Methanol Production from Syngas

Abstract

Transitioning from a heavily fuel reliant economy to a sustainable future is one of the major challenges of our time. The high energy density and good storage properties of fossil fuels have made them the most important energy source for the last centuries. Moving away from fossil fuels towards greener, biomass-based energy and energy carriers is hindered by the technological gap due to the maturity of conventional processes compared to sustainable ones. This thesis focuses on a process of converting biomass into methanol. The process is a small-scale application which is mobile so it can be moved towards the source of the biomass, with the aim to reduce transportation costs. The specific focus lies on the conversion of the gasification-derived syngas into methanol. For this an extensive literature study was conducted to find suitable technologies and process kinetics. Aspen Plus® with the integration of Excel was used to model the chosen technologies. The process was divided into three unit operations. The methanol reactor unit with a recycle stream, the CO2-removal unit to prepare the gas for the reactor unit and a H2-recycle unit to increase the utilisation of the hydrogen. Each unit operation was modelled separately to study the influence of their parameters and to determine which parameters have the largest influence. Finally, when integrating all unit operations within one model, these selected parameters were used to determine the operating conditions and process design.

The aim of the developed model was to predict and improve the process for different applications with integrated hydrogen supply from renewable sources. The disadvantage of utilising renewable energy sources for the production of hydrogen is the intermittent supply of electricity for the electrolysis of hydrogen. Therefore the process needs to be able to accommodate different levels of hydrogen production. The first case is the base case without any hydrogen input. It is given a syngas-input and the CO2-removal unit runs at full capacity. Building upon this model, the behaviour of the system for hydrogen supply integration was modelled in the second and third case. The second case introduces additional hydrogen and therefore the CO2-removal unit can be turned down. The third application adds CO2, which was removed in case one, to the system and increases the hydrogen input. The study of these processes shows, that the operating pressure of the methanol reactor unit has a very large influence on the energy requirements of the process but also on the production of methanol. In respect to the power and cooling requirements of the process a low pressure is favoured but much larger quantities of methanol can be produced at higher pressures. With the chosen designs for the cases a respective methanol production of 47.6 t/d, 96.8 t/d and 180.6 t/d is reached. The integration of hydrogen leads to two major concerns for the process. The integration requires much larger equipment due to higher flowrates and the quality of the product decreases as a higher CO2/CO-ratio produces more water. The thesis served its purpose by developing a model of the process which can be further used to optimise the process on a techno-economic level.