HCl - Direct Synthesis
The large scale production of hydrochloric acid, as a desired primary finished product, is almost always integrated in a large scale chlor-alkali facility. In the chlor-alkali industry, salt solution is electrolyzed producing chlorine (Cl2), sodium hydroxide, and hydrogen (H2). The pure chlorine gas can be re-combined with the hydrogen gas, forming hydrogen chloride gas.
Cl2 + H2→ 2 HCl
The reaction takes place in what is commonly referred to as an acid burner. The resulting hydrogen chloride gas is absorbed in demineralized water, resulting in hydrochloric acid. The product resulting from this process is often called burner grade HCl.
HCl - Organic Synthesis
The largest volume production of hydrochloric acid is a by-product of the formation of chlorinated and fluorinated organic compounds, chloroacetic acid, and PVC. These production processes very often also consume large quantities of HCl. In the chemical reactions, hydrogen atoms are replaced by chlorine atoms, whereupon the released hydrogen atom recombines with the spare atom from the chlorine molecule, forming hydrogen chloride. Fluorination is a subsequent chlorine-replacement reaction, producing again hydrogen chloride.
R-H + Cl2→ R-Cl + HCl
R-Cl + HF → R-F + HCl
The resulting hydrogen chloride gas is either reused directly, or absorbed in water, resulting in hydrochloric acid of various grades. The HCl resulting from these processes is most commonly referred to as co-product or by-product acid.
There are three primary electrolytic process technologies in practice today. The names associated with the technology refer to the means by which the positive side of the electrolytic cell is separated from the negative side. The three technologies are mercury cell production, diaphragm cell production, and membrane cell production. Nearly 70% of all the caustic soda produced in the US today is made via the diaphragm cell process. In the rest of the world, membrane call technology accounts for almost 70% of all production. Any new capacity built today will utilize the membrane call technology.