A product that does not have alternative production paths is typically the determining product because it is simply the only way to produce it. An example is the wheat grain that can only come from a wheat field. All other by-products are therefore dependent co-products (by-products). Here are two examples to better clarify the concept:
1. The chlor-alkali process
The chlor-alkali industry uses brine (saltwater) to produce chlorine, sodium hydroxide (NaOH or caustic soda), and hydrogen. An electric current is passed through the brine to form gaseous hydrogen on the negative electrode and gaseous chlorine on the positive electrode, leaving a solution of sodium hydroxide.
The balanced process is as follows: 2NaCl+2H2O→2NaOH+Cl2+H2.
The described activity has a single product without an alternative production path, namely chlorine, which cannot be easily stored and is generally sold locally. The chlor-alkali process is practically the exclusive production route for chlorine. Although currently sodium hydroxide is exclusively produced by the chlor-alkali process, it is a more flexible product that can be stored and transported over long distances. There is both a well-known substitute (sodium carbonate, used in wood pulp production, paper, water treatment, and in some chemical sectors as a neutralizing agent) and an alternative production path (the causticization process, where NaOH is produced from lime and soda, currently uneconomical due to the significant input of NaOH caused by the global increase in chlorine demand). When market conditions change, the existence of the two alternatives (substitution and causticization) will provide both a floor and a ceiling to the price of NaOH (Wesnæs and Weidema, 2006).
The third by-product, hydrogen, is produced in relatively small quantities by the activity (27g per kg of chlorine), providing only about 3% of the global hydrogen market. The main production path for hydrogen is steam reforming of natural gas, which is the most competitive process to meet a change in hydrogen demand.
Therefore, we can conclude that both sodium hydroxide and hydrogen are dependent co-products for the chlor-alkali process.
Source:
Wesnæs M, Weidema B P (2006). Long-term market reactions to changes in demand for NaOH. Study for Novozymes. Copenhagen: 2.-0 LCA consultants.
2. Oxidation of manganese dioxide
Manganese dioxide is an inorganic compound with the formula MnO2, which is a precursor to KMnO4 (potassium permanganate) and K2CO3 (potassium carbonate). Potassium permanganate is a powerful oxidizing agent and is used as such in many chemical reactions. It is used as an antiseptic and in antifungal treatment, as well as to extend the shelf life of some fruits. It is also widely used in wastewater treatment and against invasive species like zebra mussels. Potassium carbonate is a white salt, soluble in water, where it forms a strong alkaline solution. The "oxidation of manganese dioxide" activity (present in the ecoinvent database) has two co-products: potassium permanganate and potassium carbonate. Manganese dioxide (MnO2) is melted with potassium hydroxide and heated with an oxygen source to obtain potassium manganate:
2 MnO2 + 4 KOH + O2 → 2 K2MnO4 + 2 H2O
Boiling the manganate solution in the presence of carbon dioxide yields potassium permanganate and potassium carbonate:
3 K2MnO4 + 2 CO2 → 2 KMnO4 + 2 K2CO3 + MnO2
Although there are other possible production methods, the described activity is the only industrial production route for potassium permanganate. Regarding potassium carbonate, the currently predominant production process is the electrolysis of potassium chloride followed by carbonation using carbon dioxide (Open Chemistry Database - PubChem 2015b, Wikipedia 2015):
2KOH + CO2 → K2CO3 + H2O
Since potassium permanganate has no other industrial production routes, while potassium carbonate has another one, we can identify potassium permanganate as the determining product, while potassium carbonate as a dependent co-product of the "oxidation of manganese dioxide" activity.
Sources:
Open Chemistry Database – PubChem (2015a) – Potassium Permanganate. National Center for Biotechnology Information. http://pubchem.ncbi.nlm.nih.gov/compound/potassium_permanganate (accessed June 8, 2015)
Open Chemistry Database – PubChem (2015b) – Potassium carbonate. National Center for Biotechnology Information. http://pubchem.ncbi.nlm.nih.gov/compound/potassium_carbonate (accessed June 8, 2015)
Weidema B P (2014), Example –oxidation of manganese dioxide.
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