Microwave Drying of Paper Documents

A new advanced microwave drying technology has been developed using a continuous dryer combined with an air cooler. Principle of the new technology consist in using microwave absorption filters made from special porous ceramics which act as both absorbent and transparent material for microwaves. Ceramic slabs reduce intensity of microwave radiation to such a extent which prevents overheating or creating hot-spots or other kinds of damages including red-heating of metallic objects. The technology has been tested on 2200 flooded books and documents of historical value within one month. All books and documents were recovered in high quality without any damage of gold printing, collored reproductions, metallic containing objects and other components sensitive to microwaves. Moreover, the technology also proven to kill moulds and their spores and therefore an additional disinfection was not necessary. INTRODUCTION In recent years Central Europe suffered from extensive flooding. In August 2002 in Prague about 70 libraries and institutions were flooded. Thanks to enormous help of volunters 3 thousands of cubic meters of documents, books and archival records were washed and frozen in plastic bags waiting for drying and restoration. Drying of all these documents would take decades using current drying technology, e.g. vacuum freezing (lyophilization) or hot wet air drying. In last decades we can see an increased interest in using microwaves to augment or replace existing conventional drying methods with microwave systems [1,2]. The concept of using the microwave drying has always been appealing and is currently used for drying of textile, leather, cotton, ceramic, tobacco, wood, pharmaceuticals, etc. It is known that microwave drying provides extremely high rate and it is energy saving compared to the current conventional technologies. However, the microwave drying of paper materials has met many technical problems. For instance, local heat damage of paper caused by metal objects presents a serious problem. Moreover, microwave field causes irreversible damage of the documents when so called hot-spots are created within the body of document. Metallic objects turn into lightening rods and they heat paper to extreme temperature. Therefore, according this method, they had to be detected an excluded from the drying process [3]. Similarly gold printed letters can be damage and evaporated by microwaves. Consequently some companies, e.g. BATELLE, Germany, decided to stop this research and after one year of running microwave drying they turned back to conventional drying technique using reduced pressure and heating [4]. According to our knowledge, the microwave drying technology of paper materials has not been comercialized up to now. PRINCIPLES AND MECHANISM OF DRYING Conventional drying Conventional drying process proceeds from the surface to the center and it depends on thermal conductivity of material which is in case of paper very low. The limit factor is the speed of diffusion of water to the surface, where is evaporated. Regarding this capillary diffusion, conventional drying is slow, especially at the end of drying process, Fig. 1. Figure 1. Mechanism of conventional drying Microwave drying Microwave drying proceeds by completely different mechanism, i.e. in completely opposite direction. Microwaves interact directly with the water molecules, create an internal heat in the center producing vapor pressure and therefore pressure gradient which „pumps“ water to the surface where is removed by hot air, Fig. 2. Figure 2. Mechanism of microwave drying PROBLEMS OF MICROWAVE DRYING After extensive flood in 2002, a project to develop fast and efficient drying of paper documents has been started to provide dried documents in a high quality. For this goal the following problems had to be eliminated: 1. Overdrying which causes reduction of lifetime of documents 2. Creation of hot-spots, i.e. local superheating leading to burning of paper 3. Red-heating of metallic objects (staples, wire stitches etc.) 4. Evaporating or burning of gold printing 5. Damage of coloured drawings and photographs 6. Elimination of non-homogeneity in interaction of microwave with different materials like glue, leather, plastics etc. The problem of non-homogeneity of microwave field is generally known and it is a cause of most unsuccessful trials. In addition, it is almost impossible in a batch system to stop the drying in a right moment because if there is not enough moisture, fast excessive drying of paper starts immediately, Figs. 3,4. Figure 3. Example of document damage by overheating and hot-spots in a batch dryer Figure 4. Example of document damage by red-heating of metal staples in a batch dryer DESCRIPTION OF ADVANCED MICROWAVE DRYING TECHNOLOGY To solve out all the above problems of drying of paper documents we had to come with a completely new idea. We have installed on the bottom and on the top of the documents a special accessories, called absorption filters. They are special porous ceramic materials which partially absorb microwaves but they are also partially transparent for microwaves. They reduce intensity of microwave radiation to such an extent that is sufficient for drying of moisture but not sufficient for overheating or creating of hot-spots or cause other kinds of damages including red-heating of metal objects. Microwaves penetrate through the ceramic filters with a reduction of high intensity spots and are preferentially absorbed by the moisture creating internal regions of high pressure and forcing moisture to the surface. Then moisture is absorbed by porous ceramics and evaporates very fast from the surface in the form of steam even though the drying temperature is much lower than 100 C and does not exceed 60-70 C, Fig. 5.