Methods with Mass Transfer Enhancement

In a heterogeneous gas-liquid reactor system, that is where gas absorption precedes a liquid-phase reaction, the mass transfer rate has to at least equal the reaction rate. This principle can be used to determine mass transfer coefficients and/or reaction rate constants for certain kinetic regimes (see Section B 3.2.1). To determine the mass transfer coefficient, the kinetic regime must be instantaneous, and the place of the reaction must be in the film (Charpentier, 1981 Beltran and Alvarez, 1996). To determine the reaction rate constant, the kinetic regime must be fast and kLa must be known. 

An instantaneous reaction is the fastest reaction possible and no gas is transferred into the liquid bulk. This can be utilized to determine kLa, for example with the reaction of ozone with certain fast-reacting organic compounds. The reaction develops in a reaction plane located either 

The situation is characterized by the fact that both reactants are entirely consumed, so that cL = c(M) = 0 holds in the plane. Only in the latter case can kLa be determined. In the former case there is no transport of ozone into the liquid film, so that the mass transfer rate is only determined by kaa (Charpentier, 1981). The reaction rate depends on the mass transfer rate of ozone and pollutant to the reaction plane in the liquid film, but not on the reaction rate constant. Whether the reaction develops instantaneously in the liquid film depends on the experimental conditions, especially on the values of the applied ozone partial pressure p 03) and the initial concentration of M c(M)0. For example, the reaction tends toward instantaneous for low p(03) and high c(M)0. 

If an agitated cell is used, both problems are overcome because the transfer area is known and p(03) is practically constant due to continuous dosing and complete mixing of the gas-phase. 

For example, Beltran and Alvarez (1996) successfully applied a semi-batch agitated cell for the determination of kL k,a, and the rate constants of synthetic dyes, which react very fast with molecular ozone (direct reaction, kD = 5 105 to 1 108 L mol-1 s l). In conventional stirred tank reactors operated in the semi-batch mode the mass transfer coefficient for ozone kLa(03) was determined from an instantaneous reaction of ozone and 4-nitrophenol (Beltran et al., 1992 a) as well as ozone and resorchinol (l,3-c//hydroxybenzene) or phloroglucinol 

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Since we know the mass of ozone transferred has to have reacted or left the system, it is relatively easy to determine the reaction rate for slow reactions, which are controlled by chemical kinetics with this method. For kinetic regimes with mass transfer enhancement, the two rates, mass transfer and reaction rate are interdependent. Whether kLa or kD can be determined in such a system and how depends on the regime. Possible methods are similar to those described below in Section B 3.3.3 (see Levenspiel and Godfrey, 1974). 

The simplest method to measure gas solubilities is what we will call the stoichiometric technique. It can be done either at constant pressure or with a constant volume of gas. For the constant pressure technique, a given mass of IL is brought into contact with the gas at a fixed pressure. The liquid is stirred vigorously to enhance mass transfer and to allow approach to equilibrium. The total volume of gas delivered to the system (minus the vapor space) is used to determine the solubility. If the experiments are performed at pressures sufficiently high that the ideal gas law does not apply, then accurate equations of state can be employed to convert the volume of gas into moles. For the constant volume technique, a loiown volume of gas is brought into contact with the stirred ionic liquid sample. Once equilibrium is reached, the pressure is noted, and the solubility is determined as before. The effect of temperature (and thus enthalpies and entropies) can be determined by repetition of the experiment at multiple temperatures. 

The chemical method used to estimate the interfacial area is based on the theory of the enhancement factor for gas absorption accompanied with a chemical reaction. It is clear from Equations 6.22-6.24 that, in the range where y > 5, the gas absorption rate per unit area of gas-liquid interface becomes independent of the liquid phase mass transfer coefficient /cp, and is given by Equation 6.24. Such criteria can be met in the case of absorption with... 

On the enhancement of mass transfer, Tamir [5] studied the absorption of acetone into water with a similar method, i.e. using a partition. The results they obtained were under suitable operating conditions and with appropriate structural parameters, the runs without partition yield absorption rates higher than those with partition by over 4 times. 

The enhancement factors are either obtained by fitting experimental results or are derived theoretically on the grounds of simplified model assumptions. They depend on reaction character (reversible or irreversible) and order, as well as on the assumptions of the particular mass transfer model chosen [19, 26]. For very simple cases, analytical solutions are obtained, for example, for a reaction of the first or pseudo-first order or for an instantaneous reaction of the first and second order. Frequently, the enhancement factors are expressed via Hatta-numbers [26, 28]. They can be used in combination with the HTU/NTU-method or with a more advanced mass transfer description method. However, it is generally not possible to derive the enhancement factors properly from binary experiments, and a theoretical description of reversible, parallel or consecutive reactions is based on rough simplifications. Thus, for many reactive absorption processes, this approach appears questionable. 

Wu et al. [19] have analyzed the intensification of industrial mixing. An important element for process intensification is to increase mixing rate, heat transfer rate and mass transfer rate. A range of methods have been reviewed by Wu et al. [19], for both conventional stirred reactors and alternative forms of reactors, all with the aim of achieving process intensification through enhanced mixing, heat and mass transfer. 

Gupta RB, Chattopadhyay P. Method of forming nanoparticles and microparticles of controllable size using supercritical fluids with enhanced mass transfer. USA, US 6620351, 2003. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

Raising the temperature increases diffusion rates, solubility of the analytes and mass transfer, and decreases the viscosity and surface tension of the solvents. These changes improve contact of the analytes with the solvent and enhance the extraction efficiency, which can be achieved more rapidly and with less solvent consumption compared with classical methods. For example, ASE reduces solvent consumption by up to 95% compared to Soxhlet extraction. The only hmitation is the thermal stability of the analyte of interest. 

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