Mixing of the gas phase

However, there are also some drawbacks associated with the use of fluidized beds. The complete mixing of the gas phase in this type of reactor decreases the process driving force. Moreover, the formation of large bubbles makes the process less efficient and difficult to handle. However, the main disadvantages of fluidized beds are the erosion of the reactor, the attrition of the sohds, and the irregular conduction between the gas and the solid phase (McCabe et al., 1983). 

Note that we are less interested in the mixing of the gas phase. This explains why the gas-phase concentration is considered to be constant, and thus its material balance is not involved in the model. 

In small lab-scale STRs almost complete mixing of the gas-phase can also be assumed whereas plug-flow or mixed behavior can be generally assumed for the gas phase in bubble columns (Marinas et. al., 1993 Stockinger, 1995 Huang et al., 1998) and packed towers (Lin and Peng, 1997). 

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. 

The scale-up of monolith reactors is expected to be much simpler. This is due to the fact that the only difference between the laboratory and industrial monolith reactors is the number of monolith channels, provided that the inlet flow distribution is satisfactory. In slurry reactors, scale-up problems might appear. These are connected with reactor geometry, low gas superficial velocity, nonuniform catalyst concentration in the liquid, and a significant back-mixing of the gas phase. 

DISTILLATION PLATE EFFICIENCY. The two-film theory can be applied to mass transfer on a sieve tray to help correlate and extend data for tray effieiency. The bubbles formed at the holes are assumed to rise through a pool of liquid that is vertically mixed and has the local composition x. The bubbles change in composition as they rise, and there is assumed to be no mixing of the gas phase in the vertical direction. For a unit plate area with a superficial velocity the moles transferred in a thin slice dz are... 

Calculate KgU for the three eonversion levels, assuming perfect mixing of the gas phase and instantaneous saturation of gas bubbles with water vapor. Compare the eoefficients with those reported for oxidation of sulfite solution. 

Drawbacks It is well known that the spray generated by a nozzle rapidly degrades due to coalescence, loss of momentum of the droplets, etc. In general, a spray is effective over not more than 1 m from the source of generation. In addition, precipitation of PTA formed is likely to inaease the resistance to diffusion of oxygen in the droplet (Section 3.3.4). This implies that for large-scale applications, short spray columns with a parallel bank of sprays located to avoid overlap wiU be required. The problem of back mixing of the gas phase is similar to that in a stirred reactor operated above... 

A majority of applications of venturi loop systems (Table 8.1) pertain to situations in which the gas-phase reactant is a pure gas. Further, the operating pressures are also high ( 1 MPa and above) therefore, partial pressure of the liquid under saturation conditions is also negligible. As a result, back mixing of the gas phase is inconsequential. 

Another disadvantage of the packings with vertical walls is the small radial mixing of the gas phase in them. Both di idvantages can be reduced by corrugating the sur ce of the vertical sheets. Some of the most popular designs of such packings, taken from Billet [177], are presented in Fig. 111. 

For columns in which there is a substantial flash of the feed liquid, or in which the feed is a vapor of a different composition than the internal vapor, a collector plate can be installed above the feed point. The purpose of this plate is to provide mixing of the vapor phase in the gas risers so that a more uniform vapor composition enters the rectifying section of the column. 

Siemes and Weiss (SI4) investigated axial mixing of the liquid phase in a two-phase bubble-column with no net liquid flow. Column diameter was 42 mm and the height of the liquid layer 1400 mm at zero gas flow. Water and air were the fluid media. The experiments were carried out by the injection of a pulse of electrolyte solution at one position in the bed and measurement of the concentration as a function of time at another position. The mixing phenomenon was treated mathematically as a diffusion process. Diffusion coefficients increased markedly with increasing gas velocity, from about 2 cm2/sec at a superficial gas velocity of 1 cm/sec to from 30 to 70 cm2/sec at a velocity of 7 cm/sec. The diffusion coefficient also varied with bubble size, and thus, because of coalescence, with distance from the gas distributor. 

Table II summarizes the yields obtained from the CONGAS computer output variable study of the gas phase polymerization of propylene. The reactor is assumed to be a perfect backmix type. The base case for this comparison corresponds to the most active BASF TiC 3 operated at almost the same conditions used by Wisseroth, 80 C and 400 psig. Agitation speed is assumed to have no effect on yield provided there is sufficient mixing. The variable study is divided into two parts for discussion catalyst parameters and reactor conditions. The catalyst is characterized by kg , X, and d7. Percent solubles is not considered because there is presently so little kinetic data to describe this. The reactor conditions chosen for study are those that have some significant effect on the kinetics temperature, pressure, and gas composition.

Manufacturing economics require that many devices be fabricated simultaneously in large reactors. Uniformity of treatment from point to point is extremely important, and the possibility of concentration gradients in the gas phase must be considered. For some reactor designs, standard models such as axial dispersion may be suitable for describing mixing in the gas phase. More typically, many vapor deposition reactors have such low L/R ratios that two-dimensional dispersion must be considered. A pseudo-steady model is... 

Russo, P. J. et al., J. Chem. Soc., Chem. Comm., 1982, 53-54 Polymerisation of the diyne by arsenic pentafluoride at low temperature may be explosive if too rapid mixing in the gas phase, or too rapid warming of the solids from -196°C, is permitted for 200 mg quantities. 

Frohlich, S., Lotz, M., Korte, T., Lilbbert, A., Schilgerl, K., Seekamp, M., Characterization of a Pilot Plant Airlift Tower Loop Bioreactor II. Evaluation of Global Mixing Properties of the Gas Phase During Yeast Cultivation, Biotechnol. Bioeng., 37 910 (1991a)... 

The presence of N2O at 80 K was confirmed in foiu ways first by determining a difference spectrum between 80 and 120 K (when it desorbed). Two N(ls) peaks (Fig. 10) (one at 402 eV and the other at 406 eV) and one 0(1 s) peak (at 531 eV) were lost on warming the intensities of the N(ls) peaks were identical. Second, the difference spectrum was shown to be identical with N2O molecularly adsorbed at 80 K on a Cu(lll) surface. Third, a mass spectrum analysis of the gas phase on warming from 80 K showed the presence of N2O and last, helium-induced valence-level spectra at 80 K were consistent with a NO-N2O mixed adlayer (44, 45). 

As a reaction medium, solvents are used to bring reactants at suitable concentrations. For endothermic reactions, heat can be supplied readily by heating the solvent for exothermic reactions, the solvent can act as a heat sink. If necessary, surplus heat can be removed by allowing the solvent to boil. After the reaction, products have to be freed of solvent. As diffusion in solids is slow, solid-solid reactions are slow at room temperature. To get reactants to mix at a molecular level requires use of the gas phase. 

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