Coputational Modeling of Microalgae Culture Using a Helical Photobioreactor Kraipat Cheenkachorn 1,*, Nattakan Choosri 2, Anoa Chutrapukdeekul 2, Tawiwan Kangsadan 2 1 Departent of Cheical Engineering, 2 The Sirindhorn International Thai-Geran Graduate School of Engineering, King Mongkut s University of Technology North Bangkok, Thailand * Corresponding author. Tel: +66 29132500, Fax: +66 25870024, E-ail: kpc@kutnb.ac.th Abstract: - This study presented a coputational odeling of icroalgae culture using a helical photobioreactor. The objective of this work was to study the fluid dynaics based on theoretical study and phenoenon of CO 2 consued by icroalgae within a helical photobioreactor. The COMSOL Multiphysics Version 3.5a software was used to siulate this syste. Helical photobioreactors were odeled with three different designs, coposing of coil with an inclination of 5 ๐, 10 ๐ and 15 ๐ based on ground level. The verified odel was siulated for the cultivation of Chlorella sp. at constant teperature and light intensity at 32 o C and 3,000 lux, respectively. By varying CO 2 concentrations (15% and 20% CO 2 ) the ability to reproduce and grow was deterined. The odel could predict the best of coil inclination and the best of CO 2 concentrations in the feed to be 5 and 15%, respectively. Under this condition, the bioass production rate of 355.68 g/l was calculated fro the result of this odel. Key-Words: - icroalgae, Chlorella sp., helical photobioreactor,co 2 consuption rate 1 Introduction Biodiesel is considered one of the ain alternatives to fossil fuels. Several reasons that ake biodiesel an attractive energy include renewability, highly biodegradable, low toxicity, lower eissions copared to petroleu-based diesel, copatibility to existing diesel engines, good lubricity, etc. Biodiesel can be used as a pure fuel (B100) or blended with a regular diesel at different copositions such as B5 (5% Biodiesel and 95% regular diesel), or in higher concentrated blends such as B10 and B20. There are any ways in which biodiesel can be produced via the transesterification reaction. The ost coonly used ethods are conventional batch or continuous process using either hoogenous or heterogeneous catalysts. Several techniques have been developed to increase the production perforance. For exaple, a supercritical ethod, which is a catalyst-free ethod, uses supercritical ethanol at high teperatures and pressures in a continuous process to obtain high purity and yield of the product [1]. In the ultrasonic reactor ethod, the ultrasonic waves cause the reaction ixture to produce and collapse bubbles constantly [2]. Mixing and heating are siultaneously carried out to drastically reduce the reaction tie. Current research is also being directed into using coercial icrowave as a heating source to provide intense localized heating, which leads to an efficient and cost-copetitive ethod [3]. The use of enzyes as a catalyst for the transesterification has also been studied to obtain very good yields [4]. However, this ethod is still in the developent stage. Biodiesel is generally produced fro either transesterification or alcoholysis processes, which involve reacting vegetable oils or anial fats catalytically with a short-chain aliphatic alcohols (typically ethanol or ethanol). Depending on the regional availability, the sources of coercial biodiesel include soybean oil (the United States), canola oil (Europe), pal oil (South East Asia), jatropha oil (India), etc. Waste cooking oil and anial fats are also widely used as base stocks. Since these raw aterials are food crops, the use of these raw aterials for non-food applications ISBN: 978-1-61804-026-8 300
ay address controversial issues. Several studies were attepting to use non-food crops such as algae [5-6]. The advantages of producing biodiesel fro algae include rapid growth rate, sulfur-free oil, no toxicity, and high oil contents in soe species. Aong the potential icroalgae, Chlorella sp. gains a lot of attention because of its high growth rate and oil content. The objective of this work was to study the fluid dynaics based on theoretical study and phenoenon of CO 2 consued by icroalgae within a helical photobioreactor. The effects of CO 2 concentrations (15% and 20% CO 2 by ole in the feeding air) and the inclination angle were studied. The CO 2 uptake and bioass productivity were also calculated. 2 Methodology 2.1 Helical Photobioreactor In the present work, a helical tubular photobioreactor was siulated, under steadystate conditions, for incubating Chlorella sp. The inclination angle was varied fro 5, 10, and 15 based on ground level, as shown in Fig. 1. The inner diaeter of the tube was 3 c. The light intensity was kept at 3,000 lux. The teperature was set at 32 C. The total flow rate of broth within the helical photobioreactor was 0.636 L/in. The concentrations of carbon dioxide in the feed air were varied between 15% and 20%, which are equivalent to 6 and 8 ol/ 3, respectively. (a) 5 (b) 10 (c) 15 Fig. 1. The inclination angle 2.2 Governing Equations The Navier-Stokes equation was used to describe the flowing characteristics of fluid under steady-state condition inside the helical photobioreactor as shown in Eq. (1). 2 gh P V (1) where = density, h = height of fluid, V = velocity, and = dynaic viscosity of fluid. The boundary conditions were ade as follows: (1) The properties of the broth was assued to those of water. (2) The inlet velocities of the broth and CO 2 were 1.5 10-5 /s and 2.5 10-5 /s, respectively. (3) No slip condition at the wall surface and unifor flow were assued. Under steady-state condition, the CO 2 consuption by Chlorella sp. was described by Eq.(2). DCO 2-H2 O CCO C 2 COu R 2 CO2 (2) where = diffusivity of CO 2 in water, = CO 2 concentration, R A = consuption rate of CO 2 and u = fluid velocity. The following boundary conditions were ade: (1)The broth contains no CO 2 at the inlet. (2) The CO 2 concentration at the feeding position was varied between 15% and 20%. The growth rate of icroalgae, proposed by Prokop and Ericson [7], was described by Eq.(3). dx dt (3) X where = specific growth rate and X = icroalgae concentration. For CO 2 concentration of 15%, specific growth rate () proposed by Monod [7] was given as C K A C (4) CO2 CO2 where = axiu specific growth rate, K A = half-saturation constant for growth. For CO 2 concentration of 20%, specific growth rate () proposed by Michael and Kargi [30] was given as ISBN: 978-1-61804-026-8 301
(5) K C A CO 2 1 1 CCO K 2 I where K I = an inhibition constant. 2.3 Coputing software The COMSOL Multiphysics Version 3.5a software was used for the siulation while Aspen Plus 2006.5 was used to predict fluid characteristics of fluid within the helical tubular photobioreactor. (a) 5 3 Results and Discussion 3.1 Effect of inclination angle on the CO 2 uptake The CO 2 uptake by Chlorella sp. along the axial distance fro the CO 2 feeding point of the helical tubular photobioreactor was shown in Fig. 2-4. As seen fro the figure, a short lag period was observed after its feeding point. The CO 2 uptake showed a rapid increase and, then, it reached the peak and dropped to zero at about a distance of 1 after the CO 2 feeding point. The sae trend was observed for all inclination angles. This iplied that CO 2 was used up for the growth. Since the consuption of CO 2 by the algae was assued to be the first-order, the 20% CO 2 concentration showed higher CO 2 uptake than 15% CO 2. The effect of the inclination angle deonstrated that the inclination angle of 15 showed the highest CO 2 uptake followed by 10, and 5, respectively. This can be explained by the fact that the CO 2 uptake is directly proportional to the velocity of the fluid within the tube. Higher inclination angle results in higher velocity. (b) 10 (c) 15 Fig. 2. Effects of the inclination angles on the CO 2 uptake (steady-state condition, 32 C, 3,000 lux, 1-day cultivation) ISBN: 978-1-61804-026-8 302
3.2 Effect of inclination angle on icroalgal culture To investigate the effect of CO 2 concentration and the inclination angle on growth, Chlorella sp. in the helical tubular photobioreactor was odeled for the 1-day incubation under a steady-state condition at 32 C, 3,000 lux. Table 1 shows the bioass production obtained fro the odel. The 15% CO 2 concentration showed higher bioass productivity than that of 20% CO 2 for all inclination angles although the CO 2 uptake of 20% CO 2 was higher. This is due to the fact that, at the aeration of 20% CO 2, Chlorella sp. increased ost rapidly at the early period. Then, the growth of icroalgae fell off. After the rapid increase of cultured cells, the growth of the culture was inhibited because of the lack of CO 2, which was used as carbon source for the growth of icroalgae. It was worth to ephasize that the specific growth rate for 15% and 20% CO 2 aeration was perfored using different equations as shown in Eq. (4) and (5). Fig. 3 showed the specific growth rate versus tie for different inclination angles and CO 2 concentrations. Those figures indicated that the specific growth rate of 15% CO 2 was higher than that of 20% CO 2 and then fell off rapidly after about ten hours of incubation. The sae trend was observed for all inclination angles. At constant CO 2 concentration, the inclination of 5 showed the highest yield of bioass followed by those of 10 and 15, respectively. This was because higher velocity of fluid in the photobioreactor with the inclination of 10 and 15 resulted in environental stress leading to lower growth of the culture, as reported by Chiu et al. [8]. Table 1. Bioass production of Chlorella sp. in the helical tubular photobioreactor Inclination Bioass (g/l) %CO 2 5 10 15 15% CO 2 1.8525 1.8220 1.8148 20% CO 2 1.6786 1.6638 1.6456 (a) 5 (b) 10 (c) 15 Fig. 3. Effects of the inclination angles on the specific growth rate (steady-state condition, 32 C, 3,000 lux, 1-day cultivation) Table 2 suarizes the bioass productivity fro the odel of the helical tubular photobioreactor under different CO 2 concentrations and inclination angles. Bioass productivity at 15% CO 2 and the inclination ISBN: 978-1-61804-026-8 303
angle of 5 showed the highest value of 355.68 g/l. At 15% aeration, no significant difference was observed for the bioass productivity at the inclination angle of 10 and 15. At 20% aeration, the inclination angle showed no significant effect on the bioass productivity. Table 2. The bioass productivity of the Chlorella sp. In the helical tubular photobioreactor (steady-state condition, 32 C, 3,000 lux, 1-day cultivation) Inclination Bioass Productivity (g/l) %CO 2 5 10 15 15% CO 2 355.68 349.82 348.44 20% CO 2 322.29 319.45 315.96 4 Conclusion A coputational odeling of a culture of Chlorella sp. using a helical photobioreactor was perfored. The CO 2 concentration and the inclination angle were varied to deterine the effect of CO 2 uptake, specific growth rate, and productivity under a steady state condition, 32 C, 3,000 lux, 1-day cultivation. The odel showed that the inclination of 5 and 15% CO 2 concentration had the highest growth rate and productivity. Under this condition, bioass of 355.68 g/l can be produced. Acknowledgeent The authors gratefully acknowledge the contribution of the Departent of Cheical Engineering, King Mongkut s University of Technology North Bangkok for financial support. Noenclature g gravitational constant.... s -2 h height P static pressure...pa T teperature.k fluid velocity..s -1 u velocity coponent in x-direction.s -1 diffusivity of CO 2 in water.. 2 s -1 concentration of CO 2. ol -3 R A consuption rate of CO 2.. ol -3 s -1 dynaic viscosity...kgs -1 density..kg -3 specific growth rate. day -1 X icroalgae concentration...gl -1 axiu specific growth rate.. day -1 K A half-saturation constant for growth. gl -1 K I an inhibition constant... gl -1 References: [1] Ilha Z. and Saka S., Two-step supercritical diethyl carbonate ethod for biodiesel production fro Jatropha curcas oil. Bioresource Technology, 101, 2010, p. 2735-2740. [2] Ji J., Wang J., Li Y., Yu Y., and Xu Z., Preparation of biodiesel with the help of ultrasonic and hydrodynaic cavitation. Ultrasonics, 44, 2006, p.e411-e414. [3] El Sherbiny S.A., Refaat A.A., and El Sheltawy S.T., Production of biodiesel using the icrowave technique. Journal of Advanced Research, 1, 2010, p. 309-314. [4] Hernández-Martín E. and Otero C., Different enzye requireents for the synthesis of biodiesel: Novozy 435 and Lipozye TL IM. Bioresource Technology, 99, 2008, p. 277-286. [5] Chisti Y., Biodiesel fro icroalgae: Review Article. Biotechnology Advances, 25, 2007, p. 294-306. [6] Ahad A.L., Mat Yasin N.H., Derek C.J.C., and Li J.K., Microalgae as a sustainable energy source for biodiesel production: A review. Renewable and Sustainable Energy Reviews, 15, 2011, p. 584-593. [7] Prokop A. and Erickson L.E., Photobioreactors. in Eds. Asenjo, J.A. and Merchuk J.C., Bioreactor Syste Design. 2 nd ed. Marel Dekker1995. P.620 ISBN: 978-1-61804-026-8 304
[8] Chiu S.Y., Kao C.Y., Chen C., Kuan T., Ong S.,and Lin S., Reduction of CO2 by a high-density culture of Chlorella sp. in a seicontinuous photobioreactor. Bioresource Technology, 99, 2008, p. 3389-3396. ISBN: 978-1-61804-026-8 305