Wetting incomplete catalyst

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). 

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... 

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... 

No maldistribution of gas or liquid in three-phase processes. Regarding application of the BSR concept to gas/liquid/solid processes, an important advantage of the BSR is that adjacent strings do not (necessarily) touch. Because of the liquid surface tension, liquid will not spill over from one BSR string to another. Consequently, the initial liquid distribution is maintained throughout the BSR module. This feature is especially advantageous when incomplete catalyst wetting (which results from liquid maldistribution in traditional, randomly packed trickle-flow reactors) would lead to hot spots and decreased selectivity. 

Trickle-bed reactors are widely used in hydrotreating processes, i.e., hydrodesulfurization of gasoline and diesel fuel, in petroleum refining, chemical, petrochemical, and biochemical processes. The knowledge of hydrodynamic parameters is vital in the design of a TBR because the conversion of reactants, reaction yield, and selectivity depend not only on reaction kinetics, operating pressure, and temperature, but also on the hydrodynamics of the reactor. Special care is also required to prevent flow maldistribution, which can cause incomplete catalyst wetting in some parts... 

The theories of dilution techniques have been explained in detail by various researchers. The selection of the proper size of diluent is very important. Equal volumes of diluent and catalyst were used for this comparison. For the case of an undiluted bed, only commercial size catalyst is packed in a small-scale reactor. The wall effect is very significant and in this case causes channeling of liquid. Because of the high void space inside the catalyst bed, the liquid holdup in the catalyst bed is also very low. As a whole, there is incomplete wetting of catalyst and only partial utilization is achieved in this case. Besides this, an appreciable amount of axial backmixing is present in the undiluted catalyst bed. When a larger size of diluent is used, it cannot enter the void space between the catalyst particles. Thus, it does not increase liquid holdup and hence only partial utilization of catalyst is also obtained. However, the addition of diluent increases the bed height, which in turn reduces liquid axial dispersion to some extent. When the diluent size is smaller, it can enter the narrow void space between the catalyst particles and can increase the liquid holdup... 

The performance of trickle-bed reactors may be affected by many factors, such as interphase mass transfer, intraparticle diffusion, axial dispersion and incomplete catalyst wetting. Therefore, knowledge about these influenced factors is important for their mathematical description by an unsteady-state reactor model. Until now, the literature analysis shows the experimental and theoretical understanding of trickle-bed reactors under unsteady-state-operation conditions has improved, but not considerably. The following studies are focused on the trickling regime under unsteady-state-operation conditions. 

At low liquid flow rates, flow maldistributions such as channeling, bypassing and incomplete catalyst wetting may occur. These adversely affect the reactor performance. 

Paraskos, J. A., J. A. Prayer and Y. T. Shah. Effect of Holdup, Incomplete Catalyst Wetting and Backmixing During Hydroprocessing in Trickle-Bed Reactors. Ind. Eng. Chem. Process Des. Dev. 14 (1975) 315-322. 

The purpose of this paper is to summarize previous interpretations of the effect of incomplete catalyst wetting on trickle-bed performance and to develop a model for the effectiveness factor for partially wetted catalyst pellets. In the case of a reaction... 

Most of the previously used expressions to account for incomplete catalyst wetting in trickle-beds are summarized in Table I. All of these, with the exception of the last one, are based on the assumptions of a) plug flow of liquid, b) no external mass transfer limitations, c) isothermal conditions, d) first order irreversible reaction with respect to the liquid reactant, e) nonvolatile liquid reactant, f) no noncatalytic homogeneous liquid phase reaction. 

Vapor (mostly H2) and liquid (oil) are passed cocurrently downward over a fixed bed of small catalyst particles. The liquid flows over the particles in films and rivulets the vapor flows through the remaining voids. As discussed below, these hydrodynamical conditions may lead to incomplete catalyst wetting, axial dispersion, and restricted Interphase mass transfer and may therefore result in Incomplete catalyst utilization. Since the catalyst is fairly expensive and the conditions of temperature and pressure require expensive reactor vessels, there Is considerable incentive to ensure that maximum utilization of the catalyst is obtained. 

Incomplete Catalyst Wetting. It has been widely reported in the literature that liquid contacting is not complete in trickle flow reactors All of the catalyst particles may not... 

Interphase Mass Transfer. There are a number of interphase mass transfer steps that must occur in a trickle flow reactor. The mass transfer resistances can be considered as occurring at the more or less stagnant fluid layer interfaces, i.e., on the gas and/or the liquid side of the gas/llquld Interface and on the liquid side of the liquid/solid Interface. The mass transfer correlations (8) indicate that the gas/llquld Interface and the liquid/solid interface mass transfer resistances decrease with higher liquid velocity and smaller particle size. Thus, in the PDU, the use of small inert particles partially offsets the adverse effect of low velocity. These correlations indicate that for this system, external mass transfer limitations are more likely to occur in the PDU than in the commercial reactor because of the lower liquid velocity, but that probably there is no limitation in either. If a mass transfer limitation were present, it would limit conversion in a way similar to that shown for axial dispersion and incomplete catalyst wetting illustrated in Figure 1. Due to the uncertainty in the correlations and in the physical properties of these systems, particularly the molecular diffuslvities, it is of interest to examine if external mass transfer is influencing the PDU results. 

A modified process development unit apparatus has been utilized in this study in which, for the same 200 cm packed bed of catalyst, normal once-through trickle flow operation can be compared to liquid-full batch operation. In the latter, the possibilities of incomplete catalyst wetting, axial dispersion, and interphase mass transfer limitations are not present and thus, maximum catalyst utilization is obtained. Comparison of the results of the two modes of operation for the specific system of interest leads to the following conclusions ... 

Paraskos, J.A., Prayer, J.A., Shah, Y.T. 1975. Effect of holdup incomplete catalyst wetting and hackmixing during hydroprocessing in trickle bed reactors. Ind. Eng. Chem. Proc. Des. Dev. 14(3) 315-322. 

The importance of the wetting efficiency results mainly from the fact that it is closely related to the reaction yield, and more specifically to the catalyst effectiveness factor (Burghardt et al., 1995). The reaction rate over incompletely covered catalytic particles can be smaller or greater than the rate observed on completely wetted packing, depending on whether the limiting reactant is present only in the liquid-phase or in both gas and liquid-phases. 

Die difference from the real value (lm) is mainly due to the approximation made about the mass transfer coefficient as well as the complete wetting of the catalyst, as the actual wetting efficiency is 88%. Furthermore, the problem is more complicated because under incomplete wetting, the gas reactant reaches the catalyst surface more easily than the unwetted part, as Horowitz et al. found out experimentally. 

As long as fuel cells are using liquid electrolytes like phosphoric acid or concentrated caustic potash, the catalyst utilization is usually not limited by incomplete wetting of the catalyst. Provided the amount of electrolyte is sufficiently high, the hydrophilic porous particles are not only completely flooded but due to their expressed hydrophilicity are wetted externally by an electrolyte film that together with the whole electrolyte-filled hydrophilic pore system establishes the ionic contact of an electrode to the respective counterelectrode. 

Incomplete and/or ineffective wetting of the catalyst with low liquid flow-rates and low column diameter/particle size ratio (< 15/20) possibility of liquid by-passing along the reactor wall. 

At low liquid flowrates incomplete wetting of the catalyst by the liquid may occur, as illustrated in Fig. 4.19, leading to channelling and a deterioration in reactor performance 34 . 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

Another important consideration in preparing mixed-oxide catalysts is the spontaneous monolayer dispersion of oxides and salts onto surfaces of support substrates on calcination. Both temperature and duration of calcination are important here, as discussed in the reviews by Xie and Tang [63] and by Knozinger and Taglauer [64]. If this dispersion step is inadequate or incomplete, the resulting oxide layer, and any reduced metal surface from it, will not be reproducible from the same catalyst system therefore, one can then have different catalysts prepared at different times and, of course, from one laboratory to another. Spreading and wetting phenomena in preparation of supported catalysts is discussed in Section A.2.2.1.3. 

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. 

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