Enhancement of Gas-Liquid Mass Transfer

Beenackers A.A.C.M., Hofmann H., Enhancement of gas-liquid mass transfer by a dispersed second liquid phase,... 

The treatment centres around the relative rates of gas-liquid mass transfer and reaction. The reaction rate controls the overall rate if it is very slow but often it is fast, so that the overall rate is mass transfer controlled. Very fast reactions can influence the diffusion process, causing enhancement of mass transfer above the purely physical rate. This enhancement is itself a function of the reaction rate. In extreme cases Cinstantaneous reaction), mass transfer again controls the overall rate of the process. 

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). 

E will be different from 1 only if R4 is small relative to / 2, resulting in a bulk concentration of c — 0 and in a real parallel mechanism of the enhancement. The advantage of the concept of the enhancement factor as defined by eq 33 is the separation of the influence of hydrodynamic effects on gas-liquid mass transfer (incorporated in Al) and of the effects induced by the presence of a solid surface (incorporated in E ), indeed in a similar way as is common in mass transfer with homogeneous reactions. The above analysis shows that an adequate description of mass transfer with chemical reaction in slurry reactors needs reliable data on ... 

DP E F f f. Ha He AG Degree of polymerization Activation energy, enhancement factor for gas-liquid mass transfer with reaction, electrochemical cell potential Faraday constant, F statistic Efficiency of initiation in polymerization Ca/CaQ or na/nao, fraction of A remaining unconverted Hatta number Henry constant for absorption of gas in liquid Free energy change kj/kgmol Btu/lb-mol... 

Dk DP E Knudsen diffusivity Degree of polymerization Activation energy, enhancement factor for gas-liquid mass transfer with reaction. mVs ft"/s... 

Mechanically stirred hybrid airlift reactors (see Fig. 6) are well suited for use with shear sensitive fermentations that require better oxygen transfer and mixing than is provided by a conventional airlift reactor. Use of a low-power axial flow impeller in the downcomer of an airlift bioreactor can substantially enhance liquid circulation rates, mixing, and gas-liquid mass transfer relative to operation without the agitator. This enhancement increases power consumption disproportionately and also adds other disadvantages of a mechanical agitation system. 

The increase in liquid velocity enhances turbulence that helps in breaking up gas bubbles and creating more smaller-sized gas bubbles by solid particles. This leads to increases in the gas-liquid interfacial area, thereby increasing the mass transfer rate and the volumetric mass transfer coefficient. On the other hand, the holdup of solid particles and bubbles decreases with increases in liquid velocity, which has a negative effect on gas-liquid mass transfer. Beyond a critical value. 

Compared to bubble columns, airlift reactors have better liquid circulation but lower rates of mass transfer and mixing. These rates are enhanced in modified airlift reactors with perforated single or coaxial draft tubes. This enhancement is because of the breakup of gas bubbles into smaller bubbles when crossing perforated tubes. The gas-liquid interfacial area and the gas-liquid mass transfer coefficient increases. Similar effect can be achieved with the addition of packing to the riser. ... 

Buffer additives are attractive for enhancing SO2 removal and/or CaC03 utilization in lime/limestone slurry scrubbing processes for flue gas desulfurization. This work was sponsored by EPA to provide experimental data on commercial synthesis, gas/liquid mass transfer enhancement, and oxidative degradation of useful buffer additives. 

Here, the effect of reaction on gas-liquid mass transfer rate, can only be through reducing the bulk concentration of the dissolved gas, therefore increasing the overall drivitig force. However, in the case of finely powdered catalyst, the situation is better modelled by a gas-pseudo-homogeneous liquid (i.e. suspension of solid) phase, hence, leading to considerable "enhancement" of absorption rate (24). 

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. 

Another example of the OBRs use with solid particles is as a photochemical reactor with solids suspension, in this case the vortical flow patterns being used to suspend catalytic titania particles to convert organics in wastewater. The tita-nia needs to be activated by ultraviolet, and the reaction requires the presence of oxygen, so air is bubbled through. The gas-liquid mass transfer is enhanced by the oscillation of the fluid, as it increases hold-up time (bubble residence time) and reduces bubble size (increasing surface area and further increasing hold-up time). The flow patterns simultaneously ensure good exposure of the titania particles to the radiation from an axially located ultraviolet lamp. 

The particular incentive for this research was the desirability of removing the solvent in which the catalyst is dissolved - toluene - for reasons of environmental protection. If the gas-liquid reaction could be carried out under solvent-free conditions, and additionally the gas-liquid mass transfer could be enhanced, it was believed that benefits could result in several areas. The particular study reported involves the use of a homogeneous catalyst dissolved in the liquid phase. The PI arose from several sources - in the words of the researchers ... 

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

The same reaction was performed in a packed bed microreactor using supercritical CO2 [148]. The phase studies confirmed a single phase reaction mixture at 25 and 50 °C during isobaric reaction conditions of 136 bar. The single phase behavior of the reaction mixture avoids the gas/liquid mass transfer resistance. This further enhances productivity, so that the comparison with larger scale systems indicated an increase of about one order of magnitude in space time yield [148]. 

When liquid/liquid contactors are used as reacdors, values of their mass-transfer coefficients may be enhanced by reaction, analogously to those of gas/liquid processes, but there do not seem to be any published data of this nature. 

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

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