PNNL-13501 - Pacific Northwest National Laboratory

Figure 2 demonstrates the rapid thermal-swing capability
for an all-metal microscale adsorber in a series of
1-minute cycle heating and cooling curves. (In separate
tests, it was determined that the heat-exchange surfacemeasured
temperatures depicted in the figure are
representative of the zeolite bed temperature to within
1 to 2°C.) As the heat-exchange fluid flow rate was
increased from 20 to 80 mL/min, the maximum and
minimum adsorber temperatures approached the hot
(70°C) and cold (5°C) reservoir temperatures. A larger
temperature differential between adsorption and
desorption cycles increases the zeolite working capacity,
and therefore a higher adsorbent working capacity is
expected as the water flow rate is increased. This was
verified experimentally. Figure 2 also shows that the
approach to the maximum (or minimum) temperatures is
faster with increasing heat-exchange fluid flow rate. The
heating curves were fit to exponential decay functions,
and the exponential time constants were estimated. The
time constants were approximately 6, 9, and 19 seconds
for water flow rates of 80, 40, and 20 mL/min,
respectively. These data validate, from a heat transfer
perspective, the potential for rapid thermal cycling in
microscale adsorbers.
Adsorber Temperature (C)
70
60
50
40
30
20
10
0
0 20 40 60 80 100 120
Elapsed Time (s)
Heat Xfer Fluid
Flow Rate
(mL/min)
Figure 2. Effect of heat exchange fluid flow rate on
temperature profiles for a metal microscale adsorber in
1-minute adsorption and desorption cycles
Both heat and mass transfer must be effective for rapid
adsorption/desorption cycles to work well. Figures 3 and
4 demonstrate the simultaneous heat and mass transfer
effectiveness for metal, plastic, and metal-plastic
composite microscale adsorbers. Figure 3 shows
measured bed temperatures in an all-metal device for a
series of 0.5-, 1-, 3-, and 5-minute adsorption/desorption
cycles. The figure also shows the volume of gas
measured at the end of each desorption cycle (open
circles). In the tests, water flowed through the heat
exchanger at 80 mL/min, and the hot and cold reservoirs
344 FY 2000 Laboratory Directed Research and Development Annual Report
20
40
80
were set to 90° and 5°C, respectively. Pure CO2 was fed
to the zeolite at the rate of 50 mL/min during the cooling
cycles, and the feed stream bypassed the adsorber bed
during desorption cycles (Figure 1). The desorbed CO2
volume consistently reached 46 mL for longer cycle
times, fell slightly (~42 mL) in 1-minute cycles, and
dropped significantly (~22 mL) in 0.5-minute cycles. The
recovered volumes in the faster cycles were limited, at
least in part, because of the lower temperature
differentials attained in the cycles. A primary limitation
for the 0.5-minute cycles was the feed gas flow rate; only
~25 mL CO2 was delivered to the bed during the
adsorption swing. (Tests with higher feed flow rates
resulted in larger recovered gas volumes for 0.5-minute
cycles.) The theoretical working capacities, based on
measured temperature differentials, were ~52 mL for the
3- and 5-minute cycles and ~47 mL for the 1-minute
cycle. Therefore, better than 80% of the theoretical
working capacity was achieved in each of these cycles.
Factors affecting the actual working capacity are
discussed later in this section. Figure 3 also indicates
excellent cycle-to-cycle consistency—for example,
volumes evolved in 3- and 5-minute cycles late in the
sequence were identical to those measured early on.
Adsorber Temperature (C)
90
80
70
60
50
40
30
20
10
0
0 5 10 15 20 25 30 35
Elapsed Time (min)
Figure 3. Temperature profile and desorbed gas volumes
(open circles) for a metal microscale adsorber in a series of
adsorption and desorption cycles
Figure 4 compares the thermal and mass transfer
performance of three different microscale adsorption
devices during desorption cycles. In all cases, water was
delivered to the adsorber heat exchangers from a 90°C
reservoir at 80 mL/min. Figure 4a indicates a somewhat
lower rate of temperature change in the all-plastic device
than in the all-metal and metal-plastic composite devices.
However, after ~70 seconds the temperatures in the allplastic
device match those in the all-metal device, and at
longer times the temperatures in the all-plastic device
50
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25
20
15
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5
Gas Volume Desorbed (mL)