IE44811B1

IE44811B1 - Treatment of gaseous effluent

Info

Publication number: IE44811B1
Application number: IE68677A
Authority: IE
Original assignee: Lamberg Ind Res Ass
Priority date: 1976-04-09
Filing date: 1977-03-31
Publication date: 1982-04-07
Also published as: IE44811L; DE2715607A1; FR2347088B1; GB1582441A; IT1184251B; JPS52123975A; DE2715607C2; NL7703932A; FR2347088A1; SE7704130L

Description

This invention, concerns the treatment of gaseous effluent containing polluting matter which is susceptible to catalytic oxidation to render it ‘inoffensive.

Many industrial processes produce odourous or toxic 5 gases or vapours which if discharged to the atmosphere constitute either a hazard to health or a public nuisance. One solution to this problem is to collect such gases or vapours in mixture with air as they are emitted from the process and to pass the mixture through a heated reactor containing an oxidation to harmless or odourless compounds such as carbon dioxide and water.

An alternative process is to pass the mixture through a burner, but direct flame incineration requires that the large volumes of gas involved are heated to high temperatures in order to combust the contaminant. Hence considerable savings can be made in fuel costs by using catalytic combustion since catalytic reactors effect a considerable reduction in the concentration of pollutants at 15O°-5OO°C while direct flame incineration requires temperatures above this range.

However, heating large quantities of air to the $ temperature required for catalytic oxidation can still • be costly if, as is frequently the case, the concen. 25 tration of.the •contaminant is low and the heat released by its oxidation not contributing significantly to the overall energy requirement. 7 - 3 Another difficulty often encountered is that while catalytic systems can effectively remove 90%-99% of the inlet concentration of pollutant at such lower temperature, in order to obtain complete removal, which, in the case of some odours particularly, is essential, often involves raising the temperature to a level at which the economic advantage of such a system no longer applies.

The object of the present invention is to provide improved and more economic processes for the destructive oxidation of air pollutants.

According to the present invention a process for continuously treating a pollutant-containing gaseous effluent of the kind in which the pollutant is susceptible to catalytic oxidation, to render it inoffensive, is characterised by the steps of passing a major fraction of the effluent through material which will adsorb substantially all the pollutant therein and which is also an oxidation catalyst, while utilizing the remainder of the effluent to transfer, for catalytic oxidation in a catalytic reactor at an elevated temperature, pollutant which has previously adsorbed on such material, and repeatedly and appropriately changing the material'mode from an adsorbing mode to a transfer mode and bgck ngain.

In one embodiment of the invention the major fraction passes in a cool state directly to and through a first bed of such material by which the pollutant is adsorbed therefrom, and is then discharged, whilst the remainder passes first through a heater, then through a second bed of such material from which pollutant is stripped thereby and transferred therefrom and finally through said catalytic reactor, the modes of the first 44011 - 4 and second beds of material being interchanged at intervals.

In a second embodiment of the invention the major fraction passes in a cool state directly to and through a bed of such material by which the pollutant is adsorbed therefrom, and is then discharged,. whilst the remainder passes at an elevated temperature through said catalytic reactor, all whilst material with adsorbed pollutant is continuously transferred, by utilising said remainder, from said bed to said reactor, where said pollutant is oxidised, and back again.

The bed of material is preferably fluidised to facilitate a continuous overspill therefrom for entrainment by the remainder of said effluent as it passes to said reactor, and eventually for continuous return to said bed.

In a third embodiment of the invention the gaseous effluent at an elevated temperature' all passes through the catalytic reactor where most of the pollutant is oxidised and thereafter the major fraction passes, in a cool state, to and through a first bed of material by which.the pollutant remaining therein is adsorbed, and is then discharged, whilst the remainder, still at an elevated temperature, passes through a second bed of material, from which pollutant is stripped thereby and transferred therefrom, whereafter it is added to the effluent passing to the catalytic reactor, the modes of the first and second beds being interchanged at intervals.

In the first and third embodiments described the material, whilst in the transfer mode, may also function as an oxidation catalyst.

Also according to the present invention apparatus for continuously treating a pollutant-containing effluent of the kind i’n which the pollutant is susceptible to catalytic oxidation to render it inoffensive, is characterised by means for accommodating material which is capable of adsorbing substantially all the pollutant from such effluent and which is also an oxidation catalyst, a catalytic reactor, means for passing a major fraction of said effluent through a part of said material for adsorption of pollutant thereby and for simultaneously causing the remainder of said effluent to be utilised in the transference for catalytic oxidation in said reactor at an elevated temperature of pollutant from the remainder of said material, and means whereby the material mode may be repeatedly and appropriately changed from an adsorbing mode to a transfer mode and back again.

One embodiment of such apparatus comprises a heater, two containers each accommodating a bed of said material, a catalytic reactor, ducting for connecting a source of effluent to said heater and to each of said containers, ducting connecting said heater to each of said containers, ducting connecting each of said containers to said reactor, and means adapted selectively either to cause a major fraction of effluent flowing from-said source to be passed to and through one said bed for adsorption of pollutant t thereby prior to discharge therefrom and the remainder successively to and through said heater, the other said bed to strip pollutant therefrom and said reactor for discharge therefrom, or to cause a major fraction of said effluent to be passed to and through the other said bed for adsorption of pollutant thereby prior to discharge therefrom and the remainder successively to and through said heater, the one said bed to strip pollutant therefrom, and said reactor for discharge therefrom.

Another embodiment of such apparatus comprises a container for a bed of said material, in particulate form, means for facilitating the admission of effluent to said container so as to fluidise said bed, a catalytic reactor means, means connecting the upper part of said bed to said reactor means whereby overspill from the upper part of said bed when fluidised may be continuously directed to said reactor means, means connecting said reactor means to the lower part Of said bed whereby said overspill after passing through said reactor means is continuously directed to the lower part of said bed, means for introducing the major fraction of said effluent to said container, and for causing the remainder thereof continuously to entrain said overspill and transfer it to said reactor means, means for causing same to be at an elevated temperature in said reactor means and means for discharging effluent which emerges from said bed from said container .

Yet another embodiment of such apparatus comprises a catalytic reactor, a heater associated with said reactor, two containers each accommodating a bed of said material, ducting for connecting a source of effluent to the inlet side of said reactor, ducting forming connections between the outlet side of said reactor and each of said containers, cooling means in one said connection which connection is adapted to 481 j - 7 convey a major fraction of said effluent emerging from said reactor means associated with each said container adapted either to discharge effluent passing through the respective bed or re-circulate it to said reactor as required and means for causing the flow of effluent from said reactor through one said connection to be directed to one bed, and through the other said connection to the other bed, or vice versa, also as required.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a diagram of a form of apparatus which can be used in carrying out one embodiment of the process of the present invention; Fig. 2 is a diagram of a form of apparatus which can be used in carrying out a second embodiment of the process of the present invention; and Fig. 3 is a diagram of a form of apparatus which can be used in carrying out a third embodiment of the process of the present invention.

Referring first to Fig. 1, the apparatus consists of a heater 10, two adsorption beds 12, 14, one catalytic reactor 16, and associated ducting and appropriate valves, which latter may be automatically controlled.

The adsorbent of the beds 12, 14 is an oxidation catalyst, for example a metal oxide material having the properties of an oxidation catalyst or a catalyst as described in Patent Specification No. 37570.

By using an adsorbing oxidation catalyst it can be arranged that any polluting matter which is not desorbed during the desorption stage is oxidised. - 8 In operation, the flow of gaseous effluent is indicated by the arrowed lines. In the first stage the flow is indicated by full lines, a major, unheated, fraction passing through the bed 12, and a minor, heated, fraction passing through the heater 10, the bed 14 and the reactor 16. When the bed 12 is saturated and the bed 14 stripped, the flow is changed to that shown in dash lines, so that bed 12 is stripped, and bed 14 adsorbs pollutant from the effluent. in an alternative arrangement the beds may be moved from one flow line to the other. This may be conveniently accomplished by mounting the adsorbent in a cylinder divided into compartments by at least one insulated wall mounted along the axis of the cylinder. The major fraction of contaminated air passes through one compartment for adsorption while heated air passes through a second compartment for stripping. The function of the two compartments is reversed by rotating the cylinder.

In some cases, when the temperature necessary for catalytic oxidation is higher than that for stripping the adsorbent, it is necessary to position a second heater at the inlet to the catalytic reactor. This is also desirable when the unit is operated intermittently.

Reference is now made to the apparatus of Fig. 2, which is very suitable for utilising an oxidation catalyst as both adsorbent and catalyst.

Polluted air entering an adsorption chamber 18 30 fluidizes, a bed of adsorbing particulate catalyst 20 in such a way that some of it is continually falling into a receiver 22 and passing via a non-return trap «*8i, - 9 24 to an oxidation chamber 26. It is propelled through the oxidation chamber 26 by a small amount of polluted air bled from the main inlet, through a pump 28 if necessary, and heated there to a temperature sufficient to cause oxidation of the adsorbed pollutant, after which it returns to the base of the catalyst bed 20 in ......

I the adsorption chamber 18. The temperature of the catalyst material should fall to ambient before returning to the adsorption bed. If necessary a heat exchanger can be incorporated between the oxidation chamber 26 and the adsorption chamber 18 to ensure rapid cooling of catalyst leaving the latter.

In a third form of apparatus, illustrated in Fig. 3, polluted air is first subjected to catalytic oxidation at relatively low temperature to oxidise a substantial proportion of the polluting material.

The air, still slightly polluted, is then treated in a manner similar to that described, in connection with Fig. 1.

For reason of efficiency it is preferred.to use a single catalytic reactor, but with two beds of adsorbent material. When one bed is saturated, the main flow of effluent from the reactor is switched to the other bed, and the saturated bed is stripped by passing the rest over the saturated bed to heat it and remove polluting material from it, before the flow is redirected to the reactor. The main flow of effluent is cooled befofe passing over the adsorbing bed. In this way the process is also made continuous.

Referring to Fig. 3 of the accompanying drawings, the apparatus consists of a catalytic reactor 30, a heater 32, a.heat exchanger 34 for cooling purposes. - 10 a blower 3.6, two beds 38 of adsorbent material which is also an oxidation catalyst (one only being shown, the other being directly behind it in the diagram), and a system of ducting gaseous effluent around the system as will now be described.

Xn use, gaseous effluent is mixed with air and passed by the blower 36 through the heater 32 to the reactor 30 wljere 9O%-99% of polluting material is oxidised. The major part of the gas flow from the reactor 30 is then passed over the heat exchanger 34 and then conducted through the bed 38 currently in circuit, where the polluting matter remaining in the effluent is adsorbed. The remainder of the gas flow is diverted via ducting 40 to the other bed of adsorbent material, which has been taken out of the main circuit after becoming saturated with adsorbed pollutant. This hot gas heats the saturated bed and strips the adsorbed pollutant therefrom and is then returned to the reactor 30 via ducting 42.

The temperature of the reactor 30 is maintained within the range 15O°C-5OO°C. The catalyst may be, for example, a metal such as platinum, an oxide such as copper oxide, or a catalyst of the kind described in Patent Specification No. 37570. The adsorbent material may be, a metal oxide of high surface area, being also an oxidation catalyst.

It will be clear that the process can be continuous by switching from one bed of adsorbent material to the other, using the one to capture polluting material whilst the other is being stripped. Of course in time the adsorbent material will need to be regenerated or replaced.

The heat exchanger 34 may carry circulating cold water. Alternatively, where removal of the water produced by oxidation reaction on the catalyst is necessary to an extent greater than would be obtained using such a heat exchanger, then to prevent reduction in the efficiency of the adsorbent, refrigeration may be used.

Where necessary, steam may be used in removal of the pollutant from the adsorbent. This is particularly applicable to the safe removal of organic matter from a carbon bed.

The time taken to saturate a particular adsorbent bed may be determined by experiment and a timing device used to operate the process continuously. Alternatively a detector may be used to determine the saturation point of the adsorbent bed and to automatically switch the gas stream to the second adsorbent bed.

The invention will be described further by way of the following specific examples of how the processes hereinbefore more generally described may be practised.

EXAMPLE 1 The effectiveness of the process according to the present invention was tested using a manganese catalyst prepared adcorriing to Example 4. of Patent Specification No. 37570 as both adsorbent and oxidation catalyst in the apparatus shown in Figure 1. The catalyst was mixed with bentonite in the ratio 10:6 and formed into pellets, 1/8 dia, 3/16 long.

The odour to be removed was that associated with the animal rendering process which could be duplicated in the laboratory by passing air through a heated container holding some crude tallow taken from a meal press. The odour intensity was measured using the Syringe Dilution Method (ASTM Standards, Part 10, D1391-57).

Each bed contained 20 cc of catalyst and the contaminated air with an odour intensity of 100 odour units/ft was allowed to enter the adsorption bed at the rate of 4 litres/min.' After 2 hours the inlet was switched to the second adsorption bed while the first was stripped with an air flow of 0.1 litres/min at 190°C for 10 mins. The temperature of the catalytic reactor was held at 220°C. The outlet air was odourless. The process was repeated eight times and on each occasion both the stripping temperature and the catalytic reactor temperature were reduced gradually until on the ninth cycle when the stripping temperature had been reduced to 98°C and the catalytic reactor temperature to 185°C only 97 per cent of the odour was removed. On returning the stripping temperature to 100°C and the. catalytic reactor temperature to !90°c an odourless outlet was again obtained.

EXAMPLE 2 Using the same catalyst and apparatus as Example 1 the odour intensity of the inlet was increased to 1500 odour units/ft and the inlet flow to 8.4 litres/ min. After 140 minutes the inlet was switched to the second column while the first was stripped at 100°C at a flow of 0.1 litres/min for 15 minutes.

The catalytic reactor was held at 198°C. The outlet was odourless. The process was repeated for eight cycles after which the catalytic reactor temperature was reduced to 180°C. Only 98 per cent of the odour was removed. On returning the reactor temperature to 198° complete removal was restored.

EXAMPLE 3 Using the same apparatus and catalyst as for Example 1 the odour intensity of the inlet was increased to 14000 odour units/ft at an inlet flow of 4.0 litres/ min. The adsorption/stripping/catalytic oxidation cycle was carried out eight times with a stripping temperature of 97°C, stripping flow of 0.1 litres/min and catalyst temperature of 190°C, the outlet being odourless. When the catalyst temperature was then lowered to 178°C only 99 per cent removal of odour was observed. On restoring the original catalyst temperature, complete odour removal was observed.

EXAMPLE 4 Using the same apparatus and catalyst, an air stream containing 100 mg/m butyric acid flowing at 16 litres/min was admitted to the adsorption bed for 20 mins. This bed was then stripped at 220°C for 10 minutes with 0.1 litres/min flow. With the catalyst bed temperature 290°C a 97 per cent removal of the butyric acid was obtained throughout the cycle.

EXAMPLE 5 Using the same apparatus but with 10 cc of catalyst in each bed an air stream containing methyl ethyl ketone with an odour intensity of 100 odour q units/ft it 3.4 litres per minute was admitted to the adsorption bed. After 30 minutes the bed was stripped by passing air at 32O°C and 100 mls/min I through the bed. -The catalyst bed temperature was also maintained at this temperature to·, obtain complete removal of odour.

EXAMPLE 6 cc of catalyst similar to that employed in Example 1 were placed In the adsorption chamber 18 of the apparatus described with reference to Figure 2.

With an air inlet rate of 7 litres/min fluidising the bed of catalyst 1 cc of catalyst per minute fell into the receiver 22 and passed to the oxidation chamber 26 (or catalytic reactor).. When methyl ethyl ketone was added to the inlet air stream to the extent that its concentration was 0.1% v/v in the air and the temperature of the oxidation chamber was set at 350° C the outlet air remained free of methyl ethyl ketone over a period of 3 hours' operation.

EXAMPLE 7 Air containing butyric acid (100 mg/cu metre) was fed at 40 litres/hour to apparatus similar to that shown in Figure 3. The catalytic reactor 30 contained 20 co catalyst similar to that employed in Example 1. The catalyst reactor was held at a temperature of 250°C. The adsorption beds 38 contained 10 cc each of the same catalyst used as adsorbent. The temperature of the. airstream was cooled to 40°C before entering the adsorption bed. During stripping the adsorption bed temperature reached 160°C. Using an adsorption period of 1 hour on each bed the overall removal of butyric acid over six cycles was 99%.

Claims

1. A process for continuously treating a pollutantcontaining gaseous effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render t it inoffensive, characterised by the steps of passing a major fraction of the effluent through material which will adsorb substantially all the pollutant therein and which is also an oxidation catalyst, whilst utilizing the remainder of the effluent to transfer, for catalytic oxidation in a catalytic reactor at an elevated temperature, pollutant which has previously adsorbed on such material, an

2. A process as claimed in Claim 1, further characterised in that said major fraction is passed in a cool state directly to and through a first bed of such material by which the pollutant is adsorbed therefrom, and is then discharged, whilst said remainder is passed first through a heater, then through a second bed of such material from which pollutant is stripped thereby and transferred therefrom and finally through said catalytic reactor, the modes of the first and second beds of material being interchanged at intervals.

3. A process as claimed in Claim 1 further characterised in that said major fraction is passed in a cool state directly to and through a bed of such material by which the pollutant is adsorbed therefrom, and is then discharged, whilst the remainder passes at an elevated temperature through said catalytic reactor, all whilst material with adsorbed pollutant is continuously transferred by utilising said remainder from said bed to said reactor, where said pollutant is oxidised and back again.

4. A process as claimed in Claim 3 further characterised in that the bed of material is fluidised to facilitate a continuous overspill therefrom for entrainment by the remainder of said effluent as it passes to said·reactor, and eventually for continuous return to said bed.

5. A process as claimed in Claim 1 further characterised in that said gaseous effluent, at an elevated temperature, is all passed through said catalytic reactor where most of the pollutant is oxidised and thereafter the major fraction of effluent from said reactor is. passed, in a cool state, to and through a first bed of such material by which the . pollutant remaining therein is adsorbed, and is then discharged, whilst the remainder, still at an elevated temperature, is passed through a second bed of such material from which pollutant is stripped thereby, and transferred therefrom, whereafter it is added to the effluent passing to the catalytic reactor, the modes of the first and second beds being interchanged at intervals.

6. A process as claimed in any one of Claims 1, 2 and 5 in which, whilst material is in transfer mode it is also functioning as an oxidation catalyst.

7. Apparatus for continuously treating a pollutant-containing effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive, characterised by means fox accommodating material which is capable of adsorbing substantially all the pollutant from such effluent *48^ and which is also an oxidation catalyst, a catalytic reactor, means for passing a major fraction of said effluent through a part of said material for adsorption of pollutant thereby and for simultaneously causing the remainder of said effluent to be utilised in t'he transference for catalytic oxidation in said reactor at an elevated temperature of pollutant from the remainder of said material, and means whereby the material mode may be repeatedly and appropriately changed from an adsorbing mode to a transfer mode and back again.

8. Apparatus as claimed in Claim 7 comprising a heater, two containers, each accommodating a bed of said material, a catalytic reactor, ducting for connecting a source of effluent to said heater and to each of said containers, ducting connecting said heater to each of said containers, ducting connecting each of said containers to said reactor, and means adapted selectively either to cause a major fraction of effluent flowing from said source to be passed to and through one said bed for adsorption of pollutant thereby prior to discharge thereform, and the remainder successively to and through said heater, the other said bed to strip pollutant thereform and said reactor for discharge thereform or to cause a major fraction of said effluent to be passed to and through the other said bed for adsorbtion of pollutant thereby prior to discharge thereform and the remainder successively to and through said heater, the one said bed to strip pollutant thereform and said reactor for discharge thereform.

9. Apparatus as claimed in Claim 7 comprising a container for a bed of said material in particulate form, means for 'facilitating the admission of effluent to said container so as to fluidise said bed, a <ϊ481Ί - 18 catalyst reactor means, means connecting the upper part of said bed to said reactor means whereby overspill from the upper part' of said bed when fluidised may be continuously directed to said reactor means, means connecting said reactor means to the lower part of said bed whereby said overspill after passing through said reactor means is continuously directed to the lower part of said bed, means for introducing the major fraction of said effluent to said container, and for causing the remainder thereof continuously to entrain said overspill and transfer it to said reactor means, means for causing same to be at an elevated temperature in said reactor means and means for discharging effluent which emerges from said bed from said container.

10. Apparatus as claimed in Claim 7 comprising a catalytic reactor, a heater associated with said reactor, two containers each accommodating a bed of said material, ducting for connecting a source of effluent to the inlet side of said reactor, ducting forming connections between the outlet side of said reactor and each of said containers, cooling means in one said connection which connection is adapted to convey a rrtajqr fraction of said effluent emerging I from said reactor, means associated with each said contaiher adapted either to discharge effluent passing through the respective bed or recirculate it to said reactor as required and means for causing the flow of effluent from said reactor through one said connection to be directed to one bed, and through the other said connection to the other bed, or vice versa, also as required.

11. Ά .process for treating a pollutant-containing effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

12. A process for treating a pollutant-containing effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

13. . A process for treating a pollutant-containing effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

14. Apparatus for continuously treating a pollutant-conta,ining effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive substantially as hereinbefore described with reference to and as illustrated in Fig. 1 of the accompanying drawings.

15. Apparatus for continuously treating a pollutant-containing effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive substantially as hereinbefore described with reference to and as illustrated in Fig. 2 of the accompanying drawings.

16. Apparatus for continuously treating a pollutant-containing effluent of the kind in which the pollutant is susceptible to catalytic oxidation to render it inoffensive substantially as hereinbefore described with reference to and as illustrated in Fig. 3 of the accompanying drawings.