FERTILIZER INDUSTRY LECTURE (4) Natural gas as feed stock for ammonia production

1. Pharos University جامعه فاروس Faculty of Engineering كلية الهندسة Petrochemical Department قسم البتروكيماويات FERTILIZER INDUSTRY LECTURE (4) Natural gas as feed stock for ammonia production Desulfurization of natural gas: A- Direct conversion process: Direct conversion processes are used if S-recovery is an option and selective only for H2S where H2S is absorbed in alkaline solution containing an oxidizing agent which converts it to sulfur.

2. B) Indirect conversion: • For indirect conversion processes: the solid methods (adsorption) are used for removing small amounts of acid gases (where the gas flow rate is low or acid gas concentration is low). • The liquid methods (absorption) are used to remove large amounts of acid gases.

4. Indirect conversion processes Solid bed processes: • Chemical conversion (metal oxide processes) : • In this case, H2S is basically removed and the presence of CO2 does not affect the processes. When to use the metal oxide processes? • Usually, theses processes are best applied to gases containing low to medium sulfur-content feeds.

5. a) Iron sponge (dry box process) • This process is applied to sour gases with low H2S concentrations (300 ppm) operating at low to moderate pressures (50–500 psig). Carbon dioxide is not removed by this treatment. How? The basic principle of the process : The inlet gas is fed at the top of the fixed-bed reactor filled with hydrated iron oxide and wood chips. The acid gas (H2S) is adsorbed on the surface of the solid sweetening agent (Fe2O3) followed by chemical reaction between Fe2O3 and H2S. Fe2O3 + 6 H2S → 2Fe2S3 + 6H2O

7. Reaction conditions: • The reaction requires an alkalinity pH level 8–10 with controlled injection caustic soda with water and temperature of 43 ºC. • The metal oxide process can be carried out batch or continuous. For continuous process a number of vessels are used ranging from two to four. • The figure shows the two vessel processes. In this case, one of the towers is on stream removing H2S from the sour gas while the second tower is in the regeneration cycle by air blowing.

8. Bed regeneration: • A tower is operated until the bed is saturated with sulfur and hydrogen sulfide begins to appear in the sweetened gas stream. At this point the vessel is removed from service and air is circulated through the bed to regenerate the iron oxide. The bed is regenerated by controlled oxidation as: 2Fe2S3 + 3O2 → 2Fe2O3 + 6S • Some of the sulfur produced might cake in the bed and oxygen should be introduced slowly to oxide this sulfur, S2 + 2O2 → 2SO2

9. Note: • Some of the elemental sulfur produced in the regeneration step remains in the bed. After several cycles this sulfur will cake over the ferric oxide, decreasing the reactivity of the bed. Typically, after 10 cycles the bed must be removed and a new bed introduced into the vessel. The main problems associated with the iron oxide process: 1.The regeneration step, i.e., the reaction with oxygen, is exothermic 2. Some of the sulfur produced might cake in the bed. 3. The change out of the beds is hazardous

10. b) Zinc oxide: • Zinc oxide can be used instead of iron oxide for the removal of H2S, COS, CS2, and mercaptans. • However, this material is a better sorbent and the exit H2S concentration can be as low as 1 ppm at a temperature of about 300 ◦C (the high temperature increase the rate of reaction). • The zinc oxide reacts with H2S to form water and zinc sulfide: ZnO + H2S → ZnS + H2

11. Disadvantages of Zinc oxide • A major drawback of zinc oxide is that it is not possible to regenerate it to zinc oxide on site, because active surface diminishes appreciably by sintering. • Much of the mechanical strength of the solid bed is lost due to fines formation, resulting in a high-pressure-drop operation. • The process has been decreasing in use due to the above problems and the difficulty of disposing of zinc sulfide; Zn is considered a heavy metal.

12. Production of ammonia • Synthetic ammonia (NH3) has become the principal source of all nitrogen fertilizers, particularly since 1945. • At present, over 95% of all commercial fertilizer nitrogen is supplied by or derived from synthetic NH3. • The ammonia synthesis process was developed mainly by Fritz Haber starting in 1904, and by 1909 he demonstrated the process on a laboratory scale of 80 g of NH3 per hour. • Carrying out the high-temperature, high-pressure process on a commercial scale presented formidable problems with the technology and materials of construction then available. Carl Bosch, working with Haber, is generally credited with developing the process –first in a pilot plant and then on a commercial scale of 30 ton/day. • Production stared in 1913 at Germany.

13. The chemistry of the process is simple; the reaction is: N2 + 3H2 ↔ 2NH3 • The reaction is exothermic; the net heat of reaction is about 11,000 cal/g-mole at 18°C (647 kcal/kg of NH3), assuming NH3 is in the gaseous state. • The reaction does not go to completion; equilibrium conditions are such that increase in pressure favors high conversion to ammonia, while increased temperature decreases conversion; thus, a compromise must be selected between reaction rate and equilibrium values.

14. The search for a catalyst to increase the reaction rate has received many investigators. Haber’s first studies were made with an iron catalyst, which was not very active. • Later, he found that osmium or uranium was much more effective, but these elements were scarce and expensive. • After several thousand formulas had been tested, a doubly promoted iron catalyst was selected that was produced from magnetite (Fe3O4) with additions of potassium, alumina, and calcium. • Iron catalysts lose their activity if heated above 520 ºC. • Catalysts are also deactivated by contact with copper, phosphorus, arsenic,sulfur, and carbon monoxide.

15. Even with the best catalysts available, the reaction rate is a limiting factor, and a compromise must be reached between long retention time which would require a large, expensive converter and conversion efficiency.

16. Physical properties of ammonia

17. Uses of ammonia • A) Nitrogen source for production of: (15%) • plastics • Fibers • Paint • Pharmaceutical products • Refrigerant • Desulphurization and De-NOx units • B) Production of inorganic fertilizers (85%)

18. Feedstock for Ammonia Production 1-Natural Gas - As mentioned previously, natural gas is the principal feedstock for ammonia production (with a percentage of 79%) 2-Liquefied Petroleum Gas - LPG, which primarily contains butane and propane, has been used as ammo­nia feedstock in Japan. Liquefied natural gas (LNG) is also used in Japan. These materials are relatively ex­pensive but are often less expensive than naphtha. 3-Naphtha - In those areas of the world where natural gas is unavailable, naphtha became a favored feedstock for ammonia production by steam reforming. Naphtha is the lighter fraction of hydrocarbons; it boils from about 40° to 130°C with an average molecular weight of about 88 and an H:C atomic ratio of about 2.23. Straight-run naphtha is preferred to naphtha produced from higher hydrocarbons by cracking or “hydrocracking” because the latter usually contains sulfur compounds that are difficult to remove.

19. Since 1974 the price of naphtha on the world mar­ket has risen more rapidly than that of other feedstock because of the demand for naphtha for use in the manu­facture of motor fuel and petrochemicals ethylene, propylene, etc.). Therefore, several plants originally designed to use naphtha have switched to other feedstock such as natural gas. 4-Refinery Gases - Petroleum refineries and petro­chemical operations produce a variety of byproduct gases that can be used as fuel or feedstock or both. The com-position of refinery tail gas varies widely; it usually con­tains H2. CH4, and higher hydrocarbons and can be used as ammonia feedstock by steam reforming.

20. 5-Coke-Oven Gas :Only a few plants use coke-oven gas for feedstock as the amount of coke-oven gas depends on coke production which, in turn, depends on steel production. 6-Heavy Oil - Liquid hydrocarbons heavier than naph­tha are used for ammonia feedstock by partial oxidation processes. 7-Hydrogen-Rich Off-Gases - Byproduct hydrogen sources have been used for ammonia production in the past. In the 1970s, the price of light hydrocarbons began to rise, which led to other feedstocks such as hydrogen-rich off-gases being reconsidered.

21. 8-Other Feedstocks - Methanol or ethanol can be used as feedstock in steam-reforming processes, but no commercial use has been reported. Other feedstocks such as electrolytic hydrogen, coal, and fuel oil also can be used.

22. Feedstock for Ammonia Production with Suitable Processing Methods • Natural gas Steam Reforming • Liquefied petroleum gas Steam Reforming • Naphtha Steam Reforming • Refinery gases Steam Reforming • Coke oven gas Steam Reforming • Heavy oil Partial oxidation • Hydrogen-rich off-gases (Combined with other process) • Coal Gasification • Ammonia from electrolytic hydrogen ( Combined with other process)