Example - Parallel Rxns

Example - Parallel Rxns - 1 + k 2 ) p A p B dF B /dV = - r...

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1 CHE 4151 Jennings EXAMPLE PROBLEM Parallel Reactions A pilot-scale gas-phase plug-flow reactor is used to produce 1,2-dichloropropane (D) by reaction of propylene (A) and chlorine (B). C 3 H 6 + Cl 2 S)) CH 2 Cl-CHCl-CH 3 r 1 = {11.7 e - 1920 / T } p A p B (mol / L-hr) [for T in o K and p in atm.] Unfortunately, an undesirable parallel reaction produces allyl chloride (U). C 3 H 6 + Cl 2 S)) CH 2 -CH-CH 2 Cl + HCl r 2 = {20,600 e - 5610 / T } p A p B (mol / L-hr) The reactor is 8.0 m long and 5.0 cm in (internal) diameter. The reactor feed enters at a total flow rate of 0.85 mol / hr and a propylene:chlorine ratio of 4:1. The reactor temperature and pressure are held constant at 350 o K and 2 atm respectively. Calculate the selectivity achieved. ANSWER : For a plug-flow reactor, material balances may be written on propylene (A), chlorine (B), dichloropropane (D), and allyl chloride (U) in terms of molar flow rates (F) and partial pressures of propylene and chlorine: dF A /dV = - r 1 - r 2 = - ( k
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Unformatted text preview: 1 + k 2 ) p A p B dF B /dV = - r 1 - r 2 = - ( k 1 + k 2 ) p A p B dF R /dV = r 1 = k 1 p A p B dF S /dV = r 2 = k 2 p A p B A expression for the selectivity (S), defined as the ratio of F D to F U , can be derived by combining the last two equations and integrating: k 2 dF D = k 1 dF U S = F R / F S = k 1 / k 2 = 21.53 Note: solving for the fractional conversion of propylene requires integration of the material balance on A. Let x = 1 - F A / F A i . Then F B = F B i - F A i x = F A i (4 - x), F R + F S = F A i x, and F T = F A i (5 - x). Assuming that the ideal gas law is valid, p A = R T C A = R T C T ( F A / F T ) = P T (1 - x) / (5 - x) and p B = P T (4 - x) / (5 - x). Substituting into the material balance yields F A i dx / dV = (k 1 + k 2 ) P T 2 (1 - x) (4 - x) / (5 - x) 2 [ (k 1 + k 2 ) P T 2 V / F A i ] = (1/3) ln (1 - x/4) - (16/3) ln (1 - x) + x Solving by trial and error yields: x = 0.464...
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This note was uploaded on 04/29/2008 for the course CHE 4151 taught by Professor J during the Spring '08 term at FIT.

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Example - Parallel Rxns - 1 + k 2 ) p A p B dF B /dV = - r...

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