Fusion Nuclear

Nuclear fusion is a type of nuclear reaction in which to atomic nuclei combine to form a heavier nucleus, releasing energy. For a fusion reaction to take place, the nuclei, which are positively charged, must have enough kinetic energy to overcome their electrostatic force of repulsion. This can occur either when one nucleus is accelerated to high energies by an accelerating device or when the energies of both nuclei are raised by the application of very high temperatures. The later method, referred to as thermonuclear fusion, is the source of Sun’s energy, if a proton is accelerated and collides with another proton, these nuclei can fuse, forming a deuterium nucleus (one proton and one neutron), a positron, a neutrino, and energy. Such a reaction is not self sustaining, because the released energy is not readily imparted to other nuclei. Thermonuclear fusion of deuterium and tritium (one proton and two neutrons) will produce a helium nucleus and an energetic neutron that can help sustain further fusion. This is one bases of the Hydrogen Bomb, which employs a brief effort is now under way to harness thermonuclear fusion as a source of power.

Thermonuclear fusion depends on high energies, and the possibility of low-energy, low-temperature nuclear fusion has generally been discounted. Early in 1989, however, two electrochemists started the scientific world and aroused great public interest when they declared that they had achieved room temperature fusion in a simple laboratory experiment. The scientist, Stanley Pons of the University of Utah and Martin fleischmann of the University of Southampton, England, described their experiment as involving an electrochemical cell in which palladium and platinum electrodes were immersed in heavy water. They claimed that the cell produced more heat than could be accounted for by a chemical reaction alone and that they had observed certain typical fusion by product in the course of the process. According to their theory, deuterium was absorbed by the palladium electrode and fused there, releasing the extra heat.

Various laboratories around the world tried to duplicate the process, with conflicting but generally negative results. Scientists nevertheless continued to explore this possibility of “cold fusion.”

Chemical Reactor

An industrial chemical reactor is a complex device in which heat transfer, mass transfer, diffusion, and friction may occur along with chemical reaction, and it must be safe and controllable. In large vessels, questions of mixing of reactants, flow distribution, residence time distribution, and efficient utilization of the surface of porous catalysts also arise. A particular process can be dominated by one of these factors or by several of them; for example, a reactor may on occasion be predominantly a heat exchanger or a mass-transfer device. A successful commercial unit is an economic balance of all these factors.

Many successful types of reactors are illustrated throughout this section. Additional sketches may be found in other books on this topic, particularly in Walas (Chemical Process Equipment Selection and Design, Butter worths, 1990) and Ullmann (Encyclopedia of Chemical Technology (in German), vol. 3, Verlag Chemie, 1973, pp. 321–518).

The general characteristics of the main types of reactors—batch and continuous—are clear. Batch processes are suited to small production rates, to long reaction times, or to reactions where they may have superior selectivity, as in some polymerizations. They are conducted in tanks with stirring of the contents by internal impellers, gas bubbles, or Pump around. Temperature control is with internal surfaces or jackets, reflux condensers, or pump around through an exchanger.

Large daily production rates are mostly conducted in continuous equipment, either in a series of stirred tanks or in units in which some degree of plug flow is attained. Many different equipment configurations are illustrated throughout this section for reactions of liquids, gases, and solids, singly or in combinations. By showing how something has been done previously, this picture gallery may suggest how a similar new process could be implemented.

Continuous stirred tank reactors (CSTRs) are frequently employed multiply and in series. Reactants are continuously fed to the first vessel; they overflow through the others in succession, while being thoroughly mixed in each vessel. Ideally, the composition is uniform in individual vessels, but a stepped concentration gradient exists in the system as a whole. For some cases, a series of five or six vessels approximates the performance of a plug flow reactor. Instead of being in distinct vessels, the several stages of a CSTR battery can be put in a single shell. If horizontal, the multistage reactor is compartmented by vertical weirs of different heights, over which the reacting mixture cascades. When the reactants are of limited miscibilities and have a sufficient density difference, the vertical staged reactor lends itself to countercurrent operation, a real advantage with reversible reactions.

A small fluidized bed is essentially completely mixed. A large commercial fluidized bed reactor is of nearly uniform temperature, but the flow patterns consist of mixed and plug flow and in-between zones. Tubular flow reactors (TFRs) are characterized by continuous gradients of concentration in the direction of flow that approach plug flow, in contrast to the stepped gradient characteristic of the CSTR battery. They may have several pipes or tubes in parallel. The reactants are charged continuously at one end and products are removed at the other end. Normally a steady state is attained, a fact of importance for automatic control and for laboratory work. Both horizontal and vertical orientations are common. When heat transfer is needed, individual tubes are jacketed or shell-and-tube construction is used.

In the latter case the reactants may be on either the shell or the tube side. The reactant side may be filled with solid particles, either catalytic (if required) or inert, to improve heat transfer by increased turbulence or to improve interphase contact in heterogeneous reactions.

Large-diameter vessels with packing or trays may approach plug flow behavior and are widely employed. Some of the configurations in use are axial flow, radial flow, multiple shell, with built-in heat exchangers, horizontal, vertical and so on. Quasi-plug flow reactors have continuous gradients but are not quite in plug flow.

Semiflow or batch flow operations may employ a single stirred tank or a series of them. Some of the reactants are loaded into the reactors as a single charge and the remaining ones are then fed gradually. This mode of operation is especially favored when large heat effects occur and heat-transfer capability is limited, since exothermic reactions can be slowed down and endothermic rates maintained by limiting the concentrations of some of the reactants. Other situations making this sort of operation desirable occur when high concentrations may result in the formation of undesirable side products, or when one of the reactants is a gas of limited solubility so that it can be fed only at the dissolution rate.

Relative advantages and fields of application of continuous stirred and plug flow reactors may be indicated briefly. A reaction battery is a highly flexible device, although both mechanically and operationally more expensive and complex than tubular units. Relatively slow reactions are best conducted in a CSTR battery, which is usually cheaper than a single reactor for moderate production rates. The tubular reactor is especially suited to cases needing considerable heat transfer, where high pressures and very high or very low temperatures occur, and when relatively short reaction times suffice.

Sulfur

Sulfur or sulphur is a naturally occurring, yellow, water insoluble solid element. Its chemical symbol is S, its atomic number is 16 and its atomic weight is 32.064 sulfur is nonmetal and a member of the oxygen family of elements, which constitutes Group VIA of the periodic table. The discovery of sulfur predates recorded history, and the element has been used since ancient times. The early medical books of Dioscorides of Greece and Pliny the Elder mention sulfur, and fumes from burning sulfur were used in religious ceremonies and for fumigation. Alchemists recognized sulfur as a mineral substance that can be melted and burned. It was first classified as an element by Antonie Lavoisier in 1777.

Occurrence

On earth, sulfur is widely distributed in its elemental state as a secondary mineral or as a volcanic deposit, as well as in combination with a number of metals. Large sedimentary deposits of the almost pure element, mainly of Tertiary age, are found in the coastal regions of Texas and Lousiana, on the Isthmus of Tehuantepec, Mexico, and in Sicily, Sulfur occurs in some fossil fuels, most motably coal, in chemical combination with carbon and other elements, its removal is difficult. Sulfur is prevalent in the sulfide ores of iron (pyrite), zinc (sphalerite), and lead (galena). It also occurs as calcium sulfate (gypsum) and barium sulfate (barite). The magnesium and sodium sulfates are present in ocean water and in many mineral waters.

Sulfur occurs in living plant and animal tissue as part of the amino acids cysteine, cystine, and methionine. Organic sulfur compounds occur in garlic, mustard, onion, and cabbage and are responsible for the odor of skunks.