Gradientless recycle reactor

The model equations are based on the assumption that the disproportionation of EB, the present case study, is carried out in a gradientless recycle reactor (Fig. 26), where neither concentration gradients... 

The second condition can be derived from the overall mass balance (reactor) for the reacting species. As we have assumed a gradientless recycle reactor (see Fig. 26), the fluid inside the reactor is supposed to be perfectly mixed. Thus, we have an ideal CSTR for which the mass balance of species i takes the form ... 

Bertucco A, Canu P., Devetta L., Zwahlen A.G. Catalytic Hydrogenation in Supercritical C02 Kinetic Measurement in a Gradientless Recycle Reactor, Submitted for publication in Industrial and Engineering Chemistry Research, April 1996... 

Experimental verification of the models has been carried out using equipment ranging from a thermogravimetric analyzer [11], a gradientless recycle reactor [9], to a single-pellet diffusion reactor [12,13]. 

The most appropriate laboratory reactor for detailed kinetic investigations is the continuously operated, gradientless recycle reactor. A large number of different constructions is described in the literature /44/. We have developed and successfully used for many years for heterogeneously catalysed vapor phase reactions such a reactor with internal recirculation (Fig. 21) It can be operated up to 800 K and 50 bars (catalyst volume 10 cm ). 

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. 

For the following calculations it is assumed that experiments are conducted in a good recycle reactor that is close to truly gradientless. Conceptually the same type of experiment could be conducted in a differential reactor but measurement errors make this practically impossible (see later discussion.) The close to gradientless conditions is a reasonable assumption in a good recycle reactor, yet it would be helpful to know just how close the conditions come to the ideal. 

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. 

Bertucco, A., Canu, P., Devetta, L. Catalytic Hydrogenation in Supercritical C02 Kinetic Measurements in a Gradientless Internal-Recycle Reactor. Ind. Eng. Chem. Res. 1997, 36, 2626 - 2633. 

A new reactor concept for the study of catalyst deactivation is presented, it consists of the combination of an electrobalance and a recycle reactor. With the electrobalance, the coke content on the catalyst is measured continuously. The recycle reactor operates gradientlessly at high conversion, with on-line gas chromatographic analysis of the effluent. Thus, the catalyst activity and product selectivities may be coupled directly with the coke content and the coking rate on the catalyst. 

The limitation to low conversion is the major disadvantage of differential operation. This is not critical if the influence of the catalyst properties on deactivation is studied. If, on the other hand, one is interested in the mechanism and the kinetics of coke formation and in the deactivation of the main reactions, it is necessary to reach higher conversions. A solution to this problem is to combine the electrobalance with a recycle reactor. The recycle reactor is operated under complete mixing, so that the reactor is gradientless. Since in a completely mixed reactor the reactions occur at effluent conditions and not at feed conditions, a specific experimental procedure is necessary to obtain the deactivation effect of coke. 

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). 

As in any heterogeneous catalytic reaction (Chapter 7), it is necessary to use a gradientless reactor to obtain precise kinetic data. An internal recycle reactor was recently proposed by Bertucco et al. (1997) for this purpose. The data should be obtained at times shorter than needed for a perceptible onset of deactivation. 

Gradientless differential reactors allow evaluation of kinetic data practically free of distortion by heat/temperature effects. Depending on the flow, a distinction is made between reactors with outer and inner circulation (recycle reactor, continuous stirred tank reactor. Figure 4.11.1). Evaluation of kinetic measurements by means of the differential method is straightforward as the algebraic balance equation for a stirred tank reactor can be applied (prerequisite high recycle ratio R). In practice it is found that recycle ratios of more than 10 are sufficient to achieve practically ideal... 

Figure4.11.1 Gradientless laboratory reactors (a) continuous stirred tank reactor (b) recycle reactor.

Gradientless operation both with respect to temperature and concentration can be obtained by an external or internal recycle (Figure 4.11.10). In a recycle reactor the gas passes several times through the catalyst bed until the product gas leaves the reaction volume. This type of reactor is more elaborate in design, but evaluation is simpler as the rate can be determined directly. As there is a very high flow rate in... 

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. 

A Bertucco, P Canu, L Devetta, AG Zwahlen. Catalytic hydrogenation in supercritical CO2 kinetic measurements in a gradientless internal-recycle reactor. Ind Eng Chem Res 36 2626-2633, 1997. 

---