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Automatized Real-Time Control of a Sequential Encapsulation Process for Manufacturing Active Delivery Formulation
Dr. Boris Gordonov, Rhodia, Inc., Tel.: (609) 860 3252 E-mail: Boris.Gordonov@us.rhodia.com Dr. Anatoliy Gordonov, College of Staten Island/City University of New York, Department of Computer Science. Tel: (718) 982 2850 E-mail: Gordonov@postbox.csi.cuny.edu
Abstract The present paper addresses a new engineering approach to the problem of quality and cost reduction in the sequential encapsulation process. The stage of industrialization is considered as a natural continuation of the previous pilot/laboratory stage where further investigation of an industrial-scale system and optimization of its control is necessary. This idea sets different requirements for the process control system, its flexibility and accessibility for changes and tuning by the plant engineering personnel. In the developed and implemented by the authors Distributed Control System (DCS), real-time tuning of the blending process parameters, which is part of a control algorithm, is dictated by a particular stage of the polymerization process. The employment of the developed by the authors real-time process control system makes it possible to improve the quality of the coated product and stability of production.
The recent years have been marked by a noticeable growth in the development and use of the so-called Controlled/Sustained Release products for a variety of applications in industry, agriculture, and medicine. This includes new sustained release coated pharmaceutical formulations, polymer coated granular fertilizers and agrochemicals, etc. The application of a protective coating on a granular/tableted active component renders new superior properties to formulations in terms of performance, stability, economic, and environmental benefits.
Introduction An encapsulated Controlled Release Formulation (CRF) consists of a soluble core (see Fig.1) and a protective continuous coating. The latter typically consists of an insoluble polymer with a definite permeability to the solvent (usually water). The release mechanism includes the following stages (Fig.1): diffusion of the solvent (water) vapor from the environment through the protective coating; adsorption of the solvent by the soluble active material with formation of a saturated solution; development of osmotic pressure inside the coated granule; and gradual release of the solution to the environment by diffusion through the protective membrane. In some cases, release of a soluble compound is preceded by sustained decomposition of the protective coating in the environment (microbiological degradation, solubility triggered by change of acidity, etc.). An important characteristic of a CRF is the duration of the release that is controlled mainly by the thickness and permeability of the coating film, solubility of the active ingredient, and environmental conditions (temperature, pH, moisture content). Industrial technology of encapsulation/coating includes different techniques such as powder coating (the coating material is in a solid powder form) or spray coating (the coating material is in a liquid form). Spray coating is usually accomplished in the fluidized bed, rotating pan or a tumbling drum. The coating material is solidified on the surface of the particles to be coated in the form of a thin continuous film. This solidification occurs due to either solvent evaporation, melt solidification or in-situ reaction/polymerization of the coating material. The application of the coating material on the solid particles during the coating process can be accomplished by a continuous addition (spraying) or applying the coating material in sequential layers. The latter approach (widely used in all kinds of painting) has proved itself as providing good control over the coating quality. The sequential coating process is widely used in the coating applications involving in-situ polymerization of two- or multi-component systems, e.g. for polyurethane film formation. A typical process for sequential application of a two-component coating composition (Fig.2) includes a coating tumbling drum (3) with a feeding system for all coating material ingredients: (tanks 4a,b and pumps 5a,b). The material to be coated is usually preheated (1) (e.g. in a fluidized bed) and weighed (2). After the coating process has been completed, the product is forwarded to a conditioning and cooling step (6) (application of a conditioning material to prevent caking). Finally, the material is forwarded to storage/packing.
Problem Despite apparent simplicity of the technological process, the application of a continuous defect-free and uniform thin film of a protective material on the surface of commercial soluble granulated materials (e.g. fertilizers) constitutes an engineering challenge. It should be emphasized here that even microscopic defects in the coating result in almost immediate (catastrophic) release of the soluble active ingredient and loss of controlled/sustained release properties of the formulation. The task becomes even more complicated when granules to be coated are not spherical and have surface defects. It has been further found that more aggressive conditions of an industrial scale encapsulation process contribute to deterioration of the coating quality and noticeable economic losses due to production of a lower quality material. The indicated above problem is usually addressed through modification of the coating composition and/or quantity of the coating applied. This leads to increasing the cost of industrial–scale production as compared to that of pilot or laboratory production and does not eliminate sensitivity of the coating quality to variations in shape and surface smoothness of a granular substrate.
Real-Time Control of a Coating Process The present paper addresses a new engineering approach to the problem of quality and cost reduction in the sequential encapsulation process. The stage of industrialization is considered as a natural continuation of the previous pilot/laboratory stage where further investigation of an industrial-scale system and optimization of its control is necessary. This idea sets different requirements for the process control system, its flexibility and accessibility for changes and tuning by the plant engineering personnel. Control parameters of the industrial process are usually determined in the preceding stages of the project - through the laboratory and pilot-scale study. The function of the control system at a full-scale facility in this case is merely to insure maintaining the predetermined set of parameters in the preset range as accurately as possible. In the developed and implemented by the authors Distributed Control System (DCS), real-time tuning of the blending process parameters, which is part of a control algorithm, is dictated by a particular stage of the polymerization process. The utilization of the developed by the authors real-time process control system makes it possible to improve the quality of the coated product and stability of production.
The structure of the Control system is presented in Fig. 3. The architecture of the system is based on a Personal computer (1) equipped with an extension card (2) for increasing the number of communication RS-232 ports to 8. The choice of the architecture has been dictated by economic considerations, relatively small scale of the industrial encapsulation plant, and the benefit of utilizing the same hardware and software for the laboratory, pilot-scale, and full-scale production units. A simple design and wide market availability of the components used make it possible for the chemical plant engineering personnel to successfully build, use, and expand the system without involving external contractors. The data acquisition system allows continuous reading from weight indicators (3) connected to the load cells of the feeding tanks and hoppers. A bi-directional communication with a motor-control center (4) equipped with variable speed drives (frequency converters) can be accomplished through a PLC (5). As an alternative, a RS-485 multi-drop link has been used with addition of a communication module to each variable speed drive. The monitoring status of all electrical motors and their remote control are accomplished through this link. In the simplest mode, the control system identifies and analyzes if each drive is "enabled", "running" or "stopped", "running at a speed" or it is in the "acceleration"/"deceleration" mode. The important monitored parameters are the rotation speed as well as a momentary value of load current or power. These last parameters allow us to evaluate the load on the drive and are directly related to the stage of the encapsulation process in the unit. Control of digital single-phase outputs (alarm, small membrane pumps, etc.) is implemented using Code operated power switches (6 and 7) connected directly to a RS-232 PC port. This link operates as one-directional and does not allow us to monitor the actual status of simple single-phase appliances. Additional monitoring capabilities can be acquired through installation of suitable electrical or mechanical sensors and reading the corresponding input signal (e.g. a relay input indicating that the device is "on" or "off") to the PC through PLC or additional PC relay input card. Availability on the market of different control and data recording devices with network communication capabilities makes it possible to easily expand the control system by the plant engineering personnel. In the developed system, monitoring gear pumps pressure has been added at a later stage. Connecting the control computer to the company’s network makes it also possible to share the production report information with the respective engineering and business management personnel. Network communication enables us to include additional data acquisition and control devices into the system. The structure of the software is presented in Fig. 4. The database part of the control package includes: (a) Setup of General process parameters, (b) Coating process menus, (c) Recording particular production run to the Process archive (a history database). The process database part of the software is responsible for generating reports to the plant production management and engineering personnel. The process control core of the software implements the process control sequence according to the process menu. The interface with the operator includes the following features: alarms, indication of the major process parameters and status of the electrical drives, setting and changing the process menus and general settings, manual control of the pumps, feeders, and other equipment. The manual control option allows the operator to take over control in case of emergency. The control program detects some of these emergency conditions. The program is executing real-time data acquisition and preliminary analysis. The analysis includes detection of fault readings, interruption in communication with a peripheral devices and some abnormal process conditions which result in generation of doubtful readings. The control program communicates with the motor control center driver. Digital outputs are executed through generating control codes and sending them to the code operated relay switches.
Real-Time Control and Optimization of the Coating Process The described flexible control system has been developed and first implemented on the laboratory/micro-pilot scale and then was up-scaled to the industrial unit. This makes it possible to continue further development and optimization of the coating process at the plant facility giving outstanding results in reducing the start-up time and improving the process. It has been shown that with the help of the described above flexible control system it is possible to find and implement the way to increase the duration of release through improving noticeably the quality of the coating without increasing its thickness. To demonstrate the latter, one needs to understand the mechanism of a film defect formation during the coating process. Let us consider a sequential polymer coating process in a tumbling drum unit. Each layer application goes through several stages:
After adding the liquid components in each layer, an aggressive mechanical agitation is employed to facilitate mixing and uniform distribution of a liquid coating material over the solid surface. While solidifying, the coating composition passes a gel-point when the film that is being formed still possesses definite stickiness and low mechanical hardness. During the optimizing stage of the plant-scale process, it has been found that high shear stress which accompanies an intensive blending operation plays a detrimental role for the coating quality in the vicinity of the gel-point. The authors believe that decreasing a high shear stress at this point can noticeably decrease the formation of the coating film defects. For instance, this can be accomplished by decreasing the speed of drum rotation. A typical time pattern of this control parameter is demonstrated in Fig.5. Nevertheless, accurate determination of the gel point may require real-time monitoring of the condition of the tumbling bed in the unit. This can be implemented through a control system by reading and analyzing torque on the unit shaft that is reflected in changes of the consumed power or load current readings from the drum drive. The architecture of the control system has allowed us to accomplish this control function by implementing minor software changes by the plant personnel.
Conclusion Our experience has demonstrated that custom developed, flexible, and open to the operational plant personnel control systems used to control a technological process from the laboratory/pilot stage to a full-scale facility can provide definite benefits in shortening start-up time, process optimization, and improving the quality of the products. As a result, the manufacturer gets noticeable economic benefits.
Figure 1: Release Mechanism for the coated granule/tablet
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