Coke porosity

The physical properties of coke can be categorized into three groups (1) the static properties considered for a lump of coke (porosity), (2) the static properties of bulk coke (specific gravity, bulk density, and size), and (3) the mechanical properties (shatter, stability and hardness, and high-temperature strength). 

Coke porosity was also studied by mercury porosimetry true (helium) and apparent (mercury) densities were measured. For the determination of the helium density a Micromeritics Autopicnometer 1320 was used. Apparent density to mercury was determined in a Carlo Erba Macropores Unit 120. 

Figure 1 shows Ihe variation of coke porosity obtained by microscopic image analysis with oxidation time for the three series of cokes. 

Fig. 1. Variation of coke porosity determined by image analysis with oxidation time of parent coal.

High Temperature Carbonization. When heated at temperatures in excess of 700°C (1290°F), low temperature chars lose their reactivity through devolatilization and also suffer a decrease in porosity. High temperature carbonization, at temperatures >900° C, is, therefore, employed for the production of coke (27). As for the low temperature processes, the tars produced in high temperature ovens are also sources of chemicals and chemical intemiediates (32). 

The catalysts used in this CCR commercial service must meet several stringent physical property requirements. A spherical particle is required so that the catalyst flows in a moving bed down through the process reactors and regenerator vessel. These spheres must be able to withstand the physical abuse of being educated and transferred by gas flow at high velocity. The catalyst particles must also have the proper physical properties, such as particle size, porosity, and poresize distribution, to achieve adequate coke combustion kinetics. 

Values of Pp and dp are droplet density, g/cm, and droplet diameter, cm Ig is the gas viscosity, P. All other terms were defined previously. Table 14-19 gives values of J calculated from experimental data of Jackson and Calvert. Values of J for most manufactured packing appear to fall in the range from 0.16 to 0.19. The low value of 0.03 for coke may be due to the porosity of the coke itself. 

A petroleum coke with round grains is available specifically for borehole cathodic protection applications The round grains ensure high porosity and enable gas to escape, allowing the coke to sink to the base of the borehole. This material hjis a higher bulk density than petroleum coke (1 185 kgm" ) which enables it to sink to the bottom of the borehole, yet a lower fixed carbon content (93%), with higher ash (2-06%) and sulphur (5-3%) contents. The resistivity of this material is quoted as 0-1 ohmm. 

Porosity It should be porous as this facilitates oxygen contact with the carbon of coke. A factor most important for accomplishing complete combustion at a high rate. 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

Varying KOH ratio in the mixture is a very effective way of controlling porosity development in resultant activated carbons. The trend in the pore volume and BET surface area increase seems to be similar for various precursors (Fig. la). It is interesting to note, however, a sharp widening of pores, resulting in clearly mesoporous texture, when a large excess of KOH is used in reaction with coal semi-coke (Fig. lb). Increase in the reaction temperature within 600-900°C results in a strong development... 

The treatment of semi-coke or mesophase (3 1 mixture) for 2 h at 600°C gives a material of pore volume VT about 0.7 cm3/g and surface area Sbet about 1700 m2/g which is typically microporous with rather narrow micropores (average size LD below 1.2 nm). Activated carbons produced within the temperature range of 700-800°C have fairly similar porosity characteristics, VT about 1 cm3/g and Sbet near 2500 m2/g. It is interesting to note that within the wide temperature range of 600-800°C the bum-off is at a reasonably low level of 20-23 wt%. 

Even if 5A zeolite is widely used in iso-paraffin separation from an n/iso paraffin mixture, the adsorbent is affected by a slow deactivation mainly due to coke formation inside the molecular sieve porosity. Its aging phenomenon decreases its sorption properties. According to previous studies, 5A zeolite deactivation results essentially from heavy carbonaceous compound formation in a-cages blocking the 5A zeolite microporosity [1-2]. 

Propylene cokage experiments followed by gravimetry have shown that higher is the 5A zeolite calcium content, higher are the cokage kinetics and carbon content inside the pores (Fig. 1). The total carbon contents retained in the porosity after desorption at 350°C of physisorbed propylene are 14.5% and 11% for 5A 86 and 5A 67 samples respectively. These carbon contents are relatively important and probably come from the formation of heavy carbonaceous molecules (coke) as it has been observed by several authors [1-2], The coke formation requires acid protonic sites which seems to be present in both samples but in more important quantity for the highly Ca-exchanged one (5A 86). 

It is seen that the intracrystalline MgO induces pore blockage in a fraction of the pore system and alters the porosity as well as D0, and/or r with the latter factors contributing most to the reduced diffusivity. In contrast, the coke modifier appears to affect mainly the surface-to-volume ratio and suggests that the effective surface area, number of available entrance ports, is reduced by two orders of magnitude. 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

Another coke formed in a FCC unit is occluded or residual coke. In a commercial unit this coke corresponds to coke formed on catalyst porosity and its content depends on textural properties of the catalyst (pore volume and pore size distribution) and the stripping system capacity in the reaction section. Finally on the FCC catalyst rests some high-molecular weight of nonvaporized hydrocarbons. These molecules do not vaporize or react at the reactor conditions and accumulate in the catalyst pores like a soft carbonaceous residue with high hydrogen content. 

Production. Silicon is typically produced in a three-electrode, a-c submerged electric arc furnace by the carbothermic reduction of silicon dioxide (quartz) with carbonaceous reducing agents. The reductants consist of a mixture of coal (qv), charcoal, petroleum coke, and wood chips. Petroleum coke, if used, accounts for less than 10% of the total carbon requirements. Low ash bituminous coal, having a fixed carbon content of 55—70% and ash content of <4%, provides a majority of the required carbon. Typical carbon contribution is 65%. Charcoal, as a reductant, is highly reactive and varies in fixed carbon from 70—92%. Wood chips are added to the reductant mix to increase the raw material mix porosity, which improves the SiO (g) to solid carbon reaction. Silica is added to the furnace in the form of quartz, quartzite, or gravel. The key quartz requirements are friability and thermal stability. Depending on the desired silicon quality, the total oxide impurities in quartz may vary from 0.5—1%. 

---