Summary
Generally, the larger the evaporating flask the greater
is the evaporation rate. This is due to the greater turbulence
inside a larger flask, enlarging the active
surface and leading to a higher evaporation output. On
the other hand, smaller flasks are advantageous when
the residue needs to be collected for further quantitative
analyses. Moreover, larger flasks generally allow
easier handling.
Introduction
After decades of building rotary
evaporators, there is still potential to optimize the evaporation process. The
choice of the adequate glass accessories is therefore an important aspect in
increasing the evaporation output. For that reason it is interesting to examine
the influence of the evaporating flask size on the evaporation output.
The main impacts of the flask’s
size are:
- Due to increased velocity at the perimeter of larger flasks, the solvent inside the flask is more agitated, enlarging the active surface area of the solvent.
- Due to a larger glass wall surface, more heat energy transfers from the heating bath, through the evaporation flask, to the solvent.
- Larger flasks are more likely to prevent foaming over and the negative consequences of boiling delays and bumping.
- Smaller flask are more suitable for quantitative collection, for example, if the user is interested in the residue that remains after the evaporation
- Smaller flasks offer more flexibility when handling the rotary evaporator in the vertical axis.
Apart from this, larger conventional
evaporating flasks automatically have thicker glass walls which, in turn,
lowers the efficiency of the evaporation performance as there is obviously less
heat transfer through thicker glass walls [1].
Experiment
The flask size certainly affects
the evaporation process. For that reason it is interesting to examine the influence
of the evaporating flask size on the evaporation output. The aim of the
following experiment was to analyze the influence of the size of the
evaporating flask 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 realization
of the experiment, the evaporation process was performed using different sized
evaporating flasks (from 250 mL up to 4 L). 200 mL solvent were used for the
flasks up to 1 L, 750 mL for the flasks of 1 L and larger.
Results
To evaluate the outcomes of the
evaporation performance, the evaporation outputs of the two 1 L flasks were set
to 100 % and the outcomes were divided into two groups, according to the amount
of the solvent, and put in relation to the 1 L flasks.
As illustrated in the graphic above,
the evaporation rate increases with larger flask size. If the evaporation
output of the 1 L evaporating flask is put at 100 %, the 2 L flask achieved 147
% and the 4 L flask 209 % output. The 250 mL flask obtained only 38 % of the
output compared to the 1 L flask.
Interpretation
As seen from the experiment, the
larger the evaporating flask, the higher the evaporation output. For instance,
the evaporation rate using a 4 L flask is significantly higher compared to a 1
L flask.
This is due to the correlation
between the surface area inside the evaporating flask with the flask’s volume.
Therefore, with increasing flask volume, the active surface area is increased,
raising the evaporation output.
Recommendation
The evaporating flask size should
be selected to be as large as possible while still being suitable for the
application. Basically, a larger evaporating flask provides sufficient greater
interior surface area for increased evaporation performance. However, in the
case the residual needs to be quantitatively collect for further analyses, the
flask should be small enough not to risk losing the sample, as it will be more
difficult to transfer the residue out of a larger evaporating flask. Therefore,
the optimum flask size has to be adjusted to the respective sample and
application. In general, it is appropriate to select a flask that accommodates
at least twice the starting sample volume.
BUCHI’s high quality glassware assortment offers evaporating flask
sizes from 50 mL up to 5 L and a broad variety of accessories and glass
assemblies which allow optimized operation. Moreover, the lift of the
Rotavapor® R-300 (Máy cô quay chân không Buchi R-300) is movable 220 mm in the vertical axis and the heating bath
can be moved horizontally. [2] This enables evaporating flasks up to 5 L (Heating
Bath B-305) to be deeply inserted into the heating bath and adjusted to each
individual distillation situation. Nevertheless, BUCHI also offers customized glassware to meet special customer
requirements.
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