Catalyst wetting

The usual precaution was observed of keeping the catalyst wet with the solution being filtered to prevent ignition of the filter paper. 

In the production of polyvinyl chloride by the emulsion process, the percentages of catalyst, wetting agent, initiator, and solvent all affect the properties of the resultant polymer. They must be carefully metered into the reaction vessel. The vinyl chloride used must also be very pure. Either the scope must specify that the purchased raw material shall meet certain specifications, or some purification equipment must be installed so that the required quality can be obtained. 

This fraction could be viewed as a fixed-bed wetting efficiency. If he t = s and hyt= 1, the bed is completely filled with fluid (fully wetted). The term is analogous to the catalyst wetting efficiency/w for trickle beds (Section 3.7.3). However, this equality is valid solely in fixed beds where a single fluid flows through. The active volume of the solid, which is occupied by the fluid, amounts to the fixed-bed volume occupied by the fluid minus the volume occupied by fluid ... 

The catalyst wetting efficiency of the external catalyst surface can be calculated at atmospheric pressure using the correlation of El-Hisnawi et al. (1981 Wu, 1996) ... 

The value of Z/dv used in the experiments is 188.98, and thus the plug-flow assumption is valid in the whole region of working residence time. Furthermore, from the point of view of catalyst wetting, it is clear that the appropriate experimental point that we can use is the one of 60 s space-time where x = 0.44 and fw = 1. Then, from eq. (5.351),... 

Failing to identify the limiting reactant can lead to failure in the scale-up of trickle-bed reactors (Dudukovic, 1999). Gas-limited reactions occur when the gaseous reactant is slightly soluble in the liquid and at moderate operating pressures. For liquid-limited reactions, concurrent upflow is preferred (packed bubble columns) as it provides for complete catalyst wetting and thus enhances the mass transfer from the liquid phase to the catalyst. On the other hand, for gas reactions, concurrent downflow operation (trickle-bed reactors), especially at partially wetted conditions, is preferred as it facilitates the mass transfer from the gas phase to the catalyst. The differences between upflow and downflow conditions disappear by the addition of fines (see Section 3.7.3, Wetting efficiency in trickle-bed reactors). 

Metal catalyst, wetted with not less than 40 per cent water or other suitable liquid, by mass finely divided, activated or spent 1378... 

Possibility of operating partially or wholly in the vapour phase by varying the liquid flowrate according to catalyst wetting, heat of vaporization, and mass-transfer resistances in the liquid phase. 

Hydrodynamic parameters that are required for trickle bed design and analysis include bed void fraction, phase holdups (gas, liquid, and solid), wetting efficiency (fraction of catalyst wetted by liquid), volumetric gas-liquid mass-transfer coefficient, liquid-solid mass-transfer coefficient (for the wetted part of the catalyst particle surface), gas-solid... 

The above issues associated with prediction of trickle-bed reactor performance has motivated a number of researchers over the past two decades to perform laboratory-scale studies using a particular model-reaction system. These are listed in Table I. Although a more detailed summary is given elsewhere (29), a general conclusion seems to be that both incomplete catalyst wetting and mass transfer limitations are significant factors which affect trickle-bed reactor performance. 

Several forms of incomplete catalyst wetting were visually observed and reported in previous studies. These observations include i) dry areas on a portion of the catalyst surface... 

Some of the remaining studies did not necessarily observe incomplete catalyst wetting, but included this concept either directly as an adjustable parameter in the model to fit the observed conversion versus liquid mass velocity data,(7,9,13, 16-18), or indirectly via use of a correlation for liquid-solid contacting established for non-porous absorber column packings (11,19-20). 

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 ). 

A few reactor models have recently been proposed (30-31) for prediction of integral trickle-bed reactor performance when the gaseous reactant is limiting. Common features or assumptions include i) gas-to-liquid and liquid-to-solid external mass transfer resistances are present, ii) internal particle diffusion resistance is present, iii) catalyst particles are completely externally and internally wetted, iv) gas solubility can be described by Henry s law, v) isothermal operation, vi) the axial-dispersion model can be used to describe deviations from plug-flow, and vii) the intrinsic reaction kinetics exhibit first-order behavior. A few others have used similar assumptions except were developed for nonlinear kinetics (27—28). Only in a couple of instances (7,13, 29) was incomplete external catalyst wetting accounted for. 

S catalyst wetting may occur. These adversely affect the reactor performance. 

Mears24 suggested that the fact that (4-6) correlated the data was fortuitous. He questioned the validity of Eq. (4-5) for the packed-bed trickle-bed reactor, since this equation was derived from the data taken for the flow over a string of spheres. He argued that the dependence of reactor performance on velocity in pilot-scale reactors is due to incomplete catalyst wetting at low flow rates. For a first-order reaction, he modified Eq. (4-4) as... 

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

The dependence of ln(Cj/C0) on liquid velocity was verified by Skripek and Ballard50 for VGO desulfurization at 45 < GL < 150 g h 1 cm-2. In a series of articles, Paraskos et al.,37 Montagna and Shah,29 and Montagna et al.30 evaluated the applicability of holdup and incomplete catalyst-wetting models to the desulfurization, demetalization, and denitrogenation reactions for a variety of residue and gas oils. Paraskos et al.37 showed that although log log plots of In (Cj/C0) versus 1 /LHSV for the desulfurization, demetalization (i.e., nickel and... 

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).

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. 

Another approach to evaluate the performance of a trickle-bed reactor (particularly a pilot-scale reactor) is to incorporate the RTD with intrinsic kinetics. Since the liquid holdup, catalyst wetting, or the degree of axial dispersion can all be obtained from the RTD, this approach is not exclusive of the ones described above. For a first-order reaction, if the residence-time distribution E(t) and the degree of conversion are known, they can both be related by an expression... 

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. 

Mears,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. 

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