The flow behaviour of inorganic - wood fibre slurries in pressurised pipes

Understanding the flow behaviour of inorganic-wood fibre slurries is important for developing new process equipment for the cement fibreboard industry. Little is reported in the technical literature and generally slurry flow knowledge is limited to a few engineers within the industry. Pipe friction loss characteristics and the settling behaviour of inorganic-wood fibre slurries were studied and data were obtained in pressurised horizontal pipes ranging from 25 to100mm diameter at flow velocities up to 8m/s. The inorganic solids studied were cement and fine silica of size range 10 to 150 m. Solids concentrations ranged from 5-20% and fibre concentrations from 0- 2%. Wood pulp fibre suspensions at low fibre concentrations form a structured carrier medium with the ability to support fine particulate solids. Unlike fibre-free suspensions, no permanent stationary deposit formed and therefore no minimum settling velocity exists. At low flow rates particles are trapped in the fibre plug and the friction loss is above water. At high flow rates the particles are still supported but the fibres dislodged from the central plug core damp turbulence and friction losses for the cement-silica-fibre system are less than water (drag reduction). The overall flow behaviour is similar to and consistent with previous data reported for coal-fibre slurries. Fibre concentration has a significant affect on the onset of drag reduction and friction loss increases with fibre concentration as with conventional fibre suspensions. Pipe diameter has a minimal effect on the onset of drag reduction but friction loss decreases with diameter as with conventional fluids

Plug flow versus mixed flow modelling of a pressure screen

The plug flow and mixed flow methods of modelling a screen are analysed in terms of screen length, internal fibre concentration and screen separation performance. Experimental data for consistency changes in screens of narrow length are incorporated into the analysis and from this data an equation for determining the real thickening in the screen and the fibre concentration profile along the screen is derived. Lastly, thicken equations are related to common equations used for modelling overall screen performance. The work directly links screen performance equations to passage ratio measurements from narrow length screen sections

The effect of flotation cell shape on deinking behaviour

Studies were undertaken to investigate the deinking behaviour of different shaped deinking cells of the same volume. For comparative purposes, most oprational variables were kept constant, and the same injector was used throughout the study. The position of the injector, however, was varied in some cases to go along with the particular cell shape being studied. Three types of cell shapes were studied, (1) cylindrical with tangential air injection, (2) rectanular with vertical injection, and (3) rectangular with horizontal injection. Eucalyptus/toner slurries and news/mag wastepaper slurries were deinked. Flow patterns in the cells and the corresponding deinking efficiencies were measured. It was found that strong and excessive re-circulatory flows within the cells could under certain conditions be a major factor in reducing brightness lift. Vertical injection into a rectangular cell gave stable flow patterns, non-wavy froth removal and sustained brightness lift for a wide range of feed and airflow rates. Horizontal injection into a similar rectangular shaped cell exhibited quite different characteristics. High brightness lift was possible for certain conditions and not for others. Wavy froth and excessive recirculation flow patterns varied with feed and airflow. The cylindrical cell with tangential injection gave stable circulatory flow and stable froth removal at low flow rates but was unable to deink at high flows

Carbon emissions reduction and net energy generation analysis in the New Zealand electricity sector through to 2050

Carbon Emissions Pinch Analysis (CEPA) and Energy Return On Energy Investment (ERoEI) analysis are combined to investigate the feasibility of New Zealand reaching and maintaining a renewables electricity target of above 80% by 2025 and 2050, while also increasing electricity generation at an annual rate of 1.5%, and with an increase of electricity generation in the distant future to accommodate a 50% switch to electric vehicle transportation. To meet New Zealand’s growing electricity demand up to 2025 the largest growth in renewable generation is expected to come from geothermal generation (four-fold increase) followed by wind and hydro. To meet expected demand up to 2050 and beyond, including electric vehicle transportation, geothermal generation will expand to 17% of total generation, wind to 16%, and other renewables, such as marine and biomass, will make up about 4%. Including hydro, the total renewable generation in 2050 is expected to reach 82%

California’s Renewables Portfolio Standard (RPS) requires 33% renewable electricity generation by 2020 - Dream or Reality?

Progress on California’s Renewable Portfolio Standard (RPS), which requires 33% of all retail electricity sales to be served by renewable energy sources by 2020, excluding large hydro, is reported in this paper. The emerging renewable electricity mix in California (CA) and surrounding states which form the Western Electricity Coordination Council (WECC) is analysed using the Carbon Emission Pinch Analysis (CEPA) and Energy Return on Energy Invested (EROI) methodologies. The reduction in emissions with increased renewables is illustrated and the challenge of maintaining high EROI levels for renewable generation is examined for low and high electricity demand growth. The role of the California government in facilitating progress towards a more sustainable renewable electricity future is also highlighted. The investigation shows that wind and solar PV collectively form an integral part of California reaching the 33% renewables target (excluding large hydro) by 2020. Government intervention of tax rebates and subsidies, net electricity metering and a four tiered electricity price has accelerated the uptake of renewable wind and solar PV. Residential uptake of solar PV is also reducing overall California electricity grid demand. Emphasis on new renewable generation is stimulating development of affordable wind and solar technology in California which has the added benefit of enhancing social sustainability through improved employment opportunities at a variety of technical levels

An investigation of milk powder deposition on parallel fins

One method to reduce the energy consumption of industrial milk spray dryers is to recover waste heat from the exhaust dryer air. A significant challenge associated with this opportunity is the air contains a small amount of powder that may deposit on the face and surfaces of a recuperator. This paper introduces a novel lab based test that simulates powder deposition on a bank of parallel plate fins at exhaust dryer air conditions. The fin bank acts like the face of a typical finned tube row in a recuperator. The aim of this study is to look at how deposition on the front of fins is affected by the air conditions. Results show similar characteristics to other milk powder deposition studies that exhibit a dramatic increase in deposition once critical stickiness levels are reached. As powder deposits on the face of the fins, the pressure drop across the bank increases until eventually an asymptote occurs, at which point the rates of deposition and removal are similar. For very sticky conditions, deposition on the face of the fins can cause a rise in the pressure drop by as much as 65%. The pressure drop has also been successfully related to the percentage of open frontal area of the fins with and without deposition. Deposition inside and at the rear of the fin bank was found to be minimal

Area targeting and storage temperature selection for heat recovery loops

Inter-plant heat integration across a large site can be achieved using a Heat Recovery Loop (HRL). In this paper the relationship between HRL storage temperatures, heating and cooling utility savings (heat recovery) and total HRL exchanger area is investigated. A methodology for designing a HRL based on a ΔTmin approach is compared to three global optimisation approaches where heat exchangers are constrained to have either the same Number of Heat Transfer Units (NTU), Log-Mean Temperature Difference (LMTD) or no constraints (actual global optimum). Analysis is performed using time averaged flow rate and temperature data. Attention is given to understanding the actual temperature driving force of the HRL heat exchangers compared to the apparent driving force as indicated by the composite curves. The cold storage temperature is also varied to minimise the total heat exchanger area. Results for the same heat recovery level show that the ΔTmin approach is effective at minimising total area to within 5 % of the unconstrained global optimisation approach. The study also demonstrates the efficiency of the ΔT min approach to HRL design compared to the other methods which require considerable computational resources

Integration of solar heating into heat recovery loops using constant and variable temperature storage

Solar is a renewable energy that can be used to provide process heat to industrial sites. Solar is extremely variable and to use it reliably thermal storage is necessary. Heat recovery loops (HRL) are an indirect method for transferring heat from one process to another using an intermediate fluid (e.g. water, oil). With HRL’s thermal storage is also necessary to effectively meet the stop/start time dependent nature of the multiple source and sink streams. Combining solar heating with HRL’s makes sense as a means of reducing costs by sharing common storage infrastructure and pipe transport systems and by lowering nonrenewable hot utility demand. To maximise the value of solar in a HRL, the means of controlling the HRL needs to be considered. In this paper, the HRL example and design method of Walmsley et al. (2013) is employed to demonstrate the potential benefits of applying solar heating using the HRL variable temperature storage (VTS) approach and the conventional HRL constant temperature storage (CTS) approach. Results show the VTS approach is superior to the CTS approach for both the non-solar and solar integration cases. When the pinch is around the hot storage temperature the CST approach is constrained and the addition of solar heating to the HRL decreases hot utility at the expenses of increased cold utility. For the VTS approach the hot storage pinch shifts to a cold storage pinch and increased heat recovery is possible for the same exchanger area without solar. With solar the VTS approach can maintain the same heat recovery while also reducing hot utility still further due to the presence of solar, but only with additional area. When the pinch is located around the cold storage temperature, solar heating can be treated as an additional heat source and the benefits of CTS and VTS are comparable

Design and operation methods for better performing heat recovery loops

Inter-plant integration via a heat recovery loop (HRL) is an economic method for increasing total site process energy efficiency of semi-continuous processes. Results show that both the constant storage temperature approach and variable storage temperature approach have merit. Depending on the mix of source and sink streams attached, it may be advantageous to change the operation of an existing HRL from a constant temperature storage to a variable temperature storage. To realise the full benefits of this change in operation, a redistribution of the existing heat exchanger area may be needed

Internal fibre length concentration in a pressure screen

Localised axial consistency profiles within a pressure screen of a Pinus Radiata kraft pulp are reported. Axial Samples were also analysed using a Kajaani FS-200 to obtain fibre length distribution data. Localised consistency in the feed annulus was found to vary considerably and the consistency was found to be less than the feed consistency over some portions of the screen (annular dilution). Changes in consistency along the accept side was fairly constant although subtle changes were observed. Pulp passage ratios for both the bulk and individual fibre length fractions were calculated using the consistency profiles and fibre length data. In all cases fibre passage decreased along the screen length. Fibre passage was affected by a position effect which is comprised of two factors: flocculation effects, and flow and rotor effects. Fibre fractionation efficiency was found to increase along the length of the screen. Mechanisms that account for the observed annular dilution, passage ratio and efficiency changes are proposed. These involve flow of both fluid and fibre in the forward and reverse directions across the screen plate, increased flocculation in the feed annulus and the slip velocity between incoming pulp and the rotor tip