基于金属氢化物高温蓄热的氢热耦合传递机理

目前大多数可再生能源如太阳能具有间歇性和不稳定性的问题,因此高效蓄热技术成为了发展太阳能的一个关键途径。金属氢化物高温蓄热技术作为热化学蓄热中最有前途的方法之一,受到了人们的广泛关注。为了实现金属氢化物高温蓄热技术的工程应用,明确其氢热耦合传递机理至关重要。本研究采用数值模拟的方法,通过建立反应器的多物理场耦合模型,讨论了不同时刻下床层内部参数的分布,得到了反应锋面的形成和移动机理以及非均匀反应的形成机理;此外,结合反应器内部氢压、接触热阻和床层热阻的变化规律,明确了不同阶段下金属氢化物高温蓄热技术的控制环节;最后,依据金属氢化物高温蓄热技术的工程应用挑战,提出了相应的研究策略。     

耦合相变储热的金属氢化物反应器吸氢过程模拟

基于金属氢化物储氢反应,建立了相变材料蓄热的固体储氢反应器模型,模拟研究了吸氢压力等操作参数及相变材料的相变温度、固(液)态导热系数、相变潜热等物性参数对固体储氢反应器工作过程的影响. 结果表明,相变材料的固态导热系数和相变潜热对固体储氢反应器性能的影响较小,相变温度和液态导热系数对反应器性能影响较大. 相变温度越低,液态导热系数越大,储氢反应器性能越好. 在使用最优的相变材料储能时,提高充入氢气的压力可加快反应速率,强化相变材料的传热,有助于进一步优化反应器的储氢性能.     

Numerical analysis of hydrogen absorption in a P/M metal bed

In a porous metal bed, two-dimensional heat and mass transfer are analyzed for hydrogen absorption in the cylindrical coordinates. For high cooling rates an annular geometry is selected for the porous metal bed, and the bed is cooled by a fluid on both the internal and the external surfaces. The absorption process is analyzed numerically for the porous LaNi₅ P/M metal bed. Variations of the metal hydride temperature and hydrogen/metal atomic ratio are calculated in the radial and axial directions using a computational fluid dynamics (CFD) program. It is observed that the hydride formation takes places near the cold boundaries. The results are compared with the numerical results given in the literature for different geometries.     

A comparison of conventional and optimized thermally coupled reactors for Fischer–Tropsch synthesis in GTL technology

In this research, the conditions at which a thermally coupled reactor – containing the Fischer–Tropsch synthesis reactions and the dehydrogenation of cyclohexane – operates have been optimized using differential evolution (DE) method. The proposed reactor is a heat exchanger reactor consists of two fixed bed of catalysts separated by the tube wall with the ability to transfer the produced heat from the exothermic side to the endothermic side. This system can perform the exothermic Fischer–Tropsch (F–T) reactions and the endothermic reaction of cyclohexane dehydrogenation to benzene simultaneously which can save energy and improve the reactions' thermal efficiency. The objective of the research is to optimize the operating conditions to maximize the gasoline (C₅⁺) production yield in the reactor's outlet as a desired product. The temperature distribution limit along the reactor to prevent the quick deactivation of the catalysts by sintering at both sides has been considered in the optimization process. The optimization results show a desirable progress compared with the conventional single stage reactor. Optimal inlet molar flow rate and inlet temperature of exothermic and endothermic sides and pressure of exothermic side have been calculated within the practicable range of temperature and pressure for both sides.     

基于金属氢化物固态氢源的氢燃料电池动力系统特性的研究

以基于金属氢化物的固态储氢技术，与质子交换膜燃料电池(PEMFC)耦合，搭建了基于金属氢化物固态氢源的氢燃料电池动力系统试验台，测试了吸氢压力、放氢温度、氢流量等关键操作参数对氢燃料电池动力系统性能的影响。结果表明，当吸氢压力大于等于0.60 MPa时，固态储氢反应器放氢流量稳定的时间最长可达4500 s以上。当放氢温度大于60℃时，储氢反应器能完全释放氢气，且放氢时间基本相同。放氢流量越小，氢燃料电池动力系统稳定工作的时间越长。     

硼氢化钠水解制氢技术研究进展

随着石化能源的日益枯竭,氢能成为解决当前能源危机的一种新能源。制氢的方式多种多样,由于金属氢化物在储氢容量上具有其他材料无法比拟的优势,因此,金属氢化物制氢技术得到了迅速发展。硼氢化钠就是一种典型的金属氢化物,硼氢化钠水解制氢技术作为一种安全、方便的新型制氢技术,已成为当前燃料电池氢源研究中的热点之一。介绍了硼氢化钠制氢原理;综述了硼氢化钠水解制氢技术的优点、影响产氢速率的因素;对硼氢化钠制氢技术的装置进行了举例说明;指出了目前此技术所存在的问题;概述了此技术的应用与发展前景。     

The electrochemical impedance of metal hydride electrodes

The electrochemical impedance responses for different laboratory type metal hydride electrodes were successfully modeled and fitted to experimental data for AB₅ type hydrogen storage alloys as well as one MgNi type electrode. The models fitted the experimental data remarkably well. Several AC equivalent circuits have been proposed in the literature. The experimental data, however, could not always be satisfactorily approximated. The approximation model presented here exhibits smooth fit to the experimental results for all frequencies in the whole range from 10 kHz to 0.1 mHz. Equivalent circuits, explaining the experimental impedances in a wide frequency range for electrodes of hydride forming materials mixed with copper powder, were obtained. Both charge transfer and spherical diffusion of hydrogen in the particles are important sub processes that govern the total rate of the electrochemical hydrogen absorption/desorption reaction. To approximate the experimental data, equations describing the current distribution in porous electrodes were needed. Indications of one or more parallel reduction/oxidation processes competing with the electrochemical hydrogen absorption/desorption reaction were observed. The impedance analysis was found to be an efficient method for characterizing metal hydride electrodes in situ.     

Optimal design of a thermally coupled fluidised bed heat exchanger reactor for hydrogen production and octane improvement in the catalytic naphtha reformers

The present study combines simultaneously the definition of fluidisation and process intensification (thermally coupled heat exchanger reactor) concept and determines the optimum operational conditions in both sides of the reactor, using Differential Evolution (DE) optimisation approach. The exothermic hydrogenation of nitrobenzene to aniline takes place in a set of tubular reactors which is placed inside the naphtha reactors and thermally handle the endothermic reaction of reforming. A single objective function consists of four terms including aromatic mole fraction of the reformate and hydrogen production from each reactor in the endothermic side as well as the total molar flow rate of aniline and nitrobenzene conversion in the exothermic side is defined. Seven decision variables such as inlet temperature of exothermic and endothermic sides, exothermic molar flow rates for the first and the second reactors and the number of tubes are considered during the optimisation procedure. Temperature constraints have been considered in both sides during the optimisation in order to reduce the possibility of rapid catalyst deactivation by sintering. Results show approximately 464.4 and 598.9 kg/h increase in aromatic and aniline production rates in optimised thermally coupled fluidised bed naphtha reactor (OTCFBNR) compared with non‐optimised case (TCFBNR), respectively. Such a theoretical study is necessary prior to designing new pilot plants and revamping industrial units. © 2011 Canadian Society for Chemical Engineering     

CFD Modeling of Reactor Concepts to Avoid Carbon Deposition in Pyrolysis Reactions

Methane pyrolysis in an externally heated tubular reactor inevitably ends with clogging of the reactor. Thin molten metal wall films protect the walls from carbon depositions. A falling film reactor and a rotating film reactor are investigated by CFD simulation. The results show the considerable advantages of a rotating film reactor compared to the vertical film reactor. An alternative route for carbon production from methane is the implementation of an exothermic chlorination reaction. The tubular reactor concept involves the inflow of inert gas at the reactant inlet and through porous walls to ensure that the reaction takes place in the center, thus, largely reducing carbon deposits.     

新型储氢材料研究进展

氢能作为一种环保可再生的新型能源,生产技术逐渐走向成熟,成本大幅度下降,将迎来快速发展的机遇期。氢能被广泛利用的关键在于是否能够实现高效储存。本文重点讨论了四类新型储氢材料,即金属络合氢化物储氢材料、碳纳米管储氢材料、沸石以及新型沸石类材料、有机液态储氢材料。文章指出：金属络合氢化物储氢材料储存压力低但循环稳定性差;碳纳米管储氢材料已经有很长的发展历史,安全性高且易脱氢,然而目前对其储氢机理认识不够成熟;沸石以及新型沸石类材料价格低廉,但是对反应条件的要求高;有机液态储氢材料被认为是大规模储存和运输的可行选择,然而昂贵的成本和苛刻的反应条件限制了其发展。文章指出后续需要改进并开发具有较高存储容量和具有经济价值的储氢材料。     

A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst.Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times.In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this ‘back-conversion’ can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855-872.     

Thermal modeling of activated carbon based adsorptive natural gas storage system

A homogeneous, isotropic porous matrix of activated carbon inside a portable steel cylinder is considered as the adsorption bed for natural gas (NG) which is idealized as pure methane for the purpose of simulation. The heat and fluid flow inside the porous adsorption bed are modeled using a volume averaging technique and Darcy-Brinkman formulation. The effective thermal conductivity of the activated carbon-methane system is calculated as a function of uptake according to the Luikov model. Heat generation due to the exothermic process of adsorption is considered. The governing equations are solved using an implicit finite volume method for the given boundary conditions. Three different models of adsorption are considered, namely (i) a no-flow model, (ii) flow model with uniform adsorption and (iii) a flow model with local adsorption. For each of these models, transient temperature profiles in the adsorption bed during the charging process are obtained, and the corresponding mass adsorption potentials are calculated. Parametric studies are performed to investigate the effects of gas inlet temperature and rate of charging on the maximum bed temperature and the time required to fill the cylinder.     

储氢材料的研究分析

氢能作为一种可再生能源,为了使其有效的利用的关键技术是开发出安全、经济、便携的存储体系.综述了几种储氢材料,如金属氢化物、配位氢化物、碳基纳米材料以及石墨烯储氢材料,同时简要介绍了石墨烯的几种制备方法及其性质.最后对该领域的研究方向进行了总结和展望.     

耦合传质的羰基化固定床反应器传热模拟分析

固定床反应器中进行强放热反应时, 反应器的热点温度对操作参数变化敏感,容易引起飞温,导致转化率下降,影响催化剂寿命。为强化羰基化固定床反应器内热质传递与化学反应的协同性,建立考虑颗粒内扩散影响的羰基化固定床反应器拟均相一维传热模型,考察操作参数对床层热点温度、反应转化率、床层温升的影响。不仅体现传热传质和反应的协同作用,而且影响关系明晰、求解方便。为保证反应转化率,本实验条件下确定催化剂颗粒直径小于等于1.5 mm。反应器入口温度/冷却剂油温既要满足床层热稳定性需求,又要使反应转化率和床层温升都在合理范围内。模拟结果表明在床层入口温度升高的同时,可通过降低冷却剂油温获得良好的反应转化率和较小的床层温升。在此基础上,考察入口环氧乙烷浓度对反应转化率和床层温升的影响。本研究可为固定床反应器满足转化率要求、床层合理温升而选择催化剂颗粒直径、床层入口温度、冷却剂油温和床层入口浓度等操作参数提供计算依据。     

甲烷催化燃烧反应工艺研究进展

甲烷催化燃烧是一种清洁高效的甲烷燃烧技术,在节能减排中具有重要的应用价值。从催化剂、反应工艺和过程强化等方面对近年来甲烷催化燃烧技术进行综述,重点介绍颗粒催化剂固定床反应工艺、整体式催化剂反应工艺、流化床反应工艺和吸放热耦合反应工艺研究进展。用于固定床反应器的颗粒催化剂主要为负载型贵金属催化剂和非贵金属氧化物催化剂。贵金属催化剂活性好,起燃温度低,适合低浓度甲烷的催化燃烧。非贵金属氧化物催化剂耐高温性好,适合较高浓度甲烷燃烧体系。整体式催化剂的甲烷催化燃烧反应工艺中,最常用的是蜂窝陶瓷和金属合金等整体式催化剂的多段式催化燃烧反应器的设计。设计直接采用多段式整体催化剂,催化剂的位置不同,发挥的催化作用也不同。流化床催化燃烧装置具有燃烧过程接触面积广、热容量大和换热效率高等特点,可有效避免传统的固定床催化燃烧反应工艺存在的问题,非常适合应用于低浓度甲烷的催化燃烧过程。利用甲烷催化燃烧强放热的特点,将催化燃烧产生的热量进行时间或空间的耦合,可以开发出吸-放热耦合反应工艺。其中,固定床催化反应器中的流向变换强制周期操作作为一种高效的过程强化技术,在节约反应器成本的同时,可以提高反应热量的利用率。     

Electrolyte migration through electrochemical membranes: Potential source of error in batch electrochemical cells

Electrochemical membranes (ECMs) and porous electrodes have gained much attention in a broad range of applications including water and wastewater treatment, energy production and storage, and carbon dioxide capture. Lab scale batch experiments (electrochemical stirred cells) are the baseline for developing ECMs and porous electrodes. We observed electrochemical dissolution of metal fasteners (alligator clips), used to hold porous conductive and non-conductive membranes in batch electrochemical cells, despite being kept outside the electrolyte. The electrolyte migrated through the porous membranes by the action of capillary forces, forming a closed electrochemical circuit with the metal fasteners. This unexpected leaching can lead to misleading results for electrochemical experiments on porous electrodes and ECMs. In this study, we compared (1) porous membranes versus non-porous electrodes, (2) hydrophilic versus hydrophobic membranes, and (3) conductive versus non-conductive membranes in their ability to cause capillary wetting-induced corrosion of metal fasteners. We proposed a simple solution for the problem: separating the metal fasteners from the porous membrane electrode with a non-porous conductive graphite foil, which keeps the electrochemical circuit open. We have validated this solution and propose it as a standard method for experiments using porous electrodes and electrically conductive membranes.     

聚合釜传热性能的实验研究及数值模拟

基于CFD模拟与传热实验相结合的方法对5 L夹套聚合釜的传热性能进行研究。建立聚合釜的液固耦合稳态传热模型，获得釜内流体、夹套内流体及金属固体域内温度分布。开展传热实验对模拟结果进行验证，各对比点温度的最大相对误差在1%~5%范围内。通过模拟获得釜内外壁面传热系数及总传热系数，并关联出釜侧及夹套侧 Nu的经验式。结果表明：釜内流体温度分布方差始终在0.002以下，固体域内和传热边界层温度梯度较大，传热边界层厚度约3.8 mm；实验范围内，入口温度和反应放热量对釜内温度的影响显著，入口流速次之，搅拌转速影响最弱；夹套侧传热系数远小于釜侧传热系数，提高夹套侧传热系数是提升传热性能的关键；实验用聚合釜外表面散热量与内外温差呈正比，比例系数约为3.031 W·K ^-1。     

Activated carbon fibres and composites on its base for high performance hydrogen storage system

To obtain high hydrogen sorption capacity and reduce the time of the cycle (adsorption/desorption) in the gas storage system a new composite material (metal hydride particles on the activated carbon fibre matrix) was suggested. Different adsorbent materials such as activated carbon fibre “Busofit”, granular activated carbon and new composite sorbent (metal hydride La_0.5Ni₅Ce_0.5 particles on the activated carbon fibre “Busofit”) were tested. Effect of the carbon sorbent nature and metal hydride content is important to choose the optimal sorbent bed.In this paper, a thermally regulated storage system for hydrogen was numerically analyzed and experimentally validated. A two-dimensional transient model was used to analyze the influence of the thermal control on the operating characteristics of the flat sectional vessel. The evolution of the temperature, pressure and volumetric density of hydrogen inside the vessel during the charging/discharging is discussed. It was shown that heat pipe based thermal control of the process increase the efficiency of the hydrogen storage. Such vessels are interesting to be applied in fuel cells used for vehicle or dual-fuel engine car (hydrogen/gasoline, hydrogen/methane).     

SIMULATION OF HYDRATION/DEHYDRATION OF CaO/Ca(OH)2 CHEMICAL HEAT PUMP REACTOR FOR COLD/HOT HEAT GENERATION

ABSTRACT

A chemical heat pump (CHP) utilizes reversible reactions involving significant endothermic and exothermic heats of reaction in order to develop a heat pump effect by storing and releasing energy while transforming it from chemical to thermal energy and vice versa. In this paper, we present a mathematical model and its numerical solution for the heat and mass transport phenomena occurring in the reactant particle bed of the CHP for heat storage and cold/hot heat generation based on the CaO/Ca(OH)₂ reversible hydration/dehydration reaction

Transient conservation equations of mass and energy transport including chemical kinetics are solved numerically subject to appropriate boundary and initial conditions to examine the influence of the mass transfer resistance on the overall performance of this CHP configuration. These results are presented and discussed with the aim of enhancing the CHP performance in next generation reactor designs.     

Simulation of activity loss of fixed bed catalytic reactor of MTO conversion using percolation theory

In this investigation, a reactor model for prediction of the deactivation behavior of MTO's porous catalyst in a fixed bed reactor is developed. Effect of coking on molecular transport in the porous structure of SAPO-34 has been simulated using the percolation theory. Thermal effects of the reaction were considered in the model and the temperature profile of the gas stream in the reactor was predicted. The predicted loss in catalyst activity with time-on-stream was in very good agreement with the experimental data. The resulting coke deposition and gas temperature profiles along the length of reactor suggested a reaction front moving toward the outlet of the fixed bed reactor at the operating experimental conditions of 1 h⁻¹ and 723 K for methanol space velocity and inlet temperature, respectively. Effects of space time, coordination of Bethe network, and effective diffusivity of component in reaction mixture on the reactor performance are presented.