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Three-Phase Fluidization Systems 585

The development of three-phase industrial-scale reactors in the 1950’s primarily occurred in the chemical process industry through the use of slurry bubble column reactors. In the 1950’s, slurry bubble column reactors were successfully utilized in the chemical process industry for the catalytic synthesis of organic chemicals and polyolefins. The slurry bubble column reactors used for catalytic reactor applications emerged in various designs and were operated under a wide range of flow conditions. Examples of reactor applications in commercial production (Germain et al., 1979) included hydrogenation of glucose to sorbitol, benzene to cyclohexane, butynediol to butenediol, esters to fatty alcohols, aluminum and ethylene to Ziegler alcohol (ALFOL process), and ethylene polymerization (Solvay process).

In the 1960’s, following research and development beginning in the late 1950’s, the three-phase fluidized bed reactor was first used commercially for hydrotreating petroleum resids. The technologies developed at this time are known as the H-Oil process and the LC-Fining process and are still currently in commercial operation. The three-phase fluidized bed reactors used in the hydrotreating/hydrocracking operations are commonly referred to as an ebullated bed. The term ebullated bed was first defined by P. W. Garbo in the patent of Johanson (1961) to describe a gasliquid contacting process in contrast to the common industrial term fluidized bed where particles are in fluidization induced by the gas phase alone. Recent developments in the use of ebullated beds for hydrotreating/ hydrocracking include the Texaco T-STAR process, introduced in the early 1990’s (see Sec. 4.2).

The development of three-phase reactor technologies in the 1970’s saw renewed interest in the synthetic fuel area due to the energy crisis of 1973. Several processes were developed for direct coal liquefaction using both slurry bubble column reactors (Exxon Donor Solvent process and Solvent Refined Coal process) and three-phase fluidized bed reactors (H- Coal process). These processes were again shelved in the early 1980’s due to the low price of petroleum crudes.

The 1970’s also brought about increased use of three-phase systems in environmental applications. A three-phase fluidized bed system, known as the Turbulent Bed Contactor, was commercially used in the 1970’s to remove sulfur dioxide and particulates from flue gas generated by coal combustion processes. This wet scrubbing process experienced several

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operational difficulties including excessive pressure drop, and its use was discontinued in the early 1980’s. Today, however, wet scrubbing systems are still in use for flue gas desulfurization, although the operational mode of these systems is mainly as slurry spray columns rather than as fluidized beds. A second important environmental application of three-phase systems to come out of the 1970’s is the area of biological wastewater treatment. This area of three-phase research has grown rapidly since the 1970’s and continues to grow in the 1990’s with increasing environmental pressures placed on the chemical process industry. Accounts of the historical development of fluidized bed bioreactor application in wastewater treatment can be found in Fan (1989), and Heijnen et al. (1989).

The 1980’s and the early 1990’s have seen the blossoming development of the biotechnology field. Three-phase fluidized bed bioreactors have become an essential element in the commercialization of processes to yield products and treat wastewater via biological mechanisms. Fluidized bed bioreactors have been applied in the areas of wastewater treatment, discussed previously, fermentation, and cell culture. The large scale application of three-phase fluidized bed or slurry bubble column fermentors are represented by ethanol production in a 10,000 liter fermentor (Samejima et al., 1984), penicillin production in a 200 liter fermentor (Endo et al., 1986), and the production of monoclonal antibodies in a 1,000 liter slurry bubble column bioreactor (Birch et al., 1985). Fan (1989) provides a complete review of biological applications of threephase fluidized beds up to 1989. Part II of this chapter covers the recent developments in three-phase fluidized bed bioreactor technology.

The design and scale-up of three-phase fluidization systems is a challenging endeavor requiring a sound knowledge of all facets of reactor engineering and the underlying chemical engineering technologies (Tarmy and Coulaloglou, 1992). Research efforts in the 1990’s, using state-of- the-art imaging techniques, have begun to look inside small scale threephase systems to better understand the fluid mechanics and phase interactions occurring in the system which can lead to better predictive tools for the design of three-phase reactor systems. While these studies provide insight into the hydrodynamics of the system, an important area in the industrial application of three-phase reactor systems which requires further study is the coupling of the hydrodynamics and the reaction kinetics. The complex reaction mechanisms occurring at all scales in three-phase

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reacting systems need to be studied in relation to the hydrodynamic behavior transpiring at the various scales. These areas of study require improvements in experimental techniques and instrumentation to provide the necessary data required in the verification of any predictive models.

New applications and novel reactor configurations or operational modes for three-phase systems are continually being reported. These include the operation of a three-phase fluidized bed in a circulatory mode (Liang et al., 1995), similar to the commonly applied gas-solid circulating fluidized bed; the application of a three-phase fluidized bed electrode that can be used as a fuel cell (Tanaka et al., 1990); magnetically stabilized three-phase fluidized beds; centrifugal three-phase reactors; and airlift reactors.

This chapter provides an overview of recent and nontraditional industrial applications of three-phase fluidization systems. Fan (1989) has provided comprehensive tables on examples of three-phase fluidization applications as well as several chapters on common industrial applications, and readers are referred there for further information. Shah (1979) and Deckwer (1992) also provided industrial examples of three-phase fluidization systems as well as details on reactor engineering and transport phenomena of these systems. The current chapter is divided into two parts, the first focusing on chemical process applications, and the second considering the biological applications of three-phase fluidization systems. In Part I, two nontraditional industrial applications of three-phase systems are highlighted; these are smelting operations and operations in the processing of pulp and paper, including cooking of wood chips to produce pulp and flotation for recycling paper. An overview of recent developments in several classic applications of three-phase fluidization systems in the chemical process industries, including hydrotreating/hydrocracking using ebullated bed technology and hydrocarbon synthesis using slurry bubble column, are also presented in Part I. Part II of this chapter delineates the recent progress in the application of three-phase fluidization systems to the rapidly expanding biotechnology area. The current status of three-phase bioreactor research, the important differences between three-phase chemical and biological fluidized systems, and design considerations for three-phase bioreactors are covered.

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Part I: Smelting Reduction, Paper Processing, and

Chemical Processing

2.0SMELTING REDUCTION

2.1Introduction

Smelting reduction of iron ore is an intensive area of research for the replacement of the classical ironmaking process (coke ovens, agglomeration, blast furnaces). Smelting reduction has been defined in several ways; however, the most common definition of these processes in the recent literature is the production of hot metal from iron ore without the use of metallurgical coke (Fine et al., 1989; Oeters et al., 1994). Smelting reduction offers several advantages over blast furnace operation, which are listed in Table 2, but the process may still require another five to ten years before making a significant impact on the worldwide production of iron, primarily because the economics of the smelting reduction process are still uncertain. The key characteristics of smelting reduction as an alternative to the older blast furnace process is the ability to directly use iron ore and coal, which eliminates the need for preprocessing the iron ore and is significant for countries such as the United States that have large reservoirs of coal and the ability to handle an increased load of scrap metal, which is consistent with the worldwide environmental trend of reuse, reduce, and recycle. The development of various smelting reduction processes is occurring simultaneously worldwide. Table 3 summarizes the smelting reduction processes under development and in production around the world.

Smelting reduction processes can be classified into melter-gasifiers and iron-bath reactors (Oeters et al., 1994). In the melter-gasifier process, the coal is combusted to carbon monoxide (CO) and hydrogen (H2) to provide the heat to melt iron pellets previously reduced from iron oxide pellets. The iron oxide pellets are reduced via direct reduction using the off-gas from the combusted coal. The most fully developed smelting reduction process is a melter-gasifier known as the Corex process, which is in operation in South Africa (Flickenschild, 1991). The iron-bath

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reactors, however, seem to be the smelting reduction process of choice for current and future applications. Iron-bath reactors consist of three layers or zones—at the bottom of the reactor is the liquid metal bath, which contains the reduced iron product; on top of the liquid metal is the slag layer, which contains all of the solid material in the reactor, including the iron ore and coal, and is where the combustion of the coal and reduction of the iron ore occurs; the upper portion of the reactor is primarily a gaseous region where secondary or post-combustion of the gases, produced during the initial combustion of the coal in the slag layer, occurs. In iron-bath reactors, final reduction of the iron ore and post-combustion of coal occur simultaneously, while the off-gas from the post-combustion is used for the pre-reduction of iron ore in a separate reactor, generally a gas-solid fluidized bed or circulating fluidized bed. The focus of this section will be on iron-bath reactors, which are unique three-phase systems consisting of molten metal as the liquid phase and iron ore, coal, and slag particles as the solid phase. The gas phase consists of bottom injected oxygen, air or nitrogen, used for agitation of the molten metal bath, and top injected oxygen for post-combustion.

Table 2. Advantages of Smelting Reduction over Coke Oven and Blast Furnace

·Elimination of coke oven by direct use of coal

·Direct use of fine ores, do not require agglomeration plant

·Lower investment cost as a result of above factors

·Lower operating costs due to cheaper raw materials and lack of preparation plants

·Lower level of emissions

·Higher specific productivity

·Increased flexibility in production and operation

·Improved controllability

·Possibility for direct alloying of steel melts

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Table 3. Smelting Reduction Processes Under Development Throughout

the World

2.2Principles of Smelting Reduction

Smelting reduction in iron-bath reactors consists of two major steps, pre-reduction of the iron ore in a fluidized bed reactor or prereduction reactor (PRR) and smelting reduction in a smelting reduction furnace (SRF), see Fig. 1. A brief summary of the components and chemical reactions in the SRF are given in Fig. 2. The coarse coal particles are charged directly into the furnace, while lump and fine iron ore are injected into the iron-carbon slag layer or simply added to the top of the vessel at an approximate temperature of 800°C after pre-reduction by the off-gas. The iron ore is reduced at temperatures between 1400–1600°C. The gaseous products from the combustion of coal and reduction of iron oxide contain large amounts of carbon monoxide and hydrogen. A layer of slag on the liquid iron bath protects the liquid iron product from reoxidation. The predominant components of the slag layer are lime, silica, and iron oxide. On the surface of the slag and in the upper portion of the furnace, the burning of the carbon monoxide and hydrogen with oxygen and/or air injected through a top penetrating lance is called post-combustion. The

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2.3Post-Combustion and Heat Transfer in SRF

The energy required in smelting reduction in iron-bath reactors for heating the bath, for melting scrap material, for reducing iron oxide, and for gasifying coal is supplied primarily by the combustion of carbon or coal to carbon monoxide (Zhang and Oeters, 1991a). The total energy available in the coal is not entirely utilized because of incomplete combustion of the coal. Therefore, to improve the energy efficiency in the furnace, carbon monoxide and hydrogen produced from the combustion of coal in the slag layer undergo further combustion to carbon dioxide and steam in the upper portion of the furnace providing additional heat that can be transferred back into the slag layer and iron-bath. A measure of the post-combustion is the post-combustion ratio (PCR), which is one of the important process parameters in smelting reduction processes, defined by:

%CO + % CO2 + % H2 + %H2 O

Higher post-combustion ratios imply that more heat is generated which can be utilized in carrying out the smelting operations. Typical postcombustion ratios are in the range of 40–60% (Brotzmann, 1989; Ibaraki et al., 1990; Romenets, 1990; Takahashi et al., 1992; Katayama et al., 1993b). As in any process, however, not all the heat generated by the postcombustion reactions is transferred back to the bath; thus a second parameter known as the heat transfer efficiency (HTE) is defined as the portion of the heat generated by post-combustion transferred to the bath.

With this definition, a heat transfer efficiency of 100% implies that the temperature of the off-gas will be the same as the temperature of the bath. The HTE reported in the literature are in the 80–90% range (Ibaraki et al., 1990; Takahashi et al., 1992; Katayama et al., 1993b). Several authors (i.e., Gou et al., 1993, and Gudenau et al., 1993) have indicated that this definition has limitations because the heat losses to the furnace walls

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(refractories) and the top penetrating gas lance are not taken into account. The losses are accounted for in the HTE definition proposed by Gudenau et al. (1993) given in Eq. (3).

Several studies have investigated the relationship between PCR and HTE with the results, in general, demonstrating an inverse relationship (Zhang and Oeters, 1991b; Farrand et al., 1992; Hirata et al., 1992; Katayama et al., 1992; Gudenau et al., 1993). Ideally, high levels of PCR coupled with high levels of HTE are desirable; however, to achieve high levels of PCR, the amount of oxygen fed to the reactor must increase. This can lead to reoxidation of the iron droplets in the slag layer and may increase the off-gas temperatures to the point of damaging the refractory material of the furnace. The increase in the off-gas temperature can be minimized with an increase in the heat transfer to the slag layer; however, the mechanisms of heat transfer may limit the degree of post-combustion. The factors affecting the relationship between PCR and HTE are numerous and complicated (including amount and contents of coal); further studies in theoretical analysis and experimental evaluation are required.

The main aspects of heat transfer in a SRF are the heat transfer from the post-combustion gas to the slag layer and from the slag layer to the iron bath. In the mixing process of the slag layer, slag and iron particles are ejected into the gas space where they acquire heat from the post-combus- tion gas. The particles, after a short time in the gas space, fall back into the slag layer with the iron contained in the particles melting due to a rise in the temperature of the particles, and accumulating in the molten iron bath. The heat is transferred in this way from the post-combustion gases in the upper portion of the reactor to the liquid iron bath at the lower portion of the reactor. Heat transfer from the gas phase to particulate phase consists of convective and radiative transfer. Oeters et al. (1994) stated that, for conditions where the post-combustion took place by the top blown gas penetrating the slag layer, the main resistance to heat transfer was between the gas phase and slag, and that the radiative transfer between the gas and slag was the most important heat transfer mechanism. Katayama et al. (1992) reported that under actual operating conditions, 30% of the heat