Effect of polyethylene pipe on wax deposition under different temperature and flow rate conditions

Wax deposition is an important but troublesome boulder in petroleum industry that has always plagued pipeline engineering operators. In this study, wax deposition in the polyethylene pipes has different behaviors comparing wax deposition in stainless steel pipes such as smaller weight of wax deposits and lower wax content. After wax deposition experiments, some important parameters, including diffusivity and temperature gradient, were measured and calculated to explore the differences between two material pipes. Moreover, those factors can make a significant impact on radial mass flux. A series of results in polyethylene pipes suggests that higher diffusivity enhances diffusion flux of wax in the radial direction. But it was found that radial concentration gradient accounted for a larger proportion. These factors are the relationship of competition and cooperation and the combination of them results in different performances of wax deposition between polyethylene and stainless steel pipes.     

Study on wax precipitation characteristics of wax deposit in pipes

Wax deposit properties are a significant concern in pipeline pigging during waxy crude oil transportation. In the present work, the impacts of flow conditions and oil properties on the wax precipitation characteristics of wax deposits are investigated. A flow loop apparatus was developed to conduct wax deposition experiments using four crude oils collected from different field pipes. The differential scanning calorimetry (DSC) technique was employed to observe the wax precipitation characteristics of crude oil and wax deposit. The results show that the wax content and the wax appearance temperature (WAT) of the deposits increase with shear stress and radial temperature gradient, and decrease with radial wax molecule concentration gradient near the pipe wall. The DSC tests on the wax deposits revealed that the deposit wax content is strongly correlated to the oil wax content. Furthermore, an empirical correlation was developed to predict the wax content and the WAT of the wax deposit. Verification of the empirical correlation using the different oils indicated that the average relative error of the wax content prediction and average absolute error of WAT prediction were 13.2% and 3.6°C, respectively.     

An aging theory-based mathematic model for estimating the wax content of wax deposits using the Fick's second law

In this study, a mathematic model is proposed for estimating the wax content of wax deposits. The proposed model was built based on the diffusion of wax molecules and counter-diffusion of oil molecules and described using the Fick's second law, allowing for the stacking fraction and orientational order of precipitated wax crystals and the tortuosity of diffusion path of de-waxed oil molecules during the counter-diffusion. The calculated results were verified by comparing with the flow-loop wax deposition experimental results. Dependence of radial position, deposition duration, bulk temperature, and wall temperature were investigated. These factors significantly affected the wax content during wax deposition. Good agreements were observed between the predictions and experimental results. The variation trends of wax content affected by various aspects are consistent with the existing studies.     

原油管道蜡沉积物径向性质研究进展

蜡沉积物性质对原油管道清管方案的制定有重要影响,是原油流动保障领域的研究热点之一。本文回顾了近年来关于管道蜡沉积物径向特性的研究成果,对当前实验研究手段和方法进行了系统的对比分析;从蜡沉积物组成、析蜡特性、宏观形态与微观结构、力学特性四个方面深入阐述了对管道蜡沉积物径向性质的认识与结论,分析了其内在影响因素和作用机理;评述了蜡分子扩散系数及径向含蜡量分布预测模型的理论基础和存在缺陷;提出了未来的研究方向：加快研发更加精确的机械取样装置,深入研究沉积物微观结构特性对宏观流变性的影响机理并建立二者之间的定量关系,建立考虑多孔网状结构中蜡分子扩散动力学的蜡沉积物径向性质预测模型。     

原油组成对原油管道结蜡规律的影响

利用旋转式动态结蜡装置,对不同组成的人工油样(胶质和沥青质含量不同或蜡含量不同)进行了不同条件下的实验研究.通过对管壁沉积物取样并利用差式扫描量热法(DSC)和四组分法进行分析,探究了不同原油组成对结蜡的影响规律.结合胶质和沥青质在结蜡过程中的作用机理,发现胶质和沥青质的存在虽然总会削弱蜡分子的迁移动力,阻碍蜡分子的沉积,但在含量较小时会协同蜡分子的沉积作用,而当含量较大时会以粘壁的形式附着于管壁.对蜡含量不同原油的结蜡规律研究发现:原油所含蜡分子的碳数越高,结蜡层中的蜡含量就越少.但由于碳数较高的蜡分子具有相对较长的碳链,更容易与原油中的胶质和沥青质发生共晶作用,故更易与胶质和沥青质一起沉积于壁面.     

An experimentally validated heat and mass transfer model for wax deposition from flowing oil onto a cold surface

A transport model is proposed for wax deposition onto a cold finger from flowing wax-containing oils. The model solves transient energy and mass balances simultaneously for a reversible first-order kinetic rate for precipitation of pseudo-single-component wax, and the effects of yield stress using a critical solid wax concentration to withstand flow-induced stress at the deposit-fluid interface. The model can predict the time evolution of the deposit thickness, and the spatial and temporal evolution of temperature and wax concentration as validated using cold finger experiments. It was found that for high wax content oils, deposit thickness growth is dominated by heat transfer. For low wax content oils that are unable to gel, the thickness growth is slow and accompanied by occasional sloughing. Regardless of the mechanism controlling the growth, mass transfer cannot be neglected as wax diffusion into the deposit continues to take place after the deposit has stopped growing.     

A fundamental model of wax deposition in subsea oil pipelines

Wax deposition in subsea pipelines is a significant economic issue in the petroleum industry. A mathematical model has been developed to predict the increase in both the deposit thickness and the wax fraction of the deposit using a fundamental analysis of the heat and mass transfer for laminar and turbulent flow conditions. It was found that the precipitation of wax in the oil is a competing phenomenon with deposition. Two existing approaches consider either no precipitation (the independent heat and mass transfer model) or instantaneous precipitation (the solubility model) and result in either an overprediction or an underprediction of deposit thickness. By accounting for the kinetics of wax precipitation of wax in the oil (the kinetic model), accurate predictions for wax deposition for both lab‐scale and pilot‐scale flow‐loop experiments with three different oils were achieved. Furthermore, this kinetic model for wax precipitation in the oil was used to compare field‐scale deposition predictions for different oils. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Wax deposition modeling of oil/gas stratified smooth pipe flow

Wax deposition modeling is complicated under oil/gas two‐phase pipe flow and therefore remains poorly understood. One‐dimensional empirical heat and mass transfer correlations are unreliably for deposition modeling in stratified flow, due to non‐uniform deposit across the pipe circumference. A mathematical model has been developed to predict the deposit thickness and the wax fraction of deposit in oil/gas stratified pipe flow using a unidirectional flow analysis of non‐isothermal hydrodynamics and heat/mass transfer. The predictions for wax deposition are found to compare satisfactorily with experimental data with three different oils for single phase and oil/gas stratified pipe flow. In particular, the reason that the deposit forming a crescent shape at the cross section of pipe observed in different experiments is revealed, based on the non‐uniform circumferential distributions of two most important parameters for the wax deposition, diffusivity at oil–deposit interface, and the solubility gradient at the oil–deposit interface at different time. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2550–2562, 2016     

沥青质引发的蜡油体系结蜡层分层现象及分层规律

利用自主研发的Couette结蜡装置,对蜡含量相同的油样1(不含沥青质)和油样2[含0.75%(质量分数)沥青质]进行结蜡实验,并研究其结蜡层的分层现象和分层规律。通过对油样1和油样2结蜡表层和底层的宏观形貌、DSC放热、析蜡量、蜡晶微观形貌的分析发现:油样1结蜡层无明显分层现象,而油样2结蜡层分层现象明显,沥青质的加入导致了结蜡层的分层。与结蜡表层相比,油样2结蜡底层的析蜡点、蜡含量与沥青质含量显著升高,蜡晶形貌发展为致密的类球状大蜡晶。油样2结蜡表层沉积量随壁温的升高、油壁温差和转速的增大而减小;结蜡底层沉积量随壁温升高而减小,随油壁温差和转速的增大而增大;总的蜡沉积量随壁温的升高和转速的增大而减小,随油壁温差的增大先增大后减小。     

Influence of wall thickness on the creep rupture performance of polyethylene pipe

A thermal gradient is inherent to the solidification of extruded profiles such as pipes. Depending on the magnitude of the gradient, varying levels of residual stresses are frozen‐in during the pipe fabrication step. In this investigation, the residual stress state of a polyethylene (PE) pipe was altered by changing its wall thickness. Creep rupture testing of the subject pipes of varying wall thickness provided an insight on how residual stresses influences both ductile and brittle fracture processes. POLYM. ENG. SCI., 47:516–521, 2007. © 2007 Society of Plastics Engineers.     

聚乙烯蜡工艺技术进展和应用前景探讨

综述了国内外聚乙烯蜡的工艺技术进展情况,分析了聚乙烯蜡在涂料、油墨及色母粒等领域的应用情况,对今后我国聚乙烯蜡的应用前景进行了探讨。     

硬脂酸钙对石蜡、硬脂酸润滑作用的影响及其理论分析

详细地论述了硬脂酸钙（CaSt2）对石蜡、聚乙烯蜡（PE蜡）、硬脂酸（HSt）润滑作用的影响,并进行了理论分析,提出了＂润滑膜强度＂及＂铆钉作用＂概念.只用非极性石蜡、PE蜡时,其润滑膜的抗撕裂强度及其黏附强度较低,强极性CaSt2与PVC树脂表面及金属表面形成化学吸附络合键,从而起到＂铆钉作用＂,增加了石蜡、PE蜡润滑膜的强度,提高了石蜡、PE蜡的润滑作用.通过对金属皂等极性润滑剂化学结构的理论分析,笔者认为：金属皂类等极性润滑剂可以视为油溶性表面活性剂（润湿剂）,并用表面活性剂能降低PVC树脂界面张力的规律解释了CaSt2对石蜡、PE蜡、HSt润滑作用的影响.     

不同加热温度和降温速率对含蜡原油析蜡点的影响

利用差示扫描量热法(DSC)对原油的析蜡特性进行分析,并得到了析蜡过程的热谱图和dh/d T-T曲线。以大庆原油为研究对象,对dh/d T-T曲线进行分析,探讨了加热理温度、温降速率对析蜡点的影响。结果表明:加热温度高于溶蜡点温度时,析蜡点随温变化小;加热温度低于溶蜡点温度,析蜡点随温度的降低先增大后减小。温降速率越大,析蜡点越低。     

Preparation of nano‐encapsulated polyethylene wax particles for color toner by in situ emulsion polymerization

Polystyrene‐acrylate resin coated polyethylene (PE) wax composite particles used for color toner were prepared by in situ emulsion polymerization. The laser particle analyzer and transmission electron microscope (TEM) were applied to show the size and morphology of composite particles, and thermogravimetric analysis (TGA) was used to characterize the thermal properties of the nanoparticles. With the increase of emulsifier triton x‐100 (TX‐100), the size of composite particles decreased at first and then increased and the shell thickness increased gradually. When the mass ratio of TX‐100 to monomers was 1:10, the core–shell structure of the latex particles was the clearest. The homopolymer of styrene (St) and copolymer of styrene‐butyl acrylate (St‐BA) showed few differences on the size of the composite particles, but a significant difference on the thermal performance. The FTIR spectra and dissolution experiments of PE wax/P(St‐BA) composite particles provided supporting evidence for encapsulation. With the increasing percentage of polyethylene wax, the particles size got larger and emulsion shelf stability became worse. The size and the shelf stability got optimized when the reaction temperature was 80 °C. The average size of the particles initiated by 2,2‐azobisisobutyronitrile (AIBN) was smaller and the size distribution was narrower than those of initiated by potassium persulfate (KPS). © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44399.     

蜡沉积厚度随时间变化规律的模型研究

在分析蜡沉积过程的基础上, 基于一定的假设条件, 建立了蜡沉积厚度随时间变化的模型。通过提取蜡沉积厚度随时间变化曲线的数据, 拟合了模型中的待求常数, 并对模型计算值和实验值的吻合程度进行了分析。结果表明:对数模型、指数模型及动平衡模型均能反映蜡沉积厚度随时间的变化趋势, 对数模型与实验值的吻合程度最好;指数模型和动平衡模型所得的结果与实验结果的吻合程度相当, 在沉积初期, 指数模型的吻合程度高于动平衡模型, 但当沉积时间接近实验规定时间时, 动平衡模型的吻合程度高于指数模型;所建立的模型简单实用, 能够反映沉积厚度随时间变化的增长快慢趋势, 不同实验数据应用时仍需对各模型的计算精度进一步验证, 从而使得选用模型能更好地符合实际。     

Investigation of inhibitors efficacy in wax deposition mitigation using a laboratory scale flow loop

Statistically reliable wax inhibition experimental data was obtained by utilizing the newly built laboratory scale flow loop. The comb‐shaped inhibitors are found more effective in decreasing the thickness compared to the linear inhibitor under the same conditions. Moreover, this becomes more predominant as the chain length of inhibitors increases. Interestingly, even though all the inhibitors decreased the deposit thickness, the wax content increased significantly. Besides, the longer chain length (PI‐B) of the inhibitor results in a higher wax content. Since the combination of growth and aging influenced by the presence of inhibitor significantly, paraffin inhibition efficiency (PIE) based on the wax mass was proposed to quantitatively assess the inhibitors. Based on the PIE, PI‐B, and PI‐C have more inhibition efficiency than PI‐A. Therefore, when selecting wax inhibitor, one should be aware about the strength of the deposit gel in addition to the reduction of the deposit mass. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4131–4139, 2016     

甲醇合成结蜡原因和处理方法

甲醇合成生产普遍以铜基催化剂为主,此催化剂下生产甲醇的同时伴有高碳链的碳氢化合物和石蜡生产。当正常生产过程中石蜡过多时会造成甲醇气液分离器、水冷器、阀门管道、甚至合成塔出口的堵塞,严重时会造成停车清蜡。因此研究石蜡产生的机理以及如何避免和减少石蜡的产生是很有必要的。     

Thermal fractionation and properties of different polyethylene/wax blends

The influence of paraffin‐wax type and content on the properties of its blends with HDPE, LDPE, and LLDPE was investigated. Melt‐mixing of HDPE with wax gave rise to completely miscible blends for both 10 and 20% wax contents. A wax content of 30% gave rise to a partially miscible blend. These observations were supported by the thermal fractionation (stepwise cooling) results. Melt‐mixing of LDPE with hard paraffin wax gave rise to a partially miscible blend for all wax contents investigated, while complete miscibility was observed for the 10% oxidized hard paraffin wax containing blend. Complete miscibility was observed for all the LLDPE/A1 wax blends, with A1 wax as an oxidized hard paraffin wax. This indicates possible cocrystallization of this wax with LLDPE, which was also evident from the thermal fractionation curves. LLDPE blends with hard paraffin wax were, however, partially miscible for all wax contents. All the observations were supported by the surface free energy results. It is further clear from the thermal fractionation results that the presence of wax changed the crystallization behavior of LDPE and LLDPE. Changes in the tensile properties are explained in terms of the miscibility and proposed morphologies of the blends. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2225–2236, 2007     

Increasing the firmness of wax-based oleogels using ternary mixtures of sunflower wax with beeswax:candelilla wax combinations

Oleogels were prepared with 5% wax in soybean oil using mixtures of beeswax (BW) and candelilla wax (CLW) with ratios of 10:90, 30:70, 50:50, and 60:40 BW:CLW, and the same series where 10% of the total wax was substituted with sunflower wax (SFW). The hypothesis that SFW would increase the firmness of the oleogels without affecting the melting properties was tested. Firmness of one-wax oleogels decreased from SFW > CLW > BW. Oleogels with 50:50 BW:CLW and 60:40 BLW:CLW had equal firmness to pure 5% SFW oleogels. SFW significantly increased oleogel firmness and reduced the softening that occurred between 4°C and 22°C. Increased firmness was also found with rice bran wax and behenyl-behenate (C44) addition, but not with wax esters with chain lengths ranging from 30 to 40 carbons (C30 to C40). By differential scanning calorimetry, SFW significantly decreased the melting point of oleogels with 10:90 and 30:70 BW:CLW mixtures but significantly increased the melting point of those with 50:50 and 60:40 BW:CLW mixtures. However, the solid fat content melting curves were not significantly influenced by SFW addition. These results indicate that mixed wax oleogels had greater hardness and elasticity, and that the long chain wax esters contributed by SFW helped to improve the strength of oleogels without negatively affecting their melting properties.     

The overall crystallization behavior of polyethylene/wax blends as phase change materials for thermal energy storage: A mini review

This review paper deals with the overall crystallization behavior of polyethylene/wax blends as phase change materials (PCMs) for thermal energy storage with the determination of their thermal properties. The addition of molten wax to the polyethylenes decreases the crystallization and melting temperatures of the blends. However, incorporating fillers to the polyethylene/wax blends can either decrease or increase the crystallization and melting temperatures of the composites depending on the filler type. The normalized enthalpy values of linear low-density polyethylene showed no significant change when increasing the wax content. On the contrary, the normalized enthalpy values of the wax in the blends were lesser than that of pure wax and increased with increasing wax content. Since the wax in the blend had a lower crystallinity compared to pure wax, this influences its effectiveness as a PCM for thermal energy storage. The effect of different polyethylenes on the wax morphology gave rise to enhance phase separation when wax was blended to high-density polyethylene as compared to the other polyethylenes. On the contrary, the effect of various waxes on the morphology of polyethylene resulted in different morphologies due to the molecular weight of the wax used and the structure of the polyethylene chain. The addition of fillers to the polyethylene (PE)/wax samples resulted in enhanced phase separation. The overall isothermal crystallization rate and the equilibrium melting temperature of PEs in the PEs/wax blends were depressed by wax addition due to the wax dilution effect.