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A4. Industrial systems
The principles on which energy analysis was based have been described in detail elsewhere 8 and, with a few refinements, these same principles are still used in life-cycle calculations. The basic concept underlying the methodology is that of the system.
Although industry is primarily concerned with products, it is the production system that is of importance in this type of analysis. An industrial system is defined as any collection of operations which, when acting together, perform some defined function. Here it is the emphasis on the function that is important because if any two systems are to be compared then they must be performing equivalent functions. For this reason, a system whose function is to produce one kg of polyethylene cannot be compared to a system whose function is to produce one kg of polypropylene. It is therefore meaningless to compare a system which produces polyethylene with one that produces polypropylene and come up with the answer that polyethylene is better or worse than polypropylene. In a wider context, it is this concept of systems that prevents comparisons of materials on the basis of 1 kg of material A is better than 1 kg of material B.
Schematically, any life-cycle industrial system can be represented as shown in Figure 2 where the collection of operations is enclosed within the box. The outline of the box denotes the system boundary and separates the system from its surroundings - the system environment. The system environment acts as the source of all inputs to the system and the sink for all outputs from the system. For a complete life-cycle system, the only outputs will be waste materials which is why no output products are shown in Figure 2.
Figure 2.
Schematic diagram of a life-cycle system.
The physical description of the system, or inventory, is a quantitative description of all flows of materials and energy across the system boundary either into or out of the system itself. Note that this definition of a system is identical to that used in conventional thermodynamics and much of the formalism of conventional thermodynamics is directly applicable in this type of analysis. As a consequence, it is necessary to devise relatively few new procedures to manipulate the data. Many of the arbitrary decisions introduced by some analysts in recent years arise from a lack of appreciation of the close ties between life-cycle analysis and thermodynamics.
Because the system is a physical system, it must obey all of the physical laws. That is, it must obey the law of conservation of mass and the laws of thermodynamics. These laws provide a useful check on the validity of any description because if any of the laws are violated, then the description is invalid.
In some instances the system boundary can be identified as a physical boundary. It may, for example, enclose all of the operations carried out on a particular piece of machinery or in a specific factory. On the other hand, it frequently cannot be so identified. For example, a transport operation can be represented by a system whose function is to change the geographical location of a material. Alternatively, a complex industrial process may be analyzed into a set of simpler operations, none of which exist in isolation in reality but for the purposes of the analysis are treated as if they do.
Since any collection of operations can be treated as a system, it follows that there can be no such thing as a correct system. It is therefore imperative to define the system with some precision and the system definition must form part of the final answer. Many of the apparent discrepancies occurring in the literature arise because the system being described is not properly specified. The specification of a system cannot be defined solely in terms of the output product (if there is one; in a true life-cycle analysis, the only output will be waste). Not only must the function be specified but also other pertinent factors such as the inputs, any peculiarities of the internal process route, the country and year to which it refers, and so on, must all be identified. The aim, ideally, should be to allow the reader to duplicate the system in exactly the same way as the description of a laboratory experiment should allow the reader to set up the necessary equipment and perform the practical work. Only when the system is described in such detail, can the reader be sure that the system he is visualizing is the same as that intended by the author.
Within any extended industrial system, the data describing the overall performance will usually be derived from a number of different operators, each of whom will be taking the output from an upstream operation and processing it into a product for the next operation downstream. As a result, large systems must be sub-divided into a set of sub-systems such that each subsystem encompasses the activities of a single operator.
The choice of sub-systems is usually determined by the availability of data and the overall system is analyzed only to such a level of detail that the component sub-systems correspond to operations for which data can be obtained. Analyzing to a greater level of detail is pointless since performance data will not be available. In naphtha cracking, for example, it is seldom possible to separate the performance of furnace, quench tower and separation stages since the only data that are readily available are for the whole of the cracking plant. These sub-systems will be similar to the life-cycle system of Figure 2 but with two important differences; the input side may contain inputs from other up-stream processes and the output side will show one or more output products.
It is also important to remember that when an extended system is subdivided into a set of sub-systems, these sub-systems all possess the same characteristics as any other system. That is, their function must be carefully specified and, because they too are physical systems, they must also obey the standard scientific laws.
8 I Boustead & G F Hancock. Handbook of Industrial Energy Analysis. Ellis Horwood, Chichester/John Wiley, New York. ISBN 0-85312-064-1. (1979).
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