External Utilization

The vapour of evaporation can be fed into a steam system or can be supplied to another consumer. However, the capacity of the evaporator always depends on the consumed vapour quantity. 

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If the total heat consumed is from an external utility (e.g., mains steam), then a high efficiency is desirable, even perhaps at the expense of a high capital cost. However, if the heat consumed is by recovery from elsewhere in the process, as is discussed in Chap. 15, then comparison on the basis of dryer efficiency becomes less meaningful. 

After maximizing heat recovery in the heat exchanger network, those heating duties and cooling duties not serviced by heat recovery must be provided by external utilities. The outer-most layer of the onion model is now being addressed, but still dealing with targets. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

Constraints (10.3) and (10.4) are expressions of the external utility requirement for unit j, which requires heating, when operating in a standalone and integrated mode, respectively. 

Constraints (10.15) states that the amount of cold duty required by unit j at any point along the time horizon of interest is comprised of external cold utility and cold duty from heat integration with another unit j. Constraints (10.16) is similar to constraints (10.15) and applies to unit j requiring heating. Constraints (10.17) is a feasibility constraints, which ensures that in the absence of heat integration all the heat duty requirements of either unit j or j are satisfied by external utilities. The upper bound on the amount of heat exchanged between unit j and unit j will always be the minimum of the required cold and hot utilities as captured in Constraint (10.17 ). 

The objective function for the literature example is the maximization of profit, which is defined as the difference between revenue and operating cost. The operating cost consists of consumed external utility costs. 

The case study was solved using the uneven discretization of time formulation presented in this chapter. The mathematical model for the scenario without heat integration (standalone mode) involved 88 binary variables and gave an objective value of 1060 rcu. This value corresponds to the production of 14 t of product and external utility consumption of 12 energy units of steam and 20 energy units... 

Table 8.4 reports the analytical data and main features relevant to such a system. Interestingly, dihydrogen was collected at a pressure of 0.53 MPa, which allows its distribution in external utilization circuits. 

Labor skills and availability Raw materials availability External utilities availability Technical services providers... 

The above flowsheet has a major drawback inefficient use of energy. Indeed, the heating and cooling make use exclusively of external utilities. It is obvious that energy can be saved by appropriate combination of hot and cold streams. This could be found by using intuition or previous experience, and then checking the solution by simulation. However, this approach requires extensive work and can never guarantee that the solution is the best. 

Is the power to be generated on site, or drawn from an external utility, or a combination of both ... 

Figure 15.5 Simplified representation of a heat exchange network, showing hot streams, cold streams, heat exchanges (numbered pairs), and external utilities such as heating (H) or cooling (C).

Other earth movements such as sink holes, landslides, freeze/thaw heaving, coastal erosion, and settUng External utility failures... 

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