Mixture of MD-methylpolysiloxanes as heat carrier fluid

Abstract

The present invention relates to a methylpolysiloxane mixture comprising a methylpolysiloxane having Me3Si chain end groups (M) and Me2SiO units (D), wherein the molar M:D ratio in the methylpolysiloxane mixture is from 1:5.5 to 1:15, and the sum of the proportions of all cyclic methylpolysiloxanes is from 25 to 55% by mass. The present invention also relates to the use of said mixture as a heat transfer fluid.

Description

Technical Field

[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% by mass of cyclic methylpolysiloxanes, and to their use as heat carrier fluids. Background Art

[0002] Siloxanes, in particular methylpolysiloxane mixtures, are often used as heat transfer fluids due to their high thermal stability, their wide liquid range and their low temperature dependence of viscosity. WO 2014 / 001081 describes methylpolysiloxane mixtures suitable as heat transfer fluids (HTF) for high temperatures. In the methylpolysiloxane mixtures, the quantitative ratio of the Me3Si chain end groups (M) to the sum of the Me2SiO units (D) is at least 1:2 and at most 1:10.

[0003] Measurements have shown that methylpolysiloxane mixtures having an M:D ratio of 1:4, which are currently used as HTF, turn into a supercritical state at the desired maximum operating temperature below 425 °C. This has a negative impact on the performance of the HTF, since the heat transfer properties of the HTF become poorer due to the transition into the supercritical range. For example, the heat capacity or even the density decreases. Table 1 shows that between 399.6 and 450 °C, the density decreases due to the transition into the supercritical state.

[0004] Table 1: Density curve of an equilibrated methylpolysiloxane mixture with M:D = 1:4 (bold font = non-supercritical; standard font = supercritical)

[0005] T[℃] <![CDATA[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

[0006] US 3694405, Example 17 describes the equilibration of a methylpolysiloxane mixture having a molar M:D ratio of 1:5.3. Summary of the Invention

[0007] The present invention relates to a methylpolysiloxane mixture comprising methylpolysiloxanes having Me3Si chain end groups (M) and Me2SiO 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% by mass.

[0008] The factor that determines the critical point of the methylpolysiloxane mixture is the molecular composition. Methylpolysiloxanes with a relatively low molar mass, especially linear methylpolysiloxanes such as MM(Si2), MDM(Si3), MDDM(Si4), etc., and cyclic methylpolysiloxanes such as D3, D4, D5, etc., turn into the supercritical state at relatively low temperatures (see Table 2).

[0009] Table 2: Selected pure substance data for linear and cyclic siloxanes (database ASPEN DB-PURE28) (up to Si8 and up to D8, the desired operating temperature of heat transfer oils with a critical temperature below 425 °C)

[0010]

[0011] Under thermal stress, linear end-capped methylpolysiloxanes rearrange and they come to equilibrium. Regardless of the starting composition, the result is a methylpolysiloxane mixture of linear siloxanes (Si2, Si3, Si4, etc.) and cyclic siloxanes (D3, D4, D5, etc.) that is in thermal thermodynamic equilibrium. The position of this thermal equilibrium is caused by the maximum operating temperature to which the methylpolysiloxane is exposed, and by the molar M:D ratio of the methylpolysiloxane mixture. The position of the equilibrium depends on temperature. At high temperatures, for example at 425 °C, equilibrium is reached within 1 - 2 months (sustained load). At lower temperatures, another equilibrium is reached; however, at 400 °C, it takes even 2 - 4 months to establish equilibrium. Therefore, in the actual operation of heat transfer fluids, especially in CSP power plant operation, equilibrium will always be reached after some time at the highest maximum operating temperature because the rate constant for reaching equilibrium at higher temperatures is greater than that at lower temperatures (equivalent to the reverse reaction / re-equilibration). In addition, the residence time of the heat transfer oil in actual CSP power plant operation at the maximum operating temperature is high (from the receiver end to the evaporator), and in the evaporator the heat transfer oil is very rapidly cooled to 300 °C. At 300 °C, equilibrium is established only extremely slowly.

[0012] Surprisingly, it has been found that in the equilibrium state, the 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, especially 1:5.8 to 1:9, has a composition (see Table 3) that does not turn into the supercritical state up to 425 °C (see Table 4).

[0013] In a preferred embodiment, 46 to 65% by mass, preferably 50 to 60% by mass, especially 52 to 58% by mass of the methylpolysiloxanes in the methylpolysiloxane mixture are selected from methylpolysiloxanes Six, where x > 8, and Dy, where y > 8.

[0014] Due to the problems described above when transitioning to the supercritical state, when using methylpolysiloxane as a heat transfer fluid at the desired application temperature of 425 °C, it is advisable to use only methylpolysiloxanes with a molar M:D ratio of at least 1:5.5, preferably 5.6, especially 5.8. Additionally, from an application perspective, methylpolysiloxanes with too high a molar M:D ratio should not be used either, because this means an increase in the average chain length of the heat transfer fluid and thus also an increase in viscosity. This has a negative impact on the operation of the heat transfer system, because thereby, the circulation of the heat transfer fluid is only possible, in particular, at a relatively high pump capacity. It is also known and described in DE102014209670 and DE102015202158 that the shelf life of Si-HTF is determined by the formulation of trifunctional siloxane units (so-called T units). Through the formed branches, the molecules of the HTF crosslink, which means an increase in the viscosity of the Si-HTF and ultimately it can no longer be pumped. The longer the chain or the higher the molecular weight of the MD-Si-HTF (i.e., the higher the molar M:D ratio), the fewer branched T units that must be thermally formed for the HTF molecules to crosslink with each other. Therefore, the reasonable and economical use of high-temperature Si-HTF is limited to a maximum molar M:D ratio of 1:15.

[0015] For all linear methylpolysiloxanes (Six) from Si2 to Si22, the arithmetic mean of x weighted by mass ratio (preferably determined analogously to the gas chromatography described below) is preferably between 2.3 and 3.6, particularly preferably between 2.5 and 3.5.

[0016] For all cyclic methylpolysiloxanes (Siy) from D3 to D17, the arithmetic mean of y weighted by mass ratio (preferably determined analogously to the gas chromatography described below) is preferably between 1.7 and 3.5, particularly preferably between 1.9 and 3.1.

[0017] Preferably, the sum of the proportions of all cyclic methylpolysiloxanes (preferably determined analogously to the gas chromatography described below) is at least 26% by mass and at most 50% by mass, particularly preferably at least 27% by mass and at most 32% by mass.

[0018] 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, and especially preferably between 5 and 15 mPa·s.

[0019] The methylpolysiloxane mixture can be present in a unimodal, bimodal or multimodal distribution (monomodal, bimodal or multimodal distribution, similar to the gas chromatography described below and the retention time determination according to the application), and at the same time, the distribution can be narrow or wide. The methylpolysiloxane mixture according to the invention preferably has a bimodal, trimodal or multimodal distribution. The methylpolysiloxane mixture according to the invention particularly preferably has a multimodal distribution at 425 °C. Considering the distribution of the linear siloxane and the cyclic siloxane separately, a unimodal distribution results in each case.

[0020] The methylpolysiloxane mixture according to the invention preferably contains less than 500 ppm of water, particularly preferably less than 200 ppm of water, and very particularly preferably less than 50 ppm of water, the amounts being based on mass in each case.

[0021] The methylpolysiloxane mixture according to the invention can be prepared by preparing, mixing and metering methylpolysiloxanes Six or Dy or any mixture of such methylpolysiloxanes into each other in any order, optionally repeating several times, optionally also alternately or simultaneously. By a suitable method, such as distillation, the methylpolysiloxane or methylpolysiloxane mixture can also be removed again. In this case, the composition of the methylpolysiloxane mixture according to the invention is controlled by the amounts of methylpolysiloxanes Six and Dy used or removed.

[0022] The method can be carried out at room temperature and atmospheric pressure, but can also be carried out at elevated or reduced temperature and at elevated or reduced pressure.

[0023] The methylpolysiloxane mixture according to the invention can also be prepared by hydrolyzing or co-hydrolyzing suitable chlorosilanes, alkoxysilanes or mixtures of chlorosilanes or alkoxysilanes, and then removing by-products therefrom, such as chlorinated hydrocarbons or alcohols and, if necessary, excess water. Optionally, additional methylpolysiloxane can be added to the resulting methylpolysiloxane mixture, or it can be removed by a suitable method, such as distillation. The method can be carried out at room temperature and atmospheric pressure, but can also be carried out at elevated or reduced temperature and at elevated or reduced pressure. In this case, the composition of the methylpolysiloxane mixture according to the invention is controlled by the ratio of the amounts of silanes or methylpolysiloxanes used and optionally removed again.

[0024] The methylpolysiloxane mixture according to the invention can also be prepared by heating pure methylpolysiloxanes Six and Dy or any mixture of such methylpolysiloxanes to a temperature at which the rearrangement process mentioned takes place, thus obtaining a methylpolysiloxane mixture with an altered composition. This composition may correspond to the equilibrium composition at this temperature, but this is not necessarily the case. The heating can be carried out in an open or closed system, preferably under a protective atmosphere. The method can be carried out at atmospheric pressure, but can also be carried out at elevated or reduced pressure. The heating can be carried out without a catalyst or in the presence of a homogeneous or heterogeneous catalyst (such as an acid or a base). The catalyst can then be deactivated or removed from the siloxane mixture, for example by distillation or filtration, but this is not necessary. The methylpolysiloxane or methylpolysiloxane mixture can also be removed again by a suitable method, such as distillation. In this case, 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 again, the temperature and type (open or closed system) and the duration of the heating.

[0025] The three methods 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 silicon industry or can be prepared by synthetic methods known from the literature.

[0026] The methylpolysiloxane mixture according to the invention can contain dissolved, suspended or emulsified additives to increase their stability or to influence their physical properties. Dissolved metal compounds, such as iron carboxylates, as radical scavengers and oxidation inhibitors, can increase the service life of the heat carrier. Suspended additives, such as carbon or iron oxides, can improve the physical properties of the heat carrier, such as heat capacity or thermal conductivity.

[0027] Preferably, in the methylpolysiloxane mixture, the sum of the proportions of all methylpolysiloxanes Six and Dy is at least 95% by mass, particularly preferably at least 98% by mass, and especially preferably at least 99.5% by mass, based on the total mixture.

[0028] The methylpolysiloxane mixtures according to the invention can be used as heat transfer fluids, preferably as heat transfer fluids (HTF) for high temperatures, especially in solar thermal installations, in particular in parabolic trough and Fresnel power plants. They can also be used as heat transfer fluids in the chemical, pharmaceutical, food and metal industries, and as working fluids in power plants, especially in solar thermal power plants. The methylpolysiloxane mixtures are preferably used at temperatures from 350 °C to 500 °C, particularly preferably from 380 °C to 450 °C, especially from 400 °C to 430 °C. At temperatures above 200 °C, use is preferably carried out in a protective atmosphere to prevent oxidative decomposition. Detailed description

[0029] Examples

[0030] Equilibration of methylpolysiloxane mixtures

[0031] Under thermal stress, linear endblocked methylpolysiloxanes rearrange (equilibrate). Regardless of the starting composition, the result is a methylpolysiloxane mixture which is in thermal thermodynamic equilibrium. The position of this thermal equilibrium is determined by the maximum operating temperature to which the methylpolysiloxane is exposed and the molar M:D ratio (M: Me3SiO 1 / 2 chain end group; D: Me2SiO 2 / 2 chain extending unit) of the methylpolysiloxane mixture. In order to obtain methylpolysiloxane mixtures with a composition comparable to the operation of CSP power plants, in each case 150 g of a methylpolysiloxane mixture with a defined molar M:D ratio was weighed into a 250 ml steel ampoule under a nitrogen atmosphere, degassed (3 × 20 mbar, 3 minutes each time), and sealed under an argon atmosphere (1 bar). Subsequently, the steel ampoule was stored at 425 °C for 2 months to reach the thermodynamic equilibrium of the methylpolysiloxane mixture present at 425 °C. The ratio of M to D did not change as a result ( 29 Si-NMR). In contrast, the molecular composition of the methylpolysiloxane mixture had (reached equilibrium). The methylpolysiloxane mixtures thus obtained were used for further investigations (GPC, GC, heat capacity measurements).

[0032] Composition of methylpolysiloxane mixtures:

[0033] Gel permeation chromatography (GPC)

[0034] The composition of the methylpolysiloxane mixture was determined by GPC. Instrument: Iso Pump Agilent 1200, autosampler Agilent 1200, column oven Agilent 1260, detector RID Agilent 1200, column Agilent 300×7.5mm OligoPore exclusion 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 28500D Mp 25200D, Mp 10500D, Mp 5100D, Mp 4160D, Mp 1110D, Mp311D). Evaluated as area%.

[0035] Gas chromatography (GC)

[0036] The composition of the methylpolysiloxane mixture was determined by GC. Instrument: Varian GC-3900 gas chromatograph, column VF-200ms 30m×0.32mm×0.25μm, carrier gas helium, flow rate 1 ml / min, syringe CP-1177, split ratio 1:50, detector FID 39Xl 250 °C. Evaluated as area%. Calibration has shown that the area% corresponds to mass%.

[0037] The composition of the methylpolysiloxane mixture was determined by a combination of GPC and GC data. Since in GPC, 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 one peak, the proportions of the individual compounds were determined and considered by GC. Thus, the contents of Si2 - Si8 and D3 - D8 can be determined. All high boilers from Si9 and from D9 are collectively designated as "Six(x>8)+Dy(y>8)". Six is a linear methylpolysiloxane and Dy is a cyclic methylpolysiloxane. The data are expressed as area%. Calibration has shown that the area% corresponds to mass%.

[0038] Measurement of the ratio of M to D( 29 Si-NMR)

[0039] M (Me3SiO 1 / 2 - chain end) and the ratio of D groups (-Me2SiO 2 / 2 - chain links) were determined by nuclear magnetic resonance spectroscopy( 29 Si-NMR; Bruker Avance III HD 500( 29Si: 99.4 MHz) spectrometer with a BBO 500 MHz S2 probe; inverse gated pulse sequence (NS = 3000); 150 mg of methylpolysiloxane mixture in a 4×10 -2 molar solution of Cr(acac)3 in 500 μl of CD2Cl2.

[0040] Heat capacity:

[0041] The heat capacity was determined by kinetic differential scanning calorimetry (DSC) using a SENSYS evo instrument from SETARAM. The heat capacity was determined from 25 °C to 450 °C in 5 - 10 °C steps by a stepwise method. In each case, 70 mg was weighed into a 160 μl gold crucible from the methylpolysiloxane mixture to be investigated under a nitrogen atmosphere. No pressure formed by heating in the ampoule (the autogenous pressure of the methylpolysiloxane mixture) was detected. The accuracy of the measurement was confirmed by the heat capacity determination of sapphire.

[0042] Table 3: Mass composition of the methylpolysiloxane mixture (M:D ratio from NMR; Si2 - Six and D3 - Dy contents determined by GC / GPC):

[0043]

[0044] Table 4: Heat capacity measurements with critical point limits (drop in Cp value between bold and normal font):

[0045]

[0046] The example shows that a transition to the supercritical phase has occurred before the desired operating temperature of 425 °C at a molar M:D ratio of 1:4.00. At molar M:D ratios from 1:5.80 to 1:8.99, it is shown that the transition to the supercritical phase occurs only above 425 °C.

Claims

1. A methylpolysiloxane mixture comprising a methylpolysiloxane having Me3Si chain end groups M and Me2SiO units D, wherein the molar M:D ratio in the methylpolysiloxane mixture is from 1:5.8 to 1:8.99, and the sum of the proportions of all cyclic methylpolysiloxanes is from 25 to 55% by mass, and wherein 51.2% to 54.1% by mass of the methylpolysiloxane in the methylpolysiloxane mixture is selected from linear methylpolysiloxanes Six, where x > 8, and cyclic methylpolysiloxanes Dy, where y > 8.

2. The methylpolysiloxane mixture according to claim 1, wherein for all linear methylpolysiloxanes Six from Si2 to Si22, the arithmetic mean of x weighted by the mass proportion is from 2.3 to 3.

6.

3. The methylpolysiloxane mixture according to claim 1, wherein for all cyclic methylpolysiloxanes Dy from D3 to D17, the arithmetic mean of y weighted by the mass proportion is from 1.7 to 3.

5.

4. The methylpolysiloxane mixture according to any one of claims 1 - 3, which has a bimodal, trimodal or multimodal molar mass distribution.

5. Use of the methylpolysiloxane mixture according to any one of claims 1 to 4 as a heat transfer fluid.

6. The use according to claim 5, which is for a solar thermal device.

7. The use according to claim 5, which is at a temperature of from 350 °C to 500 °C.

8. The use according to claim 6, which is at a temperature of from 350 °C to 500 °C.

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