Drying and gas or vapor contact with solids – Process – With fluid current conveying or suspension of treated material
Reexamination Certificate
2002-05-16
2004-06-29
Bennett, Henry (Department: 3743)
Drying and gas or vapor contact with solids
Process
With fluid current conveying or suspension of treated material
C034S436000, C034S505000, C034S576000, C034S228000, C528S503000
Reexamination Certificate
active
06754979
ABSTRACT:
The invention relates to an apparatus for drying and heat-treating pellets with a stream of inert gas, to a process for drying polycondensate pellets in the solid phase using the apparatus of the invention, to a system for drying and postcondensing polycondensate pellets in the solid phase with at least one apparatus of the invention, and also to a use.
Polycondensates, predominantly prepared by melt polycondensation, have to be dried and generally subjected to posttreatment to remove monomers and oligomers, and also to postcondensation to increase molecular weight.
A proven device for drying and postcondensing polyamide pellets is the tower dryer described in DE-A 2 530 304, for example, also termed an annealing tower. The tower dryer is a tall cylindrical device through which polycondensate pellets are passed from the top to the bottom and a hot inert gas, frequently nitrogen, is passed in countercurrent. The polycondensate pellets are generally moist, since in an upstream process step they have been subjected to water washing in order to reduce the content of oligomers and monomers. What takes place in the tower dryer is firstly drying of the pellets and secondly heating of the pellets to a product-specific temperature for solid-phase condensation. The stream of inert gas emerging at the head of the tower dryer is cleaned of dust and, by cooling below the dew point, of moisture. It is passed back into the tower dryer after heating and, if desired, after removal of traces of oxygen. The location at which the inert gas is fed into the tower dryer is primarily a function of the diameter of the tower: specifically, the bed height above the gas entry has to be sufficient to ensure sufficiently uniform distribution of the stream of inert gas. In practice, the lower limit for the ratio of bed height to tower diameter is from 1.3 to 2. Substantially uniform distribution of the stream of inert gas is a precondition for very uniform drying of the pellets, and this in turn is important for very low variation in the viscosity number from grain to grain. The large bed height in the drying zone, relative to the tower diameter, is a result of the requirement for substantially uniform distribution and means that, particularly for high drying or annealing temperatures, the residence times for the pellets are many times greater than the residence time required by the kinetics of drying. In addition, the high pressure loss requires a corresponding blower rating, with relatively high energy costs. Another disadvantage of the tower dryer is that the amount of inert gas is restricted by the fluidization point of the pellets. In order not to exceed the fluidization point, therefore, high-capacity systems have to be designed with correspondingly large tower diameters. However, these large tower diameters make it more difficult to achieve plug flow in the tower dryer, and this flow is important for low variation in viscosity number from grain to grain. The large tower diameters, taken together with the uniform distribution of the stream of inert gas, also increase the abovementioned energy costs.
Another disadvantage of carrying out drying and annealing in a single device, for example a tower dryer such as that described in DE-A 2 530 304, is low flexibility in relation to changes in throughput or residence time, and also in relation to the preparation of products with different molar mass and/or under different annealing conditions.
DE-A 4 326 105 therefore proposes a device for preheating and drying upstream of the tower dryer, thus decoupling the drying process and the annealing process. This proposal provides two fluidized-bed devices upstream of the tower device, the first of these having an effervescent fluidized bed and the second having a fluidized bed with plug-flow characteristics. In continuously operating fluidized beds, the product to be dried is generally placed on an inflow plate, frequently a perforated plate, a bar screen or a specifically designed plate, where the hot drying gas passes through the product from the bottom to the top. The gas velocity set here is such that the product is held in flotation, but not pneumatically conveyed or discharged. The moving product behaves like a liquid. If the product is fed continuously on one side, it flows out from the discharge opening on the other side. The gas therefore has a number of tasks: it is firstly responsible for fluidizing and secondly serves as a medium for heat transfer and mass transfer. The intensive heat transfer and mass transfer in fluidized-bed devices makes them fundamentally suitable for rapid heating and drying. However, in the case of products with slow drying kinetics, for example nylon-6, the gas flow needed for product transport is insufficiently utilized, and therefore the use of fluidized-bed devices in such cases is not very cost-effective.
A further disadvantage of fluidized-bed devices is that the product residence time distribution is broader than in the tower dryer, due to well-developed fluidization. As a result, the moisture content of the product is non-uniform at the exit from the fluidized-bed device, and therefore at the entrance to the downstream holding device. The solid-phase condensation in the holding device for further increasing the molar mass downstream is highly susceptible to non-uniform moisture content of the pellets entering. This results in variations in relative viscosity from grain to grain, and these impair product quality, for example for high-speed spinning. Another disadvantage of fluidized-bed devices is the relatively severe mechanical stress placed on the pellets due to fluidization and the resultant formation of dust.
It is an object of the present invention, in contrast, to provide a device which serves for drying and heat-treating pellets with a stream of inert gas, and which is more cost-effective, and which ensures that the product residence time distribution is narrow, that the flow rate of inert gas is markedly increased due to lack of any dependency on a fluidization point, and consequently that capacity is markedly increased. In addition, a process should be provided which decouples the drying process and the annealing process and avoids the disadvantages of the known processes.
We have found that this object is achieved by an apparatus for drying and heat-treating pellets with a stream of inert gas. The apparatus of the invention has a cuboid base unit with a feed for the pellets in its upper part and an outlet in its lower part, with a feed on one lateral surface and with an exhaust on the opposite lateral surface of the cuboid base unit for the stream of inert gas, and also with distribution and retention equipment in the region of the feed and, respectively, of the exhaust for the stream of inert gas, where each of these extends essentially over the entire vertical cross section of the cuboid base unit.
We have found that the process object of the invention is achieved by means of a process for drying polycondensate pellets in the solid phase, by
a. in one or more devices, drying the polycondensate pellets and heating the same at between a minimum temperature of 105-120° C. and a maximum temperature 2° C. below their softening point,
b. then treating the same in a holding device, and
c. then cooling the same in a cooling device, and selecting an apparatus of the invention, at least as the device(s) in stage a.
Pellets usually have the form of grains with a particle size of from 1.5 to 5 mm, in particular from 2 to 3 mm. The pellets may be cylindrical, spherical or lenticular. The pellets used according to the invention are preferably polycondensate pellets, in particular polyester pellets or polyamide pellets.
Polycondensates are polymers prepared by condensing monomers with elimination of low-molecular-weight compounds. Polycondensates, preferably polyesters or polyamides, are known to the skilled worker and have been described many times in the literature, and no more detailed information is therefore needed here.
It is preferable to use semicrystalline polycond
Ludwig Alfons
Pille Ragnhild
Pipper Gunter
BASF - Aktiengesellschaft
Keil & Weinkauf
Nguyen Camtu
LandOfFree
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