Investigating the gelling behaviour of ‘waxy' paraffinic mixtures during flow shutdown

The flow of waxy or paraffinic crude oils in a pipeline could be shutdown for a variety of reasons, resulting in their cooling and subsequent gelling. Gel formation from a multicomponent wax-solvent mixture during flow shutdown was investigated experimentally and analyzed with a transient heat-transfer model based on the moving boundary problem formulation. The gelling experiments were performed with a 0.10 g/g wax-solvent mixture in a flow-loop apparatus, following the formation of a steady-state deposit layer in turbulent flow regime, at two initial wax-solvent mixture temperatures, with a constant coolant temperature, and for different shutdown times. The gel formation was found to be a fast process, which continued until the gel fully occupied the deposition tube. Gas chromatographic analyses of the deposit samples (under sheared cooling) and the gel samples (under static cooling during flow shutdown) indicated significant differences in the composition and the total wax content. The deposit samples showed an enrichment of heavier paraffins, whereas the composition of gel samples was comparable to that of the original wax-solvent mixture. The predictions from the transient model showed that a lower initial oil temperature, a lower coolant temperature, and a smaller pipe diameter would result in a faster blockage of the pipe. The predictions from the moving boundary problem formulation agreed well with the flow shutdown data, which further confirmed that the solid and gel formation from wax-solvent mixtures is modelled satisfactorily as a heat transfer process.     

Investigation of wax deposit ‘sloughing’ from paraffinic mixtures in pipe flow

Deposit ‘sloughing’ from ‘waxy’ crude oils has been described in the literature as a possible mechanism, leading to partial or complete dislodging of the deposit from the pipe wall due to changes in flow parameters. A bench‐scale flow loop apparatus was used to investigate ‘sloughing’ with prepared single‐phase ‘waxy’ mixtures of a multicomponent paraffinic wax dissolved in a multicomponent solvent. Experiments were performed to study the changes in the deposit‐layer thickness due to step increments in the ‘waxy’ mixture flow rate, the mixture temperature, and the coolant temperature. It was observed that the deposit‐layer thickness decreased with an increase in each of the three parameters; however, a complete or sudden dislodging of the deposit‐layer did not occur in any of the experiments. A steady‐state heat‐transfer model was used to predict the variation in the deposit mass or thickness due to changes in the selected parameters. In each case, the step‐wise decrease in the deposit thickness, as observed experimentally, was predicted to be caused by changes in the thermal resistance and/or thermal driving force.     

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     

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     

A review of heat-transfer mechanism for solid deposition from “waxy” or paraffinic mixtures

Summarized in this review are a large number of experimental and modelling studies for advancing the heat-transfer-based mechanism for solid deposition from “waxy” or paraffinic oils and mixtures. This comprehensive heat-transfer approach is entirely different from a more popular molecular-diffusion mechanism. It has evolved from numerous publications, over three decades, which explored topics related to thermodynamic, rheological, crystallization, solid deposition, and shutdown and deposit-aging behaviour of prepared multicomponent paraffinic mixtures of varying compositions to simulate “waxy” crude oils. These investigations covered a wide range of compositions, temperatures, and cooling rates—under static, sheared, laminar and turbulent conditions—in both the hot and cold flow regimes. The heat-transfer mechanism for wax deposition is based on (partial) freezing or liquid-to-solid phase transformation process, for which steady-state and unsteady-state mathematical models have been developed and validated with extensive laboratory data. Furthermore, a shear-induced deformation model for the deposit aging phenomenon has been developed and validated; it is based on a partial release of the liquid phase from the incipient gel, thereby causing an enrichment of heavier alkanes and a corresponding depletion of lighter alkanes in the deposit. A successful analogy with the ice deposition process has confirmed the wax deposition process to be also controlled by heat transfer, without involving any other mechanism for wax deposition. All of these previous studies confirm that wax deposition is predominantly a thermally driven process.     

A fundamental wax deposition model for water‐in‐oil dispersed flows in subsea pipelines

Water‐in‐oil dispersions frequently form in subsea oil pipeline transportation and their presence affects the wax deposition rate in subsea pipelines. A fundamental model for wax deposition on the wall of water‐in‐oil dispersed phase flow pipelines has not been developed. Dispersed water droplets can affect the heat and mass transfer characteristics of wax deposition and alter the deposit growth rate. In this study, wax deposition from water‐in‐oil dispersed flows is comprehensively modeled using first principles of heat and mass transfer. The role of the dispersed water phase on the heat and mass transfer aspects of wax deposition is analyzed. The developed model predicts different effects of the water volume fraction and droplet size on the wax deposition rates in laboratory flow loop experiments and in field scale wax deposition processes. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4201–4213, 2017     

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.     

Modeling of a Continuous Crystallization Process for Solvent Dewaxing

Abstract

A steady 1-D parallel flow model was established for a continuous solvent dewaxing process; the model equations were numerically solved to obtain the wax supersaturations, amount of crystals, crystal sizes and crystal size distributions in the wax-oil-solvent solution along the flow direction. The effects of the operating conditions, including temperature distributions, multiple dilutions, and the composition of dewaxing solvent, the system's characteristics, including nucleation constant, nucleation order, and wax compound, and the crystallizer dimensions on the formed wax crystals were examined and discussed in detail. The system's nucleation constant was determined by an experimentally obtained recovery of wax; further predictions are in acceptable agreement with the results from a pilot plant.     

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.     

Deposition of paraffin wax from kerosene in cooled heat exchanger tubes

Studies of the deposition of wax from wax/kerosene mixtures onto cooled simulated heat transfer tubes has demonstrated that, following an initial deposition taking a relatively short period of time, the heat transfer resistance fluctuates with time. Theoretical estimates of temperature distribution in the experimental flowing wax/kerosene system suggest that the cloud point temperature of the mixture falls within the laminar boundary layer. Since the cloud point is considered the temperature at which particles of wax appear, the conditions within the boundary layer will influence deposition. Increased flowrate and temperature decrease the number of particles able to deposit while the number is increased by greater wax concentration in the bulk solution.     

On the characterisation of wax deposition in an oscillatory baffled device

Deposition from viscous fluids in industrial processes is a major nuisance during production of many useful materials ranging from oil to foodstuffs. In a cold‐wall scenario rapid deposition of gel‐like material causes restriction, processing delays and potentially reduction in product quality. One well‐known example of this phenomenon is paraffin wax deposition, which is one of the greatest challenges in the petroleum industry, where the main implication is the blockage of oil. The aim of this work was to investigate the effect of applying oscillation on wax deposition, with the main focuses on the effects of oscillation frequencies and amplitudes, as well as the volume of the wax‐oil mixture. Wax deposit formation was measured gravimetrically. The results indicate that baffles and oscillation reduce wax deposition; increasing both oscillation frequency and amplitude result in a decrease in wax deposition; and the degree of reduction is greater with changing oscillation amplitude than with frequency. It was also found that deposition on the column wall rather than the baffles was the main contribution to the total wax deposit. Copyright © 2006 Society of Chemical Industry     

管道结蜡层温度分布计算研究

通过对试验环道上测试段相关数据的测试,对比分析测试段管壁有沉积物和无沉积物时测试段管壁内温度分布的不同表达式,根据能量方程建立了无量纲温度分布方程,利用贝赛尔函数推导出原油在层流状态下沉积层表面温度分布、沉积层热流密度、油流温度分布等相关温度场的计算公式,为研究蜡沉积规律,定性分析管路结蜡层厚度和制定合理的清管方案奠定了理论基础.     

Phase Transformation and Rheological Behaviour of Highly Paraffinic “Waxy” Mixtures

An experimental investigation was undertaken for the rheology and phase transformation of prepared solutions comprising a paraffin wax dissolved in n‐dodecane or n‐hexadecane. The liquid‐solid phase transformation in wax‐solvent mixtures was investigated through the measurement of wax appearance/disappearance temperature (using cross polar microscopy, differential scanning calorimetry, viscometry and a visual method), pour point temperature and crystallization temperature. The results were utilized to prepare a temperature‐composition phase diagram for the wax+n‐C₁₆H₃₄ pseudo‐binary system. The effects of composition, temperature, cooling rate and shear rate were studied on the rheology of wax‐solvent mixtures. A correlation was developed for the apparent viscosity of wax‐solvent mixtures.     

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。     

Modelling wax deposition of diesel in sequential transportation of product oil pipeline using optimized back propagation neural network

Contamination of gasoline by wax deposit of diesel is a severe problem in sequential transportation of product oil pipelines in cold areas. However, most works on wax deposition are focused on crude oil. In response, this paper aims to investigate wax deposition from a unique perspective of diesel oil in sequential transportation. To this end, a cold finger apparatus was designed and constructed. It is found that the wax deposition rate of diesel oil increases with oil temperature and wax content, and decreases with cold finger temperature. A non-monotonic variation trend is observed against shear stress. To predict diesel wax deposition rate, a back propagation (BP) neural network optimized by bald eagle search (BES) algorithm is proposed. Grey relational analysis (GRA) is employed to get the highly relevant factors as input parameters of the developed model. Prediction accuracy and generalization ability of the BES-BP model is experimentally verified. This work will be helpful to schedule the transportation program of product oil to avoid contamination of gasoline by diesel wax deposit.     

管道蜡沉积模型中扩散系数的研究

分子扩散是管道蜡沉积的主要驱动力,蜡分子的径向扩散质量流(J=D?(dC/dr))决定了蜡沉积层增长速率,其中关键参数为蜡分子在原油中的扩散系数D。原油为多组分复杂混合物,扩散过程复杂,不利于定量研究。从简单的二元烷烃体系入手,研发了Taylor分散法测试装置,并总结了国内外学者的理论和实验研究,针对n-C18+n-C7体系进行比较分析,为深入研究多元蜡分子扩散现象提供了基础。     

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     

Separation of fatty acids from binary melts using physical vapour deposition (PVD)

BACKGROUND: The use of fatty acid mixtures, natural biochemical compounds, will be extended to various chemical industries for the production of a wide variety of products, and various mixtures of fatty acids are necessary for production. Separation of a binary fatty acid mixture of lauric acid and myristic acid using physical vapour deposition (PVD) on a cold quartz crystal resonator is examined. The extremely small amount of deposits can be measured with the quartz crystal resonator. The vapour phase is prepared by vaporizing a calculated composition of melt according to the vapour‐liquid equilibrium (VLE). RESULTS: The composition of lauric acid in the melt and the melt temperature were utilized as operating variables in the PVD. The growth rate of deposit increases when melt temperature and the composition of lauric acid in the melt are increased. The composition of lauric acid in the deposit is significantly lower than that of the melt of 19% lauric acid, but the composition of lauric acid in the deposit is much higher than that of the melts of 50% and 75% lauric acid. CONCLUSION: The distribution coefficient of lauric acid between solid and vapour phases can be correlated as a function of the growth rate of deposit. The possibility of separation of fatty acid mixtures by PVD is suggested experimentally and theoretically. Copyright © 2008 Society of Chemical Industry     

Cross‐Flow Microfiltration of Fine Particles Suspended in Polymeric Aqueous Solution

Abstract

The filtration characteristics of cross‐flow microfiltration of fine particles suspended in polymeric aqueous solution are studied. Polymethylmethacrylate (PMMA) submicron particles are suspended in polyacrylamide (PAA) aqueous solution to prepare the suspensions used in experiments. Effects of operating conditions, such as cross‐flow velocity, filtration pressure, and PAA concentration, on the filtration flux and the cake properties are discussed. The results show that an increase in cross‐flow velocity or filtration pressure causes the filtration flux to be higher, but the filtration flux decreases with an increase in PAA concentration. Since the flow behavior indices of three prepared suspensions are almost the same, the average specific filtration resistance of cakes under various cross‐flow velocities and PAA concentrations remain almost constant; and then the cake mass plays a major role in determining the filtration resistance and the filtration flux. A force balance model is derived for particle deposition on the membrane surface. Once the empirical coefficients are obtained from experimental data, the filtration flux at pseudo‐steady state can be predicted accurately.     

聚丙烯酸十八酯降凝剂对合成蜡油结蜡特性影响的研究

采用自主研发的Couette结蜡装置研究了聚丙烯酸十八酯（POA）降凝剂对合成蜡油体系结蜡特性的影响。通过对结蜡层表面样（远离结蜡筒）和底部样（靠近结蜡筒）的宏观观察、DSC放热、气相色谱及蜡晶微观结构的分析发现：POA的加入降低了蜡油体系的结蜡速率,加快了蜡油体系的老化速率,且在一定浓度范围内（50～200 μg·g^-1）导致了径向不均质蜡沉积结构的形成,从结蜡层表面到底部含蜡量逐渐升高,但在较高加剂浓度（400 μg·g^-1）时径向不均质蜡沉积结构消失;POA的加入使得结蜡层表面样和底部样的临界碳数（CCN）都由C₂₄升高到C₂₅,但结蜡层底部样与表面样相比低碳数正构烷烃（≤ C₂₅）有所减少,高碳数正构烷烃（≥ C₂₆）有所增加;随着油样中POA浓度的增大,结蜡层表面样与底部样的蜡晶形貌由针状蜡晶逐渐转变为片状蜡晶,且蜡晶尺寸逐渐变大,结构更为致密。