Figure 1. Side-by-side view of the three columns. (A) The traditional packed column with Heli-Pack packing. (B) The plate-type layered (PTL) device alongside sample layers of liquid wick and vapor shim. (C) The additively manufactured porous honeycomb (AMPH) column.
All three columns were built with an active height of 25.4 cm and were operated at 1 atm. Swagelok fittings and additional tubing were welded to each column to provide a liquid inlet, an overhead vapor outlet, and a bottoms liquid outlet. The oxygen-rich reboiler liquid was withdrawn, and the composition determined using a residual gas analyzer. The reboiler liquid level was maintained by monitoring the temperature at different reboiler positions, and reboiler power was supplied via a 20 W electrical heater attached to the outside or the reboiler.
Previous cryogenic experiments with PNNL MCD microwick devices were conducted in an insulated cold box, with the required operating temperatures maintained by periodically spraying liquid nitrogen inside the box. This required significant quantities of liquid nitrogen; one 24‑hour experiment consumed multiple 180 L dewars. It also made simultaneous control of both the cold box temperature and the condenser duty difficult. To avoid these difficulties, in the present work each distillation system was placed inside a vacuum can, which provides more efficient insulation. A Stirling Cryocooler (SunPower CryoTel® GT) was used as a cooling source, rather than liquid nitrogen.

Traditional Random Packing (TRP)

The traditional packed column was a vertical cylinder with an inside diameter of 1.09 cm. It was filled with stainless steel Heli‑Pak packing distributed randomly. Liquid entered near the top of the column and flowed downward through the packing. This TRP device is representative of a typical small, packed column and served as a benchmark against which to compare the other two columns.

Plate-Type Layering (PTL)

The PTL column was built in the shape of a rectangular prism, with inner dimensions of 1.9 cm x 1.9 cm. It used an arrangement similar to that of the previous PNNL microwick device; however, unlike previous PNNL microwick experiments this device was operated in a vertical, as opposed to horizontal, direction. Internally, it was configured with a specific arrangement of three different types of materials: thicker vapor channels (0.05 cm), thinner liquid wicks (0.01 cm), and fine screens (~38 μm). The vapor channels and liquid wicks were expanded metal screens, and the materials were layered as shown in Figure 2, with each layer extending from the top to the bottom of the column.