Reaction kinetics, plant-scale

The design of a catalytic reactor requires the knowledge of the reaction rate and product selectivity as a function of the operating conditions. Rate expression based on the intrinsic reaction kinetics allows scaling up of laboratory data to a pilot plant and further to an industrial unit Without reliable kinetics, optimimi reactor design can be difficult or even impossible to achieve. 

Experimental analysis involves the use of thermal hazard analysis tests to verify the results of screening as well as to identify reaction rates and kinetics. The goal of this level of testing is to provide additional information by which the materials and processes may be characterized. The decision on the type of experimental analysis that should be undertaken is dependent on a number of factors, including perceived hazard, planned pilot plant scale, sample availability, regulations, equipment availability, etc. 

The ROTOBERTY internal recycle laboratory reactor was designed to produce experimental results that can be used for developing reaction kinetics and to test catalysts. These results are valid at the conditions of large-scale plant operations. Since internal flow rates contacting the catalyst are known, heat and mass transfer rates can be calculated between the catalyst and the recycling fluid. With these known, their influence on catalyst performance can be evaluated in the experiments as well as in production units. Operating conditions, some construction features, and performance characteristics are given next. 

In many cases, two identical reaction systems (e.g., a pilot plant scale and a full-scale commercial plant) exhibit different performances. This difference in performance may result from different flow patterns in the reactors, kinetics of the process, catalyst performance, and other extraneous factors. 

Ideally, measurements on a pilot- or full-scale plant can be based on known reaction kinetics. If the kinetics are unknown, experimental limitations will usually prevent their accurate determination. The following section describes how to make the best of a less-than-ideal situation. 

There are several factors that may be invoked to explain the discrepancy between predicted and measured results, but the discrepancy highlights the necessity for good pilot plant scale data to properly design these types of reactors. Obviously, the reaction does not involve simple first-order kinetics or equimolal counterdiffusion. The fact that the catalyst activity varies significantly with time on-stream and some carbon deposition is observed indicates that perhaps the coke residues within the catalyst may have effects like those to be discussed in Section 12.3.3. Consult the original article for further discussion of the nonisothermal catalyst pellet problem. 

The global rates of heat generation and gas evolution must be known quite accurately for inherently safe design.. These rates depend on reaction kinetics, which are functions of variables such as temperature, reactant concentrations, reaction order, addition rates, catalyst concentrations, and mass transfer. The kinetics are often determined at different scales, e.g., during product development in laboratory tests in combination with chemical analysis or during pilot plant trials. These tests provide relevant information regarding requirements... 

A manufacturing precast for producing ortho-phthalate otters derived from alkyl acid ortho-phthalatos and olefins has boon developed and demonstrated on the pilot plant scale. Process variables Include choice of reactants, stoichiometry, reaction kinetics, recycle of recovered materials and the fate of the perchloric add catalyst. Seme physical properties of the ortho-phthalate esters have been determined and severed of the esters have been evaluated as plasticizers for polyvinyl chloride. The composite data show that the acid-olefin esterification process provides commercially acceptable plasticizers for polyvinyl chloride. 

As the scale of operation increases, the effect of the heat consumption by the plant typically declines. Therefore, the extent to which the kinetics of the runaway reaction is influenced by the plant is reduced. For plant scale vessels, the ())-factor is usually low (i.e., 1.0-1.2) depending on the heat capacity of the sample and the vessel fill ratio. Laboratory testing for vent sizing must simulate these low -factors. If the laboratory ( )-factor is high, several anomalies will occur ... 

Kinetic Study of the Phenolysis Reaction. With the demonstration that all of the already outlined deficiencies of ammonium lignin sulfonates as a phenol replacement can be reduced by phenolysis, attention was turned to consideration of the construction of a pilot plant scale continuous tube reactor. This is needed in order to prepare the large amounts of phenolyzed lignin sulfonates required for resin synthesis and testing under plywood production conditions. 

In reaction engineering, kinetic models are used to predict reaction rates at specified conditions of temperature and the partial pressures or concentrations of reactants and products. The emphasis must be, therefore, upon accuracy of prediction, even at the expense, if need be, of mechanistic rigor. For this reason, kinetic models for design purposes should be developed using the same pellet size and geometry as will be used in the commercial process, and over the ranges of temperature and component partial pressures expected for it. Finally, the kinetics should be studied at realistic plant-scale gas velocities so that the data are not influenced by physical transport phenomena like heat- and raass-transfer. 

The choice of experimental reactor is important to the success of the kinetic modeling effort. The short bench-scale reaction tubes sometimes used for studies of this sort give little or no insight into best mathematical form of the kinetic model, conduct the reaction over varying temperatures and partial pressures along the tube, and inevitably operate at velocities that are a small fraction of those to be encountered in the plant-scale reactor. Rate models from laboratory reactors of this sort rarely scale-up well. The laboratory differential reactor suffers from velocity problems but does at least conduct the reaction at known and relatively constant temperature and partial pressures. However, one usually runs into accuracy problems because calculated reaction rates are based upon the small observed differences in concentration between the reactor inlet and outlet. 

This is not to say that the fundamental approach to reaction kinetics is automatically the best in every situation. At least today, if the scale is small, the process likely to be short-lived, the chemistry complicated, and timing more important than cost, the work to elucidate the mechanisms may not be warranted or entail unacceptable delay. An empirical scale-up then is preferable. In industry, the fundamental approach is at its best and fundamental kinetics in greatest demand if the scale is large and construction of successive plants over years to come is envisaged. This book has been written chiefly with such applications in mind. 

Even when the laboratory test reactor is intended to be representative in a reaction kinetic sense only (thus waiving the demand for correspondence in terms of pressure drop and hold-ups), the process performance data can be affected by differences in mass transfer and dispersion caused by scale reduction. When interphase mass transfer and chemical kinetics are both important for the overall conversions, the above test reactor, which is a relatively large pilot plant reactor, cannot be further reduced in size unless one accepts deviations in test results. 

---