A factory or manufacturing plant is an industrial site consisting of buildings and machinery, or more a complex having several buildings, where workers manufacture goods or operate machines processing one product into another. Factories arose with the introduction of machinery during the Industrial Revolution when the capital and space requirements became too great for cottage industry or workshops. Early factories that contained small amounts of machinery, such as one or two spinning mules, fewer than a dozen workers have been called "glorified workshops". Most modern factories have large warehouses or warehouse-like facilities that contain heavy equipment used for assembly line production. Large factories tend to be located with access to multiple modes of transportation, with some having rail and water loading and unloading facilities. Factories may either make discrete products or some type of material continuously produced such as chemicals and paper, or refined oil products. Factories manufacturing chemicals are called plants and may have most of their equipment – tanks, pressure vessels, chemical reactors and piping – outdoors and operated from control rooms.
Oil refineries have most of their equipment outdoors. Discrete products may be final consumer goods, or parts and sub-assemblies which are made into final products elsewhere. Factories may make them from raw materials. Continuous production industries use heat or electricity to transform streams of raw materials into finished products; the term mill referred to the milling of grain, which used natural resources such as water or wind power until those were displaced by steam power in the 19th century. Because many processes like spinning and weaving, iron rolling, paper manufacturing were powered by water, the term survives as in steel mill, paper mill, etc. Max Weber considered production during ancient times as never warranting classification as factories, with methods of production and the contemporary economic situation incomparable to modern or pre-modern developments of industry. In ancient times, the earliest production limited to the household, developed into a separate endeavour independent to the place of inhabitation with production at that time only beginning to be characteristic of industry, termed as "unfree shop industry", a situation caused under the reign of the Egyptian pharaoh, with slave employment and no differentiation of skills within the slave group comparable to modern definitions as division of labour.
According to translations of Demosthenes and Herodotus, Naucratis was a, or the only, factory in the entirety of ancient Egypt. A source of 1983, states the largest factory production in ancient times was of 120 slaves within 4th century BC Athens. An article within the New York Times article dated 13 October 2011 states: "In African Cave, Signs of an Ancient Paint Factory" –... discovered at Blombos Cave, a cave on the south coast of South Africa where 100,000-year-old tools and ingredients were found with which early modern humans mixed an ochre-based paint. Although The Cambridge Online Dictionary definition of factory states: a building or set of buildings where large amounts of goods are made using machines elsewhere:... the utilization of machines presupposes social cooperation and the division of labour The first machine is stated by one source to have been traps used to assist with the capturing of animals, corresponding to the machine as a mechanism operating independently or with little force by interaction from a human, with a capacity for use with operation the same on every occasion of functioning.
The wheel was invented c. 3000 BC, the spoked wheel c. 2000 BC. The Iron Age began 1200–1000 BC. However, other sources define machinery as a means of production. Archaeology provides a date for the earliest city as 5000 BC as Tell Brak, therefore a date for cooperation and factors of demand, by an increased community size and population to make something like factory level production a conceivable necessity. According to one text the water-mill was first made in 555 A. D. by Belisarius, although according to another they were known to Pliny the Elder and Vitruvius in the first century B. C. By the time of the 4th century A. D. mills with a capacity to grind 3 tonnes of cereal an hour, a rate sufficient to meet the needs of 80,000 persons, were in use by the Roman Empire. The Venice Arsenal provides one of the first examples of a factory in the modern sense of the word. Founded in 1104 in Venice, Republic of Venice, several hundred years before the Industrial Revolution, it mass-produced ships on assembly lines using manufactured parts.
The Venice Arsenal produced nearly one ship every day and, at its height, employed 16,000 people. One of the earliest factories was John Lombe's water-powered silk mill at Derby, operational by 1721. By 1746, an integrated brass mill was working at Warmley near Bristol. Raw material went in at one end, was smelted into brass and was turned into pans, pins and other goods. Housing was provided for workers on site. Josiah Wedgwood in Staffordshire and Matthew Boulton at his Soho Manufactory were other prominent early industrialists, who employed the factory system; the factory system began widespread use somewhat when cotton spinning was mechanized. Richard Arkwright is the person credited with inventing the prototype of the modern factory. After he patented his water frame in 1769, he established Cromford Mill, in Derbyshire, England expanding the village of Cromford to accommodate the migrant workers new to the area; the factory system was a new way of organizing labour made necessary by the developm
Air pollution occurs when harmful or excessive quantities of substances including gases and biological molecules are introduced into Earth's atmosphere. It may cause diseases and death to humans. Both human activity and natural processes can generate air pollution. Indoor air pollution and poor urban air quality are listed as two of the world's worst toxic pollution problems in the 2008 Blacksmith Institute World's Worst Polluted Places report. According to the 2014 World Health Organization report, air pollution in 2012 caused the deaths of around 7 million people worldwide, an estimate echoed by one from the International Energy Agency. An air pollutant is a material in the air that can have adverse effects on the ecosystem; the substance can be liquid droplets, or gases. A pollutant can be of man-made. Pollutants are classified as secondary. Primary pollutants are produced by processes such as ash from a volcanic eruption. Other examples include carbon monoxide gas from motor vehicle exhausts or sulphur dioxide released from the factories.
Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants interact. Ground level ozone is a prominent example of secondary pollutants; some pollutants may be both primary and secondary: they are both emitted directly and formed from other primary pollutants. Substances emitted into the atmosphere by human activity include: Carbon dioxide – Because of its role as a greenhouse gas it has been described as "the leading pollutant" and "the worst climate pollution". Carbon dioxide is a natural component of the atmosphere, essential for plant life and given off by the human respiratory system; this question of terminology has practical effects, for example as determining whether the U. S. Clean Air Act is deemed to regulate CO2 emissions. CO2 forms about 410 parts per million of earth's atmosphere, compared to about 280 ppm in pre-industrial times, billions of metric tons of CO2 are emitted annually by burning of fossil fuels. CO2 increase in earth's atmosphere has been accelerating.
Sulfur oxides – sulphur dioxide, a chemical compound with the formula SO2. SO2 is produced in various industrial processes. Coal and petroleum contain sulphur compounds, their combustion generates sulphur dioxide. Further oxidation of SO2 in the presence of a catalyst such as NO2, forms H2SO4, thus acid rain; this is one of the causes for concern over the environmental impact of the use of these fuels as power sources. Nitrogen oxides – Nitrogen oxides nitrogen dioxide, are expelled from high temperature combustion, are produced during thunderstorms by electric discharge, they can be seen as a plume downwind of cities. Nitrogen dioxide is a chemical compound with the formula NO2, it is one of several nitrogen oxides. One of the most prominent air pollutants, this reddish-brown toxic gas has a characteristic sharp, biting odor. Carbon monoxide – CO is a colorless, toxic yet non-irritating gas, it is a product of combustion of fuel such as natural coal or wood. Vehicular exhaust contributes to the majority of carbon monoxide let into our atmosphere.
It creates a smog type formation in the air, linked to many lung diseases and disruptions to the natural environment and animals. In 2013, more than half of the carbon monoxide emitted into our atmosphere was from vehicle traffic and burning one gallon of gas will emit over 20 pounds of carbon monoxide into the air. Volatile organic compounds – VOCs are a well-known outdoor air pollutant, they are categorized as either non-methane. Methane is an efficient greenhouse gas which contributes to enhanced global warming. Other hydrocarbon VOCs are significant greenhouse gases because of their role in creating ozone and prolonging the life of methane in the atmosphere; this effect varies depending on local air quality. The aromatic NMVOCs benzene and xylene are suspected carcinogens and may lead to leukemia with prolonged exposure. 1,3-butadiene is another dangerous compound associated with industrial use. Particulate matter / particles, alternatively referred to as particulate matter, atmospheric particulate matter, or fine particles, are tiny particles of solid or liquid suspended in a gas.
In contrast, aerosol refers to gas. Some particulates occur originating from volcanoes, dust storms and grassland fires, living vegetation, sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes generate significant amounts of aerosols. Averaged worldwide, anthropogenic aerosols—those made by human activities—currently account for 10 percent of our atmosphere. Increased levels of fine particles in the air are linked to health hazards such as heart disease, altered lung function and lung cancer. Particulates are related to respiratory infections and can be harmful to those suffering from conditions like asthma. Persistent free radicals connected to airborne fine particles are linked to cardiopulmonary disease. Toxic metals, such as lead and mercury their compounds. Chlorofluorocarbons – harmful to the ozone layer; these are gases which are released from air conditioners, aerosol sprays, etc. On release into the air, CFCs rise to the stratosphere.
Here they come in contact with other gases and
Plate heat exchanger
A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids are spread out over the plates; this facilitates the transfer of heat, increases the speed of the temperature change. Plate heat exchangers are now common and small brazed versions are used in the hot-water sections of millions of combination boilers; the high heat transfer efficiency for such a small physical size has increased the domestic hot water flowrate of combination boilers. The small plate heat exchanger has made a great impact in domestic hot-water. Larger commercial versions use gaskets between the plates, whereas smaller versions tend to be brazed; the concept behind a heat exchanger is the use of pipes or other containment vessels to heat or cool one fluid by transferring heat between it and another fluid. In most cases, the exchanger consists of a coiled pipe containing one fluid that passes through a chamber containing another fluid.
The walls of the pipe are made of metal, or another substance with a high thermal conductivity, to facilitate the interchange, whereas the outer casing of the larger chamber is made of a plastic or coated with thermal insulation, to discourage heat from escaping from the exchanger. The plate heat exchanger was invented by Dr Richard Seligman in 1923 and revolutionised methods of indirect heating and cooling of fluids. Dr Richard Seligman founded APV in 1910 as the Aluminium Plant & Vessel Company Limited, a specialist fabricating firm supplying welded vessels to the brewery and vegetable oil trades; the plate heat exchanger is a specialized design well suited to transferring heat between medium- and low-pressure fluids. Welded, semi-welded and brazed heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required. In place of a pipe passing through a chamber, there are instead two alternating chambers thin in depth, separated at their largest surface by a corrugated metal plate.
The plates used in a plate and frame heat exchanger are obtained by one piece pressing of metal plates. Stainless steel is a used metal for the plates because of its ability to withstand high temperatures, its strength, its corrosion resistance; the plates are spaced by rubber sealing gaskets which are cemented into a section around the edge of the plates. The plates are pressed to form troughs at right angles to the direction of flow of the liquid which runs through the channels in the heat exchanger; these troughs are arranged so that they interlink with the other plates which forms the channel with gaps of 1.3–1.5 mm between the plates. The plates are compressed together in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids; the plates produce an large surface area, which allows for the fastest possible transfer. Making each chamber thin ensures that the majority of the volume of the liquid contacts the plate, again aiding exchange; the troughs create and maintain a turbulent flow in the liquid to maximize heat transfer in the exchanger.
A high degree of turbulence can be obtained at low flow rates and high heat transfer coefficient can be achieved. As compared to shell and tube heat exchangers, the temperature approach in a plate heat exchangers may be as low as 1 °C whereas shell and tube heat exchangers require an approach of 5 °C or more. For the same amount of heat exchanged, the size of the plate heat exchanger is smaller, because of the large heat transfer area afforded by the plates. Increase and reduction of the heat transfer area is simple in a plate heat-exchanger, through the addition or removal of plates from the stack. All plate heat exchangers look similar on the outside; the difference lies on the inside, in the details of the plate design and the sealing technologies used. Hence, when evaluating a plate heat exchanger, it is important not only to explore the details of the product being supplied but to analyze the level of research and development carried out by the manufacturer and the post-commissioning service and spare parts availability.
An important aspect to take into account when evaluating a heat exchanger are the forms of corrugation within the heat exchanger. There are two types: chevron corrugations. In general, greater heat transfer enhancement is produced from chevrons for a given increase in pressure drop and are more used than intermating corrugations. To achieve improvement in PHE's, two important factors namely amount of heat transfer and pressure drop have to be considered such that amount of heat transfer needs to be increased and pressure drops need to be decreased. In plate heat exchangers due to presence of corrugated plate, there is a significant resistance to flow with high friction loss, thus to design plate heat exchangers, one should consider both factors. For various range of Reynolds numbers, many correlations and chevron angles for plate heat exchangers exist; the plate geometry is one of the most important factor in heat transfer and pressure drop in plate heat exchangers, however such a feature is not prescribed.
In the corrugated plate heat exchangers, because of narrow path between the plates, there is a large pressure capacity and the flow becomes turbulent along the path. Therefore, it requires more pumping power than the other types of heat exchangers. Therefore, higher heat transfer and less pressure drop are targeted; the shape of plate heat exchanger is important for industrial applications that are affected by pressure drop. Design calculations of a plate he
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways; the common types of flowmeters that find industrial application can be listed as below: a) Obstruction type b) Inferential c) Electromagnetic d) Positive-displacement flow meters, which accumulate a fixed volume of fluid and count the number of times the volume is filled to measure flow. E) Fluid dynamic f) Anemometer g) Ultrasonic h) Mass flowmeter. Flow measurement methods other than positive-displacement flowmeters rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of fluid over a known area. For large flows, tracer methods may be used to deduce the flow rate from the change in concentration of a dye or radioisotope. Both gas and liquid flow can be measured in volumetric or mass flow rates, such as liters per second or kilograms per second, respectively; these measurements are related by the material's density.
The density of a liquid is independent of conditions. This is not the case for gases, the densities of which depend upon pressure, temperature and to a lesser extent, composition; when gases or liquids are transferred for their energy content, as in the sale of natural gas, the flow rate may be expressed in terms of energy flow, such as gigajoule per hour or BTU per day. The energy flow rate is the volumetric flow rate multiplied by the energy content per unit volume or mass flow rate multiplied by the energy content per unit mass. Energy flow rate is derived from mass or volumetric flow rate by the use of a flow computer. In engineering contexts, the volumetric flow rate is given the symbol Q, the mass flow rate, the symbol m ˙. For a fluid having density ρ, mass and volumetric flow rates may be related by m ˙ = ρ Q. Gases change volume when placed under pressure, are heated or are cooled. A volume of gas under one set of pressure and temperature conditions is not equivalent to the same gas under different conditions.
References will be made to "actual" flow rate through a meter and "standard" or "base" flow rate through a meter with units such as acm/h, sm3/sec, kscm/h, LFM, or MMSCFD. Gas mass flow rate can be directly measured, independent of pressure and temperature effects, with thermal mass flow meters, Coriolis mass flow meters, or mass flow controllers. For liquids, various units are used depending upon the application and industry, but might include gallons per minute, liters per second, bushels per minute or, when describing river flows, cumecs or acre-feet per day. In oceanography a common unit to measure volume transport is a sverdrup equivalent to 106 m3/s. A primary flow element is a device inserted into the flowing fluid that produces a physical property that can be related to flow. For example, an orifice plate produces a pressure drop, a function of the square of the volume rate of flow through the orifice. A vortex meter primary flow element produces a series of oscillations of pressure.
The physical property generated by the primary flow element is more convenient to measure than the flow itself. The properties of the primary flow element, the fidelity of the practical installation to the assumptions made in calibration, are critical factors in the accuracy of the flow measurement. A positive displacement meter may be compared to a stopwatch; the stopwatch is started when the flow starts, stopped when the bucket reaches its limit. The volume divided by the time gives the flow rate. For continuous measurements, we need a system of continually filling and emptying buckets to divide the flow without letting it out of the pipe; these continuously forming and collapsing volumetric displacements may take the form of pistons reciprocating in cylinders, gear teeth mating against the internal wall of a meter or through a progressive cavity created by rotating oval gears or a helical screw. Because they are used for domestic water measurement, piston meters known as rotary piston or semi-positive displacement meters, are the most common flow measurement devices in the UK and are used for all meter sizes up to and including 40 mm.
The piston meter operates on the principle of a piston rotating within a chamber of known volume. For each rotation, an amount of water passes through the piston chamber. Through a gear mechanism and, sometimes, a magnetic drive, a needle dial and odometer type display are advanced. An oval gear meter is a positive displacement meter that uses two or more oblong gears configured to rotate at right angles to one another, forming a T shape; such a meter has two sides, which can be called A and B. No fluid passes through the center of the meter. On one side of the meter, the teeth of the gears close off the fluid flow because the elongated gear on side A is protruding into the measurement chamber, while on the other side of the meter, a cavity holds a fixed volume of fluid in a measurement chamber; as the fluid pushes the gears, it rotates them, allowing the fluid in the measurement chamber on side B to be released into the outlet port. Meanwhile, fluid
Shell and tube heat exchanger
A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, is suited for higher-pressure applications; as its name implies, this type of heat exchanger consists of a shell with a bundle of tubes inside it. One fluid runs through the tubes, another fluid flows over the tubes to transfer heat between the two fluids; the set of tubes is called a tube bundle, may be composed of several types of tubes: plain, longitudinally finned, etc. Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes and the other flows inside the shell. Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa; the fluids can be either gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this way, waste heat can be put to use.
This is an efficient way to conserve energy. Heat exchangers with only one phase on each side can be called one-phase or single-phase heat exchangers. Two-phase heat exchangers can be used to heat a liquid to boil it into a gas, sometimes called boilers, or cool a vapor to condense it into a liquid, with the phase change occurring on the shell side. Boilers in steam engine locomotives are large cylindrically-shaped shell-and-tube heat exchangers. In large power plants with steam-driven turbines, shell-and-tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water, recycled back to be turned into steam in the steam generator. There can be many variations on the tube design; the ends of each tube are connected to plenums through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes. In nuclear power plants called pressurized water reactors, large heat exchangers called steam generators are two-phase, shell-and-tube heat exchangers which have U-tubes.
They are used to boil water recycled from a surface condenser into steam to drive a turbine to produce power. Most shell-and-tube heat exchangers are either 2, or 4 pass designs on the tube side; this refers to the number of times. In a single pass heat exchanger, the fluid goes out the other. Surface condensers in power plants are 1-pass straight-tube heat exchangers. Two and four pass designs are common because the fluid can exit on the same side; this makes construction much simpler. There are baffles directing flow through the shell side so the fluid does not take a short cut through the shell side leaving ineffective low flow volumes; these are attached to the tube bundle rather than the shell in order that the bundle is still removable for maintenance. Counter current heat exchangers are most efficient because they allow the highest log mean temperature difference between the hot and cold streams. Many companies however do not use two pass heat exchangers with a u-tube because they can break in addition to being more expensive to build.
Multiple heat exchangers can be used to simulate the counter current flow of a single large exchanger. To be able to transfer heat well, the tube material should have good thermal conductivity; because heat is transferred from a hot to a cold side through the tubes, there is a temperature difference through the width of the tubes. Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresses occur during operation; this is in addition to any stress from high pressures from the fluids themselves. The tube material should be compatible with both the shell and tube side fluids for long periods under the operating conditions to minimize deterioration such as corrosion. All of these requirements call for careful selection of strong, thermally-conductive, corrosion-resistant, high quality tube materials metals, including aluminium, copper alloy, stainless steel, carbon steel, non-ferrous copper alloy, nickel and titanium. Fluoropolymers such as Perfluoroalkoxy alkane and Fluorinated ethylene propylene are used to produce the tubing material due to their high resistance to extreme temperatures.
Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid cross-contamination and loss of pressure. The simple design of a shell and tube heat exchanger makes it an ideal cooling solution for a wide variety of applications. One of the most common applications is the cooling of hydraulic fluid and oil in engines and hydraulic power packs. With the right choice of materials they can be used to cool or heat other mediums, such as swimming pool water or charge air. One of the big advantages of using a shell and tube heat exchanger is that they are easy to service with models where a floating tube bundle is available. Standards of the Tubular Exchanger Manufacturers Association, 9th edition, 2009 EN 13445-3 "Unfired Pressure Vessels - Part 3: Design", Section 13 ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Part UHX Boiler or Reboiler EJMA Fired heater Fouling or scaling Heat exchanger NTU method as an alternative to finding
Sintering is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction. Sintering happens in mineral deposits or as a manufacturing process used with metals, ceramics and other materials; the atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is chosen as the shaping process for materials with high melting points such as tungsten and molybdenum; the study of sintering in metallurgy powder-related processes is known as powder metallurgy. An example of sintering can be observed when ice cubes in a glass of water adhere to each other, driven by the temperature difference between the water and the ice. Examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the forming of a hard snowball by pressing loose snow together.
The word "sinter" comes from the Middle High German sinter, a cognate of English "cinder". Sintering is effective when the process reduces the porosity and enhances properties such as strength, electrical conductivity and thermal conductivity. During the firing process, atomic diffusion drives powder surface elimination in different stages, starting from the formation of necks between powders to final elimination of small pores at the end of the process; the driving force for densification is the change in free energy from the decrease in surface area and lowering of the surface free energy by the replacement of solid-vapor interfaces. It forms lower-energy solid-solid interfaces with a total decrease in free energy occurring. On a microscopic scale, material transfer is affected by the change in pressure and differences in free energy across the curved surface. If the size of the particle is small, these effects become large in magnitude; the change in energy is much higher when the radius of curvature is less than a few micrometres, one of the main reasons why much ceramic technology is based on the use of fine-particle materials.
For properties such as strength and conductivity, the bond area in relation to the particle size is the determining factor. The variables that can be controlled for any given material are the temperature and the initial grain size, because the vapor pressure depends upon temperature. Through time, the particle radius and the vapor pressure are proportional to 2/3 and to 1/3, respectively; the source of power for solid-state processes is the change in free or chemical potential energy between the neck and the surface of the particle. This energy creates a transfer of material through the fastest means possible; the pore elimination occurs faster for a trial with many pores of uniform size and higher porosity where the boundary diffusion distance is smaller. For the latter portions of the process and lattice diffusion from the boundary become important. Control of temperature is important to the sintering process, since grain-boundary diffusion and volume diffusion rely upon temperature, the size and distribution of particles of the material, the materials composition, the sintering environment to be controlled.
Sintering is part of the firing process used in the manufacture of pottery and other ceramic objects. These objects are made from substances such as glass, zirconia, magnesia, beryllium oxide, ferric oxide; some ceramic raw materials have a lower affinity for water and a lower plasticity index than clay, requiring organic additives in the stages before sintering. The general procedure of creating ceramic objects via sintering of powders includes: Mixing water, binder and unfired ceramic powder to form a slurry. All the characteristic temperatures associated with phase transformation, glass transitions, melting points, occurring during a sinterisation cycle of a particular ceramics formulation can be obtained by observing the expansion-temperature curves during optical dilatometer thermal analysis. In fact, sinterisation is associated with a remarkable shrinkage of the material because glass phases flow once their transition temperature is reached, start consolidating the powdery structure and reducing the porosity of the material.
Sintering is performed at high temperature. Additionally, a second and/or third external force could be used. A used second external force is pressure. So, the sintering, performed just using temperature is called "pressureless sintering". Pressureless sintering is possible with graded metal-ceramic composites, with a nanoparticle sintering aid and bulk molding technology. A variant used for 3D shapes is called hot isostatic pressing. To allow efficient stacking of product in the furnace during sintering and prevent parts sticking together, many manufacturers separate ware using ceramic powder separator sheets; these sheets are available in various materials such as alumina and magnesia. They are additionally categorized by fine and coarse particle sizes. By matching t
Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst, not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only small amounts of catalyst are required to alter the reaction rate in principle. In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway with a lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst reacts to form a temporary intermediate, which regenerates the original catalyst in a cyclic process. A substance which provides a mechanism with a higher activation energy does not decrease the rate because the reaction can still occur by the non-catalyzed route. An added substance which does reduce the reaction rate is not considered a catalyst but a reaction inhibitor. Catalysts may be classified as either heterogeneous. A homogeneous catalyst is one whose molecules are dispersed in the same phase as the reactant's molecules.
A heterogeneous catalyst is one whose molecules are not in the same phase as the reactant's, which are gases or liquids that are adsorbed onto the surface of the solid catalyst. Enzymes and other biocatalysts are considered as a third category. In the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations; the effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons or promoters. Catalyzed reactions have a lower activation energy than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. However, the detailed mechanics of catalysis is complex. Catalysts may bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause dissociation of reagents to reactive forms, such as chemisorbed hydrogen in catalytic hydrogenation.
Kinetically, catalytic reactions are typical chemical reactions. The catalyst participates in this slowest step, rates are limited by amount of catalyst and its "activity". In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A nanomaterial-based catalyst is an example of a heterogeneous catalyst. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid–liquid system or sublimate in a solid–gas system; the production of most industrially important chemicals involves catalysis. Most biochemically significant processes are catalysed.
Research into catalysis is a major field in applied science and involves many areas of chemistry, notably organometallic chemistry and materials science. Catalysis is relevant to many aspects of environmental science, e.g. the catalytic converter in automobiles and the dynamics of the ozone hole. Catalytic reactions are preferred in environmentally friendly green chemistry due to the reduced amount of waste generated, as opposed to stoichiometric reactions in which all reactants are consumed and more side products are formed. Many transition metals and transition metal complexes are used in catalysis as well. Catalysts called. A catalyst works by providing an alternative reaction pathway to the reaction product; the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. The disproportionation of hydrogen peroxide creates oxygen, as shown below. 2 H2O2 → 2 H2O + O2This reaction is preferable in the sense that the reaction products are more stable than the starting material, though the uncatalysed reaction is slow.
In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available. This reaction is affected by catalysts such as manganese dioxide, or the enzyme peroxidase in organisms. Upon the addition of a small amount of manganese dioxide, the hydrogen peroxide reacts rapidly; this effect is seen by the effervescence of oxygen. The manganese dioxide is not consumed in the reaction, thus may be recovered unchanged, re-used indefinitely. Accordingly, manganese dioxide catalyses this reaction. Catalytic activity is denoted by the symbol z and measured in mol/s, a unit, called katal and defined the SI unit for catalytic activity since 1999. Catalytic activity is not a kind of reaction rate, but a property of the catalyst under certain conditions, in relation to a specific chemical reaction. Catalytic activity of one katal of a catalyst means one mole of that catalyst will catalyse 1 mole of the reactant to product in one second. A catalyst may and will have different catalytic activity for di