Fluidised reactor

Riley, H. L. Trans. Inst. Chem. Eng. 37 (1959) 305. Design of fluidised reactors for naphthalene oxidation a review of patent literature. 

Jones, A.G., Wachi, S. and Delannoy, C.C., 1992b. Precipitation of calcium carbonate in a fluidised bed reactor. In Fluidization VII. Eds. O.E. Potter and D.J. Nicklin, New York Engineering Foundation, pp. 407M14. 

The BP Chemicals polymer cracking process is based at Grangemouth in Scotland and uses mixed plastics as the raw material. The reactor uses a fluidised bed which operates at 500 °C in the absence of air, and under these conditions the plastics crack thermally to yield hydrocarbons. These vaporize and are carried away from the bed with the fluidising gas. Solid impurities such as metals from PVC stabilisers accumulate in the bed or are carried away in the hot gas to be captured by a cyclone further along in the plant. PVC decomposes to HCl and this is neutralized on a solid lime absorbent to yield CaCl2 which is disposed of in landfill. The purified gas is cooled to condense most of the hydrocarbon which can be employed as commercially useful distillate feedstock. The light hydrocarbons which are less easy to condense are compressed, reheated and recycled as fluidising gas. 

Before MPW is fed into the process, a basic separation of the non-plastic fraction and size reduction is needed. This prepared feedstock is then introduced in the heated fluidised bed reactor which forms the core of the process. The reactor operates at approximately 500 °C in the absence of air. At this temperature, thermal cracking of the plastics occurs. The resulting hydrocarbons vapourise and leave the bed with the fluidising gas. Solid particles, mainly impurities formed from, e.g., stabilisers in plastics, as well as some coke formed in the process mainly accumulate in the bed. Another fraction is blown out with the hot gas and captured in a cyclone. 

By cooling the gas is condensed and then available as hydrocarbon feedstock for other processes (some 85% of the MPW input). The light hydrocarbon gas (15% of the MPW input) that remains after cooling is compressed, reheated and returned to the reactor as fluidising gas. It can also be used as a fuel for the cracking process, though other recovery options are being studied as well. 

The results of research into the fluidised bed pyrolysis of plastic wastes are reported, with reference to determining the optimum process conditions for the process with respect to the reactor behaviour. The study investigates the effects of process variables such as bed temperature, polymer feed rate, bed hold-up, fluidising velocity, and size of inert material. Findings illustrate the importance of the knowledge of the hydrodynamics of the fluidised bed and of the interactions between bed and polymer particles in the design and operation of the reactor. 15 refs. 

The thermal cracking of a light ffaction of mixed plastics waste was carried out in a fluidised bed reactor and the fractions obtained were analysed by elemental analysis, gas chromatography and ashing. The main components of the waste were PE and PP with a small amount of PS and the bed was fluidised by pyrolysis gas, nitrogen or preheated steam. Experiments conducted at different temperatures and residence times were compared by calculating the crack severity for each experiment. The results obtained revealed that the amounts of ethene and propene obtained by pyrolysis with steam were comparable with those obtained using a commercial steam cracker. 

The application of a selective pyrolysis process to the recovery of chemicals from waste PU foam is described. The reaction conditions are controlled so that target products can be collected directly from the waste stream in high yields. Molecular beam mass spectrometry is used in small-scale experiments to analyse the reaction products in real time, enabling the effects of process parameters such as temperature, catalysts and co-reagents to be quickly screened. Fixed bed and fluidised bed reactors are used to provide products for conventional chemical analysis to determine material balances and to test the concept under larger scale conditions. Results are presented for the recycling of PU foams from vehicle seats and refrigerators. 12 refs. 

A stirred-tank model has been proposed, (Daly, 1980), to model the mixing behavior of an air-solid, spouted, fluidised-bed reactor. The central spout is modelled as two tanks in series, the top fountain as a further tank and the down flowing annular region of the bed as 6 equal tanks in series. It is assumed that a constant fraction of the total solids returns from each stage of the annular region into the central two tank region, as depicted below. 

Aniline is manufactured by the hydrogenation of nitrobenzene in a fluidised bed reactor. The reactor operates at 250 °C and 20 bar. The reactor vessel is approximately 3 m diameter and 9 m high. Suggest suitable materials of construction for this reactor. 

In the chemical process industries the designer will normally be concerned with the second type catalytic reactors. Industrial packed-bed catalytic reactors range in size from small tubes, a few centimetres diameter, to large diameter packed beds. Packed-bed reactors are used for gas and gas-liquid reactions. Heat-transfer rates in large diameter packed beds are poor and where high heat-transfer rates are required fluidised beds should be considered. 

The essential features of a fluidised bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat-transfer rates and good mixing. Heat-transfer coefficients in the order of 200 W/m2oC to jackets and internal coils are typically obtained. The solids may be a catalyst a reactant in fluidised combustion processes or an inert powder, added to promote heat transfer. 

Fluidisation can only be used with relatively small sized particles, <300 gm with gases. A great deal of research and development work has been done on fluidised bed reactors in recent years, but the design and scale up of large diameter reactors is still an uncertain process and design methods are largely empirical. 

The principles of fluidisation processes are covered in Volume 2, Chapter 6. The design of fluidised bed reactors is discussed by Rase (1977). 

CLC consists of two fluidised bed reactors, namely reducer and oxidiser. In the reducer reactor, fuel is fed along with the metal oxide containing oxygen, which is transferred from the metal oxide to the reactor as the combustion occurs (Figure 7). A flue gas containing over 99%v/v of C02 can be obtained by a simply condensation stage because of the fact that the exhaust gas at the reducer outlet is primarily formed by C02 and water vapour. This stream is then sent to further compression and permanent storage. 

Calcium looping consists of two fluidised bed reactors, namely carbonator and calciner. In the... 

Approximately 3g samples of the coal fractions and of the whole coal were then reacted separately with 25 - 30 ml of tetra-lin at 450°C in a type 316 stainless steel, sealed reactor, 13 cm high by 2 cm diameter. The reactor was heated by plunging it into a preheated fluidised sand bath after 4 hours it was removed and quenched rapidly. 

Schoutens, G. EL, Guit, R. P., Zieleman, G. J., Luyben, K. C. A. M., and Kossen, N. W. F., A Comparative Study of a Fluidised Bed Reactor and a Gas Lift Loop Reactor for the IBE Process Part I. Reactor Design and Scale Down Approach, J. Chem. Tech. Biotechnol., 36 335 (1986a)... 

In this example, a fluidised biofilm sand bed reactor for nitrification, as investigated by Tanaka et al. (1981), is modelled as three tanks-in-series with a recycle loop (Fig. 1). With continuous operation, ammonium ion is fed to the reactor, and the products nitrite and nitrate exit in the effluent. The bed expands in volume because of the constant circulation flow of liquid upwards through the bed. Oxygen is supplied external to the bed in a well-mixed gas-liquid absorber. 

Fig. 1 B iofilm fluidised-bed recycle loop reactor for nitrification.

The fluidised bed reactor is modelled by considering the component balances for the three nitrogen components (i) and also for dissolved oxygen. For each stage n, the component balance equations have the form... 

Continuous Multicomponent Distillation Column 501 Gas Separation by Membrane Permeation 475 Transport of Heavy Metals in Water and Sediment 565 Residence Time Distribution Studies 381 Nitrification in a Fluidised Bed Reactor 547 Conversion of Nitrobenzene to Aniline 329 Non-Ideal Stirred-Tank Reactor 374 Oscillating Tank Reactor Behaviour 290 Oxidation Reaction in an Aerated Tank 250 Classic Streeter-Phelps Oxygen Sag Curves 569 Auto-Refrigerated Reactor 295 Batch Reactor of Luyben 253 Reversible Reaction with Temperature Effects 305 Reversible Reaction with Variable Heat Capacities 299 Reaction with Integrated Extraction of Inhibitory Product 280... 

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