Reactors in Series Flashcards
What type of reactor cannot occur in series?
Batch
What is the general purpose of putting reactors in series?
Maximise conversion or throughout.
For 3 reactors in series, write the equations for the moles of A after each reactor for xA conversion.
nA1 = nA0 - xA1nA0 nA2 = nA0 - xA2nA0 nA3 = nA0 - xA3nA0
What is the effect on residence time when putting CSTRs in series?
More molecules will have the same residence time at 1 given time, eventually forming a singular peak distribution.
What is the effect on residence time when putting PFRs in series?
In a fixed infinitesimal volume, there is constant flow in and out of this volume to neighbouring fixed volumes - resembling a CSTR at steady state. A PFR is a infinite CSTR forming a singular peak distribution.
How do you calculate the volume for a CSTR?
V/nA0 = 1/rA xA
For a positive order reaction, does a small or large CSTR operate at a faster rate.
Smaller - higher concentration at exit conditions.
Does a PFR operate at a faster or slower rate than a CSTR of the same size?
PFR operates at faster rate.
Why is a reactor in series preferred over in parallel?
Higher overall conversion as conversion in reactor 1 is fed into reactor 2..
If the same final conversion was achieved in a parallel and series reactor model then which would have the higher reaction rate?
They would have the same as a CSTR operates at exit conditions. However the first reactor in series would have a lower 1/rA therefore higher rate & throughput.
How should the size be arranged for a series for a first order reaction in a CSTR to maximise conversion?
Conversion is independent of size order. Best results are 2 equal sized.
How do you calculate conversion for a 1st order CSTR reactor in series?
xA = tk/(tk+1) 1-xA = 1/(tk+1) = CAi/CA(i-1)
How should the size be arranged for a series for a zero order reaction in a CSTR to maximise conversion?
Rate is independent of concentration so size does not matter.
How should the size be arranged for a series for a negative order reaction in a CSTR to maximise conversion?
Larger reactor should be placed first to keep conversion high & concentration low. Lower concentration = faster rate.
How should the size be arranged for a series for a positive higher order reaction in a CSTR to maximise conversion?
Smaller reactor first to keep conversion low & concentration high. First reactor should have a high rate - smallest reactor.
Generally, should a CSTR or PFR be put first in series?
PFR first as higher concentration & reaction time.
For a zero order reaction, should a CSTR or PFR be placed first in series?
Rate is independent of concentration so order does not matter.
For a higher order reaction, should a CSTR or PFR be placed first in series?
PFR first maximises rate & throughput.
For a negative order reaction, should a CSTR or PFR be placed first in series?
CSTR first higher concentration has lower rate.
For a first order reaction, should a CSTR or PFR be placed first in series?
Conversion independent of inlet concentration - order does not matter.
How do you calculate conversion in a PFR?
xA = 1 - e(-kt)
How do you connect a series of PFRs?
Becomes one large PFR. Change of shape only important for volume.
How do you calculate the volume for a PFR?
V = [nA0/rA dxA] (xA,0)
How do you calculate the number of CSTR reactors required in series for a given conversion xA?
V = nA0 xA/rA Vk/nA0u(t) = xA/(nA0(1-xA))^2 Solve for xA -> nA1 = nA0 - xAnA0 Substitute Vk/nA1u(t) = xA/(nA0(1-xA))^2 Repeat until desired xA.
How do you calculate the conversion after the 20th CSTR reactor in series?
CA20/CA0 = 1/(tk+1)^20 = 1 - xA
How do you calculate the conversion after the 20th PFR in series?
Act as one big PFR.
CA/CA0 = e (-kt) = 1-xA
How do you calculate the increased throughput in series compared to parallel for n reactors?
Parallel; 0 = nA0 - nA - rA nV u(t) = nVk/(CA0/CA -1) u(t) = nVk/(1/(1-xA) - 1) Series; 0 = nA0 - nA - rA V u(t) = Vk/(sqrt(CA0/CA2) - 1) u(t) = Vk/(sqrt(1/(1-xA)) - 1) COMPARE; Series/Parallel = [Vk/(sqrt(1/(1-xA)) - 1)]/nVk/(1/(1-xA) - 1)
