Nitrogen-methanol atmospheres – storage and control

Understanding the physical parameters of methanol, and the different measuring techniques, will enable precise methanol (and atmosphere) control.

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In this column, I will discuss the storage and control of methanol to create nitrogen methanol atmospheres.

Introduction

Nitrogen-methanol atmospheres are used frequently when natural gas is unavailable. While excellent and repeatable production of endothermic atmospheres is possible, there are some caveats that must be understood to produce an endothermic atmosphere of the proper composition.

A typical nitrogen-methanol system is usually composed of a methanol storage tank, cryogenic nitrogen tank, vaporizer, and methanol delivery system (pump or pressure). A control panel comprised of a series of flowmeters is also included. This is shown in Figure 1.

Methanol Delivery Systems

In early nitrogen-methanol systems, nitrogen was used to pressurize an underground methanol tank to “push” the methanol to the furnace control panel for each furnace, then to each furnace. Often, due to distances involved, pressure inside the methanol tank had to be maintained at approximately 60 psi (414kPa). This caused several problems. First, a large amount of nitrogen was consumed over and above that required for normal processing. This was the result of bleeding the pressurized methanol tank during filling, and the additional nitrogen required to pressurize the tank as the methanol level dropped. In addition, due to the high vapor pressure of methanol, the nitrogen-filled space above the methanol became saturated with methanol. This saturated nitrogen became an additional fire hazard when the methanol tank was vented for any reason.

Figure 1: Typical nitrogen-methanol installation.

A second problem with a pressurized methanol delivery system is that methanol absorbs nitrogen readily, especially at the pressures used. This can become exaggerated as the methanol tank level lowers, and furnaces are taken offline. As the pressurized methanol is pushed to the control panel and furnace, pressure drops occur at each pipe elbow, valve, or flowmeter. At each of the pressure drops, nitrogen will come out of solution in the form of bubbles, making control difficult. The problem of nitrogen bubbles, apparent at the methanol flowmeter, made precise control difficult. Replacing the pressurized system with a pump, and only using low-pressure nitrogen to inert the top part of the tank reduces the amount of nitrogen dissolved in methanol.

One problem with venting the methanol tank, whether as a blanket or used to push the methanol to the furnace, is that the vented nitrogen contains a high percentage of methanol. From the low flashpoint of methanol, this would be considered a volatile organic compound (VOC). These vapors can be collected, and the methanol recovered, using the cold nitrogen to condense the methanol. This nitrogen can be routed back to blanket the system.

Regarding the nitrogen system, it is very important that the vaporizers on the nitrogen tank are sized appropriately for the maximum purge flow rate for all furnaces. This is critical in case of an extended power failure to properly purge the furnaces with atmosphere, and maintain a positive pressure in the furnaces as the temperatures in the furnaces fall. This also provides protection of work in the furnaces and helps minimize oxidation of parts.

Methanol Tank Storage & Installation

Methanol storage tanks can be either above ground, or below ground. Where depends on either local fire or environmental regulations. Many fire codes (at least previously) required underground storage tanks. These tanks at, at a minimum, were double walled, with intrinsic sensing. Modern environmental regulations require above ground storage, with containment. In many respects, the storage of methanol is like that of gasoline.

For larger underground tanks, once the pit has been dug, before the tank is covered with dirt and asphalt, it should be immediately filled with methanol. It should also be fully strapped down with concrete piers on a concrete stab. Failure to do so before filling the tank with methanol may result in the tank floating out of the pit if a heavy rain or ground water fills the pit (don’t ask me how I know this).

Methanol is routinely stored in above-ground, internally baffled tanks. These tanks must be properly grounded to avoid ignition from static electricity. Nitrogen is used to inert the space above the liquid methanol.

Methanol burns with an invisible flame. First responders should have remote heat detection capability. The installation is recommended to have some sort of remote temperature sensing capability.

Methanol storage should be in a dedicated area, away from buildings. This area should be specifically marked off and placarded as a hazardous area. Safety measures should be readily available for exposure, spillage, and fire.

Methanol Control

In most nitrogen-methanol control panels, rotameters are used to control the amount of methanol and nitrogen. In a rotameter, a float is installed in a vertical tube and the fluid flows around the float. The float is raised due to the flow. This measurement technique is dependent on viscosity and temperature.  Most flow meters are calibrated for a specific fluid, at a specific temperature and pressure. For liquids, the pressure is critical because liquids are incompressible.

However, viscosity is important. Viscosity is strongly influenced by temperature, and methanol shows a large variation in dynamic viscosity as a function of temperature (Figure2).

Figure 2: Variation of dynamic viscosity of methanol as a function of temperature.
Figure 3: Correction factor of indicated flow, for a methanol flowmeter calibrated at 70°F (21.1°C).

This means that at temperatures other than the calibrated temperature, the indicated flow rate of the methanol is incorrect. Depending on the incoming methanol temperature, the variation can be very significant. A scale factor, determined by taking the viscosity at the calibration temperature, divided by the viscosity at another temperature, can then be applied to the displayed reading on the flow meter. An example of the correction factor for different incoming temperatures of methanol is shown in Figure 3.

To minimize the variation of temperature, and resulting changes in viscosity, the use of a small chiller can be used to ensure that the temperature is maintained at the calibration temperature. This will ensure that precise metering of methanol can be accomplished.

Conclusions

In this article, we have described some of the issues with storage and control of methanol used to create nitrogen-methanol endothermic atmospheres. Safety during storage and having appropriate engineering controls is important to protect people and equipment. Understanding the physical parameters of methanol, and the different measuring techniques, will enable precise methanol (and atmosphere) control.

Should you have any questions regarding this column, or suggestions for any future columns, please contact the author, or the editor. 

References

  1. D. Aronowitz, D. W. Naegeli and I. Glassman, “Kinetics of the Pyrolysis of Methanol,” J. Phys. Chem., vol. 81, no. 25, pp. 2555-25559, 1977.
  2. D. Aronowitz, R. J. Santoro, F. L. Dryer and I. Glassman, “Kinetics of the oxidation of methanol: Experimental results semi-global modeling and mechanistic concepts,” Symposium (International) on Combustion, vol. 17, no. 1, pp. 633-644, 1979.
  3. P. E. Liley, “Thermodynamic properties of methanol,” Chem. Eng. , vol. 11, no. 59-60, 1982.
  4. Methanol Institute, Methanol Safe Handling Manual, 5th ed., Alexandria, VA, 2023.
  5. NFPA, “NFPA 30: Flammable and Combustible Liquids Code,” National Fire Protection Association, 2024.
  6. Occupational Safety and Health Administration, CFR 49 1910.119: Process safety management of highly hazardous chemicals.