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 Production of Isopropanol
  • Production of Isopropanol
  • The first commercial production of (IPA, or 2-propanol) by the addition of water to propene was done in 1930 by Standard Oil of New Jersey (USA). This was also the first example of the manufacture of a petrochemical from a refinery product.

    In the classical process, propene hydration with H2SO4 took place in the liquid phase. Along with this process, which is till operated today, other processes bascd on propene have also been established involving, however, a single-stage catalytic hydration in the gas, trickle, or liquid phase. Depending on the mode of operation, various acidic catalysts can be employed, such as:
    1. supported heteropoly acids or mineral acids in the gas phase,
    2. acidic ion-exchangers in the trickle phase,
    3. water-soluble heteropoly acids containing tungsten.

    The older, two-step sulfuric acid process is still operated by BP and Shell. A Deutsche Texaco nlant used this technoloey until 1986, when it was convertcd to direct hydration over an acidic ion exchanger. The H2O addition to propene in the H2SO4 process takes place indirectly via the sulfuric acid monoester. In the second step, the acid content is lowered to less than 40% by dilution with steam or water, and the ester is hydrolyzed:

    The resulting dilute H2SO4 is then concentrated. At the same time, the higher boiling organic byproducts are burned, usually with the addition of small amounts of HNO3. In commercial implementation, two modifications are common. The strong acid process, which is two-step, has separate reactors for absorption and hydrolysis. The absorption is done with 94% H2SO4 at 10- 12 bar and 20°C. The weak acid process is single stage, and is carried out in a bubble reactor with only 70% H2SO4; the pressure and temperature must be increased to 25 bar and 60-65 °C. The isopropanol selectivity reaches more than 90% and the byproducts are and acetone.

    The catalytic gas-phase hydration of propene takes place in a manner similar to ethanol manufacture from ethylene:

    In contrast to ethylene, the protonization of propene occurs much more readily during the first stage of the reaction as the resulting secondary propyl carbenium ion is more stable than the primary ethyl carbenium ion. Therefore, higher conversions are attainable with propene than with ethylene.

    In this exothermic reaction, the equilibrium is displaced towards the desired product by high pressure and low temperatures. However, the catalyst requires a certain minimum temperature to be effective, so it is not possible to benefit fully from the thermodynamic advantages of low temperature.

    Suitable catalysts include WO3/SiO2 combinations (heteropoly acids), which have been used by IG Farben. Isopropanol was manufactured from 1951 until the end of the 1970s by ICI in a unit with a capacity of 48000 tomes per year, using WO3/SiO2 with ZnO as promoter. In Germany, Hüls (formerly Veba) uses H3PO4 on a SiO2 support in a 110000 tonne-per-year (1993) plant. Typical operating conditions are 270°C and 250 bar for the ICI process, and 170-190°C and 25-45 bar in the Hüls process.

    In the gas-phase process, isopropanol selectivity is 97% and therefore higher than in the H2SO4 liquid-phase process. The workup of the aqueous solutions is conducted in a manner similar to that described below. The disadvantages of both gasphase processes are low propene conversion (5-6%) and high plant costs due to pressurized operation and gas recycles.

    Selectivity to isopropanol is 92- 94%, with 2-4% diisopropyl ether as well as alcohols of the C3H6 oligomers formed as byproducts. The catalyst life is at least 8 months, and is essentially determined by the hydrolytic degradation of the SO3H groups of the ion-exchanger.

    After removal of the low boiling substances, an isopropanol/ H2O azeotrope is distilled from the aqueous reaction product which originally contained 12- 15% isopropanol. is added to the distillate, and further distillation gives anhydrous alcohol.

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