Parametric influences of ultrsound in homogeneous and heterogeneous mixtures for biofuel and biorefinery applications

[Truncated abstract] The conversion of lignocellulosic biomass for biofuel and biorefinery applications is limited by the pretreatment step which is aimed at separation of the three components, lignin, cellulose and hemicellulose. Here, ultrasound was considered for the augmentation of pretreatment of lignocellulose. More specifically, the parametric influences of ultrasound for biofuel and biorefinery applications were investigated. The literature lacked an evaluation of the parametric effects on the ultrasonic treatment, especially with regards to fluid flow, biomass loading and size, and chemical additions. Firstly, the effect of two types of fluid flow on sonochemical activity in a homogeneous solution was investigated. Initially fluid flow was evaluated using overhead stirring at four frequencies, 40, 376, 995 and 1179 kHz, and then circulatory flow was investigated at 376, 995 and 1179 kHz. Overhead stirring was found to increase sonochemical activity at 40 kHz attributed to a decrease in bubble coalescence and an increase in the active bubble population. At 376, 995 and 1179 kHz bubble coalescence was supposed to decrease, reducing the active bubble population, yet sonochemical activity was not always decreased. Then, the circulatory flow at 376 and 995 kHz resulted in an increase in sonochemical activity at lower flow rates and at 376 kHz, at the higher flow rate. The observed increases in sonochemical activity were attributed to an increase in asymmetrical bubble collapse in the travelling wave dominant field, which increased the participation of the bulk solution in sonochemical activity. However, at 1179 kHz, no increase in sonochemical activity was observed, attributed to an increase in cavitational threshold from the increased non-linearity of the bubble collapse. The parametric effects on ultrasonic treatment of wheat straw as a model lignocellulose were then examined. Firstly, ultrasonic frequency, reactor configuration (stirred versus still), biomass loading and particle size were investigated. Treatments at 40 and 995 kHz favoured fractionation over treatment at 376 kHz. At 40 kHz delignification was improved whereas treatment at 995 kHz improved carbohydrate solubilisation. Delignification was attributed to mostly the physical effects of ultrasound and subsequently did not benefit from downsizing below 0.5 mm. In addition, delignification was augmented with stirring, surmised to be from the prevention of lignin condensation with increased transient cavitation in the stirred reactor. The increased carbohydrate solubilisation was attributed to the increased radical production at 995 kHz, and subsequently was highest at the lowest particle size range. The optimal loading was determined to be 1/20 (g/ml) for carbohydrate solubilisation, delignification and fractionation. Secondly the effect of ultrasonic pretreatment in different chemical environments; hydrogen peroxide, peracetic acid and acetic acid was examined. Delignification was not improved by ultrasound, although ultrasonic pretreatment was able to improve the overall purity of the solid residue. The chemical treatments were conjectured to be effected from sonochemical interactions which altered the pretreatment mode. The role of fluid flow was found to effect the bubble population and lignin condensation in an ultrasonic field...