Study of Liquid–Solid Mass Transfer and Hydrodynamics in Micropacked Bed with Gas–Liquid Flow

The volumetric liquid–solid (L-S) mass transfer coefficient under gas–liquid (G-L) two-phase flow in a silicon-chip-based micropacked bed reactor (MPBR) was studied using the copper dissolution method and was related to the reactor hydrodynamic behavior. Using a high-speed camera and a robust computational image analysis method that selectively analyzed the bed voidage around the copper particles, the observed hydrodynamics were directly related to the L-S mass transfer rates in the MPBR. This hydrodynamic study revealed different pulsing structures inside the packed copper bed depending on the flow patterns established preceding the packed bed upon increasing gas velocity. A “liquid-dominated slug” flow regime was associated with an upstream slug flow feed. A “sparse slug” flow regime developed with an upstream slug-annular flow feed. At higher gas velocity, a “gas continuous with pulsing” regime developed with an annular flow feed, which had similar features to the pulsing flow in macroscale packed beds, but it was sensitive and easily destabilized by disturbances from upstream or downstream pressure fluctuations. The volumetric L-S mass transfer coefficient decreased with increasing gas velocity under the liquid-dominated slug flow regime and became rather less affected under the sparse slug flow regime. By resolving the transition from the liquid-dominated slug flow to the sparse slug flow and capturing the onset of the gas-continuous with pulsing regime, we gained new insights into the hydrodynamic effects of G-L flows on the L-S mass transfer rates in a MPBR.

Fig. S1: Copper particle size distribution: a) fraction of 53-63 m, used for the measurement of liquid-solid (L-S) mass transfer coefficients with gas-liquid (G-L) two-phase flow; b) fraction of 63-75 m, used for the experiments with single liquid-phase flow.

Micropacked bed packing procedure
The copper particles were packed in the micropacked bed reactor using a simple clamp and a vacuum pump with a standardised procedure for reproducible packing. The outlet of the microreactor was connected to the clamp, which was in turn connected to a vacuum pump. The liquid inlet port of the microreactor was blocked using tape. A small amount of glass beads was first packed to form a ~2 mm bed through the gas inlet port. Approximately 1 mg of copper particles was then introduced into the microchannel. The microreactor was taken off the clamp and weighed before and after packing the copper particles. At each packing step, the particles were loaded through the inlet port slowly and under vacuum with gentle tapping of the clamp, until no change in the bed length was observed. Finally, more glass beads were packed to form a 5-6 mm bed section upstream of the copper bed. Unloading of the particles was performed by following the same procedure, but by connecting the gas inlet port to the clamp and applying vacuum. The particles were sucked out of the channel and a few drops of water were added to clean the channel and dislodge any lodged particles.

Copper particle micropacked bed before and after experiment
Fig . S2 shows the copper packed bed before and after the experiment conducted for measuring the reaction rate of the copper dissolution at a liquid superficial velocity of 0.0149 m/s (highest liquid velocity used in this work) and nitrogen superficial velocities of 0, 0.07, 0.15, 0.3, 0.45 m/s. The particle size and shape did not show significant changes on the axial and radial position after the experiment. The average particle size changed from 56 to 50 m and the bed length decreased by about 17% (from 960 to 800 m).

Fig. S2
: Pictures of the copper particle micropacked bed before and after experiment.

Effect of copper particle size and bed length reduction on the calculation of volumetric mass transfer coefficient
To evaluate the effect of the change of copper particle size and bed length (as shown in Fig.  S2) on the calculation of the volumetric mass transfer coefficient, the following assumptions were used: 1) The packing density did not change significantly due to the change in particle size (this was justified by measuring the packing density of two particle size ranges: 38-46 m and 53-63 m in the microchannel. The packing density was ~3.5% less for the smaller particle bed. 2) The packing length of copper particles was short compared to that of glass beads upstream of the copper bed, and the hydrodynamics was not significantly affected by the reduction of copper particle size and the copper bed length. 3) The copper particle size and the bed length decreased linearly with the consumption of copper. The total copper consumed in five measurements was calculated and used to calculate the reduction of the bed length and particle size. The calculated volumetric mass transfer coefficients with and without the consideration of the reduction of the bed length and particle size are listed in Table S1. It can be seen that the ′ 2 ℎ , and ℎ ′ when taking account of the bed length and particle size change are about 1% lower than those without taking account of the bed length and particle size change.

Observation of pulsing behaviour in the micropacked bed
Side-by-side comparison of the raw high-speed videos of G-L flow (liquid superficial velocity, ul = 0.0075 m/s) in the micropacked bed (dp = 58 µm) recorded at 1000 fps (exposure time = 0.996 ms) and the difference between two consecutive frames in each dataset are shown in the following Supplementary Videos: Video S10 (ie1c00089_si_011.mp4) 4.19 Video S11 (ie1c00089_si_012.mp4) 8.48 The brightness and contrast of the original footages have been adjusted to aid better visualisation and the video playback speed is 20 fps. The G-L flow is from bottom to top in the frames.

Quantitative flow pattern mapping characterised by the relative change in the void area during the gas-liquid flow
Video S1 (ie1c00089_si_002.mp4): showing the relative change in the void area during the G-L flow (liquid superficial velocity, ul = 0.0075 m/s, gas superficial velocity, ug = 3.23 m/s) in the MPBR (dp = 58 µm) of the dataset Video S9 (ie1c00089_si_010.mp4). The video playback speed is 8 fps.