Process gas-liquid mass transfer

Packed columns have been used in chemical industry for more than a century. The packing in a packed column provides a large surface area over which the gas contacts the liquid and the gas-to-liquid (and vice versa) mass transfer occurs (Fig. 3). As a result, the gas-liquid mass transfer process in a packed column is very efficient. Research and development on packed column has received much attention. One of the earlier and original literatures reported by Leva covers the principles of gas-liquid mass transfer in a packed bed, as well as packed column design and column internals. Recent reported literatures on gas absorption in a packed column from Fair et al., Strigle, and Billet are excellent references on transport phenomena in a packed column and packed column design. 

A reaction taking place on the liquid-solid interface could ther fore result in appreciable rate enhancement only if a significant number of solid particles are present at distances from the gas-liquid interface less than 10 cm. This in turn would require particle diameters no more than 10 cm, an unrealistically low value. It therefore appears that, whenever the reaction takes place at the liquid-solid interface, no significant rate enhance ment will be observed for the gas-liquid mass transfer process the latter will essentially proceed in the slow-reaction regime. 

A1 Taweel, A.M., Yan, J., Azizi, R, Odebra, D. and Gomaa, H.G. (2005). Using in-line static mixers to intensify gas-liquid mass transfer processes. Chemical Engineering Science, Vol. 60, pp. 6378-6390. 

In slurry reactors, enhancement of the gas-liquid mass transfer process can occur due to both a fast chemical reaction and the... 

A comprehensive analysis dealing with the various asymptotic cases of gas-liquid mass transfer in series with various particle conversion mechanisms in the bulk has been presented recently by Doraiswamy and Sharma [l] Such a model has been successfully applied to oxydesulfurization of coal [l5l]. As far as we know, no analysis has been presented as yet for the case where the particles are small with respect to the gas-liquid film for mass transfer, and consequently may enhance the gas-liquid mass transfer process. According to our experience it plays a role in a new process that we recently developed for the concentration of hydrogen from lean gas mixtures with a slurry containing finely hydridable metal part-Icles [16,139,152-154], (Fig. 23). 

Yu YH (2003) Interfacial disturbance in gas liquid mass transfer process. MS dissertation, Tianjin University, Tianjin, China (in Chinese)... 

The gas-liquid mass transfer process of CO2 absorbed by quiescent ethanol is Rayleigh unstable (Ra > 0) and Marangoni stable Ma < 0). The absorption process is initiated at some local points to create concentration gradient at the interface and also establish the density gradient between interface and the bulk liquid. Thus, the condition of specified disturbance points (higher concentration points) at the interface is necessary as shown in die following sections. 

Fu further proposed more precise random disturbance model [21] by considering the position, size, and duration of concentration disturbance that should be randomly varying in the real gas-liquid mass transfer process. In this model, a probability P and a coefficient of disturbance size f are introduced to express randomness of concentration disturbance at the liquid surface. The probability P at any point in the interface represents the probability of the occurrence of concentration disturbance at that point. The distribution size Cr is proportional to the degree approach (denoted by of interfacial concentration C, to the concentration of saturated liquid Cs as follows ... 

Qualitative and, hopefully, quantitative estimates of how the process result will be measured must be made in advance. The evaluations must allow one to estabhsh the importance of the different steps in a process, such as gas-liquid mass transfer, chemical reac tion rate, or heat transfer. 

FIG. 18-28 Usually, the gas-liquid mass-transfer coefficient, K, is reduced with increased viscosity. This shows the effect of increased concentration of microbial cells in a fermentation process. 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

Several reported chemical systems of gas-liquid precipitation are first reviewed from the viewpoints of both experimental study and industrial application. The characteristic feature of gas-liquid mass transfer in terms of its effects on the crystallization process is then discussed theoretically together with a summary of experimental results. The secondary processes of particle agglomeration and disruption are then modelled and discussed in respect of the effect of reactor fluid dynamics. Finally, different types of gas-liquid contacting reactor and their respective design considerations are overviewed for application to controlled precipitate particle formation. 

Sada, E., Kumazawa, H., Lee, C. and Fujiwara, N., 1985. Gas-liquid mass transfer characteristics in a bubble column with suspended sparingly soluble fine particles. Industrial and Engineering Chemistry Process Design and Development, 24, 255-261. 

Kolbel et al. (K16) examined the conversion of carbon monoxide and hydrogen to methane catalyzed by a nickel-magnesium oxide catalyst suspended in a paraffinic hydrocarbon, as well as the oxidation of carbon monoxide catalyzed by a manganese-cupric oxide catalyst suspended in a silicone oil. The results are interpreted in terms of the theoretical model referred to in Section IV,B, in which gas-liquid mass transfer and chemical reaction are assumed to be rate-determining process steps. Conversion data for technical and pilot-scale reactors are also presented. 

Gas-Liquid Mass Transfer. Gas-liquid mass transfer within the three-phase fluidized bed bioreactor is dependent on the interfacial area available for mass transfer, a the gas-liquid mass transfer coefficient, kx, and the driving force that results from the concentration difference between the bulk liquid and the bulk gas. The latter can be easily controlled by varying the inlet gas concentration. Because estimations of the interfacial area available for mass transfer depends on somewhat challenging measurements of bubble size and bubble size distribution, much of the research on increasing mass transfer rates has concentrated on increasing the overall mass transfer coefficient, kxa, though several studies look at the influence of various process conditions on the individual parameters. Typical values of kxa reported in the literature are listed in Table 19. 

Nore, O., Briens, C., Margaritis, A., and Wild, G., Hydrodynamics, Gas-Liquid Mass Transfer and Particle-Liquid Heat and Mass Transfer in a Three-Phase Fluidized Bed for Biochemical Process Applications, Chem. Eng. Sci., 47 3573 (1992)... 

Hydrogen transfer from the gas phase to the liquid phase becomes rate limiting with very fast hydrogenations (or with insufficient agitation). The observed reaction rate is then equal to the rate of gas-liquid mass transfer of hydrogen and becomes first order in hydrogen and independent of substrate concentration. The activation energy decreases to that of a diffusion process. 

In our development studies, Endeavor (5 mL) and Buchi (IL) reactor systems were used to screen catalysts and to evaluate the impurity profile under various process conditions. Elydrogenation kinetic studies were carried out using a 100 mL EZ-seal autoclave with an automatic data acquisition system to monitor the hydrogen uptake and to collect samples for HPLC analysis. Standard conditions of 5 g of aldehyde in 25 mL ethyl acetate and 25 mL methanol with 0.5 g of 5%Pd/C Engelhard Escat 142 were used in this investigation. For the Schiff s base formation and subsequent hydrogenation, inline FTTR was used to follow the kinetics of the Schiff s base formation under different conditions. Tables 1 and 2 show the changes in the substrate concentration under different conditions. Both experiments were carried without any limitations of gas-liquid mass transfer. 

The analysis of the trickle-bed runs indicate that intraparticle mass transfer resistance is very significant. Gas-liquid mass transfer may also have a significant resistance. This is an important consideration in the decision process of using a slurry or a trickle-bed reactor. 

Another class of applications is the high shear mixers used to break up agglomerates of particles as well as to cause rapid dissolving of solids into solvents. A further type includes the catalytic processes such as hydrogenation, in which there is a basic gas-liquid mass transfer to be satisfied, but in addition, effective mixing and shear rate on the catalyst particle fluid film as well as degradation must be considered. 

As mentioned previously, axial flow impellers are typically used for solids suspension. It is also typical to use radial flow impellers for gas-liquid mass transfer. In combination gas-liquid-solid systems, it is more common to use radial flow impellers because the desired power level for mass transfer normally accomplishes solids suspension as well. The less effective flow pattern of the axial flow impeller is not often used in high-uptake-rate systems for industrial mass transfer problems. There is one exception, and that is in the aeration of waste. The uptake rate in biological oxidation systems is on the order of 30 ppm/hr, which is about to the rate that may be required in industrial processes. In waste treatment, surface aerators typically use axial flow impellers, and there are many types of draft tube aerators that use axial flow impellers in a draft tube. The gas rates are such that the axial flow characteristic of the impeller can drive the gas to whatever depth is required and provide a very effective type of mass transfer unit. 

The heart of the pilot plant study normally involves varying the speed over two or three steps with a given impeller diameter. The analysis is done on a chart, shown in Fig. 36. The process result is plotted on a log-log curve as a function of the power applied by the impeller. This, of course, implies that a quantitative process result is available, such as a process yield, a mass transfer absorption rate, or some other type of quantitative measure. The slope of the line reveals much information about likely controlling factors. A relatively high slope (0.5-0.8) is most likely caused by a controlling gas-liquid mass transfer step. A slope of 0, is usually caused by a chemical reaction, and a further increase of power is not reflected in the process improvement. Point A indicates where blend time has been satisfied, and further reductions of blend time do not improve the process performance. Intermediate slopes on the order of 0.1-0.4, do not indicate exactly which mechanism is the major one. Possibilities are shear rate factors, blend time requirements, or other types of possibilities. 

In this case study we will model, simulate and design an industrial-scale BioDeNOx process. Rigorous rate-based models of the absorption and reaction units will be presented, taking into account the kinetics of chemical and biochemical reactions, as well as the rate of gas-liquid mass transfer. After transformation in dimensionless form, the mathematical model will be solved numerically. Because of the steep profiles around the gas/liquid interface and of the relatively large number of chemical species involved, the numerical solution is computationally expensive. For this reason we will derive a simplified model, which will be used to size the units. Critical design and operating parameters will be identified... 

Parameters k and k2 can be easily related to the hydrodynamic conditions (flow rate, stirring rates) and to the current density by empirical equations. The influence of the current density can also be related to the reagent dose for parameter k and to the bubble generation for parameter k2 (the flow rate of cathodically generated hydrogen is proportional to the current density). Thus, this semiempirical model considers easily and simultaneously the gas-liquid mass transfer, the collections of solid particles in electroflotation processes, and the effect of the current density. 

Chiyoda and UOP jointly developed an improved methanol carbonyl-ation process on the basis of this supported rhodium complex catalyst the process is called the Acetica process. This process for the production of acetic acid has found several industrial applications in Asia. The process description emphasizes the use of a three-phase reactor, a bubble column, or gas-lift reactor. The reactor column contains a liquid, a solid catalyst, and a bubbling gas stream containing CO efficient dissolution of the gas in the liquid is ensured by the design, which minimizes gas-liquid mass transfer resistance. 

Interfacial Area The effective area of contact between gas and liquid is that area which participates in the gas-liquid mass-exchange process. This area may be less than the actual interfacial area because of stagnant pools where liquid reaches saturation and no longer participates in the transfer process. 

The hydrodynamic parameters involved in BCR design and scale-up are mainly dependent on adjustable operational conditions, physico-chemical properties and geometrical sizes. In general, little arbitrary variations are possible with respect to changes in chemical processes. Though a large amount of data has been reported the parameters which characterize the gas-liquid mass transfer properties are still subject to considerable error and unreliabilities. Only for aqueous systems... 

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