Surface reaction residence time

The desorption of arsenate previously sorbed onto Fe- or Al-oxides or onto an Andisol containing 42% of allophanic materials (Vacca et al. 2002) by phosphate has been demonstrated to be affected by time of reaction, residence time of arsenate onto the surfaces and the pH of the system (Pigna et al. 2006 Pigna et al. 2007, unpublished data). Figure 9 shows the desorption of arsenate at pH 6.0 (phosphate/arsenate molar ratio of 4) when phosphate was added onto the soil (Andisol) sample 1, 5 or 15 days after arsenate (surface coverage of arsenate about 60%). After 60 days of reaction, 55% of arsenate was desorbed by phosphate when the residence time of arsenate onto the surfaces of the Andisol was 1 day, but 35 and 20% of arsenate was desorbed by phosphate with increase in the residence time up to 5 and 15 days. Further, it was also observed that by keeping the... 

The global reaction rates for heterogeneously catalysed reactions comprise several elementary processes such as diffusion, adsorption/desorption and surface reaction. Thus, the situation becomes more complex, as described above. Due to mass transfer and phase boundaries, the overall reaction rate is completely different from the intrinsic reaction rate on active sites on the catalyst surface. The residence time behaviour in a reactor under SCF conditions has to be considered (for modelling in SCCO2 and SCH2O, see Refs. 32, 33). Supercritical fluids have the potential to affect some of these elementary steps, in particular mass transfer and... 

Because the characteristic of tubular reactors approximates plug-flow, they are used if careful control of residence time is important, as in the case where there are multiple reactions in series. High surface area to volume ratios are possible, which is an advantage if high rates of heat transfer are required. It is sometimes possible to approach isothermal conditions or a predetermined temperature profile by careful design of the heat transfer arrangements. 

Studies of inelastic scattering are of considerable interest in heterogeneous catalysis. The degree to which molecules are scattered specularly gives information about their residence time on the surface. Often new chemical species appear, whose trajectory from the surface correlates to some degree with that of the incident beam of molecules. The study of such reactive scattering gives mechanistic information about surface reactions. 

At still higher temperatures, when sufficient oxygen is present, combustion and "hot" flames are observed the principal products are carbon oxides and water. Key variables that determine the reaction characteristics are fuel-to-oxidant ratio, pressure, reactor configuration and residence time, and the nature of the surface exposed to the reaction 2one. The chemistry of hot flames, which occur in the high temperature region, has been extensively discussed (60-62) (see Col ustion science and technology). 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

An industrial chemical reacdor is a complex device in which heat transfer, mass transfer, diffusion, and friction may occur along with chemical reaction, and it must be safe and controllable. In large vessels, questions of mixing of reactants, flow distribution, residence time distribution, and efficient utilization of the surface of porous catalysts also arise. A particular process can be dominated by one of these factors or by several of them for example, a reactor may on occasion be predominantly a heat exchanger or a mass-transfer device. A successful commercial unit is an economic balance of all these factors. 

Tubular reactors have been the main tools to study continuous flow processes for vapor or gas-phase reactions. These are also used for reaction in tv o flowing phases over a solid catalyst. When the catalyst is in a fixed bed, the contact between the liquid on the outside surface of the particulate is uncertain. For slurry-type solid catalyst the residence time of the catalyst or the quantity in the reactor volume can be undefined. 

Measurements of the true reaction times are sometimes difficult to determine due to the two-phase nature of the fluid reactants in contact with the solid phase. Adsorption of reactants on the catalyst surface can result in catalyst-reactant contact times that are different from the fluid dynamic residence times. Additionally, different velocities between the vapor, liquid, and solid phases must be considered when measuring reaction times. Various laboratory reactors and their limitations for industrial use are reviewed below. 

When solid particles are subject to noncatalytic reactions, the effects of the reaction on individual particles are derived and then the results are averaged to determine overall properties. The general techniques for this averaging are called population balance methods. They are important in mass transfer operations such as crystallization, drop coagulation, and drop breakup. Chapter 15 uses these methods to analyze the distribution of residence times in flow systems. The following example shows how the methods can be applied to a collection of solid particles undergoing a consumptive surface reaction. 

Equations (11.54) and (11.55) apply to any distribution of particle residence times provided the linear consumption rate is constant. They do not require that the fluid phase is perfectly mixed, only that the consumption rate is strictly controlled by the surface reaction. For the special case of... 

The characteristic times on which catalytic events occur vary more or less in parallel with the different length scales discussed above. The activation and breaking of a chemical bond inside a molecule occurs in the picosecond regime, completion of an entire reaction cycle from complexation between catalyst and reactants through separation from the product may take anywhere between microseconds for the fastest enzymatic reactions to minutes for complicated reactions on surfaces. On the mesoscopic level, diffusion in and outside pores, and through shaped catalyst particles may take between seconds and minutes, and the residence times of molecules inside entire reactors may be from seconds to, effectively, infinity if the reactants end up in unwanted byproducts such as coke, which stay on the catalyst. 

At high temperatures desorption prevails, implying that the coverages of all species are small and that the surface is nearly empty. This does not mean that the reaction can not take place, but the residence time of any species on the surface before it desorbs or reacts is short. Since the surface is nearly empty, we can set 6 1 and obtain ... 

To avoid the consecutive reaction of the desired product to CO2, the catalyst has a low surface area and minimal porosity, to ensure a short residence time in the catalyst bed. 

Catalysts and their carriers are provided in micro channels by various means and in various geometric forms. In a simple variant, the catalyst itself constitutes the micro-reactor construction material without need for any carrier [2-A], In this case, however, the catalyst surface area equals that of the reactor wall and hence is comparatively low. Accordingly, applications are typically restricted to either fast reactions or processing at low flow rates for slow reactions (to enhance the residence time). 

If we assume that t0= 10 13 s (vibrational frequency)-1, then for a heat of adsorption AH of 40 kJ mol-1 and a surface temperature of 295 K the residence time zsurf is 3 x 10-6 s and for 80 kJ mol-1 it is 102 s as T decreases the value of the surface residence time increases rapidly for a given value of AH. Decreasing the temperature is one possible approach to simulating a high-pressure study in that surface coverage increases in both cases the reaction, however, must not be kinetically controlled. 

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