The solids deposition from one-phase and two-phase waxy mixtures (comprising a multicomponent paraffinic wax dissolved in a multicomponent solvent, and water) was studied using a cold finger experimental apparatus. The deposition experiments were performed with a 10 mass % wax solution, containing 0, 10, 20, and 30 vol % water, with two different rates of agitation, and for nine different deposition times ranging from 30 s to 24 h. The water content of the deposit was found to be not related to the water content of the waxy mixture. The short-duration experiments showed the deposition process to be very fast, with more than half of the deposition process completed in 30 s. Following a rapid rate of deposition initially, the deposit mass was observed to increase slowly to reach steady state at about 12 h. The deposit mass decreased with an increase in the agitation speed. The deposition data were modeled satisfactorily with a steady-state heat-transfer model and an unsteady-state model based on the moving boundary problem formulation. The results confirmed the liquid−deposit interface temperature to be equal to the wax appearance temperature (WAT) of the wax solution throughout the deposition process, i.e., for all deposition times. The results of this study provide further confirmation that the solids deposition process can be described adequately with an approach based solely on heat-transfer considerations. ■ INTRODUCTIONThe precipitation and deposition of solids are of significant importance in the production, transportation, and processing of "waxy" or paraffinic crude oils because wax deposition can damage oil reservoir formations and wells and cause blockage of pipelines and process equipment. Solids deposition in pipelines and process equipment causes an increase in pressure drop, an increase in pumping power requirement, and/or a decrease in efficiency. Challenges associated with solids deposition are more severe in cold environments, especially in subsea conditions, where seabed temperatures can be as low as 4°C. 1 With increasing prevalence of deep-water−oil recovery, 2 crude oil is transported over longer distances with an increase in exposure to low temperatures. Such solids deposition problems are expected to become worse and so will the control and remediation costs associated with them.Waxes are mixtures of long-chain hydrocarbons with carbon numbers ranging from 18 to 65. 3 Waxes occur in significant proportions in paraffinic crude oils. These waxes are less soluble in the crude oil at lower temperatures and tend to crystallize and deposit on cooler surfaces. The temperature at which the first wax crystals start to appear in the crude oil during cooling is known to as the wax appearance temperature (WAT) or the cloud point temperature (CPT). It is pointed out that the wax disappearance temperature (WDT, measured while heating a sample) has been shown to provide a better representation of the true solid−liquid phase transformation temperature or the liquidus temperature. 4 Although somewhat higher ...

The deposition of solids from two-phase waxy mixtures (comprising a multicomponent paraffinic wax dissolved in a multicomponent solvent and water) was studied under turbulent flow conditions in a flow-loop apparatus, incorporating a cocurrent double-pipe heat exchanger. The deposition experiments were performed with 6 mass % wax solutions, containing 0, 5, 10, 15, 20, 25, and 30 vol % water, at different flow rates over 5600 < Re < 25 300, and at different hot and cold stream temperatures. In the bench-scale apparatus, the deposit was formed rapidly such that a thermal steady state was attained within 10−20 min in all experiments. The water content of the deposit was found to be not related to the water content of the waxy mixture. The deposit mass was found to decrease with an increase in Re, the waxy mixture temperature, and/or the coolant temperature. The deposit mass also increased as the water content of the waxy mixture was increased to about 10 vol % and decreased thereafter. The deposition data, analyzed with a steady-state heat-transfer model, indicated that the liquid−deposit interface temperature was close to the wax appearance temperature of the waxy mixture. The average thermal conductivity of the deposit was estimated to be 0.38 W m −1 K −1 . Overall, the results of this study confirm that the deposition process from waxy mixtures is primarily thermally driven.

Solids deposition from "waxy" mixtures under turbulent flow in a pipeline was modeled as a moving boundary problem involving liquid−solid phase transformation. The developed model is applicable for the "hot flow" regime (i.e., with the mixture temperature above its wax appearance temperature, WAT) and the "cold flow" regime (i.e., with the mixture temperature below its WAT, resulting in solid particles suspended in the liquid phase). A recently proposed correlation for the wax precipitation temperature (WPT) as a function of the wax concentration and the cooling rate was used to predict the transition from the "hot flow" regime to the "cold flow" regime. Predictions obtained for both radial and axial deposit growth in the pipeline with time in the "hot flow" and "cold flow" regimes were found to be in agreement with the trends observed in the laboratory deposition results reported in the literature. The predicted deposit thickness in the axial direction increased under the "hot flow" regime, reached a maximum as the liquid temperature approached the WAT of the wax−solvent mixture, and decreased subsequently under the "cold flow" regime. The axial location for the transition from the "hot flow" regime to the "cold flow" regime was predicted to shift with changes in the inlet mixture temperature, pipe wall temperature, and Reynolds number. The predicted maximum deposit thickness was also impacted by these variables. The predictions in this study indicate that solids deposition in pipelines carrying "waxy" mixtures could be decreased by maintaining the flow under the "cold flow" regime. This study shows that solids deposition from "waxy" mixtures can be modeled satisfactorily as a thermally driven process involving partial solidification.

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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. K E Y W O R D S aging, cold flow, heat transfer, solid deposition, waxy crude oil 1 | INTRODUCTION Crude oils are complex mixtures of hydrocarbons, including high molar mass alkanes or paraffins, which are referred to as waxes. Waxes contain carbon numbers ranging from 18-65. [1] Crude oils that contain a significant proportion of high molecular weight paraffins or waxes are referred to as paraffinic or "waxy" crude oils. Waxes tend to crystallize and deposit on cooler surfaces because of their decreased solubility in the crude oil at lower temperatures. The highest temperature at which the first crystals start to appear, upon cooling of a "waxy" crude oil, is called the wax appearance temperature (WAT), which is also referred to as the cloud point temperature (CPT). The precipitation and deposition of solids are of significant importance in the production,