Measures Other Than Conversion | top |
Uses:
A. Membrane reactors
B. Multiple reaction
Liquids: Use concentrations, I.E. CA
1. For the elementary liquid phase reaction carried out in a CSTR, where V, vo, CAo, k, and Kc are given and the feed is pure A, the combined mole balance, rate laws, and stoichiometry are:
There are two equations, two unknowns, CA and CB
Gases: Use Molar Flow Rates, I.E. FI
2. If the above reaction, ,carried out in the gas phase in a PFR, where V, vo,CAo,k, and Kc are given and the feed is pure A, the combined mole balance, rate laws, and stoichiometry yield, for isothermal operation (T=To) and no pressure drop (DP=0) are:
Use Polymath to plot FA and FB down the length of the reactor.
For isothermal microreactors, we use the same equations as a PFR as long as the flow is not laminar. If the flow is laminar, we must use the techniques discussed in chapter 13. See example 4.8 of the text.
University of Washington Transport Effects in Microreactors siteMembrane Reactors | top |
Membrane reactors can be used to achieve conversions greater than the original equilibrium value. These higher conversions are the result of Le Chatelier's Principle; you can remove one of the reaction products and drive the reaction to the right. To accomplish this, a membrane that is permeable to that reaction product, but is impermeable to all other species, is placed around the reacting mixture.
Example: The following reaction is to be carried out isothermally in a membrane reactor with no pressure drop. The membrane is permeable to Product C, but it is impermeable to all other species.
For membrane reactors, we cannot use conversion. We have to work in terms of the molar flow rates FA, FB, FC.
Polymath Program
Mole Balances |
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Rate Laws |
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Combine |
Polymath will combine for you-- Thanks Polymath...you rock! |
Parameters |
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Solve |
Polymath |
Here are links to example problems dealing with membrane reactors. You could also use these problems as self tests.
Semibatch Reactors | top |
Semibatch reactors can be very effective in maximizing selectivity in liquid phase
reactions.
to Selectivity
The reactant that starts in the reactor is always the limiting reactant.
1. Molar Basis | |||
2. Concentration Basis | |||
3. Conversion |
For constant molar feed: | ||
For constant density: | ||
Use the algorithm to solve the remainder of the problem.
Example: Elementary Irreversible Reaction
Consider the following irreversible elementary reaction:
-rA = kCACB
The combined mole balance, rate law, and stoichiometry may be written in terms of number of moles, conversion, and/or concentration:
Conversion | Concentration | Number of Moles |
Polymath Equations:
Conversion | Concentration | Moles |
d(X)/d(t) = -ra*V/Nao |
d(Ca)/d(t) = ra - (Ca*vo)/V |
d(Na)/d(t) = ra*V |
ra = -k*Ca*Cb |
d(Cb)/d(t) = rb + ((Cbo-Cb)*vo)/V |
d(Nb)/d(t) = rb*V + Fbo |
Ca = Nao*(1 - X)/V |
ra = -k*Ca*Cb |
ra = -k*Ca*Cb |
Cb = (Nbi + Fbo*t - Nao*X)/V |
rb = ra |
rb = ra |
V = Vo + vo*t |
V = Vo + vo*t |
V = Vo + vo*t |
Vo = 100 |
Vo = 100 |
Vo = 100 |
vo = 2 |
vo = 2 |
vo = 2 |
Nao = 100 |
Fbo = 5 |
Fbo = 5 |
Fbo = 5 |
Nao = 100 |
Ca = Na/V |
Nbi = 0 |
Cbo = Fbo/vo |
Cb = Nb/V |
k = 0.1 |
k = 0.01 |
k = 0.01 |
Na = Ca*V |
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X = (Nao-Na)/Nao |
Polymath Screenshots:
Conversion | Concentration |
Consider the following reversible reaction:
Everything is the same as for the irreversible case, except for the rate law:
At equilibrium, -rA=0, then
See Also:
Web Module on Reactive Distillation
You Rate Some Wetlands Critical Thinking Questions
* All chapter references are for the 1st Edition of the text Essentials of Chemical Reaction Engineering .