PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN99044/1372
Microscale Adsorption for Energy and Chemical Systems
Scot D. Rassat, Donald P. Mendoza, Dean W. Matson, Dustin D. Caldwell
Microscale gas adsorption is useful for a wide array of energy and chemical processing applications. Applications
include carbon dioxide and carbon monoxide scrubbing for fuel processing in fuel cells and for carbon management in
combustion processes.
Project Description
With rapid cycling and continual regeneration of an
adsorbent in a microscale adsorber, the mass of sorbent
needed to treat a given volume of feed gas can be reduced
significantly from conventional processing schemes. In
this study, microscale adsorbers were successfully
designed, fabricated, and tested in rapid thermal-swing
adsorption experiments. The devices incorporated a
single adsorbent channel and integrated heat exchangers.
Several metal, plastic, and metal-plastic composite
adsorbers were fabricated. Plastics (polyimides) were
used to reduce adsorber mass and thermal capacity
relative to all-metal units. Adsorption and desorption
cycles of about 1-minute were readily attained with the
various devices. Adsorber temperature and carbon
dioxide desorption rate data suggest that the
adsorption/desorption cycle frequency is heat-transfer
limited, not mass-transfer limited. We also observed that
the adsorbent in the metal-plastic adsorbers heated more
quickly than in the all-metal and all-plastic devices and,
as a result, released the stored volume of carbon dioxide
more quickly during desorption (heating). Using
conventional zeolite 13X adsorbent, the test devices were
very effective at capturing carbon dioxide. Adsorbent
working capacities as high as 93% of theoretical values
were obtained for all-metal units and up to 62% of
theoretical was measured for plastic-containing adsorbers.
The variation in working capacity for the devices is
thought to be due to differences in preconditioning and
partial water poisoning of the adsorbent.
Introduction
Adsorption is one of many important industrial gas
purification technologies applicable to the separation of a
wide range of gas species (Kohl and Nielsen 1997).
Carbon dioxide (CO2) capture and sequestration is of
concern, and novel adsorption processes are of interest for
this application (Reichle et al. 1999). The reduction in
adsorbent mass achievable in rapidly cycled microscale
342 FY 2000 Laboratory Directed Research and Development Annual Report
adsorbers is of potential benefit in all gas collection or
purification applications where size and mass are a
premium, such as man-portable systems, vehicle
applications, or analytical applications. Commensurate
with reduced adsorbent mass, the size and mass of other
adsorber components shrink as well. A further reduction
in overall device mass can be achieved by incorporating
relatively lightweight materials, such as thin metals or
plastics. Lightweight and low heat capacity materials can
also reduce the excess thermal mass of an adsorber. In
the preferred limit, only the adsorbent would be cooled
and heated during thermal cycles. All other thermal mass
that is heated or cooled during thermal cycling is energyinefficient.
We have developed and evaluated a series of
metal, plastic, and metal-plastic composite microscale
adsorbers seeking to minimize thermal mass.
Approach
In conventional adsorption-based gas purification
processes, adsorbent bed loading and unloading cycles are
typically on the order of hours (Kohl and Nielsen 1997).
In microscale devices, the cycle times can be reduced to
minutes or less, resulting in a time-scaled reduction in the
mass of adsorbent needed to process an equivalent gas
volume. The faster cycle times allow a small adsorbent
mass to be regenerated more frequently and used more
effectively. For example, a process requiring 25 kg
adsorbent in an 8-hour cycle would use only 0.1 kg
adsorbent in a 2-minute cycle.
We have developed microscale adsorbers for rapid
thermal-swing adsorption processes, as depicted
schematically in Figure 1. When the adsorbent bed is
cooled, the capacity for the target gas species (such as
CO2) is relatively high, and when the bed is heated, some
fraction of the sorbed species is evolved from the
adsorbent. The difference in adsorbent capacity between
the cooled and heated states represents the working
capacity per cycle. The theoretical working capacity may
be determined from adsorption isotherms (Trent 1995).