Disintegration Of Nannochloropsis Sp. Cells In A Turbine Bead Mill With Ultrasonic Assistance

Biological cells like microalgae are rich in fatty acids and other high-value nutrients.Microalgae also have many advantages such as convenient cultivation,short growth cycle,absorption of CO₂ and reduction of greenhouse effect.They have become a useful biological resource in some commercial applications.It is necessary for the extraction of useful intracellular substances to disrupt the cell wall effectively.The common existing cell disruption methods are bead milling,high-pressure homogenization,ultrasonication,enzymatic cell lysis,and chemical treatment.However,most of the methods are difficult to realize large-scale industrial applications besides bead milling.Bead milling is one of the most viable cell disruption methods for large-scale industrial applications so far,however,it also has some applicable problems like high energy consumption and unsatisfactory disruption efficiency.In this paper,a new ultrasound-assisted turbine bead milling method was proposed.Effects of main factors on cell disruption and cell disruption mechanisms were discussed through experiments and numerical simulation.In Chapter 3,an improved mode turbine stirring bead mill was used to disrupt Nannochloropsis sp.cells.Effects of some major factors?i.e.,circumferential speed,bead size,cell concentration,disintegration time,bead filling rate,etc?on the cell disintegration were investigated via single-factor experiments.The parameters of cell disintegration were optimized via an orthogonal experiment.The results show that increasing circumferential speed,disintegration time and bead filling rate,as well as reducing bead size and cell concentration properly,are conducive to the cell disruption.Based on the cell disintegration percentage and suspension viscosity as objective functions,the optimum parameters are the circumferential speed of 2.3 m s^-1,the bead size of 0.3-0.4 mm,the bead filling rate of 70vol.%,the disruption time of 30 min and the cell concentration of 27.2 g L^-1.In Chapter 4,an ultrasound-assisted bead milling method was proposed for cell disintegration.Ultrasonic waves were introduced during the process of disrupting cells in the turbine stirred bead mill.The effects of ultrasound?i.e.,ultrasonic power and ultrasonic interval time?on the cell disintegration and suspension viscosity at different disintegration time and cell concentrations were investigated.The results show that the viscosity of suspension can be reduced in the presence of proper ultrasound.The fluidity of cell suspension and cells dispersion can be improved during the disintegration due to the viscosity-reduction effect with ultrasound,then enhancing the cell-disruption efficiency in the turbine mill.Compared to the case without ultrasound,the disintegration time for the cell disintegration percentage of>95%reduces from 35 min to 20 min,and the energy consumption of the whole disintegration process decreases from 8.573 MJ kg^-1 to 6.783 MJ kg^-1?for dry weight of Nannochloropsis sp.?for the Nannochloropsis sp.suspension with the cell concentration of 90.7 g L^-1 disrupted in the mill with ultrasound?i.e.,power of 150W,and interval time of 5 s?.Based on the analysis of fatty acids and chlorophyll a of samples dirupted under different conditions,ultrasound can facilitate the EPA extraction,and ultrasound at a lower power?i.e.,150 W?has little damage on either fatty acids or chlorophyll a.In addition,compared to alumina and glass beads,the use of zirconia beads in the mill is beneficial to the effective disruption of cells,and subsequent release and extraction of chlorophyll a,but has a slight negative impact on the fatty acid composition.In Chapter 5,a population balance model?PBM?was introduced to simulate the disruption behavior of Nannochloropsis sp.cells,and the disruption mechanism was discussed.The simulated results show that ultrasound can improve the cell disintegration efficiency in the turbine mill.It is indicated that cells with large sizes are more easily to be disrupted,and they are mainly disrupted by compressive and shear effects in the initial disruption stage.However,cells with small sizes are more difficult to be disrupted,and shear gradually becomes the main disruption mechanism as the process proceeding.Ultrasound can effectively disperse cells and reduce the viscosity of microalgae suspension,thus improve the fluidity and enhance the compressive effect during the disruption of small cells besides shear effect.Thereby,the cell disruption efficiency is improved.Finally,this dissertation gives the main conclusions obtained through experimental studies and simulation analysis,and some prospects for future work.