The effect of suspended wax crystals in wax-solvent mixtures on the solid deposition process in the cold flow regime was investigated experimentally and analyzed with a steady-state heat transfer model. A bench-scale flow-loop apparatus, incorporating a concentric-cylinder heat exchanger, was used to measure solid deposition, in the cold flow and hot flow regimes, from wax-solvent mixtures under turbulent flow conditions. The deposition experiments were performed with two wax-solvent mixtures, at two flow rates, with two coolant temperatures, at 8 wax-solvent mixture temperatures, and for several deposition times. The role of wax crystals on the deposition process was investigated by repeating some of the deposition experiments with a pre-filtered wax-solvent mixture. In all experiments, the deposit was formed rapidly such that a thermal steady-state was attained within 30 min. The deposit mass increased with decreasing the mixture temperature in the hot flow regime, reached a maximum as the mixture temperature became equal to the WAT, and then decreased linearly to zero in the cold flow regime as the mixture temperature approached the coolant temperature. Also, the deposit mass decreased with an increase in the Reynolds number and the coolant temperature. The data and predictions confirmed the solid deposition to be a thermally-driven process. The experimental deposit mass results in the cold flow regime, supported by model predictions, were identical for the unfiltered and filtered mixtures, which showed that the suspended wax crystals do not affect the deposit mass or thickness.
The process of solid deposition from wax–solvent mixtures was compared with that of ice deposition from liquid water by means of experiments using a cold-finger apparatus and a transient mathematical model based on the Stefan “moving boundary problem” formulation. Two agitation speeds of 250 rev min–1 (Reynolds number of 4100) and 500 rev min–1 (Reynolds number of 8200), with two coolant temperatures of T f – 4 °C and T f – 7 °C, at four water temperatures (from T f + 3 °C to T f + 0.7 °C), and for 11 deposition times between 30 s and 8 h, were used in the ice-deposition experiments. The wax-deposition experiments were undertaken using a 10 mass % wax–solvent multicomponent mixture, at an agitation speed of 250 rev min–1 (Reynolds number of 1400), with a constant coolant temperature of wax appearance temperature (WAT) – 12 °C, at four mixture temperatures (from WAT + 6 °C to WAT), and for 17 deposition times ranging from 2 s to 48 h. Both the ice-deposition and the wax-deposition processes were remarkably similar. Both of these phase-change systems were extremely rapid during the first few minutes. A higher deposit mass was achieved by lowering the liquid water temperature, the coolant temperature, and the agitation speed. The experimental results from this investigation, supported by those from previous studies, indicated that a higher deposit mass is achieved with lowering of the liquid mixture temperature, the coolant temperature, and the agitation speed. The results of both sets of experiments were consistent with predictions from the Stefan moving boundary problem framework, which considers both of these phase-change processes to be governed only by the heat-transfer steps involved in the freezing of a liquid. This study confirms that the solid deposition from wax–solvent mixtures is described adequately based entirely on heat-transfer considerations.
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,
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