Introduction to Adsorption

Basics, Analysis, and Applications

Chi Tien

• Publisher: Elsevier
• Year: 2018
• Print ISBN: 9780128164464
• eBook ISBN: 9780128175125
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria chimica e biochimica

Introduction to Adsorption: Basics, Analysis, and Applications presents adsorption basics that are relevant and essential to its application, including data analysis, interpretation and design calculations. The book deliberately keeps background information to a minimum, instead comprehensively covering adsorption of liquid solutions, the difference between equilibrium individual solute uptake and surface excess, a general discussion of adsorbate uptake mechanisms and uptake rate expression, uptake steps, performance models and their generalizations, application of performance models, and design methods based on the constant behavior assumption and unused bed length concept.- Includes adsorption basics and their applications- Discusses gas adsorption equilibrium and equilibrium of liquid adsorption- Gives the various steps of adsorbate uptake and their combination to yield adsorbate uptake rate expression- Presents both rational and empirical design for adsorption processes- Highlights common mistakes found in recent adsorption publications

Chapter 1

Introduction

Abstract

Adsorption as a separation process is widely applied in the manufacturing economy and everyday life. Adsorption performance is strongly influenced by mass transfer of the species between the solution and the adsorbent surfaces and the adsorption reaction rate. Therefore, adsorption can be considered an equilibrium-diffusion reaction process. This chapter outlines the basic operating principle of adsorption and then draws a comparison between adsorption and other separation processes. The modes of operation of adsorption processes are described and, finally, different applications of adsorption are presented.

Keywords

Adsorption; Sorption; Absorption; Equilibrium-diffusion reaction; Continuous-flow tank; Fixed-bed adsorption; Moving-bed adsorption

As a separation process, adsorption is widely applied in our manufacturing economy and in our daily life. Adsorption operations exploit certain solids’ ability to preferentially concentrate specific substances from solutions (gaseous or liquid) onto their surfaces. Thus, by contacting fluids with such solids, the desired objective of purification or separation may be achieved.

The extent of adsorption of a given situation is reached once equilibrium is established between the adsorbent and its contacting solution. In practice, adsorption performance is also strongly influenced by the mass transfer of the species between the solution and the adsorbent surfaces and the adsorption reaction rate. Technically, adsorption is, therefore, an equilibrium-diffusion-reaction process.

1.1 Adsorption as a Sorption Process

The basic operating principle of adsorption: the preferential concentration of species onto surfaces of adsorbing solids also operates in two other processes; namely, chromatography and ion exchange. In fact, adsorption, ion exchange, and chromatography are often grouped together under the title of “sorption processes” in engineering textbooks. Similar to most adsorption operations, chromatography operates in fixed-bed mode, but is devised for separating liquid mixtures through an intermittent feed of the solution to be separated, followed by the passage of an elution solution. In ion exchange, the solid substance used contains charged groups that interact with the charged ions present in the liquid solution. If one views adsorption as an exchange process involving a fictitious species, the equivalence between adsorption and ion exchange becomes obvious. In fact, much of the information presented in this volume may be applied to ion exchange as well.

1.2 Comparisons With Other Separation Processes

1.2.1 Adsorption Versus Absorption

Because of their similarity in spelling, the two terms; adsorption and absorption, are often used interchangeably by lay people. However, there are significant differences between them. Gas absorption is an operation in which a gas mixture is brought into contact with a liquid for the purpose of dissolving one or more components of the mixture into the liquid. Absorption, therefore, is a bulk phenomenon, and the extent of separation is limited by the solubilities of the gases involved. In contrast, adsorption is a surface phenomenon, and the extent of adsorption is limited by the relevant adsorption isotherm relationship.

Absorption may be carried out by passing the gas and liquid streams through a packed column concurrently or counter-currently. The operation consists of two moving phases (gas and liquid) and a stationary phase (column packing), which provides the interfacial area for liquid/gas contact. In fixed-bed adsorption, the fluid to be treated passes through a bed packed with adsorbent. The process involves two phases, a moving fluid and a stationary solid phase of adsorbents. Absorption, therefore, may be treated as a steady-state process, while adsorption in a fixed-bed operation is an inherently non-steady state. As a result, the computational effort required for the design of fixed-bed adsorption is more extensive than that of absorption. This point will be discussed later.

A cartoonist's version of the difference between adsorption and absorption is shown in Fig. 1.1.

Fig. 1.1 Difference between absorption and adsorption.

1.2.2 Adsorption Versus Distillation

Distillation, like adsorption and absorption, also belongs to the equilibration-diffusion category of separation processes, and is used for the separation of homogeneous liquid mixtures. However, unlike adsorption or absorption, separation by distillation is effected by using energy instead of material as an agent of separation.

Distillation is perhaps the most widely used separation process in processing engineering and operates on the principle of the difference in volatilities of substances to be separated. In a hypothetical study comparing distillation versus adsorption, Ruthven (1984) showed that for separating an A-B mixture, the use of distillation becomes impractical if the relative volatility of A to B is less than 1.2. To separate light gas mixtures, adsorption was found to be preferential to cryogenic distillation, even when the relative volatility is high.

1.2.3 Adsorption Versus Deep-Bed Filtration

Deep-bed filtration is a process designed for the removal of fine particles from diluted fluid suspensions. Its operation is carried out by passing the suspension to be treated through a column packed with granular or fibrous substances (filter media). Generally speaking, deep-bed filtration and fixed-bed adsorption share many common features, such as equipment configuration and modes of operation. Because of their similarities, the words ‘adsorption’ and ‘filtration’ are often used interchangeably. The removal of submicron colloidal particles from fluid to solid surfaces may be described as either filtration or deposition (Hirtzel and Rajagopalan, 1985). Carbon columns used to remove dissolved organic solutes in water treatment are often referred to as ‘carbon filters’ by water engineers. Similarly, the term ‘charcoal filter’ is used to denote cartridges filled with granular activated carbon for personal protection.

In spite of these similarities, the analogy between deep-bed filtration and fixed-bed adsorption is limited. A major difference between them resides in the fact that in deep-bed filtration, removal of particles from the suspension to be treated results in particle deposition over the exterior surfaces of the filter media. In contrast, the adsorbed dissolved species in fixed-bed adsorption covers mainly the interior surfaces of adsorbents. As stated before, the extent of separation achieved in adsorption is limited by the adsorption equilibrium relationship. On the other hand, particle retention in deep-bed filtration depends strongly upon the nature of particle-collector interaction forces, but there is no clear-cut limit on the extent of deposition (Tien and Ramarao, 2007). As adsorption processes may cease operation once the adsorbents become saturated, for deep-bed filtration, due to increasing particle retention, the pressure drops required for maintaining a specified throughput increase with time. The duration of operation is limited by the maximum allowable pressure drop.

1.3 Operation Modes of Adsorption Processes

Separation by adsorption is effected by contacting solutions to be treated with selected adsorbents. There are numerous ways of bringing about fluid/solid contact, as shown in Fig. 1.2. A brief description of is given as follows:

1. a. Adsorption in agitated vessels. Batch adsorption in agitated vessel represents perhaps the simplest way of bringing about fluid/adsorbent contact. A fixed amount of adsorbent of a known state is added to a volume of solution of a known solute concentration in a closed vessel. Agitation is provided by rotating stirrers in order to insure that adsorbent particles are fully suspended, and the adsorbate concentration is kept uniform throughout the solution. The data collected are the temporal evolution of the solute concentration of the solution. While batch operation is not suitable for treating large volumes of solution (such as water supplies), in general, batch adsorption test data are often used in characterizing new adsorbents for applications.
2. b. Adsorption in continuous-flow tanks. This type of operation is often used in waste water treatment. Adsorbents in powdered form, such as activated carbon, are added directly to a particular step of a treatment process (biological or physio-chemical) for the purpose of removing a particular species of...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• Chapter 1: Introduction
• Chapter 2: Adsorbents
• Chapter 3: Adsorption Equilibrium Relationships, Isotherm Expressions, Their Determinations, and Predictions
• Chapter 4: Adsorbate Uptake and Equations Describing Adsorption Processes
• Chapter 5: Batch Adsorption Models and Model Applications
• Chapter 6: Fixed-Bed Adsorption Models and Fixed-Bed Design Calculations
• Author’s Suggestions on Adopting Introduction to Adsorption in Teaching
• Index