Lean composite stream

Next, a global representation of all process lean streams is developed as a lean composite stream. First, we establish Ns/> lean composition scales (one for each process MSA) that are in one-to-one coirespondence with the rich scale according to the method outlined in Section 3.5. Next, the mass of pollutant that can be gained by each process MSA is plotted vei us the composition scale of that MSA. Hence, each i xx ess MSA is represented as an arrow extending between supply and target compositions (see Fig. 3.5 for a two-MSA example). Ihe vertical distance between the arrow head and tail is given by... 

Next, both composite streams are plotted on the same diagram (Fig. 3.7). On this diagram, thermodynamic feasibility of mass exchange is guaranteed when the lean composite stream is always above the waste composite stream. This is equivalent to ensuring that at any mass>exchange level (which comesponds to a horizontal line), the composition of the lean composite stream is located to the left of the waste composite stream, asserting thermodynamic feasibility. Therefore,... 

Figure 3.1 lb Construction of the lean composite stream for the two process MSA s of the benzene recovery example. 

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. 

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. 

The gas stream shown in Table 8-10 is fed to an isothermal absorber operating at 90°F and 75 psia. 90% of the n-butane is to be removed by contact with a lean oil stream consisting of 98.7 mol% non-volatile oil and the light components shown in Column 2 of Table 8-10. Estimate the composition of the product streams and the required number of theoretical stages if an inlet rate of 1.8 times the minimum is used. 

Normally, the lean composite curve is prepared in the same way. However, in this example, the curve is that for the one internal MSA in stream LI. It is simply copied from Figure 11.6a, but shifted... 

As for heat integration in Section 10.2, many additional observations are noteworthy in connection with the rich and lean composite curves. One is that the slopes of the composite curves always decrease at the inlet concentration of a stream and increase at the outlet concentration of a stream. It follows that points at which the slope decreases are candidate pinch points, and furthermore, that one of the inlet concentrations is always a pinch concentration, when a pinch exists. Hence, to locate a potential pinch concentration, one needs only to examine the Met concentrations of the streams. 

For the purpose of this study, a contactor column with two theoretical stages and with the feed composition shown in Table 2 was simulated. The concentration of the lean TEG stream was 99.0 weight % TEG, and it was assumed the TEG temperature was the same as the feed gas. The feed gas was saturated with water at feed conditions. For each contactor pressure and temperature, the lean TEG circulation ratio was varied. 

The flow directions in a PSA process are fixed by the composition of the stream. The most common configuration is for adsorption to take place up-flow. AH gases with compositions rich in adsorbate are introduced into the adsorption inlet end, and so effluent streams from the inlet end are rich in adsorbate. Similarly, adsorbate-lean streams to be used for purging or repressurizing must flow into the product end. 

The two condensate Hquids must be used to provide reflux and distiUate streams. NormaHy, the reflux ratio, r, is chosen so that r = L jD > (j). This requires that the reflux rate be greater than the condensation rate of entrainer-rich phase and that the distiUate rate be correspondingly less than the condensation rate of entrainer-lean phase. This means that the distiUate stream consists of pure entrainer-lean phase, ie, Xj = x, and the reflux stream consists of aU the entrainer-rich phase plus the balance of the entrainer-lean phase. Thus, the overall composition of the reflux stream, Hes on the... 

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... 

As has been previously mentioned, the minimum TAC can be identified by iteratively varying e. Since the inlet and outlet compositions of the rich stream as well as the inlet composition of the MSA are fixed, one can vary e at the rich end of the exchanger (and consequently the outlet composition of the lean stream) to minimize the TAC of the system. In order to demonstrate this opdmization procedure, let us first select a value of e at the rich end of the exchanger equal to 1.5 X 10 and evaluate the system size and cost for this value. 

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. 

Having determined the individual loads of all process streams for all composition intervals, one can also obtain the collective loads of the waste and the lean streams. The coUective load of the waste streams within the itth interval is calculated by summing up the individual loads of the waste streams that pass through that interval, i.e. 

As has been mentioned earlier, the CID generates a number Ni , of composition intervals. Within each interval, it is thermodynamically as well as technically feasible to transfer a certain mass of the key pollutant from a waste stream to a lean stream. Furthermore, it is feasible to pass mass from a waste stream in an interval to any lean stream in a lower interval. Hence, for the J th composition interval, one can write the following component material balance for the key pollutant ... 

Conversely, immediately below the pinch, each lean stream has to be brought to its pinch composition. At this composition, any lean stream can only operate against a waste stream at its pinch composition or higher. Since a MOC design does not permit the transfer of mass across the pinch, each lean stream immediately below the pinch will require the existence of at least one waste stream (or branch) at the pinch composition. 

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