Continuously stirred tank reactor semi-batch reactors

Situation I was discussed in Chapter 3. Situations II and m will be discussed in the next sections. Note that both situations can occur in laminar and in turbulent flow conditions. In the following we shall concentrate primarily on the meso-mixing in continuous stirred tank reactors (CSTR), since the mixing effects in these reactors are relatively simple to visualize. The effect of meso-mixing on reactions in semi-batch reactors will be discussed thereafter. 

Stirred-tank reactors can be operated in batch, semi-batch, or continuous mode. In batch or semi-batch mode ... 

Drawing heavily from prior experience in hydrogenation of nitriles (7-10) and of ADN to ACN and/or HMD (11), in particular, we decided to restrict the scope of this investigation to Raney Ni 2400 and Raney Co 2724 catalysts. The hydrogenation reactions were initially carried out in a semi-batch reactor, followed by continuous stirred tank reactor to study the activity, selectivity, and life of the catalyst. 

The choice of a reactor is usually based on several factors such as the desired production rate, the chemical and physical characteristics of the chemical process, and the risk of hazards for each type of reactor. In general, small production requirements suggest batch or semi-batch reactors, while large production rates are better accommodated in continuous reactors, either plug flow or continuous stirred tank reactors (CSTR). The chemical and physical features that determine the optimum reactor are treated in books on reaction engineering and thus are not considered here. 

Figure 1-2 Operating parameters necessary for ozone mass balance(s) on a continuous-flow stirred tank reactor (for operation in semi-batch mode Ol - 0).

Different types of reactors are applied in practice (Figure 1.14). Stirred tank reactors (STR), very often applied for homogeneous, enzymatic and multiphase heterogeneous catalytic reactions, can be operated batchwise (batch reactor, BR), semi-batchwise (semibatch reactor, SBR) or continuously (continuous strirred tank reactor, CSTR)... 

The ideal reactors mostly studied in chemical engineering are the batch or semi-batch reactor, the plug flow reactor (PFR) and the continuous flow stirred tank reactor (CSTR). Abatch reactor has no input or output of mass, and the stirring of the reactor avoids temperature or concentration gradient in the reaction medium. The reaaion rate is uniform and can be considered everywhere equal to the average value. 

The polymerization process should significantly be able to alter the LCB topology. In a continuous system, such as the stirred tank reactor (CSTR) system, both monomer and macromonomer concentrations are constant after the steady state conditions are achieved. Monomer concentration is also constant in a semi-batch polymerization with continuous monomer feed but the macromonomer concentration increases as the polymerization proceeds. 

Probably the main advantage of using a stirred tank reactor is the relatively easy temperature control that is possible with this reactor type. This applies both to the semi-batch and the continuous operation modes. 

A semi-batch reactor is more difficult to analyze mathematically because at least one of the reactant or product species enters or leaves the system boundaries, thus specific applications should be modeled [1,5]. However, the most typical application for a semi-batch reactor is the presence of one reactant initially contained in a stirred tank reactor and a second reactant continuously added to the reactor, with no flow out of the reactor. The addition of a gas to participate in a liquid-phase reaction is one of the more common situations involving a semi-batch reactor, especially because the rate of addition of the gas can be controlled to keep its partial pressure essentially constant as well as providing quantitative information about the rate of reaction. In addition, there is frequently little or no change in the volume of the liquid phase. Well-mixed autoclave reactors coupled with gas pressure controllers, mass flow meters and computers can nicely provide continuous, real-time rate data related to heterogeneous catalysts used in such gas/liquid systems [6-8]. Again, it must be emphasized that experiments must be performed and/or calculations made to verify that no heat or mass transfer limitations exist. 

Batch, or semi-batch, reactors are often used for suspension polymerization on an industrial scale. Dispersions in tubular flow reactors are difficult to maintain and a continuous stirred tank would produce drops containing partially polymerized material that would coalesce in the receiving equipment. However, new types of flow reactors are being developed for suspension polymerization (see Section 5.4.1). 

Key PFR = Plug Flow Reactor, BSTR = Batch Stirred-Tank Reactor, (S)BSTR = (Semi)Batch Stirred -Tank Reactor, SBSTR = Semibatch Stirred-Tank Reactor, CSTR = Continuous Stirred-Tank Reactor, TBR = Trickle-Bed Reactor. 

This section is concerned with batch, semi-batch, continuous stirred tanks and continuous stirred-tank-reactor cascades, as represented in Fig. 3.1 Tubular chemical reactor systems are discussed in Chapter 4. 

The technical feasibility of a relatively low-pressure (less than 1000 psig) and low-temperature (less than 100°C) process for the hydrogenation of depolymerized (ammonolysis) Nylon-6,6 and/or a blend of Nylon-6 and -6,6 products has been described. While Raney Co 2724 showed little or no sign of deactivation during the semi-batch hydrogenation of the ammonolysis products, before and after C02 and NH3 removal, Raney Ni 2400 showed signs of deactivation even in the presence of caustic. Raney Co 2724 proved to be an effective and robust catalyst in a continuous stirred tank reactor study. 

The depolymerized Nylon used in the hydrogenation process was obtained by the ammonolysis of a mixture of Nylon-6 and Nylon-6,6 (described elsewhere, see reference 2). Hydrogenation reactions were conducted in 300 cc stirred pressure vessels. For semi-batch reactions hydrogen was constantly replenished to the reactor from a 1L reservoir to maintain a reactor pressure of 500 psig and all of the reactions were conducted with the same operating parameters and protocol. In continuous stirred tank studies hydrogen flow was controlled using... 

Polymer production technology involves a diversity of products produced from even a single monomer. Polymerizations are carried out in a variety of reactor types batch, semi-batch and continuous flow stirred tank or tubular reactors. However, very few commercial or fundamental polymer or latex properties can be measured on-line. Therefore, if one aims to develop and apply control strategies to achieve desired polymer (or latex) property trajectories under such a variety of conditions, it is important to have a valid mechanistic model capable of predicting at least the major effects of the process variables. 

For the semi-batch stirred tank reactor, the model was based on the following assumptions the reactor is well agitated, so no concentration differences appear in the bulk of the liquid gas-liquid and liquid-solid mass transfer resistances can prevail and finally, the liquid phase is in batch, while hydrogen is continuously fed into the reactor. The hydrogen pressure is maintained constant. The liquid and gas volumes inside the reactor vessel can be regarded as constant, since the changes of the fluid properties due to reaction are minor. The total pressure of the gas phase (P) as well as the reactor temperature were continuously monitored and stored on a PC. The partial pressure of hydrogen (pnz) was calculated from the vapour pressure of the solvent (pvp) obtained from Antoine s equation (pvpo) and Raoult s law ... 

Biochemical reactors can be operated either batchwise or continuously, as noted in Section 1.5. Figure 7.1 shows, in schematic form, four modes of operation with two types of reactors for chemical and/or biochemical reactions in Uquid phases, with or without suspended solid particles, such as catalyst particles or microbial cells. The modes of operation include stirred batch stirred semi-batch continuous stirred and continuous plug flow reactors (PFRs). In the first three types, the contents of the tanks arc completely stirred and uniform in composition. 

Figure 7.1 Modes of reactor operation (a) batch reactor, (b) semi-batch reactor, (c) continuous stirred-tank reactor, and (d) continuous plug flow reactor.

An attractive property of monolithic reactors is their flexibility of application in multiphase reactions. These can be classified according to operation in (semi)batch or continuous mode and as plug-flow or stirred-tank reactor or, according to the contacting mode, as co-, counter-, and crosscurrent. In view of the relatively high flow rates and fast responses in the monolith, transient operations also are among the possibilities. 

Knowledge of these types of reactors is important because some industrial reactors approach the idealized types or may be simulated by a number of ideal reactors. In this chapter, we will review the above reactors and their applications in the chemical process industries. Additionally, multiphase reactors such as the fixed and fluidized beds are reviewed. In Chapter 5, the numerical method of analysis will be used to model the concentration-time profiles of various reactions in a batch reactor, and provide sizing of the batch, semi-batch, continuous flow stirred tank, and plug flow reactors for both isothermal and adiabatic conditions. 

Batch, semi-batch and continuous emulsion polymerizations are usually carried out in stirred tank reactors, where agitation by a stirrer is necessary. The type of stirrer chosen and its stirring speed can often affect the rate of polymerization, the number of polymer particles and their size distribution (PSD), and the molecular weight of the polymer produced. However, the effect of stirring on emulsion polymerization has never been the main research parameter in research programs [241]. This is probably due to the conflicting results obtained so far by various researchers. 

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