ACETOXYLATION CATALYST BED PACKING SCHEDULE TO REDUCE HOTSPOTS AND MITIGATE THERMAL EXCURSIONS
Publication Date: 2017-Jun-23
The IP.com Prior Art Database
A concept of a ¿staged bed¿ in a Vinyl Acetate Monomer (VAM) production process is presented, whereby the reactor tube is charged with two or more different catalysts, differing in activity as well as potentially differing in size and geometry. In one embodiment, staged bed may contain a less active catalyst in the inlet region, followed by a more active catalyst further in the catalyst tube. By reducing the catalytic activity of the initial catalyst bed regions (where the reactant concentrations and reaction rates are relatively high), and increasing the activity of the later catalyst bed regions (where the reactant concentrations and reaction rates are relatively low), hot spot intensity and over-oxidation to undesired byproducts are advantageously reduced. Early catalyst deactivation is thereby also reduced. In another embodiment, catalyst particle size and geometry in the inlet region are selected in such a way as to minimize pressure drop and maximize heat removal rate in that region, thereby advantageously reducing hot spot intensity, minimizing over-oxidation, and minimizing premature catalyst deactivation.
It is common when operating a long, tubular reactor packed with catalyst, whether it be a single reactor tube, or thousands of tubes bundled into a tube sheet, to have a distinct and pronounced temperature profile along the catalyst bed. This phenomenon is particularly relevant when considering oxidation and acetoxylation reactions, where a hydrocarbon feed is mixed with oxygen at the inlet of the reactor tube, and the reaction occurs continuously as the reactants proceed along the reactor tube. In the case of oxidation processes, a unique packing “schedule” is often utilized, whereby the active catalyst is diluted with an inert material to temper the oxidation reaction heat release in certain zones of the reactor. While this approach is effective, and at times cost efficient, it is time consuming, and must be done with care to maintain reproducibility from one reactor tube to the next. Irreproducible tube packing can lead to excessive hot spots in regions of the catalyst tube and thermally induced early catalyst deactivation, particularly in large scale multi-tube commercial reactor installations.
The Vinyl Acetate Monomer (VAM) production process, which involves acetoxylation of ethylene, has historically employed the undiluted reactor tube approach, with tubes packed with catalyst of identical size, geometry and activity. This has rendered the tube packing process easier, but limits the degree of control the operator has regarding the degree and intensity of the reaction, and the associated heat release, along the reactor tube.
In the VAM production process, because ethylene, acetic acid, and oxygen are at their highest concentrations at the reactor inlet, a rapid increase in catalyst bed temperature may occur over the initial 10-25% of the reactor length. When excessive, the increased catalyst bed temperature in this region can cause over-oxidation of the feed and reaction products, leading to undesirable products, most notably COx. In addition to the associated...