orals 5th Asia-Pacific NMR Symposium 2013

A method for studying the dewatering of wood using supercritical CO₂ (scCO₂) has been previously described [1]. A MR compatible high-pressure cell (PEEK, 20 MPa) and associated ¹H/¹³C RF coil were designed and built to operate in a Bruker Biospec 7T system [2]. FLASH imaging [3] was used to acquire ¹H images while chemical shift imaging of a single slab (6.3 mm) was performed using a 7x7 matrix reconstructed to 32x32 for a 20x20 mm FOV. 1800 averages were acquired with a spectral width of 29 kHz.

¹H images during the first and fourth pressure cycles is shown in Figure 1 [4]. These show the distribution and redistribution of water during scCO₂ drying both above and below the supercritical pressure (P_crit 7.4 MPa, T_crit 30.1 °C). 

Fig. 1 ¹H image series (A) initial 0.1 MPa, (B) 6 MPa, (C) 20 MPa, (D) 15 MPa, (E) 6 MPa, (F) 0.1 MPa of cycle 1. (G) 20 MPa, (H) 6 MPa, (I) 0.1 MPa after 4 cycles.

The ¹³C chemical shift image (Figure 2) shows the ingress of supercritical CO₂ during the first and fourth pressure cycle. During the first cycle when the wood is green (fully water saturated) ingress of scCO₂ was confined to the periphery of the specimen. After cycle 4 when the specimen had dewatered to close to the fibre-saturation point and there was little lumen water remaining, the scCO₂ was able to fully penetrate.

Fig. 2 ¹³C chemical shift images during supercritical CO₂ dewatering of a green radiata pine sapwood specimen at 20 MPa after (A) one cycle and (B) four cycles.