Circulating Fluidized Beds

Solid particles are often of great interest in the chemical process industry, mineral processing, pharmaceutical production, energy-related processes, etc. In some cases the particles serve as catalysts for reacting gases and/or liquids. In other cases, as in ore processing, the particles must be chemically converted. In still other processes the particles must undergo physical transformation, as in drying of particulate solids.

Hydrodynamics of circulating fluidized beds deal, on the one hand, with the dynamics of gas—solid suspensions over a certain solid concentration range (voidage: 0.7–0.999) and, on the other, with the hydrodynamic characteristics of particular types of gas—solid contacting devices. From a scientific viewpoint, the clustering nature of dilute suspensions, which was first detected from the large gas—solid slip (i.e. relative) velocity, should be the essential point of interest. From an engineering viewpoint, the major hydrodynamic issues are the effects of such design factors as column diameter, wall shape, gas distributor design, exit structure, solid separation and recycling devices, as well as operating conditions, on the performance of circulating systems. The engineering and scientific aspects are closely interrelated. For instance, the behavior of dilute gas—solid suspensions could not have been found so easily and studied so systematically without the development of circulating devices (i.e. riser and downcomer systems) and the hydrodynamic behavior of gas and solids in a circulating apparatus is not independent of the essential nature of suspensions. However, in the development of our hydrodynamic understanding of circulating fluidized beds, paradigm shifts have occurred between the above two aspects, causing controversy.

The dispersion of gas phase in the riser of a circulating fluidized bed exerts great, sometimes even crucial, influence on the performance of a CFB gas—solids reactor. Incomplete mixing of secondary air streams into the gas—solids suspension rising from the bottom region of a CFB combustor may lead to hydrocarbon emissions from sub-stoichiometric zones as well as NO _x emissions from over-stoichiometric zones. Optimal axial and radial dispersion of the gas phase may be crucial in achieving high conversion and/or high selectivity in some gas-conversion processes such as catalytic cracking. An adequate understanding of the gas mixing process may shed light on other issues, such as the evolution of volatiles from fuel particles or the optimal distribution of feed points of a reactant. It is therefore essential for practical design as well as for successful modelling of reactor behaviour.

Circulating fluidized beds (CFB) exhibit very complex hydrodynamics, caused by interactions between the gas and solid phase. The motions of gas and solids are driven by many mechanisms that are difficult to identify and to describe.

Experimental studies of risers have shown that these systems are often characterized by complex flow phenomena such as non-uniform spatial distribution of particles, large slip velocities between the phases, and the existence of several possible pressure gradients and solids holdups for specified values of gas and solid flow rates (see Chapter 2 — Hydrodynamics). The particle concentration profile influences the distribution of residence times of particles and may lead to recirculation of particles against the direction of their net motion. These effects are critically important in predicting the conversion in systems in which the particles react chemically with the gas or catalyse a gaseous reaction. Hence, an accurate understanding of the mechanism responsible for the cross sectional distribution of solids is necessary to predict the performance of these systems. Empirical correlations have generally proven unsuccessful as they are typically limited to the database used to develop them and ignore the effect of radial non-uniformity of the basic variables. However, fundamentally based models have made some progress in this direction. These models can be used to predict how various parameters vary as system conditions change. This is very important, especially to determine the effects of scale-up, design and optimization. A review of these models is the subject of this chapter.

A cyclone is a device that separates particulate solids from a fluid stream by a radial centrifugal force exerted on the particles. This force separates the solids from the gas by driving the solids to the cyclone wall, where they slide to the bottom outlet and are collected. Cyclones are widely used in conjunction with fluidized beds to remove solids from exit gas streams. Cyclones have no moving parts, are relatively inexpensive to construct, and maintenance costs are low.

Although chemistry is the initial driving force for the development of chemical processes, in many instances the key to successful process operation is how well the solids transport systems have been designed. This is especially true in circulating fluidized bed (CFB) processes, because these processes are dependent upon rapid and reliable circulation of solids.

Circulating fluidized beds involving combustion or exothermic reaction commonly require heat transfer to the bed walls. The heat transfer helps to control the bed temperature and serves as a primary means to generate steam or hot water from the bed.

Combustion systems, ranging in size from small-scale transportable incinerators Anderson et al., 1989) to 300 MWe boilers, are one of the main applications of circulating fluidized bed contacting. Circulating fluidized bed combustion (CFBC) systems first gained popularity in the late 1970s, with almost simultaneous development of technologies by Ahlstrom Pyropower in Finland, Lurgi in Germany, and Studsvik Energiteknik in Sweden. Each brought a slightly different design concept to the table, but all were clearly CFB concepts. Since their introduction, CFB combustors have grown in size so that atmospheric circulating fluid bed combustion systems for power generation can be considered in applications up to 400 MWe (see Chapter 11). Atmospheric pressure CFBC systems also find application as part of advanced power generation cycles such as air-blown gasification (Sage et al., 1995), while pressurized circulating fluid beds are being developed both for standalone power plants (Koskinen et al., 1995; Provol and Matousek, 1995), and integrated into cycles such as IGCC (Integrated Gasification Combined Cycle) (Abdulally and Alkan, 1995).

Since the 1970s, circulating fluidized bed (CFB) technology has been applied to combustion and steam generation. It has become the technology of choice for clean firing of solid fuels. The success of CFB boilers is mainly due to their fuel flexibility and environmental factors. The advantages include high combustion efficiency, low NO _x and SO₂ emissions, and the ability to burn a wide variety of fuels including very low grade fuels. In developed countries, stringent environmental regulations have helped promote the use of CFB technology, while in developing countries, the flexibility of burning a variety of fuels has attracted CFB customers.

The aim of this chapter is to provide the reader with some insight into the procedure for determining whether a circulating fluidized bed (CFB) system is appropriate for a given process. The approach is to examine a number of commercially significant examples (other than combustion, which is dealt with elsewhere in this book) and to explore the features that make the CFB option more attractive than other forms of reactor for gas—solid reactions. The following applications are examined: This list is not intended to be exhaustive — its purpose is merely to illustrate a number of key aspects of the CFB and how these can be put to good use. The inclusion of a non-CFB process at the end of this list is deliberate in that it illustrates the inappropriateness of CFB technology in some circumstances. Other forms of reactor have features that can lead to competitive advantage when used correctly. It is up to the process engineer to keep an open mind.

Fluid catalytic cracking, or FCC, was the first application of fine-powder fluidization. Interestingly, the first commercial FCC unit (Esso’s Model I in 1942) had both the reactor and regenerator in the form of entrained circulating fluid beds. Over 50 years later, FCC is still the major application of fluidization with over 350 FFC units operating worldwide, and with new ones coming on stream every year. FCC units convert heavy fuel oil and petroleum residue to lighter products. Major FCC products are gasoline, diesel fuel, heating oil, heavy cycle oils, which are used as bunker fuels or converted to carbon black, and light gases such as propene and butenes, which are converted to alkylate gasoline, methyl-tertiary-butyl ether, and are used for the production of petrochemicals.

The scale-up of circulating fluid bed reactors has been a continuing activity in the chemical process industries for over half a century. Despite that record, such scale-up is still not an exact science, but is rather that mix of physics, mathematics, witchcraft, history and common sense that we call engineering. The purpose of scale-up efforts is not to achieve fundamental and total knowledge of the process, but rather to minimize the possibility of expensive errors in reaching commercial operation. Better physical data, more realistic models and more exact equations are always desirable. Nevertheless, the basic goal is not ever-increasing accuracy of calculation but rather the recognition and management of uncertainties and risk.

Reactor models are idealized human constructs that attempt to capture the essence of the behaviour of the flow, mixing and contacting of reacting species and phases in order to be able to predict reactor conversions, yields and dynamic responses. Accurate modeling also requires an adequate representation of the chemical kinetics, while other factors such as heat transfer and thermodynamics may also play significant roles. Models are more complex for multi-phase reactors than for single-phase reactors as they must take into account the mixing behaviour within each individual phase, as well as the exchange of reactants and products between the phases. Each of these aspects is directly influenced by the flow patterns throughout the entire reactor.

This chapter on future prospects of circulating fluid beds discusses three major topics. The introduction addresses the issue of why CFBs have gained commercial success in some applications, and why they have replaced low-velocity fluid beds (LFB), or fixed beds in many catalytic and non-catalytic applications. In some cases, CFB processes have been designed to solve the shortcomings of an LFB, only to be replaced eventually by improved, lower cost LFB processes. One such example, Fischer—Tropsch synthesis, is discussed briefly in section 17.2, and in more detail in Chapter 14. The third topic of this chapter addresses a new emerging brand of CFBs, the downflow short contact time contractor, or ‘downer’, discussed also in Chapter 16. This novel unit operation is gaining a commercial foothold, and may feature prominently in CFB’s future prospects.