Reactor performance selectivity

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. 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

The major process parameters at selected periods in the four experiments are listed in Tables II, IV, VII, and VIII. Carbon recoveries ranged from 63 to 91%. Most of the losses occurred in connection with the recycle compressor system, and they decreased correspondingly the volume of product gas metered. Such losses, however, did not affect significantly the incoming gas to the main reactor or reactor performance. 

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. 

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

For the practical use of this CO removal reactor, the microchannel reactor should be operated carefully to maintain operating temperature ranges because the reaction temperature is critical for the microchannel reactor performance such as CO conversion, selectivity and methanation as disclosed in the above results. It also seems that the present microchannel reactor is promising as a compact and high efficient CO remover for PEMFC systems. 

Simulation studies are also conducted for a dispersed PFR and a recycle reactor at 260 °C, 500 psig and feed with DCPD=0.32 mol/min, CPD=0.96mol/min and ethylene=3.2mol/min. Peclet number (Pe) or the recycle ratio is selected as a variable parameter for the dispersed PFR or for the recycle reactor, respectively. Conversion approaches to that of PFR over Pe=50 as can be seen in Fig.4. It is also worth mentioning that the reactor performance is improved with recycle if the residence time is low. 

GP 1[ [R 1[ A change from aluminum to platinum as construction material results in reduced micro-reactor performance concerning oxidation of ammonia, decreasing N2O selectivity by 20% [28]. This is explained by the lower thermal conductivity of platinum, which causes larger temperature differences (hot spots) within the micro channels, i.e. at the catalyst site, e.g. due to insufficient heat removal from the channels or also by non-uniform temperature spread of the furnace heating. 

OS 34] [R 17] [R 19] [R 26] [P 23] By performing naphthalene nitration with fuming HNO3 in micro reactors, the selectivity for mononitronaphthalenes can be significantly enhanced (Figrue 4.48) [98]. The selectivity could be raised to more than 95%, independent of the microreactor used. 

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. 

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. 

Multiple reaction systems further complicate our fives. Recall that selectivity issues can dominate reactor performance, and rninirnally we have to solve one equation for each... 

The basic idea is to examine operating parameters to find the optimum combination of them for optimum performance. A short list of the most important might include the following Fj, Cjo, Cj, v, V, T, Tq, u, P, and, of course. For catalytic processes additional variables include D, d, Sg, e, shape, and catalyst chemical properties such as chemical composition, activity, and selectivity. Most catalytic reactors operate with significant mass transfer limitations because one usually wants to raise the temperature until mass transfer becomes noticeable in order to attain the highest rate possible. In all cases one determines the effects of these variables on reactor performance. 

As well as the operating conditions inside the reactor, the design features have a powerful influence on reactor performance. The type of reactor selected has an influence on efficiencies, on corrosion endurance, solids-operation feasibility, or even reactor reliability. The most important SCWO reactor configurations are listed in the following. 

The presence of liquid phase introduces engineering complications the interactions between transport phenomena, both for mass and energy, and intrinsic kinetics play a vital role in determining reactor performances, both for activity and selectivity, catalyst stability, etc,.. 

The mass balances [Eqs. (Al) and (A2)] assume plug-flow behavior for both the gas/vapor and liquid phases. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the reactor performance, for example, due to back-mixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for CD processes, experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK are performed (168). 

---