Void-forming pellets

As an alternative to blowing agents, it is also possible to introduce lightweight void-forming pellets or prill , manufactured from ultrafine ceramic foam. These are strong, insulating, and flame resistant, with a density of 100-200 kg m . Grades (such as Tecpril, from Filtec) are stable to 1050°C and exhibit no... 

As an alternative to blowing agents, in 1980 lightweight void-forming pellets or manufactured from ultrafine ceramic foam p = 100-200 kg/m have been introduced for PP. Teepull from Filtec are stable to 1050°C, do not generate smoke or hazardous gases and resist attack by most chemicals. They are used to form foams. 

DRI can be produced in pellet, lump, or briquette form. When produced in pellets or lumps, DRI retains the shape and form of the iron oxide material fed to the DR process. The removal of oxygen from the iron oxide during direct reduction leaves voids, giving the DRI a spongy appearance when viewed through a microscope. Thus, DRI in these forms tends to have lower apparent density, greater porosity, and more specific surface area than iron ore. In the hot briquetted form it is known as hot briquetted iron (HBI). Typical physical properties of DRI forms are shown in Table 1. 

Density, bulk Ratio of weight to volume of a solid material including voids but more often refers to loose form (bulk) material such as pellets, powders, flakes, compounded molding material, etc. 

A catalyst for cracking cumene is available commercially in the form of 0.35 cm diameter pellets. These pellets have a specific surface area of 420 m2/g and a void volume of 0.42 cm3/g. If the apparent first-order rate constant for this reaction is 1.49 cm3/sec-g catalyst at 412 °C, determine the effectiveness factor of the catalyst. 

A catalytic fixed bed reactor is a (usually) cylindrical tube that is randomly filled with porous catalyst particles. These are frequently spheres or cylindrical pellets, but other shapes are also possible. The use of rings or other forms of particles with internal voids or external shaping is on the increase. During single-phase operation, a gas or liquid flows through the tube and over the catalyst particles, and reactions take place on the surfaces, both interior and exterior, of the particles. 

In a contact sulphuric acid plant the secondary converter is a tray type converter, 2.3 m in diameter with the catalyst arranged in three layers, each 0.45 m thick. The catalyst is in the form of cylindrical pellets 9.5 mm in diameter and 9.5 mm long. The void fraction is 0.35. The gas enters the converter at 675 K and leaves at 720 K. Its inlet composition is ... 

Wakao and Smith [20] originally developed the random pore model to account for the behaviour of bidisperse systems which contain both micro- and macro-pores. Many industrial catalysts, for example, when prepared in pellet form, contain not only the smaller intraparticle pores, but also larger pores consisting of the voids between compressed particles. Transport within the pellet is assumed to occur through void regions... 

To determine the effect of adsorption on the secondary porous structure of the pellets and on the external surface of the crystals on their size variation, benzene adsorption was measured. The voids or micropores of the zeolite NaA are inaccessible to benzene molecules. Adsorption occurs only on the external surface of the crystals and in the secondary porous structure formed by the gaps between the contacting zeolite crystals. In Figure 3 the relative pressure is plotted on the z-axis for better comparison. Variations in the sizes of the pellets of zeolite NaA up to p/p9 = 0.93 are negligibly small, and the points coincide with the x-axis. Consequently, adsorption on the external surface of crystals and at the sites of their contact—i.e., in transitional and macropores of the secondary porous... 

The method relies on the properties of monodispersed latex/silica spheres to assemble, through colloidal interactions, into a well-ordered, face-centered-cubic colloidal crystal upon centrifugation, sedimentation, electrophoresis, oscillatory shear or pressing in the form of pellets. Following pre-assembly of the colloidal crystal template, the precursor is infiltrated into the empty octahedral and tetrahedral interstitial sites that exist between the spheres. After conversion of the precursor to the desired material inside the voids, the template is removed leaving... 

A significant increase in catalytic activity as compared to the limiting values, shown in Figure 8.1, can be achieved by the use of bidisperse porous structures. Such catalyst pellets are formed by compressing, extruding or in some other manner compacting finely powdered mkroporous material into a pellet. Ideally the micropores are due to the porosity in the individual microparticles of catalyst. The macropores result from voids between the microparticles, after pelletization or extrusion. In such catalysts, most of the catalytic surface is contained in the micropores, since S llre. The bidisperse structure is illustrated in Figure 8.2 compared to monodisperse particle. 

Example 8-6 The mercury-penetration data given in Table 8-4 were obtained on a 0.624-g sample of a uranium dioxide pellet formed by sintering particles at 1000°C for 2 hr. Since the particles were nonporous, the void space was entirely between the... 

Tlie catalyst bed consists of the catalyst particles and the bulk gases passing through the voids of the bed. The reactants diffuse from the bulk gas to the surface of the catalyst pellet, then through its pores where it is chemisorbed and reacts forming the products which desorb and diffuse back into the bulk of the fluid. 

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