Reactor performance conversion

At high conversion, it is not only the product gas but also the residuals that are of special interest for the analysis of reactor performance. Conversion can also be determined by the residual amount of fuel. The carbon (C) balance can be appHed as follows ... 

In using a spreadsheet for process modeling, the engineer usually finds it preferable to use constant physical properties, to express reactor performance as a constant "conversion per pass," and to use constant relative volatiHties for distillation calculations such simplifications do not affect observed trends in parametric studies and permit the user quickly to obtain useful insights into the process being modeled (74,75). 

The distribution of residence times of reactants or tracers in a flow vessel, the RTD, is a key datum for determining reactor performance, either the expected conversion or the range in which the conversion must fall. In this section it is shown how tracer tests may be used to estabhsh how nearly a particular vessel approaches some standard ideal behavior, or what its efficiency is. The most useful comparisons are with complete mixing and with plug flow. A glossary of special terms is given in Table 23-3, and major relations of tracer response functions are shown in Table 23-4. 

The space velocity SV is often used with conversion to descrihe die overall reactor performance. It is common in die petroleum and petrochemical industries to plot conversion against space velocity to descrihe die effect of feed rate on die performance of a flow system. 

The computer simulation study of the operation of the tubular free radical polymerization reactor has shown that the conversion and the product properties are sensitive to the operating parameters such as initiator type, jacket temperature, and heat transfer for a reactor of fixed size. The molecular weight-conversion contour map is particularly significant and it is used in this paper as a basis for a comparison of the reactor performances. 

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. 

Figure 8.1 includes a curve for laminar flow with 3>AtlR = 0.1. The performance of a laminar flow reactor with diffusion is intermediate between piston flow and laminar flow without diffusion, aVI = 0. Laminar flow reactors give better conversion than CSTRs, but do not generalize this result too far It is restricted to a parabolic velocity profile. Laminar velocity profiles exist that, in the absence of diffusion, give reactor performance far worse than a CSTR. 

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. 

CO concentration at the outlet of each zone was continuously measured using a CO analyzer (Shimadzu CGT-7000). To evaluate the performance of the reactors, the conversion of CO for the PBR (Xco) with 4g of catalyst and the time-average conversion of CO for the SCMBR (Tea) with 2g of catalyst in each zone were calculated and compared. It should be noted that the CO concentration wave used for Eq. (1) was obtained whrai the system is at cyclic steady state (after 30 min of operation). 

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. 

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. 

To understand how such computer packages function, consider the simple flowsheet in Figure 13.13a. This involves an isomerization of Component A to Component B. The mixture of A and B from the reactor is separated into relatively pure A, which is recycled, and relatively pure B, which is the product. No byproducts are formed and the reactor performance can be characterized by its conversion. The performance of the separator is to be characterized by the recovery of A to the recycle stream (rA) and recovery of B to the product (rB). 

Different reactor networks can give rise to the same residence time distribution function. For example, a CSTR characterized by a space time Tj followed by a PFR characterized by a space time t2 has an F(t) curve that is identical to that of these two reactors operated in the reverse order. Consequently, the F(t) curve alone is not sufficient, in general, to permit one to determine the conversion in a nonideal reactor. As a result, several mathematical models of reactor performance have been developed to provide estimates of the conversion levels in nonideal reactors. These models vary in their degree of complexity and range of applicability. In this textbook we will confine the discussion to models in which a single parameter is used to characterize the nonideal flow pattern. Multiparameter models have been developed for handling more complex situations (e.g., that which prevails in a fluidized bed reactor), but these are beyond the scope of this textbook. [See Levenspiel (2) and Himmelblau and Bischoff (4).]... 

In Section 11.1.3.2 we considered a model of reactor performance in which the actual reactor is simulated by a cascade of equal-sized continuous stirred tank reactors operating in series. We indicated how the residence time distribution function can be used to determine the number of tanks that best model the tracer measurement data. Once this parameter has been determined, the techniques discussed in Section 8.3.2 can be used to determine the effluent conversion level. 

In this section, representative results are reviewed to provide a prospective of reactor modeling techniques which deal with bed size. There probably is additional unpublished proprietary material in this area. Early studies of fluidized reactors recognized the influence of bed diameter on conversion due to less efficient gas-solid contacting. Experimental studies were used to predict reactor performance. Frye et al. (1958) used... 

The models proposed by Wu et al. [36] and by Lin and Leu [45] refer to continuous conversion processes by immobilized bacteria the first to a fixed mixed culture entrapped into PVA beads operated in a fluidized bed, and the second to BAC of P. luteola operated in a packed bed. Results of these models highlight the role of mass transport phenomena and biophase granule size on reactor performance. 

A system of N continuous stirred-tank reactors is used to carry out a first-order isothermal reaction. A simulated pulse tracer experiment can be made on the reactor system, and the results can be used to evaluate the steady state conversion from the residence time distribution function (E-curve). A comparison can be made between reactor performance and that calculated from the simulated tracer data. 

To validate the numerical work and to study the phenomena that play a role in fixed bed combustion, experiments with a fixed bed reactor (Fig. 8.8) were performed [15]. Essentially, the reactor consists of an insulated metal tube filled with biomass. The biomass is ignited at the top, while air is supplied at the bottom of the reactor. The conversion front can be tracked with thermocouples. A mass balance is used to record the conversion of the biomass. As the results of the previous section are for coal conversion, the two sets of results can not yet be compared directly. 

Chromatographic fixed-bed reactors consists of a single chromatographic column containing a solid phase on which adsorption and reaction take place. Normally a pulse of reactant is injected into the reactor and, while traveling through the reactor, simultaneous conversion and separation take place (Fig. 3). Since an extensive overview of the models and applications of this type of reactor was presented by Sardin et al. [ 132], only a few recent results will be discussed here. Most of the practical applications have been based on gas-liquid systems, which are not applicable for the enzyme reactions, but a few reactions were also reported in the liquid phase. One of these studies, performed by Mazzotti and co-workers [ 141 ], analyzed the esterification of acetic acid into ethyl acetate according to the reaction ... 

The reformer reactor performance as an ammonia cracker was evaluated. The experiments were conducted using a reformer feed composed of 6 seem ammonia. The reactor was heated with the electric heaters to determine the heater power required to achieve high conversion. In these experiments, approximately 97% of the ammonia feed was converted to hydrogen at 900 °C (approximately 1.8 W) when operating at atmospheric pressure. - °... 

So far, all of the material presented has been discussed in the absence of any numerical examples. At this point, we introduce such an example the initial calculations will be used subsequently as a basis for further examples and, in this way, it will be possible to see how raw tracer data can be processed. Eventually, predictions will be made of what conversion can be expected when a reaction with known kinetics takes place in the system from which the tracer information was gathered. In the examples which involve tracer data, it should be emphasised that only in the most carefully engineered equipment could data of the accuracy quoted be obtained. In real situations, tracer mass balances may close inadequately and predictions of reactor performance must be treated with appropriate caution. 

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