CA2009091A1

CA2009091A1 - Process for the desorption of adsorbents

Info

Publication number: CA2009091A1
Application number: CA002009091A
Authority: CA
Inventors: Joachim Moller; Hans-Joachim Voss
Original assignee: Individual
Current assignee: Dieter Hafemeister Umwelttechnik & Co GmbH
Priority date: 1989-02-02
Filing date: 1990-02-01
Publication date: 1990-08-02
Also published as: DE3903154A1; DE59006652D1; EP0381002A1; ATE109379T1; EP0381002B1; DD291706A5

Abstract

ABSTRACT
A process for the desorption of adsorbentia, such as activated carbon, is provided in which process a desorption medium is led through the adsorbent, and thereafter adsorbates entrained by the desorption medium are separated from the desorption medium, wherein a hot, dry purge gas that is decontaminated by a chemical purification process after flowing through the adsorbent is employed as the desorption medium so that the method can be simplified.

Description

A process and an apparatus for the desorption of adsorbents The invention concerns a process for the desorption of adsorbents, such as activated carbon, in which a-~desorption medium is led through the adsorbent, and thereafter the adsor~ates or materials entrained by the desorption medium are separated therefrom.
Furthermore, the invention concerns an apparatus for carrying out the process.
It is known that materials or groups of materials adsorptively bound to the surface of an adsorbent, such as activated carbon, can be stripped by means of water vapour from the surface of the activated carbon and led off with the water vapour. The water vapour that is contaminated in this way must be subsequently treated in lS such a way that the entrained materials or groups of materials can be separated from the water vapour and disposed of. This decontamination of the water vapour is very expensiveO
It is the object of the invention to design a process of the type specified at the begimling in such a way that overall the desorption of the adsorbent is simplified and economically arranged.
This ob~ect is achieved according to the invention in that a hot, dry gas that is decontaminated by a chemical purification process after flowing through the adsorbent is employed as the desorption medium.
Through the employment of a dry gas as the desorption medium, it is possible for a chemical purification process known per se, such as, e.g, a separation process by means of highly reactive thin layers, to be used to decontaminate the dry gas, whereby the desorption of the adsorbent is greatly simplified overall. Drying of the adsorbent with respect to water vapour as the desorption medium is eliminated, and, in addition to the simple course of the process, the chemical purification process yields the possibility of a controlled or selective decontamination of the dry ' . ' `':

purge gas.
Advantageous embodiments of the invention are specified in the following description and in the further claims.
An illustrative embodiment of the invention is explained in more detail below with reference to the drawing, which shows an apparatus for carrying out the process in a schematic presentation.
Designated by 1 is a closed chamber, which is provided with sealable feed inlets (not shown), in which there is situated an adsorbent that is to be decontam-inated, for example activated carbon or a mixed ad~or-bent, such as expanded bentonite~activated carbon. The activated carbon can be introduced into the chamber 1 in bulk or in the form of activated carbon filters. A fan or blower 2 conveys a prepared purge gas via a line 3 into the chamber 1, in which the purge gas flows through the adsorbent, thereby stripping the materials or groups of materials bound thereto, and discharges from the chamber l via an outlet line 4.
A gas mixture, e.g. carbon dioxide or nitrogen with a rare gas, in particular axgon, is employed as the purge gas. It is preferred to mix with the natural a~bient air appro~imately 80% by volume of dry nitrogen and approximately 20~ by volume of a rare gas, in partic-ular argon, approxLmately in the ratio of 4:1. This previou~ly set composition of the purge gas can be held in readiness~ for example at 5, and fed into the piping ~ystem.
From the chamber 1, the purge gas passes via the outlet line 4 to a mixing device 6, from which a line 7 leads to a cooler 8, downstream of which is connected a condensate separator 9 and a heating device 10. The line 7 leads from the heating device lO to a volume divider ll, to which there are connected a line 3 returning to the chamber 1 and a line 13 leading to a gas purification device 12. A predetermined partial volume, corresponding to the setting at the volume divider 11, of the contaminated purge gas coming from the chamber 1 is conveyed by a fan 14 into the gas purifica-tion device 12, in which the adsorbates contained in the purge gas are separated from the purge gas. An outlet into the environ-ment for decontaminated or purified purge gas is indicated at 15. The purge gas purified in the gas purifica~ion device 12 can be fed entirely or in a partial flow via a line 16 to the mixing device 6.
The mixing device 6 can for example, be constructed without control of the cross-sectional area of the passage in such a way that the lines 4 and 16 open into the line 7. By contrast, the volume divider 11 is constructed controllably in such a way that the volume flow arriving through the line 7 can be entirely or partially deflected into the line 3, whereas, in a corresponding way, the connection of the line 13 can be shut off or can be cleared for a specific partial flow.
The process according to the invention is carried out by means of the described apparatus as follows~ The piping system 3, 4, 7, 13, 16 filled with purge gas from the storage vesQel S is firstly preheated with the condsnser switched off to an operating temperature of, for example, 100C by means of the heating device 10.
Thereafter, the chamber 1 is charged with contaminated adsorbent and resealed, a certai.n proportion of ambient air penetrating into the chamber 1. In this connection, the mixer device 6 and the volume divider 11 can be shut off from the lines 3 and 4. It is likewise possible for shut-off valves (not represented~ in the lines 3 and 4 in front of and behind the chamber 1 to be closed for the charging process. After the charging of the chamber 1, the firs~ step is for the oxygen which has penetrated during charging into the closed system to be rarefied to such an extent by feeding purge gas from the storage vessel S that an oxygen fraction ofl for example, 1 to 3 % by volume, preferably approximately 1 to 1.5 % by volume, is established, it being possible for excess gas to be led off through the volume divider 11 via the line 13 and the outlet 15 at -the gas purification device 12.
After this preparatory stage, the purge gas, : ~ .
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retaining only a low oxygen fraction, is scarcely contaminated any longer by adsorbates from the chamber 1. The next step is, via the heating device 10, to heat the purge gas and thus also the adsorbent in the chamber 1 to the operating temper~ture, the purge gas being conveyed only through the lines 3, 4 and 7 by the fan 2.
During this heating process there is a precontamination of the purge gas with adsorbates from the chamber l. This warming step of the purge gas can be combined with the previously described rarefaction of the oxygen fraction, or can overlap with this rarefaction process.
During the warming of the purge gas and of the adsorbent in the chamber 1, the purge gas is preferably led through the adsorbent in ~he chamber 1 at low speed, that is to say in conjunction with a low volumetric flow, a relatively high temperature is transferred to the adsorbent, instead of heating the adsorbent or the activated carbon by means of a high volumetric flow and a lower temperature.
After a sufficient precontamination of the purge gas, as determined by appropriate sensors, the actual de~orption of the contaminated adsorbent contained in the chamber 1 is started in such a way that with fllrther conveyance of the purge gas through the lines 2, 3, 4 and 7 by means of the fan 2 the volume divider 11 is set in such a way that a partial flow of the contaminated purge gas is led from the line 7 into the gas purification device 12, decontaminated there and returned, at least partially, at the mixer device 6, via the line 16 into the circuit of the purge gas through the lines, 3, 4 and 7. Consequently, there is a gradual reduction in the contamination of the purge gas in the circuit of the lines 3, 4 and 7 corresponding to the reduction in the contamination of the adsorbent in the chamber 1, so that during the entire desorption process a precontamination of the purge gas in relation to the heavier contamination of the adsorbent in chamber 1 is maintained. In this connection, the difference between the precontamination of the purge gas and the degree of contamination of the ..

adsorbent is kept slight.
To compensate process-induced volume changes, inert gas or the prepared purge gas composition is added at S in the decontaminated region, so that a permanent pressure compensation is obtained in the closed system.
During this desorption process, the purge gas is cooled in the cooler 8 in the region of the line 7, whereupon the condensate thereby resulting is collected in the condensate separator 9 and led off from time to time. This condensate can, however, be passed into a layer-forming vessel 17 (described later). The purge gas dried in this way is reheated in the heating device 10 to the temperature required in chamber 1. The volume divider 11 divides the volumetric flow from the line 7 in accordance with the setting, preferably controlled by a computer installation, into appropriate partial flows to the fans 2 and 14.
If an increase in volume occurs due to the reaction behaviour of the various purge gas components, the purge gas excess is released purified at 15 to the external air after passing through the gas purification device 12.
It is ensured that purge gas which is unpurified, i.e. loaded with pollutant, camlot be led off into the external air.
The purge gas in the line 16 is pollutant-free and warm from reaction, and is enriched with inert gas or purge gas at S, insofar as this is required by the course of the process.
~he purge gas at the end of the controllable volume divider section, which is monitored by appropriate sensors, between the mixing device 6 and the volume divider 11 is thinned of pollutant due to the fact that decontamina~ed or pollutant-free purge gas from the gas purification device 12 is added at the mixing device 6 to the purge gas from the chamber 1, which is contaminated or loaded with a high pollutant content. It is a function of the volume divider 11 to charge the chamber 1 with the purge gas fraction precontaminated in accordance with the . ' :,"' :`~

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relationship to the state of contamination of the adsorbent in the chamber 1. The precontamination of the purge gas must be higher the greater the adsorptively bound total contamination of the adsorbent. In other words, the controllable volume divider 11 always sets a precontamination of the purge gas in the chamber 1, which bears a predetermined relationship to the state of contamination of the adsorbent of the chamber 1, or i~
less than the contamination of the adsorbent by a predet-ermined slight amount. In the course of the desorptionprocess, the volumetric flow through the line 3, as set, at the volume divider 11, will always exhibit a residual contamination which becomes ever slighter, since the volumetric fraction transported through the line 13 is continually purified in the gas purification device, and returned via the line 16 into the circuit.
The gas purification device 12 can be constructed in a different way as a chemical separating device for the adsorbates in the purge gas. For example, the dry-powder process or a fluidised bed process can be providedat 12 for separating the adsorbates. Preferably, a separation process by means o e highly reactive thin layers is provided, along which layers the purge gas loaded with pollutants flows. A suitable process ox a suitable apparatus is described in German Paten~
3/344,875. In the case of the illustrative embodiment represented in the drawing, this separating de~ice is shown schematically according to the aforementioned German Patent. This gas purification device 12, which is known per se, and designated below as a drying chambPr reactor, requires a dry gas circuit 18 for the precon-ditioning of the reaction layers adsorbed for the preconditioning from the fresh layer former, it being expedient to employ as the carrier gas an inert gas resembling the purge gas additive in its composition, or the purge gas mixture from the storage vessel 5. The closed circuit 18 is moved at a constant delivery rate by a fan 19, ~he dry gas being cooled in a cooler 20 to below the dew point o the reaction humidity, and the resultant condensate is separated in a condensate collector 21 and brought once again to the working temperature in a subsequent gas heater 22. The condensate from khe condensate collector 21 can likewise be added to the layer-forming vessel 17.
In the case of the represented illustrative embodiment, the cooling power required for the condensers 8 and 20 is generated in an evaporator 24 cooled by air by means of a fan 23, and transported via a heat-transfer oil circuit 25, to the condensers 8 and 20.
The highly reactive layer former required for the continuous dry layer production in the drying chamber reactor 12 is passed into the vessel 17 from a conditioning unit (not represented) and led up to the substrate plates in the drying chamber reactor 12 via a fine plate filter 26, in which the reaction products are separated off. Reaction products are collected in the storage vessel 17 and led off for disposal.
Vaxious modifications or auxiliary devices can be provided in the process according to the invention. ~hus, in the case of the purging process that runs in a com-pletely closed system and in which the purge gas is circulated by a forced conveyance by means of the ~an 2, a desired humidity value can be set, for example, by injecting water at the heating device lO, by means of which the temperature of the purge gas that is required for the desorption is set.
The concentration o the purge ga~ employed can be monitored before entry into the contaminated adsorbent by appropriate sensors, in such a way that an uncon-trolled thermal oxidation of the adsorbent cannot occur even after partial or complete stripping of the adsorbate.
The change in the concentration of the purge gas after exit from the chamber 1 can be detected by in-strumentation in such a way that the type and capacity of the layer former in the storage vessel 17 can be set, according to the difference values as established, to the changing requirements of separation. Selective '.

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, decontamination of the purge gas can be undertaken by setting the layer former appropriately.
During the described process, it is possible briefly to heat the purge gas after termination of the desorption of the adsorbent in the chamber 1 in such a way that biological, bacterial and viral colonisations of the adsorbent can be decomposed. These decomposition products can be removed by flushing with the use of a known pxocess, for example the Integro-Reakt process.
In the course of the process described above, the purge gas is precontaminated in that the purge gas is led through the contaminated adsorbent before desorption starts and after being heated to a predetermined temperature. In this case, a partial flow of fresh or purified purge gas can be added to the purge gas after it has flowed through the contaminated adsorbent, and in this way the degree of the precontamination of the puxge gas can be set to the degree of contamination of the adsorbent.
It is, however, also possible to undertake the precontamination of the purge gas by adding a gaseous additive before entry to the closed purge circuit. The gaseous additive i metered into the inert gas on the inlet side of the chamber 1, for example between the volume divider 11 and the fan 2, and it then passes together with the purge gas through the contaminated adsorbent, whereupon it is subsequently separated propor-tionally in the gas purification device 12 tog~ther with adsorbed contaminations.
The addition of the gaseous additive selected for the precontamination of the purge gas is carried out for a brief, predetermined tLme interval by continuous volumetric metering at the start of the back-flushing process when the back-flushing temperature has been reached at the purge gas inlet to the chamber 1. The metering can be carried out in such a way that the degree of the precont2mination of the puxge gas is set to the degree of contamination of the adsorbent. In this case, it is no longer necessary for a partial flow of fresh or purified purge gas to be added to the precontaminated purge gas in order to set the degree of the precon-~amination to the degrea of contamination of the adsorbent.
For the purpose of precontamination of the purge gas, gaseous additives are selected whose addition behaviour to the adsorbent to be reactivated is com-parable to that of the primary contamination, such as, eg, trichloromethane tchloroform), in the case of adsorbents contaminated with dichloromethylene (dye solvents), and whose reaction behaviour with the layer former in the drying chamber reactor or in the gas purification device 12 is known and controllable, such as decomposition into hydrogen chloride, formic acid and carbon monoxide as intermediate reaction products (Hien's reaction), and common salt and soda as decomposition end products, the intermediate reaction products favouring the decomposition or the inclusion of the primary contaminations entrained in the purge gas.

'

Claims

1. A process for the desorption of adsorbentia, such as activated carbon, in which a desorption medium is led through the adsorbent, and thereafter adsorbates entrained by the desorption medium are separated from the desorption medium, wherein a hot, dry purge gas that is decontaminated by a chemical purification process after flowing through the adsorbent is employed as the desorption medium.

2. The process as claimed in claim 1, wherein before starting the desorption the purge gas is heated to a predetermined temperature and precontaminated.

3. The process as claimed in claim 2, wherein the purge gas is precontaminated by being led through the contaminated adsorbent.

4. The process as claimed in claim 2, wherein before starting desorption the purge gas is precontaminated by the addition of a gaseous additive.

5. The process as claimed in claim 2 and 3, wherein after the precontamination a partial flow of fresh or purified purge gas is added to the purge gas and in this way the degree of precontamination of the purge gas is set to the degree of contamination of the adsorbent before starting the desorption.

6. The process as claimed in the preceding claims, wherein the purified or pollution-free purge gas is diverted from the chemical purification stage and added to the circuit of the contaminated purge gas.

7. The process as claimed in the preceding claims, wherein the contaminated purge gas is cooled in a condenser, resulting condensate being separated and then reheated by a heating device to the operating temperature before it is once again led through the adsorbent.

8. The process as claimed in the preceding claims, wherein volume changes during the course of the process are compensated by the feeding of prepared purge gas, while excess purge gas is released into the environment in purified form after leaving the chemical purification stage.

9. The process as claimed in the preceding claims, wherein after termination of the desorption of the adsorbent the purge gas is heated briefly to an elevated temperature in order to decompose biological, bacterial and viral colonisations.

10. The process as claimed in the preceding claims, wherein a mixture of carbon dioxide or nitrogen with inert gas is employed as the purge gas.

11. The process as claimed in the preceding claims, wherein the purge gas consists of nitrogen and argon.

12. The process as claimed in the preceding claims, wherein the purge gas consists of approximately 80%
nitrogen by volume and approximately 20% argon by volume.

13. The apparatus for carrying out the process according to the preceding claims, wherein a gas purific-ation device (12) is connected via a volume divider (11) to a circuit with a chamber (1) for receiving the con-taminated adsorbent, the outlet of which gas purification device can be connected to the circuit of the purge gas through the chamber (1).

14. The apparatus as claimed in claim 13, wherein the purge gas circuit has a cooler (8) with a downstream condensate separator (9) and a heating device (10), the heating device (10) being connected downstream of the volume divider (11) for sub dividing the volumetric flow in the purge gas circuit.

15. The apparatus as claimed in claims 13 and 14, wherein a circuit (13, 16) for purified purge gas, by means of which a partial flow of purified purge gas is fed to the contaminated purge gas, is connected to the purge gas circuit (3, 4, 7).

16. The apparatus as claimed in claims 13 to 15, wherein the gas purification device (12) is constructed as a drying chamber reactor in which highly reactive thin layers are flowed over by the contaminated purge gas in order to decontaminate the purge gas.