Adipic Acid Production: Process Modeling of the Benchmark and Two Electrochemical Approaches

Abstract

Adipic acid (AA), a critical precursor for nylon-6,6 and other industrial applications, is currently produced at over 3 million tons annually, with projected revenues reaching $6 billion by 2030. However, an alternative to the dominant industrial process, accounting for 95% of global production, has been sought due to inefficiency and significant environmental impact. This conventional route relies on petroleum-derived benzene as a feedstock and involves multiple hydrogenation and oxidation steps, notably utilizing nitric acid, which produces stoichiometric amounts of nitrous oxide (N₂O), a potent greenhouse gas. Furthermore, the high energy feedstocks like benzene, hydrogen, and nitric acid contribute significantly to Scope 3 emissions.

Given these concerns, there is a strong incentive to develop more energy-efficient and environmentally suitable processes. This thesis addresses the gap in comprehensive techno-economic assessments of emerging alternatives, which often overlook practical implementation challenges such as downstream separation, feedstock pretreatment, and overall carbon footprint.

The overarching research question guiding this study is: "How do various electrochemical based alternatives to current AA production compare on an economic and emissions basis from a process systems modeling prospective?". To this end, two promising alternatives were selected for evaluation through the key performance indicators of; profitability in the form of minimum selling price (MSP), emissions based on kg CO₂, and material efficiency based on the excess ratio of theoretical main feedstock to actual main feedstock.

The first modeled process was that of the conventional route. This was done to ensure a consistent feedstock price component in the final adipic acid cost across all assessed production methods, thereby providing valuable validation for modeling assumptions. Furthermore, it serves as a benchmark for comparing the economic and emissions performance of various electrochemical-based alternatives, offering insights into their relative strengths and weaknesses. The results attained were consistent with those of literature, with a minimum selling price of $1.58/kg

The first alternative route employs an electrocatalytic oxidation cell to replace the nitric acid oxidation step of the conventional process. Experimental work of previous researchers was used to create an approximate model of the cell and the electrodialyzer used for the recovery of KOH electrolyte. This was implemented within Apsen Plus along with upstream and downstream processing. The resulting model and subsequent TEA predicted an adipic acid price of $2.33/kg or a 45% increase over the results of the conventional route. However, assuming the use of renewable electricity, the CO₂ equivalent emissions dramatically reduced by half when compared to the conventional process.

The second alternative was the use of biomass based fermentation and subsequent electrochemical oxidation to produce a adipic acid alternative of similar value to industry. Once again, the experimental work of previous researchers was used to predict a final minimum selling price of around $3.97/kg; however, these results are highly susceptible to variations in input parameters. Both alternatives showed lower emissions when compared to the conventional process.