6. Algorithm for Isothermal Reactor Design^*

Topics

1. Measures Other Than Conversion
2. Membrane Reactors
3. Semibatch Reactors

Measures Other Than Conversion

Uses:    

A. Membrane reactors
B. Multiple reaction

Liquids: Use concentrations, I.E. C_A

1. For the elementary liquid phase reaction carried out in a CSTR, where V, v_o, C_Ao, k, and K_c are given and the feed is pure A, the combined mole balance, rate laws, and stoichiometry are:

There are two equations, two unknowns, C_A and C_B

Gases: Use Molar Flow Rates, I.E. F_I

2. If the above reaction, ,carried out in the gas phase in a PFR, where V, v_o,C_Ao,k, and K_c 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 F_A and F_B down the length of the reactor.

Stoichiometry for Measures Other than Conversion

Gas Phase PFR

Liquid Phase CSTR

Use Creative and then Critical Thinking

What Four Things are Wrong With this solution?

Microreactors

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.

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 F_A, F_B, F_C.               

Polymath Program

Mole Balances

Rate Laws
Stoichiometry Isothermal, no pressure drop

Combine	Polymath will combine for you-- Thanks Polymath...you rock!
Parameters
Solve	Polymath

"What four things are wrong with this membrane reactor solution?"

Here are links to example problems dealing with membrane reactors. You could also use these problems as self tests.

Membrane Type 4 Home Problem (Heterogeneous)

Membrane Type 4 Home Problem (Homogeneous)

Membrane Type 7 Home Problem

Membrane Type 8 Home Problem

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.

Three Forms of the Mole Balance Applied to Semibatch Reactors:

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:

-r_A = kC_AC_B

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:

Polymath Screenshots:

Critical Thinking Questions

Equilibrium Conversion in Semibatch Reactors with Reversible Reactions

Consider the following reversible reaction:

Everything is the same as for the irreversible case, except for the rate law:

Where:

At equilibrium, -r_A=0, then

Semibatch:  A → B Acid Catalyzed

See Also: 

Web Module on Reactive Distillation

Web Module on Wetlands

You Rate Some Wetlands Critical Thinking Questions

Object Assessment of Chapters 5 and 6

Polymath Book Problems

A. Chapter 6 PBR ODE Solver Algorithm

PBR ODE Solver Polymath File

B. Chapter 6 Semibatch ODE Solver Algorithm

Semibatch ODE Solver Polymath File

Objective Assessment of Chapter 6

^* All chapter references are for the 1st Edition of the text Essentials of Chemical Reaction Engineering .

top