| Since the second oil crisis,the third-generation biofuels derived from microalgae have been regarded as the best alternative to fossil fuels.Additionally,microalgae can also fix CO2 in flue gas and treat substances such as ammonia nitrogen in sewage through photosynthesis.However,the concentration of microalgae biomass obtained during the large-scale cultivation process is too low to use directly and further harvesting is required to achieve higher concentration for later utilization.Among the microalgae harvesting methods,flocculation is considered to be the most economical and reliable harvesting method for producing biofuels due to the advantages of wide application,low energy consumption and easy scale up.The flocculation process of microalgae mainly contains the adsorption of microalgae and gravity sedimentation of the flocs.During the adsorption process,the stable electric double layer structure of microalgae would be destroyed with the help of van der Waals force,electrostatic force and hydration force.However,the adsorption mechanism of microalgae and water soluble flocculant is not clear,which leads to the difficulty of strengthening the flocculation process with targeted methods.Therefore,the surface characteristics of microalgae cells and the flocculation process mechanism are need to be explored to effectively strengthen the harvesting performance and energy utilization of microalgae,especially for the production of biofuels.Based on the the surface characteristics and the flocculation mechanism of microalgae,the objective of this study is to improve the adsorption and settling performance of the microalgae.To fufill this purpose,firstly,the surface characteristics and the energy barriers between microalgae cells were analyzed through the theory of extended Derjaguin Landau Verwey Overbeek(e DLVO).The collision model between microalgae cells was established through the lattice Boltzmann(LB)method and successfully eliminated the energy barriers by adjusting the p H.Secondly,the e DLVO theory was applied to analyse the adsorption energy between microalgae cell and cell covered with cationic starch(CCS)and the influence mechanism of flocculant addition,p H and salinity during the adsorption process.A two-particles and multi-particles adsorption model of microalgae cells and CCS were established based on LB method and the thermodynamic characteristics of the adsorption reaction were analysed.Then,clay with large-density was used as a carrier of flocculants to adjust the effective density of microalgae flocs for strengthening the settlement performance.The effects of clay diameter and concentration on the floc effective density,settlement characteristics,flocculation efficiency and diameter distribution characteristics were analyzed.Next,the use of low-density electrolytic bubbles was proposed to carry the flocs to the surface of the suspension.The flotation characteristics of electrolytic bubbles and flocs were analysed.The adhesion process between bubbles and flocs,pore structure of flocs and effect of cationic starch on the later fermentation performance of microalgae biomass were also explored.Finally,a continuous cultivation and harvesting photobioreactor was proposed,in which,the flocculant was mixed with the disturbance caused by aeration into the microalgae suspension during the cultivation process.The structure of the photobioreactor was optimized through the analysis the flow field of the photobioreactor and the harvesting and growth characteristics of the microalgae were analysed.The main achivements obtained are as follows:The e DLVO theory was applied to analyse the energy barrier between microalgae cells in microalgae suspension.In order to eliminate the energy barrier,the p H of the microalgae suspension was adjusted and the electrostatic repulsion between microalgae cells was successfully weakened and the phenomenon of microalgae self-flocculation was appeared.Aiming at exploring the adsorption process,a two-particle collision model between microalgae cells was established with lattice boltzmann method.The result shows that microalgae cells could attach to each other during the collision at p H 3that in the shear flow with low Reynolds number.For the self-flocculation efficiency of microalgae,the highest efficiency was acheived at p H 3,which was 76.6%higher than that of p H 7.Based on the e DLVO theory,the LB collision models of two-particle and multi-particle for CCS and microalgae cells was established.The result shows that the increase of the e DLVO adsorption force would enhance the adsorption efficiency of microalgae and the particle size of the flocs.These simulation results were consistent with the experimental results.For the adsorption efficiency,when the salinity of the suspension increased,the adsorption force between CCS and microalgae cells would be weakened and it resulted in the reduction of adsorption efficiency.For the floc size,when the p H of suspension was decreased from 7 to 3,the weakened adsorption force led to the decreas of floc equivalent diameter from 0.59 mm to 0.21 mm.Besides,the thermodynamic characteristics of the adsorption process between cationic starch and microalgae was spontaneous and exothermic,which meant the adsorption performance of microalgae increased with the declined temperature.When the temperature was 278K,the adsorption rate and equilibrium capacity were the largest and they were 8.74×10-6mg g-1 min-1 and 9148.1 mg g-1,respectivelyIn order to further strengthen the settling performance of microalgae,high density clay was introduced as the carrier of flocculant according to the Stokes'law that the settling velocity of microalgae flocs was proportional to the effective density.The experimental data shows that the effective density and settling velocity of microalgae flocs could be significantly increased with the addition of clay.When the dry weight ratio of clay(80?m)to cation starch was 2,the average effective density of microalgae flocs was increased 2.2 times.While,the equivalent diameter of flocs was reduced and,as a result,the average settling velocity of flocs was increased by 1.3 times.Additionally,the adsorption efficiency of microalgae was also improved with the addition of clay due to the tear effect.With the help of the buoyancy force generated by the low-density electrolytic microbubbles,the flotation characteristics and final biomass concentration of microalgae were enhanced after the attachement between the bubbles and the flocs.The effective density of microalgae flocs was significantly reduced with these low density electrolytic bubbles.For instance,the effective density of flocs with an equivalent diameter of 0.88 mm increased 20.3 times,when the input voltage was 6 V.As a result,the settling velocity of flocs increased from 1.3mm s-1 to 29.3mm s-1(rising velocity),with an increase of 21.5 times.The application of electrolytic bubbles also made the structure of microalgae flocs more compact.The wet volume and porosity of the flocs were 76.9%and 49.5%the value of the flocculation.The compact floc structure and the electrolytic bubbles within the flocs finally raised the biomass concentration of microalgae to 63.6 g L-1 with a 3.7 times increment compared with the flocculation.A continuous photobioreactor coupling microalgae cultivatin and harvesting was constructed.The flocculant and microalgae would be mixed with the disturbance caused by CO2 aeration during the cultivation process and the generated flocs would be filtered and trapped through the membrane.The improvement of light restriction can increase the yield of microalgae biomass by 12.3%.Besides,a high harvesting efficiency close to that of magnetic stirring(500 rpm)can be achieved by reducing the flocculant concentration(3 g L-1).The subsequent membrane filtration can increase the concentration of microalgae biomass by 1.9 times to 39.7 g L-1.In addition,the cationic starch also improved the ratio of carbon to nitrogen for microalgae biomass.Without any pretreatment,the fermentation period of cationic starch and microalgae biomass was 22.1%shorter than that of pure microalgae biomass. |
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