Post on 24-May-2018
The Engineering of
Chemical Reactions
Chapter 3. Single reactions in continuous
isothermal reactors
Learning Objective & Overview
1. To understand the CSTR and PFTR
2. To understand batch processes are also ideal to measure rates and kinetics
in order to design continuous processes
Learning Objective
1. Mass balance & Energy balance
2. Conversion, X
3. CSTR and PFTR
4. Chemical reactors in series
Overview
Chapter 3. Single reactions in continuous
isothermal reactors
The Continuous Stirred Tank Reactor
Chapter 3. Single reactions in continuous
isothermal reactors
• CSTR (Continuous Stirred Tank Reactor)
• ‘stirred tank’ or ‘backmix reactor’
• In this situation the crucial feature is that the composition is identical
everywhere in the reactor and in the exit pipe
• Mass balance : .... GenOutInAccum
jojo CF 0
jj CF
V
The Continuous Stirred Tank Reactor
Chapter 3. Single reactions in continuous
isothermal reactors
ⅰ) Steady state :
reactor residence time :
ⅱ)
I. Steady state vs. non-Steady state
II. ↔
Conversion in a constant-density CSTR
Chapter 3. Single reactions in continuous
isothermal reactors
If,
If ,
νj = 1
• Irreversible Reactions
Conversion in a constant-density CSTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Irreversible Reactions
<Example 3-1>
sol’n
X
X
X
X X
Conversion in a constant-density CSTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Irreversible Reactions
<Example 3-2>
,
Sol’n
, conversion is 90%,
,
Conversion in a constant-density CSTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Fractional Conversion, X
Example 3-1
( Assumed )
Conversion in a constant-density CSTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Reversible reactions
<cf.> first-order reaction
The Plug-Flow Tubular Reactor
Chapter 3. Single reactions in continuous
isothermal reactors
• PFTR
0 LZ Z+△Z
0jF jF
The Plug-Flow Tubular Reactor
Chapter 3. Single reactions in continuous
isothermal reactors
• PFTR
0 z L
jj CF 000 jj CF
jtjj CuACzF )(
Tube of length L ; the molar flow rate of species j is Fj
The Plug-Flow Tubular Reactor
Chapter 3. Single reactions in continuous
isothermal reactors
• Mass balance
.... GenOutInAccum
, ( = volumetric flow rate,
D = diameter)
Δ
Δ
The Plug-Flow Tubular Reactor
Chapter 3. Single reactions in continuous
isothermal reactors
To assume Steady-state
,
Therefore, the mass balance on species j becomes
We next make a Taylor series expansion of the difference in Cj between z and z+dz
and let dz → 0, keeping only the lead term.
The Plug-Flow Tubular Reactor
Chapter 3. Single reactions in continuous
isothermal reactors
• Note again that this expression assumes
ⅰ) plug flow ~ no dispersion
ⅱ) steady state ~ no accumulation
ⅲ) constant density ~ =constant
ⅳ) constant tube diameter ~ At=constant
ⅴ) a single reaction ~ no summation in r
• This equation is not appropriate if all five of these conditions are not met
Taking the limit and dividing by
Conversion in a Constant-Density PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
After separation we obtain the differential equation
,
• Irreversible Reactions
Conversion in a Constant-Density PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Irreversible Reactions
<Example 3-3>
, , ,
,
sol’n
cf. 18min for CSTR
cf. 72 for CSTR
Conversion in a Constant-Density PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Irreversible Reactions
<Example 3-3>
, , ,
,
sol’n
How long a 2cm diameter tube would be required for this conversion and
what would be the fluid velocity?
∴
Conversion in a Constant-Density PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
• While the PFTR reactor volume is much smaller than the CSTR for this
conversion, the PFTR tube length may become impractical, particularly
when pumping costs are considered.
• Batch :
• PFTR :
Conversion in a Constant-Density PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Irreversible Reactions
<Example 3-4>
sol’n
, Conversion 90%
Comparison between Batch,
CSTR, and PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
cf.) first-order irreversible reaction, ρ= constant, steady-state
Comparison between Batch,
CSTR, and PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Comparisons of Possible Advantages (+) and Disadvantages (-) of Batch, CSTR, and PFTR
Batch CSTR PFTR
Reactor size for given conversion + - +
Simplicity and cost + + -
Continuous operation - + +
Large throughput - + +
Cleanout + + -
On-line analysis - + +
Product certification + - -
Comparison between Batch,
CSTR, and PFTR
Chapter 3. Single reactions in continuous
isothermal reactors
• Ratio of Residence Times and Reactor Volumes in CSTR and PFTR versus Conversion for a
First-Order Irreversible Reaction
0.0 1.0
0.5 1.44
0.9 3.91
0.95 6.34
0.99 21.5
0.999 145
00 AAA CCCX /)( PFTRCSTR /
• X ↑, ↑ pC /
Chapter 3. Single reactions in continuous
isothermal reactorsThe 1/r Plot
AAo CC AAo CC 0
r
1
• From the preceding arguments it is clear that the PFTR usually requires a
smaller reactor volume for a given conversion, but even here the CSTR may
be preferred because it may have lower material cost(pipe is more expensive
than a pot).
Chapter 3. Single reactions in continuous
isothermal reactorsSemibatch Reactors
• It is of course possible to add feed or withdraw product continuously in a
“batch” process, and we call this a semibatch reactor.
if
: CSTR + Batch reactor
cf.) now the volume V of the reactor contents increases linearly with time
must be solved numerically
Chapter 3. Single reactions in continuous
isothermal reactorsVariable-density reactor
reactants : A, steady state
∴
:
• CSTR
Chapter 3. Single reactions in continuous
isothermal reactorsVariable-density reactor
∴
• PFTR
Chapter 3. Single reactions in continuous
isothermal reactorsVariable-density reactor
AC
AoC
)(
1
ACr
X
)(
1
Xr
0AF
V
• vs.
Chapter 3. Single reactions in continuous
isothermal reactorsVariable-density reactor
<Example 3-6>
, ,
Ideal gas
Chapter 3. Single reactions in continuous
isothermal reactorsVariable-density reactor
<Example 3-6>
, ,
Chapter 3. Single reactions in continuous
isothermal reactorsVariable-density reactor
1
10 15 20 25
0.6
0.8
5 30
0.2
0.4
00
V (liters)
X
PFTR
•
nB = 2
nB = 0.5• •
nB = 1
1
40 60 80 100
0.6
0.8
20 120
0.2
0.4
00
V (liters)
X
CSTR
140
nB = 0.5• ••
nB = 2nB = 1
• Plot of conversion versus reactor volume V for the reaction 𝐴 → 𝑛𝐵𝐵, 𝑟 = 𝑘𝐶𝐵 ,
with ideal gases for 𝑛𝐵 = 2,1, and1
2. The times are all close until the
conversion becomes large, when the product dilutes the reactant for 𝑛𝐵 = 2and slows the reaction.
Chapter 3. Single reactions in continuous
isothermal reactorsVariable-density reactor
• The constant-density approximation is frequently used even when it does
not apply exactly because it is much simpler to solve the equations, and the
errors are usually not large.
• We note finally that none of these formulations gives an accurate
description of any reactor in which there is a pressure drop in gases flowing
through the reactor.
• Another rather complex type of problem involves nonideal gases and gas
mixtures because then only numerical solutions are possible.
Chapter 3. Single reactions in continuous
isothermal reactorsSpace Velocity and Space Time
ⅰ) gas hourly SV (GHSV)
ⅱ) liquid hourly SV (LHSV)
0v
VST =
STv
VSV
1
0
== SV = space velocity
ST = space time
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
,,,AoC 1AC
2AC
3AC
AnC
1
2
3
4
• CSTRs in series
Tanks-in-series (cf. Levenspiel)
Sketch of ideal chemical reactors in series with 𝐶𝐴𝑛, the product from
reactor n, which is also the feed into reactor n+1
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
• CSTRs in series
For first-order kinetics with equal-volume CSTR reactors(and therefore for
all τs equal)
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
• CSTRs in series
n→∞ : total τ of CSTR = τ of PFTR
τ i
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
• CSTRs in series
<Example 3-8>
, residence time = τ, conversion = 90%, Equal volume CSTRs in series
What residence times and reactor volumes will be required for n=1,2,3, and 4.
Sol’n
We rearrange the equation for n equal volume CSTRs in series,
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
• CSTRs in series
<Example 3-8>
, residence time = τ, conversion = 90%, Equal volume CSTRs in series
What residence times and reactor volumes will be required for n=1,2,3, and 4.
τ decreases with n to approach the
PFTR for which τ = 4.61min
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
• CSTRs in series
<Example 3-8>
, residence time = τ, conversion = 90%, Equal volume CSTRs in series
What residence times and reactor volumes will be required for n=1,2,3, and 4.
The reactor volumes are 𝑉 = 𝜈𝜏𝑛, which are 72, 34.6, 27.7, and 24.9
liters, respectively. An infinite number of CSTRs in series would
require the total volume of a PFTR, which is 4 × 4.61 = 18.4 liter s
to run this reaction to this conversion.
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
• PFTR + CSTR
PFTR + CSTR
CSTR + PFTR
For first-order kinetics
i) PFTR + CSTR ii) CSTR + PFTR
For two equal-volume reactors(𝜏1 = 𝜏2) with first-order kinetics the
expression are identical for both configurations.
Chapter 3. Single reactions in continuous
isothermal reactorsChemical Reactors in Series
In general it is a common strategy to use a CSTR first where the conversion is low and
then switch to a PFTR as the conversion become high to minimize total reactor volume.
The total residence time from CSTR+PFTR in series is indicated by the 1/r plots
PFTR + CSTR CSTR + PFTR
Chapter 3. Single reactions in continuous
isothermal reactorsAutocatalytic reactions
A+B → B+B,
The rate of the forward reaction is enhanced by the concentration of a product.
Chapter 3. Single reactions in continuous
isothermal reactorsAutocatalytic reactions
P
F
T
R
If → 𝜏 = ∞
CAoCAo - CA CAo - CACAo
PFTR
CSTR
C
S
T
R
Chapter 3. Single reactions in continuous
isothermal reactorsAutocatalytic reactions
Fermentation
Combustion reactions
▶ sugar + enzyme → alcohol + 2enzyme(A) (B)
▶
▶A + R → 2R
radical
Chapter 3. Single reactions in continuous
isothermal reactorsReversible Reactions
• For a reversible reaction the rate goes to zero before the reaction reaches
completion, and 1/r therefore goes to infinity.
AACC
0 AACC
0
0ACAeC
r r
1
0
1
r
0r
AeC 0AC0 0
Irreversible
reversible
Irreversible
reversible
Chapter 3. Single reactions in continuous
isothermal reactorsReversible Reactions
What is Residence of CSTR and PFTR ?X=0.5
sol’n
CSTR
<Example 3-9>
Chapter 3. Single reactions in continuous
isothermal reactorsReversible Reactions
What is Residence of CSTR and PFTR ?X=0.5
sol’n
PFTR
<Example 3-9>
Chapter 3. Single reactions in continuous
isothermal reactorsTransients in Continuous Reactors
• non-steady state
In the case of CSTR (𝑉, 𝜐0 = constant)
Chapter 3. Single reactions in continuous
isothermal reactorsTransients in Continuous Reactors
• non-steady state
In the case of CSTR (𝑉, 𝜐0 = constant)
Chapter 3. Single reactions in continuous
isothermal reactorsTransients in Continuous Reactors
• Solvent replacement
no reaction
at t = 0, CA = CAi
(Note carefully here the difference between 𝐶𝐴𝑖 and 𝐶𝐴0)
Chapter 3. Single reactions in continuous
isothermal reactorsTransients in Continuous Reactors
• Reaction
ODE
: first-order irreversible reactions
at t = 0, CA = CAi
, : CSTR of steady state
Chapter 3. Single reactions in continuous
isothermal reactorsTransients in Continuous Reactors
• PFTR
process of induction
.... GenOutInAccum
Divide by ,
∴
Chapter 3. Single reactions in continuous
isothermal reactorsTransients in Continuous Reactors
0AC
AsC
0
AC
0t
0t 0t
0 0t
0AC
AsC
AC
Possible transients in CSTR reactors. left picture shows the situation starting with
pure solvent (𝐶𝐴 = 0) in the reactor initially, 𝐶𝐴 = 𝐶𝐴0 at t=0 with no reaction. right
picture shows the situation where the reactor initially contains pure reactant at 𝐶𝐴0,
and at t=0 reactant flow at 𝐶𝐴0 is begun and eventually approaches a steady-state
concentration 𝐶𝐴𝑠
• CSTR