A thermodynamic approach to the design and synthesis of plant utility systems

Abstract

A systematic, thermodynamically oriented method for design and synthesis of plant utility systems is proposed. Heat requirements are satisfied in preference to power requirements. Taking a combined gassteam cycle as the most complex plant utility system, several significant new findings are obtained:1. T h e thermal efficiency of a gas turbine cycle can be a function of heat input ratio exclusively. 2.A reheating steam cycle may not improve the overall thermal efficiency. 3. The improvement of overall thermal efficiency due to regenerative heating of the feedwater will be effective only when the thermal efficiency of steam generator is greater than the overall thermal efficiency. Systems can be constructed by taking all constraints into account to get the best maximum overall thermal efficiency design reachable. A case study is illustrated to describe this method.The effectiveness of an energy conservation effort would eventually depend upon the design of a plant's utility system. The final objective of energy utilization improvement is to reduce the purchase of outside energy sources. A good design of a plant utility system should be able to match the plant's heat and power variations and keep its operation always at the highest thermal efficiency. There is little discussion in the literature concerning how to systematically design a good plant utility system. A more fundamental study of the system's intrinsic properties is therefore required.Mathematical optimization methods, such as linear programming (Petroulas and Reklaitis, 1984;Nishio and Johnson, 1977) and mixed-integer linear programming (Papoulias and Grossmann, 1983)) have been proposed for utility system design. Although a suitable formulation of the mathematical representation of a system can easily solve the problem, the usefulness of these approaches is detracted by the heavy dependence on mathematical calculations. Moreover, they are not powerful enough to increase essential understanding of the characteristics of solution. In addition, an inappropriate objective function may hinder the desired real solution, and the final unique numerical optimal solution of LP and MILP methods is generally not convincing enough in engineering practice. Nishio et al. (1984) have modified the above method with a decision tree to determine system structure in the solution space; however, this modification is still seriously constrained by the number of variables.To overcome these drawbacks, a solution procedure based on the thermodynamic analysis will be taken up here. Nishio et al. (1980) have proposed a similar approach to this problem. Although a useful strategy for the system design was constructed on the basis of systematic use of heuristic rules derived to decrease the loss of available energy as much as possible, this method still limits its discussion to the steam cycles. No fundamental treatment between steam cycles and gas turbine cycles has been given.In this paper, the design and synthesis problems will be concerned with steam cycles and simple and reg...

A systematic approach to the synthesis and design of flexible site utility systems

“…Total site analysis (Dhole and Linnhoff, 1992;Raissi, 1994) identifies acceptable candidates where TCE reduces the size of optimisation, using efficiency as a guide in the selection since the energy cost of a utility system is an overwhelming factor in the analysis of each year's cost. The strategy has been applied successfully in utility system design (Chou and Shih, 1987) and heat exchanger network design (Linnhoff and Turner, 1981).…”

“…These different types need be considered for all different sizes, different operating conditions (primarily pressure levels), and different combinations of the primal structures. Industries that use a very large amount of thermal energy, such as the petrochemical, food processing and the paper and pulp (Chou and Shih, 1987), enforce practices whereby heat requirements are satisfied first and power demands are matched exactly. The work follows an approach consistent with such practices.…”

“…The major difference between the simple and regenerative gas turbine cycles is the addition of a recuperator for heat exchange between the turbine outlet and the compressor outlet as shown in Chou and Shih (1987), the types of the gas turbines are screened by the characteristic value of power to heat ratio, P /H . Table 1 shows the ratio corresponding to each gas turbine cycle.…”

“…It next expands in the turbine to the exhaust pressure. Following Chou and Shih (1987), the reheat cycle can improve the overall thermal efficiency only if the thermal efficiency contributed by the reheat part is greater than that of the remaining parts. The required large heat-exchange area and the increased complexity in system design detract from the gain in efficiency due to reheating.…”

Optimization of energy plants including water/lithium bromide absorption chillers

“…In the "rst group, the works of Chou and Shih (1987) and Heui-Seok et al (1998) could be included, the main drawback of this type of methods is that capital costs are not considered explicitly. In the second group of methods based on thermodynamics, an exergetic analysis is performed and economic aspects are included assigning a monetary value to each exergy #ow rate and considering also capital costs.…”