Methanol synthesis

Pagina 1

METHANOL SYNTHESIS – NOTES KINETICS AND THERMODYNAMICS Why should we expect a maximum trend of the curve at constant conversion ? In general we have to deal with the backward reaction, the net reaction rate is given by forward – backward reaction rate (𝑘⃗ = forward rate constant). 𝑟𝑛𝑒𝑡 = 𝑘⃗ 𝑓1(𝑝𝐶𝑂,𝑝𝐻2) − 𝑘⃖⃗ 𝑓2(𝑝𝐶𝐻3𝑂𝐻) This curve passes through a maximum because of the progressively important growing role of the backward reaction at increasing T. we have a dynamic evolution of composition such that we start with just the 2 reactants in the vessel and then we progressively build up the product, thus backward reaction becomes more and more important; in the end we reach thermodynamic equilibrium (forward and backward rate are equal). Fixing conversion means fixing composition of the system, so we fix 𝑓1(𝑝𝐶𝑂,𝑝𝐻2) and 𝑓2(𝑝𝐶𝐻3𝑂𝐻), what changes is the different temperature dependence of the two rate constants: they have different sensitivity because of the different activation energy. Activation energy for a rate constant is measuring the sensitivity with the T. Activation energy of forward rate constant is lower than backward rate constant. Lower activation energy means lower T sensitivity, weaker T dependence. At increasing temperature 𝑘⃗ and 𝑘⃖⃗ are growing, but 𝑘⃖⃗ grows more. At increasing T net reaction rate will ideally tend to zero, we get equilibrium temperature. Rate constants have a specific relationship at equilibrium, 𝐾𝑒𝑞 = 𝑘⃗ 𝑘⃖⃗ . REACTORS: • MULTITUBOLAR: heat of reaction is continuously removed by water (cold water at the inlet, steam at the outlet). We need high surface/volume ratio to maximize heat transfer, thus small tubes are used (dt=4 -5 cm) . multitubolar reactor keeps as homogeneous as possible temperature profile, it approximates better than other…