Semi-Continuous Flow Reactors

In this chapter, we first consider uses of batch reactors, and their advantages and disadvantages compared with continuous-flow reactors. After considering what the essential features of process design are, we then develop design or performance equations for both isothermal and nonisothermal operation. The latter requires the energy balance, in addition to the material balance. We continue with an example of optimal performance of a batch reactor, and conclude with a discussion of semibatch and semi-continuous operation. We restrict attention to simple systems, deferring treatment of complex systems to Chapter 18. 

In this way, the reaction rate constants k, to can be found by matching semi-batch product trajectories to experimental results. This kinetic information can then be utilised to evaluate various forms of reactor design, particularly for plant scale continuous flow reactors. 

Potential of biogas/methane generation of waste could be also determined experimentally in batch tests, semi-continuous-flow or continuous-flow reactors. Batch tests are based on measurement of the amount of biogas (and the concentration of CH in biogas) isolated during the fermentation of the portion of material placed in a closed vessel. Parameters determined in the batch test are called biogas potential or Biochemical Methane Potential (BMP). A detailed review of conditions in which these tests are conducted is presented in the work made by Angehdaki et al. (2009). 

In continuous and semi continuous-flow systems waste go into reactor continuously or at certain intervals. The introduction of a new portion of feedstock is accompanied by a simultaneous discharge of an equivalent quantity of the batch of reactor. Part of the material leaves the reactor without being completely decomposed. Parameters determined in these systems are called biogas/methane yield. Some results of laboratory estimation of biogas production from waste are presented in Table 2.5. 

The influence of inhibitor on the performance of a semi-continuous reactor can be, in some ways, similar to both batch and continuous systems. A dead time is usually observed upon addition of the initial charge. When the secondary stream flow is started after some reaction of the initial charge, additional inhibitor flows into the reactor and the initiation rate drops. When this programmed addition is stopped the initiation rate increases sometimes enough to cause temperature control problems. 

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. 

It becomes necessary to incorporate a total mass balance equation into the reactor model, whenever the total quantity of material in the reactor varies, as in the cases of semi-continuous or semi-batch operation or where volume changes occur, owing to density changes in flow systems. Otherwise the total mass balance equation can generally be neglected. 

Both flow terms are zero for the case of batch reactor operation, and the outflow term is zero for semi-continuous or semi-batch operation. 

In this chapter the simulation examples are described. As seen from the Table of Contents, the examples are organised according to twelve application areas Batch Reactors, Continuous Tank Reactors, Tubular Reactors, Semi-Continuous Reactors, Mixing Models, Tank Flow Examples, Process Control, Mass Transfer Processes, Distillation Processes, Heat Transfer, and Dynamic Numerical Examples. There are aspects of some examples which relate them to more than one application area, which is usually apparent from the titles of the examples. Within each section, the examples are listed in order of their degree of difficulty. 

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. 

This bioreactor mode refers to a tank fermenter operated semi-continuously. The rate of the feed flow, F0, may be variable, and there is no outlet flow rate from the fermentor. As a consequence of feeding the reactor volume will change with respect to time. 

An exothermic reaction involving two reactants is run in a semi-continuous reactor. The heat evolution can be controlled by varying the feed rate of one component This is done via feedback control with reactor temperature measurement used to manipulate the feed rate. The reactor is cooled by a water jacket, for which the heat transfer area varies with volume. Additional control could involve the manipulation of the cooling-water flow rate. 

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.

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).

Reactors can be operated either in a batch or continuous-flow mode. The combination, batch with respect to the liquid and continuous-flow with respect to the gas, is called semibatch. Often this fine distinction is ignored and it is commonly referred to as batch. The majority of ozonation experiments reported in the literature have been performed in one-stage semi-batch heterogeneous systems, with liquid phase reactor volumes in the range VL = 1-10 L. Most full-scale applications are operated in continuous-flow for both phases. 

The continuous-flow set-up can use either a reaction, similar to semi-batch, to remove the oxygen transferred or two reactors in series. The ozone or oxygen is removed from the liquid in the first reactor by stripping or vacuum degassing and then it flows into the absorber. After having passed through the absorber, the liquid can be recycled or discharged. More information on full-scale application of this method in municipal waste water treatment plants can be found in Redmon (1983) and ASCE (1991). 

In semi-batch operation, when the initial charge of A has been consumed, the flow of B is interrupted, the products discharged, and the cycle begun again with a fresh charge of A. If required, however, the advantages of semi-batch operation may be retained but the reactor system designed for continuous flow of both reactants. In... 

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. 

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