Steam activation

Further evidence pointing in the same direction was provided by Pierce, Wiley and Smith, who found that on steam activation of a particular char at 900°C the saturation uptake increased three-fold, yet the isotherm was still of Type I. They argued that even if the width of the pores was only two molecular diameters before activation, it would increase, by removal of oxides, during the activation so that the second Type I isotherm would correspond to pores more than two molecular diameters wide. (The alternative explanation, that activation produced new pores of the same width as the old, seems unlikely.)... 

Steam Active Reforming Steam-blowing process Steam ejectors Steam explosion Steam gasification Steam generator... 

The Phillips Steam Active Reforming (STAR) process catalyticaHy converts isobutane to isobutylene. The reaction is carried out with steam in tubes that are packed with catalyst and located in a furnace. The catalyst is a soHd, particulate noble metal. The presence of steam diluent reduces the partial pressure of the hydrocarbons and hydrogen present, thus shifting the equHibrium conditions for this system toward greater conversions. 

The development of microporosity during steam activation was examined by Burchell et al [23] in their studies of CFCMS monoliths. A series of CFCMS cylinders, 2.5 cm in diameter and 7.5 cm in length, were machined from a 5- cm thick plate of CFCMS manufactured from P200 fibers. The axis of the cylinders was machined perpendicular to the molding direction ( to the fibers). The cylinders were activated to bum-offs ranging from 9 to 36 % and the BET surface area and micropore size and volume determined from the Nj adsorption isotherms measured at 77 K. Samples were taken from the top and bottom of each cylinder for pore sfructure characterization. 

Table 1. Micropore sfructure development during steam activation for CFCMS...

The micropore volume varied from -0.15 to -0.35 cmVg. No clear trend was observed with respect to the spatial variation. Data for the BET surface area are shown in Fig. 14. The surface area varied from -300 to -900 mVg, again with no clear dependence upon spatial location withm the monolith. The surface area and pore volume varied by a factor -3 withm the monolith, which had a volume of -1900 cm. In contrast, the steam activated monolith exhibited similar imcropore structure variability, but in a sample with less than one fiftieth of the volume. Pore size, pore volume and surface area data are given in Table 2 for four large monoliths activated via Oj chemisorption. The data in Table 2 are mean values from samples cored from each end of the monolith. A comparison of the data m Table 1 and 2 indicates that at bum-offs -10% comparable pore volumes and surface areas are developed for both steam activation and Oj chemisorption activation, although the process time is substantially longer in the latter case. 

SEM examination of the steam activated PAN fiber monoliths showed the fiber diameter to be significantly reduced during the activation process, suggesting the fibers are consumed radially by a gasification process of the external surface [28]. 

STAR [Steam Active Re-forming] A catalytic reforming process for converting aliphatic hydrocarbons to olefins or aromatic hydrocarbons. Hydrocarbons containing five or fewer carbon atoms are converted to olefins. Those containing six or more are dehydrocy-clized to aromatic hydrocarbons. The reactions take place in the vapor phase, in a fixed catalyst bed containing a noble metal catalyst, in the presence of steam. Demonstrated on a semi-commercial scale and offered for license by Phillips Petroleum Company. The first commercial plant was built for Coastal Chemicals in Cheyenne, WY, in 1992 another for Polibutenos Argentinos in 1996. 

It is well known that the acid-base properties of the support are very important in catalyzing such transformations [262]. In order to improve the catalyst lifetime, two aspects have to be considered in modifying the support enhancing the ability to steam activation and minimizing the activity towards oligomer formation. 

The adsorbing capacity of charcoal may be greatly increased by "steam activation." The principal action of the steam passing over the very hot charcoal is to widen the pores as the result of the reaction... 

Figure 4.14 shows the SAXS plots for the ACFs prepared by C02 and steam activation. All the scattering curves have the same trend, and the main difference between them is that the intensity increases with the burn-off degree due to the development of porosity. In order to estimate the mean pore size from the SAXS data, the general approach based on Guinier equation has been used (see Equation 4.50). Table 4.3 presents the Guinier radii for the ACF and the original fiber. These values are quite similar to those obtained in a previous study done with ACF [87], Table 4.3 also contains the pore width calculated for spherical particles (see Equation 4.51). 

FIGURE 4.14 SAXS plots for ACFs prepared by (a) C02 and (b) steam activation. (From Lozano-Castello, D., et al., Studies in Surface Science and Catalysis, Characterisation of Porous Solids V, vol. 128, Elsevier Science, the Netherlands, 523-532, 2000. With permission.)... 

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