Principles Of Unit Operations 2nd Ed Foust
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In a steady-state system, in which no material is added or removed, the mass balance is not only an expression of the balance between mass flows in, and out, but also of the mass of the material constituting the system. In a transient or batch operation, the mass balance is an expression of the variation in mass with time. The derivation of the mass balance in a transient system is based on the assumption that the system is in steady state at the start and again at the end of the transient cycle. The principles required for the derivation of steady-state processes are not substantially changed by the addition of the transient variable. The derivation of the mass balance in a transient system requires the addition of the assumption of constant mass in the system. In this way, the transient process must be analyzed in three ways: steady state, transient, and steady state plus transient time. The large majority of chemical engineering operations are in steady state. The transient stages are so brief that they are essentially indistinguishable from the steady state. The steady-state cycles, with no transient, are considered in the introduction. The times required for the transient stages are often not large or important, and their analysis usually is not included in the analysis of steady-state operations. Batch operations are an exception to this. The analysis of the transient stages is included in the analysis of batch operation. In some transient processes, the mass is added to the system during the transient stages; the time during which the mass is added is the transient time. The mass is removed from the system during the transient stages; the time during which the mass is removed is the transient time. These times are called the mass transient times. In batch operations, if the mass is added, the transient times are called the mass transients, and if the mass is removed, the transient times are called the mass removals. For many chemical engineering pro- cesses, the reaction is the only transient stage; the time requiredfor the reaction is the reaction transient time. The reaction is followed by a transient stage in which the products are removed, followed by another transient stage in which the reaction products are removed and the reactants are removed. The transient times are called the mass removals plus the reaction removals. In batch operations, it is often convenient to express the mass removed during the transient stages in terms of the product of the volume removed in the transient stage and the concentration of the product in the material being processed. This expression is called the transient volume.
The fundamental chemical unit operation is thereaction, or, as it is often called, the chemical change. It is more general and covers a wider rangeof reactions than the elementary physicochemical unit 0aeration. It is formed by the combination of substance;s in a state of reaction; which has the same meaning as the chemical reactionis used in physics. 827ec27edc