Ethanol production in a membrane bioreactor

Pilot plant trials were conducted in a corn wet mill with a 7000-L membrane recycle bioreactor (MRB) that integrated ceramic microfiltration membranes in a semi-closed loop configuration with a stirred-tank reactor. Residence times of 7.5–10 h with ethanol outputs of 10–11.5% (v/v) were obtained when the cell concentration was 60–100 g/L drywt of yeast, equivalent to about 10⁹−10¹⁰ cells/mL. The performance of the membrane was dependent on the startup mode and pressure management techniques. A steady flux of 70 L/(m²·h) could be maintained for several days before cleaning was necessary. The benefits of the MRB include better productivity; a clear productstream containing no particulates or yeast cells, which should improve subsequent stripping and distillation operations; and substantially reduced stillage handling. The capital cost of the MRB is $21–$34/(m³·yr) ($0.08–$0.13/[gal·yr]) of ethanol capacity. Operating cost, including depreciation, energy, membrane replacement, maintenance, labor, and cleaning, is $4.5–9/m³ ($0.017–$0.034/gal) of ethanol.