Absorption of CO2 into aqueous solutions of MEA and AMP in a wetted wall column with film promoter

The process for removing CO₂ from a gas mixture can be achieved by absorption into amines through of different conventional techniques. But, the report of mass transfer parameters that can be used for process design is limited. Thus, a study of CO₂ absorption process into monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP) and their respective blend were developed with the aim of evaluating the absorption performance and of determining a design parameter for a single-amine absorption system. The experimental tests were carried out in a wetted wall column employing a film promoter, a thin stainless steel woven. The experimental device was operated in countercurrent and the different tests were developed at 298 K, atmospheric pressure, a constant gas load (10% CO₂) and three different liquid loads. The results show that the interfacial area is influenced by increasing the liquid load. The K_Ga values of MEA and the blend based on it are higher in relation to AMP. Moreover, the results of the thermodynamical-kinetic parameter show that this absorption process is independent of liquid load, characteristic of rapid chemical reactions. The K_Ga value for the different MEA:AMP mixtures shows that it increases when the MEA proportion is increased in the blend.     

Kinetics of absorption of carbon dioxide in aqueous piperazine solutions

In the present work the absorption of carbon dioxide into aqueous piperazine (PZ) solutions has been studied in a stirred cell, at low to moderate temperatures, piperazine concentrations ranging from 0.6 to , and carbon dioxide pressures up to 500 mbar, respectively. The obtained experimental results were interpreted using the DeCoursey equation [DeCoursey, W., 1974. Absorption with chemical reaction: development of a new relation for the Danckwerts model. Chemical Engineering Science 29, 1867-1872] to extract the kinetics of the main reaction, 2PZ+CO₂→PZCOO^-+PZH⁺, which was assumed to be first order in both CO₂ and PZ. The second-order kinetic rate constant was found to be at a temperature of , with an activation temperature of . Also, the absorption rate of CO₂ into partially protonated piperazine solutions was experimentally investigated to identify the kinetics of the reaction . The results were interpreted using the Hogendoorn approach [Hogendoorn, J., Vas Bhat, R., Kuipers, J., Van Swaaij, W., Versteeg, G., 1997. Approximation for the enhancement factor applicable to reversible reactions of finite rate in chemically loaded solutions. Chemical Engineering Science 52, 4547-4559], which uses the explicit DeCoursey equation with an infinite enhancement factor which is corrected for reversibility. Also, this reaction was assumed to be first order in both reactants and the second-order rate constant for this reaction was found to be at 298.15 K.     

Development of a tool, using CFD, for the assessment of the disinfection process by ozonation in industrial scale drinking water treatment plants

Foreseen standards regarding microorganism content for drinking water require assessment of the capability of existing plants to reach the upcoming requirements. This paper presents the development of a tool to assess this capability in a commonly encountered key step of water disinfection: ozonation. In this paper, this tool is applied to the test case of an ozonation channel of the Belgian drinking water producer Vivaqua. This tool is based on a mathematical model of the momentum and mass transport phenomena in an ozonation channel. The gas–liquid flow is coupled to ozone mass transfer and kinetics describing the ozone and microorganisms concentrations decay. The degradation of Bacillus subtilis spores, as a representative of resistant microorganisms, is implemented in the model. The model takes explicitly into account the bubble size variation and its impact on mass transfer. Bubbles sizes and kinetics parameters are estimated based on dedicated experiments. The model is partially validated by comparing simulations results, obtained using computational fluid dynamics, to experimental residence time distributions, residual ozone concentration and Bacillus subtilis spores degradation efficiency measurements obtained on the studied ozonation channel. It is shown that, at the industrial scale, bubble diameter variation has a significant impact on ozone concentration in the liquid at the reactor exit. Using the tool, it is also shown that, the ozonation channel of Vivaqua can be used to achieve degradation of resistant microorganisms but only with its maximal flow rate and concentration of ozone injection. Moreover, at low operating temperature, some microorganisms that present latency towards reaction with dissolved ozone might hardly be destroyed.     

Simultaneous absorption of mercury and chlorine in sulfite solutions

Simultaneous absorption of mercury (Hg) and chlorine (Cl₂) into aqueous sulfite/bisulfite (0 to S(IV)) at pH 4.7 and 5.7 was measured in a wetted wall column. Experiments were performed at ambient temperature and pressure using 5- Cl₂ and 46 ppb Hg. Absorption was modeled using the theory of mass transfer with chemical reaction. At the gas/liquid interface, chlorine oxidizes the elemental Hg to a more soluble form. The rate constant for the reaction of mercury and chlorine was determined to be . Mercuric chloride and sodium hypochlorite also enhanced Hg absorption. The addition of sodium chloride did not affect the rate of Hg/Cl₂ absorption with S(IV). When no S(IV) was present, the chloride significantly enhanced Hg absorption. Possible reaction pathways are discussed. These results are relevant in the simultaneous removal of chlorine, sulfur dioxide, and elemental mercury from flue gas. A model was developed to predict the expected Hg removal in a limestone slurry scrubber. Mercury removal decreases as the S(IV) concentration increases. The process feasibility will depend on the SO₂/S(IV) concentration of the scrubber, the desired Hg removal, and the amount of Cl₂ which can be tolerated.     

Theoretical Aspects Of The Kinetics Of Competitive Ozone Reactions In Water

Competition of direct and decomposition reactions that ozone can undergo in water when a given compound is present is studied according to concepts of absorption theories. The importance of different parameters, such as pH, rate constants, mass transfer coefficients and concentration of reactants, is considered to clarify which reaction prevails. Parameters like Hatta numbers (film theory) or diffusion and reaction times (surface renewal theories) are key factors to check the competition of these reactions.     

Modeling CO2 absorption into concentrated aqueous monoethanolamine and piperazine

Dugas and Rochelle (2011) measured CO₂ mass transfer rates in 7–13 molal aqueous monoethanolamine (MEA) and 2–12 molal piperazine (PZ) from 40 to 100 °C over a large range of CO₂ loading. They observed that the liquid phase mass transfer coefficient (k′_g) was almost independent of amine concentration and temperature. In this paper models are created to explain this behavior.CO₂ reaction rates in MEA and PZ are represented with termolecular (base catalysis) kinetics with activity-based rate expressions. Solving the activity-based rate expressions with a shell balance and implementing diffusion resistance using film theory yielded an expression for the liquid phase mass transfer coefficient, k′_g. Parameters in the k′_g expression were estimated from existing literature data. Two pre-exponential rate constants (k_MEA and k_PZ) are the only parameters that were adjusted to match experimental data.Estimates from independent sources of parameters in the model for k′_g fully account for the observed effects of CO₂ loading, temperature, and amine concentration. The k′_g expressions match the 93 wetted wall column experimental rates measured by Dugas and Rochelle (2011) with average deviations of 13% for MEA and 19% for PZ. The mass transfer expressions also match experimental data obtained by other researchers.This model shows that complex rate behavior in MEA and PZ systems can be fully explained using existing literature data. It also shows that the MEA and PZ systems can be represented by termolecular kinetics on an activity basis.     

单元操作的过程强化

过程强化研究渐成热点,过程技术和设备是化工过程强化的两个方面。单元操作是化工过程基本操作,其过程的强化对化工过程有较大的影响。本文综述了化工原理课程中各主要单元操作的过程强化。     

Determination of diffusion coefficient from transitory uptake or release kinetics: Incidence of a recirculation loop

The determination of the diffusion coefficient of a solute in a solid matrix (polymers, gels, adsorbents) is often determined based on batch kinetics experiments performed when a given amount of matrix is put into a stirred cell containing a defined volume of diluting fluid (gas or liquid). There is, however, a strong trend to develop smaller and smaller volume set-ups (due to, for instance, the cost of either the solute or the matrix in biological or pharmaceutical fields). Furthermore, a continuous monitoring of the solute concentration in the release (or exhaustion) medium is obviously attractive and frequently applied, in place of a sequential manual sampling protocol. This option demands, however, most often a continuous recirculation loop to be implemented on the cell volume, so that a continuous detector (such as spectrophotometric, conductimetric or refractive index) can be used. The objective of this work is to analyze the influence of such a recirculation loop, which may affect significantly the hydrodynamics (fluid residence time distribution) as well as the dynamics (delayed signal) of the experimental kinetics, in order to evaluate the impact on the diffusion coefficient determination. Numerical simulations covering a broad range of situations have been performed, and an experimental validation on a system consisting of alginate hydrogel beads as a model matrix and pullulan molecules (molecular weight ranging between 730 and 880 000) as model solute is reported. Practical guidelines are finally proposed in order to estimate, through ab initio shortcut methods, the error induced by the recirculation loop, based on explicit experimental parameters (cell/loop volume, recirculation loop residence time, solute diffusion time constant).     

Kinetic study of hydrogen sulfide absorption in aqueous chlorine solution

Hydrogen sulfide (H₂S) is currently removed from gaseous effluents by chemical scrubbing using water. Chlorine is a top-grade oxidant, reacting with H₂S with a fast kinetic rate and enhancing its mass transfer rate. To design, optimize and scale-up scrubbers, knowledge of the reaction kinetics and mechanism is requested. This study investigates the H₂S oxidation rate by reactive absorption in a mechanically agitated gas–liquid reactor. Mass transfer (gas and liquid sides mass transfer coefficients) and hydrodynamic (interfacial area) performances of the gas–liquid reactor were measured using appropriated physical or chemical absorption methods. The accuracy of these parameters was checked by modeling the H₂S absorption in water without oxidant. A sensitivity analysis confirmed the robustness of the model. Finally, reactive absorption of H₂S in chlorine solution for acidic or circumneutral pH allowed to investigate the kinetics of reaction. The overall oxidation mechanism could be described assuming that H₂S is oxidized irreversibly by both hypochlorite anion ClO⁻ (k = 6.75 × 10⁶ L mol⁻¹ s⁻¹) and hypochlorous acid ClOH (k = 1.62 × 10⁵ L mol⁻¹ s⁻¹).     

Intensification of Deep Hydrodesulfurization Through a Two-stage Combination of Monolith and Trickle Bed Reactors

Deep hydrodesulfurization (HDS) is an important process to produce high quality liquid fuels with ultra-low sul-fur. Process intensification for deep HDS could be implemented by developing new active c...     

Kinetic Study of the Ozonation of Some Industrial Wastewaters

Kinetic studies of the ozonation of two wastewaters released by distillery and tomato processing plants have been carried out. Once it has been assumed that an irreversible gas-liquid reaction is developed between ozone and the matter present in the water, the film theory concept was applied to this system for kinetic determinations. The evolution of the organic and inorganic matter with ozonation time has been followed by the chemical oxygen demand. The procedure allows the determination of the rate coefficients of ozone with the wastewaters treated. According to the results obtained, ozone is consumed through fast reactions which take place near the water-gas interface during an initial period. This period is used to determine the rate coefficients. Then, at more advanced ozonation times, the reactions become slower and hence they take place in the bulk of the water, articularly for the case of tomato wastewaters. Values of the rate coefficient allow us to establish both the kinetic regime of absorption and to compare the reactivity of ozone with the wastewaters and single compounds.     

The Role of Mass Transfer in the Kinetics of Chemical Oxidation of Anthracine Slurry to Anthraquinone in a Batch Stirred Tank Reactor

The kinetics of chemical oxidation of anthracine powder by acidified dichromate in a batch agitated vessel stirred by a 45° four pitched blade turbine impeller was studied under different conditions. Variables studied were impeller rotation speed, solution physical properties and temperature. The rate of anthracine oxidation was found to increase with temperature according to the Arrhenius equation with an activation energy of 3.98 cal/mole. The rate of anthracine oxidation was found to increase with 0.56 power of impeller rotation speed. The value of the activation energy and the sensitivity of the rate of oxidation to stirring lend support to the diffusion‐controlled nature of the reaction. The data were correlated by the equation: Sh = 0.5 · 10^–3 Sc^0.33 · Fr^0.33 Implication of the above equation for the operation of industrial reactors was noted.     

THE EFFECTS OF THE SOLUBILITY LAW ON MASS TRANSFER IN ANNULAR LIQUID JETS

Underpressurized, annular liquid jets have been studied under steady and transient conditions in order to assess the effects of Henry's and Sievert's solubility laws on mass transfer. It is shown that, in general, Henry's solubility law predicts a higher, steady state mass absorption rate than Sievert's law. The rate at which the under-pressurized, annular liquid jet tends towards an equilibrium, non-pressurized configuration is higher for Henry's than for Sievert's solubility law. Both this rate and the steady state mass absorption rate increase as the Weber number, the ratio of the solubility at the inner interface to that at the outer one, the product of the gas constant times the temperature and the solubility of the gases surrounding the jet, the ratio of the product of the pressure of the gases surrounding the annular jet times the annular jet's mean radius at the nozzle exit to the liquid surface tension, and the nozzle exit angle are increased, and as the Froude and mass Peclet numbers, the annular jet's thickness-to-mean radius ratio at the nozzle exit and the initial pressure of the gases enclosed by the annular jet are decreased.     

Intensification of biodiesel synthesis using metal foam reactors

This study presents a technology for continuous and high-efficiency alkali-catalyzed biodiesel synthesis using a metal foam reactor combined with a passive mixer. A metal foam reactor with higher pore density produces smaller droplets that result in higher efficiency of biodiesel synthesis. Compared with conventional stirred reactors, the time for high methyl ester conversion can be shortened remarkably by the use of metal foam reactors. Experimental results reveal that a metal foam reactor of 50 pores per inch exhibits an energy consumption per gram biodiesel of 1.01 J g⁻¹, merely 1.69% and 0.77% of energy consumption of the zigzag micro-channel and conventional stirred reactors, respectively. Moreover, biodiesel yield per reactor for the metal foam reactor is approximately 60 times that of the zigzag micro-channel reactor, thus overcoming the problem of numbering up an excessive number of reactors in the application. These results indicate the great potential of metal foam reactors in small-fuel biodiesel processing plants for distributive applications.     

Catalytic oxidation of methanol to formaldehyde: an example of kinetics with transport phenomena in a packed-bed reactor

In the present work the partial oxidation of methanol to formaldehyde has been studied as an example of strongly exothermic reaction affected by internal diffusion in order to deep the topic of mass and heat transfer in packed-bed catalytic reactors both at particle level, introducing the calculation of the effectiveness factor for complex reactions network, and at reactor level, for what concerns long range gradients of composition and temperature. The aim of the work is to stress the impact of the use of rigorous numerical methods, today possible for the high performances reached by the computers, in the solution of a simultaneous set of many differential equations that are necessary to describe completely the mentioned system. A complete mathematical model of the particle and the reactor is presented and a solution strategy is reported for the chosen reaction by considering a reliable kinetic law and evaluating related parameters from experimental data reported by the literature. Calculation results are reported for both particle internal profiles and reactor simulation. The described approach can easily be extended to many other devices and reactors geometry such as, e.g., the ones used in the field of environmental catalysis.     

Kinetic Regime Changes in the Ozonation of 1,3-Cyclohexanedione in Aqueous Solutions

The reaction between ozone and 1,3-cyclohexanedione in aqueous solutions at different pH was investigated from the point of view of the film theory. The rate of ozone absorption is accompanied by a second order irreversible reaction with 1,3-cyclohexanedione. The kinetic regime of absorption was found to change from fast (the reaction being of pseudo first order) to instantaneous, according to the ozone partial pressure applied. The former kinetic regime allows the determination of the reaction rate constant, while the latter leads to the volumetric mass transfer coefficient of the system.     

Mechanistic aspects of oxalic acid oxidation by photocatalysis and ozonation

Oxalic acid has been oxidised in acidic aqueous solutions (pH 3) using photocatalysis and ozonation alone or coupled. The simultaneous presence of ozone, titanium dioxide and near UV irradiation increases the oxidation rate of oxalic acid to values greater than those deriving from the single contributions of photocatalysis and ozonation. In particular in the present paper ozonation alone, heterogeneous photocatalysis and also combined ozonation with heterogeneous photocatalysis have been used for the oxidation of oxalic acid at acidic pH in the presence of TiO₂ Degussa P25. A likely mechanism, able to explain both the homogeneous and heterogeneous processes, is discussed.     

Azo Dye Ozonation Film Theory Utilization for Kinetic Studies

The ozonation kinetics of three azo dyes (Direct Yellow 27, Direct Blue 1 and Acid Black 52) in aqueous solution has been studied. Two types of reactors have been used, an agitated tank for studying the influence of variables and the stoichiometry, and an agitated cell for kinetic measurements. Both the film theory and the modes of ozone action on organic matter have been considered for obtaining the kinetic rate constants. A model of ozone absorption in the fast pseudo–m–th ozone order kinetic regime with two parallel reactions fits satisfactorily the experimental results. From it, the kinetic constants of both reactions have been evaluated.     

MASS TRANSFER EFFECTS ON THE ETCH RATE OF GAAS IN FLOW SYSTEMS

In industrial wet etching reactors, the fluid contacts the substrate surface as a spray of flowing stream, thus introducing mass-transfer resistances to the reaction rate. The etching of gallium arsenide in H₂O₂-NH₄OH-H₂O solutions was studied using an open-channel flow reactor to simulate the industrial conditions. The etch rate was always lower than that obtained under kinetic control, and the dependence of etch rate on H₂O₂ concentration shifted closer to first order. From the calculation of the ratio of rate constant to mass-transfer coefficient, the reaction-rate and mass-transfer resistances were both significant in this system. When the mass-transfer coefficient was calculated from equations for flow past a flat plate, the prediction of etch rate was good, particularly when the starting length for velocity boundary layer development ahead of concentration boundary layer development was taken into account. Another approach for the calculation of mass-transfer coefficient, based on the assumptions for flow between parallel plates, best represented the relative insensitivity of etch rate to fluid velocity.