ANALYSIS OF TRACE LEVEL MERCURY IN GASEOUS SAMPLES
Hemant Tyagi
DMQC, Indian Oil Corporation Ltd. Panipat Refinery Naphtha Cracker Laboratory
Email: [email protected]
Abstract:
Mercury is a potential poison for the ethylene recovery plant. DCU off gas and FCC dry gas
will be the feed for proposed ethylene recovery plant at IOCL Panipat Refinery Naphtha
Cracker Complex. In view of the Licenser specifications (max. 100ppb mercury in the feed)
testing of mercury is essential in DCU off gas and FCC dry gas samples while selecting
guard bed design for ethylene recovery plant. Testing of mercury in liquid samples is not a
difficult task. Test methods are available for the analysis of mercury in liquid samples by
AAS or ICPMS techniques. But no test method is available for testing of mercury in gaseous
samples at ppb level. It is mainly due to non availability of gaseous sampling testing over
ICPMS. In the present work we have developed a sampling assembly for scrubbing of
mercury of gaseous samples in concentrated nitric solution. In this sampling assembly,
mercury of gaseous sample gets converted into mercury (II) salts and then analyzed over
ICP MS technique.
Chemical reaction:
Mercury reacts with oxidizing acids such as concentrated nitric acid and form mercury (II)
salts along with the NOx formation.
2Hg + 6HNO3 → 2Hg(NO3)2 + NO2 + NO + 3H2O
Safety information:
1. This reaction is exothermic and can be violent. Hot mercury emits significant
amounts of mercury vapour (the main hazard associated with metallic mercury).
Mercury (II) salts are highly toxic as well.
2. Note that NO is a highly toxic gas, as is NO2. Exposure to nitrogen oxides produces
delayed symptoms like lungs will fill with fluid and one could die if not get medical
attention.
3. Proper ventilation is a must.
Sampling assembly: Consists of following:
1.
2.
3.
4.
Sample inlet (Connected with sample point directly or through sampling bomb)
Gas rotameter (to quantify the gas content)
Coil immersed in boiling water (boiling water is used to avoid deposition of mercury)
Scrubbers having 100 ml. (5% nitric acid solution) immersed in ice bath (to balance
the effect of exothermic reaction). Scrubbers connected in series for complete
reaction of mercury with nitric acid.
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5. Outlet (connected with around 20 m discharge pipe and disposed at the safe place.
Outlet of discharge pipe was scrubbed in water to minimize the hazard, if any)
4
3
2
5
1
Figure No. 1: Sampling assembly for mercury analysis in gaseous samples
For the analysis of mercury scrubbing of gas samples was done for around 02-03 hours
continuously at the rate of 80-90 Litre/hour gas flow.
Experimental work: Mercury analysis was done in scrubber solution no. 1 and 2 by
Inductively Coupled Plasma Mass Spectrometer ICPMS (Agilent 7700 series) instrument
working with following conditions.
Before acquisition:
Uptake speed: 0.50 rps
Uptake time: 45 sec
After acquisition (Rinse port):
Rinse speed: 0.50 rps
Rinse time (sample): 30 sec
Rinse time (STD): 30 sec
After acquisition (Rinse vial):
Step 1
Step 2
Step 3
Rinse Vial:
1
1
1
Rinse speed (rps):
0.50
0.00
0.00
Plasma condition:
RF power: 1550W
RF matching: 1.8V
Carrier gas flow: 1.12 L/min
Nebulizer pump: 0.1 rps
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Results:
During this exercise following samples were analyzed:
Name
of
sample
1
DCU off gas
2
DCU off gas
3
DCU off gas
4
FCC dry gas
5
FCC dry gas
Note: Density of DCU off
ppb (by weight)
S. No.
Sampling date and time
19.10.2015, 11.00-12.00 hours
27.10.2015, 11.15-12.15 hours
27.10.2015, 12.30-13.30 hours
11.02.2016, 12.00-14.00 hours
11.02.2016, 14.00-16.00 hours
gas is approximately 1 Kg/m3 so
Mercury content (ppb) by
volume
0.229
0.207
0.213
0.00
0.00
value of mercury will be same in
During the experiment calibration curve was drawn with 0.5, 0.8, 2.0 ppb mercury standard
and 1.0, 2.0, 5.0 ppb mercury standards. Snapshots of calibration details and sample
analysis drawn from the instrument are tabulated below:
Screen shot No.1: Calibration curve and analysis of FCC sample of 12.02.2016
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Screen shot No.2: Calibration curve and analysis of DCU sample of 27.10.2015
Screen shot No.3: Calibration curve and analysis of DCU sample of 19.10.2015
Conclusion: With this developed sampling system analysis of mercury can be done at ppb
level in gaseous samples by using ICPMS technique.
Benefits: This newly developed sampling assembly and testing method saved around 14
lakh rupees (Actual testing cost quoted in a tender of IOCL by a private party as a single
offer) as a onetime measure.
Reference: Test method USA EP 29
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