close

Вход

Забыли?

вход по аккаунту

?

Large-Scale Industrial Plant for Extraction with Supercritical Gases.

код для вставкиСкачать
Large-Scale Industrial Plant for Extraction with Supercritical Gas~s[**]
By R. Eggers[*]
Criteria for the design and construction of plant for obtaining carrier material or extracts
are discussed for the example of the extraction of natural products with supercritical carbon
dioxide. The parameters to be established and the course of the process in a large-scale
plant are indicated. Plant engineering and optimization must be attuned to specific processtechnological and construction problems.
1. Introduction
In order to exploit the phenomenon of good solubility
of difficultly volatile substances in compressed supercritical
gases, increasing attention has been devoted to the planning
of processes and equipment and the design of suitable largescale plant.
High-pressure extraction with carbon dioxide represents
a potential method for the separation of thermally unstable
materials such as natural products. The resulting advantages
cannot be attained by classical separation processes. The
design and construction of such plant for the extraction of
natural products with supercritical C O will
~ be reviewed below
with due emphasis on the important features.
The operating pressures and temperatures are further fundamental parameters for the design of an extraction plant with
the use of supercritical gases. Apart from process-technologi~dl
data (such as the required input and output of energy) the
above fundamental parameters also determine the plant d e s i ~
characteristics. One problem that should not be underestimated is the precise knowledge of the material properties
of the solvent, the starting material, and the extract which,
in many cases because of the absence of literature data, can
only be obtained by laboratory determinations.
Pressure drops may be calculated on the basis of models,
but if the starting material is unusual with regard to shape
or consistency they must be determined in preliminary experiments.
2. Plant Design Criteria
3. Process Description
The concept of a plant for high-pressure (HP) extraction
of natural products must be based upon the desired aim
of production with due consideration of the starting materials.
Two fundamental possibilities should in principle be distinguished :
a) Currier ~ u t e r sepurut~on.
~u~
This is the situation where
the starting material constitutes the final product after the
removal of certain components by an extraction process.
Examples are the production of decaffeinated coffee and of
nicotine-free tobacco.
b) ~ x t r u c m
f u t e r ~ usepurution.
~
Here the carrier material
is regarded as a virtually valueless matrix from which the
extract is obtained, for example hop extract, cocoa butter,
and extracts of spices.
In order to provide a numerical basis for the plant dimensions and for detailed design of the structural components,
it is necessary that a number of process data be known or
determined:
a) Output capacity;
b) The mode of operation of the plant (continuous production or occasional shutdowns);
c) The bulk density of the material to be extracted;
d) The mass ratio of C O and
~ the material to be extracted;
e) The extraction time.
If the above-listed data are known or specified, it becomes
possible to calculate the required extraction volumes and
hence the size of the vessels and pipe cross-sections.
[*] Dr. R. Eggers
Thyssen lndustrie AG
Postfach 6320, D-5~10Witten 6 (Germany)
[**] From a paper given at the S y ~ p o s i u mon “Extraction with Supercritical
Gases”~held at Essen on 5th June 1978.
A n ~ ~C
w h. ~ ml .n t . Ed. E n ~ l 1. 7 , 7 5 1 ~ 7 S 4( 1 9 7 ~ ~
Products dissolved in a supercritical gas may be separated
out ~y pressure reduction andlor by changing the temperature[’ -41. Separation by changes in temperature will not be
discussed here, because this entails the loss of important constituents in the case of thermolabile natural products.
Plant for separating solute from soIvent by pressure reduction is, in principle, characterized by the following main stages:
I ) Extractor vessel in the extraction stage;
2) Pressure reduction at a throttle;
3) Demixing container in the separation stage;
4) Pressure increase in a pump or a compressor.
The principal course of events in a one-stage process is
described in greater detail below. It may be readily followed
by means of Figures I and 2 and by referring to a block
diagram and the associated flow chart.
P r e p u r u f ~ ~o~
n ~~e f ~ u t e r ~toube
~ extructed. The material
may be comminuted (chopped or ground) or it may be treated
by swelling with steam or water.
C h u r ~ i nof~ the plunt. The quantity of solvent required
for the extraction is determined precisely by the pressure
and temperature conditions.
~ e u t to
~ ~~e
n ~ e x ~ r u c f ~ otemperuture.
n
Heating is carried
out isochorically by pumping the solvent through the stearnheated heat exchanger Wl using the pump P2 until the desired
process conditions-supercritical pressure and supercritical
temperature-have been attained in the extraction vessel A.
Exfrucfion.The supercritical gas extracts the soluble components from the carrier material and is recycled viu the
throttle, the separator vessel B, the heat exchanger W2, the
pumps P3 and PI, and the heat exchanger W l . The pressure
reduction causes a precipitation of the extracted material in
751
extracted material. Finally the extraction residue is removed
from the vessel A and the extract from the vessel B.
4. Thermodynamic Aspects
t~
1
n
1
1
Preparation
an*
char~in~
1
illi in^ the p l a n t
1
M i ~ i nan^
~ ~ e a + t ~~ cn o~n ~ i + i o n ~
of s u ~ e r c r i i i c aea~ tract
In order to obtain the thermodynamic data for a given
step of the process it is particularly useful to have a phase
diagram for the solvent used in the extraction. Many diagrams
are available for C O ~but
, they do not agree in all respect^^^^^'.
Temperature-entropy (~ ~)diagrams are particularly suitable,
because in these diagrams the heat energy supplied and
removed in reversible processes is represented by areas.
In order to illustrate the course of events in a
diagram
(Fig. 3), let us consider the extraction/demixing cycle, using
representative values for the phase data.
~~
i~~
Eytraction
I
Separation nf e ~ t r a c ~
ract fro^
~
C O z - reco~erv
Filter
Fig. 1. Block diagram for extraction using supercritical CO* with the aim
of extract isolation.
P-
1
Fig. 2. Flow chart for obtaining the extract using supercritical C O ~ .A,
extraction vessel; B, demixing vessel; F, filter; S , sorption vessel; R, CO*-recovery with working tank; PI, pressure build-up pump; P2, mixing pump;
P3, preliminary pressure pump; W l , heater; W2, condenser; FRIC, mass
flow governor; FqlC, quantity governor.
the separator vessel, from where it may be continuously
removed by means of a bottom outlet. The medium must
be completely liquefied in the condenser W2, so that suction
can be applied from the liquid gas pump PI and to make
its circulation possible. The pressure rise to the extraction
pressure in PI is followed by raising the temperature to the
extraction temperature in the heat exchanger W l . This completes the cycle.
Provision is made during extraction for periods of stirring
the charge to prevent the formation of channels through which
the supercritical gas could pass freely in the carrier material,
since this would lead to local limitation of mass transfer.
Recovery. The solvent is returned to the working vessel
from the extraction system via filter F, in order to prevent
any contamination of the contents of this vessel with the
752
Fig. 3. Flow chart and phase diagram for a single~stepcyclic extraction
and demixing process. ~~~. ~ , ~ i n ~ r e s s iino nP i ;
heating up in
W i under constant pressure;
~. ~idiabaticpressure release; ~. ~.
evaporation of the liquid fraction;
~~,condensation and supercool in^
in W2 under constant pressure; q, heat flow; w,, power supply; /i, enthalpy.
~~.
~
~
The path from point 3 to point 4 corresponds to the adiabatic
pressure release in the demixing vessel, the extract then separating out and undergoing precipitation. At point 4 the C O ~
is present as a wet vapor. The liquid fraction is evaporated
by supplying heat, so that only the vapor without the extract
is removed from B (point 5). This vapor is condensed in
heat exchanger W2 and the liquid phase is supercooled (point
I). The solvent must then be compressed to the extraction
pressure in pump P i (point I to point 2). The solvent is
~ n a l l yheated at a constant pressure to the extraction temperature by supplying heat to Wl (point 2 to point 3).
It must also be established to what extent the process
data would be modified if data for the charged phases actually
present were to be used rather than those for pure C O ~ .
The thermodynamic consideration has the advantage that
the solvation energies of the components that pass into solution
are negligibly small compared with the enthalpy of the flowing
medium.
A n ~ e Chem.
~ . l n t . E ~ E.n ~ l 17.
. 75~-754 ( ~ 9 7 8 )
5. Plant Technology and Optimization
Although the basic plant concept varies according to the
starting material, it is possible to specify certain components
and combinations of equipment common to all designs and
for which the following characteristics are important:
- vessel design for high pressures (100 to 300 bar), with an
allowance for an increasing stress
- compliance with the requirements of foodstuffs legislation,
i. e. corrosion-resistant internal surfaces and surfaces that
are easy to clean or decontaminate
- temperatures generally not exceeding 100°C
- the bulk density of the starting material is sometimes extremely low.
The vessel-filling devices must be adapted to the widely
varying properties of the natural materials under consideration. Granular material is relatively simple to handle, but
loose bulk material, which can be compacted only with difficulty, presents certain problems. It will be necessary to examine
possible designs for continuous operation in the latter case.
Pretreatment such as grinding may be recommended if the
material suffers no detrimental effects. Materials requiring
special care are suitably placed in baskets that can be prepared
in individual batches; this is generally appropriate for all
natural products in which the leaf o r the fiber structure must
be preserved unchanged.
The design of closures is determined by the method of
charging and discharging. Granular materials may be fed
in or removed through the top or bottom of the vessel using
a small funnel; in the case of baskets it must be possible
to fully open the vessels. In such cases it is particularly necessary to provide for fast closure; there are comparatively few
examples of the latter with the required dimensions in highpressure vessel technology. One such design is shown in Figure
4.
Fig. 4.A quick method of closure for a high-pressure vessel
Pipe sections in which there is a risk of extract separation
along the internal surface require the provision of external
heating, for example by means of electric heating mantles.
The degree of automation of the plant is important. The
following settings must be suitably selected and controlled
Angew. Chem. I n t . E d . Engl. 17, 751-754 ( 1 9 7 8 )
in a plant in which the extraction is carried out with supercritical gases: quantities, mass flows, temperatures, pressure, and
charge levels.
For the construction of a production plant the optimum
working conditions must be known; these are determined
on a pilot plant. Such a pilot plant must permit maximum
possible variation of the operating conditions, in order to
provide adequate data on the yield and quality of extract,
and on the extraction times.
173
3.0
35
5
4.0
4.5
[kJ kg-'K-'J --+
5.0
Fig. 5. Phase range for the extraction of natural products with supercritical
COZ.
The T , s diagram for COz (see the shaded area within the
supercritical region of Figure 5) clearly demonstrates the need
for an extensive operating range of a pilot plant for the extraction of natural products in order to permit optimization of
the operating conditions in the final production plant. Having
thus determined the operating conditions and times for the
Fig. 6. Pressure (-)
and temperature (----) in the extraction vessel as a
function of time. A, charging of the extraction vessel: B. filling the plant
with solvent; C, heating up to extraction conditions; D, removal of the
extract and separation; E, venting down to tank pressure; F, evacuation
to residual pressure; G, venting of the residual gas; H, removal of carrier
material and extract; I, regeneration of the adsorbant.
individual process steps, it becomes possible to plot the course
of the pressure and temperature uersus time; an example of
753
this is given in Figure 6 for the single-step extraction process
that has already been discussed,
[3] 0. l‘itzthum, P. ~ u ~ e rDBP
r , 212761 1 (1971) (Br. Pat. 1336511 (l973)),
Studiengesellschaft Kohle.
Rosef~us,0. ~irzthurn,P . ~ u b e r t DAS
,
2141205 (1971), Hag AC.
[4]
[S] ~. Cramer, Chem.-1ng.-Tech. 27, 484 (t955).
m j ~ .
Properties of ~ a r h o Dioxn
[6] M . P . ~ u ~ a ~ o ~ ~i c~ ~t .~ Therrnophysical
ide. Collet’s, London 1968.
[7] N . B. ~ ~ r ~ Tables
a ~ fon ithe~ Therrnophysical
~
Proper~iesof Liquids
and Cases. 2nd Edit. Hemisphere Publishing, London 1955.
[S] IUPAC: International Tables o~the Fluid State Carbon Dioxide. Pergamon Press, Oxford 1973.
~
Received: June 14, 1978 [A 235 IE]
Cerman version., Angew. Chem. 9(t, 799 (197~)
Translated by Express Translation Service, London
[l] ~. Z ~ . ~ eDOS
l , 1493 190 (1963), Studiengesellschaft Kohle.
P . Hubert,
S i r f ~DBP
,
2127618 (1971) (Br. Pat. 1 3 ~ 8 5 8 1
[2] 0. Vi~z~hurn,
(l975)), Hag AG.
~
~
C 0M MU N I CAT10 N S
Fig. I . Schematic diagram of the apparatus. 1) Solvent reservoir; 2) high-pressure pump for up to 500 bar (HPLC pump); 3) excess-pressure valve; 4) ~ a n o meter for up to 600bar; 5) T-piece; 6) pre-heater capillary (diameter 1.6mm);
7) reactor (V= 50m1), 8) valve; 9) heat exchanger (1.6mm-diameter capillary);
10~
collector; l i ) GC furnace.
Thermal Degradation of ~elluloseand hit in in Supercritical Acetone
By Peter KO11 and J i i r ~ e nM e t = ~ e r [ * ]
In the thermal degradation of polysaccharides rapid removal
of the primary products from the reaction zone is a decisive
prerequisite for the use of this reaction on a preparative or
industrial scale as regards obtaining a high-yield uniform product pattern. Customary pyrolysis methods lead, particularly on
a large-scale, to secondary reactions accompanied by strong
carbonization. This cannot be entirely avoided, even if the
reaction is carried out ~n vacuo; moreover, the pyrolysis is
then rendered dif~cultby the poor transfer of heat.
To overcome this problem, we have for the first time exploited the good dissolution properties of compressed gases in
the supercritical statef1’in a flow apparatus. With this method
we have been able to detect-to our knowledge for the first
time-the primary product of the thermal degradation of chitinfz], namely 2-acetamido-l,6-anhydro-2-deoxy-~-~-glucop y r a n ~ s ~an
~ limportant
,
amino sugar used as starting substance for the synthesis of physiologically active oligosacchaand to isolate it on a preparative scale. Furthermore,
cellulose could be degraded to the extent of 98%.
The apparatus used (Fig. 1~was largely made up of HPLC
equipment; a GC furnace was used for heating. Acetone
(~ = 508.5 K, P~ =47 bar) proved to be especially suitable as
aprotic solvent. In a typical experiment, 18 g of microcrystalline
cellulose(Merck; dry weight 17.1 g) was transferred to a preparative HPLC column and treated at a pressure of 25Obar
with acetone at an average flowrate of4.5 ml/min. The temperature was slowly increased from 250’C at the beginning of
the experiment to 340’C at completion of the degradation
(after IOh). There remained an extraction residue of only
0.36 g (2.1 %). Concentration of the acetone solution ~n vucuo
gave 18.3g of a dark syrup, which was shown by thin-layer
chromatography to consist mainly of low molecular products.
The mass balance exceeded loo%, since small amounts of
difficultly volatile condensation products of acetone are also
formed during the reaction. However, since evolution of gases
during the reaction is very slight, it can be assumed that
approximately 98 % of the cellulose is in fact liquefied. The
main products could be identified, in particular, as anhydrosugars by comparison with authentic materials. The following
amounts (based on consumed cellulose) were determined by
quantitative gas chromatography of the acetates (Carlo Erba
Fractovap 2300, column 2m XE60, T=473 K, injection block
523 K, 20 ml He/min, triacetyl-l,6-anhydroga~actofuranose
as
internal standard):
38.8 % l,6-anhydro-~-~-glucopyranose(“glucosan”), 4.3 %
l,6-anhydro-~-~-glucofuranose,
4.0 % 1,4 : 3,6-dianhydro~-~glucopyranose, cu. I % ,6-anhydro-3,4-dideoxy-~-n-glycerohex-3-enopyranos-2-ulose.
The yield of glucosan can be regarded as excellent when
compared with the yields obtained on vacuum pyrolysisf’I.
The cellulose is extraordinarily gently degraded by acetone,
~
[*] Prof. Dr. P. KOII, Dr. J. Metzger
Fachbereich 4 (Naturwissenschaften) der Universitat
Ammerlander Heerstrasse 6 7 ~ 9 9D-2900
,
Oldenburg (Germany)
754
A n ~ e w Chem.
.
l n t . Ed. E n ~ l 17
. ~ 1 9 7 N~o). 10
Документ
Категория
Без категории
Просмотров
4
Размер файла
431 Кб
Теги
scala, large, gases, supercritical, extraction, industries, plan
1/--страниц
Пожаловаться на содержимое документа