Jul 7, 2003 - TDA Research Inc. ⢠Wheat Ridge, CO 80033 ⢠www.tda.com. Catalysts for the Selective Oxidation of Ammonia to Nitrogen. 33rd International ...
Catalysts for the Selective Oxidation of Ammonia to Nitrogen 33rd International Conference on Environmental Systems Vancouver BC July 7, 2003 David T. Wickham Jeffrey Lind
TDA Research Inc. • Wheat Ridge, CO 80033 • www.tda.com
Overview z
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Background of the Vapor Phase Catalytic Ammonia Removal (VPCAR) System. TDA approach to the identification of a selective catalyst. Results of our Phase I Project. Preliminary reactor design for the full scale VPCAR unit.
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The Need for a New Water Purification System z
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The cost of delivering a payload to Mars is much higher than to low Earth orbit. Therefore missions to Mars place higher emphasis on reducing launch weight. Thus, water purification systems currently used on the ISS may not be suitable for use on a Mars mission.
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Comparison of Water Purification Methods z
The ISS uses the Water Recycle System (WRS) to purify water. z z z
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Employs expendable adsorption beds. Resupply requirement is 0.32 kg/person day. For a remote destination, resupply costs could exceed $800 M (960 day – 6 crew) (Flynn and Borchers 2000).
For Mars missions. z z z
Emphasis is on eliminating resupply requirement. VPCAR system uses distillation to purify water. Requires more energy, but does not rely on expendables. TDA
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Technical Challenges Associated with VPCAR z
Distillation does not eliminate volatile components such as ammonia (NH3 SMAC = 7 mg/m3, 9.2 ppm, Perry 1998).
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These must be oxidized in the catalyst bed – platinum catalysts are effective.
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Unfortunately, commercial platinum catalysts convert NH3 to NO and NO2 (NOX).
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NO SMAC = 5.0 mg/m3 (3.7 ppm) and NO2 SMAC = 0.94 mg/m3 (0.45 ppm).
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The goal of our Phase I is to identify catalysts that do not produce NOX. TDA
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Schematic of the VPCAR System Compressor
Oxygen Oxidation Reactor
Evaporator
Feed stream Bleed stream Simplified from Flynn and Borchers, 2000
Condenser
recycle
Product
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Oxidation of NH3 z
Selective oxidation of ammonia to nitrogen. 2 NH3 + 3/2 O2 Æ N2 + 3 H2O
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Oxidation of ammonia can also produce NO and NO2. 2 NH3 + 5/2 O2 Æ 2 NO + 3 H2O 2 NH3 + 7/2 O2 Æ 2 NO2 + 3 H2O We must identify a selective catalyst to eliminate NO or NO2 (NOX). TDA
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Steps in Ammonia Oxidation 2 NH3 + 3/2 O2 Æ N2 + 3 H2O
N 2 + 2 H2 O
2NH3 O2 H2O N2 H2O Desorption of product(s).
Chemisorption of reactants H O H
NN
H O H
H2O N2 H2O
Surface reaction between mobile adsorbed species.
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Why NOX is Produced During Ammonia Oxidation NH3
NOX + H2O
O2
H
O O O N
O
O
O O O N O O O O O O O O H
Catalyst surface
In excess oxygen, there is a high probability that N and O will react to form NO or NO2.
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100
25
80
20
VoCat 350 - Pt / Al2O3 NH3 = 10 ppm 40% Relative Humidity Balance air GHSV = 17,000 h-1
60
40
15
10
20
0 100
% Selectivity to NOX
Percent NH3 Conversion
Previous Data on VoCat Shows High NOX Formation
5
120
140
160
180
200
220
Temperature / C From Wright et al. 1997
240
260
280
0 300
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TDA’s Approach to Preparing a Catalyst That Does not Produce NOX z
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Prepare a catalyst with two types of active surfaces in close contact with each other. Nitrogen adsorbs primarily on one surface. Oxygen is concentrated on the second active site. Reduces the reaction between oxygen and nitrogen, eliminating NOX. TDA
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Simplified Schematic of Catalyst that is Selective for Nitrogen Production H 2O
O O O H O O O N H O O OO Surface A
N2
N
Surface B
Separate adsorbed O from adsorbed N TDA
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Phase I Test Plan z
Prepare matrix of 25 catalysts consisting of platinum on modified supports. z z
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0.25 to 3 wt% Pt; 3 to 15 wt% modifier. Measure activity for ammonia oxidation and selectivity for NOX.
Test in the presence of high water concentrations. Generate kinetic data to arrive at rate expression, which can be used to size the oxidation reactor. TDA
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Automated Test Rig TC Catalyst bed Tube furnace NO/NOx analyzer Computerized Process Control YIC
FIC
800 ppm NH3/He
Oven NH3 analyzer
Heated lines 0 - 200 sccm YIC
FIC
He
S
FTIR
0 - 200 sccm YIC
FIC
O2
S
0 - 50 sccm
S
Syringe pump for water injection.
VENT
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3% Modifier Results in Low Activity 100 90
Percent NH3 Conversion
80
0.25 wt% Pt 0.5 wt% Pt 2.0 wt% Pt 3.0 wt% Pt
70 60 50
3% Modifier
40
NH3 = 50 ppm H2O = 8% O2 = 10% GHSV = 25,000 h-1
30 20 10 0 100
150
200
Temperature / C
250
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Catalysts with 10% Modifier Were the Most Active 100 90
Percent NH3 Conversion
80 0.25 wt% Pt 0.5 wt% Pt 2.0 wt% Pt 3.0 wt% Pt
70 60 50 40 30
10% Modifier
20 10 0 100
NH3 = 50 ppm H2O = 8% O2 = 10% GHSV = 25,000 h-1
150
200
Temperature / C
250
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15% Modifier Reduced the Catalyst Activity 100 90
Percent NH3 Conversion
80 70 60
15% Modifier
50 40
0.25% Pt 0.5% Pt 1% Pt 2% Pt 3% Pt
30 20 10 0 100
150
200
Temperature / C
250
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NOX Levels over 1 ppm at 200°C with 3% Modifier 5
NOX in Effluent / ppm
4
3
2
3% Modifier
0.25 wt % Pt 0.5 wt% Pt 2.0 wt% Pt 3.0 wt% Pt
Required temp
NH3 = 50 ppm H2O = 8% O2 = 10% GHSV = 25,000 h-1
.
for NH3 conversion
1
0 100
150
200
250
Temperature / C
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Very Little NOX Production at 150°C with 10% Modifier 5
NOX in Effluent / ppm
4
3
2
0.25 wt% Pt 0.5 wt% Pt 2.0 wt% Pt 3.0 wt% Pt
10% Modifier
NH3 = 50 ppm H2O = 8% O2 = 10% GHSV = 25,000 h-1 Required temp
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for NH3 conversion 1
0 100
150
200
Temperature / C
250
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Reducing Oxygen Concentration Lowers NOX
NOX in Effluent / ppm
4
80
3% Pt - 10% Modifier NH3 = 50 ppm H2O = 8% Temperature = 200°C GHSV = 25,000 h-1
3
60
2
40
1
20
0
0 0
2
4
6
Percent Oxygen in the Feed
8
Percent Ammonia Conversion
100
5
10
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Comparison of TDA Catalyst to VoCat 350 z
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Best TDA catalyst consisted of 3% Pt 10% Modifier. VoCat 350 commercial oxidation catalyst consisting of Pt/Al2O3. We compared the activity of these catalysts at higher space velocities and in the presence of water.
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VoCat Conversion is Affected by Water 100
Percent NH3 Conversion
SV = 20,000 h-1; 0% water SV = 40,000 h-1; 0% water SV = 40,000 h-1; 1.8% water
80
60
40
20 100
VoCat 350 NH3 = 50 ppm O2 = 10% 125
150
175
200
225
250
Temperature / C
275
300
325
350
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High NOX at Temperatures Required for 100% Conversion 15
SV = 20,000 h-1; 0% water SV = 40,000 h-1; 0% water SV = 40,000 h-1; 1.8% water
PercentPPM NH3NH Conversion 3
12
9
VoCat 350 NH3 = 50 ppm O2 = 10%
6
3
0 100
125
150
175
200
225
250
Temperature / C
275
300
325
350
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Conversion on TDA Catalysts Higher than Obtained on VoCat Percent NH3 Conversion
100
80 SV = 40,000 h-1; 0% water SV = 40,000 h-1; 1.8% water
60
3% Pt 10% Modifier NH3 = 50 ppm O2 = 10%
40
20 100
125
150
175
200
225
250
275
300
Temperature / C
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Low NOX at Temperatures Producing 100% Conversion 15
Percent NH3 Conversion
12
SV = 40,000 h-1; 0% water SV = 40,000 h-1; 1.8% water
9
3% Pt 10% Modifier NH3 = 50 ppm O2 = 10%
6
3
0 100
125
150
175
200
225
Temperature / C
250
275
300
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Tests to Characterize Activity of the TDA Oxidation Catalyst z
Characterize reaction kinetics. z
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Measure dependence of reaction rate on oxygen and ammonia partial pressure. Determine dependence of reaction rate on reaction temperature.
Use data to calculate conversion in the full scale VPCAR system.
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Rate Equation
Rate = v exp [-Ea/RT] * PNH3m * PO2 n Solve or m,n Ea, and v.
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Rate is About 0.6 Order in Oxygen Pressure
-17.5
Ln Reaction Rate
-17.8
Slope = n = 0.6
-18.1
-18.4
-18.7
-19
5
5.5
6
6.5
Log Oxygen Concentration
7
7.5
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Similar Analysis on Ammonia -18.9
Ln Reaction Rate
-19.2 0.703x - 21.829 Slope =y =m = 0.7
-19.5
-19.8
-20.1
-20.4 2
2.5
3
3.5
Ln Ammonia Concentration
4
4.5
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Rate Changes Rapidly above 150°C 1.0E-07
Reaction Rate / moles NH3 /g cat s
9.0E-08 8.0E-08
3% Pt - 10% Modifier NH3 = 75 ppm O2 = 623 ppm GHSV = 96,000 h-1
7.0E-08 6.0E-08 5.0E-08 4.0E-08 3.0E-08 2.0E-08 1.0E-08 0.0E+00
0
50
100
150
Temperature / C
200
250
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Temperature Dependence of the Ammonia Oxidation Reaction -16
-17
Ln Reaction Rate
-17
Activation Energy = 25.0 kcal/mole
-18
-18
-19
3% Pt - 10% Modifier NH3 = 75 ppm O2 = 623 ppm GHSV = 96,000 h-1
-19
-20
0.0021
0.00215
0.0022
0.00225 1/T (K)
0.0023
0.00235
0.0024
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Rate Equation
Rate = v exp [-Ea/RT] * PNH3m * PO2 n m = 0.6, n= 0.7 Ea = 25.0 kcal/mole v = 1.95 E9 moles/g cat s atm1.2
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Operating Conditions in the Full Scale System* z
Catalytic reactor is 3.8-in ID x 16-in long.
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Operating pressure = 92 torr.
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Nominal water flow = 13.2 lb/h.
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O2 flow = 0.13 lb/h (723 ppm at 0.82 atm).
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Operating temperature = 250°C.
* From Hamilton Sundstrand
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Preliminary Reactor Design 80
Reactor 3.8-in ID x 16 in long GHSV = 42,000 h-1 (ambient cc/cc cat h) Pressure = 92 torr NH3 = 75 ppm O2 = 623 ppm
NH3 Concentration / ppm
70 60 50 40
T = 180°C 30
T = 200°C 20
T = 220°C
10 0 0
2
4
6
8
10
Reactor Length / in
12
14
16
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The Catalyst will Oxidize HC’s at Higher Temperature 90 80
Percent Conversion
70 60 50
3% Pt 10% Modifier Feed = 100 ppm phenol GHSV = 29,000 h-1 O2 = 1000 ppm H2O = 2%
40 30 20 10 0
290
300
310
320
330
340
350
360
Temperature / C
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Summary and Conclusions z
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TDA has identified a catalyst that is very active for ammonia oxidation and produces very little NOX. The data suggest that the optimum catalyst composition is 3%Pt and 10% modifier. The catalyst is more active than the commercial oxidation catalyst and produces less NOX. A preliminary reactor design for the full scale VPCAR shows that the catalyst will achieve 100% conversion at 195°C (< the design temperature). Very little NOX should be produced under these conditions. TDA
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Acknowledgements z
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NASA SBIR Office, Contract No. NAS102034. John Fisher, Contract Monitor.
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