Reactor performance yield

In describing reactor performance, selectivity is usually a more meaningful parameter than reactor yield. Reactor yield is based on the reactant fed to the reactor rather than on that which is consumed. Clearly, part of the reactant fed might be material that has been recycled rather than fresh feed. Because of this, reactor yield takes no account of the ability to separate and recycle unconverted raw materials. Reactor yield is only a meaningful parameter when it is not possible for one reason or another to recycle unconverted raw material to the reactor inlet. By constrast, the yield of the overall process is an extremely important parameter when describing the performance of the overall plant, as will be discussed later. 

The following details establish reactor performance, considers the overall fractional yield, and predicts the concentration profiles with time of complex reactions in batch systems using the Runge-Kutta numerical method of analysis. 

Reactor Performance Measures. There are four common measures of reactor performance fraction unreacted, conversion, yield, and selectivity. The fraction unreacted is the simplest and is usually found directly when solving the component balance equations. It is a t)/oo for a batch reaction and aout/ciin for a flow reactor. The conversion is just 1 minus the fraction unreacted. The terms conversion and fraction unreacted refer to a specific reactant. It is usually the stoichiometrically limiting reactant. See Equation (1.26) for the first-order case. 

GP 10] [R 18]The best HCN yield of 31% at a p-gauze platinum catalyst (70 ml h methane 70 ml h ammonia 500 ml h air 1 bar 963 °C) is much better than the performance of monoliths (Figure 3.49) having similar laminar flow conditions [2]. A coiled strip and a straight-channel monolith have yields of 4 and 16%, respectively. The micro-reactor performance is not much below the best yield gained in a monolith operated under turbulent-flow conditions (38%). 

OS 80] [R 7] [P 60] The acid-catalyzed dehydration of of 1-hexanol to hexene was conducted in a micro reactor made of PDMS, which also contained a heahng fimc-hon [19, 138], Sulfated zirconium oxide was coated as catalyst on the top plate of the micro reactor. A yield of 85-95% was obtained by-products could not be detected. This performance exceeds those of conventional reactors (30%). 

GL 13] [R 1] [P 12] As a function of residence time, conversion increases linearly from 30 to 81% at selectivities from 79 to 67% [6]. The associated yield increase is non-linear and seems to approach a plateau (Figure 5.21). Hence residence times much larger than 14 s are not suited to increase reactor performance. 

It is clear from the presented data that the yield and selectivity in a large semibatch reactor can be improved compared to those in a small batch reactor that has much better heat-transfer capability. This has been achieved by decreasing the rate of heat evolution, which has been obtained by lowering the instantaneous concentration of reactant A. The results also indicate that the dosing policy can have a very significant influence on reactor performance. 

The design methods de.scribed above rely on correlations of the overall reactor average quantities obtained from experimental tanks of different scales. The most important deficiency of these methods is that local effects are not taken into consideration, while these might be responsible for the overall reactor performance. Accordingly, if none of the above scale-up criteria is found satisfactory (see e.g. data of Middleton et ai, 1986) a more fundamental approach must be applied, although not necessarily as complex as the one presented in Section 5.4.S.2. Such an approach was presented by Paul et al. (1971) who found that the yield of the desired intermediate in a system of consecutive reactions (iodination of L-tjrosine) correlates reasonably with fluctuations of the velocity, So, these fluctuations could be chosen as a criterion for scale-up of the reactor. The average value for u in the upper part of the tank was evaluated from ... 

Consecutive reactions, isothermal reactor cmi < cw2, otai = asi = 0. The course of reaction is shown in Fig. 5.4-71. Regardless the mode of operation, the final product after infinite time is always the undesired product S. Maximum yields of the desired product exist for non-complete conversion. A batch reactor or a plug-flow reactor performs better than a CSTR Ysbr.wux = 0.63, Ycstriiuix = 0.445 for kt/ki = 4). If continuous operation and intense mixing are needed (e.g. because a large inteifacial surface area or a high rate of heat transfer are required) a cascade of CSTRs is recommended. 

Optimise the reactor performance (space-time yield SPTB) by modification of TIMEON. Compare results obtained with those in Sec. 2.4.1.1. 

In order for a membrane reactor to produce yields of HCHO greater than in a plug flow reactor, the membrane must be permselective for this species. The more permselective the membrane is to formaldehyde the better the membrane reactor performs until the formaldehyde is approximately one thousand times more permeable than methane. At this limit, the concentration of HCHO is essentially equal on both sides of the membrane at all times. No further improvement is possible by increasing the diffusivity of the formaldehyde further because there is... 

Over 30 man-years of effort were involved in developing the model, which is named KINPTR, an acronym for kinetic platinum reforming model. Since its development, KINPTR has had a major impact in Mobil s worldwide operations. It can be accessed by personnel at each of Mobil s locations throughout the world. Input requirements are simple and convenient making it very user friendly. Only feed characteristics, product quality targets, process configuration information, and process conditions are required for input. Output is informative and detailed. Overall and detailed yields, feed and product properties, and reactor performance data are given in the output. 

In the Dupont process, cyclohexane is reacted with air at 150 °C and 10 atm pressure in the presence of a soluble cobalt(II) salt (naphthenate or stearate). The conversion is limited to 8-10% in order to prevent consecutive oxidation of the ol-one mixture. Nonconverted cyclohexane is recycled to the oxidation reactor. Combined yields of ol-one mixture are 70-80%.83,84,555 The ol-one mixture is sent to another oxidation reactor where oxidation by nitric acid is performed at 70-80 °C by nitric acid (45-50%) in the presence of a mixture of Cu(N03)2 and NH4V03 catalysts, which increase the selectivity of the reaction. The reaction is complete in a few minutes and adipic acid precipitates from the reaction medium. The adipic acid yield is about 90%. Nitric acid oxidation produces gaseous products, mainly nitric oxides, which are recycled to a nitric acid synthesis unit. Some nitric acid is lost to products such as N2 and N20 which are not recovered. 

Figure 2.11 gives a Matlab program that performs these sizing calculations. Results for the base case feedrate, a 50% conversion, and a 320 K reactor temperature yield a coolant exit temperature of 314 K and a log-mean temperature difference of 13.5 K. The heat transfer area of the coil is 81.44 m2, compared to a jacket area of 63.4 m2. The vessel... 

Step 9. The basic regulatory strategy has now been established (Fig. 10.2). We have some freedom to select several controller setpoints to optimize economics and plant performance. If reactor inlet temperature sets production rate, the setpoint of the total toluene flow controller can be selected to optimize reactor yield. However, there is an upper limit on this toluene flow to maintain at least a 5 1 hydrogen-to-aromatic ratio in the reactor feed since hydrogen recycle rate is maximized. The setpoint for the methane composition controller in the gas recycle loop must balance the trade-off between yield loss and reactor performance. Reflux flows to the stabilizer, product, and recycle columns must be determined on the basis of column energy requirements and potential yield losses of benzene (in the overhead of the stabilizer and recycle columns) and toluene (in the base of the recycle column). Since the separations are easy, in this system economics indicate that the reflux flows would probably be constant. 

Model structure is shown in Figure 5. Process variables, unit constants (such as heat transfer coefficients), and feed streams are described on input or as selected by the optimization routine. Then, heat and material balances are performed using an assumed alkylate yield and isobutane consunq>tion. These results form a set of reaction conditions irtiich are used in correlations to calculate reactor performance. The heat and material balance calculations are repeated if reactor performance differs significantly from that used in the previous calculation. Operating incentives are then conqmted and may be used in the optimization routine to select new values of the optimization variables. 

Unfortunately, most combinations of steps of different orders lead to rather unwieldy mathematics as far as reactor performance is concerned [G1.G10]. However, one important general facet of such networks warrants special attention and is easily demonstrated with the simplest case, the network 5.26. Of interest are the yield ratio of the two products and the selectivity of conversion to one of them. For the network 5.26, the instantaneous yield ratio is seen to be... 

In porous composite membranes, the support layer(s) can play an important role in the reactor performance. This, for example, is the case with consecutive reactions such as partial oxidations where intermediate products are desirable. Harold et al. [1992] presented a concept in which two reactants are introduced to a two-layer membrane system from opposite sides ethylene on the membrane side while oxygen on the support side. The mass transfer resistance of the support layer lowers the oxygen concentration in the catalytic zone and directs the preferred intermediate product, acetaldehyde, toward the membrane side. Thus the support layer structure enhances the yield of acetaldehyde. 

Another important question is how main and secondary reactions are described. In practice, it is frequently the case, for time and cost reasons, that a complete kinetic model cannot be developed and experiments in a reaction column are needed to verify the achievable purities and yields. With the results of these experiments, a scale-up problem exists. This is because in these experiments, reaction and mass transfer interact with one another. The product removal capability has to be designed with respect to separation performance and the property of converting the starting materials that show the characteristics of a reactor performance with different scale-up rules. 

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