Reactor moving-bed

Moving-bed reactors are preferred when there is a need for continuous catalyst regeneration. In this operation, fresh catalyst is fed from the top of the reactor, and it moves in the downflow direction by gravitational forces. Spent catalyst leaving the reactor at the bottom is usually replaced in the continuous mode. While the catalyst movement is downward, reactive mixture flow can be cocurrent or countercurrent to that of the catalyst flow. 

The freshly regenerated catalyst enters the top of the reactor and then moves through the reactor as a compact packed bed. The catalyst is coked continually as it moves through the reactor until it exits the reactor into the kiln, where air is used to bum off the carbon The regenerated catalyst is lifted from the kiln by an airstream and then fed into a separator before it is returned to the reactor. The catalyst pellets are typically between 1 and in. in diameter. 

Used for reactions with moderate rate of catalyst decay. 

AZChE Monogr. Sex, 75(11), 4 (1979). With permission of the AIChE. Copyright 1979 AIChE. All rights reserved.] 

Dividing by AH letting AW approach zero, and expressing the flow rate in terms of conversion gives 

The activity, as before, is a function of the time the catalyst has been in contact with the reacting gas stream. The decay rate law is 

MBR technologies overcome the problem of shutting down the operation every time the catalyst is completely deactivated. The main feature of MBRs is that they combine plug-flow mode operation of FBRs with the possibility to replace portions of spent catalyst during time-on-stream (Furimsky, 1998). Therefore, they are well suited for handling tough feeds rich in metals and asphaltenes. 

The term moving-bed arises from the mode in which the spent catalyst is replaced. The catalyst bed is displaced periodically downward by gravitational forces. The fresh catalyst enters at the top of the reactor, and the deactivated catalyst leaves the reactor through the bottom. Liquid flow can be supplied either cocurrently or countercurrently with respect to the movement of the bed. The rate of deactivation determines how frequently the catalyst is replaced. Commonly, catalyst replacement is a batch operation and it is done once or twice a week (Gosselink, 1998). 

In comparison to FBRs, MBRs offer a much favorable catalyst activity distribution along the reactor (Sie, 2001). The periodical addition of fresh catalyst in MBRs increases the overall HDM and HDAs performance. Contrary to FBRs, the substantial amount of metals and coke deposits on the catalyst particles is disposed through the bottom of the reactor during operation. This feature of MBRs allows for 

Operating at higher pressure (200MPa) and temperature (dOO C- SO C) than typical FBR units (Furimsky, 1998). Thus, MBRs are more tolerant to metals and other contaminants than FBRs even with the same type of catalyst and at more severe conditions. However, the catalysts used in MBRs must have improved mechanical properties to resist severe grinding and abrasion. 

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The OLEFLEX process uses multiple side-by-side, radial flow, moving-bed reactors connected in series. The heat of reaction is suppHed by preheated feed and interstage heaters. The gas-phase reaction is carried out over a catalyst, platinum supported over alumina, under very near isothermal conditions. The first commercial installation of this technology, having an annual capacity of 100,000 t, was made in 1990 by the National Petrochemical Corporation in Thailand. A second unit, at 245,000 t capacity, has been built in South Korea by the ISU Chemical Company (70). 

Types ofSCT Catalysts. The catalysts used in the SCR were initially formed into spherical shapes that were placed either in fixed-bed reactors for clean gas apphcations or moving-bed reactors where dust was present. The moving-bed reactors added complexity to the design and in some appHcations resulted in unacceptable catalyst abrasion. As of 1993 most SCR catalysts are either supported on a ceramic or metallic honeycomb or are direcdy extmded as a honeycomb (1). A typical honeycomb block has face dimensions of 150 by 150 mm and can be as long as one meter. The number of cells per block varies from 20 by 20 up to 45 by 45 (39). 

For example in paint shops, TCE evaporates and causes air pollution. The contaminated air has 250 ppm TCE in it and this can be fed to a moving bed reactor at 300°C that is charged with OXITOX (Chovan et al, 1997) The kinetics must be studied experimentally. The experimental setup is shown in Figure 4.5.1 and the following description explains the recommended procedure. In the experimental unit shown, the feed is contained under pressure in a gas cylinder. Two percent of the feed is saturated by TCE and diluted with the rest of the feed. The rate is calculated as ... 

Some units also use steam to strip remaining hydrocarbons and oxygen from the catalyst before being fed back to the oil stream. In recent years moving-bed reactors have largely been replaced by fluidized-bed reactors. 

An oven can be used instead of an oil bath. If the amount of polymer to be post-condensed is large, a tumble dryer (rotavapor apparatus) or moving bed reactor can be used. 

If a very high viscosity is required, die granulated polymer can be postcondensed in die solid state at 160-190° C for 4-24 h. The postcondensation step can be done batchwise in large revolving reactors or can be carried out in a continuous manner using tall moving-bed reactors. Surprisingly, die water concentration is not critical to the rate of postcondensation. The method employed for PA-6 is similar to tiiat for PA-6,6 (Example lc). As a result of a postcondensation step for 24 h at 190°C, the i/inh in 85% formic acid is increased from 0.8 to 1.35 (Afn = 18,000-35,000). 

A simulated countercurrent moving bed reactor for oxidation of CO at low concentration over P1/A1203... 

Fig. 1 Concentration profiles of CO, Oj and Hj inside a countercurrent moving bed reactor.

In this study, a simulated countercurrent moving bed reactor (SCMBR) with four parts by switching the inlets and outlets of the parts cyclically is employed in order to avoid abrasion occurring from the movement of a solid catalyst. Based on the above concepts, we focused on the performance of a SCMBR for the oxidation of CO at low concentration in absence of H2 over Pt/AlaOs catalyst adsorbent. For the first stqj of the overall nractor draign, the performance of a SCMBR is experimentally investigated and compared with that of a PBR for the reaction. 

Performance of a simulated countercurrent moving bed reactor, SCMCR is experimentally investigated for oxidation of CO at low concentration in the absence of hydrogen over Pt/AljOs catelyst/adsorhent. The time-average conversion of CO obtained in the SCMBR was higher than the conversion of CO obtained from a conventional PBR for all over the tested range (period = 2-15 min). For the next step, the effects of operating variables on its performance are planed for both CO oxidation in the absence of H2 and Hz-rich gas system. 

Reactors in which the solid catalyst particles remain in a fixed position relative to one another (fixed bed, trickle bed, and moving bed reactors). 

If larger particles are used in a moving-bed reactor, there is some sacrifice over temperature control and fluid-solid exchange. However, the pressure drop is much less than in bubbling fluidized beds, and erosion by particles is largely avoided. Furthermore, the fluid-solid contacting is close to ideal, and so performance is enhanced. 

In a simulated moving bed reactor, as its name already indicates, the movement of the solid phase is simulated. This is achieved by using a set of fixed-bed reactors (columns) connected in series and periodically switching the feed and withdrawal points from one column to the other. A schematic presentation of a simulated moving bed chromatographic reactor is shown in Fig. 8. 

The first cracking catalysts were acid-leached montmorillonite clays. The acid leach was to remove various metal impurities, principally iron, copper, and nickel, that could exert adverse effects on the cracking performance of a catalyst. The catalysts were first used in fixed- and moving-bed reactor systems in the form of shaped pellets. Later, with the development of the fluid catalytic cracking process, clay catalysts were made in the form of a ground, sized powder. Clay catalysts are relatively inexpensive and have been used extensively for many years. 

In this process, propane, and a small amount of hydrogen to control coking, are fed to either a fixed bed or moving bed reactor at 950—1300° F and near atmospheric pressure. Once again the catalyst, this time platinum on activated alumina impregnated with 20% chromium, promotes the reaction. In either design, the catalyst has to be regenerated continuously to maintain its activity. 

Discussion. Fixed bed cracking reactors as well as commercial moving bed reactors operate under steady state or pseudo-steady state conditions ( ). Observed selectivity (eg., ratio of yield of branched to n-paraffin) in a steady state catalytic reactor is independent of space velocity (1, 17). The selectivity depends on intrinsic rate constants and diffusivities of the reacting species which depend on temperature. Thus, the selectivity observations reported here are applicable to commercial FCC units operating at space velocities different from that employed in this study. 

Moving bed reactors for oil recovery from shale is one example of this kind of operation. Another somewhat analogous operation is the multistage counterflow reactor, and the four- or five-stage fluidized calciner is a good example of this. In all these operations the efficiency of heat utilization is the main concern. 

We will develop the rest of this chapter assuming that the catalyst is in a sohd phase with the reactants and products in a gas or liquid phase. In Chapter 12 we will consider some of the more complex reactor types, called multiphase reactors, where each phase has a specific residence time. Examples are the riser reactor, the moving bed reactor, and the transport bed reactor. 

This shows how catalytic reactions compare with other interfacial reactions. In a fixed bed reactor the catalyst (in phase ) has an infinite residence time, which can be ignored in the expressions we derived in previous chapters. For a moving bed reactor in which catalyst moves through the reactor, we have a true multiphase reactor because the residence time of the catalyst phase is not infinite. 

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