Selectivity CSTRs

Unique for CSTR Phases liquid, gas-liquid, hquid-liquid, hquid-catalytic solid, gas-liquid-catalytic solid, gas-liquid -biosohd. Capacity 0.0001-100 L/s and usually > 0.4 L/s volumes 1-1000 000 L. Autothermal reactions. Usually if the concentration of reactants is low, and need low concentration of reactants for selectivity. CSTR is larger and more expensive than PFTR. For multiphase, STR are characterized by high liquid holdups holdup of the reactive phase is important if the reaction is slow Ha < 1 phase ratio is easy to control. Adiabatic, CSTR usually gives higher productivity for exothermic reactions than for STR batch or PFTR. Use for large capacity, otherwise batch. Heat recovery is easier in CSTR than in a batch STR. 

Continuous-Flow Stirred-Tank Reactor. In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydrauHc agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations, ie, multiple specialty product requirements and mechanical seal pressure limitations. The CSTR is the idealized opposite of the weU-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, Hquid-, and soHd-flow behaviors. 

Selectivity A significant respect in which CSTRs may differ from batch (or PFR) reaclors is in the product distribution of complex reactions. However, each particular set of reactions must be treated individually to find the superiority. For the consecutive reactions A B C, Fig. 7-5b shows that a higher peak value of B is reached in batch reactors than in CSTRs as the number of stages increases the batch performance is approached. 

Kelkar and McCarthy (1995) proposed another method to use the feedforward experiments to develop a kinetic model in a CSTR. An initial experimental design is augmented in a stepwise manner with additional experiments until a satisfactory model is developed. For augmenting data, experiments are selected in a way to increase the determinant of the correlation matrix. The method is demonstrated on kinetic model development for the aldol condensation of acetone over a mixed oxide catalyst. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

Since a CSTR operates at or close to uniform conditions of temperature and composition, its kinetic and product parameters can usually be predicted more accurately and controlled with greater ease. The CSTR can often be operated at a selected conversion level to optimize space-time yield, or where a particular product parameter is especially favored. 

In this short initial communication we wish to describe a general purpose continuous-flow stirred-tank reactor (CSTR) system which incorporates a digital computer for supervisory control purposes and which has been constructed for use with radical and other polymerization processes. The performance of the system has been tested by attempting to control the MWD of the product from free-radically initiated solution polymerizations of methyl methacrylate (MMA) using oscillatory feed-forward control strategies for the reagent feeds. This reaction has been selected for study because of the ease of experimentation which it affords and because the theoretical aspects of the control of MWD in radical polymerizations has attracted much attention in the scientific literature. 

There are two important types of ideal, continuous-flow reactors the piston flow reactor or PFR, and the continuous-flow stirred tank reactor or CSTR. They behave very diflerently with respect to conversion and selectivity. The piston flow reactor behaves exactly like a batch reactor. It is usually visualized as a long tube as illustrated in Figure 1.3. Suppose a small clump of material enters the reactor at time t = 0 and flows from the inlet to the outlet. We suppose that there is no mixing between this particular clump and other clumps that entered at different times. The clump stays together and ages and reacts as it flows down the tube. After it has been in the piston flow reactor for t seconds, the clump will have the same composition as if it had been in a batch reactor for t seconds. The composition of a batch reactor varies with time. The composition of a small clump flowing through a piston flow reactor varies with time in the same way. It also varies with position down the tube. The relationship between time and position is... 

Selectivity in a PFR > selectivity in a CSTR which also has exceptions. 

Equation (2.23). When operating at tmax, the CSTR gives a lower yield of B and a lower selectivity than a PFR operating at its t ix-... 

The general rule is that combinations of isothermal reactors provide intermediate levels of performance compared with single reactors that have the same total volume and flow rate. The second general rule is that a single, piston flow reactor will give higher conversion and better selectivity than a CSTR. Autocatalytic reactions provide the exception to both these statements. 

The temperature profile down the reactor is the issue. The CSTR is isothermal but selectivity is inherently poor when the desired product is an... 

Range("A" CStr(ip) ). Select ActiveCell.FormulaRlCl = Rtime Range("B" CStr(ip)).Select ActiveCell. FormulaRlCl = y(2)/(y(l) + y(2))... 

Range ("A" CStr (j j ) ). Select ActiveCell.FormulaRlCl=Itotal... 

Range("B" CStr(jj)).Select ActiveCell.FormulaRlCl = aout... 

Output Outputs Results to Excel Cells Range (" B" CStr (IntPrint +1) ). Select ActiveCell.FormulaRlCl= z Range (" C" CStr (IntPrint + 1) ). Select ActiveCell.FormulaRlCl = WSty Range (" D" CStr (IntPrint + 1) ). Select... 

Caption Prints Column Headers Range("B" CStr(1)).Select ActiveCell.FormulaRlCl="Length"... 

Range("E" CStr(1)). Select ActiveCell.FormulaRlCl="CummLn"... 

Fig.2. Effect of temperature on the conversion and selectivity in a CSTR at SOOpsig.

The kinetic parameters associated with the synthesis of norbomene are determined by using the experimental data obtained at elevated temperatures and pressures. The reaction orders with respect to cyclopentadiene and ethylene are estimated to be 0.96 and 0.94, respectively. According to the simulation results, the conversion increases with both temperature and pressure but the selectivity to norbomene decreases due to the formation of DMON. Therefore, the optimal reaction conditions must be selected by considering these features. When a CSTR is used, the appropriate reaction conditions are found to be around 320°C and 1200 psig with 4 1 mole ratio of ethylene to DCPD in the feed stream. Also, it is desirable to have a Pe larger than 50 for a dispersed PFR and keep the residence time low for a PFR with recycle stream. 

Such improvements in conversion were reported for the oxidation of ethanol by hydrogen peroxide to acetic acid. This is a well-studied reaction, carried out in a continuous stirred-tank reactor (CSTR). Near-complete conversion (> 99%) at near-complete selectivity (> 99%) was found in a micro-reaction system [150]. Processing in a CSTR resulted in 30-95% conversion at > 99% selectivity. 

If the process is carried out in a stirred batch reactor (SBR) or in a plug-flow reactor (PFR) the final product will always be the mixture of both products, i.e. the selectivity will be less than one. Contrary to this, the selectivity in a continuous stirred-tank reactor (CSTR) can approach one. A selectivity equal to one, however, can only be achieved in an infinite time. In order to reach a high selectivity the mean residence time must be very long, and, consequently, the productivity of the reactor will be very low. A compromise must be made between selectivity and productivity. This is always a choice based upon economics. 

Parallel reactions, oai = om2, a i = am = 0, Ei > E2. The. selectivity to the desired product increases with temperature. The highest allowable temperature and the highest reactant concentrations should be applied. A batch reactor, a tubular reactor, or a cascade of CSTRs is the best choice. 

The next two steps after the development of a mathematical process model and before its implementation to "real life" applications, are to handle the numerical solution of the model s ode s and to estimate some unknown parameters. The computer program which handles the numerical solution of the present model has been written in a very general way. After inputing concentrations, flowrate data and reaction operating conditions, the user has the options to select from a variety of different modes of reactor operation (batch, semi-batch, single continuous, continuous train, CSTR-tube) or reactor startup conditions (seeded, unseeded, full or half-full of water or emulsion recipe and empty). Then, IMSL subroutine DCEAR handles the numerical integration of the ode s. Parameter estimation of the only two unknown parameters e and Dw has been described and is further discussed in (32). 

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