Sc-CO2 impregnering van houtpreserveermiddels
Le Roux, Cornelius Johannes
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The feasibility of impregnating two types of wood (pine, poplar) with two different preservatives (creosote, CCA) by means of supercritical carbon dioxide (sc-CO2) as carrier was investigated in view of the actuality of treatment of wooden poles in electricity distribution networks and in the agricultural sector. Two important quantities were determined prior to performing impregnation runs. The impregnation capacity of the two types of wood was measured by estimating gravimetrically the amount of water and mercury porosimetrically the amount of mercury that can be taken up by the wood. The results were highly comparable and could be used to predict the amount of preservative that could potentially be impregnated. These figures Served as benchmark for efficient impregnation. The solubility of the preservatives in sc-CO2 was measured by means of a supercritical extractor (Isco) in static mode as this quantity was expected to play an important role in the mechanism of impregnation. The impregnation of specially prepared wood samples was performed in a locally manufactured sample holder which fitted an existing supercritical extractor (Varian). The impregnation experiments on laboratory scale were extended to pilot plant scale by using a larger supercritical extractor (Swiss Nova). The wood samples could be impregnated only to a limited extent (creosote: 12% m/m for poplar, 25% m/m for pine; CCA: 7% m/m for poplar, 19% m/m for pine), since microscopic analysis revealed that neither of the two preservatives was capable of occupying the pores of the wood but bound to the wood vessels instead. The pressure dependence of the impregnation suggested that the process determining parameters were pressure and solubility for creosote impregnation and pressure only for CCA impregnation. This conclusion could be supported by surface response graphs. A break strength test was applied to all impregnated samples in order to confirm that the ultrastructure of the wood was not damaged by the high pressures involved. Impregnation was repeated with N2 as carrier, and the results confirmed the superiority of sc-CO2 as an impregnating agent.