Sintering in Fluidized Bed
Processes for cleaner and more efficient energy generation from feed stocks such as coals, lignites, peats, and waste liquors use fluidized beds that are operated at high temperatures and pressures. These processes involve systems that are multi-phase and have complex chemical reactions. Research work has tackled a number of aspects, including mechanical engineering aspects of the reactors, reaction chemistry and products, characterization and physical properties of the ash, fouling by ash deposits and the phenomenon of defluidization by agglomeration or sintering of the ash particles. It is with this latter aspect, the phenomenon of defluidization, that this contribution is concerned. Defluidization is also a problem in a number of other elevated temperature fluidized bed production processes, including size enlargement by agglomeration, fluidized bed processes for poly-olefin production and metallurgical processes.
Defluidization occurs when the particles in the bed adhere. When two particles touch, material at the point of contact migrates forming a neck that is strong enough to withstand the disruptive forces in the fluidized bed. Two categories of adhesion can be discerned. The first type is visco-plastic sintering and it occurs with glassy materials. With these materials, migration is limited by the ability of the material to flow. With increasing temperature the viscosity of the material is reduced and hence the material flows and the size of the neck is increased. At some point, the necks are sufficiently large and strong enough to cause defluidization. The second type occurs when a large quantity of liquid is formed by melting or by chemical reaction. With an increase in temperature, a phase boundary may be crossed bring about rapid defluidization. The liquid formed has a relatively low viscosity and defluidization is caused by the amount of liquid present. This section will primarily focus on visco-plastic sintering.
Model systems, in which chemical reactions do not occur, have been used to investigate the relationship between process conditions and the tendency to defluidize due to visco-plastic sintering. Materials used have included low-density ploy-ethylene and poly-propylene, soda glasses, metals, and inorganic crystalline salts. Under sintering conditions, the fluidizing velocity has to be increased above the minimum fluidizing velocity, Umf, to a higher velocity, U; at which pronounced bubbling occurs. A fluidizing velocity, Umfs, may be defined as a velocity just sufficient to prevent sintering. This characterizes a different fluidization state from Umf, since the in the latter the bed is in an expanded but not bubbling state.
This new form was called hydraulic die-forming. Hydraulic stems from the Greek word hydro meaning water and aulos meaning tube (McCreight, 2004). In today’s society, hydraulic pressing and stamping of dies are used for everything from making small pots and pans to the more extravagant automobile body models. Another use that has recently developed is the more artistic use of, producing form in silver and gold. In order to achieve this smaller, cheaper scale of die-forming experiment were conducted by Richard Thomas and Ruth Girard, which eventually led to the development of the pourable epoxy steel...
Have you ever wondered why the plastic bag that you left on the porch during winter cracks or breaks more easily than when you left it during summer time but a piece of wood which was left just like the plastic bag has no effect whatsoever? This is because of a phenomenon, which only happens to polymers, known as the glass transition. For each polymer, there is a certain temperature at which the amorphous polymers undergo a second order phase transition from a rubbery and viscous amorphous solid to a brittle and glassy amorphous solid called the glass transition temperature, Tg.1 When the polymer, or in this case, the plastic bag, is cooled below their glass transition temperature, it becomes hard and brittle like a glass but when it is used above their glass transition temperatures, it might have a different effect than when used at room temperature or below the glass transition temperature as normally, different types of polymers like clothes, food packaging, insulations for wires, etc. are either used above their glass transition temperatures or
-creates a strong surface and can be used as a thin layer for glazes or a thick layers for impasto
al. 1996). this dense produce is more thermally unbalanced than mulch or coal, mainly as of grass biomass. Little warming degrees and extensive residence eras will decrease the reactivity of the burn. On the other hand, great heating rates give to upsurge the reactivity of the subsequent char, creating it advanced appropriate for more thermal usage for example gasification or combustion (Hallgren, et. al. 1996).
Because of its ability to break down self-associative tendency of water , it may also reduce the number of water molecules entrapped between the polymeric chains, increasing the degree of postoperative polymerization In addition to post-curing increase in bond strength, the relative decrease in free water would eliminate or decrease the hydrolytic degradation of adhesive in or above the hybrid layer
Coal Combustion Residue, or coal sludge, is the byproduct of burning coal in order to make energy. It is a combination of fly ash, bottom ash, coal slag, and flue gas. According to the watchdog group “sourcewatch.org” the ...
nucleation. When nucleation in the liquid phase is catalyzed by foreign surfaces other than the material to be crystallized (such as by dust particles or wall surfaces), it is described as primary heterogeneous nucleation. In solutions in which crystals already exist, secondary nucleation is thought to be by far the most significant source of nuclei. The supersaturation at which secondary nucleation rises is much lower than that which gives rise to primary nucleation.( Garside and Davey ,1980; Garside, 1985; Nyvlt et al., 1985); Fig. (I.4) shown A simple scheme for nucleation .
The plastic obtained from polyethylene can be pressed into varying shapes, ranging from the simplest to the most complicated. The ethylene market is indirectly driven by the increased polyethylene consumption for the production of several plastic components. In 2011, the global propylene market is valued at more than $ 90 billion and significant growth is anticipated in the coming years. Acrylic fibers and coatings, PVC plasticizers and coatings, polyurethane resins, epoxy resins and propylene polymers are applied on polycarbonates and solvents, that are used in the production of propylene. The automotive industry is the biggest end-user of polypropylene. The enormous expansion of ethylene and propylene production will contribute to the growth of the ethylene and propylene market, along with the rising demand for downstream products from India, China, and the Middle East. Tight environmental regulations, fluctuations in raw material prices and political uncertainties in crude oil producing regions are the main factors limiting the ethylene and propylene market [9].
The Olefins II Unit makes hydrocarbons from naphtha or natural gas using furnaces. After distillation, the p...
Coal is one of the world’s most abundant fossil fuels. Coal was formed during the Carboniferous Period when dead plant material was buried and subjected to high pressure and heat. Coal is classified by moisture content and composition. There are four d...
In commercial processing, the burning takes place in large concrete or steel silos with very little oxygen, and stops before it all turns to ash. It is said that the “procedure leaves black lumps and powder which is about 25% of the original weight. When ignited, the carbon in charcoal merges with oxygen and forms carbon dioxide, carbon monoxide, water, other gases, and significant quantities of energy.” It packs more potential energy per ounce than raw wood. Stated by Goldwyn that the char combust steadily, hot and it produces less smoke and lesser unhazardous
The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram. Because it is well explained in earlier volumes of Metals Handbook and in many elementary textbooks, the stable iron-graphite diagram and the metastable Fe-Fe3 C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium for different combinations of carbon concentration and temperature. We distinguish at the low-carbon and ferrite, which can at most dissolve 0.028 wt% C at 727 oC and austenite which can dissolve 2.11 wt% C at 1148 oC. At the carbon-rich side we find cementite. Of less interest, except for highly alloyed steels, is the d-ferrite existing at the highest temperatures. Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + d-ferrite. In heat treating of steels the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names. These include: the carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content. The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite particles dispersed with a ferrite matrix after extended holding close to A1. The Fe-C diagram is of experimental origin. The knowledge of the thermodynamic principles and modern thermodynamic data now permits very accurate calculations of this diagram.
Acid mine drainage refers to water (leachate, drainage or seepage) that has come into contact with oxidised rocks or overburden that contains sulphide material (coal, zinc, copper, lead). (Keller, 2000; U.S.G.S.; U.S.E.P.A., 2002). A common sulphide is pyrite, or iron disulfide (FeS2), and throughout this essay it will be pyrite that will be the primary sulphide considered. Acid mine drainage is not a new phenomenon, early mining techniques utilized gravity to avoid water pooling, resulting in the water becoming polluted by acid, iron, sulphur and aluminium (U.S.E.P.A., 2002). It is most commonly associated with coal mining, especially with soft coal, coal that has high sulphur content. The pyrite that is present in coal seams will be accessible after surface mining when the overlying surfaces are removed or in deep mines that allow oxygen access to the previously inaccessible pyrite-containing coal (D.E.P. 1, 1997). After pyrite is exposed to air and water, sulphuric acid and iron hydroxide are formed, creating an acidic runoff (D.E.P. 1, 1997; 2 2002).
Biomass gasification is a process by which biofuel is produced. It has been used for over 180 years but in the last decades it has been reconsidered as an interesting technique due to the fact that oil supplies are decreasing. As mentioned before, gasification is a thermal process. Heat is added up in order to convert the organic mass to biofuel. The biomass usually undergoes drying, pyrolysis, partial oxidation and reduction. Nowadays the configurations used for gasification are three: fixed bed gasifier, fluidized bed gasifier and entrained bed gasifiers. The simplest configuration is the
Agglomeration of the materials so that the briquette stay in the compressed state for use in energy production.