Lattice Boltzmann Simulation Of Flow And Mass Transfer In Porous Media With Photo Bioreaction

Due to the over-exploitation of traditional fossil energy by human, as well as theunestablishment of the new energy system, this causes global environmental pollution,energy crisis and severe damage of ecological environment. The substainabledevelopment of economy in future requires the development and the use of cleanrenewable energy instead of the fossil energy, so as to build the harmonious relationbetween human and nature. In many alternative energy sources, hydrogen energy is themost ideal energy at present, attributed to its advantages of high energy content, goodstability of combustion, cleanness and high efficiency. In many hydrogen productionmethods and technologies, the photo biohydrogen production method is a newtechnology that the microorganisms degrade the organic matter for their own growth,and meanwhile generate hydrogen energy. The biohydrogen production method hasadvantages of mild reaction conditions, low energy consumption, proper treatment ofthe contradiction between energy and environment. Therefore, the photo biohydrogenproduction technology has been recognized as a green energy technology with a broadapplication prospect.Recently, lots of experimental studies have been carried out, and theimmobilized-cell method based on biofilm and gel granule immobilized-cell has beenapplicated to the biohydrogen production, which can improve the biomass in the reactorand the stability of operation. Meanwhile, many researchers have implemented somenumerical methods to investigate the flow and mass transfer in the bioreactor.Unfortunately, these numerical methods are mainly on macro-scale, and it is difficult toreflect the flow and mass transfer on micro-scale, and achieve good stability and highnumerical precision for complex boundaries. In last decades, the lattice Boltzmannmethod (LBM) on mesoscale has emerged, and it has some advantages, such as simplealgorithm, nature parallelism and ease of handling with the complex boundary. Thisstudy is based on the biofilm bioreactor filled with tube bundles or packed particles withphotosynthetic bacteria (PSB) biofilm, and packed bed reactor filled with porousgranule immobilized PSB-cell, and the lattice Boltzmann method is used to investigatethe flow and mass transfer in these porous media with photo-bioreaction. The mainstudies and conclusions in this paper are as follows:①First, the detail derivation of the transformation from consecutive Boltzmann equation to the lattice Boltzmann equation descrete on time, space and velocity, ispresented. Then, through Chapman-Enskog expansion, the macro equations can beobtained from the lattice Boltzmann equation, which establishes the relationship ofmodel parameters on meso-scale and macro physical quantities. The lattice Boltzmannmodel for mass transfer and the boundary treament methods are properly designedaccording to the physical problem in this study. Additionally, the feasibility of thelattice Boltzmann method is validated by simulations of some cases.②The2D lattice Bolzmann mass transfer model is implemented to simulate theflow and mass transfer for substrate solution around a cylinder and a porous media oftube bundles with photo bioreaction surface. The flow and concentration fields areobtained under various conditions, and the effects of influent velocity, tube spacing andtube arrangement are investigated by these parameters of average drag coefficient,average Sherwood number and substrate consumption efficiency. The numerical resultsdemonstrate that the increasing influent velocity leads to high average Sherwoodnumber, while low average drag coefficient and substrate consumption efficiency. Withdecreasing tube spacing, these parameters both increase. For staggered arrangement oftube bundle, the average Sherwood number as well as substrate consumption efficiencyincrease, but the average drag coefficient decrease somewhat, implying that thisarrangement is benefit for the mass transfer and bioreaction.③The flow and mass transfer of substrate solution around a particle and porousmedia of packed particles with photo-bioreaction surface are simulated with3D latticeBoltzmann model for mass transfer, which tends the2D to3D study. In the simulation, amulti-block strategy is coupled to improve the computational efficiency. The effects onflow and concentration fields by illumination intensity, influent velocity and particleaccumulation mode are investigated. Furthermore, the hydrogen productionperformance is evaluated. The numerical results indicate that when illuminationintensity is6000lx, the hydrogen production performance achieves maximum; withincreasing influent velocity, the hydrogen production performance decreases; thechoosing of body-centered structure particle accumulation mode leads to a betterhydrogen production performance, but lower average drag coefficient.④The lattice Boltzmann model at Representative elementary volume (REV) scaleis used to simulate the flow and mass transfer of substrate solution through a porousgranule immobilized PSB-cell for photo biohydrogen production. In the simulation, themulti-block model is coupled with the lattice Boltzmann model, and the effects of illumination intensity, influent velocity, permeability and porosity of porous granule areinvestigated. The numerical results show that for6000lx, the light energy conversionefficiency is highest, and thus the hydrogen production performance reaches maximum;with increasing influent velocity, the hydrogen production performance decreases; withincreasing permeability of porous granule, the hydrogen yield increases, while thesubstrate consumption efficiency decreases; with increasing porosity of porous granule,the hydrogen yield slightly decreases, and substrate consumption efficiency increases,and they tend to be stable for porosity over0.5.⑤The bioreaction in a photo bioreactor with a porous granule immobilizedPSB-cell is investigated with lattice Boltzmann model coupled with the multi-blockmodel at pore-scale. In the simulation, the porous structure of the granule is generatedby Quartet Structure Generation Set,(QSGS). The effect of porosity on flow, masstransfer and hydrogen production performance is studied, and the LB numerical resultsand experimental results have a good agreement. Furthermore, a multi-scale latticeBoltzmann model is proposed to simulate the photo bioreaction in packed bedbioreactor filled with porous granule immobilized PSB-cell, which can obtain the flowand mass transfer in the porous granule on micro-level, as well as save thecomputational cost. Furthermore, the light attenuation in bioreator is considered in thesimulation. The numerical results indicate that the light attenuation mainly occurs in theporous granule, and the output illumination intensity between the LB numerical solutionand the experimental result has a good agreement. Additionally, the hydrogenproduction performance is evaluated, and they agree well with the experimental results,which prove that the multi-scale lattice Boltzmann is viable for simulation of photobioreaction system.