Apparatus for gas or liquid admixture

Methods and apparatus for continuously mixing fluid processing media with pulp suspensions are disclosed. The method includes supplying pulp suspensions to the central portion of the substantially planar radial face of a cylindrical rotor, feeding a fluid processing medium to the pulp suspension as it approaches the rotor face, forming an annular gap at the axial outer surface of the rotor, so that the rapid rotation of the rotor causes the pulp suspension and fluid processing medium to flow radially outward along the planar face and to pass axially through the annular gap, and effecting turbulence between the pulp suspension and the fluid processing medium at the entrance to that gap. The apparatus disclosed includes a cylindrical rotor having a substantially planar radial face and an axial outer surface, a housing surrounding the rotor including a pulp suspension inlet facing the center of the radial face of the rotor, and an outlet proximate to its axial outer surface, an annular gap between the axial outer surface and the housing along a predetermined portion of the axial outer surface of the rotor, a processing medium inlet for feeding the fluid processing medium to the pulp suspension inlet, and turbulence creating fingers located at the periphery of the substantially planar radial face of the rotor to create turbulence in the pulp suspension at the entrance to the annular gap.

FIELD OF THE INVENTION 
The present invention relates to apparatus for mixing processing fluids 
with pulp suspensions. More particularly, the present invention relates to 
apparatus for continuously mixing processing liquids or gases with pulp 
suspensions. 
BACKGROUND OF THE INVENTION 
Mixing processes in general, and chemical mixing processes in particular, 
play an important role in substantially the entire cellulose processing 
industry. For example, during the bleaching of cellulose pulps, it is 
absolutely essential to obtain good chemical mixture in order to realize 
satisfactory delignification and/or bleaching thereof. The obtaining of 
such proper mixing of bleaching chemicals enables one to realize 
homogeneous bleaching results as well as good utilization of chemicals at 
the lowest possible reaction temperatures and shortest possible reaction 
times. 
The "mixing problem" which to the present date has appeared to be the most 
difficult to solve in the cellulose industry so as to obtain acceptable 
results has been the mixing of chemicals to pulp suspensions at mean pulp 
or filter concentrations (i.e., of from about 5 to 20%). The mixing of 
bleaching chemicals with pulp suspensions of low concentrations, (e.g., 
below 5%, such as in conventional chlorination) and the addition of 
bleaching chemicals in a gaseous state at high concentrations (e.g., above 
20%, as in gaseous phase bleaching), are, however, well-known and 
thoroughly tested methods which have not given rise to any appreciable 
mixing problems. 
One problem which has been encountered during such mixing at low pulp 
concentrations, however, is that in view of the large liquid volumes 
present high pump energy is required, and in addition large amounts of 
emissions are effected from corresponding bleaching plants. 
On the other hand, operations at high pulp concentrations (such as above 
20%) means that exclusive equipment is required in order to obtain a high 
degree of dewatering, and the bleaching chemicals must exist as gaseous 
media during processing for known reasons. So-called gaseous phase 
bleaching at high pulp concentrations (such as above 20%) has been and 
still is being carried out on a technical scale for bleaching with gaseous 
chemicals such as oxygen, ozone, chlorine, ammonia, and chlorine dioxide. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, apparatus has now been developed 
for the homogeneous and efficient mixing of both gaseous and liquid media 
with pulp suspensions at mean concentrations such as from 5 to 20%. In 
accordance with this: apparatus intensive mixing with momentary admixture 
of chemicals to the pulp suspension is obtained. In this context, 
momentary admixture is understood to mean that the mixing device (the 
mixer) is a proper sense "lacks" retention time, and the chemicals and 
pulp suspension at all times are added continuously and simultaneously to 
the mixer. In other words, the mixer has no significant balancing effect 
with respect to "more rapid" concentration variations, for example, as is 
often the case with conventional low concentration mixers. The present 
invention is thus based upon the fact that in order to obtain sufficient 
admixture of a liquid or gaseous chemical, such as oxygen gas, chlorine 
gas, chlorine dioxide water, or a mixture of chlorine and chlorine 
dioxide, with a pulp suspension, the principal requirement is that fibers 
in the suspension are well exposed, and that thereafter the chemicals are 
added to these free fibers as uniformly as possible. 
Thus, in accordance with the method of using the apparatus of the present 
invention it has been found that a fluid (i.e., gas or liquid) processing 
medium may be continuously mixed with a pulp suspension by supplying the 
pulp suspension to the central portion of the substantially planar radial 
face of a cylindrical rotor, feeding the fluid processing medium to the 
pulp suspension as it approaches the rotor face, forming an annular gap at 
the axial outer surface of the rotor, so that the rapid rotation of the 
rotor causes both the pulp suspension and the fluid processing medium to 
flow radially outward along the planar radial face and to pass axially 
through the annular gap, and effecting turbulence between the pulp 
suspension and the fluid processing medium at the entrance to that gap. 
In a preferred embodiment of the method of using the apparatus of the 
present invention, the fluid processing medium is fed to the pulp 
suspension at the center of the substantially planar radial face of the 
rotor. Preferably, additional turbulence is effected in the pulp 
suspension within the annular gap itself. More preferably, the pulp 
suspensions employed will have a concentration of from about 5 to 20%. 
In accordance with the apparatus of the present invention it has been found 
that fluid pulp media and pulp suspensions can be continuously mixed by 
employing an apparatus including a cylindrical rotor including a 
substantially planar radial face and a cylindrical axial outer surface, a 
housing surrounding the rotor, including a pulp suspension inlet facing 
the center of the substantially planar radial face of the rotor and an 
outlet proximate to the cyndrical axial outer surface of the rotor, gap 
means for forming an annular gap between the axial cylindrical outer 
surface of the rotor and the housing along a predetermined portion of the 
axial outer surface of the rotor, processing medium inlet means for 
feeding the fluid processing medium to the pulp suspension inlet, and 
turbulence creating means located at the periphery of the substantially 
planar radial face of the rotor so as to create turbulence in the pulp 
suspension at the entrance to the annular gap. In one embodiment of the 
apparatus of the present invention the gap means comprises an annular 
stator affixed to the housing. 
In another embodiment of the apparatus of the present invention the fluid 
processing medium inlet means feeds the fluid processing medium to the 
center of the substantially planar radial face of the rotor. 
In another embodiment of the apparatus of the present invention the outlet 
of the housing is located tangentially with respect to the rotor, and 
adjacent to the exit from the gap. 
In a preferred embodiment of the apparatus of the present invention the 
housing includes a collecting space located axially with respect to the 
rotor on the side of the housing facing the rotor face so that materials 
in the pulp suspension unable to pass through the gap can collect in that 
collecting space. In another embodiment of the apparatus of the present 
invention, turbulence creating means are also located within the gap 
itself, preferably affixed to the cylindrical axial outer surface of the 
rotor and/or the gap forming means, such as the stator. 
In yet another embodiment of the apparatus of the present invention the 
rotor includes a plurality of cylindrical axial outer surfaces and the gap 
forming means includes a plurality of gap forming members alternating with 
the plurality of cylindrical axial outer surfaces of the rotor so as to 
form a plurality of such gaps.

DETAILED DESCRIPTION 
Referring to the figures, in which like numerals refer to like portions 
thereof, the apparatus shown in FIG. 1 includes a cylindrical housing 1, 
in which cylindrical rotor 2 rotates. The rotor 2 is supported in an 
external bearing housing 3 and driven by a motor (not shown). The inlet 4 
to the mixer housing is centrally located relative to the cylindrical 
rotor 2 while the outlet 5 is tangentially located on the cylindrical 
housing 1. An inlet 6 for the addition of chemical media is located 
symmetrically within inlet 4 and opens into the center 7 of the rotor 2. 
When the diameter of rotor 2 is relatively great with respect to the 
diameter of the inlet 4, inlet 6 can open into the center 7 of the rotor 
2, although not necessarily, while still opening into pulp inlet 4. 
Between the cylindrical rotor 2 and the stator ring 8 attached to the 
housing 1, a circular or annular gap 9 is obtained. The gap 9 may be 
defined outwardly by a portion of the housing 1 itself instead of by a 
specially designed stator ring 8. The height h of the gap should be from 
about 1 to 30 millimeters, preferably between about 2 and 10 millimeters, 
and most preferably between about 3 and 5 millimeters. The length of 1 of 
the gap should be such that an efficient mixture is obtained in the gap. 
That is, 1 should exceed h by several times, suitably between 3 and 25 
times, preferably between 5 and 20 times, and most preferably between 10 
and 15 times. At the outer periphery of the rotor 2 a number of cleaning 
fingers 10 are located. A space 11 is designed at the bottom of housing 1 
to act as a scrap trap. In place of cleaning finger 10 other members such 
as semi-spherical projecting portions or the like may be provided at the 
periphery of rotor 2 for effecting turbulence in the pulp suspension. 
The apparatus of the present invention operates as follows. Pulp at a 
concentration of up to a maximum of about 20% is supplied continuously to 
the mixer through inlet 4. In view of the rotation of the cylindrical 
rotor 2, a shearing field is formed between the pulp and the rotor 2, that 
field being capable of causing the pulp at a certain pressure drop to pass 
the relatively narrow gap 9 between the cylindrical rotor 2 and the stator 
8. By the action of an intensive shearing field both at the entrance into 
and within the gap, the fibers in the pulp suspension are efficiently 
exposed. Having passed through the gap, in this manner the pulp is then 
pressed out of the mixer through outlet 5. 
In the same continuous manner as the pulp suspension is fed into the mixer, 
the chemical or chemicals to be mixed with the pulp suspension are also 
fed into the mixer through chemical inlet 6. In view of the fact that the 
chemicals are charged to the center of the rapidly rotating rotor 2 (the 
rotor 2 generally rotates at from 500 to 1500 r.p.m., preferably about 750 
r.p.m.) a uniform and homogeneous distribution of these added chemicals is 
obtained radially outward along the planar cylindrical front surface to 
the outer edge of the rotor and to the gap. These added chemicals are thus 
distributed about the gap, and each "pulp layer" forced through the gap is 
apportioned with accurately equal amounts of such chemicals. 
By adding these chemicals to the center of the rapidly rotating rotor, as 
shown in FIG. 1, in addition to such uniform chemical distribution to the 
pulp suspension within the gap, the further advantages obtained in that 
the shearing force between the rotating smooth front surface of the rotor 
2 and the pulp suspension is substantially reduced in view of the 
formation of the chemical layer closest to the rotor surface. This 
phenomenon is particularly noticeable when employing gaseous chemicals 
such as chlorine or oxygen gas. Thus, in this manner the friction between 
the pulp suspension and the rotor 2 is reduced, and it also becomes 
possible to utilize a greater part of the energy employed for useful 
mixing work both at the entrance to and within gap 9 itself. 
The aforementioned cleaning fingers 10 in addition to acting as "scrap 
ejectors" to the scrap trap 11, also serve as fiber exposing and mixing 
means when the pulp and chemicals are entering into the gap itself. In 
order to improve the mixing within the gap at substantial gap heights h, 
such as above 5 millimeters, different turbulence-forming members can be 
employed on the rotor 2, and on stator 8, respectively, as is shown in 
FIGS. 3 through 6. The object in these cases is to increase the energy 
turnover in the gap, i.e., the transfer of energy from the rotor 2 through 
the pulp layer to the stator 8, so as to obtain an increase in mixing 
capacity. These turbulence forming members may, for example, have the form 
of pins 12 or strips 13 extending about the rotor and, respectively, the 
stator. 
Pins 12, which may also have forms other than those shown in the drawings, 
such as being semi-spherical in shape, etc., can be provided on the rotor 
2, or on the stator 8, or on both. In the latter case, they should extend 
past each other. Strips 13 extending around the entire periphery of the 
rotor may include a single strip or several in number, and should be 
located on both the rotor 2 and the stator 8. They preferably extend 
sufficiently into the gap so that their pins are located on about the same 
diameter. 
One method of increasing the capacity of such a mixer is to design the 
rotor and the stator so that several gaps are formed as shown in FIGS. 7 
and 8. For example, from three to seven gaps may be formed, preferably 
from three to five gaps, and most preferably three gaps. In this manner, 
the open area, and thus the mixer capacity, can be increased within a 
given gap height. In order to render it possible to distribute the 
chemicals which have been charged (either in a gaseous or liquid state) to 
the center of the rotor, and uniformly over several gaps located 
concentrically outside one another, a number of "spokes" 14 are provided 
on the rotor 2 in front of the gaps. These spokes 14, in addition to 
acting as scrap ejectors and fiber exposers, also serve as turbulence 
formers and distributors of the chemicals uniformly over the different 
gaps. A front side of rotor 2 may be plain or tapered, in which case the 
pointed end can face towards or away from inlet 4. The rear face of rotor 
2 may be smooth or can be provided with ribs, elevations, or the like, so 
as to prevent stagnation of the suspension behind the rotor. 
In order to obtain an optimum mixture in all respects, it has been found 
that the ratio between the gap length l and the gap height h and the rotor 
diameter should be selected for specific rotor speeds and production 
levels through the mixer. For example, in the case employing a rotor with 
three circular gaps where h equals 4 millimeters, 1 equals 50 millimeters, 
and the rotor diameter is 500 millimeters, a capacity of 450 tons per 24 
hours has been measured at a pulp concentration of from 8 to 12%. At a 
rotor speed of 750 r.p.m., an evaluation of the mixer was substantially 
carried out in a pilot plant for the oxygen gas delignification at an 
average pulp concentration (i.e., from 5 to 20%), and most particularly at 
about 10%. During the evaluation of the mixer, the reaction kinetics for 
the oxygen gas delignification at average pulp concentrations was compared 
with the kinetics for oxygen gas delifnification at about 30% pulp 
concentration. By employing the mixer designed described herein, an 
astonishingly good result was obtained. The bleaching result obtained by 
this mixer was, in all respects, as good as that at oxygen gas 
delignification at 30% pulp concentration (gaseous phase bleaching).