Multiple determining products from combined production
In combined production the amounts of the co-products can be varied independently, and all products are therefore determining products, independently of each other.
For some co-products it is quite clear that they may be separated completely, so that each co-product can be produced alone, allowing separate description of this pure production. Typical examples of this kind are services, like transport of goods.
However, the combined production of most co-products is typically done to take advantage of some benefit of co-production, e.g. use of the same capital equipment, reduction in energy or raw material consumption, or reduction in waste generation. Since a description of the separate pure production would not encompass these advantages, it will not provide a correct reflection of the actual conditions. Therefore the modelling must include the specific consequences (changes in exchanges of the unit process) that result from changing the output of the co-product of interest while keeping the output of the other co-products constant. In this way, the benefits of the combined production will be inherent to the description and will be reflected in the outcome. This is effectively the same as subdividing the activity according to physical causalities, since the modelling can be repeated independently for each of the combined co-products, resulting in one dataset per co-product.
In general, a physical parameter can be identified, which – in a given situation – is the limiting parameter for the co-production. It is the contribution of the co-product of interest to this parameter that determines the consequences of the studied change.
If you by mistake treat a combined production as a joint production – and thus proceed to perform a system expansion as is described elsewhere on these pages, you will discover that you end up with the same results – so no real harm done (other than a bit more work on your part).