Energetic consumptions

It is therefore to be anticipated that the energetic consumption of the iodine section, when evaluated with a thermodynamic model taking into account recent experimental information, would be higher than the figures given above. 

According to this model the complete mineralization of phenol at BDD from a solution containing 0.5 gL-1 of reactant requires 18kWhkgCOD-1 when a is equal to 0.37 and the cell voltage is 3.6 V. Similar value of energetic consumption was also calculated by Kraft et al. (2003). 

H2O2 photolysis is usually performed with low- or medium-pressure mercury vapor lamps. Almost 50% of the energetic consumption is lost in the form of heat or emissions less than 185 nm, which are absorbed by the quartz jacket. Generally, cheap germicidal lamps are used however, as H2O2 absorption is maximal at 220 nm, it is more convenient to use Xe/Hg lamps that—although more expensive—emit in the 210-240 nm range. 

Heuristic 2 (Slope of the vapour pressure curves). The relative volatility may be improved by selecting an appropriate operating pressure. Low pressures enhance the relative volatility, but increases the energetic consumption. 

Table 7.29 indicate that the results obtained by shortcut calculations are quite different with those predicted by heuristics (see Table 7.27). It can be seen the sequences can be classified in two groups higher energetic consumption (3, 8, 9) and lower energetic consumption (11,12). The best is the sequence 12 where the first split is indirect removing the most plentiful as bottoms. The second is the sequence 11 that has the same first split. Sequence 12 is only slightly better because 50/50 second split. Third is the sequence 9 based on 50/50 first split, but well behind the first two in term of total duty. The other two sequences gives very close results.

To find an explanation Table 7.30 present comparative results regarding the sequences 9 and 12. It can be observed that the sequence 9 handles the most difficult separation D/E as the last, while sequence 12 treated it as the first. We would expect the first solution to be more advantageous. The results shows practically the same duty, 16.15 versus 16.76, although we would expect much more energetic consumption when the four components ABCD are taken as overhead distillate instead just D. Actually, the presence of the other components has as effect a considerable reduction of the minimum reflux, 3 Instead 12 Thus, minimum reflux calculation based only on key components would give false predictions. This result reinforces the statement made before that the minimum vapour flow in sequencing should be determined by accurate methods. 

The above sequencing methods valid for zeotropic systems cannot be applied in the case of mixture with strong non-ideal character and displaying distillation boundaries, as those in the case of breaking azeotropes. Fortunately, the sequencing problem in this case has a different character. Most of the separations of multi-component non-ideal mixtures can be reduced by appropriate splits to the treatment of ternary mixtures, for which two or three columns are normally sufficient. The separation sequence follows direct or indirect sequence. The energetic consumption due to the recycle of entrainer dominates the economics. From this viewpoint preferred is that sequence in which the entrainer is recycled as bottoms. Hence, in azeotropic distillation the main problem is the solvent selection and not columns sequencing. 

At the first sight, the direct sequence should be more favourable as energetic consumption, because of the large amount of acetone in the initial mixture. On the other hand, the indirect sequence should give higher purity acetone, because the second split is a zeotropic binary acetone/benzene. Note that in both sequences the entrainer is recycled as bottoms. 

In Indirect sequence, the first split is much easier (35 trays), and the amount of entrainer is lower (0.08). In the second split, however, the situation is less favourable. Equilibrium diagram y-x shows difficult separation of pure acetone, because very low relative volatility with respect to benzene. However, the same high purity of 99.8% acetone can be obtained. It is important to note that the feed should be placed near to the reboiler, so that the second column is practically a stripper. The recycled entrainer benzene may contain a small amount acetone, which helps to get high purity products. The energetic consumption is slightly below the direct sequence, because low amount of entrainer. Hence, contrary to expectations, the indirect sequence has better indices, both as hardware and energetic consumption. 

The heat balance of the streams in the Table 10.1 shows an excess of 1000 kW. However, adding 1000 kW cold utility is not sufficient. The second law of Thermodynamics requires a minimum temperature difference between hot and cold streams. Consequently, the real energetic consumption is much higher. 

As reference we consider the direct separation scheme. The design of columns was done in Aspen Plus by means of shortcut methods followed by rigorous simulation. The energetic consumption depends on the reflux ratio. We assume that the optimal R/Rmi is 1.3, and column pressures of 2 and 1 bar with 0.2 bar pressure drop. Table 11.2 presents the results. Note that the initial feed temperature is 298 K, and therefore the reboiler duty of the first column includes feed preheating. 

The quality of the intermediate DCE must fulfil strict purity specifications. Low impurity levels imply high energetic consumption, but higher impurity amounts are not desired for operation. The intermediate DCE is conditioned mainly in the distillation column S2. In the bottom product the concentration of the two bad impurities E and E must not exceed an upper limit, of 100 and 600 ppm, respectively, while the concentration of the good impurity E must be kept around optimal value of 2000 ppm. Because these impurities are implied in all three reaction systems through recycles that cross in the separation system, their inventory is a plantwide control problem. The problem is constraint by technological and environmental constraints, as mentioned. 

With the current available technology for cement manufacturing, the burning phase is, as well known, and among all the others integrating the process, the one which Involves a greater energetic consumption, namely for the effect of Its thermal component. 

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