Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Mass transfer efficiency in distillation, absorption and stripping depends on both
thermodynamic efficiency and hydrodynamic behaviour. Thermodynamic efficiency is
dependent on the system kinetics while hydrodynamics is the study of fluid flow behaviour.
The focus of this thesis is the hydrodynamic behaviour in tray columns, which affects
entrainment. In order to isolate hydrodynamic behaviour from the thermodynamic
behaviour that occurs inside sieve tray columns, investigations are conducted under
conditions of zero mass transfer. When the gas velocity is sufficiently high to transport liquid
droplets to the tray above, entrainment occurs. The onset of entrainment is one of the
operating limits that determines the design of the column and thus impacts on the capital
cost. By improving the understanding of the parameters that affect entrainment, the design
of the tray and column can be improved which will ultimately increase the operability and
capacity while reducing capital costs.
Existing correlations predicting entrainment in sieve tray columns are based on data
generated mainly from an air/water system. Previous publications recommend that more
testing should be performed over larger ranges of gas and liquid physical properties. An
experimental setup was therefore designed and constructed to test the influence of the
following parameters on entrainment:
1. gas and liquid physical properties
2. gas and liquid flow rates
3. tray spacing
The experimental setup can also measure weeping rates for a continuation of this project.
The hydrodynamic performance of a sieve tray was tested with air and water over a wide
range of gas and liquid flow rates and at different downcomer escape areas. It was found
that the downcomer escape area should be sized so that the liquid escaping the downcomer
always exceeds a velocity of approximately 0.23 m/s in order to create a sufficient liquid
seal in the downcomer. For liquid velocities between 0.23 and 0.6 m/s the area of escape
did not have an effect on the percentage of liquid entrained. It was also established that
entrainment increases with increasing gas velocity. The rate at which entrainment increases
as the gas velocity increase depends on the liquid flow rate. As soon as the liquid flow rate
exceeded 74 m3/(h.m) a significant increase in entrainment was noted and the gas velocity
had to be reduced to maintain a constant entrainment rate. This is because the increased
liquid load requires a longer flow path length for the froth to fully develop. The
undeveloped froth, caused by the short (455 mm) flow path, then creates a non-uniform
froth that is pushed up against the column wall above the downcomer. Consequently, the froth layer is closer to the tray above resulting in most of the droplets ejected from the froth
reaching the tray above and increasing entrainment. By reducing the gas velocity, the froth
height and ejecting droplet velocity is reduced, resulting in a decrease in entrainment.
The results from the experiments followed similar trends to most of the entrainment
prediction correlations found in literature, except for the change noted in liquid flow rates
above 74 m3/(h.m). There was, however, a significant difference between the experimental
results and the correlations developed by Hunt et al. (1955) and Kister and Haas (1988).
Although the gas velocities used during the air/water experiments were beyond the
suggested range of application developed by Bennett et al. (1995) their air/water
correlation followed the results very well.
The entrainment prediction correlation developed by Bennett et al. (1995) for non-air/water
systems was compared with the experimental air/water results to test for system
uniformity. A significant difference was noted between their non-air/water prediction
correlation and the air/water results, which motivates the need for a general entrainment
prediction correlation over a wider range of gas and liquid physical properties.
Based on the shortcomings found in the literature and the observations made during the
experiments it is suggested that the influence of liquid flow path length should be
investigated so that the effect on entrainment can be quantified. No single correlation was
found in the literature, which accurately predicts entrainment for a large range of liquid
loads (17 – 112 m3/(h.m)), high superficial gas velocities (3 – 4.6 m/s) and different gas and
liquid physical properties. It is therefore recommended that more work be done, as an
extension of this project, to investigate the influence of gas and liquid physical properties on
entrainment (under zero mass transfer conditions) for a large range of liquid (5 – 74
m3/(h.m)) and gas (2 – 4.6 m/s) flow rates. In order to understand the effect of droplet drag
on entrainment, tray spacing should be varied and increased to the extent where droplet
ejection velocity is no longer the mechanism for entrainment and droplet drag is responsible
for droplet transport to the tray above.
Since it is difficult and in most cases impossible to measure exact gas and liquid loads in
commercial columns, another method is required to measure or determine entrainment.
Since liquid hold-up was found to be directly related to the entrainment rate (Hunt et al.
(1955), Payne and Prince (1977) and Van Sinderen et al. (2003) to name but a few), it is
suggested that a correlation should be developed between the dynamic pressure drop
(liquid hold-up) and entrainment. This will contribute significantly to commercial column
operation from a hydrodynamic point of view.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4237 |
| Date | 03 1900 |
| Creators | Uys, Ehbenezer Chris |
| Contributors | Knoetze, J. H., Burger, A. J., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 170 p. : ill. |
| Rights | University of Stellenbosch |
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