Industrial Practices
Ethylene oxide is produced industrially by the oxidation of ethylene with either pure oxygen or air. The primary difference between the two processes is the sequencing of absorbers and desorbers in the separation train. Ethylene and oxygen (or air) are reacted at 10-30 atmospheres and 400-500F in a fixed bed catalytic reactor. The catalyst beds consists of large bundles of many tubes that contain supported silver catalyst spheres or rings. The tubes are 6-12 meters long and 20-50 millimeters in diameter. In the oxygen process, the reactor off-gas is fed to CO2 scrubbers, then to ethylene oxide scrubbers which absorb the ethylene oxide into the liquid phase. The ethylene oxide is recovered from the liquid in a desorber and distilled to remove water. In the air process, no CO2 scrubbing is used because the gas purge of inerts (mostly N2) is sufficient to remove the CO2. The reactor operates at a low conversion, however, so unreacted ethylene is fed to a secondary fixed bed reactor, then separated. Ethylene oxide purity is typically greater than 99.5%. Figure 1 is a process flow diagram (PFD) showing combined pure oxygen- and air-based systems.
Figure 1. Simplified process plow for ethylene oxidation. [5]
Catalyst pellets are designed to favor selective oxidation (epoxidation) over total oxidation (combustion) by limiting the availability of active sites. Silver is supported on pure aluminum oxide with pore diameters ranging 0.5-50 micrometers and specific surface area less than 2 m2/g. The motivation for designing this catalyst is that a less active catalyst will promote the partial oxidation of ethylene to ethylene oxide, but it will promote neither the total oxidation of ethylene nor the subsequent oxidation of ethylene oxide.
Catalyst is operated with alkali metal promoters, usually cesium, and chlorine-containing inhibitors. Alkali metal promoters are dissolved in the catalyst by adding an alkali metal salt to the catalyst mixture during manufacture. Chlorine inhibitors are adsorbed onto the catalyst in the reactor by adding a small amount of a chlorine-containing gas, such as dichloroethane, to the reactor. The amount of chlorine added must be tightly controller, however, since high coverage of chlorine will deactivate the catalyst.
The main drawback to using silver catalyst is that, although its initial selectivity ranges 79-83%, as it ages its selectivity deteriorates, and there are no generally applicable methods of regeneration. Silver catalyst may age due to abrasion, deposition of carbon-containing compounds, and recrystallization of the silver. The life span of the catalyst is 2-5 years, and due to this limitation new technologies are being investigated. One of the most important uses of ethylene oxide is the production of ethylene glycol. So, although no new technology is available for the production of ethylene oxide, new processes are being investigated that can produce ethylene glycol directly from ethylene. Increases in the cost of the ethylene could mean that only significant increases in catalyst selectivity and life-span could maintain an economically viable ethylene oxide process.