METHYLPOLYSILOXANE MIXTURES AS A HEAT-CARRIER FLUID

Abstract

A methylpolysiloxane mixture along with uses and methods for operating a solar thermal power station (or CSP plant) utilizing the same. The use for the methylpolysiloxane mixture includes providing a mixture (a) wherein the methylpolysiloxane mixture includes a linear methylpolysiloxanes MD.sub.xM, wherein x is an integer with 0≤x≤100, and wherein the mixtures have a molar M:D ratio of 1:15.5 to 1:30; or (b) wherein the methylpolysiloxane mixture includes a linear methylpolysiloxanes MD.sub.xM, wherein x is an integer with 0≤x≤80 and cyclic dimethylpolysiloxanes D.sub.y where y is an integer≥3, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is 10-95 wt %, and wherein the mixtures have a molar M:D ratio of 1:10.5 to 1:30. The methylpolysiloxane mixture is used as a heat transfer fluid in a CSP plant with operating temperatures in a range of 300 to 500° C.

Claims

1-13- (canceled)

14. A use for a methylpolysiloxane mixture, comprising: (a) wherein the methylpolysiloxane mixture comprises a linear methylpolysiloxanes MD.sub.xM, wherein x is an integer with 0≤x≤100, and wherein the mixtures have a molar M:D ratio of 1:15.5 to 1:30; or (b) wherein the methylpolysiloxane mixture comprises a linear methylpolysiloxanes MD.sub.xM, wherein x is an integer with 0≤x≤80 and cyclic dimethylpolysiloxanes D.sub.y where y is an integer≥3, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is 10-95 wt %, and wherein the mixtures have a molar M:D ratio of 1:10.5 to 1:30; and wherein the methylpolysiloxane mixture is used as a heat transfer fluid in solar thermal power stations (CSP) with operating temperatures in a range of 300 to 500° C.

15. The use of claim 14, wherein with respect of the methylpolysiloxane mixtures: (a) wherein the mixtures have a molar M:D ratio of 1:15.5-1:25; or (b) wherein the mixtures comprise linear methylpolysiloxanes MD.sub.xM wherein x is an integer with 0≤x≤29, and cyclic dimethylpolysiloxanes D.sub.y where y is an integer with 3≤y≤0, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is in a range of 60-80 wt %, and wherein the mixtures have a molar M:D ratio of 1:11 to 1:20.

16. The use of claim 14, wherein with respect of the methylpolysiloxane mixtures: a) wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is in a range of 0-1 wt %, wherein the number average M.sub.n of the mixture is in a range from 400 to 3000 g/mol, and wherein the weight average M.sub.w of the mixture is in a range of 1000 to 5000 g/mol; or b) wherein the mixtures comprise linear methylpolysiloxanes MD.sub.xM wherein x is an integer with 0≤x≤29, and cyclic dimethylpolysiloxanes D.sub.y where y is an integer with 3≤y≤0, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is in a range of 60-80 wt %, and wherein the mixtures have a molar M:D ratio of 1:11 to 1:20 and the number average M.sub.n of the mixture is in a range from 100 to 2000 g/mol and wherein the weight average M.sub.w of the mixture is in a range from 100 to 6000 g/mol.

17. The use of claim 14, wherein the mixtures contain at most 150 ppm of T groups and at most 100 ppm of Q groups.

18. The use of claim 17, where the mixtures contain at most 100 ppm of T groups and no Q groups.

19. A methylpolysiloxane mixture, comprising: linear methylpolysiloxanes MD.sub.xM wherein x is an integer with 0≤x≤80 and cyclic dimethylpolysiloxanes D.sub.y where y is an integer≥3, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is 10-95 wt %, and wherein the mixture has a molar M:D ratio of 1:10.5 to 1:30.

20. The mixture claim 19, wherein the mixture comprises linear methylpolysiloxanes MD.sub.xM where x is an integer with 0≤x≤29, and cyclic dimethylpolysiloxanes D.sub.y where y is an integer with 3≤y≤0, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is in a range of 60-80 wt %, wherein the mixture has a molar M:D ratio of 1:11 to 1:20, wherein the number average M.sub.n of the mixture is in a range of 100 to 2000 g/mol and wherein the weight average M.sub.w of the mixture is in a range of 100 to 6000 g/mol.

21. The mixture of claim 20, wherein the number average M.sub.n of the mixture is in a range of 200 to 1600 g/mol and wherein the weight average M.sub.w of the mixture is in a range of 200 to 2200 g/mol.

22. The mixture of claim 20, wherein the number average M.sub.n of the mixture is in a range of 250 to 1400 g/mol and wherein the weight average M.sub.w of the mixture is in a range of 250 to 2000 g/mol.

23. The mixture of claim 19, wherein the mixture contains at most 150 ppm of T groups and at most 100 ppm of Q groups.

24. The mixture of claim 23, wherein the mixture contains at most 100 ppm of T groups and no Q groups.

25. A method for operating a CSP plant, comprising the steps of: providing methylpolysiloxane mixture comprising linear methylpolysiloxanes MD.sub.xM wherein x is an integer with 0≤x≤80 and cyclic dimethylpolysiloxanes D.sub.y where y is an integer≥3, wherein the sum of the fractions of all cyclic dimethylpolysiloxanes D.sub.y is 10-95 wt %, and wherein the mixture has a molar M:D ratio of 1:10.5 to 1:30; utilizing the methylpolysiloxane mixture as a heat transfer fluid; and increasing the temperature gradually during startup of the plant until the operating temperature is reached.

26. The method of claim 25, wherein the gradual startup comprises the following steps: a) establishing a start temperature which is 100° C. to 200° C. below the maximum operating temperature but is at least 100° C.; b) holding the start temperature until a constant operating pressure is maintained for at least 3 hours; c) increasing the operating temperature by a value in a range from 5 to 150° C.; d) holding the temperature until a constant operating pressure is maintained for at least 3 hours; and e) repeating steps c) and d) until the maximum operating temperature is reached.

Description

EXAMPLES

Measurement Methods

1. Determining the Composition of the methylpolysiloxane Mixtures

Gas Chromatography (GC)

[0051] The composition of the methylpolysiloxane mixtures was determined by GC. Instrument: Agilent GC-3900 gas chromatograph, column MXT5 (60 m×0.28 mm, 0.25 μm), carrier gas hydrogen, flow rate 1 ml/min, injector CP-1177, split 1:50, detector FID 39XI250° C. Evaluation in area percent; calibration (siloxanes and n-hexadecane) showed that the values in area % correspond to the same values in weight %.

[0052] Based on: Analysis of Large Linear and Cyclic Methylsiloxanes and Computer Calculation of the Chromatographic Data (Journal of Chromatographic Science 1966, 4, 347-349).

High-Performance Liquid Chromatography (HPLC)

[0053] The composition of the methylpolysiloxane mixtures was determined by HPLC. Instrument: Agilent LC System Series 1100, degasser ERC 3215α, detector Agilent ELSD 385 with Burgner Research MiraMist® PTFE atomizer (40° C. evaporation temperature, 90° C. atomizer temperature, at 1.2 standard liters/min), column Accucore C30 (50 mm×4.6 mm, 2.6 μm), linear solvent gradient of [methanol/water (75:25 v/v)]:acetone, beginning with 50:50 to 100% acetone within 160 min at a flow rate of 2 ml/min. Evaluation in area %. Calibration showed that the values in area % correspond to the same values in weight %.

[0054] Based on: Separation of linear and cyclic poly(dimethylsiloxanes) with polymer high-performance liquid chromatography (B. Durner, T. Ehmann, F.-M. Matysik in Monatshefte Chemie 2019, 150, 1603; https://doi.org/10.1007/s00706-019-02389-4). The quantitative composition of the methylpolysiloxane mixtures was determined by combining the GC and HPLC data. This was done by utilizing the overlap region of the two methods for the constituents from Si10 to Si20 and from D10 to D19, respectively, and performing in each case an integral comparison of Si.sub.x to Si.sub.x+i and D.sub.x to D.sub.x+i, respectively, in the aforesaid ranges. In the range of equal intensity ratios, the data were combined and were continually supplemented and standardized with the aid of the above-stated intensity factors. Calibration showed that the values ascertained in area % correspond to the same values in weight %.

Gel Permeation Chromatography (GPC)

[0055] The composition of the methylpolysiloxane mixtures, and also number average M.sub.n, weight average M.sub.w and polydispersity, were determined by GPC. Instrument: Iso Pump Agilent 1200, autosampler Agilent 1200, column oven Agilent 1260, detector RID Agilent 1200, column Agilent 300 mm×7.5 mm OligoPore cut-off 4500D, column material highly crosslinked polystyrene/divinylbenzenes, eluent toluene, flow rate 0.7 ml/min, injection volume 10 μl, concentration 1 g/l (in toluene), PDMS (polydimethylsiloxane) calibration (Mp 28 500 D, Mp 25 200 D, Mp 10 500 D, Mp 5100 D, Mp 4160 D, Mp 1110 D, Mp 311 D). Evaluation in area %.

2. Measuring the M to D Ratio (.SUP.29.Si NMR)

[0056] The proportion of M groups (Me.sub.3SiO.sub.1/2— chain ends) and D groups (Me.sub.2SiO.sub.2/2— chain links) was determined by nuclear magnetic resonance spectroscopy (.sup.29Si NMR; Bruker Avance IN HD 500 (.sup.29Si: 99.4 MHz) spectrometer with BBO 500 MHz S2 probe; inverse gated pulse sequence (NS=3000); 150 mg of methylpolysiloxane mixtures in 500 μl of a 4×10.sup.−2 molar solution of Cr(acac).sub.3 in CD.sub.2Cl.sub.2.

3. Measuring the Viscosity

[0057] The viscosity was determined using a Stabinger SVM3000 rotary viscometer from Anton Paar at 25° C. (standard) and also in the temperature range from −40° C. to +90° C.

4. Ascertaining the Critical Temperature

[0058] The critical temperature was determined by analyzing the densities in the CSP-relevant temperature range from 300 to 450° C. The fluids (50 ml each) were heated to temperatures between 50 and 450° C. for this purpose in a high-pressure and high-temperature measuring cell from LTP GmbH and loaded with pressures of 10 to 50 bar via a pressure cylinder. The respective pressure interval was analyzed at constant temperature. The respective density was determined from the resultant change in volume of the fluid under defined pressure relative to the measuring cell volume. The error of the method lies between 1% and 5%. A collapse in density reveals the critical temperature of the fluids under analysis.

5. Methylpolysiloxane Mixtures

[0059] Different methylpolysiloxane mixtures with defined M:D ratio were used and analyzed (cf. Tables 2 and 4): [0060] CE1 (not inventive, M:D=1:4)=linear polydimethylsiloxane having a viscosity of around 5 mPa*s, available commercially from Wacker Chemie AG as HELISOL® 5A [0061] Example 1 (M:D=1:15.5)=substantially linear polydimethylsiloxane having a composition as in Table 2. [0062] Example 2 (M:D=1:18)=substantially linear polydimethylsiloxane having a composition as in Table 2. [0063] Example 3 (M:D=1:13.5), prepared from 33.1 parts by weight of WACKER® AK5 (available from Wacker Chemie AG) and 66.9 parts by weight of a mixture of cyclic compounds D composed of 0.4 part by weight of D3, 58.1 parts by weight of D4, 32.8 parts by weight of D5 and 8.7 parts by weight of D6. The corresponding cyclic compounds are available commercially. [0064] Example 4 (M:D=1:17)=prepared from 28.0 parts by weight of WACKER® AK5 and 72 parts by weight of a mixture of cyclic compounds D composed of 0.4 part by weight of D3, 58.1 parts by weight of D4, 32.8 parts by weight of D5, and 8.7 parts by weight of D6. The corresponding cyclic compounds are available commercially.

6. Equilibration of methylpolysiloxane Mixtures

[0065] In each case 2-2.3 liters of the respective methylpolysiloxane mixture with defined M:D ratio were introduced into a stainless steel autoclave (5.4 liters total volume, with analog and digital pressure transducer and jacket resistance heating with temperature sensor). Gastight sealing of the autoclave followed. After multiple vacuum degassing (3×20 mbar, 3 minutes in each case) the mixtures were blanketed with an argon atmosphere (1 bar). The autoclave was heated at 425° C. (internal temperature) for 30 days in order to obtain the thermodynamic equilibrium of the methylpolysiloxane mixtures.

[0066] This did not result in any alteration to the M:D ratio (verified by means of .sup.29Si NMR), but the equilibration did alter the molecular composition of the methylpolysiloxane mixtures. The equilibrated methylpolysiloxane mixtures obtained accordingly were used for further analysis (GC, GPC, HPLC, viscosity) (cf. Tables 3 and 4).

TABLE-US-00002 TABLE 2 Composition of the starting mixtures Starting mixtures wt % CE1 a) E1 a) E2 a) E3 a) E4 a) M:D 1:4 1:15.5 1:18 1:13.5 1:17 Si2 D3 0.272 0.591 Si3 0.000 0.000 D4 0.000 0.004 0.008 38.700 44.648 Si4 0.000 0.000 0.001 0.000 0.000 D5 0.006 0.010 0.023 21.807 20.675 Si5 8.447 0.006 0.009 2.741 2.307 D6 0.784 0.011 0.292 6.075 5.792 Si6 9.442 0.024 0.031 3.046 2.574 D7 0.184 0.011 0.024 0.593 0.947 Si7 9.051 0.128 0.106 2.904 2.456 D8 0.061 0.013 0.026 0.031 0.020 Si8 8.488 0.390 0.267 2.741 2.316 D9 0.031 0.021 0.034 0.017 0.013 Si9 7.819 0.889 0.529 2.526 2.135 D10 0.019 0.063 0.045 0.015 0.010 Si10 7.095 1.588 0.857 2.295 1.941 D11 0.013 0.095 0.055 0.000 0.009 Si11 6.375 2.268 1.151 2.066 1.748 D12 0.012 0.111 0.054 0.000 0.000 Si12 5.683 2.787 1.387 1.848 1.562 D13 0.012 0.033 0.057 0.000 0.000 Si13 5.028 3.093 1.564 1.640 1.386 D14 0.012 0.032 0.067 0.000 0.000 Si14 4.425 3.243 1.687 1.448 1.223 D15 0.013 0.030 0.062 0.000 0.000 Si15 3.874 3.285 1.783 1.272 1.074 D16 0.017 0.024 0.058 0.000 0.000 Si16 3.384 3.231 1.832 1.109 0.937 D17 0.048 0.021 0.050 0.000 0.000 Si17 2.924 3.189 1.955 0.971 0.801 D18 0.040 0.015 0.036 0.000 0.000 Si18 2.567 3.322 1.964 0.840 0.700 Si19 0.003 3.229 2.019 0.724 0.602 Si20 2.198 3.199 1.907 0.639 0.536 Si21 1.849 2.943 2.000 0.581 0.459 Si22 1.705 3.018 2.018 0.460 0.438 Si23 1.554 2.885 2.122 0.449 0.355 Si24 1.329 2.806 2.149 0.426 0.328 Si25 1.039 2.668 2.030 0.385 0.313 Si26 0.903 2.505 1.904 0.376 0.298 Si27 0.890 2.617 2.005 0.347 0.291 Si28 0.731 2.379 2.066 0.327 0.261 Si29 0.726 2.311 1.970 0.327 0.256 Si30 0.634 2.170 1.940 Si31 0.582 2.189 2.032 Si32 2.117 1.936 Si33 1.963 2.110 Si34 1.799 2.024 Si35 1.705 1.928 Si36 1.689 2.003 Si37 1.611 1.814 Si38 1.574 1.779 Si39 1.439 1.865 Si40 1.391 1.779 Si41 1.299 1.725 Si42 1.210 1.667 Si43 1.144 1.697 Si44 1.072 1.737 Si45 1.010 1.648 Si46 1.042 1.636 Si47 1.010 1.492 Si48 0.809 1.570 Si49 0.935 1.485 Si50 0.752 1.559 Si51 0.714 1.338 Si52 0.675 1.296 Si53 0.666 1.327 Si54 0.605 1.280 Si55 0.570 1.184 Si56 0.540 1.102 Si57 0.595 1.083 Si58 0.546 1.014 Si59 0.485 1.050 Si60 0.493 1.098 Si61 0.456 0.970 Si62 0.429 0.882 Si63 0.423 0.845 Si64 0.431 0.805 Si65 0.379 0.740 Si66 0.373 0.713 Si67 0.364 0.672 Si68 0.384 0.672 Si69 0.383 0.664 Si70 0.324 0.662 Si71 0.340 0.640 Si72 0.370 0.601 Si73 0.334 0.634 Si74 0.348 0.520 Si75 0.345 0.504 Si76 0.546 Si77 0.598 Si78 0.487 Si79 0.442 Sum of 1.25 0.49 0.89 67.24 72.11 cyclic compounds

TABLE-US-00003 TABLE 3 Equilibrium composition of the mixtures equilibrated at 425° C. equilibrated 1 month @ 425° C. wt % CE1 b) E1 b) E2 b) E3 b) E4 b) M:D 1:4 1:15.5 1:18 1:13.5 1:17 Si2 2.800 0.557 0.313 0.239 0.172 D3 2.717 3.573 4.385 2.332 2.328 Si3 4.871 1.016 0.612 0.456 0.344 D4 15.569 20.590 23.206 14.046 14.275 Si4 5.617 1.312 0.803 0.626 0.488 D5 8.349 11.827 13.709 8.258 8.508 Si5 5.712 1.479 0.912 0.740 0.571 D6 2.051 3.683 4.180 2.681 2.813 Si6 5.501 1.578 0.987 0.825 0.637 D7 0.770 1.065 1.203 0.785 0.807 Si7 5.218 1.655 1.054 0.898 0.697 D8 0.397 0.464 0.493 0.291 0.312 Si8 4.843 1.708 1.110 0.976 0.762 D9 0.310 0.334 0.321 0.175 0.178 Si9 4.424 1.731 1.143 1.036 0.816 D10 0.101 0.310 0.278 0.127 0.141 Si10 3.980 1.728 1.162 1.086 0.865 D11 0.044 0.080 0.102 0.109 0.122 Si11 3.545 1.710 1.174 1.129 0.907 D12 0.019 0.047 0.074 0.113 0.125 Si12 3.147 1.709 1.203 1.159 0.937 D13 0.001 0.044 0.070 0.125 0.140 Si13 2.764 1.667 1.224 1.185 0.969 D14 0.011 0.041 0.070 0.142 0.154 Si14 2.420 1.627 1.240 1.204 0.989 D15 0.004 0.040 0.066 0.000 0.000 Si15 2.111 1.574 1.201 1.219 1.007 D16 0.005 0.040 0.061 0.000 0.000 Si16 1.833 1.560 1.189 1.204 1.017 D17 0.000 0.031 0.052 0.000 0.000 Si17 1.585 1.555 1.099 1.263 1.046 D18 0.000 0.016 0.031 0.000 0.000 Si18 1.419 1.320 1.164 1.235 1.283 Si19 1.157 1.420 1.176 1.263 1.065 Si20 0.984 1.337 1.042 1.288 1.285 Si21 0.923 1.294 1.088 1.273 1.168 Si22 0.745 1.340 0.997 1.605 1.123 Si23 0.612 1.182 1.060 1.329 1.232 Si24 0.610 1.206 0.996 1.310 1.153 Si25 0.569 1.254 1.078 1.295 1.178 Si26 0.437 1.234 1.041 1.329 1.239 Si27 0.421 1.018 1.112 1.374 1.502 Si28 0.423 1.143 0.855 1.554 1.306 Si29 0.349 1.027 0.901 1.540 1.509 Si30 0.339 0.906 0.919 1.510 1.478 Si31 0.290 0.903 1.050 1.532 1.484 Si32 0.893 0.838 1.447 1.473 Si33 0.880 0.867 1.408 1.238 Si34 0.848 0.888 1.380 1.395 Si35 0.830 0.782 1.381 1.411 Si36 0.746 0.874 1.302 1.348 Si37 0.751 0.757 1.303 1.327 Si38 0.733 0.968 1.225 1.279 Si39 0.742 0.827 1.197 1.256 Si40 0.657 0.858 1.177 1.266 Si41 0.679 0.770 1.120 1.203 Si42 0.647 0.771 1.091 1.155 Si43 0.638 0.758 1.093 1.148 Si44 0.583 0.724 1.045 1.081 Si45 0.575 0.790 1.021 1.050 Si46 0.514 0.687 0.972 1.049 Si47 0.519 0.627 0.913 1.032 Si48 0.527 0.657 0.907 1.024 Si49 0.501 0.697 0.872 0.980 Si50 0.469 0.570 0.864 0.961 Si51 0.507 0.498 0.834 0.962 Si52 0.464 0.638 0.820 0.939 Si53 0.437 0.653 0.760 0.874 Si54 0.440 0.488 0.768 0.879 Si55 0.469 0.472 0.737 0.910 Si56 0.503 0.468 0.688 0.815 Si57 0.372 0.490 0.657 0.800 Si58 0.401 0.523 0.653 0.800 Si59 0.353 0.485 0.624 0.779 Si60 0.383 0.481 0.606 0.719 Si61 0.426 0.614 0.737 Si62 0.461 0.594 0.707 Si63 0.547 0.694 Si64 0.520 0.653 Si65 0.523 0.616 Si66 0.515 0.607 Si67 0.476 0.602 Si68 0.468 0.576 Si69 0.462 0.557 Si70 0.456 0.545 Si71 0.437 0.520 Si72 0.435 0.508 Si73 0.402 0.476 Si74 0.410 0.473 Si75 0.408 0.443 Sum of the 30.35 42.18 48.30 29.18 29.90 cyclic compounds

TABLE-US-00004 TABLE 4 Overview of the mixtures before and after equilibration Cyclic Molar com- Pressure M:D pounds/ M.sub.n/g/ M.sub.w/g/ Viscosity/ at 425° C./ Critical ratio wt % mol mol Polydispersity mPas filling level temperature/° C. Starting mixtures CE1 a) 1:4 1.25 862 954 1.11 5.1 — — E1 a) 1:15.5 0.49 1793 2549 1.42 19.5 — — E2 a) 1:18 0.89 2450 4077 1.66 33.7 — — E3 a) 1:13.5 67.2 390 568 1.46 4.8 — — E4 a) 1:17 72.1 333 439 1.32 3.4 — — Equilibrated mixtures CE1 b) 1:4 30.4 427 746 1.75 3.21 23 bar/  400° C. 44% E1 b) 1:15.5 42.2 541 1507 2.78 8.92 15.8 bar/  440° C. 44% E2 b) 1:18 48.3 557 2027 3.64 11.2 15.0 bar/ >450° C. 47% E3 b) 1:13.5 29.2 618 2488 4.03 11.7 16.1 bar/ >450° C. 45% E4 b) 1:17 29.9 667 4988 7.47 14.5 15.9 bar/ >450° C. 48%

[0067] As a result of the equilibration in the laboratory experiment, the initial mixtures become methylpolysiloxane mixtures which have a composition comparable to the CSP power station operation.

[0068] As a consequence of this, the viscosity drop of mixtures E1 and E2 is much more pronounced (CE1: reduction by 38%, E1: reduction by 62%, E2: reduction by 65%) than hitherto known. At the same time the vapor pressure of the equilibrated mixtures is lower than for the low molecular mass oil of the comparative example (E1: 15.8 bar, E2: 15.0 bar; CE1: 23 bar), although the mixtures E1 and E2 form significantly more low-boiling cyclic compounds (E1: 42.18 wt %, E2: 48.3 wt %; cf. CE1: 30.4 wt %). Mixtures E3 and E4 show an opposing trend in terms of viscosity: the viscosity rises during equilibration, but remains below a value of 20 mPa*s. The vapor pressure of the equilibrated mixtures E3 and E4, however, is likewise lower than for the low molecular mass oil of the comparative example.

[0069] The measurements additionally show that all of the methylpolysiloxane mixtures analyzed are still subcritical in the region of the operating temperature.

[0070] It was found that a startup operation in which the operating temperature of the heat transfer fluid utilized is brought gradually up to the desired maximum operating temperature of the plant prevents the maximum operating pressure not being exceeded in the equilibration phase.