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UBC Theses and Dissertations

Drying and co-pelletization of microalgae with sawdust Hosseinizand, Hasti


Post-harvest handling of microalgae following mechanical dewatering is challenging due to the high moisture content of biomass (about 65-75% wet basis). Therefore, thermal drying is applied to decrease the moisture content to the safe value for storage, handling, and transportation. After drying, handling of dried microalgae powder is still difficult because of its low bulk density and possibility of blocking the flow of material inside handling equipment. Pelletization improves microalgal material characteristics by making high-density and homogenous pellets. The first goal of this research is to study the thin-layer drying mechanism of microalgae Chlorella at the temperature range of 40-140° C. The second goal is to study densification mechanism of pure and mixed microalgae with pine sawdust. The specific species used in this experimental investigation is Chlorella vulgaris. In the studied temperature range, microalgae drying from an initial moisture content of 65% wet basis occurred in the falling-rate period with no constant-rate phase. The results revealed that diffusion is the controlling mechanism in microalgae drying and all the water is entrapped in algal cells. This confirms the industrial experience that further mechanical dewatering to remove water is not effective. It was also understood that although the drying rate at 100-140° C is the highest, 60 and 80° C are the optimum drying temperatures to preserve microalgae surface color and chemical composition. Pelletization of pure Chlorella occurred in two distinct regions of particles’ rearrangement and particles’ deformation. However, there was no clear separation between the two regions when pure sawdust was pelletized. Adding microalgae Chlorella to sawdust resulted in a decrease in densification energy and improvement in pellets’ properties, i.e. higher durability, density, and heating value, lower porosity, moisture adsorption, and pellets’ expansion. The results indicated that adding microalgae to sawdust eliminates the need for high pelletization temperature and force. The temperature of 75° C and maximum force of 2500 N, which are considered as moderate conditions, were adequate for making pellets containing microalgae with desirable characteristics because of the presence of natural binders in microalgae. Working at a low/moderate temperature and force improves the process economics by decreasing energy consumption.

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