ChE 321 – Design Project 2

The Scenario:

Professor Lite’s research group has continued to study their new catalyst that increases the rate of the reaction

(1)    Toluene + Methanol   p-Xylene + Water

They quickly noticed, however, that the rate of p-xylene production was less than the rate at which toluene was consumed.  Many experiments later, they concluded that the following side reactions were also occurring on the catalyst.

(2)   p-Xylene    m-Xylene

(3)    p-Xylene + Methanol  Trimethylbenzene + Water

(4)    m-Xylene + Methanol  Trimethylbenzene + Water

with k1 = 697 exp(-3180/T)         {mol/h-g-atm2}

k2 = 1.04×105 exp(-7200/T) {mol/h-g-atm}

k3 = 112 exp(-1500/T)         {mol/h-g-atm2}

k4 = 335 exp(-1620/T)         {mol/h-g-atm2}

KPXY = KP = 0.070 exp(2500/T)      {1/atm}

Having heard about these results, ABC Chemicals has contracted UND to develop a catalytic reactor system for producing p-xylene using their promising new catalyst.

Process costs can be significantly reduced by avoiding high pressure and temperature conditions, and by minimizing the amount of unwanted products and unreacted feed that must be separated from the p-xylene.

Bulk chemical pricing information for feed and product compounds is listed below to help you make a preliminary economic analysis.  (Warning- these prices, as with other parameters in this project description, have been adjusted to provide a better learning experience. You should not use them for design work outside of this class)

Chemical                 Price ($/lb)

Methanol                 0.25

Toluene                   0.45

p-Xylene                 0.85

m-Xylene                0.55

Trimethylbenzene    0.30

Your Assignment:

Use numerical simulations (Aspen Plus) to determine the best reactor system and conditions for maximizing gross margin (Total Sales of Product – Total Cost of Reactants).

Write a short project report for the engineers at ABC Chemicals documenting your findings.

It should include the following information:

Criteria for optimization

Proposed system design (reactor type, reactor + separator + ??, how connected? feed, product, recycle streams? Explain and justify your design compared to other alternatives – no calculations or simulations required, just describe qualitatively)

Recommended operating conditions (reactor T, P, feed flows, catalyst weight, and resulting outlet flows)

Discussion of the effects of operating variables (what happens when T increases/decreases and why, etc.)

Preliminary profitability estimate. You only need to consider the gross margin between feed and product streams. You do not need to include other operating costs, equipment costs, etc. (No, not realistic, but you’ll have opportunities to learn and practice that in Plant Design)

Supporting equations, calculations, figures, etc. used in your analysis (only those necessary to illustrate your findings)

Sample Aspen Plus input/output documentation

Aspen Plus HELP

See Aspen Plus Tutorials (Tutorial 6 for reactors, Tutorial 3 for sensitivity)

The catalyst density in the reactor is 1000 kg/m3.

This means your rate constants have the same value when expressed in units of (/g) or (/ft3).

Variables you will want to change are Feed Composition, Feed Temperature, Reactor Pressure and Reactor Volume, and Reactor Type (CSTR or PFR).

Note that for every set of T, P, and feed composition conditions there will be an optimum Reactor Volume!

You may/should include separators. Since the focus of this assignment is not to model the separators you should use the Sep tool and assume perfect separation.  Use 1 Sep block to remove each component.

Use the Sensitivity tool to vary multiple parameters.