External MSAs

The candidate lean streams can be classified into Nsp process MSAs and Nse external MSAs (where Nsp + Nse — Ns). The process MSAs already exist on plant site and can be used for the removal of the undesirable species at a very low cost (virtually free). The flowrate of each process MSA that can be used for mass exchange is bounded by its availability in the plant, i.e.,... 

As can be seen from Fig, 3.7, the pinch decomposes the synthesis problem into two regions a rich end and a lean end. The rich end comprises all streams or parts of streams richer than the pinch composition. Similarly, the lean end includes all the streams or parts of streams leaner than the pinch composition. Above the pinch, exchange between the rich and the lean process streams takes place. External MSAs are not required. Using an external MSA above the pinch will incur a penalty of eliminating an equivalent amount of process lean streams from service. On the other hand, below the pinch, both the process and the external lean streams should be used. Furthermore, Fig. 3.7 indicates that if any mass is transferred across the pinch, the composite lean stream will move upward and, consequently, external MSAs in excess of the minimum requirement will be used. Therefore, to minimize the cost of external MSAs, mass should not be transferred across the pinch. It is worth pointing out that these observations are valid only for the class of MEN problems covered in this chapter. When the assumptions employed in this chapter are relaxed, more general conclusions can be made. For instance, it will be shown later that the pinch analysis can still be undertaken even when there are no process MSAs in the plant. The pinch characteristics will be generalized in Chapters Five and Six. 

Optimizing the use of flie external MSA The pinch diagram (Fig. 3.12) demonstrates that below the pinch, the load of the waste stream has to be removed by the external MSA, S3. This renders the remainder of this example identical to Example 2.2. ThereftKc, the optimal flowrate of S3 is 0.0234 kg mol/s and the optimal outlet composition of S3 is 0.(X)85. Furthermore, the minimum total annualized cost of the benzene recovery system is 41,560/yr (see Fig. 2.13). 

Figure 3.17 also indicates that 0.0124 kg phenol/s are to be removed using external MSAs. The following section discusses screening the three MSAs to select the one that yields a MOC. 

Cost estimation and screening external MSAs To determine which external MSA should be used to remove this load, it is necessary to determine the supply and target compositions as well as unit cost data for each MSA. Towards this end, one ought to consider the various processes undergone by each MSA. For instance, activated carbon, S3, has an equilibrium relation (adsorption isotherm) for adsorbing phenol that is linear up to a lean-phase mass fraction of 0.11, after which activated carbon is quickly saturated and the adsorption isotherm levels off. Hence, JC3 is taken as 0.11. It is also necessary to check the thermodynamic feasibility of this composition. Equation (3.5a) can be used to calculate the corresponding... 

Since it is thermodynamically feasible for any of the three external MSAs to remove the remaining phenolic load (0.0124 kg phenol/s), one should select... 

So far, an MOC solution has been identified through a two-stage process. First, the use of process MSAs is maximized by constructing the pinch diagram with the lean composite stream composed of process MSAs only. In the second stage, the external MSAs are screened to remove the remaining load at minimum cost. 

Suppose that the process does not have any process MSAs. How can a lean composite line be developed The following shortcut method can be employed to construct the pinch diagram for external MSAs. A more rigorous method is presented in Chapter Six. 

Filture 3.18b Constructing the pinch diagram for external MSAs. 

Toluene is to be removed from a wastewater stream. The flowrate of the waste stream is 10 kg/s and its inlet composition of toluene is 5(X) ppmw. It is desired to reduced the toluene composition in water to 20 ppmw. Three external MSAs are considered air (S2) for stripping, activated carbon (S2) for adsorption, and a solvent extractant (S3). The data for the candidate MSAs are given in Table 3.6. The equilibrium data for the transfer of the pollutant from the waste stream to the yth MSA is given by... 

Figure 3.19a Screening external MSAs for the toluene-removal example,...

We are now in a position to incorporate material balance into the synthesis procedure with the objective of allocating the pinch point as well as evaluating excess capacity of process MS As and load to be removed by external MSAs. These aspects ate assessed through the mass-exchange cascade diagram. 

Three additional mass-load paths S3-R2-S2, S3-R1-S1, S3-R1-S2 ) can be employed to shift removal duties from the process MSAs to the external MSA until all the waste load (0.104 kg phenol/s) is removed by the external MSA... 

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. 

Two external MSAs may be considered for removal of chlorine ions activated carbon (Si) and ion exchange resin (Si). The equilibrium relation for removing chlorine by these MSAs is given by y =... 

A process lean stream and an external MSA are considered for removing H2S. The process lean stream, S1, is a caustic soda solution which can be used as a solvent for the reactive separation of H2S. An added bonus for using the process MSA is the conversion of a portion of the absorbed H2S into Na2S, which is needed for white-liquor makeup. In other words, H2S pollutant is converted into a valuable chemical which is needed in the process. The external MSA, S2, is a polym ic adsorbent. The data for the candidate MSAs are given in Table 8.2. The equilibrium... 

Three major sources in the kraft process are responsible for the majority of the H2S emissions. These involve the gaseous waste streams leaving the recovery furnace, the evaporator and the air stripper, respectively denoted by R), R2 and R3. Stream data for the gaseous wastes are summarized in Table 8.8. Several candidate MSAs are screened. These include three process MSAs and three external MSAs. The process MSAs are the white, the green and the black liquors (referred to as Si, S2 and S3, respectively). The external MSAs include diethanolamine (DBA), S4. activated carbon, Sj, and 30 wt% hot potassium carbonate solution, S6. Stream data for the MSAs is summarized in Table 8.9. Syndiesize a MOC REAMEN that can accomplish the desulfurization task for the three waste streams. 

The solution to this case study is to use cycle oil (a process MSA) followed by a synthesized external MSA (heptane) as shown in Fig. 12.2. 

For lean streams, you are asked to enter supply composition, target composition, maximum MSA flowrate, solute distribution (m), constant (b), cost, and epsilon (minimum composition difference). If an MSA has no upper limit to flowrate, enter a laige number or leave the flowrate as zero in this input held. Essentially all external MSAs fall into this category, while many internal MSAs are only available in certain quantities. 

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