Aspen Reactor

Aspen has been a tough program to crack. Following the tutorials turned out to be more work than we thought. We had to simulate the reactor kinetics of the cumene production using our hand calculations as a basis for our operating specifications. We used a mixer and a simple Rplug packed bed reactor model. We stuck with our 350 C temperature as well as the 35 bar pressure.

Aspen Plus overall seems to be a more intuitive program than chemcad but it still has its flaws. Setting up the reactor and specifying the components: propylene, benzene, cumene, and dissopropylbenzene. The SRK method was used as this was as suggestion that was read some where.

First run of aspen yielded nonsensical results. It showed that our propylene was comopletely reacted and we acheived 100% conversion. This was impossible due to all the text books drilling in our heads how achieving 100% conversion would need insane reactor volumes or specifications that aren't economically possible. This is generally why we assume conversions less than ideal (there is a reason why we call it ideal after all)generally around 98-60% at least. We realized quickly that our preexponential factors or reactor kinetics must be wrong. The little trick in Aspen is the need to specify the preexponential factor in SI units. Changing this and doing some basic SI conversion yielded acceptable results.

We then did a design specification to get the actual size of the reactor at 95% conversion. This was the key size of the reactor that everythign would be based on the 95% conversion.

We also ran a sensitivy analysis to determine the effects of pressure and temperature on reactor size. We also showed how temperature effects selectivity and how pressure had little to no effect on selectivity which makes sense.

There are the following results and we figured an optimal range of reactor size, temperature, and pressure that we would like to operate the reactor in order to get our correct design specifications.








From the graphs one could see how as the pressure increases the reactor volume decreases. However, there is a trade off between high pressure, reactor volume, and the cost of each. The optimal range seems to be between 25-45 bar for trying to keep the reactor volume around 30-10 cubic meters.

The T vs Volume chart highlights as well how the lower the temperature the greater the volume needs to be and there is associated cost between higher temperature and reactor volume. Since the selectivity increases with lower temperature chart 3. There is a need to balance both these variables to achieve the necessary design specifications.

These charts provided a calming feeling that we're at least on track with everything and just need to begin the next phase of our design. Our next step in our process would be to fire up a heat exchanger (excuse the pun) and a distillation column and do the same procedures with obtaining 99% product of cumene.


~RSD