Hydrogen was probably made by Paracelsus in the sixteenth century, and was described by Johann Baptista van Helmont in 1625. It's not only the gas with the greatest intrinsic lifting power (once called "levity"), hence very important for airship development, it's also an extremely important industrial chemical.

Before the twentieth century, the principal uses of hydrogen were in ballooning and in the oxyhydrogen torch. Later, it was used to hydrogenate and reduce other chemicals. Hydrogenation of oils was introduced in 1897–1913, and the Haber-Bosch process for manufacture of ammonia from nitrogen and hydrogen in 1913. Water gas (a hydrogen-carbon monoxide mixture) was used to make methanol in 1922 and hydrocarbons (Fischer-Tropsch process) in 1935.

Hence, there will be many parties interested in finding ways to produce it cheaply, in acceptable purity, on a large scale.

The impurities will vary depending on the nature of the production process, but they typically include carbon monoxide and dioxide, nitrogen, oxygen, water vapor, hydrogen sulfide, carbon disulfide, arsine, phosphine, silane and methane. (Ellis 598). These impurities reduce lift (1% air reduces lift by 1%) and some of them attack the gas cell envelope (Greenwood 234).

Under a pressure of one atmosphere, at a temperature of 20^oC (68^oF), one pound of hydrogen gas will occupy 191.26 cubic feet (so 1000 cubic feet is 5.23 pounds), and one kilogram will occupy 11.94 cubic meters (one cubic meter is 35.2 cubic feet). A 10^oC increase in temperature will cause it to expand by 3.4%, and the corresponding temperature drop will contract it by the same percentage.

It's interesting to survey which methods of manufacturing hydrogen are mentioned in known or likely Grantville literature:

McGHEST: McGraw-Hill Encyclopedia of Science and Technology

CCD: Condensed Chemical Dictionary

MI: Merck Index

C&W: Cotton & Wilkinson, Advanced Inorganic Chemistry

EB11: Encyclopedia Britannica, 11th edition (19110
EBCD: Britannica 2002 Standard Edition CDROM, based on the Encyclopedia Britannica, 15th ed. (1998).

The provided information is minimal; details will need to be worked out. And Grantville literature definitely doesn't even list all of the methods that have been used since the nineteenth century; it's possible that some of the overlooked ones will be rediscovered.

Offord, "A Trans-Atlantic Airship, Hurrah" (Grantville Gazette 36) discussed three of these methods: "electrolysis of water, the action of acid on metal, and . . . forcing steam over red hot iron." He rejected electrolysis as requiring too much energy and acid-metal as not producing hydrogen as fast as steam-iron.

In canon, Kevin and Karen Evans, "No Ship for Tranquebar, Part Three" (Grantville Gazette 29) says that the Royal Anne carries a portable hydrogen production system that can be used in Tranquebar to refill the gas cells. This system involves "spraying water on red-hot iron," i.e. flash steam. The Grantville balloonist, Marlon Pridmore, mistakenly believes that this apparatus was used in the American Civil War. While John Wise attempted to use it in 1861, it "proved to be too cumbersome and expensive for practical use." (Haydon 7). Instead, the Union adopted the acid-iron reaction. (Tunis, Crouch). However, steam-iron apparatus was used briefly during the 1790s, and it proved to be a suitable technology for large-scale, stationary hydrogen plants.

While the steam-iron reaction is certainly a plausible basis for a hydrogen generator, I believe that it would be productive to consider the alternatives. I break these down into those for field use and those for large-scale production; note that the steam-iron process is considered in the second category, consistent with early-twentieth century practice.

Pay attention to the gas production rates; airships are big and it takes a long time to fill them. If an airship is 1,000,000 cubic feet (half the size of the Royal Anne in A Ship for Tranquebar), then at 1,000 cubic feet/hour, it would take 1,000 hours of nonstop operation. Just to complicate matters, that assumes no losses; Wilcox says that production must be at least 50% in excess of the airship capacity.

Field Production of Hydrogen

In the first attempt to deploy a balloon during the American Civil War, James Allen had his balloon inflated with city gas in Washington and then transported the inflated balloon by wagon. However, this was really not practical. One of Allen's balloons was, while wagon-tethered, blown by a gust of wind into a telegraph pole, and later John Wise had an inflated balloon caught by roadside trees. (Fanton).

The airships of the 1632 universe are likely to be far larger than these nineteenth century military balloons, and thus even less amenable to being transported by road in inflated form. Hence, the hydrogen must be either made at, or brought to, their launch site.

All of the methods described in this section have been used in the field. They are portable (they produce a considerable amount of hydrogen relative to the weight of the reactants other than water), but expensive to operate. In the late-nineteenth and early-twentieth century, armies used them only for operations remote from railroad support, as otherwise it was easier to use compressed hydrogen shipped from stationary plants. (AGLJ).

The following table compares them from a reactant portability standpoint:

(1) Greenwood 234 @20^oC; (2) Teel (various) (@40^oF.

The underlying logic of the above table is that water is probably available locally and hence needn't be transported. The table unfortunately doesn't include the weight of the apparatus itself. The apparatus would be conveyed by wagon, truck, rail car or ship. It also doesn't include the weight of fuel if heat must be supplied, e.g., to make steam.

Note that hydrogen produces about 72 pounds/1000 cubic feet of lift, so carrying the reactants around so you can make more hydrogen at your destination, to refill the airship, is a losing proposition unless you are using hydrolith or Maricheau-Beaupre processes.

Steam-iron is not on Greenwood's list of portable processes, despite its use in the Napoleonic Wars (see below), but by my calculations, you would need 97 pounds of iron. Of course, if there's no fuel available locally, that will have to be brought, too.

Acid (vitriol; wet) process. This was the first process used to manufacture hydrogen for ballooning. In essence, a hydrogen-containing acid is reacted with a metal. Usually, the acid is sulfuric acid and the metal is iron. The reaction was described by Turquet de Mayerne in 1650, but it may well have been known pre-RoF—supposedly Paracelsus knew of it (Rand 34).

Formally speaking, the reaction (with stoichiometric quantities indicated in parentheses) is:

H₂SO₄ (98 grams) + Fe (56 grams) -> H₂ (2 grams) + FeSO₄ (152 grams).

(Note that if you keep the ratios the same, you may change the units to kilograms or pounds or tons.)

Unfortunately, the method only produces 2 grams of hydrogen for every 154 grams of reactants. And please note that the above assumes pure reactants, and in even the mid-eighteenth century, the sulfuric acid was only 35–40% pure. (Wikipedia/Sulfuric Acid). It can only be purified by simple distillation to 78%.

The metal-acid reaction is also cumbersome and dangerous for military-expedient field production, because of the acid that must be carted around. On April 11, 1862, the single line tethering General Porter's balloon broke, having been damaged by an acid spill, resulting in an unplanned free ballooning experience (Crouch 375). In 1830, on the brig Vittoria, the balloonist's "carboys of sulfuric acid were accidentally broken by the rolling of the ship, and caused a fire that resulted in damage amounting to some 80,000 francs." (Haydon 17). During the Spanish-American War, it was reported that if the acid were kept in glass carboys, the stoppers were often knocked out or the necks of the carboys broken during transport over rough roads. Lead-lined iron cylinders proved more convenient, but then the lining had to be perfect, to avoid acid leakage. (Maxfield).

The transport safety issue could be addressed by producing the sulfuric acid at the launch site by the seventeenth-century Glauber process. That is, you use saltpeter and steam to convert sulfur to sulfuric acid. This can be done in glass or lead-lined chambers. Of course, you then have to cart sulfur, saltpeter, the reaction chamber, and fuel for the boiler.

Alternatively, you can use the original Philips 1831 version of the contact process. This needs just sulfur, a platinum catalyst, and heat, not saltpeter, so it's more portable. For availability of platinum, see my prior chemical and mineralogical articles. Note that platinum catalysis is poisoned by arsenic impurities in the sulfur.

There are other problems with hydrogen production from the metal-sulfuric acid reaction. Iron is likely to contain sulfur, which reacts to form hydrogen sulfide. Depending on the carbon, phosphorus, sulfur, arsenic and silicon content, it may form significant amounts of methane, phosphine, hydrogen sulfide, arsine and silane. (Teed 41, Molinari 133).

Zinc (an iron substitute) is perhaps less liable to contribute significant impurities, but it and sulfuric acid may both contain arsenic, which will form arsine. (Englehardt 124; Greenwood 230; Teed 42). All of the cited impurities necessitate purification treatments; note that acids will attack the envelope. At a minimum, you will want to pass the gas through water.

While more zinc than iron is required (the weight ratio of metal to acid must be 0.66:1 rather than 0.57:1), and in the seventeenth century zinc would be the more expensive metal. In the early-twentieth century the byproduct zinc sulfate was of greater commercial value than iron sulfate. (Teel 42). Iron sulfate may be used in manufacturing other iron compounds and iron gall ink, as a mordant, and as a developer in the collodion process. Zinc sulfate may be used to remedy zinc deficiency in soil, to coagulate viscose rayon into fibers, in the manufacture of lithopone, in zinc plating, and as a mordant, preservative, and corrosion inhibitor.

Also, zinc is higher on the reactivity series, and hence hydrogen production is likely to be faster. (And still more reactive metals, such as aluminum, will react even with water, and the acid can be dispensed with—see below.)

Still, the iron-acid reaction appears to have been respectably rapid in practice. On April 5, 1862, Lowe arrived at the lines shortly after noon and ascended at 5:20 (Crouch 375). So, in five hours, he inflated a balloon, and the smallest of his balloons was 15,000 cubic feet. (358).

As the reaction progresses, the metal is covered with the sulfate, protecting it from further reaction. In the nineteenth century, mechanical and hydraulic devices were devised to scrape or wash off this coating. (Molinari 133).

The vitriol process was first used for ballooning by Jacques Charles in 1783; he reacted 1100 pounds of iron filings with 550 pounds of acid. It took three days and nights to produce enough hydrogen to fill a balloon with a capacity of less than 1400 cubic feet. (Sander). Note that Charles used too much iron and not enough acid, a mistake that someone with modern high school chemistry wouldn't make. It's possible that he had problems exposing all of the iron to the acid.

In 1785, Aime Argand produced hydrogen for the Blanchard-Jeffries crossing of the English Channel. Jeffries paid 100 guineas for materials, most of which was "spent on the most expensive item, the acid." Argand's work area was about 100 feet in diameter. He placed fifty pounds of "parings of iron plates" and one hundred pounds of "cast iron trimmings" in the bottom of each of twenty-six 54-gallon half-barrels, and added hundred pounds of sulfuric acid to each vessel. That implies use of 3,900 pounds of iron and 2,600 pounds of sulfuric acid to produce. the required 9,000 cubic feet of hydrogen. The half-barrels were capped with an upended tub with a central pipe; hydrogen rose through this pipe into a leather hose, which conveyed it to a larger barrel for "purification and cooling before transfer to the balloon." Occasionally, the half-barrel was opened and the iron stirred around with an iron rod, to expose fresh surface. (Crouch 81).

The acid process was also used by Union balloonists during the American Civil War. Thaddeus Lowe designed the army's portable gas generators. The generator was a strongly braced wooden box 11 feet long, 5 feet high, and 3 feet wide, which meant that it was able to fit in a standard wagon body. There was a manhole on top and a rear door. It was acid-proofed inside, with shelves to hold 3,300 pounds of iron filings, submerged in three feet of water. Sulfuric acid was introduced through a rooftop copper funnel, and the produced gas rose through a six-inch hose. This hose was connected to the cooler box, which was five feet long and has a smaller box inverted inside. The cooler box was partially filled with water; the gas entered the cooler box underwater and bubbled up, around cooling baffles, eventually escaping into a second hose. This conducted the gas to the purifier box, which was similarly constructed, but contained a limewater solution. (That would absorb carbon dioxide.) A third, twelve-inch hose ran from the purifier box to the balloon. (Crouch, 358; Tunis 88–89).

A British observer reported in 1862 that the iron was inexpensive since "any old iron" would do, and that the sulfuric acid "in large quantities is cheap, and with proper precautions, very easy to carry." (Templer 174).

The acid-iron process was used to fill the giant (118 feet diameter, 882,900 cubic feet) captive balloon erected by Giffard for the 1878 Paris Exhibition; 190 tons acid and 80 tons iron were consumed. (Baden-Powell 741).

In 1883, if 250 pounds of iron were used to make 1,000 cubic feet of gas, the cost of production was 1£ 5s per 1,000 cubic feet. A good portable apparatus filled a 6,000 cubic foot balloon in four hours. (Powell).

In the Andree North Pole expedition of 1893, zinc was used; the costs in kroner were estimated to be 1950 for the apparatus (producing 5300 cubic feet/hour), 3,000 for the raw materials (zinc and sulfuric acid, in 20% excess), and 1600 for the technical expert's salary. However, the expert decided to use wrought iron instead of zinc. (Capelloti 149). Nonetheless, the 1896 British manual of military ballooning favored zinc. (Taylor 169). In the 1632 universe, zinc is a rather rare commodity, and so our aeronauts will almost certainly use iron.

The acid process was again used by the 1907 Wellman expedition. My source mentions a number of interesting points, including that perfume is added to the hydrogen stream (by passage through sponges filled with muronine) so leaks are readily detectable, and the gas is piped through coke (to dry it), caustic soda (to remove residual acid), potassium permanganate (to remove arsine, stibine and phosphine?) and calcium of lime (to remove carbon dioxide?). (Capelloti 152ff).

Nonetheless, even in the early-twentieth century, the acid process was considered too expensive for large-scale industrial production, unless the hydrogen was simply a byproduct of producing a salable metallic salt. (Ellis 515). For example, in 1904, figuring zinc at 9.75 cents/kg and sulfuric acid at 1.75, 1 kg zinc and 2 kg acid would theoretically produce 32 grams (about 360 liters) hydrogen for 12.25 cents. Taking into account that the metal-acid reaction is usually incomplete, the electrolytic method (see Large Scale Production below) could produce the same volume with 800 ampere-hours (2 kwh @ 2.5V), with a then cost of power of 0.5 cents (hydroelectric) or 2.5 cents (coal/steam)(Englehardt 125). Ellis (518) adds that "the operating cost of an electrolytic plant [in 1917] is one-fifth that of a zinc-acid plant and there are no acid-eaten hydrogen pipes or freeze-ups in winter."

Base-metal process. This is alluded to by EB11/Hydrogen, which recommends reacting sodium or potassium hydroxide with zinc or aluminum, or zinc with an ammonium salt other than nitrate. The zinc-sodium hydroxide reaction produces hydrogen of high purity, but it has arsine (from the zinc) and caustic soda impurities.

Teel (44) says that the reaction of zinc with magnesium hydroxide has been used for ballooning.

In the Russo-Japanese War, the Russians reacted 30% caustic soda with aluminum scrap. They transported twenty-four generators and six coolers (the reaction generated a lot of heat) with the aid of fifteen horses, and this setup was sufficient to fill a 400 cubic meter balloon in thirty minutes. (Molinari 134).

Base-Carbon. A base like caustic soda may be reacted with coal to produce hydrogen:

4NaOH + C -> Na₂CO₃ + Na₂O + 2H₂.

The coal may generate methane, arsine or hydrogen sulfide impurities. (Teel 60).

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Alkali (Alkaline) Metal-Water. The most common reaction is

2Na (46 g) +H₂O (18 g)->2NaOH (62 g) +H₂ (2g).

The reaction of water with sodium is much more vigorous than that with iron; the water need not be provided in the form of steam. In fact, the reaction had to be slowed down, for example, by supplying the water as a fine spray, or incorporating the sodium into a briquette with an inert binder. (Taylor 127).

Since water would be available in the field, only the sodium, a light metal, had to be transported. The catch was that metallic sodium was expensive—5s/pound in 1883, so 1,000 cubic feet of hydrogen would cost 22£. (Powell). Also, sodium was dangerous to transport, because of its reactivity with water.

Other alkali metals, such as lithium, could be used in placed of sodium; 22.5 pounds of lithium hydride, reacted with equivalent water, would produce 1,000 cubic feet hydrogen. (Roth 30). They are, if anything, more expensive than sodium so these are strictly laboratory methods.

The same is true of the alkaline earth metals, of which magnesium will probably be the cheapest.

Aluminum amalgam-water. If a small amount of mercury (or mercuric salt) is added to aluminum powder, to make an amalgam, the latter will react with water to form aluminum oxide, hydrogen, and pure mercury. The latter may be reused to make more amalgam. One pound of aluminum yielded 20.5 cubic feet hydrogen. (Taylor 129).

This procedure was practical in the early-twentieth century, thanks to the Hall-Heroult electrolytic process for making aluminum.

The Mauricheau Beaupre "activated aluminum" variant involves adding water to a mixture of fine aluminum filings, mercuric chloride, and mercuric cyanide. One kilogram solid mixture, so reacted, yields 1.3 cubic meters in about two hours. The apparatus required is minimal. (Ellis 525). The aluminum must not contain copper (Teel 70).

Hydrolith Process. This was another field expedient, exploiting the reaction

CaH₂ (42 grams) +2H₂O (36 g) -> Ca(OH)2 (56 g) +2H₂ (4 g).

So only 55 pounds of calcium hydride is needed to obtain 1,000 cubic feet hydrogen. The calcium hydride would be made at base, from a calcium salt (oxide, chloride) and hydrogen in presence of a reducing agent (sodium, magnesium).

The French used this system in the early-twentieth century; the calcium hydride was carried on latticed trays, immersed in water; the hydrogen rose up. This gas was contaminated with water vapor, which was removed by passing it over dry calcium hydride. (Taylor 128). You also need to remove ammonia, and heat evolution can be a problem. A typical six generator wagon produces 15,000 cubic feet/hour. (Greenwood 229).

A related, speculative process uses lithium hydride:

LiH₂ (9 grams) +4H₂O (72g) -> 4 LiOH (75g)+ 3H₂ (6g).

Note the enormous yield of hydrogen relative to the amount of lithium hydride. This would be great for the field. The ratio is good enough so it's feasible (from a weight, not necessarily a safety standpoint) to bring the lithium hydride on board for use at a destination to make more hydrogen. It has even been suggested that the reaction could be used to produce hydrogen while in flight, reacting the hydride with water ballast (warning: this can be a violent reaction!), and then dropping the lithium hydroxide. (Teel 67). But the cost of lithium hydride, which is made by reacting lithium metal with hydrogen; is prohibitive (even 1992 price was $72/kg—Kirk-Othmer).

Silicol Process. The basic reaction was

2NaOH+Si+H₂O->NaSiO₂+2H₂.

It was first proposed in 1909, and became a popular military field expedient, especially on ships. Not only were the ingredients quite safe to transport, the produced hydrogen was of "very high purity." (Taylor 143).

Initially, commercially pure silicon was used, but this was replaced by the cheaper ferrosilicon, which was used for deoxidizing steel and introducing silicon into alloys. Ferrosilicon may be made by reducing sand (silica) with coke in the presence of iron. The ferrosilicon typically contains small amounts of phosphine, arsine, and hydrogen sulfide. (Greenwood 227), as well as air. (Teel 45). The silicon content has to be over 80% for reasonable effectiveness, and particle size affects the production rate. (Teel 50). The caustic soda must be neither too dilute nor too strong. (Teel 52).

In addition, there is an explosion hazard. The ferrosilicon dissolves only slowly in cold solution, and thus can accumulate. But the reaction produces heat, and as the solution gets hotter, the accumulated ferrosilicon is attacked, leading to rapid evolution of hydrogen. (Teel 57).

A transportable plant can produce 60–120 cubic meters/hour, whereas stationary plants of up to 300 capacity have been constructed. (Ellis 523) [The typical portable plant was mounted on a three ton truck and produced 2,500 cubic feet/hour; the largest portable apparatus produced 14,000 cubic feet/hour. The reaction has also been used for stationary production at up to 50,000 cubic feet/hour. (Greenwood 226).

For the 1929 British R100 airship, "249 tons of caustic soda and 183 tons of ferro-silicon produced 8,610,705 cu.ft of hydrogen (20.3 tons) and 929 tons of sludge [sodium silicate]." (Wilcox). The R100 had a gas capacity of about 5,000,000 cubic feet, so the gas produced was substantially in excess of the capacity. Wilcox's information about production rate is somewhat contradictory. He says that the plant could produce 60,000 cubic feet/hour, but that the highest daily production was 500,000 cubic feet. Also, that it took ten days to fill fourteen of the R100's fifteen gas bags.

An alternative reaction that can use the same apparatus exploits the reaction of aluminum with sodium hydroxide, and was used by the Russians in the Russo-Japanese War. (Taylor 145–6). It can produce 10 cubic meters/hour. (Ellis 523).

Hydrogenite process. This starts with a compressed block of a mixture ("hydrogenite") of silicon, caustic soda, and soda lime, kept in an air-tight container. To use, the container is placed in a water jacket, a match or a red hot wire is applied to a small hole in the lid. The silicon is oxidized to silica, a heat-releasing action. This heat makes possible the reaction

Si+Ca(OH)₂+2NaOH->Na₂SiO₃+CaO+2H₂.

The heat turns the water to steam and eventually this is permitted to enter the generator, increasing yield by a reaction of the silicol type.

While it requires that 50% more material be provided than for the silicol process, much less water is needed, which would be advantage for desert use. (Taylor 168). A production rate of 150 cubic meters/hour is possible(Ellis 521ff). The portable wagon-based apparatus of the French army, featuring six generators grouped around a central washer, produced 5000 cubic feet/hour. (Greenwood 228).

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Another "dry" method (by Majert and Richter) involves heating a mixture of zinc dust and slaked lime to redness, but the Prussian army deemed it too slow (it took 2–3 hours to fill a balloon). (AGLJ).

Large-Scale Production

Some processes are best suited to production of hydrogen on a large scale and at a low cost. Unless the airship hangar happens to be near the manufacturing plant, the gas will have to be compressed and shipped in containers (which must be returned empty), which increases the cost.

Steam-Carbon. First, water gas (a mixture of carbon monoxide and hydrogen) is produced by reacting red-hot coke or coal with steam at 800 or 1000^oC (2002McGHEST):

H₂O (18 grams) + C (12 grams) -> H₂ (2 grams) + CO (28 grams).

Just making steam, by itself, consumes fuel. According to EB11/Railways, the faster you burn coal, the lower the efficiency. With Indiana block coal (13000 BTU/lb):

Those are for a 1900 locomotive boiler. and a stationary plant might have a higher efficiency. Additional coal would need to be burnt to superheat the steam to the required temperature. The increase in coal consumption to achieve 100^oC superheat is 5.5%, for 150, 8.3%, and for 200, 11%. (Stovel 1475). (Superheated steam is more efficient than ordinary steam, however, in terms of the heat content of the steam relative to that of the coal burnt to produce it. (Babcock 137ff).

With the Baldwin experimental locomotive 60,000 (1926), designed for high efficiency, evaporation declined from 10 to 6.5 pounds water per pound of dry coal, as firing rate increased from 30 to 150 lb/ft² grate/hr. and superheat increased from 180^oF to 257^oF. (Pennsylvania RR, Fig. 19).

If you burn carbon in air, the hydrogen will be contaminated with nitrogen from the air. This can be avoided by burning pure oxygen into carbon monoxide, but then you must provide the oxygen somehow.

The process can be operated on a mostly continuous basis; occasionally clinker must be removed. (Teel 81). Water gas has impurities, such as hydrogen sulfide and ash (84).

Water gas in turn can undergo this shift reaction, discovered by Felice Fontana in 1780:

CO (28 grams) + H₂O (18 grams) -> CO₂ (44 grams) + H₂ (2 grams)

Since exposure to CO (carbon monoxide) is dangerous, naturally there was interest in conducting the steam-carbon reaction in such a manner as to minimize its formation, i.e., to obtain the mixture of carbon dioxide and hydrogen:

2H₂O + C -> CO₂ + 2H₂.

Gillard found that this could be accomplished by use of an excess of steam. The carbon dioxide can be removed on a batch basis (see below), but unfortunately it proved "very difficult to carry this out in practice on a large scale. . . ." (Sander).

BAMAG worked at a low temperature (at which the reaction equilibrium is favorable), but with catalysts (typically nickel) to speed up the reaction. This results in what is reportedly the cheapest method of producing hydrogen (1 shilling/9 pence per 1000 cubic feet), but unfortunately the product contained 4% nitrogen, a serious disadvantage for aeronautical use. (Greenwood 162). 2002McGHEST suggests a reaction at 350^oC over an iron oxide catalyst.

Griesheim-Elektron instead disturbed the water gas equilibrium by "absorbing" the carbon dioxide with lime or other alkali. Cost of production (1912) was 2s/2s.5p–2s/9p per 1000 cubic feet for a moderate size plant. While the process can be carried out at a lower temperature than the steam-iron process below, reducing maintenance costs, "the handling of the large amounts of lime presents some difficulty." (Greenwood 167ff).

Of course, we can eschew the shift reaction, and remove the carbon monoxide with an "absorbing agent" or by liquefaction. EB11/Carbon notes that it is "rapidly absorbed by an ammoniacal or acid (hydrochloric acid) solution of cuprous chloride," but the resulting hydrogen is only 80% pure. (Sander) Later, Frank and Caro thought of employing heated calcium carbide. This conveniently "absorbed" not only carbon monoxide, but also carbon dioxide and nitrogen, and in the process produces graphite and calcium cyanamide. (Sander; Elis 597).

Liquefaction (Linde-Frank-Caro method) at -200^oC works well, but small-scale plant costs are high (Ellis 460) and concerns have been expressed about the dangers of working with compressed carbon monoxide (595). In 1912, a plant producing 3500 cubic feet hydrogen/hour cost about 13,000 pounds, and had a cost of hydrogen production of 3 to 4 shillings per 1000 cubic feet. (Greenwood 174).

A little more explanation of liquefaction may come in handy. A gas can only be liquefied if cooled below its critical temperature; at that temperature, it must be compressed to the critical pressure; at lower temperatures, lesser pressures are needed for condensation.

(Teel 114ff).

It can be seen that cooling water gas to -200^oC ...