JPH09169972A - Working medium for absorption heat pump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a working medium for an absorption heat pump capable of eliminating occurrence and advance of corrosion of equipment for a long period of time, in an aqueous solution composition comprising lithium bromide and lithium iodide as main components, a combination of a molybdate and an alkali hydroxide as an inhibitor and a reducing agent. SOLUTION: This working medium comprises a highly concentrated aqueous solution containing lithium bromide and lithium iodide as main components, sodium hydrogensulfite as a reducing agent and a molybdate and an alkali hydroxide as inhibitors. The added amount of the alkali hydroxide is <=0.06N.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-concentration aqueous solution containing lithium bromide and lithium iodide as main components, containing molybdate and alkali hydroxide as inhibitors (corrosion inhibitors), and containing a reducing agent. The present invention relates to a working medium for an absorption heat pump. Incidentally, the term absorption heat pump in the present specification, unless otherwise specified, in a broad sense, that is, an absorption refrigerator for utilizing cold water obtained, and an absorption heat pump in a narrow sense for utilizing obtained hot water. Both shall be meant.

[0002]

2. Description of the Related Art Various working media have been proposed so far for use in absorption heat pumps, but in Japan, a system consisting essentially of water and lithium bromide (water-LiBr system) is actually used. Is used.

The water-LiBr type working medium has been conventionally adopted because it is excellent in stability, corrosiveness, price, etc. However, there is a performance limit due to the crystal limit, and the crystal limit is relaxed. If this is possible, the concentration range of the absorbing solution can be widened, which reduces the circulating amount of the solution, downsizing the solution pump, saving power, etc. It can be downsized, and if it has a large mitigation effect, it will not only have the possibility of air cooling, but it can also obtain hot water of higher temperature when used as a water heater, and further reduce the evaporation temperature of the evaporator. Therefore, cold water having a lower temperature can be obtained as the water cooler, and many performance improvements such as the use of a low temperature heat source having a lower temperature as the water warmer can be expected.

For this reason, various attempts and proposals for relaxing the crystal limit have been made for this system, and for example, the addition of zinc bromide or zinc chloride to the aqueous solution of this system has been studied and proposed. However, in the system to which these are added, the solution itself becomes acidic and exhibits not only extremely strong corrosiveness, but also a dilute solution of about 10% by weight or less causes a precipitate due to the formation of zinc hydroxide.

In this respect, Japanese Patent Publication No. 61-52738 discloses a system comprising water and lithium bromide as described above.
By adding lithium iodide to lithium bromide, and adjusting the quantitative ratios of lithium bromide to 70 to 99 mol% and lithium iodide to 1 to 30 mol%, the vapor pressure drop is lower than that of the lithium bromide aqueous solution. This is because the crystallization temperature is large and the crystallization temperature is low, and the use of this absorption liquid makes it possible to improve the performance of the absorption cooling / heating machine and suppress the occurrence of problems such as solidification of the absorption liquid.

In Japanese Patent Publication No. 5-28749, lithium bromide, lithium iodide and lithium chloride are contained in an absorption liquid used in an absorption refrigerator comprising a generator, a condenser, an evaporator and an absorber. In a weight ratio, lithium bromide 1: lithium iodide 0.1 to 1.0: lithium chloride 0.
An absorption liquid for an absorption refrigerator, which is composed of an aqueous solution in which an absorbent containing a mixture mixed in a ratio of 05 to 0.50 is dissolved in water as a refrigerant, has been proposed, which has a high concentration and a low crystallization temperature. He said that he was able to provide the absorption liquid.

However, the present inventors have found that the above water-L
Further detailed exploration of the composition of the iBr-based absorbent,
After exploring and measuring the vapor pressure, crystallization line, solution temperature, etc. extensively and in detail including the existence of a permissible range that can be put to practical use even under severe operating conditions, and further detailed examination from various viewpoints, Unexpectedly, a unique range very effective as an absorbent in a completely different place from the composition range set in the above publications for a system containing lithium bromide, lithium iodide and lithium chloride as main components. It was discovered that there was a problem, and it was developed and proposed earlier (Japanese Patent Application No. 6-9574).
No. 9), according to this, the crystal limit is greatly relaxed and it can withstand actual practical use.

The absorption heat pump is basically a generator,
It is composed of a condenser, an evaporator and an absorber. Each of these devices and equipment is composed of various materials such as mild steel and other carbon steel-based materials, copper-based alloys such as copper and cupro-nickel, and recently stainless steel-based materials as alternative materials when these carbon steel-based materials cannot be used. However, the problem of corrosion to these materials must be taken into consideration together with the problems of crystallization of the absorbent component and formation of precipitates.

In the absorption heat pump, it is necessary to keep the entire system in a completely airtight state in order to maintain smooth operation.
This is also very important for the corrosion protection of the system at the same time, but nevertheless, when water is used as the refrigerant and lithium bromide, lithium iodide, lithium chloride, etc. are used as its absorbent, this working medium Has a corrosive property with respect to iron-based materials such as copper-based and mild steel which are the main constituent materials of the above-mentioned various devices constituting the absorption heat pump, and therefore it is usually necessary to add an inhibitor for corrosion prevention.

Examples of this inhibitor include chromates such as lithium chromate, molybdates such as lithium molybdate, tungstates, nitrites, nitrates,
Azole salts, amines and the like have been proposed, and according to JP-A-1-174588, "a lithium halide salt aqueous solution containing lithium iodide or the like" has been pointed out as an absorbent having particularly strong corrosiveness. Measures are considered when using.

According to this publication, in the above absorbing solution, the conventional inhibitor alone was not sufficient for the corrosion inhibiting effect, but this problem was solved by adding an antimony compound, particularly diantimony trioxide. . And here, as the action of the added antimony compound, reducing the liberated halogen to ions and suppressing the liberation of halogen, the added antimony compound is adsorbed on the surface of the copper and steel material in the absorber,
It is pointed out that there are two points of forming a dense protective film and preventing elution of iron and steel.

As described above, in the above publication, the "lithium halide salt aqueous solution containing lithium iodide or the like" is targeted, and the specific compound as described above is effective in suppressing corrosion. However, regarding the aqueous solution composition containing lithium bromide and lithium iodide as the main components, which has been developed and proposed above, and the aqueous solution composition containing lithium chloride therein, among the inhibitors known so far, actually It turns out that few are valid.

Therefore, the inventors of the present invention have proposed, for example, caustic alkali + chromate, caustic alkali + nitrate, caustic alkali + inhibitors for inhibitors for preventing corrosion of iron-based or copper-based metallic materials constituting an absorption heat pump. We re-examined many combinations of antimony trioxide, caustic alkali + organic compounds and others, and found that the combination of molybdate and alkali hydroxide was effective only as a specific inhibitor. (Japanese Patent Application No. 6-234406).

[0014]

The limitation of the inhibitors for the above aqueous solution compositions to such specific combinations is that lithium iodide, of the main components thereof, is the other two components, namely lithium bromide and lithium bromide. It is also considered that it originates from the fact that it has different properties such as being more easily oxidized than lithium chloride, but in the present invention, the above-mentioned lithium bromide and lithium iodide are the main components, and molybdate and hydroxide are used as inhibitors. Contains in combination with alkali,
Furthermore, in an aqueous solution composition containing a reducing agent, whether the alkali concentration due to the alkali hydroxide has a strong influence on the occurrence or non-existence of equipment corrosion. The inventors have found that it is necessary to keep the value below a predetermined value and reached the present invention.

[0015]

That is, the present invention provides a high-concentration aqueous solution containing lithium bromide and lithium iodide as main components, containing molybdate and alkali hydroxide as inhibitors, and containing a reducing agent. And a working medium for an absorption heat pump, characterized in that the alkali hydroxide is contained at a concentration of 0.06 N or less.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Lithium hydroxide is preferably used as the alkali hydroxide. In the present invention, alkali hydroxide such as lithium hydroxide is contained at a concentration of 0.06 N or less so that corrosion is eliminated even at an operating temperature of about 220 ° C. or higher, and a non-condensable gas caused by the corrosion. Can occur at all or almost none. As an example, when the lithium hydroxide content is 0.02 N or less, the internal pressure of the container does not increase due to corrosion even at a high temperature of 250 ° C. for a long period of time.

Of the above molybdates, lithium molybdate is particularly advantageous, and the addition amount thereof is 50 PPM.
Below that, there is virtually no effect, while its solubility is 500.
Since it is PPM, it can be used in the range of 50 to 500 PPM. The reducing agent acts to suppress the corrosion of iron-based materials such as carbon steel and stainless steel and copper-based materials such as cupronickel by virtually eliminating the presence of iodine in the aqueous solution, but as this reducing agent, sodium bisulfite is used. , Sodium thiosulfate, sulfur, etc. are not particularly limited, but sodium bisulfite can be used more preferably.

[0018]

EXAMPLES Examples of the present invention will be described below, but it goes without saying that the present invention is not limited to these examples. In this embodiment, the container test depending on the alkali concentration is described first, and then the crevice corrosion test is described. In the container test, changes over time in the container pressure are measured.This is because if the inner surface of the test container is corroded, non-condensable gas is generated and the pressure inside the container rises. This is a guideline for the extent of the existence.

<< Container Test >> As a container material, NSS-S
CR (manufactured by Nisshin Steel: austenitic stainless steel)
With an inner diameter of 83 mm and a height of 1
An 80 mm cylindrical container was prepared. On the other hand, a liquid having the following components (A) to (C) was prepared as a test liquid. These are basically "69% LiBr-LiI system +5
00ppm NaHSO ₃ + 300ppm Li ₂ MoO ₄
+ XN (x normalization) LiOH ”, to which a trace amount of octyl alcohol was added as a solubilizing agent, and the concentration xN of LiOH was changed.

(A) LiBr + LiI [LiBr / L
iI weight ratio = 1 / 0.7]: 69 wt% + NaHSO
₃ : 500 ppm + Li ₂ MoO ₄ : 300 ppm +
LiOH: 0.1 N aqueous solution (hereinafter referred to as test solution A). (B) LiBr + LiI [LiBr / LiI weight ratio =
1 / 0.7]: 69 wt% + NaHSO ₃ : 500 p
_{pm + Li 2 MoO 4: 300ppm} + LiOH:
A 0.05 N aqueous solution (hereinafter referred to as test solution B). (C) LiBr + LiI [LiBr / LiI weight ratio =
1 / 0.7]: 69 wt% + NaHSO ₃ : 500 p
_{pm + Li 2 MoO 4: 300ppm} + LiOH:
0.02N aqueous solution (hereinafter referred to as test solution C).

The test method was as follows. The cylindrical container was evacuated, and 500 ml of each of the test solutions A, B, and C was contained in each container. These accommodating containers were maintained at a predetermined temperature, the temperature was lowered to room temperature at regular intervals, and the internal pressure was measured. The presence / absence of this internal pressure rise, and the degree of the pressure rise in the presence thereof, correspond to the amount of non-condensable gas generated. If the pressure rises, it is indicated that the amount of non-condensable gas is generated. 1 and 2 show the test liquid A and the test liquid C at temperatures of 200 ° C. and
The result when it hold | maintained at 230 degreeC and 250 degreeC is shown.

As is clear from FIG. 1, LiOH is less than 0.
In case of 1N, 800 hours (h
Even after the passage of r), the pressure in the container slightly increases and there is almost no change. However, at a holding temperature of 230 ° C, the pressure inside the container gradually increased (215 torr after a lapse of 800 hours), and at a holding temperature of 250 ° C, the pressure inside the container rapidly increased after the start of the test for 700 hours. After that, it has reached 510 torr. Thus, when the LiOH concentration in the absorbing liquid is as high as 0.1 N, almost no non-condensable gas is generated at an operating temperature of about 200 ° C.,
At a temperature of 230 ° C., non-condensable gas is gradually generated, and at a temperature of 250 ° C., the pressure rises from the beginning, indicating that the corrosion has continued.

FIG. 2 shows the results when the test solution C, that is, the LiOH concentration in the solution was 0.02 N.
At 00 ° C. and 230 ° C., there is no change in the internal pressure of the container even after the elapse of 800 hours, and it is clear that the corrosion of the internal surface of the container is completely suppressed in this way. At a temperature of 250 ° C., a slight pressure increase was observed after 600 hours until 700 hours, and there was no pressure increase up to 600 hours, indicating that the generation of non-condensable gas was sufficiently suppressed. ing.

Further, FIG. 3 shows the test liquid A (LiOH =
0.1N), the test liquid B (LiOH = 0.05N) and the test liquid C (LiOH = 0.02N) were used to measure the change in the pressure inside the container at 250 ° C., which is the most severe temperature among the above temperature conditions. It is measured with time. As shown in FIG. 3, in the case of LiOH = 0.1N (x = 0.1 in xN LiOH in FIG. 3), the pressure inside the container increased from the beginning of the test, and reached 450 torr after 500 hours. It shows that it is not suitable as an absorbent under high temperature conditions.

On the other hand, LiOH = 0.05N (see FIG.
X = 0.05) in medium xN LiOH) and LiOH
= 0.02 N (x = 0.0 in xN LiOH in FIG. 3).
In the case of 1), 400 when LiOH = 0.05N
Although there are some signs of slight pressure increase before and after the passage of time, 5
Even after the lapse of 00 hours, there was virtually no change in the pressure inside the container, indicating that the corrosion of the inner surface of the container was almost completely suppressed.

<Gross corrosion test> N as a material for the test piece
SS-SCR (manufactured by Nisshin Steel Co., Ltd., austenitic stainless steel) was prepared and processed into a crevice test piece. 5 (a) and 5 (b) show the outline of the clearance test piece. By spot welding a 50 mm × 20 mm plate (reference numeral 1 in FIG. 5) and a plate 20 mm × 20 mm (reference numeral 2 in FIG. 5) having a thickness of 5 mm with a spot diameter of about 5 mmφ (reference numeral 3 in FIG. 5), A spot welding specimen having a metal-metal clearance (gap) was produced. In addition, the reason why the test piece is such a "clearance" test piece in this test is that even in the case of stainless steel, an oxygen concentration battery is formed due to a difference in ventilation between the metal and the like. This is because the part where the oxygen supply is insufficient at the joint part or the lower part of the adhered substance becomes the anode and corrosion (crevice corrosion) progresses.

The crevice corrosion generation potential (Vcrev) was measured as follows. After degassing a titanium autoclave container having an internal volume of 1 liter and subjected to Teflon lining with nitrogen, the test liquid C (LiOH = 0.02N) was injected, and the above-mentioned crevice test piece was dipped into the liquid (immersion surface area in liquid). = About 10 cm ² ), the upper part of the container was capped and deaerated using high-purity nitrogen gas, and then the inside was made into a nitrogen atmosphere to be hermetically sealed.

Then, the vessel was heated to 220 ° C. inside, and this state was continued for 24 hours, and then the potential was kept constant at a predetermined potential, and the change in current with time was measured. As the evaluation method, the state of the clearance test piece after the above experiment and the change with time of the current were evaluated. As a result, the crevice corrosion generation potential (Vcrev) in the test liquid C was -150.
It can be regarded as mV (vs Ag / AgCl) (see FIG. 4).

The spontaneous potential (Esp) was measured as follows. Internal volume 1 with the same Teflon lining as above
After degassing a liter titanium autoclave container with nitrogen, the test liquid C (LiOH = 0.02N) was injected,
The above clearance test piece is dipped in this (immersion surface area in liquid =
The container was capped at about 10 cm ² ) and degassed with high-purity nitrogen gas, and then the inside was sealed with a nitrogen atmosphere. Then, the container was heated to a temperature of 220 ° C. inside, and this state was continued for 2 weeks (336 hours) to measure the change with time in the spontaneous potential. FIG. 4 shows the above test results.

As shown in FIG. 4, the self-potential gradually weakened with time from -420 mV (vs Ag / AgCl) slightly immediately after the start of the test to 340 mV (vs after 40 hours).
Ag / AgCl), and rises and falls somewhat slowly on the way, but after 200 hours, −320 mV (vs Ag /
The value before and after AgCl) is maintained. Crevice corrosion generation potential (Vcrev): -150 mV (vs Ag / AgC
It was found that 170 mV was less than 1) even after 200 hours, and the test liquid C (LiOH = 0.02N) has excellent performance against corrosion (and crevice corrosion).

[0031]

As described above, a high-concentration aqueous solution containing lithium bromide and lithium iodide as main components, containing an inhibitor consisting of a combination of molybdate and alkali hydroxide, and containing a reducing agent. The working fluid for absorption heat pumps is extremely effective as a working absorbing liquid in the region of low alkalinity, that is, where the concentration of alkali hydroxide is low, does not cause corrosion for a long time even at high temperatures with respect to equipment components such as iron. Can be applied to.

[Brief description of the drawings]

FIG. 1 Alkalinity of absorbing solution 0.1N in SCR container
The figure which shows the time-dependent change of the pressure in a container in the case of.

FIG. 2 Alkalinity of absorbing solution in SCR container 0.02
The figure which shows the time-dependent change of the pressure in a container in case of N.

[Fig. 3] Absorber alkalinity 0.02 in SCR container
The figure which shows the time-dependent change of the pressure in a container in N, 0.05N, and 0.1N.

FIG. 4 shows NS in an absorbing solution with LiOH = 0.02N.
The figure which shows the time-dependent change (temperature 220 degreeC) of the spontaneous potential of S-SCR (SUS304 steel).

FIG. 5 is a diagram showing an outline of a spot crevice test piece (SCR spot welding test piece) used in an example << crevice corrosion test >>.

[Explanation of symbols]

 1 SCR steel plate (large) 2 SCR steel plate (small) 3 Spot weld

Claims (4)

[Claims] 1. A high-concentration aqueous solution containing lithium bromide and lithium iodide as main components, a molybdate and an alkali hydroxide as inhibitors, and a reducing agent, wherein the alkali hydroxide is 0.06N. A working medium for an absorption heat pump, characterized in that it is contained in the following concentration. 2. The working medium for an absorption heat pump according to claim 1, wherein the alkali hydroxide is lithium hydroxide. 3. The molybdate is lithium molybdate, and the addition amount thereof is 50 to 500 PPM.
Or the working medium for an absorption heat pump according to item 2. 4. The working medium for an absorption heat pump according to claim 1, 2 or 3, wherein the reducing agent is sodium hydrogen sulfite.