Technology

The BASF Dow process consists in the epoxidation of propylene into propylene oxide (PO) with hydrogen peroxide (HP), thus referred to as the HPPO process. Dow acquired its PO technology from its 2001 purchase of EniChem's polyurethane business. BASF had been exploring HP-based routes to PO since the mid-1990s; Dow and BASF began collaborating in 2003^[1].

The oxidation reaction depicted above^[2] is performed with a solid catalyst in a tubular reactor. Reaction in methanol as essential solvent is catalyzed by a proprietary TS-1 Zeolite arranged in a fixed bed. To reach high HP conversions at high PO selectivity in a production process without violating kinetic laws, a 2-step reactor concept is employed with a design point of about 90% HP conversion in the first step at below 90°C and 30 bar in liquid phase using methanol as solvent^[3].

Around reaction and reactors, total HPPO process scope also has to take care of propylene surplus, solvent recycle and product purification for a technical realization. Polymer grade or chemical grade propylene fulfil the process needs^[3].

• The output from the first reactor consists of methanol, water, PO, by-products, unreacted propene and HP. The yield achieved is 85% with a PO selectivity based on HP of 95%^[1].
• The output is depressurized into a column operating at atmospheric pressure, at a bottom temperature of 69 ° C. PO is distilled off together with propene and some methanol^[1].
• The bottom product is fed to a second reactor operating under the same conditions as the first reactor. After feeding additional propene, the final HP conversion realized in the finishing reactor is above 99 %^[3].
• After the epoxidation reactor(s), a first column separates the lights, mainly propene, propane, nitrogen (used as ballast to avoid the formation of flammable mixtures in the epoxidation reactor) and oxygen, the latter deriving from the decomposition of HP^[1].
• Specifically, the off-gas stream is compressed (16atm) with cooling (35 °C) and fed to a stripping column under pressure, to obtain an olefin-containing bottom stream (potentially also containing propane) and an off-gas stream with a low hydrocarbon content, containing nitrogen, oxygen and further volatile by-products^[1].
• The small Propylene surplus off-gas stream is recycled to the main expoxidation reactor after removal of Oxygen traces – for safety reasons^[1,3].
• The following is an example of the stream outlet composition from the epoxidation reactor with the subsequent separation of the light-boiling compounds: unreacted propene 0.01 mass%, formaldehyde 0.01%, acetaldehyde 0.03%, PO 9.45%, methanol 71.97%, water 17.54%, glycol ethers 0.43%, propene glycol 0.05%, heavy boilers remainder^[1]
• The crude PO product from each reaction step is removed from the reaction mixture by distillation^[1,3].
• PO is separated from methanol by extractive distillation, using water (or in another embodiment, propene glycol) as the extractors. PO is distilled overhead from the extractive column as top stream, while the bottom stream contains methanol and water^[1].
• The vapors of the top stream are compressed, and the condensation heat is returned to the vaporizer employed in the extractive distillation column^[1].
• The Methanol is recycled^[1].
• The final water stream is tested for traces of glycols and Methoxipropanols prior to being discharged to the wastewater treatment unit. For 1 tonne of propylene Oxide produced, only stoechiometric amounts (300 kg) of water would contribute to the coproduct mass balance^[3].

With optimized process parameters and TS-1 Catalyst, the overall HP conversion is 99.4%, and the overall PO selectivity is 95-96%; PO conversion in the process is nearly quantitative and the PO yield based on HP is 94-95%^[1,3].

References

1. Dr. Darrin Lew, 4^th Dec 2023, From the Dream Reaction to the Real Process the Implemented HPPO Process, Sustainable Chemistry.

2. The Essential Chemical Industry - online, 26^th Jan 2017, Propene (Propylene).

3. Bassler P., Weidenbach M. and Goebbel H., (2010), The new HPPO Process for Propylene Oxide: From Joint Development to Worldscale Production, 21, 571-576, DOI: 10.3303/CET1021096