The following article describes the cleaning up process for coal gas so that it was fit to use in the home – it also provided a way of reclaiming valuable chemicals. The article comes from the South Metropolitan Gas Co’s house journal ‘Co-partnership Journal” in 1902/3. The photocopy is very poor quality and some parts (including the exact date!) are missing or illegible. The pictures are very poor quality but I have included the best ones. It basically describes the process used at East Greenwich Gas Works. Of interest is that the process was patented (but no invented) by Frank Hills whose East Greenwich Chemical Works was bought by South Met. after his death and is the Phoenix Works mentioned in the article. The Livesey Washers mentioned were devised by a young George Livesey at the Old Kent Road Gas Works
THE PURIFICATION OF COAL GAS
Last month some of us learned and some of us heard again how gas is made by the carbonisation of coal. In our interest to learn more about retort houses and the nature of the work performed in them, abruptly left the gas as it passed on its way to the exhausters. But the story of gas manufacture does not end in the retort house, and we will now endeavour to follow the gas on its passage through the works plant to the gasholder
The gas that leaves the coal and passes up the ascension pipe would not be recognised as even a distant relative of the cleanly and labour-saving domestic fuel which we sell. In fact, there can be no more apt description of it at this point than its colloquial name of “smoke”. Dirt has been described as matter out of place, and much the same might be said of the impurities present in the crude coal gas. Tar, ammonia, water, sulphur, naphthalene and the other impurities present are all excellent and useful substances in their right places, but in no instance is that place in the gas. It is the object of gas purification not only to remove all these, but to obtain them in a form
Properly speaking, purification commences in the hydraulic main where much of the tar and some of the water from the gas is deposited. As, however, the gas is still hot when it leaves the hydraulic main, it carries with it a considerable quantity of water vapour and some tar in addition to its other impurities. Before being cooled, the gas passes through the exhausters, which give to it sufficient pressure to enable it to continue its journey, and from the outlet of the exhausters it passes directly to the condensers.
Modern gas condensers are steel structures containing a number of steel partitions which divide the condensers up into numerous compartments. Cold water flows through some compartments and gas through adjacent ones, so that each steel partition receives heat from the gas on one surface and passes it through to the cold water which flows past its < opposite surface. Fresh cold water enters at the end of the condensers from which the gas leaves, and the heated water, which is still cooler than the incoming gas leaves form the end at which the hot gas enters. This principle of counter current flow which we meet in the condensers is utilised also in the ammonia extraction and naphthalene extraction plants which come later in the story.
When water-cooled condensers were first employed, the rapid cooling of the gas caused solid constituents of the tar to form into compact masses which, from time to time, blocked the gas passages and made necessary, the very unpleasant task of cleaning out by hand. The Carpenter condensers used by the Company have for many years been freed from this trouble by the simple expedient of washing the gas cooling surfaces with a gentle stream of tar in which these solid obstructions are readily soluble.
The process of condensation cools the gas down to atmospheric temperature and causes the deposition of nearly all the water carried and of most of the remaining tar. As the gas contains considerable quantities of ammonia, the water condensed from the gas also contains ammonia, and this condensate, together with that from the hydraulic main, is known as virgin liquor to distinguish it from the liquor made by washing the gas with fresh water.
(words missing) harder to destroy than a London particular. The dual task of removing this most persistent fog and of commencing. The extraction of the remaining ammonia is performed by the Livesey washers. Imagine a large mushroom with a head that is square instead round, and
in the top surface of this head punch a very large number of small, holes until it resembles a well-used pin-cushion. Now invert the mushroom in a tank of water and blow gas down through the stalk. The result will be foam of very fine bubbles of gas similar to those which, when this principle is applied in a Livesey washer, rid the gas of the last traces of tar.
From the Livesey washers the gas passes to the scrubbers,where tall-cylindrical towers packed with thin wooden boards stand on edge. Into the top (words missing) be kept down to the minimum quantity necessary for complete ammonia extraction, it has to be evenly spread over the top of the Scrubber packing.
To do this” use is made of the ingeniously simple “Barker’s mill”. This consists of a length of pipe closed at each end and pivoted in the’ middle so that it’ may rotate horizontally. On one side of the ‘pivot, holes are drilled in the pipe looking in one direction, while on the other side of the pivot similar holes look in the opposite direction. When water is admitted to the pipe, fine jets of water shoot out from these holes. Those who have used a fire hose will realise the force with which a jet of water pushes back on the hole from which it issues. It is this force exerted upon the pivoted pipe that causes it to rotate while the water sprays from it. Mirrors fixed to the mill on the top of the scrubber tell the plant attendant that all is well with the spreader.
By the time that the water has reached the bottom of the scrubber it has become weak ammoniacal liquor. It is then pumped to the top of a second scrubber where it extracts more ammonia from gas which has just left the Livesey washers. (words missing) to its final strength. We may now leave the tar and liquor to the tar busters or tar and liquor plant attendants and, having seen the tar safely sent off to Ordnance Wharf and the liquor to Phoenix Wharf, we will follow the gas to the oxide purifiers.
On entering these the gas contains about one per cent by volume of hydrogen sulphide, one-eighth of one per cent of prussic acid gas, and one-thirtieth of one per cent of carbon bisulphide. Hydrogen sulphide is a gas which is famous for justifying the schoolboys’ name of “stinks” as applied to the science of chemistry. Both it and the other two impurities are highly poisonous. The sulphur compounds when burned give rise to sulphur dioxide which is, if breathed, extremely irritating to the lining of the throat and nose, and would rapidly destroy metalwork, curtains and, book bindings if this impure gas were burned in the home.
The oxide purifiers are rectangular boxes of cast iron or reinforced concrete about 4 or 5 feet deep. Hardwood grids are used, to support the oxide of iron which absorbs the impurities; they are generally arranged so as to give three separate layers of oxide one above another in each box. Four such boxes form a stream of primary purifiers and, by a suitable arrangement of valves, gas may be passed first into anyone of the four and then, in succession, through the other three. The iron oxide used comes from two sources. Some of it is a natural peat-like deposit found underlying grassland in Holland, and the remainder is a by- product from the refining of bauxite (from which mineral aluminium is produced).
The chemistry of hydrogen sulphide removal has been pictorially conceived in a delightful manner by Friedrich Lux, but these two pictures of his, reproduced here; do not quite complete the story. (these photocopied prints are not good enough to scan, sorry). The chivalrous iron knights are not perpetually charged with guarding the sulphur demons. A small measured quantity of air is admitted with the gas, and thus a fresh supply , of oxygen chaperones for the hapless hydrogen maidens enables the knights to bind the demons, eight at a time, and .releases them for further deeds of gallantry. The bound sulphur demons are harmless to the gas and accumulate in the iron oxide until, when the purifying mass contains about hall its weight of sulphur (words missing)
At the Products Works matter is put in its right place, and the sulphur which was an impurity in the gas is converted to use as sulphuric acid. Some of this acid is caused to combine with the ammonia which reached Phoenix Wharf as liquor, the combination of these two impurities producing the valuable fertiliser Metro Sulphate.
While some of the iron oxide has been engaged in arresting, hydrogen sulphide, other portions have done battle with prussic acid gas, and, by an interesting series of chemical changes, have converted it to Prussian, blue-the product which fills the wash-day blue bag.
The majority of gas companies do not trouble to remove carbon bisulphide, but the Company has shown how, by the co-operation of engineer and chemist, a process for the removal of this objectionable compound can be made a practical proposition. Sulphur in this form is not readily absorbed by any suitable reagent. If, however, gas free from hydrogen sulphide is heated, in the presence of nickel, the carbon bisulphide is converted to hydrogen sulphide; and this can then be removed by a second (words missing) but to carry it through cheaply, efficiently and without interruption for every hour of every day has provided problems requiring much careful thought and experiment for their solution.
On leaving the secondary purifiers the gas contains no tar, less than one forty-thousandth part of carbon bisulphide, and immeasurably small traces of other impurities, but it still is not pure enough to send on to the District. Mention has been made of the solid substances in the tar which used to block early types of condensers. There is one of these substances, naphthalene, which passes right through the purification system and is found in small quantities even at the outlet of the secondary purifiers.
In the form of moth balls, and as a starting-point in the manufacture of dyestuffs, naphthalene is very useful. If sent on to the District in the gas, however, if exercises its prerogative of crystallising in pearly white flakes in gas consumers service pipes. These crystalline flakes are quite beautiful in appearance, but, it needs only one of them to lodge in a gas tap or the bend of a service pipe for the supply of gas through that tap or pipe to fail. (words missing)