Reactor gradientless

At steady state, the mass balance of an arbitrary component, according to the principles presented in Chapter 5, becomes 

FIGURE A9.7 Configuration alternatives for a gradientless reactor, (a) A stationary catalyst basket (Berty reactor), (b) a rotating catalyst basket (Carberry reactor), and (c) a recycle reactor. 

Using the component concentrations, the balance equation (Equation A9.16) can be rewritten as 

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In the petrochemical industry close to 80% of reactions are oxidations and hydrogenations, and consequently very exothermic. In addition, profitability requires fast and selective reactions. Fortunately these can be studied nowadays in gradientless reactors. The slightly exothermic reactions and many endothermic processes of the petroleum industry still can use various tubular reactors, as will be shown later. 

In gradientless reactors the catalytic rate is measured under highly, even if not completely uniform conditions of temperature and concentration. The reason is that, if achieved, the subsequent mathematical analysis and kinetic interpretation will be simpler to perform and the results can be used more reliably. The many ways of approximating gradientless operating conditions in laboratory reactors will be discussed next. 

Jankowski et al (1978) discuss in detail the great variety of gradientless reactors proposed by several authors with a pictorial overview in their paper. All of these reactors can be placed in a few general categories (1) moving catalyst basket reactors, (2) external recycle reactors, and (3) internal recycle reactors. 

When the mass transfer resistances are eliminated, the various gas-phase concentrations become equal a/(/, r, z) = j(r, z) = a(r, z). The very small particle size means that heat transfer resistances are minimized so that the catalyst particles are isothermal. The recycle reactor of Figure 4.2 is an excellent means for measuring the intrinsic kinetics of a finely ground catalyst. At high recycle rates, the system behaves as a CSTR. It is sometimes called a gradientless reactor since there are no composition and temperature gradients in the catalyst bed or in a catalyst particle. 

Suppose a gradientless reactor is used to obtain intrinsic rate data for a catalytic reaction. Gas-phase concentrations are measured, and the data are fit to a rate expression using the methods of Chapter 7. The rate expression can be arbitrary ... 

Now the surface reaction rates alter the gas-phase reactant concentrations. Cutlip (38) has studied CO oxidation over Pt/Al203 in a gradientless reactor under conditions often leading to complete conversion. The feed gas alternated between 2% CO and 3% 02 in argon. Figure 9 shows some typical results. Clearly there is no hope of simulating such data by anthing but a complicated computer model. 

Cutlip and Kenney (44) have observed isothermal limit cycles in the oxidation of CO over 0.5% Pt/Al203 in a gradientless reactor only in the presence of added 1-butene. Without butene there were no oscillations although regions of multiple steady states exist. Dwyer (22) has followed the surface CO infrared adsorption band and found that it was in phase with the gas-phase concentration. Kurtanjek et al. (45) have studied hydrogen oxidation over Ni and have also taken the logical step of following the surface concentration. Contact potential difference was used to follow the oxidation state of the nickel surface. Under some conditions, oscillations were observed on the surface when none were detected in the gas phase. Recently, Sheintuch (46) has made additional studies of CO oxidation over Pt foil. 

An experimental gradientless reactor (similar to that in Figure 1.2), which acts as a CSTR operating adiabatically, was used to measure the rate of oxidation of SO2, to S03 with a V2O5 catalyst (Thurier, 1977). The catalyst is present as a fixed bed (200 g) of solid particles within... 

In general, if heterogeneous catalytic reactions are to be conducted isothermally, the reactor design must provide for heat flow to or from the particles of catalyst so as to keep the thermal gradients small. Otherwise, temperatures within the catalyst bed will be non-uniform. The differential reactor and the various forms of the gradientless reactors are advantageous in this regard. 

A mathematical model for the unsteady-state heterogeneous catalytic reaction in a gradientless reactor is... 

In this paper three zeolite catalysts from Sud-Chemie AG (USY-zeolite Si/Al 2.3-2.5, H-ZSM-5 Si/Al 15 and H-mordenite Si/Al 10) have been investigated in a gradientless reactor under supercritical conditions using the disproportionation of ethylbenzene (EBD) as test reaction and butane or pentane as inert. A previous publication reported investigations on those three catalysts at normal pressure and the details about the geometry of the three zeolites [6]. 

The computer controlled experimental equipment consists of a feeding system, a gradientless reactor, a gas chromatograph for the analysis of the reaction mixture as well as data processing and control unit consisting of two PCs. A detailed description of the experimental setup can be found in reference [1],... 

Smith, W. D., Paper presented at Symposium on Progress towards Gradientless Reactors in Catalysis, 63rd Annual AIChE Meeting, Chicago, 111., 1970. 

The distinction between instantaneous and cumulative yield ratios or selectivities becomes immaterial in gradientless reactors (continuous stirred-tank at steady state or differential once-through reactors) or if the instantaneous values do not vary with conversion. 

A guideline for choosing a suitable method is to avoid approximations as much as possible. Thus, plots of concentration, or a function of concentrations, versus time or reactor space time are preferred for evaluation of experiments with batch, tubular, and differential recycle reactors, in which concentrations are directly measured and rates can only be obtained by a finite-difference approximation (see eqns 3.1, 3.2, 3.5, 3.6, and 3.8). On the other hand, plots of the rate, or a function of the rate, versus concentration or a function of concentrations serve equally well for evaluation of results from CSTRs or differential reactors without recycle (gradientless reactors), where concentrations and rate are related to one another by algebraic equations that involve no approximations (see eqns 3.3, 3.4, or 3.7). 

It has become customary to classify evaluation methods as "differential" or "integral." These terms stem from a time when practically all experiments were conducted in batch reactors, so that rates had to be found by differentiation of concentration-versus-time data, and the calculation of concentrations from postulated rate equations required integration. The terms do not fit the work-up of data from gradientless reactors such as CSTRs, in which rates and concentrations are related to one another by algebraic equations requiring no calculus, and are therefore avoided here. 

The interested reader might also want to use the method based on eqn 3.11 for comparison. With point-by-point evaluation he would find reactions orders ranging from 0.55 to 1.41, pointing toward first order but demonstrating a sensitivity to experimental errors even in evaluation of results from gradientless reactors. 

The CO oxidation over noble metal catalyst has been studied most intensively under both steady-state operation and periodic operation. Cutlip [47] studied the CO oxidation on a Pt/Al203 catalyst in a gradientless reactor. The observed time-averaged reaction rates... 

Mahoney, J.A. Robinson, K. Meyers, E. Catalyst evaluation with the gradientless reactor. CHEMTECH 1978, Dec, 758. 

The catalytic activity of the catalyst samples obtained were characterized in the selective reduction of NO with methane and propane-butane mixture by conversion of NO to N2 (N2O) which was determined in a gradientless reactor with chromatograph analysis of the products. The NO concentration was determined with a gas analyzer with a chemiluminescence detector [8]. 

To study the kinetics of immobilized enzymes a recirculation reactor may be used. This reactor allows one to perform kinetic measurements with defined external mass transfer effects, reached by establishing a high flow rate near the catalyst, minimizing mass transfer resistance. The reactor behaves as a differential gradientless reactor allowing initial-rate kinetic measurements to be made. 

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