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CPB 415 –Chemical Kinetics and Reactor Design

Semester Project

Objective: 

Design a reactor to produce 10000 metric tons/year of maleic anhydride from butane.

Reaction Details:

Maleic anhydride is an industrially important commodity chemical used primarily in the production of polyester resins.  The current commercial process to produce maleic anhydride employs a vanadium-phosphorous-oxide catalyst (VPO) to selectively oxidize butane:

C₄H₁₀ + 3.5 O₂ g C₄H₂O₃ + 4 H₂O   DelH_R= -1236 kJ/mol

However, side reactions to produce CO and CO₂ are also important:

C₄H₁₀ + 6.5 O₂ g 4 CO₂ + 5 H₂O  DelH_R= -2656 kJ/mol

C₄H₁₀ + 4.5 O₂ g 4 CO + 5 H₂O   DelH_R= -1526 kJ/mol

Reaction Kinetics

Reaction kinetics have been obtained from the literature (Sharma, 1991) that are to be used for your design.  The above reactions have been rewritten as the following three reactions:

C₄H₁₀ + 3.5 O₂ g C₄H₂O₃ + 4 H₂O

C₄H₂O₃ + p O₂ g (6-2p) CO + (2p-2) CO₂ + H₂O

C₄H₁₀ + n O₂ g (13-2n) CO + (2n-9) CO₂ + 5 H₂O

Where n and p are coefficients obtained from the kinetic study, n=5.5, p=1 for your design.

Reaction rates for these three reactions are given by:

r1=k₁P₁^a1/(1+K₂P₂)

r2=k₂P₂/(1+K₂P₂)²

r3=k₃P₁^a3

where P1 = partial pressure of butane and P2 = partial pressure of maleic anhydride.

The following kinetic parameters are suggested (rate constants at 400^oC):

k₁=0.96 x 10^-6 kmol/kg s atm^0.54

k₂=0.29 x 10^-5 kmol/kg s atm

k₃=0.15 x 10^-6 kmol/kg/s atm^0.54

E1=E3=93100 J/mol

E2=155000 J/mol

a1=a3=0.54

K₂=310 atm^-1

Reactor Configuration

The reactor to be designed is a packed bed reactor with multiple reaction tubes with external cooling.  External cooling is to be provided by a mixed salt.  It is assumed that the temperature of the salt is constant.  The tube diameter is to be 30 mm OD to maximize heat transfer.  Heat transfer is a significant issue in this reactor due to the large exothermicity of the oxidation reactions.

Design Basis

A reactor is to be designed with a total production of 10000 metric tons per year, which would be considered a world-class plant.  The reactor will be in operation 95% of the time.  The feed will consist of 1.7% butane in air.  Total inlet pressure is 1.9 atm, and the total molar flow rate is 100 x 10^-3 kmol/hr.  Pressure drop should be considered using the Ergun equation.  The reactor should operate with a conversion of 85%.  The following information may be helpful for your design:

Overall heat transfer coefficient (based on outside area) = 106 W/m² K

Catalyst diameter = 3 mm

Void fraction (e) = 0.45

r_B=3.11 x 10³ kg/m³

Changing the catalyst diameter by a factor of 10 did not affect reactor performance

External mass transfer limitations are negligible

DelH_f (maleic anhydride) = -3.942 x 10⁵  J/mol

Deliverables

You are to consider your design in several parts, and satisfy the following requirements:

1. Model the maleic anhydride reactor assuming an inlet temperature of 300^oC and a salt temperature of 380^oC, calculating the number of reactor tubes and mass of catalyst required.  The exiting composition and graphs depicting partial pressures, total pressure, and temperature as functions of reactor length should be reported.

2. What safety concerns are there for this reaction?  How could you design your reactor to account for these concerns?  For example, you might consider the effects of varying the salt temperature on the resulting temperature profiles.

Please show the results of your simulations in a brief report.  This report should include  a list of the assumptions you made, the equations used to simulate the reactor, and the results (plots listed above should be included).  You should also address the safety concerns, and the size of the reactor needed for the given production rate.  Please note that you are expected to do a detailed design, so many of the assumptions you have made in the past (isobaric, mean heat capacities, etc.) should not be made.  Please see Dr. Hohn if you have questions about what a reasonable assumption is.

Write out all equations you will need to solve the semester project (see Project module for details on the project). This should include all differential equations (mole balances, energy balance, momentum balance) and all additional required equations (i.e. polynomials for heat capacities, equations to relate partial pressure to molar flow rates). It is not required, but would be a good idea, to also report all parameter values (i.e. coefficients for heat capacity polynomials, enthalpies of formation, etc.)

Complete a base-case design of the maleic anhydride reactor. You should turn in your code and plots of molar flow rates, temperature, and pressure as functions of catalyst weight in the reactor.