Rheology Modification and Enzyme Kinetics of High Solids Cellulosic Slurries

Chemical additives that reduce the yield stress and viscosity of pretreated corn stover slurries and also enhance the kinetics and overall conversion of cellulose during enzymatic saccharification were explored. Additives included polymers, proteins, and nonionic, anionic, and cationic surfactants. Rheological measurements assessed changes in the yield stress of the suspensions, and enzymatic saccharification experiments were conducted to assess the effect of the additives on enzyme kinetics. For high-solid slurries with an insoluble solids content of about 20%, a 3- to 4-fold reduction in the yield stress was observed upon addition of 2% (w/w) cetylpyridinium chloride (CPCl), cetyl trimethylammonium bromide (CTAB), sodium dodecylbenzene sulfonate (NaDBS), and sodium dodecyl sulfonate (SDS). However, the presence of bovine serum albumin (BSA) at the same concentration doubled the yield stress. Although NaDBS and SDS were both very effective at reducing the yield stress, their presence was very detrimental to the saccharification kinetics, cutting cellulose conversions from 80% to less than 20% over one week due to chemical inhibition of the enzymes. However, the surfactants CPCl and CTAB synergistically reduced the yield stress and increased the relative extent of cellulose conversion by up to 35% during the first 24 h of saccharification. The presence of BSA slightly reduced the extent of cellulose conversion. It is hypothesized that the increased rate of saccharification observed with the presence of CPCl and CTAB and the decreased rate observed with BSA are associated with the respective increases and decreases in the suspensions’ yield stresses, which in turn may affect the uniformity of mixing within the saccharification reactors. Of the modifiers tested, CPCl and CTAB appear to be the most efficacious, as they both reduce the yield stress at concentrations as low as 0.1% (w/w) and improve the kinetics of enzymatic saccharification. Lastly, the economic implications of rheology modifiers in a hypothetical lignocellulosic biomass-to-ethanol pilot facility are discussed.