Types of Gas Diffusers

Contacting ozone gas with water can be achieved with every kind of gas diffuser, which is made of a material resistant to ozone. Ring pipes, porous diffusers and porous membranes, injector nozzles as well as static mixers can be employed. The different types of diffusers are mainly characterized by the diameter of the bubbles produced, e. g. micro (dB = 0.01 — 0.2 mm), small (dB 1.0 mm) or big (dB - 2.5 mm) bubbles (Calderbank, 1970 Hughmark, 1967). 

Ring pipes with 0.1-1.0 mm i. d. holes are common types of diffusers in lab-scale stirred tank reactors. Fine porous plate diffusers (dP = 10-50 pm) have also often been used, in STRs (e. g. Beltran etal., 1995) as well as bubble columns (e. g. Stockinger, 1995 Saupe, 1997). 

Coarse (dP = 50-100 pm) porous disks are the most frequently applied diffusers in large-scale drinking water treatment systems (Masschelein, 1994). They are seldom used in industrial waste water treatment applications. The reason is that blocking or clogging can easily occur, e. g. by means of precipitation of chemicals, like carbonates, aluminum or ferrous oxides, manganese oxides, calcium oxalate or organic polymers. This is also valid for ceramic filter tubes, which are sometimes used as mass transfer systems in drinking water applications. 

Two-phase gaslwater injector nozzles are mostly used in pilot- or full-scale bubble column applications (Krost, 1995) or in specialized, newly developed reactor types. An example is the Submerged Impinging Zone Reactor (IZR) (Gaddis and Vogelpohl, 1992 Air Products, 1998), which is constructed for very high mass-transfer rates. 

Normally, it makes little sense to apply such systems in lab-scale ozonation experiments, since the high mass transfer rates are only achieved at high gas flow rates which because of the typical operation characteristics of EDOGs accordingly means low ozone gas concentrations. An appropriate field of application was, however, presented in the study of Sunder and Hem pel (1996) who operated a tube-reactor for the ozonation of small concentrations of perchloroethylene. An injector nozzle coupled with the highly efficient Aquatector ozone-absorption unit was installed in front of the tube-reactor. Both the gas and liquid were partially recycled in this system. According to the authors more than 90 % of the ozone produced was absorbed in demineralized water and dissolved ozone concentrations ranged up to 100 pmol L-1 (cL = 5 mg L-1, T= 20 °C). 

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Water or waste water ozonation - regardless of the scale of equipment - is mostly performed in directly gassed systems, where the ozone containing gas is produced by an electrical discharge ozone generator and is introduced into the reactor by some type of gas diffuser. Since two phases, the gas and the liquid, are required for the oxidation reaction to proceed as it does, they are also called heterogeneous systems. 

Stirred tank reactors (STR) are the most frequently used reactors in lab-scale ozonation, partially due to the ease in modeling completely mixed phases, but they are very seldom used in full-scale applications. There are various modifications with regard to the types of gas diffusers or the construction of the stirrers possible. Normally lab-scale reactors are equipped with coarse diffusers, such as a ring pipe with holes of 0,1-1.0 m3 diameter. The k/ a-values are in the range of 0.02 to 2.0 s (see Table 2-4 ), which are considerably higher than those of bubble columns. From the viewpoint of mass transfer, the main advantage of STRs is that the stirrer speed can be varied, and thus also the ozone mass transfer coefficient, independently of the gas flow rate. 

The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid three-phase boundary. COj reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. 

Water and thermal management issues should be handled with great care in new types of gas diffusion layer (GDL) e.g., earbon cloth or metallic-carbon cloth for better electron transfer and mass transport of the reactants and products. A well defined fluid flow path in GDL and electron transfer through solid metallic... 

A special type of gas diffusion membrane (GDM) was coated with Pt or Pd nanoclusters, and showed enhanced properties with respect to commercial materials for the gas-phase electrocatalytic reduction of CO2 to fuels [113]. 

Interstellar molecules are grouped by their location in three types of clouds diffuse, dark, and black clouds [1]. The diffuse clouds are characterized by low gas density, predominately... 

Figure 10.4 Calculated activation energy of gas diffusion for counter-diffusion CVD modified MFI-type (circle symbols) zeolite membranes and that of counter-diffusion CVD...

There is a special case sometimes encountered with gas coolers, which leads to a simplified calculation procedure. This is the case in which hot gas contacts a nonvolatile cooling liquid. In such a situation the mass transfer can be negligible, and for some systems the latent heat of vaporization may be so small that the diffusion heat load can be neglected. It is possible then to neglect mass transfer altogether in this type of gas cooling operation. 

Foam films are usually used as a model in the study of various physicochemical processes, such as thinning, expansion and contraction of films, formation of black spots, film rupture, molecular interactions in films. Thus, it is possible to model not only the properties of a foam but also the processes undergoing in it. These studies allow to clarify the mechanism of these processes and to derive quantitative dependences for foams, O/W type emulsions and foamed emulsions, which in fact are closely related by properties to foams. Furthermore, a number of theoretical and practical problems of colloid chemistry, molecular physics, biophysics and biochemistry can also be solved. Several physico-technical parameters, such as pressure drop, volumetric flow rate (foam rotameter) and rate of gas diffusion through the film, are based on the measurement of some of the foam film parameters. For instance, Dewar [1] has used foam films in acoustic measurements. The study of the shape and tension of foam bubble films, in particular of bubbles floating at a liquid surface, provides information that is used in designing pneumatic constructions [2], Given bellow are the most important foam properties that determine their practical application. The processes of foam flotation of suspensions, ion flotation, foam accumulation and foam separation of soluble surfactants as well as the treatment of waste waters polluted by various substances (soluble and insoluble), are based on the difference in the compositions of the initial foaming solution and the liquid phase in the foam. Due ro this difference it is possible to accelerate some reactions (foam catalysis) and to shift the chemical equilibrium of some reactions in the foam. The low heat... 

When the size of the pores is much smaller than the molecular mean free paths in the gas mixture, collisions of gas molecules with the pore wall are more frequent than inter-molecular collisions. This type of gas transport is also known as Knudsen diffusion. In this case each molecule acts independently of all the others and each component in a mixture behaves as though it were present alone. The movement of each molecule can be conveniently pictured as a random walk between the walls of pores. This leads to the following expression for the molar flux of species A ... 

It has been shown by Chavarie and Grace (15) that the decomposition of ozone in a fluidized-bed is best described by Kunii and Levenspiel s model (16) but that the Orcutt and Davidson models (17) gave the next best approximation for the overall behaviour and are easier to use and were chosen for the simulation. They suppose a uniform bubble size distribution with mass transfer accomplished by percolation and diffusion. The difference between the two models is the presumption of the type of gas flow in the emulsion phase piston flow, PF, for one model and a perfectly mixed, PM, emulsion phase for the other model. The two models give the following expressions at the surface of the fluidized bed for first-order reaction mechanism ... 

Depending on type of gas and type of membrane material, the importance of these two variables, D and S, will vary. Both the diffusion and the solubility coefficient for the gas are temperamre dependent, while a pressure dependency is only observed for... 

It should be noted that the problem of very slow diffusion in and out of a bulky solid matrix makes any type of gas extraction impractical and the results dubious. In other words, these restrictions apply not only to the MHS approach but also to dynamic head-space processes in general. 

It has long been realized that the impossibility, or rather the improbability, of reversing spontaneous processes is based on the inability to deal with individual molecules or small groups of molecules. If a device were available which could distinguish between fast- ( hot O slow-moving ( cold ) molecules, it would be possible to produce spontaneously a temperature gradient in a gas. Similarly, if a device could discriminate between two different types of gas molecules, a partial unmixing, which is the reverse of diffusion, could be achieved. However, the fact that no such devices are known is in accordance with the second law of thermodynamics. 

For porous membranes the mass transport mechanisms that prevail depend mainly on the membrane s mean pore size [1.1, 1.3], and the size and type of the diffusing molecules. For mesoporous and macroporous membranes molecular and Knudsen diffusion, and convective flow are the prevailing means of transport [1.15, 1.16]. The description of transport in such membranes has either utilized a Fickian description of diffusion [1.16] or more elaborate Dusty Gas Model (DGM) approaches [1.17]. For microporous membranes the interaction between the diffusing molecules and the membrane pore surface is of great importance to determine the transport characteristics. The description of transport through such membranes has either utilized the Stefan-Maxwell formulation [1.18, 1.19, 1.20] or more involved molecular dynamics simulation techniques [1.21]. 

Rebour et al. (1997) review the literature describing gas diffusion in a porous medium as a double porosity process. In this model, gas diffusion is affected by the increase in water viscosity when in the close vicinity of clay minerals. This produces an environment in which the gas diffusion rate is expected to be variable in the porous network depending on the local tortuosity and grain-size distribution. In modeling this type of system, diffusion is considered to occur along a direct pathway. These fast routes interconnect slow regions, into and out of which gas also diffuses. Experimental work by the same authors (Rebour et al. 1997) determines Rf = 200 for a clayey marl from Paris basin Callovo-Oxfordian sediments that have a porosity and permeability of 23% and 10 m, respectively. 

There are several types of gas sensors in current use. Semiconductor-type sensors are sensitive to either a single gas or group of gases that modify electrical properties of the solids. A classical example is ZnO whose n-type conductivity changes proportionally to /- o 4, although this effect only becomes measurable at high temperatures due to diffusion limitations. 

There are mainly two designs of gas diffusion separators, i.e., the sandwich type and the tubular type, the schematic diagrams of which are shown in Fig. 5.1 and 5.2. A common feature of the two designs is that they have separate channels for a donor and an acceptor stream separated by a membrane which is permeable to the gaseous analyte species. The difference between the two designs is in the form of the membrane. 

Fig. l A typical sandwich-type membrane gas-diffusion separator, a, side view A, B, plastic blocks with F. threaded fittings and G, engraved grooves M, microporous gas-diffusion membrane, b, top view showing position and configuration of a straight channel groove, c, a simplified schematic presentation of the sandwich-type gas-diffusion separator D, donor stream A, acceptor stream M, membrane. 

Linares et al.[49] proposed a FI system with on-line gas-diffusion for the simultaneous determination of carbon dioxide and sulphur dioxide in wines. The two gaseous constituents were separated from the acidified sample in a sandwich-type membrane gas diffusion separator, and collected in an acceptor stream. Two detectors, one potentiome-tric, responsive to both analytes, and the other photometric, responsive only to sulphur dioxide (after reaction with a p-rosaniline-formaldehyde solution) were connected in series to determine the two constituents in the acceptor. The method was applied to the determination of carbon dioxide and sulphur dioxide in different types of fruity wines and the analytical results were in good agreement with those obtained by standard methods. 

ABSTRACT With the increase of mine exploitation depth and appliance widely of large-scale full-mechanized equipment, coal block gas emission has been one of the most gas effusion source. Base on unsteady diffusion theory and mass transmission fundamental, the mathematical and physical model of gas diffusion through coal particles with third type boundary condition was founded and its analytical solution was obtained by separate variableness method. The characteristics of gas through coal particles was analyzed according as mass transmission theory of porous material. The results show that the Biot s criterion of mass transmission can reflect the resistance characteristic of gas diffusion and the Fourier s criterion of mass transmission can represent the dynamic feature of diffusion field varying with time. 

R.H.-independent signal output has been achieved in thefour-probe type sensor shown in Fig. 36.4, where two additional Ag probes are inserted in the proton conductor bulk (AA) beneath the Pt electrodes. One of the Pt electrodes is covered by a layer of AA sheet, which acts as a sort of gas diffusion layer. The short-circuit current flowing between the two Pt electrodes is proportional to H2 concentration but dependent on R.H., just as in the previous amperometric sensor. On the other hand, the difference in potential between the two Ag probes (inner potential difference, AE g) with the outer Pt electrodes short-circuited is shown to be not only proportional to H2 concentration but also independent of R.H. as shown in Fig. 36.3b and Table 36.2. This mode of sensing has no precedence, and is noted as a new method to overcome the greatest difficulty in using proton conductor-based devices, i.e. their R.H. dependence. 

Experimentally we can check whether the flow is under the Knudsen flow regime by calculating bVMT for various gases and temperatures. If it does not vary with temperature or the type of gas, the mechanism is due to the Knudsen diffusion. Adzumi (1937) studied the flow of hydrogen, acetylene and propylene through a glass capillary of radius and length of 0.0121 and 8.7 cm, respectively, and found that at low pressures (less than 0.03 torr) the permeability is independent of the mean pressure. 

The effective thermal conductivity k in the dried material has been found to vary significantly with the total pressure and with the type of gas present. Also, the type of material affects the value of k (SI, Kl). The effective diffusivity D of the dried material is a function of the structure of the material, Knudsen diffusivity, and molecular diffusivity (Kl). 

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