Dehydration of highly viscous polyol (1,2,4-butanetriol) using microwave-induced sweep gas membrane distillation (MIMD) on nanocarbon-immobilized membranes

A challenge facing the synthesis of bioderived platform chemicals such as polyols and polyacids via fermentation processes is their separation from dilute aqueous streams. This study presents microwave-induced membrane distillation (MIMD) as a method for concentrating viscous 1,2,4-butanetriol (BT) at concentrations that can be sourced from fermentation broth. MIMD and sweep gas membrane distillation (SGMD) processes were employed to concentrate BT feed solutions utilizing nanocarbon-based membranes, namely the carbon nanotube immobilized membrane (CNIM) or the graphene oxide immobilized membrane (GOIM). Microwave heating (MWH) was utilized to elevate the temperature of the feed system and was found to be superior to conventional heating (CH) in terms of flux, mass transfer coefficients and thermal efficiency. Our findings reveal that GOIMs exhibited an 11.5% higher dehydration rate compared to CNIMs. Initial water flux reached 14.1 kg m⁻² h⁻¹, albeit this value decreased thereafter as the concentration of BT increased, thus limiting mass transfer coefficients due to increased viscosity, which reached 11.41 mPa s at 90 wt% and 80 °C. Overall, MWH substantially alleviated this issue, leading to a flux as high as 15.7 kg m⁻² h⁻¹ and a 35% improvement in mass transfer coefficients over CH. The overall thermal efficiency of BT concentration reached 74.5% for GOIM-based MIMD with a specific energy consumption (SEC) of 263 kW h m⁻³, which showed a 6.5% reduction compared to CH.