Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Field of the Invention
This invention relates to purification of air or gas and more
particularly refers to a new and improved method and apparatus for the
selective adsorption of individual vaporous or gaseous impurities from air
or other gases.
Description of the Prior Art
One problem of gas purification technology, especially of air
purification technology, is the separation of vaporous or gaseous impurities
from a stream of gas. It is known to solve such problems by means of adsorp-
tion techniques. It is likewise known to utilize adsorption filters for
this purpose. It is also known to accomplish the separation of such im-
purities by thermal combustion which may take place in the flame, as well as
by means of a catalyst. It is also known to store adsorbable impurities in
an adsorption filter and to recover them after desorption, for instance
through condensation, or to burn them after the necessary combustion air
has been added. A disadvantage of these methods is that there is no selec-
tive effect and thus, mixtures of vapors or gaseous impurities can be pre-
cipitated but not selectively separated. The problem of separation is con-
cerned with practical applications as for example in painting technology.Separating the paint solvents used is not only a necessity with respect to
keeping the air clean but the recovery subsequent to the separation can
result in substantial economic advantages if these recovered solvents can be
reintroduced into the painting process at low cost. In the exhaust air of
painting lines, there are, in addition to solvents, also softeners with a
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higher boiling point than the solvents. The recovery of the softener or
plasticizer can as a rule be dispensed with. Recovery of the softeners is
generally uneconomical and may cause contamination of the solvents sought
to be recovered.
Summar~ of the Invention
Ln object of the present invention is to provide an efficient system
of selective separation of vaporous or gaseous impurities from gases and the
recovery of valuable impurity components.
With the foregoing and other objects in view, there is provided in
accordance with the invention method for the selective adsorption in adsorp-
tion filters of a mixture of vaporous or gaseous impurities of different
volatilities in a stream of air or gas which includes passing the stream of
air or gas containing the mixture of vaporous or gaseous impurities of dif-
ferent volatilities through at least two adsorption filters connected in
tandem, maintaining the temperature of adsorption in the first adsorption
filter at a temperature sufficiently high to selectively adsorb the impuri-
ties of low volatility with passage of the stream containing impurities of
higher volatility through the first adsorption filter, and passing the stream
containing impurities of higher volatility through at least the second
adsorption filter at a temperature lower than the temperature in the first
adsorption filter to selectively adsorb the impurities of higher volatility.
In accordance with the invention there is provided apparatus for
the selective adsorption in adsorption filters of a mixture of vaporous or
gaseous impurities of different volatilities in a stream of air including
at least two adsorption filters connected in tandem, means for maintainin
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the temperature of adsorption in the first adsorption filter at a temperature
sufficiently high to selectively adsorb the impurities of low volatility
with passage of the stream containing impurities of higher volatility through
the first adsorption filter, and means for maintaining the temperature of
adsorption in the second adsorption filter at a temperature lower than the
temperature in the first adsorption filter to selectively adsorb impurities
of higher volatility.
Brief Description of the Drawings
Other features which are considered as characteristic for the
invention are set forth in the appended claims. Although the invention is
illustrated and described herein as embodied in method and apparatus for the
selective adsorption of vaporous or gaseous impurities from other gases, it
is nevertheless not intended to be limited to the details shown, since various
modifications may be made therein without departing from the spirit of the
invention and within the scope and range of equivalents of the claims.
The invention, however, together with additional objects and
advantages thereof will be best understood from the following description
when read in connection with the accompanying drawings, in which:
Figure 1 diagrammatically illustrates one embodiment for selectively
adsorbing impurities of different volatilities from gas containing them; and
Figure 2 diagrammatically illustrates a simplified form of
selectively adsorbing and desorbing impurities.
Detailed Description of the Invention
The separation by means of adsorption is carried out in such a
manner that at least two sorption filters are connected in tandem and are
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operated at different temperatures, with the temperature of the gas to be
purified being reduced from one sorption stage to the other by interposed
coolers.
Referring to Figure 1, the gas to be purified is fed to the sorption
filter 21 through the feedline 1 at the temperature corresponding substan-
tially to the conditions under which the gas collects. At the start of
operation, the valve 211 is opened, and the valves 214 and 215 are closed.
If the temperature of the gas entering feedline 1 is to be changed before
it enters the sorption filter 21, a first heat exchanger 31 is connected into
the line 1. This heat exchanger 31 has a fluid such as steam or cold water
flowing through it to raise or lower the temperature of the gas entering
through feedline 1 and passing through heat e~changer 31 to the temperature
desired for the first sorption stage 21. The gas is selectively purified
in the first gas sorption filter 21 by adsorption of low volatile impurities
in the gas. The adsorption temperature in filter 21 is sufficiently high to
permit passage through filter 21 of the air or gas and substantially all of
the highly volatile impurities but the temperature should be sufficiently
low to retain substantially all the low volatile component. The gas from
sorption filter 21 leaves through the valve 212, while the valve 213 is
closed. The gas then flows through the heat exchanger 32 which brings it to
the temperature at which the adsorption in the second stage is to take place,
which temperature is lower than the temperature of adsorption in filter 21.
The gas then flows through the open valve 221 to the second sorption filter
22, wherein impurities of higher volatility are adsorbed, and it leaves this
purification stage through the open valve 222. This may be followed further
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by as many sorption stages with the associated coolers and corresponding
valves as desired. Figure 1 shows the first two stages.
The purified gas is pumped by the blower 4 through the entire
cooler-sorption filter chain and is conveyed via the gas discharge 5 for
further processing or discharge into the open atmosphere. Blower 4 may be
dispensed with if the gas entering feedline 1 is under sufficient pressure
to overcome pressure drop and propel it through the entire cooler-sorption
filter chain.
In the individual sorption filters 21, 22 and in those which may
follow the second filter but are not shown in Figure 1, the materials are
stored and become loaded at the temperature prevailing in each stage due to
the respective adsorption isotherms. It is unavoidable that initially sub-
stances, for instance, of lower boiling point and higher volatility which
properly should be separated only in one of the following stages, are ad-
sorbed in the first stage. However, with advancing loading, these are dis-
placed by~the impurities of lower volatility which are to be separated in
this stage. Thereby, if the individual sorption filters are saturated to a
predetermined level with the selected impurity, a nearly uniform sorbate can
be removed from the sorbent by an inert gas desorption in known manner, as
20 described, for example, in United States Patents 3,853,985; 3,905,783 and
3,930,803. For this purpose, the inert gas generator 6 may be connected to
the inert gas inlet valves of the sorption filters 213, 233 and, as appli-
cable, in a corresponding manner for further stages via a line 62. As with
this procedure the stage to be desorbed is interrupted for the passage of
gas because the valves for feeding-in the gas to be purified, 2113 221 etc.
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and those for the discharge of the gas purified in this stage, 212, 222 etc.
are closed, parallel-connected alternate sorption filters, not shown can in-
sure continuous operation if during desorption the alternate filters are
switched into adsorption, and during the desorption of the parallel sorption
filter the previously desorbed filter is switched back to adsorption. m us
it is possible to alternate filters in each stage of adsorption. As a
practical matter, however, we have found such measure need not be employed
in the separation stage for low volatile impurities since such stage will
usually run for extended periods, as long as 10 days or more, without re-
quiring desorption, generally because the quantity of such impurities in thegas is small. m e highly volatile components of the impurities of the gas
which are separated at a relatively low temperature require more frequent
desorption, usually every two or three days.
The inert gas from the stages which are being desorbed leaves the
sorption filters via valves 214 in the first stage, 224 in the second stage
or via the corresponding ones in the other stages and is sent through a
collecting line 7 and returned to the inert gas inlet by a blower 8. The
inert gas generated in the inert gas generator 6 is admixed with the returned
inlet gas from blower 8. In the cooler 9 which advantageously precedes the
blower 8, an adjustment to the temperature is made so that after being mixed
with the hot inert gas, the gas mixture has the temperature required for the
desired desorption conditions. The gas so conditioned serves for the desorp-
tion and the flushing of the desorbate from the sorption filter, as described,
for example, in German Published Prosecuted Application P 22 48 267.
At the points A and B, the excess gas coming from the stage being
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desorbed can be fed to the process to be associated with that stage, after
the corresponding valves 215 for A, and 225 for B, etc. are opened. Thus,
after-combustion of the desorbate, for example, may follow at A, while a
recovery system for the desorbate follows at B. If all substances selec-
tively separated in the individual stages are fed to the same post-treatment
process, the removal of the excess gas from the inert gas loop can be limited
to one take-off point. A further advantageous implementation of the method
relates to the cooling of the adsorbers 21, 22 etc. which are highly heated
during the desorption. In order to prevent ignition if combustible sorbents
such as activated carbon are used, an auxiliary protective gas generator 10
may keep the entire inert gas system including the just desorbed adsorber
under pressure in such a manner that the volume reduction of the inert gas
due to the lowering of the temperature is more than compensated. This
auxiliary protective gas generator lO can be used for inertization, of course,
also prior to a desorption.
In the simplest case the separation of two components is involved
which are contained as impurities in a stream of gas. For this purpose the
gas to be purified, which flows in via the line 21, is conditioned, according
to the schematic process of Figure 2, if it arrives at an arbitrary tempera-
ture, in a preceding heat exchanger 22 in such a manner that it assumes the
temperature at which the first sorption process takes place. This condi-
tioning in the first heat exchanger for possible cooling of thegas can also
be replaced by cooling through humidification in a humidification cooler.
If the gas to be purified arrives at a suitable temperature, the conditioning
can be omitted, of course. The gas which has been brought to the suitable
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temperature for adsorption is now fed to the first gas sorption stage 23, the
valves 231 and 232 being open.
The gas purified of the first impurity now flows to a cooler 24, in
which the temperature of the gas is reduced so that at this temperature an
adsorption isotherm advantageous for the adsorption of the second impurity
is obtained. This cooling is advantageously performed in a heat exchanger.
The thus conditioned gas now flows to the second adsorber 25 in which the
second component is adsorbed until this adsorber 25 is saturated, i.e. charged
to a predetermined level with impurities, which level is below the point at
which the impurities may be carried through the adsorber. If the concentra-
tions of the two impurities are greatly different, the time of saturation is
first reached by that adsorber which separates-out the impurity present with
the highest concentration. In order to make it operative again, it must be
desorbed. In the simplest case, the entire system is desorbed for this
purpose; this procedure is particularly advantageous if the adsorbed impuri-
ties are further processed together. According to the invention a desorp-
tion loop is used for this purpose, in which the desorption temperature can
be adjusted. This setting of the temperature becomes possible by providing
in the desorption loop gas cooling which takes place in the existing cooler
24. With this procedure it is to be noted, however, that the impurities
separated in the first adsorber 23 require a higher desorption temperature
than those which are adsorbed in the adsorber 25. For the sake of simpli-
fication, desorption by the counterflow principle is therefore dispensed
with here and both adsorbers 23 and 25 are desorbed with a Mow in the same
direction after the valves 231 and 252 are closed. Through this connection
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it is possible to dispense with the cooler in the inert gas loop and to reduce
the capital expenditure. m e desorption starts with the feeding of inert gas
by the inert gas generator 28. In special cases it may be advantageous to
perform a preceding inertization by means of the auxiliary protective gas
generator 29. The inert gas loop blower 30 transports the inert gas in the
closed circuit, with the valves 233 and 253 open. At any desired point of
the desorption loop the desorbate-containing excess gas produced is taken off.
This point should advantageously be arranged in the loop where overpressure
of suitable magnitude is present. The excess gas flows to the post-combustion
device 31, but it can also be replaced by a condensation stage or by another
gas utilization stage. In the heat exchanger 24 the inert gas conducted in
the loop is adapted to the desorption temperature for the adsorber 25, while
an adjustment or control of the freshly generated inert gas from the inert
gas generator 28 allows adjustment of the desorption temperature for the
adsorber 23. Gas samples may be taken at various points in the plant during
operation, as is common practice, to make certain of smooth operation. The
temperature of adsorption to selectively remove a single component may be
readily determined by sampling a gas stream leaving the adsorption stage,
prior to or at the start of an operation, to make certain that the component
to be desorbed does not carry over into the gas but remains behind in the
adsorption filter. Adjustment of the temperature upwards or downwards can
then be made to effect selective adsorption. We have found that generally
selective adsorption is covered not solely by a single temperature, but by
a range covering a spread of 5 C to as much as 10 C or more. Of course the
temperature of adsorption need be determined only once for each gas composi-
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tion to be purified.The following examples illustrate the present invention.
Example 1
In a production installation, work pieces are coated with polyvinyl-
chloride (PVC) containing trichloroethylene (Tri) and Di(2-ethylhexyl)phtha-
late or dioctylphthalate (DOP). During the subsequent drying, DOP and Tri
are liberated. The DOP, as softener, comes here from the PVC and the Tri
is the solvent.
The exhaust air from the installation contains on the average 0.12
g/m3 DOP and 5.5 g/m3. The temperature of the exhaust gas varies between
60 and 110 C.
(a) For purposes of comparison, purification of the exhaust air was
carried out in the usual manner by passing it through activated-
carbon adsorbers, but failed due to the following difficulties:
When the adsorbers were operated at the prevailing temperature,
only very short service life times could be achieved by the ad-
sorbers for adsorbing the trichloroethylene. After the tempera-
ture of the gas leaving the adsorber was lowered by coolers to
40C, a dense plume of condensed DOP, which was not precipitated,
was suspended in the gas leaving the adsorber.
(b) Pilot tests were carried out to demonstrate the efficacy of the
present invention. A partial air stream of 250 m /h was taken off
and led to two activated-carbon adsorbers with 20 kg activated
carbon each. The adsorbers were connected in tandem and a cooler
interposed, which cooled the air stream from 95C to 35 C.
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In this manner, the first adsorber was able to reach a service life
of 250 hours, until the formation of fog behind the cooler indicated the
break-through of the DOP.
The adsorption time of the second adsorber reached three hours.
In this manner, DOP and Tri were separated practically completely. As a
further advantage it turned out that the regeneration could now also be
adapted very well to the pure substances which were now present separately
in two adsorbers. While the first adsorber had to be desorbed only every
250 hours, albeit at 350 C, the second stage could be desorbed every three
hours under mild conditions with inert gas or steam at temperatures below
200C.
It should further be mentioned that the cooler between the adsorbers
remains free of wetting by heavy volatile substances, as the latter are sep-
arated-out in the first adsorber. The carry over of low volatile substances
into a cooler tends to plug-up the cooler.
Since the relatively small amounts of DOP produced every 250 hours
do not appear to make recovery economical, the desorbate of the first adsorber
is burned.
The desorbate of the second stage is cooled, condensed and separated
from the water. The second stage can be operated without interruption by
employing two adsorbers connected in parallel and which adsorbers are alter-
natingly loaded and desorbed. Thus, the trichloroethylene accumulated during
adsorption can be recovered practically completely.
Example 2
In the production of synethetic resins, relatively large amounts
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of ethyl alcohol (ethanol) and small amounts of phenol are released.
Adsorptive purification of the exhaust gas containing these im-
purities has heretofore failed because of the same difficulties as in Example
1. At higher adsorption temperatures, the ethanol could be held back only
to an insufficient degree and, upon cooling, parts of the phenol, which was
not separated by the adsorber, condensed to form a bluish fog. This led to
great annoyance in the neighborhood due to odors from the exhaust gas.
In accordance with the invention, two adsorption stages with inter-
mediate cooling were connected in tandem in such a manner that the first
stage adsorbs the phenol from the vaporous phase at 85C and the second stage
reaches sufficient take-up capacity for ethyl alcohol at 30 C. As in Example
1, the second stage is desorbed with inert gas (or also with steam) at tempera-
tures below about 300 C and the ethanol recovered. The first stage is sub-
jected after an extended period of operation (l month) to a reactivating
desorption at 750 C, as the customary desorption leaves excessively large
residual loading.
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