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- Volume 19, Issue 3, 2014
South African Journal of Chemical Engineering - Volume 19, Issue 3, 2014
Volume 19, Issue 3, 2014
Source: South African Journal of Chemical Engineering 19, pp 1 –8 (2014)More Less
The rate of drying of zinc sulphate hexahydrate was measured in a 300 mm i.d. fluidised bed dryer at air temperatures of 20-200°C. The final product at these conditions is zinc sulphate monohydrate. The effect of particle size and content of crystallization water on minimum fluidisation velocity, and the rate of dehydration on the air temperature were investigated for zinc sulphate heptahydrate. A simple model has been developed to correlate the drying rate with the conditions of fluidisation. This model was used for a design of a pilot plant vibro-fluidised dryer. The dryer was 4 m long and 0.5 m wide with plenum consisting of four chambers. Each chamber had its own supply of air with temperature control to ensure the right regime of fluidization and drying rate for the required product quality. The rate of dehydration was governed by two process variables: the driving force, and the excess gas velocity U-Umf, which can be taken as the approximate bubble flow rate through the bed. On the basis of laboratory data the flow rate of air into individual chambers was adjusted to keep the excess gas velocity equal. Temperatures in individual chambers of the vibro-fluidized dryer were adjusted to prevent dissolution of crystals in the first chamber by the liberated water. The capacity of the dryer was 280 kg/h of zinc sulphate monohydrate.
Source: South African Journal of Chemical Engineering 19, pp 9 –21 (2014)More Less
Fluidization regime is dependent on factors such as particle size, particle density, particle geometry, column geometry, gas density, gas viscosity together with gas velocity. A significant amount of research has been dedicated to the measurement and analysis of pressure fluctuations in a fluidized bed. Identification of each regime could be achieved through time-series analysis of the pressure fluctuation in the time domain, frequency domain as well as state space domain.
The purpose of this work was to focus on the time-series analysis for the characterisation of the different fluidization regimes (particulate, bubbling, slugging and turbulent) in a gas-solid fluidized bed through the use of spectral analysis. The mathematical tool known as the Fast Fourier Transform (FFT) was used to analyse and interpret the pressure fluctuation in the fluidized bed. Experimental measurements were conducted in a single transparent glass column with a column height and diameter of 128 cm and 11 cm respectively. Two different kinds of solids were used: sand particles (particle size: 0.30 mm) and glass beads (particle size: 2.28 mm). The sampling frequency used for pressure measurements in this work was fixed at 500 Hz with a sampling time of 30 minutes.
Results indicated that factors such as bed height, choice of material as well as gas velocity had a strong influence on fluidization regimes. The dominant frequency characteristics were much more evident for the slugging and turbulent regime respectively. It was further observed that a higher bed height led to more pronounced frequency peaks which allowed for easy differentiation between the various fluidization regimes.
Production of synthetic gas from liquid hydrocarbons and high temperature desulphurization using zinc oxide for Fischer-Tropsch processSource: South African Journal of Chemical Engineering 19, pp 22 –34 (2014)More Less
In this study, a laboratory scale unit was used to produce and clean synthetic gas (syngas) containing 1.0-2.0 mole % H2S from a liquid hydrocarbon fuel consisting of 86% methanol, 14% 2-propanethiol, and oxygen gas as the oxidant. Experiments were performed to determine whether the use of the liquid hydrocarbon mixture as fuel in the laboratory could actually produce H2S containing syngas with a H2:CO ratio of 2:1. Desulphurization experiments were done by varying reaction temperatures (350°C and 550°C) and sorbent particle sizes (1.18-1.70 mm and 1.70-2.00 mm) at atmospheric pressure using sorbent with varying surface areas (5 m2/g and just above 22 m2/g) at a constant gas hourly space velocity of 655 h-1. Gas chromatography (GC) analysis specific to sulphur compound detection showed consistent production of H2S during gasification. In addition to this the composition of the produced syngas was validated through a GC fitted with a thermal conductivity detector (TCD) and there was good agreement giving the expected ratio of 1.97:1 of H2 and CO. Small particle size sorbent had the highest sulphur absorption capacity with retention times of 340 min at 350°C and 285 min at 550°C using 5 m2/g sorbent surface area.
Source: South African Journal of Chemical Engineering 19, pp 35 –47 (2014)More Less
The responsible disposal of wastewater sludge is a problem facing municipalities in South Africa. Coal and Waste Utilisation (Pty) Ltd (CWU) has been operating the Fluidized Bed Reactor (FBR) incinerator plant at the KwaMashu Wastewater Treatment Works (KWTW) in Durban, KwaZulu-Natal, which treats both raw and digested de-watered sludge. The facility operates on a 24/7 basis and incinerates sludge at temperatures in excess of 850°C, producing a stable ash product. As sewage sludge is classified as hazardous waste material, CWU has the required permits to ensure the strict environmental management requirements are met. This includes gas emissions, ash disposal, correct equipment operation and an ongoing local monitoring committee. This publication focuses on the selection and combustion performance of renewable supplementary biomass fuels, including chipped wood from pallets, pine bark, pine sawdust and bagasse. The results are compared to previous operation with coal as a reference scenario. The study forms part of the project's broader aim to utilise renewable energy sources for power generation, in conjunction with the sludge destruction process. It is significant research, as the technology can be applied to many municipalities in South Africa as a sludge disposal solution. The selection criteria formulated for a suitable biomass fuel were in terms of fuel calorific value, physical properties, fluidisation/combustion characteristics and cost. It was found that chipped wood and pine bark were best suited for the KWTW facility, with sawdust a possibility as very large amounts would be required. For chipped wood and pine bark, the cost of production was lower compared to coal (at equivalent production rates), and decreased CO emissions compared to coal. This supports the FBR technology as a sustainable solution for sludge disposal.
Source: South African Journal of Chemical Engineering 19, pp 48 –61 (2014)More Less
A major advantage of circulating fluidized bed combustors (CFBC) is the high flexibility according to fuel type, composition and specific properties like particle size, moisture and morphology. On the other hand, flow and transport characteristics of large scale fluidized beds are far from ideal mixing conditions. Hence, fuel flexibility design for new or existing plants is a multi-parameter optimization problem with numerous aspects from storage and feeding equipment over optimum feeding location to the effect on fuel burn-out, emissions and wear. The combination of plant audits, specific measurements (e.g. primary air nozzle pressure drop as a measure for flow distribution and bed flow pattern) and the application of advanced computational fluid dynamics are presented in this contribution. Specific fuel flexibility challenges and solutions considering
- air staging,
- wear reduction and
- limestone injection for direct desulphurization
- coal/RDF mixture,
- up to 40% substitution of coal with wood pellets and/or wood chips and
- 100% biomass with different fuel specifications.
Source: South African Journal of Chemical Engineering 19, pp 62 –71 (2014)More Less
Most solid particles of practical interest are irregular in shape. In fixed-bed and fluidized-bed reactors, shape of particles can influence bed permeability, gas flow distribution, diffusion of mass and heat and reaction rates. The most popular empirical factor to describe non-spherical shapes of particles is sphericity. In this study sphericity of Highveld coal particles in the range 4.7-106 mm and chars formed by devolatilization of this coal in the range 3.4-45 mm was determined using 3-D laser scanning technique. It was found that the relative variation in sphericity of the coal particles does not exceed 15% and, therefore, it can be possible to describe Highveld coal particles using an average value of sphericity calculated for all size classes. This average value is 0.793 (standard deviation 0.034) and it is very close to the sphericity of particles in the +37.5-53.0 mm size class (øS = 0.798). In contrast to this, the sphericity of devolatalized coal particles were observed to decrease slightly with increasing particle diameter, assuming an average value of 0.749 (standard deviation 0.042).
Pulverized coal versus circulating fluidized-bed boilers - perspectives and challenges for South AfricaSource: South African Journal of Chemical Engineering 19, pp 72 –81 (2014)More Less
Power generation in South Africa has historically and continues to depend on the use of pulverized coal combustion. The pulverized coal boilers are used by the majority of power stations around South Africa. Although other types of technologies, such as circulating fluidized bed (CFB) are available which are not used for this purpose in South Africa. CFB technology is versatile with the advantage of fuel flexibility compared to pulverized coal technology; it is able to burn coal of high ash and sulphur content and low heating values without affecting the emissions of the greenhouse gases; furthermore it burns other types of solid fuels such as low quality coal; lignite and coal blends. This paper addresses the advantages, limitations and challenges of both technologies with the focus on the behaviour of selected South African coals in CFB. The study was carried out using Russian coal of a high calorific value and low ash content as a reference. We investigated the emission trends of CO, CO2, SOχ and NOχ, HCl and other volatiles. It was found that the emissions was below the limit which didn't exceed 250 ppm for SO2 with a combustion efficiency of 98%.
Source: South African Journal of Chemical Engineering 19, pp 82 –89 (2014)More Less
The hydrodynamics of fluidization in a vertically orientated annulus, manufactured from Perspex tubes, was measured optically. Air was used for the fluidization of sieved river sand using multiple air injectors orientated tangentially to the annulus walls. Instead of achieving the desired induced rotation of the fluid bed, jet formation and spouted bed behaviour at superficial gas velocities above the minimum fluidization velocity was observed. To study the hydrodynamic behaviour of the annular fluidized bed in more detail, a representative 2D computational fluid dynamics (CFD) model was developed using COMSOL Multiphysics. It was concluded that although the 2D CFD model could be used as a first estimate, the hydrodynamics of the annular fluidized bed should rather be more accurately modelled in 3D.