Mathematical modelling of the mixing and flow inside a float glass furnace towards reducing the colour change time
Date
2018
Authors
Lotter, Bernardus Johannes Philippus
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Abstract
The transition between two different glass colours in a float furnace is a costly
manufacturing procedure as the glass properties are out of specification during
this period. This transition period is determined by the control and
understanding of the mixing characteristics for the specific float glass furnace.
Initiatives to reduce costs and waste production have led to increased focus on
reducing the colour change time. Published research on the mixing
characteristics in a glass tank gave reasonable approximations of the response
curve. However, the actual response curves of modern float glass furnaces
deviate from these systems. Therefore, the aim of this investigation was to
create a mathematical mixing model that can approximate the residence time
inside the furnace based on the glass flows. This mathematical mixing model
was used to reduce the duration of the colour change, with respect to the iron
concentration.
The first step was to create a validated CFD model that would provide in depth
knowledge and understanding of the flow cells in the furnace. Subsequently a
mathematical one or two parameter mixing model was developed for the
furnace.
The results from this study indicate that the colour change can be separated into
four parts. A one parameter mixing model with five to eight perfect mixers in
series can describe the first stage of the colour change, where the inlet iron
concentration is set at a certain percentage higher than the final required iron
concentration. The number of perfect mixers in series depends on the barrier
depth with five mixers used for 0.41 m barrier depth and eight mixers in series
for 0.56 m barrier depth. The second stage of the colour change when the barrier
is removed causes a change in the furnace mixing characteristics and a sudden increase of iron concentration at the outlet of more than 0.1 mass % Fe2O3
within 4 hours. The third stage follows this short period by an increase in the
iron outlet concentration that can be modelled by three perfect mixers in series
when the barrier is out and the inlet iron concentration is reduced to the required
outlet iron concentration. The fourth stage of the colour change is when the
barrier is inserted again. This stage can be modelled by a four mixer in series
model for the 0.3 m barrier depth.
The colour change time was reduced by 10 hours, with respect to the iron
concentration, through adjusting key variables during the colour change. These
were the overdope rate, overdope duration and the time when the barrier is
removed. Multiple methods were tested with the mathematic mixing model of
which two methods reduced the colour change time by 10 hours. One method
was increasing the overdope rate to 180 % from the standard 130 % for a
duration of 31 hours and also removing the barrier at 31 hours. Another method
was to remove the barrier at 52 hours instead of 33 hours at an over dope rate of
130 % for a duration of 37 hours.
These methods will decrease the colour change time with respect to iron
concentration, but may increase the time that the optical distortion is out of
specification; as the glass may not homogenise within the required time in the
furnace. It is also noteworthy to mention that this model was created for a
specific furnace design and bubbling rate and will vary from furnace to furnace;
but the perfect mixers in series model could be applied to other furnaces by
using the response curve from that furnace.
Description
A dissertation submitted in fulfilment of the requirements for the degree of Masters of Science in Engineering to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa, 2018
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Citation
Lotter, Bernardus Johannes Philippus (2018) Mathematical modelling of the mixing and flow inside a float glass furnace towards reducing the colour change time, University of the Witwatersrand, Johannesburg, https://hdl.handle.net/10539/26998