This Small Business Innovation Research Program (SBIR) Phase I project will develop a "Fiber-loop Cavity Ring-down spectrometer" to address the critical need for an in situ means to measure contaminants in cryogenic liquids. Lacking a way to measure impurities in the liquid phase, cryogenic liquids makers and their users must rely upon gaseous samples from the headspace of the container or extracted and evaporated samples. Such methods are costly, time-consuming and tend to be error-prone. The two major hurdles in developing a concentration sensor for cryogenic applications are achieving sufficiently low detection limits and operating at extremely low temperatures. For the Phase I project, Cavity Ring-down Spectroscopy, offering very low detection limits, will be combined with optical fibers, which have been proven to work well at cryogenic temperatures. The Fiber-loop, a ring-cavity comprising a strand of optical fiber, must contain a sensing section to permit the guided light to interact with the sample. Since the sensing section ultimately determines the detection limits of the sensor, multiple fiber tapers or a fiber section without cladding will be incorporated into the loop and optimized for sensitivity. The resultant device will perform fast, accurate and sensitive measurements in cryogenic liquids.
The broader impact/commercial potential of this project serves diverse applications of cryogenic liquids, such as high-purity gas manufacture; cooling high-tech equipment, including magnetic resonance imaging (MRI); hydrogen fuel cells; frozen food; blood banks and biotechnological applications, such as freezing vaccines and execution of chemical reactions. Moisture build-up within cryogenic processing systems promotes ice and blockages, posing a safety risk. Increased levels of impurities in cryogenic liquids for high-purity gas production reduce purifier lifetimes and lead to contamination at the point of use. Hydrogen fuel cells offer cleaner and more efficient power than traditional sources, but require relatively contamination-free materials to guarantee their performance and lifetime. The purity of cryogenic liquids used to freeze food, as well as biological and medical samples, has public health implications. Here, in situ measurements of potentially harmful contaminants, such as benzene, carbon monoxide and biological species (viruses, bacteria), are critical. An effective and affordable means of monitoring contaminants in the liquid phase will improve safety, reduce waste and promote better process control.
This Small Business Innovation Research project focused on developing a Cavity Ring-Down Spectroscopy (CRDS) sensor, using silica fibers to measure trace concentrations of contaminants in cryogenic liquids. Identifying and controlling sources of impurity is critical for many cryogenic liquid applications, from frozen food to hydrogen fuel cells to MRIs. For example, in cryogenic liquid transmission and distribution, water contamination can cause icing up of filters and transfer lines, and accumulation of oxygen condensing in cryogenic liquids poses a serious risk of explosion. The majority of cryogenic liquids are utilized in the production of gases for ultra-high-purity applications, requiring tight control of impurity levels. Impurities can be introduced during transportation and delivery, and can build up over time in transfer lines, critical control components, and long-term storage facilities. Monitoring contaminants in the liquid phase from the original source, following air separation, to the point-of-use is essential for improving safety and reducing the risk of costly production losses. Despite the pronounced need, based upon both safety and process control considerations, there is currently no way to directly measure impurities in the liquid phase within a cryogenic tank or in filling stations. Standard analyzers, such as near-IR and FTIR spectrometers, cannot operate at such low temperatures. Instead, contamination is measured in the gas phase by various instruments. This approach typically requires extracting a sample from the cryogenic liquid and evaporating it, which introduces complexity, the possibility of error, and possible back-contamination from the sample extraction probe. Another option is headspace gas sampling for continuous monitoring, but the gas phase does not necessarily have the same impurity concentrations as the liquid phase due to different boiling-point temperatures of the contaminants and matrix. Therefore, the levels of contamination might be considerably larger in the liquid phase, and impurities may accumulate in the liquid phase and yet go undetected in gas phase measurement. Until recently, challenges to development of a liquid-phase analyzer stemmed from the nature of cryogenic liquids (very cold temperatures, often pressurized). Today, however, sensors based on optical fiber have been proven to work well in cryogenic liquids. Indeed, temperature sensors and fill level sensors based on fiber optics are already commercially available for cryogenic liquids and have been implemented in many applications, although no cryogenic concentration sensors have been developed so far. The main hurdle to useful concentration measurements is achieving sufficiently low detection limits, which are usually required to be at parts per million (ppm)-levels or even parts per billion (ppb)-levels. Combining the robustness of fiber sensors with the sensitivity of Cavity Ring-Down Spectroscopy, Fiber-Loop Cavity Ring-Down Spectroscopy (FL-CRDS) provides a practical means to continuously measure trace contaminants in cryogenic liquids, without the need for evaporation or headspace sampling. FL-CRDS employs a ring-cavity formed by strand of optical fiber bent into a loop with a sensing element, allowing the light to interact with the sample. Similar to mirror-based CRDS, light is coupled into the loop and its decay time is measured. In Phase I, we successfully constructed a FL-CRDS system, optimized the sensing element and measured the performance in liquid nitrogen, which showed no degradation in performance compared to room air. Thus, we demonstrated that FL-CRDS can measure contaminants in cryogenic liquids and that it is feasible to reach a detection limit of 1 ppm of water in liquid nitrogen. In sum, this novel approach has the potential to open the door to a whole new world of direct, real-time measurement critical to the control of harmful contaminants in cryogenic liquids
