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Fundamentals of Mass and Energy Balances

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Fundamentals of Energy Balances  As with mass, energy can be considered to be separately conserved in all but nuclear processes. The conservation of energy, however, differs from that of mass in that energy can be generated (or consumed) in a chemical process. Material can change form, new molecular species can be formed by chemical reaction, but the total mass ow into a process unit must be equal to the ow out at the steady state. The same is not true of energy. The total enthalpy of the outlet streams will not equal that of the inlet streams if energy is generated or consumed in the processes; such as that due to heat of reaction. Energy can exist in several forms: heat, mechanical energy, electrical energy, and it is the total energy that is conserved. In process design, energy balances are made to determine the energy requirements of the process: heating, cooling and power required. In plant operation, an energy balance (energy audit) on the plant will show the pattern of energy usage, and suggest areas for conservation and savings. CONSERVATION OF ENERGY This is a statement of the rst law of thermodynamics. An energy balance can be written for any process step. Chemical reaction will evolve energy (exothermic) or consume energy (endothermic). For steady-state processes the accumulation of both mass and energy will be zero. Energy can exist in many forms and this, to  some extent, makes an energy balance more complex than a material balance.
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