Introduction

Air separation refers to the process of separating air into its primary components. The motivation is usually to provide concentrated streams of oxygen and nitrogen. In some cases, the recovery of argon and other noble gases is of interest. For example, there is an increasing interest in developing air separation processes to collect xenon from the atmosphere for large-scale physics experiments.1 Air separation processes are also a vital part of radioxenon monitoring systems used in the International Monitoring System for detecting nuclear explosions.2 The work presented here is motivated by the desire to develop small energy-efficient air separation equipment that can be used to produce pure gasses from the atmosphere including oxygen, nitrogen, argon, and xenon.
A variety of processes are currently used for air separation, including cryogenic distillation, temperature and/or pressure swing adsorption, and membrane separation.3-7 Adsorption and membrane-based processes are often used at smaller scales but are generally less energy efficient than large cryogenic distillation plants used for large-scale production. Here we explore the miniaturization of cryogenic distillation to provide a small energy-efficient air separation method that favorably competes with other small-scale air separation processes and systems. Importantly, distillation is a versatile process that can be tuned for the collection and purification of various gas species present in air, as well as other separations important to industry.
A traditional distillation column contains a series of horizontal trays spaced at regular intervals along the column. These trays provide regions for the liquid to pool and contact the vapor phase, with more-volatile components becoming concentrated in the vapor and less-volatile ones becoming concentrated in the liquid. Alternatively, the trays can be replaced with packing material of various shapes and sizes, with vapor-liquid contact occurring more regularly along the column, rather than only at trayed intervals.
A fundamental concept in distillation columns is the ideal stage. This is the separation that is achieved when a vapor and liquid are contacted and the components in these phases are allowed to partition and come to equilibrium. The trays in a trayed column allow for vapor liquid contact; however, imperfect mixing and mass transfer constraints may prevent full equilibrium from being reached on each tray. Thus, the separation achieved in a trayed distillation column is lower than that expected if each tray was an equilibrium stage. One measure of efficiency \(\left(\eta\right)\) in a trayed column is:
\(\eta=\frac{N_{\text{stages}}}{N_{\text{trays}}}\)(1)
where \(N_{\text{stages}}\) is the number of ideal stages required to achieve the measured separation, and \(N_{\text{trays}}\) is the actual number of column trays.
In packed columns, the lack of physical trays requires a different metric for efficiency. A frequently used performance metric is the height equivalent of a theoretical plate (HETP ), or the length of packing required to achieve the equivalent of one ideal stage. It is defined in terms of the height of the packing in the column\(\left(H\right)\) and \(N_{\text{stages}}\) as shown in the equation below:
\(\text{HETP}=\frac{H}{N_{\text{stages}}}\). (2)
One means of reducing the HETP (i.e., increasing efficiency) is to increase the interfacial area between the liquid and vapor phases. Another method for reducing the HETP is to decrease the time required for molecules to diffuse and equilibrate between the liquid and vapor phases. Because diffusion in liquid is slower than gas phase diffusion, diffusion through the liquid can be the rate-limiting step that slows the approach to equilibrium. Liquid phase diffusion time can be minimized by maintaining well-dispersed high area liquid flow paths throughout the column that are as thin as possible. Distillation devices that seek to maintain these channels at widths below around one millimeter have come to be known as “microchannel distillation” (MCD) devices. The present work is focused on examining the efficiency of two different MCD device concepts and comparing the performance to a more traditional column packing.
MCD has already been applied to various systems to achieve low HETP values. For example, Ziogas et al.8 used traditional machining and stainless steel layering to achieve an HETP of 1.08 cm in the separation of iso-octane from n-octane. MacInnes et al.9 used centrifugal forces to achieve an HETP of 0.53 cm in the separation of 2,2‑dimethylbutane from 2‑methyl‑2‑butane. For a more comprehensive list, refer to various literature reviews.10‑11
Additive manufacturing (AM, or 3D printing) is an enabling technology for new distillation column and packing designs. Features can be constructed down to the micrometer scale, and structures can be designed that allow for intimate contact between vapor and liquid phases. Some exploration of AM with distillation has already been performed. Mardani et al.12 constructed a coil-shaped distillation column using AM and applied it to the separation of cyclohexane from n-hexane. Neukäufer et al.13 began by designing various structured packings via AM, and then applied some of these14 to achieve HETP values of 20-25 cm in the separation of cyclohexane and n-heptane. The column used had a height of 2.45 m and a diameter of 50 mm.
Pacific Northwest National Laboratory (PNNL) has previously demonstrated the ability to carry out effective separation in MCD devices using patented microwick technology.15-18,19-23 This technology employs thin, porous wicks that are ~100 μm thick, and are alternately stacked between adjacent vapor channels. The liquid in these columns flows by surface tension forces (capillarity), rather than gravity. PNNL first demonstrated distillation with this technology to remove heavy sulfur species from JP8 jet fuel24 at temperatures above 200°C, with estimated HETP values of 1.8 cm.25
One challenge that has been observed with MCD has been the difficulty of maintaining low HETP values at low temperature. For example, Velocys, Inc.26 applied MCD to the separation of hexane from cyclohexane at temperatures around 70°C and achieved an HETP less than 1 cm. When the same device was applied to the separation of ethane and ethylene at around ‑10°C the HETP values doubled. TeGrotenhuis and Powell18 applied the microwick technique to a horizontal column to separate 3‑methylheptane from n‑octane at temperatures around 120℃, with reported HETP values as low as 0.33 cm (\(N_{\text{stages}}\) ~ 31). When Bottenus et al.15 used the same device to perform cryogenic distillation of propane and propylene at around ‑50°C, the HETP value increased by a factor of three to 1.0 cm. A subsequent study by Bottenus et al.16 reported HETP values as low as 0.42 cm (\(N_{\text{stages}}\) ~ 60) in the separation of the different carbon isotopes in methane, but performance was still inferior to the theoretical limit of 0.1 cm.
In this work the efficiency of cryogenic air separation is tested using three different small-scale distillation columns. The performance of a random packed column is compared to the performance of two microchannel distillation columns that use thin wicking structures and gas flow channels to achieve process intensification. The HETP values for each column are compared.