Daizo Kunii

Circulating fluidized-bed reactors

Abstract This paper develops a flow and contacting model to represent a CFB. Best estimates of contacting efficiencies are presented for the turbulent, fast fluidized, and pneumatic transport regimes of the CFB. Material balances are presented, ending up with conversion equations for first-order solid-catalyzed gas-phase reactions. Four examples show how to use this model and show its predictions.

The K-L reactor model for circulating fluidized beds

In this paper we present a model for determining the performance behavior of a CFB reactor. This model is not sophisticated and does not require computer calculations, but is realistic and convenient for engineers engaged in the development of new processes. A number of parameters must be evaluated to use this model. This paper shows what they are, and thus suggests where future research of CFB should be. Four examples in the different flow regimes illustrate the use of this model.

Effect of exit geometry on the vertical distribution of solids in circulating fluidized beds. Part I: solution of fundamental equations; Part II: analysis of reported data and prediction

Abstract In Part I, a set of differential equations is solved to account for the partial separation and reflux of solids from the bed exit, giving working equations to predict the vertical distribution of solids throughout the freeboard. This reflux is accounted for by an additive term to the vertical distribution of solids in the same vessel without reflux. Part II analyzes recent experimental data on vessel end effects by this model, and examples are presented to illustrate how to apply the derived theoretical equations for prediction of circulating fluidized bed operations.

Novel rotary gas/solid contactor

Abstract The heart of this novel rotary contactor is the inclusion of a longitudinal partition plate and a series of guide plates. The contractor is kept horizontal, not inclined, and the placement of the guide plates can cause the solids to move in one direction or in the other. With this freedom of movement, one can achieve plug flow or recirculation of solids or any combination of these. Another advantage of this device is that it can hold more solids than the conventional unit, 20–40% versus 10%. The performance equations of this contactor are developed, and its uses in industry today are noted.

Performance of Rotary Reactors, Indirect Heating

This chapter presents the procedure for designing a rotary retort heated by an electric heater. It presents the prediction for the performance of a novel rotary reactor. It also highlights that hot solids, heated to their maximum temperature, move into a coaxial inner cylinder and are transferred toward its outlet. The hot solids give their heat to cold solids, moving in the annular space between two cylinders. Heat exchange takes place between the hot and cold solids. Partition plates, positioned on the outer surface of the inner cylinder, scrape the solids sticking to the inner surface of the retort. To apply a rotary reactor for larger capacity, a wider surface area is needed to exchange the heat between two flows of solids. In the case of heating the rotary retort by combustion gas, hot gas from the burning flame is sent into heating compartments composed of a refractory wall and then heats the rotary retort. In this case, flue gas goes out from its bottom. A number of horizontal tubes are positioned in the retort, from which the air is sent into the rotating layer of catalyst to oxidize the residual carbon. A rotary retort has been applied for the thermal cracking of solid waste materials because the maximum temperature of the retort can be precisely controlled and there is far less heat loss from the outer surface of the insulation cover made of low density refractory material.

Thermal Decomposition and Conversion of Composite Pellets

This chapter focuses on the process of thermal decomposition of inorganic and/or carbonaceous substances and reduction of inorganic compounds in large-scale rotary reactors. It reveals that in some cases a small amount of substance is eliminated from the product solids by heating them to an adequate temperature. Elimination of dioxins in fly ash and of silanol from ultra-pure silica are two examples of this case. The shrinking core model is applied for decomposition of limestone, dolomite, and magnesite. Heat conduction through the product blanket controls the shrinking rate of the core instead of a reaction with a gaseous reactant. Decomposition of limestone needs considerable thermal energy. When a particle of organic substance is suddenly fed into a region at high temperature, it is decomposed very quickly, issuing a variety of gaseous and vapor species. Thermal cracking of hydrocarbon solids finishes at high temperature within a minute. In contrast, cracking of coal and/or biomass at intermediate temperature, i.e., 400–500 °C, is of practical importance to produce char of suitable quality for further utilization. The slow rate of temperature rise is necessary to obtain suitable char for production of active carbon. The reduction of composite pellets, which are composed of ferro-chromium ore and coke, is important as a preliminary process for the production of ferro-chromium. At 1300 °C, it was found that the grain size of both the ore and coke did not affect the reduction rate very much. The existence of CO 2 around the pellet deteriorates its conversion considerably.

Movement of Solids in Rotary Cylinder

This chapter discusses the movement of solids in rotary cylinder giving useful equations to predict the flow rate of solids as well as the volumetric fraction of solid. It reveals that in rotary cylinder solids rotate just like a rigid body and then collapse to slide down along a flat surface, the inclination angle of which is nearly the same as their repose angle. Due to this movement, the solids are well mixed within the cross-section, which is perpendicular to the rotation axis. This results in uniform mixing of solids and then uniform temperature. This movement of solids is crucial for any process in a rotary reactor, but slipping of the main body of solids should be prevented. Once slipping of solids occurs, solids do not mix at the cross-section, which causes deterioration of the performance of the rotary reactor. Slipping of solids occurs when the inner surface is smooth and slippery and when the volumetric fraction of bulk solids is small. The volumetric fraction of the bulk layer is increased to prevent slipping. The flow patterns of solids can be realized by the application of screw cylinders. A screw plate is fixed to its outer cylinder, and they rotate together. Solids are transferred into two screw cylinders but in opposite directions, establishing sound re-circulation of solids.

Heat Transfer in a Rotary Reactor, Direct Heating

This chapter discusses the heat transfer mechanism in a rotary reactor including thermal conduction from the inner surface of the reactor to the rotating solids. The heat necessary for thermal processing at high temperature in a rotary reactor is usually supplied by combustion of fuel by applying a suitable burner. The optimum fuel is selected from gas, liquid, and pulverized solid fuel mainly based on economic optimization, in accordance with its location and environmental requirements. The combustion of fuel from the burner is substantially controlled by turbulent mixing and diffusion of combustible gas with the air stream. A sheet of intense vortexes is generated along the discontinuous interface. At the interface between two kinds of vortexes the molecular diffusion of the gaseous reactant is very rapid, leading to an instantaneous reaction. In some rotary reactors, combustible gas issues from the rotating layer of solids and is burned by the air, which is sent from outside the reactor. It also reveals that radiant heat absorbed in combustion gas around the flame is converted to thermal energy, which is emitted as an infrared ray from the gas to the rotating solids and inner wall surface. In a rotary reactor, solids are heated by direct contact with the hot wall surface. The inner wall surface functions as a kind of regenerator, changing the surface temperature periodically during the rotation. In a conventional rotary dryer, lifters are usually positioned to enhance heat transfer between the solids and the flowing gas.

Conversion of Solids in Rotary Reactors

This chapter discusses the conversion process of solids in rotary reactors. It reveals that when the gaseous reactant diffuses within the rotating layer of solids, the overall rate of solid conversion is found to be extremely slow on a practical scale. Injecting the gaseous reactant directly into the rotating layer of solids enhances the rate of conversion appreciably. On a rotary reactor, where rotating tube tuyeres are positioned, gaseous reactant percolates through the rotating layer of bulk solids and contacts with solids. When fine solids are charged, they are apt to partially fluidize and the surface of the rotating bulk solids is flattened. Gaseous reactant percolates through the rotating layer of solids, achieving cross-flow contact. This is needed to send the gaseous reactant selectively to tubes that are just immersed in the rotating layer of solids. The design and development of a satisfactory rotary manifold, which can completely prevent leakage of reactant gas, are of the utmost importance. In the case of endothermic reaction, the small fraction of solids does not appreciably enhance the total rate of conversion. When small solids fall in a reactor, where an exothermic reaction takes place between gas and solids, the temperature of the solids rises rapidly to its stable value. Once the above phenomenon occurs in the rotary reactor, the overall rate of conversion increases enormously.

Application of a Rotary Reactor for the Re-utilization of Solid Wastes

This chapter discusses the application of a rotary reactor for the re-utilization of solid wastes. It also presents new processes for the re-utilization of solid wastes. It reveals that in this new process, metals are selectively collected and sent to metallurgical plants as raw materials with high purity. Plastics and biomass are separated to make so-called refuse derived fuel. The gasification of such stock supplies clean fuel gas to the local community. This chapter also highlights a novel rotary reactor in which drying, thermal cracking of chips, and activation of char were carried out consecutively in a single cylinder. Here, steam is injected into a rotating layer of char. To burn combustible gas without contacting the active char, air is issued from orifices to impinge on the upper wall surface. Biomass is an excellent feed stock for producing clean fuel gas, which can be sent to a gas engine or turbine for power generation. A considerable amount of char is formed, which is converted to valuable products such as active carbon. A rotary reactor is also used to produce olefins from very heavy oil such as asphalt. It also helps Pyrox to produce hydrogen for fuel cell cars where they could find enough space for landfill. It also helps in producing a mixture of plastics and spent pulp from a paper-pulp plant. Another application is to gasify car shredder dust for fuel gas. Low grade coal can be utilized in rotary reactors to generate power.