Constant conversion reactor system

The same ligand system was used in the allylic alkylation of allyl trifluoroacetate with sodium diethyl-2-methylmalonate showing a more or less constant conversion over 8 h (20 exchanged reactor volumes). This is in contrast to peripheral functionalized dendrimers (Section 4.4.2), which deactivated at longer reaction times. 

These performance equations, Eqs. 17 to 19, can be written either in terms of concentrations or conversions. For systems of changing density it is more convenient to use conversions however, there is no particular preference for constant density systems. Whatever its form, the performance equations interrelate the rate of reaction, the extent of reaction, the reactor volume, and the feed rate, and if any one of these quantities is unknown it can be found from the other three. 

In a variable-density reactor the residence time depends on the conversion (and on the selectivity in a multiple-reaction system). Also, in ary reactor involving gases, the density is also a function of reactor pressure and temperature, even if there is no change in number of moles in the reaction. Therefore, we frequently base reactor performance on the number of moles or mass of reactants processed per unit time, based on the molar or mass flow rates of the feed into the reactor. These feed variables can be kept constant as reactor parameters such as conversion, T, and P are varied. 

The molar flow rate of a species in a flow reactor is Fj = vCj. The batch reactor is a closed system in which v = 0. The volumetric flow rate is ti, while thelinear velocity in a tubular reactor is u, We usually assume that the density of the fluid in the reactor does not change with conversion or position in the reactor (the constant-density reactor) because the equations for a constant-density reactor are easier to solve. 

The conversion in 1 hr is 65% in a batch reactor. Calculate (1) the rate constant and specify the rate equation, (2) the time at which the rate passes through the maximum, and (3) the type of optimum reactor system needed for the plant to process 200 m3/hr. What would be the reactor volume in this system ... 

Fundamental deactivation data are more difficult to obtain than fundamental catalytic reaction rate data because the latter must be known before the nature of the deactivation function can be determined. This is largely due to the kinds of reactors that are used to study deactivation. Many of the usual difficulties experienced in trying to get fundamental deactivation data can be obviated by using a reactor system in which the conversion and hence the compositions of the major components remain constant both in time and in space within the reactor. A description of an apparatus of this type and its utilization to study the deactivation of a real catalytic reaction are presented in this paper. The problem of determining the initial activity in a rapidly deactivating system is also discussed. 

Industrially, catalyst activity maintenance is often screened via "temperature increase requirement" (TIR) experiments. In these experiments, constant conversion is established and the rate of temperature increase required to do so is used as a measure of the resistance of the catalyst to deactivation. However, this type of operation may mask the effect of particle size, temperature, temperature profile, and heat of reaction on poison coverage, poison profile, and the main reaction rate. This masking may be particularly important in complicated reactions and reactor systems where the TIR experiment may produce positive feedback. 

In many large-scale reactors, such as those used for hydrotreating, and reaction systems where deactivation by poisoning occurs, the catalyst decay is relatively slow. In these continuous-flow systems, constant conversion is usually necessary in order that subsequent processing steps (e.g., separation) are not upset. One w to maintain a constant conversion with a decaying catalyst in a packed or fluidized bed is to increase the reaction rate by steadily increasing the feed temperature to the reactor. (See Figme 10-26.)... 

Consider a second-order reaction being carried out in a real CSTR that can be modeled as two different reactor systems In the first system an ideal CSTR is followed by an ideal PFR in the second system the PFR precedes the CSTR. Let T, and each equal 1 min. let the reaction rate constant equal 1.0 m /kmol - min. and let the initiai concentration of liquid reactant. equal 1 kmol/mL Find the conversion in each system. 

Ogunye and Ray (1971a,b) have formulated the optimal control problem for tubular reactors with catalyst decay via a weak maximum principle for this distributed system. Detailed numerical examples have been calculated for both adiabatic and isothermal reactors. For irreversible reactions, constant conversion policies are found to not always be optimal. A practical technique for on-line optimal control for fixed bed catalytic reactors, has been suggested by Brisk and Barton (1977). Lovland (1977) derived a simple maximum principle for the optimal flow control of plug flow processes. 

The dimensionless time r in Example 2.5 does not depend on the arbitrarily chosen fbatch- Instead, fbatch cancels out in the conversion from t to t so that t is scaled by a natural time constant for the system,. This effectively eliminates ki as a parameter while scaling by fbatch does not. Whether the reactor is batch or continuous, it is always possible to use the reciprocal of a rate constant as a characteristic time. The quantity has units of time when ki is a first-order rate constant. Thus kt is a dimensionless reaction time. Similarly a kit is a dimensionless reaction time when... 

This starting condition need not be isothermal along the reactor but it is best to start with an isothermal profile and zero-conversion at the inlet. The simplest way of achieving this is to allow an equilibration time before each run, with the feed entering at a constant and low-enough temperature. The feed temperature and composition as well as the external temperature of the reactor should that be held constant until the system reaches a steady-state. We assume that the feed undergoes no conversion in the preheater, before it enters the reactor, under any of the experimental conditions encountered. Two more conditions are required. 

The information required here is not concentration versus time, but rate of reaction versus concentration. As will be seen later, some types of chemical reactors give this information directly, but the constant-volume, batch systems discussed here do not [ What does it profit you, anyway —F. Villon], In this case it is necessary to determine rates from conversion-time data by graphical or numerical methods, as indicated for the case of initial rates in Figure 1.25. In Figure 1.27 a curve is shown representing the concentration of a reactant A as a function of time, and we identify the two points Cai and Ca2 for the concentration at times q and t2- The mean value for the rate of reaction we can approximate algebraically by... 

The temperature-dependent physical constants in the mass balance (i.e., the kinetic rate constant and the equilibrium constant) are expressed in terms of nonequilibrium conversion x using the linear relation (3-42). The concept of local equilibrium allows one to rationalize the definition of temperature and calculate an equilibrium constant when the system is influenced strongly by kinetic changes. In this manner, the mass balance is written with nonequilibrium conversion of CO as the only dependent variable, and the problem can be solved by integrating only one ordinary differential equation for x as a function of reactor volume. 

Figure 4. Flow diagram of reactor system for constant conversion of dextrose...

Equilibrium constraints and catalyst deactivation lead naturally to optimization problems. Simple maximization, dynamic programming, maximum principles, and other techniques can be used to solve the optimization problems. Optimization is usually carried out with respect to operating conditions. As pointed out repeatedly in this chapter, it should also be done with respect to reactor size. Perhaps the most powerful optimal policy for a reactor affected by deactivation is that of a constant conversion. However, it is not usually applicable to realistic systems and more work is needed for simple and yet general policies applicable to realistic systems. Means for estimating the extent of deactivation from process measurements and their use for optimization is another area that needs further work since detailed knowledge of deactivation is usually unavailable. An extensive review of reactor... 

Example 4-5. Why is the equilibrium conversion lower for the batch system than the flow system Will this always be the case for constant volume batch systems For the case in which the total concentration Ctd is to remain constant as the inerts are varied, plot the equilibrium conversion as a function of mole fraction of inerts for both a PFR and a constant-volume batch reactor. The pressure and temperature are constant at 2 atm and 340 K. Only N 04 and inert 1 are to be fed. 

Continuous stirred tank reactors are used commercially for solution, bulk (mass), and emulsion polymerization of vinyl monomers. In bulk homogeneous polymerization processes (e.g., polystyrene), the reactor system usually consists of a single CSTR or multiple CSTRs and an extruder-type devolatilizer to remove unreacted monomer, which is then recycled to the reactor. As monomer conversion increases, the viscosity of the polymerizing fluid increases and the overall heat removal efficiency decreases. When styrene is polymerized in bulk in a stirred tank reactor, monomer conversion is limited to about 30-40% due to an increasing viscosity of the polymerizing fluid above this conversion level. However, the overall monomer conversion can be very high because unreacted monomer is constantly recycled to the reactor. 

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