Chapter 2 - Systems, Devices and Processes

Material, Energy, and Entropy Balances

To analyze devices and processes, mass and energy balances are made on devices and processes to account for what is going “in” to the process and what is coming “out” of the process. These balances are applied to ensure the conservation of mass and energy for any device or process: matter can be neither created nor destroyed. A mass balance is also called a material balance.

All processes can be imagined as a collection of systems that cause changes in mass and energy to occur, under the

Analysis of Devices and Processes

There are many types of devices and processes that are used with fluids to carry out a desired task. Six of the most common types of devices are (i) batch reactor or pressurized vessel, (ii) expander or turbine, (iii) compressor, (iv) pump, (v) valve, and (vi) heat exchanger. The material, energy, and entropy balances are applied to each of these devices in the following sections.

Practical Process I: Transcritical CO₂ System for Heating Hot Water

Carbon dioxide can be used as a working fluid in a heat pump [HP1] to make hot water more efficiently than if natural gas or electricity were used. The EcoCute hot water system shown in Figure 2.12 is a good example of a heat pump system [HP2]. A compressor increases the pressure of CO₂ vapor and the pressure increase is accompanied by a temperature increase of the gas. The hot CO₂ exchanges energy with cold water in a heat exchanger to produce hot water. During the heat transfer, CO₂ is

Practical Process II: Flavor Extraction with Supercritical CO₂

Supercritical CO₂ can be used as an extraction solvent to separate many flavors, spices, and aroma from their natural matrices [SFE1]. Some examples of products that have been obtained with supercritical CO₂ extraction are basil, black pepper, black tea, capsaicin, caraway, cassava, cinnamon, clove, coffee, cumin, garlic, ginger, green tea, nutmeg, oregano, paprika, rosemary, saffron, sage, and turmeric [SFE2].

A possible flavor extraction process that uses supercritical CO₂ is shown in Figure

Practical Process III: Fine Particle Formation with Supercritical H₂O

Supercritical water can be used as a reaction solvent to crystallize many types of micron-sized (μm or 10^− 6 m) and nano-sized (nm or 10^− 9 m) fine particles [MS1]. With supercritical water as the reaction solvent, the method of synthesis is fast and continuous. The method produces narrow particle size distributions that are not easily formed by other techniques [[MS2], [MS3], [MS4], [MS5]].

Figure 2.19 shows a simplified configuration for a process to produce fine particles with supercritical

Chapter Summary

Objective 1: to learn how to apply mass, energy, and entropy balances to chemical systems.

Material balance. Mass of a chemical system is conserved. Mass entering the system will equal mass leaving the system for steady-state conditions. The terms in the material balance are those of the system mass and the streams entering and leaving the system. It is important to define the system clearly and to label all streams, that is, 1, 2, 3, etc. For systems at steady-state conditions, the time

End of the Chapter Problems

Energy balance

1. 2.1

   Mixing of aqueous streams. A continuous flow of warm water at 60 °C is needed. A hot water stream at 98 °C and a cold water stream at 20 °C are available for mixing to produce the stream. Pressure is atmospheric and the mixing process may be assumed to be adiabatic. Determine the hot and cold water flow rates to obtain 0.5 kg/s of warm water.

2. 2.2

   Lost work of mixing of aqueous streams. A hot water stream at 98 °C is mixed with a cold water stream at 20 °C in various amounts to obtain a stream that has a

End of the Chapter Projects

Projects are a good way to gain a deeper understanding of the devices and how they interaction with one another. Also, its fun to think of new designs and to try to come up with some possible specifications. It is recommended that projects be done in groups of two students with one engineering student and one multidiscipline or non-engineering student. Projects can also benefit groups by being done in groups of three or four students depending on class size. For the projects, a recommended

Report Format

Title page. This should give the project title and focus, the project leader, the group members’ names and the date of the report.

Abstract. This should contain the following sentences: (i) one sentence of background information; (ii) one sentence stating the project objective in the form of, “The objective of this project was to…”; (iii) three to five sentences of factual findings related to the objective. The sentences should be prioritized by imporance to the design. Technical content can be