IMPACT OF THE HELIUM SHORTAGE ON PROCESS GAS CHROMATOGRAPHS

Transcription

1 IMPACT OF THE HELIUM SHORTAGE ON PROCESS GAS CHROMATOGRAPHS Al Kania Siemens Industry, Inc Hollister Road Houston, TX KEYWORDS ON-LINE ANALYSIS, PROCESS ANALYZER, HELIUM, CARRIER GAS, GAS CHROMATOGRAPH, ANALYZER SHELTER ABSTRACT A global helium shortage continues to squeeze stock to very low levels forcing users of helium to stretch their limited supplies. The impact is being felt in nearly every corner of society from helium used for medical scanning devices to the use of helium for party balloons. The helium shortage is also impacting process gas chromatographs that use the helium as a carrier gas. Helium has been the carrier gas of choice for many users in North America due to its inherent safety compared to hydrogen carrier. Not only are process GC users experiencing sharply higher prices; many are reporting the inability to acquire chromatograph-grade helium in the quantities they need for the site in a timely manner. As a result, many existing helium carrier gas users are converting their process GCs to hydrogen carrier gas. This paper will discuss the impact of the helium carrier gas shortage on process gas chromatographs as well as the shelter design ramifications of converting existing GCs to hydrogen carrier gas. Session 07-2: Page 1

2 INTRODUCTION In order to perform the chromatographic analysis, process gas chromatographs (GCs) require a carrier gas to transport the sample being measured through the separation columns to the detector(s). Depending on the components being measured, the measuring ranges and other analytical variables, the carrier gases tend to be helium, hydrogen or nitrogen. Helium s chemical properties, as well as inert nature made it, by far, the most popular choice in the U.S. for process GCs. Outside the U.S., hydrogen was the most common due to the high price of helium as well as the superior chromatographic properties that hydrogen has over helium. However, over the past few years, the price and availability of helium has steadily declined for reasons explained below. Some user sites have even been reporting that their helium suppliers are rationing their limited supplies of helium. As a result, users of process GCs have been inquiring about the possibility of converting their GCs from a helium-based system to a hydrogen-based system. BACKGROUND INFORMATION ON U.S. HELIUM GAS SUPPLIES Helium is one of the most abundant chemical compounds in the universe, second only to hydrogen. Yet on Earth, it is surprisingly rare to find. Helium s extremely low density makes it lighter than air resulting in most helium on Earth, floating out of atmosphere over the eons. Nonetheless, helium s unique chemical properties (1) have resulted in its extensive use throughout our society: Extremely low boiling point Low density High thermal conductivity Chemical and radioactive inertness Helium uses range from filling birthday party balloons, floats for a parade, cooling of cat scanners in a hospital, and missile guidance systems for the military. And fortunately for those depending on plentiful supplies of helium, helium can be found trapped in certain natural gas formations such as those found in the panhandle of Texas and Oklahoma. HELIUM BECOMES A U.S. STRATEGIC ASSET It was the militaries use of helium that resulted in its designation as a U.S. national strategic asset (2). Following World War I, the National Helium Reserve was established outside of Amarillo, Texas to ensure a steady supply of helium for U.S. airships. While the role of airships in modern warfare quickly ended with advances of aviation in World War II, it was kept as a strategic asset due to its new role as an important pressurizing agent for strategic missiles during the Cold War. Session 07-2: Page 2

3 GOVERNMENT REGULATIONS LEAD TO GLOBAL HELIUM SHORTAGE However, with the fall of the Soviet Union in the 1990 s as well as political pressure to reduce big government and to pay off the deficit, Congress passed the Helium Privatization Act of This Act required the government to sell off excess reserves to the private sector at a low flat rate over the course of 18 years. This resulted in the helium market being flooded with cheap helium supplies. This not only resulted in the often wasteful use of helium by industry but also made it economically impossible for the private sector to justify stepping into helium refining industry. With the availability of cheap and plentiful helium coming to an end in 2014 and without the private sector in a position to fill that role, the Helium Steward Act of 2012 was passed to extend the sale of the government reserves as well as allowing the price to rise to a value closer to realistic market prices. A number of helium refineries are emerging around the globe to extract helium from underground sources such as Qatar, Algeria, Iran and Russia. As these plants come on-line, availability of helium will eventually stabilize but the price of helium will continue to rise. IMPACT OF CONVERTING FROM HELIUM TO HYDROGEN FOR GC CARRIER GAS The shortages of helium have been impacting the users of process analytical equipment including gas chromatographs. The use of helium as a carrier gas has been extremely popular in the United States due to its excellent chromatography properties as well as safe and inert nature when compared to hydrogen. But many domestic users of helium carrier gas are reporting limited availability of chromatography-grade helium with months-long lead times in many cases. As a result, some users are requesting that their existing process gas chromatograph installations be converted from using helium as the carrier gas to hydrogen. This is not as simple as just changing out the carrier gas bottle as the choice of carrier impacts the GC s chromatography as well has implication on the safety of the analyzer installation. IMPACT ON THE GC S CHROMATOGRAPHY The efficiency of a gas chromatograph s separation depends on a number of variables (3). The oven temperature, the liquid loading in the columns, and the column sizing all play a role in how one compound separates from another. These variables are all detailed in an equation developed by van Deemter (4) to calculate the height equivalent to a theoretical plate (HETP) for packed columns with smaller values being more desired. A simplified version of this equation is: Session 07-2: Page 3

4 HETP = A + B/ µ + C µ (1) Where: HETP = Height Equivalent to a Theoretical Plate A = A constant that accounts for the effects of eddy diffusion in the column B = A constant that accounts for the effect of molecular diffusion of the vapor in the direction of the column axis C = A constant proportional to the resistance of the column to mass transfer solute through it µ = Linear velocity through the packed column of the a non-retained compound From Equation 1, calculations of the HETP versus Linear Velocity for various common GC carrier gases can be plotted (see Figure 1). FIGURE 1. van Deemter Graph for Common GC Carrier Gases As can be seen by the above graph, the speed of components eluting from a packed column can vary significantly by changing from one carrier gas to another. So for users looking to switch from helium as a carrier gas to hydrogen can often see a reduction of the analyzer s cycle time. On average, the cycle time can be reduced 5 10% with all other variables held constant. Another aspect of the chromatography that may be impacted by the use of hydrogen is slightly improved (lower) HETP values. This can allow for slightly reduced oven temperature if desired. Lower oven temperatures can potentially reduce the stress on mechanical components in the oven as well as leading to slightly longer column life. Column life is also improved in some Session 07-2: Page 4

5 situations due to acidic sites within the walls of the column being removed due to hydrogen acting as a reducing gas. While there are a number of important advantages to using hydrogen over helium as a carrier gas, a number of other issues within the GC system need to be considered and accounted for when retrofitting an existing system. For example, if the GC is using Electronic Pressure Controllers (EPCs) for the control of carrier pressure, it may be necessary to make adjustments to the EPCs to account for the lower density of hydrogen. Similarly, for GCs using mechanical regulators for controlling the carrier pressure, those may also need adjustment or replacement. Since most process GCs use some form of column switching to minimize cycle times, the valve switching times will often need to be adjusted to compensate for components eluting faster with hydrogen as carrier. If the faster cycle time due to the use of hydrogen is not important, it may be possible to minimize the valve switching time changes by reducing the hydrogen carrier pressure. Depending on the oven temperature, column length, etc., the pressure may need to be reduced as much as 30 40% to retain the older elution times. For GCs with sample injection techniques using a sample splitter, the splitter flow rates may need to be adjusted to account for the lower density of hydrogen. Similarly, if the analyzer uses programmed temperature rates for the analysis, the ramp rates and temperature settings may need adjustment to compensate for the faster elution due to hydrogen. It may also be required to change the length of the separation columns for certain chromatographic separations. For example, if the analyzer was using a heartcut technique, the faster elution time may require longer columns to accomplish the necessary separation. If the helium carrier gas was also being used to switch sample injection and column switching valves, this will likely need to be modified since most installations would not want to use hydrogen for this purpose. So instrument air would need to be installed into the GC for this purpose. Finally, if the GC is using certain polar separation columns, there can even be changes in the order components elute. This could require a review of the entire chromatography to minimize measurement errors. It may also be desirable to review the entire chromatography scheme to see if there have been advances in chromatography techniques since the analyzer was originally installed. Over the years a number of innovative techniques have been introduced such as parallel chromatography that could significantly speed up analyzer and/or simplify the analyzer s hardware design. IMPACT ON THE GC S SHELTER INSTALLATION Even though hydrogen has many chromatography advantages over helium, helium was the overwhelming preference by process GC users due to its inherent inert nature. Since many Session 07-2: Page 5

6 process GCs are installed in analyzer shelters, the concern has been the dangers of a hydrogen leak and how that would impact the enclosed space of the shelter. These concerns are certainly valid but with modern hazardous area classification rules and standards, a wide range of solutions for the use of hydrogen are available. It is important to note that outside the U.S., hydrogen carrier is the norm so the safe use of hydrogen as a carrier gas for GCs is well established. The complication comes when converting an existing installation where the shelter was designed for helium carrier for the GCs to one based on hydrogen. Depending on the plant s safety guidelines as well as various governing agency regulations, there can be a number of changes necessary before hydrogen carrier gas is brought into the shelter since it is possible the area classification of the shelter will change to Group B. Process GC users must check with their plant safety authorities and guidelines for specific actions. The topics discussed here are merely suggestions on possible action that may or may not be required and do not constitute any specific expert advice on any specific or even hypothetical installation. With that disclaimer in mind, one of the first issues that many users consider when bringing hydrogen carrier gas into a shelter is to limit the possible flow in case of a leak of a carrier gas line inside the shelter. This can often be accomplished by installing suitable flow restrictors on the outside of the shelter on the hydrogen carrier gas line. Proper engineering standards should be referenced to determine the restrictions needed. It is also important to insure there is proper ventilation inside the shelter in the event hydrogen does accumulate. The installation of hydrogen gas sensors inside the shelter is also advised for many installations to warn if hydrogen does accumulate in the shelter. Finally, it is worth pointing out that even though helium is the carrier gas used by the process GCs, the shelter may already be set up to have hydrogen brought inside. Process GCs that have methanizers, flame ionization or flame photometric detectors would likely need hydrogen as part of the analyzer s operation. IMPACT ON LONG-TERM COST-OF-OWNERSHIP In addition to the chromatography benefits of using hydrogen as a carrier gas is that hydrogen is much cheaper than helium and the price difference continues to grow as helium gets more and more expensive. Availability of hydrogen is also very easy for most locations and not subject to the rationing being seen with helium. Below (see Figure 2) are some example returns-on-investment for converting a process GC from helium to hydrogen. These costs are averages of changes needed for the GC conversion and reflect recent differences in price between helium and hydrogen. But they do not reflect and costs associated with changes to the shelter itself. Session 07-2: Page 6

7 FIGURE 2. ROI Examples for Converting From Helium to Hydrogen Carrier CONCLUSIONS The days of plentiful and cheap helium are coming to an end. The artificially low prices that resulted from government regulations are disappearing. But with the proper analyzer shelter design, hydrogen s superior chromatography properties, low cost and easy availability is making hydrogen the carrier-of-choice for many process GC users. Converting existing process GC installations from helium to hydrogen carrier is very viable if done correctly and can even result in lower long-term operational costs due to the cheaper price of hydrogen. REFERENCES 1. Perry, Robert H., Green, Don W., Physical And Chemical Data, Perry s Chemical Engineers Handbook, Seventh Edition, McGraw-Hill Publishing, pp. 2-15, Helium Shortage, Houston Chronicle, A Hearst Newspaper, p. B6, November 21, Annino, Raymond, Villalobos, Richard, The Rate Theory and Chromatographic Efficiency, Process Gas Chromatography: Fundamentals and Applications, Instrument Society of America, pp , Van Deemter, J. J., Zuiderweg, F. J., and Klinkenberg, A., Chemical Engineering Science, Volume 5, p. 271 (1956). Session 07-2: Page 7