【3D Process Flow】--Propylene oxide process: 50 years of evolution across three routes

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【3D Process Flow】Summary Post – Useful for understanding the process flow: https://bbs.hcbbs.com/thread-5711616-1-1.html ----------------------------------------------------------Propylene oxide (PO) is one of the most important derivatives of propylene in the chemical industry; its annual production exceeds 16 million tons. Almost all polyurethane foams around us – from mattresses to car seats – use it as a raw material. From the highly polluting chlorohydrin process of the 1960s to today’s zero-waste HPPO process, these diagrams show you the complete evolution of three generations of processing technologies behind this product. 1) Comparison of the three process routes: from high pollution to zero wastewater ； There are three main routes for producing propylene oxide: the chlorohydrin method is the traditional approach, in which chlorine and lime are used to convert propylene into PO; it is a simple process, but 40–80 tons of chlorinated wastewater are generated for every ton of PO produced ； The co-oxidation method cleverly produces PO along with styrene or MTBE, eliminating the need for chlorine but requiring the processing of the by-products ； The HPPO method uses hydrogen peroxide to directly oxidize propylene, with water as the only byproduct, making it the cleanest route available at present. The essential difference among the three generations of technology lies in the choice of oxidant. 2) Chlorohydrin method: a classic but burdensome \"chlor-alkali burden\"」 ； The core of the chlorohydrin method involves two steps: first, propylene and chlorine react in water to produce chloropropanol (this step is called hypochlorination), and then lime milk is used to saponify the mixture and remove HCl, thereby obtaining propylene oxide. The problem is that 1.5 tons of chlorine and 1 ton of lime are consumed per ton of PO, while 2 tons of CaCl₂ waste and large amounts of chlorinated organic wastewater are generated. With increasingly strict environmental regulations, the construction of new chlorohydrin production facilities has been basically prohibited, yet about 40% of China’s production capacity still relies on this old method. 3) Co-oxidation method for PO/SM co-production: Using ethylbenzene peroxides to \"leverage external forces\"」 ； The essence of the co-oxidation method is to first react ethylbenzene with air to produce ethylbenzene peroxide (EBHP), and then use this peroxide to oxidize propylene, thereby obtaining both propylene oxide and methylphenylmethanol in a single reaction – the latter of which is dehydrated to yield styrene (SM). Therefore, the technical name for this process is the PO/SM co-production method; for every 1 ton of PO produced, 2.2 tons of SM are also produced as a by-product. The advantage is that no chlorine is used and there is less wastewater ； The downside is that you need a styrene market to absorb the co-products; otherwise, it’s not cost-effective. 4) HPPO process reactor: A paradigm of \"atom economy\" catalyzed by TS-1 molecular sieve ； The core of the HPPO method is a single reaction: propylene + hydrogen peroxide → propylene oxide + water, with titanium silicalite molecular sieve TS-1 as the catalyst. The reaction takes place in a fixed-bed reactor at 40–60°C and 3 MPa, under conditions that are unbelievably mild. The pore diameter of TS-1 is exactly 0.55 nm, which allows only propylene and H₂O₂ to enter and undergo epoxidation at the Ti active sites. The atom utilization rate of this route is close to 100%, with water being the almost only by-product, making it a benchmark example for the greening of chemical manufacturing. 5) Product distillation purification: from crude PO to polymer-grade product ； Regardless of the route used, the crude PO obtained contains water, methanol solvent, unreacted propylene, and trace by-products; it must undergo multiple stages of distillation to achieve a purity level suitable for polymerization (>99.97%). The typical configuration consists of four towers in series: a propylene removal tower to recover unreacted feedstock, a methanol removal tower to recover the solvent, a crude PO tower to remove heavy components, and a refined PO tower to produce the final product. The boiling point of PO is only 34°C; distillation must be carried out under pressure or at low temperatures, and any carelessness can result in the loss of the product.