Brazilian Journal of Chemical Engineering SIMULATION OF MICROALGAL GROWTH IN A CONTINUOUS PHOTOBIOREACTOR WITH SEDIMENTATION AND PARTIAL BIOMASS RECYCLING

Microalgae are considered as promising feedstocks for the third generation of biofuels. They are autotrophic organisms with high growth rate and can stock an enormous quantity of lipids (about 20 – 40% of their dried cellular weight). This work was aimed at studying the cultivation of Scenedesmus obliquus in a two-stage system composed of a photobioreactor and a settler to concentrate and partially recycle the biomass as a way to enhance the microalgae cellular productivity. It was attempted to specify by simulation and experimental data a relationship between the recycling rate, kinetic parameters of microalgal growth and photobioreactor operating conditions. Scenedesmus obliquus cells were cultivated in a lab-scale flat-plate reactor, homogenized by aeration, and running in continuous flow with a residence time of 1.66 day. Experimental data for the microalgal growth were used in a semi-empirical simulation model. The best results were obtained for 0.2 w I F F = , when R = 1 and kd = 0 and 0.05 day , with the biomass production in the reactor varying between 8 g L -1 and 14 g L, respectively. The mathematical model fitted to the microalgal growth experimental data was appropriate for predicting the efficiency of the reactor in producing Scenedesmus obliquus cells, establishing a relation between cellular productivity and the minimum recycling rate that must be used in the system.


INTRODUCTION
Microalgae have gained much attention for biofuel production due to their high capability of storing value-added energy compounds.The chemical composition of such compounds encompasses starches and highly saturated fatty acids convertible to neutral lipids, which play an important role in production of bioethanol and biodiesel.This feature, along with high growth rates and ease of cultivation, make microalgae very promising when compared to higher plants (Rawat et al., 2013;Khan;Bahadar, 2013;Silva and Bertucco, 2016).
Scenedesmus obliquus is a microalga that has been widely studied because of its high cellular productivity and accumulation of value-added energy compounds.Several works relying on its cultivation have investigated a variety of aspects, including types of culture medium and substrate, which have been tested in bench or continuous systems, also employing different intensities of light.However, studies on the efficiency of biomass recycling coupled to the photobioreactor either through simulation or experimental data are still required (Vigeolas et al., 2012;Yin-Hu et al., 2012;Baky et al., 2013;Wang et al., 2013;Wu et al., 2013;Lee et al., 2013 - The recyclin ween the recy te ( ) actor tank an om the system he SRT is co nts high proc me for the etabolize the the reactor.θ is defined as the minimum residence time that allows biomass maintenance in the system.This means that wo c θ is an operating limit, below which the biomass cannot be maintained in the system because the wash-out rate is higher than the growth rate.From the fact that 1 ( ) The minimum recycling rate ( ) min R can be determined by combining Equations ( 7), ( 8) and ( 9): Considering that the residence time in the reactor tank ( ) θ , or hydraulic retention time (HRT) is given by Equation (11): A relationship between θ and c θ can be found and written as Equation ( 12): . 1 1 From an analysis of mass balance over the system and over the settler, the substrate concentration at the exit of the reactor and the biomass concentration at the exit and recycling line of the reactor are calculated by Equations ( 13), ( 14) and ( 15): / ( ) ( 1) The simulation was performed using the proposed mathematical model, introducing experimental data of specific growth for S. obliquus.The cultivation experiments of this microalga in the flat-plate reactor allowed one to calculate the , , and

RESULTS AND DISCUSSION
The S. obliquus cultivation experiments were carried out using CO 2 and nitrate (NO 3 -) solutions as carbon and nitrogen sources, respectively, for the microalgae growth.CO 2 was pumped in excess into the system while NO 3 -solution was chosen as the limiting substrate.Table 3

Y
It is noted that the NO 3 -comsumption was approximately 79%.
The high value of biomass concentration seen in Table 3 is typical of S. obliquus, which is referred as one of the most promising microalga for biofuel production.This result is in agreement with values previously reported in the literature.For instance, Baky et al. (2012) found a dried weight of 1.651 g L -1 for S. obliquus cultivated at 25 °C in N-9 culture medium at 200 µE m -2 s -1 and 9% CO 2 in the gas line.Wang et al. (2013)           Apparent yield coefficient for substrate-tobiomass conversion (g g -1 ) V r Effective volume of the reactor (m 3 ) R min Minimum recycling rate (-)

gure 1: Opti bliquus (magn igure 2: Sche able 1: Vari oreactor.
x Y gro giv s r tw rat is the effective volume of the reactor.The concept of wash-out time, , C. E.de Farias Silva, B. Gris and A. BertuccoBrazilian Journal of Chemical Engineering where V r wo c ficients.The value of 1.66 day for θ was used in the steady-state.The other parameters used in the simulation model are listed in Table2.The variables , e