Effective catalyst wetting

53 Paraskos et al.,66 Montagna and Shah,38 and Montagna et al.59 have recently shown that ineffective catalyst wetting can cause the reactor performance to be dependent on the liquid velocity. The y used a correlation of Puranik and Vogelpohl69 for the effectively wetted surface area of the packing to explain the effects ofliquid hourly space velocity and the length of the catalyst bed on the performance of bench-scale HDS reactors. 

Several other reports have also shown the importance of effective catalyst wetting on the performance of a bench-scale trickle-bed reactor. Hartman and Coughlin37 concluded that for sulfur dioxide oxidation in qojjntercurrejQt trickle-bed reactor packed with carbon particles, the catalyst was not completely wet at low liquid flow rates (of the order of 5 x 10 4 cm s-1). Sedricks and Kenney86 found that, during catalytic hydrogenation of crotonaldehyde in a cocurrent trickle-bed reactor, liquid seeped. into dry palladium-on-alumina 

The effect of catalyst wetting on the performance of a bench-scale trickle-bed reactor was also theoretically evaluated by Sylvester and Pitayagulsarn.94,9S Using the method of moments of Suzuki and Smith,93 they developed a procedure to show the combined effects of axial dispersion, external diffusion, intraparticle diffusion, and surface reaction on the conversion for a first-order irreversible reaction in an isothermal trickle-bed reactor and evaluated the effect of catalyst wetting on these combined effects. 

At present, very little is known on how the liquid-solid contact efficiency depends upon the gas and liquid flow rates. It is knovyn that higher gas and [iquid loadings can improve the contact efficiency. However, it is not clear what effects the wettability of the particles and the heal effects occurring during the chemical reactions have on the contacting efficiency. A s maller surface tension or higher viscosity of the liquid appears to increase the contacting efficiency. 

Generally, it is believed that higher dynamic holdupleads to better contacting efficiency. LeGoff,49 however, broke down the liquid holdup into three parts (a) an isolated part (termed droplet), (h) an anisotropic part (termed rivulets), and 

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Experimental Verifications of Holdup and Effective Catalyst-Wetting Models... 

All these results indicate that although, as predicted by both the holdup and the effective catalyst-wetting models, the conversions in pilot-scale hydroprocessing reactors depend upon the liquid flow rate, and log-log plots of ln(Ci/Cc) versus either l/LHSV or L are straight lines, the slopes of these plots depend upon the nature of the feed, temperature, and the catalyst size. 

Of the holdup and the effective catalyst-wetting models, the latter one appears to be physically more realistic. As indicated earlier, the two models show a... 

Figure 4-4 Correlations of the experimental data for 36 percent KATB by the effective catalyst-wetting and holdup models (after Montagna anti Shah2 1).

As mentioned earlier, the cocurrent gas-liquid downflow and, in particular, the trickle-flow operation is one of the most widely used three-phase operations in the hydroprocessing industry. The liquid holdup in such a reactor takes on added importance because it is usually low compared to the one for cocurrent upflow under similar flow conditions. Earlier we showed that the pressure drop in a trickle-bed reactor can be related to the liquid holdup. The effective catalyst wetting, as well as the thickness of the liquid film surrounding the catalyst particles, also depends strongly on the liquid holdup. 

Physically, the effective wetting model seems to be the most appropriate one. This is supported by the fact that also hydrodesulphurization of vacuum and atmospheric residuals are better correlated by an effective catalyst wetting model than by the holdup model [60]. 

Montagna, A., Y. T. Shah. The Role of Liquid Holdup, Effective Catalyst Wetting, and Backmixing on the Performance of a Trickle-Bed Reactor for Residue Hydrodesulfurization. 

Highly Lewis-acidic chloroaluminate ionic liquids (ILs) are well known to be both versatile solvents and effective catalysts for Friedel-Crafts reactions [1,2]. Tailoring the physical and chemical properties of the ILs to the needs of a specific reaction allows for a high diversity of applications [3,4]. We could show that immobilising these ILs on inorganic supports yields very active catalysts for alkylation reactions. The immobilisation of ionic liquids leads to novel Lewis-acidic catalysts (NLACs). The methods presented include the method of incipient wetness (method 1, further on called NLAC I), which has been introduced in detail by Hoelderich et al. f5], but focus of this presentation lies on the methods 2 (NLAC II) and 3 (NLAC III). 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

It is difficult to ascertain whether the poor performance observed in pilot-scale trickle-bed reactors is due either to ineffective catalyst wetting or to the axial dispersion effects, because both these effects are physically realistic and both occur under similar operating conditions (i.e., low liquid flow, large catalyst size, and shorter beds). It should be noted, however, that the criterion for removing the axial dispersion effect is available. A similar criterion for removing ineffective catalyst wetting is, however, presently not available. 

Liquid holdup, which is expressed as the volume of liquid per unit volume of bed, affects the pressure drop, the catalyst wetting efficiency, and the transition from trickle flow to pulsing flow. It can also have a major effect on the reaction rate and selectivity, as will be explained later. The total holdup, h, consists of static holdup, h, liquid that remains in the bed after flow is stopped, and dynamic holdup, h, which is liquid flowing in thin films over part of the surface. The static holdup includes liquid in the pores of the catalyst and stagnant packets of liquid held in crevices between adjacent particles. With most catalysts, the pores are full of liquid because of capillary action, and the internal holdup is the particle porosity times the volume fraction particles in the bed. Thus the internal holdup is typically (0.3 — 0.5)(0.6), or about 0.2-0.3. The external static holdup is about... 

Incineration is cited exclusively as a method of destruction, applicable to neat compounds or waste solvents. Other thermal methods, such as molten metal salt treatment, which involves intimate contact with a molten salt, such as AI2O3 (Shultz 1985), are suitable. Chemical processes that may be effective are wet air oxidation, electrochemical oxidation, and catalytic destruction. Ketones in aqueous wastes can be altered to innocuous gases by heating at 300-460°C (572-860°F) and 150-400 atm pressure with or without catalyst. Ni and Fc203 were found to be effective catalysts in such thermal treatments (Baker and Sealock 1988). 

The periodic liquid flow changes the hydrodynamic regime and influences the external and internal catalyst wetting. Therefore the catalyst particle wetted area will be increased and decreased depending on the hquid duration. The degree of wetting will be crucially influenced. This has a strong effect on the intensity of the process, as it alters the conditions of transport of the reactants to the catalyst surface where adsorption, chemical reactions and desorption of the products take place. 

When the liquid reactant and product are non volatile, reaction can proceed only on the active sites in contact with the liquid the wetting of the catalyst particles is therefore a very important feature of such systems. When the liquid reactant and product are relatively volatile and when the heat effect of the reaction is important, the reaction can also occur on dry fractions of the catalyst, through a classical gas phase process. As the apparent rates of reaction in both phases are generally different, it is an essential but very difficult problem to determine the contribution of the two simultaneous reaction processes again the knowledge of the catalyst wetting appears to be the key of the problem. 

In dry deposition, SO2 increasingly predominates over NO2 as the major dry acid pollutant away from the source, although ozone may be a potentially critical pollutant at long distances from the sources. In areas where acid rain is dominant, sulphuric and nitric acids contribute in about a 70/30 ratio to overall acidity of rain (Section 4.4.6). It is probable that there is not a linear relationship between SO2 emissions and 804 deposition as the supply of oxidants/catalysts may be a limiting factor. Opinions differ on the degree of this effect for wet deposition (acid rain)(Section 4.3.2(iii)). 

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