Exercise 2 Text

Page 1

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 H2 ↔ CH3OH (1) Since CO2 is fed to the reactor, the Reverse Water Gas Shift takes also place: CO2 + H2 ↔ CO + H2O (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: ρୡୟ୲=1.98 g/cmଷ ε = 0.4 〈Cp୫୧୶〉 = 4.081 kJ/kg K ∆HRଵ = −23460 cal/mol ∆HRଶ = 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 CO2 0.1009 H2 0.5698 CH4 0.2358 CH3OH 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: ∆𝐺ோ଴(𝑇)=−22828+56.02⋅𝑇 [cal/mol] where T in [K] RWGS: ∆𝐺ோ,ோௐீௌ଴ (𝑇)=8514−7.71⋅𝑇 [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…