Fundamentals of Material Balances

Material balances are the basis of process design. A material balance taken over the complete process will determine the quantities of raw materials required and products produced. Balances over individual process units set the process stream flows and compositions. 

The application of material balances to more complex problems is discussed in Flow-sheeting , Chapter 4. 

Material balances are also useful tools for the study of plant operation and trouble shooting. They can be used to check performance against design to extend the often limited data available from the plant instrumentation to check instrument calibrations and to locate sources of material loss. 

A good understanding of material balance calculations is essential in process design. In this chapter the fundamentals of the subject are covered, using simple examples to illustrate each topic. Practice is needed to develop expertise in handling what can often become very involved calculations. More examples and a more detailed discussion of the subject can be found in the numerous specialist books written on material and energy balance computations. Several suitable texts are listed in the Bibliography section at the end of this chapter. 

Certain restrictions imposed by nature must be taken into account when designing a new process or analyzing an existing one. You cannot, for example, specify an input to a reactor of 1000 g of lead and an output of 2000 g of lead or gold or anything else. Similarly, if you know that 1500 Ibn of sulfur is contained in the coal burned each day in a power plant boiler, you do not have to analyze the ash and stack gases to know that on the average 1500 lb , of sulfur per day leaves the furnace in one form or another. 

The basis for both of these observations is the law of conservation of mass, which states that mass can neither be created nor destroyed. (We will not be concerned in this book with the almost infinitesimal conversions between mass and energy associated with chemical reactions.) Statements based on the law of conservation of mass such as total mass of input = total mass of output or (Ibm sulfui/day), = (Ibm sulfur/day)oui are examples of mass balances or material balances. The design of a new process or analysis of an existing one is not complete until it is established that the inputs and outputs of the entire process and of each individual unit satisfy balance equations. 

Part Two of this book, which begins with this chapter, outlines procedures for writing material balances on individual process units and multiple-unit processes. In this chapter we present methods for organizing known information about process variables, setting up material balance equations, and solving these equations for unknown variables. In Chapters 5 and 6 we introduce various physical properties and laws that govern the behavior of process materials and indicate how these properties and laws are taken into account (as they must be) in formulating material balances. 

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FUNDAMENTALS OF MATERIAL BALANCES With 1 per cent loss, polymer entering sub-system... 

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