Sustainable Process Networks for CO2 Conversion

According to various organizations, especially the Intergovernmental Panel on Climate Change, global warming is an ever-increasing threat to the environment and poses a problem if not addressed. As a result, efforts are being made across academic and industrial fields to find methods of reducing contributors to global warming, primarily greenhouse gas emissions.

Of these, carbon dioxide (CO2) is the largest source and, therefore, the reduction of the amount emitted is primary focus of developments [1]. Currently, the main method that is focused on is carbon capture and storage (CCS). There are various drawbacks to this geologic storage system: the CO2 is not eliminated, the implementation is limited due to natural phenomena, and the capturing methods are often expensive.

Thus, it is desirable to develop an alternative strategy for reducing the CO2 emissions [2]. An additional process that reduces the emissions is the conversion of CO2 into useful products, such as methanol [3]. In this work, through a computer-aided framework for process network synthesis-design, a network of feasible conversion processes that all use emitted CO2 is investigated.

CO2 is emitted into the environment from various sources: power generation, industrial processes, transportation and commercial processes. Within these there are high-purity emissions and low-purity emissions. Rather than sending these to the atmosphere, it is possible to collect them and use them for other purposes.

In this work, the first step is determining the various CO2-sources, the amounts emitted, and the corresponding compositions. These sources show large variations in amounts and concentrations. Targeting some of the largest contributors: power generation, manufacturing, chemical industry, it is possible to determine the amounts available.

Transportation and other sources are more difficult to capture and utilize further and, therefore, they are not considered in this work. Once the CO2-sources are known, it is possible to determine how to utilize these through process network optimization. It is then necessary to have the information on the conversions that are thermodynamically feasible, including the co-reactants, catalysts, operating conditions and reactions.

Research has revealed that there are a variety of reactions that fulfill the aforementioned criteria. The products that are formed fall into categories: fuels, bulk chemicals and specialty chemicals. While fuels, such as methanol (MeOH) have the largest market, this network will include a variety of thermodynamically feasible conversion paths [4].

From reviews of work previously done, there are ranges of possible products that are formed from CO2 and another co-reactant directly. Methanol, dimethyl ether, dimethyl carbonate and ethylene carbonate are just some of the possible products that can be formed. Each of these involves CO2 and a co-reactant, such as hydrogen, which may also be captured from process purge streams.

The process network evolves as some of the reactions involve products from other reactions as a reactant. Combining the possible products that can be formed and the reactants that are required yields a network of products that can be created using only the CO2 emissions and not adding any CO2 emissions through the reactions.

Using the computer-aided framework, the feasible networks are generated and simulated to verify the initial synthesis design results. The economic feasibility and sustainability are assessed to identify the final, more sustainable network. The goal is to create a network that reduces emissions by forming desirable chemical products without emitting noticeable amounts of CO2 and other greenhouse gases, and creating more energy efficient processes.