Yield, fractional Product distribution Selectivity

In this chapter, we develop some guidelines regarding choice of reactor and operating conditions for reaction networks of the types introduced in Chapter 5. These involve features of reversible, parallel, and series reactions. We first consider these features separately in turn, and then in some combinations. The necessary aspects of reaction kinetics for these systems are developed in Chapter 5, together with stoichiometric analysis and variables, such as yield and fractional yield or selectivity, describing product distribution. We continue to consider only ideal reactor models and homogeneous or pseudohomogeneous systems. 

The total isomer yield reaches a maximum of 71% for the SG (24h) sample with a 88% isomerization selectivity (Fig. 6A). The lack of shape selectivity in the methyl nonane fraction for the SG (24h) sample (Fig. 6B) is a typical behavior of large pore zeolites, while ZSM-5 catalysts present a hi er selectivity towards the 2-methyl nonane isomer at lower conversion values. The cracked product distribution gives a symmetrical curve with the maximum at C5 fraction, yet another large pore zeolite property (Fig. 6C). These results are in accordance with the previously reported data for the nanoZSM-5 catalysts and prove that the mesoporous matrix does not alter the catalytic properties of the embedded nano ciystals [11]. 

When a reactant or a set of reactants undergoes several reactions (at least two) simultaneously, the reaction is said to be a complex reaction. The total conversion of the key reactant, which is used as a measure of reaction in simple reactions, has little meaning in complex reactions, and what is of primary interest is the fraction of reactant converted to the desired product. Thus the more pertinent quantity is product distribution from which the conversion to the desired product can be calculated. This is usually expressed in terms of the yield or selectivity of the reaction with respect to the desired product. 

A high aromatics selectivity, however, requires proper catalyst selection. Zhang et al. studied the fast pyrolysis of corncob in absence and presence of a catalyst (ie, ZSM-5) [287]. The presence of the catalyst increased the yields of noncondensable gas, water, and coke, while decreasing the liquid and char yields. The catalyst induced a decrease of the oxygen content of the liquid fraction by more than 25%. These studies indicate the importance of a catalyst during biomass pyrolysis. The most important catalytic parameters affecting the product distribution are pore structure and acid site type. This was demonstrated by testing siUcalite, a material with the same pore structure as ZSM-5 but with intrinsic different acid sites, and siUca-alumina, an amorphous material with Brpnsted acid sites, in the catalytic pyrolysis of... 

The plant is used to produce two chemically different EPS -types A and B in five grain size fractions each from raw materials FI, F2, F3. The polymerization reactions exhibit a selectivity of less than 100% with respect to the grain size fractions Besides one main fraction, they yield significant amounts of the other four fractions as by-products. The production processes are defined by recipes which specify the EPS-type (A or B) and the grain size distribution. For each EPS-type, five recipes are available with the grain size distributions shown in Figure 7.2 (bottom). The recipes exhibit the same structure as shown in Figure 7.2 (top) in state-task-network-representation (states in circles, tasks in squares). They differ in the parameters, e.g., the amounts of raw materials, and in the temperature profiles of the polymerization reactions. 

Table 7 shows the yield distribution of the C4 isomers from different feedstocks with specific processing schemes. The largest yield of butylenes comes from the refineries processing middle distillates and from olefins plants cracking naphtha. The refinery product contains 35 to 65% butanes olefins plants, 3 to 5%. Catalyst type and operating severity determine the selectivity of the C4 isomer distribution (41) in the refinery process stream. Processes that parallel fluid catalytic cracking to produce butylenes and propylene from heavy cmde oil fractions are under development (42). 

Figure 7 indicates the change in relative yields of 1-olefins, internal olefins, and paraffins in the C5-Ci5 liquid products as a function of hydrogen pressure. For a selected temperature (575°C) the concentration of paraffins increases with increasing pressure until it reaches a stable level between 1500-2250 psig. On the other hand, the concentration of 1-olefins decreases sharply, while that of internal olefins increases and then remains at a stable level, with increase in pressure. Figure 8 shows a similar change in type distribution for a particular liquid fraction (C7 components). 

To obtain individual phospholipids of greater than 50-60% purity, some form of selective adsorption process is usually required. Adsorption and distribution chromatography present these options. Treatment of the alcohol-soluble lecithin with alumina yields a fraction very rich in phosphatidylcholine and free of phosphatidylethanolamine and phosphatidylinositol (167). Although these products are available only in very limited quantities for highly specialized markets, products such as a lecithin containing up to 95% PC can be obtained commercially. 

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