Large-scale hydrogen distribution: Toluene as a liquid organic hydrogen carrier = Análisis del tolueno como líquido orgánico portador de hidrógeno

Abstract

During the current times, society is becoming more aware about the climate change threat. It is more present than ever nowadays, including on the industry. There is an ongoing revolution in all industrial sectors to find alternative methods and strategies in order to achieve the desired carbon dioxide emissions reductions.
One of the many alternatives to highlight is the hydrogen. According to many projections, it is possible that this is the energy of the future. Compared to other fossil-based fuels, it is true that the direct CO2 emissions are entirely reduced. However, there is a big downside regarding this technology: It is extremely difficult to handle during the storage and transportation.
There are currently several methods to transport and store hydrogen. Some of them are similar to the classic way other industrial gases are handled. Others represent promising alternatives that are still under development. In this thesis, the alternative studied consists of a group of chemical substances, called liquid organic hydrogen carriers. These chemicals have the ease of reacting with hydrogen to form substances with a high capability to retain the hydrogen on its structure. Additionally, their storage and transportation are way simpler than for pure hydrogen. At the same time, these substances formed are able to release the hydrogen by performing the inverse reaction. Particularly, the chemical studied to help on the hydrogen development is toluene and methyl cyclohexane, the hydrogenated form.
During this project, a technical analysis was performed on the viability of using the substance with this purpose. A process simulation was performed using the program AspenPlus with a successful outcome. It includes two subprocesses: A hydrogenation process, where hydrogen and toluene are reacting to obtain methyl cyclohexane, and the dehydrogenation, where hydrogen is obtained back. These two processes have the possibility of having different geographical locations.
The hydrogen recovery after going through all the processes was very high. 99.99% of the initial hydrogen entering the hydrogenation was recovered on the other side of the process. Both reactions were performed at certain conditions to obtain a > 99% conversion. The separation after the reaction was done with a simple flash and a decent result was obtained.
The technical data of the simulation was also useful for a subsequent economic analysis, to check its viability. All the costs related to the process operation were estimated, including then the corresponding data to each subprocess. The total extra cost of hydrogen handling is 1.31 $/kg. This part only includes the cost of the hydrogenation, with a cost of 0.45 $/ kg of H2 and the dehydrogenation, with 0.86. The remaining costs of other stages of the process cannot be calculated but will be estimated to get an overall idea of hydrogen prices.
With a price range for green hydrogen from 2 to 6 $/kg of hydrogen, the conclusion is that toluene could represent an interesting alternative for solving one of the main problems from hydrogen. However, it is a developing technology with still some progress to make.