微型流化床内气体返混

微型流化床基础和应用在近几年受到越来越多的关注。针对微型流化床对气固反应分析的应用要求,利用脉冲示踪法研究了内径10 mm和21 mm两种尺寸微型流化床中的气体返混特性,具体考察了管内径、颗粒静床高度、床料颗粒粒径和气体表观流速对气体返混程度的影响。结果表明:随着床内径、颗粒静床高度和表观气速的减小和床料颗粒粒径的增大,气体在床内的返混程度减小。使用粒径约270 μm粗颗粒时,两种床径的浅层微型流化床中的气体返混程度都较小,对应的Peclet数在27以上,证明了床内气体流动接近平推流,从而为利用微型流化床最小化气体返混对反应测试的影响,获得近本征反应动力学参数提供了流动特性的保障。     

CONTROL AND DYNAMIC ANALYSIS OF A PACKED DISTILLATION COLUMN BASED ON MODELING APPROACHES

In this work, dynamic analysis and control of a packed distillation column have been utilized theoretically and experimentally. In theoretical studies, two types of mathematical models stagewise (Frank model) and partial differential approaches (back-mixing model), were used. Packed distillation uses 1400 mm packing height, and packing type is rashing ring with 20-15 mm diameter. The reboiler was made from a 13 L glass container. Reflux ratio was adjusted by an on-line computer. The system temperature was measured with six thermocouples. For control studies, the reflux ratio and the reboiler heat dutywere chosen as manipulated variables. Perturbation in feed composition was utilized as the disturbance. Decoupling multivariable dynamic matrix control (DDMC) and Nondecoupling multivariable dynamic matrix control (NDMC) of overhead and bottom compositions were applied for control studies. Performance of the control system was tested by using an integral absolute error (IAE) criterion and it was also compared with decoupling multivariable PID control (DPID) and Nondecoupling multivariable PID control (NDPID).     

Control and dynamic analysis of a packed distillation column based on modeling approaches

In this work, dynamic analysis and control of a packed distillation column have been utilized theoretically and experimentally. In theoretical studies, two types of mathematical models stagewise (Frank model) and partial differential approaches (back-mixing model), were used. Packed distillation uses 1400 mm packing height, and packing type is rashing ring with 20-15 mm diameter. The reboiler was made from a 13 L glass container. Reflux ratio was adjusted by an on-line computer. The system temperature was measured with six thermocouples. For control studies, the reflux ratio and the reboiler heat dutywere chosen as manipulated variables. Perturbation in feed composition was utilized as the disturbance. Decoupling multivariable dynamic matrix control (DDMC) and Nondecoupling multivariable dynamic matrix control (NDMC) of overhead and bottom compositions were applied for control studies. Performance of the control system was tested by using an integral absolute error (IAE) criterion and it was also compared with decoupling multivariable PID control (DPID) and Nondecoupling multivariable PID control (NDPID).     

费托合成浆态床速度分布和浓度分布的模拟及其影响规律

提出了浆态床速度分布的一维流体力学模型和合成气液相浓度分布的二维扩散模型,用于描述浆态床液相返混对费托合成反应的影响。模型计算结果与示范工厂数据符合良好。计算表明,液相返混状态对合成气转化率有显著影响。利用模型考察了空塔气速对合成气转化率和空时产率的影响规律,随气速增加,合成气转化率单调下降,空时产率先增大后缓慢降低;塔径放大后,中心液速和液相循环流率都增大,浆料返混加剧,费托合成反应转化率降低。     

返混对气-固反应特性测试和活化能表征的影响

采用冷态研究和高温反应相结合的方法,探究了气-固反应过程中返混对反应特性和动力学的影响。首先利用脉冲示踪法考察微型流化床内的气体停留时间分布规律和气体返混特性,揭示床内径D、表观气速U_g、介质颗粒粒径d_p对床内气体返混程度的影响,并分析气体流动偏离平推流的程度,得到其最大程度接近平推流的操作范围。进而,选取活性焦燃烧这一典型气-固反应,分析微型流化床反应分析仪中不同程度的返混状态下等温燃烧的反应行为和活化能演变,再现了D、U_g、d_p对反应测试结果的影响,即：随着床径减小及表观气速和介质颗粒粒径的增大,反应器内产物气体经历的返混程度减小,使得产物气体近平推流的输出并被即时检测,获得更好揭示反应本质特性的反应活化能。活化能数值随返混程度的减少而增大,且更易达到稳定反应状态。     

Implementation of Ethanol Dehydration Using Dividing-Wall Heterogeneous Azeotropic Distillation Column

In this article, the design and optimization procedures of a dividing-wall column for heterogeneous azeotropic distillation (DWC-A) using cyclohexane as an entrainer for ethanol dehydration are investigated. The proposed procedures can detect the optimal values of the design variables and thereby guarantee the minimum energy requirements, which is related to the minimum CO₂ emissions and the lowest total annual cost (TAC). Since ethanol and water form an azeotrope under atmosphere pressure, a conventional heterogeneous azeotropic distillation sequence (CHADS), including an azeotropic column and a recovery column, is usually used to perform the ethanol dehydration process. However, due to high energy requirements and equipment investments of CHADS, the TAC is at a relatively high level. DWC-A can be used to eliminate the condenser of the second column and decrease the degree of back-mixing. Both CHADS and DWC-A are simulated with Aspen Plus®, and the results show that DWC-A has an energy saving of 42.17% and the TAC reduction of 35.18% along with higher thermodynamic efficiency and reduction in greenhouse gas emissions.     

A new approach for scale-up of bubble column reactors

The reactor of choice for the Fischer-Tropsch synthesis is a slurry bubble column. One of the few disadvantages of bubble columns is the difficulties associated with their scale-up. The latter is due to complex phases’ interactions and significant back-mixing.     

Decolorization of textile basic dye in aqueous solution by ozone

In this study, the factors affecting the rate of chemical oxygen demand (COD) of a synthetic waste solution containing a water soluble basic dye [Methylene Blue] were investigated. Decolorization of the dye was achieved by ozonation. The research was conducted using a batch bubble column to take the advantage of the intensive back-mixing that prevails in bubble columns. As a result, the COD of basic dyestuff wastewater was reduced to 64.96% and decolorization was observed under basic conditions (pH 12), complete MB degradation occurring in 12 min. Ozone consumption continued for a further 16 min after which time most of the degradation reactions were complete. Kinetic studies showed that direct ozonation of the aqueous dyes represented a pseudo-first-order reaction with respect to the dye. The apparent rate constant, which increased with both the applied ozone dose and higher pH values, declined logarithmically with the initial dye concentration.     

波纹板规整填料塔液体分布

将填料单元处理成立体节点网,根据节点网内网线液流和节点液流各自的运动方式建立了填料单元的液体分布模型.通过将填料与塔壁间的液体交换规范成填料节点网与塔壁节点网间的液量传递建立了填料单元对应塔壁区的流体分布模型.提出了规整填料塔液体分布问题的边界条件.2个描述填料单元及其对应塔壁区液体分布的数学模型与2类边界条件共同构成了波纹板规整填料塔的液体分布模型.采用单纯形法对3个模型参数进行了估计.模型计算结果与实验数据一致,表明模型能合理地描述波纹板规整填料塔的液体分布性能.     

Thermodynamic optimisation of distillation columns

In this paper a methodology for thermodynamic analysis and distillation column ‘targeting’ is presented, with emphasis on the use of side condensers and side reboilers. Research in the past has been towards the establishment of a heat distribution curve, showing the way in which heat can be added or extracted across the different column sections. One major disadvantage of these profiles is that they refer to reversible columns, and cannot be used effectively to target for modifications in a real column.The main feature of the proposed methodology is the introduction of a minimum driving force, defined in terms of exergy loss distribution of the existing column, to set realisable targets for side reboiling/condensing in real columns, resulting in considerable energy savings. In addition to providing realisable targets, the new approach also provides the design engineer with information about the best location to place a side exchanger, and the required additional column modifications. The methodology can be applied using conventional column models in commercial process simulation programs, but can be significantly simplified by using reboiled and refluxed absorber models in a bespoke program. Simulation results for modified designs set by the new approach, for binary and multicomponent separations, verify the feasibility of the targets. This contrasts with previous approaches, which result in temperature shifts and heat load penalties after placing side reboilers/condensers, thus requiring additional simulation time and experienced judgement.     

Dynamic modelling of the absorber of a post-combustion CO2 capture plant: Modelling and simulations

Modelling work related to carbon dioxide (CO₂) capture technologies is of great importance with respect to the design, control, and optimization of the capture process. Development of dynamic models as such is important since there is much information embedded with the dynamics of a plant which cannot be studied with steady state models. A model for the absorption column of a post-combustion CO₂ capture plant is developed following the rate based approach to represent heat and mass transfer. The Kent–Eisenberg model is used to compute the transfer and generation rates of the species. Sensitivity of the model for different physiochemical property correlations is analyzed. The predictions of the dynamic model for the capture plant start-up scenario and operation of the absorption column under varying operating conditions in the up-stream power plant and the down-stream stripping column are presented. Predictions of the transient behaviour of the developed absorber model appear realistic and comply with standard steady state models.     

气固环流反应器内颗粒返混特性

采用光纤测量与数据处理方法,在不同操作条件下,对一套大型组合流化床燃烧器环流段内固体颗粒的局部返混及截面返混特性进行了测定. 结果表明,以颗粒局部返混比Rmix,L表征的局部返混特性规律为在环隙区的径向分布较均匀、而在其他区域(导流筒区、底部区和颗粒分流区)呈中心区小边壁区大的不均匀分布形态. 在环流反应器的整个流动区域内,Rmix,L变化范围为0~0.9,颗粒截面返混比Rmix,T变化范围为0.1~0.6.     

Review Article: The Existing Models for Simulation of Pulsed and Reciprocating Columns—How Well do they Work in the Real World?

Abstract

The purpose of this article is to review the state of art of the existing approaches to the design and analysis of pulsed and reciprocating columns, compare the merits of the theoretical models vs. the experimental “black box” method, and try to recommend a way to obtain a fruitful cooperation between researchers using different approaches. There are three main theoretical approaches: models based on the correlation of single parameters (holdup, drop size, mass‐transfer coefficient etc.), drop population balances, and computational fluid dynamics (CFD). At present, the design of new columns is based only on experimental data from piloting, sometimes with the assistance of mass‐balance simulators. And the experimental “black box approach” has proven to be efficient and quick, especially since at present no company considers the erection of a new column without some experimental verification. In the long term powerful models may enable the solution of many solvent extraction problems with reliability that does not require experimental verification. In order to be widely useful, these models have to be transformed into user‐friendly simulators that require only easily available data as input. To achieve this, there must be close cooperation between the academic world and industry. Otherwise, they will simply remain academic exercises. And at the present state of knowledge, pilot‐plant testing remains an almost inevitable preliminary step to a full‐scale column design.     

三维上流式反应器床层流动和返混特性

采用内径为280 mm的上流式反应器,以空气模拟气相、甘油和水混合溶液模拟渣油。用3种不同粒径的氧化铝球形工业催化剂颗粒为填充颗粒,考察了不同模拟物系的颗粒粒径、颗粒密度、液相黏度、不同床层的高径比和不同操作条件对上流式反应器内床层压降及其波动、床层轴向返混的影响规律。得到模拟工业运行物系和操作条件的上流式反应器床层总压降关联式,相对误差在12%以内。床层总压降均随床层高径比、颗粒密度和液相黏度增加而增大,但随颗粒粒径的增大而减小,床层压降波动随表观气速增加而增大。填充颗粒粒径越小、颗粒密度越小、高径比越大,床层内轴向返混越严重;床层内压降和轴向返混均随表观气速的增加而增大。     

Modeling of core flow in a gas-solids riser

The heterogeneous flow structure in gas-solids riser reactors is typically represented by an upward solids flow in the core region and a back-mixing downward solids flow in the wall region. The hydrodynamic and reaction characteristics in these two regions are highly different, as most reactions with fresh catalyst solids occur in the core region and mostly spent catalyst solids are found in the wall region. Gross understanding on gas-solids riser flow can be conveniently obtained from a cross-section averaged one-dimensional modeling approach, which is probably only valid for the core region. The success of such an approach, however, has to rely on the appropriate modeling of controlling mechanisms of riser flows. Our recent studies show that commonly-employed Richardson-Zaki equation overestimates the hydrodynamic forces in the dense phase and acceleration regimes; there is also a non-negligible effect of solids collision on solids acceleration, and the wall effect should be taken into account in terms of wall boundary and back flow mixing. In this paper we propose a new mechanistic modeling to describe the hydrodynamics of upward flow of solids in a gas-solids riser, with new formula of hydrodynamic phase interactions. The modeling results are validated against published measurements of pressure and solids volume fraction in a wide range of particle property, gas velocity and solid mass flux. Parametric effects of operation conditions such as transport gas velocity and solid mass flux on hydrodynamic characteristics of riser flows are predicted.     

Inviscid model for gross solid circulation and gas back mixing in fluidized beds

An inviscid model is developed to describe gross solid circulation and gas backmixing in axisymmetric fluidized beds. Two distinct cases are shown to arise:

• 1.interstitial gas moves upwards everywhere; and
• 2.interstitial gas circulates giving rise to loop formation in an annular region near the wall.

It is shown that gas back-mixing is a highly local phenomenon which explains the anomalously high “apparent wake fractions” estimated in the literature. The model predictions as well as analysis of published observations indicate the “onset” of gas backmixing near the wall to occur just above the minimum fluidization (or minimum bubbling) plint. An analytical expression derived to estimate the fraction of bed area available for fresh gas movement upwards indicates that the “dead volume” of the bed is usually greater than 50% and at times can approach even 90%.     

Effect of interfacial forces and turbulence models on predicting flow pattern inside the bubble column

The numerical approaches have been used in many studies to predict the flow pattern inside the bubble column reactors because of the difficulties that are still found in designing and scaling-up the bubble columns. This review makes an effort to show suitable interfacial forces i.e., drag force, lift force, turbulent dispersion models and virtual mass and turbulence models such as standard k–ɛ model, Reynolds Stress Model, Large Eddy Simulation to predict flow pattern inside the bubble column using Eulerian–Eulerian. The effect of various interfacial forces and turbulence models on gas–liquid velocity and gas hold-up in bubble column is critically reviewed.     

Komplementäre Modellierung in der Fluidverfahrenstechnik

Optimization of technological processes depends on the relevant process design, properly selected column internals and sufficient understanding of the process behavior. This can only be achieved with the help of accurate and reliable process models. Along these lines, the present article suggests a new modelling concept – complementary modeling – for a large class of fluid engineering processes. Due to diversity of process conditions and criteria, it is impossible to develop a unified modelling approach. Instead, an efficient combination of different modeling approaches is advantageous. The complementary modeling is discussed in detail and illustrated with several case studies.     

Auslegungskonzept für pulsierte Siebboden-Extraktoren

A concept for the design of pulsed sieve-plate extractors . A concept is presented for the design of pulsed sieve-plate extractors (PSE) according to which the column diameter is calculated via the flooding load and the column length by concentration profiles using the well known backmixing models. This concept requires some parameters to describe the hydrodynamic and mass transfer behaviour. The parameters depend on physical properties, operating conditions, and extractor geometry and subject to mutual influence. It is now possible to present correlations and models for prediction of these parameters. The mass transfer term can be determined by Sherwood correlations for single drops, using the Sauter mean drop diameter. The total mass transfer coefficient can be given as a function of column lenght. The calculated concentration profiles agree well with the measured profiles and thus the concept is confirmed.