Challenges of Gas Purity Measurements Solved
Hi there. Michael Kamphus here. I’m an application engineer for process gas analyzers here at Emerson and in this blog post I’d like to discuss how the measurement of gas purity plays an important role across multiple gas processing industries and applications, mainly for the purpose of detecting gas impurities across a particular process. For example, in chemical reactions, one has to ensure the gases are free of carbon monoxide (CO) because of the risk of poisoning precious catalysts, oxygen (O2) might oxidize catalysts, and carbon dioxide (CO2) might form carbamates or carbonates which may clog process gas lines and lead to costly repairs.
One of the most important gas purity processes is the production of syngas, which is a mixture of CO and hydrogen (H2) and used as a starting point for H2 or CO generation. Syngas is mostly produced by steam reforming of natural gas. After the initial reformer, different reaction steps are necessary to convert and clean the process gas to achieve syngas in the desired H2/CO ratio. Besides steam reforming of natural gas, other technologies generate syngas from coal gasification or wood/biomass gasification. The fertilizer industry uses syngas to produce ammonia and urea. Additionally, methanol and other hydrocarbons can be synthesized by the Fischer-Tropsch reaction, an access to liquid hydrocarbons independent from crude oil.
In air separation units, the inlet air has to be free of hydrocarbons (HC) and CO2. After separation of nitrogen (N2), O2 and argon (Ar) has occurred, the product gas streams have to be monitored for impurities such as moisture, CO2, HC and O2, to ensure product quality. These gases and gases from other industrial processes are then used for gas bottling. Bottled gasses are needed in the food and beverage industry with the carbonization of beverages, welding and shielding to improve weld characteristics, and even in medical gases.
Emerson Process Management, Rosemount Analytical offers solutions for even the most challenging gas purity applications in refineries, fertilizer plants, steel plants and gas processing facilities around the world. For example:
- The impurity measurements of CO and CO2 are done with non-dispersive infrared photometer (NDIR) measurements and detect from 0-10 ppm or 0-5 ppm, respectively.
- NDIR is also used for purity measurements of CO2 and nitrous oxide (N2O) with a max suppressed range of 98-100%. Hydrogen measurements are done with a thermal conductivity detector (TCD) making it possible to measure the H2 purity up to 98-100% or impurities of H2 in CO down to 0-1000 ppm.
- NOx, meaning the sum of nitric oxide (NO) and nitrogen dioxide (NO2) measurements, are performed with a chemiluminescence detector (CLD) as a standard. These ranges can get down as low as 0-5 ppm.
- Hydrocarbon impurities are detected with a flame ionization detector (FID) with the lowest range being 0-1 ppm.
- Oxygen, as low down a range as 0-1%, but also suppressed ranges of 20-22% and 98-100%, can be measured with a paramagnetic sensor (pO2). For O2 ranges down to 0-10 ppm, a trace oxygen sensor (galvanic fuel cell) is used.
- For H2O an aluminum oxide (Al2O3) based trace moisture sensor is integrated into the analyzer enabling us to deliver measurements on the dew point range from -100°C to -10°C or 0-100 to 3000 ppm.
The integration of different technologies can be combined into one analyzer housing; for example, a suppressed 98-100% O2 purity measurement with a 0-10 ppm CO and a 0-5 ppm CO2 impurity measurement, reducing cost for analytical equipment and making integration of the analyzer into the DCS much easier.
If you have a challenging gas purity application, have a question about process gas analyzers, or would like to share your experiences within the gas purity and process gas industry, we would like to hear from you. Post a comment and let us know!
To learn more about gas purity applications and process gas analyzers, visit www.rosemountanalytical.com/gaspurity.
Fred Midencey | April 3rd, 2012 at 6:27 pm #
Hello,
I have an obsolete Rosemount model 7C (actually have four), two of them measure 0 to 1000 ppm H2 in a background of CO. There is some < 100 ppm N2. Is there a replacement for this analyzer with the same 0-1000 ppm H2 range or better? NDIR?
Awaiting you response,
Fred Midencey
Analytical Tech.
Julio zambrano | April 9th, 2012 at 6:47 am #
Wath kind of instruments could we use to measure organic pollutants in air in the ppm range?
Bonnie Brown for Michael Kamphus | April 10th, 2012 at 9:06 am #
Fred,
Here is a first short answer. Probably we need to discuss the project in more detail later. Fred, can you give me more details, so that I can work on a more specific answer and include the local sales/service person?
0-1000 ppm H2 in CO is a very demanding application. We have a special solution for this measurement using a TCD with a flowing reference in a field housing. We use internal pressure regulators and capillaries for the measurement side and the reference side to maintain a very constant flow which is very crucial for an accurate TCD measurement with a 0-1000 ppm range. As a reference gas N2 can be used. Calibration for zero will be 100% CO, for span 1000 ppm H2 in CO. The quality of the test gas (especially zero gas) will of course affect the quality of the measurement. An “accuracy” of 10 ppm H2 can be achieved. We recommend daily zero. In our last application for a 0-1000 ppm H2 in CO we also integrated a CH4 channel for cross compensation on the H2 measurement. If you beside H2 only have N2 as a contaminant in your CO a pure TCD solution will be sufficient. For installation in hazardous area we can deliver a purged solution with a Bebco z-purge.
Some details about the current solution would be helpful:
- Zero calibration gas
- Span calibration gas
- Open/closed reference (if open reference, which reference gas is used?)
- Noise of H2 measurement
- Detection limit needed
- Daily drift of analyzer (difference between two zero calibrations)
- Hazardous area classification needed?
Bonnie Brown for Michael Kamphus | April 12th, 2012 at 6:21 am #
Julio, Michael is contacting you directly, in case you have questions – here is his reply:
Typically a ppm measurement of organic compounds is done with an FID. The lowest range we can offer is 0-1 ppm CH4. Other hydrocarbons have different response factors (eg. 2 for C2H6, 3 for C3H8). Therefore the FID is often called “carbon counter”. This is only true for saturated hydrocarbons without oxygen. Double bonds and oxygen atoms decrease the response factor. The advantage of the FID is the low range capability, disadvantage are non-selectivity and slightly higher installations costs (burner fuel is needed).
An option would be a photometric measurement. But to be sure if this option is feasible we need to know which hydrocarbons have to be monitored and the requested range. We already did solutions for a large variety of gases (e.g. benzene, toluene, styrene, …)
Joe Fong | April 30th, 2012 at 5:47 am #
Re: Application 0 to 1000 ppm H2 in a background of CO
For this application, would it make sense to add a CO measurement channel and use this to cross compensate the H2 channel?
I would assume if this is done then the calibration of the H2 channel would need to be done with 100% N2 (Zero) and 1000ppm H2 in N2 (Span) to avoid a “double cross compensation” of the H2 channel.
Bonnie Brown for Michael Kamphus | May 2nd, 2012 at 7:48 am #
As long as there is a binary mixture of H2 and CO we will get a very accurate measurement of H2 with an analyzer which is calibrated in CO background. There is no need for cross compensation with an additional CO channel. If other impurities like CH4 come into play it becomes more difficult. As CO is still the background gas with a concentration of 90%+, I would go with the H2 analyzer calibrated in CO background but depending on the CH4 concentration variation a CH4 channel for cross compensation should be added.
In principle your assumption that adding a CO channel for cross compensation requires N2 as calibration background gas for the H2 channel is correct.
BOYCE O. GRIFFITH | October 15th, 2012 at 6:45 pm #
Seeking bare NDIR CO, CO2 & H2 sensors.
Cost for 3 for circuit development work.
If successful will be buying 50 to 100
per year, perhaps more.
Boyce Griffith
Ridge Engineering Consulting
107 Beechwood LN
Oak Ridge, TN 37830-7868
Also send to:
boyceg1@gmail.com
boyce.griffith@orau.org
Rosemount Analytical | October 19th, 2012 at 1:03 pm #
Hi Boyce – Thank you so much for your inquiry. We’re following up with the Emerson Rosemount team to get you information on the sensors you’re looking for and will get back to you ASAP!