OPTIMIZATION OF PRESSURE SWING ADSORPTION EQUIPMENT PART II OPTIMIZATION OF TWO BED OXYGEN GENERATORS

Local equilibrium model described in Part I is used to stimulate both, the steady state operation conditions and transient regime after the start of PSA equipment. The effect of axial dispersion, adsorption isotherm nonlinearity and high pressure level is simulated. On the basis of numerical analysis, the operation of a classical two-bed oxygen generator is compared with two configurations having pressure equalizing step. The optimization strategy of the given PSA equipment has been proposed and the procedure is demonstrated on an example. The scaling-up condition for PSA equipments which can be described by local equilibrium model has been proposed on the basis of the theoretical analysis.     

A 3-zone model for protein adsorption kinetics in expanded beds

The adsorption behavior of expanded beds is more complex than that of fixed beds, since the adsorbent particle size, local bed voidage and liquid axial dispersion will vary axially with expanded height. Models applicable to fixed beds maybe not adequately describe the hydrodynamic and adsorption behavior in expanded beds. In this paper, a 3-zone model is developed, in which the model equations are written for the bottom zone, middle zone, and top zone of the column, respectively, and the model parameters, such as the adsorbent particle diameter, bed voidage and liquid axial dispersion coefficient, are zonal values. In-bed breakthrough curves are predicted by the 3-zone model, and tested against literature data for lysozyme adsorption on Streamline SP in an expanded bed.Model parametric sensitivity is analyzed. The effects of film mass transfer resistance, liquid axial dispersion and adsorbent axial dispersion on the breakthrough curves are weaker than that of protein intraparticle diffusion resistance for stable expanded beds. Adsorbent particle size axial distribution and bed voidage axial variation significantly affect in-bed breakthrough curves, therefore, model parameters should not be assigned uniform values over the whole column; instead the model should account for the adsorbent particle size axial distribution and bed voidage axial variation.     

Simulation of protein adsorption in a batchwise affinity chromatography with a modified rate model

A rate model was adapted to simulate the dynamics of protein adsorption. This model takes axial dispersion and film mass transfer into account where there is a nonlinear adsorption isotherm for protein. The model equations were solved with the application of orthogonal collocation method on finite elements. The model is validated with experimental adsorption of urokinase in a batchwise column chromatographic process. Adsorption kinetics and isotherm were measured in a batchwise operation. With the assumption of back mixing at the column inlet, the effect of the different flow pattern on the concentration change inside the column can be simulated with the rate model.     

Sorption dynamics in fixed‐beds of inert core spherical adsorbents including axial dispersion and Langmuir isotherm

The effects of axial dispersion and Langmuir isotherm on transient behavior of sorption and intraparticle diffusion in fixed‐beds packed with monodisperse shell‐type/inert core spherical sorbents are studied. The system of partial differential equations of the mathematical model is solved numerically using finite difference methods. Results are presented in the form of breakthrough curves for adsorption and desorption processes. Results reveal that the shape of the breakthrough curves is influenced by both hydrodynamic and kinetic factors. Hydrodynamic factor is governed by axial dispersion and is controlled by changes of Peclet number. Simulation results reveal that when linear adsorption isotherm is used, the effect of axial dispersion on breakthrough curves of the system is important for Peclet numbers smaller than 50, whereas, for Langmuir isotherm axial dispersion is considerable for Peclet numbers less than 80. In addition, effects of type of adsorption isotherms and size of adsorbents on breakthrough curves are investigated, and results are compared with existing reports in the pertinent literature. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Axial liquid dispersion in a liquid–solid circulating fluidized bed

Although axial liquid dispersion has been studied extensively in particulate fluidized beds, no data has been reported previously in a liquid–solid circulating fluidized bed (LSCFb). In this work, the axial liquid dispersions at various radial positions were measured in an LSCFB of 76 mm in diameter and 3.0 m in height using a dual conductivity probe. The results reveal that the axial liquid dispersion is affected not only by the operating conditions but by the radial positions as well. A local axial dispersion model is proposed to describe the axial liquid dispersion at various radial positions. The local axial liquid dispersion coefficients determined by the proposed model are greater at the axis than near the wall region of the riser. This nonuniformity of axial liquid dispersion is believed to be caused by the radial nonuniform distribution of liquid velocity, and bed voidage in the LSCFB can significantly affect the axial liquid dispersion.     

Fixed-bed adsorption with nonplug flow: Perturbation solution for constant pattern behavior

Adsorption of a dilute solute from a fluid in nonplug flow through a fixed bed is investigated via a perturbation approach. The continuity equation for fixed-bed adsorption with axial dispersion is solved for the constant pattern concentration profile with the axial velocity characterized by a general axisymmetric function and the system having no resistances to external or intraparticle mass transfer. The isotherm is slightly favorable (i.e., concave downward) in order to justify the assumption that axial gradients of concentration are independent of the radial coordinate in the bed, as in the classical problem of Taylor diffusion. A series expansion of a general isotherm is used to treat adsorption equilibrium. The solution reveals the formation of a radial gradient of fluid-phase concentration and breakthrough behavior at the bed outlet dependent on the nonlinearity of the isotherm and the magnitude of the nonplug-flow-velocity profile. The results can be used to predict the breadth of the breakthrough wave of many chromatographic-type processes for packed beds and slightly favorable isotherms.     

THE RELATION BETWEEN WETTING EFFICIENCY AND LIQUID HOLDUP IN PACKED COLUMNS

Liquid holdup and wetting efficiency in packed columns were determined using a tracer method. A new model was developed considering axial dispersion, and analysis was made using the moment technique. The saturated solution of KG was used as an inert tracer and total liquid holdups were determined in the first part of experiments. As an adsorbable tracer, dioxane was utilized in the second part of the work and wetting efficiencies were obtained. Adsorption equilibrium constant was determined by the experiments conducted in a liquid full bed. A model proposed between liquid holdup and wetting efficiency gave good agreement with the experimental results. Axial dispersion effects were also taken into account during the work. It was also shown that axial dispersion of liquid phase should be considered especially in the small scale trickle bed reactors. @KEYWORDS:Packed beds,Wetting efficiency,Liquid holdup,Tracer method,Moment technique,Trickle bed reactors.     

Gas-bubble hold-up and axial dispersion coefficient of emulsion phases in fluidized beds

A model for liquid mixing in a gas-liquid bubble column is extended to predict gas-bubble hold-up and axial dispersion coefficient of emulsion phases in fluidized beds. Reasonable agreement is found between the proposed model which includes no adjustable parameter and the available experimental data and correlations.     

Water and mercaptan adsorption on 13X zeolite in natural gas purification process

The aim of this work is to model the adsorption process used for mercaptan and water removal from natural gas. Three fixed beds containing Zeolite molecular sieve type 13X, are used in this plant. In this operation, two beds are in process for adsorption purposes and the other one is regenerated simultaneously. This system is also operated under isothermal condition. In modeling of this process, rate of adsorption is approximated by linear driving force (LDF) expression, and the extended Langmuir isotherm is used to describe adsorption equilibrium. The set of partial differential equations of dynamic model is solved by Crank-Nicolson method. The effect of equations of state is also studied and the best equation fitting the industrial data is selected. Also, concentration profile is presented versus bed length at various times. The influences of pressure, inlet concentration and bed height on the breakthrough time are also investigated.     

On axial dispersion in packed beds with fluid flow: Über die axiale dispersion in durchströmten festbetten

In the present work a theoretical analysis of the dependence between the axial dispersion of mass and the flow nonuniformity in packed beds is given. Three types of flow nonuniformity are defined: micro-, meso- and macroscopical. In order to describe the influence of microscopical flow nonuniformity on axial dispersion in packed beds the flow channels are approximated by equal-sized cylindrical capillaries. A step function is used for the velocity profile and the fluid in the outer region of each capillary is assumed to be stagnant. The relationship derived in this manner can describe most results of tracer experiments well; above all, it helps in understanding the differences observed between dispersion of gases and dispersion of liquids. Anomalously high dispersion coefficients, obtained during dispersion of gases through beds of fine-grained particles, are attributed to the mesoscopical flow nonuniformity and are described in a model. Finally, the dependence between macroscopical flow nonuniformity and axial dispersion in packed beds is discussed. It turns out that the influence of channelling on axial dispersion is rather insignificant. In this manner, the results of tracer experiments can be brought into accordance with velocity profiles proposed in the literature. The essential differences between the present analysis and previous works are pointed out.     

EPA和DHA在超临界流体色谱柱中的传质动力学

用超临界流体色谱 (SFC)法研究了EPA和DHA在C18上的吸附和传质性能 .建立了色谱动力学模型并获得了模型参数 ,较好地拟合了实验流出曲线 .结果表明吸附平衡、轴向弥散、流体相传质和颗粒内有效扩散都是影响SFC分离EPA和DHA的重要因素     

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.     

Modeling of vanillin production in a structured bubble column reactor

Vanillin is an important flavour. Semi-synthetic vanillin can be produced by the oxidation of lignin. Experimental studies leading to vanillin production in a batch reactor and a structured bubble column reactor (SBCR) lead us to the conclusion that the SBCR could have non-idealities such as dispersion. The radial and axial liquid-phase dispersion within the packed criss-crossing sandwich structures of Mellapak-750Y had been studied. A 2D model accounting for axial and radial velocities and dispersion was formulated and solved. The model predictions were compared with that of an experimental residence time distribution curve. The axial dispersion coefficient of the liquid phase is of the same order of magnitude as the radial dispersion coefficient. The reaction kinetics available in literature is adopted for the present study. Model for the SBCR was formulated and simulated using commercial modeling software. Simulation experiments were conducted in a SBCR. The effect of the following parameters on the yield of vanillin is studied: lignin concentration, lignin molecular weight, oxygen partial pressure and reaction temperature. It can be said that lignin molecular weight is a crucial parameter in vanillin production.     

活性炭纤维填充床内多组分竞争吸附

建立了活性炭纤维填充床内多组分竞争吸附传质动力学模型 ,采用正交配置方法求解数学模型以预测突破曲线 ,从理论上探讨了竞争吸附平衡及吸附质在填充床内的轴向弥散、纤维内扩散和纤维外对流传质等因素对强、弱吸附组分突破曲线的影响。在间歇和填充床吸附器内进行了脱除水溶液中酚类化合物的实验 ,测定了活性炭纤维吸附水溶液中苯酚和氯代苯酚的吸附等温线 (间歇吸附 )以及苯酚和氯代苯酚在活性炭纤维填充床内竞争吸附时的突破曲线 ,并与模型计算值进行了比较。结果表明 ,吸附质在活性炭纤维内扩散和纤维外对流传质阻力不是填充床内吸附过程的控制步骤 ,而轴向弥散影响显著 ,不可忽略     

Theory of liquid phase dispersion in packed columns

Dispersion in the liquid phase for trickle or two-phase flow in packed columns has two important contributions, local dispersion in the packing, and axial dispersion due to the differences in liquid flow conditions between the wall and bulk regions of packing. A three-dimensional axisymmetric formulation of dispersion within the column is analysed and compared to a one-dimensional formulation in the axial co-ordinate. It is shown that the one-dimensional formulation is correct for the limit of long dispersion times. For other times the coefficient for total dispersion within the column is found to be a function of local dispersion, differences in flow between the wall and bulk regions, and dispersion time. An analytical relationship is given between the coefficient for total dispersion, and contributing factors, in local dispersion and column flows.     

活性炭纤维吸附填充床突破曲线预测(Ⅱ)实验及模型的验证

实验测定了活性炭纤维填充床吸附水中微量铅离子的突破曲线和吸附等温平衡数据,研究了活性炭纤维填充床的吸附性能。应用活性炭纤维吸附填充床突破曲线的精确解和两种近似解对实验数据进行了模拟。结果表明,本实验条件下理论计算值与实验数据均吻合较好;简单、计算速度快的拟对数概率密度函数近似解适用于模拟活性炭纤维填充床突破曲线;由突破曲线实验数据可方便准确地估算平衡常数;活性炭纤维吸附填充床内轴向弥散对突破曲线有较大影响,是不可忽略的因素。     

On the effect of gap size between baffle outer diameter and tube inner diameter on the mixing characteristics in an oscillatory‐baffled column

We report our experimental investigation on the effect of gaps between baffle outer diameter and inner tube diameter on the mixing characteristics, in terms of mixing time and axial dispersion coefficient, in a batch oscillatory‐baffled column. Local concentration profiles are measured using conductivity probes at two locations along the height of the column. The mixing time was determined based on the equilibrium concentration concept, and the axial dispersion coefficient was obtained by solving the axial dispersion governing equation. Comparison of mixing time between the ‘push‐fit’ and ‘loose‐fit’ baffle arrangements was carried out and the results showed that the existence of a gap of various sizes between the baffle outer and the tube diameters lengthened the time at which the state of uniform mixing is achieved in such a device. © 1999 Society of Chemical Industry     

Simulated pressurization of adsorption beds

Pressurization of adsorption beds with pure inert gas, a mixture of inert gases and a binary mixture of inert and adsorbable gases is studied. Modelling of pressurization involves mass and momentum balance equations. Linear equilibrium between adsorbed phase concentration and gas phase concentration inside pores for the active species is assumed. Model equations are numerically solved by collocation on finite elements. The importance of axial pressure gradients is assessed. Numerical simulations provide information on pressurization time, penetration distance and their relation with operating conditions.     

Impulse Response Analysis for Adsorption of Ethyl Acetate on Activated Carbon in Supercritical Carbon Dioxide

Abstract

Adsorption equilibria and mass transport properties for ethyl acetate on activated carbon have been evaluated in the presence of supercritical carbon dioxide by means of the impulse response technique. A two-column system, one for dissolving solute and one for adsorption, was used to evaluate the phenomena purely in the adsorption column. The adsorption equilibrium constant was larger for higher temperature and lower pressure. The apparent heat of adsorption calculated from the temperature dependence of the adsorption equilibrium constant was 30–40 kJ/mol. The intraparticle effective diffusivity was larger for higher temperature and lower pressure. The Peclet number that relates to the axial dispersion coefficient was 0.08–0.39. These estimated parameters are in good agreement with those obtained from desorption measurements reported in the literature.     

Modeling of Size Exclusion Parametric Pumping

Abstract

Sephadex G and Biogel P, well-known gels in size exclusion chromatography, show a large change in their elution behaviors as temperature changes. These phenomena were exploited to separate the binary model solutes Blue Dextran 2000 and nickel nitrate. A series of batch size exclusion parametric pumping experiments in the direct thermal mode was carried out previously to separate binary mixtures. In this paper the experimental separations in batch size exclusion parametric pumping are compared with the predictions of both a local equilibrium model and a dispersion model. The dispersion model, which includes an axial dispersion term, gives a better fit of the experimental data. Experimentally obtained adsorption isotherms are used in the dispersion model to predict the separation performance of nickel nitrate in the Sephadex gel column. The resulting simultaneous partial differential equations are solved by quasilinearization of the equations followed by numerical integration. For both models, all parameters were estimated independently.