Rheological manipulation of flocculated algal slurries to achieve high solids processing

Hydrothermal hydrolysis of algal biomass for biofuels production: A review

Hydrothermal hydrolysis is an energy-efficient and economical pretreatment technology to disrupt the algal cells and hydrolyze the intracellular compounds, thereby promoting the biofuels production of fermentation. However, complex reaction mechanisms, unpredictable rheological properties of algal slurry, and immature continuous reactors still constrain the commercialization of such a process. To systematically understand the existing status and lay a foundation for promoting the technology, the chemical mechanism of hydrothermal hydrolysis of algal biomass is elaborated in this paper, and the influences of temperature, residence time, total solid content, and pH, on the biomethane production of hydrolyzed algal biomass are summarized. Besides, a comprehensive overview of the rheological behavior of algal slurries is discussed at various operational factors. The recent advances in flow, heat and mass transfer model coupling with the generic kinetics model in continuous reactors and the application of energy-saving strategies for efficient algal biomass pretreatment are detailed reviewed.

Interplay between interfacial behaviour, cell structure and shear enables biphasic lipid extraction from whole diatom cells (Navicula sp.)

HYPOTHESIS

Bacillariophyceae (i.e., diatoms) are an important class of algae with potential use in the production of proteins and lipids including long-chain ω-3 polyunsaturated fatty acids. Biphasic extraction of microalgae lipids using water-immiscible solvents such as hexane, can avoid the excessive energy required to distil solvents from water, but generally requires energy-intensive rupture of the cells. The unique cell structure and surface chemistry of diatoms compared to other microalgae species might allow biphasic lipid extraction without prior cell rupture.

EXPERIMENTS

The kinetics of biphasic lipid extraction from intact Navicula sp. cells was investigated during low-shear and high-shear mixing, and with prior or simultaneous application of ultrasound (20 kHz at 0.57 W/mL). Dynamic interfacial tension measurements and electron microscopic analysis were used to investigate lipid extraction in relation to interfacial behaviour and cell structure.

RESULTS

High yields (>80%) of intracellular lipids were extracted from intact cells over the course of hours upon low-shear contacting with hexane. The cells associated with and stabilised the hexane-water interface, allowing hexane to infiltrate pores in the frustule component of the cell walls and access the intracellular lipids. It was shown that mucilaginous extracellular polymeric substances (EPS) bound to the cell walls acted as a barrier to solvent penetration into the cells. This EPS could be removed by prior ultrasonication. Biphasic extraction was greatly accelerated by shear applied by rotor-stator mixing or ultrasound. High-shear could remove mucilaginous EPS from the cell surfaces to facilitate direct contact of the cell surface with hexane and produced smaller emulsion droplets with increased surface area. The combination of high-shear in the presence of hexane resulted in the in-situ rupture of the cells, which greatly accelerated lipid extraction and allowed high yields of neutral lipid (>95%) to be recovered from freshly harvested cells within less than 5 min. The study demonstrated the ability of shear to enable simultaneous cell rupture and lipid extraction from a diatom alga based on its cell structure and interfacial behaviour.

Thermally coupled dark-anoxia incubation: A platform technology to induce auto-fermentation and thus cell-wall thinning in both nitrogen-replete and nitrogen-deplete Nannochloropsis slurries

Nitrogen-deprived Nannochloropsis cells invested their fixed carbon into the accumulation of triacylglycerol and cell wall cellulose (thickness of N-replete cell walls = 27.8 ± 5.8, N-deplete cell walls = 51.0 ± 10.2 nm). In this study, the effect of nitrogen depletion on the ability of the cells to weaken their own cell walls via autolysis was investigated. Autolytic cell wall thinning was achieved in both N-replete and N-deplete biomass by incubating highly concentrated slurries in darkness at 38 °C. The incubation forced cells to anaerobically ferment their intracellular cellulose and resulted in 30-40% reduction in cell wall thickness for both biomass types. This wall depletion weakened the cells and increased the extent of cell rupture by mechanical force (from 42 to 78% for N-replete biomass, from 36 to 62% for N-deplete biomass). Importantly, autolysis did not adversely impact the amino acid content of protein-rich N-replete biomass or the fatty acid content of lipid-rich N-deplete biomass.

Flow behavior analysis of Chlorella Vulgaris microalgal biomass

The processing volume of bioengineering operations requires flow properties of algal mass for effective processing techniques. Chlorella Vulgaris microalgae cultured at 25 °C in Tap media under continuous illumination was considered. It showed an exponential phase of growth up to 8 days and then a stationary phase of growth from 8 days to 15 days. The rheological properties of microalgae biomass during the growth represented power law model. Microscopic analysis showed the influence of shearing on variation of algal structure from clusters to complete cell separation. The flow properties supported the microscopy analysis showing the shear thickening property at high shear rates and shear thinning nature at low shear regime. Optimal power required for the agitation of biomass based on the variations of non-Newtonian viscosity were predicted by considering the vessel geometry.

Emulsifying properties of ruptured microalgae cells: Barriers to lipid extraction or promising biosurfactants?

A systematic investigation of the emulsifying properties of ruptured algae cells was performed for the first time. The slurry of ruptured algae cells was separated into different biomass fractions, namely the cell debris, the delipidated debris, the serum, and the lipid. The interfacial interactions of these biomass fractions with a nonpolar solvent (e.g. hexane or hexadecane) were characterized using pendant drop tensiometry and interfacial shear rheology. The stability of the different emulsions (formed by the different biomass fractions) was tested using analytical centrifugation. The extracted lipid was an excellent surfactant that reduced the interfacial tension, however, it was not effective at stabilizing the emulsions. The protein-rich serum produced a strong interfacial film that stabilized the emulsions against coalescence during centrifugation. The cell debris stabilized the emulsions to a lesser extent by adsorbing to the droplet surface, presumably via interactions with hydrophobic extracellular polymeric substances (EPS). However, neither the serum nor the cell debris were very effective surfactants, and required the presence of the lipid fraction to produce small emulsion droplets. When present together, the components exhibited competitive interfacial adsorption, which influenced emulsion stability. In particular, the interruption of the protein film by the presence of lipid or cell debris reduced the stability of the emulsions. This study provides a new mechanistic understanding of emulsification during wet lipid extraction from microalgae that will be useful for determining strategies to improve solvent recovery. The results also suggest potential for developing effective bioemulsifiers or biosurfactants from fractionated microalgae biomass for commercial application.

Rheological properties of microalgae slurry for application in hydrothermal pretreatment systems

Herein, the rheological properties of microalgae slurries (Chlorella pyrenoidosa) under different mass fractions and temperatures (293-373 K) were reported, which can significantly affect the energy requirement of hydrothermal pretreatment process and hence the performance of fermentation processes. The experiment results showed that the microalgae slurry had a shear-thinning phenomenon, suggesting that microalgae slurry is a non-Newtonian fluid. For the first time, the phenomenon of variable flow behavior index under varying shear rates was discovered. In addition, the viscosity and yield stress of the microalgae slurries decreased from 293 to 343 K; above 343 K, they increased with the temperature. Scanning electron microscopy analyses showed that the surface of the microalgae cells became rougher when the temperature increased from 343 to 373 K. Finally, the equations of the viscosity of the microalgae slurry, based on the temperatures and shear rates, were obtained.

Nitrogen deprivation of microalgae: effect on cell size, cell wall thickness, cell strength, and resistance to mechanical disruption

Nitrogen deprivation (N-deprivation) is a proven strategy for inducing triacylglyceride accumulation in microalgae. However, its effect on the physical properties of cells and subsequently on product recovery processes is relatively unknown. In this study, the effect of N-deprivation on the cell size, cell wall thickness, and mechanical strength of three microalgae was investigated. As determined by analysis of micrographs from transmission electron microscopy, the average cell size and cell wall thickness for N-deprived Nannochloropsis sp. and Chlorococcum sp. were ca. 25% greater than the N-replete cells, and 20 and 70% greater, respectively, for N-deprived Chlorella sp. The average Young’s modulus of N-deprived Chlorococcum sp. cells was estimated using atomic force microscopy to be 775 kPa; 30% greater than the N-replete population. Although statistically significant, these microstructural changes did not appear to affect the overall susceptibility of cells to mechanical rupture by high pressure homogenisation. This is important as it suggests that subjecting these microalgae to nitrogen starvation to accumulate lipids does not adversely affect the recovery of intracellular lipids.