| Lignin is the second most abundant natural polymer in nature,which has the advantages such as wide source,low cost and stable stucture,etc.However,due to its heterogeneity,large size,poor dispersion,the high value utilization is seriously restricted,resulting in a waste of resources.In recent years,producing lignin nanoparticles(LNPs)to replace traditional inorganic nanoparticles as functional additives has attracted high attention.LNPs are derived from green sources,being bio-renewable and bio-degradable,which not only have the advantages of high specific surface area of nanoparticles,but also contain more active functional groups,having chemical reaction activity on the surface,and inherent amphiphilic nature,so they show high application potential in many fields.The utilization of LNPs is one of the most promising directions to expand the industrial application of lignin,which also provides a meaningfiil new way to promote the sustainable development of related technologies and products.For example,the application of nanoparticles in the field of enhanced oil recovery(EOR)has attracted much attention in recent years.Nanofluids composed of nanoparticles and surfactants have become a research hotspot because of their remarkable effects on improving oil recovery.However,traditional nanoparticles usually have high cost,and are difficult to be degraded,which can cause some damages to the reservoir,and their environmental friendliness might be not ideal,so the large-scale application is limited.As a natural source green amphiphilic nanoparticles.LNPs deserve being paid more attention because of their important prospects and application potential in the field of EOR due to their unique advantages.At present,the preparation methods of LNPs still have many problems such as high economic and energy cost,wide size distribution,and poor dispersion stability.Therefore,it is meaningfiil to develop green,low-cost and size-controllable preparation methods for LNPs to explore their applications.For the purpose to solve the above problems,in this paper,LNPs were prepared by antisolvent method using industrial alkali lignin as raw material through introducing assistance of ultrasonic wave and surfactant.The effects of ultrasonic conditions,types and concentrations of surfactants on the self-assembly,performance and size of nanoparticles were investigated,and the formation mechanism of LNPs modulated by surfactants was discussed.Based on this,lignin-based nanofluids that can be dispersed stably at high temperature and high salt were prepared by using high temperature resistant surfactants and LNPs.Its properties,such as reducing the interfacial tension of oil and water,changing the wettability,stabilizing the emulsion and reducing the adsorption capacity of surfactants,were investigated,and the efficiency of enhancing the oil recovery was investigated by microscopic physical model flooding experiments.The application prospect of LNPs in EOR was revealed.This paper is mainly divided into two parts:(1)Preparation of lignin nanoparticles.Using industrial alkali lignin as raw material,tetrahydrofiiran as solvent,aqueous solutions of anionic surfactants sodium dodecyl sulfate(SDS),sodium alpha-olefin sulfonate(AOS),cationic surfactants cetyl-trimethyl ammonium bromide(CTAB),amphoteric surfactant cocamidopropyl betaine(CAB)and nonionic surfactant Tween 80(TW80)as antisolvent were used to prepare LNPs,with assistance of ultrasound method.The effects of ultrasonic strength,type and concentration of surfactants on the particle size and size distribution of LNPs were investigated.The results showed that irregular aggregates were formed when CTAB solution is used as antisolvent,and the nanoparticles with regular morphology cannot be obtained.Smooth spherical nanoparticles were prepared by SDS,AOS,CAB and TW80 solution as antisolvent.When the surfactant concentration is below critical micellar concentration(cmc),the prepared LNPs are smaller in size and more evenly dispersed.When the surfactant concentration is higher than cmc,the particle size becomes larger and the dispersion gets wider.By measuring Zeta potential and equilibrium surface tension,the mechanism of self-assembly of LNPs modulated by surfactants was discussed.It was found that the surfactant molecules bind with lignin units mainly through hydrophobic interaction.The electrostatic and polar interaction between the polar groups of surfactants affected the driving force of self-assembly of LNPs,thus affecting the morphology,size and surface properties of LNPs.The surface properties and basic structure of LNPs nanoparticles prepared by AOS solution as antisolvent were investigated by FT-IR,TGA,TEM and AFM.It was found that the precise control of size and morphology of the produced LNPs was realized by systematically changing the initial concentration,ultrasonic conditions and antisolvent dripping rate,etc.The prepared LNPs have good dispersion stability,which lays a foundation for further exploring the properties and applications of LNPs.(2)Construction,properties and performance of LNPs/surfactant composite nanofluids.On the basis of the above work,LNPs were prepared with AOS solution as antisolvent,and AOS/LNPs nanofluids were formed by mixing with AOS solution of appropriate concentration.Furthermore,non-ionic surfactant coconut diethanol amide(6501)was introduced to prepare AOS/6501/LNPs composite nanofluids.The salt and temperature resistance of the different nanofluids was investigated.The results showed that.AOS/LNPs nanofluids could disperse stably at a salinity of 10,000 ppm at 60℃,and AOS/6501/LNPs nanofluid are stable at 20,000 ppm salinity,no aggregation and precipitation phenomena are oberved.Compared with the single AOS surfactant solution and AOS/6501 composite surfactant solution,the surfactant/LNPs nanofluids have further improved activity to reduce the oil-water interfacial tension.AOS/LNPs nanofluids can reduce the oil-water interfacial tension to 10-2 mN/m at a salinity of 5000 ppm.For AOS/6501/LNPs nanofluids,the interfacial tension of oil and water can be reduced to 10-3 mN/m,which meets the requirements of interfacial activity of oil displacement system for tertiary oil recovery.The study showed that LNPs formed a stable adsorption layer due to its amphiphilic synergism with surfactants at the oil-water interface,thereby not only significantly improved the interfacial activity of the nanofluid,but also distinctly improved the ability of the nanofluid to stabilize the emulsion,and further improved the spreading ability of the nanofluid on the quartz surface and the ability to peel off oil droplets,which all befit the flooding efficiency of crude oil,and provide sufficient guarantee for enhanced oil recovery.The good efficiency of nanofluid to endance oil recovery is proved by micro-model oil displacement experiment.In addition,QCM-D test shows that LNPs can effectively reduce the adsorption loss of surfactant on quartz surface,which provides a novel technical way for EOR technology to reduce cost and increase efficiency.To sum up,the preparation method of LNPs was studied in this paper,which laid a good foundation for the performance research and application expansion of LNPs.By lignin-based nanoparticles was prepared using surfactant solution as antisolvent,and LNPs/surfactant nanofluids was constructed,of which the stability and related properties to enhance oil recovery under harsh conditions such as high temperature and high salt were explored,which not only revealed the application prospect of LNPs in EOR,but also provided a new idea for the application of LNPs.It is of great significance to expand the high value application of lignin,and might provide a meaningful new way for the innovation and sustainable development of EOR technology. |
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