Entropy Generation Analyses Of Transfer And Conversion Processes In Photobioreactors

In past few decades, energy shortage and environment pollution caused by overexploitation of fossil fuels has become a major threaten to human beings, which promotes the development of renewable energy to be a hot research topic. Attributing to the advantages of low energy consumption, renewable, and environmentally friendly, photosynthetic microbial energy has become one of the most promising green energy. However, the microbial energy conversion efficiency is not high enough to put it into application. As a reaction field of microbial energy conversion, photobioreactor plays a crucial role in the microbial conversion process because it can directly affect the properties of heat and mass transfer as well as the fluid flow. Therefor, it is necessary to optimize the structure and operation parameters for improvement of the microbial energy conversion efficiency. Entropy analyses have been widely used in various energy conversion systems and chemical engineering processes, while few examples were found in microbial energy conversion systems. As an energy system involved in fluid flow, heat and mass transfer and biochemical reaction, as well as the transfer efficiency and irreversible losses are important factors for the efficiency of photobioreactors. In this study, the transfer and conversion processes in photobioreactors were simulated. Based on the simulation results, entropy generation analyses were used to optimize the geometries and operation conditions of the photobioreactors.In this study, the velocity, temperature and concentration distribution as well as reaction rates of three types of photobioreactors,(biofilm photobioreactor, entrapping particles photobioreactor and suspension photobioreactor), were simulated. Based on the field distribution, the entropy generation rate caused by irreversible transfer were calculated and the influences on substrate degradation efficiency, products formation rate and entropy generation rate, such as flow rate, substrate concentration, temperature, light intensity, p H, aeration rate and the geometries of the reactor, were studied. At last, a new parameter combined entropy generation rate and reaction rate was proposed to evaluate the photobioreactor, namely, with the minimal entropy generation rate for per unit of substrate degradation(with target of substrate degradation) or product formation(with target of products formation). The optimal operation conditions were obtained base on the evaluation result. The main conclusion of this study are as follows:(1) The simulation and entropy generation rate in the transfer and conversion process of the biofilm photobioreactor were conducted. The results showed that the entropy generation rate caused by mass and heat transfer were the main part and viscous dissipation can be neglected for the biofilm photobioreactor. The entropy generation caused by mass transfer has a major influence on the total entropy generation when operation conditions changed, but entropy generation caused by heat transfer is not remarkable. So enhancing mass transfer can significantly reduce irreversible loss. For the biofilm photobioreactor in this study, the optimal operation conditions were obtained at 30°C, 70 m L/h of culture medium flow rate and 70 mol/m3 of glucose concentration.(2) Through the simulation of the velocity, temperature, concentration and entropy generation distribution in single entrapping particles of packed bed reactor, main conclusions are obtained as follows. Mass transfer resistance is mainly focused on the entrapping particles, but not in the main flow region. The entropy generation caused by glucose mass transfer decreases with the increase of the initial glucose concentration. Along with the increases of the initial temperature, the entropy generation first increases and then decreases, with maximum entropy generation obtained at 30°C. When the flow rate increases, the entropy generation caused by mass transfer decreases slowly and finally levels off. When the p H and light intensity increase, the entropy generation rate first increase and then decrease, and reaches the highest point at p H of 7 and light intensity of 6000 lx, respectively. Combining entropy generation, glucose degradation and hydrogen production rate, the following optimal operation conditions were obtained: c0=60 mol/m3, T0=30°C, qv=4 m L/h, p H=7, I0=6000 lx.(3) Two kinds of packing form for entrapping particles, i.e., body-centered cubic packing and face-centered cubic packing, were studied in this paper. The results showed that all parameters of the two packing form follow similar trends. The glucose dagradation and hydrogen production rate both reach the maximum value at c0 of 60 mol/m3, T0 of 30°C and Re of 0.6, but the reaction rate for face-centered cubic packing form is 6.8% higher than body-centered cubic packing. Entropy generation caused by glucose transfer first increases and then decreases with the increase of initial temperature. However, when glucose concentration and Re number increase, the entropy generation caused by glucose transfer decreases monotonously. The entropy generation for each cubic meter of reactor in face-centered cubic packing form is higher than that in body-centered cubic packing form.(4) Compared to the flat plate photobioreactor without partition plate, the circumfluence in the reactor with a partition plate is stronger, thus the mixture effect of culture medium is enhanced. The entropy generation caused by viscous flow increases with the increase of aeration rate and the length of partition plate. The average gas volume fraction decreases when the aeration rate increases and the length of the partition plate decrease.