Solid-Liquid Separation Technologies in the Conversion of Bagasse to Liquid Fuel

• Main Author: Miller, Keith
• Other Authors: Kochergin, Vadim
• Format: Others
• Language: en
• Published: LSU 2010
• Subjects: Biological & Agricultural Engineering
• Online Access: http://etd.lsu.edu/docs/available/etd-04072010-124421/

Development of liquid biofuels has entered a new phase of large scale pilot demonstrations. A number of plants in operation or under construction face the engineering challenges of creating a viable plant design, scaling up, and optimizing various unit operations. It is well-known that separation technologies account for 50-70% of both capital and operating costs. Processes vary in terms of selection of unit operations; however, solid-liquid separations are likely to be a major contributor to the overall project costs. A typical process for ethanol production from biomass includes several solid-liquid separation steps. The nature of biomass derived materials makes it either difficult or uneconomical to accomplish complete separation in a single step. Material balance models were developed for two bagasse-to-ethanol processes utilizing alkaline-pretreatment, and applied to evaluate the sensitivities of the process yields to separation performance. This aided in setting realistic efficiency targets for solid-liquid separations. Results from material balance calculations revealed that 10% of solid feed material can be lost to liquid streams, with an equivalent process yield reduction. Both filtration and sedimentation processes were found to have low separation efficiencies, due to small particle sizes, low density, and the fibrous nature of bagasse. Because of low concentrations of suspended solids in the liquid stream (0.1-0.15%), recovery of solids by centrifugation may require high capital and operating costs. The efficiency of a dissolved air flotation process (DAF) for recovery of suspended solids from liquid stream derived from dilute-ammonia pretreatment process was investigated. DAF was evaluated for suspended solids recovery from the liquid stream obtained from alkaline pretreated cane bagasse. A continuous bench scale DAF clarifier was constructed and tested. The effect of additives at various chemical addition rates, air-to-solids ratios and hydraulic loadings on the DAF process was determined. Small quantities of lime were found to enhance flotation of particles and minimize the use of flocculants. Recoveries of suspended solids were in the range of 50-57% and were accompanied by a greater volume reduction than could be achieved by conventional sedimentation. The DAF process effectively concentrated solids from 0.1% in the feed material to 8-9% in the floated fraction.