rXXXX American Chemical Society A dx.doi.org/10.1021/ef100979y | Energy Fuels XXXX, XXX, 000–000 ARTICLE pubs.acs.org/EF Breaking of Water-in-Crude Oil Emulsions. 4. Estimation of the Demulsifier Surfactant Performance To Destabilize the Asphaltenes Effect Juan Carlos Pereira,* , †, ‡ Jos  e Delgado-Linares, † Cesar Scorzza, † Miguel Rond  on, §, ^ Sandra Rodríguez, ^ and Jean-Louis Salager † † Laboratorio FIRP, Universidad de Los Andes, M erida, Venezuela ‡ Laboratorio de Petr oleo, Hidrocarburos y Derivados, Universidad de Carabobo, Valencia, Venezuela § Tertiary Oil Recovery Project, University of Kansas, Lawrence, Kansas 66045, United States ^ Universidad Simon Bolivar, Caracas, Venezuela ABSTRACT: Surfactant molecules are tested as water-in-crude emulsion breakers to attain the quickest separation rate in the so- called “proportional regime”. A concept of demulsifier performance is proposed on the basis of the required demulsifier con- centration to offset the effect of a given amount of asphaltenes. The experimental evidence allows one to rank the tested products and relate their performance to their hydrophilicity and molecular weight. Some evidence indicates that the presence of acids in the crude makes it easier to break emulsions and suggests that so-called “extended surfactants” can significantly shorten the demulsifying process. ’ INTRODUCTION The breaking of the water-in-crude emulsions is still a tech- nical challenge in the petroleum industry. 1 As discussed in the previous papers of this series, 2-5 the crude oil dehydration optimization may be seen as finding a synergistic effect between the natural (lipophilic) surfactants (referred to as asphaltenes in what follows) and the added demulsifier (usually a hydrophilic surfactant mixture). It has been known for more than two deca- des 6 that the proper dehydrant additive is such that the overall amphiphilic mixture at the interface, i.e., the asphaltenes and added demulsifier, exhibits an equal affinity for the oil and water phases according to the general phenomenology. 7,8 This physico- chemical circumstance corresponds to the so-called optimum formulation, which exhibits a very deep minimum in emulsion stability. 6-12 Fan et al. 13 established a relationship between the HLB and the concentration of demulsifier and its dehydrant effect. Pe~ na et al. 14 found a relationship between systematic changes in the demulsifier and the stability of water-in-crude emulsions. The affinity of the interfacial mixture depends on the hydro- philicity of the asphaltenes and demulsifier species and their pro- portions at the interface (indicated as X). For the sake of simpli- city, the reasoning is carried out using the hydrophilic-lipophilic balance (HLB) concept of the amphiphiles. The characterisitic hydrophilic-lipophilic balance of the interfacial surfactant mix- ture at optimum formulation (HLB opt ) must be determined experimentally; however, it is generally close to 10, as a first approximation, according to a previous discussion. 2 If “A” (for asphaltenes) and “D” (for demulsifier) are the subscripts that describe the two amphiphilic species, then the optimum formu- lation is attained when the following relationship is satisfied at the interface: X A HLB A þ X D HLB D ¼ HLB opt ð1Þ According to the general phenomenology, the quickest phase separation is attained when X D satisfies eq 1. In practice, the persistance of the emulsion is measured as a function of the con- centration of the demulsifier (C D ), which is added to the system, and C D * is the optimum demulsifier value, which, in this case, corresponds to the minimum emulsion stability. The value of X A and X D in the interfacial mixture are not known; however, at low asphaltenes concentration, it was shown that they depend directly on the overall concentration of asphaltenes (C A ) and demulsifier (C D ) originally introduced in the bulk phases. The previous papers 2,3 have reported the demulsifier optimization as a function of the asphaltenes content, by means of the dilution of the crude oil with a solvent such as cyclohexane. They have shown that, when the concentration of asphaltenes (C A ) is higher than a so-called “threshold” T (typically 1000 ppm), then the value of C D * is essentially constant, regardless of the asphaltenes concentration. This result is consistent with the presence of a thick layer of asphaltenes gathered at the interface, with only the first layer interacting with the demulsifier. The remaining asphal- tenes are acting out of the interface, according to some form of segregation or aggregation. 5 What was reported as a new insight in the previous articles 2,3 was the fact that, below that threshold, the concentrations C A and C D * are proportional; i.e., if there is twice as much asphaltenes in the oil phase, twice as much demulsifier is needed in the water phase. This range was called the proportional regime, 3 since it corresponds to the fact that, at optimum formulation, X D */X A = X D * /(1 - X D *)= k interf . Because most of the amphiphilic species taken into consideration in C D and C A are finally adsorbed on the large interfacial area of the emulsion, it has been assumed 3 that Received: July 29, 2010 Revised: December 5, 2010