(108b) Teaching Process Simulation Concepts in Aspen Plus Using the Ammonia Synthesis Loop

Formative process modeling assignments can greatly enhance students’ confidence and competence in effectively using process modeling software prior to the capstone design project. The ammonia synthesis loop is a relevant and simple process with three components that can be used to demonstrate setting up reactor models and recycle loops using process modeling software. The ammonia process includes a reactor (PFR w/ catalyst), two heat exchangers, a compressor, and a vapor-liquid separation.

In order to increase student confidence and competence using Aspen Plus, students in an introductory process design course were assigned a 2-week project in which they were tasked to produce a simulation of the ammonia synthesis loop starting from pure nitrogen and hydrogen reactants. Since the single pass conversion of nitrogen and hydrogen to ammonia is low, recovering and recycling unused reactants is critical, challenging students to set up a recycle stream. Through this assignment, which was proceeded by a module on “Aspen Basics”, students would be independently building a process model in Aspen Plus for the first time.

Regarding the reactor modeling part of the assignment, students gained experience in setting up a Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic model for the ammonia synthesis reaction. They modeled the packed bed reactor using a PFR model with catalyst. Tutorials were included for how to input the LHHW rate expression. One of the deliverables for the assignment was to report on the reactor dimensions that they would prescribe using heuristics and the component composition profiles along the length of the reactor.

One of the most valuable aspects of this assignment was that students got experience in building the recycle structure of their flowsheets. Setting up recycle streams in Aspen can be tricky, and Aspen often yields convergence errors due to components building up in (or being depleted from) the process. To help illustrate and navigate these potential mass balance convergence issues, students were presented with a “simulated storage model”, which when integrated into their process model could help identify the amount of raw materials needed to be fed into the process when in continuous operation in order to meet a target production goal. Additionally, this “simulated storage model” could help identify inadequacy of separations leading to holdup of components in the recycle loop, as well as any components that may need to be purged from the process. Ultimately, this simulated storage model was to be deleted and replaced with a mixer (to combine the feed and recycle stream) and a purge stream if necessary.

As a part of the students’ report on their process modeling efforts, they were expected to present:

1) the flow rate, composition, temperature, and pressure of the ammonia product (target flow rate was defined and varied for each student, and the purity specification for the product was > 99.5 wt% ammonia) ,

2) the flow rate, composition, temperature, and pressure of the hydrogen and nitrogen inputs,

3) the energy requirements of each unit operation, and

4) the length of the reactor and how they arrived at this result.

A direct assessment of students’ ability to meet expectations with regards to these deliverables, as well as an indirect assessment of outcomes by surveying students on the impact of the assignment, was used to gauge the success of the assignment in helping students to be more confident and competent using Aspen Plus as a process modeling tool.