Industrial gases used in steel industry


Industrial gases used in steel industry

The term “Industrial gas” refers 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. Speciality 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.

Industrial gases

Fig 1 Industrial gases

Oxygen

Oxygen (O2) is an active component of the atmosphere making up 20.94 % by volume or 23 % by weight of the air. It is colorless, odorless and tasteless. Oxygen 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 approximately 1.1 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 at Tab 1.

Oxygen is produced in large quantities and at high purity as a gas or liquid by cryogenic distillation and in smaller quantities as a lower purity gas (typically about 93%) by adsorption technologies such as pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA or VSA).

Oxygen is the second largest consumed industrial gas.  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. Oxygen’s reactivity is used in steel processing and in welding and cutting of steel.

The largest user of oxygen is the steel industry.  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 and open hearth furnaces as well as to replace coke with other combustible materials.  During the steel making process, 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 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 would 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 required to make a given amount of product.  Reducing the amount of hot gases vented to the atmosphere from combustion processes may also decrease 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 must 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 blast furnace gas contents are higher than the safe values.

Nitrogen

Nitrogen (N2) is a colorless, odorless and tasteless gas which makes up 78.09% (by volume) of the air.  It is nonflammable 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 colorless liquid that is lighter than water. Properties of nitrogen are at Tab 1.

It is commonly thought of and used as an inert gas; but it is not truly inert.  It forms nitric oxide and nitrogen dioxide with oxygen, ammonia with hydrogen, and nitrogen sulfide with sulfur.  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 steelmaking, 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 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).

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 vaporized 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, as a shield gas in the heat treatment of iron and steel. It is used in different laboratories for testing and 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

Argon (Ar) is a monatomic, colorless, odorless, tasteless and nontoxic 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.  They are monatomic gases with a totally filled outermost shell of electrons.  The terms noble and inert have been used to indicate that their ability 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 approximately 1.4 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 at Tab 1.

Argon is important for its total inertness in particular at high temperatures. It is the most abundant and 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.  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, requires many stages of distillation.

For many decades, the most common argon recovery and purification process used many 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 and 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 unconsumed 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, most new plants now utilize an all cryogenic distillation process for argon recovery and purification.

Argon, besides its chemical designation Ar, is sometimes referred to as PLAR (pure liquid argon) or CLAR (crude liquid argon).  Crude argon is usually thought of as an intermediate product in a facility that makes pure argon, but it may be a final product for some lower capacity air separation plants which ship 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 steel making. 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, and 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

Hydrogen (H2) is a colorless, odorless, tasteless, flammable and nontoxic gas at atmospheric temperatures and pressures. 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, approximately 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 commonly 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, odorless liquid that is only one-fourteenth as heavy as water. Liquid hydrogen is not corrosive or particularly reactive. When converted from liquid to gas, hydrogen expands approximately 840 times. Its low boiling point and low density result in liquid hydrogen spills dispersing rapidly. Properties of hydrogen are at Tab 1.

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 hydrocarbon 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 hydrocarbon reforming units, in particular those which employ an auto thermal generation process, 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 about 982 deg C with the aid of a catalyst – which results in a mix of 75 % hydrogen and 25 % mononuclear 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 manufacture of iron.

Hydrogen is mixed with inert gases to obtain a reducing atmosphere, which is required for many applications in the steel industry, such as in laboratories, heat treating steel and welding.  It is often used in annealing stainless steel alloys and magnetic steel alloys.

Large quantities of hydrogen are used to purify argon that contains trace amounts of oxygen, using catalytic combination of the oxygen and hydrogen followed by removal of the resulting water.

Tab 1 Properties of industrial gases

Gas

Oxygen

Nitrogen

Argon

Hydrogen

Property

Unit

Chemical symbol

O2

N2

Ar

H2

Molecular weight

32

28.01

39.95

2.02

Boiling point @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 @ 0 deg C and 1.033 Kg/Sq cm

Specific gravity

Air = 1

1.113

0.9737

1.39

0.06998

Specific heat (Cp)

Kcal/Kg deg C

0.2197

0.2486

0.125

3.427

Density

Kg/Cum

1.429

1.2506

1.7837

0.08988

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

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

Density

Kg/Cum

436.1

314.9

535.6

30.12