Theoretical comparison of packed bed and fluidized bed membrane reactors for methane reforming

In this theoretical work the performance of different membrane reactor concepts, both fluidized bed and packed bed membrane reactors, has been compared for ultra-pure hydrogen production via methane reforming. Using detailed theoretical models, the required membrane area to reach a given conversion and the prevailing temperature profiles have been compared. The extent of mass and heat transfer limitations in the different reactors has been evaluated, and strategies to decrease (or avoid) these limitations have been proposed for the fluidized bed membrane reactor concept. The results show that the packed bed membrane reactor requires in some conditions double membrane area with respect to the fluidized bed membrane reactor.     

Mathematical model for gas–liquid two-phase flow and biodegradation of a low concentration volatile organic compound (VOC) in a trickling biofilter

A theoretical model is established for predicting the biodegradation of a low concentration volatile organic compound (VOC) in a trickling biofilter. To facilitate the analysis, the packed bed is simplified to a series of straight capillary tubes covered by the biofilm in which the liquid film flow on the surface of biofilm and the gas core flow in the center of tube. The theoretical formulas to calculate liquid film thickness in the capillary tube are obtained by simultaneously solving a set of hydrodynamic equations representing the momentum transport behaviors of the gas–liquid two-phase flow under co-current flow and counter-current flow. Subsequently, the mass transport equations are respectively established for the gas core, liquid film, and biofilm with considering the mass transport resistance in the liquid film and biofilm, the biochemical reaction in the biofilm, and the limitation of oxygen to biochemical reaction. Meanwhile, the surface area of mass transport in the capillary tube is modified by introducing the active biofilm surface area, namely the specific wetted surface area available for biofilm formation. The predicted purification efficiencies of VOC waste gas are found to be in good agreement with the experimental data for the trickling biofilters packed with ?8 mm, ?18 mm, and ?25 mm ceramic spheres under the gas–liquid co-current flow mode and counter-current flow mode. It has been revealed that for a fixed inlet concentration of toluene, the purification efficiency of VOC waste gas decreases with the increase in the gas and liquid flow rate, and increases with the increase in the specific area of packed materials and the height of packed bed. Additionally, it is found that there is an optimal porosity of packed bed corresponding to the maximal purification efficiency.     

Interfacial thermodynamics and X-ray diffraction of hydrolysis products in multiphase reacting flow of the Cu–Cl cycle

This paper examines the chemical equilibrium and gaseous product fraction of a multiphase gas–solid flow involving hydrolysis of copper (II) chloride and steam in a packed bed reactor. The effectiveness of the water splitting process has a significant impact on the efficiency of the thermochemical copper–chlorine (Cu–Cl) cycle for hydrogen production. A thermodynamic analysis of the HCl fraction in the gaseous effluent is presented, along with a predictive model of solid particle conversion based on the reaction temperature and pressure. Experimental results are presented for a horizontal packed bed reactor which exposes a low steam flow rate to a large volume of solid CuCl₂ in the packed bed reactor. The predicted and experimental results demonstrate that an HCl fraction above 0.3 can be achieved within the hydrolysis reactor, thus allowing effective integration between hydrolysis and downstream electrolytic hydrogen production steps in the Cu–Cl cycle, without costly HCl/steam separation processes.     

In situ visualization of biofilm formation in a microchannel for a microfluidic microbial fuel cell anode

A novel in situ approach is proposed to visualize biofilm formation in the microchannel for the microfluidic microbial fuel cell (MMFC) anode, which could reflect a more precise biofilm formation during start-up process in real-time. A microchannel reactor was designed and fabricated based on a transparent indium-tin-oxide (ITO) conductive membrane. In situ visualization of biofilm formation under various anolyte flow rates was captured by a phase contrast microscope combined with a custom long working distance objective. The results show that no steady biofilm is formed on the surface of anode under low flow rate of 50 μL min^?1 because of the insufficient nutrient supply. With increasing the anolyte flow rate, more attached bacteria on the anode surface and denser biofilm are observed in the microchannel. Less bacteria are attached on the surface of anode along flow direction due to the entrance effect. However, denser biofilm leads to larger mass transfer resistance of the anolyte and product in biofilm. Therefore, a superior bioelectrochemical performance is yielded for the biofilm formed under a moderate flow rate during start-up process.     

Effect of hydraulic retention time on a continuous biohydrogen production in a packed bed biofilm reactor with recirculation flow of the liquid phase

The present paper reports on results obtained from experiments carried out in a laboratory-scale anaerobic packed bed biofilm reactor (APBR), with recirculation of the liquid phase, for continuously biohydrogen production via dark fermentation. The reactor was filled with Kaldnes^® biofilm carrier and inoculated with an anaerobic mesophilic sludge from a urban wastewater treatment plant (WWTP). The APBR was operated at a temperature of 37 °C, without pH buffering. The effect of theoretical hydraulic retention time (HRT) from 1 to 5 h on hydrogen yield (HY), hydrogen production rate (HPR), substrate conversion and metabolic pathways was investigated. This study indicates the possibility of enhancing hydrogen production by using APBR with recirculation flow. Among respondents values of HRT the highest average values of HY (2.35 mol H₂/mol substrate) and HPR (0.085 L h^?1L^?1) have been obtained at HRT equal to 2 h.     

Evaluation of process performance on biohydrogen production in continuous fixed bed reactor (C-FBR) using acid algae hydrolysate (AAH) as feedstock

In this study, a novel inoculation method to mitigate the inhibition of 5-hydroxymethylfurfural (5-HMF) is proposed. Acid algae hydrolysate containing 1.5 g 5-HMF/L and 15 g hexose/L hexose was fed to a continuous fixed bed reactor (C-FBR) partially packed with hybrid-immobilized beads. The inoculation method enabled a high rate of H₂ production, due to the reduction of 5-HMF inhibition and enhanced biofilm formation. Maximum hydrogen production was achieved at a hydraulic retention time of 6 h with a hydrogen production rate (HPR) of 20.0 ± 3.3 L H₂/L-d and a hydrogen yield (HY) of 2.3 ± 0.4 mol H₂/mol hexose _added. Butyrate and acetate were the major soluble metabolic products released during fermentation. Quantitative real-time polymerase chain reaction analysis revealed that Clostridium butyricum comprised 94.3% of the total bacteria, which was attributed to the high rate of biohydrogen production.     

Influence of CuO/ZnO/Al2O3 wash-coating slurries on the steam reforming reaction of methanol

A micro-channel reactor for methanol steam reforming is a candidate to supply hydrogen on-site to fuel cells. Micro-channel beds wash-coated with poor quality slurry formulations lead to poorer performance than packed beds using pellet catalysts. This study explored the morphology, X-ray Diffraction (XRD) spectrum, BET surface area and activity of wash-coating catalyst layers from a series of catalyst slurries. All catalyst slurries were prepared from the commercial MDC-3 catalyst. Hydrogen production using wash-coating catalyst layers was performed under packed bed conditions. The results reveal that the solubility level of the MDC-3 catalyst during the slurry preparation process affected the activity of methanol steam reforming. It is difficult to reconstruct the original fine structure, as the MDC-3 catalyst had a higher solubility status after slurry preparation. The volume of the micro-channel catalyst bed was approximately 0.3 cm³. It can supply hydrogen to fuel cells that can produce approximately 8 W with 80% H₂ utilization and 60% fuel cell efficiency.     

Performance of gas-phase photocatalytic reactors on hydrogen production

Three types of high-performance photocatalytic reactors were developed for gas-phase photocatalytic hydrogen (H₂) production from hydrogen sulphide (H₂S) and effective photocatalytic decomposition of gaseous H₂S at a very low concentration is investigated. In this paper, three lab-scale photocatalytic reactors viz., packed bed photocatalytic reactor, catalyst coated fixed bed photocatalytic reactor and catalyst dispersed photocatalytic reactors were developed to study the performance of reactors on hydrogen production. The novel photocatalyst (CdS + ZnS)/Fe₂O₃ and the optimized catalyst dosage, H₂S gas flow rate, pollutant concentration, light irradiations were used. The experimental result indicates that packed bed photocatalytic reactor can effectively splits the H₂S into hydrogen (i.e. 98%) and rapidly decompose H₂S toward zero concentration than the other two reactors. Hence the bench-scale photocatalytic reactor was fabricated in packed bed reactor and the maximum hydrogen conversion achieved from hydrogen sulphide was found to be 98%.     

Classification of combustion regimes in a packed bed of particles based on the relevant time and length scales

This paper identifies distinctive regimes for the combustion of particles in a packed bed. The study is based on the relevant time and length scales for combustion and transport in both the solid fuel and the gas phase within the voids in a packed bed. Characterizing the combustion of a particle and a packed bed by the dimensionless groups, the Damköhler number, and the Thiele modulus, four different combustion regimes are identified. Two of them resemble the well-stirred reactor with uniform reaction throughout the packed bed, whereas the particles may follow the kinetically or transport-limited reaction path. For the remaining regimes a conversion front propagates with its characteristic velocity through the packed bed, independent of whether the combustion of particles within the conversion front follows the reacting or the shrinking core mode.     

Effect of surface modification of anode with surfactant on the performance of microbial fuel cell

Surface modification of anode using surfactant has great influence on the electrical performance of a microbial fuel cell (MFC). In this study, the effect of surface‐modified exfoliated graphite used for anode fabrication on a cube‐type MFC batch reactor was examined. The surface exfoliated graphite was modified with 5‐mM anionic surfactant, sodium dodecyl sulfate. Anaerobic sludge used as inoculum containing 70% (v/v) of artificial wastewater and 30% (v/v) of seed sludge in an anode chamber and air cathode was used in cathode side. Anode modification was explored as an approach to enhance the start‐up and improve the performance of the reactor. Scanning electron microscopy was used to evaluate the morphology and activity of electrochemically active bacteria. In the study, the start‐up time of MFC required to approach stable voltage was substantially reduced, and the maximum stable voltage was higher than the control. In addition, the activation resistance of the MFC was considerably reduced, and the maximum power density (1640 mW/m²) was 20% higher than control. However, when the surface of exfoliated graphite was modified with over 10‐mM anionic surfactant, some negative effects on start‐up time, activation resistance and maximum power density were observed. This modification also enhanced the bacterial attachment and biofilm formation on the modified anode surface. The result suggested that surface modification anode with surfactant is effective for electrical responses achieved in the MFC. Copyright © 2015 John Wiley & Sons, Ltd.     

Natural gas steam reforming for hydrogen production over metal monolith catalyst with efficient heat-transfer

Heat transfer performance of the natural gas (NG) steam reforming in a reactor bed with metal monolith catalyst has been evaluated in comparison with that in the conventional packed bed with pellet catalysts. 2%Ru/Al₂O₃ catalyst with high intrinsic activity has been wash-coated on metal monolith substrates or used as it was for the packed bed application. The prepared metal monolith catalyst has been applied for NG steam reforming to increase heat-transfer efficiency. Under the same degree of temperature gradient from the furnace wall to the catalyst bed, the heat flux obtained in the monolithic bed reactor was about twice higher than that in the packed bed reactor. Maximum heat transfer coefficient achieved in this study for the former was 0.65 kW/m² K, while that for the latter was 0.3 kW/m² K. This is mainly due to enhanced heat-transfer via metal monolith catalyst.     

Comparison of conventional and spherical reactor for the industrial auto-thermal reforming of methane to maximize synthesis gas and minimize CO2

Auto-thermal reforming (ATR), a combination of exothermic partial oxidation and endothermic steam reforming of methane, is an important process to produce syngas for petrochemical industries. In a commercial ATR unit, tubular fixed bed reactors are typically used. Pressure drop across the tube, high manufacturing costs, and low production capacity are some disadvantages of these reactors. The main propose of this study is to offer an optimized radial flow, spherical packed bed reactor as a promising alternative for overcoming the drawbacks of conventional tubular reactors. In the current research, a one dimensional pseudo-homogeneous model based on mass, energy, and momentum balances is applied to simulate the performance of packed-bed reactors for the production of syngas in both tubular and spherical reactors. In the optimization section, the proposed work explores optimal values of various decision variables that simultaneously maximize outlet molar flow rate of H₂, CO and minimize molar flow rate of CO₂ from novel spherical reactor. The multi-objective model is transformed to a single objective optimization problem by weighted sum method and the single optimum point is found by using genetic algorithm. The optimization results show that the pressure drop in the spherical reactor is negligible in comparison to that of the conventional tubular reactor. Therefore, it is inferred that the spherical reactor can operate with much higher feed flow rate, more catalyst loading, and smaller catalyst particles.     

Performance simulation and experimental confirmation of a mini-channel metal hydrides reactor

A theoretical and experimental study about a proposed mini-channel reactor was carried out to enhance heat transfer performance for metal hydrides applications, such as hydrogen storage, hydrogen compression and chemical heat pumps. The configuration of the reactor and working principles are described in detail. The predicted hydride bed temperature profiles in the reactor are compared with the experimental data from the performance test system, and a reasonable agreement is observed. The simulation of the hydrogen adsorption and desorption processes in a mini-channel reactor packed with LaNi₅ is conducted, and the influences of some important parameters, e.g. the bed thickness, the number of the mini-channels, hydrogen supply and discharge pressure are analyzed. Comparing with the traditional reactors, such as tubular reactor and disc reactor, the mini-channel reactor has some obvious advantages, therefore can be recommended for applications.     

Hydrogen production by immobilized Rhodopseudomonas palustris in packed or fluidized bed photobioreactor systems

The production of biohydrogen via photofermentation has been shown to have a low environmental impact and can often be integrated into wastewater treatment systems. However, currently, photofermentation has low production rates in comparison to industrial hydrogen production processes, and therefore requires improvement. One route for enhancing hydrogen productivity is the development of improved photobioreactor (PBR) systems. The aim of this study was to compare the hydrogen productivity of Rhodopseudomonas palustris under planktonic, and immobilized cell conditions, with the reactor operating either as a packed bed or a fluidized bed. The fluidized bed PBR achieved a maximum specific hydrogen production rate and substrate conversion efficiency of 15.74 ± 2.2 mL/g/h and 43% respectively, outperforming the conventional planktonic culture and the packed bed PBR. This work demonstrates a significant improvement in productivity over planktonic photofermentation, as well as demonstrating the use of immobilized cells under reactor conditions not usually associated with photosynthetic systems.     

CFD simulation and experimental study for two-phase flow through the trickle bed reactors, sock and dense loaded by trilobe catalysts

In this study single and two phase flow through the trickle bed reactor loaded with a two different loading manner, sock and dense, have been investigated numerically and experimentally. The CT-scan imaging and an image processing code have been used for investigation of radial porosity distribution of trilobe catalyst in sock and dense loading procedure in trickle bed reactors and two different correlations have been proposed. These correlations were used in a single and two phase CFD code for prediction of pressure drop of gas flow in dry and prewet trilobe catalyst packed bed and also pressure drop and dynamic liquid holdup for two phase flow. In addition, these variables were studied experimentally with a laboratory scale trickle bed reactor. The results of CFD simulations show a very good agreement with experimental data.     

Numerical simulations of HI decomposition in packed bed membrane reactors

Numerical simulations to develop an understanding of transport processes inside PBMRs (packed bed membrane reactors) and to evaluate effectiveness of PBMRs in increasing the conversion of HI (hydrogen iodide) decomposition reaction of IS (iodine–sulfur) thermochemical cycle are reported. The computational approach used in the simulations has been validated using the data reported for HI decomposition in a packed bed reactor (PBR). The validated computational approach has been used for parametric studies. Effects of different parameters (temperature, pressure, space velocity, membrane permeability, permselectivity, packed bed porosity and reactor diameter) on HI conversion are reported. The parameters having the maximum impact on the conversion are identified. The findings show that using a PBMR instead of a PBR leads to significant enhancement in conversion and the parameters having high impact on conversion are wall temperature, feed temperature, reactor diameter and packed bed porosity. Based on the findings of parametric studies, ranges of the parameters having maximum impact on conversion are suggested, e.g. the reactor wall temperature is recommended to be in the range of 690–700 K, the bed porosity is recommended to be in the range of 0.2–0.4.     

渣油加氢技术进展

在重质原油加工利用上,加氢路线较脱碳路线的重油转化深度高、资源利用率高、经济效益好,但反应条件苛刻,流程复杂,能耗与投资比脱碳路线高。随着原油劣质化及日益严格的环保要求,渣油加工选择加氢路线会越来越多。固定床加氢工艺是通过不同床层的不同类型催化剂,对重油中的金属杂原子和硫、氮元素进行脱除以及对重组分进行改质,技术最为成熟。目前移动床加氢技术主要用作固定床工艺的前置反应器系统,是移动床与固定床的组合工艺。沸腾床加氢裂化原料油适应性广,反应器内温度均匀,催化剂可在线加入和排除,运行周期长,传质传热好,渣油转化率高,装置操作灵活。渣油悬浮床加氢裂化在建装置不多,然而其渣油原料转化率和轻油收率都比延迟焦化和沸腾床加氢裂化高得多,工业应用前景乐观。当渣油原料Ni+V小于120μg/g时,固定床渣油加氢是首选;为延长装置运转周期,可在固定床反应器前增加移动床反应器;加工高残炭、高金属含量减压渣油,沸腾床渣油加氢裂化技术是首选;悬浮床渣油加氢裂化为未来加工更重、更高金属含量及残炭的渣油做好了技术准备。     

Biomass purge strategies to control the bacterial community and reactor stability for biohydrogen production from winery wastewater

Hydrogen production is a viable alternative for the valorization of agro-industrial effluents, such as winery wastewater (WW). One limitation of using new substrates is system stability, where high organic loading rates favor high productivities. Using a new reactor configuration of packed bed reactors, different biomass purge strategies were evaluated in parallel reactors to maintain the stability in the long-term feeding WW, along with a community evaluation. A frequently programmed purge of biofilm and suspended biomass (every 7–8 days) resulted in the most stable hydrogen productivity, 930 mL H₂ L^?1d^?1, unlike only purging biofilm or not programing the purge (<650H₂ L^?1d^?1). The long-term stability is explained by a proper balance of the microbial genera in the reactor, Lactobacillus, Pectinatus, and Clostridium. This work proposes an innovative reactor configuration and operation strategy using WW, where a programmed biomass purge helps control the lactate and hydrogen-producing microbial groups.     

光合细菌生物膜制氢反应器的产氢特性

对光合细菌生物膜制氢反应器的产氢性能进行了实验研究,探讨了光照度、光谱和底物浓度对反应器产氢性能的影响.实验结果表明:光合细菌生物膜反应器内最适底物浓度为0.12mol/L,最佳光照度为5000 lx,过高或过低的底物浓度和光照度均对光合细菌产氢具有抑制作用;而光的波长对光合细菌产氢的最适底物浓度和最佳光照度均无影响.当底物浓度为0.12mol/L,光照度为5000 lx,波长为590 nm时,反应器的产氢率达到最高,为3.54mg/h.     

CFD modeling and experimental validation of sulfur trioxide decomposition in bayonet type heat exchanger and chemical decomposer for different packed bed designs

The growth of global energy demand during the 21st century, combined with the necessity to master greenhouse gas emissions has lead to the introduction of a new and universal energy carrier: hydrogen. The Department of Energy (DOE) Nuclear Hydrogen Initiative was investigating thermochemical cycles for hydrogen production using high-temperature heat exchangers. In this study a three-dimensional computational model of high-temperature heat exchanger and decomposer for decomposition of sulfur trioxide by the sulfur–iodine thermochemical water-splitting cycle with different packed bed designs has been done. The decomposer region of the bayonet heat exchanger also called as silicon carbide integrated decomposer (SID) is designed as the packed bed region. Cylindrical, spherical, cubical and hollow cylindrical pellets have been arranged inside the packed bed. The engineering design of the packed bed was very much influenced by the structure of the packing matrix, which was governed by the shape, dimension and the loading of the constituent particles. Staggered and regular packing methods are used for packing the pellets in the packed bed region. The numerical model is created using GAMBIT and fluid, thermal and chemical analyses were performed using FLUENT. The decomposition percentage of sulfur trioxide is found for the packed bed region with different pellets and the numerical results obtained is compared with the experimental results. A comparison is made for the decomposition percentage of SO₃ for the packed bed approach and the porous media approach.