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Class 10 – Wednesday, September 15th

Multiple Reactions, Yield, & Selectivity

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 + H2OCH4 + 2 O2 → CO2 + 2 H2O

100 mol/s containing equimolar amounts of methane and oxygen are fed to the reactor. Assuming that the limiting reactant has a fractional conversion, how many degrees of freedom are there?

a) 0 b) 1 c) 2 d) 3

Multiple Reactions - In most chemical processes, various reaction pathways may occur for the reactants. Therefore we define two terms to describe the degree to which a desired reaction predominates:

Moles of desired product formedYield =Yield = ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

Moles that would have been formed if there were no side reactions and the limiting reactant had reacted completely

Moles of desired product formedSelectivity = ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

Moles of undesired product formed

In combustion reactions, you often have complete combustion (forming H2O and CO2) and incomplete combustion (forming H2O and CO). If 10 moles of methane are fed to a reactor, where the fractional conversion is 0.5, forming 3 moles of CO2 and 2 moles of CO, what is the percentage yield ?

CH + 2O CO + 2H OCH4 + 2O2 → CO2 + 2H2OCH4 + 3/2O2 → CO + 2H2O

a) 60 %b) 30 %c) 50 %d) There is no yield because combustion reactions

do not have desired products

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Molecular species balances:

For a given reaction, we can balance using molecular species, as we did with non-reactive systems. What is the general balance for a steady-state reactive system using molecular species?p

a) Input = outputb) Input + generation = outputc) Input = output + consumptiond) Input + generation = output + consumptione) Input + generation – output – consumption =

accumulation

In the Deacon process for the manufacture of chlorine, HCl and O2 react to form Cl2 and H20. Sufficient air (79 % N2 and 21 % O2) is fed to provide 35 % excess oxygen and the fractional conversion of HCl is 85 %. Calculate the mole fractions of the product stream components using molecular species balances.

2HCl + ½ O2 → Cl2 + H2O

How many species will be in the product stream?a) 3b) 4c) 5d) 6

Sufficient air is fed to provide 35 % excess oxygen and the fractional conversion of HCl is 85 %. Calculate the mole fractions of the product stream components using molecular species balances.

2HCl + ½ O2 → Cl2 + H2O

What is the general balance for HCl?ga) Input = outputb) Input + generation = outputc) Input = output + consumptiond) Input + generation = output + consumptione) Input + generation – output – consumption =

accumulation

Sufficient air is fed to provide 35 % excess oxygen and the fractional conversion of HCl is 85 %. Calculate the mole fractions of the product stream components using molecular species balances.

2HCl + ½ O2 → Cl2 + H2O

What is the general balance for Cl2?g 2a) Input = outputb) Input + generation = outputc) Input = output + consumptiond) generation = outpute) 0 = output + consumption

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Sufficient air is fed to provide 35 % excess oxygen and the fractional conversion of HCl is 85 %. Calculate the mole fractions of the product stream components using molecular species balances.

2HCl + ½ O2 → Cl2 + H2O

What is the general balance for N2?g 2a) Input = outputb) Input + generation = outputc) Input = output + consumptiond) generation = outpute) 0 = output + consumption

Degree of freedom analysis for molecular species balances:

# unknown variables+ # independent chemical reactions- # independent molecular species balances p p- # other information, choice of basis if needed, etc

Sufficient air (79 % N2 and 21 % O2) is fed to provide 35 % excess oxygen and the fractional conversion of HCl is 85 %. Calculate the mole fractions of the product stream components using molecular species balances.

2HCl + ½ O2 → Cl2 + H2O2HCl + ½ O2 → Cl2 + H2O

How many degrees of freedom do you have?

a) 0b) 1c) -1d) 2

Sufficient air (79 % N2 and 21 % O2) is fed to provide 35 % excess oxygen and the fractional conversion of HCl is 85 %. Calculate the mole fractions of the product stream components using molecular species balances.

2HCl + ½ O2 → Cl2 + H2O

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Sufficient air (79 % N2 and 21 % O2) is fed to provide 35 % excess oxygen and the fractional conversion of HCl is 85 %. Calculate the mole fractions of the product stream components using molecular species balances.

2HCl + ½ O2 → Cl2 + H2O