Reactor, fluidized bed

In a fluidized-bed reactor, fluid is introduced at many points and at a sufficiently high velocity that the upward flow through a bed of particles causes the particles to lift and 

The fluidized-bed reactor was originally developed for catalytic cracking in petroleum 

The fluidized bed occupies the lower part of the vessel, and is supported by a grate containing many openings through which air, entering at the bottom, flows to bring about fluidization. The greater part of the vessel is freeboard, in which lower gas 

Design aspects of fluidized-bed reactors are considered in Chapter 23. 

In a fluidized bed relatively small particles of catalyst are sustained in -a vertical tube by the upward motion of the reacting fluid. The range of 

Flnidized-bed reactors are featured by presenting both temperature and composition homogeneity. This is a remarkable advantage for the cracking of polymers due to their low thermal conductivity and high viscosity that usually lead to the appearance of temperature 

Fluidized-bed reactors are used in a number of applications ranging from catalytic cracking in the petroleum industry to oxidation reactions in the chemical industry. A number of advantages and disadvantages may be tabulated for this type of reactor, but we can shorten this somewhat by saying that most applications involve reactions in which catalyst decay is prominent and a continuous circuit is required for catalyst regeneration, or reactions where close control of operating conditions, particularly 

1 Simple Fluid Mechanics Minimum Fluidization and Entrainment 

For the determination of the entrainment velocity, assuming that there is no particle-particle interaction, we may use a force balance based on Stoke s law 

Practical operating conditions for fluidized beds must obviously be lower than this value, often about one-half of u,. The entrainment velocity is strongly dependent upon particle diameter so that in a given operation if a range of dp is involved and there is to be no elution of solid from the bed, then the size distribution must be 

Determination of the minimum fluidization velocity is a little more complicated, but still straightforward. The well-accepted correlation for pressure drop of a fluid flowing through a packed bed, from Ergun, we repeat here, but written in terms of the minimum fluidization velocity. 

For the design of a fluid bed reactor, we need to have a reasonably accurate description of the bubble pattern in the bed. In section 45.1.4 and in the Appendix to section 4.5.1 some relations were presented for describing bubble behaviour and mass transfer. We will consider two cases relatively slow and relatively rapid chemical reactions. 

For a first order reaction, or for second order reaction with excess B quasi first order). Da follows, respectively, from 

Note that when the particles are porous, the specific particle surface area has to be multiplied by the ratio of the internal and external surface areas, times the effectiveness factor see after eq. (5.57). 

When the volume of the apparent diffusion layer around the bubbles cannot be neglected, one needs eq. (5A. 10) for predicting E (see Appendix). The parameter d can be found from eq. (5A.11), where e is the bubble fraction in the bed, and Sh is the Sherwood number for the bubbles Sh - dj (see section 5.4.4). 

Alternatively, one can resort to the use of moving- or fluidized-bed systems in which the adsorbent, and sometimes also the catalyst, is continuously withdrawn from the reactor to undergo an external regeneration. Ideally, one tries to achieve countercurrent adsorbent flow pattern to optimize utilization of the adsorptive capacity. The major problems of such arrangements are those of solids handling (e.g., gas-tight 

In view of the extensive number of publications on the subject of adsorptive reactors (see Tab. 7.2), the neglect with which regeneration processes have been treated is surprising and short-sighted, since it is this section of the cycle and its expedient integration in the overall process that often holds the key to success. 

Four basic regenerative procedures - or hybrids thereof - can be used in conjunction with adsorptive reactor operation pressure swing, temperature swing, elution, and reaction. 

In this chapter the characteristics of fluidized gas-solid suspensions are described, and the basic designs of fluidized bed reactors are sketched. Several modeling approaches that have been applied to described these units are outlined. 

Jakobsen, Chemical Reactor Modeling, doi 10.1007/978-3-540-68622-4 10, Springer-Verlag Berlin Heidelberg 2008 

As mentioned in Section 2.2 (Fixed-Bed Reactors) and in the Micro activity test example, even fluid-bed catalysts are tested in fixed-bed reactors when working on a small scale. The reason is that the experimental conditions in laboratory fluidized-bed reactors can not even approach that in production units. Even catalyst particle size must be much smaller to get proper fluidization. The reactors of ARCO (Wachtel, et al, 1972) and that of Kraemer and deLasa (1988) are such attempts. 

This term is restricted here to equipment in which finely divided solids in suspension interact with gases. Solids fluidized by liquids are called slurries. Three phase fluidized mixtures occur in some coal liquefaction and petroleum treating processes. In dense phase gas-solid fluidization, a fairly definite bed level is maintained in dilute phase systems the solid is entrained continuously through the reaction zone and is separated out in a subsequent zone. 

Fluid catalytic vessels are very large. Dimensions and performance of a medium capacity unit (about 50,000 BPSD, 60kg/sec) are shown with the figure. Other data for a reactor to handle 15,000 BPSD are a diameter of 25 ft and a height of 50 ft. Catalyst holdup and other data of such a reactor are given by Kraft, Ulrich, and O Connor (in Othmer (Ed.), Fluidization, Reinhold, New York, 1956) as follows  

Flue gas plus solids density, cyclone inlet 

000 BPSD 250tons 100tons 2.5 fps 28.0 Ib/cuft 0.5 Ib/cuft 24.0tons/min 7.0tons/min 2.0tons/day 

Typical Data for IC1 Quench Converters of Various Sizes 

Catalyst loss rate, design expectation 2.0 tons/day 

Typical Dila for ICI Quencfa Converters of Various Sizes 

Flue gas plus solids density, cyclone inlet 0.5 Ib/cuft Catalyst circulation rate, unit 24.0tons/min 

B - Gas exit to heal recovery C — Gas exit D - Direct by pass 

In addition to this introduction, the chapter primarily addresses two types of catalytic reactors. Although there are numerous types of catalytic units, this chapter solely reviews fluidized bed reactors and fixed bed reactors. Details on other reactors are available in the literature The remaining sections of this chapter include  

From a force balance perspective, as the flow rate upward through a packed hed is increased, a point is reached at which the ftictional drag and buoyant force is enough to overcome the downward force exerted on the hed by gravity. Although the hed is supported at the bottom by a screen, it is finee to expand upward, as it will if the velocity is increased above the aforementioned minimum fluidization velocity. At this point, the catalysts are no longer supported by the screen, but rather are suspended in the 

The terminal settling velocity can be evaluated for the case of flow past one catalyst particle in the bed. By superimposition, this case is equivalent to that of the terminal velocity that a catalyst particle would attain flowing through a fluid. Once again, a force balance can be applied and empirical data used to evaluate a friction (drag) coefficient 

At intermediate velocities between the minimum fluidization velocity and the terminal velocity, the bed is expanded above the volume that it would occupy at the minimum value. Note also that above the minimum fluidization velocity, the pressure drop stays essentially constant. 

There are two modes of fluidization. When the fluid and solid catalyst densities are not too different, or the particles are very small, the bed is fluidized evenly. This is called smooth fluidization, and is typical of liquid-solid systems. If the fluid and solid densities are significantly different, or the catalyst particles are large, the velocity of the flow must be relatively high. In this case, fluidization is uneven, and the fluid passes through the bed mainly in large bubbles. These bubbles burst at the surface, spraying the solid catalyst above the bed. Here, the bed has many of the characteristics 

Apart from the operational drawbacks stated earlier, capital and operating expenses involved in an FBR exceed those of a PBR of equivalent capacity due to requirements of larger vessel volume for handling fluidization and of installing gas purification and solid circulation components. Chaotic nature of the operation also calls for a tedious preliminary study of the process of interest at the pilot scale that should be followed by a labor and cost-intensive scahng-up stage, all of which eventually increase the capital cost of the commercial FBR unit. 

Although not as widely used as a gas-sofid PBR, FBR remains as the only choice for processes such as FCC and high-temperature Fischer-Tropsch (HTFT) synthesis, both of which have key roles in the petroleum processing and petrochemical industries. FCC is a critical step in petroleum refining and involves catalytic breakdown of heavy gas oil molecules into 

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Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

The performance of fluidized-bed reactors is not approximated by either the well-stirred or plug-flow idealized models. The solid phase tends to be well-mixed, but the bubbles lead to the gas phase having a poorer performance than well mixed. Overall, the performance of a fluidized-bed reactor often lies somewhere between the well-stirred and plug-flow models. 

Figure 2.8 A fluidized-bed reactor allows the catalyst to be continuously withdrawn and regenerated as with the refinery catalytic cracker.

By contrast, if the reactor is continuous well-mixed, then the reactor is isothermal. This behavior is typical of stirred tanks used for liquid-phase reactions or fluidized-bed reactors used for gas-phase reactions. The mixing causes the temperature in the reactor to be effectively uniform. 

Where there are large volumes of contaminated water under a small site, it is sometimes most convenient to treat the contaminant in a biological reactor at the surface. Considerable research has gone into reactor optimization for different situations and a variety of stirred reactors, fluidized-bed reactors, and trickling filters have been developed. Such reactors are usually much more efficient than in situ treatments, although correspondingly more expensive. 

Recent advances in Eischer-Tropsch technology at Sasol include the demonstration of the slurry-bed Eischer-Tropsch process and the new generation Sasol Advanced Synthol (SAS) Reactor, which is a classical fluidized-bed reactor design. The slurry-bed reactor is considered a superior alternative to the Arge tubular fixed-bed reactor. Commercial implementation of a slurry-bed design requires development of efficient catalyst separation techniques. Sasol has developed proprietary technology that provides satisfactory separation of wax and soHd catalyst, and a commercial-scale reactor is being commissioned in the first half of 1993. 

Another hydrogenation process utilizes internally generated hydrogen for hydroconversion in a single-stage, noncatalytic, fluidized-bed reactor (41). Biomass is converted in the reactor, which is operated at about 2.1 kPa, 800°C, and residence times of a few minutes with steam-oxygen injection. About 95% carbon conversion is anticipated to produce a medium heat value (MHV) gas which is subjected to the shift reaction, scmbbing, and methanation to form SNG. The cold gas thermal efficiencies are estimated to be about 60%. 

More recently, Sasol commercialized a new type of fluidized-bed reactor and was also operating a higher pressure commercial fixed-bed reactor (38). In 1989, a commercial scale fixed fluid-bed reactor was commissioned having a capacity similar to existing commercial reactors at Sasol One (39). This effort is aimed at expanded production of higher value chemicals, in particular waxes (qv) and linear olefins. 

FIOR Process. In the FIOR process, shown in Figure 5, sized iron ore fines (0.04—12 mm) are dried in a gas-fired rotary dryer. A skip hoist dehvers the dry fines to lock hoppers for pressurizing. The fines pass through four fluidized-bed reactors in series. Reactor 1 preheats the ore to 760°C in a nonreducing atmosphere. Reactors 2, 3, and 4 reduce the ore at 690—780°C. At higher (ca 810°C) temperatures there is a tendency for the beds to defluidize as a result of sticking or hogging of the reduced material. 

The iron carbide process is alow temperature, gas-based, fluidized-bed process. Sized iron oxide fines (0.1—1.0 mm) are preheated in cyclones or a rotary kiln to 500°C and reduced to iron carbide in a single-stage, fluidized-bed reactor system at about 590°C in a process gas consisting primarily of methane, hydrogen, and some carbon monoxide. Reduction time is up to 18 hours owing to the low reduction temperature and slow rate of carburization. The product has the consistency of sand, is very britde, and contains approximately 6% carbon, mostly in the form of Ee C. 

Fluidized-bed reactor systems put other unique stresses on the VPO catalyst system. The mixing action inside the reactor creates an environment that is too harsh for the mechanical strength of a vanadium phosphoms oxide catalyst, and thus requires that the catalyst be attrition resistant (121,140,141). To achieve this goal, vanadium phosphoms oxide is usually spray dried with coUoidal siUca [7631-86-9] or polysiUcic acid [1343-98-2]. Vanadium phosphoms oxide catalysts made with coUoidal sUica are reported to have a loss of selectivity, while no loss in selectivity is reported for catalysts spray dried with polysUicic acid (140). 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

The chlorination is mostly carried out in fluidized-bed reactors. Whereas the reaction is slightly exothermic, the heat generated during the reaction is not sufficient to maintain it. Thus, a small amount of oxygen is added to the mixture to react with the coke and to create the necessary amount of heat. To prevent any formation of HCl, all reactants entering the reactor must be completely dry. At the bottom of the chlorination furnace, chlorides of metal impurities present in the titanium source, such as magnesium, calcium, and zircon, accumulate. 

Fluidized bed dehydrogenation technology is more prevalent in the former Soviet Union. A continuous fluidized-bed reactor system is used with a... 

This reaction takes place in a fluidized-bed reactor or a specially made furnace called a Mannheim furnace. This method was last used in the United States in the 1980s. In another process, SO2, O2, and H2O react with NaCl. 

Chloride Process. In the chloride process (Fig. 3), a high grade titanium oxide ore is chlorinated in a fluidized-bed reactor in the presence of coke at 925-1010°C ... 

In oxychlorination, ethylene reacts with dry HCl and either air or pure oxygen to produce EDC and water. Various commercial oxychlorination processes differ from one another to some extent because they were developed independentiy by several different vinyl chloride producers (78,83), but in each case the reaction is carried out in the vapor phase in either a fixed- or fluidized-bed reactor containing a modified Deacon catalyst. Unlike the Deacon process for chlorine production, oxychlorination of ethylene occurs readily at temperatures weU below those requited for HCl oxidation. 

Fig. 18. Anaerobic wastewater treatment processes (a) anaerobic filter reactor (b) anaerobic contact reactor (c) fluidized-bed reactor (d) upflow anaerobic...

Sasol uses both fixed-bed reactors and transported fluidized-bed reactors to convert synthesis gas to hydrocarbons. The multitubular, water-cooled fixed-bed reactors were designed by Lurgi and Ruhrchemie, whereas the newer fluidized-bed reactors scaled up from a pilot unit by Kellogg are now known as Sasol Synthol reactors. The two reactor types use different iron-based catalysts and give different product distributions. 

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