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/Title (Impact of Mixing Parameters on Homogenization of Borax Solution and Nucleation Rate in Dual Radial Impeller Crystallizer)
/Author ( A . K a u n i , M . o s i , N . K u z m a n i)
/Creator (WASET Zend Publishing System)
/ModDate (D:20160705130259+02'00')
/Subject (Interaction between mixing and crystallization is often
\r\nignored despite the fact that it affects almost every aspect of the
\r\noperation including nucleation, growth, and maintenance of the
\r\ncrystal slurry. This is especially pronounced in multiple impeller
\r\nsystems where flow complexity is increased. By choosing proper
\r\nmixing parameters, what closely depends on the knowledge of the
\r\nhydrodynamics in a mixing vessel, the process of batch cooling
\r\ncrystallization may considerably be improved. The values that render
\r\nuseful information when making this choice are mixing time and
\r\npower consumption. The predominant motivation for this work was
\r\nto investigate the extent to which radial dual impeller configuration
\r\ninfluences mixing time, power consumption and consequently the
\r\nvalues of metastable zone width and nucleation rate. In this research,
\r\ncrystallization of borax was conducted in a 15 dm3 baffled batch
\r\ncooling crystallizer with an aspect ratio \(H/T\) of 1.3. Mixing was
\r\nperformed using two straight blade turbines \(4-SBT\) mounted on the
\r\nsame shaft that generated radial fluid flow. Experiments were
\r\nconducted at different values of N/NJS ratio \(impeller speed/
\r\nminimum impeller speed for complete suspension\), D/T ratio
\r\n\(impeller diameter/crystallizer diameter\), c/D ratio \(lower impeller
\r\noff-bottom clearance/impeller diameter\), and s/D ratio \(spacing
\r\nbetween impellers/impeller diameter\). Mother liquor was saturated at
\r\n30°C and was cooled at the rate of 6°C/h. Its concentration was
\r\nmonitored in line by Na-ion selective electrode. From the values of
\r\nsupersaturation that was monitored continuously over process time, it
\r\nwas possible to determine the metastable zone width and
\r\nsubsequently the nucleation rate using the Mersmann’s nucleation
\r\ncriterion. For all applied dual impeller configurations, the mixing
\r\ntime was determined by potentiometric method using a pulse
\r\ntechnique, while the power consumption was determined using a
\r\ntorque meter produced by Himmelstein & Co. Results obtained in
\r\nthis investigation show that dual impeller configuration significantly
\r\ninfluences the values of mixing time, power consumption as well as
\r\nthe metastable zone width and nucleation rate. A special attention
\r\nshould be addressed to the impeller spacing considering the flow
\r\ninteraction that could be more or less pronounced depending on the
\r\nspacing value.)
/Keywords (Dual impeller crystallizer, mixing time, power\r\nconsumption, metastable zone width, nucleation rate.)
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WASET Zend Publishing System
2016-07-05T13:02:59+02:00
2016-01-05T11:40:01+02:00
application/pdf
Impact of Mixing Parameters on Homogenization of Borax Solution and Nucleation Rate in Dual Radial Impeller Crystallizer
merve
Interaction between mixing and crystallization is often<br />
ignored despite the fact that it affects almost every aspect of the<br />
operation including nucleation, growth, and maintenance of the<br />
crystal slurry. This is especially pronounced in multiple impeller<br />
systems where flow complexity is increased. By choosing proper<br />
mixing parameters, what closely depends on the knowledge of the<br />
hydrodynamics in a mixing vessel, the process of batch cooling<br />
crystallization may considerably be improved. The values that render<br />
useful information when making this choice are mixing time and<br />
power consumption. The predominant motivation for this work was<br />
to investigate the extent to which radial dual impeller configuration<br />
influences mixing time, power consumption and consequently the<br />
values of metastable zone width and nucleation rate. In this research,<br />
crystallization of borax was conducted in a 15 dm3 baffled batch<br />
cooling crystallizer with an aspect ratio (H/T) of 1.3. Mixing was<br />
performed using two straight blade turbines (4-SBT) mounted on the<br />
same shaft that generated radial fluid flow. Experiments were<br />
conducted at different values of N/NJS ratio (impeller speed/<br />
minimum impeller speed for complete suspension), D/T ratio<br />
(impeller diameter/crystallizer diameter), c/D ratio (lower impeller<br />
off-bottom clearance/impeller diameter), and s/D ratio (spacing<br />
between impellers/impeller diameter). Mother liquor was saturated at<br />
30°C and was cooled at the rate of 6°C/h. Its concentration was<br />
monitored in line by Na-ion selective electrode. From the values of<br />
supersaturation that was monitored continuously over process time, it<br />
was possible to determine the metastable zone width and<br />
subsequently the nucleation rate using the Mersmann’s nucleation<br />
criterion. For all applied dual impeller configurations, the mixing<br />
time was determined by potentiometric method using a pulse<br />
technique, while the power consumption was determined using a<br />
torque meter produced by Himmelstein & Co. Results obtained in<br />
this investigation show that dual impeller configuration significantly<br />
influences the values of mixing time, power consumption as well as<br />
the metastable zone width and nucleation rate. A special attention<br />
should be addressed to the impeller spacing considering the flow<br />
interaction that could be more or less pronounced depending on the<br />
spacing value.
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