Plastics Engineering - May 2014 - (Page 37)

Figure 3: The flow diagram, sometimes referred to as a cradle-to-grave materials flow diagram, of a recycling scheme showing both materials and energy flow in and out of the system for a product life cycle (reprinted with permission of the publisher from Curran1). Outputs of the system are the collection of releases to the environment (i.e., land, water, or air) as well as discarded product. the quantification of energy and raw material requirements, air emissions, waterborne effluents, solid waste, and other environmental releases throughout the life cycle of a product is referred to as life-cycle inventory (lci). Impact analysis is an attempt to quantify the effects of the environmental loadings identified in the inventory phase. it mainly refers to the pollution of the ecosystems and the effects on human health. Improvement assessment is a systematic evaluation of the opportunities to reduce the environmental burden associated with energy and raw materials use and environmental releases throughout the whole life cycle of the product or process. this assessment may include both quantitative and qualitative measures of improvement, such as changes in the product or process design, use of raw materials, consumer use, and waste management. the life-cycle system concept for a product life cycle is illustrated in Figure 3. this diagram is often referred to as a cradle-to-grave materials flow diagram. the boundaries for the lci encompass the acquisition of raw materials, manufacture of intermediate materials, manufacture of the product being studied, use of the product, and the final disposition. recycling or reuse of the product is part of the lci analysis. reuse here means the direct reuse of the product for its initial intended use, such as a beverage bottle being placed back into service. the product could be separated from a mixed stream, ground, and returned to the product manu- facturing step. in the figure is also shown the return of the product all the way back to the materials manufacturing step, which is referred to as feedstock recycling. Furthermore, the product can be incinerated to recover the inherent energy associated with the polymer. in the diagram various types of recycling options are illustrated. the use of energy, as shown for each step in Figure 3, carries with it the input of energy resources as well as the inputs and outputs for processing these energy resources into usable fuels. the inputs and outputs for transportation of materials between process steps are implied by the arrows showing the flow of materials between steps. Mass & Energy Flow in Recycling Before any recycling scheme is introduced, it is essential that the ultimate goal be specified. if the goal is not specified, then it is impossible to judge whether the recycling scheme has achieved its purpose. if the goals are to reduce the consumption of raw materials or the generation of solid waste, then it is clear that recycling should be practiced. if the goal is to reduce energy consumption, then the process must be examined very closely. the basis for making the decision is centered on the use of material and energy balances. several types of recycling schemes exist: (1) closed-loop recycling with no losses in the recycling loop; (2) closedloop recycling when losses occur in the recycling loop; and (3) open-loop recycling. to illustrate how mass and energy balances are employed in the analysis of recycling schemes, www.plasticsengineering.org | www.4spe.org | MaY 2014 | Plastics EnginEEring | 37 http://www.plasticsengineering.org http://www.4spe.org

Plastics Engineering - May 2014

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