This column is based on “Propylene Oxide from Propylene and Chlorine – Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/propylene-oxide-e11a.
Propylene oxide (PO) is a highly reactive commodity compound that serves as a starting material for several widely used products, from polymers and solvents to industrial fluids. Historically, PO has been produced by the so-called chlorohydrin process, which is further described in this article. Currently, new production technologies that generate less waste are starting to replace the chlorohydrin production process.
The following describes a typical chlorohydrin process for PO production from propylene and chlorine (Figure 1).
Hypochlorination. Initially, fresh, gaseous polymer-grade propylene and chlorine are mixed with water to form an aqueous solution. Such compounds react to produce a propene-chloronium complex, an intermediate that reacts with water to yield hydrochloric acid and propylene chlorohydrin (PCH) isomers. The gaseous effluent from the reactor is fed to a separator, which separates a solution containing PCH from the vent gas. The PCH solution is directed to the epoxidation stage, while the vent gas is passed through a caustic scrubber and released.
Epoxidation. The PCH solution and a caustic solution (from a chlor-alkali unit) are fed to a saponifier, where a dehydrochlorination reaction takes place. Here, the PCH is converted to propylene oxide, while organic impurities are stripped from the brine. The brine from the saponifier bottom is directed to a treatment step, while a stream containing PO from the saponifier overhead is routed to purification steps.
Purification. The PO-rich stream is distilled in a first column to obtain a crude propylene oxide stream, which is removed from the column overhead. A solution containing dichloropropane (DCP) byproduct from the column bottom is decanted, yielding a DCP-rich stream and water. In a second column, residual light ends are stripped off from the crude PO stream and burned for fuel. The column’s bottom product is fed to a third column, which further removes impurities from the PO. A 99.9 wt.% PO, withdrawn from column overhead, is condensed and routed to storage facilities. Heavy ends from the column bottom are burned for fuel.
The DCP-rich stream is treated with an acid-caustic-water wash, for the neutralization and removal of epoxidation byproducts, and then distilled for the removal of heavy components.
The brine stream from the epoxidation step and the wastewater from the first column are treated by biodegradation and filtration. The treated brine is returned to the chlor-alkali facility.
There are two main routes for PO production — one based on the dehydrochlorination of propylene chlorohydrin with a base, and the other based on the oxidation of propylene by an organic hydroperoxide (Figure 2).
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce propylene oxide was about $2,360 per ton of propylene oxide in the second quarter of 2014. The analysis was based on a plant constructed in the U.S. with capacity to produce 500,000 metric ton per year of propylene oxide.
Edited by Scott Jenkins
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