Wastewater treatment using photo-impinging streams cyclone reactor: Computational fluid dynamics and kinetics modeling

A photo impinging streams cyclone reactor has been used as a novel apparatus in photocatalytic degradation of organic compounds using titanium dioxide nanoparticles in wastewater. The operating parameters, including catalyst loading, pH, initial phenol concentration and light intensity have been optimized to increase the efficiency of the photocatalytic degradation process within this photoreactor. The results have demonstrated a higher efficiency and an increased performance capability of the present reactor in comparison with the conventional processes. In the next step, residence time distribution (RTD) of the slurry phase within the reactor was measured using the impulse tracer method. A CFD-based model for predicting the RTD was also developed which compared well with the experimental results. The RTD data was finally applied in conjunction with the phenol degradation kinetic model to predict the apparent rate coefficient for such a reaction.     

Computational fluid dynamics (CFD) modeling of UV disinfection in a closed-conduit reactor

The use of ultraviolet (UV) disinfection in water treatment is governed by several factors, including flow field, fluence rate field, and microbial inactivation kinetics. In this study, a computational fluid dynamics (CFD) model was developed for UV disinfection in a closed-conduit reactor where an improved low-Reynolds number k–ε model was used to calculate flow field and a modified P-1 model was employed to obtain the fluence rate field. The Chick–Watson model was adopted to characterize the inactivation of microorganisms. Commercial CFD software FLUENT 6.3 was employed to solve the governing equations. The predicted flow field agreed well with experimental data obtained by digital particle image velocimetry (DPIV) (Liu et al., 2007) in terms of velocity field. The proposed CFD model was also evaluated by comparing current predictions to bioassay test data, and reasonable agreement was obtained in terms of effluent log inactivation. The impact of wall reflection of the light on the fluence rate field and the viable microorganism concentration field was investigated. The effect of wall reflection of the light on effluent log inactivation was also investigated under different water qualities and lamp power conditions. The results showed that at higher inactivation levels, the effect of wall reflection was more influential.     

柴油自热重整制氢反应及反应器

设计了带预热段的绝热管式反应器(A型)和带喷嘴及预热段的绝热管式反应器(B型),设计了相应流程并组建了柴油自热重整制氢装置,以直馏柴油为原料,研究了2种反应器内的柴油自热重整制氢反应行为。研究结果表明:2种结构的反应器均能用于柴油自热重整制氢,采用带喷嘴的绝热管式反应器可以确保雾化及气化效果良好,柴油热裂解生成甲烷的反应有助于柴油制氢过程。     

Computational fluid dynamics modeling of biomass catalytic fast pyrolysis in bubbling fluidized reactor: Effects of catalyst parameters on process performance

In this study, a computational fluid dynamics mathematical model has been developed for catalytic fast pyrolysis (CFP) of biomass based on multiphase flow, transfer process, and biomass pyrolysis reactions in a bubbling fluidized bed reactor. The multiphase fluid flow, and the inter-phase momentum and energy transfer processes are modeled with Eulerian multiphase formulas, representing the flows of gases and solids (catalyst and biomass) within the reactor. The biomass CFP reactions are described by using a two-stage, semi-global model. Specified secondary tar catalytic cracking process, which considers both intrinsic reaction rates and mass-transfer process, is embedded to the developed model by user-defined function. The model simulation results of pyrolysis product yield and distribution are compared with the experimental data with close agreement. The model is then employed to investigate the effects of structural properties of catalyst, such as specific internal area, average size of active sites, pore diameter, and tortuosity, on products yields and composition. The tar cracking process by the selected catalyst is proposed and the influences of adsorption capability of tar molecule on catalyst surface and external film mass transfer are also analyzed. The developed model can be solved with short computational time and thus it can be employed for further research and engineering designs of the catalytic pyrolysis of carbonaceous materials.     

On the global dynamics of an autothermal reactor stabilized by linear feedback control

The global dynamics of an autothermal reactor stabilized at an (open-loop) unstable steady-state by a single proportional controller is studied as a bi     

Simulation of autothermal reforming in a staged‐separation membrane reactor for pure hydrogen production

Steam methane reforming with oxygen input is simulated in staged‐separation membrane reactors. The configuration retains the advantage of regular membrane reactors for achieving super‐equilibrium conversion, but reaction and membrane separation are carried out in two separate units. Equilibrium is assumed in the models given the excess of catalyst. The optimal pure hydrogen yield is obtained with 55% of the total membrane area allocated to the first of two modules. The performance of the process with pure oxygen input is only marginally better than with air. Oxygen must be added in split mode to reach autothermal operation for both reformer modules, and the oxygen input to each module depends on the process conditions. The effects of temperature, steam‐to‐carbon ratio and pressure of the reformer and the area of the membrane modules are investigated for various conditions. Compared with a traditional reformer with an ex situ membrane purifier downstream, the staged reactor is capable of much better pure hydrogen yield for the same autothermal reforming operating conditions.     

On controlling an autothermal fixed-bed reactor at an unstable state—1: Steady-state and dynamic modeling

Models of distributed chemical reaction systems which are both accurate and wieldy are extremely difficult to obtain. This paper discusses traditional methods for deriving models of fixed-bed reactors, i.e. where the attempt is made to accurately account for steady-state and dynamic characteristics only. It illustrates the development of such models for the complex case of a tubular autothermal reactor with internal countercurrent heat exchange which exhibits multiple steady states for certain ranges of operating conditions.A two-dimensional dynamic model of the reactor is constructed, and then simplified to a linear state-variable form suitable for dynamic analysis by using successively: (i) a double collocation procedure to discretize the equations in both the radial and axial directions; (ii) a linearization technique around a specified steady-state profile; and (iii) the assumption of quasi-steady state for the coolant temperature and for the concentration of the reacting mixture.The dynamic analysis includes evaluation of the eigenproperties of the linearized model. The degree of instability of the reactor is correlated to some design variables and properties of the reaction. The sensitivity of the model to changes in parameters or operating conditions is also investigated. Finally, a shortcoming of the traditional modeling approach with its total emphasis on steady-state and dynamic properties is discussed. It is noted that the internal structure of a model also must be considered carefully if it is to be useful for controller design. A sequel paper discusses this virtually unrecognized point, again using the autothermal reactor system for example purposes.     

Computational fluid dynamics and electrostatic modeling of polymerization fluidized-bed reactors

Electrostatics plays an important role in gas-solid polymerization fluidized-bed reactors. Agglomeration of polymer particles can occur due to either electrostatic and/or thermal effects, and can lead to reactor operability problems if not properly mitigated. In this work a first-principles electrostatic model is developed and coupled with a multi-fluid computational fluid dynamic (CFD) model to understand the effect of electrostatics on the bulk polymer, polymer fines, and catalyst particles. The multi-phase CFD model for gas-solid flow is based on the kinetic theory of granular flows and the frictional theory. The electrostatic model is developed based on a fixed, size-dependent charge for each type of particle (catalyst, polymer fines and polymer). The combined CFD model is first verified using simple test cases and then applied to a pilot-plant-scale polymerization fluidized-bed reactor. The multi-phase CFD model is applied to reproduce qualitative trends in particle segregation and entrainment due to electrostatic charges observed in experiments.     

Computational fluid dynamics applied to high temperature hydrogen separation membranes

This work reviews the development of computational fluid dynamics (CFD) modeling for hydrogen separation, with a focus on high temperature membranes to address industrial requirements in terms of membrane systems as contactors, or in membrane reactor arrangements. CFD modeling of membranes attracts interesting challenges as the membrane provides a discontinuity of flow, and therefore cannot be solved by the Navier-Stokes equations. To address this problem, the concept of source has been introduced to understand gas flows on both sides or domains (feed and permeate) of the membrane. This is an important solution, as the gas flow and concentrations in the permeate domain are intrinsically affected by the gas flow and concentrations in the feed domain and vice-versa. In turn, the source term will depend on the membrane used, as different membrane materials comply with different transport mechanisms, in addition to varying gas selectivity and fluxes. This work also addresses concentration polarization, a common effect in membrane systems, though its significance is dependent upon the performance of the membrane coupled with the operating conditions. Finally, CFD modeling is shifting from simplified single gas simulation to industrial gas mixtures, when the mathematical treatment becomes more complex.     

Machine learning-assisted multiscale modeling of an autothermal fixed-bed reactor for methanol to propylene process

The current commercial multistage reactor for methanol to propylene (MTP) process suffers from poor propylene selectivity and catalyst efficiency, mainly because of the low inlet methanol concentration and long residence time. In this work, we proposed an autothermal co-current flow reactor for MTP process, where the reaction heat is continuously removed through heat exchange with cold reactants, thus single-stage reactor can be used with higher methanol inlet concentration. The reactor feasibility was investigated by a three-dimensional multiscale model, in which the diffusion–reaction interaction inside catalyst particle was described by a neural network model trained by machine learning. With the feeding methanol fraction increasing to 30%, propylene selectivity reaches 82.27% while the space velocity approaches 2.68 g_MeOH g_cat⁻¹ h⁻¹ at 99.97% methanol conversion, about 1.4 and 3.8 times those of a commercial multibed reactor, respectively. With proper catalyst bed dilution, the reaction temperature is well controlled between 700 and 754 K.     

生物反应器内不同桨叶组合计算流体力学模拟

运用计算流体力学(CFD)数值模拟方法研究在相同工况下(搅拌桨转速为400 r/min、通气速度为0.86 vvm和操作温度为15 ℃),4种不同桨叶组合方式对5 L气液生物反应器流场、氧传质系数kLα、空气体积分数、气含率(体积分数)和功率的影响,评判各桨叶组合综合性能.计算结果表明:1号方案上下档均为径向桨,具有最...     

Computational fluid dynamics modeling of high temperature air combustion in an heat recovery steam generator boiler

This paper reports a numerical study on the possibility of using high temperature air combustion (HiTAC) technique in the heat recovery steam generator (HRSG) boiler of the Fajr Petrochemical Complex, Iran. For this purpose a theoretical fuel nozzle which operates in HiTAC mode of combustion has been installed and modeled using the computational fluid dynamics (CFD) technique. By aim of establishing an efficient heat transfer rate to the boiler’s tubes, the proper nozzle location and an optimum mass flow rate of fuel have been found. The results show that by using this modification it is possible to increase the steam temperature up to 37 percent.     

自热制氢反应器研究进展

介绍了同心圆式反应器、板式反应器、壁反应器、微通道反应器在自热重整反应制氢中的特点。同心圆式反应器的传热是控制步骤,为强化传热而开发了空间形状不同和流体经过反应器不同腔体的先后顺序不同的反应器;板式反应器易于组装、拆卸和放大,而且热效率也比较高,是目前十分活跃的研究领域,重点在于操作参数和设计的优化及其高效壁载制氢催化剂的研制;壁反应器的反应表面和换热表面不分离,具有较高的热量耦合效果;微通道反应器具有优越的传热性能,但对加工和流体的性质有比较苛刻的要求。另外,不同燃料制氢机理的研究及其过程参数的稳态、瞬态模拟,为反应器的设计提供了理论依据。而制氢过程并行单元的研究为系统的集成奠定了基础。最后,指出开发板式壁反应器以及开展其在CO变换、净化方面的研究有较好的发展前景。     

Computational fluid dynamics modeling of immobilized photocatalytic reactors for water treatment

A computational fluid dynamics (CFD) model for the simulation of immobilized photocatalytic reactors used for water treatment was developed and evaluated experimentally. The model integrated hydrodynamics, species mass transport, chemical reaction kinetics, and irradiance distribution within the reactor. The experimental evaluation was performed using various configurations of annular reactors and ultraviolet lamp sizes over a wide range of hydrodynamic conditions (350 < Re < 11,000). The evaluation showed that the developed CFD model was able to successfully predict the photocatalytic degradation rate of a model pollutant in the analyzed reactors. In terms of hydrodynamic models, the results demonstrated that the laminar model performs well for systems under laminar flow conditions, whereas the Abe‐Kondoh‐Nagano low Reynolds number and the Reynolds stress turbulence models give accurate predictions for photoreactors under transitional or turbulent flow regimes. The performed analysis confirmed that degradation rates of organic contaminants in immobilized photocatalytic reactors are strongly limited by external mass transfer; as a consequence, the degradation prediction capability of the CFD model is largely determined by the external mass transfer prediction performance of the hydrodynamic models used. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Synthesis of model-based controllers for an autothermal reactor

Model-based controllers for a bench scale autothermal tubular packed-bed reactor have been formulated using the Internal Model Control (IMC) approach. The Structural Dominance Analysis technique has been used in developing the reduced-order models. Controller performance at robust and sensitive steady states have been assessed through simulations and experiments. Both PI and model-based controllers can regulate reactor operation at robust steady states, but only third order IMC controllers are able to regulate reactor operation at the sensitive steady state.     

Computational fluid dynamics modeling of rice husk combustion in a fluidised bed combustor

Computational fluid dynamics (CFD) modeling was carried out to determine the trajectories and residence time of burning rice husk particles in the fluidised bed combustor (FBC) at different secondary air flowrates. In FBC, the intra and extra-particle mass transfer resistance of the oxidising agent plays a major role in determining the combustion rate because of high temperature processing. Moreover, factors such as turbulence and retention time determine the reaction rate. In actual combustion experiments, these two factors could not be observed or determined distinctly, thereby hindering any further improvements in operating parameters or combustor design in order to maximise the efficiency of particle combustion. This hitch was solved through the application of (CFD) modeling. The modeling results offered significant insights into the trajectory and mass loss history of the rice husk particle combustion. The actual experimental results also showed agreement with the modeling results.     

C_5热二聚反应器计算流体力学研究

为了了解现有C5装置所用中空管式反应器内的流场是否为反应所期望的平推流,文中结合理论分析及计算流体力学模拟软件ANSYS CFX 14.5,采用SST湍流模型,发现其入口处存在边界层分离及再附现象。分别对在入口处增加挡板、筛板等轴向阻尼内件和旋转进料的径向阻尼内件进行CFX模拟,并引进一个与速度有关的物理量Jk来表征返混的程度。结果表明:轴向阻尼内件形式在减少返混程度作用上效果不理想,甚至有加重反混的现象,在所研究的轴向阻尼内件中,迷宫式进料效果最优。结果同时表明采用径向阻尼内件旋转进料器可以较大幅度减小反应器内返混程度。     

Computational fluid dynamics simulation of a novel bioreactor for sophorolipid production

This paper describes three-dimensional computational fluid dynamics(CFD) simulations of gas–liquid flow in a novel laboratory-scale bioreactor contained dual ventilation-pipe and double sieve-plate bioreactor(DVDSB)used for sophorolipid(SL) production. To evaluate the role of hydrodynamics in reactor design, the comparisons between conventional fed-batch fermenter and DVDSB on the hydrodynamic behavior are predicted by the CFD methods. Important hydrodynamic parameters of the gas–liquid two-phase system such as the liquid phase velocity field, turbulent kinetic energy and volume-averaged overall and time-averaged local gas holdups were simulated and analyzed in detail. The numerical results were also validated by experimental measurements of overall gas holdups. The yield of sophorolipids was significantly improved to 484 g·L~(-1)with a 320 h fermentation period in the new reactor.