Flocculation rate measurements

The preceding treatment relates primarily to flocculation rates, while the irreversible aging of emulsions involves the coalescence of droplets, the prelude to which is the thinning of the liquid film separating the droplets. Similar theories were developed by Spielman [54] and by Honig and co-workers [55], which added hydrodynamic considerations to basic DLVO theory. A successful experimental test of these equations was made by Bernstein and co-workers [56] (see also Ref. 57). Coalescence leads eventually to separation of bulk oil phase, and a practical measure of emulsion stability is the rate of increase of the volume of this phase, V, as a function of time. A useful equation is... 

Flocculation rate limitation. The adsorption step was rate limiting for the overall flocculation process in this system. Polymer adsorption rate measurements for dispersed systems reported in the literature (2,26) do not lend themselves to direct comparisons with the present work due to lack of information on shear rates, flocculation rates, and particle and polymer sizes. Gregory (12) proposed that the adsorption and coagulation halftimes, tA and t, respectively, should be good indications of whether or not the adsorption step is expected to be rate limiting. The halftimes, tA and t, are defined as the times required to halve the initial concentrations of polymer and particles, respectively. Adsorption should not limit the flocculation rate if... 

Emulsion stability of SEDDS is usually good because the droplets are small and have narrow size distributions. However, stability can be measured by determining the flocculation rates, degree of separation, or changes in the diameter of droplets formed on dilution over time during storage under various conditions. 

The Flocculation Rate Constant. In principle Equation 1 contains all of the information to determine both v and kB from a single lightscattering experiment as has been demonstrated earlier with the clotting of fibrin( 3). However, the present turbidity measurements suffer from multiple scattering ), which makes an estimate of v from the previously proposed plot of (Mw — M0)/M0t vs. t2 unreliable. Therefore v was estimated with the catalytic constants recalculated from Gamier s data... 

Here, co is the original concentration of droplets at the beginning. The quantity d(l/c) /dt, which should be equal to kf or k f, is used as a measure of the initial flocculation rate. 

The present paper deals with kinetics of coagulation of Phthallylsulfathiazole stabilized xylene in water emulsion in the presence of some cationic detergents. Rate of flocculation, rate of coalescence and rate of creaming have been determined. To estimate the stability of the present systems their zeta potentials have been measured and stability factors calculated. Temperature effect on the system was also studied. 

Steady-state shear stress-shear rate measurements are by far the most commonly used method in many industrial laboratories. Basically, the dispersion is stored at various temperatures and the yield value and plastic viscosity r are measured at various intervals of time. Any flocculation in the formulation should be accompanied by an increase in tr and r. One rapid technique for studying the effect of temperature changes on the flocculation of a formulation is to perform temperature sweep experiments, running the samples from perhaps 5 to 50 °C. Any trend in the variation of tr and r with temperature can quickly provide an indication of the temperature range at which a dispersion will remain stable, since during that temperature range cr and r will remain constant. 

Often the van der Waals attraction is balanced by electric double-layer repulsion. An important example occurs in the flocculation of aqueous colloids. A suspension of charged particles experiences both the double-layer repulsion and dispersion attraction, and the balance between these determines the ease and hence the rate with which particles aggregate. Verwey and Overbeek [44, 45] considered the case of two colloidal spheres and calculated the net potential energy versus distance curves of the type illustrated in Fig. VI-5 for the case of 0 = 25.6 mV (i.e., 0 = k.T/e at 25°C). At low ionic strength, as measured by K (see Section V-2), the double-layer repulsion is overwhelming except at very small separations, but as k is increased, a net attraction at all distances... 

The effect of mechanical treatment on floe behavior is illustrated in Figure 5. In one work (40), identical slurries were treated with varying doses of the same polymer. At each dosage, it can be assumed that the same type of floe formed at the same rate. However, the dosage response was completely different depending on which parameter of the flocculated slurry was measured. Thus the term optimal flocculation caimot be appHed to any flocculant—substrate combination if the soHd—Hquid separation process or process parameter is not specified. 

The design of the sludge-blanket clarifiers used primarily in the water industry is based on the jar test and a simple measurement of the blanket expansion and settling rate (12). Different versions of the jar test exist, but essentially it consists of a bank of stirred beakers used as a series flocculator to optimize the flocculant addition that produces the maximum floc-setfling rate. Visual floc-size evaluation is usually included. 

Automated controls for flocciJating reagents can use a feedforward mode based on feed turbidity and feed volumetric rate, or a feed-back mode incorporating a streaming current detector on the flocculated feed. Attempts to control coagulant addition on the basis of overflow turbidity generally have been less successful. Control for pH has been accomplished by feed-forward modes on the feed pH and by feed-back modes on the basis of clarifier feedwell or external reaction tank pH. Control loops based on measurement of feedwell pH are useful for control in apphcations in which flocculated sohds are internaUy recirculated within the clarifier feedwell. 

Flowmeters These are used to measure flocculant addition, underflow, and feed flow rates. For automatic control, the more commonly used devices are magnetic flowmeters and Doppler effect flowmeters. 

Flocculation values achieved from turbidity measurements using the light scattering technique showed improvement with nonylphenol ether carboxylic acid (4 mol EO) in particular. The oil solubilization rate has been found to be proportional to the surfactant micellar size [190]. 

Studies of flow-induced coalescence are possible with the methods described here. Effects of flow conditions and emulsion properties, such as shear rate, initial droplet size, viscosity and type of surfactant can be investigated in detail. Recently developed, fast (3-10 s) [82, 83] PFG NMR methods of measuring droplet size distributions have provided nearly real-time droplet distribution curves during evolving flows such as emulsification [83], Studies of other destabilization mechanisms in emulsions such as creaming and flocculation can also be performed. 

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