Chapter 3

Industrial Gases

Three inorganic gases, nitrogen, oxygen, and carbon dioxide, appear inthe top 50 chemicals. A fourth gas, hydrogen, would also be included if itwere not for the large amounts of captive use of hydrogen to manufactureammonia, which makes it difficult to estimate hydrogen production. It isconvenient to discuss all four at this time in our study of inorganicchemicals. Two of them, nitrogen and hydrogen, are used to produceammonia, which in turn has important derivatives that will be discussed inthe next chapter. Not all four major gases are manufactured by the samemethod. Nitrogen and oxygen, obtained by the liquefaction of air, will bediscusses first. Next, carbon dioxide and hydrogen, made by the process ofsteam-reforming of hydrocarbons, will be considered.

1. NITROGEN

1.1 Manufacture

The large-scale availability of nitrogen, oxygen, and argon fromliquefaction of air began about 1939-40. A 90% recovery is now feasible forthese three major components in air. Nitrogen makes up 78% of all air,oxygen 21%, and argon 0.9%. Two major processes are used, differing onlyin the way in which the expansion of air occurs. The Linde-Frankl cycle isbased on the classic Joule-Thompson effect of a gas, which means that thereis a tremendous cooling effect of a gas when it is rapidly expanded, eventhough no external work is done on the system. Alternatively, the Claudeprocess employs an expansion engine doing useful work on the gas. The

Figure 3.1 Liquefaction of air.

temperature is reduced because of the removal of energy. This process ismore efficient than relying on the Joule-Thompson effect.

Fig. 3.1 outlines the liquefaction of air. Air is filtered to removeparticulates and then compressed to 77 psi. An oxidation chamber convertstraces of hydrocarbons into carbon dioxide and water. The air is then passedthrough a water separator, which gets some of the water out. A heatexchanger cools the sample down to very low temperatures, causing solidwater and carbon dioxide to be separated from the main components.

Most of the nitrogen-oxygen mixture, now at -1680C and 72 psi, entersthe bottom of a fractionating column. An expansion valve at this pointcauses further cooling. The more volatile nitrogen rises to the top of thecolumn as a gas since nitrogen (bp = -1960C, 770K) has a lower boilingpoint than oxygen (bp = -1830C, 9O0K), and the column at 830K is able toseparate the two. The oxygen stays at the bottom of the column as a liquidbecause it is less volatile.

A small amount of nitrogen-oxygen mixture after being recooled in theheat exchanger is shunted to the main expander valve (operated by the Joule-Thompson effect or by a Claude engine). This extremely cold gas isrecycled into the heat exchanger to keep the system cold. Some argonremains in the oxygen fraction and this mixture can be sold as 90-95%oxygen. If purer oxygen is required, a more elaborate fractionating columnwith a greater number of plates gives an oxygen-argon separation. Oxygencan be obtained in 99.5% purity in this fashion.

Argon can be obtained from a middle fraction between nitrogen andoxygen and redistilled. A small amount of hydrogen can be added to reactwith any remaining oxygen to give oxygen-free argon. Not only argon, butother rare gases, neon, krypton, and xenon, can also be obtained in

SolidH2O, CO2

-1530C72 psi

Some N2, O2

-1870C7.5 psi

Expander valveor Claude engine

Air Filter Compressor Oxidationchamber

Waterseparator

N2 gasbp-196°C

770K

O2 liquidbp-183°C

9O0K

Fractionatingcolumn and

expander valve

-19O0C, 830K7psi

Heatexchanger

Table 3.1 Merchant Uses of Nitrogen

Chemicals 33%

Oil & gas extraction 14

Electronics 13

Primary metals 11

Petroleum refining 10

Food industry 5

Glass 2

Rubber & plastics 1Miscellaneous 11

Source: Chemical Economics Handbook

separations. Helium is not obtained from liquefaction of air. It occurs inmuch greater concentrations (2%) in natural gas wells and is isolated in thepetroleum refinery.

1.2 Uses

By far the largest use of nitrogen is in ammonia synthesis. However, thisuse is not included here because it is "captive," that is, the same companyimmediately reuses the gas internally to make another product, in this caseammonia. This nitrogen is not isolated, sold, or inventoried. Only"merchant" use is included in Table 3.1. Chemicals manufactured withnitrogen include many reactions where nitrogen is the inert blanketingatmosphere to prevent reaction with oxygen and minimize the possibility offire and explosion for reactions sensitive to oxygen. Oil and gas extractionis a fast-growing use of nitrogen. This application is called advanced oilrecovery (AOR) or enhanced oil recovery (EOR), where nitrogen maintainspressure in oil fields so that a vacuum is not formed underground whennatural gas and oil are pumped out. It competes with carbon dioxide in thisapplication. In the electronic industry nitrogen is an important blanketingand purge gas in the manufacture of semiconductors and integrated circuits.It is used in liquid form for cryogenic (low temperature) testing. In primarymetals manufacture it is an inert atmosphere for making steel, blanketing thepowdered coal and other fuel for the furnace. Petroleum refining makes useof it for its inert atmosphere and the food industry uses liquid nitrogen forfreezing.

Table 3.2 Merchant Uses of Oxygen

Primary metals production 49%

Chemicals & gasification 25

Clay, glass, & concrete products 6

Petroleum refineries 6

Welding & cutting 6

Health sciences 4

Pulp & paper 2

Water treatment 1Miscellaneous 1

Source: Chemical Economics Handbook

2. OXYGEN

The manufacture of oxygen is described along with that of nitrogen.Both are formed from the liquefaction of air. Oxygen gas is colorless,odorless, and tasteless, but it is slightly blue in the liquid state. Up to99.995% purity is available commercially. It is commonly used fromseamless steel cylinders under 2,000 psi pressure. A 1.5 cu ft cylinder holds15 Ib of oxygen, equivalent to 244 cu ft at standard temperature andpressure.

Table 3.2 gives the uses of oxygen. The steel industry and other primarymetals production prefers to use pure oxygen rather than air in processingiron. The oxygen reacts with elemental carbon to form carbon monoxide,which is processed with iron oxide so that carbon is incorporated into theiron metal, making it much lower melting and more pliable. This material iscalled fusible pig iron. Common pig irons contain 4.3% carbon and melt at113O0C, whereas pure iron has a melting point of 15390C. The followingequations summarize some of this chemistry.

2C + O2 *• 2CO

Fe2O3 + 3CO ^ 2Fe + 3CO2

2CO ^C(inFe) + CO2

Oxygen also removes sulfur, phosphorus, silicon, and other impurities inthe iron. Steel is a mixture of several physical forms of iron and ironcarbides. Properties are controlled by the amount of carbon and other

elements present, such as manganese, cobalt, and nickel. Since the steelindustry uses approximately half of all oxygen, the production of oxygen isvery dependent on this one use.

Gasification involves partial oxidation of hydrocarbons to producesynthesis gas, a mixture of carbon monoxide and hydrogen. This will bediscussed under the section on hydrogen manufacture. Chemicals madefrom oxygen include ethylene and propylene oxide, titanium dioxide,acetylene, vinyl chloride, and vinyl acetate. These are discussed in latersections of this book. Welding and cutting with an oxygen-acetylene torch iscommon in industry. The health sciences use oxygen to ease patients'breathing. Pulp and paper bleaching and sewage treatment and aeration areother examples of oxygen's broad importance to many industries that affectour everyday lives.

3. HYDROGEN

3.1 Manufacture

Hydrogen does not actually appear in the top 50. One reason is that mostof it is captive and immediately reused to make ammonia, hydrogenchloride, and methanol—three other chemicals with high rankings. Since itis a feedstock for these chemicals it is even more important than these threeand we will study its manufacture in detail. Hydrogen is our first example ofa "petrochemical" even though it is not organic. Its primary manufacturingprocess is by steam-reforming of natural gas or hydrocarbons.Approximately 80% of the hydrogen used for ammonia manufacture comesfrom this process.

3.1.1 Reactions

A variety of low molecular weight hydrocarbons can be used asfeedstock in the steam-reforming process. Equations are given for bothmethane (natural gas) and propane. The reaction occurs in two separatesteps: reforming and shift conversion.

Methane

Reforming CH4 + H2O ^ CO + 3H2

Shift conversion CO + H2O ^ CO2 + H2

Propane

Reforming C3H8 + 3H2O ^ 3CO + 7H2

Shift conversion 3CO + 3H2O *» 3CO2 + 3H2

The reforming step makes a hydrogen:carbon monoxide mixture that isone of the most important materials known in the chemical industry. It iscalled synthesis gas and is used to produce a variety of other chemicals. Theold method of making synthesis gas was from coke, but this gave a lowerpercentage of hydrogen in the mixture, which was called water gas or bluegas.

C + H2O *• CO + H2

Higher H2:CO ratios are now needed, and thus the newer hydrocarbonfeedstocks are used. Coal gives a 1:1 ratio of H2:CO, oil a 2:1 ratio, gasoline2.4:1, and methane 4:1.

Note that in the second step, the shift conversion process (also known asthe carbon monoxide or water gas shift reaction), more hydrogen is formedalong with the other product, carbon dioxide. A variety of methods is usedto make carbon dioxide, but this process is the leading method.

3.1.2 Description

Fig. 3.2 diagrams the steam-reforming process. The hydrocarbon

AirNa2S (optional)

Steam

CO2

Figure 3.2 Steam-reforming of hydrocarbons.

C 5 H 3 S Heater Desulfurizer NaOHscrubber

Reformingfurnaces

Steam

MethanatorTrace CO2

CO2adsorber Cooler Shift

converter

Heatexchanger

Figure 3.3 The primary reformer for methane conversion to carbon monoxide andhydrogen. (Courtesy of Solatia Inc., Luling, LA)

feedstock, usually contaminated with some organosulfur traces, is heated to37O0C before entering the desulfurizer, which contains a metallic oxidecatalyst that converts the organosulfur compounds to hydrogen sulfide.Elemental sulfur can also be removed with activated carbon absorption. Acaustic soda scrubber removes the hydrogen sulfide by salt formation in thebasic aqueous solution.

H2S + 2NaOH *> Na2S + 2H2O

Steam is added and the mixture is heated in the furnace at 760-98O0C and600 psi over a nickel catalyst. When larger hydrocarbons are the feedstock,potassium oxide is used along with nickel to avoid larger amounts of carbonformation. There are primary (Fig. 3.3) and secondary (Fig. 3.4) furnaces insome plants. Air can be added to the secondary reformers. Oxygen reactswith some of the hydrocarbon feedstock to keep the temperature high. Thenitrogen of the air is utilized when it, along with the hydrogen formed, reactsin the ammonia synthesizer. More steam is added and the mixture enters theshift converter (Fig. 3.5), where iron or chromic oxide catalysts at 4250Cfurther react the gas to hydrogen and carbon dioxide. Some shift convertershave high and low temperature sections, the high temperature section

Figure 3.4 A secondary reformer converts the last of the methane. (Courtesy of SolutiaInc., Luling, LA)

converting most of the CO to CO2 relatively fast, the low temperaturesection completing the process and taking advantage of a more favorableequilibrium toward CO2 at the low temperatures in this exothermic reaction.Cooling to 380C is followed by carbon dioxide absorption withmonoethanolamine. The carbon dioxide is desorbed by heating themonoethanolamine and reversing this reaction. The carbon dioxide is animportant by-product. Alternatively, hot carbonate solutions can replace themonoethanolamine.

HO-CH2-CH2-NH2 + H2O + CO2 ^ HO-CH2-CH2-NH3+ + HCO3'

A methanator converts the last traces of carbon dioxide to methane, a lessinterfering contaminant in hydrogen used for ammonia manufacture.

Figure 3.5 A shift converter reacts carbon monoxide and water to give carbon dioxide

and more hydrogen. (Courtesy of Solutia Inc., Luling, LA)

3.2 Uses

Table 3.3 gives the total uses of hydrogen. Ammonia production is byfar the most important application, followed by methanol manufacture.Hydrogenations in petroleum refineries are an important use. Many otherindustries utilize hydrogen. Miscellaneous uses include hydrogenation offats and oils in the food industry, reduction of the oxides of metals to the freemetals, pure hydrogen chloride manufacture, and liquid hydrogen as rocketfuel.

Table 3.3 Total Uses of Hydrogen

Ammonia 40%

Methanol 10

Other chemicals 6Petroleum refining 4

Miscellaneous 40

Source: Chemical Economics Handbook

4. CARBON DIOXIDE

4.1 Manufacture

Over 90% of all carbon dioxide is made by steam-reforming ofhydrocarbons, and much of the time natural gas is the feedstock. It is animportant by-product of hydrogen and ammonia manufacture.

CH4 + 2H2O *» 4H2 + CO2

A small amount (1%) of carbon dioxide is still made from fermentationof grain. Ethyl alcohol is the main product.

Another 1% is recovered as a by-product of ethylene oxide manufacturefrom ethylene and oxygen. When the oxidation goes too far some carbondioxide is formed.

Other small amounts are obtained from coke burning, the calcination oflime, and in the manufacture of sodium phosphates from soda ash andphosphoric acid.

Table 3.4 Uses of Carbon Dioxide

Liquid and Solid:

Food industry 51%

Beverage carbonation 18

Oil and gas recovery 11

Chemical manufacture 10

Metalworking 4

Miscellaneous 6

Gas:Oil and gas recovery 83Chemical manufacture 17

Source: Chemical Economics Handbook

C(coke) + O2 ^ CO2

CaCO3 A » CaO + CO2

Na2CO3 + H3PO4 ^ Na2HPO4 + CO2 + H2O

4.2 Uses

Carbon dioxide is a gas at room temperature. Below -780C it is a solidand is commonly referred to as "dry ice." At that temperature it sublimesand changes directly from a solid to a vapor. Because of this uniqueproperty, as well as its non-combustible nature, it is a common refrigerantand inert blanket. Table 3.4 shows the uses of carbon dioxide in all itsforms: liquid, solid, and gas. Refrigeration using dry ice is especiallyimportant in the food industry. Beverage carbonation for soft drinks is avery big application. In oil and gas recovery carbon dioxide competes withnitrogen as an inert atmosphere for oil wells.

5. ECONOMICS OF INDUSTRIAL GASES

U.S. production of industrial gases is given in Fig. 3.6. Hydrogen is notincluded because so much of its production is captive, so as to make itsproduction profile meaningless. Nor is the amount of nitrogen used to makeammonia included. Even without this captive nitrogen, notice the muchsteeper nitrogen production curve, especially in the late '70s and '80s. In1980 nitrogen was ranked fifth in chemical production. It is now second.

NitrogenOxygenCarbon Dioxide

Bill

ions

of

Pou

nds

Year

Figure 3.6 U.S. production of gases. (Source: Lowenheim and Moran and Chemicaland Engineering News)

HydrogenOxygenNitrogen

Cen

ts/H

undr

ed C

ubic

Fee

t

Year

Figure 3.7 U.S. prices of gases. (Source: Chemical Economics Handbook}

Nitrogen and carbon dioxide were two of the fastest growing chemicals inthe 1970s and 1980s especially because of their uses in oil and gas recovery.Oxygen on the other hand had a more linear growth, no doubt due to thesuffering steel industry, in these years. Nitrogen passed oxygen inproduction in 1982, but oxygen is once again very close to nitrogenproduction in the 1990s and has made a comeback in this decade. Prices fornitrogen, oxygen, and hydrogen are given in Fig. 3.7. Carbon dioxide variesconsiderably in price depending on its form, dry ice or gaseous. Note thathydrogen is much more expensive, coming from expensive hydrocarbons,compared to nitrogen and oxygen, which share equivalent prices because ofbeing manufactured together. Hydrogen prices were especially high in the1970s and '80s, as were all petrochemicals derived from oil. The key in gaspricing is the important shipping charges, which are not included here. Theyare very expensive since a heavy container must be used to withstand thehigh pressures of even light weights of gases.

Suggested Readings

Austin, Shreve 's Chemical Process Industries, pp. 106-124.Kent, Riegel's Handbook of Industrial Chemistry, pp. 442-457, 1073-1084.Thompson, Industrial Inorganic Chemicals: Production and Uses, pp. 233-

256.Wiseman, Petrochemicals, pp. 141-148.