Noninvasive Chemical Concentration Analyzer (NCCA) Preserves Process Stream Purity

For years the chemical processing industry has needed a noninvasive, real-time method to measure the concentration of chemicals in process. The NCCA, developed by NMT scientists, uses Fourier transform analysis of high-frequency acoustic signals from ultrasonic sensors attached to the outside of a pipe or small tank to determine the concentrations of chemicals contained within. By taking concentration measurements without actually contacting the fluid, the NCCA prevents contamination of product streams, reduces or eliminates risks for workers who handle hazardous or radioactive materials, and decreases costs by eliminating the need to cut into the pipe or tank as required to install an invasive sensor.

Many industries seek to reduce worker exposure to toxic, radioactive, or other hazardous process streams and to preserve the purity of process streams during monitoring procedures. An important means of achieving both goals is the use of a process monitor that noninvasively determines concentrations of materials in sealed fluid containers, such as pipes and tanks, and provides reliable real-time analytical measurements. In Building PF-4 this is especially important because of the possibility that the inside of the container is radioactively contaminated.

The NCCA is a novel ultrasonic method for evaluating chemical concentrations noninvasively and in real time. It is based on the determination of the velocity of sound through a fluid inside a container, such as a pipe or tank. The velocity of sound is a concentration dependent parameter for binary aqueous or organic mixtures; measurements of sound velocity for many binary mixtures are well-known and can be used to determine the concentrations of such mixtures as they pass through a pipe or while they are stored in a tank. The NCCA analyzes the Fourier transform of the swept high-frequency acoustic spectrum after transmission through the container and its chemical fluid. For fluids of complex mixtures (such as gasolines and insecticides), this diagnostic technique cannot determine individual components, but it can determine the characteristic sound velocity of the total mixture. The mixture can then be compared to a norm, and its consistency can be validated.

For empty pipes and tanks, the acoustic response at low frequencies is strong, complex, and difficult to interpret. At ultrasonic frequencies greater than 100 kilohertz (or higher depending upon the geometry), these structural modes of the container become so closely spaced and weak that the spectrum (system response as a function of frequency) looks like "white noise." However, when the container is filled with liquid, the ultrasonic spectrum changes, exhibiting a series of equally spaced peaks in the midst of the container's white noise as shown in the upper panel of Fig. 3. These resonant peaks arise from standing waves (those acoustic reflections within the fluid that are in phase and additive) in the fluid between the walls of the container. The frequency between any two of these peaks, v, is given by v = c/2l, where c is the velocity of the sound of the fluid, and l is the inside diameter of the container. An accurate deter-mination of the velocity of sound from the high-frequency ultrasonic spectrum requires an accurate measurement of the frequency between the peaks and accurate knowledge of the diameter of a container. The resonant peaks of sound filtered through a container exhibit a significant amount of superimposed structure, which is removed during Fourier transform analysis. Because the diameter of the container does not change with time, the calibration of l needs to be accomplished only once by measuring a fluid whose ultrasonic frequency is known.

A simple software package that Fourier-transforms the transmitted high-frequency acoustic response allows our inexpensive diagnostic system to be insensitive to the low-frequency acoustic signature associated with containment vessel geometry and composition. Using sensors glued to the outside of a 6-inch-diameter pipe, we have measured the fundamental frequency, and therefore the sound velocity, reproducibly to 7 parts in 100,000. To reach this accuracy requires concurrent temperature measurement to compensate for temperature-dependent velocity changes. Velocity measurements using the NCCA in nitric acid are compared to velocity measurements using a commercial instrument. Typically for nitric acid a velocity change of 1 m/s corresponds to a change in concentration of less than 0.1 molar. For the most favorable concentrations, we have demonstrated the ability of the NCCA to analyze nitric acid and sodium chloride solutions to 0.01-molar accuracy.

Figure 3. The NCCA noninvasively measures the concentration of chemicals in a tank or pipe. High-frequency acoustic signals are processed by a computer before and after transmission through the container and its chemical fluid. The computer display shows the ultrasonic sine wave swept through a preset frequency range.

The analyzer consists of a PC, a specialized PC card that simultaneously generates a swept high-frequency signal and detects system response, and two ultrasonic trans-ducers attached to the outside of a pipe or tank . Software written by Dave Sanchez of NMT-6, for LabWindows, takes the Fourier transform of the data and determines the sound velocity of the fluid. We use algorithms that are specific to the chemicals known to be present in the fluid to calculate the concentration from the values obtained for the sound velocity. The analyzer can provide real-time concentration measure-ments for a wide variety of chemical systems and containers. It has been tested on pipes from 0.5 inch to 8 inches in diameter and on a rectangular tank whose dimensions are 6 inches by 24 inches by 12 inches. We currently have databases for nitric acid, hydrochloric acid and sodium hydroxide.

Sensors that must be immersed in the fluid they will assay add to the cost of monitoring and pose the risk of contamination. With its sensors attached to the outside of the process pipe or tank, the NCCA reduces cost by eliminating the need for special piping or flanges for sensor mounting and associated precision welding and construction. Further, its provision for external sensor mounting precludes the need for special construction materials that resist corrosion when immersed in the fluid. The NCCA can be used on any existing container geometry (arbitrary length, diameter up to 2 feet, any thickness, and any cross section) and composition (so long as it transmits high-frequency acoustic signals).

Chemical processing of nuclear material will continue to be required as an important part of global waste management efforts. The immediate benefits of this new analytical technique are a reduced risk of worker exposure to radioactive and hazardous materials; reduced cost of instrumentation, installation, and maintenance; and improved process efficiency. These substantial benefits can also be realized in a whole host of industries that handle or process hazardous or toxic materials. Strict regulation of industry requires that worker exposure to chemical hazards during processing be minimized. The NCCA can contribute to this effort even as it reduces assay costs by tracking concentrations in real time and eliminating traditional sampling methods. Industries such as chemical manufacturing, petrochemicals, agricultural chemicals, and photochemicals handle hazardous materials in large quantities and could benefit from this instrument.

D. Kirk Veirs, Noah G. Pope, David E. Sanchez, Vincente D. Sandoval are the developers of this diagnostic technique.