Floating bed reactor

The present invention concerns a floating bed reactor which is used for biological purification of fibre-containing liquid suspensions and for simultaneous clarification. The reactor comprises a floating bed (8) composed of finely divided particles floating in an ascending flow, above it being formed a fibre agglomerate layer (9) which binds to itself fibres present in the suspension being treated. What is essential in the invention is that the removal of fibre agglomerate from the reactor has been arranged with the aid of an open-top collecting vessel (11) placed in the reactor and disposed to collect fibre agglomerate in itself so that the top margin of the agglomerate layer (9) positions itself in the reactor substantially on level with the top end of the collecting vessel. The agglomerate then moves into the collecting vessel (11) by effect of a trailing vortex, for the generation of which the top end (13) of the vessel is bordered by a flow guiding member projecting laterally therefrom, or encircling it. The reactor of the invention is particularly suitable for treating cellulose fibre-containing effluents of the wood processing industry.

The present invention concerns a floating bed reactor intended to be used 
for biological purification of liquid suspensions containing fibrous solid 
matter, and for their simultaneous clarifying, said reactor comprising 
members for conducting the liquid to be treated through the reactor as a 
flow ascending upwards from below, a particle mass floating in the 
ascending flow, and members for removing the fibre agglomerate 
accumulating above the floating bed. 
Liquid suspensions containing fibrous solid matter, which are particularly 
produced in processes of the cellulose and paper industry, have been 
clarified mainly with clarifiers acting on the settling principle. In the 
international patent application PCT/FI No. 82/00065, a procedure has 
however been disclosed for clarifying such suspensions and in which a 
floating bed reactor is used. According to the procedure, above the finely 
divided particles floating in the reactor is formed a fibre agglomerate 
layer which binds to itself fibrous matter from the flow passing through 
the bed. When effluents of the wood-processing industry are treated in a 
floating bed reactor of this type, biological purification is achieved 
with the reactor on the side of clarification, based on the activity of a 
microorganism that has become attached to the surface of the floating 
particles. 
Since the fibre agglomerate layer in the floating bed reactor may 
continuously increase in the course of the biological purification and 
clarification process, it is necessary to attend to removing the 
agglomerate, in order to prevent blocking of the reactor. In the design 
disclosed in the international patent application PCT/FI 82/00065, the 
floating bed reactor has been provided with one or more agglomerate 
discharge lines, which may start from the side of the reactor or from the 
middle of the reactor space. A drawback of this design is, however, that 
much water escapes from the reactor together with the agglomerate of very 
low solid matter content. In addition, the surface of the agglomerate 
layer rises very close to the top end of the reactor, whereby some of the 
agglomerate may enter the circulation pipe by which liquid is circulated 
from the upper part of the reactor down to its lower end in order to 
obtain appropriate floatation of the finely divided particles. 
Such entrainment of agglomerate with the circulation is harmful because the 
agglomerate disturbs the floatation and may in addition cause blocking in 
the flow guiding members located in the lower part of the reactor. 
The object of the present invention is to provide a design for removing the 
fibre agglomerate from the floating bed reactor so that the drawbacks 
mentioned are avoided. The invention is characterized in that the reactor 
comprises an open-top collecting vessel past which the ascending flow 
passes and which has been arranged to collect in itself fibre agglomerate 
accumulating above the floating bed, whereby the collecting vessel, when 
it operates, maintains the upper margin of the agglomerate layer 
substantially on level with the top end of the collecting vessel, and said 
top end of the collecting vessel being bordered by a flow guiding member 
projecting therefrom laterally, or encircling it, producing a trailing 
vortex in the ascending flow, which boosts the guiding of fibre 
agglomerate over the rim of the collecting vessel and into the vessel. 
In the floating bed reactor of the invention the advantage is gained that 
the top margin of the agglomerate layer can be maintained on a certain 
level which lies below the end of the circulation pipe. This prevents 
agglomerate from entering the circulation and eliminates the harmful 
effects consequent thereupon. In addition, with the aid of the collecting 
vessel the agglomerate can be compacted so that its solid matter content 
increases, whereby the quantity of water departing from the reactor 
together with the agglomerate will be less than before. 
As taught by the invention, the top margin of the fibre agglomerate 
collecting vessel may be provided with a laterally protruding or other 
equivalent projection which boosts the guiding of agglomerate over the rim 
into the vessel. Alternatively, the collection of agglomerate can be 
boosted by means of a separate guiding member encircling the upper rim of 
the collecting vessel with a given spacing. In either case, the guiding of 
the agglomerate into the collecting vessel is based on a trailing vortex 
produced in the ascending flow, which pushes agglomerate up over the 
collecting vessel, where the surface load is zero, whereby the agglomerate 
is enabled by gravity effect to sink into the vessel. 
The fibre agglomerate collecting vessel of the invention is preferably 
shaped to be tapering at the lower end, and placed in the reactor 
relatively high up in such manner that its location in the flow 
cross-section of the reactor is symmetric with reference to the centre of 
this cross-section. Such shaping and placement of the collecting vessel 
conforms best to the flow rising from down upwards in the reactor and 
allows adequate space for the agglomerate layer to form therein. 
Furthermore, it is possible that the floating bed reactor of the invention 
comprises several fibre agglomerate collecting vessels, their location 
symmetric in the flow cross-section of the reactor with reference to the 
centre of this cross-section. For instance, in a reactor with circular 
cross-section one collecting vessel may be placed in the centre of the 
reactor, in the horizontal plane, 2nd the rest of the collecting vessels 
therearound in annular arrangement. Instead of the last-mentioned, or in 
addition to them, for collecting vessel may also be used a gutter attached 
to the inside of the reactor wall and running annularly around the reactor 
.

In FIG. 1, a floating bed reactor according to the invention is depicted, 
in which is purified and clarified fibre-containing effluent of the 
cellulose industry with the aid of finely divided particles floating in 
the ascending effluent flow. The floating particles may be carbon, sand, 
resin or other equivalent material, and they serve as substrate for a 
microorganism effecting the biological purification of the effluent. Above 
the floating bed is formed an agglomerated fibre bed which clarifies the 
water by retaining in itself a predominant part of the cellulose fibres 
present in the water. 
The floating bed reactor just presented is composed of a vertical tank 1 
with circular cross-section and its lower part 2 having been shaped to be 
conical. The effluent to be treated enters the lower end of the reactor by 
the inlet pipe 3, and the treated, purified and clarified water is removed 
from the top end of the reactor into the exit pipe 4. In addition, the 
reactor comprises a circulation pipe 5, by which water to be treated is 
circulated from the upper part of the reactor to the lower end thereof in 
order to maintain the desired flow rate in the floating bed. 
The effluent supplied into the reactor through the inlet and circulation 
pipes 3 and 5 first enters the conically widening space 6 in the lower 
part 2 of the reactor, which contains a horizontal perforated plate 7 
serving as a guiding member. The purpose with the perforated plate 7 is to 
distribute the flow as uniformly as possible over the cross-section area 
of the reactor and to stabilize the flow by reducing the vortices 
occurring therein. 
The floating bed 8 constituted by finely divided floating particles, in 
which biological purification of the effluent takes place, is located in 
the reactor above the conically widening space. On top of the floating bed 
8 has been formed an agglomerated fibre bed 9, which clarifies the water 
by retaining in itself cellulose fibres present in the water. The purified 
and clarified water accumulates in a layer 10 over the agglomerate layer 9 
and departs from the reactor through the exit pipe 4. 
Since fibre material continuously enters the reactor with the water that is 
being treated, and is retained in the fiber agglomerate layer 9, it is 
necessary to attend to removal of the fibre material in order to prevent 
excessive growth of the agglomerate layer in the reactor. To this end 
there has been placed in the upper part of the reactor, slightly below the 
end of the circulation pipe 5, an agglomerate collecting vessel 11 which 
is provided with a drain pipe 12 starting on the bottom of this vessel. 
The collecting vessel 11 has been designed to be conical of its lower part 
and placed, in horizontal direction, in the centre of the reactor, whereby 
the disturbance of the ascending flow in the reactor by this vessel is 
minimized. The collecting vessel is open at the top, whereby the top 
margin of the fibre agglomerate layer 9 positions itself in the reactor on 
level with the top margin 13 of the collecting vessel and the agglomerate 
moves over the rim into the vessel by effect of the trailing vortices 
occuring in the ascending flow. This is because the vortices move the 
agglomerate to an elevation over the open collecting vessel 11, where the 
surface load is zero, whereby the agglomerate is free to sink into the 
vessel. In the collecting vessel 11 the agglomerate is compacted so that 
its solid matter content may vary in the range 0.1-1.0%, while at the same 
time the solid matter content of the agglomerate in the bed 9 outside the 
vessel is 0.2-0.3%. 
The generation of trailing vortices transferring the agglomerate into the 
collecting vessel 11 can be boosted with the aid of appropriate shaping of 
the top margin 13 of the vessel. In FIGS. 3-5, three different profiles of 
the rim 13 of the collecting vessel are presented, in each of them the rim 
protruding laterally from the wall of the vessel. In FIG. 3 the rim is 
composed of U section, in FIG. 4 of L section, and in FIG. 5 of V section. 
In FIG. 6 is presented an alternative design, in which the top margin 13 of 
the collecting vessel 11 has been encircled with a guiding member 14 
separated therefrom. The agglomerate may then go into the collecting 
vessel 11 either through the gap between the vessel's rim 13 and the 
guiding member 14, or over the guiding member 14. 
When the reactor is operated, the collecting vessel need not absolutely be 
continuously emptied: one may allow the vessel to fill up in the intervals 
so that the top margin of the fibre agglomerate layer 9 is enabled to rise 
somewhat higher than the top end 13 of the vessel. This is because the top 
margin of the agglomerate layer 9 can be made to return into register with 
the top end 13 of the vessel 11 simultaneously as emptying of the vessel 
is started. Care should however be taken when regulating the level of the 
top margin of the agglomerate layer 9 that the agglomerate layer will at 
no stage rise to such height that agglomerate would be entrained in the 
circulation. 
The collecting of fibre agglomerate and its removal from the reactor may 
take place not only with the aid of one collecting vessel 11 but also with 
a plurality of parallel collecting vessels 11. In FIG. 2 is presented a 
construction in which altogether five collecting vessels 11 are placed 
side by side in such manner that one vessel is located in the centre of 
the reactor 1, and the other four vessels (two of them visible in the 
figure) are placed symmetrically on different sides of the first-mentioned 
vessel. On the other hand, in FIGS. 7 and 8 is presented a construction 
comprising two collecting vessels, one of the vessels 11 being placed in 
the centre of the reactor 1 and the other vessel 11' consisting of a 
groove attached to the inner surface of the reactor wall and running 
annularly around the reactor. Functionally, these designs are fully 
equivalent with that presented above. 
It is obvious to a person skilled in the art that various embodiments of 
the invention are not confined to the examples presented and may vary 
within the scope of the claims stated below. For instance, the collecting 
vessel may have any desired shape which conforms with appropriate 
streamlining to the flow passing by the vessel. Moreover, when several 
collecting vessels are used, not all of them must necessarily be located 
at the same elevation in the reactor. For instance, in the design of FIGS. 
7 and 8 the upper rim of the groove-resembling collecting vessel running 
around the reactor may be located slightly higher than the collecting 
vessel in the centre of the reactor.