The results of this testing indicate that the agitated low-temperature plasma reactor system successfully converted carbon, hydrogen, and nitrogen into gaseous products at residence times that were about ten times shorter than those achieved by stationary processing. The inorganic matrix present was virtually unchanged by the processing technique. It was concluded that this processing technique is feasible for use as part of a close-looped processing resource management system

Hetland, Melanie D.

Jones, Frank A.

Rindt, John R.

Sauer, Randal S.

English

NASA Technical Reports Server

N91-22174
LOM-TENPERATUREPLASNATECHNOLOGYAS PARTOF A CLOSED-LOOP
RL_OURCENANAGENENTSYSTEH
Melanie D. Hetland, John R. Rindt, Frank A. Jones, and Randal S. Sauer
Universityof North Dakota Energy and EnvironmentalResearch Center _
Grand Forks, ND 58202
ABSTRACT
More comprehensiveresourcemanagement systemsmust be developed if the United
States is to continue extraterrestrialexplorationinvolvingextended lengths
of time in space. Testing was performed to nbtain additional information
! concerning the feasibilityof using a low-temperatureplasm_ reactor as the
centerpiece of a closed-loop processin9 (CLP) resource management system.
Low- and high-ash carbon, freeze-dried human feces, sunflower, and plastic
were proccLsed in a low-temperatureoxygen plasma reactor system in both
stationary and agitated modes. Conversionof carbon, hydrogen, and nitrogen
to gaseous species was determined for each of the test materials. Sample
agitation greatly reduced the residence times required for species
conversion. Virtually complete carbon conversionwas achieved at residence
times ranging from 2 to 8 hr. The inorganic matrix was unchanged by the
processing technique. Based upon the results of this testing, scale-up and
further testing of this technologyare warranted.
INTRODUCTION 1
More comprehensive resource management systems must be developed if the
United States is to continue extraterrestrialexploration involvingextended
lengths of time in space. Three types of resource management systems are
available for this type of mission: 1) systems in which consumables such as
oxygen and food are not recycled, 2) totally closed-loopsystems in which all
consumablesare rLcoveredand reused in some manner, and 3) a combinationof I
and 2. The quantity of consumablematerial to be recycled (i.e., 50% or 90%)
and/or ferried back and forth from Earth will be based primarilyon the mass,
volume, and energy requirementsof the closed-loopprocessing (CLP) system.
An extraterrestrialbase will require a CLP system capable of integratingall
functions necessary to support life in a remote setting, including
manufacturingand biologicalby-productprocessing.
A low-temperatureptasma reactor is capable of oxidizing or reducing the
organic components of a stream while leaving the inorganic matrix
unaffected. In effect, separation and conversion take place in a single
reactor. The organic fraction of plants, human waste, and plastics can be
convertcJ to gases, while the inorganic fraction remains virtually
Jnchanged. The inorganic materials can then be recycled directly to other
operations or subsystems.
The feasibility of using this technology as part of a Ct.P resource
management system can be measured by _'eactorsize, efficiency, and energy
requirements, as well as by the capabilityof the reactor to be closely
444
.!
_. t_
__ " ........................ _Ik"_ ,................. _,---_, ..... _ _ __
1991012826-444
https://ntrs.nasa.gov/search.jsp?R=19910012861 2020-03-19T17:53:31+00:00Z
integrated into the closed-loop system. Stationary-bed batch tests
were performed previously at EERC to determine conversion rates and i
efficienciesfor a variety of organic wastes (ref. I). The results of these
..............._ _,,__=_,,,u,uyxheid _romJse as part ot a CLP resource :
management system, with its ultimate potential dependent upon lowering
residence time requirements. The current research focused on determining if
the r_sidence times could be reduced to a point where it is likely that scale- '!
up and reactor system optimizationcould result in a small, e ergy-efficient J
system capable of being closely integrated into a closed-loop resource
management system.
METHOD AND MATERIALS
Equipment
The low-temperature plasma tests were performed using three pieces of
equipment: a radio frequency (rf) generator, _ reactor center, and a vacuum
system. The rf generator and the reactor center were manufactured by the
InternationalPlasma CorpoTation. The rf generator can generate up t_ 500 W
of rf pewer at a frequencyof 13.6 MHz. The reactor center houses the reactor
chamber, the electrodes that generate the plasma field, and a control panel.
The floor plan of the work area supportingthis system is shown in Figure 1.
The control panel is used to regulate _he gas flow and the vacuum in the
reactor chamber. It also balances the load on the r* generator. Tne quartz
reactor chamber is eight inches long and fou inches in diameter and has been
modified to contain the hardware necessary for agitationduring operation. A
vacuum pulls the plasma-producingqas through the chamber, while the rf energy
is transmitted across the electrodes surrounding the sides of the reactor
chamber. The reactor chamber is shown in Figure 2.
As part of earlier research, the reactor chamber was fitted with three types
of agitation. The first was a fritted quartz cylinder attached to a rotary
coupling (the reactor agitation system originally proposed for this
testing). Samples were placed inside this cylinder and tumbled during
processing. Two other methods of sample agitatlonduring processingwere also
establishedwith the existing reactor system.
The second device consisted of a pair of rotating aluminum trays. The trays
were each constructedwith a lip on one longitudinaledge and were aligned es
shown in Figure 3. They were attached to the rotary coupling and fitted
insiCe the quartz reactor chamber. At this small scale, some of the particle
sizes of the samples tested were not large enough to overcome the
electrostaticforces which held them to the aluminum plates. To overcome this
problem, the system wa_: "internally" agitated. The method chosen was
essentiallya ball mill. An inert solid (sandor silica gel) was added to the
test material to provide a mixing, crushing action. It also ensured that some
type of sample would be present at extinctionfor analysis.
The third tumbling device consisted of a thin blade of aluminum which was
attached to the rotary coupling. The blade continuouslyscraped the reactor
chamber walls, thereby agitating the ;ample, as shown in Figure 4. This
_ method also made use of the addition of an inert solid to the test materials
for the purpose of providingfurthermixing action.
445
-- - • Illll I -
........- ........................""' "" - 1991012826-445
I IIF--lO.,M.,.,tcooT,.o.I I IT
C Bag -4---
1 j LControl Panel
ORIGINAL PAGE
BLACK AIN.£_WHITE Pi--IOTOGi,V-_I_h
Figure 1. (a) Flc_r plen of low-temperatureplasma system work area.
(b) Low-temperatureplasma system control area.
C_'(JG]N_L t _UE '"
of Pooegmj_y
446
F ,II ..... 11111 .... IIII I , " "_' " - ' ............ m-:-- .... rr _____
1991012826-446
ORIGINAL PAGE
BLACK AND WHLTF. [-'.rIOTOGBAPH .t
ORIGli_'I-4LPAGE IS
OFPOOW
W
I IN
Figure 2. Low-temperdL,;replasma system quartz reactor chamber.
buring the tests, nominel reactor chamber pressurewas held at 67 to 107 N/m2,
rf power was set at 100 W, and the gas flow rate was less than 0.1 cc/min.
ExperimentalMetho<l
Irrespectiveof the agitationmethod used, sand or silica gel was added to the
test material in a mass ratio of 9:1. The composite sample was then loaded
into the reactor. When the aluminum trays were used, the sQmple was placed on
the bottom tray. As the trays rotated,the sample _as dumped from one tray to
another, falling through the plasma as it did so. At the completion of the
test, the trays were removed and the sample collected. Any sample that had
fallen off of the trays was reclaimed from the bottom of the reactor chamber
af,dadded back to the product container_
In the case of the method involvingthe rotating aluminum blade, the sample
was loaded onto, and collected frnm, the bottom of the reactor chamber. This
method was used for the majorityof the kinetics tests involvingagitation.
To provide a yardstick against which to measure the results of the tests
involvinb agitation, tests were also performed in a stationarymode. The same
quartz reactor chamber was used for these tests, but without agitation.
Instead, the sample was loaded onto quartz trays which were placed inside the
reactor chamber.
Quality control measures take_ during this study are described in reference 2.
447
t
........... III -_ ............ n -- ' -- _ • ' m,1 ........ - ' " ' " -
1991012826-447
feed). These test points were not included in the plots. As expected,
conversion to gaseous products generallytook place more rapidly when samples
w_ _nit_f_A althoughthe differencebetweenth_ rP_ult_ of the aaitated and
stationarytests was more pronouncedfor some of the test materials.
Figure 5 shows the conversionof carbon in low-ashcarbon as a function of
time. In this instance, there was a rather dramatic difference between the
results of the stationaryand agitated tests. This can be compared with the
conversion of carbon in high-ashcarbon as a functionof time, shown in Figure
6. The same trend Is noted in that higher conversionlevelswere attained for
the high-ash carbon sample when it was processed in the agitated reactor
configuration. However, under stationaryconditions,carbon conversions were
higher for the high-ash carbon than for the low-ashcarbon.
Of the five test materials, it was more difficult to obtain a
representativesample of freeze-driedhuman fecal material fo_ analysis due to
the variations in particle size within the compositesample. As such, only
one viable test point was obtained for the agitated tests. Conclusions can
still be drawn from the plot shown in Figure 7. From the fecal material data
point, it appears that the carbon conversionrate was enhanced by agitatien
and was of the same order of magnitude as that observedwith carbon.
The results of processing the sunflower samples are presented in Figure
8. Again, agitation enhanced the conversion of carbon to gaseous species.
The carbon conversions obtained for the sunflower samples were higher than
those of any other materials tested within the one-hour kinetics evaluation
series.
Conversions of hydrogen and nitrogcn to gaseous products were also
determined for the sunflower samples and are shown in Figures 9 and 10,
respectively. As was. the case with the carbon_ agitation hastened the
conversion of both the hydrogen and the nitrogen. Figure 10 indicates that
the nitrogen conversion rate was more rapid during the first 20 min of
agitated processing.
The plastic processed consistedof ground plexiglas. During the agitated
processing,the particles had a tendencyto melt together,making it difficult
to contact the sample with the plasma. However, one viable test point was
obtained for the i_gitatedprocessing(60 min). As Figure 11 shows, this value
is less than the 57% conversion obtained during stationaryprocessing for 60
min. This is most likely due to the fact that the sample did not agglomerate
during the stationaryprocessing.
Samples of each of the test materials were processed to extinction using
the rotating aluminumplate reactor configuration. Due to the tendency of the
silica gel to stick to the reactor plates as a res_Jltof static electricity,
the processing was performed using sand as the inert matrix. The product
samples could not be analyzed for carbon, hydrogen, and nitrogen content
because not enough test material remained. The time necessary for complete
extinction varied from sample to sample, with the plastic requiring
approximately8 hr and the remainingsamples requiring3 hr or less. The five
test materials are shown before and after processing in Figures 12 through 16.
448
1991012826-448
'I
b
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
C
Figure 3. (a) Diagram of rotatin9 tray agitationsystem.
(b) Rotary coupling.
(c) Rotating tray _gitationsystem.
449 Ot,h"'_JINALPAGE IS
Poo_ .c_uJu'ry .
1991012826-449
• °';#';° ° i
Figure 4. Diagramof aluminum blade agitationsystem.
Test Matrix
The tests were performedusing oxygen plasma and the followingmaterials:
1. Low-ash carbon
2. High-ash carbon
3. Freeze-driedhuman fecal material
4. Sunflowerroots, stalk, and head
5. Plastic (ground plexiglas)
Tests were performed in both stationary and agitated modes _or times of 20,
40, 60 min, and to extinction (less than 3 hr in most cases). The rotating
aluminum blade method of agitation was used for the testing, except for the
extinction tests, which were performedusing the aluminum plata reactor. When
possible, the product sampleswere analyzedfor carbon, hydrogen,and r.itrogen
content.
ANALYTICALRESULTSANO!HT_PRETATION
Carbon, hydrogen,and nitrogen (CHN) contentswere determined for the five
feed samples and 29 of ,he products. The theoreticallimits of calculations
involving these results e the same as the detectability limits of the
analytical instru_ntatior,(i.e.,±0.5% for carbon and ±0.1% for hydrogen and
nitrogen).
Conversionsof the carbon, hydrogen,and nitrogen to gaseous species were
determined using the CHN information. The conversion data were plotted as
functions of residence tim° for e_se of interpretation. In some cases,
negative conversionswere calculated (i.e., the analytical results indicated
that more carbon was present in the product than had been present in the
450
",,%
1991012826-450
100% "t
" Stationary
,- 60%
o • Agitated
p_
e 50%
=>
O
o 4O%
10%
_o _ i l i l _ l T I , r : :
0 0.4 0.8 1.2 1.6 2 2.4 2.8
Time (hr)
Figure 5. Carbon conversion in low-ash csrbon sample as a
function of time.
100%
9O%
8O%
70%
¢-
o 609(
_ • AgitatedO
:> /c 50%Oo 4O%
2O%
10%
0o_ T r i i i 1 t t I I _ _ I i "
0 0.4 0.8 1.2 1.6 2 2.4 2.8
"l]me(hr)
Figure 6. Carbon conversion in high-ash carbon sample as a
function of time.
451
...."'""'"'"'........................ 1991012826-451
........_.................... _ ___ ______.___
i • PreviousWork
70% • Stationary
Agitated
8O%
-!5O%
8 40%
30%
20%
10%
% ' _ I : I I T
0 4 8 12 16 20
Time (hr)
Figure 7. Carbon conversion in feces sample as a function of time.
9O%
• Stationary
80% • Agitated
70%
_" 60%
o
50%
0
L) 40%
30%
20%
10%0% -
0 0.4 0.8 1.2 1.6 2 2.4 2.8
Time (hr)
Figure 8. Carbon conversion in sunflower sample as a
function of time,
452
1991012826-452
::P ' .100%
+o++ i
80%
f
70% • Agitated /J
._ 60%
50%
C
8 40%
30%
20%
10%
o%
0 20 40 60
Time (rain)
Figure 9. Hydrogen conversion in sunflower sample as a
function of time,
10o%
90%
i_ Stationar,/80% I
I * Agitated7o% L
c $0%O
> 50%
0
0 40%
30%
20%
10%
0%
0 20 40 6O
Time (rain)
Figure 10. Nit_-ogen conversion in sunflower sample as a
function of time.
453
i" "_,, ,,.
.,,..,'
1991012826-453
100% "1
90%
80%
70% _ • Stationary_
Agitated /60% •
• 50%
t-
O
o 40%
30%
2O%
10o_
0%
0 2 4 6 8
Time (hr)
Figure11. Carbonconversioninplasticsampleas a
Function of time.
ORICINAL _ __=r- t ,*._ L..
BLACK AND WHITE PHOTOGRAPH
LOW ASH CARBON
Figure 12. Low-ash carbon sample before and after
processing to extinction.
454
,,.
=,,
f ......................... _ n II II ........ ,,. .....
1991012826-454
tli(;ll ASIi ('ARBON
,-FEED EXTINCnON
Figure 13. High-ashcarbon sample before and after
processingto extinction.
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
FEED t_XTIN(qlON
Figure 14. Freeze-driedfeces sample before and after
processingto extinction.
455
1991012826-455
ORIGINAL PAGE :I
BLACK AND WHITE PHOTOGRAPh -_
-,_
St IN I-I.()Wt:.R
,' EXTINCTION
Figure 15. Sunflower sample before and after
;: processing to extinction.
PL_"I[C
FEED
Figure 16. Plastic sample before and after
processing to extinc*ion.
456
bl (I
I I III I I I • II .......... i ,.
1991012826-456
The results obtained during this researchwere compared to those obtained
during earlier bench-scale testing at EERC (rPf. !). The comparison is
su,,_arizedi, Table [. lhe products o_ the agitatedruns were very similar in
conversion and solubility characteristicsto the products of the stationary
tests performed during earlier work. The ranges of residence times required
for complete carbon conversion are indicated in Figure 17. As the figure
shows, the residence times required to completely convert the carbon were
reduced dramaticallyby agitation, in one case by a Factor of 12.
Early in the testing, it was observed that the carbon conversion rates
were related to particle size as viewed by gross examination. As a result,
select samples were submitted for size analysis. The carbon conversion rates
of the three samples analyzed were not related solely to particle size as
determined by small particle analysis. This discrepancy suggests that the
conversion rate may be related instead to other physical parameters such as
surface area and/or physical ash or residue formationor structurerelative to
the reactant species. Sample preparationwill therefore be a necessary area
of research in definingefficienciesfor this system application.
Samples of the inertmatrices (i.e., sand and silica gel) taken before and
after p essing were analyzed using x-ray diffraction and x-ray Fluores-
cence. _ne results of these analyses indicated that the processing had not
caused any significantmorphologicalchanges.
CONCLUSIONS
The results of this testing indicate that the agitated low-temperature
plasma reactor system successfullyconvertedcarbon, hydrogen, and nitrogen to
gaseous products at residence times that were approximatelyten times shorter
than those achieved by stationary processing. The inorganic matrix present
was virtually unchangedby the processingt_chnique. It is concluded that, at
this stage of development, thi_ processing technique is feasible for use as
part of a CLP resource management system. Further investigationto optimize
reactor configuration,energy requirements,and scale-up are warranted based
upon the results of this work.
REFERENCES
i. "Applicationof Low-TemperaturePlasma Reactors as a Resource Mar,agement
SubsysteF,_in Closed-LoupProcessingApplications," Proposal submitted to
NASA on August 13, 1987, by the Advanced Technologies Group of the
Un,versityof North Dakota Energy Research Center.
2. Hetland, M.D., Rindt, J.R., Jones, F.A., and Sauer, R.S., "Further
Investigation of the Feasibility of Applying Low-Temperature Plasma
Technologyto a Closed-LoopProcessingResourceManagementSystem," F_nal
report submitted to NASA, Contract No. NASW-4455, Project No. 10-49501
(April 1990).
457
(
1991012826-457
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_'_ _ (...3 t. _1 (IJ .e- ,
i_l 4.1 ::E. f.. ,--- .,-
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r_ ,_ 0 0 .Z3 "" E
_ _ >_ = _
:3 l.- ,-- ,'_ f,. L _. .,-- .,-- tO "O E
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458
'- .......... IIII ..... "__. -- iI l ---- l ....
1991012826-458
100%.
9O%
7O%
C
._o 60%
c0
o
4O%
30%
2O%
10%
O%
0 20 40
Time (hr)
Figure 17. Ranges of residence times required for complete
carbon conversion.
459
1991012826-459


Low-temperature plasma technology as part of a closed-loop resource management system

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