Studies On Harvesting Of Chlorella By Air Flotation

Harvesting is an important step of producing biodiesel by microalgae. How to harvest the microalgae efficiently and economically proves to be one of the bottlenecks of algal biodiesel industrialization because of the small cells and the low cultivation density. The traditional harvesting methods such as sedimentation, filtration and centrifugation, which are low efficiency or high energy consumption, are not suitable for the industrialized production.Flocculation, dissolved air flotation and dispersed gas flotation were applied to harvest Chlorella in this paper. The influencing parameters were investigated in the experiment. The statics and dynamics theories about the harvesting of Chlorella by air flotation were studied. In addition, the dynamic model of harvesting Chlorella by air flotation was established.In the experiment of harvesting Chlorella by flocculation, eight kinds of flocculants:agarophyte, polyacrylamide, zinc sulfate, alum, aluminum sulfate, calcium hydroxide, ferric sulfate, ferric trichloride were used. The effects of the dosage of flocculant, flocculation time on recovery were studied, then the recycle of nutrient solution after flocculation was investigated. The results showed that the four kinds of flocculants:aluminum sulfate, calcium hydroxide, ferric sulfate and ferric trichloride, could harvest Chlorella effectively. About90min after adding0.8g,0.8g,0.5g, and0.3g aluminum sulfate, calcium hydroxide, ferric sulfate and ferric trichloride into1L culture, the recovery ratio of Chlorella could reach89.7%、91.7%、88.9%and92.3%, respectively. The supernatants after harvesting the algae by flocculation were reused to cultivate Chlorella. Chlorella could grow in the four supernatants, but Chlorella could grow better in the supernatant of calcium hydroxide and ferric trichloride than in the two other supernatants.In the experiment of harvesting Chlorella by dissolved air flotation, the effects of the dosage of N-Cetyl-N-N-N-trimethylammonium bromide (CTAB), pH, the flow rate of chlorella culture dissolved air (CCDA), saturation time and saturation pressure were investigated. The results showed that the recovery of Chlorella increased firstly and then decreased with the increase of the dosage of CTAB, pH or the the flow rate of the CCDA. When the dosage of CTAB was0.04g/L culture, pH was10or the flow rate of CCDA was40L/h, the recovery ratio could reach the peak. With the increase of saturation time, the recovery ratio of Chlorella increased firstly and then remained stable. But with the increase of saturation pressure, the recovery ratio of Chlorella increased firstly and then decreased slightly. Appropriate recovery condition of this experiment was that the dosage of CTAB was0.04g/L culture, pH was8, saturation time was25h, saturation pressure was0.6MPa and the increase of CCDA is40L/h. Under the operating conditions, the recovery of Chlorella could reach70.5%.In the experiment of harvesting Chlorella by dispersed gas flotation, the effects of the dosage of CTAB, pH, air flow, the pore diameter of gas distributor, the height of liquid layer in the separation column and the addition of ethanol were investigated. The results showed that increasing the dosage of CTAB, increasing the height of liquid in the separation column, reducing the pore diameter of gas distributor or adding ethanol were conductive to harvesting Chlorella. pH had great influence on the recovery of Chlorella. The recovery ratio could reach the peak value at pH5and pH10, and the corresponding recovery ratio were64.4%and64.8%respectively. The effect of the initial concentration of Chlorella was also investigated in the experiment. It was discovered that high initial concentration of Chlorella could achieve a low recovery of Chlorella. When the initial concentration of Chlorella was2.7×107cells/ml Chlorella liquid, the dosage of CTAB was0.06g/L Chlorella liquid, pH was9, air flow rate was100ml/min and gas distributor pore size was between5-15μm, the recovery rate could reach more than90%.The statics and dynamics theories of the process of harvesting Chlorella by air flotation were studied, and the dynamic model of harvesting Chlorella by air flotation was established. The dynamic model accorded with the first-order kinetics, and matched the experimental dates of harvesting Chlorella by dispersed gas flotation.