Published December 2002
Epoxidation of propylene with hydrogen peroxide (HP) is regarded as an attractive route to propylene oxide (PO) as it does not produce byproducts like conventional PO technologies do, but relatively high market prices of HP have been an obstacle to the route's commercial acceptance. However, recent technologies for HP production by direct combination of hydrogen and oxygen, and integration of such HP production facility with a PO plant is claimed to offer savings in purification and shipping costs of HP, and thus in the overall PO production cost. Elimination of alkylanthraquinones and alkylanthrahydroquinones from the HP-PO process is said to be another cost lowering factor. BASF and Bayer have patented such processes. BASF reportedly is jointly developing a HP-PO process with Dow and a world-scale PO plant may come onstream as early as 2006. Another agreement for commercial collaboration was recently signed by BASF with Solvay. Degussa and Krupp Udhe have also joined their hands to work on the commercial development of a HP-PO process. Apart from the HP-PO route, a few other PO technologies producing little or no byproducts are said to be nearing commercialization. Sumitomo is reported to be building a plant in Japan, which would employ direct oxidation of propylene with an oxidation source. PO, at present, is commercially made via chlorohydrin and hydroperoxide technologies.
This Review analyses a HP-PO process patented by BASF. In the BASF's two-stage integrated approach for PO production, HP is produced by the direct combination of hydrogen and oxygen in 6 – 7 wt% concentration in an alcoholic solution, preferably methanol. The catalyst system is comprised of alternately laid corrugated and flat steel nets made from fine wires, and rolled into a cylindrical monolith. The monolith is impregnated with a palladium salt. The reactor vessel is flooded with methanol and hydrogen is bubbled in it as fine droplets from multi-leveled feed points. Enriched oxygen is fed from the bottom of reactor. Oxygen to hydrogen ratio in the reactor is within safe limits. Hydrogen conversion is 76% and selectivity to HP is 82 mol%. Water is another byproduct. Reaction conditions are 104 – 122°F (40 – 50°C) and 735 – 800 psia (50 – 54 atmosphere). Reactor temperature is maintained by external cooling of reactor contents.
In the second step, dilute HP-alcohol solution from above is introduced in a fixed-bed epoxidation reactor. Make-up propylene, recycle propylene and HP from product purification stage, are fed into the reactor. The reaction, catalyzed by titanium silicalite, takes place at 104 – 122°F (40 – 50°C) and 300 psia (20.4 atmosphere). HP per-pass conversion is initially 96% but drops down to 63% after 400 hours. PO selectivity is 95 mol%. Propylene per-pass conversion is 39.8%.
Our estimates show that a 220-million lb/yr (100,000 metric ton/yr) grassroots HP-PO plant based on the direct H2 and O2 combination technology may give capital savings of up to 16% when compared with the conventional hydroperoxidation technologies. However, operating cost of such a plant is much higher than that of latter. The new process cannot compete with chlorohydrin technologies, which have the lowest capital investment, though their operating economics are close to those of HP-PO process.