Stirred flow system

A condition achieved in a multistep process when the rate of formation of unstable intermediates is time-invariant, or nearly so. 2. In stirred flow systems, the steady-state condition refers to the procedure in which the concentration of all components is kept constant, via additions to and withdrawals from the system. 

Table 7.3 lists the various NPD for the well-stirred batch mixer, the plug-flow system with recirculation, and the well-stirred flow system. Figures E7.12a and E7.12b plot the distribution in the batch mixer and the continuous batch mixer. Note that in batch mixers the distribution widens considerably with increasing mean number of passages, and the distribution is much skewed toward low numbers of passages in the flow batch system. Therefore, two mixers with the same mean passages may have very different distributions. 

The stationary-state heat release rate may also be interpreted from the measured temperature excess in well-stirred flow systems. The energy conservation equation for a well-stirred flow system is similar to equation (6.13) but an additional term is required to represent heat transport via the outflowing gases (a-Cp(T- Tafltres) as shown in equation (4.4). The inflowing gases are assumed to be pre-heated to the vessel temperature, Ta- Under constant pressure conditions, normally applicable to flow reactors, Cp replaces C, and A.H replaces AU in equation (6.13). The heat release is obtained from a summation of the product of individual reaction rates and their enthalpy change (-AH)jRj) in equation (5.4)). 

Fig. 6.10. Idealized (p-T ) ignition diagram which is typical of that obtained for the combustion of many hydrocarbons in oxygen or air in closed vessels and well-stirred flow systems. Ignition, oscillatory cool-flame and slow reaction regions are shown.

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

Fig. 14. Calcium response of Sf-9 insect cells subjected to different values of e in a stirred bioreactor equipped with a 5.1 cm diameter 6-bladed Rushton impeller (closed circles) or in the capillary flow system (open squares). Error bars for stirred bioreactor are standard deviation for each experiment but for the capillary, data are hard to discern [99]...

An alternative to the stirred tank system is a column-type device which provides for constant fluid flow through a powder bed. The mass transport process was shown to be primarily determined by the length and cross-sectional area of the cylinder and the fluid flow rate [36],... 

There are a variety of limiting forms of equation 8.0.3 that are appropriate for use with different types of reactors and different modes of operation. For stirred tanks the reactor contents are uniform in temperature and composition throughout, and it is possible to write the energy balance over the entire reactor. In the case of a batch reactor, only the first two terms need be retained. For continuous flow systems operating at steady state, the accumulation term disappears. For adiabatic operation in the absence of shaft work effects the energy transfer term is omitted. For the case of semibatch operation it may be necessary to retain all four terms. For tubular flow reactors neither the composition nor the temperature need be independent of position, and the energy balance must be written on a differential element of reactor volume. The resultant differential equation must then be solved in conjunction with the differential equation describing the material balance on the differential element. 

A chemical reaction is being studied in a laboratory scale steady-state flow system. The reactor is a well-stirred 1000 cm3 flask containing an aqueous solution. The reactor contents (1000 cm3 of solution) are uniform throughout. The stoichiometric equation and data are given below. What is the expression for the rate of this reaction Determine the reaction order and the activation energy. 

Figure 7. (a) Flow diagram of the optical fibre continuous-flow system for bioluminescence and chemiluminescence measurements S, sample C, carrier stream PP, peristaltic pump IV, injection valve W, waste FO, optical fibre FC, flow-cell, (b) Details of the optical fibre biosensor/flow-cell interface a, optical fibre b, sensing layer c, light-tight flow-cell d, stirring bar. 

The flow systems are generally of two types, viz. (i) plug flow (Fig. 7.1), in which there is no stirring in the reactor and (ii) stirred flow, in which there is stirring in the reactor to effect complete mixing within the reactor. 

In the steady-state approach to determining the rate law, solutions containing reactants are pumped separately at a constant flow rate into a vessel ( reactor ), the contents of which are vigorously stirred. After a while, produets and some reactants will flow from the reactor at the same total rate of inflow and a steady state will be attained, in which the reaction will take place in the reactor with a constant concentration of reactants, and therefore a constant rate. This is the basis of the stirred-flow reactor, or capacity-flow method. Although the method has been little used, it has the advantage of a simplified kinetic treatment even for complex systems. 

Stirring speed, stirring time, and phase ratio. From this data, the stage efficiency in a flow system may be estimated. Optimization of these parameters can result in providing sufficient data for the most economic design... 

The simplest form of flow system is the continuously fed well-stirred tank reactor or CSTR, represented schematically in Fig. 1.11. The behaviour of typical autocatalytic systems in a CSTR will be considered in chapters 4 and 5, but here we may quickly examine how multistability can arise, even with only one overall chemical reaction. We will take a CSTR in which just the... 

In chapters 2-5 two models of oscillatory reaction in closed vessels were considered one based on chemical feedback (autocatalysis), the other on thermal coupling under non-isothermal reaction conditions. To begin this chapter, we again return to non-isothermal systems, now in a well-stirred flow reactor (CSTR) such as that considered in chapter 6. 

Reactor type For the highest relative yield of P a batch or tubular plug-flow reactor should be chosen. If a continuous stirred-tank system is adopted on other grounds, several tanks should be used in series so that the behaviour may approach that of a plug-flow tubular reactor. 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

Using Eq. 7.3.16, it is possible to derive all the interrelationships of the RTD functions, which are listed in Table 7.1. The two extreme flow systems with respect to RTD are the plug flow system, which exhibits no distribution of residence times, and the continuous stirred tank (CST), which exhibits perfect back-mixing and has the following RTD function ... 

Figure 2.5 (a) Stirred vessel of a flow system, (b) Four sets of positions of inlet and outlet of a flow system, (c) Mixing capacity change with impeller rotational speed for four sets of positions of inlet and outlet of a flow system. 

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