reaction rates ratio Case 3: E D >E U k D and r D ↑ with temperature than Ku Keep temperature as high as possible Case 4: E U >E D Operate at low possible temperature

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6.3 Reactions in series For the sequence where B is the desired product If the first reaction is slow and the second reaction is fast it will difficult to produce B If the first reaction is fast and the second reaction is slow large yield of B can be achieved For series reactions space time (in flow reactor) and real time in batch reactor is the most important variable

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The reactions can be rewritten as Applying on A and B mole balance, rate law, stoichiometry, combine and evaluate algorithm

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Optimum yield of B (maximum C B ) At this maximum value of C B τ, W and X can be solved for as follows

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6.4 Complex Reactions The algorithm for complex reactions is 1.Number each reaction 2.Write mole balance on each and every species 3.Write the net reaction rate for each species 4.Write rate law for one species in every reaction 5.Relate the reaction rates for each species 6.Combine the rates in terms of conc 7.Write stoichiometry (conc in terms of flow rates) 8.Write pressure drop in terms flow rates 9.Combine and solve ODE

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The net reaction rate for species j is the sum of all reaction rates in which j appears Where q is the number of reactions Rate law is required for one species in each reaction For the reaction The reaction rate of each species can be related to each other as

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For liquid phase For gas phase Then Where represents the net rate of formation

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6.5 Multiple Reactions in PFR/PBR Combining mole balance, rate laws, and stoichiometry for species 1 to j in the gas phase Coupled j ODEs must be solved simultaneously using numerical package (Polymath)

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6.6 Multiple Reactions in CSTR Recall the design CSTR equation For q gas phase reactions with N species, the following set of algebraic equations These equations must be solved simultaneously using numerical package (Polymath)

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