Process Analysis - The Importance of Mass and Energy Balances

Process Analysis - The Importance of Mass and Energy Balances 

In carrying out any analysis, it is important to ensure that all units of measurement used are consistent. For example, mass may be given in kg (kilogrammes), in lb (pounds) or in any other units. If two quantities are given in different units, one quantity must be converted to the same unit as the other quantity. Any book on chemical engineering (or physics and chemistry) will have conversion tables for standard units. 

There are seven fundamental quantities that are typically used to describe chemical processes, mass, length, volume, force, pressure, energy and power, although some of these can be described in terms 

Chemical engineering uses some standard notation for many of the quantities we will encounter in process analysis. These are summarised in Table 1 where T is time, M is mass and L is length. 

In describing processes, the variables that describe the condition of a process fall into two categories  

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When calculating energy balance around a boiler it is important also to consider fuel and air inlet temperatures. In general fuel temperature can be neglected in boiler efficiency analysis, at least it is inside standard conditions. This temperature is more important for the performances of the fuel line and atomization process. The situation is different for inlet air its temperature can have an important impact on boiler efficiency due to the flow mass involved (compare Figure 34.4). In case the inlet air is preheated by exhaust smoke, the boiler efficiency expression doesn t change with respect to Equation 34.1. In case the air is preheated by an external source, sensible enthalpy of air has to be considered in both the denominator and the numerator. 

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