tmhChE
Chemical
- Jan 26, 2007
- 2
We are studying the redesign and replacement of a 35 year old wet scrubber system for removing NO from arc jet processes. Arc jets heat air to very high temperatures with an electric discharge arc, and then the gas flows through a converging diverging nozzle to produce a supersonic flow for testing space craft heat shield materials for atmospheric entry applications. See section on Arc Jet Complex in this Wikipedia article for background information
It is actually at a lower than usual operating temperature of around 4000 K where the most NO forms, and this can theoretically be as high as 80,000 ppm NO. Arc jets do not run 24 hours per day. Runs can be as short as a few minutes with perhaps 3 runs per day, and the total arc jet running time for a year might be around 50 hours. The current system is based on wet scrubbing with dilute caustic (NaOH), and the most prominent part of the existing facility is a 75 ft diameter sphere that functions as a spray chamber where the long residence time allows for significant oxidation of NO to NO2, which is more easily absorbed than NO. The sphere is followed by a packed tower. I do not have access to chemical process modeling software so I have been doing calculations by hand to model the existing system. I have found kinetics data as a function of temperature for the third order oxidation reaction of NO to NO2 in the presence of oxygen. We have excess oxygen available for oxidation in our waste stream because the source is from the arc heating of air, not a combustion process. I have also found in the literature some information about the performance of packed towers for the absorption of NO/NO2, but I have not found information about how the effectiveness of absorption is affected by temperature. Temperature can be a significant factor for us because the waste stream comes from a Steam Vacuum System (SVS) so it can be as hot as 195 deg F and saturated with water vapor, so it heats up the scrubber system. Prior to the actual arc jet run the SVS is operating in standby mode for sometimes up to a couple of hours, where it continually discharges steam to the scrubber system and heats it up, even before the arc jet run where the NO is produced. Just to give you an idea of a worst case waste stream that the current scrubber would at least theoretically be expected to handle, its flow rate would be 1.5 kg/s air (dry basis), 80,000 ppm NO (dry basis), and saturated with water vapor at 195 deg F which gives 1.5 kg water vapor/kg dry air. If each run is 30 minutes long and there are 3 runs in one day and 10 lbs of NOx is allowed to be emitted in one day, then the scrubber system would have to have an efficiency of 99.6%. Please note that the vast majority of runs during a year are at a very much less severe loading than this worst case scenario.
e=1-((286,000)(10))/((1.5)(80,000)(90)(60)) = 0.996
I doubt the existing system would be capable of this, but this should give you an idea of what we might be trying to design against. On top of this, there is the future prospect of having a larger arc jet reactor that could at least double if not triple this waste load (i.e. a flow rate of 4.5 kg/s air).
Keys issues include:
How to handle the removal of heat from the system if it must be maintained at a temperature considerably lower than 195 deg F to ensure reasonable performance of the system.
Whether designing the spray chamber so that the waste stream flows through it in more of a plug flow reactor (PFR) mode rather than a continuous stirred tank reactor (CSTR) mode as the existing sphere likely acts, would significantly reduce the size of the spray chamber necessary.
Whether oxidation enhancement should be provided with the addition of pure oxygen, or some other means such as an oxidizing scrubber solution.
Any comments or feedback from the forum would be helpful. I am especially interested in any information about how the performance of a packed tower absorbing NO/NO2 with dilute caustic would be expected to be a function of temperature. I started working on this project 2 years ago when I lead a group of 5 chemical engineering students in developing a preliminary design as a part of their plant design course. The little bit of contact with vendors that we made at that time gave me the impression that our application here is very unusual. The project sat dormant for a couple of years but is now being picked up again with more seriousness.
It is actually at a lower than usual operating temperature of around 4000 K where the most NO forms, and this can theoretically be as high as 80,000 ppm NO. Arc jets do not run 24 hours per day. Runs can be as short as a few minutes with perhaps 3 runs per day, and the total arc jet running time for a year might be around 50 hours. The current system is based on wet scrubbing with dilute caustic (NaOH), and the most prominent part of the existing facility is a 75 ft diameter sphere that functions as a spray chamber where the long residence time allows for significant oxidation of NO to NO2, which is more easily absorbed than NO. The sphere is followed by a packed tower. I do not have access to chemical process modeling software so I have been doing calculations by hand to model the existing system. I have found kinetics data as a function of temperature for the third order oxidation reaction of NO to NO2 in the presence of oxygen. We have excess oxygen available for oxidation in our waste stream because the source is from the arc heating of air, not a combustion process. I have also found in the literature some information about the performance of packed towers for the absorption of NO/NO2, but I have not found information about how the effectiveness of absorption is affected by temperature. Temperature can be a significant factor for us because the waste stream comes from a Steam Vacuum System (SVS) so it can be as hot as 195 deg F and saturated with water vapor, so it heats up the scrubber system. Prior to the actual arc jet run the SVS is operating in standby mode for sometimes up to a couple of hours, where it continually discharges steam to the scrubber system and heats it up, even before the arc jet run where the NO is produced. Just to give you an idea of a worst case waste stream that the current scrubber would at least theoretically be expected to handle, its flow rate would be 1.5 kg/s air (dry basis), 80,000 ppm NO (dry basis), and saturated with water vapor at 195 deg F which gives 1.5 kg water vapor/kg dry air. If each run is 30 minutes long and there are 3 runs in one day and 10 lbs of NOx is allowed to be emitted in one day, then the scrubber system would have to have an efficiency of 99.6%. Please note that the vast majority of runs during a year are at a very much less severe loading than this worst case scenario.
e=1-((286,000)(10))/((1.5)(80,000)(90)(60)) = 0.996
I doubt the existing system would be capable of this, but this should give you an idea of what we might be trying to design against. On top of this, there is the future prospect of having a larger arc jet reactor that could at least double if not triple this waste load (i.e. a flow rate of 4.5 kg/s air).
Keys issues include:
How to handle the removal of heat from the system if it must be maintained at a temperature considerably lower than 195 deg F to ensure reasonable performance of the system.
Whether designing the spray chamber so that the waste stream flows through it in more of a plug flow reactor (PFR) mode rather than a continuous stirred tank reactor (CSTR) mode as the existing sphere likely acts, would significantly reduce the size of the spray chamber necessary.
Whether oxidation enhancement should be provided with the addition of pure oxygen, or some other means such as an oxidizing scrubber solution.
Any comments or feedback from the forum would be helpful. I am especially interested in any information about how the performance of a packed tower absorbing NO/NO2 with dilute caustic would be expected to be a function of temperature. I started working on this project 2 years ago when I lead a group of 5 chemical engineering students in developing a preliminary design as a part of their plant design course. The little bit of contact with vendors that we made at that time gave me the impression that our application here is very unusual. The project sat dormant for a couple of years but is now being picked up again with more seriousness.
