A new process and equipment have been developed for reducing the emissions of hazardous nitrogen oxides (NOx), and eliminating a hazardous waste stream. In this process, a waste gas stream of nitrogen and NOx, arising from spacecraft-propellant operations, is reacted (scrubbed) with a dilute solution of hydrogen peroxide (scrubber liquor). The resulting liquor is treated simultaneously with potassium hydroxide during the scrubbing operation to produce potassium nitrate. The overall effect of the process is to absorb NOx from the waste gas stream into the scrubber liquor where NOx is converted into an aqueous solution of potassium nitrate, which can be used as a fertilizer.

In the original application for which the process and equipment were developed, NOx is an undesired gaseous effluent from the handling of nitrogen tetroxide, a hypergolic propellant oxidizer used in space shuttle and Titan rockets. The process and equipment could also be adapted to removal of NOx from flue gases, ventilation streams from metal-pickling operations, and other gaseous effluent streams.

A Comparison of the current oxidizer scrubber process with the new proposed scrubber process shows the advantage of the modified configuration.

As in a typical scrubber process, the improved process has a scrubber liquor pumped to the top of a tower and sprayed down through packing, while the gaseous mixture rises through the column and is vented after being scrubbed by the falling liquor. The concentration of hydrogen peroxide (H2O2) is controlled to 1.0 percent in the liquor storage tank by automatic addition of 35-percent H2O2. Also, the pH is maintained at 7.0 by the automatic addition of potassium hydroxide solution.

Scrubber liquor is continually sampled during scrubbing to determine the pH and hydrogen peroxide concentration. A commercial controller monitors pH and adds 45-percent potassium hydroxide solution, as needed. The potassium hydroxide reacts with nitric acid produced in the liquor, as nitrogen tetroxide is scrubbed. As the liquor is recycled back to absorb more oxidizer, the concentration of potassium nitrate builds up to a maximum of 15 weight percent, after which it is stored in a separate tank for shipment to fertilizer use areas.

A new invention, the NOx control system including the hydrogen peroxide controller, repeatedly takes small samples of the scrubber liquor and measures the peroxide content. A programmable logic controller activates switches, valves, and pumps to control the rate of addition of a 35-percent hydrogen peroxide solution to maintain a 1-percent peroxide concentration in the bulk liquor. The hydrogen peroxide controller (see figure) is based on the chemical reaction between hydrogen peroxide and sodium hypochlorite, ordinary household bleach:

H2O2 + NaOCI → H2O + NaCl + O2

This reaction takes place in a closed vessel, so that the increase in pressure from generation of O2 can be measured as an indication of the amount of H2O2 in the sampled scrubber liquor.

The improved process is intended to replace an existing process in which the scrubber liquor is a 25-weight-percent solution of sodium hydroxide. When all impacts of both processes are considered, the improved process is found to cost less, to eliminate a hazardous waste stream, to lower NOx emissions, and to produce a valuable liquid fertilizer instead.

This work was done by Dale E. Lueck of Kennedy Space Center and Clyde F. Parrish and Ronald G. Barile formerly of I-Net.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
Kennedy Space Center; (407) 867-6225

Refer to KSC-11884.

NASA Tech Briefs Magazine

This article first appeared in the October, 1998 issue of NASA Tech Briefs Magazine.

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