ChE 206 chapter 7 - CHAPTER 7 ENERGY AND ENERGY BALANCE In...

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CHAPTER 7 ENERGY AND ENERGY BALANCE
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In the past the cost of energy was often insignificant part of the overall process cost, and gross operational inefficiencies were tolerated. In recent years, however, a dramatic decrease in the availability of natural gas and petroleum has raised the cost of energy severalfold and has intensified the need to eliminated unnecessary energy consumption. As an engineer designing a process, one of the principal jobs would therefore to be account carefully for the energy that flows into and out of each process unit and to determine the overall energy requirement for the process . You would do this by writing energy balances on the process, in much the same way that you write material balances to account for the mass flows to and from the process and its units.
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Typical problems that may be solved using energy balances 1. How much heat is required to convert 2000kg of water at 30 to steam at 180 ? 2. A highly exothermic chemical reaction A B takes place in a continuous reactor. If a 75% conversion of A is to be achieved, at what rate must heat be removed from the reactor to keep the contents at a constant temperature?
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7.1 FORMS OF ENERGY: THE FIRST LAW OF THERMODYNAMICS The total energy of a system has three components: 1. Kinetic energy : Energy due to the translational motion of the system as a whole relative to some frame of reference (usually the earth’s surface) or to rotation of the system about some axis. In this text, we will deal only with translational kinetic energy. 2.Potential energy : Energy due to the position of the system in a potential field (such as a gravitational or electromagnetic field). In this text, we will deal only with gravitational potential field.
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3. Internal energy : All energy possessed by a system other than kinetic and potential energy, such as energy due to the motion of molecules relative to the center of mass of the system, to the rotational and vibrational motion and the electromagnetic interactions of the molecules, and to the motion and interactions of the atomic and subatomic constituents of the molecules.
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Suppose a process system is closed , meaning that no mass is transferred across its boundaries while the process is taking place. Energy may be transferred between such a system and its surroundings in two ways: 1. Heat : energy that flows as a result of a temperature difference between a system and its surroundings. 2. Work : energy that flows in response to any driving force (other than a temperature difference), such as a force, a torque, or a voltage. The first law of thermodynamics ( the law of conserva- tion of energy ) – the principle that underlies all energy balance calculations which states that energy can neither be created nor destroyed .
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7.2 KINETIC AND POTENTIAL ENERGY The kinetic energy 2 2 2 1 2 1 u m E mu E K K = = The gravitational potential energy gz m E mgz E P P = =
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Example7.2-1 Kinetic Energy Transported by a Flowing Stream Water flows into a process unit through a 2-cm ID pipe at a rate of 2.00m
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ChE 206 chapter 7 - CHAPTER 7 ENERGY AND ENERGY BALANCE In...

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