Suspension characteristics

A study was made of the comparative stabilities at various exposures of an upset stomach remedy suspension. This product consisted of a dispersion of bismuth subsalicylate and phenylsalicylate in an aqueous system. Methyl-cellulose and magnesium aluminum silicate were selected as the suspending agents, because the presence of polyvalent metallic ions precluded the use of hydrocolloids affected by these ions. In addition, it was found that methyl-cellulose contributed a demulcent effect. The viscosity, as well as the suspension characteristics of the combination of protective colloids used, was of a synergistic nature. These colloids formed a thixotropic system. The thixotropy undoubtedly aids in stabilizing this system. In order to make this product palatable and impart elegance, color and flavor were added. Sample 1 (with protective colloids) showed no separation, while sample 2 (without... 

Refining is the most important of all the processes to which fibres are subjected, in terms of developing pulp suspension characteristics and final sheet properties, and a great deal of research has been carried out into understanding the process more fundamentally. Whilst there is still much controversy about certain aspects of the refining process and its effects upon the fibres, a number of things are widely accepted. Firstly the primary cell wall, which does not... 

Schultz, R. K., Miller, N. K., Christensen, J. D. Method for evaluation of suspension characteristics relevant to metered dose inhaler formulations. Pharm Res 7(9, suppl.) 5 - S (1990). 

Kneule and Weinspach (1967) divided suspension characteristics into two regimes. They postulated... 

Kneule and Weinspach (1967) also measured the suspension characteristics of numerous stirrer types and agitated vessels. They found the optimum stirrer diameter d, and distance from the bottom H, to be given by dT/d, = 3.0-3.5 and Hj/d, = 0.3-0.5. The optimum shapes for the vessel bottom are hemispherical and elliptical a flat vessel bottom is unsuitable for particle suspension. For a vessel with an elliptical bottom, baffles, and a propeller stirrer installed at HJd = 0.2-0.8 pumping the liquid toward the floor, the constant b in Eq. (3.22) has the value b = 3.06. For a turbine stirrer with six paddles and Hj/d, = 0.3, the value is b = 1.21. In order to keep the particles in the same material system in suspension, the propeller stirrer must therefore operate at a rotational speed (3.06/1.21)1/2 = 1.59 times higher than a turbine stirrer of the same size. 

For CFC-based suspension formulations, a surfactant was typically included. A variety of surfactants were used in these systems, e.g., lecithin, oleic acid, sorbitan trioleate. " All these surfactants were freely soluble in the CFC propellants and allowed for a degree of control over the suspension characteristics. Rates of flocculation, sedimentation, and creaming could be controlled and deposition on the internal container components was minimized. The transition to HFA-based MDIs has created significant issues in that none of the surfactants, previously used with the CFC products are soluble in HFA propellants alone. Some formulations have still used these surfactants, but the addition of a cosolvent (ethanol) has been required to solubilize the surfactant. 

In addition to HFA formulations using the traditional surfactants plus cosolvents, there has also been some research work to identify novel surfactant molecules for use with HFA propellants. As yet none of these research programs have successfully yielded molecules with suitable properties to control the suspension characteristics whilst also retaining a suitable toxicology profile. 

An interesting alternative approach to suspension formulations in HFA propellants is via engineered drug substance particles. Porous drug substance/ excipient particles can be produced via spray drying techniques. It is theoretically possible to match the density of the particles with that of the liquid propellant to produce a formulation with desirable suspension characteristics. ... 

Mass-transfer characteristics on large-scale equipment seem to be quite similar, but the fluidfoil impellers tend to release a larger-diameter bubble than is common with the radial-flow turbines. The blend time is one-half or one-third as long, and solid-suspension characteristics are better so that there have been notable improved process results with these impellers. This is particularly true if the process requires better blending and there is solid suspension. If this is not the case, the results from these impellers can be negative compared to radial-flow turbines. 

If A/S is kept constant, thus eliminating the effect of this parameter upon the removal efficiency of coagulated suspensions, one finds relationships between suspension characteristics and removal effect similar to those described for sedimentation units. 

A logical next step is the formulation of correlation functions between suspension characteristics (as presently described by a calculated collision efficiency value), specific tank parameters (as presently expressed in terms of the dispersion number, derived from tracer analysis), and solids separation... 

When different criteria for describing the suspension state are in use, these must also be taken into consideration in the formulation of the suspension characteristics. Depending on which criterion is used in the determination of the critical stirrer speed Wx , which can be represented by x for the 1-s criterion, for the particle layer height criterion h = 0.9 or by a required distribution quality the process numbers are correspondingly designated e.g. Re or Fr. ... 

Suspension Characteristics with dp as the Characteristic Particle Dimension... 

Kneule and Weinspach [284] in a comprehensive research study have derived the suspension characteristics for the l-s criterion with a number of stirrer types in baffled and unbaffled vessels with differently shaped bottoms (dished, hemispherical, truncated cone, flat) by using 2 tank sizes (D = 0.37 and 2.07 m) and different solid particles p 2630 to 11.100 kg/m dp = 1.1-10 mm) with mass fractions Pn, of 0.02 to 0.25. As an example, experimental data are presented in Fig. 5.6, which were obtained with an open double turbine (12-paddle stirrer with paddle height b of b/d = 0.5 from EKATO) in baffled tanks with dished bottoms (D = 370 mm D/d = 3.25 h/d = 0.5 circles D — 370 mm filled triangles D = 2.07 m). 

Fig. 5.6 The suspension characteristic for the 1-s criterion for a double turbine (partly covered on both sides), Type EKATO (for geometry see the original publication) in the transition and turbulent flow range from [284]... 

Lamade [315] has evaluated the suspension characteristics for three different enameled stirrers (PFAUDLER impeller, pitched-blade, paddle stirrers) in the turbulent flow range. He found a process relationship of the form ... 

Suspension Characteristic with Wss as the Characteristic Particle Property... 

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