GB2051761A

GB2051761A - Method and apparatus for the removal of oxidizable pollutants from gases

Info

Publication number: GB2051761A
Application number: GB8018433A
Authority: GB
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 1979-06-06
Filing date: 1980-06-05
Publication date: 1981-01-21
Also published as: DK156814C; JPS55165131A; SE8003958L; DE3021174A1; NL8003288A; DK234879A; JPH0347884B2; DK156814B; FR2458308B1; FR2458308A1

Abstract

The method comprises: a) conducting the polluted gas (e.g. air) at a temperature of from 0 to 250 DEG C through a bed of particulate adsorbent- catalyst which adsorbent-catalyst comprises an adsorbent which has a high specific internal surface area and is impregnated with a material active at elevated temperature as an oxidation catalyst; and b) at intervals regenerating the said adsorbent-catalyst by increasing the temperature to a temperature of from 250 to 350 DEG C whereby the catalytic oxidation of the contaminating material accumulated on and in the said adsorbent catalyst is initiated. Apparatus for performing the above method is also described.

Description

SPECIFICATION Method and apparatus for the removal of oxidizable pollutants from gases The present invention relates to a method for the removal of oxidizable, especially gaseous, pollutants from gases, particularly from poiluted air.

During the production processes in a number of industries (such as printing works, dyestuff and paint factories, food and foodstuff processing plants, chemical works and a large number of small workshops) airborne substances and gases are generated which are oxidizable and which may be malodorous, inflammable and/or toxic. These substances are removed by air extraction and for economic reasons are often discharged directly into the atmosphere without a preceding purification. Increasingly severe regulations against environmental pollution and increasing public demand are to an increasing degree rendering necessary the purification of such exhausted air, even in those cases where it has not previously been necessary.The purification of such extracted air flows is expensive and technically difficult because it is often a question of large amounts, for example 100,000 Nm3 per hour, which usually contain small amounts, for example below about 1.5 g per Nm3, of the substances to be removed. (The abbreviation Nm3 is used herein to denote normal cubic metres; that is cubic metres as measured at 0 C and 760 mm Hg pressure).

Known methods for the purification of extracted air flows can be divided roughly into four categories: washing, adsorption, thermal oxidation, and catalytic oxidation.

Washing is typically carried out with water, often with added chemicals which react with the undesired substances in the extracted air. The drawback of this method is that, apart from being expensive, the real problem of rendering the undesired substances harmless, is only partially solved since they are merely transferred to the water.

Adsorption is most frequently made onto activated charcoal. The greatest problem in this method is that the regeneration of the activated charcoal, necessary to make the method sufficiently economic, cannot be carried out quite satisfactorily. The reason for this is that regeneration must be carried out in an oxygen-free atmosphere, which typically will be an atmosphere of superheated steam. Many of the substances to be removed from the carbon tend to form polymerisation products which clog the pores of the carbon particles.

A partial removal of these polymerisation products can be carried out by maintaining temperatures in the oxygenf ree atmosphere in the range of 700-800 C but at such high temperature a decomposition of the activated charcoal takes place. Other adsorbents such as molecular sieves and refractive ceramic oxides, e.g.

alumina, Al2O3, may also be employed. However, molecular sieves are considerably more expensive than activated charcoal and refractive ceramic oxides have lower adsorption capacity than activated charcoal.

However, one advantage of these incombustible adsorbents is that they can be regenerated with air.

Thermal oxidation is distinguished by being a simple and comparatively dependable method. The disadvantage of the method is that the heating or the large masses of air to 700-800 C in a combustion chamber requires large amounts of energy. Usually the air is heated by the direct firing with oil or possibly gas. It is even possible to utilize part of the heat present in the large masses of air, the method is still expensive to work.

Catalytic oxidation differs from thermal oxidation in that the oxidation does not take place in a combustion chamber but in a catalyst bed. The advantage of this method is that the catalytic oxidation can take place at a temperature of 250-350 C whereby the energy consumption is substantially reduced. It is a drawback of the catalytic oxidation that certain catalyst types become poisoned by the contact with such substances as hydrogen sulfide, H2S, and sulfur dioxide, 502. It is particularly important that, even if the energy consumption by the catalytic oxidation is substantially less than by thermal oxidation, it is still considerable when large masses of air are to be heated.As the amounts of the mentioned impurities are small, the oxidation of these impurities does not supply sufficient energy adequately to heat the mass of exhaust air.

It is one object of the invention to overcome or reduce the drawbacks attached to the known methods and provide a method for purification of gases and air of oxidizable pollutants so as to achieve an efficient purification with a substantially reduced energy consumption compared with the known thermal and catalytic methods of oxidation.

According to one aspect of the present invention we therefore provide a method for the removal of oxidizable pollutants from a polluted gas, in particularfrom polluted air, which method comprises; (a) conducting the polluted gas at a temperature of from 0 to 250 C through a bed of particulate adsorbent - catalyst which adsorbent-catalyst comprises an adsorbent which has a high specific internal surface area and is impregnated with a material active at elevated temperature as an oxidation catalyst; and (b) at intervals regenerating the said adsorbent - catalyst by increasing the temperature to a temperature of from 250 to 3500C whereby the catalytic oxidation of the contaminating material accumulated on and in the said adsorbent - catalyst is initiated.

A particularly favorable embodiment of the present of the present invention involves further subjecting the gas leaving the said adsorbent-catalyst during the regenrationthereof to a subsequent purification, preferably by catlytic oxidation and advantageously using an adsorbent-catalyst as used above in steps (a) and (b).

It is thus a great advantage of the method that one may take the polluted air and conduct the treatment without the need for preheating.

Therefore the majority of the gas to be purified by the method of the present invention passes through the adsorbent-catalyst substantially at the temperature at which it exists priorto purification (although where the polluted gas is at a relatively high temperature, there may-actually be some loss of heat) and consequently no energy is used to heat it. If desired, where the polluted gas is hot, part of the heat of the gas may be retrieved before the adsorption by passing the gas through a heat exchanger. During the passage through the adsorbent-catalyst the pollutants or the major part thereof are absorbed onto the adsorbent catalyst and where the gas is air, it maybe led away and reteased-intotnesurroundingsin a substantially purified condition.As the adsorbent-catalyst becomes more and more saturated with the impurities, a break-through of impurities to the surroundings will occur; therefore when the concentration of undesired substances has reached a permitted or acceptable maximum value, the adsorbent-catalyst may be regenerated by increasing the temperature to that needed for the catalytic oxidation, normally to within the range of 250-350"C, whereupon, when oxygen is present, oxidation of the contaminants on the adsorbent-catalyst commences. However it should be realized that during the catalytic oxidation the temperature may well rise further.

However it should also be realised that, in the case where the polluted gas does not contain oxygen (as would be the case the polluted gas is generated by a process carried out under an inert atmosphere, such as for example nitrogen or helium), then the regeneration of the adsorbent-catalyst should involve the introduction of sufficient oxygen or oxygen-containing- gas for the oxidation of the impurities on the adsorbent-catalyst to be effected.

Accordingly, heat need only be supplied in the periods when catalyst regeneration is to be carried out.

Under certain circumstances, as will be explained later in the specification, it is even possible, once the apparatus for carrying the method has started operation, for the apparatus to function without further supply of heat or even with a heat gain.

Conveniently, the adsorbent-catalyst used in the method of the present invention will be a porous ceramic carrier having a large specific internal surface area (e.g. 70-250 m2/g, preferably 100~20Q m2/g), impregnated with a substance which will act at elevated temperature as an oxidation catalyst.The porous carrier in-itself will act as an adsorbent, but only to a small degree if at all as an oxidation catalyst Particularly suitable are a series of ceramic materials in the form of oxides, particularly of the elements in groups 11,111, and IV in the Periodic Table Examples of suitable materials are aluminium oxide, Al2O3, hereinafter called alumina, magnesium aluminium spinel, MgAI204, and silicon dioxide, hereinafter called silica. It has been found that y-alumina is a particularly suitable material but oxides of Ti and Zr and similar ceramic oxides also can be used. The carrier may of course comprise a mixture of two or more of the said materials.

The carrier, as mentioned above, advantageously is impregnated with a material which is active as an oxidation catalyst at elevated temperature. As such material, metals of group still in the Periodic Table (as shown in "Handbook of Chemistry and Physics", 53th ed., 1972-3) and compounds thereof, especially oxides, are suitable for use; particularly suitable being the platinum-based catalysts. Other suitable materials are copper and those metals of group Vb,Vlb, and Vllb and oxides thereof and especially preferred are oxides of copper, chromium, manganese, iron, vanadium and cerium.

The catalysts mentioned have excellent catalytic activity and stand regeneration well. It has suprisingly been found, however, that copper chromite, CuO.Cr2O3, combines a high adsorption power for the oxidizable pollutants that frequently occur in practice, and a high activity as an oxidation catalyst. According to the invention there is therefore employed with particular advantage and adsorbent-catalyst consisting of y-alumina impregnated with copper chromite.

A particular advantage of this adsorbent-catalyst is its ability to adsorb large amounts of polymerisable compounds, such as for instance styrene. Many polluting compounds such as for example styrene will undergo a partial polymerisation after the adsorption on the adsorbent-catalyst, and copperchromite has the ability to accelerate this polymerisation. Such a polymerisation will contribute to increase the adsorption capacity and thereby will prolongthe periods during which adsorption can take place without regeneration.

When the adsorbent-catalyst in its property as adsorbent is close to saturation with the impurities, which is easily ascertained by the fact that the purification becomes less efficient, the temperature is increased to cause catalytic oxidation of the adsorbed impurities. The circumstance that part of the adsorbed substances may possibly have been polymerized will not make the regeneration difficult In the regeneration the temperature can be increased, preferably as stated to 250-350"C, in any expedient manner.Thus it is possible to recirculate hot gas from the regeneration or to add heat directly to the adsorbent-catalyst bed, e.g. by the aid of electric heater elements Generally however it will be most convenient to insert, upstream of the bed, relative to the flow direction of the polluted gas, a combustion chamber fired with for instance oil or gas.

The gas leaving the adsorbent-catalyst bed during the regeneration will generally contain undesired components in contradistinction of that leaving it during the adsorption. Therefore, according to the invention, it may be convenient to subject this exhaust gas to a subsequent purification. The subsequent purification may take place according to any one of-the above-mentioned main methods, i.e. washing, adsorption, thermal or catalytic oxidation, the choice of method being dependent amongst other factors on the undesirable substances leaving the bed during the regeneration. If these substances are oxidizable it is most convenient according to the invention that the said subsequent purification takes place by catalytic oxidation.In this way it is possible to use another catalyst than that used as adsorbent-catalyst However with a view to rendering the apparatus and operation as simple as possible, it is most practical to use as catalyst the same material as the adsorbent-catalyst employed in the adsorption. The particular advantage of carrying out the said subsequent purification as a catalytic oxidation is that under certain circumstances it makes it possible to carry out the method in an apparatus which does not consume energy supplied from outside the operation, apart from that present in the gas to be purified, and which may even produce useful heat.

The invention also relates to an apparatus for carrying out the method described.

According to a further aspect of the present invention we therefore also provide apparatus for the removal of oxidizable pollutants from a polluted gas, which apparatus comprises, in combination: (a) at least one reactor containing a bed of an adsorbent which at elevated temperature is active as an oxidation catalyst; (b) a feed line to convey the polluted gas to the said reactor; (c) a discharge line to convey substantially decontaminated gas from the said reactor; and (d) means whereby the said adsorbent-catalyst may at intervals be regenerated within the said reactor; and, if desired (e) means whereby gases emerging from the said reactor (a) during the regeneration of the adsorbent-catalyst therein may be further subjected to a subsequent purification.

The means for the periodical regeneration of the adsorbent-catalyst preferably is a combustion chamber with a connected burner, inserted in the feed line.

According to the invention the means for subsequent purification of gas emanating from the reactor during regeneration of the adsorbent-catalyst preferably is provided by a second combustion chamber with a burner, inserted in a branch of the discharge line and connected to a second reactor containing an oxidation catalyst.

The purified air emanating from the subsequent purification reactor may either be recirculated to the discharge line or be transported seperately to a stack. Under certain circumstances a part of this purified air may be conveyed to the reactor with the adsorbent-catalyst. This may be the case when, in a particularly advantageous embodiment, apparatus comprising several reactors containing adsorbent-catalyst is employed in such a manner that at any time one of these reactors is in the regeneration phase while other reactors are adsorbing the impurities. Such an embodiment is particularly advantageous when the content of organic matter is more than 0.5 g per m3 of the hot gas since it makes possible the utilisation of the calorific value represented by the oxidizable substances for heating purposes, e.g., where in this manner the entire apparatus may be autothermal or even exothermal.The circumstances of this are explained in detail hereinafter but such an apparatus according to the invention expediently comprises in combination: (a) two or more primary reaction chambers each containing a bed of an adsorbent-catalyst material, said reaction chambers being connected in parallel to a feed line for polluted gas; (b) a line from the said feed line to a subsequent purification section; (c) discharge lines in parallel from each of the said primary reaction chambers; (d) lines in parallel from each of the said primary reaction chambers to the said subsequent purificaton section;; (e) the said subsequent purification section comprising a combustion chamber connected to a burner, a line from the said combustion chamber to a secondary reaction chamber, the said secondary reaction chamber reaction containing a bed of an oxidation catalyst and being provided with a discharge line therefrom; and (f) the said discharge line from the said secondary reaction chamber having a first branch to discharge gas therefrom, a second branch to act as a recirculation line being connected to the said line from the combustion chamber to the secondary reaction chamber, and third branches to the said primary reaction chambers.

The method and the apparatus according to the present invention will be illustrated more fully hereinafter with reference to the Examples and the drawings, in which: Figure land Figure 2 show flow diagrams of two different embodiments of an apparatus according to the present invention for carrying out the method according to the invention, Figure 3 shows an experimental apparatus in which certain experiments to illustrate the method according to the invention have been carried out, and Figure 4 and Figure 5 show graphs illustrating the results of such experiments.

In the plant illustrated in Figure 1 (where auxiliary details such as pumps, valves and control means have been left out for the sake of clarity) the polluted gas enters via a line 10 into the purification apparatus and passes a chamber 12 where a burner 14 is connected. Most of the time, viz. while the undesirable substances are adsorbed onto the adsorbent, burner 14 is not in operation, only being turned on when the adsorbent is to be regenerated. Thereafter the gas enters a reaction chamber 16 where adsorbent-catalyst is placed. The manner in which the adsorbent-catalyst 18 is placed does not consitute any part of the invention. However, it is always desirable to place it in such manner that the pressure drop in reactor 16 is as small as possible.

The air being processed is conveyed away from reaction chamber 16 via a pipe 20 which divides into two to provide a pipe 22 leading to a stack, and a second pipe 30 leading to a subsequent purification section. The means 10-22 are those normally required for carrying out the method according to the invention. In normal operation, i.e. during adsorption, the purified gas is passed directly from reactor 16 via pipes 20 and 22 to the stack. During regeneration the burner 14 is switched on and heats the gas entering chamber 12, preferably to a temperature of 250-450"C, so that the bed 18 of adsorbent-catalyst, preferably copper chromite impregnated onto porous particles of y-alumina, will become active as an oxidation-catalyst and the adsorbed, oxidizable impurities will be burned off.During this the discharge gas from reactor 16 may also be conducted away to the stack via pipes 20 and 22, but preferably the discharge gas is conveyed via pipe 30 to the subsequent purification section from which the discharged purified gas is conducted via a pipe 40 to a pupe 42 leading to pipe 22 or to the stack. The subsequent purification in the apparatus shown in Figure 1 is a catalytic oxidation process. The gas is heated directly in a combustion chamber 32 by means of a burner 34 at the desired temperature and is subsequently passed to a reaction chamber 36 in which a catalyst 38 is placed. With the aid of an oxidation catalyst 38, the content of undesired components of the gas is reduced to the desired level.The oxidation catalyst 38 in reaction chamber 36 may be selected independently of the adsorbent-catalyst 18 in reactor 16 but, for practical reasons, it is particularly advantageous to employ the afore-mentioned copperchromite/alumina adsorbent-catalyst both in reactor 16 and reaction chamber 36.

A calculation Example, given as Example 1 hereinafter, serves to provide a more explictdescription of the mode of action of the invention in an apparatus as shown in Figure 1.

Another embodiment of the plant according to the invention is shown in Figure 2 which likewise is much simplified by the omission of a number of details, inter alia various blowers, pumps, valves and the entire control apparatus, as these do not have any direct bearing on the invention and their use and function will be obvious to the expert.

The polluted gas enters the apparatus via a line 10 which divides into two lines 42 and 44. A minor part of the gas is conveyed via line 42 to the subsequent purificaton section. The major part is conveyed from line 44 through branches 4611 46all ..... to several reactors containing adsorbent-catalyst 18. The plant on the figure has six reactors denoted 481,4811,48111, 48V 48 V, and 48V. The number of reactors determines the length of the regeneration period relative the length of the adsorption period in the reactors. The number of reactors 461.... will depend, among other factors, on the volume of gas to be treated and the degree of pollution thereof.

The purified gas is conveyed from the reactors via lines 501,5011.... which are all united in a common pipe 22 which leads the purified gas to a stack. In practice, at all times during operation, five reactors are in the adsorption phase and one in the regeneration phase.

The gas for the regeneration is conveyed to the reactors 481.... from a line 52 with branches 541, 5411 for the individual reactors. A series of other pipes 56', 5611.... leads the gas from the reactors into a common line 58 conducting the gas to the subsequent purification section.

From line 58 the gas emerging from a reactor wherein the adsorbent-catalyst 18 is being regenerated flows into a combustion chamber 60 inserted into line 58 and having connected thereto a burner 62; these two members normally are only in function when the operation of the apparatus commences and the gas passes directly through them without heating when the apparatus is in full operation. From the combustion chamber 60 which accordingly during normal operation is merely a part of line 58, the gas flows through the remainder of line 58 to a junction 64 where a desired amount of hot recirculation gas is added from a line 66.

In this manner there is formed in a line 68 a stream of gas at the desired temperature for input to a reaction chamber 76 containing an oxidation catalyst. The purified gas is conducted from chamber 76 via a line 70 to a junction 72. The said recirculation gas is conducted therefrom as beforehand through line 66 and a part of the gas is exhausted through a line 74. The purified stream of gas from chamber 76 will have a considerable temperature (400-5000C) for which reason the portion led away via line 74 ordinarily will be utilized for heating purposes. At junction 72 the hot, purified gas is admixed with polluted gas from line 42 to form the gas flow which via line 52 passes to one of reactors 48' .... for use during the regeneration phase.

A substantial advantage of the apparatus as shown in Figure 2 is that the burner 62 is only used when the apparatus commences operation. During normal industrial operation accordingly there is no energy supplied to the burner or to anywhere at all in the apparatus. On the contrary it is possible to utilize in the apparatus the calorific value of the substances in the polluted gas since the heat of the gas flowing from junction 72 typically will be utilized, either as hot air for drying ovens or for heating water.

Recirculation line 66 moreover has a particular significance. If it did not exist the temperature in the reaction chamber (where the catalyst may be an oxidation catalyst and conveniently the same as employed as adsorbent catalyst 18) would vary strongly and in a complicated manner as it will be explained further hereinafter.By suitable regulation of the amount of recirculating gas in line 66 and the amount of gas to the reactor being regenerated, e.g. 48", the temperature in reaction chamber 76 may be kept comparatively constant, which partly is of interest in connection with the utilization of the gas in line 74 for heating purposes, partly may be important for the catalyst; certain catalyst and particularly carrier materials might sinter or have their activity otherwise decreased if the temperature rises to too high a level, but this may efficiently be prevented by regulating the amount recirculated. Conversely, the subsequent purification in chamber 76 may be unsatisfactory if the temperature becomes too low, in which case it is likewise possible to regulate the amount recirculated.

A calculation example to elucidate the operation of the apparatus as illustrated in Figure 2 is provided in Example 2 hereinafter.

A small pilot plant in which some experiments with polluted air were carried out is outlined in Figure 3.

Atmospheric air is led into the plant via a line 90. The amount of air is measured in a flowmeter 78 and is controlled by means of a valve 80. From the flowmeter the air is conducted via a line 82 into a cylindrical preheater 84 wherein an electric heating coil is situated. By controlling the current through the heating coil it is possible to adjust the temperature to a desired level, leading the the heated air away from the preheater in line 10. A line 88 conducts polluting liquid to a pump 92 which, via a line 94, conducts the liquid into line 10 where evaporation takes place. Line 19 conducts the air, now polluted, into a reactor 16. In the upper part of the reactor a heating coil 96 is situated, permitted adjustment of the temperature of the polluted gas.In the lower part of the reactor an adsorbent-catalyst 18 is placed in a cylindrical bed 18 having a height up to 450 mm and a diameter of 73 mm, corresponding to the interior diameter of the reactor. Along the axis of the bed a heat well 98 extends from the bottom to the top of the bed. In the heat well there is a movable thermo-element with which it is possible to determine the temperature at various depths of the catalyst bed.

The air emerging from the catalyst bed is led from the reactor via line 20. With the aid of a line 22 and a control valve 100 it is possible to conduct a part of this emergent air to analysis in a continuously operating hydrocarbon analyzer 102. The hydrocrabon analyzer employed was a flame ionization analyzer; in this case a Beckmann Model 400 Hydrocarbon Analyzer.

Example 1 This is a calculated Example wherein a purification apparatus as shown in Figure 1 is used for decontaminating discharge gas from the preparation of styrene-containing polymers. In reaction chamber 16 there is placed 22 m3 of adsorbent in the form of particles having a size of 3-6 mm. The adsorbent consists of y-alumina impregnated 20% by weight with CuO.Cr2O3. The amount of catalyst in reactor 36 depends on the volume of air during regeneration. In the present Example it is stipulated that it is possible to decrease the volume of air to 10-20% of the volume during normal operation. In this manner the volume of catalyst in reactor 36 is reduced to 3 m3. The catalyst is identical with the adsorbent-catalyst 18 in reactor 16.

45,000 Nm3 per hour of the discharge gas having a content of styrene of 0.25 g per Nm3 is conducted to the purification plant. The content of styrene in the decontaminated gas conducted away through the stack will increase slowly with the time from an initial value of about 0; after about 40 hours operation the styrene content in the decontaminated gas will have increased to about 10 mg/Nm3. At this time about 20 g of styrene have been adsorbed per kg of adsorbent. The regeneration typically will last 2-3 hours. The adsorbed styrene has some calorific value, causing the temperature in the adsorbent to increase during regeneration. In extreme cases the temperature may be so high that the adsorbent loses its adsorptive properties, e.g. because of loss of specific surface area, and/or loses its catalytic properties, e.g. because the sintering.An efficient regeneration ensuring that the temperature in the adsorbent does not exceed 8500C may be carried out by reducing the air flow to 6,000 Nm3 per hour. The emergent air is conducted to the subsequent purification section via line 30. The temperature in combustion chamber 12 is maintained at 2500C with the aid of the burner 14. After 45 minutes the volume of air is increased to 10,000 Nm3 per hour and the temperature is increased to 270 C. After another 45 minutes, burner 14 is switched off and the air flow is increased to 20,000 Nm3/hour. 45 minutes later the regeneration is considered to be complete. The air flow is increased to 45,000 Nm3/hour and the air is conducted via lines 40 and 42 to the stack.During the entire regeneration, the inlet temperature to reactor 30 is maintained at 3000C, if necessary with the aid of the burner 34. An adsorption and regeneration cycle lasts about 42 1/4 hours and the total energy consumption corresponds to 310 kg of oil of which 110 kg of oil were used in burner 12 and 200 kg of oil in burner 34; in this connection it is observed that the discharge air from reactor 16 during regeneration is of some calorific value.

Example 2 This is a calculated Example for apparatus as shown in Figure 2. 25,000 Nm3/hour of air emanating from the drying ovens of an offset printing house were conducted to the apparatus. The polluted air contains 1.5 g hydrocarbons per Nm3 and has a temperature of 130 C. The hydrocarbons, which have been used as solvent for printing inks, consist of about 20% aromatic hydrocarbons and 80% aliphatic hydrocarbons and have a boiling point range of about 240 to about 270'C. The polluted drying air is conducted to the plant via line 10.

A part of this air flow of v0 Nm3 per hour for use in the regeneration is led away via line 42. The remaining 25,000 - v0 Nm3 is conducted via line 44to those of reactors 48 which are in the adsorption phase and is distributed with (25000 - vo)!5 Nm3/hour to each of the five reactors. Each reactor 48 contains 1500 kg of the same adsorbent-catalyst as in Example 1. The subsequent purification reactor 76 likewise contains 1500 kg of the same adsorbent-catalyst.

The duration of the adsorption period is limited by two considerations. Firstly, it must not be so long that the hydrocarbon concentration in the air stream from the adsorption becomes greater than the permissible value. Secondly there should not be adsorbed more than about 15 g of hydrcarbons per kg of adsorbent-catalyst. By remaining below this value, the maximum temperature in the adsorbent-catalyst will not exceed 800-850 C.

Fixing the duration of the adsorption period at 175 minutes there will be adsorbed 12.8 g of hydrocarbons per kg of adsorbent-catalyst, which ensures that the maximum temperature in the adsorbent-catalyst will not exceed the desired value during regeneration, viz. a value which can be assumed with certainty not to destroy the catalyst. During the adsorption, the hydrocarbon concentration in the decontaminated gas conducted to the stack will be about 2 mg/Nm3 in the first 100 minutes and will increase to 5 mg/Nm3 at the end of the adsorption. Since the number of reactors is six, the regeneration period will be 35 (175/5) minutes.

When v1 denotes the amount of air conducted via line 52 to regeneration, v2 denotes the amount of air circulated in line 66 past reactor 76, T1 denotes the temperature in line 52 to reactors 48, T3 denotes the temperature in line 68 to reactor 76, Table 1 below shows how the regeneration of the adsorbent in a reactor 48 proceeds.

During the first 12 minutes of the regeneration there is obtained a temperature in the line 54 in question of about 318"C by admixing 1000 Nm3/hour(vO) of contaminated air from line 42 with 2000 Nm3/hour (v1-v0) decontaminated hot air from line 70 at junction 72. At the temperature obtained the adsorbed hydrocarbons in the inlet layer in the relevant reactor 48 will be ignited and burn as a result of the catalytic properties of the catalyst-adsorbent. The hydrocarbons from the polluted gas used for the regeneration will become oxidized together with part of the adsorbed hydrocrabons.The remainder of the adsorbed hydrocarbons, because of the temperature increase, will become desorbed and will be conducted with the effluent air to reactor 76 where an oxidation takes place. 5-10 minutes after the commencement of the regeneration the hydrocrabon concentration in the air will have increased to a maximum of 2-3 g/Nm3. In the first 15-20 minutes, when the hydrocarbon content in the air in line 58 is large, the temperature will be comparatively low. To ensure sufficient oxidation in reactor 76 the air must be heated to at least 260 C, which is obtained by recirculating hot air over the reactor via line 66. The temperature in the air emerging from reactor 76 will have a constant value of about 41 80C despite the temperature variations in line 58.This is partly due to the fac that the catalyst in reactor 76 has a certain capacity as heat buffer and partly that hot air is recirculated via line 66. The temperature in line 74 therefore also will be almost constantly 418 C. The volume of decontaminated air in line 74 will be identical with vO.

12 minutes after the commencement of the regeneration, the addition of hot air from line 70 at junction 72 is stopped and at the same time the amount of polluted air in line 42 is increased. The remainder of the regeneration is carried out with 130 C hot, polluted air at a flow rate of 4000 Nm3/hour. The circulation over reactor 76 can be reduced to 3500 Nm3/h since the temperature of the air emerging from the reactor 48 in question will have begun to rise. This temperature will reach a maximum of about 600 C about 25 minutes after the commencementothe regeneration.At the same time the temperature of the decontaminated air emerging from reactor 76 will also reach a maximum of about 600 C. The hydrocarbon content in line 58 will have dropped to below 20 mg/Nm3 after 15-20 minutes of regeneration.

Burner 62 does not come into function during the regeneration but is only used at the start of operation of the apparatus and accordingly there is no energy consumption for heating air during the normal operation of the plant, which is a great advantage of this construction. The hot, decontaminated air which is conducted away via line 74 has as mentioned a high heat content. By cooling the air stream to 1 300C an amount of heat corresponding to the calorific value of 27 kg of oil per hour may be recovered.

TABLE 1 Number of minutes from the commencement of the regeneration 0-12 12-20 20-25 25 25-30 30-35 V0(Nm3/h) 1000 4000 4000 4000 4000 4000 v (Nm3/h) 3000 4000 4000 4000 4000 4000 v2(Nm3/h) 4000 3500 3500 3500 3500 3500 T1("C) 318 130 130 130 130 130 T2(#) 130 200 450 600 450 350 T3(0C) 290 300 450 600 450 350 Hydrocarbon concentration in line 58 (g/Nm3) max 2-3 1-0.02 < 0.01 < 0.01 < 0.01 < 0.01 Example 3 In the first experiment with the experimental apparatus as shown in Figure 3 there was used air polluted with styrene. The adsorbent-catalyst was the aforementioned copper chromite-y-alumina adsorbent -catalyst, in the form of balls with a diameter of 3-6 mm.

Per hour 4.4 Nm3 of air containing 0.22 g of styrene per Nm3 and having a temperature of 500C were conducted into the catalyst bed. The amount of adsorbent-catalyst was 1.67 kg corresponding to 1.8 litres.

The styrene content in the exit stream from the reactor was measured in the hydrocarbon analyzer as ppm by volume methane equivalents, hereinafter denoted by C1. It can be mentioned that 100 mg styrene per Nm3 corresponds to 179 ppm by vol C1. In the first 30 hours of the adsorption period, the styrene content in the decontaminated air was measured at below 20 ppm by vol C1. After 40 hours operation the content rose to 30 ppm by vol C1 and after 45 hours the styrene content had risen to 50 ppm by vol C1. The adsorption period thereby had ceased and the regeneration began.The flow was reduced to 1.1 Nm3 per hour while at the s#metime the temperature of the air flowing to the catalyst bed was increased to 300 C. The adsorbed matter at the entrance to the bed was ignited at about 2800C and a hot zone thereafter moved down the bed.

The hot zone reached its highest temperature of 7000C about 150 mm down the bed. The hot zone reached the exit of the reactor after about 1 hour. The styrene content in the air discharged from the reactor at the commencement of the regeneration was 30 ppm by vol of C1. During the first approximately 17 minutes of the regeneration period the styrene content rose to abve 1000 ppm by vol of C1, which is the upper limit that the hydrocarbon analyzer could measure. After about 40 minutes the styrene content again decreased to below 1000 ppm by vol of C1 and during the subsequent 10 minutes the styrene content dropped to 20 ppm by vol of C1. The regeneration thus altogether lasted 50 minutes. The experiment was repeated three times and the result essentially was as described hereinabove.Calculations on the experimental data show that the adsorption capacity at the operational conditions employed is about 25 g of styrene per kg of the adsorbentcatalyst.

After 45 hours adsorption operation a sample of a few grams of adsorbent-catalyst was taken out and analyzed. The sample contained 33 g of organic carbon per kg. The sample thereafter was heat-treated at 2000C in nitrogen. After having been maintained at this temperature for 2 hours the sample was analyzed anew. The content of organic carbon was determined at 15 g per kg adsorbent-catalyst. It must be assumed that this amount of organic carbon corresponds to styrene adsorbed in a more or less polymerized form.

Example 4 In this series of experiments, consisting of four individual experiments, the reactor contained 1.6 litre (1.5 kg) of the adsorbent-catalyst placed in a bed of 40 cm height. The air flow to the absorbent-catalyst bed in all the experiments was 4.0 Nm3/h. The air was polluted with a hydrocarbon mixture having boiling point range 240-270 C, typically employed as solvent in Heatset offset-printing.

The temperature and hydrocarbon content in the air at the inlet (Z= 0) to the adsorbent-catalyst bed is shown in Table 2 below.

TABLE 2 Experiment No. Temperature, C Hydrocarbon content, g per Nm3 1 155 1.40 2 125 1.40 3 122 0.78 4 100 0.91 The hydrocarbon concentration in the outlet stream from the reactor was measured on the hydrocarbon analyzer as ppm by vol. of C1. Figure 4 shows a graph of the hydrocarbon concentration (HC) in ppm by vol.

of C1 as a function of the time in hours for the four experiments. In the graph a line is drawn which shows the concentration 20 mg C per Nm3, which is frequently the permitted maximum limit for emission to the atmosphere.

From Figure 4 it can be seen that the adsorption capacity in the temperature interval investigated is highly temperature dependent since a small temperature increase or decrease will cause a strong decrease or increase, respectively, of the adsorption capacity.

Before the series of experiments the adsorbent-catalyst had been used in 10 adsorption and regeneration cycles in which the maximum temperature during the regenerations was in the range 900-920 C; the limit of 800-850 Cfixed hereinbefore thus gives a good safety margin. During 10 further adsorption and regeneration cycles the adsorption capacity was measured. There was no sign of any significant decrease of the adsorption capacity.

In experiment 2 the adsorption period was terminated after 6 1/2 hours and the regeneration was commenced. At this moment the adsorbent-catalyst, according to calculations, had adsorbed a total of 39 g of hydrocarbon corresponding to 26 g per kg. The process of the regeneration, which was carried out with clean air, is shown in Figure 5. The time from the commencement of the regeneration is shown in minutes in the abscissa. The left ordinate shows the content of methane equivalents in the air emerging from the reactor, and the right ordinate the temperature measured at varying depths (Z) in the adsorbent-catalyst bed.

Since the length of the bed is 40 cm, Z=0 corresponds to the inlet layer of the bed and Z=40 corresponds to the outlet layer of the bed. The temperature at z=0 is identical with the temperature of the air used for the regeneration; the temperature at Z=40 is identical with the temperature of the air leaving the reactor.

From Figure Sit can be seen that the temperature of the regeneration air during the first approximately 10 minutes was about 2900C and that the temperature in the remaining time of the regeneration was about 100 C. From the Figure it is seen that the highest temperature in the bed, viz. about 9300C, was measured at the 10 cm depth.

Corresponding regenerations with smaller amounts of adsorbate show that about 15 g hydrocarbon per kg adsorbent will give a maximum temperature in the bed of about 825"C.

The hydrocarbon concentration in the emergent air decreased, as also may be seen from the Figure, to below 10 ppm by vol. C, after about 20 minutes. The regeneration was terminated after 30 minutes. In the last 10 minutes of the regeneration the adsorbate was oxidized completely on the adsorbent-catalyst. During the entire regeneration period ordinarily around half of the adsorbate will become oxidized on the adsorbent-catalyst while the remainder will be carried with the emergent air to the subsequent purification section.

Claims

1. A method for the removal of oxidizable pollutants from a polluted gas, which method comprises: (a) conducting the polluted gas at a temperature of from 0 to 250 C through a bed of particulate absorbent-catalyst which absorbent -catalyst comprises an adsorbent which has a high specific internal surface area and is impregnated with a material active at elevated temperature as an oxidation catalyst; and (b) at intervals regenerating the said adsorbent-catalyst by increasing the temperature to a temperature of from 250 to 350 C whereby the catalytic oxidation of the contaminating material accumulated on and in the said adsorbent-catalyst is initiated.

2. A method as claimed in claim 1 wherein the said polluted gas is polluted air.

3. A method as claimed in either of claims 1 and 2 wherein the said adsorbent-catalyst comprises y-alumina impregnated with copper chromite.

4. A method as claimed in any one of claims 1 to 3 wherein the gases leaving the adsorbent-catalyst during the regeneration thereof are further subjected to a subsequent purification.

5. A method as claimed in claim 4 wherein the said subsequent purification is effected by catalytic oxidation.

6. A method as claimed in claim 5 wherein the adsorbent-catalyst used in the said subsequent purification is an adsorbent-catalyst as used in the steps of adsorption (a) and regeneration (b) described in claim 1.

7. A method as claimed in claim 1 substantially as herein described.

8. Apparatusforthe removal of oxidizable pollutants from polluted gas, which apparatus comprises, in combination: (a) at least one reactor containing a bed of an adsorbent which at elevated temperature is active as an oxidation catalyst; (b) a feed line to convey the polluted gas to the said reactor; (c) a discharge line to convey substantially decontaminated gas from the said reactor; and (d) means whereby the said adsorbent-catalyst may at intervals be regenerated within the said reactor.

9. Apparatus as claimed in claim 8 wherein the said means (d) is provided by a combustion chamber with a burner attached thereto.

10. Apparatus as claimed in either of claims 8 and 9 further comprising means whereby gases emerging from the said reactor (a) during regeneration of the adsorbent-catalyst therein may be further subjected to a subsequent purification.

11. Apparatus as claimed in claim 10 wherein the means forthe said subsequent purification are provided by a combustion chamber connected to a branch of the said discharge line (c) and to a burner, a second reactor containing therein a bed of an oxidation catalyst and connected by a discharge line to the above combustion chamber and itself provided with a discharge line for emergent gases.

12. Apparatus as claimed in claim 8 comprising in combination: (a) two or more primary reaction chamber each containing a bed of an adsorbent-catalyst material, said reaction chambers being connected in parallel to a feed line for polluted gas; (b) a line from the said feed line to a subsequent purification section; (c) discharge lines in parallel from each of the said primary reaction chambers to the said subsequent purification section; (e) the said subsequent purification section comprising a combustion chamber connected to a burner, a line from the said combustion chamber to a secondary reaction chamber, the said secondary reaction chamber containing a bed of an oxidation catalyst and being provided with a discharge line therefrom; and (f) the said discharge line from the said secondary reaction chamber having a first branch to discharge gas therefrom, a second branch to act as a recirculation line being connected to the said line from the combustion chamber to the secondary reaction chamber, and third branches to the said primary reaction chambers.

13. Apparatus as claimed in claim 8 substantially as herein described.

14. Apparatus as claimed in claim 8 substantially as herein described with particular reference to the Figures and Examples. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~