From process integration to eco-industrial systems

Abstract

Process integration methods were first used to develop heat recovery strategies in industrial plants for purposes of energy conservation (Linnhoff et al. 1982). Not surprisingly, commercial interest in the early process integration tools such as thermal pinch analysis coincided with the global energy crises in the late 1970s. These early tools allowed analysts to determine energy targets for industrial facilities and also provided heuristics for the detailed design of heat recovery networks. More recently, the field of process integration has been extended to include procedures for the conservation of material inputs for process plants. El-Halwagi (1997) gave a comprehensive description of many such techniques for efficient use of such resources based on the analogy between heat and material flows. Much work has been done in particular on the minimization of water use in industry. The seminal work of Wang and Smith (1994) pioneered the conceptual, insightbased pinch analysis school of thought. Meanwhile, mathematical programming approaches were also applied for similar problems, for example by Takama et al. (1980). An early review by Bagajewicz (2000) describes both pinch analysis and optimization based approaches, although many more advances have since been made in the field of industrial water reuse. In any case, the central premise in the field of process integration is the cascading of energy or material streams from high-quality applications to low-quality ones, and the bulk of the scientific literature in this field covers the development of design methodologies for various problem scenarios. Meanwhile, the field of industrial ecology has emerged based on a parallel concept. The field is founded on emulating cyclic flows found in natural ecosystems in order to minimize the use of resources and the generation of wastes in industrial systems (Graedel and Allenby 1995; Korhonen 2004). As in process integration, such cyclic flows are possible through cascading material flows through applications of progressively lower grades. Hence, the analogies between the two fields are inevitable, except that in process integration, the exchange of streams occurs among process within an industrial plant, while in industrial ecology such exchanges are made between industrial plants. The potential for design methods in the former discipline to be used in the latter field was first proposed by Spriggs et al. (2004). In particular, many pinch analysis procedures allow energy or material consumption targets to be identified; these figures then serve as useful benchmarks for designers, who must then identify waste stream exchange network configurations that allow such targets to be achieved, or at least approached. Exactly how far can the analogy be taken? It has been noted by many observers that eco-industrial systems which have implemented such symbiotic networks have often emerged spontaneously or at least with minimal direction from any centralized authority (Korhonen 2004; Posch 2004; Koenig 2005). Thus, the optimization-based ethos underlying the field of process integration may not be entirely applicable to industrial ecology. The main stumbling block is that each plant or organization that might participate in eco-industrial symbiosis has its own interests, which may not coincide with those of its potential partners. In contrast, process integration is applied within a single organization, which implies the existence of a centralized authority to direct any system-level changes. At best, conventional process integration tools may be used in a manner analogous to the application of pinch analysis for targeting; they can provide planners with an idea of the R. R. Tan (&) De La Salle University-Manila, Manila, Philippines e-mail: tanr_a@dlsu.edu.ph