Lecture 3 Industrial Gases+ Ammonia 2019

Inorganic Industries Engineering ‫هندسة الصناعات غريالعضويه‬ Lecture 3 Dr.: Sameh Araby El-Mekawy

Outlines Industrial Gases Manufacture of Ammonia

Industrial Gases  The term “industrial gases” is “a collective term for combustible and non-combustible gases generated on an industrial scale, such as several (acetylene, hydrogen, carbon dioxide, nitrogen, oxygen and argon)  Their production is a part of the wider chemical Industry (where industrial gases are often seen as "specialty chemicals").  There are seven major companies whose combined gas-related revenue accounted for over 75% of the global market at the end of 2005: AL: Air Liquide (French gas company) BOC: BOC Gases (UK gas company) AP: Air Products and Chemicals, Inc. (US gas company) Praxair: Praxair, Inc. (US gas company) Linde: Linde Gas (German gas company) TNS: Taiyo Nippon Sanso Co. (Japanese gas company) Airgas: Airgas, Inc. (major US distributor)

Uses of industrial gases Hydrogen 1) As fuel for automobiles: Hydrogen fuel is a zero-emission fuel which burns on reaction with oxygen. It is an exciting concept which aims to power the automobiles by use of hydrogen instead of petroleum fuels. 2) Deuterium for electricity generation: the isotope of hydrogen is used to make heavy water (D2O). This heavy water is used in nuclear reactors as a coolant. 3) Large quantities of hydrogen are used to hydrogenate oils to form fats, for example to make margarine. 4) It is also used to remove sulfur from fuels during the oil-refining process. 5) In the chemical industry it is used to make ammonia for agricultural fertilizer (the Haber process) and cyclohexane and methanol, which are intermediates in the production of plastics and pharmaceuticals.

Carbon dioxide  Widely used in the food industry for applications such as removing the caffeine from coffee beans to make decaffeinated coffee.  Carbon dioxide can be obtained as a solid in the form of dry ice by allowing the liquified CO2 to expand rapidly.  It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue (other than incidental frost from moisture in the atmosphere).  Dry ice is used as a refrigerant for ice-cream and frozen food. Gaseous CO2 is extensively used to carbonate soft drinks.  Being heavy and non-supporter of combustion it is used as fire extinguisher.

Oxygen 1) The greatest commercial use of oxygen gas is in the steel industry.

2) Large quantities are also used in the manufacture of a wide range of chemicals including HNO3and H2O2. 3) It is also used to make epoxyethane (ethylene oxide), used as antifreeze

and to make polyester, and chloroethene, the precursor to PVC. 4) Oxygen gas is used for oxy-acetylene welding and cutting of metals. 5) A growing use is in the treatment of sewage and of effluent from

industry.

Nitrogen 1)

It is used to make fertilizers, nitric acid, nylon, dyes and explosives. To make these products, nitrogen must first be reacted with hydrogen to produce ammonia. This is done by the Haber process. 150 million tonnes of ammonia are produced in this way every year.

2) Nitrogen gas is also used to provide an unreactive (inert) atmosphere. It is used in this way to preserve foods, and in the electronics industry during the production of transistors and diodes. Large quantities of nitrogen are used in annealing stainless steel and other steel mill products. Annealing is a heat treatment that makes steel easier to work. 3) Liquid nitrogen is often used as a refrigerant. It is used for storing sperm, eggs and other cells for medical research and reproductive technology. It is also used to rapidly freeze foods, helping them to maintain moisture, colour, flavour and texture.

 Cryogenics: The branches of physics and engineering that study very low temperatures, how to produce them, and how materials behave at those temperatures.  Cryogens, like liquid nitrogen, are further used for specialty chilling and freezing applications. Some chemical reactions, like those used to produce the active ingredients for the popular statin drugs, must occur at low temperatures of approximately -100 °C. Special cryogenic chemical reactors are used to remove reaction heat and provide a low temperature environment. The freezing of foods and biotechnology products, like vaccines, requires nitrogen in blast freezing or immersion freezing systems.

Manufacture of Industrial Gases

Raw Materials

AIR

Composition of dry atmospheric air

 Nitrogen, oxygen and argon are almost exclusively recovered from atmospheric air.  Atmospheric Air is comprised of a mixture of invisible permanent and variable gases as well as suspended microscopic particles (both liquid and solid) 1) Permanent Gases – Form a constant proportion of the total atmospheric mass  78.08% Nitrogen (N2)  20.95% Oxygen (O2)  <1% Argon (Ar) 2) Variable Gases – Distribution and concentration varies in space and time  Water vapor (H2O) 0 to 4%  Carbon Dioxide (CO2) 0.038%  Methane(CH4) 0.00017%  Ozone(O3) 0.000004% 3) Aerosols – An aerosol is a suspension of fine solid particles or liquid droplets, in air typically ranging in size from 0.001 to 100 μm. (e.g. dust)

Intake Air purification  The first step in processing air is removing contaminants from the air stream. Before distillation the air should be purified from different impurities and components. Failure to do this results in the buildup of these contaminants in the liquid air distillation column inside the cold box. The worst of these is acetylene, which can decompose explosively.  Acetylene is a very hazardous reactive contaminant. Because acetylene has a low solubility in LOX, if it enters the cold box it concentrates in LOX and precipitates out as a solid at concentrations as low as 4 ppm to 6 ppm (depending on the LOX pressure). The solid is relatively unstable and requires little energy to ignite.  However, any hydrocarbon is hazardous because it can be exposed to pure oxygen in parts of the system. Mixing any hydrocarbon with pure oxygen creates a combustion explosion hazard.  Trace contaminants can be put into three main categories based on the potential problems they cause in the ASU (plugging, reactive, or corrosive) as shown in Table 2.  Plugging contaminants concentrate, precipitate out as a solid, or both in the ASU process. While plugging is an operating problem, it can also lead to dry boiling or pool boiling, which can in turn concentrate the reactive contaminants to form flammable mixtures. The plugging contaminants of most concern are water, carbon dioxide, and nitrous oxide.  Corrosive contaminants (acid gases and ammonia) can react with equipment and piping causing operating problems and impacting equipment life.

Air purification technologies 1. Removal of acidic compounds like H2S, Carbon dioxide and oxides of nitrogen can be carried out by using Caustic scrubbers (air is washed by solution of the sodium hydroxide or potassium hydroxide). 2. Clearing air from acetylene: Air clearing from acetylene because it’s very dangerous for the air separation plant so it’s important to clear air from it because if acetylene aggregation it will be explosion. Acetylene has low part pressure in the air so it can’t distant in heat exchanger and in regenerator and it’s aggregation in liquid. Acetylene has low solubility in air, oxygen and nitrogen so it can be very easy clean in SiO2.H2O filters. 3. An electrostatic precipitator (ESP) is a filtration device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit. 4. Spraying air with water cleans the air of water-soluble substances, dusts and vaporized air compressor lubricants.

Air separation technologies Air separation technology is based on the fact that the fundamental components of air all have different physical properties and air separation is therefore realized through, for example, distinguishing between molecule sizes, distinguishing between difference in diffusion rates through certain materials, adsorption preference special materials have towards certain gasses of the atmosphere and difference in boiling temperatures. The most common methods to perform gas separation are:

1) Pressure swing adsorption (PSA) 2) Membrane separation 3) Cryogenic separation (has a share of far more than 90% on the

worldwide production)

Air separation technologies  These methods have common characteristics such as 1) they use air as a feedstock; 2)

they are physical processes - no chemical reactions are involved;

3) the products are consistently high quality and contain no unwanted contaminants

 Cryogenic separation is applied whenever high purity, large quantities, liquid products or argon is required.  Membrane and adsorption plants have a high load range and can be started and

powered up to full production within a few minutes.  A cryogenic plant needs about 2 hours for the start from cold condition until the beginning of production of oxygen and nitrogen.

1)Pressure swing adsorption (PSA)

Schematic representation of Pressure Swing Adsorption method (PSA).

 PSA provides separation of oxygen or nitrogen from air without liquefaction.

The process operates around ambient temperature; a zeolite (adsorbent bed) (molecular sponge) is exposed to high pressure air, As soon as the pressure is

reduced again, the gas is desorbed (released). This additionally frees up the adsorbent, ready for the next cycle.  The basis of PSA is that when gases are put under high pressure, the gases are

attracted to solid surfaces and hence are adsorbed. This is a proportional relationship, in that the higher the pressure, the more gas is adsorbed.  These systems rely on the zeolites to trap nitrogen and hence produce oxygen

with high purity  Different gases can be adsorbed by using different solid particles, based on what

the gases are more easily attracted to, so this method typically has relatively high product purity

2) Membrane separation

Schematic representation of Membranes for Air separation

 Separation of gases with membranes relies on the different affinities of one or more

gases towards the membrane material, causing one gas to be permeate faster (or slower) than others. The membrane could be composed of Microporous Organic Polymers,

Zeolites or Ceramic.  Air is directed into a vessel and put in contact to the membrane material which is at the interface with another vessel. The mixture is allowed to diffuse into the second vessel under a pressure gradient which promotes the mass transport through the membrane separating the retentate (slower gas) from the permeate (faster gas).

 Advantages: the most valuable is the high cost-efficiency (both for the mechanical simplicity of the system and for low-energy regeneration) in fact, they do not require

thermal regeneration in their operation.  Disadvantages: Selectivity for a required gas component. This means that high

permeable membranes have low selectivity, requiring several run for a good separation, and highly selective membranes have low permeability, meaning long operational times.

3) Cryogenic distillation process

Schematic representation of Cryogenic distillation method

 Cryogenic air separation process is one of the most popular air separation process, used frequently in medium to large scale plants.  In the cryogenic gas processing, various equipment is used like the distillation columns, heat exchangers, cold interconnecting piping etc. which operate at very low temperatures and hence must be well insulated. These items are located inside sealed "cold boxes". Cold boxes are tall structures with either round or rectangular cross section. Depending on plant type, size and capacity, cold boxes may have a height of 15 to 60 meters and 2 to 4 meters on a side.

 Cryogenic air separation is the most common and standard technology used for the separation of air into its constituents.

 Pure gases can be separated from air by first cooling it until it liquefies, then selectively distilling the components at their various boiling temperatures.  The process can produce high purity gases but is energy-intensive.

 This process was pioneered by Carl von Linde in the early 20th century and is still used today to produce high purity gases.

 Principle Cryogenic air separation technology is based on the fact that the different constitute gases of air all have different boiling points and by manipulating immediate environment in terms of temperature and pressure the air can be

separated into its components

Several important principles are involved in the process and are summarized below: • When work is done on air, by compressing it, it becomes hotter. • When compressed air is expanded through and opening or valve it becomes cooler. • When air is expanded in a turbine it does work on the rotors and cools by approximately ten times as much as in simple expansion. • When a mixture of liquids is in equilibrium with its vapour, the vapour above the liquid is richer in the more volatile component (i.e. more of the lower boiling liquid vapourises). • The boiling point of a liquid is lower at lower pressure.

Process description The cryogenic air separation flow diagram given below does not represent any particular plant and shows in a general way many of the important steps involved in producing oxygen, nitrogen, and argon as both gas and liquid products

Block diagram of cryogenic air separation plant

Process description There are various thermodynamic processes needed in cryogenic air separation, of which fundamental ones are: air compression, air purification, heat exchanging, distillation and product compression. Air compression  Atmospheric air is pre-filtered (to remove dust), and compressed using a centrifugal compressor to a pressure required.  Since the compressor heats up the air, it is cooled in inter stage coolers and watercooled after cooler to condense any water vapors.

Air purification  Air is then processed through a pre-purification unit. During this process, compressed air is generally passed through a pre- purification unit which removes any remaining water vapors, as well as carbon dioxide to avoid freeze of water vapors and carbon dioxide in the cryogenic equipment.  The air passes through a molecular sieve adsorber. The adsorber contains zeolite and silica gel-type adsorbents, which trap the carbon dioxide, heavier hydrocarbons, and any remaining traces of water vapor. Periodically the adsorber is cleaned to remove the trapped impurities. This usually requires two absorbers operating in parallel, so that one can continue to process the air-flow while the other one is flushed

Heat exchanging for cooling  After purification, air passes through a plate- fin heat exchanger where it is cooled to a temperature at which it is partially liquid. This cooling is done in brazed aluminum heat exchangers which allow the exchange of heat between the incoming feed air and cold products and waste gas streams exiting the separation process. The exiting gas streams are warmed to close-to-ambient air temperature and incoming feed air liquefied.  The air is passed through heat exchangers for further cooling and for removal of water vapor and carbon dioxide by freezing. Distillation  Partially liquefied air is sent to the double column distillation system integrated with crude argon column, where it is separated into O2, N2 and crude argon fractions.  The distillation is the heart of the cryogenic air separation process.  The air stream which is part liquid and part gas enters the base of the highpressure fractionating or distillation column. As the air moves up the column, it loses additional heat. The oxygen continues to liquefy, leading to the formation of oxygen-rich mixture in the bottom of the column, and other gases like nitrogen and argon flow to the top as a vapor.

Product compression  The gases are sometimes pressurized to meet user requirement and supplied by pipeline to large industrial users adjacent to or nearby to the production plant or stored as liquid.

Distillation of air 1. Air has to be condensed into a liquid. This happens at -200oC.

Colder at the top

4. Nitrogen boils at -196oC so it can be removed from the top of the column as a gas.

2. This is done by compressing the air

-200oC

3. At carbon dioxide and water are solids so can be easily taken out. Warmer at the bottom

5. At -185oC oxygen is still a liquid so can be taken out the bottom of the column.

Manufacture of Ammonia: NH3  Nitrogen is the starting point for an important group of compounds. First, nitrogen is combined with hydrogen to make ammonia (NH3). The production of ammonia is sometimes called industrial nitrogen fixation.  The formation of ammonia from nitrogen and hydrogen is very difficult to accomplish. The two elements do not easily combine.  German chemist Fritz Haber in 1905 found that nitrogen and hydrogen would combine if they were heated to a very high temperature with a very high pressure. He also found that a catalyst was needed to make the reaction occur. The catalyst he used was iron metal, though other metals are sometimes used

 Reactivity of Nitrogen: The dissociation energy of the N≡N bond is very large(946 kJ mol) and dissociation of nitrogen molecules into atoms is not readily effected until very high temperatures, being only slight even at 3000 K. It is this high bond energy coupled with the absence of bond polarity that explains the low reactivity of nitrogen, in sharp contrast to other triple bond structures such as -C ≡ N,-C ≡ C-.  Nitrogen is an inert molecule and will only react with other elements, including oxygen, at very high temperature. Yet nitrogen and oxygen form an array of oxides in which nitrogen exhibits a whole range of oxidation state from +1 to +5: N2O, NO, N2O3, NO2, N2O4, and N2O5. At high temperature, nitrogen gas also reacts with H2, Li, the Group 2A elements, B, Al, C, Si, Ge, and many transition elements.

PROPERTIES OF AMMONIA A colourless liquefied gas Has a pungent smell and is irritating to eyes and lungs

Is a gas at room temperature (b.p. -33.4°C) Is non flammable Is toxic

Is corrosive Is considered dangerous for the environment Ammonia is soluble in aqueous solutions

Ammonia is a gas at room temperature and pressure

Haber Process Ammonia, NH3, is produced commercially by the Haber Process. N2(g) + 3H2(g)

Fe 460 oC 200 atm

2NH3(g)

DH = -92 KJmol-1

Raw Materials  N2(g) is taken from the air via a process of Cryogenic separation.  H2(g) is obtained from either the catalytic steam reforming of natural gas (methane) or naphtha, or the electrolysis of brine at chlorine plants  Modern ammonia-producing plant first converts natural gas (i.e., methane) or LPG (liquefied petroleum gases such as propane and butane) or petroleum naphtha into gaseous hydrogen. The method for producing hydrogen from hydrocarbons is known as steam reforming.

CH4 + H2O ⇌ CO + 3 H2 ΔH = 206 kJ.mol-1

Modern Method of Manufacturing Ammonia While all ammonia plants use this basic process, details such as operating pressures, temperatures, and quantities of feedstock vary from plant to plant. The manufacturing process consists the following stages as shown from the block diagram

Desulphurization  Hydrocarbon feedstocks contain sulphur in the form of H2S andCS2  The catalyst used in the reforming reaction is deactivated (poisoned) by sulphur.  The problem is solved by catalytic hydrogenation of the sulphur compounds as shown in the following equation: H2+RSHRH + H2S(g)  The gaseous hydrogen sulphide is then removed by passing it through a bed of zinc oxide where it is converted to solid zinc sulphide: H2S+ZnO  ZnS+H2O

Primary (Steam) Reforming.  Reforming is the process of converting natural gas or naptha (CnH2n+2) into hydrogen, carbon monoxide and carbon dioxide.  Steam and natural gas are combined at a 3:1 ratio. This mixture is preheated and passed through catalyst-filled tubes in the primary reformer.  Catalytic steam reforming of the sulphur-free feedstock produces synthesis gas (hydrogen and carbon monoxide). Using methane as an example: CH4 + H2O ↔ CO + 3H2

∆H = 49.2 kcal/mol

Secondary reformer  From the primary reformer, the mixture flows to the secondary reformer.  Air is fed into the reformer to completely convert methane to CO in the following endothermic reaction. Ni/15-20atm/1000-1100oC CH4 + Air CO + H2O + N2

Shift Conversion.  The carbon monoxide is converted to carbon dioxide with the assistance of catalyst beds at different temperatures. CO+H2O → CO2+H2  This water-gas shift reaction is favorable for producing carbon dioxide which is used as a raw material for urea production. At the same time more hydrogen is produced.

Purification (Methanation)  CO is an irreversible poison for the catalyst used in the synthesis reaction, hence the need for its removal  The synthesis gas is passed over another catalyst bed in the methanator, where remaining trace amounts of carbon monoxide and dioxide are converted back to methane using hydrogen. CO+3H2CH4+H2O

CO2+4H2  CH4+2H2O Note that the first equation is the opposite of the reformer reaction  Methane is an inert gas with respect to ammonia catalyst, while CO2 and CO can poison the catalyst.

Ammonia Converter.  After leaving the compressor, the gaseous mixture goes through catalyst beds in the synthesis converter where ammonia is produced with a three-to-one hydrogen-to-nitrogen stoichiometric ratio.  Not all the hydrogen and nitrogen are converted to ammonia. The unconverted hydrogen and nitrogen are separated from the ammonia in the separator and re-cycled back to the synthesis gas compressor and to the converter with fresh feed.

Kinetics A catalyst (iron) is used to speed up the rate of reaction and to lower the high activation energy in breaking the N2 triple bond. However, if the temperature is too high, it begins to get destroyed and must be changed more regularly.

The reaction mechanism, involving the heterogeneous catalyst, is believed to be as follows: 1. N2 (g) → N2 (adsorbed) 2. N2 (adsorbed) → 2N (adsorbed) 3. H2 (g) → H2 (adsorbed)

4. H2 (adsorbed) → 2H (adsorbed) 5. N (adsorbed) + 3H (adsorbed)→ NH3 (adsorbed) 6. NH3 (adsorbed) → NH3 (g)

Ammonia Separation The removal of product ammonia is accomplished via mechanical refrigeration or absorption/distillation. The choice is made by examining the fixed and operating costs. Typically, refrigeration is more economical at synthesis pressures of 100 atm or greater. At lower pressures, absorption/distillation is usually favored.

Effect of temperature and pressure on Haber Process In the Haber Process for the production of ammonia, based on the reversible reaction: N2(g) + 3H2(g) 2NH3(g) it is observed that: • As the total pressure increases, the amount of ammonia present at equilibrium increases. • As the temperature decreases, the amount of ammonia at equilibrium increases.

Haber Process N2(g) + 3H2(g)

2NH3(g)

USES OF AMMONIA  Ammonia is also used for the production of plastics, explosives, nitric acid HNO3(via the Ostwald process) and intermediates for dyes and pharmaceuticals.  Around 85% of the ammonia is used in fertilizer manufacture:  Ammonium salts are used as fertilizers because they contain nitrogen in a form that plants can use. The fertilizer Nitram is ammonium nitrate and is made from a solution of ammonia in water and nitric acid in an acid-base reaction: NH3(aq) + HNO3(aq)

NH4NO3(aq)

Similar neutralization reactions with phosphoric(V) and sulphuric acids: 2NH3(aq) + H2SO4(aq)

(NH4)2SO4(aq)

HEALTH AND SAFETY  The occupational health and safety issues associated with ammonia production and storage are: Fire/explosion injuries Poisoning Suffocation  NH3 is a toxic gas that is highly irritating to eyes and lungs. Exposure to high doses can be fatal. Plants must be well ventilated and contain readily available breathing apparatus.  NH3 reacts readily and explosively with a wide range of chemicals such as acids. Fires and explosions may occur in the parts of the plant where hydrogen is produced, requiring careful design and safety features.  NH3 boils at –33°C and hence liquid ammonia can cause frost bite and severe burning.  As ammonia is highly soluble in water, it is extremely toxic to the environment, both in its gaseous form and when dissolved in water.  Workers involved in the handling of liquid ammonia storage and transport need to wear impervious gloves, face shields, and rubber boots and aprons.  CO gas produced during reforming is toxic and exposure to this gas must also be carefully monitored.  The site used for ammonia production is often connected directly to the sites of plants synthesizing other chemicals, such as urea and nitric acid, thus minimising the hazards and costs associated with transport of the chemical.