Solids Deposition from Wax–Solvent–Water “Waxy” Mixtures Using a Cold Finger Apparatus

Abstract

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 ...

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

“…The WAT of the 'waxy' mixture was 28 8C. [21,22] Each 13-L batch of the wax-solvent mixture was prepared by mixing appropriate amounts of wax and solvent, and heated to about 70 8C and stirred for 2-3 h until a homogenous solution was achieved. Note that each batch of the prepared 'waxy' mixture was sufficiently large to avoid any significant change in its composition, due to depletion in C 20 þ components during the deposition experiments, and a new batch of the 'waxy' mixture was used after completing 6 deposition experiments.…”

“…[35] The density and viscosity of the 'waxy' mixture were estimated from the correlations reported previously. [21,22]…”

“…[18,[26][27][28][29] In the heat-transfer approach, the liquid-deposit interface temperature is taken to be equal to the WAT of the wax solution throughout the deposition process. Batch cooling studies under static and sheared conditions [30,31] and bench-scale flow-loop studies [15][16][17][19][20][21][22] have confirmed the interface temperature (T d ) to remain constant, and close to the WAT, during the gelling and deposit-growth periods. It is also pointed out that the heat-transfer approach has been validated with experimental measurements under both the 'hot flow' (i.e.…”

“…The heat-transfer modelling approach considers the deposit formation and growth to be a partial solidification or freezing process along with crystallization. [15][16][17][18][19][20][21][22][23][24][25] According to the heattransfer approach, the deposition occurs because of a temperature difference between the hot 'waxy' mixture temperature and the colder surroundings. The rate of heat transfer depends on the thermal resistances and the thermal driving force between the bulk temperature of the 'waxy' mixture and the pipe-wall temperature.…”

“…T h < WAT) regimes. [15][16][17][19][20][21][22] The key observations from experimental and modelling studies, [15][16][17][19][20][21][22] based on the heat-transfer approach, are: (1) the deposition process starts soon after the pipe-wall is exposed to a temperature below the WAT and continues at a relatively fast pace until a steady-state is achieved, (2) the steady-state liquiddeposit interface temperature, T d , is always equal to the WAT of the liquid phase of the 'waxy' mixture, (3) the concentration of heavier paraffins in the deposit increases with time (referred to as 'deposit aging'), (4) an increase in the coolant temperature yields a smaller deposit-layer thickness, (5) an increase in the 'waxy' mixture temperature yields a smaller deposit-layer thickness, (6) an increase in the 'waxy' mixture flow rate, or Reynolds number (in both laminar and turbulent flow regimes), yields a smaller deposit-layer thickness, (7) even though the rate of heat transfer depends on the temperature difference between the 'waxy' mixture and the coolant, (T h -T c ), the deposit-layer thickness does not depend on it, and (8) the deposit-layer thickness depends on the temperature difference between the 'waxy' mixture and the WAT, (T h -WAT), and between the WAT and the coolant temperature, (WAT-T c ), such that it increases with a decrease in (T h -WAT) and/or an increase in (WAT -T c ).…”