Chapter Seventeen - Heterogeneous Reactions - Introduction Flashcards
(22 cards)
What are the two complicating factors that must be accounted for beyond what is normally considered in homogeneous systems?
For these systems there are two complicating factors that must be accounted for beyond what is normally considered in homogeneous systems. First, we have the complication of the rate expression, and second the complication of the contacting pattern for two-phase systems.
What is the course’s focus?
The second half of reaction engineering treats the kinetics and design of chemical reactors for heterogeneous systems of various kinds, each chapter considering a different system.
What will the rate equation generally incorporate?
The rate expression in general will incorporate mass transfer terms in addition to the usual chemical kinetics term.
What must be considered in the rate equation for heterogeneous reactions?
Since more than one phase is present, the movement of material from phase to phase must be considered in the rate equation.
Are these mass transfer terms identical for the different kinds of heterogeneous systems?
These mass transfer terms are different in type and numbers in the different kinds of heterogeneous systems; hence, no single rate expression gas general application.
How many steps are involved in the Burning of a carbon particle?
From Figure 17.1, we see that two steps in series are involved - mass transfer of oxygen to the surface followed by reaction at the surface of the particle.
Tell how many rate steps are involved when air bubbles through a tank of liquid which contains dispersed microbes and is taken up by the microbes to produce product material.
(Draw the corresponding figure). From 17.2 we see that there are up to seven possible resistance steps, only one involving the reaction. How many you choose to consider depends on you and on the situation.
What is the first step to get an overall rate expression?
To get an overall rate expression, write the individual rate steps on the same basis (unit surface of burning particle, unit volume of fermenter, unit volume of cells, etc.) (Write the different forms of rate available and how they are linked)
What is the second step?
Now put all the mass transfer and reaction steps into the same rate form and then combine.
How do we combine for steps in series?
r overall = r1 = r2 = r3
How do we combine rate for steps in parallel?
r overall = r1 + r2
When it is easy to combine steps in series?
Consider steps in series. In general, if all steps are linear in concentration, then it is easy to combine them. However, if any of the steps are nonlinear, then you will get a messy overall expression.
How can you bypass a nonlinear step?
You may try to bypass this nonlinear step in one of various ways. Approximating the rA versus CA curve by a first-order expression is probably the most useful procedure.
What do we look to eliminate in combining rates? Solve Example 17.3
In combining rates, we normally do not know the concentration of materials at intermediate conditions, so there are the concentrations that we eliminate in combining rates.
When is the addition of resistances to obtain an overall resistance permissible?
It so happens that the addition of resistances to obtain an overall resistance is permissible only when the rate is a linear function of the driving force and when the processes occur in series.
Example 17.4
Solve the Example Again
What about contacting patterns for two-phase systems?
There are many ways two phases can be contacted, and for each the design equation will be unique. Design equations for these ideal flow patterns can be developed without too much difficulty.
What can we do when real flow deviates from ideal flow patterns?
When real flow deviates considerably from these, we can do one of two things: we may develop models to mirror actual flow closely, or we may calculate performance with ideal patterns which “bracket” actual flow.
What is fortunate in the case of heterogeneous systems? Name the different ideal flow patterns.
Fortunately, most real reactors for heterogeneous systems can be satisfactorily approximated by one of the five ideal flow patterns.
What is one notable exception?
Notable exceptions are the reactions which take place in fluidized beds. There, special models must be developed.
What is essential in reactor design and scale up?
In reactor design and scale up, it is essential to select a flow model which reasonably represents our setup.
What is a mistake that happens too often in reactor design and scale-up?
Too often we put a little thought here, carelessly picking a nonrepresentative model and then doing computer calculations to the nth degree of accuracy. A simple reasonable model is much better than a precise and detailed which does not represent the contacting. Often choice of a good flow model and the knowledge of how the flow pattern changes with scale up spells the difference between success and failure
