Colloids and Surfaces A: Physicochemical and Engineering Aspects
Evaluation of the efficiency of polyethylenimine as flocculants in the removal of oil present in produced water
Graphical abstract
Introduction
Secondary and enhanced oil recovery generate large amounts of produced water, which is one of the largest waste streams associated with oil production [1]. The oil can be present in four different forms in the produced water (also called oily water): free oil, dispersed oil, emulsified oil and dissolved oil [2]. The emulsified oil constitutes the fraction of interest of this work and, like the other fractions, must be treated for final disposal purposes, guided by the current local environmental legislation.
One of the steps of treating the emulsified fraction of oily water is the flocculation process, in which colloidal particles are aggregated together by contact between them [3]. This process usually occurs with the aid of chemical additives known as flocculants. Usually, the flocculants used for the treatment of produced water are water-soluble polymers, which can be classified in terms of their charge: cationic, anionic or non-ionic. The main characteristic of flocculant polymers is their molar mass and, in the case of the polymer being cationic or anionic (also called polyelectrolytes), their charge density can also be considered as an important property [4].
Polyethylenimine (PEI) is a polymer used in various industry segments, for example in the preparation of polymer complexes in pharmaceutical formulations, controlled drug release, in the introduction and transfer of genes from one cell to another, bacterial cell aggregation, in the manufacture of paper, amongst other uses [[5], [6], [7], [8]]. Although PEI has been modificated with ethylene oxide and propylene oxide and tested to act as a demulsifier for oily water treatment by means of phase separation [9], no reports have been found in literature on its use as a flocculant polymer for oily waters from oil recovery processes. Winnik et al. [10] reported that at pH = 10, PEI is practically an uncharged polymer. However, at pH = 6.6, PEI is a strong polyelectrolyte, presenting about 20 to 40% of its amino groups protonated. It is also reported by Sharma et al. [6] that charged polymers associate with oppositely charged surfactants through electrostatic interactions. Protonated amine groups confer to the polymer charge neutralization properties [11].
Wang et al. [12] studied the interaction between linear and branched PEI with the surfactant sodium dodecyl sulfate (SDS), which is an anionic surfactant with high affinity for PEI. Among their findings, they realized that when the SDS concentration is equal to the critical micelle concentration (CMC), visible precipitates start to form, and when the SDS concentration is higher than the CMC, the interactions are even stronger, leading to complete precipitation and difficulty in solubilizing the polymer again in the solution.
Given this scenario, this work aims to study and evaluate the action of PEI as a flocculating polymer, increasing its interaction with oil droplets using sodium dodecylbenzene sulfonate (SDBS), an anionic surfactant that tends to be allocated at the interface Oil / water, due to the amphiphilic nature of this class of molecules. For this, tests of removal efficiency were carried out in a Jartest and in a Dissolved Air Flotation system (DAF), built to carry out this work.
In DAF, the air is dissolved in water by pressurizing water in a pressure vessel and then depressurized into a flotation chamber. The sudden drop of pressure through the inlet valve in the tank leads to the formation of small bubbles of air (between 20 and 100 μm), that are released from the saturated water, which attaches on the oil droplets and aid in the separation process [13].
Section snippets
Materials
For the preparation of the synthetic oily water, saline water containing 33,000 ppm salts, 441 ppm of potassium chloride (KCl), 708 ppm of calcium chloride dihydrate (CaCl2.H2O), 2626 ppm of magnesium chloride hexahydrate (MgCl2.6H2O), 29,250 ppm sodium chloride (NaCl) and distilled and deionized water. The salts are from Vetec Quimica (São Paulo, Brazil), with 99% purity. Petroleum used to prepare synthetic oily water was donated by the Petrobras Research Center (CENPES) and presented API 16
Determination of 1H-NMR Spectra of polyethylenimines (PEI LW and Pei HW)
The 1H-NMR Spectra of both polyethylenimines are shown in Fig. 3.
PEI LW Spectra (Fig. 3a) has shown two chemical shifts, one located at δ = 284 and another at δ = 289. These chemical shifts are related to the alpha hydrogens of CH2 groups bonded to primary, secondary and tertiary amines. According to Chamberlain [18], the chemical shifts of alpha hydrogens bonded to primary amines are usually in the range of 2,7 < δ < 3,0 ppm, while the alpha hydrogens bonded to secondary and tertiary show
Conclusion
In this work the removal efficiency of high molar mass polyethylenimine (PEI HW) and low molar mass polyethylenimine (PEI LW) along with an anionic surfactant SDBS were evaluated, applied to the produced water treatment. A cationic reference polymer was used as a standard removal efficiency parameter. PEI LW and PEI HW were evaluated in Jartest systems and in dissolved air flotation systems.
It was observed that the higher the concentration of SDBS in the medium, the greater the removal
Acknowledgments
This work was supported byConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPQ), by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), by Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) and by PETROBRAS.
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