© philadelphia scientific 2003 philadelphia scientific catalyst 201: catalysts and poisons from the...
TRANSCRIPT
© Philadelphia Scientific 2003 Philadelphia Scientific
Catalyst 201: Catalysts and Poisons from
the Battery
Harold A. Vanasse
Daniel Jones
Philadelphia Scientific
© Philadelphia Scientific 2003 Philadelphia Scientific
Presentation Outline
• A Review of Catalyst Basics • Hydrogen Sulfide in VRLA Cells• Catalyst Poisoning• Filter Science• A Design to Survive Poisons• Catalyst Life Estimates
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Catalyst Basics
• By placing a catalyst into a VRLA cell:– A small amount of O2 is prevented from
reaching the negative plate. – The negative stays polarized.– The positive polarization is reduced. – The float current of the cell is lowered.
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Catalyst Basics
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Catalysts in the Field
• 5 years of commercial VRLA Catalyst success.
• A large number of cells returned to good health.
• After 2-3 years, we found a small number of dead catalysts.– Original unprotected design.– Indicated by a rise in float current to
pre-catalyst level.
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Dead Catalysts
• No physical signs of damage to explain death.
• Unprotected catalysts have been killed in most manufacturers’ cells in our lab. – Catalyst deaths are not certain.– Length of life can be as short as 12 months.
• Theoretically catalysts never stop working …. unless poisoned.
• Investigation revealed hydrogen sulfide (H2S) poisoning.
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H2S Produced on Negative Plate
• Test rig collects gas produced over negative plate.
• Very pure lead and 1.300 specific gravity acid used.
• Test run at a variety of voltages.
• Gas analyzed with GC.
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Test Results
• High concentration of H2S produced.
• H2S concentration independent of voltage.
• H2S produced at normal cell voltage!
0
100
200
300
400
500
600
2.25 2.35 2.45 2.55 2.65 2.75
Cell voltage (V)
H2S
con
cent
ratio
n (p
pm)
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H2S Absorbed by Positive Plate
Material to be tested
Reactor
H2 +
H2S
10
0 p
pm
GC
H2 Gas with 100 ppm of
H2S
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Test Results
• Lead oxides make up positive plate active material.
• Lead oxides absorb H2S.
Test Material
Amount (grams)
Breakthrough Time
(minutes)
Empty 0.0 0.01
PbO 2.2 120
PbO2 2.0 360
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H2S Absorbed in a VRLA Cell
H
2 +
H2S
10
0 p
pm
H2 Gas with 100 ppm of
H2S
VRLA cell
GC
2.27V
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Test Results
• H2S clearly being removed in the cell.
• 10 ppm of H2S detected when gassing rate was 1,000 times normal rate of cell on float!
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30
Time (hours)
H2S
co
nce
ntr
ati
on
in
th
e o
utl
et
ga
s (p
pm
)
0
20
40
60
80
100
120
140
160
Inle
t ga
s flow
rate
(ml/m
in)
H2S Concentration (ppm)
Gas Flowrate (ml/min)
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GC Analysis of VRLA Cells
• Cells from multiple manufacturers sampled weekly for H2S since November 2000.
• All cells on float service at 2.27 VPC at either 25°C or 32° C.
• Results:
– H2S routinely found in all cells.
– H2S levels were inconsistent and varied from 0 ppm to 1 ppm, but were always much less than 1 ppm.
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H2S in VRLA Cells
• H2S can be produced on the negative plate in a reaction between the plate and the acid.
• H2S is absorbed by the PbO2 of the positive plate in large quantities.
• An equilibrium condition exists where H2S concentration does not exceed 1 ppm.
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How do we protect the Catalyst?
• Two possible methods:– Add a filter to remove poisons before they
reach the catalyst material.– Slow down the gas flow reaching the
catalyst to slow down the poisoning.
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Basic Filter Science
• Precious metal catalysts can be poisoned by two categories of poison:– Electron Donors: Hydrogen Sulfide (H2S)
– Electron Receivers: Arsine & Stibine
• A different filter is needed for each category.
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Our Filter Selection
• We chose a dual-acting filter to address both types of poison.– Proprietary material filters electron
donor poisons such as H2S.
– Activated Carbon filters electron receiver poisons.
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Slowing Down the Reaction
• There is a fixed amount of material inside the catalyst unit.
• Catalyst and filter materials both absorb poisons until “used up”.
• Limiting the gas access to the catalyst slows down the rate of poisoning and the rate of catalyst reaction.
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Microcat® Catalyst Design
• Chamber created by non-porous walls.
• Gas enters through one opening.
• Microporous disk further restricts flow.
• Gas passes through filter before reaching catalyst.
Gas / Vapor Path Porous Disk
FilterMaterial
Catalyst Material
Housing
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How long will it last?
• Theoretical Life Estimate
• Empirical Life Estimate
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Theoretical Life Estimate
• Microcat® catalyst theoretical life is 45 times longer than original design. – Filter improves life by factor of 9.– Rate reduction improves life by factor of 5.
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Empirical Life Estimate:
• Stubby Microcat® catalysts developed for accelerated testing. – 1/100th the H2S absorption capacity of
normal.– All other materials the same. – Placed in VRLA cells on float at 2.25 VPC &
90ºF (32ºC).– Two tests running.
• Float current and gas emitted are monitored for signs of death.
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Stubby Microcat® Catalyst Test Results
• Stubby Microcats lasted for:– Unit 1: 407 days.– Unit 2: 273 days.
• Translation: – Unit 1: 407 x 100 = 40,700 days = 111 yrs– Unit 2: 273 x 100 = 27,300 days = 75 yrs.
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Catalyst Life Estimate
• Life estimates range from 75 years to 111 years.
• We only need 20 years to match design life of VRLA battery.
• A Catalyst is only one component in battery system and VRLA cells must be designed to minimize H2S production. – Fortunately this is part of good battery
design.
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Conclusions
• Catalysts reduce float current and maintain cell capacity.
• VRLA Cells can produce small amounts of H2S, which poisons catalysts.
• H2S can be successfully filtered.
• A catalyst design has been developed to survive in batteries.