FI103481B - Method and apparatus for removing contaminants from a gas stream m - Google Patents

Description

103481

The present invention relates to a method for removing impurities from a gas stream, and to a device for carrying out the method. The method is intended in particular to remove impurities present in low concentrations in process air streams.

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In many processes, the gases to be removed include gaseous and solid substances which, although discharged to the environment, are harmful to the environment. To eliminate these contaminants from the gas streams in question, microbiological filters have proved to be useful. In microbiological filters, the contaminants contained in the gas are brought into contact with the microbial culture, which microbial or microbes have the ability to degrade substances in the gas stream. The gas purification apparatus operating on this principle is described e.g. in WO 93/07952.

The apparatus described in this publication has a porous, mineral wool plate substrate provided for microbes 25 through which a gas stream to be purified is passed.

·· The living conditions of the microbes in the medium are monitored, for example. by introducing an aqueous liquid into the medium, • · ·; ·. which contains the nutrients the microbes need. According to one embodiment, the nutrient solution is injected into the gas stream to be purified before passing the gas through a filter material serving as a growth medium. This apparatus is said to be effective in gas purification, for example in an application having • ·: ·· ': 35 purified exhaust gas from mineral wool manufacture, including e.g. mineral wool binder pe-. rusmonomers, has a purification efficiency over phenol of over 95% and over 98% for ammonia. As a mechanical filter used for this purpose, purification efficiencies of about 20% for both phenol and ammonia can be achieved. With respect to formaldehyde, such a filter will, in part, be slightly more efficient due to the chemical reactions therein, whereby about 50% purification efficiency can be achieved.

However, both types of filter mentioned above have a problem with clogging due to the fact that the gas to be purified is passed through the filter. Process gases, along with gaseous chemical compounds to be removed, always inevitably contain solid particles, which over time block the filter. Alternatively, the gas must be subjected to pre-purification steps to remove solid particles from the gas stream. However, reasonable hardware solutions do not yet achieve satisfactory results. As an example, it may be mentioned further that the exhaust gas from mineral wool manufacture, which in spite of pre-purification steps, such as a slow settling flow, contains a binder as an aerosol when entering an actual filter, such as a microbiological filter, as well as fine fibers.

'·' 25 Further disadvantages of known device types can still be considered ... • the need for manure over the available cleaning surface, which • in practice significantly reduces the loading capacity of these • * • * ···. The space requirement is dictated by the basic function of the device, according to which the gas stream 30 to be purified is passed through a filter. In this case, the filter will run *. ·. for example, as an effective microbial growth medium • · · only one of its surfaces.

The above-mentioned problems have been improved by the method of the invention, the essential characteristics of which are referred to in the appended claim 1. The essential characteristics of the device implementing the method are in turn given in the appended claim 7.

The process of the invention utilizes the above-mentioned ability of microbes to degrade substances used in many process gases and utilize them for nutrition. Above all, a method for removing organic compounds from airborne process gases by means of an aerobic microbial or microbes is suitable.

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The microbial strain or strains to be used in the process must, of course, be selected according to the process and the compounds contained in its gases. The selection of microbes that degrade various chemical compounds is known per se and is within the skill of the art and will not be discussed in further detail herein.

The method provides conditions for efficiently contacting the impurity compounds in the gases with the microbes, as well as conditions for microbial growth to take place in an advantageous manner, whereby their action to degrade and utilize the impurities is also maximized.

25 ·· · An essential feature of the process is that the gas stream to be cleaned is never forced into any kind of · ·; * · · Through the filter surfaces, but the gas may flow freely • · ·: / ·: in the immediate vicinity of the microbial culture bearing surfaces, i.e. in contact with the microbial culture. This avoids the risk of rapid clogging in known filtering methods, extends the service intervals of the device, · · · and achieves greater efficiency, improved reliability * and longer service life.

35 ..... To the first part of the method, namely the impurities of the gases; The introduction of microbes into the microbial pathway involves two steps of absorption: the absorption step, in which the majority of the contaminants are transferred from the gas to be purified to the finely divided aqueous liquid, and the adsorption step, i.e. the liquid to microbial 5 transfer. The efficiency of the absorption step (gas / liquid) is determined by the amount of liquid used per amount of purified gas, assuming, of course, that the distribution of the liquid into the gas is treated in the most efficient way possible, with as fine droplets as possible.

A volume ratio of 0.01 to 0.02 volume ratio of liquid to gas can be considered as a guide, which ratio provides sufficient material transfer under normal operating conditions for the purification of most of the exhaust gases involved.

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The adsorption step, i.e. the migration of material from the liquid under the influence of microbes, is in most cases a more determining factor for the efficiency of the process than the migration from the gas to the liquid. Adsorption can only be perfectly successful if sufficient time is allowed for the impurities to come into contact with the active microbes, i.e. if the residence time in the apparatus is long enough. This places strict demands on the material of the microbial media. , le, shape and placement such that a sufficiently long residence time is achieved without increasing the size of the equipment ··· uneconomically.

Since the gas flow in the process of the invention is not · · · s.! ϊ forced through microbial growth surfaces, the surface need not be uniform but may be divided into small discrete parts. These parts can be placed in the equipment as much as possible in terms of dwell time and space use. In experiments performed it has been found that by using equipment according to the method of the invention, a capacity of 4-5 times higher can be achieved, or the like; ·; ·· | lower space requirements than similar known ones; ·. ·. 103481 5 forced gas flow microbiological purification methods.

On the other hand, an additional aspect of the invention preferably affects the adsorption power. According to this aspect, the aqueous liquid sprayed into the gas stream to be purified contains microbes used for purification, whereby the microbes are contacted with the impurities as early as possible and efficiently.

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Furthermore, it should be noted that the method of the invention and the device implementing it also provide an additional efficiency effect on material transfer by keeping the gas to be purified in contact with the microbial culture for an extended period of time. The transfer of impurity residues is thus also possible directly from the gas to the microbes.

In a preferred embodiment of the invention, the aqueous liquid containing impurities separated from the gas stream is made to flow over the substrate surface. This function ensures maximum utilization of the substrate surface. As the fluid flows on the substrate surface, the microbes on the substrate come into contact with the constituents of the fluid and utilize them according to their capacity. The content of impurities is naturally highest at the top of the pouring surface, and • · J * ·· decreases as the surface goes down. The microbial growth on the upper surface of the substrate surface thus receives, as far as possible, nutrients added to the aqueous liquid as well as nutrition as impurities. The welfare of this microbial crop is therefore as good as possible in this respect. Instead, the microbial growth at the bottom of the surface receives a variety of nutrients and nutrients, which controls the amount of growth at the lower surface. The underside of the surface thus forms a potential load on the culture medium reserve. for peak peaks, as well as the aging surface of • · 6 103481 for it. The eutrophication of the lower crop can thus also be used to assess the need for surface replacement.

The structure of the medium as well as the conditions in the medium also influence the maintenance of vital microbial vital functions. The above refers to nutrients, and nutritional conditions. Aerobic microbes suitable for gas purification can be given roughly 100: 10: 1 carbon, nitrogen and phosphorus ratios of basic nutrients.

10 An effort should be made to maintain the ratio by directing nutritional supplements, for example, with an aqueous liquid injected into the gas, if the substances introduced by the gas stream to be purified do not maintain the ratio. In addition, care must be taken to ensure that there is sufficient hap-15 pea in the growing environment. The exhaust gas to be purified will normally contain a sufficient amount of oxygen, but it is not excluded that additional oxygen, for example external air, is introduced into the media. Similarly, moisture content and temperature are important. With regard to moisture content, the 20 basic guidelines may be that the relative humidity of the air in the substrate should be at least 50%. The temperature should be between 15 and 50 ° C. Unless this can otherwise be maintained, the process streams are cooled. . VA / heated. The pH of the growth conditions is also important for microbial growth and this must be taken into account by providing, if necessary, a pH adjustment. The usable pH of the · · * ··· depends on the gas being purified and its purification 103481 7

The invention will be explained in more detail by means of the accompanying drawings, in which:

Fig. 1 schematically illustrates a filter unit useful in carrying out the invention, in vertical section,

Figure 2 schematically illustrates another filter unit useful in practicing the invention, also in vertical section,

Figure 3 shows an exemplary flow diagram for an apparatus for carrying out the method of the invention.

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Figure 4 schematically shows another filter unit for carrying out the invention in vertical section, and Figure 5 shows another filter unit for carrying out the invention in vertical section.

The filter unit of Figure 1 exhibits microbial growth. , a parallel, spaced-apart plate used as a substrate at an angle of about 45 ° ·· ·. The gas to be purified is fed into the system via ♦ ·· 2. The gas to be cleaned is sprayed * ♦ · This aqueous liquid is passed through conduit 4. The gas entering through the I 2 thus contacts the aqueous liquid downstream and flows downwardly into the apparatus and encounters the upper surface of the media 1. In this situation, the liquid settles on the surface of the growth medium, and the gas continues to flow in the spaces between the plates as flushes in the direction of contact with the microbial growth on the surfaces of the plates, and exits together through 3.

• · · • · • 8 103481

The aqueous liquid sprayed from the nozzles of the conduit 4 among the gas to be cleaned settles on the upper edge of the growth plates 1, wets the plate, and starts to run along the upper surface of the plate towards the lower edge as the plate 5 reaches sufficient moisture. With the plates 1 disposed in the oblique position as mentioned above, the plate conveniently shields the underlying plate so that the sprayed liquid can only settle in the upper area of the plates. The aqueous liquid flows over the entire surface of the substrate plate 10 and further into the collector space below the plate, where it is discharged through one, preferably for recycling.

Figures 4 and 5 show alternative placement options for the substrate-15 belts.

The apparatus of Fig. 2 is a variant in which the substrate plates 1 are replaced by hollow cylinders 20 7 of open portions of suitable porous material which are positioned vertically in the chamber 9 of the apparatus. . The aqueous liquid to be sprayed into the gas to be cleaned is led from the nozzles of the pipe 4 in the same manner as in the device of Fig. 1.

... The upper ends of the cylinders may be provided with a «balancing» wire fabric 8 or the like. In this embodiment, • - * * ·· gas-sprayed liquid flows on the inner and outer surfaces of the cylinders: T: and, after being in use for some time, forms a liquid film on these surfaces. The material of the cylinders is advantageous. The mineral wool is similar to the one of Fig. 1. Moisture conveys nutrients and impurities in the substrate to • • ·

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* · For use by robes.

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An aqueous liquid which has been discharged from the top down to the surface of the cylinders 7 and which has delivered its contents to microbes is similarly removed as in the device of Fig. 1 for recycling or disposal.

In order to enlarge the growing surface, cylinders of different diameters can be used in the embodiment of Fig. 2, whereby the packing density is increased on the cross-section of the chamber 9.

10 In all embodiments, the gas flow direction may also be reversed.

Figure 3 shows an apparatus for implementing the method according to the invention in its entirety. The gas to be cleaned is introduced as a process exhaust gas, for example as a result of said mineral wool manufacturing gas, into 2 series and partly parallel cleaning units L and K. The number of cleaning units can be increased or decreased as needed.

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The aqueous liquid 4 injected into the gas to be cleaned is led in a substantially closed circuit to the purifiers L and K and back to the tank 9. The reservoir 9 maintains a substantially constant volume of liquid, whereby the water discharged with the purified gas is replaced by fresh water. Fresh water can be added as a supplement% ·· ♦ ,,, · also as a supplement to the removal of contaminated water. The nutrients required by the microbes 4 * f * ·· are added to the water drawn from the tank 7, preferably with a control over the composition of the solution. The pH of the solution is likewise monitored. In the example, the addition of phosphoric acid is used to adjust the pH, which is. also suitably provides a phosphorus supplement. Otherwise, the • «« nutrient solution contains e.g. K2HPO4, MgSO4, CaCl2 and FeSO4.

«J I f» i \, '· 35 Example (I. · ·

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*,, ·, The apparatus of Figure 3 was used as a pilot scale v 10 103481 in a practical application for purifying exhaust gas from mineral wool production. The gas to be purified was taken as a suitable by-pass from the actual exhaust gas stream to give a maximum load of the purification plant of 1000 m3 / hour per purification unit. The purification unit had a cross-sectional area of 1 m2 and a structure as shown in Figure 1 with the substrate consisting of mineral wool plates 30 mm thick and 100 kg / m3. The panels were placed obliquely on tinplate 10 at 30 mm apart and had a total surface area of 40 m2. The total length of the trial period was 17 weeks, during which time the purification efficiency of the microbioligsten units was examined for phenol, formaldehyde, ammonia and total hydrocarbon content. Gas concentrations 15 and properties ranged within the following ranges: Temperature, ° C 20-47

Fiber content, mg / m3 0-20

Drops of resin, mg / m3 50-100 Phenol, mg / m3 10-30

Formaldehyde, mg / m3 10-30

Ammonia, mg / m3 20-90 'With respect to chemical removal, the values given in the table below are 25 l. At the end of the trial period, there was no sign of clogging in the cleaning unit * ... ·.

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Table I; reduction of chemicals

Test Load Temperature Liquid / Phenol Formal Ammonia m3 / h1m2 space ° C degassing dehyde% deg% removal ______% __% 1 500__42__0,014> 95 72-100 13-59 5 1 800__38__0,013 96-99 ___ 52 -66 2 500__28__0,015 97-98 ___ 96-98 2,760__33__0,012 96-98 82-92 86-92 2 970__34__0,010 87 79__90 2 1000_96_ 10

The process of the invention and the apparatus implementing it can be used to purify the exhaust gases of many different processes. The exhaust gas from mineral wool manufacture is exemplified above. Other applications can be found in the food industry, agriculture, the forest industry, and the chemical industry in general.

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Claims (8)

A method of microbiologically removing aerosol contaminants from a gas stream such that in the gas stream to be purified, aqueous liquid, the aqueous liquid which has been in contact with the gas stream, is conducted to moisten the surface of a culture support. containing a microbe resistant and the gas stream is passed over the surface of the culture substrate and in contact with it, characterized in that when aqueous liquid is fed to the gas stream 'to be purified, a volume flow ratio' ··· 'for liquid and gas of the order 0 is used. , 01 - 0.02. 2. A method according to claim 1, characterized in that microbes which break down the gas's pollutants are supplied with the aqueous liquid which is carried into the gas stream. • · · • · · • · ·. 3. A process according to claim 1 or 2, characterized in that the aqueous liquid containing the compounds is allowed to flow freely along the microbial culture substrate, whereby the desired run-off time is controlled by the structure of the substrate. 103481 Apparatus for carrying out the method according to claim 1, which comprises a gas washing chamber (9), gas inlet / outlet connections (2,3) in the upper and lower portions of the chamber, liquid medium (4) for cleaning the chamber. An upper portion and an outlet connection (5) for liquid in the lower portion of the chamber, and a cultivation substrate under the influence of liquid-emitting fluids for a microbial which breaks down the pollutants of the gas, which culture substrate (1, 7) for microbes is placed in the chamber. so that it allows the flow of gas from the inlet connection to the outlet connection in the direction of the surface of the culture substrate and in its immediate vicinity, characterized in that the culture substrate is mineral wool, whose volume weight is between 35-200 kg / m3, 5. Device according to claim 4, characterized in that the cultivation base consists of a plurality of separate essentially parallel disks (1) spaced apart from each other. 20 6. Device according to claim 4, characterized in that the cultivation base is made up of cylinders (7), which are preferably hollow and open at their ends. 7. Apparatus according to any one of claims 4-6, characterized in that the surfaces of the microbial culture substrate are arranged in a relative plane to the horizontal plane in a 30 ° · 90 °: s, preferably at an angle of 40-60 °: · · · · · ♦ · · · · · · · · · · · · · ♦ · · • · ♦