CSTR Subject

In previous studies, the main tool for process improvement was the tubular reactor. This small version of an industrial reactor tube had to be operated at less severe conditions than the industrial-size reactor. Even then, isothermal conditions could never be achieved and kinetic interpretation was ambiguous. Obviously, better tools and techniques were needed for every part of the project. In particular, a better experimental reactor had to be developed that could produce more precise results at well defined conditions. By that time many home-built recycle reactors (RRs), spinning basket reactors and other laboratory continuous stirred tank reactors (CSTRs) were in use and the subject of publications. Most of these served the original author and his reaction well but few could generate the mass velocities used in actual production units. 

Suppose you have two CSTRs, one with a volume of 2m and one with a volume of 4m. You have decided to install them in series and to operate them at the same temperature. Which goes first if you want to maximize production subject to a minimum conversion constraint Consider the following cases ... 

Erlang with time delay expC-t Vd + t2s/n)n The last item is of a PFR and an n-stage CSTR in series. More complex combinations are the subject of problems P5.01.33, P5.03.10, P5.03.02 and... 

The chemical reactor is the unif in which chemical reactions occur. Reactors can be operated in batch (no mass flow into or out of the reactor) or flow modes. Flow reactors operate between hmits of completely unmixed contents (the plug-flow tubular reactor or PFTR) and completely mixed contents (the continuous stirred tank reactor or CSTR). A flow reactor may be operated in steady state (no variables vary with time) or transient modes. The properties of continuous flow reactors wiU be the main subject of this course, and an alternate title of this book could be Continuous Chemical Reactors. The next two chapters will deal with the characteristics of these reactors operated isothermaUy. We can categorize chemical reactors as shown in Figure 2-8. 

We stiU must use a PFTR in many chemical processes, and we must then determine how to program the cooling or heating to attain a temperature profile in the reactor close to that desired. The subject of this chapter is the proper temperature management to attain desired operation of a PFTR. In the next chapter the nonisothermal CSTR will be considered specifically. 

Note several features of these solutions. First the C (t) and T(t) solutions have identical shapes (but T increases as Ca decreases for the exothermic reaction) for the adiabatic reaction in these reactors or in any adiabatic reactor. If we plot Cao Ca versus T from these solutions we obtain Figure 5-8, because, by the previous arguments, this must be a straight line for any single reaction in any reactor as long as parameters do not depend on temperature or composition. Second, note that the CSTR in this example requires a shorter residence time for a given conversion than a PFTR. Third, note that the CSTR exhibits multiple values of Ca and T for a range of x. This situation is physically real and will be the subject of the next chapter. 

The existence and properties of these steady states and their impHcations in the design of CSTRs is the major subject of this chapter. 

Flow pattern Next one decides whether a batch or continuous reactor is suitable and, if flow, whether a mixed of unmixed reactor is preferred. Initially one may do calculations for PFTR and CSTR to bracket all flow patterns. This is the subject of Chapters 3 and 4. The choice of catalyst and heat removal method will be very important in deciding the best flow pattern. 

To summarize, whenever one needs to model a chemical process, the first step is to predict the performance of the PFTR or CSTR for assumed kinetics, the subjects of Chapters 2 through 4. If the reactors are not isothermal, we need to add the complexities of Chapters 5 and 6. 

A solution of the reaction-diffusion equation (9.14) subject to the boundary condition on the reactant A will have the form a = a(p,r), i.e. it will specify how the spatial dependence of the concentration (the concentration profile) will evolve in time. This differs in spirit from the solution of the same reaction behaviour in a CSTR only in the sense that we must consider position as well as time. In the analysis of the behaviour for a CSTR, the natural starting point was the identification of stationary states. For the reaction-diffusion cell, we can also examine the stationary-state behaviour by setting doi/dz equal to zero in (9.14). Thus we seek to find a concentration profile cuss = ass(p) which satisfies... 

Although the early literature described the application of a tubular reactor for the production of SBR latexes(1), the standard continuous emulsion polymerization processes for SBR polymerization still consist of continuous stirred tank reactors(CSTR s) and all of the recipe ingredients are normally fed into the first reactor and a latex is removed from the last one, as shown in Figure 1. However, it is doubtful whether this conventional reactor combination and operation method is the most efficient in continuous emulsion polymerization. As is well known, the kinetic behavior of continuous emulsion polymerization differs very much according to the kind of monomers. In this paper, therefore, the discussion about the present subject will be advanced using the... 

The two-phase theory of fluidization has been extensively used to describe fluidization (e.g., see Kunii and Levenspiel, Fluidization Engineering, 2d ed., Wiley, 1990). The fluidized bed is assumed to contain a bubble and an emulsion phase. The bubble phase may be modeled by a plug flow (or dispersion) model, and the emulsion phase is assumed to be well mixed and may be modeled as a CSTR. Correlations for the size of the bubbles and the heat and mass transport from the bubbles to the emulsion phase are available in Sec. 17 of this Handbook and in textbooks on the subject. Davidson and Harrison (Fluidization, 2d ed., Academic Press, 1985), Geldart (Gas Fluidization Technology, Wiley, 1986), Kunii and Levenspiel (Fluidization Engineering, Wiley, 1969), and Zenz (Fluidization and Fluid-Particle Systems, Pemm-Corp Publications, 1989) are good reference books. 

The optimization attempted to minimize total reactor volume, subject to maintaining the downstream pH between 6.5 and 7.5 and to the second CSTR pH recovering to within. 005 pH of the setpoint 30 minutes after a load change. The latter requirement was imposed to ensure reasonable damping. 

At the end of each of the two sets of pulse-mode reaction runs, the catalyst was subjected to a progressive heating (10°C/min) up to 550°C. Both catalyst samples released H2O, VP and HEP in the order, but the C02-poisoned sample in much lower amount. To confirm these findings some additional experiments by CSTR under standard conditions were carried out after treating two samples of activated Y84 catalyst with pure CO2, either at room temperature or at 160°C. After four hours of reaction the conversion of HEP dropped by 8.4 and 25.9% with respect to the unpoisoned catalyst, for the two samples, respectively. It is then evident that the activity of the catalyst is greatly reduced when its basic sites are poisoned by CO2. 

Propagation of Errors. A quantity of interest may be a function of several variables subject to measurement. The quantity could be, say, the reaction rate in a CSTR, calculated from measurements of a concentration and a flow rate. Uncertainties in both measurements affect the uncertainty of the calculated rate. 

The complex 0 contains nothing at all and is a mathematical artifice, not a species. [The pseudo-steps and thus the augmented network are not subject to the rules of stoichiometry and thermodynamics. For example, the CSTR network 14.1 violates microscopic reversibility in that it allows net circular reactions.]... 

Devise a recycle scheme that minimizes the combined amounts of nitric and sulfuric acids that must be discarded subject to the constraint that the combined acid concentration in the aqueous phase in the first CSTR must be at least 25 wt %. The nitric acid comes as a 68 wt % solution. The sulfuric acid comes as a 98 wt. % solution, and the fuming sulfuric acid is the equivalent of 114%. 

Figure 13.4 shows a liquid reaction vessel, subject to two feed streams and one outlet stream. It is the liquid analogue of the gas reactor described in Section 13.10. We shall again assume that there are M reactions involving N chemical species. The reactions occur continuously, and we shall assume that the stirrer effects perfect mixing. The gas above the liquid in the CSTR may consist of an inert gas blanket, perhaps under some form of pressure control, or it may be atmospheric air. The gas pressure may experience some small variability around the chosen pressure setpoint in the former case, while in the second case the pressure will undergo a slow, diurnal change of a few per cent at most. 

The initiation and propagation mechanism of the anionic polymerization of styrene using NBL has been the subject of considerable investi tion and has been found to be very complex [71]. This is due to the associated complexes of the initiator and polystyryllithium as well as cross association between the two species. The simultaneous initiation and propagation that occurs in CSTR processes adds further complications. Try berg and Anthony [72] simplified this problem by initiating the polymerization in a separate reactor. They con-added the low molecular weight polystyryllithium solution to a CSTR polymerization reactor. 

Figure 6.1 illustrates the fact that for various ranges of kinetic and reactor parameters it is possible for the mass and energy conservation relations for a CSTR to be in stable balance at more than one condition. This may imply that there are other balance conditions that are unstable the point needs to be examined. Which of the stable balances is attained in actual operation may be dependent on the details of startup procedure, for example, which are not subject to the control of the designer. Thus, it is important to investigate reactor stability using unsteady-state rather than steady-state models. 

In the analysis of thermal effects on CSTR uniqueness (given at the beginning of the last section), it was seen that different criteria resulted depending on the nature of the condition selected [i.e., equation (6-43) or equation (6-48)]. This suggests the inverse question, are there any other approaches to such an analysis and, if so, are their results subject to the same variability ... 

A certain industrial reactor is well modeled by a sequence of N CSTRs with equivalent holding times, i. The catalyst in this reactor is subject to deactivation, so that the rate constant of the first-order irreversible reaction being carried out decreases with time according to... 

G. Barnett, R. Hawks, and R. Resnick [/. Ethnopharmacol., 3, 353 (1981)] of the National Institute on Drug Abuse have studied cocaine pharmacokinetics in the human body. Cocaine was given to human subjects by intravenous administration. Gas chromatography-mass spectroscopy was used to determine the concentration of this drug in blood plasma samples. From the measurements reported below, determine whether the data are consistent with the response of a single CSTR to a pulse injection of tracer. Also determine the mean residence time of the material in the body. 

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