Mass Balances for Mixtures

Here we make the steady-state assumption that j the concentration is not changing with time, so the algebraic sum of the flows of pollutant in and out must be zero  

0 = flow rate of pollutant into city air from upwind + flow rate of pollutant into city air from city - flow rate of pollutant out of city air at downwind edge of city (3.27) 

The pollutant flow rates are expressed as concentrations (e.g., kilograms per cubic meter) times volumetric flow rates (e.g., cubic meters per second), so 

Equation 3.28 says that the pollutant concentration in the city is equal to that in the air entering the city (the background concentration) plus a term [qLl VH) which indicates how much the pollutantj concentration has been increased by the emissions from the city itself. This is the box model or proportional or rollback equation, which has played a very important role in the formulation of air pollution regulations in thelUnited States [2].  

Example 3.8. Our paint shop will use a special paint that has benzene as a solvent. In the course of an 8-h day, the paint evaporates 200 kg (4401b) of benzene q= 200kg/8h). The shop dimensions are 10mx4mx4m. To protect the health of our workers, we must limit the concentration of benzene in the shop air to less than or equal to the industrial hygiene standard for benzene [3], which is 30mg/m. If we wish to keep the concentration c of benzene in the shop at or below this permitted concentration, how large a flow of ventilating air must we supply I 

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We next consider the mass balances for a mixture. In this case while total mass is conserved, there will be a change af mass of some or all species if one or more chemical reactions occur. For this case, it is more convenient to develop the mass balance for mixtures on a-molar basis, as chemical reaction stoichiomentry is much easier to write on a molar basis than on a mass basis. In this chapter we will consider only the ca.se of a single chemical reaction in later chapters the more general case of several chemical reactions occuring simultaneously will be considered. 

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