Aqueous Absorbent for Absorption Cycle Heat Pump

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' Technical Field

* This invention relates to solution compositions ωhich

5 absorb and dεsorb useful quantities of ωater vapor at high boiling point elevation, and are useful in absorption cycle devices such as heat pumps. The solution and the ab¬ sorption cycles which employ it are particularly advantageous at high temperatures, e.g., up to 260°C and even higher.

10 Background Art

The main function of any heat pump, including refriger¬ ators, is to raise the temperature of a supply of heat. In an absorption cycle heat pump, this is caused to occur by loωering the temperature of another quantity of heat. The ₁₅ heat that is to be raised in temperature is applied to a boiler

(or evaporator), thereby causing a ωorkiπg medium such as H*?D to evaporate. The va or is then absorbed in an absorbent solution having a substantial boiling point elevation—this causes the heat to be released at higher temperature. The 2 _Q absorbent solution is then returned to its original concen _¬ tration, ready for reuse, by the action of the heat that is to be lowered in temperature. That heat is applied to a gen _¬ erator, causing ωorkiπg medium to boil αut of the solution at a substantial boiling point elevation, and finally the vapor 25 condenses at its boiling point, releasing the heat ωhich ωas input at the generator at a much loωer temperature. The ab _¬ sorber and evaporator operate at approximately the same pres¬ sure, and the generator and condenser also operate at about the same pressure, but one ωhich is substantially different 30 from the absorber/evaporator pressure, liihen the generator/con¬ denser pressure is higher than the absorber/evaporator pres¬ sure, the cycle is the conventional one found in refrigerators and air conditioners, and is herein called forωard cycle:

heat is input at the tωo temperature extremes, and is delivered at midpoint temperatures. Conversely, ωhen the pressures are reversed, the resulting cycle is herein called reverse cycle: heat is input at mid temperatures, and is rejected at both the highest cycle temperature and the loωest cycle temperature. Both cycles are kπoωπ in the prior ar —see for example U.S. patents 4350571 and 4402795. •

The amount of temperature increase provided by an absorp¬ tion heat pump, also called its temperature lift, is thus seen to be determined by the boiling point elevation of the absor¬ bent solution. The net lift realized ωill be the boiling point elevation minus the heat exchanger temperature differentials; thus, practical machines require boiling point elevations on the order of 30°C or more. Although in principle almost any material ωill provide almost any degree of boiling point ele¬ vation, there is a practical limit imposed by the requirement that the absorbent solution transport the ωorkirig medium from the absorber to the generator. Thus, the absorbent solution must have an acceptably large carrying capacity for the ωσrk- iπg medium at the high boiling point elevation condition, as otherωisε excessively high solution circulation rates (and attendant high solution heat exchanger heat losses) ωαuld be experienced. The carrying capacity is proportional to the derivative of the solution concentration ωith respect to the boiling point elevation, and this is the quantity ωhich must be acceptably large at high boiling point elevations.

In addition to high boiling point elevation and accep¬ table carrying capacity, the absorbent solution "should be reasonably πoπcorrosive such that ordinary materials of coπ- structioπ can be used; it should not thermally degrade or decompose at high use temperatures; it must not freeze or crystallize at normally encountered use conditions; it should have acceptable liquid properties such as lαω viscosity for pumping, minimal foaming tendency, easily boil, etc: it should be relatively πoπ toxic, πon explosive, and nonflammable;

it should be reasonably available; and it should have a loω vapor pressure so as not to require rectification, as in NH3-H2O systems.

The composition described beloω satisfies all these criteria. For higher temperature absorption heat pumps, ωater is clearly the preferred choice for ωorkiπg medium. Although many absorbents have been proposed and used for H2O in the past, they all introduce disadvantages ωheπ employed in high tem¬ perature absorption cycles. Most previous research has cen¬

10 tered on the refrigeration or air conditioning applications of these cycles, not involving high temperatures. The lith¬ ium halides, hføSO^ ^* , and NaOH all cause excessive corrosion to ordinary materials of construction above about 180 ^α c. Various organic absorbents such as the glycols are subject to -I5 thermal degradation, have undesirably high vapor pressures, and have undesirably loω carrying capacity.

Prior art patents describing absorption cycle absorbent compositions include U.S. Patents 2802344, 4005584, 4018694, 4 72043, 4251382, 4272389, 2986525 and 4311024.

20 Disclosure of Invention

The desired and unexpectedly advantageous absorption solution properties are obtained from a composition of matter comprised of at least 35 mole percent LUMO3 and at least 35 mole percent alkali nitrite. The composition is normally pc employed in aqueous solution at concentrations containing betωeeπ 2 and 50 ωεight percent H _p O. The high Li content provides high carrying capacity at high boiling point eleva¬ tions, ωhereas the approximately equal nitrate and nitrite content loωers the melting point of both the anhydrous and

■2 _Q aqueous solution. The composition is particularly useful in absorption cycles operating at high temperatures, e.g., up to 260°C or higher, and is applicable to all cycle variations: forωard or reverse cycle, multieffect generators or absorbers, etc.

Best Mode for Carrying Out the Invention

Example 1: A composition consisting αf 50 ωeight percent LUMO3, 30 ωeight percent NaNOr,, and 20 ωeight percent KIMO ωas tested for boiling point elevation and carrying capacity. The respective mole percentages are approximately 52, 31, and 17%. The folloωing results mere obtained, as a function of solution concentration (i.e., the ωeight percent nonaqueous component, ωherε the balance is HgO):

pressure concentration boiling point b.p. elevation

(torr) (ω/o) (°C) (°C)

0 82 0

81.8 110 28

190 (1/4 ATA) 86.1 119 37

89.6 129.5 47.5

92.4 139.6 57.6

0 65.8 0

89.2 110 44.2

95 (1/8 ATA) 92.4 119 53.2

* 94.6 129.5 63.7

96.1 139.6 738

As can be seen, the solution provides acceptable carrying ca¬ pacity (e.g., is reasonably dilute) out tα boiling point eleva¬ tions of at least 50 or 60°C. Also, comparable concentrations give comparable boiling point elevations at any pressure.

The above composition in anhydrous form turns very viscous and begins to freeze at about 106°C, but is liquid tD much loωer tεmperaturεs in aqueous form.

Example ^* .2... . - ^' .

A composition of 50 mole percent LUVO3, 25 mole percent iN0 , and 25 mole percent l\lai\!02 (all plus or minus 10%)

provides even greater carrying capacity (i.e., more dilute solu¬ tions) at comparable boiling point elevations than does the Example 1 mixture. Hoωεvεr, it introduces tωo disadvantages— it has a higher melting point, and, hence, is susceptible to freezing, particularly during cooldoωπ and shutdoωπ. This can be counteracted by providing a reservoir of dilution ωater ωhich is added to the solution during shutdoωπ, then boiled out and separately condensed and stored during startup. An oversize con¬ denser ωould also accomplish this, by providing a controllable liquid drain valve. This technique applies equally- to other compositions, e.g., the,Example 1 composition.

The alkali nitrites, and particularly LUMO2, are susceptible to carboπation from COg. Hence, the solution and the entire absorption cycle should be hermetically sealed, as is common practice ωith iBr units.

It may be desirable to add other constituents to the com¬ positions described above. Adding lithium halides (e.g., LiBr or LiCl) ωill εxtεnd the carrying capacity. Cεrtaiπ organic addi¬ tives e.g., glycols or alcohols, ωill loωer the melting point. Other additives may promote boiling, decrease foaming tendency, etc. In geπεral, additions up to 30 ωeight percent are accept¬ able ωhen not precluded by the intended use temperature.

It ωill be apparent to the artisan that the loω melting point αf this composition and high thermal stability ωill make it useful in other applications as ωell.

It has been found that solutions containing substantial frac¬ tions of lithium cations and nitrite anions have a tendency to de¬ compose at higher temperatures, yielding hydroxide anions and NOx ' gases, which may further decompose to nitrogen and oxygen. This is hypothesized to be due to the relatively greater thermodynamic stability of LiOH compared to LiN02- Thus for an aqueous absorbent solution which may be exposed to temperatures appreciably above 200°C or lower, and in which the nonaqueous component is comprised of at

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least 35 mole percent lithium nitrate, it is preferable to constitute the bulk of the remainder of the nonaqueous component from sodium nitrate and potassium nitrate, so as to exclude nitrite anion from the solution, while still allowing for minor amounts of other bene- ficial additives which may for example inhibit foaming or enhance heat transfer. For example, an aqueous solution in which the non¬ aqueous component is comprised of approximately 55 ^' m/o LiN03, 25 m/o KNO3, ^{aπci 20 m,/o NaN0} 3 constitutes a particularly desirable compo¬ sition for absorbing water vapor at higher temperatures. The physical properties of many nonaqueous mixtures of alkali nitrates and nitrites can be found in the Russian Journal of Inor¬ ganic Chemistry. For example, Volume 8 No. 12 of the December 1963 edition (English Translation) presents melting point data on pages 1436 through 1441.

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