A flowsheeting approach to integrated life cycle analysis

The objective of life cycle analysis is to determine the total environmental impact of any given product. The paper addresses the problem of branched production chains incorporating multi-product plant and inter-process recycle (or reuse). We show that in such chains no product can be assigned an unambiguous environmental impact. Furthermore, the traditional approaches to LCA can lead to some environmental impacts being assigned multiple times and others omitted altogether. The paper introduces the important mass-balance principle that the sum total of all actual environmental impacts should equal the sum total of impacts assigned to the range of products. We describe a method for ensuring material balance in integrated LCA over a multi-product branched production chain. The method is illustrated by application to a desulphurization process. The method allows operators to assign environmental cost to any product according to their own judgement. The judgement is akin to assigning costs to individual products from a multi-product facility. Nevertheless, no matter how the judgement is applied, material balance must be maintained. It is noted that nearly all production chains include multi-product facilities. Environmentally, multi-product facilities are frequently superior because they minimize waste production. However, in traditional LCA, such processes may score badly because their full environmental cost is assigned to more than one product stream. The methods put forward correct the imbalance. A note on recycle and reuse. The chemical industry differentiates between recycle and reuse. "Recycle" is applied to reuse within the same process. For example, unconverted raw material recycled to a reactor. The process design ensures that the recycled material is used on site without transportation, and that the production is exactly balanced with the use. "Reuse" is use of the waste from one process as an input to another process. Reused material may need to be transported and may need preliminary processing before it can be reused. Under this definition, domestic recycled waste is actually reused waste (such as reused waste paper). We follow the convention of differentiating between reuse and recycle. (C) 2006 Elsevier B.V. All rights reserved.