Fixed bed reactors play an important role in the production processes associated to industrial sectors such as: oil refining, petrochemicals, environmental protection, fertilizers and heterogeneous catalytic process in general. In these reactors the catalyst plays the key-role, thanks to its capacity to speed up the desired chemical reactions under relatively mild pressure and temperature conditions (at least, milder than those required when it is not present).
However, the prominent role played by the catalyst can lead us to forget about its loyal squire, the ceramic and alumina supports, whose most common form of presentation is the inert ball. Selection and arrangement of the innert support are aspects that must be carefully considered so that the catalytic bed finally performs as expected. That is why we have decided to start the publication of MERYT monthly bulletins dealing with this subject – we hope you will find it interesting!
Supports were originally introduced in fixed-bed reactors with a fundamental goal: to ensure that the catalyst hardly moves inside the reactor once it has been loaded and distributed homogeneously. By remaining fixed, both the formation of preferential flow paths and dead zones are avoided. Dead zones hardly contribute to the progress of the desired reactions and may raise the local temperature, causing the catalyst to deteriorate. In addition to that, keeping the catalyst in place helps to reduce the loss of individual particles and fine dust, which is generated by attrition.
You may consider that many substances could perform the support function effectively. However, the number of candidates goes down when the temperature of the reactor varies over a wide range (due to wide heating-cooling thermal cycles), or when sudden depressurization or rapid variations in linear velocity can take place. All of them are common events in the conversion units at refineries, for example.
The most used inert support consists of silica (SiO2) and alumina (Al2O3), or alumina only, balls with high sphericity and similar dimensions. These balls are available in various standard sizes such as: 1/8″, 1/4″, 3/8″, 1/2″, 5/8″, 3/4″, 1″, 1¼”, 1½” and 2 inches.
Inert balls display good mechanical properties (minimizing fracture and wear during loading) and have very low thermal expansion coefficients (so that both its shape and spatial arrangement remain about the same with thermal cycles); they are also chemically inert in most scenarios. In addition to that, regular cavities between the spheres (with void fractions around 0.45) help to distribute the flow evenly with low pressure drops (less than 10 millimeters of water column per 100 mm of layer)
As indicated above, inert balls can be made of silica and alumina or can also be high purity alumina in its Alpha-crystallization form. In the first case, the most common type corresponds to a Al2O3 content of 20-30% wt. (on dry basis) and a (Al2O3+SiO2) content higher than 90%. This ceramic material is composed of potassium feldspar, quartz and a specific type of clay, kaolin. Since the ceramic mixture has a relatively low sintering temperature (<1300ºC) and kaolin is a relatively abundant and cheap mineral, this type of balls is the most economical to manufacture.
At the opposite end, high purity alumina balls contain more than 99% Alpha-Al2O3. Due to their composition and high sintering temperature (1500ºC), these balls exhibit the best properties in terms of chemical resistance, resistance to thermal shock and maximum operating temperature (1650ºC), and they hardly contain any impurities. They are mainly used for the industrial synthesis of ammonia and in highly reactive olefinic processes (such as the alkylation of benzene by HF catalysis, for example).
The most important properties for inert balls are presented in table 1. Reported limits correspond to different ceramic and high alumina supports as supplied by MERYT.
Bulk density refers to the density of the inert balls once they are placed in the reactor, including their void space. This bulk value should be used to estimate the effective volume occupied by the layers of support, as opposed to the density of the ceramic material itself, which is always higher (from 2 up to 4 kg/l).
Table 1. Main properties of ceramic and alumina balls.
Water absorption is calculated based on the average wt. percentage increase for the population of ceramic balls. It is quite sensitive to the alumina content and to the specific surface of the balls; the larger the size of the balls, the lower its relative water absorption.
Acidic and alkaline resistances are usually quantified as the relative loss of weight in the support when it remains at a well-defined acidic or caustic medium for a specific time. Notice that alkaline resistance increases with alumina content.
Inert balls usually work under compression. Due to their spherical shape, they can withstand high mechanical stress. Moreover, mechanical resistance greatly increases with size.
Resistance to thermal shock can be measured by exposing the support to a given number of heating-cooling cycles and inspecting to check if any breakage has occurred. For comparison purposes, maximum and minimum temperatures should be similar, but also heating and cooling rates should be comparable. Ramps of at least 15ºC/min are common practice.
Overall, it can be concluded that the content of alumina in the ceramic support must be specified based on three key parameters: operating temperature, alkaline resistance and compressive strength. Other properties, except for water absorption, are relatively independent of alumina. All the values presented in table 1 represent high quality ceramic balls.
To summarize we would like to highlight the remarkable role that the inert support, loyal and hard worker squire, plays in maintaining the adequate disposition of the lord to whom it serves, the fixed and uncontaminated catalytic bed. Inert balls, arranged in stratified layers of different sizes, are a good option to fight against tough rivals like cyclic thermal oscillations, flow variations and possible reactor depressurizations. Higher or lower content of alumina should be selected as a function of specific temperature, pressure and corrosiveness. To finish with, it is important to emphasize that these recommendations are valid not only for catalysts support but can also be applied to adsorbents.
At MERYT we would feel very happy to discuss with you any question or inquiry you may have regarding this interesting subject of inert supports.
Best regards and see you soon!