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270 CHAPTER PROCESS SYNTHESIS BY THE HIERARCHICAL APPROACH Express the formation rate of by-products in terms of design variables (conversion, molar ratio, etc.) Determine the flow rates for reactants in excess and not recycled, and include them in outlet streams Determine the flow rates of impurities entered with some reactant streams Calculate outlet flow rates for impurities in purge or bleed streams EXAMPLE 7.3 OVERALL MATERIAL BALANCE OF THE HDA PROCESS Figure 7.7 shows the Input/Output structure of the HDA process Input streams are toluene and hydrogen Outlet streams are benzene, diphenyl and purge Toluene is pure, but hydrogen has 5% methane The design decisions are: (1) not purify the feed, (2) recycle hydrogen and (3) consider a purge stream for getting out the methane In a first approach building an ideal I/O material balance is useful, at 100% selectivity and stoichiometric feeds, since it set upper limit of the material efficiency The result is that for producing 100 kmol/h (7800 kg/h) benzene one needs 100 kmol/h toluene and 105.26 kmol/h hydrogen One gets also 105.26 kmol/h methane Now, let consider the formation of by-products, as well as the possibility to feed some reactants in excess The selectivity of the main reaction is given by the following relation (Douglas, 1988): À 0:0036 (7.1) S¼ ð1 À xT Þ1:55 where xT is the toluene conversion to benzene Graphical representation (Figure 7.8) shows that the selectivity is over 98% up to a conversion of 0.6, but it declines rapidly after Clearly, the reaction conversion is an important design variable The next step is formulating the material balance as function of dominant design variables, toluene conversion and hydrogen excess in feed In this case, it is possible to examine the problem analytically The notations given in Figure 7.7 are: • PB, PD, PM, PG: molar rate of benzene, diphenyl, methane and purge flow • FT, FH: molar feed of toluene and hydrogen • yFH, yPH: hydrogen mole fraction in feed and purge The following equations describe the material balance for a given production rate PB: FT ¼ PB S (i) PM ¼ PB S (ii) PG yPH Purge PM H2 + CH4 Benzene FH yFH HDA process Toluene FT FIGURE 7.7 Input/Output structure of the HDA process Diphenyl PB PD 7.5 INPUT/OUTPUT ANALYSIS 271 0.98 0.96 Selectivity Conversion range 0.94 0.92 0.9 0.88 0.86 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Conversion of toluene 0.8 0.9 FIGURE 7.8 Selectivity of as function of conversion PD ¼ yFH FH ¼ PB À S S (iii) PB PB À ð1 À SÞ + PG yPH S 2S (iv) PB ¼ ð1 À yPH ÞPG S (v) ð1 À yFH ÞFH + The combination of Equations (iv) and (v) gives the purge flow rate for given production, selectivity and input of hydrogen: PG ¼ FH + PB À S S Combining Equations (v) and (vi) give the following relation for the make-up hydrogen: 1ÀS PB À ð1 À yPH Þ FH ¼ SðyFH À yPH Þ (vi) (vii) Because hydrogen concentration in feed yFH is fixed, it may be concluded that the purge concentration yPH might be another design variable However, Equation (vii) indicates that yPH cannot be set independently of the hydrogen feed This result is important for the process control strategy Controlling both make-up hydrogen and hydrogen concentration in purge is not possible for the simple reason that the problem is over-specified at steady state Table 7.4 presents the material balance for the following design variables: toluene conversion 0.75, selectivity 0.969 and hydrogen excess of 40% Comparison with the simplified analysis shows an increase of material consumption with 4.3% This is due to the formation of the diphenyl by-product, as well as to the increase in the purge rate Continued 272 CHAPTER PROCESS SYNTHESIS BY THE HIERARCHICAL APPROACH Table 7.4 Preliminary Input/Output Material Balance for the HDA Process Input Output Component Hydrogen Toluene H2 CH4 Benzene Toluene Diphenyl Total (kmol/h) Mw Total (kg/h) 140 7.37 0 147.37 2.7 397.89 0 103.20 103.20 92 9494.70 Total Benzene Diphenyl Purge Total 9892.60 0 100 0 100 78 7800 0 0 1.60 1.60 154 246.65 38.40 110.57 0 148.97 12.39 1845.94 9892.60 The question that arises is if we could bring the material consumption close to the stoichiometric requirements The answer is yes, by installing a membrane device for separating the methane and recycling the hydrogen This simple example demonstrates that important design decisions can be taken early at the Input/Output level Material balance analysis depends on the achievable performance of the reaction system Impurities present in raw materials or formed by secondary reactions generate environmental problems that must be fixed This aspect will be analysed in more detail in the next example 7.5.4 ECOLOGICAL ISSUES The Input/Output structure is also the place for an early evaluation of the environmental performance of a process This analysis should be combined with material and economic indices for the assessment of various alternatives In this section, we follow the approach developed by Allen and Shonnard (2002) When the conceptual design phase is finished, a more detailed analysis can be done in a separate section of the project devoted to HSE issues, as developed in Chapter 16 Here, we point out only some aspects regarding the evaluation of alternatives from this viewpoint A first method is the calculation of an overall Environmental Index based on absolute threshold limit value (TLV) of the toxicity of reactants and products weighted by the absolute values of the mass stoichiometric coefficients wi: X Environmental index ðTLVÞ ¼ jwi j  TLV (7.2) TLV is the time-averaged (8 h day, 40 h week) concentration level in air (ppm) of a chemical component to which a worker can be exposed without adverse effect TLV values are established by a non-governmental organisation, the American Conference of Governmental Industrial Hygienists, at www.acgih.org The TLV values may be found in official reports, or by Internet sources (DEHS, University of Minnesota) TLV equivalent measures have been developed in other countries ... excess of 40% Comparison with the simplified analysis shows an increase of material consumption with 4.3% This is due to the formation of the diphenyl by-product, as well as to the increase in the. .. in the purge rate Continued 272 CHAPTER PROCESS SYNTHESIS BY THE HIERARCHICAL APPROACH Table 7.4 Preliminary Input/Output Material Balance for the HDA Process Input Output Component Hydrogen... structure is also the place for an early evaluation of the environmental performance of a process This analysis should be combined with material and economic indices for the assessment of various alternatives