[curiouscat]

Residence time of a fluid in a reactor (tube / plug flow) is inversely related to its velocity. i.e. lesser the velocity more the residence time.

Is there a practical limit how low this velocity may be kept? Normally, in fluid flow calculations there's a limit on max velocity due to erosion or pressure drop worries. What's the minimum limit? I've seen 0.5 m/sec quoted during heat exchanger design due to fouling worries.

But for reactors could one go lower? Assume: Liquid single phase flow.

0.5 m/sec would imply a max reaction  residence time of 30 sec. in a 15 m long reactor.

[Woopy]

I imagine that there are economic factors that play into this more than physical. If you have your chemical sit in a CSTR all day, you might as well make it a batch reactor for maximum conversion, or at least semibatch. The time it spends in the reactor and converting for too long means that you can't produce any more without another reactor, which can be money wasted, which requires economic analysis.

[curiouscat]

I imagine that there are economic factors that play into this more than physical. If you have your chemical sit in a CSTR all day, you might as well make it a batch reactor for maximum conversion, or at least semibatch. The time it spends in the reactor and converting for too long means that you can't produce any more without another reactor, which can be money wasted, which requires economic analysis.

Yes, but the economic factors (in my example at least) become limiting far after the engineering ones.

e.g. 5 tube reactors of 1" size with 0.1 m/sec flow would already give more throughput than our current batch setup.