Solid/Liquid Separation: Equipment Selection and Process Design

Steve Tarleton, Richard Wakeman

• Publisher: Elsevier Science
• Year: 2006
• Print ISBN: 9781856174213
• eBook ISBN: 9780080467177
• Topic: Technology & Engineering
• Subtopic: Industrial & Technical Chemistry
• Index: Technology & Engineering

In this volume, the third in a set specifically written for the industrial process and chemical engineer, the authors provide the detailed information on filtration equipment and media which allows the reader to then consider the pre-treatment of suspensions, selection of the most appropriate equipment for the task, data analysis and the subsequent design of the processes involved for particular separations. The result is a comprehensive book which is designed to be used frequently and referred to regularly in order to achieve better industrial separations.Successful industrial-scale separation of solids from liquids requires not only a thorough understanding of the principles involved, but also an appreciation of which equipment to use for best effect, and a start-to-finish plan for the various processes involved in the operation. If these factors are all correct, then successful separations should result. - Part of 3-volume set - Unique approach to industrial separations - Internationally-known authors

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Solid/liquid separation equipment

Publisher Summary

This chapter attempts to provide a descriptive overview of the generic equipment types and the main alternatives available to the design engineer. It highlights the advantages and disadvantages of equipment, and provides quantitative values whenever possible. Gravity thickeners and clarifiers represent a class of solid-walled separator, where gravitational forces are used to raise the concentration of a suspension through sedimentation to produce a thickened sludge with a clear liquid as overflow. The rate of sedimentation should be as high as reasonably possible to both increase throughput and reduce floor area. Sedimentation rates are often artificially increased by the addition of coagulants or flocculants. Cyclones designed for use with liquids are referred to as hydrocyclones, hydraulic cyclones, or hydroclones. The basic principle employed to effect either concentration or classification of the solids is centrifugal sedimentation, which is caused by introducing the feed suspension tangentially into the unit; they are particularly attractive for many applications because they have no moving parts. Centrifuges are designed to either thicken or filter suspensions, which leads to two general classes of machine: sedimenting centrifuges and filtering centrifuges.

Over the years manufacturers have developed many generic forms of solid/liquid separator. The need to compete in the marketplace and gain a competitive edge, however, has led manufacturers to develop a plethora of variants. Rather than detailing all of these variants, this chapter attempts to give a descriptive overview of the generic equipment types and the main alternatives available to the design engineer (see Figure 1.1). The advantages and disadvantages of equipment are highlighted and an effort has been made to provide quantitative values whenever possible. More guidance values for the operational parameters of filters are shown in Appendix A, while greater details of recent developments in several equipment types are presented in Wakeman and Tarleton (2005b). In addition to the more specific references given throughout this chapter, the interested reader is also referred to the texts by Dickenson (1997), Kirk-Othmer (1980), Matteson and Orr (1987), Perry and Green (1984), Purchas (1981), Purchas and Wakeman (1986), Rushton et al (1996), Svarovsky (1990), Schweitzer (1997) and Wills (1992).

Figure 1.1 Broad classification chart showing the forms of solid/liquid separator described in this chapter.

General descriptions and typical equipment uses are shown under each main heading in addition to the solids concentration and particle size found in a typical feed. The process ratings used in the Filter Design Software® (FDS) package are described in Chapter 5. The ratings give relative values between 0 and 9 for cake dryness (and state), washing performance, liquid product clarity and crystal breakage, where 9 represents the best performance currently available; a ‘-’ indicates that either a rating is not applicable or the equipment is not capable of performing the operation. For instance, the ‘1 S, 2,5,9’ ratings shown for the circular basin thickener in Section 1.1.1 signify a wet solids discharge in the form of a slurry (‘C’ designates a cake and ‘N’ designates that solids are not recoverable), poor washing performance, near average liquid product clarity and minimal breakage of the solid product. Such representation allows potentially suitable equipment to be numerically ranked. Simulation procedures for many of the filters described are presented in Chapters 6 and 7.

1.1 Gravity thickeners and clarifiers

Gravity thickeners and clarifiers represent a class of solid-walled separator, where gravitational forces are used to raise the concentration of a suspension through sedimentation to produce a thickened sludge with a clear liquid as overflow. The rate of sedimentation should be as high as reasonably possible to both increase throughput and reduce floor area. Sedimentation rates are often artificially increased by the addition of (relatively expensive) coagulants or flocculants. The cross-sectional area plan of a thickener controls the time available for sedimentation and is important in determining clarification capacity. The physical depth of a separator controls sludge thickening time and is an important parameter in determining thickening capacity. Thickeners and clarifiers can be designed to operate in either batch or continuous mode, although most commercial operations utilise the latter. More specific details of design are available in Akers (1974), Dixon (1979), Fitch (1966, 1975), Gough (2005), Hasset (1965), Kos (1974), Kynch (1952), Talmage and Fitch (1955), Wakeman and Tarleton (2005a) and Yoshioka et al (1957).

1.1.1 Circular basin thickener

Typical uses: Larger scale thickening and deliquoring of solids from relatively dilute suspension.

FDS process ratings: 1 S, -, 5, 9 (settling tank or lagoon); 1 S, 2, 5, 9 (thickener).

Typical particle size and feed concentration range: 0.1–500 μm and <20% w/w.

A circular thickener comprises a relatively shallow, open-top cylindrical tank with either a flat bottom or a bottom shaped in the form of an inverted cone (see Figures 1.2 and 1.3). The feed mixture is gently and continuously introduced to the feedwell in which exists a pool of settling suspension along with any additional coagulant or flocculant. With settling and thickening proceeding, clear liquid (the overflow) is removed via an annular weir at the top of the unit and solids sludge (the underflow) is removed from a ‘well’ at the bottom. Slowly rotating arms (or rakes) mounted on a central drive head aid the thickening process by directing thickened solids towards the we...

Table of contents

• Cover image
• Title page
• Table of Contents
• Preface
• Chapter 1: Solid/liquid separation equipment
• Chapter 2: Filter media
• Chapter 3: Pretreatment of suspensions
• Chapter 4: Data acquisition, analysis and scale-up
• Chapter 5: Selection, data analysis and simulation by computer software
• Chapter 6: Process design for batch separations
• Chapter 7: Process design for continuous separations
• Nomenclature
• Bibliography
• Appendix A: Variable ranges for filter cycle calculations
• Appendix B: Correlations for cake washing and gas deliquoring
• Appendix C: Definitions and conversions for concentration
• Appendix D: Troubleshooting filter operation
• Appendix E: Comparisons between experimental data and design equation predictions
• Index