Ind. Eng. Chem. Res. 2005, 44, 5373-5379
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Regeneration of Coked Catalysts in a Polytropic Reactor C. Finol, M. Mene´ ndez, and J. Santamarı´a* Department of Chemical and Environmental Engineering., University of Zaragoza, 50009 Zaragoza, Spain
A new procedure for the oxidative regeneration of coked fixed-bed catalytic reactors is proposed in which oxygen is simultaneously introduced at several locations along the reactor. Mathematical simulation indicates that, with this method, the heat generation is more evenly distributed in the bed, and, as a consequence, the temperatures reached during regeneration can be substantially reduced. Laboratory-scale experiments have confirmed model predictions, showing that oxygen distribution in a polytropic-feed reactor (i.e., a reactor with several inlets) is an effective way to reduce catalyst deactivation by sintering. 1. Introduction The regeneration of coked fixed-bed catalytic reactors poses a classical reaction engineering problem where suitable operation strategies must be developed to avoid the formation of hot spots leading to the permanent deactivation of the catalyst by sintering. In a classical oxidative regeneration process, the coke deposited in the catalyst is burned off using a low concentration of oxygen in the feed to the reactor. Typically, a hot front develops and travels down the reactor as coke is progressively removed from a given reactor region. The process continues until elimination of the coke deposits, and the progress of the regeneration can be followed by monitoring the evolution of temperature at different positions in the bed.1,2 The location of the hot front corresponds to the zone where coke combustion is occurring, and, in this zone, very high temperatures may be reached, causing irreversible sintering of the catalyst and/or damage to the reactor. However, the temperature can be kept below a certain value by controlling the concentration of oxygen in the feed to the reactor. Because regeneration is often accompanied by the development of steep temperature gradients, the temperature is usually measured at different positions in the reactor, and the highest temperature reading is used as the control signal to regulate the oxygen concentration. Thus, catalyst regeneration is often quoted as a typical case of “auctioneering control”.3 Most of the research on the regeneration of fixed-bed reactors studies the development of temperature profiles in the process.1,4,5 Some attempts to limit temperature increases have involved the use of steam6,7 or coke pretreatments.8 Flow reversal has also been advocated as a means to limit the temperatures reached during regeneration.9 However, the usual system that is applied to avoid high temperature increases still involves maintaining a sufficiently low oxygen concentration, a procedure that strongly increases the time required for regeneration. In this work, we present a new regeneration strategy based on the use of a polytropic reactor, i.e., a reactor with several feeds along its length that allow dosing of one of more reactants. Polytropic reactors have been used to distribute the oxygen feed in selective oxidation * To whom all correspondence should be addressed. Phone: +34 976 761153. Fax: +34 976 762142. E-mail: iqcatal@ unizar.es.
Figure 1. Reactor scheme used for regeneration. Legend is as follows: 1, catalyst bed; 2, inert ceramic packing; 3, feed inlets; 4, thermocouple positions in the bed; 5, gas exit; 6, feed preheating coils; and 7, thermocouples in the gas inlets.
reactions (for example, see the work of Santamarı´a et al.,10 Finol et al.,11 Lu et al.,12-14 and Al-Sherehy et al.15). In many selective oxidation systems, it has been shown that oxygen distribution may lead to a significant improvement of selectivity and, hence, improvement of the yield to the desired reaction product. A polytropic reactor could also be used to distribute heat generation more evenly during the regeneration of fixed beds deactivated by coke. Rather than having coke combustion concentrated in a small region of the reactor, as is the case with a single oxygen feed, a polytropic reactor could, in principle, allowed to use the same amount of oxygen distributed at several locations in the reactor, thus distributing the heat generation. The validity of this concept has been investigated in this work, both theoretically and experimentally. 2. Experimental Section The same experimental system has been used for the coke-producing reaction (n-butane dehydrogenation on a commercial Cr2O3/Al2O3 catalyst with 19% Cr2O3 in 4 mm pellets) and for the regeneration using N2/O2 mixtures. In the case of regeneration, both single feed and polytropic feeds have been used. 2.1. Catalyst and Reactor. Figure 1 shows a scheme of the reactor used for dehydrogenation (coking) and regeneration experiments. It is a stainless-steel tube 450 mm long, with an inner diameter (i.d.) of 40 mm, one conventional (axial) inlet, and four lateral inlets. The
10.1021/ie0491443 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/23/2005
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effective catalyst bed length was 300 mm, because the first 80 mm and the last 70 mm of tube were packed by inert ceramic beads. The first lateral inlet is located approximately at the interface between the ceramic beads and the catalyst bed, and the other three lateral inlets are located at 70-mm intervals. Gas can be fed independently to each of the five reactor inlets, using mass-flow controllers (Brooks). Flow control accuracy (0.2%) was especially important, in regard to the mixtures of N2 and O2, which had to give a specific oxygen concentration. Before entering the bed, each of the gas streams pass through a serpentine pipe immersed in a sand fluidized bed, which guarantees a homogeneous temperature in all the gas feeds. A thin axial stainless-steel sheath allows displacement of a movable thermocouple that can be used to measure the temperature at a given location. This has been used during the coking experiments to check temperature homogeneity in the reactor. During regeneration, however, the axial stainless-steel sheath was used to introduce a set of thermocouples that would continuously register the temperature at fixed reactor positions. The temperatures at six different positions equally spaced inside the catalytic bed were recorded simultaneously, using a computer-controlled data acquisition system. To approach a pseudo-adiabatic behavior, the stainless-steel tube was heated on the outside using a set of external electrical resistances. These were fine-tuned using potentiometers, so that the heat supply would approximately compensate for the losses along the reactor. 2.2. Regeneration Procedure. It is well-established that most coking processes in fixed-bed reactors result in a pattern of decreasing (parallel-type coking) or increasing (series-type coking) coke concentration along the reactor. However, the focus of this work is the comparison of the evolution of temperatures during regeneration of fixed-bed reactors for the cases of single and multiple oxygen feeds to the reactor. If coke concentration in the bed changes significantly, the interpretation of the temperature evolution results would be complicated by the interplay between the relative positions of the lateral inlets and the pattern of coke concentration in the bed. To avoid this, the experiments have been performed in such a way that the initial coke concentration is the same in each experiment and at every position in the bed. To this end, several dehydrogenation runs were performed, after which the coked catalyst (∼420 g) was discharged from the reactor and kept aside. Individual batches presented slightly different coke contents; therefore, the contents of different batches (∼5 kg of catalyst) were thoroughly mixed together to give a coke content of 3.3 wt %, as determined by repeated experiments in a thermobalance (C. I. Instruments). After the reactor was loaded with the coked catalyst, the heating systems (a fluidized bed, to heat the gases, and an external electrical resistance, to compensate for heat losses) and the temperature reading system were started. After stable values of the desired temperature had been achieved throughout the reactor (780 ( 10 K) under a N2 flow of 1500 cm3(STP)/min, the desired amount of oxygen was introduced and regeneration was started. Experiments have been conducted with a constant total flow rate (1650 cm3 (STP)/min) and different
oxygen concentrations (in the range of 7.27%-8.5%) in the feed, with a fixed oxygen concentration and different total flow rates (1650 and 2145 cm3 (STP)/min) and with single and polytropic feeds. Regeneration is considered complete when the temperature of all thermocouples has returned to values close to the starting point. This was confirmed by performing analyses of residual coke in the regenerated catalyst particles. 3. Results and Discussion 3.1. Simulation of the Regeneration Process in Conventional and Polytropic Reactors. As a first assessment of the advantages of operation with polytropic reactors, the evolution of temperature profiles during regeneration of coked fixed-bed reactors has been simulated both for conventional and polytropic feed operation. The simulation of the conventional (i.e., single feed) regeneration of coked fixed-bed reactors has been attempted in the past with varying degrees of complexity (for example, see the work of Froment and Bischoff,16,17 Byrne et al.,4 Santamarı´a et al.,18 Acharya et al.,5 and Brito et al.2). In this case, the shrinking-core model used by Borio et al.19 has been adapted, with some supplementary assumptions, which are specified below: (i) The temperature difference between the particle surface and the surrounding gas can be neglected. (ii) The temperature can be considered to be homogeneous inside the catalyst pellet at the particle sizes considered (equivalent spherical diameter of
