| Biomass is a protential renewable resource,which can be used for easing the energy crisis.Microalgae,with advantage of fast growing and organic-rich,can be converted to biofuels by microbial fermentation to obtain a high economic benefit.However,the compact structures of microalgae cell wall hinder the organics to release out from the microalgae cells,severely reducing the performance of fermentation.Thus pretreatment of microalgae biomass is required to release out the organics from microalgae cells before the fermentation processes.Currently,a promising pretreatment technology is hydrothermal hydrolysis,which has a high economy efficiency and can be used for large scale application.Therefore,the continuous hydrothermal hydrolysis process of microalgae was investigated,the rheological properties,convective heat transfer and continuous hydrothermal hydrolysis properties were analyzed.Microalgae slurry showed shear thinning non-Newtonian properties at both low temperature and subcritical temperature.The apparent viscosity of microalgae slurry under subcritical conditions first drastically increased,and then gradually decreased with the increasing temperature,which is quite different with the variation trend at low temperatures(20~70°C).The main reason was that the hydrothermal reaction(starch gelatinization and protein denaturation)happened at such temperature.In addition,the temperatures corresponding to the maximum apparent viscosities were different at different shear rates,which was probably due to the combined effects of starch gelatinization and protein denaturation at high temperatures.And the effects of biochemical reaction were different at various shear rates.Apparent viscosity of microalgae slurry increased with the increasing mass fraction at subcritical temperature,which was similar to that at low temperature.While the increase extent of apparent viscosity with the increasing mass fraction was different,due to the biochemical reaction in the hydrothermal hydrolysis process.Finally,the empirical correlations of the apparent viscosity,density,heat conductivity coefficient,and specific heat capacity were obtained.At the mass fraction of 5,10 wt.% and flow rate ranging 6~14 L/h,the microalgae slurry was in laminar flow.The convective heat transfer increased with the increasing flow rate,while the length at entry decreased.When the mass fraction of microalgae slurry increased from 0.5 to 10 wt.%,the convective heat transfer slightly increased first and then decreased.It was proibably because that the submicroscopic particulate moved in the radial direction(from tube wall to center),which caused the non-uniform heat conduction and enhanced the convective heat transfer.However,when the mass fraction was higher than 5 wt.%,the viscosities of microalgae slurry dramatically increased with the increasing mass fraction.The increased viscosities led to the decrease of the generalized Reynolds number,which caused the decrease of convective heat transfer coefficient.In addition,the Nusselt number increased with the increasing Reynolds number,Prandtl number and the decreasing power law index.The main reason was that the shear thinning properties of microalgae became obviously with the decreasing power law index,which enhance the convective heat transfer.Finally,the empirical correlations of convective heat transfer were obtained,which can be used for design of continuous hydrothermal pretreatment reactors in large-scale application.The release out of organics from cells and hydrothermal reactions was investigated.The yields of carbohydrate firstly increased and then decreased with the increasing outlet temperature,while the yields of protein increased with the increasing temperature.With the increasing temperature,the cells were gotten more heavy damage.Thus the process that organics released out from the microalgae cells became more easily,which led to the increasing yield of carbohydrate and protein in aqueous.When the temperature continued to increase,Maillard reaction creeped into carbonylation by utilizing the formyl,ketone groups of reducing sugar from the hydrolysis of carbohydrate and the free amino groups of protein,which let the yield of carbohydrate decrease.The yields of carbohydrate with the increasing retention time slightly increased and then slightly decreased with the increasing of retention time.The yield of protein from microalgae first increased with the increasing retention time.The increase of organincs yield from microalgae with the retention time was probably because that more organics dissolved into aqueous with the gradual release out of organics from microalgae cells with the increasing retention time.With the further increase of retention time,the reducing sugar of carbohydrate that dissolved into the aqueous participated the Maillard reaction with protein at a low rate,resulting in the slight decrease of carbohydrate yield in aqueous.The radial temperature distribution of microalgae slurry affected by mass fraction and flow rate during heating process had effects on damage of microalgae cells,and furtherly influenced the yields of organics(carbohydrate,protein)in aqueous.The effect degrees of different factors on the organics yields in aquous were investigated via response surface methodology.The factors corresponding effect degrees on the carbohydrate yields from high to low were mass fraction,outlet temperature of heating part,and retention of heat preservation zone.And the factors corresponding effect degrees on the protein yields from high to low were outlet temperature of heating part,mass fraction,and retention of heat preservation zone.In addition,mass fraction of microalgae and outlet temperature of heating part had combined effects on the organics yield,which furtherly confirm that the mass fraction had effects on the organics yields in aqueous through changing the radial temperature distribution.Finally,the empirical correlations of organics yields were obtained,which can be used to estimate the yields of organics of microalgae slurry in continuous hydrothermal pretreatment reactors. |
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