Surface area reference materials

To control their products and to calibrate their instruments many manufacturers of powders or porous materials produce standardised materials for their own needs (ref. 5). For public use such reference materials are certified, stored and offered by national and international standardisation institutions and by some commercial distributors (Tab. 1). Disregarding the difficulties in storage a variety of particle size and surface area reference materials are available (Tab. 2) and more are under consideration. Lists of the many industrial materials suitable for tests are included in ref. 9. The Institute for Inorganic and Analytical Chemistry of the University Mainz has developed single crystals of zeolites and aluminophosphates which may be used as reference materials in the micropore range (ref. 6). 

The validity of the simplifying assumption is usually within the variance of surface area determinations but needs to be checked for the particular system either by calibration against the standard BET procedure or by using surface area reference samples of the same material (see Section 11.2.1.6.B). 

Thus the key parameters influencing the corrosion rate under deposits will be the deposit porosity, which determines the available surface area of material for corrosion, and the deposit tortuosity, which along with porosity will modify the fluxes of diffusing species within pores. Readers interested in a more extensive discussion are referred to other sources (17). Here we concentrate on a brief discussion of electrochemical methods of investigating the properties of deposits as a basis for eventual modeling. 

Activated charcoal is a finely divided form of amorphous carbon and is manufactured from organic materials (e.g. peat, wood) by heating in the presence of reagents that promote both oxidation and dehydration. Activated charcoal possesses a pore structure with a large internal surface area microporous materials exhibit pores <2nm wide, macroporous refers to activated charcoals with a pore size >50nm, and mesoporous materials fall in between these extremes. The largest internal surface areas are found for microporous materials (>700m g ). The ability of the hydrophobic surface to adsorb small molecules is the key to the widespread applications of activated charcoal. (Comparisons should be made with the porous structures and applications of zeolites see Sections 13.9 and 26.6.)... 

Brunauer, Emmett, and Teller (BET method) measures surface area of material based on the adsorption of gaseons nitrogen at the temperature of liquid nitrogea For a full information how this method is carried out, one may refer to ISO 5794/1 Annex D or ASTM C1274-12. 

Striking confirmation of the conclusion that the BET area derived from a Type IV isotherm is indeed equal to the specific surface is afforded by a recent study of a mesoporous silica, Gasil I, undertaken by Havard and Wilson. This material, having been extensively characterized, had already been adopted as a standard adsorbent for surface area determination (cf. Section 2.12). The nitrogen isotherm was of Type IV with a well defined hysteresis loop, which closed at a point below saturation (cf. F, in Fig. 3.1). The BET area calculated from it was 290 5 0 9 m g , in excellent agreement with the value 291 m g obtained from the slope of the initial region of the plot (based on silica TK800 as reference cf. p. 93). 

Example 4 Calculation of Sample Weight for Surface Moisture Content An example is given with reference to material with minimal internal or pore-retained moisture such as mineral concentrates wherein physically adhering moisture is the sole consideration. With this simphfication, a moisture coefficient K is employed as miiltipher of nominal top-size particle size d taken to the third power to account for surface area. Adapting fundamental sampling theory to moisture sampling, variance is of a minimum sample quantity is expressed as... 

RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

Refer to catalogues or websites of catalyst support producers and compile an overview of commercially available support materials and the range of surface areas per gram in which these are available. 

Chemical deactivation. In chemical deactivation the active surface area changes by strong chemisorption of impurities in the feed, by blocking of active sites by heavy products formed in parallel or sequential reactions, etc. The most important chemical causes of deactivation are poisoning by impurities in the feed and deposition of carbonaceous material, usually referred to as coke . 

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