Figure 4. (A) CAD model
of the portion of the MCD system contained within the vacuum can. (B)
CAD model of the custom-built condenser. (C) Image of the MCD system
with the vacuum can lid lying on a flat surface, when running the
orientation is inverted with the column hanging down from the lid.
The incoming air is dried, and CO2 removed using
adsorption cartridges containing 13X molecular sieve material. Mass flow
controllers are used to control the flowrate of the incoming air and the
bottoms stream exiting the reboiler. The condenser—shown in
Figure 4(B)—contains a heat transfer surface custom-machined from a
copper block contained in a stainless-steel housing. It was situated
around and attached to the cold head of the cryocooler, a Sunpower
CryoTel® GT 16 W unit. During testing, the cryocooler cold head operated
around 77 K (‑196°C), at which temperature it provided 16 W of lift (per
manufacturer specifications). The feed entered the bottom of the
condenser and the nitrogen rich gas exited to top of the condenser.
To provide thermal insulation, the heat exchanger, column, and condenser
were all enclosed within the vacuum can. The pressure inside the can was
maintained below 10‑4 torr, and the pieces of
equipment inside the vacuum can were each wrapped in 5‑10 layers of
500 DM cryogenic laminate (multi-layer insulation). Swagelok and flange
fittings were used for all connections. Type K thermocouples were placed
on the inlet and outlet of the various components to monitor
temperatures throughout the system. Opto 22 software was used to monitor
the temperatures, flowrates, and pressures inside the vacuum can, as
well as the pressure of the process fluid in the distillation system.
The power inputs to the cryocooler and the reboiler were also modulated
using Opto 22. A vacuum pump was connected to the reboiler for
continuous removal of liquid. The flow rate was measured by a mass flow
controller and verified with a Mesa Labs Definer 220‑L DryCal. A small
sample of the reboiler vapor was periodically withdrawn and sent to the
Dycor residual gas analyzer for compositional analysis. The residual gas
analyzer pulls samples at vacuum pressures, so the gas was assumed to be
ideal. Consequently, the total pressure \(P\) was assumed to equal the
sum of the individual species partial pressures \(P_{i}\), and the
species mole fractions \(y_{i}\) were assumed to be directly
proportional to the species partial pressures (i.e.,\(y_{i}=\frac{P_{i}}{P}\)).
Process Flow
Atmospheric air was first processed through the adsorption cartridges.
The CO2-free dry air is then pre-cooled in the
recuperator inside the vacuum can. The cooled dry air then flows into
the condenser and was partially condensed. The uncondensed
vapor—mostly nitrogen—flows back through the heat exchanger. The
condensed liquid collects in the bottom of the condenser and then flows
into the column, passes down through the column, and is eventually
collected in the reboiler. Along the way, it interacts continuously with
the upward moving vapor produced in the reboiler. The less volatile
components—primarily oxygen and argon but also other heavy components
such as xenon—became concentrated in the liquid, while the more
volatile nitrogen becomes concentrated in the vapor. A portion of the
liquid in the reboiler is periodically withdrawn and sampled.
Operation
During startup, the reboiler is turned off while waiting for the system
to cool down to operating temperatures. This cooling process took a
couple of hours. All three columns were tested under similar conditions
with maximum reflux to compare separation efficiency. For these tests,
the feed air flow rate was fixed at 1 SLM (standard volumetric flow) and
the flow rate of the bottoms liquid product was kept below 0.01 SLM
(also standard volumetric flow). The AMPH device was also tested to
determine the maximum flow of oxygen-rich product (minimum mole fraction
0.90) that could be produced by the equipment. For these tests the feed
air flow rate was fixed at 5 SLM and the bottoms liquid product flow was
adjusted from 0.140 to 0.400 SLM in increments of 0.054 SLM.
Modeling
The MCD system was modeled using process simulation software, the
flowsheet for which is shown in Figure 5. This is a very simple
model, consisting of only a heat exchanger to model the recuperator and
a distillation column to model the MCD column.