Recent research in chemical plant operation shows increasing interest in dynamic process operation as part of designed operating strategy for reasons such as increased dependency on renewable energy, and process intensification. Conventional analyses of fixed bed reactors are developed for steady state optimization and may not be adequate for dynamic operation. In fact, the important metrics and targets in dynamic process design are not entirely clear. The first objective of this article is to provide a state-of-the-art survey categorize types of dynamic operation, and rank the available common modelling and analytical tools suitable for quantification of dynamic process variables. The article then examines a case study of 1D and 2D model differences in a methanol steam reforming reactor. The case study shows model prediction differences of up to 15% for conversion, and up to 50% for CO concentration at the outlet during extreme load changes. The study concludes that the complexity of analytical and numerical techniques for dynamic processes is notably higher compared to steady state analyses, but appropriate tools and procedures are currently lacking. ... Read more

Hydrogen economy is spreading across the maritime sector in response to increasingly stringent regulations for shipping emissions. The challenging on-board hydrogen logistics are often mitigated with hydrogen carriers such as methanol. Research on methanol reforming to hydrogen for fuel cell feed is conducted mostly in steady state, overlooking dynamic reactor operation and its effects on the power production system. Forced reactor operations induce fluctuations of CO content in the reformate potentially harmful to the PEM fuel cell, and drops in methanol conversion causing inefficient operation. In present research, simulations with a physical 2D unsteady model of a packed bed methanol steam reforming reactor resulted in methanol conversion drop durations of up to a minute. Additionally, temporary increases of CO content up to 112% were observed. Throughput ramp ups most notably impact the conversion, while ramp downs negatively affect selectivity. The investigation on reactor geometry concludes that larger tube diameters increase transient time and CO spikes, while they decrease with reactor length. Amplified unsteady effects are also observed with larger changes in input process variables. The results imply that heat transfer rate to the reactor are most often the detrimental factor for transient effects and durations in practice. Following this work, inclusion of realistic heating methods is recommended, instead of uniform tube temperatures used in present simulations. Heating system characteristics are necessary for realistic evaluation of the methanol reformer constraint on fuel cell feed demand in fully integrated systems. ... Read more

A growing concern is associated to the greenhouse gases emissions of superyachts, consequently alternative fuels are introduced to the market. For the yachting decarbonisation, this work focuses on hydrotreated vegetable oil (HVO) and methanol. Nevertheless, the uncertain global availability of these fuels can undermine the operations of ocean-crossing superyachts. Thus, a multi-fuel system is installed allowing for fuels switchover and built-in flexibility. Moreover, non-dedicated tanks are installed for the alternative storage of HVO and methanol to make optimal use of the tanks’ capacity. However, the alternative storage of HVO and methanol causes mutual fuels’ contamination. The lack of standards and research on accepted fuels impurity makes full fuels’ separation relevant to be explored. In this work, to avoid degradation of dual-fuel engines or fuel cells, gravity-settling tanks and disc-bowl centrifuges were studied to separate HVO-methanol mixtures. Shake tests were conducted on HVO-methanol mixtures to quantify the separation time and relative concentrations to obtain complete gravity separation. The gravity tests revealed methanol traces in HVO for all the tested mixtures within the 1 hour-3 days observation time, due to the low-density difference between the fuels. This makes the use of gravity-settling tanks impractical onboard for quasi-instantaneous fuels supply to the converters. A mathematical model was developed for disc-bowl centrifuges to assess the separator performance and separation time. Furthermore, the centrifuge was sized by providing the separator working conditions for varying engine modes. Moreover, spin tests were conducted to validate the mathematical model. The model showed that full separation is achievable with a larger centrifuge compared to existing designs. The larger design is due to the low-density difference between the fuels. The maximum separation time ranges from 5-10 minutes. Nevertheless, all the tested mixtures with the spin tests failed at achieving a state of full separation due to the dilution of a certain residual volume in the continuous liquid. The discrepancy between the mathematical model and the spin test results can lie in the neglected diluted phase of the dispersed fuel in the continuous liquid in the mathematical model. However, the mathematical model is a good tool to simulate the dynamic behaviour of the dispersed droplets. Consequently, the onboard use of a centrifuge for separating HVO-methanol mixtures should be evaluated by quantifying the concentration of the fuels’ mixture entering the separator tailored per yacht. Furthermore, tests on dual-fuel engines or fuel cells are recommended to establish tolerable limits of fuel’s contamination. ... Read more