Heat Exchange of Tubular Surfaces in a Bubbling Fluidized Bed

By Oleksandr Redko and Andriy Redko

Heat Exchange of Tubular Surfaces in a Bubbling Boiling Bed bridges the gap surrounding the study of a boiling bed of large particles with smooth and ribbed pipes, as well as pipe bundles. The book's authors combine results from experimental studies with their varied practical experience in fields of boiling bed applications across various disciplines such as chemical, pharmacological, metallurgical and power engineering industries. This book provides readers with a deep practical understanding of how to calculate the heat engineering parameters of ribbed pipe bundles in a boiling bed, along with the hydrodynamics of the boiling bed.Researchers and experts involved in the design, development and operation of boiling bed apparatus will follow step-by-step methods and procedures to gain knowledge of the hydrodynamic and heat exchange elements of the boiling bed which can be applied to their own settings. The effect of gas velocity, size and properties of the dispersed material, the geometric characteristics of the pipe bundle is also presented, alongside data on the effect of high temperature and high pressure of gas in a dispersed system on heat exchange intensity.

• Covers the design of apparatus and devices with a boiling bed in various industries
• Includes criteria equations for calculating heat exchange, as well as data for the calculation of furnace devices
• Presents the structure analysis of the boiling bed with submerged pipe bundles and the calculation of the hydrodynamic resistance of the boiling bed

• Language: English
• Publisher: Academic Press
• Release date: Aug 12, 2023
• ISBN: 9780128232811

Oleksandr Redko

Redko Oleksandr is Doctor of technical sciences and Professor, department of Heat, gas supply, ventilation and using WHR at the Kharkiv National University of Construction and Architecture. He defended his Dr. S thesis on the problem of intensification of heat processes in a boiling bed and is currently researching processes of low-grade fuel combustion in a boiling bed.

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Heat Exchange of Tubular Surfaces in a Bubbling Fluidized Bed

Oleksandr Redko

Department of Heat Gas Supply, Ventilation and Use of Thermal Secondary Resources, Kharkiv National University of Civil Engineering and Architecture, Kharkiv, Ukraine

Andriy Redko

Department of Heat Gas Supply, Ventilation and Use of Thermal Secondary Resources, Kharkiv National University of Civil Engineering and Architecture, Kharkiv, Ukraine

Table of Contents

Cover image

Title page

Copyright

Dedication

Preface

Acknowledgment

Chapter 1. Fluidization: general characteristics

1.1. Solid particle properties

1.2. Particle size distributions

1.3. Fluidization regime

1.4. Minimum fluidization, BFB limits

1.5. Bed expansion: porosity and layer height

1.6. Hydrodynamics of a polydisperse two-component BFB

1.7. Mathematical modeling of the BFB hydrodynamics

Nomenclature

Chapter 2. Hydrodynamic characteristics of the flow around dispersed medium bodies

2.1. Structural and gas-dynamic conditions at the surface of a body located in a fixed bed, which is the prehistory of a fluidized state

2.2. Flat surface flow pattern by a heterogeneous fluidized bed

2.3. Geometrical parameters of the gas cavity and the boundary zone arising near the plate

2.4. Gas-dynamic characteristics of the gas cavity and the boundary zone

2.5. Peculiarities of the flow around a ball, cylinder, and bodies of another surface shape

2.6. Hydrodynamics of a fluidized bed in the interpipe space of staggered arrangement and in-line of pipe bundles

2.7. Analysis of the causes of gas cavity and boundary zone formation when flowing bodies with the fluidized medium

Nomenclature

Chapter 3. Void fraction, gas speed in wall layer, and external heat exchange

3.1. Void fraction and gas velocity in the wall layer

3.2. Instantaneous local rate of external heat exchange

3.3. Average intensity of the heat transfer process from the body immersed in the fluidized bed

3.4. Methods for intensifying the process of external heat exchange in a fluidized bed

Nomenclature

Chapter 4. Heat exchange of fined pipes and their bundles immersed in a fluidized bed

4.1. Techniques and experimental facilities

4.2. Characteristics of the geometric parameters of fined pipes and used dispersed materials

4.3. Geometrical parameters of fined pipes

4.4. Comparison of total and local thermal modeling methods

4.5. Heat exchange of single fined pipes horizontally placed in a fluidized bed of large particles

4.6. Effect of thermal conductivity of fluidizing gas and fin material on heat transfer

4.7. Comparison of heat exchange efficiency of different types of pipe fining in the fluidized bed

4.8. Heat transfer, aerodynamic resistance, and erosional wear of fined pipe bundles in a fluidized bed of large particles

4.9. Fined pipe bundle efficiency

4.10. Heat transfer of smooth and fined pipe bundles in a high-temperature fluidized bed

4.11. Components of complex heat exchange in a coarse high-temperature fluidized bed

4.12. Local heat exchange of fined pipes in a fluidized bed

4.13. Heat transfer of fined pipes in a fluidized bed under pressure

4.14. Heat transfer of fined pipes in a fluidized bed under vibration

4.15. Heat transfer of fined pipes in a pulsating fluidized bed

4.16. Heat transfer of fined pipes in a wetted fluidized bed

Nomenclature

Chapter 5. Combustion of solid fuel in boilers with fluidized bed

5.1. Boiler diagrams with fluidized bed

5.2. Hot water boiler with a fined heating surface immersed in a fluidized bed

5.3. Combustion of low-grade solid fuels, coal enrichment waste in fluidized bed furnaces, and water–coal suspensions

5.4. Combustion of solid fuels in the pulsating FB

5.5. Combustion of biomass and wood waste in furnaces with a boiling bed

5.6. Simulation of solid fuel combustion processes in a vortex furnace

Nomenclature

Index

Copyright

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Dedication

For my wife Nataliia, my son Hryhorii, and my daughter Elisabeth

Andriy Redko

For my wife Rayisa

Oleksandr Redko

Preface

The authors have been working on the study of heat and mass transfer processes in the bubble fluidized bed (BFB), their intensification in the combustion of biomass and solid fuel waste in boilers, and the thermal treatment of building materials for many years.

Experimental research results were obtained more than 30 years ago. They are published either in conference proceedings which are not available in the open literature or in hard-to-reach publications.

The results of experimental studies are published in Ukraine in the following monographs: O. Redko. Heat transfer in furnaces with a fluidized bed. Kiev (1990); V. Korolev, O. Redko, A. Khudenko. Intensification of external heat transfer in a fluidized bed. Kharkov (2002); A. Redko. Heat and mass transfer characteristics of the process of hydrothermal treatment of gypsum in a fluidized bed under pressure. Makeevka (2005).

Modern literature has insufficiently published research data on the problem of intensification of external heat exchange in the BFB, which served as the motivation for the authors to write this book.

Chapter 1 focuses on the basic problems of the hydrodynamics of a free BFB and the mechanism of interaction between gas and solid particles. The basic fundamental correlations for the BFB are given and the boundaries of the existence of fluidization modes are determined. Particular attention is paid to the hydrodynamics of polydisperse BFBs. A special attention is paid to the hydrodynamics of polydisperse BFB.

Chapter 2 is aimed at analyzing the BFB structure near submerged heat exchange surfaces. The mechanism of heat transfer and modern research methods are analyzed. A comparison of engineering methods for calculating the efficiency of heat exchange and various models is made. The methods of intensification of external heat exchange in the BFB are analyzed. The features of heat transfer of large particles in the BFB during biomass burning are indicated. Comparison of fined surfaces of pipes of various types and their thermal efficiency is performed.

Chapter 3 provides methods for modeling the heat transfer of surfaces in the BFB and scaling issues. Data on the geometrical parameters of fined pipes and BFB materials as well as descriptions of experimental setups are presented.

Chapter 4 presents the results of an experimental study of the heat exchange of fined pipes with different fins. Correlations generalizing experimental data are given, a comparison of the heat exchange efficiency of various types of fins is made, and the influence of the thermophysical properties of gas and the material of fins is shown as well. Experimental data on the heat exchange of pipe bundles and the hydraulic resistance of large particles in the BFB are presented. An assessment is made and erosion wear of BFB fined pipe bundles is predicted. The characteristics of fined surfaces are given.

The results of the experimental study of the heat exchange of fined and smooth pipes at high temperatures in the BFB and high pressure are highlighted. The components of complex heat exchange in the coarse-grained BFB at high temperatures are estimated. Experimental data on the heat transfer of fined pipes under the conditions of pipe vibration and BFB pulsation are presented.

Chapter 5 presents the results of heat exchange of pipe bundles in the BFB when burning solid fuels—coal and coal waste. The effect of heat removal from the BFB on the environmental characteristics of boilers is shown. The results of the experimental studies of combustion of various fuels and fuel waste in the BFB are presented. Also, particular attention is paid to coal wastes, coal–water suspensions, and wood wastes. The results of the numerical study of biomass-peat combustion processes in a vortex furnace of oncoming flows of an air–fuel mixture and secondary air are presented. The possibility of using vortex combustion technology in the thermal circuits of boilers with a circulating fluidized bed is shown as well.

The results of computational fluid dynamics modeling in a vortex furnace with counterflows of peat, coal, and wood waste combustion are presented. Moreover, the thermal diagram of a circulating BFB with a vortex furnace, as well as a diagram of a cogeneration power plant, is shown.

Acknowledgment

The authors express their gratitude to several people from Elsevier for their encouragement and support from the beginning of this project, including Fahmida Sultana and the production team.

The authors thank their translator, Ms. Nina Tychina, who turned the Ukrainian text into English, and To Yuriy Burda, Ph.D., for his assistance in formatting the manuscript.

Chapter 1: Fluidization

general characteristics

Abstract

In chemical, petrochemical, and power engineering, high-tech equipment with a dispersed heat carrier is being created. In the petrochemical and oil refining industries, technological equipment with finely dispersed catalytic and adsorption facilities is used. The modern understanding of complex computational schemes for multicomponent multiphase flows of gas–solid particle systems in combination with chemical reactions, catalytic and adsorption processes is still insufficiently complete. Over the past 30 years, a large number of experimental and model studies have been carried out using laboratory and industrial installations. Various models based on the fundamental mathematical principles of fluid mechanics can provide answers to questions that arise during operation and design. However, such models are not readily available for engineering practice due to their complexity and computational problems, while they contain many parameters that require experimental identification. In recent years, significant progress has been made in this direction. In engineering practice, cell models and approximated empirical correlations which determine the hydrodynamic and heat transfer characteristics of BFB systems with sufficient accuracy continue to be used.

Keywords

BFB technology; Bed expansion; Catalyzer; Experimental correlation; Fluidization; Hydrodynamic characteristics; Particle properties; Voidage

In chemical, petrochemical, and power engineering, high-tech equipment with a dispersed heat carrier is being created. In the petrochemical and oil refining industries, technological equipment with finely dispersed catalytic and adsorption facilities is used [1–19].

The processes of drying and burning a dispersed material, heat treatment of parts in mechanical engineering, the processes of combustion, gasification, and pyrolysis of solid fuels are largely determined by the aerodynamic characteristics and organizational structure of the monodispersed layer material [20–30].

Hydrodynamic processes of two-phase flows in polydispersed BFB systems are described by a system of differential equations, which is determined by the structure of the dispersed system and the mechanism of heat and mass exchange, gasification, and fuel combustion.

The modern understanding of complex computational schemes for multicomponent multiphase flows of gas–solid particle systems in combination with chemical reactions, catalytic and adsorption processes is still insufficiently complete.

1.1. Solid particle properties

The properties of solid particles determine the hydrodynamic characteristics of fluidization beds.

The main characteristics of solid particles are geometric (size, size distribution, and shape of particles) and physical (density and density distribution for polydisperse fluidized beds, equivalent diameters) ones.

1.1.1. Particle size

Equivalent diameters of a nonspherical particle include a sieve diameter. The equivalent diameter is determined by Martin's, Feret's, and Souter's diameters.

Sieve diameter of a particle is defined as the width of the minimum square aperture that the particle can pass through. There are two widely used sieve standards, i.e., Tyler standard and ASTM (US). A detailed analysis on the particle size can be found in Refs. [4,21,31].

Souter [32] developed a method to measure particle size as the diameter of a sphere that has the same volume to surface area ratio as the given particle:

(1.1)

The sander mean diameter is defined in terms of the surface diameter.

The volume diameter is defined as the diameter of a sphere having the same volume as the given particle:

(1.2)

where and are the external surface and volume of the particle.

Different definitions of the equivalent particle diameter (Table 1.1).

1.1.2. Particle shape

Shape factor is a useful quantitative parameter. The sphericity of a particle is defined as the ratio of the surface area of an equal-volume sphere to the actual area of the particle [33]:

(1.3)

where and are the volume and surface area of the particle.

It was shown in Ref. [37] that for widely different loose materials, the following holds:

The coefficient for a cylinder with different ratio h⁄d (from 0.66 to 30) varies from 0.220 to 0.860 [21].

The value of for some substances is given in Ref. [19]. The shape factor value ranges from 0.3 to 0.73 [32] as shown in Fig. 1.1.

Figure 1.1  Published data and correlation of the effect of sphericity on voidage [143].

Table 1.1

In Ref. [141], the dependence between voidage and sphericity is proposed:

(1.3a)

at .

Another equation was proposed in Ref. [142]:

(1.3b)

1.1.3. Hydrodynamic diameter (Stokes diameter)

The hydrodynamic diameter (Stokes diameter) is thus defined as the equivalent diameter of a sphere with the same density and terminal velocity as the particle in a fluid of the same density and viscosity [4,21,31].

Dynamic (Stokes) diameter of a particle according to Refs. [26,48,49] is:

(1.4)

where is an equivalent dynamic diameter.

1.1.4. Particle density, bulk density, porosity, and void fraction

The density of the particles is the ratio of particle mass to its actual volume, excluding internal pores. The particle density is normally less than its material density.

The bulk density is the ratio of the solid mass to the bulk volume, which includes the void of the packing ( ). Bulk density is less than particle density.

The porosity of a particle describes the fraction of void space in the material, where the void may contain any fluid. It is defined by the ratio:

(1.5)

where is the volume of void space; is the total or bulk volume of material, including the solid and void components. Porosity is a fraction between 0 and 1.

Bubbling fluidized bed (BFB) porosity (voidage) can be determined as a fraction of the volume of voids among the particles in the bed volume by the following function:

(1.6)

where is particle density; is bulk density .

Void fraction of the fluidized bed (ε) is expressed as the ratio between the volume of the particles and the volume of the bed.

1.2. Particle size distributions

In polydisperse BFBs, particles can differ in particle diameter, their surface area, and shape as well as physical and chemical properties [21,26,27,52,60,61].

A particle size density function can be defined based on either the number or the mass of the particles within a given size range.

Three typical distributions in particle size, namely normal distribution (Gaussian distribution), log-normal distribution, and Rosin–Rammler distribution are widely used [21,28].

Normal (Gaussian) distribution is a probability density function that has the following form:

(1.7)

where A is the normalizing constant; is the root-mean-square deviation; is the arithmetic mean diameter:

(1.8)

is a standard deviation:

(1.9)

The cumulative distribution function of the particle size of   is expressed as:

(1.10)

1.2.1. Log-normal distribution

The density function of the log-normal distribution can be determined by the function [34]:

(1.11)

where is a geometric standard deviation; is a geometric mean diameter:

(1.12)

or

(1.13)

1.2.2. Rosin–Rammler distribution

The density function of a Rosin–Rammler distribution [35] is:

(1.14)

where and are constants.

The Rosin–Rammler distribution can be presented in the following form:

(1.15)

or

(1.16)

When calculating polydisperse BFBs, the specific surface area of particles is used instead of the equivalent diameter [59,63,74–76].

1.3. Fluidization regime

The following hydraulic modes are distinguished in the solid–gas dispersed system: a fixed bed, a bubbling bed, a turbulent bed, fast fluidization, and pneumatic transport.

Particles referred to group A and partially to group B are subject to the fluidization regime according to Geldart's classification [2,4,11,12,29–31] (Fig. 1.2).

Geldart [29] was the first to comprehensively study the behavior of different materials in the course of fluidization and suggest a classification of particulate solids according to the density and size of particles. Geldart's classification (Fig. 1.2) divides solid particles into three groups [26,29,30]:

1. Group A comprises materials with particles of small mean size and low density (ρp<1400 kg/m³). During fluidization of these materials, homogenous fluidization can be attained with substantial bed expansion before the occurrence of bubbles. Bubble rise velocity exceeds the interstitial gas velocity in the emulsion phase. A maximum bubble size, however, does appear to exist.

Figure 1.2  Geldart's classification for solid particles with gas fluidization [ 8,29] (A is an aeration group; B is a sandy group; C is a cohesive (nonfluidized) group; D is a gushing group).

2. Group B includes numerous materials with particles of medium size and medium density. Ordinary river or sea sand is a typical representative of the group. Bubbles occur immediately after the minimum fluidization velocity has been reached. The bubble rising velocity is greater than the interstitial gas velocity in the emulsion phase. There is no evidence of a maximum bubble size.

3. Group C includes highly cohesive, fine powders which do not fluidize easily. They are prone to bed channeling, and group D comprises loose materials with extremely dense and coarse particles. Their important feature is that the bubbles rise slowly, much more slowly than the interstitial gas velocity in the emulsion phase.

As the gas velocity increases through the dispersed material layer, several successively changing modes are observed (Fig. 1.3): dense layer (I), homogeneous fluidization (II), bubble fluidization (III), turbulent layer (IV), and fast fluidized bed (V). The shaded area (Fig. 1.3) is realized in installations without continuous loading of a dispersed material [94]. The transition from the dense layer (I) to homogeneous fluidization (II) occurs at a certain speed, called the critical one . A homogeneous layer is meant by a fluidized bed with a uniform distribution of particles along the height and a sharply defined upper boundary. Theoretically, particles in such a layer can be considered to be immobile, assuming that the medium is filtered between them with a constant speed in time and coordinates.

In the work [130], J. Yerushalmi and N.T. Kankurt relate the transition from a bubble fluidization to a turbulent one with both dynamic pressure fluctuations at any layer point and pressure fluctuations along the height of the layer. At the velocity of the fluidizing agent U c , the pressure fluctuations reach a maximum value and this means the onset of a bubble (bubbling) fluidization. At a speed of U k , these oscillations begin to decrease, and this means the onset of a turbulent fluidization. In Ref. [130], the transition rates U c and U k are determined depending on the parameter , which is chosen arbitrarily but included in the Geldart classification [120], because it defines two of the most important properties of particles, namely their density and size.

Figure 1.3  Diagram of fluidization modes [128].

Fine particles, 7.24 <  < 26.6, are studied in Ref. [130]. The maximum weighted-average diameter is for quartz sand (d p = 268 μm), and the minimum one is for dicalite 4200 (d p = 33 μm).

Fig. 1.4 shows the dependences of U c and U k on , which are close to straight lines.

Fig. 1.5 shows the ratios between the velocities U c and U k and the velocity of particle « floating» (U fl , ε = 1) in the function of the parameter . The plots of U c /U fl and U k /U fl in the function of are very indicative. These ratios approach 10 or more for very small particles and decrease sharply with increasing particle size and density [131]. The authors [130,132] indicate that the transition rates U c and U k decrease with a decrease in the layer size.

The turbulent fluidization exists depending on the change in the velocity of the fluidizing agent U k to the transport velocity U tr . The transport velocity U tr can be considered as the boundary dividing the vertical movement of the gas–solid particles system into two groups of states. When a fluidizing agent velocity is less than the transport velocity U tr , a turbulent and bubble fluidization regime is observed, in which the bed has motions relative to the apparatus walls. When the velocity of the fluidizing agent is greater than the transport velocity U tr , the layer transport mode is observed. A fast fluidized bed is a two-component heterogeneous gas–fine solid particles system, characterized by a sufficiently high concentration of solid particles, a significant change in the concentration of particles along the height of the apparatus, aggregation of particles, backward mixing of a solid, and a sliding velocity value that is an order of magnitude higher than the hovering speed of individual particles.