ABSTRACT
In this book the chemical plant is focused upon. Therefore, the present chapter emphasizes chemical reactors for the chemical process industry. But it should be made clear that structured packings and catalysts also have a large potential in consumer products. Chemical reactors form the heart of a (petro-)chemicals production plant. Given the large variety of plants it is no surprise that a wide variety of chemical reactors are used. Catalytic reactors can be roughly divided into random and structured reactors. It is useful to start with a summary of the major basic concerns (apart from high activity, selectivity, etc.) for catalytic reactors:
Catalyst quality on a microscopic length scale (quality, number of active sites)
Catalyst quality on a mesoscopic length scale (diffusion length, loading, profiles)
Ease of catalyst separation and handling
Heat supply and removal
Hydrodynamics (regimes, controllability, predictability)
Transport resistance (rate and selectivity)
Safety and environmental aspects (runaways, hazardous materials, selectivity)
Costs
On each of these, random and structured reactors behave quite differently. In terms of costs and catalyst loading, random packed-bed reactors usually are most favorable. So why would one use structured reactors? As will become clear, in many of the concerns listed, structured reactors are to be preferred. Precision in catalytic processes is the basis for process improvement. It does not make sense to develop the best possible catalyst and to use it in an unsatisfactory reactor. Both the catalyst and the reactor should be close to perfect. Random packed beds do not fulfill this requirement. They are not homogeneous, because maldistributions always occur; at the reactor wall these are unavoidable, originating form the looser packing there. These maldistributions lead to nonuniform flow and concentration profiles, and even hot spots can arise (1). A similar analysis holds
for slurry reactors. For instance, in a mechanically stirred tank reactor the mixing intensity is highly non-uniform and conditions exist where only a relatively small annulus around the tip of the stirrer is an effective reaction space.
