Compositions – Vaporization – or expansion – refrigeration or heat or energy... – With lubricants – or warning – stabilizing or anti-corrosion...
Reexamination Certificate
1999-08-30
2001-03-20
Douyon, Lorna M. (Department: 1751)
Compositions
Vaporization, or expansion, refrigeration or heat or energy...
With lubricants, or warning, stabilizing or anti-corrosion...
C252S069000, C062S112000, C422S013000
Reexamination Certificate
active
06203718
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Corrosion inhibitors for steel absorption cycle components using aqua ammonia working fluid.
2. Description of the Related Art
Various corrosion inhibition protocols are described in the prior art which protect mild steel in NH
3
—H
2
O absorption cycle service, and limit non-condensable gas formation.
McKelvy and Isaacs in Bureau of Standards Technologic Paper 180 (1920) disclosed the importance of eliminating carbonates, and also that adding 0.28 weight percent (w/o) of sodium or potassium dichromate (based on water content) stops any gas generation.
When it was later found that the chromate was depleted over time, greater amounts were added, until solubility limit problems were encountered. U.S. Pat. No. 2,580,983 disclosed provision of relatively insoluble SrCrO
4
or PbCrO
4
in tablet form, in conjunction with up to 2 w/o NaOH, to effect slow release of the chromate.
Alkali hydroxides have routinely been used for pH adjustment in conjunction with other inhibitors in LiBr absorption systems.
This has also been done in aqua ammonia absorption systems, for example, NTIS PB 95-226445 discloses 0.19 w/o NaOH plus 1 w/o sodium metasilicate as inhibitor. That amount of NaOH in the charge of distilled water produces a pH of 12.67 at 25° C. The NaOH alone was found to be at least as effective as the silicate in inhibiting the 10 w/o NH
3
solution.
Other investigators also advocate using alkali hydroxides as inhibiitor. U.S. Pat. No. 5,725,793 discloses alkali metal hydroxide concentrations in the range of 2 to 10 w/o (based on H
2
O) as being effective for both corrosion inhibition and also for COP enhancement owing to reduced or eliminated need for rectification. This range of concentrations will yield pH in the range of 13.1 to 14.4 in the charged distilled water at 25° C.
U.S. Pat. No. 5,811,026 discloses inhibiting aqueous ammonia with a strong base (alkali metal hydroxide) at a concentration between 0.015 Normal and 0.2 N at 25° C., relative to the charge distilled water. This corresponds to a pH in the range of 12.2 to 13.3. The inhibitor is disclosed as effective for aqueous NH
3
concentrations down to about 1 w/o, although the autoclave test results reported are all at 10 w/o NH
3
.
Modern advanced GAX cycles (U.S. Pat. Nos. 5,097,676, 5,024,063, and 5,572,885) must concentrate the water absorbent up to fairly high purity, with substantially less than 1% residual NH
3
, in order to achieve high performance. The prior art teachings of high caustic content are found to cause excessive corrosion and non-condensable gas generation at those conditions. Hence, an improved corrosion inhibitor is needed for advanced aqua ammonia absorption cycles. Thus, the objective of this invention is to provide a corrosion-inhibited aqua ammonia solution which not only inhibits corrosion and non-condensable gas generation in the high ammonia concentration environments of prior art generators (e.g., >5 w/o NH
3
) but also in the generators of advanced absorption cycles wherein the NH
3
concentration is less than 1%, and may be less than 0.1%.
DISCLOSURE OF INVENTION
The above and further useful objects are attained by supplying a carefully controlled amount of caustic as inhibitor in the aqua ammonia solution, an amount much smaller than the minimum effective amount specified in prior applications. In particular, the absorption working fluid is charged with an alkali metal base in the concentration range of 0.0003 to 0.0063 moles per liter of water. When the alkali metal base is the hydroxide, this is equivalent to adjusting the pH of the charge water to between 10.5 and 11.8 at 25° C.
Further actions or additives are disclosed for use when necessary to counteract initial use gas generation and/or very high temperature operation. In particular, the gas generation accruing during initial use is counteracted by either or both of:
charging pure water which is de-oxygenated and de-carbonated;
pre-oxidizing the steel apparatus to establish the desired Fe
3
O
4
film.
Very high temperature (>160° C.) corrosion is further counteracted by very small additions of chromate and/or G
e
O
2
.
REFERENCES:
patent: 2580983 (1952-01-01), Widell
patent: 5547600 (1996-08-01), Downey
patent: 5548971 (1996-08-01), Rockenfeller et al.
patent: 5725793 (1998-03-01), Keller et al.
patent: 5783104 (1998-07-01), Kujak
patent: 5811026 (1998-09-01), Phillips et al.
patent: 96/01296 (1996-01-01), None
McKelvy, E.C. et al. “Causes and Prevention of the Formation of Noncondensible Gases in Ammonia Absorption Refrigeration Machines”, Technologic Papers of the Bureau of Standards, No. 180, Washington Oct. 25, 1920 pp. 1-10.*
Gaskell, David., “Introduction to Metallurgical Thermodynamics”., Scripta Publishing Co., Washington, D.C., 1973, p. 269.*
Cacciola, G. et al., “Theoretical Performance of an Absorption Heat Pump Using Ammonia-Water—Potassium Hydroxide Solution”, Heat Recovery Syst. Chp, 1990, vol. 10, No. 3, pp. 177-185.*
Brass, M. et al., “Ammonia-Water-Lye (NAOH, KOH) as New Working Substances for Absorption Refrigerating Machines”, Fortschr. Ber. VDI, Reihe 3, 1988, 544 (Technische Sorption SPRC ZESSE), pp. 280-289.*
Brass, M., “Thermophysical Material Data of Ammonia—Water Alkali (NaOH, KOH) Systems as New Working Substances for Absorption Refrigeration Processes”, Forts Chr. Ber. VDI, Reihe 3, 1998, 565, 1-XIII, pp. 1-138.
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