Vessel for the treatment of particulate materials

Disclosed is a substantially vertically disposed vessel for the continuous gravity plug flow of solid particles and countercurrent flow of fluid, the improvement comprising the vessel having at least one fluid inlet and a fluid distributing member communicating therewith and extending therefrom for the introduction of fluid into the vessel, the fluid distributor being of a generally inverted V-shape to provide for the substantially uninterrupted mass plug flow of particles past said distributor and discharge of gas out the bottom.

TECHNICAL FIELD 
The present invention relates generally to a vessel for the treatment of 
particulate materials in which particles move continuously by gravity in 
plug flow through the vessel, and are contacted by fluid flowing 
countercurrent thereto. The vessel according to this invention is 
especially useful for the solid state polymerization of polyester. It is 
also useful as a heat exchanger between solid particles and a fluid. 
BACKGROUND OF THE INVENTION 
Although the present invention is useful in various processes in which a 
continuous plug, gravity flow of particles is to be contacted by a fluid, 
it has particular application in the solid state polymerization of 
polyester, in which the polyester is first formed to a low molecular 
weight in a melt phase, is solidified, formed into particles, and further 
polymerized in the solid state to a higher molecular weight. 
Many reactor designs employ internal diffusers for introduction of inert 
gas. These diffusers along with their supports are expensive and may 
interfere with the uniform flow of pellets through the reactor. In 
addition, the structural integrity of internal diffusers and supports 
cannot be guaranteed without a proper understanding of mass flow of 
solids. 
Reactors may also use a sparge ring around the circumference of the vessel 
either by itself or in combination with internal diffusers. While a sparge 
ring may avoid interference with pellet flow, it may be expensive to 
fabricate and if not properly designed offers a place for fines to 
collect. 
U.S. Patents relating to the solid state polymerization of polyester, and 
reactors for conducting the polymerization, include U.S. Pat. Nos. 
4,276,261; 3,405,098; 4,064,112; 3,960,817; 3,544,525 and 3,756,990.

DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the vertically disposed vessel 10 comprises a top 
section indicated generally at 12, an intermediate, generally tubular 
section 14 and a converging bottom section 16, all of the sections merging 
together for the continuous gravity plug flow of particles downwardly and 
the upward countercurrent flow of fluid. The top section 12 is provided 
with at least one particle inlet 18 for the particulate material and at 
least one fluid outlet 20. In a vertically disposed vessel in accordance 
with this invention, the inlet 18 for the particulate material is at 
substantially the uppermost part and the outlet 22 for the particulate 
material is at substantially the lowermost part. The centerline extending 
through the vessel from the inlet to the outlet is preferably 
substantially vertical. The bottom section 16 is provided with a particle 
outlet 22. The vessel has at least one fluid inlet 23, 24, and 25. Also, 
the top section 12 may be provided with particle distribution devices (not 
shown) known in the art to distribute the incoming particulate material. 
Preferably, the inlet 18 and outlet 22 for particulate material will be 
provided with feeders (not shown) for the continuous flow of particles 
into and out of the vessel at the same rate so that a bed of particles is 
formed in the reactor. The top of the bed will normally be lower than the 
top of the reactor. If desired, the vessel may be provided with a 
conventional jacket (not shown) for temperature control purposes. 
The intermediate section 214 is generally tubular (preferably cylindrical) 
and merges into the generally converging (normally conical) bottom section 
16. 
According to the present invention, a means is provided for introducing 
fluid into the vessel (fluid distributing member or members) such that the 
uniformity of the fluid flow is improved without hindering the uniformity 
of the gravity plug flow of the particulate material. An examination of 
Darcy's equation for fluid flow through porous media reveals that the 
pressure drop is proportional to the product of the superficial velocity 
and the path length of the fluid flow. Thus improved uniformity of flow is 
obtained by improved uniformity of the path length for the fluid flow 
through the particulate material. To this end, the fluid distributing 
member or members communicating with fluid inlets 23, 24 and 25 will have 
sides steep enough for the mass flow of particles by gravity and are 
provided with an open area at their bottom so the fluid will be discharged 
substantially downward in the direction of particle flow. This design 
provides no cavity or surface for the accumulation of fines, gives good 
uniformity of path length for the fluid flow, allows mass flow of the 
particulate material and is a simple, inexpensive fabrication. 
The cross-section for such members is preferably of a generally inverted 
V-shape (FIGS. 4,5, and 6.) The actual shape of the fluid distributing 
member or members is obtained by translating a cross-section of the 
preferred shape along one or more axes that are substantially horizontal. 
One embodiment is shown in FIGS. 1 and 2 as two, horizontal intersecting 
members spanning the vessel interior at right angles near the bottom. 
Rotating a cross-section of the preferred shape about an axis that is 
substantially vertical, results in the embodiments shown in FIGS. 7 to 10. 
The fluid distributing members may be located anywhere along the height of 
the reactor but are preferably positioned near the bottom of section 14 or 
in section 16, most preferably, low in the bottom section 16 provided the 
fluid flow rate does not cause fluidization of the bed. 
The fluid distributing members are positioned such that the apex of the V 
is upward in the vertically disposed vessel, i.e., the apex is pointed 
directly vertical so that both of the sides 42 and 44 make an angle 
".alpha." of between about 65 and about 85 degrees with a horizontal line 
46. The preferred angles, designated ".alpha.", will vary according to the 
characteristics of the particles in the vessel. That is, properties such 
as shape, size, propensity to agglomerate (such as powder), stickiness, 
etc., should be considered and the angles chosen for the mass flow of the 
particles. For example, for the solid state polymerization of polyethylene 
terephthalate, angles ".alpha." are preferably about 70 to about 75 
degrees. It is preferred that the apex be nearly a knife-edge, but this is 
not required. 
The angles ".alpha." are chosen for mass flow of particles in the vessel 
and along the intersection of members with each other or the vessel wall. 
Mass flow is defined as movement of particles through a vessel such that 
generally all of the contents of the vessel are in motion. Consequences of 
this type flow are that product is in motion relative to vessel walls and 
internals, and in the normal case of a symmetrical flow channel or (as in 
the case of the vessel described herein) multiple, identical, symmetrical 
channels, product velocity at any point in a horizontal plane is generally 
the same and particles discharge from the vessel generally in the order in 
which they entered the vessel. Ideally, there are substantially no 
stagnant regions and all particles experience the same range of conditions 
moving from inlet to discharge. In reality, short periods of stagnation 
may occur, and it may take 10-20 hours for a particle to pass through the 
vessel, making it appear that little or no movement is occurring. 
Mass flow will occur when channel walls are steep enough and smooth enough 
to accommodate the kinematic angle of friction between the flowing solid 
and the channel wall, and the effective angle of internal friction of the 
solid itself at that point. In other words, for a given product, in a 
vessel of a given material of construction and surface finish, there is a 
critical angle or wall steepness above which mass flow will occur. 
Particles being treated in the vessel may be of any conventional size and 
shape. Shapes may be spherical, cubical, cylindrical, shapes such as 
described in U.S. Pat. No. 5,145,742, etc. Although size is not important, 
the particles should not be small enough to show a tendency to cake. 
Normally, in the solid state polymerization of polyethylene terephthalate, 
sizes will be at least 2.times.2.times.2 mm, and not over about 
9.times.9.times.9 mm. 
The fluid entering the vessel will normally be an inert gas such as, for 
example, nitrogen. The gas, however, may be reactive, depending on the 
treatment being given to the particles. The gas may be heated, cooled, or 
at room temperature as desired, with any desired flow rate usually below 
that which would cause fluidization of the particles. 
The vessel according to the present invention is especially suitable for 
use as a solid state reactor or heat exchanger for polyester such as 
polyethylene terephthalate and copolymers thereof. Especially preferred is 
an embodiment having two horizontally disposed intersecting manifolds 
crossing the vessel at right angles, as shown in FIGS. 1-3. 
FIGS. 7-10 illustrate different embodiments of the invention. In FIGS. 7 
and 8, the vessel bottom section 16 is provided with a single, centrally 
located distributor 50, the sides 52 of which are sloped as described 
above. The fluid inlet is at 54. In FIGS. 9 and 10, a ring or circular 
distributor 56 having sloped sides 58 in accordance with this invention is 
illustrated. The present invention provides no cavity or surface for the 
accumulation of fines, gives good uniformity of path length for the gas 
and particle flow and allows for mass flow of solid particles. 
As an example of the solid state polymerization of polyethylene 
terephthalate, polyester particles having an inherent viscosity (I.V.) of 
about 0.6, a size of 4.times.4.times.2 mm and an OH:COOH end group ratio 
of 6.3:1, are charged from a silo to a mixer equipped with agitating 
blades and jacket-heating. The particles are heated in a continuously 
operating mixer and then crystallized in conjunction with a continuous 
nitrogen sparge at a nitrogen to granulate weight ratio of 2:1. The 
particles are then continuously fed, under an airtight seal; to a 
vertical, jacket-heated, tubular reactor as shown in the drawings and as 
described herein where it forms a bed. Nitrogen at a weight ratio of 
nitrogen to granulate of 2:1 and at a temperature of about 200.degree. C. 
is passed in countercurrent flow through the particulate bed. After a 
solid state polycondensation time of 8 hours, the final granulate having 
an I.V. of about 0.7 emerging from the tubular reactor. 
I.V. is measured at 25.degree. C. using 0.5 g of polymer per 100 mL of a 
solvent consisting of 60% by weight phenol and 40% by weight 
tetrachloroethane. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.