Reaction pt 2

What assumptions are made for ideal reacors?

perfectly mixed

uniform temperature/concentration

steady state

What causes non-ideal behaviour?

channeling

bypass

dispersion from turbulent trasnport

dead volume

non-ideal inlet/outlet conditions

diffusion from concentration gradient

Ideal CSTR

uniform reactant concentration throughout the vessel

Real CSTR

•Relatively high reactant concentration at the feed entrance

•Relatively low concentration in the stagnant regions, called dead zones (usually corners and behind baffles)

•Some material bypasses the intended flow path and has a lower residence time (short circuit)

Ideal PFR

all reactant and product molecules at any given axial position move at same rate in the direction of the bulk fluid flow. Velocity is constant and channel length is the same for all streamlines

Real PFR

fluid velocity profiles, turbulent mixing, & molecular diffusion cause molecules to move with changing speeds and in different directions (channeling)

Relatively low concentration in the stagnant regions, called dead zones (usually corners and behind baffles)

What are the consequences of non-idealities

•reduced conversion

•Product quality is affected e.g. molecular weight distribution is varied.

•unpredictable

•Reactor volume and length will be bigger/longer.

Residence time distribution (RTD)

The amount of time molecules spend in the reactor. Different residence time means different reaction times

Tracer Experiment

Used to determine RTD. A tracer is injected at the feed point and a detector measures the concentration of the tracer leaving the reactor.

Wha is the area under Ct vs t graph

The area under these curves are proportional with the …amount of tracer  if there is no hold up/no leak/ adsorbing on catalyst etc.

This means area under the curves of C(t) and E(t) are equal.

what does E(t) represent?

Probability of a molecule will reside in the reactor for time t

what’s the difference between RTD for ideal and non ideal reactors?

RTD can be calculated for ideal reactors, but must be measured for real reactors

Space time (tau)

Reactor volume / volumetric flowrate

when is mean residence time equal to space time

perfectly mixed reacors (ideal)

•No density change

• No backmixing

PFR pulse input response

sharp peak at space time for ideal reactor

CSTR pulse input response

sharp peak with a tail at t = 0

what are some potential solutions for non ideal reactors?

•adding or deleting baffles

•adding or changing the position of mixing blades

•repacking of catalyst particles.

how does space time affect pulse input curve

•Large t - slow decay of the output transient C(t), & E(t)

•Small t - a rapid decay of the transient C(t), & E(t)

bypass space time compared to ideal

higher

dead volume space time compared to ideal

lower

what happens when a tracer is recycled through the reactor

Recycle acts like dilution. Thus, peak heights are reduced

how many peaks does an E(t) curve for a PFR with bypass have

2 peaks

why can’t RTD be used on its own to characterise a reactor

Same RTD may belong to different reactor configurations with different conversions.

how can you properly characterise a reactor

use an adequate model of non-ideal reactor flow pattern and knowledge of the quality of mixing

why do you need to know degree of mixing?

because concentration affects conversion in higher order reactions

When is the dispersion model used?

to describe nonideal tubular reactors.

In this model, there is an axial dispersion of the material, which is governed by an analogy to Fick’s law of diffusion, superimposed on the flow

What can cause axial dispersion?

generated by molecular diffusion, turbulent mixing & non-uniform velocities, which gives rise to distribution of residence times in the reactor.

what does Damkohler (Da) number show?

rate of consumption of A by reaction / rate of transport of A by convection

what does reactor Peclet (Per) number show?

rate of transport by convection / rate of transport by diffusion or dispersion

How can you find Pe number

laminar flow with radial and axial molecular diffusion

correlation from the literature for pipes and packed beds

experimental tracer data.

Two parameter model

real reactor modeled as combinations of ideal reactors

alpha

System V / V

beta

bypass flowrate / flowrate

what do you plot for 2 parameter model

ln(CT0/(CT0-CT)) against t

plot should be linear