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4.8 Microreactors (Micro Process Engineering)
A typical channel width might be 100 m with a length of 20,000 m (2 cm). The resulting high surface area-to-volume ratio (~10,000 m 2/m3) reduces or even eliminates heat and mass transfer resistances often found in larger reactors. Consequently, surface catalyzed rxns can be greatly facilitated, hot spots in highly exothermic reactions can be eliminated, and in many cases highly exothermic reactions can be carried out isothermally. Other advantages include shorter residence times and narrower residence time distributions.
Fig: Microreactors: All-in-One Extractor Ref: web.mit.edu
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Fig: Microreactors. Ref: www.carbogen-amcis.com
Fig: Microplant with reactors, valves, and
mixers.
Fig: Microreactor with Heat Exchanger. Courtesy of Ehrfeld, Hessel, & Lowe. Microreactors: New Technology for Modern Chemistry (Wiley-VCH. 2000).
4.8 Microreactors (Micro Process Engineering)
Production in microreactor systems can be increased simply by adding more units in parallel. For example, the catalyzed reaction required only 32 microreaction systems in parallel to produce 2000 tons/yr of acetate!
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Microreactors are also used for the production of specialty chemicals, combinatorial chemical screening, lab-on-a-chip, & chemical sensors. In modeling microreactors, we will assume they are either in plug flow (dFj/dV = rj) or in laminar flow (segregation model Chap. 13)
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Example 4-7 Gas-Phase Reaction in a Microreactor Molar Flow Rates
The gas-phase reaction which follows an elementary rate law 2NOCl 2NO + Cl2 is carried out at 425°C & 1641 kPa (16.2 atm) in a microreactor system using a bank of ten microreactors in parallel. Each microreactor has 100 channels. Each channel has a cross-sectional area of 0.2 mm 2 & a length of 50 mm (volume of 10-5 L). In this system, 85% conversion is achieved and 20 tons of NO per year is produced by processing 0.0226 mol/s of pure NOCl, or 2.26X10 -5 mol/s per channel. The rate constant is: k = 0.29 L/mol .s at 500 K with E = 24 kcal/mol. Plot the Fj = f (V ) down the length of the reactor.
Example 4-7 Gas-Phase Reaction in a Microreactor
Molar Flow Rates
Solution: 2NOCl 2NO + Cl2 2A
2B + C
T = 425°C
P = 1641 kPa
yNOCl,0 or yA0 = 1
-rA = kCA2
k = 0.29 L/mol.s at 500 K with E = 25 kcal/mol
Each channel: Ac = 0.2 mm 2
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Z = 50 mm
V = 10-5 L
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2NOCl 2NO + Cl2
Solution:
1. Mole balance
2A
2B + C
2. Rate law
-r A = kC A2 k = 0.29*exp[-E/R(1/T-1/500)]
where E = 24x10 3 cal/mol
3. Stoichiometry Gas phase with T = T 0 and P = P 0
Relative Rates
Concentrations
Total Flow Rates
4. Combine 5. Evaluate
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Using Polymath
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2NOCl 2NO + Cl2 2A
2B + C
4.9 Membrane Reactors
The term membrane reactor describes a number of different types of reactor configurations that contain a membrane. List 3 reasons for using “membrane reactors”.
To increase X when the rxn is thermodynamically limited
To increase the selectivity when multiple rxns are occurring
To obtain pure products.
Thermodynamically limited rxns are rxns where the equilibrium lies far to the left (i.e.. reactant side) and there is little X . What will happen if T is increased during an exothermic rev. rxn?
So should I carry out the rxn at low T ? What could happen?
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Increasing T will only drive the rxn further to the left (lower the X ) Decreasing T will result in a slow reaction rate and, therefore, low X .
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4.9 Membrane Reactors
Is it favorable to run an endothermic rev. rxn under high T ?
Are there any drawback for increasing the T excessively?
Yes, increasing T will move the rxn to the right to favor a higher X .
Yes, for many rxns, these higher T induce undesired side rxns and cause the catalyst to become deactivated.
The membrane reactor is a technique for driving reversible rxns to the right toward completion in order to achieve very high X. This can be achieved by having one of the rxn products diffuse out of a semipermeable membrane surrounding the rxn mixture. As a result, the reverse rxn will not be able to take place, and the rxn will continue to proceed to the right toward completion.
4.9 Membrane Reactors
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Mention the main types of the membrane reactors.
Inert Membrane Reactor (IMR): the reactor is packed with catalyst while the membrane serves as a barrier to the reactants & some of the products.
Catalytic Membrane Reactor (CMR): the catalyst is deposited directly on the membrane, and only specific rxn products are able to per meate through the membrane.
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Inert Membrane Reactor
Catalytic Membrane Reactor (CMR)
Fig. 4-13 Ceramic Membrane Reactors
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CRE Algorithm for MRs
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We use similar algorithm, but with slight modification. We use the reactor volume instead of the catalyst weight. Write hydrogen mole balance for the shown segment of the membrane reactor.
How do we quantify RB?
Through molar flux of B:
Where kC’ is the overall mass transfer coefficient in m/s and CBS is the concentration of B in the sweep gas channel (mol/L). The kC’ accounts for all resistances to transport:
the tube side resistance of the membrane
the membrane itself
the shell (sweep gas) side resistance.
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Example 4-8 Membrane Reactor
According to the US Department of Energy (DOE), an energy saving of 10 trillion BTU per year could result from the use of catalytic membrane reactors as replacements for conventional reactors for dehydrogenation reactions such as the dehydrogenation of ethylbenzene to styrene:
and of butane to butene: The dehydrogenation of propane is another reaction that has proven successful with a membrane reactor: All the preceding dehydrogenation reactions above can be represented symbolically as: and could take place on the catalyst side of an IMR or inside a CMR.
Consider the following dehydrogenation reaction : which is to be carried out in a catalytic membrane reactor (CMR). The equilibrium constant for this reaction is quite small at 227°C (KC = 0.05 mol/l). The membrane is permeable to B – which is H2 – but not to A and C. Pure gaseous A enters the reactor at 8.2 atm and 227°C at a rate of 10 mol/min. Consider the rate of diffusion of B out of the reactor per unit volume of reactor,RB , to be proportional to the concentration of B (i.e.. RB = kC.CB , where kC represents the overall mass transfer coefficient). The reactor volume is 400 liters. The specific reaction rate, k, and the transport coefficient, kC, are k = 0.7 min-1 and kC = 0.2 min-1, respectively. (a) Perform differential mole balances on A, B, and C to arrive at a set of coupled differential equations to solve. (b) Plot the molar flow rates of each species as a function of the reactor volume. (c) Calculate the conversion at the exit of the reactor. (d) What would be the achieved conversion if the reaction was carried out in a conventional PFR. (e) Compare the conversions in both cases and explain the obtained results.
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CMR X = 0.567
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PFR X = X eq = 0.447
4.10 Unsteady-State Operation of Stirred Reactors
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Situations, studied in this chapter, where we have unsteady state operation include:
Batch Reactor: CA vs. t
Startup of a CSTR: to determine t necessary to reach steady-state operation.
Semibatch Reactor: CA or X vs. t
analytical solutions to the differential equations arising from the mole balance of these rxn types can be obtained only for zero- and firstorder rxns. ODE solvers must be used for other rxn orders.
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4.10.1 Startup of a CSTR
If a CSTR is to be started up by introducing the feed & without having an overflow stream (no exit), then this is a semibatch operation. The startup of a fixed volume CSTR under isothermal conditions is rare, but does occur occasionally. We can, however, carry out an analysis to estimate the time necessary to reach steady-state operation. Feed
AB
C A0
Startup of a CSTR in semibatch mode
AB Feed
Product
C A0
C A & C B
Startup of a constant volume CSTR
4.10.1 Startup of a CSTR
Consider the liquid phase reactive system shown below where:
The reactor is well-mixed (no spatial variation in rA)
= 0 & V = V0
Assume the reactor is initially filled with the solvent only. Write a general mole balance equation for the given system.
AB Feed
Product
0,
,
C A0, F A0
F A, F B, C A, C B
Startup of a constant volume CSTR
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4.10.1 Startup of a CSTR
The general mole balance equation for the given system is: FA0 - FA rAV
dt
Conversion does not have any meaning in startup because one cannot distinguish between the NA present due to unreacted A molecules & these due to accumulation within the CSTR. Rewrite the balance equation in terms of concentration. C A0 -C A r A
dN A
dC A dt
Feed 0,
AB
C A0
Product ,
C A & C B
Fur most first-order systems, steady state is achieved in 3 to 4 space times. Startup of a constant volume CSTR
4.10.2 Semibatch Reactors
Besides the startup of a CSTR in semibatch mode prior to achieving constant V , there are two basic types of semibatch operations:
Removal of a product (reactive distillation)
Addition of a reactant Consider the rxn:
A + B C + D
Feed C A0 & C B0
Startup of a CSTR in semibatch mode
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A + B C + D
4.10.2 Semibatch Reactors
A + B C + D
The first type of semibatch reactor is reactive distillation. Here reactants A and B are initially charged together and one of the products is vaporized and withdrawn continuously. What are the benefits of removing one of the products (e.g. C) in this manner? 1.
2.
For an equilibrium rxn, this will shift the equilibrium toward the right, increasing the final X . This will further concentrates the reactants, and thereby producing an increased rate of rxn & decreased processing time.
&
4.10.2 Semibatch Reactors
Examples of rxns carried out in this type of rxn include acetylation rxns & esterification rxns in which water is removed. E.g. (1): Salicylic acid is acetylated to form aspirin (acetyl functional group )
E.g. (2): Esterification of acetic acid in excess ethanol to form ethyl acetate (ester)
Rxns from: www.wikipedia.com
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4.10.2 Semibatch Reactors
In the second type of the semibatch operation, one of the reactants (e.g. B) is slowly fed to a reactor containing the other reactant (e.g. A), which has already been charged to the reactor. When this type of reactor is used? It is generally used when unwanted side reactions occur at high CB or when the reaction is highly exothermic/indothermic. In some reactions, the reactant B is a gas and is bubbled continuously through liquid reactant A. Examples of reactions used in this type of semibatch reactor operation include:
Ammonolysis
chlorination
hydrolysis
4.10.2 Semibatch Reactors
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A + B C + D
A + B D + U
One of the best reasons to use semibatch reactors is to enhance selectivity in liquid-phase reactions. For example, consider the following two simultaneous rxns:
Desired rxn:
Undesired rxn:
The instantaneous selectivity SD/R is defined as the ratio of the relative rates:
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How can we produce the most of our desired product and the least of our undesired product?
By keeping the CA high and the CB low.
How can this achieved?
This can be achieved through the use of the semibatch reactor, which is charged with pure A and to which B is fed slowly to A in the vat.
Commercial Break Now you are ready to read this article:
Published in “Chemical Engineering Progress”, An
AIChE Publication, CEP: Mar 2001. Available on Black Board.
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4.10.2 Semibatch Reactors
consider the elementary liquid-phase reaction A + B C
that is to be carried out in the shown semibatch reactor with a constant molar feed.
Write a mole balance on species A.
Three variables can be used to formulate and solve semibatch reactor problems:
the concentrations, Cj
the number of moles, Nj
the conversion, X .
4.10.3 Writing the Semibatch Reactor Equations in Terms of Concentrations
For the considered reaction, rewrite the mole balance equations for species A & B in terms of concentration. Also find V (t ) A + B C
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Remember, @ t = 0, CA=CAi & CB = 0 If the rxn order is other than zero- or first-order, or if the rxn is non-isothermal, we must use numerical techniques to solve the above Eqs.
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Problem 4-23 production of ethylene glycol from ethylene chlorohydrin
The production of ethylene glycol from ethylene chlorohydrin and sodium bicarbonate is carried out in a semibatch reactor. A 1.5 molar solution of ethylene chlorohydrin is fed at a rate of 30 mole/min to 1500 liters of a 0.75 molar solution of sodium bicarbonate. The reaction is elementary and carried out isothermally at 30oC where the specific reaction rate is 5.1 L/mol .h. The reactor can hold a maximum of 2500 liters of liquid. Assume constant density. A conversion of 96% of sodium bicarbonate is desired. 1.
Perform mole balances on the reactants.
2.
Calculate the time required to achieve the desired conversion.
3.
4.
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Is it possible to achieve the desired conversion with the given reactor and conditions? Why? If not, then what is the maximum possible conversion. Plot the concentration of reactants, the reaction rate, and the conversion as a function of time. (Fig. 1: CA, CB, & X vs t ; Fig. 2: -rB vs. t )
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Example Hydrolysis of acetic anhydride in a semibatch reactor
The kinetics of hydrolysis of acetic anhydride was studied by Haji & Erkey* using in-situ FTIR spectroscopy. O C H3C
O
O +
C O
H2O
C
2
CH3
H3C
OH
In part of the study, acetic anhydride was hydrolyzed in a semibatch operation as shown in the figure.
*Haji, Shaker; Erkey, Can. AN EXPERIMENT FOR UNDERGRADUATE LABORATORY – KINETICS OF HYDROLYSIS OF ACETIC ANHYDRIDE BY INSITU FTIR SPECTROSCOPY. Chemical Engineering Education, winter 2005.
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In the semibatch experiment, 150 ml of distilled water (W) was initially charged to the reactor while pure acetic anhydride (A) was continuously added to the reactor at a rate of 3.55 ml/min over a period of 8.17 minutes. The reaction was carried out isothermally at 35oC. Water was present in excess, therefore, it concentration was assumed to be constant during the course of the rxn. Hence the rxn was considered to be first-order in acetic anhydride. Additional information: k = 0.2752 min-1, MMA = 102.09 g/mol, A = 1.082 g/ml
Assume a constant-density system Solve for the concentration of acetic anhydride and the rate of reaction as a function of time.
4.10.4 Writing the Semibatch Reactor Equations in Terms of Conversion
Consider the reversible elementary reaction: A + B C + D in which B is fed, at constant rate, to a vat containing only A initially. Write an integral mole balance on A, B, C, & D.
NA - NA0 = - NA0X
NB-NBi = FB0dt - NA0X or NB = FB0.t - NA0X
NC = + NA0X
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ND = + NA0X
Then, write the rate law:
Then, use the stoichiometry
Finally, combine & evaluate
A + B C + D
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Finding the Equilibrium Conversion: A + B
C + D
For reversible rxns carried out in a semibatch reactor, the maximum attainable X (i.e., the equilibrium X ) will change as the rxn proceeds. Why? Because more reactant is continuously added to the reactor. This addition shifts the equilibrium continually to the right toward more product. The X e could be calculated as follows at equilibrium: where
Therefore,
Solving for t :
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Solving for X e:
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Finding the Equilibrium Conversion: A + B
C + D
For reversible rxns carried out in a semibatch reactor, the maximum attainable X (i.e., the equilibrium X ) will change as the rxn proceeds. Why? Because more reactant is continuously added to the reactor. This addition shifts the equilibrium continually to the right toward more product. The X e could be calculated as follows at equilibrium: where
Therefore,
Solving for t :
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Solving for X e:
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