Reactors, batch residence time

Chemical Kinetics, Tank and Tubular Reactor Fundamentals, Residence Time Distributions, Multiphase Reaction Systems, Basic Reactor Types, Batch Reactor Dynamics, Semi-batch Reactors, Control and Stability of Nonisotheimal Reactors. Complex Reactions with Feeding Strategies, Liquid Phase Tubular Reactors, Gas Phase Tubular Reactors, Axial Dispersion, Unsteady State Tubular Reactor Models... 

There is, however, another way of looking at a tubular reactor in which plug flow occurs (Fig. 1.15). If we imagine that a small volume of reaction mixture is encapsulated by a membrane in which it is free to expand or contract at constant pressure, it will behave as a miniature batch reactor, spending a time r, said to be the residence time, in the reactor, and emerging with the conversion aA/. If there is no expansion or contraction of the element, i.e. the volumetric rate of flow is constant and equal to v throughout the reactor, the residence time or contact time... 

It can be readily discerned that the reactor equation for the batch reactor (5.12) and the plug-flow reactor (5.13) are identical. In the former, the concentration changes with time, in the latter, with location. In contrast to the situation in the other two ideal reactors, the residence time T in a CSTR is only an average, as every volume element has a different residence time throughout the reactor. 

Case G GlaxoSmithKline Fine Chemical from Carbonyl Process (41). The fine chemical is produced in a high-heat exchange reactor. The residence time is thereby reduced by a factor of 1800( ) compared to a conventional batch reactor. The reactive content is thereby considerably reduced hence the process is safer. 

A plant operability analysis helps to establish the correct cycle time. In certain situations, the batch cycle of the entire plant is not determined by the units around the main reactor. Additional time may be needed downstream for product finishing in equipment that operates in series with the main reactor. The batch cycle time T is a composite of three contributions (1) the sum of the batch residence times t(i) in the M true batch units of the plant that operate in series (2) the sum of the residence times t(j) in the N semicontinuous trains of the plant that operate in series and (3) the sum of the downtimes t(k) in series encountered in the total batch cycle ... 

For a series reaction network the most important variable is either time in batch systems or residence time in continuous flow systems. For the reaction system A - B - C the concentration profiles with respect to time in a batch reactor (or residence time in a PFR) are given in Figure 6. 

Tower Reactor The tower reactor is convenient when working with flocculating yeast cells. The reactor consists of a cylinder provided with bottom and upper zones for feeding substrate and cells and sofid/liquid separation. The overall aspect ratio is of 10 1, with 6 1 for the reaction zone. A tower reactor does not use mechanical mixing, and is simpler to build. Cell concentrations up to 100 g/1 can be achieved with productivities 30-80 times higher than in batch reactors. The residence time is below 0.4 h and the yield up to 95% of the theoretical one. A design procedure is available [18]. 

Batch reactors—optimum residence time for series and complex reactions, minimum cost, optimal operating temperature, and maximum rate of reaction... 

The important issue of possible deactivation of noble metal catalysts by CO formed from the reverse water gas shift reaction between CO2 and H2 was investigated using high-pressure transmission FTDR. spectroscopy. It was shown that CO could be formed on Pd/Al203 when exposed to CO2 (95%) and H2 (5%) at the reaction condition (70 °C, 138 bar). This adsorbed CO evolved with time and was insignificant at short residence times, implying that short-residence time continuous reactors are preferred over batch reactors (with residence times > 20 min) to minimize the effects of possible catalyst deactivation by CO. 

This is the mass balance equation for the batch reactor. The symbol t for clock time is replaced here by the more usual symbol 0 for batch residence time. For the continuous, completely mixed reactor, it is usdiil to start from the reduced continuity equation in terms of concentrations, analogous to Eq. 7.2.b-l (but with no diffusion term) ... 

In a CSTR, each element of monomer feed has an equal chance of being withdrawn from the reactor at any instant regardless of the time it has been in the reactor. Therefore, in a CSTR, unlike in batch and tubular reactors, the residence time is variable. The contents of a well-stirred tank reactor show an exponential distribution of residence times of the type shown in Equation 10.15. 

C for annealing and 77 °C for extension of the DNA. After this, the sample flows through the 95 ° C zone again, etc. Hence, instead of cycling the temperature in time in a batch reactor, the temperature cycling is performed in space in a flow reactor. The residence time at each temperature is controlled by the length of the microchannel in the particular temperature zone. 

Each one of the fluid elements, which is a completely segregated cluster of fluid molecules, can be treated as a micro-batch reactor. The residence time 0 of a fluid element is taken as the batch reaction time to determine the conversion achieved in the fluid element. Consider a first-order reaction A—carried out in the laminar flow reactor, (-ni) = kCA is the kinetic rate equation. The rate of change of reactant concentration in a single fluid element (treated as a batch reactor) is given by... 

It is perhaps not generally realised that a switch from batch to continuous reactor design has intrinsic beneficial PI and safety implications when exothermic reactions and their associated runaway risks are involved. For batch operation, the time during which the reaction exotherm is generated is only a fraction of the batch cycle time. In order to control the reactions, it is imperative that provision is made to cope with the maximum likely heat evolution load so as to inhibit runaway. On the other hand, the heat exchanger provision for a continuous process operating at the same production rate needs to be considerably less than that for the batch equivalent, because the heat load is uniformly time distributed rather than being concentrated in a fraction of the batch residence time. Hence, continuous versions of batch processes have both safety and intensification benefits. 

For a batch reactor, the reaction time t is the natural performance measure. For flow reactors, the residence time z is used, which is defined as the ratio of the reactor volume to die volumetric flow rate at reaction conditions (V in m s ) ... 

For a batch reactor, the reaction time t is the natural performance measure. For flow reactors, the residence time r is used, which is defined as the ratio of the reactor volume to the volumetric flow rate at reaction conditions. In mixed flow reactors, r represents a mean value because the residence time of the fluid elements is distributed. Only for plug flow tubular reactors is the residence time the same for all fluid elements. For heterogeneously catalyzed or gas-solid reactions it is convenient to use a (mean) modified residence time related to the mass of catalyst or solid. 

Equation (4.5.133) is only valid for reactors with a successive decrease of the concentration with time (batch reactor) or residence time (tubular reactor) for a continuously stirred tank reaction (CSTR) see Levenspiel (1999). 

This is the mass balance equation for the batch reactor. The symbol f for "clock time" is replaced here by the more usual symbol 0 for "batch residence time". 

When our experimental reactor approaches plug flow, we can use simple equations for describing the conversion of a chemical reaction, when the kinetics are simple. In principle, the outcome of a chemical reaction is the same as in a batch reactor, if residence times are equal. A complication arises when the density of the reaction mixture is not constant, because then we do not know the contents of the reactor a priori, so the concept of residence time loses its practical purpose. 

Continuous reactors in the pharmaceutical and specialty chemical industries may not only be needed for high productivity as in other segments of the chemical industry, but additionally to solve specific reactor design problems caused by limitations in batch operation. These limitations include heat transfer, mass transfer, and mixing. Continuous reactors are also used to minimize the reacting volume of thermally potent and/or noxious reactions and to decrease the potential and exposure for catastrophic failure of a vessel. Chemical industry reactor standards such as packed bed, fluid bed, and trickle bed reactors And limited utility since this type of phase contacting can usually be achieved in a slurry reactor, where residence time distribution variations, which can lead to changes in product distributions, are eliminated. Continuous stirred tank reactor operation is used only... 

Sohition. As demonstrated in Example 3, the chain length depends on how long a chain is allowed to grow. In a batch reactor and an ideal plug-flow reactor, all chains react for the same length of tim hence, the product will be essentially monodisperse. In a CSTR and a laminar-flow tubular reactor, the residence time of chains in the reactor varies, causing a spread in the distribution. Keep in mind, however, that ideal pluf flow is a practical impossiblity, particularly with highly viscous polymer solutions. Compare these conclusions with those of Chapter X, Example 13. 

Figure 1 shows the relationship between conversion and batch residence time for three of the batch trials. The polymerization reaction proceeded quickly in the first 2-3 hours of reaction time but the rate of conversion diminishes in the later stages of the polymerization. The target conversion was 80%. A kinetic model based on the batch trial data, also shown in Figure 1, was developed in order to predict the polymerization conversion as a ftmction of residence time (axial length) in the continuous kneader reactor. 

Solution We wish to avoid as much as possible the production of di- and triethanolamine, which are formed by series reactions with respect to monoethanolamine. In a continuous well-mixed reactor, part of the monoethanolamine formed in the primary reaction could stay for extended periods, thus increasing its chances of being converted to di- and triethanolamine. The ideal batch or plug-flow arrangement is preferred, to carefully control the residence time in the reactor. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

The majority of thermal polymerizations are carried out as a batch process, which requires a heat-up and a cool down stage. Typical conditions are 250—300°C for 0.5—4 h in an oxygen-free atmosphere (typically nitrogen) at approximately 1.4 MPa (200 psi). A continuous thermal polymerization has been reported which utilizes a tubular flow reactor having three temperature zones and recycle capabiHty (62). The advantages of this process are reduced residence time, increased production, and improved molecular weight control. Molecular weight may be controlled with temperature, residence time, feed composition, and polymerizate recycle. 

Batch reactors often are used to develop continuous processes because of their suitabiUty and convenient use in laboratory experimentation. Industrial practice generally favors processing continuously rather than in single batches, because overall investment and operating costs usually are less. Data obtained in batch reactors, except for very rapid reactions, can be well defined and used to predict performance of larger scale, continuous-flow reactors. Almost all batch reactors are well stirred thus, ideally, compositions are uniform throughout and residence times of all contained reactants are constant. 

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]). 

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