Adsorption aus der Gasphase — Grundlagen und Verfahren

Adsorption from the gas phase – Fundamentals and processes . For the design of an adsorption process, both equilibrium and kinetic data are needed. These data are used for mass- and energy balances and to determine the breakthrough curve, i.e. the end of adsorption in a fixed bed adsorber. The newer results of equilibrium and kinetics studies, in particular concerning the adsorption of gas mixtures and the non-isothermal behaviour of adsorption are presented. Even today the breakthrough curve can be calculated analytically only under limiting assumptions. When the physical properties are however known a numerical calculation is possible, though it is not straightforward. In the design of an adsorption process it is therefore possible to be as precise as in other comparable thermal separation processes.     

Modellierung isothermer Festbettadsorber

Modelling of isothermal fixed bed adsorbers . The effective adsorption velocity in porous solids is determined primarily by the adsorption equilibrium coupled with bulk diffusion, Knudsen diffusion, and surface diffusion in the particles. Adsorption and mass transport may follow different mechanisms for different combinations of gaseous (fluid) and solid phases and for different pore size distributions. For the industrially important cases models were developed and discussed for isothermal fixed bed adsorbers. The balances of the different models show different mathematical structures, so that different mathematical evaluation methods are necessary for determination of the breakthrough curve. Computed results of the models are discussed. The evaluation of the breakthrough curves is needed for parameter estimation from laboratory scale experiments and for the design of industrial adsorbers with known parameters.     

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.     

More effective membrane chromatography

Adsorption in membranes with polydispersed pores gives a dispersed breakthrough curve even when mass transfer is so fast that it reaches saturation. Such a breakthrough is due to unequal flows in unequally sized pores. A theory of polydispersed pores can predict the breakthrough curves for the removal of lead ions from model solutions if the pore‐size distribution is known. Such predictions are in better agreement for lead adsorption than predictions based on mass transfer. The results suggest ways in which More effective membrane chromatography can be achieved. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3871–3878, 2015     

季铵离子化腈纶纤维对染料的动态吸附性能

对腈纶纤维进行季铵化改性,得到季铵离子化的腈纶纤维吸附剂,经过红外表征,证明了该功能化吸附剂被成功制备,并用于水中阴离子染料的去除。为模拟实际工业处理,系统研究了吸附剂对甲基橙污水的动态吸附性能,考察了甲基橙污水的初始浓度,吸附柱高度和流速对穿透曲线的影响。采用Thomas模型、Yoon-Nelson模型和BDST模型对实验数据进行拟合,预测动态吸附的容量、50%穿透时间以及初始浓度和初始速率改变后的穿透时间。预测结果与实际结果基本一致,表明这3种模型可以很好地对动态流动进行模拟。因此,这3种模型可以对实际工业污水处理进行指导。     

ADIABATIC SORPTION OF DILUTE SINGLE ADSORBATE FROM AN INERT GAS—EFFECT OF GAS-SOLID MASS AND HEAT TRANSFER COEFFICIENTS

The effects of the gas-solid mass (k) and heat (h) transfer coefficients on the column dynamics for Type II adsorption system were analyzed by numerically solving the conservation equations for a single sorbate. Langmuir equilibrium and the linear driving force model for mass transfer were assumed

The transfer zone is larger in an adiabatic column than the corresponding zone in an isothermal column even for a small rise in column temperature. Initially proportionate pattern transfer zones are formed in both the adiabatic and the isothermal columns. As the column length increases these profiles approach to the constant pattern limit. Column adiabaticity slows down the rate of approach to constant pattern limit. On the other hand, higher adsorbate concentration and high k enhance this rate. The symmetry of the breakthrough curve may be distorted in an adiabatic column. The effects of k and h on the size of the mass transfer zone are interchangeable. Consequently, estimation of k from experimental breakthrough data by assuming thermal equilibrium between gas and solid phases may be erroneous.     

Identification and Prediction of Protein Adsorption Breakthrough,Desorption, and Fractionation in a Packed Column Using a Neural Network

Abstract

A simple neural network with a three-node hidden layer has been used to identify and predict protein adsorption, desorption and fractionation profiles in a 25 × 1 cm ID Productiv^TM CM ion-exchange column. To predict the effect of flow rate on the adsorption breakthrough curve, two sets of data obtained at the maximum and minimum of the full range of flow rates used were sufficient to train the neural network which was then able to predict the effects of flow rate changes within the training range on the adsorption breakthrough curve. This training method was also applied to explore the effects of flow rate on desorption and fractionation. It was found that the network training algorithm performed satisfactorily if the flow rate data for desorption and fractionation were scaled in the form of logarithm.     

Comparison of experimental and predicted breakthrough curves for adiabatic adsorption in fixed bed

Mass and heat transfer in adiabatic fixed bed adsorbers is described by a model which considers resistance to mass and heat transfer simultaneously within the porous adsorbent particles and in the fluid flowing past the pellets. The adsorption equlibrium is described by a temperature dependent Freundlich isotherm.The mathematical model which is given in dimensionless form is solved numerically for different values of the dimensionless parameters.An example of predicted concentration and temperature breakthrough curves for an adiabatic integral bed is given. This prediction is performed solely on the basis of data from an isothermal differential bed. The predicted breakthrough time is within 8% of the experimental value when the dimensionless parameters are assumed constant and within 2% when their concentration and temperature dependence is taken into account.     

Dynamics of CO2 adsorption on sodium oxide promoted alumina in a packed-bed reactor

CO₂ adsorption in packed-bed reactors has potential applications in flue gas CO₂ capture and adsorption enhanced reaction processes. This work focuses on CO₂ adsorption dynamics on sodium oxide promoted alumina in a packed-bed reactor. A comprehensive model is developed to describe the coupled transport phenomena and is solved using orthogonal collocation on finite elements. The model predicted breakthrough curve matches very well with experimental data obtained from a pilot-scale packed-bed reactor. Several dimensionless parameters are also derived to explain the shape of the breakthrough curve.     

Methanol Oxidation over Iron-Molybdate Catalysts: Optimization of the Process

This research aims to shed more light on the possibilities to optimize the process of selective oxidation of methanol over oxide catalysts using an adiabatic layer by determining border values of basic parameters of the process in the adiabatic layer that ensure a maximum rate of methanol oxidation at high selectivity. Laboratory experiments were performed to clarify the influence of the most important parameters of the process (temperature, space velocity and methanol concentration) on the efficiency of the adiabatic layer. An industrial experiment was carried out to verify the results obtained.     

A systematic investigation of the overall rate coefficient in the Wheeler-Jonas equation for adsorption on dry activated carbons

Modelling of the adsorption rate coefficient, k_v of the Wheeler-Jonas equation for estimating the service life of packed carbon beds, is addressed. Current methods for extracting k_v from experimental breakthrough data include approaches that introduce easily propagated errors. The weaknesses of these approaches are analyzed, and a calculation based on multiple points on the breakthrough curve is suggested. Experimental breakthrough data for a representative set of volatile organic compounds (VOCs) has been measured. A systematic investigation of factors influencing k_v, including adsorbate and carbon properties, adsorbate inlet concentration and flow velocity is performed. It is found that flow velocity and carbon particle size have the largest influence, followed by adsorbate properties related to the adsorption capacity. A simple linear empirical model for k_v, including air flow velocity, carbon particle size, and dielectric constant of the adsorbate, is presented. The model is based on the breakthrough range up to 20% of the inlet concentration, for which k_v is shown to be almost constant. The range of the model covers breathing rates valid for a respirator at different work loads. The model can be used to estimate the adsorption rate coefficient for specific carbon particle sizes and various VOCs present in workplace environments.     

Removal characteristics of some priority organic pollutants from water in a fixed bed fly ash column

The efficiency of a coal fly ash (generated from a thermal power plant) adsorption column for the removal of some priority organic pollutants, viz phenol, o‐hydroxyphenol, m‐hydroxyphenol and 4‐nitrophenol from aqueous solution has been studied. The column performance was evaluated from the concept of the formation of a primary adsorption zone and the breakthrough curve. The extent of solute removal obtained from breakthrough curve during column operation was compared with that obtained from the isotherm parameters for batch operation. The loaded solutes in the column were successfully eluted with acetone, achieving 98% recovery. In order to determine the practical applicability of the column operation the process was repeated a number of times and the variation of column capacity with number of operation cycles was evaluated. Even after six successive cycles, the column was found to retain almost 80% capacity. Copyright © 2005 Society of Chemical Industry     

Adsorption of phenanthrene on activated carbons: Breakthrough curve modeling

The modeling of breakthrough curves obtained in the adsorption of phenanthrene as model compound on different activated carbons is described. All the runs were performed in a fixed bed reactor at atmospheric pressure, with a process temperature of 150 °C and low contaminant concentrations. Therefore, experimental conditions were similar to the observed in the flue gases of energy generation systems. This work is mainly focused on the study of how adsorbent characteristics (surface area and micropore size distribution) influence the kinetics of the adsorption process. First, equilibrium values are found from the breakthrough curves and they are satisfactory fitted to the Freundlich isotherm. Using the obtained equilibrium values together with the linear driving force model as kinetic expression, the breakthrough curve modeling is achieved. It was found that this model fits all the breakthrough curves and it is a useful tool for modeling purposes. Values for the phenanthrene surface and effective diffusion coefficients are calculated and reported, and a relationship with the microporosity is found. As it was expected, it is observed that the phenanthrene molecule finds kinetic restrictions for the diffusion in those carbons with narrow microporosity, especially in those with a mean pore diameter close to the molecular size.     

活性碳纤维吸附水溶液中的二价铅离子

对活性碳纤维(ACF)吸附水溶液中的二价铅离子进行研究。分别运用静态和动态方法来考察ACF吸附Pb^2 的性能，测定了吸附等温线，探讨了吸附时间、DH值等因素对吸附的影响，并进行了ACF的再生实验。结果表明，ACF对水中二价铅离子的吸附特性良好，且吸附剂易于再生，可作为去除水中离子态重金属的优良吸附剂。     

Unusual adsorber dynamics due to S-shaped equilibrium isotherm

A fixed bed adsorption study has been performed to investigate the effect of S-shaped (Type V) equilibrium isotherm on the breakthrough patterns. The adsorption breakthrough data of benzene vapours on dealuminated zeolite Y were collected by using a lab-scale fixed bed apparatus. It was found that the adsorption system, at influent concentrations from 100 to 5,000 ppm at 303 K, yields breakthrough curves of unusual shapes. A mathematical model was applied to predict the complex breakthrough patterns at various conditions. The experimental and modelling results suggest that the unusual breakthrough curve is due to the basic shape of the equilibrium isotherm.     

Application of wave propagation theory to adsorption breakthrough studies of toluene on activated carbon fiber beds

Based on the wave propagation theory, a dynamics model that combines the nonlinear equilibrium isotherm and the linear mass-transfer equation has been developed to predict the breakthrough behaviour of toluene adsorption in a fixed bed packed with activated carbon fibers. The experimental results showed that the constant-pattern wave model using the Langmuir isotherm equation could capture the dynamic behaviour of the adsorption column. Two important parameters, the half breakthrough time (t_1/2) and the volumetric mass-transfer coefficients (k_Gα) in the model were obtained from linear fitting of the model to experimental breakthrough data. k_Gα was found to be insensitive to the initial concentration and increased with the increasing the superficial velocity. It was also observed that t_1/2 decreases with increasing the superficial velocity and the initial concentration, and increases with increasing the bed height. A sensitivity analysis showed that external mass-transfer had a much stronger influence on the breakthrough curve than internal mass-transfer, confirming that the overall mass-transfer for toluene adsorption onto activated carbon fibers in fixed bed is controlled by external mass-transfer.     

Equilibrium kinetic and thermodynamic studies of Cr(VI) adsorption onto a novel adsorbent of <Emphasis Type="Italic">Eucalyptus camaldulensis</Emphasis> waste: Batch and column reactors

Cr(VI) adsorption onto Eucalyptus camaldulensis sawdust (ECS) waste was investigated in batch and column reactors. Various parameters, including the adsorbent dose, pH, initial concentration, particle size, contact time and temperature were optimized. The maximum adsorption capacity (35.58mg g^-1, 71.16%) was achieved at pH 2.0. Data fitted well to Freundlich and Halsey’s models (R²=0.992), indicating the multilayer adsorption of Cr(VI). It obeys the pseudo-second order kinetics. Endothermic and non-spontaneous nature of Cr(VI) adsorption was observed with positive values of changes in enthalpy (9.83 kJ mol^-1), and Gibbs-free energy (1.52, 1.38, 1.24, 1.10 and 0.97 kJ mol^-1), respectively. In this column study, the breakthrough curve time increased from 670 to 1,270min by increasing the bed height from 5 to 15 cm, respectively. Column data was found well fitted to bed depth service time model. Adsorption capacity at 60% breakthrough was 2,443.636mg L^-1. The study indicates that ECS waste can be a promising adsorbent for Cr(VI) remediation from industrial effluents.     

分子筛吸附脱除芳烃中的环状烯烃

以Y型分子筛为吸附剂脱除甲苯中的环状烯烃化合物（二聚环戊二烯,DCPD）。采用NH3-TPD测试方法分析了USY和HY分子筛的酸性中心性质。通过间歇吸附实验考察了USY和HY分子筛对甲苯中DCPD的吸附脱除能力以及温度对USY分子筛吸附能力的影响,结果表明,USY分子筛的烯烃吸附量是HY分子筛的1.68倍;当温度由30℃升至50℃时,USY分子筛的吸附速率增大。采用固定床连续实验考察了温度、体积空速和原料中烯烃浓度对USY分子筛吸附效果的影响,当温度为80℃、空速为0.133h?1时,分子筛的穿透吸附量最大;分子筛处理具有较高初始浓度的原料时得到较高的穿透吸附量,且对工业焦化甲苯溶液的处理效果明显,具有较好的工业应用前景。TGA热重分析结果表明,USY分子筛对DCPD的选择性吸附能力强于甲苯。     

吸附法处理含氟水

以活性氧化铝为吸附剂,测定了ＮａＦ溶液的平衡吸附量和透过曲线。考察了停留时间、吸附质浓度对透过曲线的影响,获得了较佳的吸附过程操作条件,为工业设计放大提供了必要的基础数据。     

Relationship between breakthrough curve and adsorption isotherm of Ca(Ⅱ) imprinted chitosan microspheres for metal adsorption

In this work, an equilibrium-dispersion model was successfully established to describe the breakthrough performance of Ca(II) imprinted chitosan (Ca(II)-CS) microspheres packed column for metal adsorption, and the assumptions of Langmuir isotherms and axial dispersion controlled mass transfer process were confirmed. The axial dispersion coefficient in Ca(II)-CS microspheres packed column was found to be almost proportional to the linear velocity and fit for prediction through single breakthrough test. Sensitivity analysis for breakthrough curve indicated the axial dispersion coefficient as well as Langmuir coefficient was sensitive variable for deep removal requirement. The retrieval of the adsorption isotherms of Ca(II)-CS microspheres from breakthrough curve was fulfilled by model ing calibration. A strategy based on the correlation between adsorption isotherms and breakthrough performance was further proposed to simplify the column adsorption design using absorbents with smal/uniform size and fast adsorption kinetics like Ca(II)-CS microspheres to cut down the gap between lab and industry.