Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Fluidized bed
Reexamination Certificate
1993-01-11
2004-01-20
Johnson, Jerry D. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Fluidized bed
C422S139000, C422S140000, C422S145000, C427S213000
Reexamination Certificate
active
06680032
ABSTRACT:
The present invention relates to a new process and a new apparatus for preparing granulates. The invention also relates to granulates which are obtained by means of the new process.
There already are numerous known processes for preparing granular material by fluidised-bed granulation (compare Chem. Ing. Techn. 45, 736-739 (1973), DE-OS (German Published Specification) 2,231,445, DE-OS (German Published Specification) 2,555,917 and EP-OS (European Published Specification) 0,087,039). In these disclosed processes, which are carried out continuously, the ready-to-use granulate is obtained in one step without separate post-drying. In essence, it is possible to distinguish three different methods in this context which, however, are all based on the same granulate formation process. Thus, in each case the product to be granulated is of sprayable consistency, that is to say in the form of a melt, suspension or solution, when sprayed into a fluidised bed. The sprayed-in liquid product first wets the surface of the solid particles in the fluidised bed and then solidifies by drying on or cooling down. In this way the particles grow in shell-like fashion, and their size increases the longer they dwell in the fluidised bed. For that reason the granulate size depends critically upon the bed content.
The growth of the particles starts in the fluidised bed from nuclei which either are formed in the fluidised bed itself by non-impacting solidifying drops of spray or are produced by abrasion of solid particles already present, or which are fed into the fluidised bed from the outside. Nucleation which takes place in the fluidised bed (internal nucleation) is affected by the content of the fluidised bed in such a way that as the bed content grows there is on the one hand a decrease in the number of non-impacting droplets of spray and on the other an increase in the number of nuclei produced by abrasion.
The process which is required for producing granulates in a predetermined grain size is, in fluidised-bed granulation, the interaction of nucleus availability and granulate growth. Therefore, the process of granulation can be affected in many and varied ways. Thus, for example, the availability of nuclei can be increased by adding nuclei from the outside.
Steady-state granulation conditions have been reached when the bed content remains constant. In the steady state the mass of solid material fed into the fluidised bed must correspond to the mass of completed granulates removed from the bed. In addition to this mass balance, however, the particles need also to be in balance. That is to say, all the granulates removed from the fluidised bed must be replaced in number by new nuclei. In the known methods, a constant granulate formation process is forced on the system. The methods differ in the manner in which this constancy is obtained.
In the case of the processes disclosed in DE-OS (German Published Specification) 2,231,445 and EP-OS (European Published Specification) 0,087,039, only internally formed nuclei are used. The control of the associated low fluidised-bed content actuates a discharge element according to the principle of fill level control and hence ensures that the granulate removal matches product intake. The granulate emerging from the granulator is classified, and the resulting fines are returned into the granulator. Since, however, in the case of different throughputs through the serially connected apparatus elements of classifying line and discharge element the fill level control gets out of step, the only classifying line which can be used in this case is a classifying line where the amount of granular material which is let through matches the required throughput through the discharge element irrespective of separating efficiency. Accordingly, the classifying line employed in the methods described in DE-OS (German Published Specification) 2,231,445 and EP-OS (European Published Specification) 0,087,039 is in each case a second fluidised bed. This second fluidised bed provides only very inefficient classification of the granulates. For that reason, this process is unsuitable for producing granulates having a narrow grain size distribution.
The granulating process communicated in DE-OS (German Published Specification) 2,263,968 is in principle identical to the method described above. However, the classifying line employed in this instance is not a second fluidised bed but an efficiently separating sieve. The necessary synchronisation of the throughputs through the classifying line and the discharge element is achieved by not removing a portion of the granular material but instead grinding it and then returning it to the fluidised bed. This additional availability of nuclei needs to be compensated by reducing nucleation in the fluidised bed. The reduction of nucleation in the fluidised bed is achieved by operating the granulator at a high bed content. However, the prerequisite for this mode of operation is an abrasion-resistant granulate.—This method thus provides granulates of narrow grain size distribution. However, it is a disadvantage that a comparatively high amount of apparatus needs to be used. In addition, this method cannot be used for granulating solvent-moist or dust explosion hazard products, since the apparatus required in addition to the fluidised-bed granulator cannot be inertised nor be installed to withstand explosive pressure.
In the third alternative disclosed in DE-OS (German Published Specification) 2,555,917 for producing granulates by the fluidised-bed/spray method, the discharge element used is a countercurrent gravity classifier This classifier combines the functions of classifying line and discharge element.
Using this apparatus, only granulates which have reached the desired grain size are removed from the fluidised bed. Any variation in the number of discharged granulates has an immediate effect on the bed content. If, for example, the bed content rises, the granulates produced are too small. Consequently, the growth of granulates needs to be promoted and the feed of nuclei, which is in this case from the outside, needs to be cut back. To be able to apply effective control measures to the number of nuclei available for the granulation process it is necessary for internal nucleation to be minimised, which, in the case of abrasion-resistant granulates, can be obtained by granulation with high bed content.—In the final analysis this known method produces granulates within a narrow grain size distribution. However, it is a disadvantage that the method requires controlled outside supply of nuclei which is complicated. Furthermore, conversion of the process from a certain mean grain size to another mean grain size is associated with extensive preliminary experimental work to determine the exact settings. That is also true of the other processes described above.
A new process has now been found for the continuous preparation of granulates having narrow grain size distribution, characterised in that
a) the product to be granulated is sprayed in liquid form into a fluidised bed,
b) the proportions of fine material escaping from the fluidised bed in the off-gas are separated off and returned into the fluidised bed as nuclei for granulate formation,
c) the granulation process in the fluidised bed is influenced solely by setting the classifying gas stream in such a way that granulates are formed which are of the size predetermined by the classifying gas stream, and
d) the completed granulate is removed solely by way of one or several countercurrent gravity classifiers which are inserted into the outflow bottom of the fluidised-bed apparatus, and
e) if desired the granulates thus obtained are subjected to a thermal aftertreatment.
A new apparatus for the continuous preparation of granulates having a narrow grain size distribution has also been found. The apparatus essentially comprises a fluidised-bed reactor
which contains devices for dispersing the product supplied in sprayable form,
which further contains a system suitable for returning proportions of fine materia
Braun Burkhard
Hausmann Heinz
Karkossa Horts
Stopp Gerhard
Uhlemann Hans
Bayer AG
Doroshenk Alexa Ann
Johnson Jerry D.
Norris & McLaughlin & Marcus
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