Title: Chapter 6 Design for Single Reaction
1Chapter 6 Design for Single Reaction
- In this chapter we deal with single reaction.
These are reactions whose progress can be
described and followed adequately by using one
and only one rate expression coupled with the
necessary stoichiometric and equilibrium
expressions. - For such reactions product distribution is fixed
hence, the important factor in comparing design
is the reactor size.
26.1 Size Comparison of Single Reactors
- Batch reactor
- Batch reactor has the advantage of small
instrumentation cost and flexibility of
operation. It has the disadvantage of high labor
and handling cost, often considerable shutdown
time to empty, clean out, and refill, and poorer
quality control of the product.
3- Regarding reactor size, for a given duty and a
constant volume system, an element of fluid
reacts for the same length of time in the batch
and in the plug flow reactor. Thus, the same
volume of these reactors is needed to do a given
job. - On a long-term production basis we must correct
the size requirement estimate to account for the
shutdown time between batches.
4- Mixed versus plug flow reactor, first- and
second-order reaction - Make a comparison for large class of reactions
approximated by the simple nth-order rate law
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7Above figure shows
Remember that the essential factor for reactor
size is reaction rate, which is controlled only
by concentration of reactant A, besides
temperature. The order of reaction, conversion,
expansion factor are all related to concentration.
8- Variation of reactant ratio for second-order
reaction
9- General graphical comparison
- For a nth-order reaction(ngt0), it can be seen
that mixed flow always needs a larger volume than
does plug flow for any given duty.
106.2 Multiple-Reactor System
- Plug flow reactors in series and/or in parallel
- N plug flow reactors in series with a total
volume V gives conversion as a single plug flow
reactor of volume V. - For reactors in parallel V/F or t must be the
same for each parallel line. Any other way of
feeding is less efficient.
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13- Equal-size mixed flow reactors in series
- In plug flow, the concentration of reactant
decreases progressively through the system in
mixed flow, the concentration drops immediately
to a low value.
14Consider a system of N equal-size mixed reactors
connected in series. Density changes will be
assumed to be negligible.
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19- Second-order reactions
- With the same per-condition and same procedure,
the performance of a second-order
bimolecular-type reaction with no excess of
either reactant can be found
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24To double reactor size
Divide reactor to two
To lower conversion by increasing treatment rate
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26- Mixed flow reactors of different sizes in series
- Two types of questions may be asked
- How to find the outlet conversion from a given
reactor system? - How to find the best setup to achieve a given
conversion?
27- Finding the conversion in a given system
Note for a given system, ti is known
28- Determining the best system for a given
conversion
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33- Reactors of different types in series
34- Ideas for best arrangement
356.3 Recycle reactor
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45Overlap graph for a comparison of recycled and
equal size mixed flow reactors
46- Rough Comparison of recycled and equal-sized
mixed flow reactors in series
The data can be read from the overlap graph
47isolation
48 Inner-recycle reactor
496.4 Autocatalytic Reactions
50- Plug flow versus mixed flow reactors
- At low conversion, the mixed reactor is superior
to the plug flow reactor. - At high enough conversions the plug flow reactor
is superior.
51- Optimum Recycle Operations
- Only autocatalytic reactions need to optimize
recycle ratio. - The optimum recycle ratio is found by
differentiating - with respect to R and setting to zero
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54By selecting proper R, to make
55 56Chapter 7 Design for Parallel Reactions
- In this chapter, we discuss how to optimize the
size of a reactor and product distribution with a
pre-condition of constant volume. - The two requirements, small size and maximization
of desired product, may run counter to each
other. In such a situation an economic analysis
will yield the best compromise.
57- Qualitative Discussion About Product Distribution
- Consider the decomposition of A by either one or
two path
58- Discussion
- CA is the only factor in this equation which we
can adjust and control. - If a1gta2 or the desired reaction is of higher
order than unwanted reaction, high reactant
concentration is desirable. As a result, a batch
or plug flow reactor would favor formation of
product R and would require a minimum reactor
size.
59- If a1lta2 or the desired reaction is of lower
order than unwanted reaction, we need a low
reactant concentration to favor formation of
product R. But this would also require large
mixed flow reactor. - If a1a2 or the two reactions are of same order.
- Product distribution is fixed by k1/k2 alone and
is unaffected by type of reactor used.
60summarizing
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65- Quantitative Treatment of Product Distribution
and of Reactor Size - For convenience in evaluating product
distribution we introduce two terms
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67- For mixed flow reactor in series
- Single reactor
CR,N
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70- When there are two or more reactants involved,
the fractional yield can be based on one of the
reactants consumed, on all reactants consumed, or
on products formed. It is simply a matter of
convenience which definition is used. Thus in
general, we define as the
instantaneous fractional yield of M, based on the
disappearance or formation of N.
71- The Selectivity
- The selectivity is defined as follow
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82Chapter 8 Potpourri of Multiple Reaction
- In this chapter, we develop or present the
performance equations of some of the simpler
system and point out their special features such
as maxima of intermediates
83- 8.1 Irreversible first-order reaction in series
84- Qualitative discussion about product distribution
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86- The following rule governs product distribution
for reactions in series - For irreversible reactions in series the mixing
of fluid of different composition is the key to
the formation of intermediates. The maximum
possible amount of any and all intermediates is
obtained if fluids of different compositions and
at different stages of conversion are not allowed
to mix.
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88why?
89Overlap graph for a comparison of recycled and
equal size mixed flow reactors
90- Quantitative treatment, plug flow or batch flow
reactor
Chapter 3
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92- Quantitative treatment, mixed flow reactor
- Material balance for reactant A
- inputoutputdisappearance by reaction
- FA0FA(-rA)V
- vCA0vCAk1CAV
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97- 8.2 First-order followed by zero-order reaction
98why?
99- 8.3 Zero-order followed by first-order reaction
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101- 8.4 Successive irreversible reaction of different
order - explicit solution are difficult to obtain,
numerical methods provide the best tool for
treating such reactions. - As with reactions in parallel, a rise in reactant
concentration of favors the higher-order
reaction a lower concentration favors
lower-order reaction.
102- 8.5 Reversible reactions
- Solution of the equations for successive
reversible reactions is quite formidable even for
the first-order case thus we illustrate only the
general characteristics for a few typical cases.
Consider the reversible first-order reactions
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105- 8.6 Irreversible series-parallel reaction
Some examples shown on the text book
106- Two-step irreversible series-parallel reaction
107- Qualitative discussion about product distribution
- R is desired product, and S is unwanted.
- Image we have two beakers, one holding A and the
other holding B. - Mix them by three ways, add A slowly to B, add B
slowly to A and mix A and B together rapidly.
108- (a) Add A slowly to B
- The result is that at no time during the slow
addition will A and R be present in any
appreciable amount.
109 110- (c) Mix A and B rapidly
- Same type of distribution curve as for the
mixture in which B is added slowly to A can be
found. - In case (a), the performance is a reaction in
series - In case (b) and (c) , it looks like
111- From this discussion we propose the general rule
- Irreversible series-parallel reactions can be
analyzed in term of their constituent series
reactions and parallel reactions in that optimum
contacting for favorable product distribution is
the same as for the constituent reaction.
112- Quantitative treatment, plug flow or batch reactor
113- The solution of above differential equation is
114- Quantitative treatment, mixed flow
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117Chapter 9 Temperature and Pressure Effects
- This is the only chapter which deals with heat.
- 9.1 Single Reaction
- Brief Review
- Heats of reaction from thermodynamics
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122- Equilibrium Constants from Thermodynamics
123Equilibrium Conversion
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131- General Graphical Design Procedure
- Temperature, composition and reaction rate are
uniquely related for any single homogeneous
reaction.
132For a given feed and using conversion of key
component as a measure of composition and extent
of reaction, the XA vs. T plot has the general
shape shown below
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134- The size of reactor for a given duty and for a
given temperature progression is found as follows - 1. Draw the reaction path on the XA vs. T plot.
This is the operating line for the operation. - 2. Find the rates at various XA along this path.
- 3. Plot the 1/(-rA) vs. XA curve for this path.
- 4. Find the area under this curve. This gives
V/FA0.
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140- Optimum Temperature Progression
- We define the optimum temperature progression to
be that progression which minimizes V/FA0 for
given conversion of reactant. This optimum may be
an isothermal or it may be a changing
temperature in time for a batch reactor, along
the length of a plug flow reactor, or from stage
to stage for a series of mixed flow reactors. It
is important to know this progression because it
is the ideal which we try to approach with a real
system. It also allows us to estimate how far any
real system departs from this ideal.
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142- Heat Effects
- If the reaction is exothermic and if heat
transfer is unable to remove all of the liberated
heat, then the temperature of the reacting fluid
will rise as conversion rise. - Adiabatic Operations
- ------The heat liberated (or absorbed) by
reaction is only for heating (or cooling) the
reacting fluid. - In this situation, the conversion and temperature
shows sole relationship.
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151- Nonadiabatic Operation(not isothermal)
- Some heat add or remove from the reaction system
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153Some true cases
154Exothermic Reactions in Mixed Flow Reactor---- A
Special Problem
VR
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156- The heat generated by reaction
- The heat removed by heating the feed stream and
cooling water
157Q
Q
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159Igniting and extinguish points of a mixed flow
reactor
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169Important line CD is doubtable for optimal
value.
170- Summarizing of above examples
171- 9.2 Multiple Reaction
- In multiple reactions both reactor size and
product distribution should be considered. - Product distribution and temperature
172Chapter 10 Choose the Right Kind of Reactor
- Six rules and some examples
- The summary of the first 9 chapters
- Nothing new