Inputs section: Unit process use elementary flows and System process use product flows?

Question

Dear all

I wanted to cofirm something - please correct me if I am wrong.
Unit process use elementary flows and System process use product flows?

For example if I want to model "Production of HDPE granulate"

- In a unit process I will only consider as input the elementary flows such as "gas" , "primary energy, from windpower", "biomasse" etc. and not include "ethylene, used electricity in granulate production and process heat"??

- in the system process, I would only include as input the ethylene, electricity for production of the granulates, and process heat"? And not include any elementary flow?

Also, does it make sense to make a process with all - both elemantary flows and the other product flows as inputs?

I am quite confused how to model this. Because it is part of a bigger product - HDPE jerry can. So i am not sure what to include when I want to model the "production of HDPE granulates"

Thanks in advance, I hope it was clear.

Answer 1

There is nothing that says a unit process or system process must have only elementary flows or product flows. System processes are meant to represent the inventory of a product system (or a collection of unit processes). Ideally that system process would actually only show elementary flows plus your product of interest. If you calculate a result for a product system, there is an button on General information tab of the analysis result to "Save as LCI result," which will create a system process.

Unit processes are meant to connect to each other and thus are much more likely to have product flows, along with any elementary flows associated (e.g. CO2 emissions from a natural gas boiler [elementary flow] and natural gas input to the boiler [product flow]).

For your example, you could have a unit process for "HDPE jerry can, manufacturing" connected to process "production of HDPE granulate" via a product flow "HDPE granulate." If you can find the cradle-to-gate data for the HDPE granulate - in other words it contains all the emissions from the extraction of crude or natural gas all the way through the HDPE granulate, then "production of HDPE granulate" could be a system process and you'd probably be 90% of the way to finishing your LCA. If you needed to be able to dive into where the emissions hotspots were within the production of HDPE granulate, then you would try to use unit processes to "build up" the manufacture of HDPE granulate.

Comments

Thank you so much for your answer! It is much clear now :)