Effects of pretreatment with saturated wet air and steaming on the high-frequency vacuum drying characteristics of wood

Abstract

The pretreatment of Mongolian pine (Pinus sylvestris var. mongholica Litv.) was conducted with saturated wet air (80 °C) and atmospheric saturated steam (100 °C) before high-frequency vacuum drying. Effects of the pretreatment on moisture content (MC), drying rate, drying crack, and drying shrinkage strain were investigated. The results showed that through pretreatment, the initial MC of the test material decreased by 2.6% to 6%, and the MC distribution was uniform. The maximum difference of the untreated wood MC distribution after drying was 2%, while that within the pretreated wood was less than 1%. The drying rate also increased, as the drying rates of the untreated and pretreated wood were 0.268%/h, 0.333%/h, and 0.398%/h, respectively. The drying shrinkage strain of the sample was reduced. The drying shrinkage strain of untreated wood was between 0.044 and 0.063, while the drying shrinkage strain of pretreatment wood was between 0.013 and 0.049. The crack of the test material was also reduced, and the drying quality was improved. The pretreatment reduced the high-frequency vacuum drying stress of wood and improved the drying speed and quality.

Production and characterisation of pine wood powders from a multi-blade shaft mill

“…Due to the design of the experimental procedure, the differences in shape factors between the two groups of powders must be due to one of the following effects or combination of them: (i) the drying steps at 105 ºC after milling or (ii) the action of the MBSM blades during milling. Considering the first option, the presence of a MC gradient during drying causes internal stress and leads to wood deformation [236]. Similarly, pine wood particles showed more spherical shape and had greater porosity after rapid devolatilisation [237], although the temperature used in drying was comparatively low.…”

“…Dimensional changes of wood, i.e., shrinkage and swelling, occurs below the MC of the fibre saturation point (FSP) [238]. Drying temperature is an influential factor in shrinkage [236]. Tangential and radial shrinkage of wood veneers can be observed at 7.6 and 9.9 % and 5.4 and 5.7 % during drying at 60 and 150 ºC, respectively [239].…”

“…MBSM powders were more likely to have intact structure at their cell wall levels while HM powders appeared to exhibit more disrupted. Stresses during drying can also enhance deformation or defects in fibre walls [236] and increases the microfibril angle [243]. Both have been observed but HM powder showed comparatively greater effects.…”