Process and device for scrubbing flows of gaseous effluents loaded with polluting substances

A process and device for scrubbing flows of gaseous effluents loaded with polluting substances, wherein the effluents are caused to circulate through a device including an array of scrubber units (4a to 4n) juxtaposed in an enclosure (3), each containing materials adsorbing the substances. When the adsorbing load thereof is saturated and for the scrubber units (4) to regain the scrubbing capacity thereof, they are selectively and successively isolated by a mobile collector (8) during the time required for their desorption by heating and for the substances to be transferred by an auxiliary fluid (a fraction of the circulating effluents for example or a gas delivered selectively to the inlet of the scrubber unit to be desorbed by an auxiliary circuit), and the substances mixed with the auxiliary fluid are possibly transferred towards a reactor (10) suited for removing them. The process can be applied for concentrating and scrubbing of polluting substances such as VOCs or gas dehumidification.

FIELD OF THE INVENTION 
The present invention relates to a process and to a device for scrubbing 
gaseous effluent flows. 
The process and the device according to the invention find application 
notably in the removal of Volatile Organic Compounds, hereafter referred 
to as VOCs, or of malodorous substances. The term VOC includes the 
hydrocarbons, the chlorine, fluorine and chlorofluorine compounds, the 
NOx, the SOx, H.sub.2 S and mercaptans, NH.sub.3 and amines, including 
H.sub.2 0, and more generally all the organic and inorganic compounds that 
may be present in air at concentrations that are inconvenient on several 
accounts. 
Greater respect for the environment has become one of the major concerns of 
many industrialists who have to face more stringent regulations relative 
to polluting discharges to the atmosphere. 
BACKGROUND OF THE INVENTION 
There are many devices intended for scrubbing gaseous effluents in 
industry, that work mainly by adsorption of the polluting substances on 
adsorbing materials or by terminal separation through a filter. After 
scrubbing in contact with the adsorbent, the scrubbed effluents can be 
discharged to the atmosphere or recycled in premises at least during 
winter time so as to decrease heating costs. 
In certain types of devices, using for example activated carbon filters 
working according to the principle of cohesive force and of retention by 
chemisorption, the adsorbents are not regeneratable and they must be 
replaced as soon as the saturation threshold thereof is reached. The 
granular activated carbon used in some other filters can be regenerated at 
the plant provided that it is returned to the maker. A few days of 
activity of the polluting plant are sometimes enough to reach this 
saturation threshold when the initial polluting substance concentration is 
high. The cost of the adsorbent or the regeneration thereof quickly 
becomes prohibitive. This solution of filters with non regeneratable 
consumables is in any case unacceptable in an ecobalance since the 
pollution is in this case merely transferred from one support to another 
without being eliminated. Because of the replacement costs, of the lack of 
information and sometimes of difficulties in checking the condition of the 
devices, it often happens that the filters are not changed as frequently 
as they should. 
There are also scrubbing devices with adsorbent regeneration by desorption 
and recovery or destruction of the polluting substances. However, these 
devices are often more expensive. They are rather suitable for facilities 
generating large amounts of polluting substances. In many cases, such 
devices most often do not constitute economically acceptable solutions. 
Many facilities are therefore not adequately equipped. 
U.S. Pat. No. 3,608,273 for example or patent applications FR-A-2,659,869 
and FR-A-2,709,431 describe processes for treating fluids loaded with 
substances to be eliminated, notably consisting in passing these fluids 
into tanks through a series of spaced-out filter layers containing an 
adsorbing material such as activated carbon for example. An adsorbing 
material that is electrically conducting or made conducting by adding 
conducting particles or heating wire is used. Desorption of the adsorbate 
is performed by heating it by means of an electric current running 
therethrough or of an electro-magnetic field produced by a winding. Two 
arrays of filter layers may be used and one is used in adsorption while 
the other is regenerated. 
The claimants' patent application FR-94/06,281 describes a continuous 
scrubbing device for polluted effluents. It includes an internally 
chambered ring of vertical axis loaded with a particulate solid material 
adsorbing the polluting substances: silica, activated carbon, alumina or 
others, and continuously rotating inside a cage. A permanent circulation 
of gaseous effluents is established on the one hand between an effluent 
delivery line and an exhaust line in the central area via a first angular 
adsorption section of the ring, and on the other hand between a hot 
gaseous flow delivery line in the same area and an exhaust line via a 
second angular desorption section of the ring, where the adsorbate is 
regenerated. The lines are connected to external heat exchange and/or 
incineration means. This device can be associated with many types of 
polluting plants, but it is more particularly intended for those producing 
relatively high rates of polluted effluents (10,000 to 100,000 m3/h for 
example). 
SUMMARY OF THE INVENTION 
The device for scrubbing substances mixed with gaseous effluents according 
to the invention includes an enclosure, a first line for canalizing the 
effluents in the enclosure, scrubbing means for holding the substances 
back and for concentrating them, at least a second line for discharging 
the filtered effluents out of the enclosure and means (such as a fan) 
inside the enclosure for example, for self-contained scrubbing, or outside 
if the scrubbing device is integrated in an existing global process for 
establishing a circulation of effluents through the enclosure. 
It is characterized in that the scrubbing means comprise a fixed scrubbing 
barrier interposed across the enclosure and including a plurality of 
scrubber units placed parallel to one another, containing a material 
suited for adsorbing the substances, and a selective thermal desorption 
assembly allowing application of a desorption fluid of relatively low flow 
rate to at least one scrubber unit, comprising a mobile collector that can 
be shifted linearly in the enclosure, suited for gathering selectively all 
the desorption fluid coming from this scrubber unit, and means for heating 
the adsorbing material of each scrubber unit to be desorbed. 
The device can also comprise means such as an incineration reactor 
preferably placed in the enclosure and secured to the collector in 
translation in order to remove the substances mixed with the desorption 
fluid. 
According to an embodiment, the device includes a ventilation circuit 
partly situated outside the enclosure in order to establish selectively a 
circulation of the desorption fluid through each scrubber unit to be 
desorbed. This circuit includes for example at least one auxiliary line 
that can be shifted at the same time as the mobile collector, so as to 
send the desorption fluid towards each scrubber unit selected and/or 
possibly at least one auxiliary line, that can also be shifted at the same 
time as the mobile collector, for driving the desorption fluid coming from 
the selected scrubber unit out of the enclosure. 
According to an embodiment, the means for heating the adsorbing material 
(with resistors within the adsorbent or by Joule effect for example) are 
included in each scrubber unit. 
According to another embodiment, part of the heating means is placed 
outside each scrubber unit and may consist of a source of infrared rays, 
such as for example an incineration reactor at the outlet of the scrubber 
unit to be desorbed. 
According to an embodiment, each scrubber unit includes several perforated 
panels containing adsorbent and deviation walls intended to increase the 
surface of contact of the effluents with the adsorbing material. 
The desorption assembly preferably comprises motive means for positioning 
the collector in front of any scrubber unit to be desorbed and for 
pressing it against the unit. 
The process for scrubbing substances mixed with gaseous effluents according 
to the invention is characterized in that a circulation of effluents is 
established through a device including an array of scrubber units 
juxtaposed in an enclosure, containing each materials for adsorbing said 
substances, the various scrubber units are selectively and successively 
isolated by means of a mobile collector when the adsorbing load thereof is 
saturated, during the time required for the desorption thereof by heating 
and for the transfer of the substances by an auxiliary fluid (a fraction 
of the circulating effluents for example or a gas delivered selectively to 
the inlet of the scrubber unit to be desorbed by an auxiliary circuit), 
and the substances mixed with the auxiliary fluid are possibly transferred 
towards a reactor suited for removing them. 
The device according to the invention provides a simple solution of 
moderate cost price and upkeep cost, compatible with many facilities 
generating average polluting effluent rates (1000 to 5000 m3/h for 
example), because of many factors: 
the enclosure can be economically made from the same material as that used 
for manufacturing the main and auxiliary lines (air ducts), with a 
parallelepiped shape of constant section, that is standard and easy to 
assemble. Galvanized steel, polyester or, if need be, stainless steel can 
be used for example; 
there are few moving parts since the desorption assembly with the mobile 
collector and the optional auxiliary lines thereof can be readily 
translated by electrical, pneumatic or hydraulic means; 
the high temperature rises necessary for desorption are confined in the 
thermal reactor and in a single desorption unit. Since these scrubber 
units are thermally insulated from one another and from the enclosure 
body, thermal insulation of the enclosure is not necessary. 
Incineration of the pollutants is preferably performed inside the 
enclosure, in the immediate neighbourhood of the scrubber unit to be 
desorbed, with the advantage that the desorption heat can be partly 
provided by an infrared radiation. 
The optional desorption and incineration operations can be readily 
postponed until off-peak periods, which allows the energy expenses 
required to be reduced; 
the concentration and the nature of the polluting substances, as well as 
the temperature of the effluents, may vary within a wide range without 
harming the effectiveness of the treatment; 
since the device remains static during all the working phases, the 
enclosure being at underpressure in order to establish the circulation 
inside, no leak is possible towards the outside; 
the simplicity of construction, of use and of operation facilitates 
maintenance and limits servicing and control frequency; 
it is not necessary to double the desorption means as in conventional 
processes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As will be described in detail hereunder, the process according to the 
invention mainly consists in placing an array of juxtaposed scrubber units 
containing adsorbing materials on the path of a circulation of effluents 
to be processed and in successively isolating, by means of a collector, 
each of the units when the adsorbing load thereof is saturated, during the 
time required for the desorption thereof by heating and for the transfer, 
by means of an auxiliary fluid, of the substances held back by the 
adsorbent. The process can apply to the dehumidification of gas or to the 
scrubbing of effluents containing polluting substances. In this case, the 
process preferably comprises transferring the substances towards an 
incineration reactor placed, for example, in the immediate neighbourhood 
of the unit in the process of being desorbed. 
The polluted effluent to be processed is delivered through a duct 1 into an 
inlet chamber 2 at a first end of a rigid enclosure 3 made in one piece 
(FIG. 1), that may have a parallelepiped shape for example and be made, 
for a general use, from galvanized steel. The whole cross-section of the 
enclosure is closed by a scrubbing barrier 4 consisting of a series of 
juxtaposed scrubber units 4a to 4n (n being equal to 11 for example) 
separated from one another by layers made from a thermally insulating 
material 5. Each of these scrubber units contains an adsorbing material 
suited to the nature of the substance mixed with the effluent to be 
processed, notably in the form of polluting molecules. This adsorbing 
material may be, for example, activated carbon, zeolites, etc, as 
described in detail hereafter. 
Circulation of the effluent through scrubber units 4a to 4n is provided by 
a fan 6 placed in an extraction chamber 7 on the side of the scrubbing 
barrier 4 opposite inlet chamber 2, or possibly outside the enclosure when 
the scrubbing device is integrated in a global assembly. After passing 
through barrier 4, the scrubbed effluent is either discharged out of 
enclosure 3 to the atmosphere or recycled to the facility where it was 
sucked. 
The adsorbing material load has to be regenerated at regular intervals 
depending on the saturation rate thereof. To that effect, the device 
includes a selective thermal desorption assembly allowing selective 
isolation of at least one scrubber unit from filtering barrier 4 during 
the time required for desorption. 
This assembly includes a convergent mobile collector 8 whose inlet is 
suited for resting against the whole of the rear face of a scrubber unit 
4a to 4n. This collector is mobile crosswise along guide rails 9 under the 
action of motive means (FIG. 4). 
The desorption assembly also includes means for heating the adsorbent load 
inside or outside each scrubber unit. When the adsorbent used is granular, 
fixed embedded resistors (not shown) can be used to that effect. If the 
adsorbent is contained in a honeycomb structure, or in textile or even 
granular form of reduced thickness, it is also possible to use 
infrared-ray emitters mobile with collector 8, that can be placed in front 
of the front and rear faces of a scrubber unit to be desorbed. The thermal 
waves penetrate axially deep into the cells and the adsorbent, that is 
very porous, quickly heats up. With this type of radiation, most of the 
heat enters the adsorbent, and the heating of the air is merely due to the 
matter-gas conduction. 
When the adsorbing material is sufficiently electricity-conductive, another 
heating mode may consist in applying a potential difference between the 
front and the rear faces and in heating it by Joule effect. 
The polluting particles released by heating the material are driven out of 
the scrubber unit by a low flow rate fluid so as to obtain a high 
desorption rate at the outlet. Various means will be described hereafter 
in connection with FIGS. 2, 3. 
An incineration reactor 10 intended for the polluting substances carried 
along by the auxiliary fluid is placed at the outlet of the convergent 
collector. 
Destruction can be obtained thermally at high temperature (1000.degree. C. 
for example). In this case, a combustion chamber containing for example a 
mattress of conducting wires with a high specific surface: 1500 m2/m3 for 
example is used, these wires being incandesced by applying an electric 
current, oxidation occurring in contact with the wires. The reaction is 
sufficiently exothermic for supporting the incandescence alone and no 
electric energy supply is required any longer. 
Destruction can also be obtained at a lower temperature (of the order of 
400.degree. C.) in a catalytic oxidation reactor of a well-known type 
containing, for example, a honeycomb catalyst block, the gas oxidizing in 
contact therewith. In such a reactor, auxiliary heating means such as a 
burner, for example, are used to reach the oxidation starting temperature, 
that is thereafter supported by the oxidation autothermicity. 
The effluent entering enclosure 3 (FIG. 1) can be used as an auxiliary 
desorption fluid itself, provided that the driving power of fan 6 is 
highly reduced during the regeneration phase. This embodiment is suitable 
for example in plants where cyclic decreases in the pollution rate of the 
effluent occur (at night, for example, when this pollution is linked with 
diurnal activities for example). 
It is also possible to use a fraction of the effluent itself as an 
auxiliary desorption fluid. The regeneration rate per unit area, 
comparatively lower, is obtained by self-balancing of the circuit because 
of the greater pressure drop in the succession of elements under 
desorption in relation to the single adsorption means. This principle 
requires only one fan. 
When the pollution rate of the effluent remains too high for it to be used 
as an auxiliary fluid, the embodiment of FIG. 2 can for example be used, 
which comprises a divergent 11 whose outlet is shaped to be pressed 
against the whole of the rear face of each scrubber unit 4i, and whose 
inlet is connected to a supply pipe 12 communicating, outside enclosure 3, 
with a source of fluid (not shown) such as a fresh air fan for example. 
This divergent is preferably coupled with the collector on the other side 
of scrubbing barrier 4 and it moves therewith and with incineration 
reactor 10. 
According to the variant of FIG. 3, the incineration reactor 10 or a 
recovery device can optionally be placed outside the enclosure. In this 
case, the outlet of collector 8 is also connected to a supply pipe 13 and 
a connection circuit 14 to reactor 10. 
The motive means allowing collector 8, reactor 10 (FIG. 1) and/or divergent 
11 (FIGS. 2, 3) to be shifted include for example (FIG. 4): 
linear guide means 9 of a well-known type comprising, for example, 
cylindrical ball or felt slide rails, running rails for rollers or other 
standard guide mechanisms allowing linear displacement in a single degree 
of translation, 
motive elements of a well-known type (not shown) including, for example, 
gear wheels (resp. toothed pinions) co-operating with a chain (resp. a 
toothed belt) and an electric, hydraulic or pneumatic motor, or a rubber 
wheel rolling on a fixed race, a gear pinion rolling on a linear toothed 
rule or a pneumatic jack with rope and pulley-wheel return, or more 
generally any driving gear commonly used by the man skilled in the art, 
a longitudinal translation mechanism allowing collector 8 to be pressed 
against a scrubber unit 4i to be desorbed. Collector 8 is secured to the 
body 15 of two hydraulic (or pneumatic) jacks whose rods 16 are rigidly 
connected respectively to two rings 17 that are mounted sliding on guide 
rails 9. Return springs 18 are interposed between the body and the rod of 
each jack and have the effect of pressing permanently collector 8 against 
the rear face 4r of the scrubber unit to be desorbed. Intermittent 
withdrawal of collector 8 during the time required for the transfer 
thereof to another scrubber unit is obtained by applying to the jacks a 
hydraulic pressure provided by a generator (not shown). 
According to possible embodiment variants known to technicians, the 
mechanism may comprise for example an electromagnet with return spring or 
a feeler sliding on a fixed cam, bearing at regular intervals bosses, 
stops whose position and height are suited to the desired withdrawal. 
Stopping of collector 8 in front of the rear face of a selected scrubber 
unit can be conventionally obtained by controlling the required 
translation distance, for example by means of a control unit (not shown), 
or by using a linear displacement electric motor provided with a torque 
limiter and by raising, in front of the scrubber unit selected, a stop 
ring causing the engine to stop. Pickups suited for detecting the 
saturation of the adsorbent in each scrubber unit can be associated with 
this control unit so as to automate the regeneration cycles. 
It is for example possible to use as adsorption materials: 
natural granular zeolites, 
synthesis, dealuminized and/or graft zeolites, 
impregnated zeolites on honeycomb or metal support, 
bagged zeolites or A.C. (activated carbon), 
granular A.C., 
fabrics, felts or metal or polymeric knitted fabrics covered with zeolite 
or A.C., 
activated carbon (A.C.) fabrics. 
According to the embodiment of FIG. 5, each scrubber unit 4a to 4n includes 
several flat compartments 19 placed in parallel or according to an 
accordion pattern between the front and the rear faces 4f and 4r of 
scrubbing barrier 4, with a space between them. Each one of them is 
laterally delimited by perforated plates and filled with an adsorbing 
material such as activated carbon for example. Some inlets and outlets on 
the front 4f and rear 4r faces are closed so as to lengthen the path of 
the effluents and thereby to increase their surface of contact with the 
adsorbent. 
This flow deviation can also be obtained (FIG. 6) by means of deflecting 
plates 20 arranged slantwise between the various compartments. This layout 
is favourable to the transmission of the infrared radiation when this 
heating mode is used to desorb the adsorbent of the various compartments. 
According to the variant of FIG. 7, each scrubber unit comprises a 
plurality of flat compartments 19 arranged in a zigzag pattern so as to 
compel the effluent to flow therethrough. 
The device such as it has been described can be implemented in many fields 
and notably: 
in car body repair shops to eliminate solvents from the painting booths, 
in waste water pumping-up stations in towns, to eliminate the hydrogen 
sulfide, 
in plastics processing shops, to clean the gaseous effluents loaded with 
styrene, 
in water treating plants for the processing of smells from the sludge 
filtering facilities, 
by painters in order to gather, clean and recycle solvent emanations 
(neoprene glue) in situ, 
in the building industry and civil engineering, for the treatment of 
confined places with air recycling possible while epoxy resins or 
polyurethane foams are being applied, 
for printing works, to clean gaseous discharges loaded with VOCs, 
in food processing shops to destroy smells, 
in the food processing industry to prepare air free of pollutants (NOx, 
C.sub.2 H.sub.4), 
in emergency services where a self-contained mobile device is required, 
in all industries where purified air is required, 
in chemical production processes in order to clean gaseous effluents, 
in test laboratories, also to clean gaseous effluents prior to discharging 
them to the atmosphere, 
in mechanical workshops in order to treat working premises allowing 
authorized recycling. 
Embodiments where the device is used to scrub an effluent loaded with 
polluting substances or particles (notably VOCs) have been described. 
However, without departing from the scope of the invention, the device may 
be used to dehumidify a gas. In such a case, the load of the various 
scrubber barriers consists of a hydrophilic adsorbent: natural or 
synthesis zeolites, silicagel, silica, alumina, silica-alumina, calcium 
chloride or lithium chloride, etc, or any other hydrophilic materials with 
a large active surface.