Prediction of pyrolysis of pistachio shells based on its components hemicellulose, cellulose and lignin

The objective of this contribution is to describe thermal degradation of pistachio shells by a detailed reaction mechanism. Pistachio shells are assumed to be composed of hemicellulose, cellulose and lignin of which degradation is described by relevant kinetics based on experimental data. The mechanism yields a detailed composition of product gases, and therefore, is well-suited to predict evolution of both thermal decomposition and products. Thermal degradation is described by a system of coupled differential conservation equations for mass, momentum, species, and energy for a pistachio shell particle. The relevant conservation equations are discretised by the Finite Volume Method (FVM) and solved within the conversion module of the Discrete Particle Method (DPM). A comparison between experimental data and predicted results yielded good agreement.     

Performance of integration schemes in discrete element simulations of particle systems involving consecutive contacts

In this investigation, which is a follow-up study extending earlier work (Kruggel-Emden, Sturm, Wirtz, & Scherer, 2008), a realistic assessment of the performance of integration schemes in systems of moving particles and consecutive contacts is conducted. Linear contact models are applied throughout this work as they allow for an analytical solution of consecutive oblique impacts. The many-particle systems considered are the discharge of particles from a hopper and particle movement in a shaken container. Results for many-particle systems are robust with respect to the applied integration method and step size once particle interactions are resolved with a sufficient number of steps. The integration schemes are also evaluated based on consecutive particle/wall contacts. Integration of consecutive contacts in a discrete element framework implies repeatedly solving non-continuous systems of differential equations. Various termination conditions for the normal force models and adaptive time stepping for one-step integration methods are investigated. The effect of softened contacts on particle trajectories is discussed. Based on these insights, recommendations for the most accurate integration schemes are made.     

Characterization of solids mixing patterns in bubbling fluidized beds

Behavior of the solid phase in fluidized beds was studied by a 2D CFD-DEM approach to obtain more information on the solid mixing and circulation. Hydrodynamic parameters, including solid diffusivity, and internal and gross circulations were considered in this study. To validate the simulation, time-position data obtained by the Radioactive Particle Tracking (RPT) technique were used. It was shown that the 2D model can satisfactorily predict the axial diffusivity, while the radial diffusivity calculated based on the model is an order of magnitude smaller than the experimental one in 3D. The influence of aspect ratio of the bed, type of distributor, and inlet gas velocity on solids mixing pattern were also studied. The solids flow pattern in the bed changed considerably by increasing the aspect ratio. Different solid circulations were captured by numerical model for the two types of distributors, namely porous and injection types. The results suggested that increasing the superficial gas velocity caused rigorous internal and gross circulations, which in return, improved solids mixing and decreased deviations from well mixed state.     

Shock waves in granular materials: Discrete and continuum comparisons

Small amplitude compression and shock waves in granular materials were examined from the point of view of an analytical model and discrete element simulations. The barotropic behaviour of granular materials was discussed in terms of the mechanisms behind the formation of shock fronts. This discussion leads to the development of a one dimensional continuum model which was used together with the method of characteristics to describe the nature of shock waves. The model provides a relationship between the states of the material on either side of a shock and an equation that defines the velocity of shock waves.Discrete Element Modelling was used to demonstrate the shock forming process in granular materials and to confirm the predictions of the analytical model. During this process it is demonstrated that an assembly based on a linear contact model does not show any barotropy and consequently cannot describe dynamics of granular materials in terms of shock forming mechanisms. This observation may have important consequences in the application of the linear contact model to dynamic systems. The Hertz-Mindlin contact model did not suffer from this issue and was able to demonstrate both barotropy and shock formation.The DEM assemblies were characterised in terms of the material properties of the analytical model which allowed direct comparisons to be made. In all respects the analytical model performed well, predicting the change in state of the material and the shock wave speed with a good deal of accuracy.     

Investigation of wall stress and outflow rate in a flat-bottomed bin: A comparison of the DEM model results with the experimental measurements

The present paper provides a discrete element method (DEM) analysis of the filling and discharge processes of granular material in a 3D flat-bottomed bin. A granular aggregate of nearly round particles (20,400 pea grains, 7.2-7.8 mm in diameter) is considered. The numerical results are compared with the experimental data. The DEM analysis provides an accurate prediction of wall stress distribution and the outflow rate of discharge throughout the bottom orifice. The stress distribution developed within the granular material after filling and during the discharge phase is considered, and the transition from the active to passive stress state is discussed. This analysis aims to quantitatively predict the flow parameters related to the careful identification of the material parameters. The investigation presented may be useful for the ongoing development of DEM.     

Role of inertial lift in elutriating particles according to their density

A Reflux Classifier involves the liquid fluidization of particles into a set of parallel inclined channels. Closely spaced inclined channels promote a combination of laminar flow and a high shear rate, which in turn promote the elutriation of the particles according to their density. The hydrodynamics of the particle transport within the inclined channels was examined theoretically by combining established equations for describing the fluid flow, the terminal velocity of a particle, and the shear induced inertial lift, with no adjustable parameters. The theoretical calculations provided excellent agreement with a comprehensive experimental data set, demonstrating the significance of the inertial lift force that arises at a high shear rate under the condition of laminar flow. Complex features of the experimental data were described theoretically. This work explains how it is possible to elutriate particles according to their density, with the effects of particle size suppressed. A remarkable convergence of several criteria was found to be necessary for achieving the reported phenomena.     

Discrete particle simulation of gas fluidization of ellipsoidal particles

Fluidization is widely used in industries and has been extensively studied, either experimentally or theoretically, in the past decades. In recent years, a coupled simulation approach of discrete element method (DEM) and computational fluid dynamics (CFD) has been successfully developed to study the gas–solid flow and heat transfer in fluidization at a particle scale. However, to date, such studies mainly deal with spherical particles. The effect of particle shape on fluidization is recognized but not properly quantified. In this paper, the CFD–DEM approach is extended to consider the fluidization of ellipsoidal particles. In the simulation, particles used are either oblate or prolate, with aspect ratios varying from very flat (aspect ratio=0.25) to elongated (aspect ratio=3.5), representing cylinder-type and disk-type shaped particles, respectively. The commonly used correlations to determine the fluid drag force acting on a non-spherical particle are compared first. Then the model is verified in terms of solid flow patterns. The effect of aspect ratio on the flow pattern, the relationship between pressure drop and gas superficial velocity, and microscopic parameters such as coordination number, particle orientation and force structure are investigated. It is shown that particle shape affects bed permeability and the minimum fluidization velocity significantly. The coordination number generally increases with aspect ratio deviating from 1.0. The analysis of particle orientations shows that the bed structures for ellipsoids are not random as that for spheres. Oblate particles prefer facing upward or downward while prolate particles prefer horizontal orientation. Spheres have the largest particle–particle contact force and fluid drag force under the comparable conditions. With aspect ratio deviating from 1.0, particle–particle interaction and fluid drag become relatively weak. The proposed model shows a promising method in examining the effect of particle shape on different flow behaviour in gas fluidization.     

直流输电系统离散模型拓扑分块法数字仿真

本文以离散化模型为基础,提出了一种直流输电系统数字仿真的新方法－－分块交接变量方程法。该方法圆满地解决了应用拓扑分块技术仿真直流输电系统时遇到的如何确定各分块交接变量的难题，并且非常易于实现并行计算，特别适合于大规模、多端交直流互联系统的数字仿真。     

影响预分解窑窑尾物料填充率的因素分析

确定合理的物料填充率是稳定窑内热工制度的前提,窑尾物料填充率过高会导致窑尾漏料事故的发生.引起窑尾物料填充率变化的因素主要有入窑物料量、窑的转速、窑尾温度、预分解系统温度以及窑皮的长度和厚度、结圈等.生料易烧性发生变化,窑头用煤量过多或不完全燃烧,燃烧器使用不当等都可能导致窑皮长度和厚度的变化.     

往复式 CO2压缩机五段入口温度的优化

分析了往复式CO2压缩机五段入口温度控制难度较大的原因;采取了定期清洗压缩机酸排、增大脱硫槽容积、调整压缩机四段冷却器用水流程等优化措施.改造效果表明,CO2压缩机五段入口温度的波动范围为±2℃,达到指标要求.