Exothermic catalytic reactions

TABLE 23-5 Parameters of Some Exothermic Catalytic Reactions... 

Berty, J.M., Process and Equipment for Exothermal Catalytic Reaction in the Vapor Phase, 1969, German Patent Disclosure 1,915,560. 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acryhc acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubihty in water and retain this property after functionahzation with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by coohng the solution to 0 °C [92]. Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

Truly isothermal operation of a tubular reactor may not be feasible in practice because of large enthalpies of reaction or poor heat transfer characteristics. Nor is it always desirable, as, for example, in the case of a reversible exothermic reaction (see Sect. 3.2.4). In an exothermic catalytic reaction, it may be necessary to provide adequate means for heat transfer to prevent the development of local hot-spots on which coking may occur and reduce the catalyst activity. An excessive temperature rise may also cause the catalyst particles to sinter, thereby reducing their surface area and causing an irreversible decrease in catalytic activity. 

In practice, there is always some degree of departure from the ideal plug flow condition of uniform velocity, temperature, and composition profiles. If the reactor is not packed and the flow is turbulent, the velocity profile is reasonably flat in the region of the turbulent core (Volume 1, Chapter 3), but in laminar flow, the velocity profile is parabolic. More serious however than departures from a uniform velocity profile are departures from a uniform temperature profile. If there are variations in temperature across the reactor, there will be local variations in reaction rate and therefore in the composition of the reaction mixture. These transverse variations in temperature may be particularly serious in the case of strongly exothermic catalytic reactions which are cooled at the wall (Chapter 3, Section 3.6.1). An excellent discussion on how deviations from plug flow arise is given by DENBIGH and TURNER 5 . 

There are several aspects of thermal sensitivity and instability which are important to consider in relation to reactor design. When an exothermic catalytic reaction occurs in a non-isothermal reactor, for example, a small change in coolant temperature may, under certain circumstances, produce undesirable hotspots or regions of high temperature within the reactor. Similarly, it is of central importance to determine whether or not there is likely to be any set of operating conditions which may cause thermal instability in the sense that the reaction may either become extinguished or continue at a higher temperature level as a result of fluctuations in the feed condition. We will briefly examine these problems. 

Construct the diagrams of the effectiveness factor and the desired yield versus the Thiele modulus of the reactant A for a first-order consecutive exothermic catalytic reaction... 

If the reaction rate is a function of pressure, then the momentum balance is considered along with the mass and energy balance equations. Both Equations 6-105 and 6-106 are coupled and highly nonlinear because of the effect of temperature on the reaction rate. Numerical methods of solution involving the use of finite difference are generally adopted. A review of the partial differential equation employing the finite difference method is illustrated in Appendix D. Figures 6-16 and 6-17, respectively, show typical profiles of an exothermic catalytic reaction. 

P H Calderbjnk A D Caldwell, G Ross, The Diluted Catalyst Fixed Bed Reactor for Exothermic Catalytic Reactions, Chemw et Industne-Geme Chumque 1969, 101, 215 230... 

The nondimensional parameter /) (positive for exothermic reactions) is a measure of nonisothermal effects and is called the heat generation function. It represents the ratio between the rate of heat generation due to the chemical reaction and the heat flow by thermal conduction. Nonisothermal effects may become important for increasing values of /3, while the limit (3 - 0 represents an isothermal pellet. Table 9.1 shows the values of [3 and some other parameters for exothermic catalytic reactions. For any interior points within the pore where the reactant is largely consumed, the maximum temperature difference for an exothermic reaction becomes... 

Free clusters are ideal model systems to probe the influence of their intrinsic, size-dependent properties on the catalytic activity due to the lack of any support interactions. Free clusters are prepared from cluster sources [20] and only very low densities are obtained. They are highly unstable under normal conditions and, even under UHV conditions, exothermal catalytic reactions may lead to fragmentation without the presence of a buffer gas. Thus, free clusters may not become relevant for industrial applications. Nevertheless, they are important vehicles to gain a fundamental understanding of nanocatalysis. 

HIE) 1968 Hugo, P. Dynamic Behavior of Strongly Exothermic Catalytic Reactions in Open Gas Circulations (In German), Chem. React. Eng., Proc. 4th Eur. Symp, Pergamon Press, Oxford, England, (197 ) 459-472... 

The dynamics of exothermic catalytic reactions has been the object of investigation for almost half a century comprehensive updated studies have been recently reported [1, 2]. Whereas various models have been proposed in a large number of papers, describing mathematical and theoretical approaches, experimental data concerning these phenomena are relatively scarce, due to the difficulty of performing accurate measurements under reaction conditions. 

Infrared imaging was utilized in several studies of spatial effects in exothermic catalytic reactions over model catalysts, such as isolated particles, wafers, plates, discs [2]. Our approach has been to characterize the catalysts directly in a packed-bed microreactor, under realistic reaction conditions. In-situ measurements by infrared thermography of the adsorption properties of catalytic materials have been previously reported [6]. In the present study, the catalytic oxidation of compounds having different chemical properties was investigated by the same technique, with the aim of obtaining comparative data useful to better understand the factors governing the complex phenomena associated with catalytic combustion. 

During the oxidation of CO, CH4 and C3H8, the ignited state is characterized by a reaction front stabilized in a thin portion of the bed near the reactor inlet. This condition, corresponding to a diffusion-controlled reaction, is predicted by the known models of exothermic catalytic reactions [4], The chemical factors determining this dynamics are the heat of reaction and the activation energy. For all of the reactants considered in this study, a similar behaviour in the ignited state is observed. 

Table 3J.a-I Parameters of Some Exothermic Catalytic Reactions (after Hlavacek, Kubicek, and Marek [113]. ... 

Between 350°C and 420°C, the DTA-signal exhibited strong oscillations similar to those detected by IMR-MS in the reactor experiment (see Fig. 1). Weight changes were not detected by TG analysis. The oscillations in the DTA thus seem to arise from the highly exothermic catalytic reaction. A relation between the oscillations frequencies and the reaction temperature could not be derived. 

Local overheating of the catalyst granules is known to occur on account of exothermal catalytic reactions under the catalyst-adsorbent regeneration. This leads to the transformation of active alumina into corundum and to the reduction of specific surface area and porosity. Thus, the improvement of the catalyst thermal stability depending on that of the support is of a key importance, y-Alumina is the most thermally stable alumina since its transition to a- starts at the highest temperature, %- alumina is the least stable. The temperature of the transformation is influenced by various factors such as crystalline form, particle size and morphology, the nature of the gaseous atmosphere, additives (or impurities) etc. [35]. 

The oxidation of ethylene in air on a Pt wire is a good example by which to demonstrate the ignition behavior of exothermic catalytic reactions. The experiment was conducted as follows (Table 4.5.4). A coil consisting of a thin Pt-wire is placed in a tubular reactor. Then an ethylene-air mixture of constant temperature and pressure (303 K, 1 bar) is fed into the tubular reactor. The wire is now electrically heated until ignition (jump in temperature) occurs. The current and the voltage is measured and, thus, also the temperature of the wire as the electrical resistance depends on temperature. 

Kolodziej, A., Krajewski, W. and Dubis, A. (2001). Alternative Solution for Strongly Exothermal Catalytic Reactions A New Metal-Structured Catalyst Carrier, Catal. Today, 69, pp. 115-120. 

In a commercial fluidized bed, the presence of defluldized or semi-fluidized particles on the distributor in-between the gas inlet points is very undesirable since with "sticky" or "tacky" materials, these zones will grow and eventually plug the distributor. With exothermic catalytic reactions, these defluldized zones would lead to the formation of hot spots on the distributor. Fakhlml [60] developed a model which predicts the formation of defluldized zones and Zenz [22] suggested a distributor made of a honeycomb of nearly touching cones or hexagons to eliminate these stagnant zones. 

In the present work, a packed bed cell model is used to calculate temperature and concentration profiles in the adiabatic RFBR for exothermic catalytic reactions with interphase resistance to mass and heat transfer. In particular, differences between the RFBR and the AFBR, operated at the same space velocity, are explored with respect to uniqueness, multiplicity, and stejDility of the steady state, profile location, selectivity in parallel and series reactions, and transient behavior. 

The first issue is what should be controlled in the reactor and why. This is not a single question to answer because many things must be considered. Some of the important considerations for exothermic catalytic reactions were discussed in Chapter 20. Here, we assume that the exit temperature from the reactor, Stream 8, is the variable that must be controlled. If the reactor is designed such that the temperature increases monotonically from the inlet to the outlet, then the exit stream is the hottest point in the reactor. More commonly, there will be a tenperature bunp or warm (hot) spot somewhere within the reactor, and the exit temperature will be cooler than at the tenperature bunp. In this case, we could use a series of in-bed thermocouples to measure the tenperature profile within the catalyst tubes and use the maximum tenperature as our controlled variable. In either case, it is inportant to control the reactor tenperature, because the reaction rate and catalyst activity are both affected strongly by tenperature. 

---