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Catalyst Preparation Techniques and Equipment

1.1 Introduction

A catalyst is made up of its constituent components that are one or more active phases held together with one or more binder components. The components of a catalyst are initially in the form of loose powders. The forming of a catalyst entails using powders that serve as building blocks and making the catalyst hold together as a shaped particle. Excellent textbooks by Stiles [1] and Le Page [2] cover catalyst formation and show the typical equipment, methods, and materials used commercially. Adding catalytic components inside a particle by impregnation or by ion exchange with solutions of metals followed by using appropriate drying and calcination procedures makes catalysts useful for many processes.

The science and experience of impregnation, drying, and calcination is a vast area of knowledge represented by many excellent papers. Cited here, among a wealth of research on the subject, will be works by Neimark et al. [3], Chester and Derouane [4], Marceau et al. [5], Lekhal et al. [6, 7], and de Jong [8]. The Rutgers Catalyst Consortium headed by Professor Benjamin Glasser has been investigating manufacturing fundamentals for the last two decades. Many of their published contributions yield a solid basis for applied methods and numerical models for the many facets of catalyst manufacturing.

In the area of catalyst impregnation, I will cite Chester et al. [9], Chester and Muzzio [10], Liu et al. [11], Shen et al. [12], Romanski et al. [13-15], and Koynov et al. [16]. On the subject of metal profiles in catalyst particles, I will mention Kresge et al. [17] and Liu et al. [18-20]. For rotary calcination, I will mention Chaudhuri et al. [21, 22], Gao et al. [23], Paredes et al. [24], Emady et al. [25], and Yohannes et al. [26, 27]. Last but not least, this book cites a very interesting historical perspective of the many pioneers in heterogeneous catalysis by Davis and Hettinger [28].

The common structural properties of interest for catalysts include surface area, porosity, pore size distribution, particle shape, and particle size. A wonderful textbook in this area is by Thomas and Thomas [29].

Part of the catalyst-forming technology includes the development of the mechanical strength of the catalyst. This aspect is sometimes overlooked initially, but should not be underestimated or forgotten and taken for granted. Insufficient mechanical strength requires the changing or adjusting of recipes in order to yield the strength needed for handling and subsequent use. Changing recipes at the end of the game always brings the risk of an unforeseen detrimental impact on catalyst activity.

Catalysts come in many shapes, and the processes that allow their manufacture include extrusion, spray drying, bead forming by dripping, bead forming in granulation pans, fluid bed granulation, spheronization, and pelletizing. The process of use in the chemical and petrochemical industries dictates the size and the shape of the catalyst, while the manufacturing cost often dictates the choice of the catalyst-forming process. Extrusion and spray drying, Masters [30], yield economic manufacturing solutions when large quantities of catalyst are required, such as in the petrochemical industry.

In the automotive industry, monolith-type extruded catalysts are well established for their low pressure drop and hydrothermal stability properties. Monolith manufacturing processes have been described in the open literature and are available in textbooks, including those of Cybulski and Moulijn [31] and Satterfield [32].

Beyond the forming of a catalyst particle, many more operations are required to make a final catalyst. For instance, after the forming, drying followed by calcination at high temperatures set the matrix of a catalyst and yield the basic strength properties of the catalyst. Thereafter, the catalyst may require metal components to be introduced into its structure, followed by drying and calcination.

Catalytic metal impregnation, drying, and calcination can lead to further breakage of catalyst extrudates and a loss of catalyst and catalyst quality. Bringing an optimal recipe to fruition may require sacrificing many thousands of pounds of material together with precious plant manufacturing time.

Catalyst manufacturing plants are built with a combination of different pieces of production equipment in mind. The layout and the design of the production path needs to allow for product quality assurance and product quality control. Usually, new plants are designed based on experience with existing plants, but there is little guidance from first-principles methods based on the science of the phenomenon of breakage by collision or by stress in a fixed bed of catalyst. Often, a reference catalyst is tested in a piece of equipment and the decision to go ahead with that equipment or to change it depends on the outcome of that test. Engineers and chemists with many years of experience in the catalyst manufacturing industry are valuable resources when it comes to designing new plants or modifying existing ones.

1.1.1 What are Catalysts?

Catalysts are solid bodies with a porous structure that allows for the fantastic ingress of reagents and egress of products. This accessibility exposes the bulk fluid to enormous surface areas and densities of the catalytic sites located inside these catalytic bodies. Maximizing surface area and accessibility is always one of the goals, but it is necessary to balance this goal with the mechanical strength of the catalyst and the cost of the catalyst in order to obtain a commercially viable product.

Catalysts come in all shapes and sizes, from the micron scale in fluid catalytic cracking to the millimeter scale in hydrotreating applications to the meter scale for monoliths and coated catalysts in coal- and gas-fired applications. Catalysts are loaded in reactors as a fixed bed or as a moving bed, or are dispersed in and travel with the bulk fluid. It is important to consider the process pressure drop in reactor designs when it comes to catalyst selection.

Most if not all catalysts deactivate over time and have a limited life span that can range widely from fractions of a second to several years, depending on the application. Catalyst deactivation results in a loss in terms of conversion and/or selectivity, and also hampers throughput, which requires either a catalyst change-out or an in situ regeneration.

Catalyst abrasion and attrition are also factors that have a direct impact on both their practical use and economics.

Fouling of catalyst beds by scale also requires engineering solutions due to pressure drop considerations.

1.1.2 Catalyst Composition

Catalysts transform reagents into products, and catalyst activity dictates the rate of this transformation. It is often one of our objectives to let the products escape readily from the catalyst without undergoing secondary reactions, which enhances the selectivity of the catalyst. Needless to say, catalyst compositions are heavily guarded by patents and trade secrets. Although this book only mentions generic compositions, there are very good textbooks by Stiles [33] and Satterfield [32] that give a rich flavor of the many possibilities in catalyst composition.

A catalyst may consist of one or more active phases and one or more binders. The active phase is often ?-alumina (aluminum oxide) because of its stability, large surface area, and porosity. Other active phases include silica or zeolite (silicon oxide or crystalline silica alumina). Binders can vary, but may be alumina or silica. Base metals include cobalt, nickel, molybdenum, etc., while noble metal catalysts often include platinum or palladium.

1.2 Forming of Catalysts

1.2.1 Catalysts Formed by Extrusion

1.2.1.1 Typical Materials

Standard formulations for catalysts entail adding an appropriate amount of binder to an active material and an appropriate amount of liquid (often water). The sequence of the addition of the components can be an important factor for certain catalysts.

1.2.1.2 Mixing, Mulling, Granulation, and Kneading

Mixing, mulling, granulation, and kneading occur before the extrusion of powders. It is necessary to blend powders and "work" them with a liquid - typically water - and to bring them to the desired rheological consistency before extrusion. The recipes are ad hoc, and it is difficult to predict which combination will bring success. A term often used when preparing a batch for extrusion is the "amount of work" put into a batch during mulling. Because this phrase has different meanings, it can lead to miscommunication among crews or teams. This process goes together with what is called the "peptization" of a batch, and both essentially go hand in hand when judging whether a batch is ready to extrude.

The appearance and response of a catalyst mixture in a batch will differ from one technique to another. Mill wheels will essentially go through stages: mixing and mulling followed by granulation and kneading. Mixing refers to both the mixing of dry powders and, later, the mixing of liquids. Mulling reduces the...