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This thesis explores the use of electrodialysis, an electrical and membrane-based technology, to partially desalinate polymer-containing produced water (PFPW) generated in the oil and gas industry. Chapter 1 includes an introduction to polymer flooding, explains its water needs in terms of quality and quantity, and enlists some possible treatments to reach the desired water quality. It is also explained how, by partially desalinating PFPW while retaining the viscosifying polymer, electrodialysis would allow reusing the water for polymer flooding, while significantly decreasing the amount of new polymer to be added when preparing viscous solutions. The chapter also includes some possible constraints for the implementation and enlists the research questions that are answered in the subsequent chapters.
In Chapter 2, we assessed electrodialysis to desalinate PFPW generated in different scenarios and evaluated the reuse of the desalted water to confect the polymer-flooding solution. The experimental work involved desalting two kinds of synthetic PFPW solutions, one with relatively low salinity (TDS=5000 mg/L, brackish PFPW), and another with high salinity (TDS= 32,000 mg/L, sea PFPW), at two different temperatures, and later reusing the desalted solution to prepare viscous solutions. The analysis for the ED-step included the effects of feed composition and temperature on water transport, energy consumption, and current efficiency. It was found that the presence of polymer did not significantly influence the water transport rate nor the specific energy consumption for the seawater cases, but had a measurable effect when desalting brackish water at 20°C. It was also found that some polymer remained in the stack, the loss occurring faster for the brackish PFPW. Still, both kinds of reused PFPW probed adequate to be employed as a basis for preparing polymer solutions.
Chapter 3 addressed the selective removal of divalent cations from PFPW through a variety of operational conditions. The chapter starts by explaining how the presence of multivalent ions in PFPW hampers its recycling mainly because i) they increase the risk of scaling and reservoir souring (sulfate), ii) they interfere with the viscosifying effect of the fresh polyelectrolyte. Then, the chapter explains the experimental work, which consisted of batch experiments run in an electrodialysis-stack. Synthetic PFPW solutions containing a mixture of monovalent and divalent ions were desalted at four different current densities and three different temperatures. Additionally, the effect of the dissolved polymer on the removal was assessed by performing half of the experiments on polymer-containing solutions and half of them on solutions without it. The results demonstrated that it is possible to achieve preferential removal of divalent ions (calcium, magnesium, sulfate) through electrodialysis, especially when employing low current densities (24 A/m2) and high temperature (40 °C). The presence of polyelectrolyte did not significantly affect the removal rate of divalent ions. It was concluded that the precise application of ED to minimize concentrations of divalent ions in PFPW is a possible a more effective way for water and polymer recycling in enhanced oil recovery situations, then the use of other non-selective desalination technologies.
Chapters 4 and 5 address the influence of the feed composition on the fouling formed on anion and cation exchange membranes, respectively. The composition of the solution, which includes various dissolved salts, partially hydrolyzed polyacrylamide (HPAM), crude oil, and surfactants. Electrodialysis experiments were performed to desalinate feed solutions with different compositions, aiming to distinguish between their individual and combined effects. The solutions contained diverse mono- and divalent ions. The analysis included data collected during the desalination and characterization of the fouled AEMs by diverse analytical techniques. Chapter 4 shows that HPAM produced the most severe effects in terms of visible fouling and increase of resistance. This polyelectrolyte fouls the AEM by adsorbing on its surface and by forming a viscous gel layer that hampers the replenishment of ions from the bulk solution. Ca and Mg have a large influence on the formation of thick HPAM gel layers, while the oily compounds have only a minimal influence acting mainly as a destabilizing agent. The membranes showed scaling consisting of calcium precipitates. The effects of the gel layer were minimized by applying current reversal and foulant-free solutions. Regarding the CEMs, Chapter 5 showed that fouling was detected on most CEMs and occurred mainly in the presence of the viscosifying polyelectrolyte. Under normal pH conditions (pH~8), the polyelectrolyte fouled the concentrate side of the CEMs, as expected due to electrophoresis. Precipitation occurred mostly on the opposite side of the membrane, with different morphology depending on the feed composition.
In Chapter 6, we evaluated the application of pulsed electric field (PEF) during the electrodialysis of PFPW, that is, to supply a constant current during a short time (pulse) followed by a period without current (pause). This operation mode aimed to improve process performance by reducing fouling incidences. The experimental work consisted of ED batch runs in a laboratory-scale stack containing commercial ion-exchange membranes. Synthetic PFPW was desalinated under different operating regimes until a fixed amount of charges was passed. After each experiment, a membrane pair was recovered from the stack and analyzed through diverse techniques. The application of PEF improved the ED performance in terms of demineralization rate and energy consumption, the latter having reductions of 36% compared to the continuous mode. In general, the shorter the pulses, the higher the demineralization rate, and the lower the energy consumption. Regarding the application of different pause lengths, longer pauses yielded lower energy consumptions but also lower demineralization. Amorphous precipitates composed of polymer and calcium fouled on the anion and cation exchange membranes, independently of the applied current regime, but in a moderate amount. Finally, the chapter related the observed effects of PEF application to the electrophoresis and diffusion of HPAM. It showed that PEF is a sound option to enhance the desalination of PFPW.
Under the premise that process performance is limited by fouling occurring on the anion-exchange membranes, Chapter 7 aimed to correlate the properties of different AEMs with their performance while desalinating PFPW. The study made use of six stacks containing different homogeneous, commercially available AEMs, which were employed to desalinate synthetic PFPW during 8-days ED experiments operated in reversal mode. The AEMs recovered from the stacks were analyzed in terms of water uptake, ion-exchange capacity, permselectivity, and area resistance, and compared against virgin AEMs. Relatively small changes were measured for most of the parameters evaluated. For most AEMs, the water uptake and resistance increased, while the IEC and permselectivity decreased during operation. Ultimately, AEMs with high area resistance were linked to the fast development of limiting current conditions in the stack, so this property turned out to be the most relevant when desalinating PFPW.
Considering the most favorable conditions assessed in previous chapters, Chapter 8 addresses the experimental evaluation of different operational conditions to increase water recovery while keeping a low energy consumption. The experimental work included the evaluation of applying a continuous constant voltage operation versus the use of PEF, as well as comparing the performance of stacks composed by either aromatic or aliphatic membranes. The results were analyzed in terms of operative time, water recovery, and energy consumption and serve to indicate under which conditions specific types of membranes would be preferred. At last, the collected data was used to perform an economic analysis. It indicated that although further optimization should be possible, achieved settings already made ED desalination of polymer-flooding produced water, a sound case from an economic point of view.
Chapter 9 assessed the contribution of the desalination and the pumping energy to the total energy consumption desalinating streams with high viscosities, i.e., above the one of water (1 cP). The assessment included the influence of other important parameters, namely the salinity of the feed and the type and thickness of the spacer. It was found that the type of spacer did not significantly influence the energy required for desalination. Regarding the pumping energy, it was higher than predicted by models found in the literature, but in most cases, it was minimal compared to the energy for desalination. Only when using very thin spacers (300 µm) or very viscous feeds (12 cP) energy for pumping equalized that of desalination of feeds with 1 g/L NaCl. Thus, it was concluded that the main contributor to the energy consumption of these viscous solutions is still the desalination energy, and it is recommended to use spacers of a thickness between 450 and 720 µm to keep pumping energy low.
At last, Chapter 10 starts by recalling the main areas of investigation that were defined in Chapter 1 (influence of polymer-flooding produced water composition and conditions, removal of multivalent ions, fouling, preservation of polymer integrity) and discuss the findings presented in this thesis. Then, a full-scale polymer flooding case is introduced and based on the results of the previous chapters, an economic analysis performed. In the three scenarios analyzed, introducing a produced water desalination step would have a positive return on investment. In two of them, the payback time is sooner than six years. Finally, the chapter includes some last remarks about the investigation and recommendations for future research.
|Doctor of Philosophy
|11 Sept 2020
|Place of Publication
|Published - 11 Sept 2020
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1/03/16 → 11/09/20