MANUFACTURING METHODS:As described above, sulfuric acid is an important raw material for phosphate fertilizer production and to a much lesser extent for nitrogen and potassium fertilizers. World production of sulfuric acid was about 121 million tons in 1977 and about half of this production was used in the fertilizer production. About 58% of the worlds production was based on elemental Sulfur,25% on Pyrite and 17% on other sources. Of the other sources, the principal one was the by-product sulfuric acid recovered from smelting operations. In general terms, the sulfuric acid is produced by catalytic oxidation of sulfur dioxide to sulfur trioxide, which is subsequently absorbed in water to form sulfuric acid. In practice the sulfur trioxide is absorbed in sulfuric acid which is kept at acontrolled concentration (usually 98%) by the addition of water. There are no major variations of commercial interests on this mentioned chemistry. There are alternatives as to source of Sulfur dioxide and method of conversion to sulfur trioxide. The two most common methods for the conversion of sulfur dioxide to sulfuric acid are 1. 2. Lead Chamber Process: This is an old process and wasintroduced in Europe in near the middle of 18th century. This method uses nitrogen oxides as oxygen carrying catalysts for the conversion of sulfur dioxide to sulfur trioxide. The reactions which produce the sulfur trioxide and sulfuric acid take place within the huge lead chambers or in packed towers which may be substituted for the chambers. Chambers process produced acid of concentration less than 80%.The major disadvantage includes the limitations in throughput, quality and concentration of the acid produced. All known new plants uses the Contact process although some older Chamber process plants may still be in use. Contact Process: In the contact processes, the sulfur dioxide is converted to sulfur trioxide by the use of metal oxide catalyst. Platinum was once widely used as catalyst butbecause of it’s excessive first cost and susceptibility to poisoning, it has been largely replaced by vanadium oxide. The vanadium pentaoxide is dispersed on a porous carrier in a pellet form. The characterstics of the catalyst which can be used are mentioned as follows: 1. Porous carrier having large surface area, controlled pore size and resistance to process gases at high temperature; in pelletform if used in fixed bed and powdered form if used for fluidized bed. Ex- Alumina, silica gel, zeolites. Lead Chamber Process: Contact Process
Active catalytic agent: Vanadium pentaoxide in this case. Preparations are generally kept secret for the competitive reasons but they usually consist of adding water soluble compounds to gels or porous substrates and firing at temperaturebelow the sintering point. Promoter: Alkali and/or metallic compounds added in trace amounts to enhance the activity of the catalytic agent.
Advantages of the V2O5 catalyst 1. 2. Relatively immune to poisons. Low initial investment and only 5% replacement per year.
Disadvantages of V2O5 catalyst 1. Must use dilute SO2 input (7-10%). 2. As a catalyst it is less active and requires high oxygen orsulfur dioxide to give economic conversions 3. Requires larger converters and thus higher initial investment. Now the SO3 gas is passed to an absorption tower where it is absorbed in recirculating concentrated acid. There are many variations in the contact process depending upon the types of raw materials available and other considerations; also a number of engineering variations are in use bymany different design/construction firms offering services in this field. Main disadvantages of the contact process are that concentrated acid (98%) of high purity can be produced directly and that compact plants of quite high capacity have now become rather common place. THE PRODUCTION OF SULFURIC ACID BY CONTACT PROCESS: RAW MATERIALS: One of the early raw materials for the sulfuric acid was...