Đạt được hiệu quả chưng cất cao hơn khi sử dụng Máy cô quay chân không Buchi – Impact of rotation speed
ĐẠI DIỆN ĐỘC QUYỀN HÃNG BUCHI TẠI VIỆT NAM
Summary
The evaporation rate of a rotary
evaporator is greater than that of static distillation apparatus. Moreover, the
evaporation rate increases with higher rotation speed. This is, among other
factors, due to the greater surface area inside the evaporating flask at one
given time. Especially significant is the dissipation at lower rotation speeds.
For instance an increase in speed above 200 rpm has a relatively low influence
on the evaporation output. A rotation speed above 300 rpm can result in
mechanical problems, vibrations and spillage from the heating bath. Thus,
considering the advantages and disadvantages of different rotation speeds, the
optimum rotation speed is around 250 to 280 rpm.
Introduction
Even 50 years after the launch of
the first rotary evaporator, there is much ambiguity concerning evaporation in
the rotating evaporating flasks.
Even nowadays, evaporation and
distillation are still one of the most frequently used separation methods. In
fact, the evaporation output of a rotary evaporator during single-stage
distillation is around four times greater than that of conventional, static
distillation apparatus [1]. There are several reasons why the rotation
influences the evaporation output:
- Through the rotation, the liquid inside the heating bath is agitated, causing additional turbulence, which leads to a massive improvement of the heat transfer coefficient and, therefore, to an increase of heat transfer from the heating bath to the evaporating flask as well as from the flask to the solvent.
- Rotation greatly enlarges the active surface area inside the evaporating flask, thus accelerating the evaporation. As the solvent is heated via the heating bath, it is distributed as a thin film over the warm, inner surface of the rotating evaporating flask. This film partially vaporizes, resulting in a higher evaporation rate. With the liquid remaining at any one point of the flask wall for a short time only, rotation homogenous mixture of the sample is ensured and overheating, thus incrustation inside the flask is reduced.
- The frictional force and the centrifugal force between the wall of the rotating evaporating flask and the liquid inside the flask, result in the formation of a thin film of warm solvent spread over a larger surface.
Additional advantages are that
the forces created by the rotation suppress bumping, foaming and boiling
delays. However, the higher the rotation speed, the greater is the chance that
mechanical problems with the rotary evaporator occur and the resulting
vibration is more likely to cause deterioration of the instrument.
Experiment
Rotation certainly speeds up the evaporation.
It is of great importance to scrutinize to what extent the speed of the
rotation influences the evaporation output. For this reason, the dependence of
the rotation speed on the evaporation rate should be determined. The aim of the
following experiment was to analyze the impact of the rotation speed on the
evaporation rate of a single-stage solvent distillation. The experiment was
executed with a modified BUCHI Rotavapor® ( Máy cô quay chân không Buchi) where the rotation speed was
adjustable to 500 rpm. The experiment was executed in two steps. Firstly, in
order to get a broad overview, the evaporation output of acetone was measured
at nine different rotation speeds (from 50 up to 500 rpm). Secondly, the
evaporation output of water was measured at stillstand and six different
rotation speeds, within the actual operating range of up to 280 rpm.
The graphic above shows that an
increase of the rotation speed up to 400 rpm raised the evaporation output.
Though, the evaporation rate decreased again at speeds above 400 rpm (acetone).
The test series with water clearly showed that the dissipation was especially
significant at lower rotation speeds and relatively less above 200 rpm.
Interpretation
The experiment shows that the
maximum evaporation output is achieved at a rotation speed around 400 rpm. This
is because the rotation speed can be raised to a point where the content will
be pressed against the wall by centrifugal force and corotate with the flask.
In this case the turbulence will decrease again, which consequently results in
a lower evaporation rate. The optimal turbulence depends on the flask size, the
consistency and texture of the solvent/substance and the filling quantity.
Recommendation
The rotation speed should be
selected to produce maximum turbulence inside the flask as well as in the
heating bath, in order to reach maximum evaporation output and still guarantee
a long lasting life of the rotary evaporator.
The graphic illustrates the ideal
rotation speed range, where the problems, such as spillage, vibration,
mechanical problems, are still low. Considering the factors of maximum output
and long lifetime of the instruments, the optimum rotation speed of a BUCHI Rotavapor® ( Máy cô quay chân không Buchi) is from 250 to 280 rpm. For this
reason the Rotavapors are limited to a rotation speed of 280 rpm.
ĐẠI DIỆN ĐỘC QUYỀN HÃNG BUCHI TẠI VIỆT NAM
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