Trickle bed experiment

TABLE II. Range of operating conditions in trickle-bed experiments... 

Comparatively few kinetic experiments in trickle-bed reactors have been described in the literature. 

In continuous flow experiments, catalyst was packed into a downflow trickle-bed reactor of 30 cc bed volume. Hydrogen was passed slowly over the catalyst at atmospheric pressme and the temperature was slowly raised to the desired reduction/activation temperature and held for at least four hours. After activation, the reactor was cooled to the desired reaction temperature, the pressure was raised, and flow of an aqueous feed of glycerol and sodium hydroxide initiated along with a corresponding amonnt of hydrogen. A large set of reaction conditions was tested. 

In the remainder of this chapter the MRI data presented are images of liquid distribution within the trickle bed. In all experiments described a Bruker Spectro-spin DMX 200 spectrometer was used, equipped with a 4.7 T magnet and a... 

An exception to improved performance under flow interruption is for a group of points at r = 90 min with s = 0.01. In these experiments water flowed through the trickle bed for less than 1 min during a cycle. The oxidation rate under steady state is close to the maximum steady-state rate. Even though the scatter of the experimental data at s = 0.01 is large, all the measurements under flow interruption lie below the line representing steady state. 

A useful application of the model is to examine the S02 and 02 concentration profiles in the trickle bed. These are shown for the steady-state conditions used by Haure et al. (1989) in Fig. 25. The equilibrium S02 concentration drops through the bed, but the 02 concentration is constant. In Haure s experiments 02 partial pressure is 16 times the S02 partial pressure. At the catalyst particle surface, however, 02 concentration is much smaller and is only about one-third of the S02 concentration. This explains why 02 transport is rate limiting and why experimentally oxidation appears to be zero-order in S02. 

Several uncertainties in this periodic process have not been resolved. Pressure drop is too high at SV = 10,000 h 1 when packed beds of carbon are used. Study of carbon-coated structured packing or of monoliths with activated carbon washcoats is needed to see if lower pressure drops at 95% SO2 removal can be achieved. Stack gas from coal or heavy oil combustion contains parts-per-million or -per-billion quantities of toxic elements and compounds. Their removal in the periodically operated trickle bed must be examined, as well as the effect of these elements on acid quality. So far, laboratory experiments have been done to just 80°C use of acid for flushing the carbon bed should permit operation at temperatures up to 150°C. Performance of periodic flow interruption at such temperatures needs to be determined. The heat exchange requirements for the RTI-Waterloo process shown in Fig. 26 depend on the temperature of S02 scrubbing. If operation at 150°C is possible, gas leaving the trickle bed can be passed directly to the deNO, step without reheating. 

Reactions were carried out in batch autoclaves and in a continuous trickle bed reactor. In the latter experiments at 55-95°C a Bi2.39Ru1.61°7-y catalVst gave no evidence of deactivation, leaching by the alkaline solution, or change in bulk structure after 180 hours of operation. Adipic acid selectivities were of the order of 81-877. at complete conversion. In contrast, we have not observed, in initial experiments42, any cleavage with carbohydrates under the same conditions. 

An appropriate model for trickle-bed reactor performance for the case of a gas-phase, rate limiting reactant is developed. The use of the model for predictive calculations requires the knowledge of liquid-solid contacting efficiency, gas-liquid-solid mass transfer coefficients, rate constants and effectiveness factors of completely wetted catalysts, all of which are obtained by independent experiments. 

The on-going research at the Center for Applied Energy Research in the preparation, characterization and activity testing of these catalysts has produced a number of continuous plug-flow and trickle bed reactor runs of 20 to over 600 hours duration using different hydrocarbon feedstocks. At the completion of experiments in which the activity, defined by conversion of the feedstock, has declined, experiments to determined if the acid and/or metal function was responsible for the observed deactivation were performed. The results of these experiments are reported below. 

The main difference between parameter estimates based on continuous (micro-trickle bed reactor) and discontinuous (stirred tank reactor) experiments is the preexponential factor which is somewhat smaller for continuous operation. The parameter values reported in Eq. 1 and 2 correspond to continuous operations. This problem is analyzed elsewhere in more detail (6 ). 

Theoretical analyses of the liquid holdup are given by Kolar and Broz28 and Hutton et al.24 Kolar and Broz presented a relation between the liquid holdup and the flow rates, and physical properties of the fluids. The relationship contains three parameters, the values of which must be determined by experiment. Hutton et al.24 derived relationships between the liquid flow rate, pressure drop, and liquid holdup. At low Reynolds numbers, only gravitjLand yisgous forces were considered to beTmporfant anct from a force balance on the trickle bed, they derived a relation... 

In this chapter, first, the existing correlations for three-phase monolith reactors will be reviewed. It should be emphasized that most of these correlations were derived from a limited number of experiments, and care must be taken in applying them outside the ranges studied. Furthermore, most of the theoretical work concerns Taylor flow in cylindrical channels (see Chapter 9). However, for other geometries and flow patterns we have to rely on empirical or semiempirical correlations. Next, the modeling of the monolith reactors will be presented. On this basis, comparisons will be made between three basic types of continuous three-phase reactor monolith reactor (MR), trickle-bed reactor (TBR), and slurry reactor (SR). Finally, for MRs, factors important in the reactor design will be discussed. 

Operating experience with the sandwich structure in trickle-bed mode is as yet limited, but its behavior is sufficiently similar to that when employed as a catalytic distillation packing, in terms of liquid holdup/residence time, mass transfer, and pressure drop, to allow design of pilot plant for these parameters to be made with confidence. 

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