# 8 - Energy Balances 1 In the past the cost of energy was...

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1 Energy Balances

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2 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.
3 Typical problems that may be solved using energy balances 1. How much heat is required to convert 2000kg of water at 30 o C to steam at 180 o C? 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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4 Thermodynamics 0 - Existence of Temperature 1 - Conservation of Energy 2 - Tendency towards disorder 3 - It is impossible to attain 0 K There are only four laws which govern thermodynamics, and they are usually numbered as:
5 We can do Energy Balances because of the law of Conservation of Energy which states that energy can neither be produced nor destroyed but only transformed Whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred. Accumulation =In – Out Final system energy-initial system energy = Net energy transferred to the system First Law of Thermodynamics

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6 Close and Open Systems Closed system - matter cannot pass between system & surroundings. BATCH PROCESS Open system - matter can pass SEMIBATCH AND CONTINUOUS SYSTEMS surroundings system Open System Energy Matter surroundings system Closed System Energy
7 Since some of the thermodynamic relations that are applicable to closed and open systems are slightly different in form, it is extremely important that we recognize the type of system we are working with before we start analysing it.

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8 Lets analyse the close system first …
9 Accumulation =In – Out Final system energy-initial system energy = Net energy transferred to the system (In-Out) First Law of Thermodynamics The total energy of a system has three components

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10 Potential ( E P =mgz) Energy due to the the motion of the system as a whole relative to some frame of reference (e.g. Earth’s surface) Kinetic ( E K =1/2 mv 2 ) Energy due to the position of the system in a potential field (e.g. gravitionational or electromagnetic field)
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## This note was uploaded on 06/06/2011 for the course CHEM 3040 taught by Professor Reddy during the Spring '10 term at Taylor's.

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8 - Energy Balances 1 In the past the cost of energy was...

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