A METHOD FOR REGENERATING AN ADSORBENT

Abstract: The invention relates to a method for regenerating an adsorbent with respect to organic substances adsorbed therein. These organic substances are expelled with warm gas by desorption and are destroyed by combustion. The invention is characterized in that the warm gas is an air-combustion-gas mixture; in that the gaseous mixture exiting from the adsorbent is combusted, when applicable after adding an auxiliairy fuel, at a combustion temperature of 750-850 DEG C; in that the resultant combustion gas is first cooled by adding air and then by heat exchange with the gas mixture exiting from the adsorbent. At least one part-flow of the thus cooled mixture of air and combustion gas is then brought into contact with the adsorbent to form said gaseous mixture, subsequent to adding an amount of air needed to achieve a predetermined temperature at each moment in time.

Description: A METHOD FOR REGENERATING AN ADSORBENT The present invention relates to a method for regenerat- ing an adsorbent with regard to organic substances adsorbed therein, in which method the organic substances are expelled by desorption with warm gas and destroyed by combustion. The invention relates particularly, although not exclusively, to the regeneration of adsor- bents used to cleanse gases, such as air, of organic substances, such as solvents, deriving from industrial processes. It has long been known that air and similar gases can be cleansed of minor quantities of contaminants in the form of gaseous organic substances with the aid of adsorption techniques. The adsorbents used may be active carbon or some other combustible organic material. For instance, polymeric materials suitable for this purpose are avail- able commercially. Inorganic, non-combustible materials have also been proposed and in recent years the use of zeolites, which in principle are comprised of hydrated alkali-aluminium silicates, has been proposed. Zeolites are used generally as an extremely active drying agent and normally have hydrophiliσ properties. Certain zeo¬ lites, however, can be converted to a hydrophobic form and can then be used, for instance, as means for adsor¬ bing organic substances. When an adsorbent used to cleanse air, or other gases, from organic substances, such as solvents, becomes saturated with respect to the adsorbed substances, it is necessary to regenerate the adsorbent before it can be used again. The adsorbent is normally regenerated by heating the adsorbent so as to decrease its adsorption capacity, in the majority of cases by blowing gas through the adsorbent, forcing the adsorbed substances to leave the adsorbent. This process step is referred to as desorption. It is often desired to recover the organic substances, and the regeneration process must then be effected in a manner to ensure that the organic substances will not be destroyed. The regenerating gas proposed in many cases is an inert gas that can be used in a recirculation system, in which the desorbed substances are isolated and collected by cooling the gas departing from the adsorbent in a condenser. The regenerating process, however, creates many pro¬ blems, even in those instances when recovery of the organic substances is not necessary or desirable for one reason or another. It is then necessary to destroy effectively the desorbed organic substances, so that these substances will not pass to the surroundings. Many processes for the desorption and destruction of organic substances have been proposed and several have also been tested. For instance, CH-A-468847 and DE-A-20 41 458 propose desorption with the aid of hot air followed by combustion of the sorbed organic material. When using non-combustible adsorbents such as zeolites, the desorp¬ tion and destruction processes may be carried out essen- tially at one and the same time when the temperature of the regenerating gas is above 300 β C and contains oxygen, thereby combusting the adsorbed organic material. Such methods are described, for instance, in SE-A-8801435-2 and 8901916-0, these publications, however, indicate serious risks of overheating problems. It is proposed that these problems are solved by limiting the oxygen content and the temperature, at least during the major part of the desorption process, wherein initially de¬ sorption is effected solely with inert gas, or by dimen- sioning each bed strictly on the basis of each applica¬ tion having knowledge of the gas volume that can pass through the bed and also knowledge of the regenerating temperatures. The afore-proposed measures are necessary in order to avoid those temperature shocks which would otherwise occur upon the introduction of hot, oxygen- containing regenerating gas. It will be obvious, however, that the costs involved with each piece of expensive apparatus, the application of complicated process steps and the need of more exclu- sive inert gases or gaseous mixtures make the expanded use of effective air-cleansing processes for recovering organic impurities totally prohibitive. Consequently, in many cases, the release is continued over an unnec¬ essarily long period before being forced by the authori- ties to take steps to cleanse the emission or to reduce the emission in some other way, for instance by laying- down a certain part of the activity concerned or, in the best of cases, reconstructing the activity so as to eliminate the emission, if possible. Naturally, it is highly desirable to take the possible emission of process organic material into account at the planning stage of new industrial plants. One condition for providing the plant at the plant planning stage with the possibility, or facility, of cleansing gases of organic substances prior to releasing the gases to atmosphere is whether or not the cost of gas cleansing can be carried by the process concerned. If the process is unable to carry the costs, there will probably be no investment at all, at least not in the country origin¬ ally intended. It is therefore important that the organ¬ ic substances can be adsorbed and recovered in the simplest possible adsorption and recovery plant at investment and operating costs which are low in this particular context, although while observing the strict safety regulations that may be placed on a process involving combustible gases. This implies, among other things, that the temperatures in the major part of the regenerating system shall be so low as to enable standard metal sheet to be used for the construction of gas conduits, and fan systems, resulting in substantially lower costs than those entailed by more heat-resistant qualities. The threshold for the use of fans constructed from black metal sheet lies at 300-350°C. It must also be possible to reduce the number of heat exchangers used and the heat exchangers must be of the simplest conceiv¬ able construction, for instance conventional stainless steel flat heat exchangers. The concentrations of de¬ sorbed solvent in the regenerating system shall also be so low as to exclude the necessity of a fully closed system. The cost of heating the regenerating gas must also be kept as low as possible, which is important in view of the relatively large volumes required to prevent solvent concentrations from becoming prohibitively high. The present invention relates to a method for regenerat¬ ing adsorbents with which the extremely important mar¬ keting desiderata and construction desiderata presented above can be generally fulfilled, thereby enabling gases emanating from organic substances to be effectively cleansed at costs that are considered reasonable with regard to the industrial activity concerned. To this end, the inventive method is characterized by the fea¬ tures set forth in the following Claims. The invention thus relates to a method in which the hot gas used for the desorption of the adsorbed organic substances is comprised of an air-combustion-gas mix¬ ture. The gaseous mixture departing upon desorption of the adsorbent is then combusted, if necessary after the addition of an auxiliary fuel, for instance natural gas, this process being controlled to obtain a combustion temperature of 750-850°C. The new combustion gas thus formed is cooled in stages, first by adding air and then by heat transfer to the gaseous mixture departing from the adsorbent, whereafter, subsequent to adding an amount of air at each time point required to achieve a predetermined temperature, at least one part-flow of the thus cooled gas is brought into contact with the adsor¬ bent so as to form the warm gas-mixture used in the desorption process. The predetermined temperature is set and maintained by preprogrammed control parameters, which are in turn selected at each moment in time during the destruction process, preferably with respect to the concentration of desorbed hydrocarbons in the departing gas mixture, for instance calculated as the total hydrocarbon concentra¬ tion during the desorption process. By "air" is meant in this context also contaminated gaseous mixtures which consist essentially of air. The inventive concept is thus based on the regeneration of an adsorbent material with a gaseous mixture which contains at least sufficient oxygen to ensure that the desorbed organic substances taken-up in the gas will be combusted in the combustion stage, and also on control¬ ling the temperature of the input gaseous mixture so as to obtain a controlled concentration of desorbed solvent in the gaseous mixture leaving the adsorbent during the regeneration process. Thus, as a result of the inventive method, the desorption rate in the adsorbent is also controlled indirectly, i.e. the amount of solvent re¬ leased from the adsorbent per unit of time. This reduces the risk of overheating in the adsorbent beds that is likely to occur when using solely warm air of constant temperature, and also reduces the risk of problems that would otherwise occur with a warm front that migrates through the bed. As a result hereof, it is not necessary to place as large demands on the configuration and content of the adsorbent bed as those placed on the bed when only air is used. The invention also enables tem¬ peratures in the gas conducting system, fans, etc., to be kept low in the present context, so as to obviate the need for expensive system material grades or qualities. Furthermore, regeneration gas, e.g. inert gas, costs are obviated, with the exception of the cost of an auxiliary fuel, when used. The oxygen content of the gaseous mixture is suitably set to 15-20% in the desorption process. The gas enter- ing the desorption stage and the gas exiting from said stage are suitably subjected to a heat exchange process, thereby enabling the temperature of the exiting gas to be adjusted to about 200°C, this temperature being the maximum system temperature when organic substances are present in the gas, because of the latent risk of explo¬ sion. Combustion of the gas leaving the desorption stage is effected in a separate, closed combustion chamber with the aid of a bottom-located burner to which an auxiliary fuel, for instance a gasol-type fuel, can be introduced in addition to said gaseous mixture. The combustion gas is preferably divided into two flows, which are deliv¬ ered separately to the burner as a primary gas and a secondary gas respectively in proportions that can be controlled in a suitable manner. The combustion process is preferably controlled in response to measuring sig¬ nals and by also monitoring instant combustion tempera¬ tures. The combustion gas supplied to the burner may also be diluted if it obtains excessively high contents of organic substances, which is manifested in elevated combustion temperatures. The invention will now be described in more detail with reference to an exemplifying embodiment thereof and also with reference to the accompanying drawing which is a flow sheet illustrating said exemplifying embodiment. The illustrated system comprises a bed 1 containing absorbent material. The bed 1 is through-passed by air taken from an industrial location A, through a conduit 2. The air is sucked through the bed by a fan 3, wherein the organic substance content of the air is adsorbed in the bed and the air is then released in a cleanse state. The system also includes valves 4 which can be used to shut-down the bed 1 and, instead, to switch-in another bed (not shown) when the bed 1 is to be regenerated. In this regeneration process, a gas flow 5 is passed through the bed 1 at the same time as valves 6 are opened. The gas exiting from the bed 1 now also contains organic constituents and is passed, via a conduit 7, to a heat exchanger 8 and from there through a conduit 9 by the action of a fan 10. The system also includes in conjunction herewith a conduit 11 through which gas- diluting air is introduced into the system, via a safety valve 12. The gas is delivered to a burner 15 positioned downstream of the fan 10 in the lower part of a combus¬ tion chamber 16, via a conduit 13 and valves 14. The valves 14 A, B function to divide the gas between the various gas intakes in the burner 15. In the illustrated case, the burner 15 is also supplied with fuel 17 via a separate, central fuel intake in the burner 15. The combustion process is followed by a temperature measur¬ ing process, here illustrated by a measuring de- vice/sensor 19 which sends signals over a line 20 to a control centre 21, which in turn is able to send signals to a valve 14 via a line 22 and to the safety valve 12 via a line 23 and to a valve 24 for controlling the flow of auxiliary fuel 17, all for controlling the combustion process. The combustion gas flowing from the combustion chamber 16 is drawn by suction through the heat exchang¬ er 8, via a conduit 25, through the action of a fan 26. Immediately after exiting from the combustion chamber 16, the combustion gas is cooled in a first stage with the aid of a cold air supply, which is controlled by a valve 28 via a line 27. By adjusting the settings of two valves 29, the gas flow cooled by heat-exchange is divided so that a suitably large part-flow will be passed back to the bed 1, through a conduit 30, whereas the remainder of the gas flow is passed to a chimney or stack for release to atmosphere, via a conduit 31. Further cooling of the gas is effected in the conduit 30, by introducing cold air through a conduit 32 and a valve 33 into the gas flowing through the conduit 30. The manner in which the combustion process is controlled in the combustion chamber 16 will now be described in more detail. When the combustion temperature which is monitored and measured by the measuring device/sensor 19 exceeds the control value set in the control centre 21, for instance 800°C, the supply of auxiliary fuel is throttled contin¬ uously with the aid of the valve 24. The valve 14B, whose setting is thus adjusted parallel in sequence with the supply of auxiliary fuel is throttled at the same time through the valve 24, since the part-flow of gas mixture passing from the conduit 13 through the valve 14B is used as combustion air for the auxiliary fuel. However, both fuel and air are delivered even when the valves 24 and 14B are fully closed, so as to sustain a pilot flame which thus always burns when the plant is in operation. All gas mixture now passes to the burner 15 through the valve 14A and thus to the second¬ ary nozzle of the burner 15. If the set control value of the combustion temperature is exceeded, a signal is sent from the control centre 21 to the valve 12, via 23, which opens in response to said signal so as to allow dilution air to flow through in the amount required to lower the concentration of the desorbent solvent in the gas mixture and therewith reduce the combustion tempera¬ ture sufficiently to reach the control value. Although not shown, temperature measuring devices can be arranged in the bed 1 as an additional safety measure, these devices functioning to send a signal to close the valves 6 when the temperatures measured in the bed are excessively high. Example 1 A bed containing zeolite absorbent was regenerated in a regenerating plant in the manner aforedescribed with 3 reference to the drawing. 300 Nm /h of an air-combus¬ tion-gas mixture containing about 17.4% O and having a temperature of 100°C were passed through the bed. The regeneration gas mixture exiting from the bed now 3 •contained about 10 g/Nm organic substances and had a temperature of 100°C. The gas was subjected to a heat- exchange process, wherein the temperature of the gas rose to 200'C. This gas was divided into two part-flows 3 3 of 87.7 Nm /h and 212 Nm /h respectively. These part- flows were delivered to the burner through separate conduits. Combustion was effected in the combustion chamber and the combustion temperature was maintained at a control value of about 800°C. The combustion gas exiting from the combustion chamber contained practic¬ trated case had a prevailing temperature of -20°C, 3 wherein the cooled gas in an amount of 650 Nm /h ob¬ tained a temperature of 358°C and an 0 -content of about 11.3%. The gas was then subjected to a heat-exchange process with the gaseous mixture arriving from the bed, whereupon the temperature dropped to 310°C. Of the total 3 3 amount of cooled gas of 650 Nm /h, 546 Nm /h were deliv- 3 ered to the chimney or stack, while 110 Nm /h were 3 recycled to the bed after being cooled with 190 Nm /h external air having a temperature of -20°C, wherein the 3 gaseous mixture delivered to the bed, total 300 Nm /h, had a temperature of 100°C and contained about 17.4% 0 . Example 2 In a test series, quantities and temperatures of air- combustion-gas mixtures were controlled so that the gas exiting from the adsorbent contained varying quantities of solvent. It can be mentioned from this test series that in the case of a relatively low value of the solvent concentra- 3 tion as 1 g/Nm gaseous mixture, which occurs at the beginning of the regenerating process, an additional 3 energy input of about 0.25 Kw/Nm is required to the destruction burner in the form of auxiliary fuel. When starting up the system, this additional energy is, to a large part, consumed to heat the conduit system and fans. The solvent content is allowed to rise to a maximum of 3 about 12 g/Nm gaseous mixture as the system temperature rises, therewith lowering the auxiliary fuel requirement 3 to about 0.1 kW/Nm .

Claims: CLAIMS 1. A method for regenerating an adsorbent with respect to organic substances adsorbed therein, in which the organic substances are expelled by warm gas by desorp¬ tion and destroyed by combustion, c h a r a c ¬ t e r i z e d in that the warm gas is comprised of an air-combustion-gas mixture; in that the gaseous mixture departing from the absorbent is combusted, when applic¬ able after adding an auxiliary fuel, at a combustion temperature of 750-850°C; in that the formed combustion gas is first cooled by adding air and then by heat transfer with the gaseous mixture leaving the adsorbent; and in that at least one part-flow of the thus cooled mixture of air and combustion gas is brought into con¬ tact with the adsorbent to form said gaseous mixture, subsequent to adding a requisite amount of air to achieve a tem-perature which is predetermined for each point in time. 2. A method according to Claim 1, c h a r a c ¬ t e r i z e d in that the air-combuεtion-gas mixture contains 15-20% oxygen. 3. A method according to Claim l or 2, c h a r a c ¬ t e r i z e d in that the heat transfer between the gas exiting from the regenerating stage and the gas entering said stage is effected in a heat exchanger. 4. A method according to Claims 1-3, c h a r a c ¬ t e r i z e d in that combustion of said exiting gas mixture is controlled with respect to the possible supply of auxiliary fuel and distribution of gas mixture in burner nozzles forming part of the system. 5. A method according to Claim 4, c h a r a c ¬ t e r i z e d in that said control is effected with the aid of signals produced when measuring prevailing com¬ bustion temperatures. 6. A method according to any one of Claims 1-5, c h a r a c t e r i z e d in that the concentration of desorbed solvent in the gas mixture exiting from the adsorbent is controlled by regulating the temperature of the ingoing air-combustion-gas mixture.

Inventors: HULTEN JOHAN, OLSSON SVERKER, HULTEN, JOHAN, OLSSON, SVERKER
Applicants: VENTILATORVERKEN AB, AB VENTILATORVERKEN
Application year: 1992
Main classification: B01J20
IPC codes: B01J20