Đạt được hiệu quả chưng cất cao
hơn khi sử dụng Máy cô quay chân không Buchi – Impact of immersion angle.
Summary
The evaporation rate of a rotary
evaporator depends to a large extend on the immersion angle of the evaporating
flask. The flatter the immersion angle, the higher the evaporation rate. This
is, among other reasons, due to the fact that a flat immersion angle increases
turbulence inside the evaporating flask, hence accelerating the evaporation.
However, with a flatter immersion angle the evaporating flask cannot be as
deeply immersed in the heating bath medium compared with a steep immersion
angle. This can be an issue especially working with smaller evaporating flasks.
Generally speaking, a 40 degrees immersion angle leads to a maximum efficiency
rate without restriction in handling or risking losing parts of the sample due
to over spilling and bumping into the receiving flask.
Introduction
Only a matter of decades ago, the
rotary evaporators’ immersion angles were rather flat. Furthermore, only a few
sizes of evaporating flasks were available, features were limited and the
heating bath a separate unit, hence manually movable. Nowadays the rotary
evaporator and heating bath are most often a single unit. They are designed to
operate with a wide range of evaporating flask sizes and several different
glass assemblies are available so that the rotary evaporators are adjustable
for various applications.
 |
| Figure 1: First generation of rotary evaporators with rather flat immersion angles (Máy Cô quay chân không Buchi, 1957). |
With modern rotary evaporators,
the immersion angle is adaptable to different circumstances. The main impacts
of different immersion angle are:
- If the axis is vertical, there is almost no
mixture inside the evaporating flask and heat transmission is limited to the
immersion area of the flask. Moreover, there is only little turbulence in the
heating bath. As a consequence, only limited heat transfer occurs within the
heating bath.
- Flattening the immersion angle, leads to better
mixing of the content. A larger percentage of the rotating evaporating flask is
wetted by the heating bath medium. This generates a more effective heat
transfer for faster evaporation and prevents local overheating.
- In the case of a very flat immersion angle, an
even larger percentage of the flask is covered by the heating bath medium.
Through the rotation, a thin film of the water/oil of the heating bath is distributed
almost over the whole surface of the rotating evaporating flask. Moreover, the
heating medium is more agitated, which leads to more even heat distribution,
resulting to more efficient heat transfer. However, a flatter angle creates a
risk of the solvent that is being held in the flask, of pouring over into the
receiving flask.
A disadvantage of a flatter
immersion angle is that the evaporating flask cannot be inserted as deeply in
the heating bath. This is especially an issue with smaller flasks. Moreover,
the sample solution is, due to less volume of the flask, more likely to bump
and foam over.
Experiment
The factor of different angles
is, in comparison with other variables of the evaporation process, relatively
significant. For that reason is it worthwhile to find out the evaporation’s
effectiveness by applying different immersion angles. The aim of the following
experiment was to analyze the influence of different angles on the evaporation
rate of a solvent single-stage distillation. The experiment was executed with a
BUCHI Rotavapor® (Máy cô quay chân không Buchi).
For the experiment the evaporation output of acetone was measured
using immersion angles of 10, 25 and 35 degrees, each at a rotation speed of
50, 150 and 250 rpm.
As shown in the graphic above, an
immersion angle of 10 degrees, compared to an immersion angle of 35 degrees,
led to a significant higher output of the evaporation rate, from 119 % (at 50
rpm) up to 180 % (at 250 rpm). An immersion angle of 10 degrees, compared to an
immersion angle of 25 degrees, led to a 13 % higher evaporation output (at 250
rpm). The main increase in performance led the modification of an immersion
angle from 35 to 25 degrees. In all three position, the disparity was always
more significant at higher rotation speeds.
Interpretation
The experiment shows that the
tests with a 10 and 25 degrees immersion angles (flat and standard position)
obtained significantly more evaporation output compared to the tests with a 35
degrees immersion angle (steep position). This is for the simple reason that
due to a flatter immersion angle, the active surface area inside the
evaporating flask is enlarged, thus, accelerating the evaporation rate
significantly. Though, the impact between the settings of 10 and 25 degrees was
less noteworthy.
Recommendation
The immersion angle selected
should be as flat as possible to produce maximum turbulence inside the
evaporating flask. Moreover, in order to perform best, the flask’s immersion
depth should be set at least as deep that the level of the solvent is equal to
the level of the heating bath medium. The BUCHI Rotavapor® R-300 (Máy cô quay chân không Buchi Rotavapor R-300) is manually adjustable to seven different immersion angles within 40
degrees and the lift movable 220 mm in the vertical axis. [1] Furthermore, the
Heating Bath B-305 is movable horizontally to enable 50 mL up to 5 L
evaporating flasks to be completely inserted into the heating bath and, thus,
to be adjusted to each individual distillation situation.
The take home message is using an
immersion angle at 25 degrees guarantees most efficient evaporation rate
without jeopardize losing parts of the sample.
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