• Average water droplet size in water-oil emulsions were measured by applying NMR technique. • Effect of type and concentration of demulsifier on average water droplet size was investigated. • Influence of water volume ratio , water salinity and mixing speed was also evaluated. • Water/oil volume ratio and water salinity increased the average droplet size. A B S T R A C T The effects of water/oil volume ratio, type and concentration of demulsifier, water salinity and mixing speed on the average

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... water droplets size in water-oil emulsion are evaluated at different times through NMR measurements. The type and concentration of demulsifier have the greatest effects on the average droplets size with 38% and 31.5%, respectively. The water/oil volume ratio, water salinity and mixing speed are significant factors with 13.1%, 7.5% and 5.71%, respectively. The commercial demulsifier Break 6754 has the greater influence on the average droplets size compared to the acrylic acid. The water droplets size increases upon increasing the concentration of demulsifier, the water volume ratio and the salinity of water and decreases upon increasing the mixing speed. methods including Carr-Purcell-Meimboom-Gill (CPMG) pulse sequence, pulsed field gradient (PFG) and simulated echo (STE) and proposed a new method for determining the droplet size distribution by NMR. In this method also the short observation time model [10] and slightly modified method of Parker and Rees are combined to develop a new method for calculating the droplet size distribution of crude oil emulsions. In this article, NMR relaxation time and measurements of diffusion coefficient of the dispersed phase are used to determine the water-in-oil samples drop-let size distributions. The effect of type and concentration of demulsifier, volume water/oil ratio, water salinity and mixing speed on the average droplets size are investigated. 2. Theory The droplet size distribution is determined through two different methods depending on the sizes range of droplets. 2.1. Large droplet size If the square root of the average of molecular penetration length inside the droplet is much smaller than the radius of the drop i.e. the short observation time model is used [10]. In short observation time, only a small fraction of molecules within the droplet feels droplet wall. Ignoring this fraction of molecules, Mitra et al. [10] showed that the deviation of the observed diffusivity at time t from the liquid bulk diffusivity within the droplet is expressed as: (1) where, D(t) is the diffusion coefficient of the molecules trapped in the droplets at time t, D 0 is bulk diffusion coefficient of the bulk of dispersed phase, is the surface to volume ratio of the droplets, and Q(P,R,T) is a parameter related to the curvature of the droplet surface. For short times, the Q(P,R,T) parameter is small and thus the ratio is related to directly. If values of all other parameters are known the can be obtain from Eq. 1. For a distribution of droplets considered as the average surface-to-volume ratio of all droplets and thus, D(t) can