History of Manufactured Gas - Appliances and Machinery of The Historic Gas-works - Other Gas-Works Facilities - Purifier

Purifier

Coal gas coming directly from the bench was a noxious soup of chemicals, and removal of the most deleterious fractions was important, for improving the quality of the gas, for preventing damage to equipment or premises, and for recovering revenues from the sale of the extracted chemicals. Several offensive fractions being present in a distributed gas might lead to problems – Tar in the distributed gas might gum up the pipes (and could be sold for a good price), ammoniacal vapours in the gas might lead to corrosion problems (and the extracted ammonium sulfate was a decent fertilizer), naphthalene vapours in the gas might stop up the gas-mains, and even carbon dioxide in the gas was known to decrease illumination; thus various facilities within the gas-works were tasked with the removal of these deleterious effluents. But these do not compare to the most hazardous contaminant in the raw coal gas: the sulfuret of hydrogen (hydrogen sulfide, H₂S). This was regarded as utterly unacceptable for several reasons:

1. The gas would smell rankly of rotten eggs when burnt;
2. The gas-works and adjacent district would smell of rotten eggs when the gas-works was producing gas;
3. The gas, upon burning, would form sulfur dioxide, which would be quickly oxidized to sulfur trioxide, and subsequently would react with the water vapor produced by combustion to form sulfuric acid vapour. In a dwelling-house, this could lead to the formation of irritating, poisonous and corrosive atmospheres where and when burnt.
4. Manufactured gas was originally distributed in the well-to-do districts, as such were low-hanging fruit for the gas utility. Such persons were of a class known to possess silver goods of varying sorts. If exposed to a sulfurous atmosphere, silver tarnishes – and a sulfurous atmosphere would undoubtedly be present in any house lit with sulfuretted gas.

As such, the removal of the sulfuret of hydrogen was given the highest level of priority in the gas-works. A special facility existed to extract the sulfuret of hydrogen – known as the purifier. The purifier was arguably the most important facility in the gas-works, if the retort-bench itself is not included.

Originally, purifiers were simple tanks of lime-water, also known as cream or milk of lime, where the raw gas from the retort bench was bubbled through to remove the sulfuret of hydrogen. This original process of purification was known as the "wet lime" process. The lime residue left over from the "wet lime" process was one of the first true "toxic wastes", a material called "blue billy". Originally, the waste of the purifier house was flushed into a nearby body of water, such as a river or a canal. However, after fish kills, the nauseating way it made the rivers stink, and the truly horrendous stench caused by exposure of residuals if the river was running low, the public clamoured for better means of disposal. Thus it was piled into heaps for disposal. Some enterprising gas entrepreneurs tried to sell it as a weed-killer, but most people wanted nothing to do with it, and generally, it was regarded as waste which was both smelly and poisonous, and gas-works could do little with, except bury. But this was not the end of the "blue billy", for after burying it, rain would often fall upon its burial site, and leach the poison and stench from the buried waste, which could drain into fields or streams. Following countless fiascoes with "blue billy" contaminating the environment, a furious public, aided by courts, juries, judges, and masters in chancery, were often very willing to demand that the gas-works seek other methods of purification – and even pay for the damages caused by their old methods of purification.

This led to the development of the "dry lime" purification process, which was less effective than the "wet lime" process, but had less toxic consequences. Still, it was quite noxious. Slaked lime (calcium hydroxide) was placed in thick layers on trays which were then inserted into a square or cylinder-shaped purifier tower which gas was then passed through, from the bottom to the top. After the charge of slaked lime had lost most of its absorption effectiveness, the purifier was then shut off from the flow of gas, and either was opened, or air was piped in. Immediately, the sulfur-impregnated slaked lime would react with the air to liberate large concentrations of sulfuretted hydrogen, which would then billow out of the purifier house, and make the gas-works, and the district, stink of sulfuretted hydrogen. Though toxic in sufficient concentrations or long exposures, the sulfuret was generally just nauseating for short exposures at moderate concentrations, and was merely a health hazard (as compared to the outright danger of "blue billy") for the gas-works employees and the neighbors of the gas-works. The sulfuretted lime was not toxic, but not greatly wanted, slightly stinking of the odor of the sulfuret, and was spread as a low grade fertilizer, being impregnated with ammonia to some degree. The outrageous stinks from many gas-works led many citizens to regard them as public nuisances, and attracted the eye of regulators, neighbors, and courts.

The "gas nuisance" was finally solved by the "iron ore" process. Enterprising gas-works engineers discovered that bog iron ore could be used to remove the sulfuretted hydrogen from the gas, and not only could it be used for such, but it could be used in the purifier, exposed to the air, whence it would be rejuvenated, without emitting noxious sulfuretted hydrogen gas, the sulfur being retained in the iron ore. Then it could be reinserted into the purifier, and reused and rejuvenated multiple times, until it was thoroughly embedded with sulfur. It could then be sold to the sulfuric acid works for a small profit. Lime was sometimes still used after the iron ore had thoroughly removed the sulfuret of hydrogen, to remove carbonic acid (carbon dioxide, CO₂), the bisulfuret of carbon (carbon disulfide, CS₂), and any ammonia still aeroform after its travels through the works. But it was not made noxious as before, and usually could fetch a decent rate as fertilizer when impregnated with ammonia. This finally solved the greatest pollution nuisances of the gas-works, but still lesser problems remained – not any that the purifier house could solve, though.

Purifier designs also went through different stages throughout the years.