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.