Pinch significance

The process requires (Qup + Qlp) to satisfy its enthalpy imbalance above the pinch. If there were no losses from the boiler, then fuel W would be converted to shaftwork W at 100 percent efficiency. However, the boiler losses Qloss reduce this to below 100 percent conversion. In practice, in addition to the boiler losses, there also can be significant losses from the steam distribution system. Figure 6.336 shows how the grand composite curve can be used to size steam turbine cycles. ... 

In Chap. 12 it was discussed how the pinch takes on fundamental significance in improving heat integration. Let us now explore the consequences of placing reactors in different locations relative to the pinch. 

A low temperature of approach for the network reduces utihties but raises heat-transfer area requirements. Research has shown that for most of the pubhshed problems, utility costs are normally more important than annualized capital costs. For this reason, AI is chosen eady in the network design as part of the first tier of the solution. The temperature of approach, AI, for the network is not necessarily the same as the minimum temperature of approach, AT that should be used for individual exchangers. This difference is significant for industrial problems in which multiple shells may be necessary to exchange the heat requited for a given match (5). The economic choice for AT depends on whether the process environment is heater- or refrigeration-dependent and on the shape of the composite curves, ie, whether approximately parallel or severely pinched. In cmde-oil units, the range of AI is usually 10—20°C. By definition, AT A AT. The best relative value of these temperature differences depends on the particular problem under study. 

The variable that has the most significant impact on the economics of an extractive distillation is the solvent-to-feed (S/F) ratio. For closeboiling or pinched nonazeotropic mixtures, no minimum-solvent flow rate is required to effect the separation, as the separation is always theoretically possible (if not economical) in the absence of the solvent. However, the extent of enhancement of the relative volatihty is largely determined by the solvent concentration and hence the S/F ratio. The relative volatility tends to increase as the S/F ratio increases. Thus, a given separation can be accomplished in fewer equihbrium stages. As an illustration, the total number of theoretical stages required as a function of S/F ratio is plotted in Fig. 13-75 7 for the separation of the nonazeotropic mixture of vinyl acetate and ethyl acetate using phenol as the solvent. 

Table I lists the concentrations of nitrosodimethylamine (NDMA) and nitrosodiethylamine (NDEA) in some of the tobacco products which had greater than 0.5% nitrate content. Tobacco with less than 0.5% nitrate content, such as Bright tobaccos, yielded NDMA, NDEA and nitrosopyrrolidine (NPYR) generally below 5 ppb. The relatively high concentrations of NDMA in fine cut tobaccos and in snuff are possibly of significance in the increased risk for oral cancer among snuff dippers who use these tobacco types repeatedly each day by placing a pinch of the product directly into the gingival buccal fold (6,7).

Parison cooling significantly impacts the cycle time only when the final parison thickness is large. In thin blown articles the mold is opened when the pinched-off parts have solidified so that they can be easily stripped off thus they are the rate-controlling element in the cooling process. For fast blow molding of even very thin articles, the crystallization rate must be fast. For this reason, HDPF, which crystallizes rapidly, is ideally suited for blow molding, as are amorphous polymers that do not crystallize at all. 

Direct-oxidation processes can conveniently remove H2S from the process gas to levels of 100 ppm. At significantly greater cost, levels of perhaps 10 ppm can be achieved. The systems are characterized by relatively low sulfur-carrying capacities and, therefore, high liquor recirculation rates. They are not generally applicable to gas streams with high partial pressures of carbon dioxide the carbon dioxide also dissolves in the solution, causing a pinch in the column and a reduction of sulfur purification capability. 

A large number of suggested uses for compounds containing the 1,2,4,5-tetrazine skeleton have been recorded in the literature. It seems that most claims must be taken with a pinch of salt and it seems doubtful that any is of significant value. 

The difficulty that arises in the utility targeting task is due to two sources. First, the requirement of a strict pinch assumes that energy is the dominant cost contribution and hence lower total cost solutions in which heat is allowed to be transferred across the pinch (i.e., pseudo-pinch case) may be excluded from consideration, Second, the specification of HRAT requires iterating between several values of HRAT to determine the optimal network and hence needs a significant amount of time and effort. 

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