Peter Toma

Occurrence of Naturally Shaped Lenticular Bed Deposit and Its Influence on the Frictional Pressure Drop during Pipeline Transportation of Low Concentration Slurries

Solid particles can be transported along a pipeline in the form of trains of individually shaped lenticular deposits (LDs) when the concentration of solids is less than 1 vol% and the transport velocity is below the critical value required for full suspension. Such special bed transport, observed as rippled sand dune patterns, may occur in petroleum production lines transporting oil and gas produced from unconsolidated sand reservoirs under turbulent flow conditions, and during sediment transport by rivers and winds. The primary objective of this study was to investigate how the occurrence of lenticular bed deposits affects near-wall turbulent activities at the sand/fluid (“wave-like”) interface and frictional pressure drop during pipeline transportation of solids. Particle image velocimetry (PIV) measurements were used to quantify the velocity field, the turbulence kinetic energy (TKE), and the coherent structures associated with surface morphology change and LD formation near the bed deposits/fluid interface. A 7–8% reduction in frictional pressure drop was consistently observed during the transition from continuous sand bed to LDs. Results also indicate that the formation of naturally shaped LDs reduces the intensity and frequency of near-wall turbulent coherent structures (burst-sweep events). Moreover, TKE associated with flow over the LDs was found to be lower than that of continuous bed and water (only) flow.

Reducing the Deposition of Scale in the Evaporator of a Mechanical Vapour Recompression System for Concentration of Pulp Mill Effluents

A numerical model was developed, validated with the aid of pilot evaporator tests, and used to assess practical methods of minimizing scaling observed in a mechanical vapour recompression (MVR) plant at Millar’s Western’s Meadow Lake, Saskatchewan pulp mill. On average, 8,000 m3 /day of effluent (approximately 7 m3 /Bone Dry Tonne product) resulting from bleached-chemi-thermo-mechanical wood processing, are purified and the recovered water is returned to the plant while the effluent is concentrated to 75% TD&SS in a recovery boiler. The evaporators are used in the first stage of the feed concentration process. The system uses a heat pump principle: steam produced during the boiling of the falling liquid film is mechanically compressed and condensed on the outer surface of a vertical tube evaporator. Most of the condensation and compression heat is recovered. Reducing the rate of scale deposition and increasing the interval between two successive cleaning operations of vertical evaporators used in the MVR scheme was identified as an essential component of operation costs and given special attention. To assist the mill in assessing practical methods for achieving this goal an experimental pilot evaporator and a numerical model were developed and used first at the Alberta Research Council in Edmonton, Canada, and then at the mill location. The mill uses a different model for control and supervision of system parameters. The magnitude of the (critical) temperature difference (CTD) across the laminar sub-layer of boiling liquid film is calculated and is recommended in this paper to be used to quantify the fouling tendency. Further to recommendations resulting from previous experimental investigations [1,2] as well as in this study, the mill introduced additional process control parameters to reduce and maintain the temperature drop across the effluent boiling film to a maximum range of 2–4°C. In addition to CTD, the wall (top-bottom) axial temperature difference (ATD) has been identified as another criteria for assessing potential scale deposition during evaporation-concentration. Calculations and experimental measurements performed with the pilot evaporator [3] suggest that increasing the circulation rate of effluent pumped from the sump to feed the liquid film at the top of evaporator tubes has a positive effect on reducing the CTD and the ATD. During four months of laboratory investigations with a pilot evaporator, non-uniform liquid distribution among vertical evaporator tubes of the evaporator was observed and is discussed separately. This paper will present the model and mill observations and summarize the main results and suggested practical strategies for reducing the rate of scale deposition and improving the system economics.Copyright