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

Abstract

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,

The development of a wax layer on the interior wall of a circular pipe transporting heated oil: the effects of temperature-dependent wax conductivity

“…This approach is gaining considerable traction compared to the traditional mechanical and material diffusion theories; it is able to describe features associated with wax layer formation which have been absent from, or even contrary to, the outcomes from the mechanical and material diffusion theories. This view point is vindicated in a number of recent detailed reviews of the wax layer formation process, see, for example [4,9,12] and [10]. The current paper extends the theory developed in [1] to allow for the dependence of solid wax thermal conductivity on local temperature, which is a significant feature of solidified paraffinic wax, and which should be included as a principle component in the thermal model.…”

“…A modelling approach based on the thermal phase change mechanism as outlined in [1] shows that this, and other fundamental experimental observations can be explained without the need to include rather speculative physical mechanisms. Very recently, the extensive review by Mehrotra et al [9] has given a thorough consideration of experimental evidence, which provides significant and substantial support for the thermal phase change mechanism introduced in [1], as the principal and key mechanism in the process of wax deposition on the interior wall of pipes transporting heated oil. Most recently, a further review has been provided by Van der Geest et al [10], which again supplies detailed and critical experimental support for the thermal phase change mechanism.…”

An Overview of Flow Assurance Heat Management Systems in Subsea Flowlines

“…Wax mainly consists of hydrocarbon normally between C 18 H 38 and C 70 H 142 [25][26][27]. When the waxy crude oil temperature decreases to below its solubility limit in oil, the Wax Appearance Temperature (WAT) or cloud point might be reached [28,29]. Pour point is simply the minimum temperature that the fluid stops to flow and the crude oil becomes "frozen solid" [17,30], and the waxy crude oil properties increase with the wax content [31].…”

Application of full factorial design to screen the factors influencing the wax deposition of Malaysian crude oil

“…Furthermore, higher T values lead to an increased wax deposit rate. The cold finger's temperature was set under the WAT value to measure the temperature influence on wax deposition rate (Fan et al 2021;Mehrotra et al 2020).…”