The Chemical Industry 3. Industrial Production of NH3, HNO3, H2SO4

The Chemical Industry 3. Industrial production of NH3, HNO3, H2SO4

1 Ammonia production https://www.youtube.com/watch?v=wiwl4eoHbig&t=8s

◦ Ammonia is one of the most highly produced inorganic chemicals. There are numerous large-scale ammonia production plants worldwide, producing a total of 144 million tonnes of nitrogen (equivalent to 175 million tonnes of ammonia) in 2016. ◦ China produced 31.9% of the worldwide production, followed by Russia with 8.7%, India with 7.5%, and the United States with 7.1%. ◦ 80% or more of the ammonia produced is used for fertilizing agricultural crops. ◦ Ammonia is also used for the production of plastics, fibers, explosives, nitric acid (via the Ostwald process) and intermediates for dyes and pharmaceuticals.

2 Modern ammonia-producing plants

◦ A typical modern ammonia-producing plant first converts natural gas, liquified petroleum gas, or petroleum naphtha into gaseous hydrogen. ◦ The method for producing hydrogen from hydrocarbons is known as steam reforming. ◦ The hydrogen is then combined with nitrogen to produce ammonia via the Haber-Bosch process. https://www.youtube.com/watch?v=pzFZ9TYizaw

3 Starting with a natural gas feedstock, the processes used in producing the hydrogen are: The first step in the process is to remove sulfur compounds from the feedstock because sulfur deactivates the catalysts used in subsequent steps. Sulfur removal requires catalytic hydrogenation to convert sulfur compounds in the feedstocks to gaseous hydrogen sulfide:

H2 + RSH → RH + H2S(gas) The gaseous hydrogen sulfide is then adsorbed and removed by passing it through beds of zinc oxide where it is converted to solid zinc sulfide:

H2S + ZnO → ZnS + H2O

Catalytic steam reforming of the sulfur-free feedstock is then used to form hydrogen plus carbon monoxide:

CH4 + H2O → CO + 3H2 The next step then uses catalytic shift conversion to convert the carbon monoxide to carbon dioxide and more hydrogen:

CO + H2O → CO2 + H2 ◦ The carbon dioxide is then removed either by absorption in aqueous ethanolamine solutions or by adsorption in pressure swing adsorbers (PSA) using proprietary solid adsorption media. 4 ◦ The final step in producing the hydrogen is to use catalytic methanation to remove any small residual amounts of carbon monoxide or carbon dioxide from the hydrogen:

CO + 3H2 → CH4 + H2O

CO2 + 4H2 → CH4 +2H2O

◦ To produce the desired end-product ammonia, the hydrogen is then catalytically reacted with nitrogen (derived from process air) to form anhydrous liquid ammonia. This step is known as the ammonia synthesis loop (also referred to as the Haber-Bosch process):

N2(g)+3H2(g)⇌2NH3(g) ΔH=−92.0kJ(−22.0kcal)

5 Equilibrium Percent Concentrations of Ammonia at Various Temperatures and Pressures

Absolute pressure (atm)

Temperature, °C 1 10 50 100 300 600 1000 200 15.3 50.7 74.4 81.5 89.9 95.4 98.3

300 2.2 14.7 39.4 52.0 71.0 84.2 92.6

400 0.4 3.9 15.3 25.1 47.0 65.2 79.8

500 – 1.2 5.6 10.6 26.4 42.2 57.5

600 – 0.5 2.3 4.5 13.8 23.1 31.4

700 – – 1.1 2.2 7.3 11.5 12.9

For these reasons the conditions used by most synthetic ammonia processes tend to cluster around 100–300 atm and 400–500°C in an attempt to maximize

conversions and conversion rates while moderating compression costs. 6 ◦ Due to the nature of the (typically multi-promoted magnetite) catalyst used in the ammonia synthesis reaction, only

very low levels of oxygen-containing (especially CO, CO2 and H2O) compounds can be tolerated in the synthesis (hydrogen and nitrogen mixture) gas. Relatively pure nitrogen can be obtained by air separation, but additional oxygen removal may be required.

◦ Because of relatively low single pass conversion rates (typically less than 20%), a large recycle stream is required. This can lead to the accumulation of inerts in the loop gas.

◦ The steam reforming, shift conversion, carbon dioxide removal and methanation steps each operate at absolute pressures of about 25 to 35 bar, and the ammonia synthesis loop operates at absolute pressures ranging from 60 to 180 bar depending upon which proprietary design is used. ◦ There are many engineering and construction companies that offer proprietary designs for ammonia synthesis plants. Haldor Topsoe of Denmark, Thyssenkrupp Industrial Solutions GmbH of Germany, Ammonia Casale of Switzerland and Kellogg Brown & Root of the United States are among the most experienced companies in that field.

7 Introduction to Ammonia Production AIChE SEPTEMBER 2016 VENKAT PATTABATHULA, JIM RICHARDSON

◦ Ammonia is critical in the manufacturing of fertilizers, and is one of the largest-volume synthetic chemicals produced in the world. This article explores the evolution of ammonia production and describes the current manufacturing technologies.

◦ Most people associate the pungent smell of ammonia (NH3) with cleaners or smelling salts. However, the use of ammonia in these two products represents only a small fraction of the total global ammonia production, which was around 176 million metric tons in 2014. To appreciate where the industry and technology are today, let’s first take a look at how we got here.

8 ◦ Ammonia has been known for more than 200 years. Joseph Priestley, an English chemist, first isolated gaseous ammonia in 1774. Its composition was ascertained by French chemist Claude Louis Berthollet in 1785. In 1898,

Adolph Frank and Nikodem Caro found that N2 could be fixed by calcium carbide to form calcium cyanamide, which could then be hydrolyzed with water to form ammonia (2):

CaO + 3C ↔ CaC2 + CO

CaC2 + N2 ↔ CaCN2 + C

CaCN2 + 3H2O ↔ CaCO3 + 2NH3

◦ The production of significant quantities of ammonia using the cyanamide process did not occur until the early 20th century. Because this process required large amounts of energy, scientists focused their efforts on reducing energy requirements.

9 ◦ German chemist Fritz Haber performed some of the most important work in the development of the modern ammonia industry. Working with a student at the Univ. of Karlsruhe, he synthesized ammonia in the laboratory

from N2 and H2. ◦ Meanwhile, Walther Nernst, a professor of physical chemistry at the Univ. of Berlin, developed a process to make

ammonia by passing a mixture of N2 and H2 across an iron catalyst at 1,000°C and 75 barg pressure. He was able to produce larger quantities of ammonia at this pressure than earlier experiments by Haber and others at atmospheric pressure. However, Nernst concluded that the process was not feasible because it was difficult or almost impossible (at that time) to produce large equipment capable of operating at that pressure. ◦ Nonetheless, both Haber and Nernst pursued the high-pressure route to produce ammonia over a catalyst. Haber finally developed a process for producing commercial quantities of ammonia, and in 1906 he was able to achieve a 6% ammonia concentration in a reactor loaded with an osmium catalyst. This is generally recognized as the turning point in the development of a practical process for the production of ammonia in commercial quantities.

10 ◦ Haber realized that the amount of ammonia formed in a single pass through a converter was far too low to be of commercial interest. To produce more ammonia from the makeup gas, he proposed a recycle system, and received a patent for the concept. Haber’s recycle idea changed the perception of process engineering as static in favor of a more dynamic approach. In addition to the chemical reaction equilibrium, Haber recognized that reaction rate was a determining factor. Instead of simple yield in a once-through process, he concentrated on space-time yield in a system with recycle. ◦ BASF purchased Haber’s patents and started development of a commercial process. After testing more than 2,500 different catalysts, Carl Bosch, Alvin Mittasch, and other BASF chemists developed a promoted iron catalyst for the production of ammonia in 1910. ◦ Developing equipment that could withstand the necessary high temperatures and pressure was an even more difficult task. An early mild steel reactor lasted only 80 hours before failure due to decarbonization. Lining mild steel reactors with soft iron (which was not vulnerable to decarbonization) and adding grooves between the two liners to release hydrogen that had diffused through the soft iron liner solved this problem. Other major challenges included designing a heat exchanger to bring the inlet gas to reaction temperatures and cool the exit gas, and devising a method to bring the catalyst to reaction temperature.

11 The first commercial ammonia plant by BASF

The first commercial ammonia plant based on the Haber- Bosch process was built by BASF at Oppau, Germany. The plant went on-stream on Sept. 9, 1913, with a production capacity of 30 m.t./day. Figure 1 is a flowsheet of the first commercial ammonia plant. The reactor contained an internal heat exchanger in addition to those shown on the schematic.

12 Global production rates

◦ Ammonia production has become one of the most important industries in the world. Without the crop yield made possible by ammonia-based fertilizers and chemicals, the global population would be at least two to three billion less than it is today (3). Ammonia production has increased steadily since 1946, and it is estimated that the annual production of ammonia is worth more than $100 billion, with some plants producing more than

3,000 m.t./day of NH3.

13 ◦ In 1983, on the occasion of the 75th anniversary of AIChE’s founding, a blue ribbon panel of distinguished chemical engineers named what they believed to be the world’s ten greatest chemical engineering achievements (4). Embracing such feats as wonder drugs, synthetic fibers, and atomic energy, the citation also included the breakthrough that permitted the production of large quantities of ammonia in compact, single-unit plants. ◦ Within the past decades, chemical engineers have succeeded in creating processes that make vast amounts of ammonia at relatively low costs. As recently as 80 years ago, the total annual production of synthesized ammonia was just over 300,000 m.t. Thanks to chemical engineering breakthroughs, one modern ammonia plant can produce more than 750,000 m.t./yr. ◦ Approximately 88% of ammonia made annually is consumed in the manufacturing of fertilizer. Most of the remainder goes into the production of formaldehyde. China produced about 32.6% of the global production in 2014, while Russia, India, and the U.S. produced 8.1%, 7.6%, and 6.4%, respectively (1). While most of the global production of ammonia is based on steam reforming of natural gas, significant quantities are produced by coal gasification; most of the gasification plants are located in China.

14 Modern production processes

◦ The tremendous increase in ammonia demand from 1950 to 1980 necessitated larger, more-energy-efficient plants. Those decades also saw a change in design philosophy. Until that time, an ammonia plant was regarded as an assembly of unrelated units, such as gas preparation, gas purification, gas compression, and ammonia synthesis. New innovations and an integral design tied process units together in the most effective and efficient ways. ◦ In the mid-1960s, the American Oil Co. installed a single-converter ammonia plant engineered by M.W. Kellogg (MWK) at Texas City, TX, with a capacity of 544 m.t./day. The single-train design concept was so revolutionary that it received the Kirkpatrick Chemical Engineering Achievement Award in 1967. ◦ The plant used a four-case centrifugal compressor to compress the syngas to a pressure of 152 bar, and final compression to an operating pressure of 324 bar occurred in a reciprocating compressor. Centrifugal compressors for the synthesis loop and refrigeration services were also implemented, which provided significant cost savings.

15 MWK designed one of the first single-train, large- capacity ammonia plants.

16 The key differences between the MWK process and the processes used in previous ammonia plants included: ◦ using a centrifugal compressor as part of the synthesis gas compression ◦ maximizing the recovery of waste heat from the process ◦ generating steam from the waste heat for use in steam turbine drivers ◦ using the refrigeration compressor for rundown and atmospheric refrigeration. An integrated scheme that balanced energy consumption, energy production, equipment size, and catalyst volumes was incorporated throughout the plant.

17 ◦ Most plants built between 1963 and 1993 had large single-train designs with synthesis gas production at 25–35 bar and ammonia synthesis at 150–200 bar. Another variation by Braun (now KBR) offered slight modifications to the basic design. The Braun Purifier process plants utilized a primary or tubular reformer with a low outlet temperature and high methane leakage to reduce the size and cost of the reformer. Excess air was added to the secondary reformer to reduce the methane content of the primary reformer exit stream to 1–2%. Excess nitrogen and other impurities were removed downstream of the methanator. Because the synthesis gas was essentially free of impurities, two axial-flow ammonia converters were used to achieve a high ammonia conversion.

18 ◦ Some recently built plants have a synthesis gas generation system with only one reformer (no secondary

reformer), a pressure-swing adsorption (PSA) system for H2 recovery, and an air separation plant as the source of N2. Improvements in converter design, such as radial and horizontal catalyst beds, internal heat exchangers, and synthesis gas treatment, helped increase ammonia concentrations exiting the synthesis converter from about 12% to 19–21%. A higher conversion per pass, along with more-efficient turbines and compressors, further reduced

energy consumption. More-efficient CO2 removal solutions, such as potassium carbonate and methyldiethanolamine (MDEA), have contributed to improved energy efficiency. Most modern plants can produce ammonia with an energy consumption of 28 GJ/m.t. ◦ In addition to the design, mechanical, and metallurgical improvements made during this time, the operating pressure of the synthesis loop was significantly reduced. When the first single-train plant was built in the 1960s, it contained a high-pressure synthesis loop. In 1962, MWK received an inquiry from Imperial Chemical Industries (ICI) for a proposal to build a 544-m.t./day plant at their Severnside site. MWK proposed a 152-bar synthesis loop instead of a 324-bar loop.

19 ◦ Because the development of kinetic data for the ammonia reaction at 152 bar would take more time than MWK had to respond to the ICI inquiry, they contacted Haldor Topsøe to support their plans. Topsøe had data covering the entire pressure range of interest to MWK. In addition, they had a computer program for calculating the quantity of catalyst that was required at the lower operating pressure. Even though ICI chose Bechtel to design the plant, MWK was able to develop a flowsheet for a 544-m.t./day design with centrifugal compressors and a low- pressure synthesis loop, which some people consider the single most important event in the development of the single-train ammonia plant. ◦ Approximately twice as much catalyst was required at 152 bar as at 324 bar, an increase that seemed economically feasible. Although the converter would need twice the volume, the lower operating pressure would reduce the required thickness of the pressure shell. As a result, the weight of metal required for the converter plus the catalyst remained about the same. The lower-pressure synthesis loop also allowed the use of centrifugal compressors instead of reciprocating compressors. Another improvement was recovering heat to generate high- pressure steam for steam turbine drives.

20 Plant designs in the 21st century

◦ During the first few years of the 21st century, many improvements were made in ammonia plant technology that allow existing plants to increase production rates and new plants to be built with larger and larger capacities. Competition between technology suppliers is quite fierce. Three technology licensors — KBR (Kellogg Brown and Root), Haldor Topsøe, and ThyssenKrupp Industrial Solutions (TKIS) — currently dominate the market. Ammonia Casale, which offers an axial-radial catalyst bed design, is a market leader in revamps of existing plants.

21 Modern ammonia plants designed by KBR employ its proprietary Purifier design.

22 ◦ Most of the ammonia plants recently designed by KBR utilize its Purifier process, which combines low-severity

reforming in the primary reformer, a liquid N2 wash purifier downstream of the methanator to remove impurities and adjust the H2:N2 ratio, a proprietary waste-heat boiler design, a unitized chiller, and a horizontal ammonia synthesis converter. ◦ Depending on the configuration of the plant, energy consumption can be as low as 28 GJ/m.t. Because the secondary reformer uses excess air, the primary reformer can be smaller than in conventional designs. The cryogenic purifier (shown in Figure in light green with a light orange background), which consists of an expander,

condenser, feed/effluent exchanger, and rectifier column, removes impurities such as CO, CH4, and argon from the synthesis gas while adjusting the H2:N2 ratio of the makeup gas in the ammonia loop to the optimum level. The ammonia concentration exiting the low-pressure-drop horizontal converter is 20–21%, which reduces energy requirements for the recycle compressor. KBR also offers a low-pressure ammonia loop that employs a combination of magnetite catalyst and its proprietary ruthenium catalyst.

23 Haldor Topsøe- designed plant The syngas generation section (or front end) of a Haldor Topsøe- designed plant is quite traditional with the exception of its proprietary side-fired reformer, which uses radiant burners to supply heat for the reforming reaction. Haldor Topsøe also offers a proprietary iron-based synthesis catalyst, radial-flow converters consisting of one, two, or three beds, and a proprietary bayonet-tube waste-heat boiler. More recent developments include the S- 300 and S-350 converter designs. The S-300 converter is a three-bed radial-flow configuration with internal heat exchangers, while the S-350 design combines an S-300 converter with an S-50 single-bed design with waste-heat recovery between converters to maximize ammonia 24 conversion. ThyssenKrupp’s conventional plant

ThyssenKrupp offers a conventional plant with a unique secondary reformer design, a proprietary waste- heat boiler, radial-flow converters, and a dual- pressure ammonia synthesis loop. Today, a production rate of 3,300 m.t./day can be achieved using the TKIS dual-pressure process.

25 The Linde Ammonia Concept (LAC) The Linde Ammonia Concept (LAC) is an established technology process scheme with over 25 years of operating experience in plants with capacities from 200 m.t./day to over 1,750 m.t./day. The LAC process scheme replaces the costly and complex front end of a conventional ammonia plant with two well- proven, reliable process units: • production of ultra-high- purity hydrogen from a steam-methane reformer with PSA purification • production of ultra-high- purity nitrogen by a cryogenic nitrogen generation unit, also known as an air separation unit (ASU). 26 ◦ Ammonia Casale’s plant design has a production rate of 2,000 m.t./day. One of the key features of this design is axial-radial technology in the catalyst bed. In an axial- radial catalyst bed, most of the synthesis gas passes through the catalyst bed in a radial direction, creating a very low pressure drop. The rest of the gas passes down through a top layer of catalyst in an axial direction, eliminating the need for a top cover on the catalyst bed. Casale’s axial-radial catalyst bed technology is used in both high- temperature and low-temperature shift converters, as well as in the synthesis converter.

27 Ammonia from coal

China produces most of its ammonia from coal.

28 ◦ The basic processing units in a coal-based ammonia plant are the ASU for the separation of O2 and N2 from air, the gasifier, the sour gas shift (SGS) unit, the acid gas removal unit (AGRU), and the ammonia synthesis

unit. Oxygen from the ASU is fed to the gasifier to convert coal into synthesis gas (H2, CO, CO2) and CH4. There are many gasifier designs, but most modern gasifiers are based on fluidized beds that operate above atmospheric pressure and have the ability to utilize different coal feeds. Depending on the design, CO levels of 30–60% by volume may be produced. ◦ After gasification, any particulate matter in the synthesis gas is removed and steam is added to the SGS unit. The SGS process typically utilizes a cobalt and molybdenum (CoMo) catalyst specially designed for operation in a sulfur environment. ◦ After reducing the CO concentration in the synthesis gas to less than 1 vol%, the syngas is fed to an AGRU,

where a chilled methanol scrubbing solution (e.g., Rectisol) removes CO2 and sulfur from the synthesis gas. The CO2 overhead is either vented or fed to a urea plant. The sulfur outlet stream is fed to a sulfur recover unit (SRU). ◦ Syngas that passes through the AGRU is typically purified by one of two methods:

◦ a nitrogen wash unit to remove residual CO and CH4 from the syngas before it is fed to the synthesis loop

◦ a PSA system for CO and CH4 removal.

29 A green way to make ammonia

◦ Reverse fuel cells can use renewable power to make ammonia from air and water, a far more environmentally friendly technique than the industrial Haber-Bosch process. ◦ Renewable ammonia could serve as fertilizer—ammonia's traditional role—or as an energy-dense fuel.

30 Industrial ammonia

Most of the world’s ammonia is synthesized using Haber– Bosch, a century-old process thatis fast and fairly efficient. But the factories emit vast amounts of carbon dioxide

(CO2).

31 Gentler reactions

A reverse fuel cell uses renewable electricity to drive a chemical reaction that makes ammonia. Water reacts at the anode to make hydrogen ions (H+), which migrate to the cathode where they react with nitrogen (N2) to form ammonia. The reaction is efficient, but slow.

32 To market

Ammonia is more than fertilizer. The gas liquefies easily under light pressure and chilling and can be transported to power plants to generate carbon-free electricity. It can also be “cracked” into

H2, a valuable energy source for fuel cell vehicles.

33 Nitric acid production https://www.youtube.com/watch?v=FIxz7biiIG0

◦ In 1991, there were approximately 65 nitric acid (HNO3) manufacturing plants in the U. S. with a total capacity of 11 million tons of HNO3 per year. The plants range in size from 6,000 to 700,000 tons per year. ◦ About 70 percent of the nitric acid produced is consumed as an intermediate in the manufacture of

ammonium nitrate (NH4NO3), which in turn is used in fertilizers. The majority of the nitric acid plants are located in agricultural regions such as the Midwest, South Central, and Gulf States because of the high demand for fertilizer in these areas. ◦ Another 5 to 10 percent of the nitric acid produced is used for organic oxidation in adipic acid manufacturing. Nitric acid is also used in organic oxidation to manufacture terephthalic acid and other organic compounds. ◦ Explosive manufacturing utilizes nitric acid for organic nitrations. Nitric acid nitrations are used in producing nitrobenzene, dinitrotoluenes, and other chemical intermediates. ◦ Other end uses of nitric acid are gold and silver separation, military munitions, steel and brass pickling, photoengraving, and acidulation of phosphate rock.

34 Process Description

◦ Nitric acid is produced by 2 methods.

◦ The first method utilizes oxidation, condensation, and absorption to produce a weak nitric acid. Weak nitric acid can have concentrations ranging from 30 to 70 percent nitric acid.

◦ The second method combines dehydrating, bleaching, condensing, and absorption to produce a high- strength nitric acid from a weak nitric acid. High-strength nitric acid generally contains more than 90 percent nitric acid.

35 Weak Nitric Acid Production

Nearly all the nitric acid produced in the U. S. is manufactured by the high-temperature catalytic oxidation of ammonia as shown schematically in Figure. This process typically consists of 3 steps: (1) ammonia oxidation, (2) nitric oxide oxidation, and (3) absorption. Each step corresponds to a distinct chemical reaction.

36 Ammonia Oxidation

First, a 1:9 ammonia/air mixture is oxidized at a temperature of 1380 to 1470 oF as it passes through a catalytic convertor, according to the following reaction:

ퟒ푵푯ퟑ + ퟓ푶ퟐ → ퟒ푵푶 + ퟔ푯ퟐ푶 (1) The most commonly used catalyst is made of 90 percent platinum and 10 percent rhodium gauze constructed from squares of fine wire. Under these conditions the oxidation of ammonia to nitric oxide (NO) proceeds in an exothermic reaction with a range of 93 to 98 percent yield. Oxidation temperatures can vary from 1380 to 1650 oF. Higher catalyst temperatures increase reaction selectivity toward NO production. Lower catalyst temperatures tend to be more selective toward less useful products: nitrogen (N2) and nitrous oxide (N2O). Nitric oxide is considered to be a criteria pollutant and nitrous oxide is known to be a global warming gas. The nitrogen dioxide/dimer mixture then passes through a waste heat boiler and a platinum filter.

37 Nitric Oxide Oxidation

The nitric oxide formed during the ammonia oxidation must be oxidized. The process stream is passed through a cooler/condenser and cooled to 100 oF or less at pressures up to 116 pounds per square inch absolute (psia). The nitric oxide reacts non catalytically with residual oxygen to form nitrogen dioxide (NO2) and its liquid dimer, nitrogen tetroxide:

ퟐ푵휪 + 푶ퟐ → ퟐ푵휪ퟐ ↔ 휨ퟐ휪ퟒ (2) ◦ This slow, homogeneous reaction is highly temperature- and pressure-dependent. Operating at low

temperatures and high pressures promotes maximum production of NO2 within a minimum reaction time.

38 Absorption

The final step introduces the nitrogen dioxide/dimer mixture into an absorption process after being cooled. The mixture is pumped into the bottom of the absorption tower, while liquid dinitrogen tetroxide is added at a higher point. Deionized process water enters the top of the column. Both liquids flow countercurrent to the nitrogen dioxide/dimer gas mixture. Oxidation takes place in the free space between the trays, while absorption occurs on the trays. The absorption trays are usually sieve or bubble cap trays. The exothermic reaction occurs as follows:

ퟑ푵휪ퟐ + 휢ퟐ휪 → ퟐ휢푵휪ퟑ + 휨휪 (3) A secondary air stream is introduced into the column to re-oxidize the NO that is formed in Reaction (3). This secondary air also removes NO2 from the product acid. An aqueous solution of 55 to 65 percent (typically) nitric acid is withdrawn from the bottom of the tower. The acid concentration can vary from 30 to 70 percent nitric acid. The acid concentration depends upon the temperature, pressure, number of absorption stages, and concentration of nitrogen oxides entering the absorber.

39 There are 2 basic types of systems used to produce weak nitric acid: (1) single-stage pressure process, and (2) dual-stage pressure process.

In the past, nitric acid plants have been operated at a single pressure, ranging from atmospheric pressure to 14.7 to 203 psia. However, since Reaction 1 is favored by low pressures and Reactions 2 and 3 are favored by higher pressures, newer plants tend to operate a dual-stage pressure system, incorporating a compressor between the ammonia oxidizer and the condenser. The oxidation reaction is carried out at pressures from slightly negative to about 58 psia, and the absorption reactions are carried out at 116 to 203 psia.

In the dual-stage pressure system, the nitric acid formed in the absorber (bottoms) is usually sent to an external bleacher where air is used to remove (bleach) any dissolved oxides of nitrogen. The bleacher gases are then compressed and passed through the absorber. The absorber tail gas (distillate) is sent to an entrainment separator for acid mist removal. Next, the tail gas is reheated in the ammonia oxidation heat exchanger to approximately 392οF. The final step expands the gas in the power-recovery turbine. The thermal energy produced in this turbine can be used to drive the compressor.

40 High-Strength Nitric Acid Production

Flow diagram of high- strength nitric acid production from weak nitric acid.

41 A high-strength nitric acid (98 to 99 percent concentration) can be obtained by concentrating the weak nitric acid (30 to 70 percent concentration) using extractive distillation. The weak nitric acid cannot be concentrated by simple fractional distillation. The distillation must be carried out in the presence of a dehydrating agent. Concentrated sulfuric acid (typically 60 percent sulfuric acid) is most commonly used for this purpose. The nitric acid concentration process consists of feeding strong sulfuric acid and 55 to 65 percent nitric acid to the top of a packed dehydrating column at approximately atmospheric pressure. The acid mixture flows downward, countercurrent to ascending vapors. Concentrated nitric acid leaves the top of the column as 99 percent vapor, containing a small amount of NO2 and oxygen (O2) resulting from dissociation of nitric acid. The concentrated acid vapor leaves the column and goes to a bleacher and a countercurrent condenser system to effect the condensation of strong nitric acid and the separation of oxygen and oxides of nitrogen (NOx) byproducts. These byproducts then flow to an absorption column where the nitric oxide mixes with auxiliary air to form

NO2, which is recovered as weak nitric acid. Inert and unreacted gases are vented to the atmosphere from the top of the absorption column. Emissions from this process are relatively minor. A small absorber can be used to recover NO2. 42 Emissions And Controls

Emissions from nitric acid manufacture consist primarily of NO, NO2 (which account for visible emissions), trace amounts of HNO3 mist, and ammonia (NH3). By far, the major source of nitrogen oxides (NOx) is the tailgas from the acid absorption tower. In general, the quantity of NOx emissions is directly related to the kinetics of the nitric acid formation reaction and absorption tower design.

NOx emissions can increase when there is: (1) insufficient air supply to the oxidizer and absorber, (2) low pressure, especially in the absorber, (3) high temperatures in the cooler-condenser and absorber, (4) production of an excessively high-strength product acid, (5) operation at high throughput rates, and (6) faulty equipment such as compressors or pumps that lead to lower pressures and leaks, and decrease plant efficiency. 43 The 2 most common techniques used to control absorption tower tail gas emissions are extended absorption and catalytic reduction.

Extended absorption reduces NOx emissions by increasing the efficiency of the existing process absorption tower or incorporating an additional absorption tower. An efficiency increase is achieved by increasing the number of absorber trays, operating the absorber at higher pressures, or cooling the weak acid liquid in the absorber. The existing tower can also be replaced with a single tower of a larger diameter and/or additional trays.

In the catalytic reduction process (often termed catalytic oxidation or incineration), tail gases from the absorption tower are heated to ignition temperature, mixed with fuel (natural gas, hydrogen, propane, butane, naphtha, carbon monoxide, or ammonia) and passed over a catalyst bed. In the presence of the catalyst, the fuels are oxidized and the NOx are reduced to N2. The extent of reduction of NO2 and NO to N2 is a function of plant design, fuel type, operating temperature and pressure, space velocity through the reduction catalytic reactor, type of catalyst, and reactant concentration. Catalytic reduction can be used in conjunction with other NOx emission controls. Other advantages include the capability to operate at any pressure and the option of heat recovery to provide energy for process compression as well as extra steam.

Catalytic reduction can achieve greater NOx reduction than extended absorption. However, high fuel costs have caused a decline in its use. 44 Sulfuric acid production https://www.youtube.com/watch?v=mym1rRPX6F4 https://chemicalengineeringworld.com/sulphuric-acid-manufacturing-process/

◦ Sulfuric acid or sulphuric acid, also known as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and

hydrogen, with molecular formula H2SO4. It is a colourless and viscous liquid that is miscible with water at all concentrations. ◦ The acid in pure form is highly corrosive towards other materials, as it is an oxidant and a strong acidic nature. ◦ The pure acid is highly dehydrating, which means it will strip water away from any substance it comes into contact with. It is also hygroscopic, readily absorbing water vapor from the air. ◦ Upon addition of sulfuric acid to water (this should not be reversed), a lot of heat is released. ◦ Upon contact, pure sulfuric acid can cause severe chemical burns and even secondary thermal burns due to dehydration; even small amounts of the pure acid are dangerous. The solution of sulfuric acid in water is substantially less hazardous; the oxidative and dehydrating properties are only present in the pure acid, although the solution of the acid in water will be very acidic, and should therefore still be handled with care. 45 ◦ Sulfuric acid is a very important commodity chemical, and a nation's sulfuric acid production is a good indicator of its industrial strength. It is widely produced with different methods, such as contact process, wet sulfuric acid process, lead chamber process and some other methods.

◦ Sulfuric acid is also a key substance in the chemical industry. It is most commonly used in fertilizer manufacture, but is also important in mineral processing, oil refining, wastewater processing, and chemical synthesis. It has a wide range of end applications including in domestic acidic drain cleaners, as an electrolyte in lead-acid batteries, in dehydrating a compound, and in various cleaning agents.

46 Current Status of Sulfuric Acid Production

Sulfuric acid is produced in almost all the countries of the world with the major producers being USA, Russia, China, Japan, Finland, Brazil, India, South Korea, Australia, Indonesia, Germany, Spain, France, and Belgium. The last four countries account for 70% of the total European production. USA and Canada account for nearly 40% of sulfuric acid production.

Sulfuric acid is used directly or indirectly in nearly all manufacturing activities and is a vital commodity in any national economy. For this contribution, sulfuric acid is known as ‘King of Chemicals’.

Higher derivatives of sulfuric acid are 23–25% oleums, 65% oleums and pure SO3, as well as liquid SO2, which find applications in organic and petroleum processing.

Battery grade, pharmaceutical grade, laboratory (AR), as well as electronic grade sulfuric acids are purer and purer varieties of the acid and have specific uses. 47 Major Uses of Sulfuric Acid

1. It finds application as a dehydrating agent, catalyst, active reactant in chemical processes, solvent, and absorbent. 2. It is used in the process industries from very dilute concentrations for pH control of saline solutions to strong fuming acids used in the dye, explosives, and pharmaceutical industries. 3. It is produced and supplied in grades of exacting purity for the storage battery, rayon, alums, dye, and pharmaceutical industries, and in grades of less exacting specifications for use in the steel, heavy chemical, and super phosphate and phosphatic fertilizer industries. 4. Sulfuric acid is not a one-use product. Like a returnable steel drum, after initial use in some phases of the explosives, petroleum, and dye industries, the sulfuric acid is recovered in a form often unsuitable for use in the same process but of a strength and grade entirely suitable for use in other process industries. Large amounts of sulfuric acid are consumed in the manufacture of phosphatic fertilizers, ammonium sulfate,

etc.; for every tonne of P2O5, 3 tonnes of 100% sulfuric acid is required.

48 Processes of Manufacture of Sulfuric Acid

The processes for the manufacture of sulfuric acid can be classified on the basis of raw materials used: • Elemental sulfur • Sulfide ores • Spent acid

• Gases like H2S

Many variants are available for each process utilizing the above-mentioned starting materials.

49 Overview of Techniques Applicable to Sulfuric Acid Production

50 Single Contact Process

The SO2-containing gases, which have been carefully cleaned and dried, are oxidized to sulfur trioxide in the presence of catalysts containing alkali and vanadium oxides. The sulfur trioxide is absorbed by concentrated sulfuric acid in absorbers, preceded if necessary by oleum absorbers. In the absorbers, the sulfur trioxide is converted to sulfuric acid by the existing water in the absorber acid. The absorber acid is kept at the desired concentration of approximately 99 wt% by adding water or dilute sulfuric acid.

The single contact process is generally used with an SO2 content of inlet gases of 6–10%; in new plants, the conversion efficiency is about 98.5% as a daily average and can be upgraded to 99.1% by good design and use of specially adapted doped Cs catalyst. In existing single conversion single absorption plants, it is difficult to obtain better than 98.0% conversion; however, in some existing plants a conversion efficiency of 98.5% can be achieved with a large loading of catalyst in the last pass and operating at as low a temperature as possible (410–415 °C).

51 ◦ Sulfuric acid can be produced by reacting sulfur trioxide directly with water according to the equation below.

-1 ◦ SO3(g) + H2O(l) =>H2SO4(aq) ΔH = -130kJ mol ◦ However, so much heat is released that the reaction chamber becomes full of sulfuric acid mist which is very difficult to collect. For this reason sulfuric acid is produced in several stages known as the contact process. ◦ Stage one - Liquid sulfur is sprayed in the burner where it

reacts with dry air to produce sulfur dioxide (SO2). Water that may be present forms sulfuric acid in the converter and corrodes the wall. The air is dried using concentrated sulfuric acid. ◦ Stage two - The sulfur dioxide is oxidised to sulfur trioxide by oxygen using vanadium(V)oxide as a catalyst. ◦ Stage three - Concentrated sulfuric acid is used to dissolve http://www.dynamicscience.com.au/tester/solutio sulfur trioxide where it forms oleum (H2S2O7) in an absorption ns1/chemistry/sulfuricacid4.gif tower. ◦ Stage four - Oleum is then mixed with water to obtain sulfuric 52 acid. 53 Double Contact Process (Double Absorption)

In the double contact process, the degree of conversion obtained is about 99.5%, depending on the arrangement of the contact beds and of contact time preceding the intermediate absorber. After cooling the gases to approximately 160–190 °C in a heat exchanger, the sulfur trioxide already formed is absorbed in the intermediate absorber in sulfuric acid with a concentration of 98.5–99.5 wt%. The intermediate absorber is preceded by an oleum absorber if required. The absorption of the sulfur trioxide

brings about a considerable shift in the reaction equilibrium towards the formation of SO3, resulting in considerably higher overall conversion efficiencies when the residual gas is passed through one or two secondary contact beds. The sulfur trioxide formed in the secondary stage is absorbed in the final absorber.

In general, SO2 feed gases containing up to 12 vol.% SO2 are used for this process. The conversion efficiency in new plants can reach about 99.6% as a daily average in the case of sulfur burning.

54 55 Wet Contact Process (WCP) This process is not sensitive to the water balance and has been used to treat off-gas from a molybdenum smelter as well as being installed in two desulfurization plants (one in a flue gas desulfurization system, the other on an industrial boiler) currently under construction. An earlier version of the WCP technology was used to treat lean hydrogen sulfide gases. For all gas feeds, sulfurous components in the gas are converted to sulfuric acid without the need to dry the gas first. Roaster gases are cleaned by a combination of cyclones, bag filters, electrostatic precipitators, venture scrubber, etc. An induced draft (ID) fan is provided at suitable point to convey the gases through the plant, and to overcome the pressure drop in the scrubbing system. Cleaned gases (containing SO2) are brought up to conversion temperature prior to admission to the converter. The SO2 is converted to SO3 by the catalyst and the hot gases from the converter exit are cooled by heat exchange with the incoming cold feed gas. SO3 combines with water vapor present in the gases and the sulfuric acid formed is condensed on specially designed condensers (where acid mist formation is minimized). In addition to the usual utilities required for a sulfuric acid plant, there can be a need for an additional fuel (e.g., for oil/gas fired burners) for heating the process gas to conversion temperatures if the gas strength is low (less than 3.5–4.0% SO2) since autothermal operation is difficult at such low strengths. 56 Pressure Process

The conversion of SO2 to SO3 increases as the operating pressure is increased since there is a reduction in volume during the reaction

2SO2 +O2→2SO3 The increase can be achieved by maintaining optimum temperatures in the catalyst bed. Increasing the process gas pressure can reduce the required size of the equipment, but the higher power consumption of the blower offsets the advantage gained. Hence most SA plants are built to operate at only a little above atmospheric pressure. Lately the cold process has become economically viable at high pressure; Reduction in gas volumes can reduce both the required size of the equipment and the amount of catalyst required. Hence, a capital saving is possible, but the blower consumes more power. As already said, this can offset the advantage gained. The principal disadvantage of the pressure contact process compared with the conventional double- absorption process is that it consumes more power.