Abstract
Capillary microseparators have been gaining interest in downstream unit operations, especially for pharmaceutical, space, and nuclear applications, offering efficient separation of two-phase flows. In this work, a detailed analysis of the dynamics of gas–liquid separation at the single meniscus level helped to formulate a model to map the operability region of microseparation devices. A water–nitrogen segmented flow was separated in a microfabricated silicon-glass device, with a main channel (width, W = 600 μm; height, H = 120 μm) leading into an array of 276 capillaries (100 μm long; width = 5 μm facing the main channel and 25 μm facing the liquid outlet), on both sides of the channel. At optimal pressure differences, the wetting phase (water) flowed through the capillaries into the liquid outlet, whereas the nonwetting phase (nitrogen) flowed past the capillaries into the gas outlet. A high-speed imaging methodology aided by computational analysis was used to quantify the length of the liquid slugs and their positions in the separation zone. It was observed that during stable separation, the position of the leading edge of the liquid slugs (advancing meniscus), which became stationary in the separation zone, was dependent only on the outlet pressure difference. The trailing edge of the liquid slugs (receding meniscus) approached the advancing meniscus at a constant speed, thus leading to a linear decrease of the liquid slug length. Close to the liquid-to-gas breakthrough point, that is, when water exited through the gas outlet, the advancing meniscus was no longer stationary, and the slug lengths decreased exponentially. The rates of decrease of the liquid slug length during separation were accurately estimated by the model, and the calculated liquid-to-gas breakthrough pressures agreed with experimental measurements.
Highlights
Capillary-based separators are currently seeing a significant growth in their implementation for downstream separation of two-phase flows
The pressure difference (ΔPGL), which is the difference between the gas outlet pressure (PG at P2 in Figure 1) and the liquid outlet pressure (PL at P3 in Figure 1), was controlled with the help of BPRs and monitored using pressure sensors connected to LabView via a data acquisition board
We observed that at stable separation conditions, the advancing meniscus became pinned in the separation zone, whereas the receding meniscus approached it at a constant speed, giving rise to a linear decrease of the liquid slug length
Summary
Capillary-based separators are currently seeing a significant growth in their implementation for downstream separation of two-phase flows. Capillarity-driven separation can be exploited for their high efficiency and accurate spatiotemporal control afforded by advancements in microfluidics.[1,2] Much of its application has been in microchemical reaction systems,[3] where, for example, a gaseous component in a reaction requires a gas−liquid separation step to isolate the two phases. Such devices have been applied for liquid−liquid extraction,[4−10] gas−liquid separation in fuel cells,[11] and safe handling of toxic or carcinogenic molecules.[12] Capillarity-driven separators utilize either membranes or microfabricated capillaries to separate the phases. For a typical hydrophilic substrate (e.g., silicon or glass), the wetted phase (e.g., water) flows through these capillaries, whereas the nonwetting dispersed phase (gas) is prevented from entering the capillaries due to the capillary pressure.[19,20] Liquid−liquid separation can be achieved through a similar methodology[21] by taking advantage of the differences in the wetting properties of the two liquids
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