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The development of an in-line energy efficient polymer microwave based moisture measurement and drying system

Final Report Summary - POLYDRY (The development of an in-line energy efficient polymer microwave based moisture measurement and drying system)

The overall industrial objective of the project POLYDRY was to develop a new low cost, rapid, energy efficient microwave based moisture measurement and drying system. This would enable in-line, accurate, real-time microwave moisture measurements to control the drying process, integrated with a low cost microwave based drying system that can dry a diverse range of polymer types. These would be joined together by an intelligent control module to enable closed loop control of the continuous measure-dry process and integration with the downstream process.

The project aimed to improve the quality of life of all European Union (EU) citizens by enabling plastic processors to rapidly and effectively identify and control polymer moisture levels thereby dramatically reducing scrap and rejected products which would reduce the amount of material to be granulated thereby reducing the time operators are exposed to the dangers of granulators reducing fatal and major injuries in the plastics processing sector.

The technical objective was to reduce energy requirement of the drying phase of polymer processing, increased productivity through savings in drying time and reduced waste by eliminating faults related to residual moisture as a consequence of inadequate drying. The result of this innovation would be a more competitively priced service and supply of plastic products from EU plastic processors competing in a global market.

The technical work over the reporting period has been spread over the tasks in Work packages 1, 2, 3, and 4:
- WP 1: Enhanced scientific understanding of polymer and microwave interaction
- WP 2: Moisture content sensing development
- WP 3: Microwave drying development
- WP 4: Integration and industrial trials.

The theory of micro-waving was investigated and compared with conventional thermal processing and it has been determined that microwaves belong to the portion of the electromagnetic spectrum with wavelengths from 1 mm to 1 m with corresponding frequencies between 300 MHz and 300 GHz. The two most commonly used frequencies are 0.915 and 2.45 GHz. The theory also shows that the two main mechanisms by which microwaves produce heat in dielectric materials are ionic polarisation and dipole rotation and microwavability of thermoplastic polymers mainly depends on the dielectric constant and the tangent loss of the materials.

The material properties and chemical structure of polymers were investigated with regard to the transportation of water and their dielectric properties. Trials concerning the microwave absorption have been carried out and recommendations on microwave system requirements for moisture measurement and removal have been developed. Initial trials to prove that water molecules are bound to polymer have been carried out on a number of selected polymers. The drying techniques used were: microwave drying, conventional oven drying and traditional dehumidifying drying. All the results were plotted and the results clearly prove that micro waving polymers helps in moisture loss. Also the percentage weight was found to show some linearity with respect to time of micro waving.

Two sensor types were identified as most suitable for further development: a so called open resonance technique and a cylindrical resonance pipe. The first type is very easy to assemble; the second could achieve a better accuracy. The contact with the manufacturer TEWS / Germany was intensified who agreed to develop conjointly a system with improved accuracy so that it would fit with the projects demands.

Several polymers of different families were dried, prepared and trialled. Conventional moisture detection was carried out with these samples by Karl-Fischer-Titration and an ABONI-system to calibrate the microwave data. Some of the trials could not identify a clear relation between moisture content and microwave signal. TEWS / Germany could calibrate their systems to deliver a reasonable signal. Several tests with two systems and three types of polymer at different drying stages were carried out. At the moment an accuracy of 0.03 % moisture content can be achieved but at a very low signal level.

The development of the moisture measurement software and the drying software a Software requirement specification (SRS) document has been created to describe the two first steps 'requirement analysis' and 'high level design' of the V-cycle model which has been chosen to develop the software as this is a standard and a recognised approach to develop software. The SRS document has been used to enumerate the feature requirements of the moisture measurement and drying software, regarding the description of work and input from the partners. Preliminary Graphical user interfaces (GUI) have been built based on the features list, in order to give an idea of what the software should look like to the future end user.

A number of extensive moisture measurement trials were carried out on several polymer families; however, it was concluded that the necessary accuracy would not be achievable with a microwave based system. The consortium agreed to concentrate the work on the drying system and to use a new offline solution which was tested and resulted with the required moisture measurement accuracy, and the results are reproducible and stable.

Initial trials successfully dried different raw materials in a 'lab based' micro wave system and determined that the plastic granulate should be dried using micro waves to heat up the moisture in the plastic granulate but it should be done under vacuum and with a Infrared (IR) sensor temperature control build into the process to provide continuous measurement of the temperature in the plastic raw material.

Design ideas for the 'pilot' microwave drying system were produced and an outer chamber and inner drum concept were successfully reviewed and a three-dimensional (3D) concept model developed. The positioning of the magnetrons was critical for good distribution of the microwave field pattern. The spacing of the magnetrons and their individual alignment to the chamber was determined by using simulation software to analysis of the microwave field in the prototype 3D model.

A detailed costing spreadsheet was produced which calculated that the overall estimated cost for the build of the POLYDRY prototype system was acceptable; however, this cost will be significantly reduced when the system is designed for manufacture and volume production set-up.

Initial validation trials concentrated on the functionality tests for the systems components and fine tuning of the system that reduced the reflected power to < 2 % and achieved an efficiency of 80 % with each magnetron. Water load tests were carried out which concluded that the functionality of the microwave components had been successfully demonstrated.

The overall project objective was the development of a new low cost, in-line, rapid, energy efficient polymer microwave based moisture measurement and drying system where the key requirements would be:
- a real-time microwave moisture measurement system to control the drying process;
- an integrated low cost microwave based drying system that can dry a diverse range of polymer types;
- an intelligent control module to enable closed loop control of the continuous measure-dry process and integration with the downstream process.

The key operation targets to develop a prototype microwave drying system are outlined below:
- provide a microwave generator capable of achieving the drying of polymer granules passing through a continuous feed system;
- controlling a drying process of the specimen as a function of the changes of the monitored microwave energy;
- provide relative and absolute moisture detection down to 0.01 %, 10 times / second, for 95 % of the polymer types on the market;
- provide accurate drying for 95 % of the polymer types on the market to moisture levels of 0.01 %;
- provide sufficiently accurate process control of the moisture vaporisation process to avoid clumping of the polymer particles due to melting;
- provide drying of 100 kg of polymer per hour at a maximum output rate of 1.6 kg per minute;
- achieve polymer drying with a 50 % reduction in energy usage compared to conventional hot air drying systems;
- provide a continuous drying facility, which meets safety requirements for microwave shielding to current safety standards;
- provide continuous polymer drying facility, which can be, electronically and mechanically interfaced with existing industry standard materials handling equipment;
- support the data storage of material drying parameters for quality assurance systems and traceability requirements and has a built in reference database covering 50 of the most commonly used polymer types, while supporting the definition of a material traceability setting for 1 polymer type within 5 minutes;
- have a total cost of less than EUR 20 000 per unit to the Small and medium-sized enterprise (SME) plastic processors.