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.