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petroleum refi neries. Natural sources of nitrogen oxides include biomass burning, lightning and soil release [2].


NITROGEN OXIDES MEASUREMENTS IN HYDROCARBON GASES


Nitrogen oxides (NOx) constitute group air pollutants with economic implications in the hydrocarbon production


industry. For example in olefi ns production, NOx may form gum products when they are combined with resins and produce explosive substances in the cold box unit. Dry colorimetry offers an affordable solution for NOx monitoring


at very low concentrations. The methodology proposed for C.I. Analytics, for laboratory and on line applications, presents a detection limit of 10 ppb in hydrocarbon streams with response times near to fi ve minutes.


1. INTRODUCTION


Nitrogen oxides are a group of oxidised species relevant for the environmental chemistry. This group of oxides, commonly named NOx, is represented by nitric oxide (NO) and nitrogen dioxide (NO2


) as the most prevalent species in the air. In fact,


environmental regulatory organisms establish air quality policies as a function of NO2


concentration [1]. The NOx importance lies


in the wide participation in the chemical reactions taking place in the troposphere. NO2


reacts with air in the presence of ultraviolet


(UV) radiation to produce ozone and NO. The NO reacts with free radicals, through photochemical complex mechanisms with volatile organic compounds (VOC) present in the atmosphere. Besides, NOx in conjunction with sulphur oxides and other air pollutants contribute to the acid rain formation which has negative effects in certain ecosystems and directly affects some economic sectors.


NOx emission is a product of anthropogenic and biogenic combined activities. Anthropogenic sources are automobile emissions, electric plant boilers, incineration products, gas turbine products, cement manufacturers, nitric acid manufacturers and


Further, the presence of NOx in the environment represents a public health concern. The main source exposition to NOx is through the respiratory tract. Being exposed to high NOx levels may cause irritation of the mucous membranes, fl uid accumulation in the lungs and even death. Skin or eye contact with NO or NO2


produce


chemical burn in the tissues. In workplaces, the maximum exposure limits for NO was established at 25 ppm during an 8 hour workday, 40 hours per week and 5 ppm for NO2


for an exposure time of


15 minutes according to the Occupational Safety and Health Administration (OSHA)[3,4].


In petroleum refi neries, NOx levels are controlled since nitrogen compounds are an undesirable product. Nitrogen content in petroleum distillates may deactivate the acidic sites of the catalysts, reducing the cracking effi ciency and some of those components have a corrosive effect. Additionally, there are safety concerns about the nitrogen oxides presence in hydrocarbon streams. At specifi c conditions, nitrate explosive resins may be formed in ethene production inside the cryogenic recovery unit.


Intrinsically with the emerging technologies for enhanced oil recovery (EOR), specifi cally hydraulic fracturing operations, there are concerns about the economical viability of the projects with the least possible impact to the environment. For example, during the Marcellus shale extraction plans academic and fi eld studies have been performed to predict and correct the unconventional natural gas extraction effects in nitrogen oxides emissions in the United States [5].


Whether to comply with environmental standards or to control process effi ciency, it is mandatory to monitor the concentration of nitrogen oxides at low concentrations. Nowadays, there is a wide market of instruments able to quantify NO2


levels at very low


detection limits. Those techniques include spectroscopic methods, such as Differential Optical Absorption Spectroscopy (DOAS), Laser induced fl uorescence (LIF), Resonance Enhanced Multiphoton Ionisation (REMPI) and GC-Chemiluminescence. Although some of the analysers offer high selectivity for direct or indirect measurement of NO2


Aware of the need to develop a method that produces reliable results for NOx quantitation, C.I. Analytics developed an analyser capable of measuring NO2


at concentration levels as low as


10ppb. The principle of the analytical technique relies on the dry colorimetric method where a targeted compound produces different color intensities proportional to the concentration in the gas phase. The colorimetric detection is carried out with an Photodiode system sensing the colour changes of a chemically impregnated paper that reacts selectively to the NO2


and the fi nal


, the system components complexity as well as


an appropriate operational expertise makes the cost of the projects associated to this application prohibitive [6].


presence. The


NO present in the original gas sample or standard (for the analyser calibration purposes) is selectively oxidised to NO2


results are reported as total NOx content. Within the advantages presented by the technique, the effi ciency of the oxidation in a reasonable period of time (approximately less than fi ve minutes of analysis time) at very low concentration levels with virtually no known interferences what are most relevant. Moreover, the fast response is achieved thanks to a catalyst that operates at temperatures below 100°C to produce NO2


.


In the next section, some experimental results are presented discussing the features that make this method unparalleled to monitor NOx content at trace levels in hydrocarbon streams.


Figure 1: Calibration curve results for ppb range. The standard used is a certifi ed mixture of NO diluted with ethene.


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