Phenomenological study on heat and mass coupling mechanism of waxy crude oil pipeline transport process

Based on the phase equilibrium model of the paraffin wax precipitation in the process of oil pipeline transportation, theory and method of non-equilibrium thermodynamics were applied to obtain the linear phenomenological equations for the cross-interaction of heat and mass transfer during pipeline transport, which were derived from the irreversible entropy production rate equation. Then, the analysis of the irreversible heat flow and the mass flow were carried out, and the mathematical expressions of the phenomenological coefficient of liquid phase, the phenomenological coefficient of solid phase flow, and the heat flow phenomenological coefficient were obtained. Taking a waxy crude oil transportation pipeline in Daqing Oilfield as an example, based on the analysis of liquid–solid phase equilibrium, the irreversible linear phenomenological mechanism of heat and mass coupling in waxy crude oil pipeline transportation was analyzed in detail from three levels: phenomenological coefficients which reflect characteristic of the effect of force on flow in heat and mass transfer; thermodynamic forces which trigger heat and mass transfer; transmitted heat and mass flow density, providing a theoretical basis for the further study of the wax deposition in the process of pipeline transportation.     

Investigation of wax deposition and effective diffusion coefficient in water-in-oil emulsion system

As pipeline transportation is widely used in the petroleum industry, the problem of wax deposition is a severe threat to the safety of oil and gas transportation. In addition, the mechanism of wax deposition is very complicated due to the presence of water phase. This paper tries to clarify the effects of water fraction, temperature difference and experimental period on the wax deposition process in water-in-oil emulsion system by a series of static cold finger experiments. The experimental results reveal that the average diffusion rates decrease with increased water fraction, longer experimental period and reduced temperature difference. Furthermore, on the basis of wax deposition experiments in cold finger apparatus and radial temperature distribution simulations via Fluent, the influence of water phase on heat transfer occurring in the wax molecular diffusion process is revealed, and relationship between mass transfer and heat transfer is investigated. Additionally, the effective diffusion coefficient of wax molecules is calculated on the basis of experimental and simulation results. The calculated effective diffusion coefficients using this approach are significantly lower than the calculated results from conventional methods. This explains the remarkable disparity with previous works due to underestimating the influence of dispersed water.     

The wax deposition rate of water-in-crude oil emulsions based on the laboratory flow loop experiment

This paper aims to develop a mathematical model to predict the wax deposition rate of waxy crude emulsions, combining heat and mass transfer mechanisms. According to the flow loop experimental results, the wax deposition rate increases with the decreasing average temperature of oil/wall in a manner of linear regularity, and shows a downtrend with the increase of water cut due to diffusion resistance. An applicable model is developed regarding emulsion properties, radial temperature gradient, shear stress, and wax diffusion coefficient. In model validation, the prediction results are in good agreement with experimental data with the relative errors within 28.87%.     

Advances in the research of polymeric pour point depressant for waxy crude oil

In the pipeline transportation of waxy crude oil when the temperature of the waxy crude oil drops lower than the wax appearance temperature (WAT), wax precipitates continuously from oil phase, which brings a series of negative impacts on pipeline performance and even causes complete blockage under severe conditions in recent decades, polymeric pour point depressants have a very important practical application in depressing the pour point and improving the fluidity of waxy crude oil, thus reducing the cost of waxy crude oil pipeline transportation. The most commonly used types of the polymeric pour point depressants include ethylene-vinyl acetate (EVA) copolymers, comb copolymers, and nano-hybrid pour point depressants. In this paper, the structural character, interactive types, and the action mechanism of the polymeric pour point depressants are reviewed with their application and research progress introduced.     

Slurries and emulsions of waxy and heavy crude oils for pipeline transportation of crude oil

The high viscosity of many asphaltic crude oils and the high pour points of many waxy crude oils present significant problems in their transportation over long distances by pipeline and tanker. While heating the oils and insulating the pipelines will help alleviate the problem, there is danger associated with an extended shutdown of flow and either congealing or solidification of the oil. A possible solution which we have studied in the laboratory is the emulsification or dispersion of the oil in water or brine so that shear takes place in the continuous aqueous phase rather than the oil droplets or particles.Synthetic waxy crude oils were prepared by dissolving paraffin wax in white mineral oil at slightly elevated temperatures and then measuring the pour point. One containing 30% wax had a pour point of 43°C and was selected for preparations of the dispersions. This was emulsified in water at a temperature higher than the pour point by using a suitable surfactant as an emulsifying agent. Rheological properties were measured at various temperatures and are reported in the paper. The method shows great promise for use in countries such as China which produce significant quantities of waxy crude oil and have seasonal temperatures significantly lower than the pour point of the crude oil.     

Prediction Model Based on the Grey Theory for Tackling Wax Deposition in Oil Pipelines

1. Introduction Wax deposition from crude oil is an age-long prob- lem in the petroleum industry. This problem includes progressive precipitation and accumulation of waxes at the sand-face and perforations, tubings, surface production lines and storage tanks, thus limiting the production capacity of these facilities. Depending on the severity, wax deposition may lead to loss of production, mechanical failure of tubular equipment, increased production downtime, increased handling costs and mini…     

KINETICS OF DEPOSITION OF METAL IONS TO ACTIVATED CARBON FIBERS (ACFs) WITH FLUIDIZATION

1. INTRODUCTION Study on the deposition of metal ions on ACFs indicated that such a process consists of several consecutive steps [1]: (1) transfer of the solvated ions (metal ions) from the bulk solution to the proximity of the ACFs surface; (2) absorpt…     

Thermodynamic phase equilibria of wax precipitation in crude oils

Economic loss due to wax precipitation in oil exploitation and transportation has reached several billion dollars a year recently. Development of a model for better understanding of the process of wax precipitation is therefore very important to reduce the loss. In this paper, a new thermodynamic model for predicting phase equilibriums of crude oils is proposed. The modified SRK EOS and the UNIQUAC equations are used to describe the vapor, liquid phase and the wax, respectively. New correlations have been introduced to calculate the volume parameter, c, in SRK EOS and the heat of vaporization in UNIQUAC equation. The model can be used to describe the systems which contain paraffin, naphthene and aromatic fractions. New correlations for the enthalpies, temperatures of solid–solid transitions and fusion enthalpies of paraffins are established in this paper based on data obtained from open literature. By using the proposed modified model, the wax precipitation in hydrocarbon fluids has been predicted for three crude oil systems. The calculation results have been compared with experimental observations and those results obtained using regular solution models. It is found that wax precipitation in complex systems can be better predicted by using this new model.     

Wax inhibition by comb-like polymers: support of the incorporation-perturbation mechanism from molecular dynamics simulations

Deposition of wax on a cold surface is a serious problem in oil production. Progress in developing more effective wax inhibitors has been impeded by the lack of an established mechanism connecting the molecular structure to inhibitor efficiency. Some comb-like polymers having long alkyl side chains are known to decrease the rate of wax formation. Among several possible mechanisms, we investigate here the incorporation-perturbation mechanism. According to this mechanism, the inhibitor molecules in oil are preferentially partitioned (incorporation) toward the wax-rich (amorphous) wax deposits (soft wax), which then serves as a perturbation to slow down the ordering transition of soft amorphous wax into more stable but problematic hard wax crystals. Indeed, molecular dynamics simulations on an effective inhibitor molecule in both the oil phase and in the amorphous wax phase support the idea that the oil-to-wax partition of the inhibitor is energetically favorable. With the inhibitor molecule embedded, the structure of wax crystal is disturbed, significantly decreasing the order and significantly lowering the cohesive energy density relative to that of the pure wax crystal, supporting the slower transition from soft wax to hard wax. Thus, in the presence of an effective wax inhibitor, crystallization (formation of hard wax) is slowed dramatically, so that there is time to flush out the soft wax with a high-pressure flow inside the pipeline. This suggests design principles for developing improved wax inhibitors.     

触变性含蜡原油等温胶凝过程特性及模型研究

综合利用理论推导和流变实验方法对触变性含蜡原油的等温胶凝过程进行了详细的研究, 采用结构参数法建立了静态结构恢复模型, 并经实验结果证明该模型对多种原油的结构变化曲线均有较高的拟合精度. 同时以青海原油为例, 根据实验数据和拟合参数分析了含蜡原油等温胶凝过程特性. 结果表明, 随着温度的升高、预剪切速率的增大以及预剪切时间的增加, 原油在恒温静止过程中形成的结构强度均会减弱. 测量温度越高, 预剪切时间越长, 则原油的结构参数变化速率越大; 而预剪切速率越高, 原油结构参数变化速率在初始阶段越大, 在最终阶段反而越小, 但与未经剪切油样相比, 原油受剪切后其结构参数变化速率在整个测量时间范围内均偏大.     

KINETICS OF DEPOSITION OF METAL IONS TO ACTIVATED CARBON FIBERS（ACFs)WITH FLUIDIZATION

The diffusion and mass transfer, reaction, integration and growth processes of the metalions on the activated carbon fibers (ACFs) are discussed. Based on the diffusion film theory, thediiffusion and the integration model are developed to describe the deposition processes of metal ionsfrom the solution to ACFs in the fiuidized beds. The model of heat transfer of this process isestablished to expound the important role-played in deposition processes by the influence of thereaction heat released at ACFs surface and the non-uniform temperature distribution caused byhydrodynamics.     

Thermal behavior and solid fraction dependent gel strength model of waxy oils

Thermal behavior of waxy oils is investigated using the techniques of thermogravimetric (TG) analysis and differential scanning calorimetry (DSC). Model waxy oils and real waxy crude oils are utilized. Decomposition temperatures of waxy oils are obtained using TG analysis. The effects of thermal history, wax content, and additive on the gelation process of waxy oils are investigated using DSC. The DSC method provides a measure of wax solubility as well as solid fraction. An integration method and a computation method are utilized to predict solid fraction. In addition, wax crystallization onset points are obtained at the cooling rates ranging from 1 to 20 °C min^?1. Similarly, wax dissolution endset points are obtained at heating rates ranging from 1 to 20 °C min^?1. Extrapolated onset and endset points yield wax precipitation temperature and wax dissolution temperature, respectively. Subsequently, wax solubility curves are obtained using thermodynamic computations. A wax precipitation temperature method and a wax dissolution temperature method combine thermodynamic phase behavior with onset/endset points to predict solid fraction. Both the wax precipitation temperature method and the wax dissolution temperature method can predict solid fraction of waxy oil samples. The wax precipitation temperature method and the wax dissolution temperature method are accurate when the temperature is close to the wax appearance temperature. A heat-integration method provides accurate values of the solid fraction at temperatures significantly below the wax appearance temperature. Therefore, integration method and wax precipitation temperature/wax dissolution temperature method are combined to predict solid fraction. The effect of solid fraction on yield stress is also investigated using differential scanning calorimetry and rheometry. Finally, a new solid fraction dependent gel strength model is obtained for shut in and restart of waxy crude oil pipelines.     

Combining of intelligent models through committee machine for estimation of wax deposition

Deposition of the wax is one of the thorny issues in the petroleum industry, invoking costly problems during the transportation and production of crude oil. Owing to its devastating impacts on oil companies' economy, it is essential to develop a simple and robust strategy for the quantitative estimation of wax deposition. In this paper, support vector regression (SVR) is first proposed to estimate the amount of wax deposition. Subsequently, an artificial neural network (ANN) is developed for wax deposition prediction. Eventually, a sophisticated committee machine (CM) is constructed for combining the results of the SVR and ANN models. Optimal contribution of each model in final prediction of the wax deposit is determined through genetic algorithm in CM. Statistical error analysis shows that the CM model performs better than the individual models performing alone.     

Rheological behavior and structural interpretation of waxy crude oil gels

A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.     

降凝剂对蜡油中蜡析出与溶解影响的物理化学研究

用DSC热分析仪研究了合成蜡油的加热与冷却过程,测定了蜡油在不同蜡浓度下,添加降凝剂前后的平衡蜡溶点和析蜡点以及溶解度和饱和度,并进行了热力学分析.结果表明,含蜡油的平衡蜡溶点高于平衡析蜡点,降凝剂使平衡蜡溶点进一步升高,析蜡点进一步降低,导致含蜡油凝点较大幅度降低.在实验浓度和温度范围内,该过程符合Van′tHoff方程,降凝剂使蜡的溶解焓和溶解熵增大,析出焓和析出熵减少.降凝剂提高了蜡晶析出的临界半径,增大了成核位垒,使蜡晶析出困难.     

Surface diffusion in reversed-phase liquid chromatography

More than 40 years ago, Giddings pointed out in “Dynamics of Chromatography” that surface diffusion should become an important research topic in the kinetics of chromatographic phenomena. However, few studies on surface diffusion in adsorbents used in chromatography were published since then. Most scientists use ordinary rate equations to study mass transfer kinetics in chromatography. They take no account of surface diffusion and overlook the significant contributions of this mass transfer process to chromatographic behavior and to column efficiency at high mobile phase flow rate. Only recently did the significance of surface diffusion in separation processes begin to be recognized in connection with the development of new techniques of fast flow, high efficiency chromatography. In this review, we revisit the reports on experimental data on surface diffusion and introduce a surface-restricted molecular diffusion model, derived as a first approximation for the mechanism of surface diffusion, on the basis of the absolute rate theory. We also explain how this model accounts for many intrinsic characteristics of surface diffusion that cannot properly be explained by the conventional models of surface diffusion.     

Role of the Dissolution Mechanism in Determining Chromatographic Separation of Biogenic Amines from Their Precursors and Metabolites by HPLC-EC

Abstract

The current study was designed to elucidate the theoretical basis for chromatographic separation of biogenic amines on an octadecyl-silica (C-18) reverse phase column by determining the intermolecular forces between the solute and the stationary phase. The solutes mass transfer diffusion and the heat effect between solutes and stationary phase were assessed by a convenient method. This study demonstrates that the dissolution mechanism plays a major role in the process of chromatographic resolution of biogenic amines and their precursors and metabolites by HPLC-EC.     

含蜡原油胶凝过程特性研究

利用RS75流变仪在小振幅振荡剪切的模式下分别研究了3种不同历史条件下含蜡原油的胶凝过程, 同时通过偏光显微镜观察了不同降温速率条件下原油中的蜡晶形貌. 结果表明, 在静态降温条件下, 降温速率越大, 原油胶凝的温度越低, 原油形成的胶凝结构越弱; 并且降温速率越大, 原油在恒温静止过程中, 结构随时间恢复的速率越大, 恢复至平衡所需时间越长, 但最终的平衡结构却越弱; 在同样的降温速率下, 原油低温胶凝结构随着降温过程中剪切作用的增强而减弱, 但当历史剪切速率超过一定数值时, 原油的胶凝结构将基本不再继续变化; 在同样的历史剪切速率下, 降温速率越大, 原油在低温(31 ℃)静止初始的储能模量越小, 但最终的平衡结构却越强.     

Mechanism and kinetics of hydrated electron diffusion

Molecular dynamics simulations are used to study the mechanism and kinetics of hydrated electron diffusion. The electron center of mass is found to exhibit Brownian-type behavior with a diffusion coefficient considerably greater than that of the solvent. As previously postulated by both experimental and theoretical works, the instantaneous response of the electron to the librational motions of surrounding water molecules constitutes the principal mode of motion. The diffusive mechanism can be understood within the traditional framework of transfer diffusion processes, where the diffusive step is akin to the exchange of an extramolecular electron between neighboring water molecules. This is a second-order process with a computed rate constant of 5.0 ps(-1) at 298 K. In agreement with experiment the electron diffusion exhibits Arrhenius behavior over the temperature range of 298-400 K. We compute an activation energy of 8.9 kJ mol(-1). Through analysis of Arrhenius plots and the application of a simple random walk model it is demonstrated that the computed rate constant for exchange of an excess electron is indeed the phenomenological rate constant associated with the diffusive process.     

HZSM-5分子筛上甲醇制烯烃典型产物的传质行为研究

分子筛催化甲醇制烯烃反应(MTO)是典型的扩散主导反应过程,运用频率响应技术系统研究了几种典型产物分子(乙烯/乙烷、丙烯/丙烷、苯)在HZSM-5分子筛上的扩散行为。结果表明,频率响应法成功辨析了不同产物分子的传质规律,证实C2和C3烃分子在HZSM-5微孔孔道内具有相近的扩散速率,但由于受晶体表面阻碍效应影响不同,乙烷分子可自由进出HZSM-5分子筛孔道,而丙烷分子则受到较显著的微孔孔道扩散限制。另外,苯分子的扩散速率显著小于C2和C3分子,且苯分子受晶体表面阻抗效应的影响较小。本研究结果可用于解释HZSM-5分子筛在MTO反应中产物选择性的特点及表面结焦原因,进而从传质角度为高活性、选择性以及稳定性的高效甲醇转化制烃催化剂的定向开发提供理论指导。