Description
LEARNING OBJECTIVES
This chapter is intended to help you learn how to:
– Define a system that has clearly understood boundaries
– Define a process that has a clear beginning and end
– Identify systems and processes that fall into specific categories, such as open, closed, isothermal, and adiabatic
– Define equilibrium and steady state, including how they are distinct from each other Recognize systems and processes that are at equilibrium, at a steady state, or both Recognize the forms in which energy can be stored by matter: internal energy, kinetic energy, and potential energy
– Recognize the forms in which energy can be transferred to or from matter: work and heat
– Quantify force, pressure, temperature, work, kinetic energy, and potential energy
Thermodynamics is the study of energy, including the conversion of energy from one form into another and the effects that adding or removing energy have on a system. Thermodynamics is essential for the practice of chemical engineering. The principles of thermodynamics have a fundamental role in how chemical processes are understood, analyzed, and designed. This book is intended for readers who are being introduced to this crucial subject for the first time.
The first chapter gives an overview of how and why thermodynamics is important and introduces some fundamental concepts. In particular, two abilities that are foundational in chemical engineering thermodynamics are
1. Recognizing the forms in which energy can be stored and transferred.
2. Identifying and analyzing systems.
Every chapter of this book opens with a list of objectives like the one above. Each chapter also features a “motivational example,” where an examination of an engineering ap-plication underscores the significance of the topics presented. Normally, the motivational example immediately follows the chapter instructional objectives, but in this chapter, the motivational example is in Section 1. 2. First, we briefly explore why the field of thermody-namics as a whole is essential to the practice of chemical engineering.
The Role of Thermodynamics in Chemical Engineering
The number and variety of chemical products is staggering. Walking through the aisles of a drug store or a supermarket, you will see hundreds of different products, where chemical engineers played some role in the production of almost all of them. Consider these examples:
The 2011–2013 Alfa Aesar® catalog contains a 2210-page alphabetical list of the company’s chemical products, from Abietic Acid to Zirconium(IV) 1,1,1-trifluoro-2,4-pentanedionate.
The 2010 Physicians’ Desk Reference lists over 2400 prescription drugs sold in the United States.
In a trip to a local supermarket, one of the authors counted over 200 different household cleaning products, over 200 different hair care products, over 50 kinds of toothpaste, and over 40 different insect repellents—not to mention the multitude of processed food products.
Many of these household products contain 10 or more ingredients, each of which can itself be considered a chemical product.
These various products are manufactured in factories and chemical plants from a wide variety of raw materials. Chemical engineers are responsible for designing processes to manufacture them efficiently, economically, reliably, and (in particular) safely.
Chemical engineering plays an important role in the manufacture of products in a wide range of industries. Even across all of these different kinds of applications, all chemical manufacturing processes basically follow a universal rule of thumb: Chemical reactions convert raw materials (chemicals) into desired products (other chemicals). From a business standpoint, the product must have a greater monetary value than the raw materials being consumed (see Figure 1-1).
As illustrated in Figure 1-1, chemical reactions almost never lead to products that are pure. The mixture leaving a reactor normally contains not only the desired prod-uct but also by-products and/or unused raw materials. Consequently, some chemical and/or physical process is needed to separate the substances that leave the reactor.
But what do the boxes labeled “Chemical Reactions” and “Separation Processes” in Figure 1-1 actually represent? The specific processes depend on the answers to ques-tions such as
How many different chemical reactions are necessary?
Can all reactions be carried out in a single reactor, or is a separate piece of equipment needed for each reaction?
Figure 1-1 shows pure products leaving the Separation processes. How close to ‘pure’ is required?
How are the separations to be carried out?
Can the separations all be completed with a single piece of equipment, or are several distinct separation steps needed?
At what temperature and pressure does each piece of equipment operate?
Answering questions like these is fundamental to the practice of chemical engi-neering, and the process designer will find that every process presents unique chal-lenges and opportunities. Indeed, some products have more than one plausible route to making them, each of which could have completely different answers to these questions. Thermodynamics plays a vital role in the design of processes. The chemical en-gineer must consider thermodynamic properties when addressing questions such as
How much raw material and energy will it take to make 10 million pounds of this product annually?
What methods can be used to separate this product from any by-products and unused raw materials?
How much energy does it take to heat this process stream to the required tem-perature of 300°F?
How can the reactor conditions be optimized for the maximum production of the desired product while minimizing the production of undesired by-products?
This book introduces and illustrates the principles of chemical engineering thermodynamics by exploring the use of thermodynamics in the solution of engi-neering problems. Although focusing on examples of particular interest to chemical engineers, the text also illustrates the extreme breadth of systems for which thermo-dynamics is applicable.
Consequently, this book emphasizes examples drawn from the design or components of chemical manufacturing processes, while also exa- mining a broad range of engineering systems and problems. The next section con-siders an extremely broad and practical problem in our society: the generation of electricity.