HEAT AND MASS TRANSFER ADDITIVES FOR IMPROVED AQUEOUS ABSORPTION FLUIDS
Summary of the Invention
According to the present invention, aqueous absorption working fluids are improved by adding effective amounts of an aliphatic, cycloaliphatic or aromatic ketone or aldehyde having between 5 and 24 carbon atoms. The improved heat and mass transfer additives are used in aqueous metal salt solutions. The presence of the effective amount of ketone or aldehyde increases the rate of water vapor absorption by the absorption fluid thereby achieving important advantages and improvements in system performance. The improvements include absorber power load increases and improvements in the change of absorber fluid concentrations. Further improvements include increases in overall heat transfer coefficients and sorption fluid side film heat transfer coefficients. The advantage of such improved absorption fluid performance allows for reduction of sorber heat exchange surface areas needed to satisfy a given load resulting in reduction of absorber size and costs. These and other improvements and advantages will be evident from the following detailed description.
Detailed Description of the Invention
The specific improved aqueous absorption solutions used m the system of the present invention are aqueous solutions of metal salts. The preferred metal salts are alkali metal halides. particularly lithium bromide, lithium chloride and lithium iodide and mixtures of two or more of them. The most preferred working fluid is an aqueous solution of lithium bromide as the only substantial absorbent present in the aqueous solution as used in today's absorption chillers. In such solutions, a small amount of metal hydroxide may be present for pH corrosion control, typically between about .05 and about .15 normal. However, in addition to the preferred lithium bromide, one or more of the following salts may also be present: ZnCI2, ZnBr2. MnCI2, MnBr2, MgCI2, MgBr2, SrCl2, SrBr2,, CaCl2, CaBr2,, FeCI2,, FeBr2j, LiCI, Lil, UNO2, UNQ2, liSCN and UCIO3. The amount of salt present, whether lithium bromide alone or a mixture of two or more of the aforesaid salts is preferrbly between about 40% and about 85%, by weight. The aforasaid LiBr aqueous absorption solutions are well known to those skilled in the art as disclosed, for example, in U.S, Patent No. 3,478,530.
Ketones may also be used with aqueous solutions containing alkali metal hydroxides. The alkali metal hydroxide absorption fluids are aqueous soluitions of sodium hydroxide, potassium hydroxide or matures thereof. Preferred hydroxide compositions are those utilizing a mixture of the two hydroxides, and preferably those in which the total hydroxide concentration is between about 30% and about 60%, by weight. It has been found that optimum energy storage potentials are realized when sodium hydroxide is present between about 35% and about 75%, by weight, of the combined sodium hydroxide and potassium hydroxide weight. The amount of sodium hydroxide present in the salt muture for systems at crystallization temperatures above 30''C is 50% or above, whereas for temperatures below 30°C, the preferred amount of sodium hydroxide is at or below 50% of the salt mature. The most preferred amount of sodium hydroxide is between 40% and 55% of the combined weight of sodium hydroxide and potassium hydroxide far crystallization temperatures below 30°C. In addition to the aforesaid sodium and/or
potassium hydroxide solutions, relatively small amounts of other alkal metal hydroxides may be added as well. Thus,
the hydroxide solutions may contain up to about 50% cesium, rubidium, and/or about 35% lithium hydroxida. based
on the weight of sodium and/or potassium hydroxides. It ha:; been found that aldetiydes are not effective heat and
mass transfer additives with metal hydroxides.
The heat and mass transfer additives of the invention are aliphatic, cycloaliiyhatic. and aromatic ketones
and aldehydes having between 5 and 24 carbon atoms. Preferred additives are normally liquid at system operating
conditions, for example, between about 30°C and up to 100°C or more. Normal boiling points of 180°C or higher
are particularly preferred. The additives may be soluble or insoluble IN the brine working fluid. However, insoluble
compounds must be fuiuid at 20'* and above, whereas soluble additives may be solid as a pure compound at ambient
temperatures but must fully dissolve and not form precipitates at 20° or higher. Additives of the present invention
may be used in systems with absorber temperatures down to about 15°'C if they are selected from compounds
having a melting point less than the absorber temperature. The additives may be liquid over a wider temperature
range, both higher and lower, without adversely affecting the operation of the invention. The ketones are of the
formula RilC-CIR, wherein R, and/or R2, are aliphatic or substituted aliphatic groups of from 1 to 12 carbon atoms
or are cycioaliphatic or aromatic or substituted cycloaliphatic or aromatic groups of from 6 to 12 carbon atoms,
respectively. Where the hydrocarbon groups are alkyl groups, and wherein R1 or R2 is a methyl group, the carbon
atom of the other alkyl group attached to the carbonyl group is a primary or tertiary carbon atom. Thus, ketones
having R1 as a methyl group and the other alkyl group R2 having a secondary carbon atom attached to the carbonyl
group such as 3-methyl-2-heptanone as diseased in U.S. patent No. 3,609,087 have inferior heat and mass transfer
properties in LiBr absorption fluids as compared to ketones of the present invention, and are thus excluded, peferred
aliphatic ketones are those in which R1 and R2 are alkyl groups having from 1 to 8 carbon atoms, and wherein either
or both of the alkyl groups are substituted, preferably halogen substituted, and more preferably fluoro substituted.
Examples of preferred aliphatic ketones include 4-methyl-2-pentanone, 5-methyl-2-hexanone, 3,3-dimethyl-2-hexanone,
2-pentanone, 3-pentanone, 2,4-dimethyt-3-petanone, 2-hexanone, 4-methyl-2-hexanone, 5-methyl-2-hexanone, 2-
heptanone, 3-heptanone, 4-heptanone, 2-octanone, 3-octanane, 4-octanone, nonanone and decanone, Cycloaliphatic
ketones, and particularly cyclohexanone and alkyl and halogen substituted derivatives are also effective additives.
Aromatic ketones include acetophenone (methyl phenyl ketone) and benzophenone (phenyl ketone] and their alkyl and
halogen substituted derivatives.
The aldehydes used as heat and mass transfer additives within the scope of the invention are aliphatic
aldehydes having from 5 to 24 carbon atoms, and more preferably from 6 to 14 carbon atoms in the aliphatic or
alkyl chain, which also may he halogen substituted. Specific aliphatic aldehydes are hexanal, heptanal, octanal,
nonanal. decanal, etc. and longer chain aldehydes for example, lauryl aldehyde (C-12! and myristic aldehyde (C-14). antsaiaenyoe una suosmuieo aromatic aioenyoes including ether and
hydroxy substituted compounds such as vanillin {3-methoxy-4-Hydroxy benzaldehyde).
The ketones and aldehydes may also include one or more other functional groups, particularly halogens and
more preferably chlorine or fluorine substituted molecules as previously disclosed. In addition, the molecules may
also contain alcohai thiol, amine, and ether groups, provided such functional groups do not cause the additive to form a precipitate or otfierwise induce chemical instability in the presence of the metal salt solutions. Combinations of icetones and/or aldehydes may also be found suitable for different salts or salt combinations.
Effective additive amounts of ketorra or aldehyde added to the aqueous salt solutions for the purpose of Improving heat and mass transfer are from at least about two parts per million preferably up to about 20.000 ppm, by weight. Preferred concentrations are between about 2 ppm and 10,000 ppm and more preferably between about 5 ppm and about 5,000 ppm, although amounts in excess of this range up to 20,000 ppm or more will not adversely affect the system operation. Mixtures of the aforesaid addititres may also be used. In addition to mixtures of the ketones and aldehydes, the additives may be used with other known heat and mass transfer additives which may be present or added to the absorption fluid. Examples of such known additives are alcohols, particularly those having 6-10 carbon atoms such as 2-ethyihexanol or n-octanol and amines disclosed in U.S. Patent No. 5,419,145 and application serial No. 08J443,707, the descriptions of which are incorporated herein by reference,
According to the invention, it has been found that the presence of one or more ketones or aldehydes in the aforesaid aqueous absorption fluids results in substantially improved heat and mass transfer performance of the absorption working fluid composition. Specifically, the performance of the system improves in the following manner water cooling temperature change in the absorber, for a constant water flow rate, rises significantly; the absorber power load increases proportional to the change in water cooling temperature; the steady state process vapor pressure drops, and if brought back to its initial value to achieve a constant evaporator temperature, the change in concentration of the absorber fluid increases significantly; tlie absorber solution subcooling, i.e., the difference between the maxin\um temperature at equilibrium (saturation) and the actual solution temperature, is decreased by several degrees; the absorber heat exchanger tube (sokition sidi;) Interfax comprises a highly agitated turbulent film as compared to a generally laminar flow pattern without ^iresence of the additive; and overall heat transfer coefficients and sorption fluid fi^ heat transfer coefficients are increased.
As previously noted, water vapor absorption systems incorporating the heat and mass transfer additives in the aqueous alkali metal absorption solutions of tiie bivention include a number of different types of systems incorporating one or more absorbers in which water vapor absorption solutions are typically used. Such equipment includes absorption chaiers and refrigeratkin systems as disclosed in U.S, Patents 4,966,007. 5,038,574 ar^d 5,166,009, thermal energy storage systems as disclosed in U.S. Patent 4,823,B64, as well as multiple effect absorption systems, for example, double effect and dual ioo|:> systems disclosed in U.S. patents 3,266,266 and 4,542,626. triple effect systems disclosed in U.S. patents 5,335,515 and 5,390,509 and multiple effect systems resulting from a combination of single or double effect apparatus such as the triple effect system comprising combined single effect circuits as described in U.S. Patent Mo. 5,205,136 as well as single effect chillers and commonly used double effect chillers and heat transformers. The aqueous working fluids of the mvention may also contain a corrosion inhibitor such a chromate, nitrate, tungstata or molybdate, as disclosed for example in U.S. patents 5,186,009 and 5,335,515 or any other suitable corrosion inhibitors. Where aldehydes are present, the use of certain corrosion inhibitors may be restricted to avoid aldehyde oxidation.

To illustrate the improvement of system performance by using the heat and mass transfer additives of the present invention, the following examples are provided. In the Table, addityres within the scope of the present invention are shown by way of eiample. The results given are for a constant set of operating conditions and include: initial cooling water {"Tube") temperature of 30''C, a system water vapor pressure of 10 mhar, 7°C dew point (evaporator temperature), initial aqueous sohition concentration of 60% LiBr, initial solution flow rate of 408 grams/min, and initial solution temperature of 48°C antering the absorber, using a special bench test absorption machine. The solution flow rate used with the cyclohexanore additive is 500 gramslmin. The water and solution temperatures, flow rates and vapor pressures are taken from rnonitor readings during operation. The concentrations are determined from fluid samples taken from the machine.
The results are based on heat transfer equations well known to practitioners of the art. The absorber load dfl/dt, shown as W, is calculated from the water temperature, heat capacity of water, and water flow rates,
1
Eq (Removed)
For the heat transfer coefficients, the temperature change is treated as the log mean differential temperature, where
Eq (Removed)
(2)
The outside film heat transfer coefficient h, is calculated from
Eq (Removed)
(3)
where
Eq (Removed)
(4)
and where R is the thermal resistivity of the inside cooling w ater and copper metal tube wall. Solution subcooling is the temperature difference (AT) between the actual solution temperature leaving the absorber and the calculated absorber solution equilibrium temperature at the measured concentration and system vapor pressure values. Thus, lower subcooling temperature values reflect greater system efficiency as do increased outside film heat transfer coefficients, h,. The value calculated for solution subcooling is one measure of the efficiency of the system for water vapor absorption, with lower subcooling temperatures indicating an improvement. However, under certain
conditions, the derived subcooling numbers may be contrary to other direct measurements of water vapor absorption. in the Table some data show higher calculated solution subcoortng while also showing increased water vapor absorption rates by increases in absorber loads, and increases in heat transfer coefficients. Thus, the derived subcooling results are to be checked against other direct evidence of increased absorption rates.
TABLE
(Table Removed)
From the test results given in the Table, utilizino representative examples of ketones according to the invention, the improvements of the operating condition performance, taken from actual measurements of the laboratory bench test system as well as calculated values for flukl film heat transfer coefficients th,} are shown. Additionally, the absorber power kiad increases are shown as are the significant decreases in solution subcooGng at the operating conditions. Calculated values of film heat transfer coefficients (h,) are also increased.

WE CLAlM:
1. A water vapor absorption cycle cooling and/or heating system containing a working fluid comprising an aqueous solution of a metal salt comprising Iithium bromide, lithium chloride, lithium iodide, or mixtures of two or more thereof, and an effective additive amount of at least 2 parts per million, by weight of an aliphatic, cycloaliphatic or aromatic ketone or aldehyde having between 5 and 24 carbon atoms for increasing the rate of water vapor sorption of said working fluid, said aliphatic ketone being of the formula R1(C-0)R2 wherein R1 and R2 are the same or different alkyl and substituted alkyl groups of from 1 to 12 carbon atoms and wherein when R1 is a methyl group the carbon atom of the other alkyl group R2 attached to the carbonyl group is a primary or tertiary carbon atom,
2. A system of Claim 1 wherein said ketone and said aldehyde are insoluble or slightly soluble in said working fluid and are liquid at 100°C and at 20°C or below.
3. A system of Claim 1 wherein the additive in said working fluid is an aliphatic ketone wherein said R1 and R2 are alkyl groups having from 1 to 8 carbon atoms.
4. A system of Claim 3 wherein either or both of said alkyl groups are halogen substituted.
5. A system of Claim 3 wherein either or both of said alkyl groups are fluoro substituted.
6. A system of Claim 1 wherein said working fluid contains a mixture of said ketones.
7. A system of Claim 1 wherein said working fluid contains a mixture of said aldehydes.
8. A system of Claim 1 wherein said working fluid contains a mixture of one or more of said ketones
and one or more of said aldehydes.
9. A system of Claim 1 wherein said ketone aid said aldehyde are substantially completely soluble in said working fluid at 20°C and above.
10. A system of Claim 1 wherein said working fluid contains between 2 ppm and about 20,000 .ppm, by weight of said additive.
11. A system of Claim 1 wherein said working fluid contains between 2 ppm and about 10,000 ppm, by weight of said additive.
12. A system of Claim 1 wherein the additive is a ketone selected from the group consisting of 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, 2,4-dimethyl-3-pentanone, 2-hexanone, 3,3-dimethyl-2-hexanone, 4-methyl-2-hexanane,5-methyl-2-hexanone,2-heptanone,3-heptan(ine,4-heptanone,2-octanone,3-Dctanone,4-octanone, cyclohexanone, benzophenone and acetophenone.
13. A system of Claim 1 wherein the additive is an aliphatic aldehyde having between 6 and 14 carbon atoms.
14. A system of Claim 13 wherein said aldehyde is halogen substituted.
15. A system of Claim 1 wherein said working fluid contains lithium bromide and one or more salts
selected from the group consisting of ZnCI2,, ZnBr2. MnCI2,, MnB2, MgCl2, MgBr2,, SrCl2, SrBr2 FeCl2, FeBr2, CaCl2,
CaBr2,, LiCI, Lil, UNO2, LiNO3, LiSCN and LiCIO3.
16. A system of Claim 1 wherein said cooling and(or heating system comprises a single or double effect absorption apparatus and wherein said metal salt comprises lithium bromide.
17. A system of Claim 1 wherein said additive is a ketone, and wherein said metal salt includes sodium hydroxide, potassium hydroxide or mixtures thereof.
18. A system of Claim 1 wherein sard additive is a ketone, and wherein said metal salt is a hydroxide comprising one or more of sodium hydroxide, potassium hydroxide, cesium hydroxide, rubidium hydroxide or lithium hydroxide.
19. A system of Claim 1 wherein said additive is a ketone, said working fluid contain is addition to said leten e said i
aldehyde, an additive amount of an alcohol or an amine heat and mass transfer addithra.
20. A water vapor absorption cycle cooling andlnr heating system containing a working fluid comprising an aqueous solution of a metal salt selected from the group consisting of lithium bromide, lithium chloride, lithium iodide, and mixtures of two or more thereof, and sodium fiydroxide, potassium hydroxide and mixtures of said hydroxides, and an effective additive amount of at least 2 parts per million, by weight of an aliphatic, cycloaliphatic or aromatic ketone having between 5 and 24 carbon atoms for increasing the rate of water vapor sorption of said working fluid, said aliphatic ketone being of the formula R1(C-0)R2 wherein R1and R2 are the same or different alkyl and substituted alkyl groups of from 1 to 12 carbon atoms and wherein when R, is a methyl group the carbon atom of the other alkyl group R2 attached to the carbonyl group is a primary or tertiary carbon atom.
21. A system of Claim 20, wherein said working fluid contains in addition to said ketone, an additive amount of an alcohol or an amine heat and mass transfer additive.
22. In operation of an absorption cycle cooling and/or heating system including an absorber containing an aqueous metal salt absorption solution for absorbing water vapor therein and wherein said metal salt comprises an alkali metal halide, a method of improving the rate of water vapor absorption in said absorption solution comprising adding thereto an effecthre additive amount of at least 2 parti per million, by weight, of an aliphatic, cycloaliphatic, or aromatic ketone or aldehyde having between 5 and 24 carbon atoms and capable of increasing said rate of water vapor absorption, said aliphatic ketone being of the formula R1(C-0)R2 wherein R1 and R2 are the same or different alky! and substituted alkyl groups of from 1 to 12 :arbon atoms and wherein when R, is a methyl group the carbon atom of the other alky! group Rj attached to the carbonyl group is a primary or tertiary carbon atom.
23. A method of Claim 22 wherein said ketone and said aldehyde are insoluble or slightly soluble in said working fluid and are liquid at 100°C and at 20°C or below.

24. A method of Claim 22 wherein the additive in said working fluid is an aliphatic ketone wherein said R, and R, are alkyl groups having from 1 to 8 carbon atoms.
25. A method of Claim 22 wherein either or both of said alkyl groups are halogen substituted.
26. A method of Claim 22 wherein either or both of said alkyl groups are fluoro substituted,
27. A method of Claim 22 wherein said working fluid contains a mixture of said ketones.
28. A method of Claim 22 wherein said working fluid contains a mixture of said aldehydes.
29. A method of Claim 22 wherein said working fluid contains a mixture of one or more of said ketones and one or more of said aldehydes.
30. A method of Claim 22 wherein said ketone and said aldehyde are substantially completely soluble in said working fluid at 20 °0 and above.
31. A method of Claim 22 wherein said working fluid contains between 2 ppm and about 20,000 ppm, by weight of said additive.
32. A method of Claim 22 wherein said working fluid contains between 2 ppm and about 10,000 ppm, by weight of said additive.
33. A method of Claim 22 wherein the additive is a ketone selected from the group consisting of 2-pentanone, 3-penianone, 4-methyl-2-pentanone, 2,4-dimethy|-3-pentanone, 2-hexanone, 3,3-dimethyl-2-hexanone, 4-methyl-2-hexanone, 5-methyl-2-hexanone, 2-heptanone, 3-hept:)none, 4-heptancne, 2-octanone, 3-octanone, and 4-octanone, cyctohexanone, benzophenone and acetophenone.

34. A method of Claim 22 wherein the additive, is an aliphatic aldehyde having between 6 and 14 carbon atoms.
35. A method of Claim 22 wherein said aldehyde is halogen substituted.
36. A method of Claim 22 wherein said working fluid contains ythium bromide and one or more sahs selected from the group consisting of ZnCl2, ZnBr2, MnCl2, MnBr2, MgCl2, MgBr2, SrCI2, SrBr2, FeCl2, FeBr2, CaCl2, CaBr2, LiCI, Lil, UNO2, LiNO3, LiSCN and LiCIO3
37. A method of Claim 22 wherein said cooling andlor heating system comprises a single or double effect absorption apparatus and wherein said metal salt comprises lithium bromide.
38. A method of Claim 22 wherein said additive is a ketone, and wherein said metal salt includes sodium hydroxide, potassium hydroxide or mixtures thereof.
39. A method of Clann 22 wherin said additive is a ketone, and wherein said metal salt is a hydioxide comprising one or more of sodium hydroxide, potassium hydroxide, cesium hydroxide, rubidium hydroxide or lithium hydroxide.
40. In operation of an absorption cycle cooling and/or heating system including an absorber containing an aqueous metal salt absorption solution for absorbing water vapor therein and wherein said metal salt is selected from the group consisting of an alkali metal hafide, sodium hydroxide, potassium hydroxide and mixtures of said hydroxides, a method of improving the rate of water vapor absorption in said absorption solution comprising adding thereto an effective additive amount of at least 2 parts per mlon, by weight, of an aliphatrc, cycloaliphatic, or aromatic ketone having between 5 and 24 carbon atoms and capable of increasing said rate of water vapor absorption, said aliphatic ketone being of the formula R,(C-0)Rj wherein R1 and R2 are the same or different alkyl and substituted alkyl groups of from 1 to 12 carbon atoms and wherein when R, is a methyl group, the carbon atom of the other alkyl group R2 attached to the carbonyl group is a primary or tertiary carbon atom.
41. A water vapor absorption cycle and/or heating system substantially as herein described.
42. A method of improving the rate of water vapor absorption in an absorption solution substantially as herein described