Ring-shape catalysts, pressure drop,

The catalyst combines two essential ingredients found in eadier catalysts, vanadium oxide and titanium dioxide, which are coated on an inert, nonporous carrier in a layer 0.02- to 2.0-mm thick (13,16). Other elements such as phosphoms are also used. Ring-shaped supports are used instead of spherical supports to give longer catalyst life, less pressure drop though the reactor, and higher yields (17,18). Half rings are even better and allow more catalyst to be loaded (18). 

Solid catalysts can be subdivided further according to the reactor chosen. Dependent on the type of reactor the optimal dimensions and shapes of the catalyst particles differ. Catalysts applied in fixed beds are relatively large particles (typically several mm in diameter) in order to avoid excessive pressure drops. Extrudates, tablets, and rings are the common shapes. Figure 3.9 shows some commonly encountered particle shapes. 

The flow pattern is primarily determined by the particle Reynolds number, which is about 100 in the industrial converter (superficial velocity 0.35-0.55 Nm/s), but in order to improve the accuracy of the comparison of different catalysts, higher flow rates are also included. Although theoretical correlations can be used for extrapolating the measured pressure drops for a new shape to the industrial operation temperature, a more reliable method is to calculate the pressure drop from industrial experience for well-known shapes, e g. 10-mm ring, and assume the same relative pressure drop as in the cold measurements. 

The simplest kind of a fixed catalyst bed is a random packing of catalyst particles in a tube. Different particle shapes are in use like spheres, cylinders, rings, flat disc pellets or crushed material of a certain sieve fraction. Mean particle diameters range from 2 to 10 mm, the minimum diameter is limited primarily by pressure drop considerations, the maximum diameter by the specific outer surface area for mass and heat transfer. 

Hie most commonly found shape of catalyst particle today is the hollow cylinder. One reason is the convenience of manufacture. In addition there are often a number of distinct process advantages in the use of ring-shaped particles, the most important being enhancement of the chemical reaction under conditions of diffusion control, the larger transverse mixing in packed bed reactors, and the possible significant reduction in pressure drop. It is remarkable (as discussed later) that the last advantage may even take the form of reduced pressure losses and an increased chemical reaction rate per unit reactor volume [11]. 

Various catalyst shapes have been developed by the individual catalyst manufacturers and have progressively replaced Raschig rings, which themselves once displaced simple tablets. The shaped catalysts are applied especially in the high heatflux zone in the upper third of the tube in the lower end of the tube there would be no significant difference between their performance and that of traditional sizes and shapes, apart from a certain reduction in the pressure drop. Examples are a four-hole type (ICI... 

Reactor Operation with Fixed Bed of Catalyst Shape and dimensions of the catalyst particles should offer minimum resistance to flow of gaseous reactants even at a slight overload operation. This wUl reduce the power required to run the air blower for the plant (catalyst beds with hollow ring type catalyst have much less pressure drop as compared to the catalyst bed with solid cylindrical pellets used earlier in sulphuric acid plants). 

The typical catalyst shapes for the low load catalyst are cylinders or balls. The hollow cylinder form (rings) is preferred to obtain a lower pressure drop with respect to balls or other shapes this lower pressure drop leads to a lower energy consumption for the air compression. 

Diffusion can also be minimized by reducing the catalyst particle size. While a viable option in experimental studies, it is of limited use in a commercial reactor because of the increase in pressure drop caused by the decrease in bed void volume. However, by careful design (radial flow, horizontal converters, etc.) most modern reactors now use considerably smaller particle sizes (1-2 mm) in the high activity/high temperature beds. The voidage can also be uncoupled from the particle size by the use of shaped supports, such as Rachig rings, trilobes, etc. 

If pressure drop through the catalyst beds (or passes as they are normally called) increases then the catalyst can be removed during a normal shut-down period and sieved before being examined and replaced for future use. Catalysts in the form of rings or other shapes have now been introduced in order to minimize pressure drop problems. A normal average life of a eatalyst in sulfurie acid plants is usually more than 10 years. 

Modem catalysts are now supplied in a variety of shapes, all with the same composition. These allow longer continuous operation, at a lower pressure drop, by distributing the dust to prevent the formation of a crast. Shapes are available as rings of various diameters, often with fluted surfaces (ribs) and simple fluted extradates. Use of any shaped catalyst can also offer more than 30% reduction in pressure drop and, often, increased activity to allow more operating flexibility. The best combination of shapes to be used in particular plants is recommended by catalyst suppliers. 

A partial solution to the low activity problem was to use shorter rings to improve heat transfer although this did, of course, this did increase the pressure drop through the tubes. Hot bands could also be avoided by using alkalized catalysts in the top part of tubes to control carbon deposition. It was clear that improved reforming catalysts were required, not only with a higher, stable activity but also, as it turned out, with a different shape. 

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