Design parameters comparison of bubble column, airlift and stirred tank photobioreactors for microalgae production.

Microalgae are the most propitious feedstock for biofuel production due to their lipid and fatty acid content. Microalgae cultivation shares many features with bioreactors, such as thermal and pH regulation, feeding procedures, and mixing to enhance heat and mass transfers. Aeration and stirring speeds are important parameters to reduce the costs of producing microalgae. In this study, three different photobioreactor types (stirred tank, airlift, bubble column) were characterized and compared for microalgae production. Hydrodynamics, mass transfer, and power consumption were determined for various aeration rates (0.9, 1.2, 1.5 L/min), and stirring speeds (100, 200 rpm), and Chlorella sorokiniana growth performance was compared under the conditions that provided the highest volumetric mass transfer and the lowest mixing time. Photo-bioreactor homogenization was good as indicated by low mixing times (< 10 s). Bubble column had the highest volumetric mass transfer due to its sparger design. Gas holdup and volumetric mass transfer coefficient were found to increase with the air flow rate and stirring speed. For stirred tank, bubble column, and airlift photobioreactors, maximum specific growth rates of C. sorokiniana were 0.053, 0.061, 0.057 h-1, and biomass productivities were 0.064, 0.097, 0.072 gdw/L.day, respectively. Under the conditions tested, growth was limited by the volumetric mass transfer in the airlift and stirred tank and bubble column was the best option for producing microalgae. These findings pave way for more extensive use of these systems in producing microalgae and provide a basis to compare photobioreactors of different designs.

Potato peel waste fermentation by Rhizopus oryzae to produce lactic acid and ethanol.

Potato peel waste (PPW), a zero-value by-product generated from potato processing, is a promising fermentation substrate due to its large quantity of starch, nonstarch polysaccharides, lignin, protein, and lipid. Rhizopus oryzae is a filamentous fungus that is mainly known as a lactic acid producer and can ferment various agro-wastes. This study aimed to use R. oryzae for the fermentation of PPW. A series of batch fermentations were conducted to investigate the effects of different PPW loading rates (2%-8%) and particle sizes (0-4 mm). Under an initial PPW loading rate of 8% and particle size of 1-2 mm, the maximum ethanol (18.83 g/L) and lactic acid (3.14 g/L) concentrations, the highest ethanol (9.41 g/L·day) and lactic acid (1.89 g/L·day) average production rates were obtained. Under these conditions, the yield of ethanol and lactic acid was 0.235 g/gPPW and 0.039 g/gPPW, respectively. R. oryzae was shown to utilize PPW as a substrate to produce value-added bioproducts such as ethanol (major product) and lactic acid.

Dynamic modeling of temperature change in outdoor operated tubular photobioreactors.

In this study, a one-dimensional transient model was developed to analyze the temperature variation of tubular photobioreactors operated outdoors and the validity of the model was tested by comparing the predictions of the model with the experimental data. The model included the effects of convection and radiative heat exchange on the reactor temperature throughout the day. The temperatures in the reactors increased with increasing solar radiation and air temperatures, and the predicted reactor temperatures corresponded well to the measured experimental values. The heat transferred to the reactor was mainly through radiation: the radiative heat absorbed by the reactor medium, ground radiation, air radiation, and solar (direct and diffuse) radiation, while heat loss was mainly through the heat transfer to the cooling water and forced convection. The amount of heat transferred by reflected radiation and metabolic activities of the bacteria and pump work was negligible. Counter-current cooling was more effective in controlling reactor temperature than co-current cooling. The model developed identifies major heat transfer mechanisms in outdoor operated tubular photobioreactors, and accurately predicts temperature changes in these systems. This is useful in determining cooling duty under transient conditions and scaling up photobioreactors. The photobioreactor design and the thermal modeling were carried out and experimental results obtained for the case study of photofermentative hydrogen production by Rhodobacter capsulatus, but the approach is applicable to photobiological systems that are to be operated under outdoor conditions with significant cooling demands.

Implementation and analysis of temperature control strategies for outdoor photobiological hydrogen production.

For outdoor photobiological hydrogen production, the effective control of temperature in photobioreactors is a challenge. In this work, an internal cooling system for outdoor tubular photobioreactors was designed, built, and tested. The temperatures in the reactors with bacteria were consistently higher than those without bacteria, and were also strongly influenced by solar irradiation and ambient air temperature. The cooling protocol applied successfully kept the reactor temperatures below the threshold limit (38 °C) required for the bioprocess and provided a uniform distribution of temperature along the reactor tube length. The biomass growth and hydrogen production were similar in the reactors cooled co-currently and counter-currently. The biomass growth rate was 0.1 l/h, the maximum hydrogen production rate was 1.28 mol/m3/h, and the overall hydrogen yield obtained was 20 %. The change in the biomass was fitted using the logistic model while cumulative hydrogen production was fitted using the modified Gompertz equation.

Bioreactor design for photofermentative hydrogen production.

Hydrogen will become a significant fuel in the near future. Photofermentative production of hydrogen is a promising and sustainable process. The design, construction and successful operation of the photobioreactors are of critical importance for photofermentative hydrogen production and became a major field of research where novel technologies are developed and adapted frequently. This paper gives an overview of the design aspects related to photobioreactors giving particular attention to design limitations, construction material, type, operating mode and scale-up. Sub-components of the overall system setup such as mixing, temperature control and hydrogen collection are also discussed. Recent achievements in the photobioreactor technologies are described.

Hydrogen production by hup(-) mutant and wild-type strains of Rhodobacter capsulatus from dark fermentation effluent of sugar beet thick juice in batch and continuous photobioreactors.

Photofermentative production of hydrogen is a promising and sustainable process; however, it should be coupled to dark fermentation to become cost effective. In order to integrate dark fermentation and photofermentation, the suitability of dark fermenter effluents for the photofermentative hydrogen production must be demonstrated. In this study, thermophilic dark fermenter effluent (DFE) of sugar beet thick juice was used as a substrate in photofermentation process to compare wild-type and uptake hydrogenase-deficient (hup (-)) mutant strains of Rhodobacter capsulatus by means of hydrogen production and biomass growth. The tests were conducted in small-scale (50 mL) batch and large-scale (4 L) continuous photobioreactors in indoor conditions under continuous illumination. In small scale batch conditions, maximum cell concentrations were 0.92 gdcw/L c and 1.50 gdcw/L c, hydrogen yields were 34 % and 31 %, hydrogen productivities were 0.49 mmol/(L c·h) and 0.26 mmol/(Lc·h), for hup (-) and wild-type cells, respectively. In large-scale continuous conditions, maximum cell concentrations were 1.44 gdcw/L c and 1.87 gdcw/L c, hydrogen yields were 48 and 46 %, and hydrogen productivities were 1.01 mmol/(L c·h) and 1.05 mmol/(L c·h), for hup (-) and wild-type cells, respectively. Our results showed that Rhodobacter capsulatus hup (-) cells reached to a lower maximum cell concentration but their hydrogen yield and productivity were in the same range or superior compared to the wild-type cells in both batch and continuous operating modes. The maximum biomass concentration, yield and productivity of hydrogen were higher in continuous mode compared to the batch mode with both bacterial strains.

Photoproduction of hydrogen by Rhodobacter capsulatus from thermophilic fermentation effluent.

Rhodobacter capsulatus was used for the phototrophic hydrogen production on effluent solution derived from the thermophilic fermentation of Miscanthus hydrolysate by Thermotoga neapolitana. Pretreatments such as centrifugation, dilution, buffer addition, pH adjustment and sterilization were suggested for the effluent before being fed to the photofermentation. Batch-wise experiments showed that R. capsulatus grows and produces hydrogen on the pretreated effluent solution. Moreover, it was found that the hydrogen yield increased from 0.3 to 1.0 L/L(culture) by addition of iron to the effluent solution.