Catalyst solid pellet

The analogy is a three-dimensional pile of coal, catalyst, plastic pellets, soil particles, or other solid material. It could be in the field, plant, or lab. From each of these perspectives, consider how you would physically obtain several randomly chosen individual blocks from this group of 27. How would you access material in the middle, realizing that the material readily accessible on the top or outside may not be representative How would you define a correct sample and extract it Next, reduce the sampling dimension to 2 or 1. Define and extract correct samples. How well would this work in situations you encounter Review your methods and logic after segregating the blocks by color from top to bottom or using some other pattern. 

Fig. 10.1.3 [Kop3] Moisture transport in cylindrical catalyst support pellets from alumina with a diameter of 3.5 mm. (a) Drying profiles of water along the diameter of an initially wet piellet. Dotted lines Experimentally detected profiles. Solid lines simulated profiles. The first profile was acquired when the dry gas flow was turned on. The delay between the detection of the successive profiles is 60s. Profiles 1-7,9, 11,14, and 18 are shown, (b) Experimental profiles for water vapour sorption by an initially dry pellet containing CaC with uniform salt distribution.

Many products come in the form of a powdered solid. The solid once produced is stored in a container. It may be a barrel, a bag, or a can depending on the volume. Powdered milk is a good example so is lawn fertilizer. Catalytic solids are another. Catalysts promote the rate of chemical reaction and are used throughout the chemical and petroleum industries they are usually small solid pellets of uniform size and shape. Catalysts are not consumed in the course of the reaction they promote. Nevertheless, catalysts do eventually need to be replaced. This is either because they were poisoned or their sohd structures have become clogged with high molecular weight molecules that prevent access to the active sites. At the end of its lifetime, then, the catalyst must be replaced. The spent catalyst is removed from the reactor vessel, the reactor is cleaned, and the space left open is ready for a charge of fresh solid catalyst. 

Fluid-solid contacting towers such as a packed bed or a fluidised bed are used for carrying out two-phase catalytic reactions. Packed bed reactors are used if the catalyst is in the form of solid pellets, where as fluidised bed reactors are used if the catalyst is solid particles. 

Reaction steps occurring on a gas-solid interface at the microscopic level basically involve adsorption, surface reactions, desorption, and diffusion steps. These processes are briefly summarized in Fig. 2.3. The processes are exemplarily shown on a catalyst with pellet structure. 

However, before extrapolating the arguments from the gross patterns through the reactor for homogeneous reactions to solid-catalyzed reactions, it must be recognized that in catalytic reactions the fluid in the interior of catalyst pellets may diSer from the main body of fluid. The local inhomogeneities caused by lowered reactant concentration within the catalyst pellets result in a product distribution different from that which would otherwise be observed. 

When a model is based on a picture of an interconnected network of pores of finite size, the question arises whether it may be assumed that the composition of the gas in the pores can be represented adequately by a smooth function of position in the medium. This is always true in the dusty gas model, where the solid material is regarded as dispersed on a molecular scale in the gas, but Is by no means necessarily so when the pores are pictured more realistically, and may be long compared with gaseous mean free paths. To see this, consider a reactive catalyst pellet with Long non-branching pores. The composition at a point within a given pore is... 

Sohd Catalysts Processes with solid catalysts are affected by diffusion of heat and mass (1) within the pores of the pellet, (2) between the fluid and the particle, and (3) axially and radially within the packed bed. Criteria in terms of various dimensionless groups have been developed to tell when these effects are appreciable. They are discussed by Mears (Ind. Eng. Chem. Proc. Des. Devel., 10, 541-547 [1971] Jnd. Eng. Chem. Fund., 15, 20-23 [1976]) and Satterfield (Heterogeneous Cataly.sls in Practice, McGraw-Hill, 1991, p. 491). 

Physical Characteristics With a few exceptions, solid catalysts are employed as porous pellets in a fixed or flmdized bed. Their physical characteristics of major importance are as follows. 

These reactors for hquids and liquids plus gases employ small particles in the range of 0.05 to 1.0 mm (0.0020 to 0.039 in), the minimum size hmited by filterability. Small diameters are used to provide as large an interface as possible since the internal surface of porous pellets is poorly accessible to the hquid phase. Solids concentrations up to 10 percent by volume can be handled. In hydrogenation of oils with Ni catalyst, however, the sohds content is about 0.5 percent, and in the manufacture of hydroxylamine phosphate with Pd-C it is 0.05 percent. Fischer-Tropsch slurry reac tors have been tested with concentrations of 10 to 950 g catalyst/L (0.624 to 59.3 IbiTi/fF) (Satterfield and Huff, Chem. Eng. Sci., 35, 195 [1980]). 

Consider a gaseous reactant flowing through a bed of solid catalyst pellets. The physical steps involved are the transfer of the component... 

Table 4-4 summarizes the ratings of the various reactors. The CFSTR and the recirculating transport reactor are the best choices because they are satisfactory in every category except for construction. The stirred batch and contained solid reactors are satisfactory if the catalyst under study does not decay. If the system is not limited by internal diffusion in the catalyst pellet, larger pellets could be used and the stirred-contained solids reactor is the better choice. However,... 

The reactor volume is calculated from Mj and the bulk density of the catalyst material, (-r ) depends not only on composition and temperature, but also on the nature and size of the catalyst pellets and the flow velocity of the mixture. In a heterogeneous reaction where a solid catalyst is used, the reactor load is often determined by the term space velocity, SV. This is defined as the volumetric flow at the inlet of the reactor divided by the reaction volume (or the total mass of catalyst), that is... 

The three principal catalyst bed configurations are the pellet bed, the monolith, and the metallic wire meshes. An open structure with large openings is needed to fulfill the requirement of a low pressure drop even at the very high space velocities of 200,000 hr-1. On the other hand, packings with small diameters would provide more external surface area to fulfill the requirement for rapid mass transfer from the g .s stream to the solid surface. The compromise between these two ideals results in a rather narrow range of dimensions pellets are from to 1 in. in diameter, monoliths have 6 to 20 channels/in., and metallic meshes have diameters of about 0.004 to 0.03 in. 

The kinetics of a mixed platinum and base metal oxide catalyst should have complementary features, and would avoid some of the reactor instability problems here. The only stirred tank reactor for a solid-gas reaction is the whirling basket reactor of Carberry, and is not adaptable for automotive use (84) A very shallow pellet bed and a recycle reactor may approach the stirred tank reactor sufficiently to offer some interest. 

Equation (15) is derived under the assumption that the amount of adsorbed component transferred by flow or diffusion of the solid phase may be neglected. This assumption is clearly justified in cases of fixed-bed operation, and it is believed to be permissible in many cases of slurries or fluidized beds, since the absolute amount of adsorbed component will probably be quite low due to its low diffusivity in the interior of the catalyst pellet. The assumption can, however, be waived by including in Eq. (15) the appropriate diffusive and convective terms. 

Figure 9.21. (a) Dependence of selectivity on C2H2 conversion for various catalyst potentials. The behaviour is compared with this of BASF catalysts (b) Dependence of C2H2 conversion (closed symbols, solid lines) and selectivity (open symbols, dashed lines) on flowrate and GHSV for various catalyst potentials. 3-pellet configuration.30 Reprinted with permission from the Institute for Ionics. 

Two types of continuous flow solid oxide cell reactors are typically used in electrochemical promotion experiments. The single chamber reactor depicted in Fig. B.l is made of a quartz tube closed at one end. The open end of the tube is mounted on a stainless steel cap, which has provisions for the introduction of reactants and removal of products as well as for the insertion of a thermocouple and connecting wires to the electrodes of the cell. A solid electrolyte disk, with three porous electrodes deposited on it, is appropriately clamped inside the reactor. Au wires are normally used to connect the catalyst-working electrode as well as the two Au auxiliary electrodes with the external circuit. These wires are mechanically pressed onto the corresponding electrodes, using an appropriate ceramic holder. A thermocouple, inserted in a closed-end quartz tube is used to measure the temperature of the solid electrolyte pellet. 

As in the case of homogeneous acids as catalyst, we would also benefit from using solid ba.ses instead of dissolved bases as catalyst. A number of industrially important reactions are carried out with bases as catalyst. A well know example is the aldol condensation of acetone to diacetone alcohol, where dissolved NaOH in ethyl alcohol is u.sed as a catalyst at about 200 to 300 ppm level. However, heterogeneous pelleted sodamide can be used as a catalyst for this reaction and it obviates the problem of alkali removal from the product, which would otherwise lead to reversion of diacetone alcohol to acetone during distillation of the product mixture. 

The flow pattern of fluids in gas-liquid-solid (catalyst) reactors is often far from ideal. Special care must be taken to avoid by-passing of the catalyst particles near the reactor walls, where the packing density of the catalyst pellets is lower than in the centre of the bed. By-passing becomes negligible if the ratio of reactor to particles diameter is larger than 10 a ratio of 20 is recommended. Flow maldistributions might be serious in the case of shallow beds. Special devices must be used to equalize the velocity over the cross-section of the reactor before reactants are introduced onto the catalyst bed. 

Our very first experiments with the reactor depicted in Figure 5.4.1 were carried out with a 15% Pt-Y-Al203 single cylindrical catalyst pellet [10-12], The acquisition time of 2D images of an axial slice at that time was about 260 s. Despite this, the first direct MRI visualization of the operation of a model gas-liquid-solid reactor has revealed the existence of large gradients of the liquid phase content within the catalyst pellet upon imbibition of liquid a-methylstyrene (AMS) under conditions... 

This section discusses the techniques used to characterize the physical properties of solid catalysts. In industrial practice, the chemical engineer who anticipates the use of these catalysts in developing new or improved processes must effectively combine theoretical models, physical measurements, and empirical information on the behavior of catalysts manufactured in similar ways in order to be able to predict how these materials will behave. The complex models are beyond the scope of this text, but the principles involved are readily illustrated by the simplest model. This model requires the specific surface area, the void volume per gram, and the gross geometric properties of the catalyst pellet as input. 

Diffusion Poisons. This phenomenon is closely akin to catalyst fouling. Blockage of pore mouths prevents full use of the interior surface area of the pellets Entrained dust par ticles or materials that can react on the catalyst to yield a solid residue give rise to this type of poisoning. 

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