MD-METHYLPOLYSILOXANE BLENDS USED AS HEAT TRANSFER FLUID

Description

[0001] The present invention relates to methylpolysiloxane mixtures having a molar M:D ratio of 1:5.5 to 1:15 and 25 to 55 mass% of cyclic methylpolysiloxanes, and their use as heat transfer fluid.

[0002] Organosiloxanes, particularly methylpolysiloxane mixtures, are frequently used as heat transfer fluids due to their high thermal stability, wide fluidity range, and low temperature dependence of their viscosity. WO 2014 / 001081 describes methylpolysiloxane mixtures suitable as high-temperature heat transfer fluids (HTF). The numerical ratio of Me 3 Si chain end groups (M) to the total Me 2 SiO units (D) in the methylpolysiloxane mixtures is at least 1:2 and at most 1:10.

[0003] Measurements have shown that methylpolysiloxane mixtures with a M:D ratio of 1:4, currently used as HTFs, transition to the supercritical state below the targeted maximum operating temperature of 425°C. This has a negative impact on the performance of the HTF, as the heat transfer properties of the HTF deteriorate upon transition to the supercritical range. For example, the heat capacity and density collapse. Table 1 shows that between 399.6 and 450°C, the density collapses upon transition to the supercritical state. Table 1: Density curve of equilibrated methylpolysiloxane mixture with M:D = 1:4 (bold = not supercritical; normal pressure = supercritical) T[°C] ρ [g / cm 3< ] 27,4 0,9097 49,7 0,8914 99,9 0,8424 149,7 0,7961 199,8 0,7463 250,8 0,6963 299,7 0,6334 350,1 0,5465 399,6 0,4299 450,0 0,1999

[0004] US3694405, Example 17 describes the equilibration of a methylpolysiloxane mixture with a molar M:D ratio of 1:5.3.

[0005] The invention relates to a methylpolysiloxane mixture comprising methylpolysiloxanes with Me 3 Si chain end groups (M) and Me 2 SiO units (D), wherein the molar M:D ratio in the methylpolysiloxane mixture is 1:5.5 to 1:15 and the sum of the proportions of all cyclic methylpolysiloxanes is 25 to 55 mass%.

[0006] The location of the critical point of a methylpolysiloxane mixture is determined by its molecular composition. Methylpolysiloxanes with lower molar mass, particularly linear methylpolysiloxanes MM (Si2), MDM (Si3), MDDM (Si4), etc., and cyclic methylpolysiloxanes D3, D4, D5, etc., transition to the supercritical state at lower temperatures (see Table 2). Table 2: Selected pure substance data of linear and cyclic siloxanes (ASPEN DB-PURE28 database) (up to Si8 and up to D8, the critical temperature is below the target operating temperature of the heat transfer oil of 425°C) Si- 2 4 6 8 12 Critical temperature [°C] 245,8 326,3 380,05 415,8 478,2 D- 3 4 5 6 8 Critical temperature [°C] 281,1 313,4 346,0 372,7 416,1

[0007] Under thermal stress, linear end-capped methylpolysiloxanes rearrange and equilibrate. Regardless of the initial composition, the resulting methylpolysiloxane mixture consists of linear siloxanes (Si2, Si3, Si4, etc.) and cyclic siloxanes (D3, D4, D5, etc.), which are in thermal thermodynamic equilibrium. The position of thermal equilibrium is determined by the maximum operating temperature to which the methylpolysiloxane is exposed and the molar M:D ratio of the methylpolysiloxane mixture. The position of the equilibrium depends on the temperature. At high temperatures, such as 425°C, equilibrium is established within 1-2 months (continuous load). At lower temperatures, a different equilibrium is established; at 400°C, however, equilibrium takes 2-4 months.Therefore, in real-world operation, a heat transfer fluid, especially in CSP power plant operation, always reaches equilibrium at the highest maximum operating temperature after some time, since the rate constant for reaching equilibrium at higher temperatures is greater than the rate constant for reaching equilibrium at lower temperatures (corresponding to reverse reaction / reequilibration). Furthermore, the residence time of the heat transfer fluid in real-world CSP power plant operation at maximum operating temperature is longer (from the receiver end to the evaporator), and in the evaporator, the heat transfer fluid is cooled very quickly to 300°C. At 300°C, equilibration occurs only extremely slowly.

[0008] Surprisingly, it was found that a methylpolysiloxane mixture according to the invention with a molar M:D ratio of 1:5.5 to 1:15, preferably 1:5.6 to 1:10.5, in particular 1:5.8 to 1:9, has a composition in the equilibrated state (see Table 3) that it does not enter the supercritical state up to 425°C (see Table 4).

[0009] In a preferred embodiment, 46 to 65 mass%, preferably 50 to 60 mass%, in particular 52 to 58 mass% of the methylpolysiloxanes in the methylpolysiloxane mixture are selected from methylpolysiloxanes Six with x>8 and Dy with y>8.

[0010] Due to the problems described above during the transition to the supercritical state, when using methylpolysiloxanes as heat transfer oil at a desired application temperature of 425°C, it is advisable to only use methylpolysiloxanes with a molar M:D ratio of at least 1:5.5, preferably 5.6, and in particular 5.8. Furthermore, from an application-related perspective, methylpolysiloxanes with an excessively high molar M:D ratio should not be used, as this increases the average chain length of the heat transfer fluid and thus also the viscosity. This has a negative impact on the operation of a heat transfer system because, among other things, the circulation of the heat transfer fluid can only be achieved with a higher pumping power. Furthermore, it is known, and described in DE102014209670 and DE102015202158, that the service life of a Si-HTF is determined by the formation of trifunctional siloxane units, so-called T-units.The resulting branches cause the molecules of the HTF to crosslink, increasing the viscosity of the Si-HTF and ultimately rendering it unpumpable. The longer the chain length or higher the molecular weight of an MD-Si-HTF (i.e., the higher the molar M:D ratio), the fewer branching T units need to be thermally formed to crosslink the HTF molecules. Therefore, the sensible, economical use of high-temperature Si-HTFs is limited to a maximum molar M:D ratio of 1:15.

[0011] The mass fraction-weighted arithmetic mean (preferably determined analogously to the gas chromatographic method described below) of x over all linear methylpolysiloxanes (Six) from Si2 to Si22 is preferably between 2.3 and 3.6, particularly preferably between 2.5 and 3.5.

[0012] The mass fraction-weighted arithmetic mean (preferably determined analogously to the gas chromatographic method described below) of y over all cyclic methylpolysiloxanes (Siy) from D3 to D17 is preferably between 1.7 and 3.5, particularly preferably between 1.9 and 3.1.

[0013] Preferably, the sum of the proportions (preferably determined analogously to the gas chromatographic method described below) of all cyclic methylpolysiloxanes is at least 26 mass% and at most 50 mass%, particularly preferably at least 27 mass% and at most 32 mass%.

[0014] The viscosity of the methylpolysiloxane mixture according to the invention at 25°C is preferably below 50 mPa*s, particularly preferably below 20 mPa*s, most preferably between 5 and 15 mPa*s.

[0015] The methylpolysiloxane mixture according to the invention can have a monomodal, bimodal, or multimodal distribution (determined analogously to the gas chromatographic method described below and plotted according to retention times), and the distribution can be narrow or broad. The methylpolysiloxane mixture according to the invention preferably has a bimodal, trimodal, or multimodal distribution. The methylpolysiloxane mixture particularly preferably has a multimodal distribution at 425°C. Considering the distribution of the linear siloxanes and the cyclic siloxanes separately, a monomodal distribution results in each case.

[0016] The methylpolysiloxane mixture according to the invention preferably contains less than 500 ppm water, more preferably less than 200 ppm water, most preferably less than 50 ppm water, in each case based on the mass.

[0017] The methylpolysiloxane mixture according to the invention can be prepared by preparing, mixing, and dosing methylpolysiloxanes Six or Dy or any mixtures of such methylpolysiloxanes in any order, if necessary repeatedly, if necessary alternately or simultaneously. Methylpolysiloxanes or methylpolysiloxane mixtures can also be removed again by suitable processes, for example distillation. The composition of the methylpolysiloxane mixture according to the invention is controlled by the amounts of methylpolysiloxanes Six and Dy used or removed.

[0018] The process can be carried out at room temperature and ambient pressure, but also at elevated or reduced temperature and elevated or reduced pressure.

[0019] Methylpolysiloxane mixtures according to the invention can also be prepared by hydrolyzing or cohydrolyzing suitable chlorosilanes, alkoxysilanes, or mixtures of chlorosilanes or alkoxysilanes and subsequently freeing them from byproducts such as hydrogen chloride or alcohols and, if appropriate, from excess water. Optionally, further methylpolysiloxane can be added to the resulting methylpolysiloxane mixture or removed by suitable processes, for example distillation. The process can be carried out at room temperature and ambient pressure, but also at elevated or reduced temperature and elevated or reduced pressure. The composition of the methylpolysiloxane mixture according to the invention is controlled by the ratio of the amounts of silanes or methylpolysiloxanes used and, if appropriate, removed again.

[0020] Methylpolysiloxane mixtures according to the invention can also be prepared by heating pure methylpolysiloxanes Six and Dy or any mixtures of such methylpolysiloxanes to temperatures at which the aforementioned rearrangement processes take place, thus obtaining methylpolysiloxane mixtures with an altered composition. This composition may, but need not, correspond to the equilibrium composition at this temperature. Heating can take place in an open or closed system, preferably under a protective gas atmosphere. The process can be carried out at ambient pressure, but also at elevated or reduced pressure. Heating can take place uncatalyzed or in the presence of a homogeneous or heterogeneous catalyst, for example an acid or base. The catalyst can then be deactivated or removed from the siloxane mixture, for example by distillation or filtration, but this is not necessarily the case.Methylpolysiloxanes or methylpolysiloxane mixtures can also be removed again by suitable processes, such as distillation. The composition of the methylpolysiloxane mixture according to the invention is controlled by the ratio of the amounts of methylpolysiloxanes Six and Dy used and optionally removed, the temperature, and the type (open or closed system) and duration of heating.

[0021] The three processes described above can also be combined. They can optionally be carried out in the presence of one or more solvents. Preferably, no solvent is used. The silanes, silane mixtures, methylpolysiloxanes, and methylpolysiloxane mixtures used are either standard products of the silicone industry or can be prepared using synthesis methods known from the literature.

[0022] The methylpolysiloxane mixtures according to the invention can contain dissolved, suspended, or emulsified additives to increase their stability or influence their physical properties. Dissolved metal compounds, such as iron carboxylates, can act as radical scavengers and oxidation inhibitors to increase the durability of a heat transfer fluid. Suspended additives, such as carbon or iron oxide, can improve physical properties of a heat transfer fluid, such as heat capacity or thermal conductivity.

[0023] Preferably, in the methylpolysiloxane mixture, the sum of the proportions of all methylpolysiloxanes Six and Dy is at least 95 mass%, in particular at least 98 mass%, in particular at least 99.5 mass%, based on the entire mixture.

[0024] The methylpolysiloxane mixture according to the invention can be used as a heat transfer fluid, preferably as a high-temperature heat transfer fluid (HTF), particularly in solar thermal devices, especially in parabolic trough and Fresnel power plants. They can also be used as a heat transfer fluid in the chemical, pharmaceutical, food, and metal industries and as working fluids in heat engines, especially solar thermal engines. The methylpolysiloxane mixture is preferably used at temperatures of 350°C to 500°C, particularly preferably 380°C to 450°C, especially 400°C to 430°C. At temperatures above 200°C, use under a protective gas atmosphere is preferred to prevent oxidative decomposition. Examples Equilibration of the methylpolysiloxane mixtures:

[0025] Under thermal stress, linear end-capped methylpolysiloxanes undergo rearrangement (equilibration). Regardless of the initial composition, a methylpolysiloxane mixture is formed that is in thermal thermodynamic equilibrium. The position of thermal equilibrium is determined by the maximum operating temperature to which the methylpolysiloxane is exposed and the molar M:D ratio (M: Me3SiO1 / 2 chain end groups; D: Me2SiO2 / 2 chain extender units) of the methylpolysiloxane mixture. To obtain methylpolysiloxane mixtures with a composition comparable to that used in CSP power plant operation, 150 g of methylpolysiloxane mixtures with a defined molar M:D ratio were weighed into 250 ml steel ampoules under a nitrogen atmosphere, degassed (3 × 20 mbar, 3 minutes each), and sealed under an argon atmosphere (1 bar).The steel ampoules were then stored for two months at 425°C to maintain the thermodynamic equilibrium of the methylpolysiloxane mixtures at 425°C. The M to D ratio did not change (29< Si NMR). However, the molecular composition of the methylpolysiloxane mixtures did (equilibration). The resulting methylpolysiloxane mixtures were used for further investigation (GPC, GC, heat capacity measurement). Composition of the methylpolysiloxane mixtures: Gel permeation chromatography (GPC)

[0026] The composition of the methylpolysiloxane mixtures was determined by GPC. Instrument: Agilent 1200 Iso Pump, Agilent 1200 Autosampler, Agilent 1260 Column Oven, Agilent 1200 RID, Agilent 300 × 7.5 mm OligoPore 4500D Column Material: Highly Crosslinked Polystyrene / Divinylbenzene, eluent: toluene, flow rate: 0.7 ml / min, injection volume: 10 µl, concentration: 1 g / l (in toluene), PDMS (polydimethylsiloxane) calibration (Mp: 28500 D, Mp: 25200 D, Mp: 10500 D, Mp: 5100 D, Mp: 4160 D, Mp: 1110 D, Mp: 311D). Analysis in area percent. Gas chromatography (GC)

[0027] The composition of the methylpolysiloxane mixtures was determined by GC. Instrument: Varian GC-3900 gas chromatograph, column VF-200ms (30 m × 0.32 mm × 0.25 pm), carrier gas: helium, flow rate: 1 ml / min, injector: CP-1177, split: 1:50, detector: FID 39XL, 250°C. Analysis in area percent. Calibration has shown that the area percent corresponds to mass percent.

[0028] The composition of the methylpolysiloxane mixtures was determined by combining GPC and GC data. Since Si2 and D3, Si3 and D4, Si4 and D5, Si5 and D6, Si6 and D7, Si7 and D8, and Si8 and D9 each appear as a single peak in GPC, the ratio of the respective compounds was determined by GC and calculated. This allows the determination of the Si2-Si8 and D3-D8 contents. All high boilers from Si9 and D9 upwards are listed together as "Six(x > 8) + Dy (y > 8)". Six are linear methylpolysiloxanes, Dy are cyclic methylpolysiloxanes. Data are given in area percent. Calibration has shown that the area percent corresponds to mass percent. Measurement of the M to D ratio ( 29< Si-NMR)

[0029] The determination of the proportion of M- (chain ends Me 3 Si0 1 / 2 -) and D groups(chain links -Me 2 Si0 2 / 2 -) was determined by nuclear magnetic resonance spectroscopy ( 29< Si-NMR; Bruker Avance III HD 500 ( 29< Si: 99.4 MHz) spectrometer with BBO 500 MHz S2 probe head; inverse gated pulse sequence (NS = 3000); 150 mg methylpolysiloxane mixtures in 500 µl of a 4×10 -2< molar solution of Cr(acac) 3 in CD 2 Cl 2 . Heat capacity:

[0030] The heat capacity was determined using differential scanning calorimetry (DSC) with a SENSYS evo device from SETARAM. The heat capacity was determined using the step method in 5-10°C increments from 25°C to 450°C. 70 mg of the methylpolysiloxane mixture to be tested was weighed into 160 µl gold crucibles under a nitrogen atmosphere. The pressure developed in the capsules during heating (the inherent pressure of the methylpolysiloxane mixtures) was not detected. The accuracy of the measurements was confirmed by heat capacity determinations by Saphir. Table 3: Mass composition of the methylpolysiloxane mixtures (M:D ratio from NMR; Si2-Six and D3-Dy content via GC / GPC): M:D = 1:y 4,00 5,80 8,99 Si2 2,1 1,2 0,8 D3 2,0 2,1 2,5 Si3 3,8 2,1 1,3 D4 10,3 11,6 12,6 Si4 6,0 3,4 2,1 D5 6,2 7,7 9,0 Si5 7,1 4,3 3,0 D6 2,0 2,8 3,5 Si6 6,6 4,5 3,3 D7 0,4 0,7 1,0 Si7 6,0 4,3 3,3 D8 0,1 0,2 0,3 Si8 5,3 4,0 3,2 Six(x > 8) + Dy(y > 8 42,2 51,2 54,1 Table 4: Heat capacity measurements with limitation of the critical point (drop in the Cp value between rich and normal): T [°C] Cp M:D = 1:4.00 [kJ / kg*K] Cp M:D = 1:5.80 [kJ / kg*K] Cp M:D = 1:8.99 [kJ / kg*K] 25 1,677 1,670 1,545 50 1,722 1,711 1,613 100 1,811 1,795 1,740 150 1,899 1,878 1,856 200 1,988 1,962 1,960 250 2,077 2,045 2,053 300 2,166 2,129 2,134 350 2,255 2,212 2,203 360 2,272 2,229 2,216 370 2,290 2,246 2,228 380 2,308 2,262 2,239 385 2,317 2,271 2,245 390 2,326 2,279 2,250 395 2,335 2,287 2,256 400 2,343 2,296 2,261 405 2,152 2,304 2,266 410 2,160 2,312 2,271 415 2,168 2,321 2,276 420 2,175 2,329 2,281 425 2,183 2,337 2,285 430 2,191 2,201 2,289 435 2,199 2,204 2,201 440 2,206 2,208 2,208 445 2,214 2,211 2,214 450 2,222 2,214 2,221

[0031] The examples show that the transition to the supercritical phase occurs at a molar M:D ratio of 1:4.00 even before the target operating temperature of 425°C. At a molar M:D ratio of 1:5.80 to 1:8.99, the transition to the supercritical phase only occurs above 425°C.

Claims

1. Methylpolysiloxane mixture, comprising methylpolysiloxanes having Me3Si chain end groups (M) und Me2SiO units (D), wherein the molar M:D ratio in the methylpolysiloxane mixture is from 1:5.5 to 1:15 and the sum total of the proportions of all cyclic methylpolysiloxanes is 25 to 55% by mass.

2. Mixture according to Claim 1, in which 35 to 65% by mass of the methylpolysiloxanes in the methylpolysiloxane mixture are selected from methylpolysiloxanes Six where x>8 and Dy where y>8.

3. Mixture according to one or more of the preceding claims, in which the arithmetic mean of x, weighted by the proportions by mass, over all linear methylpolysiloxanes (Six) from Si2 to Si22 is 2.3 to 3.6.

4. Mixture according to one or more of the preceding claims, in which the arithmetic mean of y, weighted by the proportions by mass, over all cyclic methylpolysiloxanes (Siy) from D3 to D17 is 1.7 to 3.5.

5. Mixture according to one or more of the preceding claims, in which the viscosity at 25°C is below 50 mPa*s.

6. Mixture according to one or more of the preceding claims, having a bimodal, trimodal or multimodal molar mass distribution.

7. Use of the mixture according to Claims 1 to 6 as heat transfer fluid.

8. Use according to Claim 7 for solar thermal devices.

9. Use according to Claim 7 or 8 at temperatures of 350°C to 500°C.