Use of Methanol to Control Water Freezing in Propane Gas Systems

1. Use of Methanol to Control Water Freezing in Propane Gas Systems

2. Water in Propane • Water can dissolve into propane (180 ppm @ 68F) • This water can cause two types of problems: • Water solubility decreases rapidly with temperature (65 ppm @ 32F), causing water to drop out of solution with temperature decrease, leaving a water or water–rich layer at tank bottom • Water layer can increase corrosion attack on tank metal • Water can significantly reduce fuel quality on liquid withdrawal systems • Once water layer has formed, diffusion of water into liquid propane is so slow that even if propane warms or is replaced by drier fuel, water will not readily go back into solution, but will remain as lingering source of moisture • Water vapor will be in propane vapor in higher proportions than liquid water in liquid propane (K-ratio for water in propane >> 1) • Ice may be formed at vapor pressure drops (for example, in a regulator)

3. Solutions to Water in Propane • Two possible solutions 1 - Prevent excessive water in propane 2 - Use a de-icing agent (antifreeze), such as methanol • Because of storage and batch shipping, preventing water from entering propane stream is difficult – therefore marketers must either install local dryers or use de-icing agent  methanol • Guidance on the use of methanol as de-icer comes from several industry sources

4. How Much Methanol to Use? • Main goal is to prevent “out of service” calls due to regulator freeze-ups • Industry veteran Sam McTier – driving around in the 1950’s with a drum of methanol in the truck of his Buick – developed still-used “rule of thumb” of 1 pint per 100 gallons of propane • Current serviceman handbook recipes (“rules of thumb”) • RegO LP-Gas Serviceman’s Manual: 1 pint / 100 gallons of tank capacity to neutralize water left from tank hydrotest) • 1250 ppm/vol (total volume), 1560 ppm/vol based on 80% fill level (1980 ppm/mass total volume, 2470 ppm/mass 80% fill) • Fisher Controls LP-Gas Serviceman’s Handbook: 1 gallon in 1000 gallon tank to neutralize moisture in container • 1000 ppm/vol (total volume), 1250 ppm/vol based on 80% fill level (1580 ppm/mass total volume, 1980 ppm/mass 80% fill)

5. How Much Methanol to Use? • ASTM 1835-2003a, “Standard Specification for LP Gases”: Footnote NOTE X1.1 - Commercial propane and special duty propane should be produced to comply with the moisture content requirement, and de-icer additives should not routinely be used to pass dryness test requirements… A de-icer such as methyl alcohol (methanol) should not be added to these products without specific agreement and approval of the purchaser. During short-term upsets in production, or inadvertent contamination by trace water during storage or distribution, addition of 50 ppm methyl alcohol has proven to be acceptable to prevent valve freezing in normal applications. For guidance, based on historical experience and phase separation data, the maximum cumulative addition of methyl alcohol should not exceed 200 ppm by volume. • Results in 315 ppm/mass

6. Can there be too much methanol? • Anecdotal evidence: • If some is good, more is better • Can’t get too much • Methanol is rarely included on bill of lading/shipping papers – receivers don’t know how much methanol has been added at various points along the distribution chain • Methanol may • Increase corrosion when in a water/methanol layer (SCC in steel, general corrosion in aluminum) • Affect combustion of higher efficiency/low emission burners • Methanol does • Cause general degradation of elastomers (seals, seats, hoses) • With these concerns, NPGA / PERC asked: From a scientific basis, how much methanol should be used?

7. Scientific guidance • Scientific guidance on the use of methanol as de-icer • While data available, very little is available for the temperature/ concentration range that is of interest to propane marketers • Use of existing or newly-generated data ultimately requires knowledge of the quantity of water and methanol in propane – several technologies look promising in development of marketer-usable water / methanol meters • Existing data – tertiary (propane + water + methanol) system is extremely complex

8. Data • Most propane-methanol-water data generated for natural gas pipelines – higher temperatures, low propane, high methane content, generally more water • Example of data available  starts at 100F • Propane industry’s real interest is for Minnesota in February

9. Estimating amount of methanol needed Rationale for Recommended Guidelines – Method 1 • Assume amount of dissolved water in liquid propane fuel • Assume propane at 10C (50F) is saturated with water  Water in liquid propane: ~100 ppm/mass • Estimate amount of water in vapor phase • K ratio for water in propane: ~7 at 10C  concentration of water in the vapor phase: ~700 ppm/mass • Assume that to protect water/methanol layer from freezing, 50-50 methanol-water mixture is needed – this mixture will freeze -54C (-64F)  Need 700 ppm/mass of methanol vapor • Estimate methanol needed in liquid fuel • K ratio for methanol in propane estimated to be 1.5  concentration of methanol in liquid phase: ~470 ppm/mass

10. Estimating amount of methanol needed Rationale for Recommended Guidelines – Method 2 • Assume amount of dissolved water in liquid propane fuel • Assume propane at 40C (104F) is saturated with water  Water in liquid propane: ~380 ppm/mass • Estimate amount of water that separates from bulk cooling • Assume water cools to -40C (-40F), where solubility is ~20 ppm/mass  360 ppm/mass comes out of solution • Assume that to protect water/methanol layer from freezing, 50-50 methanol-water mixture is needed - therefore, 360 ppm/mass of methanol is needed in the water/methanol layer • Estimate methanol needed in liquid fuel • Using partioning ratio data for methanol-in-water to methanol-in-propane, methanol 50 times more likely to be in water layer, therefore methanol added to bulk volume will follow water  overall methanol concentration should be ~360 ppm/mass

11. Recommended methanol concentration • Two analyses, with differing assumptions, give a range of 360 and 470ppm/mass • To be conservative, increase higher number  600ppm/mass • 3 pints per 1000 gal propane • 2.5 lb methanol per 1000 gal • 0.6 lb methanol per 1000 lb propane • 0.5 kg methanol per 1000 liters propane • 0.6 liters methanol per 1000 liters propane • 0.6 kg methanol per 1000 kg propane Rates for Typical Tank Sizes

12. Alternatives to Methanol • Desired properties • Miscible in water • Relatively low molecular weight and volatility, to “follow” water vapor • Inexpensive • Relatively non-corrosive to metals, non-attacking to elastomers • Desired properties with respect to methanol • is a more effective antifreeze • is less acidic and therefore less corrosive • costs less • provides some other benefit.

13. Alternatives to Methanol • Methanol effective to -54C • Alternatives perhaps slightly less acidic • Tendency to form secondary phase less only with tetrahydrofuran • Methanol is very low cost because of large production volumes • Questions of compatibility with existing methanol in distribution chain • NO CLEAR SUCCESSORS TO METHANOL

15. Summary • Little data exist on tertiary mixture of propane + methanol + water, over the temperature and composition ranges of interest • Methanol can attack elastomers, may attack metals when combined with water • Existing recommendations on methanol usage appear to be high, some by factor of 4, and actual use may be even higher • For complete guidance on methanol use, cost-effective, easy-to-use meter for marketers is needed • No drop-in replacements for methanol offer significant, if any, advantages

16. Funding for this work was provided by the Propane Education & Research Council under Docket 11992. Final report is available at www.propanecouncil.org/industry/resLib_councilDetail.cfv?id=587&t=Research%20%26%20Development For More Information Contact: Rod Osborne 1.614.424.4833 osborner@battelle.org www.battelle.org