Modeling of a natural circulation flow reactor for methanol synthesis from renewable sources

Abstract

In 2015, 30% of the world's energy demand was met through petroleum based sources, whose burning has affected the world's climate. The Paris Climate Agreement, among other policies, has shifted the focus towards usage of renewable energy sources. Currently, the renewable market caters predominantly to meeting the electric demand, which only represents 19% of the world's energy consumption. Zero Emission Fuels (ZEF) aims to create a small-scale chemical plant that uses renewable energy sources to create synthetic methanol. This follows the vision of adapting renewable energy sources for non-electric usage. Methanol is the simplest liquid hydrocarbon at atmospheric condition; it can be used as a fuel or as a base chemical for other products. Presently, the industry reforms syngas obtained from fossil fuels to create methanol, predominantly using large-scale catalytic reactors. For production of methanol from renewable sources, Bos and Brilman developed a small-scale catalytic reactor driven by natural circulation. ZEF adapted the Brilman reactor for their chemical plant, creating a prototype of the modified Brilman reactor (MBR). The present work aims to create a model that describes the steady-state methanol production of the prototype using computational fluid dynamics (CFD) and chemical process models.

The MBR is an innovative design based on a closed loop geometry. The gases are driven by buoyant forces from a temperature difference between the high-temperature catalytic reaction zone and the low-temperature condensation zone. Heat recovery elements are used to exchange heat between the hot and cold streams as they travel in the loop. The CFD model developed on Fluent 18.2, uses a RNG k-e turbulent model, with multicomponent species transport, convective heat transfer and momentum loss effects from the catalytic zone. The boundary conditions are based on the operating conditions of the prototype. The model calculates flow-rate, and the effect of heat recovery elements inside the natural circulation loop. A chemical process model developed in COCO 3.2 is used to calculate a mass and energy balance for the two-phase behavior of the gas mixture inside the MBR.

A study of the effects of a natural circulation loop with a configuration similar to the reactor is performed, observing potential benefits by tilting the system. The combination of effects due to heat recovery and flow blockage in the reactor is studied, and the results are evaluated with experimental measurements. The chemical process model developed closely resembles the experimental characterization of the reactor. The parameters of flow-rate and internal temperatures can not be validated due to limited data available from experimental procedures.