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Chapter 6 Multiple Reactions.

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Presentation on theme: "Chapter 6 Multiple Reactions."— Presentation transcript:

1 Chapter 6 Multiple Reactions

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

3 6.1 Definitions 6.1.1 Types of reactions
There are four types of multiple reactions 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 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 triethanolamine

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

5 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

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

7 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

8 6.2Parallel Reactions We examine ways to maximize SD/U for parallel reactions

9 Keep the conc of reactant A as high as possible
Case 1: α1>α2 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: α1<α2 Keep the conc of reactant A as low as possible Diluting the feed with inerts Recyle reactor product CSTR is used

10 The sensitivity of the selectivity to the temperature can be determined from the reaction rates ratio Case 3: ED>EU kD and rD↑ with temperature than Ku Keep temperature as high as possible Case 4: EU>ED Operate at low possible temperature

11 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

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

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

14 6.4 Complex Reactions The algorithm for complex reactions is
Number each reaction Write mole balance on each and every species Write the net reaction rate for each species Write rate law for one species in every reaction Relate the reaction rates for each species Combine the rates in terms of conc Write stoichiometry (conc in terms of flow rates) Write pressure drop in terms flow rates Combine and solve ODE

15 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

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

17 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)

18 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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