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1.5 Application Examples

Mass transport processes are ubiquitous in nature and engineering practice. One can cite numerous examples, which by themselves will fill an entire book. This section presents a few examples and includes the questions that an engineer may ask in an attempt to understand or design these processes. As the content of this book unfolds, we will be in a position to answer these questions and perhaps raise yet more questions. As you will appreciate from a perusal of these examples, the analysis and modeling of mass transfer processes (the main goal of this book) is widely useful in many fields. You will be well trained to tackle problems in your chosen engineering profession and be in a good position to do research and advance this area upon completing your study of this book.

1.5.1 Reacting Systems

Heterogeneous reactions are commonly encountered in chemical processing. These systems include two or more phases, with reaction taking place mainly in one of the phases while the reactants are usually present in the other phase. Hence a prerequisite for reaction to occur is mass transfer from one phase to the other. This coupling of mass transfer and reaction has many interesting consequences, which are studied in detail in Part II of the book. A simple classification of heterogeneous reacting systems is based on the type and number of phases contacted, as shown in Table 1.3 (along with one example for each case). Most reaction engineering books include sections on heterogeneous reactions and offer varying degrees of coverage of this area (e.g., Levenspiel, 1974). The extensive (almost encyclopaedic) monograph by Doraiswamy and Sharma (1984) is an useful reference book and deals exclusively with heterogeneous reactions.

Table 1.3 Examples of Heterogeneous Reactions Based on the Phases Being Contacted

Phases Contacted

Example

Gas + solid catalyst

oxidation of ethylene

Gas + solid reactant

combustion of coal

Gas–liquid

removal of CO2 by reactive solvent

Liquid–liquid

biodiesel production

Gas–liquid + solid catalyst

removal of sulfur compounds from diesel

Gas–liquid + solid reactant

carbonation of lime

Gas–liquid–liquid

production of hydroxylamine from nitrobenzene

Two examples of reacting systems where mass transfer plays an important role are shown here. Other examples are taken up in Part II.

Catalytic Converter

A common example of mass transfer accompanied by reaction is found in an automobile. A catalytic converter consists of a set of flow channels coated with an active layer of catalyst such as platinum (Pt). This is an example of a flow system accompanied by mass transfer and chemical reaction to reduce the release of pollutants such as CO and NOx. The extent of pollutant removal depends on the flow rate of the gas, the temperature, the rate of mass transfer to the catalytic surface, and the rate of the reaction at the surface itself. The overall rate of reaction can be calculated by a mass transfer analysis and used to design the converter. The dynamic response of the system and the extent of pollutant removal during the initial cold start period (when the converter is cold) can also be found using these models.

Trickle Bed Reactor

A trickle bed reactor is an example of a three-phase reactor (gas–liquid–solid catalyst). It is similar to the packed column used in the unit operation of absorption of a gas. Here gas and liquid flow over a packed catalytic bed and a reaction occurs on the surface of the catalyst (Figure 1.4). Gas, liquid, and solid catalyst are the three phases present in the reactor. Such reactors are widely used in the chemical and petroleum industry. For example, they are used to remove sulfur compounds from diesel, an application that relies on a reactor with three phases: hydrogen, diesel, and a cobalt–molybdenum (Co-Mo) based catalyst. Today’s urban air is often cleaner thanks to the trickle bed reactor. The book by Ramachandran and Chaudhari (1983) is a good starting source on this subject and covers the various types of three-phase catalytic reactors in detail.

Figure 1.4

Figure 1.4 Schematic of a trickle bed reactor; mass transfer of species in the gas phase (e.g., hydrogen) to the liquid and then to the surface of the catalyst is followed by a surface reaction on the catalyst with a second species diffusing from the liquid. (An operation with upflow of both phases is shown here.)

1.5.2 Unit Operations

Unit operation is defined as a unified study of a particular separation technique used for a chemical engineering application. Distillation is an example in which the vapor pressure differences between two components are used to separate and purify these components. The unit operation approach takes the view that the analysis is same whether you are doing a distillation of a crude oil mixture or making brandy. Common mass transfer–based separation processes are listed in Table 1.4. All of these separations rely on interfacial mass transfer; hence the analysis and modeling of mass transport effects is a prelude to the design of such systems.

Table 1.4 Common Mass Transfer–Based Separation Processes and the Phases Being Contacted

Phase 1

Phase 2

Operation

Vapor

Liquid

Distillation

Gas

Liquid

Absorption

Liquid

Gas

Stripping

Liquid

Liquid

Extraction

Gas

Solid

Adsorption

Liquid

Solid

Adsorption

Wet solid

Gas

Drying

An example of a unit operation is liquid–liquid extraction. The schematic of a simple single-stage extraction unit is shown in Figure 1.5, and we will use this unit as a prototype example for modeling stage contactors. In this unit operation, two phases are intimately mixed in the mixer section of the contactor to create an emulsion or dispersion, which promotes a high rate of mass transfer. The two-phase mixture is then allowed to settle in the settler section, and the enriched solvent and the lean solutions are separated. Determining the degree of mass transfer that can be achieved for a specified rate of agitation is one of the objectives of the mass transfer calculation.

Figure 1.5

Figure 1.5 Schematic of a single-stage mixer-settler used in liquid–liquid extraction.

The degree of separation that can be achieved in a single-stage contactor is usually limited; hence a multistage cascade is used, with the lean solution being treated further with fresh or recycled solvent. A designer may want to know how many stages are needed to achieve a certain level of purity. Modeling of such a multistage cascade is used to provide the answer.

Part III of the book covers the application of mass transfer principles to design some of the various unit operations listed in Table 1.3. The key idea is that the modeling methodology is common to all the unit operations and can be approached in an unified format. Thus the various aspects of individual unit operation, although very important on their own, can be brought together under one umbrella of modeling of mass transfer processes. Seader, Henley, and Roper (2011) offer valuable insights into separation process principles and provide a detailed analysis of the various operations.

1.5.3 Bioseparations

Bioseparation refers to separation of products produced by biochemical reactions; the separation of products from a fermentation broth is an often-cited example. These processes have a number of distinguishing features compared to the traditional separations practiced in the bulk chemical and petroleum industry. The distillation is the main workhorse in the chemical industry, but due to the heat-sensitive nature of bioproducts many alternative separation methods are needed. Rapid extraction may also be a requirement since the product may degrade or react further (e.g., in penicillin separation). In many cases the compounds may be present in low concentrations. The target compound to be separated may have similar properties to the other compounds in the broth, so that novel separation tools are needed. Common techniques used are extraction, adsorption, chromatography, and electrophoresis, and the use of mass transfer analysis for modeling these cases is illustrated in later chapters.

The book by Harrison, Todd, Rudge, and Petrides (2003) is a good introduction to this field. The book by Seader et al. (2011) has also considerable information on this topic. A review article published by Harrison (2014) provides an introductory reading, with the author stressing the importance of understanding the basic principles and theory as a prelude to design and control of purity of products obtained in bioprocessing.

1.5.4 Semiconductor and Solar Devices

The heart of the computer you use is made of a silicon chip, but the electronic activity arises due to the fact that the chip has undergone a diffusion process to incorporate phosphorus, boron, or other dopants. Semiconductor doped with group V metals are called n-type, while those with group III metals are called p-type. A junction is formed by contacting these two types of semiconductors and acts as a diode or transistor. The electronic behavior in such systems depends on the transport of the electrons from the n to p side and transport of holes from the p to n side, together with recombination. The diffusion-reaction analysis for porous catalysts presented in Chapter 18 can be readily adapted to this system.

A number of processing steps in this field involve chemical vapor deposition, in which a species (precursor) is transported from a vapor and reacts and forms a deposit or a film on a (substrate) surface. This process again involves mass transport of reactants, with control of the deposited material’s properties being affected by the rate of transport. Oxidation of silicon to form an insulating layer is another example of mass transfer in fabrication of metal oxide semiconductor (MOS) devices. The book by Middleman and Hochberg (1993) is a classic in this area and a must-read for students who wish to get involved in this field.

1.5.5 Biomedical Applications

The focus of mass transfer analysis in the field of biomedical engineering is to bring together fundamentals of transport models and life sciences principles. Key areas where mass transport phenomena can be utilized include the following:

  • Pharmacokinetics analysis, distribution, and metabolism of drugs in the body

  • Understanding of transport of oxygen in the lungs and tissues

  • Tissue engineering, including development of artificial organs

  • Design of assistive devices such as dialysis units

Some application examples are briefly described in this book in Chapter 23. An early book by Lightfoot (1974) and the more recent books by Sharma (2010); Truskey, Yuan, and Katz (2004); and Fournier (2011) are illustrative of the mass transport applications in this field.

1.5.6 Application to Metallurgy and Metal Winning

Transport phenomena analysis and models are widely used in metallurgy and metal winning. Books by Szekely and Themelis (1991) and by Geiger and Poirier (1998) provide a number of applications in this field. Ore smelting in a blast furnace, gas–solid reactions in steel and copper making, and alloy formation by melt drop solidification are examples of applications in which mass transport principles are needed. Electrochemical processes are also used for metal winning (e.g., copper), in which transport of copper ions to the cathode is an important step in the overall process.

1.5.7 Product Development and Product Engineering

Transport phenomena are increasingly being exploited in product development. Example applications include the design of drug capsules that should provide a constant release rate and the design of polymer wrapping in food packaging to reduce oxygen diffusion. The book by Cussler and Moggridge (2001) is an useful resource for further reading in this field.

An example of drug release from a capsule is shown in Figure 1.6. In this case, if the drug has a uniform concentration inside the capsule, the release rate reaches its maximum in the beginning and decreases with time. Ideally we want a (nearly) constant rate, like that shown in the figure. The design of the pore structure of the capsule to achieve this type of steady release rate is an important application of transient mass diffusion principles.

Figure 1.6

Figure 1.6 A product design application example. The drug release rate is shown for a drug with both a uniform distribution and a tailored capsule.

1.5.8 Electrochemical Processes

Electrochemical processes have a wide range of applications, including batteries, solar cells, electro-deposition, thin films, and microfluidic devices. Transport phenomena principles are increasingly being used to design and improve these kinds of devices. The book by Newman and Thomas-Alyea (2004) is a good treatise on this subject.

In the energy sector, there is a need to store solar energy generated during nonpeak hours and to develop improved batteries for electric cars. A commonly used type of battery is the lithium-ion battery shown in Figure 1.7. Here Li ions are transported across the electrolyte separating cathode and anode. During the charging cycle, Li ions are transported and stored in the carbon matrix. During the discharging cycle, the transport takes place in the opposite direction, such that Li is stored in the metal oxide matrix. Mass transport considerations are an important component in the simulation of the performance of this device; a mass transfer–based model for this system is discussed in Chapter 24.

Figure 1.7

Figure 1.7 Schematic of a lithium-ion battery showing the various mass transfer and reaction steps occurring in the equipment. Lithium ions stored in the carbon “hotels” are released during the discharge cycle and diffuse through the electrolyte to the cathode, where they react with metal oxide matrix. This produces a current in the external circuit.

1.5.9 Environmental Applications

Transport phenomena principles and modeling have found extensive applications in environmental engineering, where they provide a modern perspective and new approach. Typical environmental problems that may addressed using the transport modeling methodology are as follows:

  • Fate and contaminant transport in the atmosphere is usually simulated by dividing the system into four (air, water, soil, and biota) or more compartments and considering transport and reaction in each of the compartment. See, for example, Figure 1.14.

  • Groundwater transport is another example. Leakage of contaminants from nuclear waste tanks into rivers could be a major problem, and some of these scenarios can be analyzed by transport models to provide information on the rate of leakage and measures needed to alleviate the problem.

  • Transport of excess nutrients to water bodies leads to algae growth and destruction of other organisms, a process known as eutrophication. The rate of transport in such systems is needed to determine further remediation actions.

  • Carbon dioxide sequestration in underground mines is contemplated as a solution to reduce the impact of global warming. Mass transfer analysis is needed to predict the leakage and long-term feasibility of this solution.

The book by Clark (1996) provides a nice introduction to some of the problems mentioned here.

Next, we discuss the general methodology involved in setting up models for mass transfer processes.

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