PhD Thesis by Helena Maria Borja Veiga: “Study of wax deposits in pipelines”

The present research provided original information to aid the understanding of the physical mechanisms governing wax deposition in pipelines. The research program addressed a number of relevant open questions in the literature regarding the formation, growth and aging of the wax deposit layer. To this end, an experimental program was devised, following a strategy of conducting simple experiments, employing lab-scale test sections with well-defined boundary and initial conditions, and using simple test fluids with known properties. Measurements were performed in a rectangular and in an annular test section, both especially designed to allow for optical measurements of the time evolution of the wax deposit thickness spatial distribution. The test sections were equipped with heat flux sensor, temperature traversing probes and deposit sampling ports which allowed for the measurement of relevant local information on the deposit, such as thermal conductivity under flowing conditions, temperature profiles within the deposit, deposit-liquid interface temperature and deposit composition. The temporal and spatial evolution of the deposit layer were measured for different values of the laminar flow Reynolds number. Excellent agreement was obtained between measured values of the deposit thickness and predictions from a numerical model previously developed in our research group. Measurements of the evolution of the deposit-liquid interface temperature have shown that the interface temperature evolves from a value equal to that of the solution wax appearence temperature, WAT, to the wax disappearance temperature, WDT, as the deposit grows to attain its steady state thickness. The temperature traversing probe was employed to obtain information on the temperature profiles within the wax deposit layer under flowing conditions. A comparison of the measured temperature profiles within the deposit with the theoretical solutions indicated the possibility of convective transport in the deposit. Measurements of the deposit thermal conductivity under flowing conditions did not reveal any effects of the imposed shear rate, for the range of Reynolds numbers investigated. Local variations of the thermal conductivity across the deposit layer indicated the presence of liquid close to the cold wall. Deposit samples were obtained and analyzed by high temperature gas chromatography, for the range of the laminar Reynolds numbers tested and for different durations of the deposition experiments. The analyzes revealed that the carbon distributions of the deposit samples presented a shift toward higher carbon numbers both, with increasing deposition time and Reynolds number, characterizing the aging process of the deposit. The carbon number distributions were seen to display an asymptotic behavior with Reynolds number, for samples obtained from the final portion of the longer deposition lengths of the annular test section.

Keywords: Wax deposition, Flow assurance, Interface temperature, Deposit thermal conductivity, Deposit aging.