Chemical Reaction Engineering II
Aim
• To introduce various types of Reactions and Reactors that are commonly used in Chemical Engineering
operations.
Objectives
• Get ability in deciding and designing the type of Reactorsthat are necessary for a particular type of
reaction in an Industry.
• They also learn mechanism and control of several type of reactions.
Unit I – Non-Ideal Reactors
The residence time distribution as a factor performance
Residence time functions and relationship between them in reactor
Basic models for non-ideal flow
Conversion in non-ideal reactors
Learning Objectives
The real reactors are always deviate from the ideal reactors because of the non-ideal flow
conditions within the reactor during the process. In the first unit you are going to study
1. The non-ideal (real) reactors and its design
2. The factors that make up the contacting or flow pattern in the non-ideal reactors.
3. Basic models used for non-ideal reactors
Ideal Reactors
1. PFR (no-axial mixing)
2. MFR (complete mixing)
Non-ideal reactors (real reactors) The reactors always deviate some degree from these ideal behaviors
called real reactors.
The deviation from ideal flow patterns may be caused by
1. Stagnant regions in the vessel (dead zone)
2. Bypassing or short-circuiting of the fluid
3. Channeling
4. Recycling of fluid
5. Due to vortices and turbulences at inlet and outlet
Factors that make up the contacting or flow pattern are:
1. The RTD – residence time distribution of fluid flowing through the vessel.
2. The state of aggregation of fluid and
3. The earliness and lateness of fluid mixing in the vessel
Residence Time Distribution Function
• The time it takes a molecule to pass through a reactor is called the residence time of the molecule in
the reactor.
• It is clear that elements of fluid taking different routes through the reactor may take different lengths
of time to pass through it.
• So there is a distribution of residence time of the fluid material within the reactor.
• The distribution of these times for the stream of fluid leaving the vessel is called the exit age
distribution, E, or the residence time distribution RTD of fluid.
In an ideal PFR – all the molecules within the reactor having the same length of time.
• In an ideal batch reactor - all the molecules within the reactor having the same length of time
• The ideal PFR and BR are the only reactors in which all the fluid elements in the reactors have the
same residence time.
• In all other reactors, the various molecules spend different times inside the reactor, Ex: MFR
• The feed introduced into the MFR at any given time completely mixed with the material already in the
reactor.
Some of the molecules entering the MFR leave it almost immediately, because the material is
continuously removed from the reactor, other molecules remain in the reactor almost forever
because all the material is never withdrawn at one time from the reactor. Many of the molecules
leave the reactor after spending a period of time which is somewhere close to the mean residence
time.
• The distribution of residence time can significantly affects the performance of reactor.
• The RTD of a reactor is a characteristic of the mixing that occurs in the reactor.
RTD Measurement
The RTD is determined experimentally by using an inert chemical called a tracer (dyes and radioactive
materials).
Characteristics of chemical tracer:
1. It should be non-reactive
2. It should be completely soluble in the system fluid
3. It should be easily detectable
4. Physical properties of the tracer should be similar to those of the system and
5. It should not adsorb on the walls or others surfaces in the vessel.
Experimental Methods for Finding E
1. Pulse input
2. Step input
3. Periodic input
4. Random input
1.The Pulse Input Experiment
• The known quantity of tracer (M kg or moles) is suddenly injected in one shot into the fluid
entering the vessel in a very short time period
• The tracer concentration in the exit stream is measured as a function of time.
• In RTD analysis, the effluent concentration-time curve is referred to as the C curve (Cpulse
curve).
• The concentration of effluent stream increases with time, reaches a maximum value and then
falls, eventually approaching zero.
Relationship between the F and E curves
• Consider that there is a steady flow of main fluid through the vessel and at time t=0, we switch
to tracer fluid and measure the rising concentration of tracer fluid in the effluent stream, the F
curve.
• At any time t>0, the tracer fluid and only tracer fluid in the effluent stream is younger than t.
Therefore,
Relation among the F,C,I and E curves in closed vessel
• The closed vessel being defined as one in which material passes in and out by bulk flow only.
• Consider steady-state flow of fluid through a vessel and a step function of tracer introduced
into the fluid entering the vessel.
• Suppose that the tracer is simply a second fluid introduced into the vessel at time t=0 in place
of the original flowing fluid.
• Then at any time t or θ>0 a material balance for the vessel gives
