Solved: Methane and oxygen react in the presence of a

Chapter 4, Problem 4.67

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Methane and oxygen react in the presence of a catalyst to form formaldehyde. In a parallel reaction, methane is oxidized to carbon dioxide and water: CH4 O2 ! HCHO H2O CH4 2O2 ! CO2 2H2O The feed to the reactor contains equimolar amounts of methane and oxygen. Assume a basis of 100 mol feed/s. (a) Draw and label a flowchart. Use a degree-of-freedom analysis based on extents of reaction to determine how many process variable values must be specified for the remaining variable values to be calculated. (b) Use Equation 4.6-7 to derive expressions for the product stream component flow rates in terms of the two extents of reaction, 1 and 2. (c) The fractional conversion of methane is 0.900 and the fractional yield of formaldehyde is 0.855. Calculate the molar composition of the reactor output stream and the selectivity of formaldehyde production relative to carbon dioxide production. (d) A classmate of yours makes the following observation: If you add the stoichiometric equations for the two reactions, you get the balanced equation 2CH4 3O2 ! HCHO CO2 3H2O The reactor output must therefore contain one mole of CO2 for every mole of HCHO, so the selectivity of formaldehyde to carbon dioxide must be 1.0. Doing it the way the book said to do it, I got a different selectivity. Which way is right, and why is the other way wrong? What is your response?