Dispersion in gas phase

Gas-liquid contactors may be operated either by way of gas bubble dispersion into liquid or droplet dispersion in gas phase, while thin film reactors, i.e. packed columns and trickle beds are not suitable for solid formation due... 

Computational fluid dynamics based flow models were then developed to simulate flow and mixing in the loop reactor. Even here, instead of developing a single CFD model to simulate complex flows in the loop reactor (gas dispersed in liquid phase in the heater section and liquid dispersed in gas phase in the vapor space of the vapor-liquid separator), four separate flow models were developed. In the first, the bottom portion of the reactor, in which liquid is a continuous phase, was modeled using a Eulerian-Eulerian approach. Instead of actually simulating reactions in the CFD model, results obtained from the simplified reactor model were used to specify vapor generation rate along the heater. Initially some preliminary simulations were carried out for the whole reactor. However, it was noticed that the presence of the gas-liquid interface within the solution domain and inversion of the continuous phase. 

Eqns. (84) to (89) present the dimensionless steady state isothermal balance equations of model <23> [33]. In model <22>, dispersion in gas phase is considered. The governing gas phase balance equation is derived as usual [6, 33] and leads to... 

Therefore, the process occurs in the diffusional regime and a liquid phase has not to be considered. Due to the large diameter of the BCR dispersion in gas phase cannot be neglected. In addition, gas flow variations must be considered. Hence, the mass balances... 

The experimental and theoretical work reported in the literature will be reviewed for each of the five major types of ga s-liquid-particle operation under the headings Mass transfer across gas-liquid interface mass transfer across liquid-solid interface holdup and axial dispersion of gas phase holdup and axial dispersion of liquid phase heat transfer reaction kinetics. 

Glaser and Litt (G4) have proposed, in an extension of the above study, a model for gas-liquid flow through a b d of porous particles. The bed is assumed to consist of two basic structures which influence the fluid flow patterns (1) Void channels external to the packing, with which are associated dead-ended pockets that can hold stagnant pools of liquid and (2) pore channels and pockets, i.e., continuous and dead-ended pockets in the interior of the particles. On this basis, a theoretical model of liquid-phase dispersion in mixed-phase flow is developed. The model uses three bed parameters for the description of axial dispersion (1) Dispersion due to the mixing of streams from various channels of different residence times (2) dispersion from axial diffusion in the void channels and (3) dispersion from diffusion into the pores. The model is not applicable to turbulent flow nor to such low flow rates that molecular diffusion is comparable to Taylor diffusion. The latter region is unlikely to be of practical interest. The model predicts that the reciprocal Peclet number should be directly proportional to nominal liquid velocity, a prediction that has been confirmed by a few determinations of residence-time distribution for a wax desulfurization pilot reactor of 1-in. diameter packed with 10-14 mesh particles. 

This study relates to a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase. The water-alcohol method provided more dispersed catalysts than the impregnation method. The Cu particles of about 20 nm showed enhanced stability and higher activity than the particles larger than 40 nm. This was correlated with the distribution of copper particle sizes shown by XRD and TEM. Compared with silver and zinc, copper is better active and stable metal. 

Foams are agglomerations of gas bubbles separated from each other by thin films (5). Mainly, the problem is concerned with one class of colloidal systems —gas dispersed in liquid—but liquid dispersed in gas, solids dispersed in liquid (suspensions), and liquids dispersed in liquids (emulsions) cannot be ignored. The dispersion of a gas into a liquid must be studied and observed by the food technologist to improve the contact between the liquid and gas phases, the agitation of the liquid phase, and most important, the production of foam 10). 

In most applications, the axial dispersion in both phases can be considered to be negligible (Smith, 1981). Moreover, no radial gradients of concentration and velocity exist for the gas or liquid. 

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

On the other hand, the assembly condition and the inter-molecule force of liquids are quite different from those of solids. In gas-continuous impinging streams with solid and liquid as the dispersed phases, respectively, some different phenomena would occur, which may affect the performance of impinging streams and thus are of concern. This chapter discusses the problem of liquid dispersion in gas-continuous impinging streams, which is related to the topic mentioned above, and introduces the related results of investigations. 

In gas-phase processes, gaseous ethylene or propylene is contacted with solid catalysts, sometimes dispersed in dry polymer powder. The industry uses two different methods of carrying out this reaction, depending on the method of heat removal. In one class of processes, a fluidized bed is used, and in the other, a mechanically agitated dry powder bed is used with evaporative cooling in vertical and horizontal autoclaves. The advantage of a gas-phase process is that no solvent recovery is necessary so that the energy requirement is less. 

Data on gas phase dispersion in three-phase sparged columns arc scarce. For two-phase systems a few correlations arc available [64, 67, 71, 72] which are shown... 

With a gas sparger and a radial turbine of the Rushton type, gas loads of up to 500 m3/(m2h) can be achieved with reasonable energy consumption. This method of dispersing the gas phase is usually employed for fast reactions or in situations where the hourly demand for the gaseous reactant is high, as in industrial fermenters. With such high gas-feed rates, the gas reactant may not react completely, so that eventually the unreacted gas may be recycled externally. 

Power or energy dissipated in the aerated suspension has to be large enough (a) to suspend all solid particles and (b) to disperse the gas phase into small enough bubbles. It is essential to determine the power consumption of the stirrer in agitated slurry reactors, as this quantity is required in the prediction of parameters such as gas holdup, gas-liquid interfacial area, and mass- and heat-transfer coefficients. In the absence of gas bubbling, the power number Po, is defined as... 

Commercial reactors are non isothermal and often adiabatic. In a noniso-thermal gas-liquid reactor, along with the mass dispersions in each phase, the corresponding heat dispersions are also required. Normally, the gas and liquid at any given axial position are assumed to be at the same temperature. Thus, in contrast to the case of mass, only a single heat-balance equation (and corresponding heat-dispersion coefficient) is needed. Under turbulent flow conditions (such as in the bubble-column reactor) the Peclet number for the heat dispersion is often assumed to be approximately equal to the Peclet number for the mass dispersion in a slow-moving liquid phase. 

As discussed in Chap. 3, there are a large number of models proposed to evaluate macromixing in a trickle-bed reactor. A brief summary of the reported experimental studies on the measurements of RTD in a cocurrent-downflow trickle-bed reactor is given in Table 6-7. Some of these experimental studies are described in more detail in a review by Ostergaard.94 Here we briefly review some of the correlations for the axial dispersion in gas and liquid phases based on these experimental studies. 

Significant literature on the axiaj dispersion in gas and liquid phases for countercurrent-flow packed-bed columns have been reported. Trickle- and bubble-flow regimes have been considered. Unlike the holdup, there is quite a discrepancy in the results of various investigators. Almost all the RTD data are correlated by a single-parameter axial dispersion model. A summary of the reported axial dispersion studies in countercurrent flow through a packed bed is given in Table 8-1. 

In this chapter, we review the reported studies on the hydrodynamics, holdups, and RTD of the various phases (or axial dispersion in various phases), as well as the mass-transfer (gas-liquid, liquid-solid, and slurry-wall), and heat-transfer characteristics of these types of reactors. It should be noted that the three-phase slurry reactor is presently a subject of considerable research investigation. In some cases, the work performed in two-phase (either gas-liquid or liquid-solid) reactors is applicable to three-phase reactors however, this type of extrapolation is kept to a minimum. Details of the equivalent two-phase reactors are considered to be outside the scope of this chapter. 

Co and Cr have been found to be incorporated into the lattice of aluminiumphosphates in a well dispersed manner [159]. Both elements assume two oxidation states in the lattice depending upon the pretreatment procedures. While it seems certain that during synthesis incorporation can be achieved and that these tetrahedrally coordinated atoms are stable in gas phase reactions, conclusive evidence is lacking that leaching is not an important side reaction in liquid phase studies. Indeed, it seems that for several reactions the highly active complexes that are leached out of the lattice and homogeneously dissolved in the reactant/solvent mixture dominate the catalytic properties. 

More fundamentally, gas-liquid systems promote reaction in the liquid phase by utilizing the following three possible modes of gas-liquid contact 1) gas bubbling through a liquid (as in bubble colunms and stirred tanks) 2) gas contacting a thin film of flowing liquid (as with packed beds or colunms) and 3) liquid droplets dispersed in gas (as in spray colunms and venturi scrubbers). 

The chemical composition, structure, and, hence the properties of products with modified surface are determined both by observing the required sequence of operations, and chosen chemico-technological parameters of process the chemical nature of reagents (volatile and solid), temperature (in stages of preparation of surface, chemisorption and desorption), concentration of reagents (in gas phase and functional groups on surfaces of substrate), hydrodynamics of the process (rate of transport and removal of reagents, mobility or stationary condition of disperse solid phase). 

Supported silver catalysts are relatively commonly used in gas phase oxidations of alcohols.74,75 Benzyl alcohol can be selectively oxidised to benzaldehyde using a 0.6% Ag/pumice catalyst76 with 100% selectivity, although its activity is less than a similar Pd material. However, a mixed Pd-Ag/pumice bimetallic increases the activity whilst retaining the 100% selectivity to benzaldehyde. The authors of this study concluded that the role of the Pd was to activate the substrate whereas the highly dispersed silver particles served to activate the oxygen. Hence, the mechanism was one of cooperation between the Ag° and Pd° sites, the alloy phase, detected by EXAFS, was considered not to play an important role. 

Generation of solid colloidal particles in aerosols has certain advantages over precipitation from homogeneous solutions described in Chapter IV. During precipitation from solutions it is usually impossible to predict a priori the shape of the resulting particles, while particles prepared by the aerosol methods are usually spherical because of the natural shape of liquid droplets dispersed in gas. Also, it was pointed out earlier (see Chapter IV) that in the case of particles of internally mixed composition, the molar ratio of constituents in the solid phase differs from that in solution [13], while in the case of aerosol technique the content of resulting solid particles is determined by the molar ratio of components in solution that is dispersed in the gas phase to form aerosol droplets. ... 

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