Surface area permeametry

Viscous flow permeametry measured near atmospheric pressure offers the advantages of experimental simplicity and a means of measuring the external or envelope area of a powder sample which is otherwise not readily available by any adsorption method. The usefulness of measuring the external surface area rather than the BET or total surface area becomes evident if the data is to be correlated with fluid flow through a powder bed or with the average particle size. 

Comparisons have been made between surface areas measured by porosimetry and gas adsorption as well as by permeametry with results ranging from excellent to poor. 

In many areas of powder technology the need to measure the powder volume or density often arises. For example, powder-bed porosities in permeametry, volume specific surface area, sample cell void volumes as well as numerous other calculated values all require accurately measured powder densities or specific volumes. It is appropriate, therefore, to introduce some discussion of powder density measurements. 

The concept of the surface diameter may be mostly used in the field of adsorption and reaction engineering, where the equivalent surface exposure area is important. The determination of the surface area depends on the method of measurements for example, permeametry can give a much lower area than does gas adsorption. The latter often includes the contribution of pore surface area, which is accessible to the gas molecules. The determination of particle surface area by gas adsorption is given in 1.2.2.4. The fundamentals of gas adsorption are further covered in 1.4.1. 

Effective permeability volume-specific surface the effective surface area divided by the effective solid volume, determined by permeametry. 

Permeametry Method This method is based on the fact that the flow rate of a fluid through a bed of particles depends on the pore space, the pressure drop across the bed, the fluid viscosity, dimensional factors such as the area of the bed, and specific surface area (S. The determination of permeability can be made either under continuous steady-state flow (constant flow rate) or under variable-flow (constant-volume) conditions. 

Volume shape coefficients may be determined from knowledge of the number, volume mean size, weight and density of the particles comprising a fraction graded between close limits e.g. by sieving. Further, if surface areas are also determined by permeametry, surface shape coefficients may... 

In this study we have compared surface areas measured by gas permeametry or gas adsorption with SSA calculated from PSD obtained by laser light scattering or from MIP curves for the case of a low specific surface area micronized DS (SSA in the range of 1 to 3 mVg). 

Correlation between specific surface areas measured using a simple and robust technique such as gas permeametry and SSA determined by more sophisticated techniques such as PSD, MIP and BET have been carried out. A good agreement between Sbf (surface measured by gas permeametry, Blaine Fisher, Sbf) and measured Sbet was obtained. This is due to the fact that the DS studied does not exhibit intraparticular microporosity (both Krypton and air can access all the surface developed by the powder). Sbf compared with estimated Sng (estimated from MIP results) and Spsd (estimated from PSD results) show a good linearity, but Shb and Spsd values are overestimated. This arises due to the simplifying approximations for particles shape included in the theoretical models for PSD and PIM... 

Specific surface area (SSA) by gas permeametry Measurements have been carried out using a Blaine Fisher permeabilimeter (95 Sub-Sieve-Sizer). 

All of the permeametry methods are based on the Carman-Kozeny equation given in Fig. 4 which relates the approach velocity u to the porosity of the powder e and the specific surface of the sample Sw. The specific surface calculated involves only the walls of the pores of the bed which are swept by the flow and it does not take into account the pores within the particles which do not contribute to the flow. The surface measured, therefore, is an envelope surface area and it can be very much smaller than the total surface area of the particles as measured, say, by gas adsorption. 

Particles consist of both internal and external surface area. The external surface area represents that caused by exterior topography, whereas the internal surface area measures that caused by microcracks, capillaries, and closed voids inside the particles. Since the chosen surface area technique should relate to the ultimate use of the data, not all techniques are useful for fine powders. The commonly used approaches are permeametry and gas adsorption according to the Brunauer, Emmet, and Teller (BET) equation [9]. Because of simplicity of operation and speed of operation, permeametry methods have received much attention. The permeametry apparatus consists of a chamber for placing the material to be measured and a device to force fluid to flow through the powder bed. The pressure drop and rate of flow across the powder bed are measured and related to an average particle size and surface area. Especially for porous powders, permeametry data include some internal surface area, thus decreasing their value. 

Surface Area Determination. Specific surface areas were required to determine the fraction of adsorbent surface covered by the virus at equilibrium. This in turn is required for adsorption free-energy evaluations. BET-N2 (21) methods were used where applicable and Kozeny (22) permeametry methods were used for confirmation. Values are listed in Table I. Although the values measured by the two methods are not directly comparable, trends shown by the two sets of values are similar. The BET method measures all surface accessible to adsorbing N2 molecules and generally is considered to be the most reliable... 

Permeametry ) Surface area measurement useful for particle sizes smaller than Gas adsorption) about 50 pm ... 

The Fisher sub-seive sizer employs permeametry in a relatively quick and simple technique to determine the volume specific surface area, 5, of a powder compact. In combination with the pore volume of the compact, V, the mean equivalent radius, r, of the macropores (>1000 A radius) in a dense green powder compact composed of micron-size particles can be estimated from... 

In cake filtration, where the surface volume diameter is of interest, methods for surface area determination are relevant (these measure only mean sizes) particularly, permeametry, gas diffusion and hindered settling methods. 

Alderbom G, Duberg M, Nystrom C. Studies on direct compression of tablets X. Measurement of tablet surface area by permeametry. Powder Technol 1985 41 49. 

In fluidised and fixed beds, knowledge of particle density is necessary for calculation of the effective void bed volume (Ergun equation Ergun 1952) and specific surface areas of solids by permeametry using the Kozeny-Carman equation (BS 4359 Pt 2 1982 Carman 1937). Kozeny-Carman equation... 

BS 4359 Pt 2 (1982) Determination of Specific Surface Area of Powder. Permeametry the Kozeny-Carman Equation. BSI, Gunnersbury, London, UK. 

As in the case of fixed pressure permeametry, if a series of measurements is going to be performed on one substance then the equipment can be calibrated using a powder of known surface area and then there is no need to assume a value for the factor B nor is it necessary to measure quantities Hp, Hp a, T], r, A, g,, provided they are all fixed. When all these quantities are fixed and calibration is achieved using a powder of known surface area, the simple relationship for calculation the unknown surface... 

Ober and Frederick [23] have extensively studied the use of the variable-pressure permeameter in the cement industry and recommend that the porosity function E / (1 - e) of equation 9.6 should be replaced by the function (0.850 — e). As explained when discussing the physical significance of the porosity function in the Kozeny-Carman equation, the replacing of 1 by the constant 0.850 is probably specific to the substances studied and cannot be recommended as a general procedure for all permeametry studies. From equation 9.5 it can be seen that, if the times of fall are too short or too long, the range of the permeameter can be altered by altering the cross-sectional area a of the manometer tube. It is not always possible to work at constant porosity, and for specific laboratory studies it is not always possible to have a standard powder of known surface area. In those cases, when all experimental conditions are standardized, equation 9.5 reduces to the form... 

Permeametry is widely used in the pharmaceutical industry and the technique has been found to give useful information on the assessment of surface area and sphericity of pellitized granules with good agreement with microscopy [26]. 

Allen and Maghafarti [38] used a modified Griffin apparatus to determine the changes in die measured surface area widi pressure. They found that the measured permeametry surface (Sj ) at atmospheric pressure was porosity dependent and selected the porosity for which this was a maximum for the variable pressure experiment. The volume speciflc surface (Sp) measured for BCR 70 quartz, determined using the Carman-Amell equation, remained constant at 3.654 m m-. The powder has a nominal size range of 1.2 to 20 pm and this value of Sy indicates a surface-volume mean diameter of 1.38 pm. 

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