Separation of Oil/Water Emulsions in Continuous Flow Using Microwave Heating Eleanor R. Binner, John P. Robinson,* Sam W. Kingman, Ed H. Lester, Barry J. Azzopardi, Georgios Dimitrakis, and John Briggs Process and Environmental Research Division, Faculty of Engineering, University of Nottingham, NG7 2RD, Nottinghamshire, United Kingdom ABSTRACT: This work studies a continuous ﬂow microwave system to enhance gravity settling of water-in-oil emulsions. Settling times were found to be dependent upon the applied power, ﬂow rate, and energy input. Power and energy input are linked to liquid ﬂow rate within the ﬂow system used in this study, so a key objective of this work was to understand the eﬀect of turbulence on the heating and separation of the ﬂowing emulsion. At high ﬂow rates (9-12 L/min), it was found that turbulence dominates, with settling times largely independent of energy input. At lower ﬂow rates (1-6 L/min), when turbulence was decreased, it was found that the settling time decreased as the power density was increased. Settling times have the potential to be less than half that of untreated emulsions and can be reduced further if turbulence can be minimized between the microwave heating zone and the settling zone in the process equipment. ■ INTRODUCTION The demulsiﬁcation of stable oil/water emulsions is a key process in the removal of water from crude oil. These emulsions are formed during the production of oil downstream from the well. Water is present with the oil, particularly when water injection is used to enhance the production rate. On leaving the well, the mixture, at high pressure, is let down through a choke valve so that it can be further processed at moderate pressures. The pressure reduction valve creates signiﬁcant shear within the ﬂuid, and it is this pressure drop that causes the emulsions to form. The emulsions can contain more than 20% water, 1 which must be separated before further downstream transport and processing can commence. As the world’s oil production comes from increasingly heavy oil reserves, these emulsions are becoming more stable and therefore costly to separate. 2 Demulsiﬁcation is based on the density diﬀerence between oil and water. Droplets of oil rise through water to the top whereas droplets of water fall through oil to the bottom. The rise and fall of droplets in a continuous medium can be quantiﬁed according to Stokes’ Law, which is given by eq 1: ρ ρ μ = − u Dg( ) 18 S 1 2 1 2 2 (1) where u S is the terminal settling velocity of the droplet (m/s), D 1 the droplet diameter (m), g the acceleration due to gravity (m/s 2 ), ρ 1 the droplet density (kg/m 3 ), ρ 2 the density of the continuous medium (kg/m 3 ), and μ 2 the viscosity of the continuous medium (kg/m·s). The separation of the two phases is governed by the settling velocity, u S . If u S is small, then a large residence time is required for the separation to occur; hence, separation vessels need to be very large. Conversely, if u S is large, then the separation occurs in a short time and separation equipment can be relatively small. Equation 1 shows that u S can be manipulated by changing the properties of the emulsion system. The densities of liquids are largely independent of changes in temperature, varying by only 6% over a 100 °C range for water; 3 hence, the two main governing parameters are the viscosity and droplet diameter. Increased settling velocities will result from an increase in droplet diameter and a reduction in the viscosity of the continuous medium. The viscosity of a substance can be reduced by increasing its temperature, typically in an exponential fashion. Droplet size can be increased by a process known as coalescence, where smaller droplets join together to produce larger ones. The probability of coalescence is a function of the droplet size distribution, number of droplets and dispersion density, and the surface properties of the interface between the droplet and the continuous medium, namely the interfacial tension. Improving the Separation of Water-in-Oil Emul- sionsEnhancing Settling Velocity. Gravity separation is a slow process requiring large residence times, and there are several ways to speed up the process. Heat treatment reduces both the interfacial tension of the oil/water and viscosity of the oil. 4 Centrifugal separation eﬀectively increases g and hence u s in eq 1. Coalescence can be promoted by using chemical additives, which lower interfacial tension, or by applying a physical force to move the droplets together, which can be achieved using ultrasound or an electrostatic force. 5 There are several disadvantages with these methods: 1. Heating the whole process stream requires a signiﬁcant amount of energy. 2. Centrifugation has high operating and maintenance costs. 3. Chemical demulsiﬁers can add a signiﬁcant cost and environmental impact. 4. Electrostatic coalescers add signiﬁcant cost and are often limited to low water concentrations. Received: April 9, 2013 Revised: May 22, 2013 Published: May 24, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 3173 dx.doi.org/10.1021/ef400634n | Energy Fuels 2013, 27, 3173-3178