We will now give three examples of multiple reactions with heat effects: Example 12-5 discusses parallel reactions, Example 12-6 discusses series reactions, and Example 12-7 discusses complex reactions.
Example 12-5. Parallel Reactions in a PFR with Heat Effects
The following gas-phase reactions occur in a PFR:
Pure A is fed at a rate of 100 mol/s, a temperature of 150°C, and a concentration of 0.1 mol/dm3. Determine the temperature and molar flow rate profiles down the reactor.
Additional information
ΔHRx1A = –20,000 J/(mol of A reacted in reaction 1)
ΔHRx2A = –60,000 J/(mol of A reacted in reaction 2)
The PFR energy balance becomes [cf. Equation (12-35)]
Mole balances:
Rates:
Rate laws
Relative rates
Reaction 1:
Reaction 2:
Net rates:
Stoichiometry (gas phase ΔP = 0):
Energy balance:
The Polymath program and its graphical outputs are shown in Table E12-5.1 and Figures E12-5.1 and E12-5.2.
Table E12-5.1. Polymath Program
Analysis: The reactant is virtually consumed by the time it reaches a reactor volume V = 0.45 dm3 and beyond this point Qr > Qg and the reactor temperature begins to drop. In addition, the selectivity remains constant after this point. If a high selectivity is required, then the reactor should be shortened to V = 0.3 dm3, at which point the selectivity is .
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