What chemical reactions takes place in purifying acetylene?

Question

Acetylene produced by water on calcium carbide method contain several impurity compound, that are harmful for welding or other purposes of acetylene and needs to be absorbed from the gas mixture before filling the gas into cylinders. Different stages is considered in a acetylene plant in order to decrease impurities. Mainly, these impurities are Phosphine, Hydrogen sulfide, Ammonia, Arsine, water and traces amount of some other compounds. Water is absorbed by several dryer stages and ammonia by a water scrubber. The other gases are absorbed in purifier stage. Purifier absorbing material can be wet or dry. One formulation sample of dry absorbing material contains following ingredients.

Diatomaceous Earth – 35.8 % (mass percentage)
(Contains approx. 35% crystalline silica (Cristobalite))

Ferrous Chloride, 35% solution in water - 62.7 %
(Contains small amounts of ferric chloride, copper, acid)

Copper Sulfate - 1.1 %

Manganese Dioxide - 0.3 %

Mercury Dichloride - 0.023 %

Can you please direct me with the chemical reactions that take place in absorbing gases and the role of each chemical ingredient?

As mercury chloride is an environmentally hazard, what will happen if we eliminate it from the formulations? Can a formulation be found, without mercury chloride?

In acetylene code of practices it is mentioned that, Purifying material can be regenerated several times before disposal. Regeneration is done by passing air through purifier for several hours. What reactions are taking place in regeneration process?

Thank you very much.

Answer 1

The main impurities of $\ce{C2H2}$ are : 1) $\ce{H2}$ destroyed by $\ce{MnO2}$, 2) $\ce{NH3}$, destroyed by copper sulfate, mercury chloride and and ferrous chloride, 3) $\ce{PH3}$, destroyed by copper sulfate into copper phosphide, 4) $\ce{H2S}$ by copper and mercury, and 5) heavier hydrocarbons adsorbed physically on diatomaceous earth. Iron(II) is here specially for removing tiniest traces of oxygen from the air. The equations are : $$\ce{2 MnO2 + H2 -> Mn2O3 + H2O}$$ $$\ce{Cu^{2+} + 4 NH3 -> [Cu(NH3)4]^2+}$$ $$\ce{HgCl2 + 2 NH3 -> HgNH2Cl + NH4^+ + Cl-}$$ $$\ce{Fe^{2+} + 2 NH3 + 2 H2O -> Fe(OH)2 + 2 NH4^{+}}$$ $$\ce{3 Cu^{2+} + 2 PH3 -> Cu3P2 + 6 H+}$$ $$\ce{Cu^{2+} + H2S -> CuS + 2 H+ }$$ $$\ce{HgCl2 + H2S -> HgS + 2 H+ + 2 Cl^-}$$ $$\ce{4 Fe^{2+} + O2 + 2 H2O -> 4 Fe(OH)^{2+}}$$

Mercuric chloride is used to remove all sorts of thioderivatives in mixtures, like thiols (sometimes called mercaptans, meaning prompt to fix mercury) and dithianes. This reaction exploits the high affinity of $\ce{Hg(II)}$ for sulfur ligands.

Edit : Mercury can be removed from the given mixture. $\ce{H2S}$ will be removed by copper, but not thiols and not to the same level.

Edit : These reactions are not reversible