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Đạt được hiệu quả chưng cất cao hơn khi sử dụng Máy cô quay chân không Buchi - Tác động của sự khác biệt nhiệt độ.
Summary
There is a direct
relationship between the heating bath temperature and the evaporation rate. The
more energy applied to the evaporation side, and at the same time removed from
the condensation side, the more efficient is the distillation. Furthermore,
sufficient cooling as well as an appropriate and stable under pressure are
crucial for efficient distillation. On the other hand, the consumption of
electrical energy is comparatively greater at higher temperatures. Moreover,
some samples are thermo-sensitive, thus exacerbating the circumstances.
Therefore the respective parameters have to be fine-tuned to the individual
sample and application. The “Delta 20 Rule” is a guideline to compromise
between high evaporation output and energy usage. For instance, using the
10/30/50 parameters is appropriate for the evaporation process in order to
bring in and to carry off the accumulated energy efficiently.
Introduction
The performance of a
rotary evaporator (Máy cô quay chân không Buchi) is limited by the input, the
amount of heat that can be added to the evaporation side, and the output, the
amount of heat that can be removed on the condensation side. Basically, energy
is imparted to the solvent in order to transform it to the vaporous state;
during the condensation cycle this energy has to be removed again within the
same length of time.
Formerly, only the energy supply
was easily controllable. The cooling temperature was rather inflexible as
mainly tap water was used as the cooling source. Moreover, the vacuum was only
roughly controllable. Nowadays, the vacuum can be adjusted very precisely and
kept stable. Furthermore, with the possibility of the modern “recirculating
chiller”, the energy supplied for cooling the condenser can be selected
accurately, typically to produce temperatures as low as -5 to 10 °C. Therefore
recirculating chillers are very effectively in cooling and the distillation can
be kept at low temperatures. The heating bath temperature, the vacuum as well
as the cooling temperature need to be adjusted to the condenser’s capacity. A
condenser is working at its optimum capacity if two-thirds of its height is
covered with condensate, hence the top third acts as a safety barrier for
"entrained" low-boiling solvent plus for pressure fluctuations. A
condenser is overloaded if condensate is seen to form downstream from the
condenser or if the vacuum pump sucks continually in order to maintain a
specific pressure. The speed of evaporation and condensation should be attuned
to maintain a balanced dynamic pressure.
When working with a
thermo-sensitive sample, a mild heating bath temperature needs to be selected
in order not to harm the compounds. In addition, a heating bath at lower
temperature is more convenient to work with. For instance, with a heating bath
temperature of 50 °C, the evaporating flask can be changed without any risk of
scalding. With higher temperatures, the vaporizing rate of the heating bath
medium (e.g. water) increases, and it thus has to be refilled more frequently.
This results in additional consumption of energy.
Experiment
The aspects of
heating and cooling are very important and determine the evaporation rate. It is
interesting to examine to what extend different heating bath temperatures
influence the evaporation output. The aim of the following experiment was to
analyze the impact of the amount of energy, in form of heat, applied to the
system on evaporation rate of a solvent single-stage distillation. The
experiment was executed with a BUCHI Rotavapor® (cô quay chân không Buchi). For
the experiment the evaporation process was executed using five different
heating bath temperatures (from 40 to 80 °C).
Achieve higher distillation efficiency when using a rotary evaporator – Impact of temperature differences.
As illustrated in the above
graphic, the higher the heating bath temperature, the higher is the evaporation
rate. The differences of the evaporation output increased more or less linearly
with the temperature rise. For instance, with a heating bath of 80 °C, the
distillation output was about four times greater compared to a heating bath
temperature of 40 °C.
Interpretation
As the temperature
of the heating bath was raised, the evaporation output increased significantly.
However, the energy consumption of the heating bath and recirculating chiller
increased, too. For instance, when using an 80 °C heating bath, it should be
remembered that much more energy has to be supplied and again removed from the
system than is the case when working at lower temperatures.
Recommendation
The heating bath
temperature and the vacuum needs to be coordinated for the condenser to work as
closely as possible to optimum condenser’s capacity without being overloaded.
For a sufficient condensation of the vapor, the cooling temperature should be
about 20 ºC lower than the vapor temperature.
BUCHI recommends that the “Delta 20 Rule” should be applied. This
rule of thumb can be applied as following: set the bath temperature at 50 °C to
yield a solvent vapor temperature of 30 °C, which is subsequently condensed at
10 ºC [1].
The “Delta 20 Rule” can also be
applied to lower heating bath temperatures for solvents with a low boiling
point or thermo-sensitive products. For example: cooling media: 0 ºC; vapor: 20
ºC; heating bath: 40 °C and the pressure lowered in order to lower the solvents
boiling temperature. A heating bath temperature above 50 °C is less easy to
handle, thus increasing the risk of accidents. Moreover, environmental and
economical issues should also be taken into account. The “Delta 20 Rule” makes
solvent removal simple and efficient. The vacuum is the only setting that has
to be changed and the pressure for each solvent can be conveniently selected
from BUCHI’s “List of solvents”. The
Heating Bath B-100 and B-305 have a heating power of 1300 watts and a standard
condenser a cooling area of 1500 cm2 [2], hence it can achieve high evaporation
output. To sum up, the “Delta 20 Rule” compromises evaporation output and
energy consumption. The optimized settings of the heating and cooling
temperatures are depending on the specific application and have to be
fine-tuned for each individual sample.
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ĐẠI DIỆN ĐỘC QUYỀN HÃNG BUCHI TẠI VIỆT NAM
Trụ sở tại TP. Hồ Chí Minh: Số 71 đường số 6 KDC Trung Sơn, Bình Hưng, Bình Chánh.
VPĐD tại HN: Nhà B7, Ngõ 23, Phố Đỗ Quang, Phường Trung Hòa, Quận Cầu Giấy.
Email: huynhthanhmt06@gmail.com