WO1992000792A1

WO1992000792A1 - Method and device for purifying air

Info

Publication number: WO1992000792A1
Application number: PCT/SE1991/000465
Authority: WO
Inventors: Ola Larsson
Original assignee: ABB Fläkt AB
Priority date: 1990-07-13
Filing date: 1991-06-27
Publication date: 1992-01-23
Also published as: PL297599A1; SE9002429L; JPH05508583A; EP0539459A1; FI930002A; CA2086927A1; AU644009B2; AU8224291A; SE9002429D0; FI930002A0; BR9106639A; SE466735B

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 (1) in a particle board or fiber board production process. The polluted air from the drier is fed through a scrubber (3) 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 According to the invention a portion of the process water from the washing cycle is withdrawn and is fed through a biological purification plant (20), designed to decompose the organic contaminants, which are dissolved in the water, and after that the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 (3) is combined with a heat exchanger (8), designed to raise the temperature of the fresh air to the drier (1) and to lower the temperature of the water to the purification plant (20).

Description

METHOD AND DEVICE FOR PURIFYING AIR.

The present invention relates to a purification and reco¬ very process of the type stated in the preamble of patent claim 1. The invention also relates to a device, designed to carry out the purification and recovery process accor¬ ding to the patent claim, which device is defined in more detail in the 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 dry¬ ing 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 formal¬ dehyde 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 formal¬ dehyde may in such a process result in a large killing of microorganisms. The consequence of this is a sharply re¬ duced purification effect for a long period of time.

Also, since the microorganisms are suspended in the wa¬ ter, a separation must be done downstreams of the ponds. This means that equipment is needed for separation and equipment designed to recirculate the obtained microor¬ ganism sludge. Of course, this makes such a process comp¬ licated, expensive and time-consuming, and consequently it is objectionable or simply unrealizable, e.g. in con- tinuous processes, such as particle board and fiber board production.

Already known art in this field is described in EP-A1- 0064 960, DE-A1-2547 675 and EP-A1-0282 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, neither 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 ncrc-^ ^ - .r' ^■•--* +- e p^resent 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 wash¬ ing liquid in the main washing phase principally only is fed through a circuit between the scrubber and the biore- actor. 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 inven¬ tion, since this cycle only is used to circulate a small flow 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 inven¬ tion, but unlike the cycles according to the present in- 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 togeth¬ er 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 :. pro¬ cess and a device, which are characterized by a satisfac¬ tory 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 eco¬ nomically advantageous way. An additional object is to, despite ?n im nv^prl cleaning of the process air as well as the process water, lower the energy requirement for the production process itself. Finally, the present in¬ vention is supposed to develop the state of the art in this field also in additional respects.

These objects car. be achieved according to the present in¬ vention by carrying out a process c: 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 desc¬ ription, reference being made to the accompanying draw¬ ing, which schematically illustrates a preferred embo¬ diment, designed to carry out a process according to the invention and regarding the principal construction of a device according to the invention respectively. 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 produc¬ tion. 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, ethanol and acetic acid, which are only a few of the principal¬ ly 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 scrub¬ ber. The air, cleaned in this way, leaves the scrubber via an outlet 6 and e.g. a chimney 7, which leads out into the atmosphere.

In accordance with a preferred embodiment of the inven¬ tion said sc^rubber is combined with a heat exchanger 8 , through which fresh air is fed into source 1 via an inlet duct 9. The Fresh air, which may have the same tempera¬ ture 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 scrub¬ ber water will be somewhat colder. These two effects re¬ sult 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 scrub¬ ber water temperature. A certain temperature stabiliza¬ tion, by means of a device (not shown) in the duct up- streams of the biological purification, may neverthe¬ less be required in order to provide the microorganisms with stable conditions .

The cleaning water leaves scrubber 3 via a coarse separa- 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 built- in pump 18 and leading preferably first to a particle se¬ parator 19, designed mainly for accompanying finer fibers and after that to a biological purification plant, gene¬ rally 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 nor-zle^ 27. v n * ar fo^ 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 microorga¬ nisms with oxygen. Duct 17 leads in the bioreactor sub¬ sequently to a separator 30 for biosludge and reaches fi- - nally via an additional pump 31 water tank 13 again.

Such a device functions in the following way: In the scrub¬ ber 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 scrub¬ ber. 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 substanti¬ ally, since also most particles are absorbed by the wa- 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 g formalde- hyde/m , as compared to e.g. 10-25 mg formaldehyde/m⁰ in the air, which flows into the scrubber. Simultaneous¬ ly the amount of particles has decreased from 40-60 mg/ m 3 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 wa¬ ter 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 wa¬ ter, having a temperature of say about 40 C, often is considerably hotter than the freε-i 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 coo¬ ling effect being obtained.

The water flow from tank 13 to purification plant 20 ge¬ nerally corresponds to the amount of liquid condensed in the scrubber plus possibly added fresh water. The formal¬ dehyde contents of this flow is often 200-300 mg/1. A wa¬ ter which has such a high amount of formaldehyde usually is considered very difficult to treat. The total COD-con- tents (COD = chemical oxygen demand) in the water has been measured and often it is between 1000 and 2000 mg/1.

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 is separated by means of e.g. flotation. Subsequently the p -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 microorga^¬ nisms in the bioreactor, connected downstreams, need according to a calculation. In the latter the microorga^¬ nisms grow on filling bodies of a type known per se, e.g. those which AB Carl Munthcr sells, called Euroform. The filling bodies are lowered into the bioreactor and can quickly and simply be exchanged, in case and when this is required.

An analysis of the biological film or. the filling bodies showed, that it contained a rnicroflcrr. , rich in species and comprising i.a. a largo amount of zooglea colonies and filament-generating bacteria. The rnicrofiora con¬ tained also a certain amount of protozoan' derable number of fungi.

The filling body material preferably is desiσned in such a way, that the finely divided air bubbles, issuing from nozzles 27, are distributed evenly in the reactor vo¬ lume. The organisms- ecompose e.g. 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 for¬ maldehyde. However, the remaining organic contents makes the rnicroflora grow, and consequently it is important to know the properties of th contaminants and to adjust the process according to the same. h short dwell time will bo sufficient, e.g. 3-8 hours, provided microorganisms are used, which are specialized as to their capacity to decompose the specific Organic contaminants encountered. The reactor size is in a case

3 with 2.5 /hour water and 250 mg formaldehyde/liter al-

3 . most 30 m including the free spaces 32 and 33, needed be-

3 low and above filling bodies 25. If the rate is 21 m /

3 hour, the reactor volume is estimated to be 150 m . Pro¬ vided the reactor is designed in this way and. the vari¬ ous processes can be controlled, the reactor can be ope¬ rated 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/1. The reduc¬ tions of COD and BOD (biological oxygen demand) arc 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 recovery- circuit. Since the microorganisms are attached tc _--o t>UD- mersed support material, the amount suspended in the wa¬ ter will be very small. No organisms have to be reintro- duced into the process. That amount which leaves the pro¬ cess is mainly killed microorganisms. Before the water is returned to the tank, said particle contents is separa¬ ted by means of a flotation process or a continuous sand filter.

By reintroducing the water directly from the water puri¬ fication step a mainly closed process is obtained as re¬ gards the used water. That amount of water, which corre¬ sponds to the amount which is condensed in the scrubber, is leaked subsequent to the water purification, down¬ streams 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- times it is also advantageous to add chemicals, which in¬ crease 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 chim¬ ney 7, can be controlled and influenced by varying either the liquid flow through scrubber 3 or the contaminant con¬ tents 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 re¬ moval 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 formaldehyde 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 accepted, pro¬ vided 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 inte¬ grated. Also, by minimizing the water flows, the reco¬ verable heat will be comparatively large. By controlling the 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 de- ^' scribed embodiments but can be modified and supplemented in an arbitrary way within the scope of the inventive idea and the following patent claims.

The control/detection described above of the amount of the remaining organic contaminants in the discharged air can be carried out by means of a control element 34, con¬ nected in the discharged air-path 6,7, the outsignal of said control element being sent to a guide element 35, de¬ signed to influence the liquid circuit through the purifi- cation plant.- Also, in order to take care of emergencies with temporarily extremely high contents of contami¬ nants in the air from the drier, duct inlet 17 to the tank and duct outlet 4 from the tank can be short-cir- ^• . cuited via a valve 36. Also, it is feasible to short-cir¬ cuit 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 pla-* ces 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 pro¬ vided downstreams of the reactor. The object of such a space is to via pump 31 temporarily increase the dis¬ charge 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

1. A cleaning and recovery method in connection with an industrial production process, particulary with reference to a drier (1) , 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 (3) , in which it is washed with water, which is fed via spraying nozzles (5) , after which the air, cleaned in this way, completely or partially, is discharged from the process, e.g. into the atmosphere through a chimney (7) , and water containing contaminants being discharged from the scrubber and at least partly recycled to the scrubber, preferably via a coarse separator (10) , 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 (3) being purified in a biological pu¬ rification plant (20) , 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 (20) are regulated independently of each other in order to ob¬ tain the required purification effect on the discharged air as well as on circulating and possibly discharged water, in that the water, which contains contaminants and has been discharged from the scrubber (3) , flows through a water tank (13) , before it is returned to the scrubber, in that the partial flow (17) , 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 (20) , which is to be returned, is returned to the same water tank .

2. A method according to patent claim 1, c h a r a c ¬ t e r i z e d in that the partial flow (17) , which is to be purified biologically, is passed through a parricJe se¬ parator (19) , 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 (30) , 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 (3) for the drier or the like (1) 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. A device designed to carry out the purification and recovery method according to patent claim 1 , comprising a source (1) , 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 (3) , 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 (7) , as well as comprising a coarse separator (10) , 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 (5) of the scrubber in the form av. spray nozzles, a portion of the coarsely cleaned scrubber liquid being fed through a closed biolo¬ gical purification plant (20) , c h 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 liquid 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 (13) , 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 (20) , 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 (3) .

5. A device according tc patent claim 4, c a r a c ¬ t e r i z e d in that said scrubber (3) is combined with a heat exchanger (8) , which raises the temperature of the air fed to the drier or the like (1) and lowers the tem¬ perature cf the discharged cleaned process air and re¬ latively also the scrubber liqui .

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 (17) , which starts from the water tank (13) and passes through the purifi¬ cation plant (20) , in the flow direction a particle sepa¬ rator (19) is first connected and downstreams of the same a mixer (21) , 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 bioreactor (24) with microorganisms, designed to decompose organic substances in the water, and finally separators (30) , particularly for killed microorganisms, a pump (18 and 31 respectively) preferably being connected upstrcams of the particle sepa¬ rator (19) and downstreams of the biosludge separator (30) 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 (25) , above which and below which free spaces (31 and 32 respectively) are placed, and in that in the lower free volume (32) ar. aeration device (26) is placed, which includes nozzles (27) , 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 (34) , connected in the dis¬ charged air-path (6,7) from the scrubber (3) 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 (20) .