On the simulation of hydrocyclones using CFD

This study reports on the development of a complete solution methodology for the simulation of a hydrocyclone. A commercial software package, Ansys 12 Fluid Dynamics, is used to solve the governing conservation equations. Turbulence is modelled using the large eddy simulation, and the discrete particle model was used to predict the particle separation. Two hydrocyclones of differing geometries are explored, and the results compared with experimental values. It is shown that there are two key factors for obtaining a reliable result. The first is the domain discretisation, and the second is the generation of a consistent initial value, including the establishment of a stable air core. Using the methodology developed, superior agreement was obtained for predicted and experimental values of pressure, velocity distribution, air core profile and separation efficiency. © 2012 Canadian Society for Chemical Engineering     

HYDRAULICS AND EFFICIENCY OF SCREEN TRAYS FOR DISTILLATION

Screen trays are proprietary devices similar to sieve trays which are used in distillation and absorption applications. The pressure drop, liquid holdup and entrainment are measured for screen trays in an air/water column and compared to results obtained with sieve trays. The effciencies of both types of trays were compared in a distillation column using systems of various physical properties (methanol/water, acetic acid/water and cyclohexane/n-heptane mixtures). It was found that the screen tray has a lower dry pressure drop, higher liquid holdup, lower entrainment and weeping, and higher jet flooding capacity than the sieve tray. However, the effciencies of both trays are similar for the systems studied. A model was developed for predicting total pressure drop for screen trays using an air/water column. The model was tested using results for methanol/water, cyclohexane/n-heptane and acetic acid/water systems at total reflux. The deviations are within +/- 20% for 90% of the data points. The effect of a bed of mesh packing on the screen tray was also studied. It was found that the packing led to a higher tray effciency, but resulted in a higher tray pressure drop.     

Prediction of gas hold‐up in a bubble column filled with pure and binary liquids

A dimensionless correlation for gas hold‐up in a bubble column filled with pure liquids and binary mixtures has been developed. The criteria of bubble coalescence in pure liquids and foamability of binary mixtures have been included successfully in the proposed correlation. For the first time, comprehensive data of gas hold‐up with respect to concentration in four binary systems are presented. The often‐reported enhancement of gas hold‐up in aqueous alcohol mixtures has been observed experimentally. The proposed correlation is able to predict successfully the trend of gas hold‐up enhancement with respect to concentration. The estimated values are found to be within 7% of the measured values.     

WALL SAMPLING FROM HORIZONTAL SLURRY PIPELINES

An experimental study of the errors associated with wall sampling from a horizontal slurry pipeline has been conducted. Experiments were performed for water-sand slurries flowing in a one inch horizontal pipeline. The effects of particle size (0.08 mm to 0.4 mm), upstream slurry solids concentration (4% to 12%, by volume), upstream bulk velocity (1.8 m/s to 4.0 m/s), sampler diameter (4 mm to 25.4 mm), sampler location, and sampling velocity ratio (defined as the ratio of the sampling velocity to the upstream bulk velocity, 0 to 4.5) on the sample solids concentration and sample particle size distribution were examined.

The experimental results show that the observed sample solids concentration and its particle size distribution are functions of the sampler location, sampler diameter and sampling velocity. For the case of sampling from the top or from the side of the pipe, the sampling efficiency (defined as the ratio of the sample solids concentration to the upstream solids concentration) was found to be always less than unity, and it increases as the sampling velocity increases. Also, the sample mean particle diameter is significantly smaller than that in the main pipe, especially at low sampling velocities. For sampling from the bottom of the pipe, the relation of the sampling efficiency to the sampling velocity ratio exhibits a maximum, and the sample mean particle diameter is very close to that in the main pipe.

The operating conditions upstream of the sampler also affect the sample solids concentration and its particle size distribution. The importance of the upstream bulk velocity, upstream solids concentration and particle size on the sample efficiency and sample particle size distribution depend on the sampler location.     

Bubble Size Distributions for Dispersed Air – Water Flows in a 100 mm Horizontal Pipeline

The bubble size distributions of air dispersed in water flowing in a 100 mm horizontal pipeline were studied. Size distributions were obtained with a high‐speed digital camera at axial positions 0.5, 15.3 and 42.3 m from the air injection point. Air was injected into the pipeline through a narrow tube extending into the pipe, perpendicular to the pipe axis. The effects of average water velocity, air concentration, and injection nozzle diameter on the evolution of bubble size distribution with axial pipe position were studied. For the lowest air concentration of 0.07%, equilibrium bubble size distributions were dependent only on water velocity. Nozzle injection diameter did not affect the downstream bubble size distributions at air concentrations of 0.07% and 0.3%. Levich's break‐up theory was found to over‐predict the experimental d_max for each test condition.     

Hydrodynamics in a gravity settling vessel: CFD modelling with LDA validation

Vertical gravity settling vessels, usually referred to as primary separation vessels (PSV), are used in separating bitumen aggregates from slurry containing sand and fine clays. The hydrodynamics in the PSV influences the separation efficiency of recovered bitumen through the overall mean flow and turbulent interaction. In order to deepen our understanding of the hydrodynamic conditions in such vessels, this paper presents a combined study of the flow field using Laser Doppler Anemometry (LDA) to measure the velocity field, and computational fluid dynamics (CFD) simulations to validate the CFD model. The investigation shows that the flow geometry has a significant influence on the overall flow pattern in such vessels. It also demonstrates that the CFD simulation is a reliable tool in capturing the complex mean flow pattern observed in experiments. Use of different turbulent models such as the standard k‐ε model and Reynolds stress model has very little effect on the mean flow field.     

Single particle impingement current transients for prediction of erosion-enhanced corrosion on 304 stainless steel

A new device was used to study the characteristics of a single particle impact on the surface of stainless steel. Maximum depassivation occurred at 30° where nearly 30% of the scar area was depassivated. The kinetics of particles motion and electrochemical response of the surface were correlated. The predicted erosion-enhanced corrosion current under slurry flow conditions based on the correlation reasonably matched the experimental results at lower sand concentrations. The predicted currents deviated from experimental values at higher sand concentrations where interactions between particles before and after impact are not negligible.     

Undergraduate design of experiment laboratory on analysis and optimization of distillation column

A two-level full factorial design and/or a central composite design are used to optimize the operation of a distillation column by manipulating three variables: the feed flow rate, the reflux ratio and the steam flow to the reboiler. The response variable of interest is a profit function where the objective of this experiment is to examine the effect of each of the three factors and their interactions on this profit function. This is accomplished in a sieve tray type continuous distillation column operated at ambient pressure to separate a methanol–isopropanol mixture. Students were asked to determine the significant variables and the optimal operating conditions for steam rate, reflux ratio and feed rate such that the profit is maximized. As a result, this experiment not only exposes students to an industrially relevant unit operation but it also helps complement the classroom teaching on design of experiment concepts.     

Double-sensor conductivity probe applied in a flotation system

This study proposed a new approach for measuring bubble size distribution, bubble mean diameter, Sauter mean bubble diameter, and gas holdup using a double-sensor conductivity probe in an air/water two-phase system bubble column. The results for the two-phase system were compared and calibrated using analyses from bubble images taken by a digital camera from the side of the column wall. Good agreement was observed between the two techniques. The same double-sensor conductivity was used in an air/water/solids three-phase system. The conductivity probe captured the change in bubble dynamic behaviour inside the pulp phase; however, the presence of the solids made it more challenging to measure. As a result, the VisioFroth commercial package, using images taken from the top of the froth layer, could be used in conjunction with the double-sensor conductivity probe to show the dynamic evolution of mineralized bubbles from the pulp zone to the froth zone in a flotation process.