## 3 Life Cycle Assessment Methodology
### Life Cycle Inventory Analysis  
In the life cycle inventory analysis, an inventory of all input (such as material, energy, resource flows) and output (such as product, waste to treatment, emission flows) data for all processes inside the system boundaries is set and as a result, a list of elementary flows (resources and emissions) is provided. Generally, process data are protected by companies; then, other different methods can be used, with differences in accuracy and data/time requirements (time is an important parameter because it is necessary to reduce the time to complete a life cycle assessment) as reported in Figure 4. Here, in addition to real plant data, the following sources can be used: database (method 0), process simulation tools (method 1), advanced process calculations (method 2), basic process calculations (method 3), stoichiometry (method 4), molecular structure models (method 5), and proxy data (method 6). In some cases, data can be omitted (method 7). In method 0, data are provided by manufacturers, environmental product declarations, company sustainability publications, industry associations, or life cycle inventory databases. In method 1, data can be provided by process simulation software such as Aspen, Hysys, Chemcad, Pro II, etc. The disadvantages of this method are the greater required time, skills in modeling, the use of modeling tools, and the fact that the quality of the life cycle inventory depends on the detailed level of the developed simulation. In methods 2 and 3, inventory data are provided by correlations, Perry and NIST databases, and process encyclopedia. These methods are used if the usable data are not enough for the application of simulation tools but can be used to set material and energy balances and quantify emissions. In method 4, data are provided by chemical reactions and stoichiometry from online databases and literature. It can be used in the absence of any process data. Although the energy requirement can be estimated by the heat of the reaction, the stoichiometric approach does not consider different factors such as the use of material and energy balances in addition to emissions, process conditions, heat losses, yields, and particular operations  
Figure 3: Definition of ecosphere and technosphere, elementary (flows exchanged between the technosphere and ecosphere) and process flows (flows inside the technosphere) [Modified after [PERSON] (2018)]. while it can be useful for fast calculations and in comparing different processes. In method 5, the FineChem web model is used to take relevant data, for example, molar weight, functional groups, and chiral centers to various measures of the environmental impact of its production. Method 6 uses the environmentally extended input-output analysis: it uses the life cycle inventory data of similar processes. In method 7, the omission of data can be used in the presence of the cut-off (an input is very low compared to others), and the irrelevant flow is not included inside system boundaries. Generally, flows are considered when their relative mass, energy content, or price is more than 1% of all flows in the analyzed process. In the life cycle inventory phase, multifunctionality can be treated when two or more functions are present where a function is a valuable product in output or an unwanted waste in input [63]. Multifunctionality occurs for the combinations of the following: the co-production of more valuable products in multiple outputs; the combination of multiple unwanted wastes in multiple inputs; and the recycling of unwanted wastes into valuable products. The problem in this case is how to allocate the environmental impact of this multifunctional process to different functional flows.