Industrial Gases. Chapter 3 1. NITROGEN. 1.1 Manufacture

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1 Chapter 3 Industrial Gases Three inorganic gases, nitrogen, oxygen, and carbon dioxide, appear in the top 50 chemicals. A fourth gas, hydrogen, would also be included if it were not for the large amounts of captive use of hydrogen to manufacture ammonia, which makes it difficult to estimate hydrogen production. It is convenient to discuss all four at this time in our study of inorganic chemicals. Two of them, nitrogen and hydrogen, are used to produce ammonia, which in turn has important derivatives that will be discussed in the next chapter. Not all four major gases are manufactured by the same method. Nitrogen and oxygen, obtained by the liquefaction of air, will be discusses first. Next, carbon dioxide and hydrogen, made by the process of steam-reforming of hydrocarbons, will be considered. 1. NITROGEN 1.1 Manufacture The large-scale availability of nitrogen, oxygen, and argon from liquefaction of air began about A 90% recovery is now feasible for these three major components in air. Nitrogen makes up 78% of all air, oxygen 21%, and argon 0.9%. Two major processes are used, differing only in the way in which the expansion of air occurs. The Linde-Frankl cycle is based on the classic Joule-Thompson effect of a gas, which means that there is a tremendous cooling effect of a gas when it is rapidly expanded, even though no external work is done on the system. Alternatively, the Claude process employs an expansion engine doing useful work on the gas. The

2 Air Filter Compressor Oxidation chamber Water separator N 2 gas bp-196 C 77 0 K O 2 liquid bp-183 C 9O 0 K Fractionating column and expander valve -19O 0 C, 83 0 K 7psi C 72 psi Some N 2, O 2 Heat exchanger C 7.5 psi Expander valve or Claude engine Solid H 2 O, CO 2 Figure 3.1 Liquefaction of air. temperature is reduced because of the removal of energy. This process is more efficient than relying on the Joule-Thompson effect. Fig. 3.1 outlines the liquefaction of air. Air is filtered to remove particulates and then compressed to 77 psi. An oxidation chamber converts traces of hydrocarbons into carbon dioxide and water. The air is then passed through a water separator, which gets some of the water out. A heat exchanger cools the sample down to very low temperatures, causing solid water and carbon dioxide to be separated from the main components. Most of the nitrogen-oxygen mixture, now at C and 72 psi, enters the bottom of a fractionating column. An expansion valve at this point causes further cooling. The more volatile nitrogen rises to the top of the column as a gas since nitrogen (bp = C, 77 0 K) has a lower boiling point than oxygen (bp = C, 9O 0 K), and the column at 83 0 K is able to separate the two. The oxygen stays at the bottom of the column as a liquid because it is less volatile. A small amount of nitrogen-oxygen mixture after being recooled in the heat exchanger is shunted to the main expander valve (operated by the Joule- Thompson effect or by a Claude engine). This extremely cold gas is recycled into the heat exchanger to keep the system cold. Some argon remains in the oxygen fraction and this mixture can be sold as 90-95% oxygen. If purer oxygen is required, a more elaborate fractionating column with a greater number of plates gives an oxygen-argon separation. Oxygen can be obtained in 99.5% purity in this fashion. Argon can be obtained from a middle fraction between nitrogen and oxygen and redistilled. A small amount of hydrogen can be added to react with any remaining oxygen to give oxygen-free argon. Not only argon, but other rare gases, neon, krypton, and xenon, can also be obtained in

3 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 1 Miscellaneous 11 Source: Chemical Economics Handbook separations. Helium is not obtained from liquefaction of air. It occurs in much greater concentrations (2%) in natural gas wells and is isolated in the petroleum refinery. 1.2 Uses By far the largest use of nitrogen is in ammonia synthesis. However, this use is not included here because it is "captive," that is, the same company immediately reuses the gas internally to make another product, in this case ammonia. This nitrogen is not isolated, sold, or inventoried. Only "merchant" use is included in Table 3.1. Chemicals manufactured with nitrogen include many reactions where nitrogen is the inert blanketing atmosphere to prevent reaction with oxygen and minimize the possibility of fire and explosion for reactions sensitive to oxygen. Oil and gas extraction is a fast-growing use of nitrogen. This application is called advanced oil recovery (AOR) or enhanced oil recovery (EOR), where nitrogen maintains pressure in oil fields so that a vacuum is not formed underground when natural gas and oil are pumped out. It competes with carbon dioxide in this application. In the electronic industry nitrogen is an important blanketing and purge gas in the manufacture of semiconductors and integrated circuits. It is used in liquid form for cryogenic (low temperature) testing. In primary metals manufacture it is an inert atmosphere for making steel, blanketing the powdered coal and other fuel for the furnace. Petroleum refining makes use of it for its inert atmosphere and the food industry uses liquid nitrogen for freezing.

4 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 1 Miscellaneous 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 to % purity is available commercially. It is commonly used from seamless steel cylinders under 2,000 psi pressure. A 1.5 cu ft cylinder holds 15 Ib of oxygen, equivalent to 244 cu ft at standard temperature and pressure. Table 3.2 gives the uses of oxygen. The steel industry and other primary metals production prefers to use pure oxygen rather than air in processing iron. The oxygen reacts with elemental carbon to form carbon monoxide, which is processed with iron oxide so that carbon is incorporated into the iron metal, making it much lower melting and more pliable. This material is called fusible pig iron. Common pig irons contain 4.3% carbon and melt at 113O 0 C, whereas pure iron has a melting point of C. The following equations summarize some of this chemistry. 2C + O 2 * 2CO Fe 2 O 3 + 3CO ^ 2Fe + 3CO 2 2CO ^C(inFe) + CO 2 Oxygen also removes sulfur, phosphorus, silicon, and other impurities in the iron. Steel is a mixture of several physical forms of iron and iron carbides. Properties are controlled by the amount of carbon and other

5 elements present, such as manganese, cobalt, and nickel. Since the steel industry uses approximately half of all oxygen, the production of oxygen is very dependent on this one use. Gasification involves partial oxidation of hydrocarbons to produce synthesis gas, a mixture of carbon monoxide and hydrogen. This will be discussed under the section on hydrogen manufacture. Chemicals made from oxygen include ethylene and propylene oxide, titanium dioxide, acetylene, vinyl chloride, and vinyl acetate. These are discussed in later sections of this book. Welding and cutting with an oxygen-acetylene torch is common in industry. The health sciences use oxygen to ease patients' breathing. Pulp and paper bleaching and sewage treatment and aeration are other examples of oxygen's broad importance to many industries that affect our everyday lives. 3. HYDROGEN 3.1 Manufacture Hydrogen does not actually appear in the top 50. One reason is that most of it is captive and immediately reused to make ammonia, hydrogen chloride, and methanol three other chemicals with high rankings. Since it is a feedstock for these chemicals it is even more important than these three and we will study its manufacture in detail. Hydrogen is our first example of a "petrochemical" even though it is not organic. Its primary manufacturing process is by steam-reforming of natural gas or hydrocarbons. Approximately 80% of the hydrogen used for ammonia manufacture comes from this process Reactions A variety of low molecular weight hydrocarbons can be used as feedstock in the steam-reforming process. Equations are given for both methane (natural gas) and propane. The reaction occurs in two separate steps: reforming and shift conversion. Methane Reforming CH 4 + H 2 O ^ CO + 3H 2 Shift conversion CO + H 2 O ^ CO 2 + H 2

6 Propane Reforming C 3 H 8 + 3H 2 O ^ 3CO + 7H 2 Shift conversion 3CO + 3H 2 O *» 3CO 2 + 3H 2 The reforming step makes a hydrogen:carbon monoxide mixture that is one of the most important materials known in the chemical industry. It is called synthesis gas and is used to produce a variety of other chemicals. The old method of making synthesis gas was from coke, but this gave a lower percentage of hydrogen in the mixture, which was called water gas or blue gas. C + H 2 O * CO + H 2 Higher H 2 :CO ratios are now needed, and thus the newer hydrocarbon feedstocks are used. Coal gives a 1:1 ratio of H 2 :CO, oil a 2:1 ratio, gasoline 2.4:1, and methane 4:1. Note that in the second step, the shift conversion process (also known as the carbon monoxide or water gas shift reaction), more hydrogen is formed along with the other product, carbon dioxide. A variety of methods is used to make carbon dioxide, but this process is the leading method Description Fig. 3.2 diagrams the steam-reforming process. The hydrocarbon Na 2 S Steam Air (optional) C 5 H 3 S Heater Desulfurizer NaOH scrubber Reforming furnaces Steam Methanator Trace CO 2 CO 2 adsorber Cooler Shift converter CO 2 Heat exchanger Figure 3.2 Steam-reforming of hydrocarbons.

7 Figure 3.3 The primary reformer for methane conversion to carbon monoxide and hydrogen. (Courtesy of Solatia Inc., Luling, LA) feedstock, usually contaminated with some organosulfur traces, is heated to 37O 0 C before entering the desulfurizer, which contains a metallic oxide catalyst that converts the organosulfur compounds to hydrogen sulfide. Elemental sulfur can also be removed with activated carbon absorption. A caustic soda scrubber removes the hydrogen sulfide by salt formation in the basic aqueous solution. H 2 S + 2NaOH *> Na 2 S + 2H 2 O Steam is added and the mixture is heated in the furnace at O 0 C and 600 psi over a nickel catalyst. When larger hydrocarbons are the feedstock, potassium oxide is used along with nickel to avoid larger amounts of carbon formation. There are primary (Fig. 3.3) and secondary (Fig. 3.4) furnaces in some plants. Air can be added to the secondary reformers. Oxygen reacts with some of the hydrocarbon feedstock to keep the temperature high. The nitrogen of the air is utilized when it, along with the hydrogen formed, reacts in the ammonia synthesizer. More steam is added and the mixture enters the shift converter (Fig. 3.5), where iron or chromic oxide catalysts at C further react the gas to hydrogen and carbon dioxide. Some shift converters have high and low temperature sections, the high temperature section

8 Figure 3.4 A secondary reformer converts the last of the methane. (Courtesy of Solutia Inc., Luling, LA) converting most of the CO to CO 2 relatively fast, the low temperature section completing the process and taking advantage of a more favorable equilibrium toward CO 2 at the low temperatures in this exothermic reaction. Cooling to 38 0 C is followed by carbon dioxide absorption with monoethanolamine. The carbon dioxide is desorbed by heating the monoethanolamine and reversing this reaction. The carbon dioxide is an important by-product. Alternatively, hot carbonate solutions can replace the monoethanolamine. HO-CH 2 -CH 2 -NH 2 + H 2 O + CO 2 ^ HO-CH 2 -CH 2 -NH HCO 3 ' A methanator converts the last traces of carbon dioxide to methane, a less interfering contaminant in hydrogen used for ammonia manufacture.

9 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 by far the most important application, followed by methanol manufacture. Hydrogenations in petroleum refineries are an important use. Many other industries utilize hydrogen. Miscellaneous uses include hydrogenation of fats and oils in the food industry, reduction of the oxides of metals to the free metals, pure hydrogen chloride manufacture, and liquid hydrogen as rocket fuel.

10 Table 3.3 Total Uses of Hydrogen Ammonia 40% Methanol 10 Other chemicals 6 Petroleum 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 of hydrocarbons, and much of the time natural gas is the feedstock. It is an important by-product of hydrogen and ammonia manufacture. CH 4 + 2H 2 O *» 4H 2 + CO 2 A small amount (1%) of carbon dioxide is still made from fermentation of grain. Ethyl alcohol is the main product. Another 1% is recovered as a by-product of ethylene oxide manufacture from ethylene and oxygen. When the oxidation goes too far some carbon dioxide is formed. Other small amounts are obtained from coke burning, the calcination of lime, and in the manufacture of sodium phosphates from soda ash and phosphoric acid.

11 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 83 Chemical manufacture 17 Source: Chemical Economics Handbook C(coke) + O 2 ^ CO 2 CaCO 3 A» CaO + CO 2 Na 2 CO 3 + H 3 PO 4 ^ Na 2 HPO 4 + CO 2 + H 2 O 4.2 Uses Carbon dioxide is a gas at room temperature. Below C it is a solid and is commonly referred to as "dry ice." At that temperature it sublimes and changes directly from a solid to a vapor. Because of this unique property, as well as its non-combustible nature, it is a common refrigerant and inert blanket. Table 3.4 shows the uses of carbon dioxide in all its forms: liquid, solid, and gas. Refrigeration using dry ice is especially important in the food industry. Beverage carbonation for soft drinks is a very big application. In oil and gas recovery carbon dioxide competes with nitrogen as an inert atmosphere for oil wells. 5. ECONOMICS OF INDUSTRIAL GASES U.S. production of industrial gases is given in Fig Hydrogen is not included because so much of its production is captive, so as to make its production profile meaningless. Nor is the amount of nitrogen used to make ammonia included. Even without this captive nitrogen, notice the much steeper nitrogen production curve, especially in the late '70s and '80s. In 1980 nitrogen was ranked fifth in chemical production. It is now second.

12 Billions of Pounds Nitrogen Oxygen Carbon Dioxide Year Figure 3.6 U.S. production of gases. (Source: Lowenheim and Moran and Chemical and Engineering News) Cents/Hundred Cubic Feet Hydrogen Oxygen Nitrogen Year Figure 3.7 U.S. prices of gases. (Source: Chemical Economics Handbook}

13 Nitrogen and carbon dioxide were two of the fastest growing chemicals in the 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 the suffering steel industry, in these years. Nitrogen passed oxygen in production in 1982, but oxygen is once again very close to nitrogen production in the 1990s and has made a comeback in this decade. Prices for nitrogen, oxygen, and hydrogen are given in Fig Carbon dioxide varies considerably in price depending on its form, dry ice or gaseous. Note that hydrogen is much more expensive, coming from expensive hydrocarbons, compared to nitrogen and oxygen, which share equivalent prices because of being manufactured together. Hydrogen prices were especially high in the 1970s and '80s, as were all petrochemicals derived from oil. The key in gas pricing is the important shipping charges, which are not included here. They are very expensive since a heavy container must be used to withstand the high pressures of even light weights of gases. Suggested Readings Austin, Shreve 's Chemical Process Industries, pp Kent, Riegel's Handbook of Industrial Chemistry, pp , Thompson, Industrial Inorganic Chemicals: Production and Uses, pp Wiseman, Petrochemicals, pp