Chemical Engineering - Industrial Organic Chemistry

Exercise 2 - Text

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INDUSTRIAL ORGANIC CHEMISTRY – EXERCISE 2 Methanol synthesis: design of the multi-stage adiabatic reactor Methanol synthesis is carried out in multi-stage adiabatic reactors with intermediate cooling in order to maintain a high reaction rate. The main reaction involved in the methanol synthesis is: CO + 2 H 2 ↔ CH 3OH (1) Since CO 2 is fed to the reactor, the Reverse Water Gas Shift takes also place: CO 2 + H 2 ↔ CO + H 2O (2) By assuming that the intermediate cooling is carried out by means of external heat exchangers, the temperature of the reacting mixture leaving the exchanger is equal to 245 °C. The maximum temperature in the catalytic bed has to be lower than 270 °C to avoid catalyst deactivation. The pressure drops along the reactors are negligible. Based on the reported data: a) write the mass and heat balance equations for the generic adiabatic bed adopting a pseudo- homogeneous model; b) evaluate the volume of the reactor and the number of beds required to obtain a molar fraction of methanol in the outlet stream equal to 0.049; c) evaluate the conversion of CO in the entire reactor Physical-chemical properties: ρa_r = 1.98 g/cm 7 ε = 0.4 〈Cp kgv 〉= 4.081 kJ/kg K ∆HR 5 = −23460 cal/mol ∆HR 6 = 9510 cal/mol Operating conditions: Absolute pressure: 65 bar Inlet temperature: 245 °C Total inlet flow rate: 5824 kmol/h Composition (mol/mol): CO 0.0540 CO 2 0.1009 H2 0.5698 CH 4 0.2358 CH 3OH 0.0042 H2O 0.0009 N2 0.0344 Kinetic equations: To evaluate the equilibrium constants, the ΔG° of reaction are reported (T in K): Methanol synthesis: ∆�������4(������) = − 22828 + 56 .02 ⋅������ [cal/mol] where T in [K] RWGS: ∆�������,����4 (������) = 8514 − 7.71 ⋅������ [cal/mol] where T in [K] Fugacity coefficients : Specie CO CO 2 H 2 H 2O CH 3OH N 2 CH 4 Fugacity coefficient 1.02 0.98 1.02 0.86 0.84 1.02 1.00 As a first approximation, the fugacity coefficients can be assumed constant with respect to temperature