How do you maximize selectivity for parallel and series reaction systems?

Parallel reactions (A -> B desired, A -> C undesired): selectivity depends on relative rates. If n1 > n2 (B formation higher order), keep CA high - use PFR or batch. If n1 < n2, keep CA low - use CSTR or dilute feed. Series reactions (A -> B -> C, B desired): selectivity maximized at intermediate conversion. Use PFR or batch with optimal residence time. Monitor and stop at peak B concentration. For CSTR, multiple tanks allow staged optimization. Temperature also affects selectivity based on activation energies.