A "unit" is defined as a combination of elements and may or may not constitute the entire process. By definition
Nu = Nr + ^ Nt i and Nu = NVu - Nu where Nu refers to new restricting relationships (identities) that may arise when elements are combined. Ncu does not include any of the restrictions considered in calculating the Nt's for the various elements. It includes only the stream identities that exist in each inter-stream between two elements. The interstream variables (C + 2) were counted in each of the two elements when their respective Ne 's were calculated. Therefore, (C + 2) new restricting relationships must be counted for each interstream in the combination of elements to prevent redundancy.
The simple absorber column shown in Fig. 13-23 will be analyzed here to illustrate the procedure. This unit consists of a series of simple equilibrium stages ofthe type in Fig. 13-22. Specification of the number of stages N utilizes the single repetition variable and
NU = Nr + ^ Ne = 1 + N(2C + 6) i since in Table 13-5 Nf = 2C + 6 for a simple equilibrium stage. There are 2(N - 1) interstreams, and therefore 2(N - 1)(C + 2) new identities (not previously counted) come into existence when elements are combined. Subtraction of these restrictions from NU gives Nu, the design variables that must be specified.
= [1 + N(2C + 6)] - [2(N - 1)(C + 2)] = 2C + 2N + 5
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