Continuous solution method and apparatus

A method is disclosed for preparing a homogeneous solution of a powder, Particularly hard-to-wet polymer powders, which includes: PA1 (1) feeding simultaneously a powder and a liquid solvent for the powder to a high-speed in-line mixer minimizing contact between said powder and said liquid solvent before said powder and said liquid solvent enter said mixer; PA1 (2) mixing said powder and said liquid solvent in said mixer to form a dispersion of said powder in said liquid solvent; and PA1 (3) dissolving said dispersed powder in said solvent. This invention is also directed to a powder/liquid delivery apparatus which minimizes the contact between a powder stream and a liquid stream, containing: PA1 (1) a column having an upper section and a lower section, said lower section having an interior diameter less than the interior diameter of said upper section, said upper section having an inlet at the top and said lower section having an outlet at the bottom; PA1 (2) a means for feeding a powder stream into said lower section and through said outlet, said powder stream having a diameter less than the diameter of the inner surface of said outlet and said powder stream having a center aligned with the center of said outlet; and PA1 (3) a means for evenly feeding liquid downwardly along the inside surface of said lower section and into said outlet.

FIELD OF THE INVENTION 
This invention relates to a method and apparatus for continuously preparing 
a homogeneous solution of powder in liquid, particularly an aqueous 
solution of hard-to-wet polymer powder. 
BACKGROUND OF THE INVENTION 
For industrial purposes, it is frequently necessary to rapidly combine 
streams of liquids and solids to form solutions on a continuous basis. The 
problems encountered in forming uniform solutions by mixing powdered or 
granulated solids with liquids have been researched extensively. However, 
no suitable means for dissolving hard-to-wet materials, such as certain 
polymers, has previously been discovered to prepare solutions of 5-95% 
dissolved solids. 
Many water soluble polymers, such as polyvinyl alcohol, hydroxyethyl 
cellulose, carboxymethyl cellulose and the like, are extremely soluble in 
water but are nevertheless very difficult to dissolve. The polymer 
particles adhere strongly to one another on wetting and tend to form 
lumps. In most traditional mixing devices, such lumps become wetted before 
the particles disperse into individual particles. The wetted surface of a 
lump becomes an impermeable film that hinders break up of the lump, and 
the lumps are carried through the mixer with the powder inside remaining 
substantially dry and unmixed with the liquid. 
Another problem encountered in forming solutions of polymer powders is the 
release of substantial amounts of dust. Depending on the type of material, 
airborne dust may present health hazards. For example, polymer dust is 
extremely slippery when wet and thus may present a safety hazard to 
workers if it forms on surfaces around the mixing device. It may also be a 
respiratory irritant. The dust may also present a process problem if it 
accumulates on the interior surfaces of the mixing equipment. If the 
surfaces are moist but not washed, a wet paste can build up to plug 
passages of the equipment. 
A traditional solution to the problems of preparing solutions of 
hard-to-wet polymer powders is a batch process. In an example of a 
traditional method, ambient temperature water is charged to a blend tank, 
and the water is agitated to form a vortex. The powder is then dispersed 
in the ambient water by gradually adding it to the vortex. The agitated 
mixture of powder and water is heated using, for example, steam sparging 
or jacket heating to a specific cure temperature. The mixture is held and 
agitated at the cure temperature for the time required to dissolve the 
powder. Polyvinyl alcohol, for example, is first formed into a slurry in 
ambient temperature water and then heated to a temperature of at least 
90.degree. C. Under these conditions, the complete dissolution of the 
slurry typically takes 30 to 60 minutes and yields no greater than a 10% 
solution. Hydroxyethyl cellulose is another hard-to-wet powder which, 
although curing at ambient temperature, requires at least two hours to 
form a complete solution. 
There are many disadvantages with the traditional method. A fundamental 
problem is that the method is inefficient, costly, capital intensive and 
time-consuming. The powder is added to ambient temperature water with high 
agitation to disperse the powder. If the water is at an elevated 
temperature, the powder clumps more readily. Once the powder is relatively 
well dispersed, the mixture must be heated and held at the higher 
temperature in order to dissolve the polymer. The mixing, heating, and 
curing cycle is slow. In addition, the space required for the blend tank 
may present a problem in installing a polymer solution system in an 
existing plant. Also problematic is that undissolved powder clumps can 
remain in the solution and result in inconsistent solution properties. 
Solution aeration due to the high speed agitation required for polymer 
dispersion and excessive foaming due to the heat-curing requirement are 
additional problems. 
DESCRIPTION OF THE PRIOR ART 
Numerous alternative solution methods have been offered to the traditional 
method. As with the traditional method, many of the alternative solutions 
use mechanical energy to disperse the solid in the solvent. For example, 
U.S. Pat. Nos. 4,440,711 and 4,551,296 disclose processes for preparing 
high-molecular weight polymer solutions in two mixing stages. In the first 
step of each process, a 2 to 15 weight % solution of a high-molecular 
weight polymer is formed in a nonvolatile solvent. The solution is 
prepared using two mixing vessels. In the first mixing vessel, a 
high-molecular weight polymer is combined with a non-volatile solvent and 
agitated to form a slurry. More intensive mixing occurs in the second 
vessel, which is equipped with helical agitator blades in an agitated 
mixing tank, to convert the slurry into a solution. Generally, the 
temperature in the second vessel is above 200.degree. C. 
U.S. Pat. Nos. 4,784,820 discloses a method for continuously preparing 
solutions of high molecular weight polymers for extrusion or solution 
spinning. The first step of the method is to form a slurry of polymer 
particles in solvent; in the examples, this slurry is formed by mixing a 
polymer powder and a solvent in an agitated drum. The slurry is pumped to 
a screw extruder and maintained in the extruder for sufficient time and at 
a sufficient temperature so as to form a homogeneous polymer solution. 
European Patent Application EP 0,183,285 also discloses the use of a screw 
extruder for this application. It teaches a method for the continuous 
preparation of homogeneous polymer solutions in which a suspension of a 
high molecular weight polymer powder and a suitable liquid solvent is 
processed in a screw extruder at elevated temperatures. The extruder is 
equipped with alternate mixing and conveying sections used to mix and 
knead the suspension in order to transform it into a homogeneous solution. 
The temperature must be above the dissolution temperature of the polymer 
and below the boiling point of the solvent; generally, a temperature 
between about 140.degree. C. and 220.degree. C. is used. The speed of 
rotation is such that mechanical shear rates of between 30 and 2000 
seconds.sup.-1 are achieved. 
Other methods use alternatives to mechanical energy to disperse the solid. 
U.S. Pat. No. 4,501,828 discloses a method for dissolving water soluble 
polymers in suitable liquid solvents to achieve very high polymer 
concentrations. Finely divided particulate polymer is mixed with frozen 
solvent particles at a temperature below the freezing point of the solvent 
to form a homogeneous blend. As the mixture warms to a temperature above 
the freezing point of the solvent, the polymer particles dissolve in the 
melting solvent. Solid solutions containing from less than 25% to as high 
as 75% polymer can be produced in this manner. 
U.S. Pat. No. 4,390,284 discloses a high kinetic energy water spraying 
apparatus for wetting powdered or granular dry material, particularly 
hard-to-wet polymers, to provide lump-free solutions containing up to 2% 
or more particulate material. A volumetric feeder drops the dry material 
through an upper conical portion of the apparatus into a lower cylindrical 
portion. The upper portion serves primarily as a receptacle for the dry 
material. As the material falls through the upper portion and into the 
lower, there is no substantial contact between the dry material and the 
liquid. However, the inner surface of the upper portion is completely 
wetted to prevent buildup of dry material on the surface. Wetting of the 
bulk of the dry material occurs in the lower portion of the apparatus. The 
lower portion is a cylindrical column equipped with spray nozzles 
positioned to produce jets of liquid that are oriented downward and 
radially inward. As the material falls through the column, it is wetted as 
it is impinged on by the high energy jets of liquid. The slope of the jets 
creates a downward air flow so that any dust from the dry material is 
drawn downward into the column. 
Japanese Patent 85-055539 discloses a method of continuously preparing 
aqueous polyvinyl alcohol solutions. An aqueous suspension of polyvinyl 
alcohol is formed and fed continuously to a steam jetter. Simultaneously, 
steam is introduced into the jetter to heat the suspension. From the 
jetter the suspension is fed to a zone consisting on long pipes and 
in-line mixers where it is held at elevated temperature and pressure for a 
specified period of time. 
Although the above-described methods offer some improvements over the 
traditional batch process for preparing solutions of hard-to-wet polymers, 
none is completely satisfactory. Many of these methods are time consuming 
and capital intensive. In addition, many of these methods are methods of 
polymer dissolution, that is, converting a polymer slurry into a polymer 
solution, and do not solve how to disperse the polymer in the solvent to 
form the initial slurry. The methods are also limited by the solution 
concentration that can be achieved, particularly for the range of 5-95% 
dissolved solids. Furthermore, the methods are limited by the temperature 
to which the slurry must be subjected. 
SUMMARY OF THE INVENTION 
This invention is directed to a method for preparing homogeneous solution 
of a powder in a liquid solvent, including: 
(1) feeding simultaneously a powder and a liquid solvent for the powder to 
a high-intensity in-line mixer minimizing contact between said powder and 
said liquid solvent before said powder and said liquid solvent enter said 
mixer; 
(2) mixing said powder and said liquid solvent in said mixer to form a 
dispersion of said powder in said liquid solvent; and 
(3) dissolving said dispersed powder in said solvent. 
This invention is also directed to a powder/liquid delivery apparatus which 
minimizes the contact between a powder stream and a liquid solvent stream, 
containing: 
(1) a column having an upper section and a lower section, said lower 
section having an interior diameter less than the interior diameter of 
said upper section, said upper section having an inlet at the top and said 
lower section having an outlet at the bottom; 
(2) a means for feeding a powder stream into said lower section and through 
said outlet, said powder stream having a diameter less than the diameter 
of the inner surface of said outlet and said powder stream having a center 
aligned with the center of said outlet; and 
(3) a means for evenly feeding liquid downwardly along the inside surface 
of said lower section and into said outlet.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to a method for preparing a homogeneous 
solution of a powder in a liquid and a powder/liquid delivery apparatus. 
For purposes of discussion, this description will refer to "powder" as the 
solid material being wetted and dissolved and "liquid" as the solvent for 
the solid material being wetted and dissolved. It is understood, however, 
that any similar powdered or granular material can be wetted and formed 
into a solution using apparatus according to the present invention. Also, 
the method and apparatus are not only suitable for forming solutions of 
hard-to-wet powders and granulated materials, such as polymers, but also 
is capable of efficiently forming solutions and mixtures of more easily 
wetted materials. 
The method of the invention for preparing a homogeneous solution of a 
powder in a liquid includes at least three steps: 
First, a powder and a liquid solvent for the powder are fed simultaneously 
into a high-intensity in-line mixer minimizing contact between the powder 
and the liquid solvent before the powder and the liquid solvent enter the 
mixer. The configuration of the powder/liquid delivery apparatus of the 
invention, such as the preferred embodiment as shown in FIG. 1, FIG. 2 and 
FIG. 3, minimizes the contact between the powder stream 20 and the liquid 
streams 21 and 22. If the powder stream and the liquid streams are in 
excessive intimate contact as they enter the mixer, clumps will form. The 
liquid flow, therefore, must be controlled such that no liquid collects in 
any substantial quantity in the lower section or outlet of the 
powder/liquid delivery apparatus. 
Next, the powder and liquid solvent are mixed to form a solution of the 
powder in the liquid solvent, if the dissolution is not time-dependent, or 
a dispersion of the powder in the liquid solvent, if the dissolution is 
time-dependent. The mixer must mix the powder and liquid solvent and then 
move the solution or dispersion out of the mixing chamber 16 of the mixer 
2 before the dispersion backs up causing excessive contact between the 
powder stream 20 and the liquid streams 21 and 22 in the powder/liquid 
delivery apparatus 1. 
Finally, the dispersed powder is allowed to dissolve in the liquid solvent, 
if the dissolution is time-dependent, as the dispersion or solution 19 
flows from the mixing chamber 16 and into a storage tank or other 
processing equipment. 
The method of this invention is suitable for forming solutions of 
hard-to-wet materials of from about 0.1% solids by weight to about 95% 
solids by weight, preferably from about 5% solids by weight to about 30% 
solids by weight. Typical hard-to-wet materials include polyvinyl alcohol, 
hydroxyethyl cellulose, carboxymethyl cellulose and the like. 
Powder/Liquid Delivery Apparatus 
The apparatus of the present invention is best understood with reference to 
the drawings. FIG. 1 illustrates schematically a first embodiment of the 
invention which includes a powder/liquid delivery apparatus 1 positioned 
above a high-intensity in-line mixer 2. The powder/liquid delivery 
apparatus and high-intensity in-liner mixer are made of conventional 
materials, such as, for example, metal, ceramic or plastic, limited only 
by the particular application for which the apparatus and mixer are used. 
For example, a plastic material may not be utilized if the liquid is a 
solvent for the plastic material as well as the powder being wetted and 
dissolved. 
The powder/liquid delivery apparatus 1 is a continuous, low residence time 
contactor where gravity provides the main driving force for flow of the 
materials through the contactor. Additional mixing occurs downstream in 
the high intensity in-line mixer 2. The powder/liquid delivery apparatus 
of the invention contains at least three parts and is best understood with 
further reference to the FIG. 2 and FIG. 3. 
The first part of the powder/liquid delivery apparatus 1 is a column 3 
having an upper section 4 and a lower section 5. The lower section 5 has 
an interior diameter less than the interior diameter of the upper section. 
The upper section 4 has an inlet 6 at the top and the lower section 5 
having an outlet 7 at the bottom. Preferably, the lower section 5 is 
tapered toward the outlet. More preferably, the lower section 5 is 
conically-shaped. Preferably, the upper section 4 is cylindrically-shaped. 
The second part of the powder/liquid delivery apparatus 1 is a means for 
feeding a powder stream into the lower section 5 and through the outlet 7. 
The resulting powder stream 23 must have a diameter less than the diameter 
of the inner surface of the outlet 7 and have its center aligned with the 
center of outlet 7. 
In FIG. 3, the powder is fed continuously into the powder/liquid delivery 
apparatus 1 through a narrow diameter hollow cylinder 8 which is aligned 
with the center of the outlet 7 of the lower section 5 of the 
powder/liquid delivery apparatus 1 and having a diameter less than the 
diameter of the inner surface of the outlet 7. The opening at the end of 
shaft 9 is positioned such that it delivers a stream of powder 23 at a 
level lower than the first source of liquid 10 and higher than the second 
source of liquid 11. Preferably, the opening at the end of shaft 9 is 
positioned such that it delivers a stream of powder 23 at a level equal to 
the horizontal plane where the upper section 4 and lower section intersect 
5. 
The third part of the powder/liquid delivery apparatus 1 is a means for 
evenly feeding liquid at any temperature downwardly along the inside 
surface of the lower section 5 and into the outlet 7. The flow of the 
liquid-feeding means must be such that no liquid collects in any 
substantial quantity in the lower section 5 or outlet 7 of the 
powder/liquid delivery apparatus 1. 
In FIG. 3, a liquid is fed continuously into the powder/liquid delivery 
apparatus 1 through two sources 10 and 11. The first source 10 provides a 
continuous stream of liquid 21 which contacts the inner surface of the 
upper section 4 of the powder/liquid delivery apparatus 1, supplied by a 
narrow diameter hollow cylinder 25, in the form of a sparge ring 12 having 
a plurality of holes and encircling the inside surface of the upper 
section 4 of the powder/liquid delivery apparatus 1. Preferably, the 
sparge ring 12 provides a mist of the liquid 21 which deposits on and 
moves uniformly down the inner surface of the upper section 4 the 
powder/liquid delivery apparatus 1 of the powder/liquid delivery apparatus 
the powder/liquid delivery apparatus 1. The second source provides a 
stream of liquid through a narrow diameter hollow cylinder 13 positioned 
such that it delivers a stream of liquid 22 into the lower portion of the 
lower section 5 of the powder/liquid delivery apparatus 1. It is preferred 
that at about 90% of the liquid flow be channeled through the second 
source 11. 
Preferably, the powder/liquid delivery apparatus 1 contains a means for 
pulling vacuum 15 on the inside of the column 3, especially under 
conditions when the liquid solvent is volatile or utilized at an elevated 
temperature. A means for pulling vacuum removes any gaseous material 
before it wets the free-falling powder and beneficially cools the 
powder/liquid delivery apparatus. The vacuum pulling means may be house 
vacuum, such as for example through a shaft 16, pulling vacuum from 1 inch 
mercury to 10 inch mercury. The vacuum-pulling means preferably located as 
high as possible above the liquid-feeding and powder-feeding means. 
Preferably, the internal surfaces of the powder/liquid delivery apparatus 
are coated with a non-stick coating, such as tetrafluoroethylene 
fluorocarbon polymers, fluorinated ethylenepropylene resins, and the like. 
High-Intensity In-Line Mixer 
The method of the invention utilizes any high-intensity in-line mixer. In 
FIG. 1, the high-intensity in-line mixer 2 is attached to the outlet 7 of 
the powder/liquid delivery apparatus 1. The high-intensity in-line mixer 2 
generally contains a mixing chamber 17 where the powder stream 23 and 
liquid streams 21 and 22 are mixed, a gear drive 18 and a motor 19. 
Preferably, the high-intensity in-line mixer 2 has multi-staged mixing 
zones. The degree of dispersion of the powder in the liquid and final 
dissolution of the powder in the liquid is increased with each successive 
stage. Suitable mixing heads which provide multistaged mixing zones 
include tandem shear turbine/stator configurations. 
The following examples illustrate specific aspects and particular 
embodiments of the invention which, however, in not to be construed as 
limited thereby. 
EXAMPLE 1. 
Preparation of Homogeneous Solutions of Polyvinyl Alcohol 
The following types of polyvinyl alcohol were utilized to demonstate that 
the method of the invention and the powder/liquid delivery system are 
useful for forming homogeneous solutions of a hard-to-wet powder: 
__________________________________________________________________________ 
Polyvinyl Alcohol 
Relative Temperature 
Grade Source Molecular Weight 
Degree of Hydrolysis 
(.degree.C.) 
__________________________________________________________________________ 
VINOL 205-S 
Air Products 
low 87-89% (partially-hydrolyzed) 
ambient 
ELVANOL 71-30 
DuPont low 99+% (fully hydrolyzed) 
90 
VINOL 540-S 
Air Products 
high 87-89% (partially hydrolyzed) 
ambient 
__________________________________________________________________________ 
The apparatus of the claimed invention as described in FIGS. 1-3 was 
utilized to form homogeneous solutions of polyvinyl alcohol without any 
time necessary for curing. Undissolved powder clumps were not present in 
the solutions formed for any of the three polyvinyl alcohol samples. Dust 
did not accumulate to any appreciable extent. 
EXAMPLE 2. 
Preparation of Homogeneous Solutions of Hydroxyethyl Cellulose 
Cellosize QP-3L hydroxyethyl cellulose from Union Carbide was utilized to 
demonstate that the method of the invention and the powder/liquid delivery 
system are useful for forming a homogeneous solution of a hard-to-wet and 
hard-to-dissolve powder. 
The apparatus of the claimed invention as described in FIGS. 1-3 was 
utilized to form homogeneous solutions of hydroxyethyl cellulose without 
any time necessary for curing at 80.degree.-85.degree. C. Undissolved 
powder clumps were not present in the solutions formed for any of the 
three hydroxyethyl cellulose samples. Dust did not accumulate to any 
appreciable extent.