Agitating – Rubber or heavy plastic working – Stationary mixing chamber
Reexamination Certificate
1999-01-07
2001-07-17
Soohoo, Tony G. (Department: 1723)
Agitating
Rubber or heavy plastic working
Stationary mixing chamber
C366S300000, C366S318000, C366S301000
Reexamination Certificate
active
06260995
ABSTRACT:
The invention relates to a mixing apparatus for the treatment of viscous substances, comprising a heatable vessel with a product inlet on one side of the vessel and a product outlet on the other side of the vessel, if appropriate an exhaust vapor nozzle as well as two or more mutually parallel, counter-rotating shafts with, located thereupon, paddles extending in the circumferential direction and blades extending along the axial direction. The apparatus permits the active axial conveyance of the mixed substance, the moving internals fully cleaning themselves kinematically during operation of the apparatus.
In the production and processing of plastics and rubber, high-viscosity liquids, which may contain fine-particle solids, are treated by using process technology. In particular, apparatuses for mixing, evaporating and reacting plastics are required. These mixers or reactors must have a good mixing action, and for example in the case of evaporating volatile components caused by a reaction-induced release, also permit rapid renewal of the free surfaces of the mixer.
For the process-technological treatment of viscous products, apparatuses which have both as large a free volume as possible (defined as the ratio of product volume to interior housing volume and as good kinematic self-cleaning as possible are required for operations such as evaporating and reacting. The free volume of the apparatus is in this case indirectly a measure of the apparatus costs, since with a large free volume the volume of the rotors decreases in comparison with the volume which can be taken up by the product.
For those process steps which are determined by the mass exchange between a liquid phase and the gas phase, the product from the interface between these two phases and the mass-transfer coefficient are speed-determining.
In the mass exchange between gas phase and high-viscosity liquids, the main controlling factor for the mass-transfer coefficient is the frequency of the renewal of the liquid surface.
Good kinematic self-cleaning is required, moreover, to avoid the formation of deposits, in which undesired secondary reactions are favored by a long dwell time if the mixer is used as a reactor. Similarly, a regular renewal of the surfaces leads to a good mass-exchange behavior in evaporation processes, such as polycondensations for example.
These two properties are achieved to a certain extent by apparatuses known from the prior art, for example the mixers in the publications U.S. Pat. No. 5,399,012, EP 0 715 881 A2 and DE 19611852 A1.
U.S. Pat. No. 5,399,012 describes a mixing apparatus with full kinematic self-cleaning. A high degree of surface renewal is achieved by the product wiping-out motion of the scrapers. The free volume can be set to over 70%, depending on the choice of the geometry parameters (housing diameter/axial spacing, number of scrapers).
The apparatus according to EP 0 715 881 A2 is also fully self-cleaning. Since the scrapers from U.S. Pat. No. 5,399,012 are, however, replaced by bars with a smaller surface, the surface renewed with each revolution is smaller.
DE 19 611 852 A1 (=U.S. Pat. No. 5,876,115) has the same advantages as U.S. Pat. No. 5,399,012, but has an additional conveyance in the axial direction.
An application for such apparatuses is represented, for example, in German Offenlegungsschrift [German Published Specification] DE 19 612 139 A1.
However, in the selection of mixing apparatuses for carrying out continuous processes, the through-flowing behavior of the mixing apparatuses is also very important. In particular for carrying out polymerization reactions, apparatuses which have a low axial backmixing of the mixed substance running through are desired, since the equivalent number of agitator vessels is then increased. Depending on the desired dwell time, an active conveyance is required.
The apparatuses according to U.S. Pat. No. 5,399,012 and European Patent Application EP 0 715 881 A2 have no device for the axial conveyance of the mixed substance, but are dependent on gravity as the driving potential for the axial transport. A dwell time below 20 minutes cannot meaningfully be achieved in the case of products with a viscosity of over 10 Pas and an equivalent number of agitator vessels of greater than 5.
Extruders have a particularly good axial conveying behavior. Their disadvantage is the small free volume, as well as the low renewal of their internal surfaces in comparison with apparatuses of equal volume according to one of the apparatuses mentioned above. For commercial reasons, they can be used only for mixing processes with a dwell time of less than 3 minutes, preferably less than 1 minute.
With the apparatus known from DE 19611852 A1, or U.S. Pat. No. 5,876,115 a novel conveying mechanism was found: a chamber formed by two scrapers and a paddle, which is fastened on a shaft, is entered by a scraper of a neighboring shaft. The product is preferably forced in the direction away from the paddle. This mixer has two serious weaknesses: the product located in the chamber largely escapes through the gaps between shafts and scrapers, without being axially conveyed. In addition, half-discs are required for securing scrapers on the shaft ends. A consideration of the kinematics of this mixer leads to the realization that an axial discharge of product is not possible, since one of the half-discs always gets in the way in the region in which the axial conveyance takes place.
The object of the invention is to provide an apparatus which is kinematically fully self-cleaning, which, by virtue of good axial conveying ability, is suitable in particular for a dwell time of less than 15 minutes and, on account of a large renewed product surface with each revolution, is suitable in particular for evaporating and degassing processes in the case of viscous liquids.
This object is achieved according to the invention by a mixing apparatus for the treatment of viscous mixed substance, comprising a housing with a product inlet and a product outlet, if appropriate an exhaust vapor nozzle, two or more shafts with, located thereupon, paddles extending in the circumferential direction and blades extending along the axial direction and a drive means for the counter-rotation of neighboring shafts, characterized
in that the blades are respectively connected both to one of the shafts and to a paddle, apart from the blades which, seen in the axial direction, are located at the ends of the apparatus,
in that on the upstream side of each paddle there is at least one outwardly conveying blade and on the downstream side of each paddle there is at least one inwardly conveying blade, apart from the paddles which, in the axial direction, are located at the ends of the apparatus,
in that, with the rotation of the shafts brought about by the drive means, the blades and paddles sweep fully over the housing and consequently fully clean it kinematically,
in that, with the rotation of the shafts brought about by the drive means, the blades and paddles sweep fully over the shafts and consequently fully clean them kinematically,
in that, with the rotation of the shafts brought about by the drive means, the blades and paddles of one shaft are swept over by blades and paddles of a directly neighboring shaft, by this shaft itself or by the housing inside wall and are consequently fully cleaned kinematically,
in that pairs of blades following each other in the circumferential direction of a shaft and at least one paddle, (which is connected to one of the blades) form a open chamber into which, with the rotation of the shafts and, a blade located on another, neighboring shaft, in engagement, moves and thereby forces most of the product out of the open chamber in the direction opposed to the surface of the paddle.
For the following description, for the sake of simplicity, the entirety of the shaft and blades and paddles connected to the shaft is referred to as a rotor. The apparatus may have two, three, four or more rotors arranged parallel to one another, respectively neighboring rotors always
Bayer Aktiengesellschaft
Norris & McLaughlin & Marcus
Soohoo Tony G.
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