Moisture content irreducible

The key to understanding dewatering by air displacement is the capillary pressure diagram. Figure 6 presents an example typical for a fine coal suspension there is a minimum moisture content, about 12%, called irreducible saturation, which cannot be removed by air displacement at any pressure and a threshold pressure, about 13 kPa. 

With impermeable woods - and heartwood - the supply of moisture from the interior eaimot keep paee with evaporation of water vapour from the surface, because mass flow of water is not possible and diffusion is a much slower process. Thus the surfaee moisture eontent quiekly falls below fibre saturation and the evaporative front starts reeeding into the wood. Figure 8.6 shows the parabolic moisture content profile for a slowly air-dried impermeable hardwood. Similarly for permeable softwoods that have been dried below the irreducible moisture content, Stamm (1964, 1967b) reported parabolic moisture profiles that are consistent with diffusion of both water vapour and bound water. 

Even with a permeable wood diffusion assumes increasing importance as the average moisture content approaches the irreducible moisture content indeed, in every part of the board where the moisture eontent approaehes this value drying is diffusion controlled. Permeable and impermeable timbers of similar densities should dry from fibre saturation at about the same rate. The behaviour of mixed heart/sapwood boards is eomplieated sinee, at first, there is both an evaporative interface near the sapwood surfaee and one in the interior at the zonal boundary between heart and sapwood. For a board with only a slither of heartwood along one face, mass flow can only move to the sapwood faee so in effeet the board appears to be twice the width than it aetually is. Pang et al. (1994) predieted that such a 50 mm thick board would dry from green to 6% moisture eontent using a 140°C/90°C schedule in 14 hours, compared to 10 hours for sapwood and 11 hours for heartwood. 

In order to predict gas deliquoring performance on a lull scale filter it is a prerequisite to determine the threshold pressure (pi), the minimum pressure difference that must be applied across a cake to effect any deliquoring whatsoever, and the irreducible saturation (SJ which is the lowest saturation achievable by fluid displacement alone. While the former can be calculated with reasonable confidence, the latter is far more difficult to predict and is best measured in the laboratory test described below. Both quantities are obtainable from the same experiment although the threshold pressure can, on occasion, be a troublesome measurement. The irreducible saturation can sometimes be inferred from the moisture content of a cake discharged from a test filter. 

It has been shown that liquid moisture transfer still exists in the sorption region and is a strong funetion of free water content. Scientists studied gas phase convective transport in the dry region, which contains irreduc-... 

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