A. Garea

Kinetics of flue gas desulphurization at low temperatures: fly ash/calcium (31) sorbent behaviour

Abstract The reaction rate equation for the flue gas desulphurization at low temperatures using fly ash/calcium sorbents has been determined in a fixed-bed reactor in the range of SO 2 concentration, 2000 ≤ C ≤ 5500 (ppmv), temperature 52 ≤ T ≤ 67°C and relative humidity 0.40 ≤ RH ≤ 0.90, working at conditions prevailing in the exhaust gases of coal-fired plants. The kinetic models discrimination procedure leads to a reaction rate expression based on the SO 2 adsorption on a nonideal surface as the rate-controlling step, in terms of an exponential influence of the calcium conversion in the reaction rate. The influence of the SO 2 concentration in the gas phase has been described by a partial reaction order equal to 1. Concerning the temperature influence, the kinetic constant does not depend on this variable, which can be explained by a sorption equilibrium step previous to the reaction, the observed kinetic constant being an apparent parameter. The relative humidity has been concluded to be the most relevant variable, with a significant influence on the kinetic behaviour of the sorbent, which can be described by the fitting of the parameter which accounts for the nonideal solid surface to the relative humidity. From the obtained conclusions, an appropriate kinetic model based on three parameters has been proposed for the kinetic description of the desulphurization reaction under study, i.e. when a sorbent based on a fly ash-Ca(OH) 2 mixture 3 1 is used in the process.

Preparation and characterization of fly ash/hydrated lime sorbents for SO2 removal

Abstract Coal combustion fly ash and its mixtures with lime and hydrated lime are being widely studied for use as sorbents in desulfurization processes. In this work, coal fly ash has been slurried with hydrated lime in a Parr pressure reactor. In order to study the preparation process of the solids, the influence of the operation variables (temperature, time, fly ash/Ca(OH)2 ratio, pressure and water/solids ratio) on the structural properties of the final sorbent, the BET-nitrogen specific surface area and the pore volume distribution are studied using a factorial design for the experiments. Results of the specific surface area were correlated by a linear expression of the fly ash/Ca(OH)2 ratio and hydration time, the variables which exhibit the main influence on the generation of specific surface area in the studied range of variables. The influence of the hydration time was studied for solids prepared at atmospheric pressure and under pressure. No influence of the studied variables on the total pore volume distribution was detected in the researched range of variables. X-ray diffraction analysis was also used for solids characterization, showing that the hydration reaction resulted in the formation of CaCO3, mullite and calcium aluminium silicate hydoxide. SEM analysis showed the macrostructural changes with the hydration reaction.

Fly-ash/calcium hydroxide mixtures for SO2 removal: structural properties and maximum yield

Abstract The structural properties and sulfation capacity of mixed solids obtained from pressurized hydration of commercial calcium hydroxide and coal combustion fly-ashes have been analyzed. Hydration experiments were planned according to a 2 5 fractional factorial design, studying the influence of temperature, time, pressure, fly-ash/Ca(OH) 2 weight ratio and water/solid weight ratio, on the BET specific surface area (5) and pore volume ( V p ) of the final solids, as well as on the dissolved calcium concentration [Ca]in the slurries. Whereas a strong dependence of the ratio fly-ash/Ca(OH) 2 on ( S ) and [Ca]functions was observed, with values of these parameters in the ranges 2.5-64.3 m 2 g −1 and 6.0–748.0 mg l −1 , respectively, no dependence was found of the studied variables on the pore volume (determined by Hg intrusion porosimetry), obtaining an average value of 1.077 cm 3 g −1 . Sulfation runs using the hydrated solids were conducted in a packed bed reactor with a pure humidified SO 2 gas phase, leading to a constant sulfation yield of 0.56 ± 0.07 mol SO 2 per mol Ca; this yield was independent of the solids specific surface area, and related only to the sorbents calcium content.

Kinetics of dry flue gas desulfurization at low temperatures using Ca(OH)2: competitive reactions of sulfation and carbonation

The kinetics of the sulfation and carbonation reactions of the sorbent Ca(OH) 2 were investigated in an isothermal packed-bed reactor for the application to the flue gas desulfurization process at low temperatures. The reaction temperature was analyzed at three levels: 55, 65 and 75°C; the relative humidity in the range between 40 and 70% and the sorbent load in the bed ranging from 200 to 600 mg. The experimental SO 2 breakthrough curves obtained were simulated and fitted to kinetic models according to least-squares minimization procedures by means of a commercial package for solving systems of PDEs. The sulfation and carbonation kinetics were described by a two parameters non-ideal adsorption model, considering a scheme of simultaneous reactions. The influence of the temperature and the relative humidity, was fitted by the kinetic parameters. The sorbent utilization profiles along the bed for sulfation and carbonation were also simulated, being the conditions at the inlet of reactor a good estimation of the sorbent activity at conditions prevailing in low-temperature desulfurization processes.

PHENOL RECOVERY WITH SLM USING “CYANEX 923”

Abstract The study of the phenol separation-concentration process with the hollow fiber supported liquid membrane technology has been performed. Mixtures of kerosene and CYANEX 923 were used as liquid membrane. The extractant CYANEX 923 is characterized by a high phenol selectivity and an extremely low solubility in the aqueous phase. The introduction of CYANEX 923 in the membrane composition decreases the extractant losses from the pores of the support. The phenol separation and simultaneous concentration process has been checked. The influences of the initial concentration of phenol in the feed solution and sodium hydroxide in the stripping phase and the membrane composition on the separation rate have been investigated in a single-pass mode. The analysis of the membrane composition influence has been performed according to the steady-state mass transfer conservation equation and the associated boundary conditions, leading to the mass transfer parameters of the process

Desulfurization yield of calcium hydroxide/fly-ash mixtures. Thermogravimetric determination

Abstract This work reports the thermogravimetric characterization of the solid products resulting from the low temperature reaction between SO2 and mixed solids containing hydrated mixtures of calcium hydroxide and fly-ashes. The latter sorbents were obtained after a pressurized hydration procedure of coal fly-ashes and commercial calcium hydroxide following a factorial design of the experiments. The analysis of the TG curves performed in a high temperature thermobalance allowed calculation of yield of the desulfurization reaction expressed as mol SO 2 mol Ca , a parameter needed for the design and optimization of desulfurization processes.

Flue gas desulfurization at low temperatures.Characterization of the structural changes in the solid sorbent

This work has analysed the evolution of the structural properties, pore volume (Vp) and surface area (S), ofthe solid sorbent during the reaction between SO2 and hydrated lime at low temperatures. Two different types of experiments have been carried out: (i) experiments directed towards the analysis of the influence of the composition of the gas phase on the structural properties of the solid; (ii) experiments directed towards the analysis of the influence of the presence of the reaction product, CaSO3 · H2O, on the structural properties. Some fluid-solid reactions are characterized by incomplete conversion as result of premature pore closure at the surface of reacting particles; however, in the case under study it has been observed that when the maximum solid conversion is reached, 0.6 mol SO2/MOl Ca, there are still pore volume and surface area in solid. On the other hand the results obtained in this work show a linear decrease of the structural parameters, S and Vp, with solid conversion; the main decrease is produced in the region of mesopores (0.2–0.009 μm) which has been demonstrated to be the effective pore region for the desulfurization reaction.

Recovery of sulfur dioxide using non-dispersive absorption

Removal of SO2 from gas emissions by selective absorption into a liquid is a common method to reduce air pollution and environmental risks. The absorption efficiency is determined by the interaction between the gases and the liquid. A great number of gas desulphurization methods have been developed where aqueous or organic solvents are used as sorbents.N,N-dimethylaniline (DMA) is an organic solvent used in the industry because its affinity with SO2. This absorption is neither too strong nor too weak, thus absorption and desorption can occur leading to a regenerative process where SO2 can be recovered. However, a direct contact between SO2 and DMA leads to several environmental problems caused by solvent evaporation and drops dragging into the gas stream.In order to increase the process efficiency and reduce environmental risks, a non-dispersive absorption process using hollow fibre membrane modules is developed in this work for a solvent zero emission process. The mass transfer into a fibre may be described by three main assumptions: gas-phase laminar flow, gas-phase plug-flow and gas-phase mixing. A numerical calculation was carried out to establish the performance of a hollow fibre membrane contactor for the removal of SO2 when water and N,N-dimethylaniline are used as sorbents in order to compare both, wetted and non-wetted operating modes.

Multiobjective optimization of membrane processes for chemicals ultrapurification

Abstract The present work is focused on the multiobjective optimization of integrated multistage reverse osmosis membrane cascades applied to the ultrapurification of chemicals for the semiconductor industry. The membrane systems were formulated adding product quality metrics to economic criteria to result a bi-criteria nonlinear programming (NLP) problem. The Pareto solutions to the problem were generated via the epsilon constraint method. On the one hand, maximum economic profit solutions corresponded with the configurations applying bypass. The bypass from the feed stream of the last stage was preferred over other proposed options. On the other hand, maximum quality solutions were obtained by low recovery rates (specifically in the last stages of the cascade) and the consequent high flow (and low metallic content) streams recirculated to previous stages.

Flue-gas desulfurization at medium temperatures. Kinetic model validation from thermogravimetric data

A thermogravimetric technique was used in the kinetic study of SO2 reaction with Ca(OH)2 in the medium temperature range (300–400°C) in order to compare results of different reaction systems. Two kinds of sample-holders were used: when a cylindrical sample-holder is used, the initial sulfation rate is mass-transfer limited; this resistance can be neglected using a specially designed ‘net’ sample-holder. From these experiments a strong decrease in the reaction rate was observed, ending with the deactivation of the sorbent at calcium utilization much lower than 1. The experimental ‘conversion versus time’ curves were compared with the simulations of the exponential kinetic model previously developed for the medium temperature desulfurization reaction in an integral sand-bed reactor: rs= ky(1-xt)exp-γ0RTexpDTx1 . This model allows a good description of the thermogravimetric sulfation data.