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MODULE 2
2.1.1 Nuclear Chemistry
2.1.1 Overview of Nuclear Chemistry Nuclear chemistry is the study of the structure of atomic nuclei and the changes they undergo. The figure above shows the components of an atom. Electrons – negatively charged particles surrounding the nucleus Protons - positively charged particles in the nucleus Neutrons – neutral particles in the nucleus Nucleus – the center of an atom composed of neutrons and protons
2.1.2 Chemical vs Nuclear Reactions
2.1.2 Chemical vs Nuclear Reactions Nuclear reactions are very different from chemical reactions.
In chemical reactions , atoms become more stable by participating in a transfer of electrons or by sharing electrons with other atoms. Bonds may be broken or formed , atoms are rearranged , and reaction rates can be changed. In nuclear reactions , it is the nucleus of the atom that gains stability by emitting particles and/or rays. Here, atoms may change into atoms of different element , and the reaction rate cannot be changed. The comparison between chemical reactions and nuclear reactions is presented in the table below. Chemical Reactions Nuclear Reactions Bonds are broken Nuclei emit particles and/or rays Atoms are rearranged Atoms change into atoms of different element Involve valence electrons Involve protons, neutrons, and/or electrons Small energy changes Large energy changes Reaction rate can be changed. Reaction rate cannot be changed
2.1.3 Nuclides and Nuclear Stability
Nuclide is a particular type of nucleus, characterized by a specific atomic number and nucleon number. Nucleon number or mass number is the number of nucleons (protons and neutrons) in the nucleus of a nuclide. The nuclide symbolism is shown below.
2.1.4 Nuclear Reactions There are two general kinds of nuclear reactions. They are nuclear decay reactions and nuclear transmutation reactions. In a nuclear decay reaction , also called radioactive decay, a spontaneous reaction under all conditions occurs wherein an unstable nucleus emits radiation and is transformed into the nucleus of one or more other elements. The products of this kind of reaction have a lower mass and are lower in energy (more stable) than the starting material that decayed. Meanwhile, in a nuclear transmutation reaction , a nucleus reacts with a subatomic particle or another nucleus to form a product nucleus that is more massive than the starting material. This type of reaction does not occur spontaneously as it needs special conditions such as the collision of a beam of highly energetic particles with a target nucleus. The following are illustrations of nuclear decay reactions : Alpha Emission or Alpha Decay It is the emission of an alpha particle ( α ) denoted by the symbol 42 He. An alpha particle has 2 protons and 2 neutrons. Its charge is +2 because of the 2 protons. Alpha decay causes the mass number to decrease by 4 and the atomic number to decrease by 2.
Note that all nuclear reactions are balanced, and the atomic number determines the element. Beta Emission or Beta Decay It is the emission of a beta particle ( β ), a fast moving electron, denoted by the symbol e-^ or^0 -1e. β has insignificant mass (0) and its charge is -1 because it is an electron. Beta decay causes no change in mass number and causes the atomic number to increase by 1. In this process, a neutron is converted to a proton and a beta particle. Gamma Emission or Gamma Decay
General Nuclear Equations Shown below are the general nuclear equations. Take note of the nuclide symbols presented previously. Below are some examples of nuclear equations. In a balanced nuclear equation, the sum of the mass number on the left side is equal to that of on the right side, and so as the atomic number. The atomic number determines the element.
2.1.5 Half-life and Radioactive Decay The rate of radioactive decay is often characterized by the half-life of a radioisotope.
At its half-life, where N = 1/2 N 0 , the equation may be simplified to
t1/2 = ln 2 / λt
Try to solve this! What percentage of the original sample has undergone decay after 1000 years, if its half-life is 2350 years?
2.1.6 Nuclear Fission and Fusion
Nuclear fission is a process in which a very heavy nucleus splits into smaller nuclei of intermediate mass. Because the smaller nuclei are more stable, the fission process releases tremendous amounts of energy.
Chain reaction releases several neutrons which split more nuclei. If controlled, energy is released slowly, like in nuclear reactors. Reactors use uranium for nuclear fuel. The uranium is processed into small ceramic pellets and stacked together into sealed metal tubes called fuel rods. Typically more than 200 of these rods are bundled together to form a fuel assembly. A reactor core is typically made up of a couple hundred assemblies, depending on power level.
Radioactive dating : Carbon–14 used to determine the age of an object that was once alive. Detection of diseases : Iodine–131 used to detect thyroid problems, technetium–99 used to detect cancerous tumors and brain disorders, phosphorus – 32 used to detect stomach cancer. Treatment of some malignant tumors (cobalt–60 and cesium–137) cancer cells are more sensitive to radiation than normal, healthy cells X-rays Radioactive tracers: used in research to tag chemicals to follow in living organisms Everyday items: thorium–232 used in lantern mantels, plutonium– 238 used in long-lasting batteries for space, and americium–241 in smoke detectors.
2.2.1 Fuel Chemistry
A fuel is any compound that has stored energy. It can also be
defined as any material that can be made to react with other
material so that it releases energy as heat energy or to be used
for work.
2.2.2 Classifications of Fuels
Fossil fuel is the product of natural changes in organic materials over millennia. Ancient marine bodies first turn into kerogen before becoming a fossil fuel. Kerogen when subjected to sufficient heat and pressure will become fossil fuel. Kerogen + pressure and heat --> fossil fuel There are three types of fossil fuels :
- Coal - It is a solid fossil fuel formed over millions of years by decay of land vegetation. When layers are compacted and heated over time , deposits are turned into coal. Coal is quite abundant compared to the other two fossil fuels.
There are four main types/ranks of coal depending on the type and amount of carbon it contains and on the amount of energy it can produce. Those are a. Anthracite contains 86%–97% carbon and generally has the highest heating value of all ranks of coal. b. Bituminous coal contains 45%–86% carbon. c. Subbituminous coal typically contains 35%–45% carbon , and it has a lower heating value than bituminous coal. d. Lignite contains 2 5%–35% carbon and has the lowest energy content of all coal ranks.
- Natural Gas - It is a gaseous fossil fuel that is versatile, abundant, and relatively clean compared to coal and oil. Natural gas mainly consists of methane (CH 4 ). The figure below shows the components of raw natural gas.
Most of its constituents are hydrocarbons but there are amounts of compounds containing nitrogen (0 to 0.5%), sulfur (0 to 6%), and oxygen (0 to 3.5%). Crude oil, in its natural state, is useless. What's useful is the various components that make it up. The name petroleum is derived from the word " petra " meaning rock and " oleum " meaning oil. Petroleum may have different names depending on its appearance and state. When it is a dark and sticky liquid, it is called " crude oil ". If it is clear and volatile, it is called " condensate ". In a semi-solid form, it is called " bitumen ", while in its solid form it is called " asphalt ". Crude oil Condensate Bitumen Asphalt Petroleum Processing Petroleum processing starts with the exploration and production in which sources are explored and developed and petroleum is produced ( upstream ). From the petroleum source, it is transported thru shipping, pipelines, or LNG terminals ( midstream ). And finally, petroleum is processed in refineries to produce different products ( downstream ).
Petroleum refining is the process of transforming crude oil into useful products such as liquefied petroleum gas (LPG), gasoline or petrol, kerosene, jet fuel, diesel oil, and fuel oils. The two main processes involved in petroleum refining are separation and conversion. Separation is carried out through fractional distillation. Crude oil is a mixture of hydrocarbons with different boiling temperatures. Therefore, these can be separated by distillation into groups of hydrocarbons that boil between two specified boiling points. Large hydrocarbons can be found at the bottom of the fractioning column as they have high boiling points while small hydrocarbons can be found higher up the column due to their lower boiling points. Hydrocarbons with lower boiling points will evaporate faster compared to those with higher boiling points. The hydrocarbons obtained from different trays in the fractionating column may be converted into more useful products through conversion processes which include:
- Isomerization - It is the alteration of the arrangement of the atoms in a molecule without changing the number of atoms.
- Reforming - It is the conversion of naphthas to obtain products of higher octane number.
- Coking - It is the conversion of the residual oil from the distillation column into low molecular weight hydrocarbon gases, naphtha, light and heavy gas oils, and petroleum coke. Shown below is the petroleum coke.
2.2.4 Petroleum Products
Petroleum or crude oil consists of different hydrocarbons which may be separated through fractional distillation and may be converted into other forms by several processes as discussed previously. Some of the petroleum products obtained from petroleum processing are:
- Liquified Petroleum Gas (LPG) - It can be propane or butane which are flammable mixtures of hydrocarbon gases used as fuel in heating appliances, cooking equipment, and vehicles. It is also obtained from natural gas.
- Gasoline or Petrol - It is a mixture of paraffins (alkanes), cycloalkanes (naphthenes) and olefins (alkenes). It is used to fuel internal combustion engines for cars, motorbikes, trucks, boats and other transport vehicles (light vehicles). Other chemicals are also added to gasoline to further stabilize it and improve its color and smell. Gasoline has an octane rating which compares the gasoline blend with the performance of pure octane hydrocarbon with eight carbon atoms. The octane rating is a measure of the resistance of gasoline to detonation (engine “knocking”) in spark-ignition internal combustion engines.
