HEAT-TRANSFER FLUIDS AND PROCESS FOR PREPARING THE SAME

Abstract: This invention relates to an aromatic heat-transfer fluid composition comprising diphenyl ether, terphenyl and at least one additional aromatic compound. An example of such a composition is a blend of terphenyl with a eutectic mixture of diphenyl ether and biphenyl. Unlike known heat-transfer fluid compositions, the present aromatic heat-transfer fluid composition has a low freezing point and is stable at high temperatures.

Description: HEAT-TRANSFER FLUIDS AND PROCESS FOR PREPARING THE SAME This invention relates to aromatic heat- -transfer fluids. Heat-trans er fluids are useful to transfer heat from a heat source to a heat sink. An example of a situation where such fluids are useful is a chemical manufacturing process where the heat-transfer fluid serves to heat and/or cool chemical reactions. An important characteristic of a heat-transfer fluid is its ability to withstand wide temperature ranges. The temperatures to which a heat-transfer fluid may be exposed range from cool temperatures due to external weather conditions or to cooling cycles and to highly elevated temperatures experienced when the process or equipment is in operation. It is important that a heat-transfer fluid is pumpable, that it can be con¬ tained in the process equipment and that it does not rapidly decompose at any of the temperatures to which it is exposed. Heat-transfer fluids useful at higher tempera- tures are known. For example, U.S. Patent 1,882,809 describes a eutectic mixture of diphenyl ether and biphenyl which is stable at high temperatures, but freezes at 54°F. U.S. Patent 3,888,777 teaches that biphenyl phenyl ethers may be added to the eutectic mixture to lower the freezing point. U.S. Patent 4,622,160 describes a heat-transfer fluid comprising a lower alkyl biphenyl and a diethyl benzene which also results in a lower freezing point. None of these mix¬ tures are without problems which include high viscosity at temperatures well above the freezing point even for those fluids with low freezing points and instability at elevated temperatures. Thus, what is needed is a heat-transfer fluid which has a low freezing point and is stable at high temperatures. The present invention is such a heat-transfer fluid composition comprising diphenyl ether, terphenyl and at least one additional aromatic compound having two or more rings either fused directly or joined by covalent bonds. It is surprising that the novel composition of this invention results in a heat-transfer fluid having both high thermal stability and a low freezing point. These heat-transfer fluids are useful in pro¬ cesses which require fluids which are thermally stable at high temperatures and also exhibit low freezing points. Examples of such processes include high temperature processes such as solar generating systems that are conducted outside or in unheated buildings in areas where the ambient temperature is sometimes relatively low. The diphenyl ether useful in the practice of this invention is well-known and available commer¬ cially. It corresponds to the following formula: The terphenyls useful in the practice of this invention are also well-known and available commercially. Alternatively, the terphenyls may be prepared by the thermal dehydrocondensation of benzene. This process produces both biphenyl and terphenyls. The terphenyls are separated by known processes such as reduced pressure distillation. Typically, the production of terphenyl results in a mixture of three isomers corresponding to the fol¬ lowing formulas: p-terphenyl, 10 o-terphenyl and 20 ra-terphenyl, 30 In the practice of this invention, it is pre¬ ferred to use the ortho isomer or a mixture of ortho and meta isomers of terphenyl. Preferably the ratio of ortho isomer to meta isomer on a weight to weight basis is at least 1:10 and no greater than 10:1. It is more preferred that the ratio of ortho isomer to meta isomer is at least 2:1 and no greater than -4:1. Due to the process by which terphenyl is produced, there is frequently a certain amount of the para isomer associated with the ortho and meta isomers. It is preferred that no more than 2 weight percent of p- terphenyl be present in the composition of this invention. It is more preferred that no more than 1 weight percent be present. In addition to the diphenyl ether and the ter¬ phenyl, the composition of this invention comprises at least one additional aromatic compound having two or more rings either fused or joined by a covalent bond. It is preferred to use aromatic hydrocarbons with no aliphatic substituents. Non-limiting examples of the additional aromatic compound include biphenyl, naphtha¬ lene, phenanthrene and anthracene. It is preferred that the additional aromatic compound be biphenyl, naphthalene or a mixture thereof and most preferred that the additional aromatic compound be biphenyl. The composition of this invention can be pre- pared easily. In one method, the various components to be used, including the desired isomers of terphenyl, are obtained commercially and blended in the desired ratios. In an alternate approach, the components including a blend of terphenyl isomers are mixed. The undesired para isomer of terphenyl is relatively insoluble in the composition and therefore much of it precipitates out of solution and can be removed. Thus, the percentage of the para isomer of terphenyl in the composition can be maintained at a low level. As stated above, the most preferred ratios of components in the composition will depend on the desired end use. The composition comprises (1) from 5 to 85 weight percent diphenyl ether; (2) from 1 to 60 weight percent terphenyl; and (3) from 1 to 35 weight percent additional aromatic compound. It is preferred that the composition comprises (1) from 35 to 55 weight percent diphenyl ether; (2) from 15 to 55 weight percent terphenyl; and (3) from 10 to 25 weight percent additional aromatic compound. Component (2) may comprise from 10 to 40 weight percent o-terphenyl and from 5 to 15 weight percent m- terphenyl. It is more preferred that the composition comprises (1) from 40 to 50 weight percent diphenyl ether; (2) from 20 to 45 weight percent terphenyl; and (3) from 12 to 20 weight percent additional aromatic compound. It should be noted that, in addition to the specified components, the composition of this invention will contain certain impurities that are normally found in the manufacture of the components of the composition. Examples of such impurities include quaterphenyls, triphenylene, and chrysene. It is preferred that the amount of these impurities does not exceed 10 weight percent of the entire composition. Additional examples of impurities which may be present in the composition of this invention include 1-methylnaphthalene, 2-methylnaphthalene, 2-methyl- biphenyl, 3-methylbiphenyl, 4-methylbiphenyl, 2-methylbiphenyl phenyl ether, 3-methylbiphenyl phenyl ether, 4-methylbiphenyl phenyl ether, fluorene, dibenzofuran, fluoranthene and 3»4-benzofluoranthene. It is preferred that the amount of these impurities does not exceed 5 weight percent and more preferred that they do not exceed 1 weight percent of the composition. The freezing point of the heat-transfer fluid compositions of this invention may vary depending on the percentages of the various components used. It should be noted that the freezing point of the composi¬ tions of this invention is the highest temperature of a freezing range. Since mixtures frequently freeze over a temperature range rather than at a single distinct temperature, care must be taken in measuring the freezing point to avoid finding an artificially depressed freezing point. For purposes of this invention, freezing points are determined by cooling the compositions well below their expected freezing point and then adding seed crystals of all the major components of the composition. The composition is kept cool until it appears to be frozen solid and then the temperature is slowly raised so that thermal equilib¬ rium and solid-liquid equilibrium are maintained until the material is completely liquid. The freezing point is determined to be the temperature at which the last solid material melts. It is preferred that the freez¬ ing point of the composition be no greater than about 40°F (5°C) and more preferred that it be no greater than 25°F (-4°C). It is also preferred that the compositions of this invention be thermally stable at temperatures of at least 650°F (345°C) and more preferably at least 700°F (370°C). It is most preferred that the compositions of this invention are thermally stable at temperatures of at least 750°F (400°C). For purposes of this invention, a composition that is thermally stable at a stated temperature will degrade to some extent at that temperature. To be thermally stable simply means that the rate of degradation at a given temperature is acceptable for the purposes for which the composition is being used. For many applications, a composition is considered to be thermally stable at a particular temperature if the rate of degradation does not exceed one percent per week at that temperature. In a particularly preferred embodiment, the heat-transfer fluid of this invention comprises diphenyl ether, terphenyl and biphenyl. The diphenyl ether and biphenyl are present as a eutectic mixture and the terphenyl is present as a mixture of the ortho and meta isomers. The eutectic mixture of diphenyl ether and biphenyl constitutes at least 50 and no more than 80 weight percent of the composition. The ter¬ phenyl constitutes at least 20 and no more than 50 weight percent of the composition. It is most preferred that the composition comprise a four- component eutectic mixture of diphenyl ether, biphenyl, o-terphenyl and m-terphenyl. Such a eutectic mixture would comprise 42 weight percent diphenyl ether, 16 weight percent biphenyl, 33 weight percent o-terphenyl and 9 weight percent m-terphenyl. 0 The heat-transfer fluids of this invention exhibit thermal stability at high temperatures along with reduced melting points. For example, in the par¬ ticularly preferred embodiment described above, the 5 composition of this invention exhibit thermal stability at 750°F (400°C) comparable to that exhibited by the eutectic mixture of diphenyl ether and biphenyl while also having a freezing point of 24°F (-4°C) which is well below that of the eutectic mixture of diphenyl 0 ether and biphenyl. The following examples are provided to further explain the invention and should not be interpreted as c limiting it in any way. Unless stated otherwise, all parts and percentages are given by weight. Example 1 0 A 57.76-g portion of a eutectic mixture of diphenyl ether and biphenyl, 33.02 g of o-terphenyl and 9.22 g of m-terphenyl were blended in a container and agitated for a short time. This mixture was then placed in a freezer at 14°F (-10°C) and allowed to thermally equilibrate. Next, seed crystals of diphenyl ether, biphenyl, o-terphenyl and m-terphenyl were added. The mixture visually appeared completely frozen a short time after the seed crystals were added. The temperature of the freezer was increased at a rate of approximately 4°F (2°C) per day until no solids remained. The temperature in the freezer at this point was 24°F (-4°C). Example 2 A 73.86-g portion of a eutectic mixture of diphenyl ether and biphenyl, 13.39 g of o-terphenyl and 12.75 g of m-terphenyl were blended in a container and agitated for a short time. This mixture was then placed in a freezer at 14°F (10°C) and allowed to thermally equilibrate. Next, seed crystals of diphenyl ether, biphenyl, o-terphenyl and m-terphenyl were added. The mixture visually appeared completely frozen a short time after the seed crystals were added. The temperature of the freezer was increased at a rate of approximately 4°F (2°C) per day until no solids remained. The temperature of the freezer at this point was 42°F (6°C). The above examples demonstrate that the addi¬ tion of terphenyl to the eutectic mixture of diphenyl ether and biphenyl results in a freezing point lower than the 54°F (12°C) freezing point for the eutectic mixture without the terphenyl additive. Further, the examples demonstrate that the percentages of the various components may be varied to obtain heat- -transfer fluids with different freezing points depending on the desired end use.

Claims: CLAIMS 1. A heat-transfer fluid composition com¬ prising diphenyl ether, terphenyl and at least one additional aromatic compound having two or more rings either fused directly or joined by covalent bonds. 2. A composition as claimed in Claim 1 comprising (1) from 5 to 85 weight percent diphenyl ether; (2) from 1 to 60 weight percent terphenyl; and (3) from 1 to 35 weight percent of the additional aromatic compound or mixture of two or more additional aromatic compounds. 3. A composition as claimed in Claim 2 comprising (1) from 35 to 55 weight percent diphenyl ether; (2) from 15 to 55 weight percent terphenyl; and (3) from 10 to 25 weight percent of the additional aromatic compound or mixture of two or more additional aromatic compounds; or (1) from 40 to 50 weight percent diphenyl ether; (2) from 20 to 45 weight percent terphenyl; and (3) from 12 to 20 weight percent of the additional aromatic compound or mixture of two or more additional aromatic compounds. 4. A composition as claimed in Claim 1 wherein the terphenyl is o-terphenyl, a mixture of g o-terphenyl and m-terphenyl, or a mixture of o- terphenyl, m-terphenyl and no greater than 2 weight percent p-terphenyl, based on the weight of the composition. 10 5. A composition as claimed in Claim 4 wherein the ratio of o-terphenyl to m-terphenyl is at least 1:10 and no greater than 10:1 or at least 2:1 and no greater than 4:1. 15 6. A composition as claimed in Claim 4 wherein the amount of p-terphenyl present on a weight basis is no greater than 1 weight percent. 20 7. A composition as claimed in Claim 1 wherein the additional aromatic compound is biphenyl or a mixture of naphthalene and biphenyl. 8. A composition as claimed in Claim 1 25 comprising from 35 to 55 weight percent diphenyl ether, from 10 to 25 weight percent biphenyl, from 10 to 40 weight percent o-terphenyl and from 5 to 15 weight percent m-terphenyl. 30 9. A composition as claimed in Claim 1 comprising a eutectic blend of 42 weight percent diphenyl ether, 16 weight percent biphenyl, 33 weight percent o-terphenyl and 9 weight percent m-terphenyl. 10. A process for preparing a heat transfer fluid composition which comprises blending (1) from 5 to 85 weight percent diphenyl ether; (2) from 1 to 60 weight percent terphenyl; and (3) from 1 to 35 weight percent of the c additional aromatic compound or mixture of two or more additional aromatic compounds. 10 15 20 25 30

Inventors: BUSKE GARY ROBERT, BUSKE, GARY, ROBERT
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1989
Main classification: C09K5
IPC codes: C09K5