Coating apparatus – Control means responsive to a randomly occurring sensed... – Temperature responsive
Reexamination Certificate
2002-03-26
2004-08-03
Pianalto, Bernard (Department: 1762)
Coating apparatus
Control means responsive to a randomly occurring sensed...
Temperature responsive
C118S058000, C118S300000, C118S688000, C118S708000, C118S712000, C427S008000, C427S213000, C427S372200, C427S421100
Reexamination Certificate
active
06770141
ABSTRACT:
TECHNICAL FIELD
The present invention relates to methods and systems for controlling evaporative drying processes. More particularly, the present invention relates to methods and systems for controlling evaporative drying processes using environmental equivalency.
BACKGROUND ART
Evaporative drying processes, such as tablet film coating, spray drying, and fluid bed processing, utilize evaporative drying to achieve a desired output product quality. For example, in tablet film coating, tablets are placed in the coating pan of a tablet coater. The coating pan is a perforated or semi-perforated cylinder, similar in appearance to the tumbler of a conventional clothing dryer. The coating pan rotates as a coating material, such as a solution or a suspension, is sprayed onto the tablets. In order to dry the coating material on the tablets, a heated gas, such as air, is pumped or drawn into the chamber through a gas inlet. The gas evaporates liquid from the coating material and exits through a gas outlet.
Some of the parameters associated wit tablet film coating are:
drying gas temperature;
dew point;
drying gas flow rate;
spray rate; and
solution/dispersion percentage of solids.
In order to achieve proper coating of tablets using conventional methods, optimal values for each of these parameters must be determined empirically. In addition, subsequent processes must be carefully controlled to ensure that the optimal parameter values are maintained.
In order to determine optimal values for tablet film coating parameters, many experiments must be performed. For example, a process technician may start coating tablets in a tablet coater using initial values for the above-listed parameters. The quality of the coating of the tablets may be analyzed to determine required adjustments in the parameters. This process is repeated until optimal values are determined for the parameters. The optimal parameter values are then programmed into a control device, such as a programmable logic controller, to control subsequent coating of tablets.
The empirical method for determining optimal parameter values is undesirable for a variety of reasons. When multiple tests are required in order to determine optimal parameter values, many hours of tablet coater operation are required. As a result, a pharmaceuticals manufacturing company may be required to slow production or purchase multiple tablet coaters in order to maintain a given production level. The increased time and/or equipment required to empirically determine optimal process parameters undesirably increases the cost of developing evaporative drying processes, such as tablet film coating.
Another problem associated with conventional development of evaporative drying processes is that conventional development of evaporative drying processes is product specific. In other words, experimental tests must be performed for each new product to determine optimal process parameters. This testing undesirably increases labor and expense associated with conventional evaporative drying processes.
Another reason that the conventional empirical method of determining optimal process parameter values is undesirable is that results may not be scalable. For example, parameter values determined for a small tablet coater may not be valid for a larger tablet coater and vice versa. As a result, new parameter values may have to be determined when the scale of a process changes. In addition, model parameters that hold true for one processing environment may not be transferrable to another processing environment. For example, parameter values for a tablet film coating process operating in one geographic area with a high relative humidity may not be transferrable to another geographic area with a low relative humidity. As a result, empirical tests must be performed in the new geographic area to determine optimal parameter values for the new area. This lack of scalability and transferability associated with conventional tablet film coating process control results in increased labor and expense.
Still another problem associated with tablet film coating is the time required to start coating tablets. For example, in convention tablet coating minutes or even hours may be required to reach operating parameter values. This increased startup time decreases production for a given tablet coater.
Yet another problem associated with conventional tablet film coating is that when one or more process parameters change during a tablet coating operation, this change may adversely affect output product quality. For example, if inlet air humidity or temperature changes during a tablet coating operation, other parameters may require adjustment during the operation in order to compensate for the changes. Such compensation may require continuous monitoring and manual adjustment by an operator throughout the tablet coating process. Thus, conventional methods for manufacturing pharmaceutical products may be labor-intensive.
“A Thermodynamic Model for Aqueous Film-Coating”,
Pharmaceutical Technology
, April 1987, by Glenn C. Ebey of Thomas Engineering, describes a dimensionless quantity, referred to as environmental equivalency (EE), that can be used to model relationships between process parameters associated with aqueous film coating. In the publication, an example is given where environmental equivalency is used to determine a new inlet air temperature for a tablet coater to produce a desired environmental equivalency value when inlet air humidity changes. The new inlet air temperature is determined as follows. First, the example states that “a good quality of coating can be obtained at an inlet air temperature of 149□F, an air flow rate of 2000 actual cubic feet per minute, a humidity ratio of 25 grains per pound mass, and a spray rate of 400 grams per minute, using a solution of 10% solids”. Based on these parameters, an EE value of 2.990 is calculated. The humidity of the processing environment changes to 125 grains per pound mass. The inlet air temperature required to maintain the same EE value is then calculated. In the example, the resulting inlet air temperature is 160□F in order to achieve the same EE value.
While the publication describes, in theory, a method for modeling film coating processes using environmental equivalency, the example reiterated above only demonstrates how to change one variable associated with a film coating process to compensate for a step change in another variable, while the remaining parameters are held constant. In a real tablet coating system, multiple parameters may change and/or require adjustment during a tablet coating operation. Such multi-variable changes and adjustments are not addressed in the publication.
Another shortcoming of the publication is that a control system for continuously adjusting process parameters to maintain EE values is not disclosed. In the example stated above, when the humidity changes from 25 to 125 grains per pound mass, a new inlet air temperature is calculated such that the EE value will be 2.9. Such calculations may be useful for a step change in humidity, such as that which occurs when a process is moved from one geographical location to another and humidity remains constant at the new location. However, in real systems, process parameters may vary sinusoidally about setpoints, as determined by time constants of the respective control systems for process parameters. Thus, it is desirable in a real system to continuously measure process parameters and use the measured values to maintain a desired EE value.
Yet another shortcoming of the publication is that it does not address preferred ranges of EE values for tablet film coating. Finally, the publication does not address the application of environmental equivalency control to evaporative drying processes other than aqueous tablet film coating, such as spray drying, fluid bed processing, or other evaporative drying processes.
In light of these difficulties, there continues to exist a long-felt need in the pharmaceuticals industry and other industries that utilize evap
Campbell Dwayne A
Marranca Joseph T.
Pope Raymond E.
Stagner Robert Allen
Pianalto Bernard
SmithKline Beecham Corporation
Strickland J. Michael
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