Conservation of Energy and Heat Balances

Energy can appear in many forms. Some of the common forms are enthalpy, electrical energy, chemical energy (in terms of AH reaction), kinetic energy, potential energy, work, and heat inflow. 

In making a heat balance at steady state we use methods similar to those used in making a material balance. The energy or heat coming into a process in the inlet materials plus any net energy added to the process is equal to the energy leaving in the materials. Expressed mathematically. 

EXAMPLE 1.7-1. Heating of Fermentation Medium A liquid fermentation medium at 30°C is pumped at a rate of 2000 kg/h through a heater, where it is heated to 70°C under pressure. The waste heat water used to heat this medium enters at 95°C and leaves at 85°C. The average heat capacity of the fermentation medium is 4.06 kJ/kg- K, and that for water is 4.21 kJ/kg K (Appendix A.2). The fermentation stream and the wastewater stream are separated by a metal surface through which heat is transferred and do not physically mix with each other. Make a complete heat, balance on the system. Calculate the water flow and the amount of heat added to the fermentation medium assuming no heat losses. The process flow is given in Fig. 1.7-1. 

Solution It is convenient to use the standard reference state of 298 K (25°C) as the datum to calculate the various enthalpies. From Eq. (1.7-1) the input items are as follows. 

The amount of heat added to the fermentation medium is simply the difference of the outlet and inlet liquid enthalpies. 

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CONSERVATION OF ENERGY AND HEAT BALANCES 1.7A Conservation of Energy... 

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