Industrial Gases used in Steel Industry
Industrial Gases used in Steel Industry
The term ‘Industrial gas’ is referred to a group of gases (Fig 1) which are specifically produced for use in a variety of industrial processes. They are distinct from the fuel gases. However, acetylene is sometimes considered as industrial gas. Specialty gases such as neon, krypton, xenon and helium are sometimes considered under the category of industrial gases. Industrial gases are produced and supplied in both gas and liquid form and transported in cylinder, as bulk liquid or in pipelines as gas. Industrial gases usually used in steel industry are oxygen, nitrogen, argon and hydrogen.
Fig 1 Industrial gases
Industrial gases are supplied in a range of different cylinders depending on the properties of the gas. Some are supplied at high pressures, while others are only available at low pressures. The properties of the industrial gas decide the method in which it is supplied to the customer. Gases such as oxygen, nitrogen, argon and hydrogen can be readily compressed into a cylinder at pressures up to 200 bars. Acetylene, because of its properties, needs to be stored in a cylinder containing a ‘porous mass’ in which the gas is held in a carrier solvent.
Industrial gas cylinders come in a range of sizes which are normally categorized by the water capacity of the container. The size, which size is the most suitable, depends on a range of factors including consumption and flow rate. Further, each cylinder is fitted with a cylinder valve tailored to suit the gas and pressure requirements. The outlet thread is determined by national standards to ensure that only regulators compatible with these requirements can be fitted. Also, for applications requiring higher capacities, industrial gases are supplied in a range of manifolded cylinder pallets, which consists of multiple cylinders connected together and palletized.
Industrial gases normally used in steel industry are oxygen, nitrogen, argon, hydrogen, and acetylene. Properties of these gases are given in Tab 1. These gases are described after the table. Further, some of specialty gases and gas mixes are used in the steel plant laboratories for instrumental analysis work.
|Tab 1 Properties of industrial gases|
|Boiling point at 1.033 kg/sq cm||Temperature||deg C||-183||-195.8||-185.9||-252.8|
|Latent heat of vaporization||kcal/kg||50.91||47.586||38.791||106.597|
|Gas phase properties at 0 deg C and 1.033 kg/sq cm||Specific gravity||Air = 1||1.113||0.9737||1.39||0.06998||0.91|
|Specific heat (Cp)||kcal/kg deg C||0.2197||0.2486||0.125||3.427|
|Liquid phase properties at boiling point and 1.033 kg/sq cm||Specific gravity||Water=1||1.14||0.808||1.4||0.071|
|Specific heat (Cp)||kcal/kg deg C||0.3989||0.488||0.2576||2.311|
|Triple point||Temperature||deg C||-218.8||-210||-189.3||-259.2||-80.8|
|Pressure||kg/sq cm abs.||0.0015||0.1275||0.7026||0.0735|
|Critical point||Temperature||deg C||-188.6||-146.9||-122.3||-239.96|
|Pressure||kg/sq cm abs.||51.44||34.66||50.02||13.41|
Air, natural atmosphere of the earth, is a non-flammable, colourless and odourless mixture of gases in which nitrogen (78 %) and oxygen (21 %) predominate. The balance of around 1 % is composed of the rare gases helium, neon, argon, krypton and xenon. The compressed (pressurized) air is used for several applications in the steel plant. It is also used for the production of oxygen, nitrogen, and argon.
Oxygen (O2) is an active component of the atmosphere making up 20.94 % by volume or 23 % by weight of the air. It is a colourless, odourless and tasteless gas. It is highly oxidizing. Oxygen reacts vigorously with combustible materials, especially in its pure state, releasing heat in the reaction process. Many reactions require the presence of water or are accelerated by a catalyst. Oxygen has a low boiling / condensing point which is -183 deg C. The gas is around 1.11 times heavier than air and is slightly soluble in water and alcohol. Below its boiling point, oxygen is a pale blue liquid slightly heavier than water. Properties of oxygen are given in Tab 1.
Oxygen is produced in large quantities and at high purity as a gas or liquid by cryogenic distillation process and in smaller quantities as a lower purity gas (typically around 93 %) by adsorption technologies such as pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA or VSA). Fig 2 shows the cryogenic and non-cryogenic production processes of gases.
Fig 2 Cryogenic and non-cryogenic production of gases
Oxygen is the second largest consumed industrial gas. The largest user of oxygen is the steel industry. Aside from its chemical name O2, oxygen is also referred to as GOX or GO when produced and delivered in gaseous form, or as LOX or LO when in its cryogenic liquid form.
Oxygen is important for its reactivity. The reactivity of oxygen is used in steel processing and in welding and cutting of steel. Steelmaking by basic oxygen furnace relies heavily on the use of oxygen. It is also used to enrich air and increase combustion temperatures in blast furnaces as well as to replace coke with other combustible materials such as pulverized coal, fuel oil, or natural gas etc. During the steelmaking process by basic oxygen furnace, unwanted carbon combines with oxygen to form carbon oxides, which leave as gases. Oxygen is fed into the steel bath through a special lance. Oxygen is also used to increase the productivity in electric arc furnaces.
Oxygen enrichment of air in industrial processes increases reaction rates, which permits greater throughput in existing equipment or the ability to reduce the physical size of equivalent capacity new equipment. Another benefit of oxygen enrichment versus use of plain air is energy savings and due to a reduction in the amount of nitrogen and other gases passing through a furnace or through a chemical process. Reducing inert gases which otherwise have to be compressed or heated can reduce energy consumption due to a decrease in gas compression requirements or a reduction in the amount of fuel needed to make a given amount of product. Reducing the amount of hot gases vented to the atmosphere from combustion processes also decreases the size and cleanup costs associated with stack gas cleanup systems.
Oxygen is used with fuel gases in gas welding, gas cutting, oxygen scarfing, flame cleaning, flame hardening, and flame straightening. In gas cutting, the oxygen is to be of high quality to ensure a high cutting speed and a clean cut.
Oxygen is also used in breathing apparatus. These apparatus are used in steel industry in those places where the contents of blast furnace gas or other gases containing carbon monoxide are higher than the safe values.
Nitrogen (N2) is a colourless, odourless and tasteless gas which makes up 78.09 % (by volume) of the air. It is non-flammable and does not support combustion. Nitrogen gas is slightly lighter than air and slightly soluble in water. Nitrogen condenses at its boiling point (-195.8 deg C) to a colourless liquid which is lighter than water. Properties of nitrogen are given in Tab 1.
Nitrogen is normally thought of and used as an inert gas. But nitrogen gas is not truly inert. It forms nitric oxide and nitrogen dioxide with oxygen, ammonia with hydrogen, and nitrogen sulphide with sulphur. Nitrogen compounds are also formed naturally through biological activity. Compounds are also formed at high temperature or at moderate temperature with the aid of catalysts.
Nitrogen is the largest consumed industrial gas. Besides the steel industry, it is used in a broad range of industries such as chemicals, pharmaceuticals, petroleum processing, glass and ceramic manufacture, metals refining and fabrication processes, pulp and paper manufacture, and healthcare etc. Aside from its chemical name N2, nitrogen is also referred as GAN or GN in its gaseous form and LIN or LN in its liquid form.
Nitrogen is produced in large quantities and at high purity as a gas or liquid by cryogenic distillation process and in smaller quantities as a lower purity gas by adsorption technologies such as pressure swing adsorption (PSA) or diffusion separation processes (permeation through specially designed hollow fibers) as shown in Fig 2.
Gaseous nitrogen is valued for inertness. It is used to shield potentially reactive materials from contact with oxygen. Liquid nitrogen is valued for coldness as well as inertness. When liquid nitrogen is vapourized and warmed to ambient temperature, it absorbs a large quantity of heat. The combination of inertness and its intensely cold initial state makes liquid nitrogen an ideal coolant for certain applications. Liquid nitrogen is also used to cool materials which are heat sensitive or normally soft to allow machining or fracturing.
Nitrogen is used in the steel industry as purging gas for pipeline purging, as a coolant in the dry quenching of hot coke, as a cooling gas in the blast furnace top, as a carrier gas for conveying of pulverized coal, as an inert gas in the bottom blowing converter, and as a shield gas in the heat treatment of iron and steel. It is used in different laboratories for testing as well as a process gas, together with other gases for reduction of carbonization and nitriding.
Shrink fitting is an interesting alternative to traditional expansion fitting. Instead of heating the outer metal part, the inner part is cooled by liquid nitrogen so that the metal shrinks and can be inserted. When the metal returns to its normal temperature, it expands to its original size, giving a very tight fit.
Argon (Ar) is a monatomic, colourless, odourless, tasteless, and non-toxic gas, present in the atmosphere at a concentration of 0.934 % by volume. Argon is a member of a special group of gases known as the rare, noble, or inert gases. Other gases in this group are helium, neon, krypton, xenon and radon. These gases are monatomic gases with a totally filled outermost shell of electrons. The terms noble and inert have been used to indicate that the ability of these gases to chemically interact with other materials is extremely weak. All members of this group emit light when electrically excited. Argon produces a pale blue violet light.
Normal boiling point of argon is -185.9 deg C. The gas is around 1.39 times as heavy as air and is slightly soluble in water. Freezing point is -199.3 deg C which is only a few degrees lower than its normal boiling point. Properties of argon are given in Tab 1.
Argon is important gas known for its total inertness in particular at high temperatures. It is the most abundant and the least expensive of the truly inert gases. It is produced normally in conjunction with the manufacture of high purity oxygen using cryogenic distillation of air (Fig 2). Since the boiling point of argon is very close to that of oxygen (a difference of only 2.9 deg C) separating pure argon from oxygen, while also achieving high recovery of both the products, needs several stages of distillation.
For many decades, the most common argon recovery and purification process used several steps which were (i) taking a ‘side draw’ stream from the primary air separation distillation system at a point in the low pressure column where the concentration of argon is highest, (ii) processing the feed in a crude argon column which returns the nitrogen to the low pressure column which produces a crude argon product, (iii) warming the crude argon and reacting the oxygen impurity in the stream (typically around 2 %) with a controlled amount of hydrogen to form water, (iv) removing the water vapour by condensation and adsorption, (v) re cooling the gas to cryogenic temperature and (vi) removing the remaining non argon components (small amounts of nitrogen and not consumed hydrogen) through further distillation in a pure argon distillation column.
With the development of packed column technology, which allows cryogenic distillations to be performed with low pressure drop, majority of new plants now utilize an all cryogenic distillation process for argon recovery and purification.
Argon, besides its chemical name Ar, is sometimes referred to as PLAR (pure liquid argon) or CLAR (crude liquid argon). Crude argon is normally thought of as an intermediate product in a facility which makes pure argon, but it can be a final product for some lower capacity air separation plants which dispatch it to larger facilities for final purification. Commercial quantities of argon can also be produced in conjunction with the manufacture of ammonia.
Argon is used where a completely non reactive gas is needed. In steel melting shop it is used for bottom blowing in the combined blowing process of steelmaking. It is used for stirring of liquid steel in teeming ladles and as a shield gas in continuous casting of steel. It is also used in AOD converter where it is blown in the molten metal along with oxygen. The addition of argon reduces chromium losses and the desired carbon content is achieved at a lower temperature.
Pure argon, as well as argon mixed with various other gases, is used as a shield gas in TIG welding (tungsten inert gas welding) which uses a non-consumable tungsten electrode, and in MIG (metal inert gas) welding which employs a consumable wire feed electrode. The function of the shielding gas is to protect the electrode and the weld pool against the oxidizing effect of air.
Hydrogen (H2) is a colourless, odourless, tasteless, flammable and non-toxic gas at atmospheric temperature and pressure. It is the most abundant element in the universe, but is almost absent from the atmosphere as individual molecules in the upper atmosphere can gain high velocities during collisions with heavier molecules, and become ejected from the atmosphere. It is still quite abundant on earth, but as part of compounds such as water.
Hydrogen burns in air with a pale blue, almost invisible flame. Hydrogen is the lightest of all gases, around one-fifteenth as heavy as air. Hydrogen ignites easily and forms, together with oxygen or air, an explosive gas (oxy-hydrogen). Hydrogen has the highest combustion energy release per unit of weight of any normal occurring material. This property makes it the fuel of choice for upper stages of multi-stage rockets.
Hydrogen has the lowest boiling point of any element except helium. When cooled to its boiling point (-252.76 deg C), hydrogen becomes a transparent, odourless liquid which is only one-fourteenth as heavy as water. Liquid hydrogen is not corrosive or particularly reactive. When converted from liquid to gas, hydrogen expands around 840 times. Its low boiling point and low density result in liquid hydrogen spills dispersing rapidly. Properties of hydrogen are given in Tab 1.
Hydrogen can be produced using a number of different processes. Thermo-chemical processes use heat and chemical reactions to release hydrogen from organic materials such as fossil fuels and biomass. Water (H2O) can be split into hydrogen and oxygen using electrolysis or solar energy. Microorganisms such as bacteria and algae can produce hydrogen through biological processes. Fig 3 shows electrolytic production of hydrogen.
Fig 3 Electrolytic production of hydrogen
Some thermo-chemical processes use the energy in various resources, such as natural gas, coal, or biomass, to release hydrogen from their molecular structure. In other processes, heat, in combination with closed-chemical cycles, produces hydrogen from feed-stocks such as water. The common thermo-chemical processes for to get hydrogen gas are (i) natural gas reforming (also called steam methane reforming or SMR), (ii) coal gasification, (iii) biomass gasification, (iv) biomass-derived liquid reforming, and (v) solar thermo-chemical hydrogen (STCH).
The electrolytic processes are carried out in electrolyzers which use electricity to split water into hydrogen and oxygen. This technology is well developed and available commercially, and systems which can efficiently use intermittent renewable power are being developed.
Direct solar water splitting, or photolytic, processes use light energy to split water into hydrogen and oxygen. These processes are at present in the very early stages of research but offer long-term potential for sustainable hydrogen production with low environmental impact. The two solar water splitting processes are (i) photo electro-chemical (PEC) process, and (ii) photo biological process.
In biological processes microbes are used. Microbes such as bacteria and micro-algae can produce hydrogen through biological reactions, using sunlight or organic matter. These technology pathways are at an early stage of research, but in the long term have the potential for sustainable, low-carbon hydrogen production. The two biological processes are (i) microbial biomass conversion process, and (ii) photo biological process.
The most common large scale process for manufacturing hydrogen is steam reforming of hydrocarbons. Other methods used for hydrogen production include generation by partial oxidation of coal or hydrocarbons, electrolysis of water, recovery of byproduct hydrogen from electrolytic cells used to produce chlorine and other products, and dissociation of ammonia. Hydrogen is also recovered for internal use and sale from various refinery and chemical streams, typically purge gas, tail gas, fuel gas or other contaminated or low-valued streams. Purification methods include pressure swing adsorption (PSA), cryogenic separation and membrane gas separation.
Some industrial processes with relatively small hydrogen requirements choose to produce their needs using compact generators. In the past, ammonia dissociation was a common technology choice. More recently, improvements in small packaged electrolytic and hydro-carbon reforming systems have made these routes to small volume hydrogen production increasingly attractive. Electrolytic production techniques can produce high purity hydrogen at elevated pressure, eliminating the need for supplemental compression. The latest generation of highly packaged hydro-carbon reforming units, in particular those which employ an auto thermal generation process and which operates at relatively low temperature and pressure, have made on site hydrocarbon reforming a viable route to hydrogen production at much lower production rates than were considered commercially feasible just a few years ago.
Hydrogen is produced by dissociation of ammonia at around 982 deg C with the aid of a catalyst. This results in a mix of 75 % hydrogen and 25 % mono-nuclear nitrogen (N rather than N2). The mix is used as a protective atmosphere for applications during bright annealing of cold rolled coils and strips. Hydrogen is also used as a reducing agent in the production of direct reduced iron (DRI).
Hydrogen is mixed with inert gases to obtain a reducing atmosphere, which is needed for many applications in the steel industry, such as in laboratories, heat treating steel, and welding. It is frequently used in annealing stainless steel alloys and magnetic steel alloys.
Large quantities of hydrogen are used to purify argon which contains trace amounts of oxygen, using catalytic combination of the oxygen and hydrogen followed by removal of the resulting water.
Acetylene is the chemical compound with the formula C2H2. It is an unsaturated hydrocarbon and the simplest of alkynes. An acetylene molecule is composed of two carbon atoms and two hydrogen atoms. The two carbon atoms are held together by what is known as a triple carbon bond. This bond is useful in that it stores substantial energy which can be released as heat during the combustion process. However, the triple carbon bond is unstable, making acetylene gas very sensitive to conditions such as excess pressure, excess temperature, static electricity, or mechanical shock.
Acetylene is a colourless and odourless gas. At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. The adiabatic flame temperature (AFT) in air at atmospheric pressure is 2534 deg C. Its specific gravity is 0.91 at 21 deg C. It is produced by the hydrolysis of calcium carbide as per the chemical reaction represented by the equation CaC2 + 2H2O = Ca(OH)2 + C2H2. These days acetylene is mainly manufactured by the partial combustion of methane or appears as a side product in the ethylene stream from cracking of hydrocarbons.
Acetylene is to be stored under special conditions because of its unstable nature. This is accomplished by dissolving the acetylene in liquid acetone. The liquid acetone is then stored in the acetylene cylinder, which in turn, is filled with a porous (sponge like) cementitious material.
Acetylene is used in steel plants for oxy-acetylene gas cutting and welding and in flame cutting machines of continuous casting machines. It is sometimes used for carburization of steel.