EP2521693A1

EP2521693A1 - Heat transfer and heat storage fluids for extremely high temperatures, based on polysulfides

Info

Publication number: EP2521693A1
Application number: EP10801583A
Authority: EP
Inventors: Hans-Josef Sterzel
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2010-01-05
Filing date: 2010-12-23
Publication date: 2012-11-14
Also published as: IL220542A0; CN102695671B; US20110163258A1; US20110163259A1; CL2012001786A1; JP2013516380A; CA2785150A1; TN2012000336A1; JP5774025B2; WO2011083053A1; AU2010340923A1; MA33950B1; KR20120125488A; CN102695671A; BR112012016661A2; MX2012007394A

Abstract

The invention relates to a composition for transporting and storing heat energy, comprising alkali polysulfides of the form (Me1(1-x)Me2x)2 Sz, where Me1 and Me2 are selected from the group of alkali metals made of lithium, sodium, potassium, rubidium, and cesium, and Me1 is not the same as Me2, and x = 0 to 1 and z = 2.3 to 3.5.

Description

Heat transfer and heat storage fluids for extremely high temperatures on the

Base of polysulfides

Liquids for transferring heat energy are used in a variety of fields of technology. In internal combustion engines, mixtures of water and ethylene glycol carry waste heat from combustion into the radiator. Similar mixtures transport the heat from solar roof collectors into heat storage. In the chemical industry, they convert the heat from electric or fossil-heated heating systems to chemical reactors or out of them to cooling devices.

According to their requirement profile, a variety of liquids is used. The liquids should be liquid at room temperature or even lower temperatures and, above all, have low viscosities. For higher operating temperatures, water is out of the question, its vapor pressure would be too high. Therefore, up to 250 ° C hydrocarbons are used, which usually consist of aromatic and aliphatic molecular proportions. In many cases, oligomeric siloxanes are used for higher temperatures.

A new challenge for heat transfer fluids is thermal solar power plants, which generate large scale electrical energy. So far, such power plants were built with a total installed capacity of about 1000 megawatts. In one embodiment, the solar radiation is focused via parabolic mirror grooves in the focal line of the mirror. There is a metal tube, which is located in a glass tube to prevent heat loss, the space between the concentric tubes is evacuated. The metal tube is flowed through by a heat transfer fluid. According to the prior art, a mixture of diphenyl ether and diphenyl is usually used here. The heat carrier is heated to a maximum of 380 to 400 ° C and thus operates a steam generator, in which water is evaporated. This steam drives a turbine, which in turn drives the generator like in a conventional power plant. Total efficiencies are achieved by 20 to 23 percent, based on the energy content of solar radiation.

For concentrating the solar radiation there are various possibilities; In addition to parabolic mirrors, one also works with Fresnel mirrors, which also concentrate the radiation on a flow-through pipe. Both components of the heat carrier, (diphenyl ether and diphenyl), boiling under normal pressure at about 256 ° C. The melting point of the diphenyl is 70 ° C, that of the diphenyl ether at 28 ° C. By mixing both substances, the melting point is lowered to about 10 ° C. The mixture of both components, (diphenyl ether and diphenyl), can be used up to a maximum of 380 to 400 ° C, at higher temperatures decomposition occurs, hydrogen gas is evolved, insoluble condensation products are deposited in pipes and containers. Of the Vapor pressure at these temperatures is about 10 bar, a pressure that is still tolerable in the art.

To obtain higher total efficiencies than 20 to 23 percent, higher steam inlet temperatures are necessary. The efficiency of a steam turbine increases with the turbine inlet temperature. Modern fossil-fueled power plants operate with steam inlet temperatures of up to 650 ° C and thus achieve efficiencies of 45%. It would be quite technically possible to heat the heat transfer fluid in the focal line of the mirror to temperatures around 650 ° C and thus also to achieve such high efficiencies; but this prohibits the limited temperature resistance of the heat transfer fluids.

Obviously, there are no organic substances that are able to endure temperatures above 400 ° C in the situation, at least until today are not known. Therefore, attempts have been made to avoid inorganic, more temperature-resistant liquids. The possibility known from nuclear technology to use liquid sodium as heat transfer fluid has been intensively tested. However, the practical use was that sodium is rather expensive, that it has to be produced with high energy expenditure by electrolysis of sodium chloride and that it reacts with traces of water already under evolution of hydrogen and thus represents a safety problem.

These problems are even more acute with the eutectic alloy of sodium and potassium (about 68 atomic percent potassium), which crystallizes only at -12 ° C.

Another possibility is the use of inorganic salt melts as a heat transfer fluid. Such molten salts are state of the art in processes that operate at high temperatures. With mixtures of potassium nitrate, sodium nitrate, the corresponding nitrites and optionally other cations such as lithium or calcium, working temperatures of up to 500 ° C. and crystallization temperatures down to 100 ° C. are achieved (US Pat. No. 7,588,694).

The fertilizer industry is capable of producing large quantities of nitrites and nitrates. However, two significant drawbacks of molten salts cause them to be used hesitantly in solar thermal power plants: As nitrates, they have a strong oxidizing effect on the metallic materials, preferably steels, at elevated temperatures, which limits their upper application temperature to the cited about 500 ° C. is. Second, the thermal stability of the nitrates is limited at higher temperatures. They decompose with the elimination of oxygen, and insoluble oxides are formed. Because of its crystalline melting point, its lowest application temperature is about 160 ° C. The addition of lithium or calcium salts achieves a further lowering of the melting point. However, the lithium salts cause greatly increased costs, the calcium content increases the

Melt viscosity at low temperatures disadvantageously. Currently, molten salts are used in solar thermal power plants as a heat storage fluid. However, in the solar field, diphenyl and diphenyl ether mixtures continue to be used, which further limits the storage temperature to about 390.degree. It has also been investigated whether water under suitable high pressure is not suitable as a heat transfer medium. But this is countered by the extremely high vapor pressure of over three hundred bar, which would make the thousands of kilometers of pipelines of a large solar thermal power plant uneconomically more expensive. The steam itself is unsuitable as a heat transfer medium and heat storage because of its comparatively low thermal conductivity and the low heat capacity per volume compared to a liquid.

Another problem arises from the fact that one strives to operate a solar thermal power plant even at night. For this purpose, significant amounts of heat transfer fluid in large, heat-insulated tanks to store.

If you want to store the heat content for a power plant with an electric power by one gigawatt for thirteen to fourteen hours, so requires the tank fillings of the order of one hundred thousand cubic meters at 600 ° C and an efficiency of 40% from the heat reservoir to the output of the generator. This means that the heat transfer medium must be very inexpensive, otherwise the investment for such a power plant becomes uneconomical. It also means that enough material of the heat carrier must be available, because hundreds of one gigawatt units are needed to supply on a large scale and to secure the base load. This ultimately depends on solving the question of economic supply of solar energy

Energy on a large scale depends on whether there is a heat transfer fluid that allows temperatures up to 650 ° C in continuous use, which has the lowest possible economically viable vapor pressure at this temperature, preferably below ten bar, which does not oxidatively attack the iron materials used, and which has the lowest possible melting point.

At first glance, these conditions could most likely be met by elemental sulfur. Sulfur is available in sufficient quantities; There are very large, rich deposits, and sulfur is produced as waste in the desulphurisation of fuels and natural gas. At present there is no use for millions of tons of sulfur.

The melting point of sulfur at just under 120 ° C is lower for use as a heat transfer fluid than that of molten salts, the boiling point of sulfur at 444 ° C in the right range, a decomposition is practically impossible. At 650 ° C, the vapor pressure of sulfur is ten bar, a pressure that is technically manageable. At 120 ° C, the viscosity of sulfur is only about 7 cPoise (7 mPas). The density of the liquid sulfur is in wide temperature ranges on average

1, 6 kg / liter, the specific heat by 1,000 joules per kg and degree or by 1,600 joules per liter and degree. It is thus below that of water by 4,000 joules per liter and degree, but above the specific heat of most common organic heat transfer fluids. (Substance data, Hans Günther Hirschberg, manual process engineering and plant construction, page 166, Springer Verlag 1999, ISBN 3540606238).

A disadvantage of elemental sulfur for use as a heat transfer fluid or Wärmespeicherflüsigkeit could be its viscosity behavior:

In the temperature range of about 160 to 230 ° C, the ring-shaped sulfur molecules polymerize ring-opening to very long chains. While the viscosity is above the melting range by 7 mPas, it rises to 23 mPas at 160 ° C to reach maximum values around 100,000 mPas at temperatures in the range of 170 to 200 ° C. The polymerization of sulfur thus generally causes an increase in viscosity, whereby the normal purified sulfur in this temperature range is generally no longer eligible, is not well suited for use as heat transfer fluid.

It was an object of the invention to provide a composition for transport and storage of thermal energy (hereinafter also referred to as "heat transfer medium / inventive heat storage medium") containing sulfur and the disadvantages described above, for example, the higher vapor pressure at elevated temperatures and especially the viscosity increase , not showing.

As a result of developments in the sodium-sulfur battery, some technically important properties of polysulfide melts have become known in the past, as shown below.

Melting point minima in the binary systems represent the compositions Na ₂ S ₃ with 235 ° C and K ₂ S _{3; 44} with 1 12 ° C, with Na ₂ S ₃ does not exist in the melt, but a mixture, mainly of Na ₂ S ₂ and Na ₂ S ₄ , is present. The lowest eutectic melting point in the (calculated) K-Na-S ternary system is a polysulfide of the composition

(K ₀ , 77Na ₀ , ₂ 3) 2 S ₃ , 74 at 73 ° C (Lindberg, D., Backman, R., Hupa, M., Chartrand, P., "Thermo-dynamic evolution and optimization of the Na-KS system "in J. Chem. Thenn. (2006) 38, 900-915).

Some references suggest that the sodium polysulfides are unstable at their melting points. More stable are the potassium polysulfides. Thereafter, K ₂ S ₄ decomposes under normal pressure at 620 ° C to K ₂ S ₃ and sulfur; K ₂ S ₃ decomposes at 780 ° C to K ₂ S ₂ and sulfur (US Patent 4,210,526).

Thus, therefore, the areas with molar sulfur contents of S ₂ to S _{3 are} particularly stable. If one looks at the phase diagrams of the binary systems, one finds for example for Na ₂ S _{2; 8} a melting point of 360 ° C, for K ₂ S _{2, 8} a melting point of 250 ° C and for the ternary polysulfide NaKS _{2, 8} a melting point of about 270 ° C.

These rather high melting points are not particularly encouraging to deal with the alkali poly- sulfides for use as a heat transfer and heat storage medium.

The viscosity behavior of the polysulfides also tends to not concentrate on this class of compounds: Upon closer examination of the melts of alkali metal polysulfides was found that the alkali metal polysulfides at temperatures below 200 ° C have increased viscosities. Thus, sodium polysulfides of the formula Na ₂ S ₃ . ₄ at 400 ° C a viscosity of 10 centipoise ("The Sodium Sulfur Battery", JL Sudworth and AR Tilley, Univ. Press 1985, pages 143-146, ISBN 0412-16490-6).

This value doubles when the temperature is lowered by 50 ° C, ie to 20 cP at 350 ° C, 40 cP at 300 ° C, 160 cP at 200 ° C, 320 cP at 150 ° C and further extrapolated 640 cP, if the polysulfide would still be liquid at 100 ° C. The final value of 640 cP corresponds to about half the viscosity of glycerol at room temperature (1,480 cP). By comparison, the viscosity of water is around 1 cP, that of olive oil around 100 to 200 cP. Often the alkali polysulfides solidify glassy and form highly viscous glasses which slowly crystallize at room temperature for days.

Finally, the corrosion behavior of the Alkalipolysulfidschmelzen also not suitable to investigate this class of compounds as encouraging for use as a heat transfer and heat storage liquid. It is known, for example, that alkali metal polysulfide melts can even quickly dissolve metallic gold with the formation of complex sulfides.

In the following, "Me" means the group of the following alkali metals of the Periodic Table of the Elements: lithium, sodium, potassium, rubidium and cesium.

It has now surprisingly been found that alkali metal polysulfides of the composition (I) (also referred to below as "alkali metal polysulfides according to the invention") wherein Me1 and Me2 are selected from the group of alkali metals consisting of lithium, sodium, potassium, rubidium and cesium, Me1 is not equal to Me2 and x = 0 to 1 and z = 2.3 to 3.5 is still at temperatures down to 130 ° C were thin, so had significantly lower melting points and viscosities than those expected from the literature.

Preferred are the polysulfides defined above in formula (I) with Me1 = potassium and Me2 = sodium, particularly preferred are the polysulfides defined above in formula (I) with x = 0.5 to 0.7 and z = 2.4 to 2.9, furthermore particularly preferred are the poly sulfides defined above in formula (I) with Me1 = potassium and Me2 = sodium and with x = 0.5 to 0.7 and z = 2, 4 to 2.9.

Particular preference is furthermore given to the polysulfides of the formulas (Na ₀ .5 to .o, 65Ko, ₅ to 8 ·, or (Na ₀ , 6K ₀ , ₄ ) 2 S ₂ , ₆ .

The observed melting temperatures were usually more than 200 ° C lower than the literature values. According to the current state of knowledge, this is attributed to the different synthesis of the alkali metal polysulfides according to the invention compared to the literature

The alkali metal polysulfides according to the invention are obtainable by the following methods. In the context of the invention, very economical synthesis routes should be followed. For this purpose, concentrated aqueous solutions of the corresponding alkali metal hydrosulfides (MeHS), for example sodium hydrogen sulfide, NaHS or potassium hydrogen sulfide, KHS, which are obtained by introducing hydrogen sulfide into the aqueous hydroxide solutions of the corresponding alkali metals Me, were reacted with sulfur according to the general formula

2 MeHS + zS> Me ₂ S _{(z +} i) + H ₂ S (Me = alkali metal, for example K, Na) with one equivalent of hydrogen sulphide escaping. This hydrogen sulfide can be recycled and used again for the preparation of the alkali hydrogen sulfides.

The water of reaction and the water of dissolution were preferably distilled off rapidly with increasing the temperature up to 500 ° C., thus obtaining the alkali metal polysulfides according to the invention.

In science, on the other hand, it is endeavored to produce polysulfides as pure as possible, the economy is generally irrelevant. Therefore, to purify the polysulfides, the alkali metals are reacted with elemental sulfur, usually in liquid ammonia, with which the considerable heat of reaction occurring in this reaction is removed. According to the current state of knowledge, the different properties of the alkali metal polysulfides according to the invention are caused by the different synthetic routes:

The anhydrous synthesis of the prior art gives very pure alkali metal polysulfides.

In the synthesis according to the invention, water and hydrogen sulfide are usually present. In very complex, temperature-dependent equilibria, according to the current state of knowledge, water and hydrogen sulfide intervene in the reaction process and Probably causes other structures and / or molecular mass distributions than in anhydrous synthesis. Under the inventive, economical process conditions undetectable, tightly bound small residues of water and / or hydrogen sulfide, hydrogen sulfides or Sulfanendgruppen may possibly be responsible for the lowering of melting point and viscosity of the alkali metal polysulfides according to the invention.

This observation leads to the solution of melting point and viscosity problems:

A further process for preparing the alkali metal polysulfides of the formula (I) according to the invention or their preferred embodiments described above is the reaction of alkali hydrogen sulfides with sulfur to form the alkali metal polysulfides according to the invention in concentrated aqueous solution and preferably their subsequent drainage by direct distillation of the water it is possible to prepare the alkali metal polysulfides according to the invention by reacting the alkali metal hydrogensulfides

MeHS + MeOH <> Me ₂ S + H ₂ 0 is reacted with alkali metal hydroxide to the alkali sulfides and the alkali metal sulfides are reacted with further sulfur to the polysulfides.

However, there is a risk in this synthesis that in the concentrated aqueous solution due to the hydrolytic back reaction, a large concentration of hydroxide ions present, which can implement in an undesirable side reaction with the sulfur of the following reaction stage to high-melting and thermally unstable AI kalithiosulfat.

6 MeOH + zS> 2 Me ₂ S ( _Z _ ₂ ) + Me ₂ S ₂ 0 ₃ + 3H ₂ 0 Alkaline thiosulfates generally increase the melting temperature, increase the melt viscosity of the alkali metal polysulfides and decompose at elevated temperatures in different reaction routes to further salts ,

Among the decomposition products of the thiosulfates are the sulfates of the alkali metals, which also generally have the disadvantageous properties of high melting temperature and viscosity as fractions in the polysulfide melt.

The synthesis route according to the invention avoids this side reaction, there are usually no excess hydroxide ions in an increased concentration.

In a further variant for the preparation of the alkali metal polysulfides according to the invention, it is possible to avoid the secondary reactions by reacting in the reaction of alkali metal hydrogen sulphide. Working with alkali hydroxide with a deficiency of alkali hydroxide and thus also avoids excess hydroxide ions. In this case, a maximum of 0.9 moles of alkali metal hydroxide is used per mole of alkali metal hydrogen sulfide. According to the molar deficiency of alkali metal hydroxide then usually there is a mixture of sulfide and hydrogen sulfide, which is reacted with the sulfur to the alkali metal polysulfides according to the invention.

In a further variant for the production of the alkali metal polysulfides according to the invention it is possible, instead of reacting the concentrated aqueous solutions of the alkali hydrogen sulfides and optionally of the sulfides in mixture with hydrogen sulfides with sulfur and dehydrating the polysulfides, the alkali metal hydrogen sulfides, optionally in admixture with sulfides, before the reaction to dehydrate with the sulfur first and react with the sulfur in a second step the dehydrated hydrogen sulfides and optionally contained therein sulfides.

As a rule, this variant involves obtaining the high-melting dry substances during the dehydration of the hydrogen sulfides or the sulfides present in a mixture with the hydrogen sulfides, which makes the production somewhat more complicated.

However, according to this process variant, alkali metal polysulfides according to the invention are obtained whose solidification temperature is 10 to 20 ° C. below those alkali metal polysulfides according to the invention of the same composition according to the first and preferred process variant.

The alkali metal polysulfides according to the invention with z = 2.3 to 3.5 are preferably used.

Contrary to the literature, the pure alkali metal polysulfides according to the invention, preferably sodium polysulfides, with these sulfur contents prove extremely temperature-stable, up to about 700 ° C.

The high temperature stability of the alkali metal polysulfides according to the invention, preferably sodium sulfides, is especially given values of z less than 3. Sulfur contents with values of z greater than 3.5 usually result in disadvantageously increased viscosities. The densities of the alkali metal polysulfides according to the invention are generally at 350 ° C. in the range from 1.8 to 1.9 g / cm ³ .

Of course, the use of cesium or rubidium as the alkali metal is suitable for the alkali metal polysulfides according to the invention. These alkali metals usually form polysulfides up to the hexasulfides.

According to the current hypothesis, the size of the ions influences the viscosity of the alkali metal polysulfides according to the invention. Thus, the larger potassium ions generally give slightly lower viscosities than the smaller sodium ions.

Apart from this, it is preferable not to add further salts, for example alkali thiocyanates, to the alkali metal polysulfides according to the invention for lowering their melting points. The thermal stability or the corrosion behavior (especially at high temperature) of the alkali metal polysulfides according to the invention can thereby be changed in a disadvantageous manner.

The heat transfer / heat storage media according to the invention usually contain the alkali metal polysulfides according to the invention in a substantial amount up to a maximum of practicality

100 wt .-%, for example in the range of 20 wt .-% to practically 100 wt .-% or 50 wt .-% to practically 100 wt .-%.

Usually, the heat transfer / heat storage media according to the invention are protected against the ingress of moisture during production, storage, transport and use. In general, therefore, the heat transfer / heat storage media according to the invention are used in a closed system of pipelines, pumps, control devices and containers. The low viscosity of the heat transfer / heat storage media according to the invention is particularly advantageous because promoted by a lower viscosity of the heat transfer and the energy required to pump the liquid through the pipes is lowered. In many cases, this can be more important than extending the temperature range downwards. The negligible vapor pressure of the heat transfer medium according to the invention

/ Heat storage media, with reduced wall thicknesses of piping and equipment, contributes to lower investment costs and avoids sealing problems.

The operation of plants, preferably those for energy production, at temperatures up to 700 ° C with the heat transfer / heat storage media according to the invention generally requires materials which are stable to sulfidation at high temperatures. As already mentioned above, it is known from the literature that sodium polysulfide melts are capable of dissolving metallic gold in the form of complex sulfides. It has been found that the heat transfer media / heat storage media according to the invention then have no particularly great corrosion potential if they contain as little volatile, distillable water as possible.

Highly suitable materials for the heat transfer medium / heat storage media according to the invention, especially at elevated temperature, are the following:

Particularly corrosion-resistant materials are aluminum and in particular aluminum-containing alloys, for example, high-temperature resistant aluminum-containing steels. Such iron materials have ferritic structure and are free of nickel. Nickel sulphides form low-melting phases with iron. The most effective alloying constituent is aluminum, which has on the surface of the material a dense, passivating oxide layer and / or sulphide Layer forms. Such an older material with 22 wt .-% chromium and 6 wt .-% aluminum, a material which is used as a heat conductor, has become known under the name Kanthai. Sulfur-resistant iron alloys contain less chromium and more aluminum, as described, for example, in EP 0 652 297 A. There, alloys of composition 12 to 18 at.% Aluminum, 0.1 to 10 at.% Chromium, 0.1 to 2 at.% Niobium, 0.1 to 2 at.% Silicon, 0.01 up to 2 at.% titanium and 0.1 to 5 at.% boron. Niobium, boron and titanium serve to precipitate a fine-grained iron aluminide (Fe ₃ Al), thus providing increased toughness with strains above 3% and improved processability.

A particularly good combination of sulphidation resistance with good processability by casting, hot working, cold working and good ductility at room temperature with elongation at break of 20% allows an alloy composition with 8 to 10 weight percent aluminum, 0.5 to 2 weight percent molybdenum, balance Iron. Silicon should not be present in the alloy, it lowers the ductility at room temperature. Shares of chromium are also not beneficial, chromium sulfide is dissolved in the melts. By alloying up to 2 percent by weight of yttrium and / or zirconium zirconium and / or yttrium oxide are also formed in the protective aluminum oxide layer, which greatly increase the ductility of the alumina and thus make the protective layer particularly stable against chipping and mechanical stresses in temperature fluctuations. In particular by zirconium oxide, the ductility of the aluminum oxide layer is advantageously increased.

With increased ductility of base material and protective oxide layer, sulfidation resistances similar to alloys with higher aluminum contents are obtained. When the temperature changes, no microcracks are formed, and the alloys are not sensitive to hydrogen. Iron alloys with even higher aluminum contents should be even more stable than polysulfide melts; However, they are no longer cold processable. They are extruded, extruded or rolled at elevated temperatures. Such alloys, which are alloys with Fe ₃ Al phases, contain 21 at.% Aluminum, 2 at.% Chromium and 0.5 at.% Niobium or 26 at.% Aluminum, 4 at.%. Titanium and 2 at.% Vanadium or 26 at.% Aluminum and 4 at.% Niobium or 28 at.% Aluminum, 5 at.% Chromium, 0.5 at.% Niobium and 0.2 at % Carbon (EP 0455 752 A). The chromium content should be kept as low as possible, it is best to dispense with the chromium as an alloying element.

Even the highest possible molybdenum content, as far as it does not reduce the room temperature ductility, should prevent the sulfidation there. Molybdenum is recommended in addition to aluminum as a housing material for sodium-sulfur batteries.

According to literature, the corrosivity of alkali metal polysulfides decreases with decreasing sulfur content. The mechanical strength of iron alloys with a high aluminum content is sufficiently high up to temperatures of 700 ° C for use with the heat carrier / heat storage media according to the invention.

The heat transfer / heat storage media according to the invention can be inexpensively manufactured from inexpensive primary products by the conventional large-scale processes of the chemical industry. The alkali metal polysulfides according to the invention can be prepared, for example, in the case of sodium or potassium, by preparing the corresponding hydroxides from sodium and potassium chloride by means of chloralkali electrolysis.

The hydrogen produced at the same time is advantageously converted with liquid sulfur to hydrogen sulphide. For this purpose, the chemical industry has developed very elegant economical continuous and non-pressurized processes which eliminate the need for storage of larger amounts of hydrogen sulphide (e.g., WO 2008/087086). It is generated with the mass flow that is needed by the following stage.

Of course, it also makes sense to use the resulting in desulfurization in the hydrogenation hydrogen sulfide.

The hydrogen sulfide is usually mixed with the alkali hydroxides to form the alkali hydrogen sulfides and then reacted with sulfur to form the polysulfides.

It is also possible to prepare the alkali metal polysulfides according to the invention by reacting concentrated aqueous solutions of ammonium sulfide (NH ₄ ) ₂ S or ammonium hydrogen sulfide NH ₄ HS or mixtures of ammonium sulfide and ammonium hydrogen sulfide with the corresponding alkali hydroxides with elimination of ammonia to give the corresponding alkali metal hydrogensulfides. Ammonia is recycled to the synthesis of ammonium sulfides. In general, this synthesis route will be carried out if ammonia sulfide and / or ammonium hydrogen sulfide are favorably available from another process, for example from the washing out of hydrogen sulfide from gases.

If it is desired to avoid the coupling production of chlorine by the chloralkali electrolysis, it is advisable to convert low-chloride potassium sulfate or sodium sulfate with chloride contents of less than 0.01% by weight into the sulfides by means of reducing agents.

In particular, potassium sulfate is produced by the fertilizer industry in quantities of millions of tons per year. Economic processes for lowering the chloride content of potassium sulfate, for example by treating the salts with water, are known (DE 2 219 704). If hydrogen is used as the reducing agent, it is possible to work in a solid state at temperatures of 600 to 700 ° C. in a rotary kiln and obtain very clean sulfides (US Pat. No. 20,690,958, DE 590 660). As catalysts for the reduction generally 1 to 5 weight percent of alkali metal carboxylates are used, preferably the formates or the oxalates. However, the most effective catalysts are alkali metal polysulfides, which only have to be added to the alkali metal sulfate at the beginning of the reduction.

It is also possible to effect the reduction of the alkali metal sulfates Me ₂ S0 ₄ directly with natural gas according to the following equation:

Me ₂ S0 ₄ + 4/3 CH ₄ > Me ₂ S + 4/3 CO + 8/3 H ₂ 0

The sulfides are advantageously dissolved in water and converted into the hydrogen sulfides by the introduction of hydrogen sulfide: the equilibrium forms in concentrated aqueous solution

Me ₂ S + H ₂ 0 <> MeHS + MeOH.

By bubbling the hydrogen sulfide this reacts with the hydroxide, and the sulfide is converted into the hydrogen sulfide after

H ₂ S + MeOH> MeHS + H ₂ 0

This results in an overall reaction: Me ₂ S + H ₂ S> 2 MeHS

Accordingly, in addition to natural gas for the production of hydrogen, for example in the steam reforming process and as an energy carrier, this synthesis requires only inexpensive mineral raw materials and the very inexpensive sulfur.

Hydrogen sulfide is circulated in this procedure and thus required in only small amounts, so that a separate process stage for the production of hydrogen sulfide is usually superfluous. Me ₂ S + H ₂ S> 2 MeHS

2MeHS + zS> Me ₂ S _{(z +} _i) + H ₂ S

Complete conversion of the sulfides to the hydrogen sulfides is also generally not necessary here. It is usually sufficient if the formation of the alkali metal hydroxides is suppressed by the addition of hydrogen sulfide and for the implementation of the alkali metal polysulfides according to the invention is a mixture of alkali metal sulfides and alkali metal bisulfides, which contains a very low concentration of alkali metal hydroxide. An advantage of the alkali metal polysulfides according to the invention is that they can be prepared in a cost-effective continuous process: the individual reaction steps proceed very rapidly and exothermically. This allows small reaction volumes to be flowed through quickly by the reactants. A well-suited method is performed as follows.

With intensive cooling, hydrogen sulfide is introduced into concentrated aqueous solutions of the alkali metal hydroxides or alkali sulfides whose concentration is 40 to 60% by weight. The reaction temperature is kept below 80 ° C. Subsequently, optionally after a step of concentrating the reaction solution to values around 50 to 80% by weight by rapid distillation, the concentrated alkali metal hydrogensulfide solution is reacted with the predetermined amount of liquid sulfur. Here, the resulting heat of reaction can be used to evaporate water. Subsequently, the water contained in the reaction mixture is rapidly evaporated while raising the temperature up to 450 ° C, if necessary with the use of a reduced pressure. The resulting stream of hydrogen sulphide, mixed with steam, is cooled and the hydrogen sulphide together with the hydrogen sulphide-containing water in the Level of the hydrogen sulfide synthesis attributed. As a rule, there are no by-products to be disposed of. All reaction steps are carried out under inert conditions. Oxygen is usually excluded because it can oxidize the polysulfides to undesirable, the melting temperature of the liquid increasing and most unstable thiosulfates, sulfites and high-melting sulfates. Reaction mixers are advantageously used as reactors, followed by residence time ranges to complete the reactions. The reaction times of the individual reactions are in the time range of 0.1 to 10 minutes. As apparatus for removing the water, such as falling film evaporator or thin film evaporator are generally used. With heat transfer / heat storage media according to the invention it is generally possible to operate solar thermal power plants with the efficiencies of fossil-fired power plants, advantageous to operate them over appropriately sized storage tanks for the hot liquid day and night, without interruption. Because of the increased efficiency, the cost of investment per kilowatt hour compared to the prior art generally falls by a factor of 1.5. The solidification point above room temperature can be constructively met with little effort by setting up the mirror and the absorber tubes with a slight slope and the heat carrier / heat storage media according to the invention from the tubes shortly before sunset in a manifold and in heat-insulated buffer tanks for the operation on stored next day in the liquid state a few degrees above the benchmark.

But you can suck the heat carrier / heat storage media according to the invention but also without significant structural gradient in the heat-insulated tanks. If care is taken in the design of the systems to ensure that there are no moving apparatus such as pumps or valves in the cooling system components, then residues of the heat transfer media / heat storage media according to the invention may freeze in these components without disadvantages and melt again later. It is advantageous to keep moving parts such as pumps or control valves above the melting temperature of sulfur by additional heating. The easiest way, however, is to slowly pump the heat transfer medium / heat storage media according to the invention after sunset through the solar field and thereby lower their temperature to 150 to 200 ° C. The pipelines usually have to be insulated very well thermally against heat losses anyway, so that the losses due to heat conduction are low, much lower than during the daytime operation. At the comparatively low temperatures, the radiation losses due to the absorber tubes located in the vacuum are also generally quite low. If the temperature of the circulating heat transfer medium / heat storage media according to the invention should decrease too far, small quantities of the hot heat carrier / heat storage media according to the invention from the corresponding storage tank are admixed with them. Advantageously, the heat transfer / heat storage media according to the invention are used as heat transfer in combination with absorber tubes, which carry a coating that allows a high absorption capacity for solar radiation with a low emission for thermal radiation in the temperature range of 150 to 250 ° C in combination.

The heat transfer / heat storage media according to the invention also allow the combination with another heat transfer fluid. So it is, for example, to operate the or the heat storage of a solar thermal power plant with its large amounts of storage medium with a very inexpensive sulfane-containing and thus low-viscosity sulfur under a pressure as a storage medium, the solar field with its absorber tubes but unpressurized with the smaller amounts of höherpreisigen to operate according to the invention Alkalipolysulfide. The energy is transferred in this case via an intermediate heat exchanger. For a further design of solar thermal power plants, the tower technology, the heat transfer medium / heat storage media according to the invention are just as suitable as for the parabolic trough construction: Tracking mirrors direct the solar radiation to the top of a tower where it impinges on the receiver and heats the heat transfer fluid in the receiver to the highest possible temperatures. The heated liquid is used for steam generation and directed for storage in large volume tanks for night operation. At sunset, you simply let the liquid down from the receiver into a storage tank. Even if you evaporate water directly in the receiver and thus operate a heat engine, there is still the problem of operating the system at night. Thus, a heat storage fluid for such types of power plants is usually essential. However, the heat transfer / heat storage media according to the invention are also suitable for all other applications of heat transfer and heat storage in the art, which require an extremely wide temperature range of the liquid phase and high temperatures. Their vapor pressure is negligibly low for the purposes of the art. In particular, the heat transfer / heat storage media according to the invention are also suitable for the transport of heat energy from the fuel elements of a nuclear reactor in a primary circuit, which can thus be operated practically without pressure and thus safely up to temperatures of 700 ° C. This would make a safe, radiation-resistant heat transfer medium available. The steam temperatures in the secondary circuit can be significantly increased and the efficiency of nuclear power plants can thus be correspondingly increased.

At high temperatures, the use of the heat transfer / heat storage media according to the invention is limited only by the stability of the materials used.

In the case of an accidental product leakage, the heat transfer / heat storage media according to the invention are far less environmentally hazardous or safety-critical than organic liquids.

Occurs little inventive heat transfer medium / heat storage medium, it is usually oxidized within a few days by the atmospheric oxygen to the mineral sulfates. At elevated temperatures, the polysulfides may ignite in humid air because the ignition temperature of the hydrogen sulfide formed by hydrolysis is 270 ° C.

The polysulfides burn with the formation of sulfur dioxide with little luminous flame. Apart from sulfur dioxide, no environmentally toxic products are produced. Sulfur dioxide and the resulting from oxidation with atmospheric oxygen sulfur trioxide are not known as greenhouse gases.

Burning alkali polysulfides can be easily quenched with water because their density is greater than that of water. The evaporating water quickly cools the polysulfide melt, the resulting water vapor simultaneously binds sulfur dioxide. Sulfur dioxide must be precipitated with water, the polysulfides dissolve easily in water. Polysulfide residues adhering to parts of the system can easily be washed off with water and without residue, leaving no encrustations behind. Polysulfides dissolved in water are also oxidized by atmospheric oxygen, usually forming sulfur and sulfates. Both the polysulfides and the sulfur can be oxidized in the soil by sulfur bacteria to sulfates.

By neutralizing a polysulfide solution with dilute acids, preferably sulfuric acid, the degradation of the polysulfides is greatly accelerated, because in addition to the sulfides Me ₂ S immediately sulfur is released after

Me ₂ S _z + acid> Me ₂ S + (z-1) S. The sulfur released is, as far as known, environmentally neutral.

embodiments

General Procedure The synthesis of the polysulfides according to the invention was carried out to demonstrate their simplicity with small amounts in test tubes.

For this purpose, commercial sodium bisulfide in a concentration of 76 wt .-% (balance: water) and sulfur were used in commercial purity.

Potassium hydrogensulfide was prepared by introducing hydrogen sulfide to saturation in 12 grams of a commercial 50% by weight aqueous potassium hydroxide solution, one mole equivalent, with cooling. In this case, a temperature of 50 ° C was not exceeded. The bulk of the solution increased by 34 grams, corresponding to one mole of hydrogen sulfide. Thus, an aqueous solution of potassium hydrogensulfide in a concentration of 49% by weight was obtained.

After weighing the alkali hydrogen sulfide and the sulfur, the atmospheric oxygen was displaced by argon and the mixture heated under argon cover from room temperature to 100 to 130 ° C. The sulfur melted, and the reaction to polysulfide began at the same time. The temperature increased adiabatically within a few seconds to values of 130 ° C to 150 ° C. Water, mixed with hydrogen sulfide, distilled off.

The temperature was further increased after a short time, within 2 to 5 minutes to values around 500 ° C in order to evaporate the water as completely as possible.

Subsequently, the temperature of the reaction product was maintained for about 2 minutes. The temperatures were measured electronically by means of a thermocouple. The lower application temperature measured on cooling was the temperature at which the melt on the thermocouple of 1.5 mm diameter when removing it from the melt just started to draw a thin thread. The corresponding viscosity was about 200 cP.

Example I 2 NaHS + 1, 8S> Na ₂ S _{2; 8} + H ₂ S

0.04 moles of sodium hydrosulfide (2.95 grams, 76 weight percent) and 0.036 mole (1.15 grams) of sulfur were weighed into a test tube and reacted according to the procedure described. The resulting red liquid of the composition Na ₂ S _{2; 8} was thin. On cooling, she began to draw threads at 140 ° C. Upon further cooling, it solidified under crystallization. The liquid was heated in the test tube to 700 ° C. The color changed to black, and initially formed few gas bubbles. It was released, at least to the eye not recognizable, no sulfur. Upon cooling, the red color returned, the properties had not changed.

An analogously prepared sodium polysulfide of the composition Na ₂ S _{3 had} a slightly higher viscosity. It began to draw on cooling at 150 ° C threads and solidified on further cooling without crystallization glassy. The sodium polysulfide Na ₂ S ₃ was prepared again, in contrast to the first procedure, however, characterized in that the sodium hydrogen sulfide was dehydrated in a first step by heating to about 350 ° C. In the second step, the sulfur was added and the mixture heated with shaking. The polysulfide thus obtained began to draw threads upon cooling at 135 ° C.

Example 2

2 KHS + 2.4 S> K ₂ S _{3; 4} + H ₂ S By analogy with Example 1, 0.04 moles of potassium hydrosulfide (5.88 grams, 49 weight percent) were reacted with 0.048 mole (1.54 grams) of sulfur.

The red liquid of composition K ₂ S _{3; 4} began to draw filaments on cooling at 150 ° C. Upon further cooling, it crystallized. When heated to about 750 ° C, it turned dark. Signs of decomposition were not observed. On cooling, she turned red again and began to draw threads at 150 ° C, indicating that she did not experience any change when heated to 750 ° C.

Example 3

KHS + NaHS + 1, 7 S> (K ₀ , ₅ Nao, 5) 2S ₂ , 7 + H ₂ S

0.02 mol of sodium hydrosulfide, 0.02 mol of potassium hydrogen sulfide and 0.034 mol of sulfur were reacted with one another analogously to Example 1. A red low viscosity liquid of the composition (Ko _{, 5} Nao _{, 5} ) ₂ S _2; 7, which on cooling at 125 ° C to draw threads and crystallized on further cooling. The liquid was heated to 700 ° C, whereby it turned dark. After cooling, it again showed the properties as before heating. Example 4

1, 5 KHS + 0.5 NaHS + 2.2 S> (K ₀ , ₇₅ Na ₀ , ₂₅ ) ₂ S _{3; 2} + 0.5 H ₂ S As in Example 1, 0.06 mole of potassium hydrogensulfide, 0.02 A red liquid of the composition (K _0.75 Na _0.25 ) ₂ S _{3; 2 was} obtained, which began to draw threads upon cooling at 125 ° C and at further cooled glassy solidified. The liquid was heated to 700 ° C and then allowed to cool again. After cooling, it began to draw threads as before at 125 ° C.

Example 5

0.04 KHS + 0.032 NaOH + 0.088 S> 0.036 (K ₀ , ₅ 55 Na ₀ , ₄ 4 ₅ ) ₂ S _{3; 2} + 0.032 H ₂ 0

+ 0.004 H ₂ S

0.032 mole (1.28 grams) of 100% sodium hydroxide was dissolved in 0.04 mole of the 49% potassium bisulfide solution (5.88 grams) with heating, which corresponds to 80% of the molar amount needed Complete potassium hydrogensulfide to the sulfide. 0.088 moles of sulfur (2.82 grams) were weighed into the homogeneous solution and the reaction mixture heated to about 600 ° C after completion of the exothermic reaction and distilling off of water and hydrogen sulfide. The red liquid began to draw filaments during cooling at 135 ° C. Upon further lowering of the temperature, the liquid solidified glassy.

In a further experiment, the polysulfide of the above composition was prepared again, but by dehydration of the reaction mixture of the potassium hydrogen sulfide and the sodium hydroxide. In the second step, the dehydrated Hydrogensulfidl sulfide mixture was reacted with the sulfur. The resulting red polysulphide began to draw on cooling at 1 15 ° C threads, with further cooling, it solidified glassy.

Example 6

0.04 KHS + 0.024 KOH + 0.0544 S> 0.032 K ₂ S _{2; 7} + 0.024 H ₂ O + 0.008 H ₂ S

As in Example 4, 0.024 mole (1.66 grams) of an 81% potassium hydroxide was dissolved in 0.04 mole of the 49% potassium hydrogensulfide while heating. The amount of potassium hydroxide corresponded to 60% of the theoretical amount of potassium hydroxide for complete neutralization of the hydrogen sulfide. In this solution, 0.0544 mol (1, 74 grams) of sulfur were weighed and the reaction mixture after the exothermic use of the reaction while distilling off water and hydrogen sulfide to about 600 ° C heated.

The red liquid began to crystallize on cooling at 190 ° C. From a variety of experiments, the following relationships emerged:

Rising potassium levels promote crystallization. The melt viscosity rises more by increasing sulfur content than by a higher sodium content.

The thermal stability is promoted by the lowest possible sulfur content.

According to the literature, as already stated, the corrosivity of the alkali metal polysulfides is reduced by lower sulfur content.

This results in the optimum composition with the highest possible sodium content at the lowest possible sulfur content. However, a proportion of potassium is needed to suppress the crystallization, all the more, the lower the sulfur content.

Optimum compositions are in the range of

(Nao bis, 65Ko bis0) 2S2 .bis2 One of these alkali metal polysulfides having the composition Does not decompose at temperatures up to 700 ° C and remains low viscous when cooling, not drawing, down to around 1 10 to 15 ° C, its melting range.

According to the cited literature on the calculated phase diagram Na ₂ S - K ₂ S - S (Lindberg et al), this composition should have a melting range of 360 to 380 ° C.

Claims

claims

1 . Composition for transportation and storage of thermal energy, comprising the alkali metal polysulfides of the formula (Me1 _{(i_} _X) Me2 _x) ₂ S _z where Me1 and Me2 are selected from the group of alkali metals consisting of lithium, sodium, potassium, rubidium and cesium and Me1 non-Me2 and x = 0 to 1 and z = 2.3 to 3.5.

2. Compositions according to claim 1, wherein Me1 is potassium and Me2 is sodium.

3. Compositions according to claim 1 to 2, wherein X = 0.5 to 0.7 and z = 2.4 to 2.9.

4. Compositions according to claim 1 to 3 of the formulas (Na ₀ .5 bis.o, 65Ko, ₅ bis 8th·,

5. Compositions according to claim 1 to 4, wherein the alkali metal polysulfides are obtainable by the reaction of concentrated aqueous solutions of alkali hydrogen sulfides with sulfur.

6. Compositions according to claim 1 to 4, wherein the alkali metal polysulfides are obtainable by reacting alkali metal hydrogen sulfides with a molar deficit of alkali metal hydroxides to alkali metal sulfides, mixed with alkali metal hydrogen sulfides, and reacting with sulfur completely to alkali metal polysulfides and, optionally under reduced pressure, the water at temperatures distilled off up to 500 ° C.

7. Compositions according to claim 1 to 4, wherein the alkali metal polysulfides are prepared by dehydrating aqueous solutions of alkali metal hydrogen sulfides, or aqueous solutions of alkali metal hydrogen sulfides which have been reacted with a molar excess of alkali metal hydroxides to alkali metal sulfides, mixed with alkali hydrogen sulfides, optionally under the application of reduced pressure, and in a second step, reacting the dehydrated alkali metal hydrogen sulphide alkali metal sulphides with liquid sulfur to form the alkali metal polysulphides.

8. A composition according to claim 6 to 7, characterized in that per mole of alkali metal hydrogen sulfide used at most 0.9 moles of alkali metal hydroxide.

Use of the compositions as defined in claims 1 to 8 in the presence of aluminum-containing materials.

Use of the compositions as defined in claims 1 to 8 in the presence of iron-based materials.

1 1. Use according to claim 10, wherein the iron-based materials contain 6 to 28 weight percent aluminum, and below 3 weight percent molybdenum and up to 2 weight percent zirconium and / or yttrium.

12. Use of the compositions as defined in claim 1 to 8 as a medium for the transport and / or storage of thermal energy.

13. Use of the compositions as defined in claim 1 to 8, for the transport and / or storage of heat energy in solar thermal power plants or in the primary cycle of nuclear power plants.

14. Use of the compositions as defined in claim 1 to 8 as heat transfer fluid, wherein one transfers their heat energy to another medium as a heat storage.

Use according to claim 14, wherein the other medium is sulfane-containing low-viscosity sulfur.

16. plants for the production of energy containing a composition as defined in claims 1 to 8.

17. Plant according to claim 16, comprising a composition as defined in claims 1 to 8 as a heat transfer and / or heat storage medium.