AU644009B2 - Method and device for purifying air - Google Patents

Description

OPI DATE 04/02/92 J AOJP DATE 12/03/92 APPLN. ID 82242 91 PCT NUMBER PCT/SE91/00465

Y

INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 5 (11) International Publication Number: WO 92/00792 BO1D 47/00, 53/00, C02F 3/00 Al (43) International Publication Date: 23 January 1992 (23,01.92) (21) International Application Number: PCT/SE91/00465 (81) Designated States: AT, AT (European patent), AU, BB, BE (European patent), BF (OAPI patent), BG, BJ (OAPI (22) International Filing Date: 27 June 1991 (27.06.91) patent), BR, CA, CF (OAPI patent), CG (OAPI patent), CH, CH (European patent), CI (OAPI patent), CM (OAPI patent), CS, DE, DE (European patent), DK, data: DK (European patent), ES, ES (European patent), FI, 9002429-0 13 July 1990 (13.07.90) SE FR (European patent), GA (OAPI patent), GB, GB (European patent), GN (OAPI patent), GR (European patent), HU, IT (European patent), JP, KP, KR, LK, LU, (71) Applicant (for all designated States except US): ABB FLAKT LU (European patent), MC, MG, ML (OAPI patent), AB [SE/SE]; S-120 86 Stockholm MR (OAPI patent), MW, NL, NL (European patent), NO, PL, RO, SD, SE, SE (European patent), SN (OAPI (72) Inventor; and patent), SU, TD (OAPI patent), TG (OAPI patent), US.

Inventor/Applicant (for US only) LARSSON, Ola [SE/SE]; SkogslyckevAgen 12, S-352 45 VAxj6 (SE).

Published (74) Agent: SIEBMANNS, Hubertus; Gotapatent AB, Box 154, With international search report.

S-561 22 Huskvarna In English translation (filed in Swedish).

64 00 (54) Title: METHOD AND DEVICE FOR PURIFYING AIR (57) Abstract A method and a device designed to clean air and water as well as to recover purified process water with reference to a drier in a particle board or fiber board production process. The polluted air from the drier is fed through a scrubber and is freed in the scrubber from the main part of solid and gaseous contaminants, before it is discharged into the atmosphere, whereas the process water is fed through a coarse separator, before it is returned to the scrubber. According to the invention a portion of the process water from the washing cycle is withdrawn and is fed through a biological purification plant designed to decompose the organic contaminants, which are dissolved in the water, and after that the process water will be returned to the washing cycle. The integration of the two cycles and the variation of the flow through the purification cycle will influence the conditioning of the washing cycle and also its washing capacity, and in this way it will be possible to maintain the remaining organic contaminants in the discharged air below a certain limit value. In order to optimize the economy of the production process as well as the cleaning process the scrubber is combined with a heat exchanger designed to raise the temperature of the fresh air to the drier and to lower the temperature of the water to the purification plant See back of page WO 92/00792 PCP/SE91/0046-5 METHOD AND DEVICE FOR PURIFYING AIR.

The present invention relates to a purification and recovery process of the type stated in the preamble of patent claim i. The invention also relates to a device, designed to carry out the purification and recovery process according to the patent claim, which device is defined in more detail in Lhe first device claim.

In industrial production processes often polluted air is discharged. In particle board and fiber board production for instance warm moist air is discharged from the drying equipment, which air is polluted with i.a. fibers and formaldehyde. Particularly the volatile impurities are causing serious problems, if economic and environmental considerations are to be shown at the same time. Other processes, in which formaldehyde is discharged are i.a.

resin and polymer preparations. The separation of formaldehyde from the polluted air is often done with a wet scrubber. In the discharged scrubber liquid the amount of formaldehyde then generally is very high.

Ordinarily this liquid is diluted with additional water in order to obtain a lower amount of formaldehyde and then of course large amounts of water must be subjected to a biological purification, which in most cases is done in large aerated ponds. The dwell time generally is 4-10 days. Possible chock loads with larger amounts of formaldehyde may in such a process result in a large killing of microorganisms. The consequence of this is a sharply reduced purification effect for a long period of time.

Also, since the microorganisms are suspended in the water, a separation must be done downstreams of the ponds.

This means that equipment is needed for separation and equipment designed to recirculate the obtained microorganism sludge. Of course, this makes such a process complicated, expensive and time-consuming, and consequently it is objectionable or simply unrealizable, e.g. in con- WO 92/00792 PCT/SE91/00465 tinuous processes, such as particle board and fiber board production.

Already known art in this field is described in EP-Al- 0064 960, DE-Al-2547 675 and EP-Al-0282 750.

However, in the first-mentioned doucment, which by the way belongs to Applicant, there are not even any indications, that for instance an "increase in the impurity contents of the air is to be remedied by any adequate counter-measures.

Also, the liquid flow from the biological purification plant is according to this document returned to the feed duct of the spraying chamber. It is true that according to this .document a stabilization tank 8 is used, in which nutrients are added. However, neithcr is to this tank.8 any liquid from the bioreactors added nor is any liquid withdrawn to the spraying chamber, i.e. this tank has no functions, which are similar to the functions according to the present invention. Finally, said document does not relate to any heat exchange in a way, which is suggested n- 4 he n-esent invention.

Document DE-25 47 675 relates to a process, in which there are just few points of similarity with the process .and the device according to the present invention, since the washing liquid in the main washing phase principally only is fed through a circuit between the scrubber and the bioreactor. It is true that there is an additional cycle for a distribution device 4, upstreams of the bioreactor, but this cycle is not really similar to the present invention, since this cycle only is used to circulate a small ilow of washing liquid to and from a sludge separator 6.

Also, distribution device 4 is not really similar to the present invention, and there is no suggestions in this document of a heat exchange between polluted discharged air and fresh air.

According to the process described in document EP-G 282 750 there are indeed two cycles like the present invention, but unlike the cycles according to the present in- WO 92/00792 PCU/SE91/00465 vention the two cycles pass according to this document through the scrubber and also they are not connected to each other in any way. Also, there is no bringing together of the liquid flow which passes through the bioreactor and the liquid flow which is returned directly to the scrubber before the recycling to the scrubber, as is done according to the present invention, but the clean as well as the polluted liquid flow are returned to the scrubber.

Also, there is no suggestion in this document, that there is a heat exchange between polluted discharged air and fresh air fed into the system.

One object of the present invention is to provide ca process and a device, which are characterized by a satisfactory cleaning of air as well as water, which pass through a production plant. Another object is to obtain such a cleaning quickly, in a realiable, controllable and economically advantageous way. An additional object is to, despite Pn imr-rnvd cleaning of the process air as well as the process water, lower the energy requirement for the production process itself. Finally, the present invention is supposed to develop the state of the art in this field also in additional respects.

These objects can be achieved according to the present invention by carrying out a process of the type described in the introduction above mainly according to the method, which is set forth in the characterizing clause of patent claim 1. These objects are also achieved by means of a device according to the first device claim.

Additional characterizing features and advantages of the present invention are set forth in the following description, reference being made to the accompanying drawing, which schematically illustrates a preferred embodiment, designed to carry out a process according to the invention and regarding the principal construction of a device according to the invention respectively.

WO 92/00792 PCT/SE91/00465 4 In the drawing a source 1 is shown, which produces warm and polluted as well as usually moist air, e.g. a drier or a hot press for particle board or fiber board production. In a practical embodiment the temperature of the air is 45-60 C and the contaminants comprise partly dust and fiber particles and partly formaldehyde, methanol and acetic acid, which are only a few of the principally existing contaminants.

From said source the polluted air flows via an outlet duct 2 to a scrubber 3, in which the air is cleaned with water, which is fed via a feed duct 4 and is finely divided by means of spray-nozzles 5 in the upper area of the scrubber. The air, cleaned in this way, leaves the scrubber 1ia an outlet 6 and e.g. a chimney 7, which leads out into the atmosphere.

In accordance with a preferred embodiment of the invention said scrubber is combined with a heat exchanger 8, through which frusn air is fed into source 1 via an inlet duct 9. The Fresh air, which may have the same temperature as the ambient air, is heated in heat exchanger 8, and in this way the heating requirement for the fed air is reduced considerably. Also, the condensation of water vapor from the polluted air is increased and the scrubber water will be somewhat colder. These two effects result to an increased extent in an absorption of volatile contaminants in the scrubber water, the cleaning effect being improved. Also, during the subsequent treatment of the scrubber water it is advantageous with a lower scrubber water temperature. A certain temperature stabilization, by means of a device (not shown) in the duct upstreams of the biological purification, may nevertheless be required in order to provide the microorganisms with stable conditions.

The cleaning water leaves scrubber 3 via a coarse separa- WO 92/00792 PCT/SE91/00465 tor 10, designed to separate mainly coarser solid par- .ticles. The cleaning water, roughly purified in this way, then is fed via a duct 11, having a built-in pump 12, to a water tank 13, to which also a fresh water-duct 14 is connected, having a built-in pump 15, as well as the above-described feed duct 4 having built-in pump 16.

Also, from tank 13 a feed duct 17 issues, having a builtin pump 18 and leading preferably first to a particle separator'19, designed mainly for accompanying finer fibers and after that to a biological purification plant, generally designated 20. The latter comprises, in the flow direction of the water, mainly a mixer 21 with connected store tanks 22 and 23, which contain nutrient salts and/ or pH-adjusting means etc. for a biological treatment of the water, which subsequently is fed into a bioreactor 24 with filling bodies 25, in and on which microorganisms are cultivated, which are to decompose at least some of the contaminants in the water, particularly formaldehyde.

The bottom of the bioreactor is connected to an aeration device 26 having no7zlec 27. whirh ;re for instance placed along the bottom of the bioreactor and fed with air via a feed duct 28 with a fan 29. From these nozzles finely divided air bubbles issue, which provide the microorganisms with oxygen. Duct 17 leads in the bioreactor subsequently to a separator 30 for biosludge and reaches fi- Snally via an additional pump 31 water tank 13 again.

Such a device functions in the following way: In the scrubber several processes take place simultaneously, as has been mentioned above. Partly heat is transferred from the heat exchanger and the hot, moist and polluted gas to the cold fresh air stream to the drier or the like. Partly an absorption of organic gaseous substances takes place to the circulating water, which is sprayed into the sci.bber. Thanks to the comparatively low temperature of the polluted air and water a satisfactory absorption of said organic gaseous substances is obtained. Simultaneously the amount of particles in the air is reduced substantially, since also .most particles are absorbed by the wa- WO 92/00792 PCT/SE91/00465 6 ter. After the described treatment the purified air is discharged through chimney 7. It now contains a small amount of said organic substances, e.g. 2-5 mc formaldehyde/m as compared to e.g. 10-25 mg formaldehyde/rr.

in the air, which flows into the scrubber. Simultaneously the amount of particles has decreased from 40-60 mg/ m to a value of below 10 mg/m3 In order to maintain the transferred amount of heat on a high level, it is important to avoid feeding more water into the scrubber than what is deemed required. This amount of water is controlled in a simple way by means of pump 16. The added water functions as a cooling medium as regards the heat exchanger. It is true that this water, having a temperature of say about 40 0 C, often is considerably hotter than the fres.; air, which is sucked into the system, but the temperature of the polluted air, which flows into the scrubber and has a temperature of 45-60 C, will nevertheless be lowered, a relative cooling effect being obtained.

The water flow from tank 13 to purification plant 20 generally corresponds to the amount of liquid condensed in the scrubber plus possibly added fresh water. The formaldehyde contents of this flow is often 200-300 mag/. A water which has such a high amount of formaldehyde usually is considered very difficult to treat. The total COD-contents (COD chemical oxygen demand) in the water has been measured and often it is between 1000 and 2000 mg/l.

However, thanks to the described and shown biological purification plant and its integration with the scrubber it is possible to simultaneously treat high contents of particularly formaldehyde in the air as well as in the water in an appropriate and economically advantageous way as regards the environment. In a first separation step the suspended amount of fiber i- separated by means WO 92/00792 PCT/SE91/00465 7 of e.g. flotation. Subsequently the pH-value of the water is adjusted to about 7 by adding e.g. sodium hydroxide through the mixer 21, through which also the required amount of nutrient salts is added, which the microorganisms in the bioreactor, connected downstreams, need according to a calculation. In the latter the microorganisms grow on filling bodies of a type known per se, e.g. those which AB Carl Munther sells, called Euroform.

The filling bodies are lowered into the bioreactor and can quickly and simply be exchanaed, ir. case and when this is required.

An analysis of the biological film or. the filling bodies showed, that it contained a microflor., rich in species and comprising i.a. a large amount, of zooglea colonies and filament-generating bacteria. The microflora contained also a certain amount of protozoanc derable number of funci.

The filling body material preferably is designed in such a way, that the finely divided air bubbles, issuing from nozzles 27, are distributed evenly in the reactor volume. The organisms-decompose e.c. the formaldehyde and the remaining organic contents. Since formaldehyde is a toxic substance when in large amounts, the decomposition of the formaldehyde will result in a negative growth of the microorganisms in such cases. However, when the formaldehydeconcentrations are comparatively low, the microorganisms will survive, when they decompose the formaldehyde. However, the remaining organic contents makes the microflorn grow, and consequently it is important to know the properties of thr contaminants and to adjust the process according to the same.

WO 0/07 PCT/SE91/0046-.

^f V/ 8 A short dwell time will be sufficient, e.y. 3-8 hours, provided .aicroorganisms are used, which are specialized as to their capacity to decompose the specific organic contaminants encountered. The reactor size is in a case with 2.5 m 3 /hour water and 250 mg formaldehyde/liter almost 30 m 3 including the free spaces 32 and 33, needed below and above filling bodies 25. If the rate is 21 m hour, the reactor volume is estimated to be 150 m 3 Provided the reactor is designed in this way and.the various processes can be controlled, the reactor can be operated with a very high load, with satisfactory results despite the short dwell times.

In the discharged water from the reactor the formaldehyde contents has been reduced to below 1 mg/l. The reductions of COD and BOD (biological oxygen demand) are more than 95 in the reactor. Consequently, the discharged water advantageously can be reused as fresh water in the process and recycled into the washing-heat recoverycircuit. Since the microorganisms are attached tc j suDmersed support material, the amount suspended in thu water will be very small. No organisms have to be reintroduced into the process. That amount which leaves the process is mainly killed microorganisms. Before the water is returned to the tank, said particle contents is separated by means of a flotation process or a continuous sand filter.

By reintroducing the water directly from the water purification step a mainly closed process is obtained as regards the used water. That amount of water, which corresponds to the amount which is condensed in the scrubber, is leaked subsequent to the water purification, downstreams separator 30, via a not shown element. During the starting of the device or during a shut down fresh water can be added through duct 14 by means of pump 15. Some- WO 92/00792 PCT/SE91/00465 9 times it is also advantageous to add chemicals, which increase the capacity of the water to absorb formaldehyde and/or causes the formaldehyde to be transformed from a gaseous phase to a water phase faster., The discharge of formaldehyde, which takes place via chimr..y 7, can be controlled and influenced by varying either the liquid flow through scrubber 3 or the contaminant contents in the scrubber liquid. Usually the liquid flow is kept constant. In case the formaldehyde contents in the gas, which is discharged through chimney 7, is increased above the allowed limit, that partial stream which is fed to the biological purification plant is increased. The removal of water, from tank 13, with a high formaldehyde contents via duct 17 increases and to the same extent the amount of purified water, returned to tank 13 via duct 41, increases. In this way the scrubber liquid in scrubber 3 will have a lower amount of contaminants than before. The physical equilibrium between the formalA.ehyde in the gaseous phase and the formaldehyde in the water phase will then be influenced in such a direction, that more formaldehyde will be absorbed by the liquid.

Biological purification plant 20 must of course have such dimensions, that the increased load will be taken care of.

However, a certain overload-can easier be acceDted, provided the incoming liquid flow has a lower contents of formaldehyde, which automaticly will result due to the above-described change.

Special advantages of the described and shown combined purification and recovery process are partly a simple control of outgoing formaldehyde-contents by adjusting the water flows and partly the possibility of minimizing the various water flows, since they are mutually integrated. Also, by minimizing the water flows, the recoverable heat will be comparatively large. By controlling WO 92/00792 P 1-CTIISE91/0046the water flows the biological purification plant can be loaded/utilized to the greatest extent.

The various parts of the device are of course controlled and guided from a common control and monitoring device, the principal construction and function of which are known per se from other technical areas.

The present invention is not limited to the shown and described embodiments but can be modified and supplemented in an arbitrary way within the scope of the inventive idc and the following patent claims.

The control/detection described above of the ai'ount of the remaining organic contaminants in the discharged air can be carried out by means of a control element 34, connected in the discharged air-path 6,7, the outsignal of said control element being sent to a guide element 35, designed to influence the liquid circuit through the purification plant. Also, in order to take care of emergencies with temporarily extremely high contents of contaminants in the air from the drier, duct inlet 17 to the tank and duct outlet 4 from the tank can be short-circuited via a valve 36. Also, it is feasible to short-circuit duct outlet 17 from the tank and fresh water duct 14 via a valve 37 in order to reduce extremely high loads on the purification plant. Of course, in arbitrary places detection and control elements can be installed in order to adequately control the process and the device respectively.

Also, the bioreactor can include a buffer space, e.g. by oversizing the bioreactor, or such a space 38 can be provided downstreams of the reactor. The object of such a space is to via pump 31 temporarily increase the discharge of cleaned water and at the same time let pump 18 continue to feed polluted water into the purification plant with an unchanged speed.

Claims (7)

1. A cleaning alid recovery method in connection with an industrial production process, particulary with reference to a drier used in particle board or fiber board production, clean or purified air from e.g. the environ- ment being fed during the production process and e.g. air polluted with e.g. particles and formaldehyde being dis- charged, which discharged air is fed through a scrubber in which it is washed with water, which is fed via spraying nozzles after which the air, cleaned in this way, completely or partially, is discharged from the process, e.g. into the atmosphere through a chimney and water containing contaminants being discharged from the scrubber and at least partly recycled to the scrubber, preferably via a coarse separator which is placed in or downstreams of the scrubber and designed to separate solid contaminants, and at least a partial flow of the water which contains contaminants and comes from the scrubber being purified in a biological pu- rification plant c h a r a c t e r i z e d in that at least a portion of this biologically purified partial flow is added to the rest of the liquid, which contains contaminants, to be recycled to the scrubber, in that the liquid flow through the scrubber and the par- tial flow through the biological purification plant are regulated independently of each other in order to ob- tain the required purification effect on the discli:ged air as well as on circulating and possibly discharged water, in that the water, which contains contaminants and has been discharged from the scrubber flows through a water tank before it is returned to the scrubber, in that the partial flow which is to be purified biologically, is withdrawn from said water tank, and in that that portion of the biologically purified par- tial flow from the biological purification plant which is to be returned, is returned to the same water WO 92/00792 PCT/SE91/00465 12 tank.

2. A method according to patent cl m 1, .c h a r a c t e r i z e d in that the partia, .low which is to be purified biologically, is passed through a particJese- parator additives such as neutralization agents and nutrient salts are added, the flow is passed through a bioreactor (20) with microorganisms and after that an ad- ditional particle separator before at least a por- tion of the partial flow, biologically purified in this way, is recycled to the circulation system for the wash- ing liquid of the scrubber.

3. A method according to patent claim 1, c h a r a c t e r i z e d in that the polluted air, in connection with the washing, also is subjected to a heat recovery process with a rise of temperature in the fresh air, fed into the scrubber for the drier or the like and with a temperature reduction in the discharged scrubber liquid to a level, which is closer to the optimal tempe- rature for the subsequent biological purification.

4. device designed to carry out the purification and recovery method according to patent claim 1, comprising a source which,belongs to an industrial production process, of air, which is polluted with e.g. fibers, par- ticles and organic substances, such as formaldehyde, e.g. a drier for particle board or fiber board production, a scrubber connected to said source and designed to wash said air, which after that is to be, completely or partially, discharged from the process, e.g. out into the atmosphere through a chimney as well as comprising a coarse separator placed at the lower end of the scrubber and designed for large particles, which separa- tor via ducts (11,4) and a pump (12 and 16 respectively) is connected to the washing device of the scrubber in the form av spray nozzles, a portion of the coarsely WO 92/00792 PCT/SE91/00465 13 cleaned scrubber liquid being fed through a closed biolo- gical purification plant c h a r a c t e r i z e d in that the scrubber liquid, which has been purified in said purification plant, is to be returned to the washing process, in that the two main liquic cycles, which con- sist of one cycle through the scrubber and another cycle through the purification plant, are mutually integrated and adjustable in order to, by means of a liquid regu- lation, obtain partly an adjustable washing of the pol- luted air and partly a maximum utilization of the biolo- gical purification plant, in that the two main liquid cycles are mutually integrated in the form of a common water tank to which preferably also a fresh water feed duct (14) with a pump (15) is connected, and in that a regulation of at least the liquid cycle, which passes through said purification plant is to influence the conditioning of the water in the tank and then also in the second cycle and finally the washing of the scrub- ber A device according to patent claim 4, c h a r a c t e r i z e d in that said scrubber is combined with a heat exchanger which raises the temperature of the air fed to the drier or the like and lowers the tem- perature cf the discharged cleaned process air and re- latively also the scrubber licuid.

6. A device according to claim 4, c h a r a c t e r i z e d in that in the feed duct which starts from the water tank (13) and passes through the purifi- cation plant in the flow direction a particle sepa- rator (19) is first connected and downstreams of the same a mixer designed to add pH-adjustment agents to the water and/or additional treatment agents for the water, e.g. nutrient salts for the microorganisms from storage containers (22,23) and after that a bioreactcr (24) with microorganisms, designed to decompose organic substances WO 92/00792 PCT/SE91/00465 14 in the water, and finally separators particularly for killed microorganisms, a pump (18 and 31 respectively) preferably being connected upstreams of the particle sepa- rator (19) and downstreams of the biosludge separator respectively, and in that the feed duct (17) downstreams of the purification plant leads back to the water tank (13).

7. A device according to claim 4, c h a r a.c t e r i z e d in that the microorganisms in the bioreactor (24) are placed on filling bodies above which and below which free spaces (31 and 32 respectively) arc placed, and in that in the lower free vclum- (32) ar. aeration device (26) is placed, which includes nozzles distributed along the reactor bottom and designed to introduce finely divided air bubbels into the reactor and to aerate the filling bodies and the microorganisms cultivated on the same.

8. A device according to claim 4, c h a r a c t e r i z e d by a control device connected in the dis- charged air-path from the scrubber and designed to detect and determine the amount of at least some of the remaining organic contaminants in the discharged air and via guide means (35) influence the liquid cycle passing through the purification plant