Note: Descriptions are shown in the official language in which they were submitted.
W~ 92/00792 PCr/SE91/0046'
~V8692 7
METHOD AND DEV I CE FOR PURI FY I ~'G AI R .
The present in~entio~ rel~tes to a purirication and reco-
very process of the type stated in the preamble of patent
claim 1. The invention also relates to a devic~, designed
to carry out thc purification and rccovc-~ 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, ~hich`air is ~olluted with i.a. fibers and
formaldchyde. Par~ c~ularl~ ~he ~olatile impuritics are
causing serious proble~s, if economic and environmental
considerations are to be sho~n at the same time. Other
processes, in which formaldehyde is discharged are i.a.
resin and polyme_ preparations. The separation of formal-
dehyde fro~ the polluted air is often done with a wet
scrubber. In the discharged scrubber liquid the amount
of formaldehyde the.. generallv is very high.
Ordinarily this liquid is diluted ~ith additional water .i~
in order to obtàin a lower amoun. of formaldehyde a..d then
of course large amo_.~s of wate- must be subjectec to a
biological puri.ication, ~h c:- in most cases is done in
large aeratea ponds. The d-;e'' 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 do~nstreams of thc ponds.
This means that equipment is needed for separation and
equipment designed to recirculate thc obtained microor-
ganism sludge. O_ course, this makes such a processcomp-
licated, expensive and time-consuming, and consequently
it is objectionable or simpiy unrealizable, e.g. in con-
W092/nO792 2 }~CT/SE~ltO~46~
'~8~2',i
tinuous processes, such as pa-ticle boa-s and fibe boarc'
production.
Already known art in this field is described ~n EP-Ai~
0064 960, DE-~1-2547 67~ and _?-.il-02~2 7;0.
However, in the first-~entionec doucmer.-, which b~ thewa~
belongs to Applicant, there are not even any indications,
that for instance an increase in the impuxity contents of
the air is to be remedied by any adequate counter-measures.
Also, the liquid flow from the biological purificatior.
plant is according to this document returned to the feed
duct of the spraying chamber. It is true tha. according
to this document a stabilization tank 8 is used, in ~hich
r.~-_rie~.t-s are added. However, neithc~- ~s to t`nis tan~ ~
ar.y liquid ~rom the bioreactors added nor i5 ar.. li_~id
wishcrat... to the sp_aying chambe-, i.e. '~hiC tank has no
functions, which are similar to th~ functior.s according
to the present invention. Finally, said document does not
relate to any heat exchange in a way, ~hich is'suggested
'; r~ e p-esent invention.
Document DE-25 47 67; relates to a process, in which there
are just few points of similaritv with the process and the
device according to the present in~!ent~on, since the wash-
ing liquid in the main washing phase prir.cip~lly only is
fed through a circuit between the scrubber and the biore- r
actor. It is true that there is ar. adaitiona' c~cle 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.
Accordins to the process described in documer.s E?-v 2~2
750 there are indeed two cycles li~e the present inver.-
tion, but unlike the cycles acco-ding to the present ir-
WO9~/00792 PCT/SE91/00~6~
2.J
vention the two c~cles pass accordinc to this document
through the scrubber and also the~ are no~ conrected to
ea^h other in an~ .. Also, the-c is no b-ir.ci-g togeth-
e- of the liquid 1O~! which passes throush the ~ioreac~or
and the liquic flo:~hich is returned directl~ o the
scrubber before the recycling to the scrubber, as is donc
according to the present invention, but the clean aswell
as the polluted liquid flow are returned to the scrubber.
Also, there is no suggestion in this document, ha~ there
is a heat exchange between polluted discharged air and
fresh air fed into the system.
.
One object o~ the ?resent in~ention-i~-to pro~ ro-
cess and a device, whic:r; are charactr-rized b~ 2 satisfac-
tory cleaning of air as we~' as water, ~hich pass through
a production plan~. Another object is to obtain such a
cleaning quickly, in a realiable~, controllable and eco-
nomically advantageous wa~. An additional objec is to,
deslite ~n i~V~d ~leaning o- the process air as well
as the process water, lower the enerci recuire~ent for r
the production process itself. ~inall~, the p-esen~ in-
vention is supposec to develop the state of the art in
this fielc also ir. adaitior.a res?ects.
These objects can be achir-~ved accordins tC the present ir.-
ver.tion by c~rr~ing out a-?rocess c- the . ~- c-^sc-_be_
in the introduction above mainly accordinc to tne 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 ofthe
present invention are set forth in the followin- desc-
ription, reference being made to the accom?an~.~ing draw-
ins, ~Ihic.~ schematicail-~- illustrates a prc~crre~ cm~o-
diment, desianed to ca~r ou~ a process accc-c ng to th-
invention and recarding the ?rin^i?al construc~ion of
device according to the~ invention res?ectivel~.
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W092/~079' PCT/SE91/00~6
~a8 69 2r~ 4
In the drawing a source l is shown, which produces ~arm
and polluted as ~ell as usuallv mois' ai-, e.c. a drie--
or a hot press for particle boar~ or fibc:- board produc-
tion. In a practical embodimer.t 'he tem~e~-ature or the
air is 45-60C anc thecontaminantscom?rise partly dus~
and fiber particles and partly formaldehyde, methanol
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 scrubbe- 3, in which the air is cleaned with water,
which is fed via a feed duct 4 and is finely divided by
means o~ s~ra~-nozzles 5 ir. the u??e~ area of th~ scrub-
be-. The air, cleane --. this W2., leaves ~he scrubber
via an outlet 6 and e.g. a chimney 7, which leads out
into the atmosphere.
In accordance with a pre~erred embodiment of the inver.-
tion said sc~ubber is combined with a heat exchanger 8,
through which fr~s1l air is fed into source l via an inlet
duct 9. The ~esh air, which may have the same temoera-
ture as the ambient air, is heated in heat exchanger 8,
and in this wa~ the heating requirement for the fec air
is reduced considerabl~. Also, the condensation o_ wate-
vapor from the polluted air is increasec ar.d tne scrub-
be_ wate. will be so~ewha~ colde-. These tw_ ef_ects re-
sult to an increased extent in an absorption o~ 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 tne ductup-
streams of the biological purirication, may neverthe-
less be requ:ire~ in order to proviae the micrOGrsanisms
with stable con~ tior.s.
The cleaning water leaves scrubber 3 via a coarse se?ara-
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W092/00792 ~ P~T/SE9l/0046~
9 2 ?
tor lO, designed to separate mainl~ coarse~- soli_ par-
ticles. The cleanin~ wate~, roughly purifiec in this
way, then is fed via a duct 11, having a buil~-in pump
12, to a water tan~ 1,, to whicr. also a fre~ water-duct
14 is connected, ha~!ing a built-in pumD 1~, as well as
the above-described feed duct 4 having built-in pump 16.
~lso, from tank 13 a feed duct 17 issues, having abuilt-
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 2~ anc 3, which contain nutrier.t salts and/
or ?H-adjusting means etc. f^- a biologica ~eatment of
the water, which subsequently is fed lr.'o a bioreactor .-;~
24 with fillina bodies 2;, in and on which microor~anisms
are cultivated, wnich are to decompose at leas. some of
the contaminants in ~he water, particularly formaldehyde.
The bottom of the bioreactor is connected to an aeration
device 26 having no~zle~ 27 ~h;~h i~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 microorga-
nisms with oxygen. Duct 17 leads in the bioreactor sub-
seauentl~ to a seDarator 30 for biosludge and reaches fi-
nally ~ia an additional pum? 3i ;ate~ tank 13 again.
Such a device functions in the following way: In the scrub-
ber several processes take place simultaneously, as ~as
been mentioned above. Partly heat is transferred from the
heat exchanger and the hot, moist and polluted gas tothe
cold fresh air stream to the drier or the like. Partly an
absorption of organic gaseous substances takes place to
the circulat:ing water, which is s~rayed into the scrub-
ber. Thanks to the comparatively low temperature of the
polluted air. and wate- a satisfac o~y 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-
W092/00792 PCT/SE93/0046
~ 6
ter. After the described treatmer.t the pu~ifiec. ai-- is
discharged through chimne~ 7. I~ now contains a s.~a;_
amount o' said o_ganic substances, e.g. 2-~ r~ orr,;u;dc--
hyàe/m3, as compared to e... 10-25 m~ ormaldeh~ae/.'
in the air, which flows into the scrubber. SimulL~n.cous-
ly the amount o- particles has decreased from 40-~ mg/
m3 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. ~his
a~ount of water is controlled in a sim-le wû- b means
o~ pump 16. The adaec wate~- functions 2S a coo'ing medium
as regards the hea. eY.changer. . is ~rue tha~ t;nis W2-
ter, having a temperature of say about 40C, o~te.. 15
considerably hotter than the fresiai-, which is sucked
into the system, but the temperature Gf the poliuted air,
which flows into the scrubber and has a temFerature o_
- 45-60C, will nevertheless be lowered, a relative coo-
ling effect being obtained.
The water flow from tank 13 to purification ?lan 20 ge-
ne-ally corresponds to the amour.- Oc li~uic condensed ir.
the scrubbe-- plus possibly adde~ fr~sh wa~ -. Tne _c-mai-
dehyde contents o this fiow is o~ten 20G-30,- m-':. ~ wa-
te~- which L.aS SU_I. a high amount o- form_ldehyce usually
is considered verv 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/l.
However, thanks to the described and shown biological
purification plant and its integration with the scrubber
i. is possib:Le to simultaneously treat high contents o'
particularly formaldenyde in the ai- as well as in the
water in an apc~oDriate and economically adventaaeou-
way as regards the environment. In a first separation
step the suspended amount Oc fibe~~ is separated by means
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W092/00~92 PCT/SE9l/~ 6
of e.s. ~lotatior.. Suascauc-.-'. th^- o'-;a`~c o~ the wa~
is adjusted to abou, ~ b~. ad~ir.y C.5. soàiu~ dro:idc
through the mixer 21, throush which al5^ the req~i~;ea
amount of nutrient salts is added, which the mieroorga-
nisms in the bioreactor, conneeted downstreams, neeà
according to a calculation. In the latte!- the mieroorga-
nisms grow on fillina bodies of a tv?c kno~n per sc,
e.g. those whieh AB Carl Munther sells, ca~led Euroform.
The filling bodies are lowered into the bioreaetor and
can quieklv ane simpl~l~ be e~;chanaed, ir. cas ar,d ~he-.
.his is req~airec~
An analvsis o the bioloaic_: _il.~ on th~ Liliinn bocies
showed, tha_ it eontaines . micrc~ n~, r~ch ir. species
and eom~rising i.a. a largc amount of zoo~lea colonies
anc -ilamer.t-~enera~ing ~a~ -ia. mhe r~;icro-lora'eor.-
tained also a eertain amount o~ ?~oto~.oa..
derable number o' -unei.
The fitling bodv matc-ia' ?re~e~-2~' is desiened ir. s~aeh
a wa~, that ~he fin.e dividrd ai- '~u~ics, ssuins from:
r,o7zles 27, are cis_ributecl eve-'. in the reaetor vo-
lume. The organisms aecom~o-e e.~. the ~orm~laehyde ana
the remainins organic ccn~e-ts. Sinee formalcehyde isa
toxic substance when in largc amounts, the deeomposition
of the formaldehyde will rcsult in a nenative growth
of the microorganisms in sueh eases. Iiowever, whcn the
formaldehydeconcentrations are eom?arativel,~ lot, thc
microorganisms will survivc, wllcn thev dccompose the-or-
maldehyde. However, the remainin~ organic contents makes
the mieroflora grot;, an~ eonsc~uertl~ it is im?e-tar.t to
knot~ the pro?erties o th~ contaminar.t~ anC. to adjust
the proeess aeeordins tG ti~c samc.
W0~2/00792 ~ PCT/SE91/0016~
~2~92~
sl1ort dwcll time ~il bc sufficien,, c.~. 3-8 hours,
provided microorganisms a~o usec, .hich are specil;ze~
as to their capacity to decomposc the specific organic
eontam~inants encountered. The reactor si~e 15 ir. a case
with 2 5 m3/hour water and 2~0 ms ormaldehyae/liter al-
most 30 m3 includins the frec spaces 3 anc 33, needed be-
low and above fillin7 bodies 25. If thc rate is 21 m3/
hour, the reactor volume is estimated to be 150 m3.Pro-
vided the reactor is designed in this way and the vari-
ous processes can be controlled, the reaeto- ean be ope-
rated with a very high load, with satisfaetory results
` despite the short dwell times.
In the disehargec water fromm the reae.or the formaldei1yce
S eontents has been redueed to below l mg/l. The redue-
tions of COD and BOD (bioloaieal oxygen demar.c) arc more
than 95 ~ in the reactor. Consequently, the dlscr.argeù
water advantageously can be reused as f~ h wate- in
the proeess and reeyeled into the washin~-heat recovery-
circuit. Since the ~icroorganisms are attached tc ~ UD-
~- mersed suppo-t materia1, tne amour.. suspended ir. th- wa-
ter will be very small. ~o orgal1isms have to be rein.ro-
dueed ir.to the process. That amour.t whieh leaves the pro-
e-ess is mainly ~illed microo_qar.isms. 2e.ore the WâtC~~iS
returned ~o the tank, said pa~ticle contents is se_ara-
; ted by means of a le,ation process o- a cor. i-uous sanc
filter.
By reintroducing the water direetly from the wate- puri-
fication step a mainly closed process is obtained as re-
gards the used water. That amount of water, whieh eorre-
sponds to the amount which is condensed in the serubber,
is leaked subsequent to the water purifieation, down-
streams separator 3Q, via a not shown element. Durin~ thestarting of the deviee or during a shut dow-~ f-esh water
can be added through duet 14 by means of ?um? 15. Some-
.
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W092/00792 PCT/SE91/00~6~ ~
~!g~927 '`
times it is also advantageous to adc chemicals, which in-
crease the capacity of the wate~- to absorb formaldehyae
and/o- causes the formaldehyde to be trans~ormed ~rom 2
gaseous phase to a ~ate- ?hase f2ster.
The discharge of formaldehyde, which takes place viachim-
ney 7, can be controlled and influenced by varying either
the liquid flow through scrubber 3 or the contaminantcon-
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
.o thQ ~iological purification pla~t is increased. There-
moval o- water, from tank 13, with a high formaldehyde
contents via duct 17 increases and to the same extent
~he amount of purified water, returned to tank 13 via
duct 41, increases. In this way the scrubbex liquid in
scrubbe- 3 will have a lower amount of cor.taminants than
before. The physical equilib~ium 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 liquic. ~`
.
Biologica1 pu~-i~ication piar.t 20 must o_ course have such
dimensions, that the increasec loac wili be taken care of.
However, a certain overloaa can easie~- be acceptec, 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 va-ious w~te- flows, since the~ are mutua'l~ inte-
grated. Also, by minimizins the water lows, the reco-
verable heat will be comparatively large. By controlling
~.
:
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W092/0~792 PCT/~E91/0046~
w~8~92~ lo
the water flows the biological purificatio.~ plantcan be
loaded/utilized to the grea-test exten-.
The various parts of the device are o~ cou~-s~ co~._rolle
and guided from a common control ar.d monitorins device,
the principal construction and func~ on 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 o-
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 ot~t5ignal 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
tan~ and duct outlet 4 from the tank can be short-cir-
cuited via a valve 36. Also, i is feasible to short-cir-
cuit duct outlet 17 from the tan:~ and fresh water duct14
via a valve 37 i.. order to reduce extremely high loacs
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 objec~ of such a
space is to via pUmD 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.
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