A broad range of thermal analysis (TA) techniques measure the effects of temperature on stability and other physical properties of polymers that are used in pharmaceuticals and pharmaceutical packaging. When TA measurements are taken, the chemical nature of the polymer should not change because of a reaction with oxygen or water vapor in the sample chamber.
To minimize the possibility of oxidation or other reactions, a high-purity inert gas such as nitrogen is passed through the sample chamber to displace the air. The DSC-1 Differential Scanning Calorimeter (Mettler-Toledo, Columbus, Ohio), for example, uses a flow of nitrogen on the order of 200 mL/minute at a pressure of two to five pounds per square inch (psi) to maintain optimum analytical conditions.
A high-pressure tank, a dewar, or an in-house nitrogen generator can supply nitrogen for thermal analysis. An in-house generator provides significant benefits compared with other approaches, including increased safety and convenience, lower cost, and diminished use of energy.
Figure 1. Nitrogen is generated from air using a hollow-fiber membrane bundle.
IMAGE COURTESY OF PARKER HANNIFIN CORP.
Figure 1. Nitrogen is generated from air using a hollow-fiber membrane bundle. It has a small diameter, so many fibers are bundled together to provide a large surface area for the permeation of oxygen and water.
Generation of Nitrogen for Analysis
In-house generation of nitrogen from ambient air requires the removal of oxygen, water vapor, and particulate matter. Nitrogen can be generated from air using a hollow-fiber membrane that permits oxygen and water vapor to permeate the membrane while the nitrogen flows through the tube. A fiber membrane has a small internal diameter, so a large number of fibers are bundled together to provide a large surface area for the permeation of oxygen and water (see Figure 1, left).
The Model N2-04 Nitrogen Generator (Parker Hannifin Corporation, Haverhill, Mass.) can provide six standard liters per minute (SLPM) of 99% nitrogen at 145 psi. Compressed air is first filtered to remove liquids and particulate matter; the filters are equipped with float drains to empty liquids that accumulate inside the filter housing. Separation of the nitrogen takes place inside the membrane bundle. The nitrogen is then directed downstream while oxygen and water molecules are ported to the atmosphere at a low pressure. A 0.01 µm membrane performs the final filtration, and the gas is delivered to the analyzer (see Figure 2, p. 27). The nitrogen has an atmospheric dew point of -58°F (-50°C), contains no particulate matter greater than 0.01 µm and no suspended liquids, is hydrocarbon- and phthalate-free, and is commercially sterile.
In-house generators can provide nitrogen with a purity as high as 99.9999%, with CO, CO2, H2O, and O2 levels at less than one parts per million, and at a flow rate and pressure compatible with thermal analysis.
Benefits of In-House Generator
An in-house nitrogen generator is safer, significantly more convenient, and less expensive than other methods. In addition, use of an in-house generator dramatically reduces environmental impact.
Because an in-house nitrogen generator will not alter the atmospheric composition of the air in the lab, it is considerably safer than a tank or a dewar. The generator separates the O2 from the N2 and vents it to the atmosphere; the 200 ml/min of N2 that is used by the TA instrument is then vented back to the room as well. The net change to the room atmosphere is zero.
In contrast, serious hazards are present when nitrogen gas is supplied to a thermal analyzer using a high-pressure gas tank or a liquid tank. If the contents of a full tank were suddenly vented into the laboratory, up to 9,000 L of gas would be released into the atmosphere. This volume would displace the equivalent amount of laboratory air, thereby reducing the breathable oxygen and potentially creating an asphyxiation hazard for laboratory occupants.
Use of an in-house generator eliminates the possibility of injury or damage that can occur when a gas tank is transported and installed. A standard gas tank is quite heavy and can pose a significant hazard to staff and facilities if the valve on a full tank is compromised during transport. In many facilities, trained technicians are used to replace gas tanks.
When a dewar flask or a high-pressure liquid tank is used, the possibility of user contact with liquid nitrogen, which has a boiling point of -196°C, must be considered. As with a high-pressure tank, a leak in the delivery system could release a significant amount of gas into the laboratory.
Roland Brunell, special projects manager at Danafilms Inc., of Marlborough, Mass., a manufacturer of films for packaging, said that safety was “the primary reason that an in-house generator was selected for the DSC used for analyzing polyethylene films.”
Figure 2. Schematic design of a N2-04 nitrogen generator.
IMAGE COURTESY OF PARKER HANNIFIN CORP.
Figure 2. Schematic design of a N2-04 nitrogen generator.
More Convenient
When an in-house generator is used, the gas can be supplied continually for 24 hours, seven days a week, with no user interaction other than routine annual maintenance. With tank gas or a dewar, on the other hand, the user must pay close attention to the level of gas in the tank and replace it periodically to ensure that the gas will not be depleted in the middle of a long series of analyses. For safety reasons, tanks are typically stored outside in a remote area, so replacing a cylinder can be time-consuming as well as inconvenient in inclement weather. Also, the analyst may need to get a qualified handler to move the tanks. A pressurized tank can be a significant hazard if the laboratory is located in a seismic zone.
If a nitrogen tank must be replaced during a series of analyses, analytical work will be interrupted to restart the system and wait for a stable baseline. In addition, if a series of automated analyses is desired, perhaps overnight, the analyst must ensure that a sufficient volume of gas is available before starting the sequence.
An in-house nitrogen generator allows for continuous operation of the thermal analyzer, and calibration only requires the measurement of a standard sample at a user-specified interval to ensure proper operation of the system. When a new tank is installed, however, the system may need recalibration to ensure accuracy, a time-consuming procedure that decreases laboratory efficiency and throughput.
The maintenance requirements for the in-house nitrogen generator are minimal. The readily accessible filters are typically replaced once year, a process that takes about 10 minutes for all three filters.
Table 1. Annual Costs of In-House Generation vs. High-Pressure Tanks (U.S. $)
IMAGE COURTESY OF PARKER HANNIFIN CORP.
Table 1. Annual Costs of In-House Generation vs. High-Pressure Tanks (U.S. $)
Lower Costs
In addition to significant improvements in safety and convenience, use of an in-house generator provides economic benefits in comparison with a gas tank or liquid nitrogen. The running cost of operating an in-house generator is extremely low, because the gas is obtained from laboratory air, with no electricity required. The running costs and maintenance for an in-house generator add up to a few hundred dollars a year for periodic filter replacement.
In contrast, the expense associated with a liquid or gas nitrogen tank is higher. The actual cost for using nitrogen gas from tanks is usually significantly greater than just the cost of obtaining the gas tank. The time involved in changing tanks, ordering tanks, maintaining inventory, and conducting related activities adds to the cost.
Hidden costs of tank gas can include the transportation demurrage and paperwork—purchase orders, inventory control and invoice payment. Additional costs are associated with the time required to transport the tank from the storage area, install the tank, replace the used tank in storage, and wait for the system to re-equilibrate after the tank has been replaced.
While the calculation of the precise cost of nitrogen gas from tanks is dependent on a broad range of local parameters as well as amount of gas used, significant savings are probable with in-house generation of nitrogen. According to Brunell of Danafilms, the payback period of the in-house nitrogen generator is about two years.
Table 1 (see left) shows a cost comparison between supplying gas with a tank vs. generating gas in house. For the analysis, we assumed that a single tank of gas is consumed weekly, that the tanks cost $60 each, and that four tanks are in house, with each tank replaced once each month. The analysis does not include incidental expenses, such as the costs associated with handling the gas tank, down time, ordering tanks, and other related activities. As this comparison shows, the cost of using an in-house generator is solely tied to maintenance (filter replacement) and is estimated to be about $1000 per year or approximately $20 per week.
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