GAS DETECTION & ANALYSIS


How aIr separatIon unIts use gas analysIs In process control


C


ryogenic air separation processes are used to divide atmospheric air into three pure gaseous


elements – nitrogen (N2), oxygen (O2), and argon (Ar).


Further separation can also produce quantities


of noble gases such as neon, krypton, and xenon. However, the first three elements listed are the products most commonly required by industry, particularly as ultra-high-purity gases for the semiconductor manufacturing sector. Accurate analysis of gas concentrations


across the process supports improved process control, safety, and optimal product quality. Purity measurements ensure the gas products are of the required specifications. For example,


the typical bulk gas composition for N2 is for the gas to contain less than 10 parts per million (ppm) of oxygen. Oxygen usually needs to be better than 99.5 per cent pure, while argon must contain fewer than 10ppm impurities.


Air sepArAtion


The process of air separation may vary according to user requirements regarding the final products, but generally follow the same common principles and stages. Ambient air is filtered to remove dust, and


then compressed. Next it undergoes cooling and is passed through molecular sieve absorbers to remove water vapor, carbon dioxide, and hydrocarbons. This purified, compressed air feed is then


cooled in a “cold box” through heat exchangers and refrigeration processes, reaching a cryogenic temperature of around -180°C. Part of the air is liquefied, creating a liquid that is


rich in O2, while the remaining gas has a much higher N2 concentration. This mixture of liquid and vapor air is


separated and fed to a separation column, which consists of a high-pressure and low-pressure column. The mixture is then separated out into oxygen, nitrogen, and argon streams. As can be surmised, there are a number


of different elements and compounds that must be measured to ensure this process operates safely and efficiently.


Key meAsurement: totAl hydrocArbons


The first point of gas analysis in the process is at the air inlet feedstock, where the concentration of total hydrocarbons (THC) is monitored to ensure the process scrubbers are able to cope with the levels present. For safety purposes, THC measurements are made in the low-pressure column, while the product streams are also monitored for


THC impurities, supporting product quality. This is typically accomplished using the Flame


Ionization Detection (FID) sensing technology, which can measure flammable THC down to parts-per-billion (ppb) levels. FID sensors detect ions formed in the


combustion of organic compounds in a sample, producing charged molecules that cause electrical conduction between two electrodes. These ions are attracted to a collector plate and induce a current upon hitting the plate. The FID measures this conduction and


produces an output which is directly proportional to the concentration of THC in the sample. A logarithmic amplifier enhances this signal,


reducing drift and thermal noise, to deliver an accurate, non-depleting measurement.


Key meAsurement: oxygen


The measurement of percentage O2 is a key parameter for process control, and also to verify


the purity of the O2 product. The main control points are at the low-pressure


column (liquid phase), distillation column (process stream), feed to the low-pressure column, and the crude argon column feed. Percentage O2 checks also support quality


measurements for the final gaseous and liquid oxygen products. Paramagnetic sensing is ideal for these applications, as it delivers fast, sensitive, and


accurate measurements of percentage O2. It is a non-depleting technology, so its performance does not deteriorate over time. At the feed to the pure argon column, a ppm


measurement of O2 is required. This is delivered by a Coulometric sensor, which offers


a highly sensitive measurement and reacts


quickly to changing O2 concentrations. This is also a non-depleting technology, and


can also be used for quality control


measurements by monitoring O2 impurities at ppb levels in the N2 and Ar product feeds. These quality measurements also play a role


in process control, as any indication that the product is insufficiently pure can be used to adjust the process accordingly to bring it back into specification.


Key meAsurement: nitrogen


For process control, N2 measurements are needed at the crude argon feed and also on the feed to the pure argon column. This provides tight control of the argon production process, improving yields. Gas analysers using Plasma Emission Detector


(PED) technology provide an effective solution for these measurements, as they can provide the required levels of detection below 1ppm. The PED sensor consists of a custom quartz


cell with transparent windows fitted with electrodes powered by a controlled radio


38 January/february 2022 | IndustrIal complIance


O2 measurements – the SERVOPRO MultiExact 4100 and SERVOPRO MonoExact


DF310E


N2 measurements – the SERVOPRO Chroma and SERVOPRO Plasma Moisture measurements – the AquaXact 1688


CO2 and N2O measurements – the SERVOPRO MultiExact 4100 As with all gas analysis products, correct


installation and commissioning are essential to their reliable performance, and expert service support throughout the lifetime of the product is highly recommended. By delivering accurate gas measurements


throughout the process, these technologies ensure that the ASU process remains under close control, for safe, efficient operation and a high-purity final product.


servomex servomex.com


frequency electromagnetic field. Multiple optical emission spectroscopy detector assemblies, surrounding the quartz cell, make selective measurements of emitted spectra of multiple gas species at the same time. This highly sensitive and selective speciation


of gases enables trace-level measurements in the ppb range.


other importAnt meAsurements


While we have outlined the key process control measurements for ASU operations, there are other gas analysis applications to consider. Water vapor measurements are essential for both safety and quality throughout the process, particularly since ice formation can block the heat exchanger leading to a build-up of pressure. This can be measured by an Aluminum


Oxide sensor, which offers highly accurate, fast-response detection of moisture concentrations at ppm levels. Carbon dioxide and nitrous oxide must also


be monitored as impurities, mainly in the low- pressure column. Gas Filter Correlation sensing, an enhanced version of Infrared technology, provides a reliable measurement for both these gases, with a real-time measurement response unaffected by background gases.


supplying A comprehensive solution


As the global expert in gas analysis, Servomex supplies a complete solution for air separation unit gas analysis, with measurements for all the process points identified above:


THC measurements – the SERVOPRO FID and SERVOPRO Chroma


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