Emulsion aging effects

Chen, J., Dickinson, E. (1999a). Interfacial ageing effect on the rheology of heat-set protein emulsion gel. Food Hydrocolloids, 13, 363-369. 

Monitoring Emulsion Aging. The surfactants used in transport emulsions may gradually lose their ability to stabilize the oil droplets. As the oil droplets coalesce, a two-phase mixture is formed, and it remains pumpable with no significant change in effective viscosity. This process is referred to as emulsion failure. An alternative to this process is inversion of the emulsion, in which a water-in-oil emulsion is formed with a potentially very high viscosity. Proper selection of the surfactant formulation can prevent the occurrence of emulsion inversion. 

There are no standard aging or stability tests in our industry. Each company or independent formulator has developed its/his own set of standards to assess product stability to higher temperatures and to use high-temperature aging as a means to project longer-term aging effects to speed up product introductions. The best approach is to test a product at multiple temperatures because in some cases (e.g., some emulsions can be more stable at a higher than at a lower temperature). 

In 1996, Pal studied the effect of droplet size and found it had a dramatic influence on emulsion rheology (62). Fine emulsions have much higher viscosity and storage moduli than the corresponding coarse emulsions. The shear thinning effect is much stronger in the case of fine emulsions. Water-in-oil emulsions age much more rapidly than oil-in-water emulsions. More recently, Lee et al. (63) and Aomari et al. (64) examined model emulsions and found that a maximum shear strain existed which occurred around 100s. ... 

Monomer conversion (79) is followed by measuring the specific gravity of the emulsion. The polymerization is stopped at 91% conversion (sp gr 1.069) by adding a xylene solution of tetraethylthiuram disulfide. The emulsion is cooled to 20°C and aged at this temperature for about 8 hours to peptize the polymer. During this process, the disulfide reacts with and cleaves polysulfide chain segments. Thiuram disulfide also serves to retard formation of gel polymer in the finished dry product. After aging, the alkaline latex is acidified to pH 5.5—5.8 with 10% acetic acid. This effectively stops the peptization reaction and neutralizes the rosin soap (80). 

Several practical formulae have been developed for estimating the effect on emulsion viscosity of changes in key variables such as temperature, water content, and droplet size distribution, in which adjusting factors for each property are obtained from empirical correlations. An illustration is provided by Rimmer et al. [763]. Such formulae may also contain a term representing changes in emulsion droplet size due to droplet coalescence that occurs with time as the emulsion moves through the pipeline ( ageing ). 

Aged emulsions containing a substantial portion of large (>200 xm) droplets exhibit a lower temperature-viscosity sensitivity, and this effect must be considered in calculating pressure gradients. Adjustment factors shown are for temperature increases (lower viscosity). The inverse of the factor applies to temperature decreases (higher viscosity). 

Effect of Aging. With increasing volume fraction of the dispersed phase, increasing droplet diameter and wider diameter distribution, the viscosity of a dispersed system increases (36). Unstable emulsions show droplet coalescence by extending the diameter distribution, accompanied by viscosity increasing, an effect, which is called "aging" (36-38). ... 

Economic pipeline transport of viscous crudes as concentrated oil-inwater emulsions has been demonstrated in at least two commercial pipelines. The present study was undertaken to learn more about the rheological characteristics of concentrated emulsions and the effect of such variables as emulsion formulation and preparation techniques, aging,and crude oil viscosity on emulsion properties. 

Two crude oils were used, a California crude with a viscosity of 24 poise at 25 C and a Canadian crude of 164 poise. Both could be emulsified by the addition of NaOH which reacted with the acids present in the crude. A series of oil-in-water emulsions containing 60% (by volume) of oil were prepared. Concentration of NaOH and NaCl and mixer speed were varied. Emulsion stability was measured as was particle size distribution and viscosity and the effect of aging on the latter two. Emulsions of the heavier crude had viscosities about 600 times smaller than the crude viscosity. 

Equations proposed by Sherman for predicting viscosities from apparent volume fractions and particle diameters were useful in analyzing the effects of formulation and preparation variables and aging on emulsion viscosities. 

This paper reports results on one phase of a program of study of concentrated oil-in-water emulsions, namely the effect of several composition variables on the apparent viscosity of concentrated (60 volume percent) crude oil-in-water emulsions. In particular, the effects of NaOH content, NaCl content, emulsion preparation technique, crude oil viscosity and aging of emulsion were studied. 

B. The Effect of Aging on Shell Crude Emulsion Properties... 

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