Plasma Gasification Process...

Plasma gasification is a multi-stage process which starts with feed inputs – ranging from waste to coal to plant matter, and can include hazardous wastes. The first step is to process the feed stock to make it uniform and dry, and have the valuable recyclables sorted out. The second step is gasification, where extreme heat from the plasma torches is applied inside a sealed, air-controlled reactor. During gasification, carbon-based materials break down into gases and the inorganic materials melt into liquid slag which is poured off and cooled. The heat causes hazards and poisons to be completely destroyed. The third stage is gas clean-up and heat recovery, where the gases are scrubbed of impurities to form clean fuel, and heat exchangers recycle the heat back into the system as steam. The final stage is fuel production – the output can range from electricity to a variety of fuels as well as chemicals, hydrogen and polymers.

Gasification has a long history in industry where it has been used to refine coal and biomass into a variety of liquid fuels, gases and chemicals. Modern clean coal plants are all gasifiers, and so were the earliest 19th century municipal light and power systems.

Plasma gasification refers to the use of plasma torches as the heat source, as opposed to conventional fires and furnaces. Plasma torches have the advantage of being one of the most intense heat sources available while being relatively simple to operate.

Plasma is a superheated column of electrically conductive gas. In nature, plasma is found in lightning and on the surface of the sun. Plasma torches burn at temperatures approaching 5500ºC (10,000˚F) and can reliably destroy any materials found on earth – with the exception of nuclear waste.

Plasma torches are used in foundries to melt and cut metals. When utilized for waste treatment, plasma torches are very efficient at causing organic and carbonaceous materials to vaporize into gas. Non-organic materials are melted and cool into a vitrified glass.

Waste gasification typically operates at temperatures of 1500˚C (2700˚F), and at those temperatures materials are subject to a process called molecular disassociation, meaning their molecular bonds are broken down and in the process all toxins and organic poisons are destroyed. Plasma torches have been used for many years to destroy chemical weapons and toxic wastes, like printed circuit boards (PCBs) and asbestos, but it is only recently that these processes have been optimized for energy capture and fuel production.

America’s Westinghouse Corporation began building plasma torches with NASA for the Apollo Space Program in the 1960s to test the heat shields for spacecraft at 5500˚C. In the late 1990s, the first pilot-scale plasma gasification projects were built in Japan to convert MSW, sewage sludge, and auto-shredder residue to energy. The Japanese pilot plants have been successful, and commercial-scale projects are under development now in Canada and other countries, by companies such as Alter NRG, from Alberta, Canada.

Uranium..

• Uranium is a very heavy metal which can be used as an abundant source of concentrated energy. 
• Uranium occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth's crust as tin, tungsten and molybdenum. Uranium occurs in seawater, and can be recovered from the oceans. 
• Uranium was discovered in 1789 by Martin Klaproth, a German chemist, in the mineral called pitchblende. It was named after the planet Uranus, which had been discovered eight years earlier.
• Uranium was apparently formed in supernova  about 6.6 billion years ago. While it is not common in the solar system, today its slow radioactive decay provides the main source of heat inside the Earth, causing convection and continental drift. 
• The high density of uranium means that it also finds uses in the keels of yachts and as counterweights for aircraft control surfaces, as well as for radiation shielding.
• Uranium has a melting point is 1132°C. The chemical symbol for uranium is U.

The Uranium Atom

On a scale arranged according to the increasing mass of their nuclei, uranium is one of the heaviest of all the naturally-occurring elements (Hydrogen is the lightest). Uranium is 18.7 times as dense as water.

Like other elements, uranium occurs in several slightly differing forms known as 'isotopes'. These isotopes differ from each other in the number of uncharged particles (neutrons) in the nucleus. Natural uranium as found in the Earth's crust is a mixture largely of two isotopes: uranium-238 (U-238), accounting for 99.3% and uranium-235 (U-235) about 0.7%.

The isotope U-235 is important because under certain conditions it can readily be split, yielding a lot of energy. It is therefore said to be 'fissile' and we use the expression 'nuclear fission'.

Meanwhile, like all radioactive isotopes, they decay. U-238 decays very slowly, its half-life being about the same as the age of the Earth (4500 million years). This means that it is barely radioactive, less so than many other isotopes in rocks and sand. Nevertheless it generates 0.1 watts/tonne as decay heat and this is enough to warm the Earth's core. U-235 decays slightly faster.

Energy from the uranium atom

The nucleus of the U-235 atom comprises 92 protons and 143 neutrons (92 + 143 = 235). When the nucleus of a U-235 atom captures a moving neutron it splits in two (fissions) and releases some energy in the form of heat, also two or three additional neutrons are thrown off. If enough of these expelled neutrons cause the nuclei of other U-235 atoms to split, releasing further neutrons, a fission 'chain reaction' can be achieved. When this happens over and over again, many millions of times, a very large amount of heat is produced from a relatively small amount of uranium.

It is this process, in effect "burning" uranium, which occurs in a nuclear reactor. The heat is used to make steam to produce electricity.

Inside the reactor

Nuclear power stations and fossil-fuelled power stations of similar capacity have many features in common. Both require heat to produce steam to drive turbines and generators. In a nuclear power station, however, the fissioning of uranium atoms replaces the burning of coal or gas.In a nuclear reactor the uranium fuel is assembled in such a way that a controlled fission chain reaction can be achieved. The heat created by splitting the U-235 atoms is then used to make steam which spins a turbine to drive a generator, producing electricity.

The chain reaction that takes place in the core of a nuclear reactor is controlled by rods which absorb neutrons and which can be inserted or withdrawn to set the reactor at the required power level.

The fuel elements are surrounded by a substance called a moderator to slow the speed of the emitted neutrons and thus enable the chain reaction to continue. Water, graphite and heavy water are used as moderators in different types of reactors.

Because of the kind of fuel used (ie the concentration of U-235, see below), if there is a major uncorrected malfunction in a reactor the fuel may overheat and melt, but it cannot explode like a bomb.

A typical 1000 megawatt (MWe) reactor can provide enough electricity for a modern city of up to one million people.

Uranium and Plutonium

Whereas the U-235 nucleus is 'fissile', that of U-238 is said to be 'fertile'. This means that it can capture one of the neutrons which are flying about in the core of the reactor and become (indirectly) plutonium-239, which is fissile. Pu-239 is very much like U-235, in that it fissions when hit by a neutron and this also yields a lot of energy.

Because there is so much U-238 in a reactor core (most of the fuel), these reactions occur frequently, and in fact about one third of the fuel's energy yield comes from "burning" Pu-239.

But sometimes a Pu-239 atom simply captures a neutron without splitting, and it becomes Pu-240. Because the Pu-239 is either progressively "burned" or becomes Pu-240, the longer the fuel stays in the reactor the more Pu-240 is in it. (The significance of this is that when the spent fuel is removed after about three years, the plutonium in it is not suitable for making weapons but can be recycled as fuel.)

From uranium ore to reactor fuel 

Uranium ore can be mined by underground or open-cut methods, depending on its depth. After mining, the ore is crushed and ground up. Then it is treated with acid to dissolve the uranium, which is recovered from solution. 

Uranium may also be mined by in situ leaching (ISL), where it is dissolved from a porous underground ore body in situ and pumped to the surface.

The end product of the mining and milling stages, or of ISL, is uranium oxide concentrate (U3O8). This is the form in which uranium is sold.

Before it can be used in a reactor for electricity generation, however, it must undergo a series of processes to produce a useable fuel.

For most of the world's reactors, the next step in making the fuel is to convert the uranium oxide into a gas, uranium hexafluoride (UF6), which enables it to be enriched. Enrichment increases the proportion of the uranium-235 isotope from its natural level of 0.7% to 4 - 5%. This enables greater technical efficiency in reactor design and operation, particularly in larger reactors, and allows the use of ordinary water as a moderator.

After enrichment, the UF6 gas is converted to uranium dioxide (UO2) which is formed into fuel pellets. These fuel pellets are placed inside thin metal tubes which are assembled in bundles to become the fuel elements or assemblies for the core of the reactor.

For reactors which use natural uranium as their fuel (and hence which require graphite or heavy water as a moderator) the U3O8 concentrate simply needs to be refined and converted directly to uranium dioxide.

When the uranium fuel has been in the reactor for about three years, the used fuel is removed, stored, and then either reprocessed or disposed of underground (see Nuclear Fuel Cycle or Radioactive Waste Management in this series).

Who uses nuclear power?

Over 13% of the world's electricity is generated from uranium in nuclear reactors. This amounts to over 2500 billion kWh each year, as much as from all sources of electricity worldwide in 1960.

It comes from some 440 nuclear reactors with a total output capacity of about 377 000 megawatts (MWe) operating in 30 countries. Over 60 more reactors are under construction and another 150 are planned.

Belgium, Bulgaria, Czech Republic, Finland, France, Hungary, Japan, South Korea, Slovakia, Slovenia, Sweden, Switzerland and Ukraine all get 30% or more of their electricity from nuclear reactors. The USA has over 100 reactors operating, supplying 20% of its electricity. France gets three quarters of its electricity from uranium.

  

 

See also Table of the World's Nuclear Power Reactors

Who has and who mines uranium?

Uranium is widespread in many rocks, and even in seawater. However, like other metals, it is seldom sufficiently concentrated to be economically recoverable. Where it is, we speak of an orebody. In defining what is ore, assumptions are made about the cost of mining and the market price of the metal. Uranium reserves are therefore calculated as tonnes recoverable up to a certain cost.

Australia's reasonably assured resources and inferred resources of uranium are 1,673,000 tonnes of uranium recoverable at up to US$130/kg U (well under the market 'spot' price), Kazakhstan's are 651,000 tonnes of uranium and Canada's are 485,000 tU. Australia's resources in this category are almost one third of the world's total, Kazakhstan's are 12%, Canada's 9%. 

Several countries have significant uranium resources. Apart from the top three, they are in order: Russia, South Africa, Namibia, Brazil, Niger, USA, China, Jordan, Uzbekistan, Ukraine and India. Other countries have smaller deposits which could be mined if needed.

Kazakhstan is the world's top uranium producer, followed by Canada and then Australia as the main suppliers of uranium to world markets - now over 50,000 tU per year.

Uranium is sold only to countries which are signatories of the Nuclear Non-Proliferation Treaty (NPT), and which allow international inspection to verify that it is used only for peaceful purposes. 

Other uses of nuclear energy

Many people, when talking about nuclear energy, have only nuclear reactors (or perhaps nuclear weapons) in mind. Few people realise the extent to which the use of radioisotopes has changed our lives over the last few decades.

Using relatively small special-purpose nuclear reactors it is possible to make a wide range of radioactive materials (radioisotopes) at low cost. For this reason the use of artificially-produced radioisotopes has become widespread since the early 1950s, and there are now over 200 "research" reactors in 56 countries producing them.  These are essentially neutron factories rather than sources of heat.

Radioisotopes

In our daily life we need food, water and good health. Today, radioactive isotopes play an important part in the technologies that provide us with all three. They are produced by bombarding small amounts of particular elements with neutrons.

In medicine, radioisotopes are widely used for diagnosis and research. Radioactive chemical tracers emit gamma radiation which provides diagnostic information about a person's anatomy and the functioning of specific organs. Radiotherapy also employs radioisotopes in the treatment of some illnesses, such as cancer. More powerful gamma sources are used to sterilise syringes, bandages and other medical equipment. About one person in two in the western world is likely to experience the benefits of nuclear medicine in their lifetime, and gamma sterilisation of equipment is almost universal.

 In the preservation of food, radioisotopes are used to inhibit the sprouting of root crops after harvesting, to kill parasites and pests, and to control the ripening of stored fruit and vegetables. Irradiated foodstuffs are accepted by world and national health authorities for human consumption in an increasing number of countries. They include potatoes, onions, dried and fresh fruits, grain and grain products, poultry and some fish. Some prepacked foods can also be irradiated. 

In the growing of crops and breeding livestock, radioisotopes also play an important role. They are used to produce high yielding, disease-resistant and weather-resistant varieties of crops, to study how fertilisers and insecticides work, and to improve the productivity and health of domestic animals.

 Industrially, and in mining, they are used to examine welds, to detect leaks, to study the rate of wear of metals, and for on-stream analysis of a wide range of minerals and fuels. 

There are many other uses. A radioisotope derived from the plutonium formed in nuclear reactors is used in most householdsmoke detectors. 

Radioisotopes are used to detect and analyse pollutants in the environment, and to study the movement of surface water in streams and also of groundwater.

Other reactors

There are also other uses for reactors. About 200 small nuclear reactors power some 150 ships, mostly submarines, but ranging from icebreakers to aircraft carriers. These can stay at sea for long periods without having to make refuelling stops. In the Russian Arctic where operating conditions are beyond the capability of conventional icebreakers, very powerful nuclear-powered vessels operate almost year-round, where previously only two months could be used each year.

The heat produced by nuclear reactors can also be used directly rather than for generating electricity. In Sweden and Russia, for example, it is used to heat buildings and to provide heat for a variety of industrial processes such as water desalination. Nuclear desalination is likely to be a major growth area in the next decade.

High-temperature heat from nuclear reactors is likely to be employed in some industrial processes in future, especially for making hydrogen.

Military sources

Both uranium and plutonium were used to make bombs before they became important for making electricity and radioisotopes. The type of uranium and plutonium for bombs is different from that in a nuclear power plant. Bomb-grade uranium is highly-enriched (>90% U-235, instead of up to 5%); bomb-grade plutonium is fairly pure Pu-239 (>90%, instead of about 60% in reactor-grade) and is made in special reactors.

Since the 1990s, due to disarmament, a lot of military uranium has become available for electricity production. The military uranium is diluted about 25:1 with depleted uranium (mostly U-238) from the enrichment process before being used in power generation.  Military plutonium is starting to be used similarly, mixed with depleted uranium.

Interesting Facts about Ammonium Nitrate

Ammonium nitrate is a chemical compound with the formula NH4NO3. It is composed of nitric acid and salt of ammonia. In room temperature, ammonium nitrate appears in a white crystalline form and it is also colorless. Its melting point is at 169.6 degrees Celsius or 337.3 degrees Fahrenheit. These crystals are rhombohedral in shape but when they are subjected to temperatures above 32 degrees Celsius, they change to monoclinic crystals.

 Ammonium nitrate image

Ammonium nitrate was said to be developed Germans which they used as fertilizers instead of Chilean Nitrates since it is a lot cheaper. Commercially, it is prepared by mixing nitric acid and ammonia salt. The reaction from the two substances combined will form Ammonium Nitrate. The kind of ammonium nitrate sold in the market contains an average of 33.5 percent of nitrogen. This compound is very soluble in water; and if the water which ammonium nitrate was dissolved at is heated, the by- product will be nitrous oxide which is commonly referred to as laughing gas.

Ammonium Nitrate and fertilizers

Ammonium Nitrate is generally used as a fertilizer. It is actually sold in the form of pellets that are coated with clay. The reason why it is very popular in agriculture is because of the high nitrogen amount in this compound. Nitrogen is a very important plant nutrient that assists in the growth and metabolic processes that the plant undergoes. Agriculturists love using ammonium nitrate since it is a cheap alternative to expensive fertilizers. It can also yield rapid growth and may increase the fruit production capacity of a plant. It may also affect the quality of green leafy vegetables since the nitrogen which is used by the plants is actually very helpful in the process of photosynthesis. Another famous use of ammonium nitrate is as an additive in explosives. Ammonium nitrate is sensitive to heat and any application of this external factor can lead to explosion. It is a strong oxidizing agent. This means that it can actually remove certain electrons from other reactants when subjected to a redox chemical reaction. This is the reason why ammonium nitrates are paired and added in combustibles like TNT and others.

Ammonium Nitrate and explosives

Aside from that, ammonium nitrate is also the main component of an explosive called ANFO which stands for Ammonium Nitrate Fuel Oil. It is an explosive mixture which is used widely in mining. ANFO is composed of 94 percent ammonium nitrate and 6 percent fuel oil. The ammonium nitrate will serve as the oxidizing agent for the fuel. Another interesting fact about this compound is that it is actually hygroscopic. A hygroscopic substance is something that can easily collect water molecules from the environment where it is placed. Because of this reason, ammonium nitrates should not be stored in humid areas since water can easily affect the compound’s explosive function. Ammonium nitrates are now regulated by the government since it is already used to create fertilizer bombs. These are improvised explosive devices that other people use in terrorism. Ammonium nitrate can be very helpful in agriculture but correct storage and handling should always be observed.

potassium cyanide. (KCN)

Potassium cyanide is a compound with the formula KCN. This colorless crystalline compound, similar in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include jewelry for chemical gilding andbuffing.

KCN is highly toxic. The moist solid emits small amounts of hydrogen cyanide due to hydrolysis, which tastes like bitter almonds. Not everyone, however, can taste this: the ability to do so is a genetic trait

Production

KCN is produced by treating hydrogen cyanide with a 50% aqueous solution of potassium hydroxide, followed by evaporation of the solution in a vacuum:

HCN + KOH → KCN + H₂O

or by treating formamide with potassium hydroxide:

HCONH₂ + KOH → KCN + 2H₂O

Approximately 50,000 tons of potassium cyanide are produced yearly.

Structure

In aqueous solution, KCN is dissociated into hydrated K⁺ ions and CN⁻. As a solid, the salt crystallizes such that the cations and anions organize like Na⁺ and Cl⁻ in NaCl. The cations and anions six-coordinate. Each K⁺ is linked to two pi-bonds of the CN⁻ as well as two links each to C and N each. Since CN⁻ is diatomic, the symmetry of the solid is lower than that in NaCl. The cyanide anions form sheets. The CN⁻ ions rapidly rotate in the solid at ambient temperature such that the time averaged shape of the CN⁻ ions is spherical.

Applications

KCN and its close relative sodium cyanide (NaCN) are widely used in organic synthesis for the preparation of nitriles and carboxylic acids, particularly in the von Richter reaction.

Potassium gold cyanide

In gold mining, KCN forms the water-soluble salt potassium gold cyanide (or gold potassium cyanide) and potassium hydroxide from gold metal in the presence of oxygen (usually from the surrounding air) and water:

4 Au + 8 KCN + O₂ + 2 H₂O → 4 K[Au(CN)₂] + 4 KOH

A similar process uses sodium cyanide (NaCN, a close relative of potassium cyanide) to produce sodium gold cyanide (NaAu(CN₂)).

Very few other methods exist for this extraction process.

Toxicity

KCN can be detoxified most efficiently with hydrogen peroxide:

KCN + H₂O₂ → KOCN + H₂O

Cyanide is a potent inhibitor of cellular respiration, acting on mitochondrial cytochrome c oxidase and hence blocking oxidative phosphorylation. This prevents the body from oxidizing food to produce useful energy. Lactic acidosis then occurs as a consequence of anaerobic metabolism. Initially, acute cyanide poisoning causes a red or ruddy complexion in the victim because the tissues are not able to use the oxygen in the blood. The effects of potassium and sodium cyanide are identical. The person loses consciousness, and death eventually follows over a period of time. During this period, convulsions may occur. Death occurs not by cardiac arrest, but by hypoxia of neural tissue.

The lethal dose for potassium cyanide is 200–300 mg. The toxicity of potassium cyanide when ingested depends on the acidity of the stomach, because it must react with an acid to become hydrogen cyanide, the deadly form of cyanide. Grigori Rasputin may have survived a potassium cyanide poisoning because his stomach acidity was unusually low.

A number of prominent persons were killed or committed suicide using potassium cyanide, including members of the Young Bosnia and members of the Nazi Party, such as Hermann Göring and Heinrich Himmler, World War II era British agents (using purpose-made suicide pills), computer scientist Alan Turing, and various religious cult suicides such as by the Peoples Temple and Heaven's Gate. Danish writer Gustav Wied and members of the LTTE involved in the assassination of Indian prime minister Rajiv Gandhi also committed suicide using potassium cyanide.

It is used by entomologists as a killing agent in collecting jars, as most insects succumb within seconds, minimizing damage of even highly fragile specimens.

Steam Jet Ejectors

Steam jet Ejectors are based on the ejector-venturi principal and operate by passing motive steam through an expanding nozzle. The nozzle provides controlled expansion of the motive steam to convert pressure in to velocity which creates a vacuum with in the body chamber to draw in and entrain gases or vapours. The motive steam and suction gas are then completely mixed and then passed through the diffuser or tail, where the gases velocity is converted in to sufficient pressure to meet the predetermined discharge pressure.

Vacuum Ejectors are used in a variety of applications in the process, food, steel and petrochemical industries. Typical duties involve filtration, distillation, absorption, mixing, vacuum packaging, freeze drying, dehysrating and degassing. Ejectors will handle both condensible and none condensible gas loads as well as small amounts of solids or liquids, however accidental entrainment of liquids can cause a momentary interruption in vacuum but this will not cause damage to the ejector.

Primary advantages over other vacuum pumps can be seen below:

• No Moving Parts - Ejectors are exceedingly simple and reliable. There are no moving parts to wear or break in a basic ejector.
• Low Cost - Units are small in relation to the work they do and cost is correspondingly low.
• Versatile - Various piping arrangements permit adapting to environmental conditions.
• Self Priming - Ejectors are self-priming. They operate equally well in continuous or intermittent service.
• Easy to Install - Relatively light in weight, ejectors are easy to install, and require no foundations. Even multi stage units are readily adaptable to existing conditions.
• Corrosion and Erosion Resistant - Because they can be made of practically any workable material, or coated with corrosion-resistant materials, ejectors can be made highly resistant erosion and corrosion.
• High Vacuum Performance - Ejectors can handle air or other gases at suction pressures as low as 3 microns HgA.

Ejectors range from Single upto Six Stage units, and can be either Condensing or Non-Condensing types. The number of Ejector stages required are usually determined by the economy of the ejectors and the level of vacuum required. The operating range for each stage of Vacuum Ejector can be seen below, also for reference 1 BarA = 760 mm HgA.

1st Stage : 810mm HgA - 30mm HgA
2nd Stage : 130mm HgA - 3 mm HgA
3rd Stage : 25mm HgA - 0.8mm HgA
4th Stage : 4mm HgA - 75 microns HgA
5th Stage : 0.4mm HgA - 10 microns HgA
6th Stage : 0.1mm HgA - 3 microns HgA

Single Stage Ejectors

Single stage Vacuum Ejectors generally cover vacuum ranges from 30mm HgA up to atmospheric pressure. To maximise performance eight different designs are available with each ejector being optimised to operate in a specific vacuum range. This allows the motive steam consumption to be kept at a minimum for the selected ejector, and also ensures that operation will be stable. All single stage ejectors are designed to discharge either at or slightly above atmospheric pressure. Sizes range from 1 Inch to 6 Inch, however large size are available if required. Standard materials of construction are carbon steel or stainless steel, both of which are fitted with a stainless steel nozzle.

Two Stage Ejectors

Staging of Ejectors is required for more economical operation when the required absolute vacuum level is reduced. Two stage Vacuum Ejectors generally cover vacuum ranges between 3mm HgA to 130mm HgA, however depending up on actual operating conditions a Single Stage may be more economical if at the upper limit of the operational envelope, or a Three Stage Ejector System if conditions are at the lower end.

In operation a two stage system consist of a primary High Vacuum (HV) Ejector and a secondary Low Vacuum (LV) Ejector. Initially the LV ejector is operated to pull vacuum down from the starting pressure to an intermediate pressure. Once this pressure is reached the HV ejector is then operated in conjunction with the LV ejector to finally pull vacuum to the required pressure.

Two stage systems can also be either Condensing or Non-condensing types. Condensers can be used as pre-condensers, inter-condensers, and after-condensers, all of which help to reduce the gas load being passed on to the next ejector stage. This helps to reduce motive steam consumption and also allows smaller ejectors to be used with in the system. Depending up on the application Non-condensing systems can also be used, however this can be less efficient than Condensing Types as each ejector must entrain the full gas load from the previous stage. This can lead to ejectors becoming large and also increases motive steam consumption. Non-condensing types are usually used where it is not feasible to install condensers, or where service is intermittent, making operating costs a secondary consideration.

Three Stage Ejectors

Three stage Vacuum Ejectors generally cover vacuum ranges between 0.8mm HgA to 25mm HgA, however depending up on actual operating conditions a Two Stage Ejector system may be more economical if at the upper limit of the operational envelope, or a Four Stage Ejector system if conditions are at the lower end.

In operation a Three Stage system consist of a primary Booster, a secondary High Vacuum (HV) Ejector, and a tertiary Low Vacuum (LV) Ejector. As per the Two Stage System, initially the LV ejector is operated to pull vacuum down from the starting pressure to an intermediate pressure. Once this pressure is reached the HV ejector is then operated in conjunction with the LV ejector to pull vacuum to the lower intermediate pressure. Finally the Booster is operated (in conjunction with the HV & LV Ejectors) pull vacuum to the required pressure.

Three stage systems are also usually of the Condensing type. Again as per the Two Stage system, condensers can be used as pre-condensers, inter-condensers, and after-condensers in order to reduce the gas load being passed on to the next ejector stage. Depending up on the application Non-condensing systems can also be used however this is less efficient than Condensing Types as each ejector must entrain the full gas load from the previous stage.

Four, Five & Six Stage Ejectors

These systems are similar to Three Stage Systems, however they include additional boosters which are equipped with Steam Jackets to prevent ice forming with in the ejectors. These systems are usually of the Condensing type to increase efficiency and reduce motive steam consumption.

A furnace is a device used for heating. The name derives from Latin fornax, oven. The earliest furnace was excavated at Balakot, a site of the Indus Valley Civilization, dating back to its mature phase (c. 2500-1900 BC). The furnace was most likely used for the manufacturing of ceramic objects

The furnace components can be divided into three categories.
1- The burners, heat exchanger, draft inducer, and venting.
2- The controls and safety devices.
3- The blower and air movement.

Mass Transfer Principles......

The whole idea behind mass transfer is Chemical potential equilibrium or for simplification, you can call it Concentration equilibrium -but it would be not very accurate term-. Anyhow, do you think that it is possible to find a place on the surface of earth that isn't occupied with air?
Of course not! All of earth's face is covered with air. OK, why do you think that air can't be depleted even if we used a vacum pressure to suck it? This is because of "Mass transfer" Air molecules -and any other molecules in the universe- tend to move from the high chemical potential point to the less one -can be expressed with concentration too-. Mass transfer process can take place in a gas or vapour or in a liquid, and it can result from the random velocities of the molecules (molecular diffusion) or from the circulating or eddy currents present in a turbulent fluid (eddy diffusion). Mass transfer rate is governed by Fick's law which expresses the mass transfer rate as a linear function of the molar concentration gradient. Na=-Dab*(dCa/dy)
where NA is the molar flux of A (moles per unit area per unit time),
Ca is the concentration of A (moles of A per unit volume),
Dab is known as the diffusivity or diffusion coefficient for A in B, and
y is distance in the direction of transfer.
It is also worth mentioning that concentration can be expressed in partial pressure in case of gases.
Diffusivity is extremely important factor in our course, There is two kinds of diffusivities, molecular diffusivity (Which is D is a physical property of the system and a function only of its composition, pressure and temperature) and Eddy diffusivity (Which is dependent on the flow pattern and varies with position). the higher the velocity of the molecules, the greater is the distance they travel before colliding with other molecules, and the higher is the diffusivity D. The higher pressure applied, the less distance a molecule travels before it colloids with another and hence, less mean free path and consequently, less diffusivity.
When the mass transfer rates of the two components are equal and opposite the process is said to be one of equimolecular counter diffusion. Such a process occurs in the case of the box with a movable partition, where each partition contains different gas. It occurs also in a distillation column when the molar latent heats of the two components are the same. At any point in the column a falling stream of liquid is brought into contact with a rising stream of vapour with which it is not in equilibrium. The less volatile component is transferred from the vapour to the liquid and the more volatile component is transferred in the opposite direction. If the molar latent heats of the components are equal, the condensation of a given amount of less volatile component releases exactly the amount of latent heat required to volatilise the same molar quantity of the more volatile component. Thus at the interface, and consequently throughout the liquid and vapour phases, equimolecular counterdiffusion is taking place.

Combined Cycle......

Combined cycle is an electric generating technology that creates additional electricity from heat exiting gas turbines. The exhaust heat from the gas turbines is routed to a conventional boiler or to a heat recovery steam generator for utilization by a steam turbine in the production of electricity.
Where natural gas is abundant and cheap, combining the Brayton (gas) and Rankine (steam) cycles to generate electricity has many advantages. Among these is natural gas, a relatively clean-burning fuel, emitting far less CO2,, mercury and particulate emissions than coal. In a combined cycle power plant, a waste heat boiler is installed onto the gas turbine exhaust stream. This heat recovery steam generator (HRSG) produces steam from hot gas exhaust to drive a steam turbine that turns an electric generator.
Flowserve pumps, valves and seals are fully proven and ideally suited for combined cycle project applications, including co-generation of electrical power and steam. These applications include heat recovery steam generator (HRSG) feed water, condensate and circulating water as well as auxiliary services...

In electric power generation a combined cycle is an assembly of heat engines that work in tandem from the same source of heat, converting it intomechanical energy, which in turn usually drives electrical generators. The principle is that the exhaust of one heat engine is used as the heat source for another, thus extracting more useful energy from the heat, increasing the system's overall efficiency. This works because heat engines are only able to use a portion of the energy their fuel generates (usually less than 50%). In an ordinary (non combined cycle) heat engine the remaining heat (e.g., hot exhaust fumes) from combustion is generally wasted.

Combining two or more thermodynamic cycles results in improved overall efficiency, reducing fuel costs. In stationary power plants, a widely used combination is a gas turbine (operating by the Brayton cycle) burning natural gas or synthesis gas from coal, whose hot exhaust powers a steam power plant (operating by the Rankine cycle). This is called a Combined Cycle Gas Turbine (CCGT) plant, and can achieve a thermal efficiency of around 60%, in contrast to a single cycle steam power plant which is limited to efficiencies of around 35-42%. Many new gas power plants in North America and Europe are of this type. Such an arrangement is also used for marine propulsion, and is called a combined gas and steam (COGAS)plant. Multiple stage turbine or steam cycles are also common.

Other historically successful combined cycles have used hot cycles with mercury vapor turbines, magnetohydrodynamic generators or molten carbonate fuel cells, with steam plants for the low temperature "bottoming" cycle. Bottoming cycles operating from a steam condenser's heat exhaust are theoretically possible, but uneconomical because of the very large, expensive equipment needed to extract energy from the small temperature differences between condensing steam and outside air or water. However, it is common in cold climates (such as Finland) to drive community heating systems from a power plant's condenser heat. Such cogeneration systems can yield theoretical efficiencies above 95%.

In automotive and aeronautical engines, turbines have been driven from the exhausts of Otto and Diesel cycles. These are called turbo-compound engines (not to be confused with turbochargers). They have failed commercially because their mechanical complexity and weight are less economical than multistage turbine engines. Stirling engines are also a good theoretical fit for this application. A turbocharged car is also a combined cycle.

Types of cracking....

In petroleum geology and chemistry, cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors. The rate of cracking and the end products are strongly dependent on the temperature and presence of catalysts. Cracking is the breakdown of a large alkane into smaller, more useful alkanes and alkenes. Simply put, hydrocarbon cracking is the process of breaking a long-chain of hydrocarbons into short ones.

More loosely, outside the field of petroleum chemistry, the term "cracking" is used to describe any type of splitting of molecules under the influence of heat, catalysts and solvents, such as in processes of destructive distillation or pyrolysis.

Fluid catalytic cracking produces a high yield of gasoline and LPG, while hydrocracking is a major source of jet fuel, diesel, naphtha, and LPG.

Initiation

In these reactions a single molecule breaks apart into two free radicals. Only a small fraction of the feed molecules actually undergo initiation, but these reactions are necessary to produce the free radicals that drive the rest of the reactions. In steam cracking, initiation usually involves breaking a chemical bond between two carbon atoms, rather than the bond between a carbon and ahydrogen atom..

CH₃CH₃ → 2 CH₃•

Hydrogen abstraction

In these reactions a free radical removes a hydrogen atom from another molecule, turning the second molecule into a free radical.

CH₃• + CH₃CH₃ → CH₄ + CH₃CH₂•

Radical decomposition

In these reactions a free radical breaks apart into two molecules, one an alkene, the other a free radical. This is the process that results in alkene products.

CH₃CH₂• → CH₂=CH₂ + H•

Radical addition

In these reactions, the reverse of radical decomposition reactions, a radical reacts with an alkene to form a single, larger free radical. These processes are involved in forming the aromatic products that result when heavier feedstocks are used.

CH₃CH₂• + CH₂=CH₂ → CH₃CH₂CH₂CH₂•

Termination

In these reactions two free radicals react with each other to produce products that are not free radicals. Two common forms of termination are recombination, where the two radicals combine to form one larger molecule, and disproportionation, where one radical transfers a hydrogen atom to the other, giving an alkene and an alkane.

CH₃• + CH₃CH₂• → CH₃CH₂CH₃

CH₃CH₂• + CH₃CH₂• → CH₂=CH₂ + CH₃CH₃

Example: cracking butane

There are three places where a butane molecule (CH₃-CH₂-CH₂-CH₃) might be split. Each has a distinct likelihood:

• 48%: break at the CH₃-CH₂ bond.

CH₃* / *CH₂-CH₂-CH₃

Ultimately this produces an alkane and an alkene: CH₄ + CH₂=CH-CH₃

• 38%: break at a CH₂-CH₂ bond.

CH₃-CH₂* / *CH₂-CH₃

Ultimately this produces an alkane and an alkene of different types: CH₃-CH₃ + CH₂=CH₂

• 14%: break at a terminal C-H bond

H/CH₂-CH₂-CH₂-CH₃

Ultimately this produces an alkene and hydrogen gas: CH₂=CH-CH₂-CH₃ + H₂

Cracking methodologies

Thermal methods

Thermal cracking was the first category of hydrocarbon cracking to be developed. Thermal cracking is an example of a reaction whose energetics are dominated by entropy (∆S°) rather than by enthalpy (∆H°) in the Gibbs Free Energy equation ∆G°=∆H°-T∆S°. Although the bond dissociation energy D for a carbon-carbon single bond is relatively high (about 375 kJ/mol) and cracking is highly endothermic, the large positive entropy change resulting from the fragmentation of one large molecule into several smaller pieces, together with the extremely high temperature, makes T∆S° term larger than the ∆H° term, thereby favoring the cracking reaction.

Thermal cracking

Modern high-pressure thermal cracking operates at absolute pressures of about 7,000 kPa. An overall process of disproportionation can be observed, where "light", hydrogen-rich products are formed at the expense of heavier molecules which condense and are depleted of hydrogen. The actual reaction is known as homolytic fission and produces alkenes, which are the basis for the economically important production of polymers.

Thermal cracking is currently used to "upgrade" very heavy fractions or to produce light fractions or distillates, burner fuel and/or petroleum coke. Two extremes of the thermal cracking in terms of product range are represented by the high-temperature process called "steam cracking" or pyrolysis (ca. 750 °C to 900 °C or higher) which produces valuable ethylene and other feedstocks for the petrochemical industry, and the milder-temperature delayed coking (ca. 500 °C) which can produce, under the right conditions, valuable needle coke, a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminium industries.

The first thermal cracking method, the Shukhov cracking process, was invented by Russian engineer Vladimir Shukhov, in the Russian empire, Patent No. 12926, November 27, 1891.

William Merriam Burton developed one of the earliest thermal cracking processes in 1912 which operated at 700–750 °F (371–399 °C) and an absolute pressure of 90 psi (620 kPa) and was known as the Burton process. Shortly thereafter, in 1921, C.P. Dubbs, an employee of the Universal Oil Products Company, developed a somewhat more advanced thermal cracking process which operated at 750–860 °F (399–460 °C) and was known as the Dubbs process. The Dubbs process was used extensively by many refineries until the early 1940s when catalytic cracking came into use.

Steam cracking

Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter alkenes (or commonly olefins), including ethene (or ethylene) and propene (or propylene). Steam cracker units are facilities in which a feedstock such as naphtha, liquefied petroleum gas (LPG), ethane, propane or butane is thermally cracked through the use of steam in a bank of pyrolysis furnaces to produce lighter hydrocarbons. The products obtained depend on the composition of the feed, the hydrocarbon-to-steam ratio, and on the cracking temperature and furnace residence time.

In steam cracking, a gaseous or liquid hydrocarbon feed like naphtha, LPG or ethane is diluted with steam and briefly heated in a furnace without the presence of oxygen. Typically, the reaction temperature is very high, at around 850°C, but the reaction is only allowed to take place very briefly. In modern cracking furnaces, the residence time is reduced to milliseconds to improve yield, resulting in gas velocities faster than the speed of sound. After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer line heat exchanger or inside a quenching header using quench oil.

The products produced in the reaction depend on the composition of the feed, the hydrocarbon to steam ratio and on the cracking temperature and furnace residence time. Light hydrocarbon feeds such as ethane, LPGs or light naphtha give product streams rich in the lighter alkenes, including ethylene, propylene, and butadiene. Heavier hydrocarbon (full range and heavy naphthas as well as other refinery products) feeds give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil.

A higher cracking temperature (also referred to as severity) favors the production of ethene and benzene, whereas lower severity produces higher amounts of propene, C4-hydrocarbons and liquid products. The process also results in the slow deposition of coke, a form of carbon, on the reactor walls. This degrades the efficiency of the reactor, so reaction conditions are designed to minimize this. Nonetheless, a steam cracking furnace can usually only run for a few months at a time between de-cokings. Decokes require the furnace to be isolated from the process and then a flow of steam or a steam/air mixture is passed through the furnace coils. This converts the hard solid carbon layer to carbon monoxide and carbon dioxide. Once this reaction is complete, the furnace can be returned to service.

Catalytic methods

The catalytic cracking process involves the presence of acid catalysts (usually solid acids such as silica-alumina and zeolites) which promote a heterolytic (asymmetric) breakage of bonds yielding pairs of ions of opposite charges, usually a carbocation and the very unstable hydride anion. Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C-C scission in position beta as in cracking, and intra- and intermolecular hydrogen transfer. In both types of processes, the corresponding reactive intermediates (radicals, ions) are permanently regenerated, and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.

Fluid Catalytic cracking

Schematic flow diagram of a fluid catalytic cracker

Fluid catalytic cracking is a commonly used process, and a modern oil refinery will typically include a cat cracker, particularly at refineries in the US, due to the high demand for gasoline. The process was first used around 1942 and employs a powdered catalyst. During WWII, in contrast to the Axis Forces which suffered severe shortages of gasoline and artificial rubber, the Allied Forces were supplied with plentiful supplies of the materials. Initial process implementations were based on low activity alumina catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in a fluidized bed.

Alumina-catalyzed cracking systems are still in use in high school and university laboratories in experiments concerning alkanes and alkenes. The catalyst is usually obtained by crushing pumice stones, which contain mainly aluminium oxide and silica into small, porous pieces. In the laboratory, aluminium oxide (or porous pot) must be heated.

In newer designs, cracking takes place using a very active zeolite-based catalyst in a short-contact time vertical or upward-sloped pipe called the "riser". Pre-heated feed is sprayed into the base of the riser via feed nozzles where it contacts extremely hot fluidized catalyst at 1,230 to 1,400 °F (666 to 760 °C). The hot catalyst vaporizes the feed and catalyzes the cracking reactions that break down the high-molecular weight oil into lighter components including LPG, gasoline, and diesel. The catalyst-hydrocarbon mixture flows upward through the riser for a few seconds, and then the mixture is separated via cyclones. The catalyst-free hydrocarbons are routed to a main fractionator for separation into fuel gas, LPG, gasoline, naphtha, light cycle oils used in diesel and jet fuel, and heavy fuel oil.

During the trip up the riser, the cracking catalyst is "spent" by reactions which deposit coke on the catalyst and greatly reduce activity and selectivity. The "spent" catalyst is disengaged from the cracked hydrocarbon vapors and sent to a stripper where it is contacts steam to remove hydrocarbons remaining in the catalyst pores. The "spent" catalyst then flows into a fluidized-bed regenerator where air (or in some cases air plus oxygen) is used to burn off the coke to restore catalyst activity and also provide the necessary heat for the next reaction cycle, cracking being anendothermic reaction. The "regenerated" catalyst then flows to the base of the riser, repeating the cycle.

The gasoline produced in the FCC unit has an elevated octane rating but is less chemically stable compared to other gasoline components due to its olefinic profile. Olefins in gasoline are responsible for the formation of polymeric deposits in storage tanks, fuel ducts and injectors. The FCC LPG is an important source of C₃-C₄ olefins and isobutane that are essential feeds for thealkylation process and the production of polymers such as polypropylene.

Hydrocracking

Hydrocracking is a catalytic cracking process assisted by the presence of an elevated partial pressure of hydrogen gas. Similar to the hydrotreater, the function of hydrogen is the purification of the hydrocarbon stream from sulfur and nitrogen hetero-atoms.

The products of this process are saturated hydrocarbons; depending on the reaction conditions (temperature, pressure, catalyst activity) these products range from ethane, LPG to heavier hydrocarbons consisting mostly of isoparaffins. Hydrocracking is normally facilitated by a bifunctional catalyst that is capable of rearranging and breaking hydrocarbon chains as well as adding hydrogen to aromatics and olefins to produce naphthenes and alkanes.

The major products from hydrocracking are jet fuel and diesel, but high octane rating gasoline fractions and LPG are also produced. All these products have a very low content of sulfur and othercontaminants.

In 1920, a plant for the commercial hydrogenation of brown coal was commissioned at Leuna in Germany. It is very common in Europe and Asia because those regions have high demand for diesel and kerosene. In the US, fluid catalytic cracking is more common because the demand for gasoline is higher.

The hydrocracking process largely depends on the nature of the feedstock and the relative rates of the two competing reactions, hydrogenation and cracking. Heavy aromatic feedstock is converted into lighter products under a wide range of very high pressures (1,000-2,000 psi) and fairly high temperatures (750°-1,500° F), in the presence of hydrogen and special catalysts.

The primary function of hydrogen is, thus: a) If feedstock has a high paraffinic content, the primary function of hydrogen is to prevent the formation of polycyclic aromatic compounds. b) Reduced tar formation c) Reduced Impurities d) Prevent buildup of coke on the catalyst. e) Convert sulfur and nitrogen compounds present in the feedstock to hydrogen sulfide and ammonia. f) High octane fuel is achieved.