Predicting the Effect of High RH on Organic Vapor Cartridge Performance

Our experiments illustrate the potentially dramatic effect of relative humidity on cartridge service life.

• By Erik Johnson
• Nov 01, 2004

OSHA regulations for the use of chemical cartridges require the establishment of change schedules based on objective information. One of the most commonly used mathematical models for estimating the service life of organic vapor (OV) cartridges was developed by Wood.¹ However, this model does not account for the potential effect of relative humidity (RH) above 50 percent on service life.

The effect of RH on service life of OV cartridges depends on the relative humidity level, the chemical concentration, volatility of the chemical and the chemical's miscibility (ability to dissolve) in water.

The severity of the effect of high RH on the performance of OV cartridges is often underestimated. Early work by Nelson demonstrated that OV cartridges preconditioned and tested at 90 percent RH had only about half the service life of cartridges preconditioned and tested at 50 percent RH.² However, these tests were conducted at a challenge concentration of 1000 ppm. Nelson's observation that humidity has an even greater effect on cartridge performance at lower concentrations commonly seen in workplaces has been widely ignored.

A paper presented by Johnson at the 2001 AICHE described the effect of RH on OV cartridges at workplace concentrations (5-1000 ppm).³ Correction factors were measured for several organic solvents representing a wide range of volatility, including n-hexane, benzene, toluene, and styrene (see Table 1). Cartridges were tested without preconditioning to mimic the dynamic competition of water and solvent vapor for active sites on fresh cartridges. Testing was done at a flow rate of 32 L/min per cartridge (equivalent to 64 L/min for a pair of cartridges) to 1 percent breakthrough.

Discussion

Figure 1. Correction factors versus solvent concentration at 75 percent relative humidity and 1 percent breakthrough.

In practice, a 50 percent RH service life estimate should be divided by the correction factor on the y-axis to determine the predicted service life at 75, 85, or 90 percent RH. For example, 25 ppm toluene at 85 percent RH would require a correction factor of about 4.

Figure 2. Correction factors versus solvent concentration at 85 percent relative humidity and 1 percent breakthrough.

For solvents not shown on the figures, the compound with the closest vapor pressure or boiling point could be used as a surrogate. Methyl ethyl ketone has a boiling point of 79.5º C and a saturation vapor pressure of about 75 mm Hg at 20º C. Therefore, the correction factors for benzene may be used.

Service life testing was also done at 65 percent RH. Compared to the tests done at higher RH, the effects were very limited. Correction factors for the most volatile solvent in this study, n-hexane, ranged from 1.9 at 25 ppm to 1.0 (no correction at all) at 1000 ppm.

Figure 3. Correction factors versus solvent concentration at 90 percent relative humidity and 1 percent breakthrough.

The tests in this study were done with water immiscible (insoluble) solvents to demonstrate worst-case RH effects. Water miscible solvents are less strongly affected by RH.⁴ At room temperature, 85 percent RH is equivalent to about 27,000 ppm water. Under these conditions, a significant amount of water will be adsorbed into the carbon pores. This allows increased loading of water miscible compounds or even compounds that are normally considered only slightly soluble in water. For example, ethylene dichloride and methyl ethyl ketone were less strongly affected by high RH than non-miscible compounds with the same chemical properties and adsorption capacity.

It should be noted that unlike OV performance, service life for cartridges designed to remove acid gases and bases may actually improve at high RH. Water vapor may improve interaction between the chemical treatment on the carbon and the acid or base contaminant. Testing of these compounds for Service Life Software™ was done at 50 percent RH to represent a challenging environment. More work needs to be done to determine what correction factors may be needed in lower RH environments.

Conclusion
These experiments illustrate the potentially dramatic effect of relative humidity on OV cartridge service life. Nelson's widely quoted observation that high humidity reduces service time by half holds true only at high contaminant concentrations.

The impact of high RH must be considered when establishing cartridge change schedules. Service life estimates calculated at low RH should be divided by the appropriate safety factor when high RH is present in the workplace. Professional judgment should be used, and users also may wish to conduct testing on the performance of OV cartridges in their work environments.

References

1. Wood, G.O. Estimating Service Lives of Organic Vapor Cartridges. Am. Ind. Hyg. Assoc. J. 55(1): 11-15 (1994).


2. Nelson, G.O., A.N. Correia and C. A. Harder. Respirator Cartridge Efficiency Studies: VII. Effect of Relative Humidity and Temperature. Am. Ind. Hyg. Assoc. J. 37:280-288 (1976).


3. Brey, L.A. and E.W. Johnson: "Prediction of the effect of high relative humidity on organic vapor cartridge performance." Paper presented at the American Industrial Hygiene Conference and Exposition, New Orleans, La. (June 2001).


4. Kawar, K.H. and D.W. Underhill. Effect of Relative Humidity on the Adsorption of Selected Water-Miscible Organic Vapors by Activated Carbon. Am. Ind. Hyg. Assoc. J. 60:730-736 (1999).

This article originally appeared in the November 2004 issue of Occupational Health & Safety.