Minimum allowable composition difference

By employing a minimum allowable composition difference of j, at the lean end of the exchanger, one can identify the minimum practically feasible outlet composition of the waste stream to be y° which is given by... 

Fi re 2.10 Minimum allowable composition difference at the lean end of a mass exchanger. 

As discussed in Section 2.5, the minimum allowable composition differences can be used to trade off fixed versus operating costs. TVpically, an increase in j leads to an increase in the MOC of the network (see Figs. 3.12 and 3.14) and a decrease in the fixed cost of the system. Hence, the minimum allowable composition differences can be iteratively varied until the total annualized cost of the system is attained (see Fig. 2.13). 

Three MSAs can be used to remove the solvent from the gaseous emission. The equilibrium data for the transfer of the organic solvent to the yth lean stream is given by y - mjXj, where the values of my are given in Table 4.2. Throughout this problem, a minimum allowable composition difference, Sy, of 0.001 (kg organic solvent)/(kg MSA) is to be used. The data for the MSAs are given in Table 1. 

The first step in determining the MOC is to construct the CID for the problem to represent the waste streams along with the process and external MSAs. The CID is shown in Fig. (6.1) for the case when the minimum allowable composition differences are 0.(K)1. Hence, one can evaluate the exchangeable loads for the two waste streams over each composition interval. These loads are calculated through Eqs. (6.2) and (6.3). The results are illustrated by Table 6.3. 

Hint vary the minimum allowable composition differences to iteratively trade off fixed versus operating costs). 

There are several gaseous wastes emitted from the process (see Dunn and El-Halwagi, 1993, and Homewoik Problem 8.5). In this exanqile, we focus on the gaseous waste leaving the multiple effect evaporators, R, whose primary pollutant is H2S. Stream data for this waste stream are given in Ihble 8.1. A rich-( ase minimum allowable composition difference, of 1.5 x 10 ° kmol/m is used. 

Up to now many researches on improving the synthesis procedures of MEN have been done successfully However, we can still observe some weakness and limitations. First, most methods proposed specify a fixed minimum allowable composition difference instead of optimizing it second, most research work are limited to the circumstance of transferring lower mass load, in which the input/output flow rates can be approximately regarded as constants third, the researches on multi-component system are few and without regarding to the interactions of components. 

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