Microwave Drying of Expanded Perlite Insulation Board Blaz Skubic, † Mitja Lakner, ‡ and Igor Plazl* ,§ † Trimo d.d., Prijateljeva 12, SI-8210 Trebnje, Slovenia ‡ Civil and Geodetic Faculty, University of Ljubljana, Jamova 2, SI-1000 Ljubljana, Slovenia § Department of Chemical Engineering, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, SI-1000 Ljubljana, Slovenia ABSTRACT: The microwave drying process intensification of new inorganic thermal insulation board, containing expanded perlite and inorganic binder, was investigated in a multimode microwave applicator, with a 1.8 kW, 2.45 GHz microwave source. Ability to generate heat inside moistened material and to avoid the problem of heat conduction to the core of the material is especially important when dealing with efficient thermal insulators. A 3D unsteady mathematical model was developed, considering Lambert’s law of exponential decay of microwave energy and nonuniform power distribution inside the microwave cavity, for temperature and drying rate predictions. The predicted temperature profiles at critical locations of the exposed board and cumulative drying rate were compared and verified with experimental data. 1. INTRODUCTION Inorganic insulation materials represent the majority of insulation products used in Europe, with approximately 60% market share. 1 The most extensively used are fibrous materials, mineral and glass wool, but others like foam glass, vermiculite, and expanded perlite also represent an important part, especially in special applications. Besides the construction industry, expanded perlite can also be used in industrial, chemical, horticultural, and petrochemical industries. 2 Perlite is obtained from pumice, which is a glassy form of rhyolitic or dacitic magma. It contains from 2 to 6% water, which causes expansion of crude ore during rapid heating to temperatures above 900 °C. 3 Water inside the ore produces steam, and this forms bubbles within the softened rock to produce a frothylike structure. 4 The process causes a volume expansion from 15 to 20 times of the original ore volume. Reports on a number of different mixtures have been published and patented, containing expanded perlite as a basic material for insulation product production, with multiple different binders, organic and inorganic. Physical and mechanical properties of perlite insulation products can vary in a density range from 110 to 240 kg/m 3 , with heat conductivity from below 0.06 W/m K to as much as 0.36 W/m K. 5-8 The conventional perlite insulation board is made of an aqueous slurry of expanded perlite particles, binder, and additives (fibers, waterproofing agents, fire retardant, etc.), that is formed into a board by a Fourdrinier process and subsequently dried. Another type of board production is suggested, with mixing expanded perlite particles and sodium or sodium-potassium silicate, pressing the mixture in a mold and drying the pressed board. Conventional and microwave techniques are possible for drying such a board. The common limitation for all conventional drying methods is that, regardless of the principal mode of heat transfer from the heating medium to the surface of the board, the transfer from the surface to the inside of the board is principally by conduction. Since such a mixture is an efficient thermal insulator, the conduction of heat into the core of the thick board is very slow, so the process for very thick boards takes a great amount of time. Microwave drying can, in that case, have an advantage over conventional hot air drying because of the direct effect on water molecules inside the material and consecutively bypassing the surface-to- center conduction stage. Microwave drying has been extensively studied in various applications, including drying of silica sludge, borax, and cotton, and in particular in food processing. 9-13 The study of drying behavior of expanded perlite insulation plates during microwave irradiation presented in this work is related to the industrial project. The technology for the production of a new lightweight building material for sandwich panels includes drying in two stages. In the first stage, the controlled microwave drying allows homogeneous cross-linking of bonding material mixed with perlite and therefore basic mechanical properties of insulation plates. At the end of the process it is necessary to provide water repellency of formed insulation plates, and it is therefore necessary to dry moistened plates again. Especially drying during the process of hydro- phobization significantly affects the economics of the industrial technology. The project research from the laboratory to industrial level has confirmed the advantages of microwave heating from a conventional heating due to the optimum time, energy, and economic efficiency. Especially in the last stages of microwave drying of a perlite insulation board, the problem of rapid temperature rise inside the board can occur. This can cause problems in controlling the temperature at the end of the process, so it becomes very important to stop the process exactly at the time all the moisture has evaporated. In order to predict the occurrence of such phenomena, it is necessary to analysis a mathematical model from which insight might be gleaned into an inherently complex physical process. 14 Mathematical modeling can predict the necessary residence time for microwave drying process and Received: August 11, 2011 Revised: January 4, 2012 Accepted: January 31, 2012 Published: January 31, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 3314 dx.doi.org/10.1021/ie201790w | Ind. Eng. Chem. Res. 2012, 51, 3314-3321