Steelmaking process is a highly carbon-intensive process. This is mainly due to the use of coke as a reducing agent in the blast furnaces to produce carbon-rich pig iron, which in turn, is used for the production of low-carbon steel in the basic oxygen furnaces. The exhaust gases from the blast and basic oxygen furnaces, which mainly contain CO and CO₂, are utilised for electricity generation, and thus, these pollutants are released to the atmosphere. One of the possible ways to treat these work’s arising gases (WAGs) is to convert them into syngas, which can then be further converted into syncrude via Fischer-Tropsch synthesis (FTS). The FTS syncrude can then be further refined and processed to produce liquid fuels such as gasoline, kerosene, diesel, etc. These synthetic fuels are sulphur-lean and are essentially capable of replacing the existing fossil-derived liquid fuels, thus contributing to curbing the carbon emissions.

The main objective of this thesis was to develop a detailed model of a multi-tubular fixed-bed reactor (MTFBR) to produce synthetic crude from syngas via Fischer-Tropsch synthesis (FTS). The model was then used to simulate a reactor that utilises the syngas obtained from the processing of work’s arising gases of an integrated steel mill to produce synthetic crude. The FTS product distribution was modelled using the kinetic model based on CO-insertion mechanism, developed by Todic et al. A basic MTFBR model was initially developed using the equations and assumptions from the fixed-bed reactor model of Todic, and the basic MTFBR model was able to produce similar results as that of the Todic’s model, with slight deviations in the temperature and pressures profiles. The basic MTFBR model was then further improved to render it comparable with the commercial FT reactors. The main improvements in the model include the dynamic extraction of thermodynamic and transport properties of the system components using Aspen Properties; calculation of dynamic vapour-liquid equilibrium, liquid holdup and catalyst effectiveness factor; and the use of improved heat transfer and pressure drop equations.

A sensitivity analysis was performed to determine the effect of design and process parameters on the performance of the MTFBR model. The most crucial design parameter was observed to be the tube diameter as it had a considerable effect on the heat management and the pressure drop in the reactor bed. The most important process parameters for the reactor were observed to be the inlet temperature and the feed flow rate. A simplified FTS gas loop process was also modelled in Aspen Plus in order to introduce a recycle stream into the MTFBR. The effect of tail gas recycle for the recovery of unreacted H₂ and CO was also studied, and it was observed that higher recycle ratios resulted in lower conversions per pass; however, overall CO conversions were observed to increase until a maximum, and then decrease thereafter. The optimum conditions for the simplified gas loop process were estimated to be with an inlet temperature of 484.5K and a total recycle of tail gas to the recovery section, for a inlet pressure of 30 bar. Optimised process conditions resulted in a CO conversion per pass of 46%, an overall CO conversion of 89%, a C₅₊ selectivity of 86.6%, a CH₄ selectivity of 6.2%, and a C₅₊ productivity of 252,540 tonnes/y. The optimised model results, in terms of C₅₊ selectivity and overall CO conversion, were also pretty much inline with the available data from the Shell SMDS plant in Bintulu. ... Read more

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. ... Read more