Simulation of mercury emission control by activated carbon under confined-bed operations

Mercury emissions from coal-fired power plants have been a great environmental and regulatory concern due to the toxic nature of mercury and the significant amount of emissions from these plants. An effective method for controlling mercury emission is to employ activated carbon to adsorb mercury from the combustion flue gas. In this study, an activated carbon mercury sorption model was applied to simulate a confined-bed mercury emission control process. Model simulations were performed to generate dynamic mercury concentration profiles and the corresponding profiles of mercury uptake by activated carbon at various bed locations under various process conditions. The simulation parameters included flue gas flow rate, inlet mercury concentration, and adsorption bed temperature. The study has demonstrated the applicability of the model for simulating the process and provided insights into the mercury control process especially the effects of flue gas flow rate, inlet mercury concentration, and activated carbon bed temperature on the process. Such information is critically needed in the design and operation of a mercury emission control process involving activated carbon adsorption.     

活性炭对丁酮的吸附动力学研究

研究了2种活性炭（木质活性炭和煤质活性炭）对丁酮的吸附,重点考察了活性炭的吸附时间、吸附温度和丁酮载气流量对丁酮吸附的影响,并用准一级、准二级、Elovich和Bangham 4种动力学模型对活性炭在不同温度条件下对丁酮的吸附行为进行了动力学拟合,确定其动力学吸附模型。实验表明：不同的活性炭对丁酮的吸附过程不同;活性炭对丁酮的吸附是一个吸附和解吸同时存在的过程,当吸附速率和解吸速率相等时,该过程达到吸附平衡;随着吸附温度的升高,活性炭对丁酮的饱和吸附量逐渐降低,说明活性炭对丁酮的吸附过程为放热反应;丁酮载气流量对活性炭吸附丁酮达到饱和的时间以及吸附速率有影响,对AC-1的最终饱和吸附量影响显著,对AC-2的最终饱和吸附量没有显著影响。这2种活性炭吸附丁酮最适宜的吸附温度均为303 K,最佳的载气流量为400 mL/min。在不同温度下对活性炭吸附丁酮的过程进行动力学分析,发现Bangham方程计算得到的相关系数R²大于0.99,因此,活性炭对丁酮的吸附动力学方程符合Bangham动力学方程。     

Removal of volatile organic compound by activated carbon fiber

Experiments were carried out to study adsorption/desorption of volatile organic compound (VOC) on the activated carbon fiber (ACF) under dynamic conditions. The primary objective was to experimentally demonstrate the suitability of ACF in effectively adsorbing VOCs from inert gaseous stream under varying operating conditions, and compare its performance vis-à-vis that of the other commercially available adsorbents, such as granular activated carbon (GAC), silica gel, and zeolites. The adsorption experiments were carried out in a fixed tubular packed bed reactor under various operating conditions including temperature (35-100 °C), gas concentration (2000-10,000 ppm), gas flow rate (0.2-1.0 slpm) and weight of the adsorbent (2-10 g). A mathematical model was developed to predict the VOC breakthrough characteristics on ACF. The model incorporated the effects of the gas-particle film mass transfer resistance, adsorbent pore diffusion and the adsorption/desorption rates within the pore. The experimental data and the corresponding model simulated results were compared and found to be in good agreement. The ACF repeatedly showed a good regeneration capability following desorption by DC electrical heating.     

Stokes flow past periodic rows of porous cylinders

The flow of a viscous incompressible fluid at low Reynolds numbers in model filters—systems of porous permeable cylinders—is studied. A single row and hexagonal and square arrays of parallel cylinders perpendicular to the flow direction are considered. The flow field outside and inside a porous cylinder is determined by solving the Brinkman and Stokes equations. The drag force on a porous cylinder versus the cylinder permeability and the distance between the axes of neighboring cylinders is calculated. It is shown that rows of porous cylinders arranged into a square array have almost no mutual hydrodynamic effect at any array spacing. The cell model is shown to be applicable to the hexagonal array of porous cylinders over a wide range of packing densities.     

Velocity variation effect in fixed bed columns: A case study of CO2 capture using porous solid adsorbents

This study shows that for a reliable evaluation of porous adsorbents for carbon capture based on the fixed bed adsorption analysis, one must consider the effect of velocity variation due to adsorption to make a fair judgment on predicting the performance of materials under flow conditions. A combined experimental and numerical study of CO₂/N₂ adsorption in fixed beds using three forms of adsorbents of amorphous powder (bulk activated carbon, AC), crystalline powder (bulk CuBTC metal‐organic framework, MOF) and crystalline pellets (pelleted CuBTC) was carried out to show the effect of velocity variation on CO₂ breakthrough curves. Significant deviations are observed in the estimated amount adsorbed calculated from fixed bed experiments when models used for interpretation the measured data consider constant gas velocity because the stoichiometric time is underestimated. We show that the difference in breakthrough times estimated in models that consider constant and variable gas velocity grows exponentially with the feed gas concentration. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2189–2197, 2018     

A non-equilibrium model for fixed-bed multi-component adiabatic adsorption

A mathematical model for adiabatic adsorption of multiple components onto a fixed bed of adsorbent is developed. The model consists of a set of coupled, hyperbolic, parial-differential equations. Heat and mass transfer resistances evident in real systems are included in the model. Solutions of the equations are obtained by numerical application of the method of characteristics. Results show non-ideal mass and thermal wavefronts and complex wave-wave interactions in adiabatic adsorption of benzene and cyclohexane onto activated carbon. Phenomena of inversions in relative adsorptivity due to temperature changes are also shown. Column performance is indicated to be greatly affected by these phenomena. Cases considered demonstrate that time on stream before breakthrough may be doubled by increasing influent and initial bed temperatures. It is shown, in contrast to results obtained fron using the non-equilibrium model, that models based on assumptions of local equilibrium cannot quantitatively describe behavior of a typical industrial adsorption system.     

Steam-activated carbons from a bituminous coal in a continuous multistage fluidized bed pilot plant

A vertical three-stage fluidized bed pilot plant, with downcomers, was designed and built in order to study the continuous process of the production of activated carbons from a high-volatile bituminous coal from the Puertollano basin (Spain), by steam activation. The pilot plant can operate with a production of up to 40 kg per day. Very good activated carbons were produced at the selected operating conditions. The effect of the following operating conditions on the reactivity and adsorption characteristics of the activated carbons was analyzed: (1) carbonization conditions (one- and two-step activation), (2) activation temperature (800–850 °C), and (3) steam gas velocity (1.5–3 times the minimum fluidization velocity). Carbonization conditions considerably affect the reactivity of the chars obtained; the faster the carbonization process, the higher the reactivity. Nevertheless, the effect of this variable on the development of porosity is not very relevant, and consequently the direct activation process is an attractive alternative to the two-step (carbonization and activation) process. On the other hand, both temperature and steam flow rate (affecting the reaction rate) have a marked effect on the development of porosity.     

Numerical simulation of toluene adsorption on activated carbon in a technical column in low concentration range

This paper deals with the modeling of toluene adsorption in a technical column of activated carbon in a low concentration range (2.6 to 3.4·10^?3 kg/m³), which is usual for environmental applications. It is also concerned with methods of numerical solutions of the nonlinear partial differential equations (PDE), a modified control volume method was used to determine the PDE. Three nonequilibrium isotherm models are developed to describe mass transfer in a fixed bed of activated carbon (Degussa WS-4). The important parts of this paper are the simulation of the maldistribution of concentration and loading of the pollutant in the technical column, as a result of the flow velocity maldistribution and the study of the mass transfer mechanisms in a technical fixed bed. It is found that in a low concentration range, toluene adsorption in a fixed bed of activated carbon (Degussa WS-4) is an external rate-controlled process, thus surface diffusion cannot be neglected. The calculated results are in good agreement with the experimental ones. The measured effect of an earlier breakthrough near the wall-near zone has been calculated using the model.     

椰壳活性炭对气态氯化汞吸附作用的对比研究

以氯化汞为目标污染物,研究了椰壳活性炭对气态氯化汞的吸附性能,并结合活性炭微结构表征以及动力学模型拟合研究了其吸附机理。结果表明,椰壳活性炭对气态氯化汞的最大吸附量35.9 mg/g,且活性炭比表面积和总孔容对其吸附氯化汞有显著影响,比表面积大、总孔容大有利于提高饱和吸附量。载气流量不影响活性炭对氯化汞的饱和吸附量,但是影响其吸附时间,增大载气流量能够缩短吸附时间。温度对吸附量和吸附时间均有影响,升高温度能够提高吸附量且缩短吸附时间。通过对吸附过程的动力学模拟,发现活性炭对氯化汞的吸附均符合班厄姆动力学模型,相关系数均大于0.99,活性炭的吸附速率与吸附量随比表面积与总孔容的增大而增大。     

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).     

Development of mesopores during activation of poly(vinylidene fluoride)-based carbon

Mesoporous carbon films were prepared from poly(vinylidene fluoride) through liquid-phase dehydrofluorination (DHF)-treatment, carbonization at a high temperature and activation in carbon dioxide gas. The adsorption capacity of the resultant carbon was investigated by using nitrogen and methylene blue as adsorbates. The maximum adsorption capacity was obtained for the activated carbon prepared by applying a slight extent of DHF-treatment. The growing process of the pores in this carbon during activation was considered based on the changes of mass, pore volume and surface area. In order to increase the pore size in the activated carbon, it was essential to increase the pore size in the carbon before activation. The application of a slight extent of DHF-treatment was effective to increase the pore size in the carbon before activation.     

Experiment and 3D simulation of slugging regime in a circulating fluidized bed

A circulating fluidized bed (CFB) is widely applied in many industries because it has high efficiency. To develop and improve the process, an understanding of the hydrodynamics inside the CFB is very important. Computational fluid dynamics (CFD) represents a powerful tool for helping to understand the phenomena involved in the process. In this study, a CFD model was developed to represent a cold model of the laboratory scale CFB which was designed to study the hydrodynamics of a CFB using commercial CFD software. The Eulerian approach with kinetic theory of granular flow was used for simulating the hydrodynamics inside the system. After proper tuning of relevant parameters, the pressure profile along the equipment from the simulation was well agreed with that from the experiment. The simulation result expresses the hydrodynamic parameters of the slug flow such as solid volume fraction, gas and solid velocities and granular temperature in the riser.     

Pollutant adsorption during activated carbon and steam regeneration in technical columns (experiments, modelling): Part 2. Influence of process parameters during steam regeneration of activated carbon beds on pollutant exit concentration

Experiments in activated carbon columns of technical scale with upstream adsorption and downstream steam regeneration showed that pollutant exit concentration of waste gas can be markedly decreased if the upper zone of the fixed bed is preheated to 120 °C before desorption and, additionally, if the bed is desorbed with superheated steam with a temperature 150 °C. With this improved steam regeneration technique a decrease in pollutant exit concentration is achieved: for example, from 20 to 3 mg m⁻³ toluene. An increase in adsorption time before pollutant breakthrough from 12 to 19 h is also noted.

During regeneration of a cold bed of activated carbon with saturated steam the particles will become wetted with condensate outside and adsorbed steam inside. These water condensation/adsorption effects are markedly reduced by preheating the particles to temperatures so far above that of steam condensation that steam adsorption is prevented too. Additionally the regeneration steam is superheated so that the temperature drop caused by heat of pollutant desorption is not so great that steam adsorption occurs.

The whole desorption-adsorption cycle in a technical scale bed, including the effects of Steam condensation/ adsorption, flux and suction of condensate, hetero-azeotropic pollutant distillation and evaporation of condensate was modelled.

Radial maldistribution effects (discussed in part 1 of this paper, Chem. Eng. Proc., 32 (1993) 359) have not been taken into account here.

The calculated flux rates of pollutant in the gas mixture leaving the bed during desorption and the pollutant breakthrough curves at the following adsorption step correlate quite well with experimental results.     

Two‐Phase Sequential Simulation of a Fluidized Bed Reformer

A two‐phase flow model is adapted in order to predict the performance of a fluidized bed reformer using the sequential modular simulator. Since there are physical and chemical phenomena interacting in the reformer, two sub‐models appear to be necessary to describe the overall model. These are the hydrodynamic and reaction sub‐models. The hydrodynamic sub‐model is based on the dynamic two‐phase model and the reaction sub‐model is derived from the literature. In the overall model, the bed is divided into several sections. At each section, the flow of the gas is considered as plug flow through the bubble phase and to be perfectly mixed through the emulsion phase. Two sets of experimental data from the literature at different hydrodynamic regimes were used in order to validate the proposed model. A close agreement was observed between the model predictions and the experimental data. The model proposed in this work may be used as a framework for the development of sophisticated models for non‐ideal reactors inside process simulators.     

Experimental breakthrough curves determined under carefully controlled conditions for the adsorption of water vapour on alumina in adiabatic fixed beds

The adsorption on activated alumina of water vapour from an air stream is studied experimentally in an adiabatic fixed bed. The experimental set-up which provides accurate results is described. The influence of particle size, gas inlet concentration and temperature, linear gas velocity and bed length on concentration and temperature at bed outlet is investigated.     

Modelling of batch fluidised bed drying of pharmaceutical granules

This paper presents a mathematical model based on a three-phase theory, which is used to describe the mass and heat transfer between the gas and solids phases in a batch fluidised bed dryer. In the model, it is assumed that the dilute phase (i.e., bubble) is plug flow while the interstitial gas and the solid particles are considered as being perfectly mixed. The thermal conductivity of wet particles is modelled using a serial and parallel circuit. The moisture diffusion in wet particles was simulated using a numerical finite volume method. Applying a simplified lumped model to a single solid particle, the heat and mass transfer between the interstitial gas and solid phase is taken into account during the whole drying process as three drying rate periods: warming-up, constant rate and falling-rate. The effects of the process parameters, such as particle size, gas velocity, inlet gas temperature and relative humidity, on the moisture content of solids in the bed have been studied by numerical computation using this model. The results are in good agreement with experimental data of heat and mass transfer in fluidised bed dryers. The model will be employed for online simulation of a fluidised bed dryer and for online control.     

MATHEMATICAL AND EXPERIMENTAL INVESTIGATION ON NOX RECOVERY IN FLUIDIZED BED ADSORBERS FOR EXHAUST GASES OF NITRIC ACID PLANTS

A mathematical model for sorption of NOx from exhaust gas of nitric acid plants by activated carbon (AC) in a fluidized bed is proposed based on two-phase flow theory of fluidization. To solve the proposed model a computer program has been developed. The output of this program reveals the effects of various parameters such as temperature, inlet gas velocity, particle diameter, and inlet gas concentration on the rate of adsorption. To evaluate the proposed model, a pilot-scale plant was erected and the obtained experimental data were compared with the values predicted by the model. A good agreement is observed between these values. Once the validity of the proposed model is checked, it could be used for obtaining the optimum operating condition for this type of bed.     

MATHEMATICAL AND EXPERIMENTAL INVESTIGATION ON NOX RECOVERY IN FLUIDIZED BED ADSORBERS FOR EXHAUST GASES OF NITRIC ACID PLANTS

A mathematical model for sorption of NOx from exhaust gas of nitric acid plants by activated carbon (AC) in a fluidized bed is proposed based on two-phase flow theory of fluidization. To solve the proposed model a computer program has been developed. The output of this program reveals the effects of various parameters such as temperature, inlet gas velocity, particle diameter, and inlet gas concentration on the rate of adsorption. To evaluate the proposed model, a pilot-scale plant was erected and the obtained experimental data were compared with the values predicted by the model. A good agreement is observed between these values. Once the validity of the proposed model is checked, it could be used for obtaining the optimum operating condition for this type of bed.     

对二甲苯在活性炭固定床上的吸附动力学

研究了对二甲苯在活性炭固定床上的吸附动力学。考察了初始浓度、气体流量、床层长度等因素对吸附透过曲线的影响。同时,采用Yoon—Nelson模型对吸附透过曲线进行线性回归分析。实验结果表明,随着初始浓度的增大,透过时间缩短,吸附量增大：气体流最对透过曲线的形状影响不大：床层长度基本不影响透过曲线;Yoon-Nelson模型可以较好的模拟固定床吸附过程。     

催化裂化汽油吸附脱硫工艺研究

在固定床吸附装置上对催化裂化汽油进行吸附脱硫实验,考察了吸附脱硫工艺条件对催化裂化汽油硫质量分数、辛烷值及吸附剂单程寿命的影响。实验结果表明,吸附脱硫适宜的工艺条件为:吸附温度360℃,吸附压力0.3 MPa,氢气流量300 mL/m in,体积空速1.0 h-1。通过对吸附脱硫实验过程中的尾气分析,对催化裂化汽油吸附脱硫的机理进行了探讨。