Residence time chemical reaction

Table 5. Characteristic timescales of several important processes involved in biouptake as estimated by their residence time (physical processes) or by their half-reaction time (chemical reactions). See also [2,165]...

Constant average rate of shear Constant mixing time Chemical reaction (constant residence time distribution and constant mean residence time)13 N D5 May be impossible to achieve due to large power requirements... 

The comparability of the two methods was further tested by attempting to use kinetic-parameter values determined by gas purge to predict breakthrough curves (BTC) obtained from the same soil/solute pairs. A BTC obtained from miscible displacement of PCE through the Eustis soil column is presented in Fig. 11-5. The values for ki and F (fraction of sorbent for which sorption is instantaneous) determined from the Eustis/PCE gas-purge experiment were used to calculate values for /3 (fraction of instantaneous retardation) and w (ratio of hydrodynamic residence time to reaction time). The simulation produced by a first-order bicontinuum model (Brusseau and Rao, 1989a) with these independently determined kinetic parameters is shown in Fig. 11-5. The model simulation compares extremely well with the experimental BTC. Such predictions for organic chemicals, where values for... 

This assumes that all chemical species have the same residence time distribution, and is very convenient to compute the reaction paths for different contacting patterns. Matsuyama and Miyauchi [16] have also considered some aspects of this. An important conclusion of Wei [15] is that for a reactor with distribution of residence times, all reactions are slowed down in comparison with those in a plug flow reactor, but the faster reactions are slowed down a great deal more than the slower ones. Consequently, the occurrence of distribution of residence times makes all reaction rates of the characteristic species nearly equal. That is, the differences between the various reaction rates are decreased, thereby decreasing the selectivity. This is similar to the diffusion effects considered in Chapter 3. 

Exercise 3.1 Time of residence and chemical reaction in a stirred reactor... 

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. 

Chemical Reaction Measurements. Experimental studies of incineration kinetics have been described (37—39), where the waste species is generally introduced as a gas in a large excess of oxidant so that the oxidant concentration is constant, and the heat of reaction is negligible compared to the heat flux required to maintain the reacting mixture at temperature. The reaction is conducted in an externally heated reactor so that the temperature can be controlled to a known value and both oxidant concentration and temperature can be easily varied. The experimental reactor is generally a long tube of small diameter so that the residence time is well defined and axial dispersion may be neglected as a source of variation. Off-gas analysis is used to track both the disappearance of the feed material and the appearance and disappearance of any products of incomplete combustion. 

Computer Models, The actual residence time for waste destmction can be quite different from the superficial value calculated by dividing the chamber volume by the volumetric flow rate. The large activation energies for chemical reaction, and the sensitivity of reaction rates to oxidant concentration, mean that the presence of cold spots or oxidant deficient zones render such subvolumes ineffective. Poor flow patterns, ie, dead zones and bypassing, can also contribute to loss of effective volume. The tools of computational fluid dynamics (qv) are useful in assessing the extent to which the actual profiles of velocity, temperature, and oxidant concentration deviate from the ideal (40). 

The equiHbrium approach should not be used for species that are highly sensitive to variations in residence time, oxidant concentration, or temperature, or for species which clearly do not reach equiHbrium. There are at least three classes of compounds that cannot be estimated weU by assuming equiHbrium CO, products of incomplete combustion (PlCs), and NO. Under most incineration conditions, chemical equiHbrium results in virtually no CO or PlCs, as required by regulations. Thus success depends on achieving a nearly complete approach to equiHbrium. Calculations depend on detailed knowledge of the reaction network, its kinetics, the mixing patterns, and the temperature, oxidant, and velocity profiles. 

Acetaldehyde can be used as an oxidation-promoter in place of bromine. The absence of bromine means that titanium metallurgy is not required. Eastman Chemical Co. has used such a process, with cobalt as the only catalyst metal. In that process, acetaldehyde is converted to acetic acid at the rate of 0.55—1.1 kg/kg of terephthahc acid produced. The acetic acid is recycled as the solvent and can be isolated as a by-product. Reaction temperatures can be low, 120—140°C, and residence times tend to be high, with values of two hours or more (55). Recovery of dry terephthahc acid follows steps similar to those in the Amoco process. Eastman has abandoned this process in favor of a bromine promoter (56). Another oxidation promoter which has been used is paraldehyde (57), employed by Toray Industries. This leads to the coproduction of acetic acid. 2-Butanone has been used by Mobil Chemical Co. (58). 

The process involving aHyl alcohol has not been iadustriaHy adopted because of the high production cost of this alcohol However, if the aHyl alcohol production cost can be markedly reduced, and also if the evaluated cost of hydrogen chloride, which is obtained as a by-product from the substitutive chlorination reaction, is cheap, then this process would have commercial potential. The high temperature propylene—chlorination process was started by SheH Chemical Corporation ia 1945 as an iadustrial process (1). The reaction conditions are a temperature of 500°C, residence time 2—3 s, pressure 1.5 MPa (218 psi), and an excess of propylene to chlorine. The yield of aHyl chloride is 75—80% and the main by-product is dichloropropane, which is obtained as a result of addition of chlorine. Other by-products iaclude monochioropropenes, dichloropropenes, 1,5-hexadiene. At low temperatures, the amount of... 

Advanced Cracking Reactor. The selectivity to olefins is increased by reducing the residence time. This requires high temperature or reduction of the hydrocarbon partial pressure. An advanced cracking reactor (ACR) was developed jointly by Union Carbide with Kureha Chemical Industry and Chiyoda Chemical Constmction Co. (72). A schematic of this reactor is shown in Figure 6. The key to this process is high temperature, short residence time, and low hydrocarbon partial pressure. Superheated steam is used as the heat carrier to provide the heat of reaction. The burning of fuel... 

In turbulent flow, axial mixing is usually described in terms of turbulent diffusion or dispersion coefficients, from which cumulative residence time distribution functions can be computed. Davies (Turbulence Phenomena, Academic, New York, 1972, p. 93), gives Di = l.OlvRe for the longitudinal dispersion coefficient. Levenspiel (Chemical Reaction Engineering, 2d ed., Wiley, New York, 1972, pp. 253-278) discusses the relations among various residence time distribution functions, and the relation between dispersion coefficient and residence time distribution. 

More often than not the rate at which residual absorbed gas can be driven from the liqmd in a stripping tower is limited by the rate of a chemical reaction, in which case the liquid-phase residence time (and hence, the tower liquid holdup) becomes the most important design factor. Thus, many stripper-regenerators are designed on the basis of liquid holdup rather than on the basis of mass transfer rate. 

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. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

For a chemically controlled process, conversion depends only on the residence time and not on which phase is dispersed, whereas the interfacial area and, consequently, the rate or mass transfer will change when the relative volumes of the phases are changed. If a reaction is known to occur in a particular phase, and the conversion is... 

Shinnar, R., Use of Residence and Contact Time Distributions in Reactor Design, Chapter 2, pp. 63-149 of Chemical Reaction and Reactor Engineering, Carberry, J. J. and Varma, A., Eds., Marcel Dekker, New York, 1987. 

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