Ideal reactors, continuously stirred tank reactor residence time

Ideal CSTR (continuous stirred tank reactor) behavior is approached when the mean residence time is 5-10 times the length of time needed to achieve homogeneity, which is accomplished with 500-2000 revolutions of a properly designed stirrer. 

There are two common types of continuous reactors continuous stirred tank reactors (CSTRs) (53), and plug flow reactors (PFRs). CSTRs are simply large tanks that are ideally well-mixed (such that the emulsion composition is uniform throughout the entire reactor volume) in which the polymerisation takes place. CSTRs are operated at a constant overall conversion. CSTRs are often used in series or trains to build up conversion incrementally. Styrene-butadiene rubber has been produced in this manner. Not all latex particles spend the same amount of time polymerising in a CSTR. Some particles exit sooner than others, producing a distribution of particle residence times, diameters and compositions. 

In an ideal continuous stirred tank reactor, CSTR, the composition and temperature are uniform throughout and the condition of the effluent is the same as that of the tank. When a battery of such vessels is employed in series, the concentration profile is step shaped if the abscissa is total residence time or the stage number. 

The arguments advanced in Sect. 3.2.3 apply equally well to a continuous stirred tank reactor. With a reversible exothermic reaction and a fixed mean residence time, t, there is an optimum temperature for operation of a continuous stirred tank reactor. Since the conditions in an ideal stirred tank are, by definition, uniform, there is no opportunity to employ a temperature gradient, as with the plug-flow reactor, to achieve an even better performance. 

To complete the model of lime dissolution, a discretized particle size distribution is defined, with [x,] being the molar concentration of size fraction i with radius For an ideal continuous stirred tank reactor (CSTR) with residence time T the equation for updating the concentrations of the solid lime size fractions is... 

The continuous production of aliphatic chiral alcohols was demonstrated by Leuchs et al. as shown in Scheme 6.20 by using continuous stirred tank reactor (CSTR). The CRED from Lactobacillus brevis was used in a biphasic system with MTBE as cosolvent to reduce aliphatic ketones with the general structure 53. It was shown that inaeasing the chain length resulted in a decreased yield due to less available substrate in the aqueous phase. The continuous process was run with a ketone concentration of 100 mmol/L and IPA was used as the reductant (1 mol/L). It was also demonstrated that 200 rpm was ideal for longevity of the enzyme. Doubling the residence time did not lead to a sigttificant increase in the space-time yield [30]. 

The following equations are written for absorption (of any gas) in a continuous-flow stirred tank reactor (CFSTR) under the assumption that the gas and liquid phases are ideally mixed (Figure B 1-2). The assumption of an ideally mixed phase can be checked by determining the residence time distribution in the reactor (e. g. Levenspiel, 1972 Lin and Peng, 1997 Huang et al., 1998). 

The simplest type of open system of interest in combustion is the continuous-flow well-stirred tank reactor or CSTR, which is an idealization of tank reactors used widely in industry. In essence, this is simply a tank into which reactants flow continuously at some known volumetric flow-rate and the reactant-intermediate-product mixture is efficiently stirred so that there are no spatial concentration or temperature gradients. In order to maintain a constant reaction volume, there is a matching volumetric outflow of the mixture from the CSTR so that molecules spend only a finite time in the reactor. This is known as the mean residence time t es and is determined by the volumetric flow-rates and the reactor volume. 

The ideal cases are the piston flow reactor (PFR), also known as a plug flow reactor, and the continuous flow stirred tank reactor (CSTR). A third kind of ideal reactor, the completely segregated CSTR, has the same distribution of residence times as a normal, perfectly mixed CSTR. The washout function for a CSTR has the simple exponential form... 

An important question for the design of continuous flow systems is When can the classic perfectly mixed assumption (ideal CSTR) be used in a continnons flow stirred tank reactor The blend time concept can be used here. If the blend time is small compared to the residence time in the reactor, the reactor can be considered to be well mixed. That is because the residence time is proportional to the characteristic chemical reaction time. A 1 10 ratio of blend time to reaction time is often used, but often, larger values result because the mixer must do other jobs, which lead to even smaller blend times. Frequently, residence time distributions are used to determine whether a reactor is well-mixed. It is usually easy to achieve well-mixed conditions in continuous flow, turbulent stirred vessels unless the reactions are very fast, such as acid-base neutralizations. Even in laminar systems the blend time can be made much less than the required residence time for the chemical reaction mainly because required residence times are so long for high viscosity reactants. For discussions of residence time distribution analysis, see Chapter 1, Levenspiel (1972), and Nauman (1982). 

Just like a batch reactor, all the reacting species in an idealized tubular reactor have exactly the same residence time. Thus, the mechanisms and kinetics presented in Chapters 3 and 4 are also applicable to emulsion polymerization carried out in a tubular reactor. On the other hand, the feed stream introduced into a continuous stirred tank reactor at any given time becomes completely mixed with the reaction mixture already present in the reaction system. As a result, a distribution of residence times of the material within a continuous stirred tank reactor is achieved. In other words, some of the recipe ingredients entering the continuous stirred tank reactor may leave it almost immediately because material is continuously withdrawn from the reactor. In contrast, other recipe ingredients may remain in the reactor almost forever because all the material is never removed from the reactor at one time. Many of the reaction species leave the reactor after spending a period of time somewhere in the vicinity of the mean residence time. The distribution of residence... 

For a few highly idealized systems, the residence time distribution function can be determined a priori without the need for experimental work. These systems include our two idealized flow reactors—the plug flow reactor and the continuous stirred tank reactor—and the tubular laminar flow reactor. The F(t) and response curves for each of these three types of well-characterized flow patterns will be developed in turn. 

Continuous Multicomponent Distillation Column 501 Gas Separation by Membrane Permeation 475 Transport of Heavy Metals in Water and Sediment 565 Residence Time Distribution Studies 381 Nitrification in a Fluidised Bed Reactor 547 Conversion of Nitrobenzene to Aniline 329 Non-Ideal Stirred-Tank Reactor 374 Oscillating Tank Reactor Behaviour 290 Oxidation Reaction in an Aerated Tank 250 Classic Streeter-Phelps Oxygen Sag Curves 569 Auto-Refrigerated Reactor 295 Batch Reactor of Luyben 253 Reversible Reaction with Temperature Effects 305 Reversible Reaction with Variable Heat Capacities 299 Reaction with Integrated Extraction of Inhibitory Product 280... 

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