There is a one more consideration . The flow in large diameter tubes is more likely to have radial and recirculation components . These components can affect the reaction rate and in any case make the plug flow model less valid .
If you are considering to use many PFR tubes then i would suggest you to use them in parallel. but it also depends on the rate of reaction, and your desired product composition as in PFR composition is different at different point. Use any system ( Either parallel or single) which gives optimum area, rate of reaction, your desired product composition etc.
We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling We Value Civility
• Positive and compassionate attitudes
• Patience while debating We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving