Gas-solid trickle-flow reactor

Oxidation of Hydrogen Sulphide to Sulphur in a Gas-Solid Trickle Flow Reactor, Ph.D. thesis. University of Twente, Enschede, The Netherlands, 1984. With permission.)... 

Verver, A.B., van Swaaij, W.P.M., Modeling of a gas-solid trickle flow reactor for the catalytic oxidation of hydrogen sulfide to elementar sulfur, Inst. Chem. Eng. Symp. Ser., 87, 177 184, 1984. 

Another interesting example of reactive adsorption is the so-called gas-solid-solid trickle flow reactor, in which adsorbent trickles through the fixed bed of catalyst, removing selectively in situ one or more of the products from the reaction zone. In the case of methanol synthesis this led to conversions significantly exceeding the equilibrium conversions under the given conditions (67). 

Methanol synthesis 98,99 Gas-solid-solid trickle-flow reactor (GSSTFR)... 

Kuczynski M. The synthesis of methanol in a gas-solid-solid trickle-flow reactor. Ph.D. dissertation, University of Twente, Enschede, Netherlands, 1987. 

Several processing alternatives have been proposed for converting synthesis gas to methanol. The main incentives are reduced energy costs due to the ability to operate at lower temperatures, lower pressures or both. The most notable of these alternatives (in terms of recent interest) have been the alkyl formate process (ref. 27) and the Chem Systems three-phase reactor approach (ref. 28). A very recent development is the use of a gas-solid-solid trickle flow reactor.which it is proposed can be retrofitted in conventional low pressure methanol synthesis plants (ref. 29). These three alternatives will be reviewed in turn. 

The Gas-Solid-Solid Trickle Flow Reactor appears to offer considerable potential for reduced energy costs in conventional methanol plants. If the claims made by the inventors are realised, it would be a simple matter to retrofit plants by replacing existing converters with reactors using this novel contacting method. 

Kuezynski, M., Oyevaar, M.H., Pieters, R.T., and Westerterp, K.R., Methanol synthesis in a countercurrent gas-solid-solid trickle flow reactor. An experimental study, Chem. Eng. Sci., 42, 1887, 1987. 

Here Sh is the modified Sherwood number defined as Sh = /Csnsdp/fl,D and We is the modified Weber number defined as We = UoLpi.dr/hlci. A graphical illustration of the above correlation is shown in Fig. 6-20. The predictions of Eq. (6-67) also agree fairly well with the data of Lemay el al.so Specchia et al.9i showed that, in a trickle-flow reactor, KLaL and Ksas are essentially of the same order of the magnitudes. They also evaluated the conditions under which the mass-transfer (gas-liquid and liquid-solid) influences significantly the performance of a trickle-bed reactor. 

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. 

GSSTFR. See reactor, gas-solid-solid trickle-flow GT. See gas, turbine GTL. See gas, -to-liquid... 

From the experimental results obtained in the trickle flow reactor and model predictions, it could be concluded that for the H2S—O2 reaction, no mass transfer limitations are to be expected below about 200°C (Figure 22.14). However, at about 300°C, both external mass transfer and diffusion within the dense solids suspension are likely to offer substantial resistance to the reaction. Ultimately, at higher temperatures, the conversion rate can be determined by gas-phase mass transfer rate only, similar to the much faster reaction of H2S with SO2 (Figure 22.14). 

When the solid is a catalyst, a trickle flow reactor may be an alternative for relatively high gas/liquid flow ratios (section 4.723). 

Slurry reactors can be classified according to the phases where the reactants are present. Table II gives an overview. The most important distinction is whether the solid phase is a reactant or a catalyst. In principle, the solids could also be inert and only present to increase mass transfer between phases as is often the case, e.g., in trickle flow reactors. In slurry reactors the introduction of solids for this purpose only is not worthwhile, with the exception of solids like zeolites and activated carbon for enhancement of mass transfer or improvement of selectivity [21, 22] but in such a system the solid is not really inert. Another example is the turbulent contactor in which large but light balls are moved by a gas flow and irrigated by a liquid phase. However, this regime falls outside the scope of the present presentation. If the solid is a reactant as well as the gas phase and liquid phase, the situation becomes rather complex nevertheless, it corresponds to many practical situations (see e.g. Shah [2]). A rather exceptional... 

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. 

In parallel with the development of high-activity catalysts, researchers are studying other types of reactors that would prevent the hot-spot phenomenon associated with the current fixed-bed reactor and/or increase the single-pass conversion. These include fluidized-bed, recirculating fluidized-bed, slurry, trickle-bed, gas-solid-solid trickle-flow, and liquid-phase reactors. Complete single-pass conversion has been demonstrated using continuous methanol removal by Kquid or solid absorbents [18,19]. 

The advantages of trickle flow reactors are low pressure drop, good contacting properties emd simple construction in comparison with tray columns. These properties are combined with general advantages of counter current operation extraction of reactants or products out of the reaction zone possible (e.g. for equilibrium reactions), efficient heat exchange between solids and gas phase, etc. The radial heat transport, wall heat transfer coefficients and scaling-up factors are not yet known. 

Some contrasting characteristics of the main lands of three-phase reactors are summarized in Table 23-15. In trickle bed reactors both phases usually flow down, the liquid as a film over the packing. In flooded reactors, the gas and hquid flow upward through a fixed oed. Slurry reactors keep the solids in suspension mechanically the overflow may be a clear liquid or a slurry, and the gas disengages from the... 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

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