Length Of Mass Transfer Zone Research Articles

Adsorption of sulphur dioxide onto activated carbon prepared from oil-palm shells with and without pre-impregnation

Adsorption of sulphur dioxide (SO2), a common gaseous pollutant, on oil-palm-shell activated carbon in a fixed bed was studied in this paper. Oil-palm shell is an abundant agricultural solid waste in tropical countries like Malaysia and Thailand. The effects of fixed-bed length, SO2 gas superficial velocity, adsorbent particle size and internal pore structure on fixed-bed performance were investigated. Some characteristic parameters such as the breakthrough time, τ0.05, exhaustion time, τ0.95, length of mass transfer zone, LMTZ, adsorptive capacity, W, and adsorption rate constant, K, were derived from the breakthrough curves. Tests of SO2 adsorption onto activated carbons prepared from oil-palm shells pre-impregnated with potassium hydroxide (KOH) and phosphoric acid (H3PO4) of different concentrations were also carried out. It was found that the fixed-bed performance was not only dependent on the operating conditions and the textural properties of the adsorbent but was also influenced by the surface chemistry of the adsorbent, which was related to the type and concentration of the impregnating agent. In general, the quality of oil-palm-shell activated carbon prepared by CO2 activation is comparable to that of a commercial product, and the samples prepared from oil-palm shell with KOH pre-impregnation are more suitable for the removal of SO2 gas.

Absorption kinetics and dynamics of gaseous impurities in helium and hydrogen by intermetallic compounds

Absorption kinetics and dynamics of various gaseous impurities in helium and hydrogen by intermetallic compounds Zr 0.8Ti 0.2Mn 2 and ZrCrFe were studied in the range of impurity concentration 10 −6–10 −2 vol.%, the temperature range 570–1170 K and at 0.3–3.3 MPa of gas pressure. Kinetic equation of the first order chemical reaction was used for evaluation of the sorption rate constants. The dependencies of the rate constant of methane and nitrogen sorption on the amount of the impurity absorbed were obtained for various temperatures of the getter and gas pressures. Length of mass transfer zone and dynamic sorption capacity were measured for dynamics of different impurities absorption from a multi-component gas mixture. The effect of various gas–solid reactions on dynamics of the impurities absorption has been observed.

Effect of the process parameters on the length of the mass transfer zone during product withdrawal in pressure swing adsorption cycles

The effects of several process parameters on the length of the mass transfer zone (MTZ) during product withdrawal in equilibrium controlled pressure swing adsorption (PSA) operations are investigated in this study. The process parameters under consideration are the adsorbent selectivity, the initial MTZ length, the product purity, and the adsorption pressure. For locally equilibrated PSA processes the analytical solution for binary mixtures is used to develop analytical expressions for the dependence of the MTZ length on the above mentioned parameters. For equilibrium controlled PSA processes with modest mass transfer limitations the evolution of the MTZ length during product withdrawal is examined through numerical simulations using a linear driving force (LDF) model. It is observed that the trends exhibited by the locally equilibrated PSA processes pertain to PSA processes with modest mass transfer limitations as well. It is found that the length of the MTZ during product withdrawal is strongly affected by the adsorbent selectivity when the selectivity is below four, and by the initial MTZ length. The length of the MTZ during product withdrawal is modestly affected by the adsorbent selectivity when the selectivity is above eight, and by the product purity. The length of the MTZ during product withdrawal is not affected at all by the adsorption pressure for locally equilibrated PSA processes. For PSA processes with modest mass transfer limitations the length of the MTZ during product withdrawal is only mildly affected by the adsorption pressure. These results enhance the theoretical understanding of the PSA processes. They can be used in PSA design to optimize the bed size and reduce the adsorbent inventory and the pressure drop.

Design of a carousel process for cesium removal using crystalline silicotitanate

AbstractA three‐column carousel process based on a pelletized form of CST powder was designed to remove radioactive 137Cs+ from SRS nuclear wastes. A multicomponent ion‐exchange equilibrium model was used to generate cesium loading data, which were fit to the Langmuir equation to obtain effective single‐component cesium isotherm parameters for representative wastes. Mass‐transfer parameters were estimated by analyzing breakthrough curves for two simulated wastes. Simulations based on a pore‐diffusion rate model were performed to determine the lengths of the mass‐transfer zone for different feed compositions and linear velocities. The length of a single segment in the carousel process was the constant‐pattern mass‐transfer zone to ensure consistent high column utilization during startup and cyclic steady state. Analysis of the dimensionless groups in the mass balance equations revealed that the mass‐transfer zone length is proportional to the particle Peclet number. The proportionality constant is a function of the waste composition and the Cs+ concentration in the waste. A higher distribution coefficient and isotherm nonlinearity result in a smaller proportionality constant. With this analysis designs can be easily adjusted for variations in isotherms, feed concentration, particle size, linear velocity, and intraparticle diffusivity.

Adsorption dynamics in a stratified convergent tapered bed

Mass transfer characteristics of a novel fixed-bed adsorber configuration with a convergent tapered geometry and reverse layering of adsorbent particles is examined in this paper from both experimental and theoretical standpoint. In the tapered convergent bed, the adsorbent particle diameters and the fluid flow velocity decrease in the direction of flow. Laboratory column studies conducted for the adsorption of trichloroethylene on activated carbon in tapered convergent beds show that the solute front sharpens as it travels along the column depth, resulting in a longer time to breakthrough compared to a conventional stratified fixed-bed adsorber. The experimental results were verified using a modified version of the homogeneous solid diffusion model (HSDM). Model simulations indicate that the mass transfer zone length in this adsorber is sensitive to the angle of taper of the bed, particle size distribution, and influent concentration. With a modest taper angle of 1.5°, this system is shown to provide increase in breakthrough times ranging from 46 to 75% over that for conventional fixed-bed adsorbers operating under same conditions. The improved bed utilization afforded by this arrangement can reduce adsorbent inventory and operating costs relative to a conventional stratified fixed-bed adsorber.

Adsorption Kinetics and Breakthrough Behaviour of Tri-n-butyl Phosphate on Amberlite XAD-4 Resin

Adsorption behaviour of tri-n-butyl phosphate (TBP) from aqueous solution (dilute HNO3) on Amberlite XAD-4 has been investigated experimentally. Factors affecting the adsorption rates and breakthrough curves have been studied in batchwise and semicontinuous fixed bed adsorber. The adsorption isotherm of TBP on Amberlite XAD-4 resin have also been determined and the Freundlich model of adsorption is used to describe adsorption equilibrium. For the case of nonlinear equilibrium a simplified method proposed by Chi Tein is used to estimate intraparticle diffusion coefficient. The intraparticle diffusion coefficient is found to be in the range of 10−8 cm2·s−1. Results of fixed bed column are used to estimate mass transfer zone (MTZ) length, length of unused bed (LUB), breakthrough capacity corresponding to fixed value of effluent concentration (5 mg/l of TBP) and degree of saturation of bed at breakthrough. The MTZ length was found to vary from 21.8 cm to 37.3cm when flow rate changed from 13.5 ml/min to 52 ml/min, whereas column utilization ranged from 65.1% for the lowest flow rate to 43.5% for the highest flow rate.

A Simple Model for Strontium Breakthrough on Zeolite Columns

Abstract The Process Waste Treatment Plant (PWTP) at the Oak Ridge National Laboratory is designed to remove radioactive contaminants, principally 90Sr, from process wastewater. Planned upgrades to the PWTP will use chabazite zeolite columns. Pilot-scale studies have shown that mass transfer zone lengths increase from 10 to about 30 cm as the superficial velocity increases from 5.5 to 22 cm/min. Calculations with a multicomponent equilibrium model showed that the distribution coefficient for strontium remains essentially constant over the process conditions, suggesting that a simple kinetic model (the Rosen long-bed solution) should adequately represent breakthrough behavior. Using a distribution coefficient of 4.87 L/g predicted by the equilibrium model, good agreement was found between experimental breakthrough curves and those calculated with the Rosen solution. This model allows prediction of bed depths and cycle times necessary to achieve the required decontamination factor at high zeolite utilizatio...

Liquid phase dehydration of ethanol solutions in a fixed bed of molecular sieves

The liquid phase dehydration of ethanol solutions with type 3A molecular sieves in fixed-bed adsorption columns has been studied under ambient conditions. The breakthrough curves for this system are well predicted by the model of Weber and Chakravorti, which considers both the effects of external film and internal pore diffusional resistances. The concept of mass transfer zone (MTZ) provides a simple design procedure for the system. The fraction ability of the adsorbent within the mass transfer zone to adsorb solute was found to have a constant value of 0.43. The MTZ length was observed to be independent of bed height. Variations of MTZ length with feed concentration and fluid velocity were evaluated.

An experimental comparison of type A molecular sieves for drying the ethanol-water azeotrope

The recovery by adsorption of ethanol from aqueous vapours of composition around that of the azeotrope has been studied using fixed beds packed with various type A molecular sieves. For similar adsorption and desorption conditions, 4A is superior to 3A molecular sieve giving higher capacities, shorter mass transfer zone lengths and greater rates of desorption. Very low water concentrations (< 0.05 wt% water) can be achieved provided that the initial bed temperature and initial water concentration in the feed are not too high. Type 5A and 10A molecular sieves are not suitable for this separation since high temperature increases which arise from the adsorption of both water and ethanol cause the ethanol to react to form undesirable by-products.

Design Considerations for GAC Treatment of Organic Chemicals

Procedures with which to determine preliminary fixed‐bed adsorber design calculations for single solutes are described. Correlations are proposed and verified to determine single‐solute isotherm and mass transfer parameters. These correlations are used in combination with a simplified version of a mass transfer model to calculate mass transfer zone lengths and the maximum amount of water that may be treated. The calculated results can then be used to select the optimum fixed‐bed adsorber operation. Comparisons of this simplified procedure with actual pilot‐plant and full‐scale data are presented to demonstrate its validity.