Isopropyl alcohol indirect hydration

Indirect Hydration. Indirect hydration is based on a two-step reaction of propylene and sulfuric acid. In the first step, mixed sulfate esters, primarily isopropyl hydrogen sulfate, but also diisopropyl sulfate, form. These are then hydrolyzed, forming the alcohol and sulfuric acid. 

Process. A typical indirect hydration process is presented in Eigure 1. In the process, propylene reacts with sulfuric acid (>60 wt%) in agitated reactors or absorbers at moderate (0.7—2.8 MPa (100—400 psig)) pressure. The isopropyl sulfate esters form and are maintained in the Hquid state at 20—80°C. Low propylene concentrations, ie, 50 wt %, can be tolerated, but concentrations of 65 wt % or higher are preferred to achieve high alcohol yields. Because the reaction is exothermic, internal cooling coils or external heat exchangers are used to control the temperature. 

Fig. 1. Indirect hydration process for the manufacture of isopropyl alcohol CBM = constant boiling mixture (61,62).

Isopropyl Alcohol. Propylene may be easily hydrolyzed to isopropyl alcohol. Eady commercial processes involved the use of sulfuric acid in an indirect process (100). The disadvantage was the need to reconcentrate the sulfuric acid after hydrolysis. Direct catalytic hydration of propylene to 2-propanol followed commercialization of the sulfuric acid process and eliniinated the need for acid reconcentration, thus reducing corrosion problems, energy use, and air pollution by SO2 and organic sulfur compounds. Gas-phase hydration takes place over supported oxides of tungsten at 540 K and 25... 

Indirect hydration, the traditional route, took advantage of readily available refinery grade propylene and cheap sulfuric acid in a quick two-step to isopropyl alcohol. Persistent catalysis research has now resulted in a direct route involving a small amount of an arcane catalyst, less energy intensity, high conversion rate, and an overall cheaper process. 

You may wonder why boch these processes produce isopropyl alcohol instead of (normal) propyl alcohoL With the exception of ethylene, direct or indirect hydration of an aliphatic olefin always produces an alcohol with the hydroxyl group preferentially attached to the double-bonded carbon with the least number of hydrogen atoms. 

Synthesis gas is also the precursor to MTBE via methanol. The process requires isobutylene as well. Ethyl alcohol is made by direct, catalyzed hydration of ethylene. The route to isopropyl alcohol historically used to be solely indirect hydration of propylene, which occurs at much lower pressures and temperatures than the direct method, but advances in catalysis now make the direct route competitive. 

Isopropyl alcohol (IPA) has been called the first petrochemical. Both historically and today, it is prepared by sulfuric acid-mediated indirect hydration of propylene (see equations below and Fig. 22.28). Originally it was the source of most of the acetone used in the world. Now, this route must compete with acetone derived from the cumene oxidation process, in which cumene is converted to equimolar amounts of phenol and acetone. Between 1959 and 1978 the amount of acetone derived from IPA dehydrogenation declined from 80 percent to 34 percent, and the amount of IPA used for this purpose declined from 47 percent in 1978 to 12 percent in 1990. 

Isopropyl alcohol also can be made by hydrogenation of acetone, but the large capacity for IPA by indirect hydration is a disincentive for this application. In the early 1990s there were idled IPA-to-acetone plants, with little likelihood of their being reactivated. A hydrogen peroxide process based on oxidation of IPA with molecular oxygen also produces acetone. 

---