Various gas mixtures that are used for the manufacture of chemical compounds mixtures of ammonia, methanol, ethylene, or hydrocarbons generally are sometimes referred to as synthesis gases. The term “synthesis gas” is, however, more specifically applicable to a mixture of nitrogen (N2) and hydrogen (H2) which is employed in the synthetic production of ammonia (N2 + 3 H2 = 2 NH3). In the early 1960s over 80% of this mixture was still being produced from solid fuels . At the present time, however, processes for making synthesis gas from natural gas and hydrocarbons are gaining ground. Of increasing importance, too, are gas mixtures that are used for the manufacture of hydrocarbons and alcohols: [Read more...]
Ammonia (NH3) is a colorless gas which can readily be liquefied by compression. The liquid is colorless and highly refractive and has a boiling point of 30 C. Because of its high heat of vaporization, ammonia is widely used in refrigeration. Most ammonia is produced by the Haber, or Haber-Bosch, process—first applied technically on a large scale by the BASF chemical works in Germany in 1913. In this process, hydrogen and nitrogen are combined at a temperature of 5000 C and a pressure of 200 atm. (approx. 3000 lb/in.2) according to the reaction 3 H2 + N2 = 2 NH3, which takes place in contact with a catalyst consisting of iron stabilized with aluminum oxide and potassium oxide. The initial materials are obtained from air and water, the oxygen being removed by means of carbon (in the form of coke) as a reducing agent. At elevated temperature, air is converted in a gas producer according to the following reaction:
4N2+O2+2 C =4 N2+2 CO
Whereby so called producer gas is formed. As a result of this reaction, in which air is blown through the gas producer, the coke charge in the latter is heated to incandescence. Then water vapor (steam) is passed through the incandescent coke and is decomposed into so-called water gas, a mixture of hydrogen and carbon monoxide, as follows:
C+H20 = H2+CO
As a result of passing steam through the coke, the temperature in the gas producer is lowered. The cycle can then be repeated by blowing air again, to yield producer gas and heat up-the coke, and so on. Hydrogen sulphide, which is present in the coke, is liable to contaminate the catalyst and therefore has to be removed. This is done in the gas washer by means of the alkazide method. The gas mixture consisting of nitrogen. hydrogen and carbon monoxide (i.e., the combination of producer gas and water gas alternately supplied by the gas producer), is drawn from the gas storage tank by the fan and subjected to what is referred to as a conversion process in order to get rid of the carbon monoxide, as this gas too is harmful to the catalyst used in the synthetic ammonia process. Conversion is effected with steam and catalytic contact with iron oxide and chromium oxide at 5000 C:
The cooled converted gas (N2, H2, CO2 and traces of CO) is compressed to 25 atm. (370 lb/in.2) by compressors and passed to the carbon dioxide washer, where about 99% of the carbon dioxide is removed by means of water under pressure. Remaining traces of carbon monoxide and dioxide are removed with ammoniacal copper (I) chloride solution. The gas mixture, compressed to 100 atm. (1470 lb/in.2), is passed through the washing tower. With correct adjustment of the mix proportions of producer gas and water gas, the right gas mixture for the synthetic ammonia process can be obtained, consisting of three parts of nitrogen to one part of hydrogen. This nitrogen-hydrogen mixture is passed through the contact reactor, which contains a system of heat-exchanger tubes and contact tubes in which the synthesis reaction takes place. On entering the reactor, the gas is preheated by absorbing the heat of reaction evolved from the gas mixture that has already reacted. Once the reaction has been started with the aid of an electric heating device, no heat from an outside source is needed to sustain it: it produces its own heat. Eleven percent of the gas introduced into the synthetic process is transformed into ammonia. The resulting gas mixture (NH3 and unreacted N2 and H2) is cooled with water in a tubular cooler and then further cooled in a low-temperature cooler to between —20° and —30° C. As a result of this refrigeration, liquid ammonia is formed. The low temperature in the last-mentioned cooler is obtained by evaporation of liquid ammonia diverted for this purpose from the production process. The recycle gas is returned to the contact reactor by a circulation pump, with additional fresh nitrogen-hydrogen mixture to compensate for the ammonia removed from the system.
One of the best-known pieces of apparatus used in nuclear-fusion research and in efforts to utilize this phenomenon for purposes of practical energy production is Zeta. It operates on the principle of the transformer. The primary winding is of the usual type; a condenser bank is discharged through it. The secondary winding is formed by plasma which is produced in an annular tube (torus). Before the condensers are discharged, the gas in the torus (e.g., deuterium) at a pressure of l0 mm is slightly ionized that is, made electrically conductive by the action of high-frequency electromagnetic waves. As soon as the discharge through the primary winding commences, a powerful current (up to 200,000 amps.) is induced in this plasma. The charge carriers move in circular paths parallel to the wall of the torus. These “current filaments,” like all electric currents flowing in parallel paths, attract one another: The ring of plasma, which initially fills the entire space within the torus, contracts and becomes detached from the wall. This phenomenon is called “pinch effect.” The resulting compression of the plasma is attended with a considerable rise in temperature. At the same time, the degree of ionization increases so greatly that the plasma becomes completely ionized. In this way the conditions in which nuclear-fusion processes can occur are established.
With the aid of Zeta it has proved possible to keep the compressed plasma tube, the so-called “pinch,” stable for periods of a few milliseconds and to reach temperatures of 5 million degrees centigrade. It has not yet, however, proved possible to make the fusion process self-sustaining.
Another type of apparatus is the Stellarator. In this device the containment and the heating of the plasma take place independently of each other. In a torus of double-loop shape, a magnetic field is produced by an electric current flowing through a winding. The magnetic-field strength increases with the distance from the axis to the wall of the torus. The plasma is thereby kept away from the wall. Heating is affected on the transformer principle, just as in Zeta (electrical resistance produces heat in the plasma functioning as the secondary winding). However, this phenomenon generates a temperature of only about I million degrees centigrade, because at elevated temperatures the electrical conductivity of the plasma increases and the resistance therefore decreases. Yet another method of raising the temperature to very high values is based on periodic variation of a magnetic field by means of a booster coil. An alternating current flowing through this coil produces periodic increases and decreases in the density of the magnetic lines of force in the torus (this effect is called “magnetic pumping”). By choosing an appropriate frequency it is thus possible to supply energy more particularly to the nuclei (as distinct from the electrons), so that the bremsstrahlung losses due to the electrons are kept low.
In a third type of apparatus, the containment of the plasma is achieved by a magnetic field which is stronger at the (externally open) ends than in the middle. The regions of higher field strength act as “magnetic mirrors”: They are able to reflect plasma particles. This type of configuration for the containment of plasma in controlled-thermonuclear-reaction experiments is referred to as a “magnetic bottle.” The initially cold plasma is compressed and heated by rapid intensification of the magnetic field. Temperatures exceeding 10 million degrees centigrade are thus attained in very small regions of the plasma.
Energy that is released as a result of fission of heavy atom nuclei (e.g., uranium) in atomic reactors has been utilized already for a good many years. Another possible method of nuclear energy production is by the fusion of the nuclei of the lightest chemical elements. Fusion of this kind called nuclear (or thermonuclear) fusion may occur, for example, when in a mixture of the two gases deuterium and tritium (which are heavy hydrogen isotopes indicated by the symbols 2H and 3H respectively) the atom nuclei collide with one another with sufficiently high energy (i.e., with sufficiently high relative velocity) In such circumstances the electrostatic repulsion operating between the nuclei (which are positively charged) can be overcome, so that the colliding nuclei “fuse” together. This phenomenon is accompanied by the release of individual nuclear components with high kinetic energy.
The principal processes that may occur in a mixture of deuterium and tritium are:
D+D = 3He+n+3.25 MeV 3He+D = 4He+p+ 18.3 MeV
D+D = T+p+4MeV T+D = 4He+n+17.6MeV
D denotes a deuterium nucleus (deuteron), T a tritium nucleus (triton), p a proton and n a neutron, while 3He and He denote helium nuclei with mass numbers equal to 3 and 4, respectively. The energy that is released is expressed in mega-electron volts (1 MeV = 4.45 > 10-20 kWh).
There are many other possible ways of achieving the fusion of light atom nuclei. In the sun and other stars there occur, besides the fusion of hydrogen nuclei, complex fusion processes involving the participation of heavy nuclei. It is these processes that produce the vast amounts of energy that arc constantly radiated into space by the stars. The devastating effect of a hydrogen-bomb explosion is also due to the energy released by fusion processes of this type: with an atomic bomb as a detonator, an explosive or uncontrolled chain reaction of nuclear fusion processes is initiated. “Taming the hydrogen bomb” in the sense of bringing these reactions under control in a so-called fusion reactor would place a tremendous new source of energy at man’s disposal. Using pure deuterium as “fuel,” it would be possible to produce about 1025 kWh of energy from the quantity of heavy water (deuterium oxide) estimated to be present in the natural waters of the earth.
As already stated, fusion processes take place only when nuclei of atoms collide at high velocities. To achieve these, the gas serving as fuel must be heated to such extremely high temperatures T that the average energy kT of the particles (k denotes the Boltzmann constant) is of the order of magnitude of the potential wall U0. Since the potential wall is to a certain extent “permeable” to the particles whose energy is less than Uo (quantum-mechanical tunnel effect), nuclear reactions already take place at kinetic-energy values of 10 to 100 keV. Hence, sufficiently large numbers of nuclei can react with one another in “thermal collisions,’ if kT= 100 keV i.e., T =100 million degrees centigrade. Nuclear reactions caused by thermal collisions are generally called thermonuclear reactions.
Since each fusion results in the release of several MeV of energy, it is possible in principle to achieve a positive energy balance even when only a small proportion of the nuclei react with one another. If the energy that is released can be kept together for some length of time, spontaneous heating of the gas may be initiated. At these extremely high temperatures the gas employed is completely ionized: i.e., all the atoms have been split up into freely moving electrons and “naked” nuclei. The gas has thus become completely ionized plasma. Because of this, it can be compressed into a relatively small space and be contained there by the action of a sufficiently strong magnetic field (approx. 100 kilogauss); (It is, of course, not possible to contain the plasma in any sort of material receptacle, as this would be vaporized at these high temperatures.) The escape of neutrons and radiation (more particularly the so-called “bremsstrahlung,”) inevitably causes losses of energy. A positive energy balance and therefore a continuation of the fusion processes can be achieved only if the energy of the electrically charged reaction products that are formed is able to make up for these losses. This is the case with a mixture comprising 50% deuterium and 50% tritium at temperatures of more than 50 million degrees centigrade (for pure deuterium it would require a temperature of 400 million degrees).