The instant invention is in the field of methods for the control of excess air in cracker furnace burners. The production of olefins by thermally cracking a hydrocarbon material, such as petroleum naphtha, is one of the most important processes in the chemical process industry. For example, ABB Corporation reportedly constructed a cracking plant in Port Arthur Texas having a capacity to produce over a million tons of ethylene and propylene per year. The cracking process is conducted in a “cracker”. A cracker usually comprises an enclosure containing tubes and a burner. Heat generated by burning a fuel heats the hydrocarbon material flowing in the tubes so that the hydrocarbon material is thermally cracked to produce, among other things, ethylene and propylene.
Ordinarily, a cracker is comprised of a radiant section and a convection section. The burner is positioned in the radiant section so that the tubes positioned in the radiant section are heated primarily by radiant heat emitted from the walls adjacent to the burner. The combustion gas from the radiant section is then directed to the convection section where heat from the combustion gas is recovered to heat tubes positioned in the convection section. An oxygen sensor, such as a zirconium oxide oxygen sensor, is ordinarily positioned in the cracker between the radiant section and the convection section to facilitate control of the air/fuel ratio of the burner. The overall efficiency of the cracker is primarily a function of the amount of excess air present in the firebox and the temperature of the exhaust gas from the cracker. It can be beneficial from an efficiency viewpoint to control the amount of air in the furnace. Carbon monoxide and smoke emissions from the cracker tend to increase when the amount of air used in the burner is reduced below the stoichiometric ratio of air-to-fuel. On the other hand, too much excess air can reduce the overall efficiency of the cracker and can result in excessive emissions of oxides of nitrogen. Therefore, accurate control of the amount of excess air used in the cracker furnace is necessary for an optimum balancing of efficiency and for the control of emissions.
The oxygen sensor of a conventional cracker furnace is a “point measurement device”, i.e., it measures oxygen corresponding to a small volume at the position where the sensor is located. Such a measurement is not representative of the oxygen concentration in the cracker furnace as a whole. It would be an advance in the art of the control of cracker furnaces if a system were developed that provided a more representative determination of oxygen in the cracker. Also, it is well known that conventional zirconium oxide sensors are subject to interferences known to affect the accuracy of the O2 measurement (such as hydrocarbons and CO gases). It would be an advance in the art of the control of cracker furnaces if a system were developed that was more immune to these interferences.
Section II.4.3, Sensors for Advanced Combustion Systems, Global Climate & Energy Project, Stanford University, 2004, by Hanson et al., summarized the development of the tunable near-infrared diode laser and absorption spectroscopy approach for the determination of oxygen, carbon monoxide and oxides of nitrogen in the combustion gas from a coal fired utility boiler, a waste incinerator as well as from jet engines. Thompson et al., U.S. Patent Application Publication U.S. 2004/0191712 A1 applied such a system to combustion applications in the steelmaking industry. It would be an advance in the art if the tunable near-infrared diode laser and absorption spectroscopy approach for the determination of oxygen, carbon monoxide and oxides of nitrogen in combustion gas were applied to thermal crackers.
The instant invention is a solution, at least in part, to the above-stated problem of the need for a more reliable and representative analysis of combustion gas from a thermal cracker furnace. The instant invention is the application of the tunable near-infrared diode laser and absorption spectroscopy approach for the determination of, for example, oxygen, carbon monoxide and oxides of nitrogen in the combustion gas from a thermal cracker furnace.
More specifically, the instant invention is a method for control of the air/fuel ratio of the burners of a thermal cracker for producing olefins which comprises a firebox portion, a bridge wall portion and a convection portion, comprising the steps of: (a) directing a wavelength modulated beam of near infrared light from two tunable diode lasers that are positioned with a line of sight through combustion gas from burners located in the firebox portion at a location in the bridge wall portion where mixing of the combustion gas is uniform, one of the tunable diode lasers being tuned to a frequency characteristic of oxygen to establish a signal for oxygen content of the combustion gas and one being tuned to a frequency characteristic of carbon monoxide to establish a signal for carbon monoxide content of the combustion gas to a pair of near infrared light detectors, one for each tunable diode laser, to generate two detector signals, one for each of oxygen and carbon monoxide; (b) analyzing the detector signals for spectroscopic absorption at wavelengths characteristic of oxygen and carbon to determine their respective concentration in the combustion gas; and (c) adjusting the air/fuel ratio of the burners (i.e. excess air in the furnace) in response to the concentrations of the oxygen and carbon monoxide of step (b).
As used herein, each of “uniform mixing” and “a location where mixing is uniform” equates to a location that does not include a recirculation zone. A preferred placement of the two tunable diode lasers locates them such that their line of sight focuses upon a location where mixing is uniform. The location preferably provides conditions consistent with those in the combustion zone such that gas concentrations for oxygen and carbon monoxide at the location represent or indicate a true air-to-fuel ratio present in the combustion zone proximate to burners contained in the combustion zone.
The method of this invention employs two tunable diode lasers (TDLs), one for each of oxygen and carbon monoxide. Skilled artisans recognize that current equipment limitations of TDLs provide some ability to vary frequency, but not enough that a single TDL can be tuned to cover frequencies as disparate as those for carbon monoxide (wavelength of number of 2325 nanometers (nm) to 2330 nm) and oxygen (wavelength of 760 nm to 764 nm). If desired, one can add one or more TDLs to measure other combustion gases such as nitrogen oxides, but doing so increases costs associated with measurement and does not provide a concurrent increase in speed or accuracy of measuring carbon monoxide and oxygen.
The two TDLs may be positioned such that they are parallel to one another or orthogonal to each other or canted such that their beams cross one another so long as their respective beams intersect a location in the bridge wall portion of the thermal cracker where mixing is uniform and thereafter enter into operative contact with an associated near infrared light detector (i.e. each TDL is paired with a near infrared light detector). Beams from the two TDLs pass directly through combustion gases at the above location without previously passing through a multiplexer or thereafter passing through a demultiplexer.
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While the instant invention has been described above according to its preferred embodiments, it can be modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the instant invention using the general principles disclosed herein. Further, the instant application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the following claims.
Number | Date | Country | |
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Parent | 12735360 | Dec 2010 | US |
Child | 14248585 | US | |
Parent | 11934210 | US | |
Child | 12735360 | US |