Non-Ideal Reactors, Residence Time Distribution & Dispersion Model Flashcards
Define space time and write the equation
The time needed to fill one reactor volume (macromixing)
tao = V/v0
Define residence time
The amount of time molecules spend in the reactor (micromixing)
State the assumptions of the residence time distribution
- Steady state
- Without reaction
- The fluid is incompressible so no density change
Why do we use the residence time distribution?
It allows the detection of non-ideal flows such as bypassing and short-circuiting by understanding the routes the fluid takes
What is the pulse experiment?
An experiment where a pulse input where N0 moles of an inert tracer is injected at t=0 and the concentration at the outlet is analysed. The vessel must be closed.
What is E(t)? What are the units?
The exit age distribution (1/time).
This shows the fraction of all the material leaving between a given time interval
Write the general equation for the exit age distribution
E (t) = C (t)/integral [C (t) dt]
Write the exit age distribution formulae in its discrete form
E (t) = C / sum [C delta (t)] = C / A
OR if the amount of tracer inputted is known:
E (t) = C v / N0
What is F (t)?
The cumulative distribution function.
This shows the fraction of fluid that leaves the reactor with an age less than a given value of t.
Write the general equation for the cumulative distribution function
F (t) = integral [E (t) dt]
How do you calculate the fraction of fluid that leaves the reactor with an age greater than a given value of t?
1 - F (t) = 1 - integral [E (t) dt]
Write the cumulative distribution function formulae in its discrete form
To find the F value for 1 specific time use:
F (t2) = F (t1) + (E (t1) + E (t2)/2) x delta (t2 - t1)
To find the fraction of fluid between 2 times (eg. 4 and 8 given in 2 min intervals):
F (t) = [ (E (4) + E (6))/2 ] x (6 - 4) + [(E (6) + E (8))/2 ] x (8 - 6)
Write the equation for the area under the C-curve
A = integral [ C(t) dt ] (infinity, 0) A = sum [ Ci delta ti ] A = N0/v
Write the equation for the mean residence time of the C-curve
t = integral [ t E (t) dt ] (infinity, 0)
substituting in for E (t)
t = integral [ t C (t) dt ] (infinity, 0) / integral [ C (t) dt ] (infinity, 0)
t = sum [ ti Ci] / sum [ Ci ]
What is the difference between the C-curve and the E-curve?
The E-curve is the normalised version of the C-curve which neglects the amount of tracer and the volumetric flow rate. This allows two sets of experimental data to be more accurately compared
Write the equation for the area under the E-curve
A = N0/v A = M/v
Write the equation for the mean residence time of the E-curve
t = integral [ t E (t) dt ] (infinity, 0)
Why would we use the E (theta) - curve? Define the units and necessary equations
To remove the effect of reactor volume
theta = ti/mean res time (t)
E (theta) = mean res time (t) x E
The units for E (theta) are dimensionless.
Write the equation for the variance of the C-curve or E-curve?
variance = integral [ ti^2 E (t) dt ] (infinity, 0) - (mean res time (t))^2
How do you diagnose reactor problems using the compartment model?
If reactor volume and volumetric flow rate is known we can use the compartment model.
Calculate V/v and compare this to the mean residence time observed from the data, if:
- mean res time (t) = V/v then the reactor has no dead spaces
- mean res time (t) ≠ V/v the reactor has dead spaces and the volume of dead space (Vd) can be calculated by:
Vp = mean res time (t) x v
Vd = V - Vp
If there is a bypass, the active volumetric flow rate (va) will be LESS than the total volumetric flow rate. To calculate the bypass volumetric flow (vb) use:
v = va + vb
Draw the curves for the following diagnoses in a PFR:
- Stagnant backwaters
- Parallel paths/channelling
- Good flow
- Internal recirculation
- Late curve
See W9 notes
What are the 2 potential reasons for the late curve diagnosis in a PFR?
- v or V has been measured incorrectly
- The tracer used is not inert
- The vessel is not closed
Draw the curves for the following diagnoses in a CSTR:
- Stagnant fluud
- Time lag
- Good flow
- Internal recirculation
- Late curve
- Shortcircuiting
See W9 notes
Outline the method for calculating a new conversion of reactant once a reactor diagnosis is improved
- Write the design equation
- Replace volume, V with Va (active volume) so that:
k Va/v = ( CA0 / (1 - XA) CA0 ) - 1 - Calculate the RHS of the above equation (this is the ration for V/Va)
- Equate k V/v = n k Va/v so that it becomes:
= n x (V/v) = number - The number calculated in 4 = (CA0/CA2) - 1 so rearrange to find CA2/CA0
- The new conversion = 1 - the number in 5.