Improving Biomethanol Synthesis via the Addition of Extra Hydrogen to Biohydrogen Using a Reverse Water–Gas Shift Reaction Compared with Direct Methanol Synthesis

Abstract

Conventionally, methanol is derived from a petroleum base and natural gas, but biomethanol is obtained from biobased sources, which can provide a good alternative for commercial methanol synthesis. The fermentation of molasses to produce biomethanol via the production of biohydrogen (H2 and CO2) was studied. Molasses concentrations of 20, 30, or 40 g/L with the addition of 0, 0.01, or 0.1 g/L of trace elements (TEs) (NiCl2 and FeSO4·7H2O) were investigated, and the proper conditions were a 30 g/L molasses solution combined with 0.01 g/L of TEs. H2/CO2 ratios of 50/50% (v/v), 60/40% (v/v), and 70/30% (v/v) with a constant feed rate of 60 g/h for CO2 conversion via methanol synthesis (MS) and the reverse water–gas shift (RWGS) reaction were studied. MS at temperatures of 170, 200, and 230 °C with a Cu/ZnO/Al2O3 catalyst and pressure of 40 barg was studied. Increasing the H2/CO2 ratio increased the maximum methanol product rate, and the maximum H2/CO2 ratio of 70/30% (v/v) resulted in methanol production rates of 13.15, 17.81, and 14.15 g/h, respectively. The optimum temperature and methanol purity were 200 °C and 62.9% (wt). The RWGS was studied at temperatures ranging from 150 to 550 °C at atm pressure with the same catalyst and feed. Increasing the temperature supported CO generation, which remained unchanged at 21 to 23% at 500 to 550 °C. For direct methanol synthesis (DMS), there was an initial methanol synthesis (MS) reaction followed by a second methanol synthesis (MS) reaction, and for indirect methanol synthesis (IMS), there was a reverse water–gas shift (RWGS) reaction followed by methanol synthesis (MS). For pathway 1, DMS (1st MS + 2nd MS), and pathway 2, IMS (1st RWGS + 2nd MS), the same optimal H2/CO2 ratio at 60/40% (v/v) or 1.49/1 (mole ratio) was determined, and methanol production rates of 1.04 (0.033) and 1.0111 (0.032) g/min (mol/min), methanol purities of 75.91% (wt) and 97.98% (wt), and CO2 consumptions of 27.32% and 57.25%, respectively, were achieved.