US2185639A - Heat transfer process - Google Patents

Description

Patented Jan. 2, 1940 NT OFFICE HEAT TRANSFER PROCESS Arthur A. Levine and Oliver W. Cass, Niagara Falls, N. Y., assignors to E. I. du Pont de Nemours & Company, Wilmington. DeL, a corporation of Delaware No Drawing. Application April 15, 1938,

Serial No. 202,288

8 Claims.

This invention relates to an improved method.

for transferring heat involving the use of certain superior aromatic organic compounds of high molecular weight and high boiling point and mix- 6 tures of those aromatic organic compounds. This application is, in part, acontinuation of our earlier application, Serial Number 63,276, filed February 10, 1936.

More specifically, the invention is concerned with an improved "'method of transferring heat at relatively elevated temperatures utilizing certain novel heat transfer compositions which consist essentially of relatively high boiling halogenated aromatic compounds and mixtures of these high boiling halogenated aromatic compounds. Under these circumstances the novel heat transfer agents utilized in our improved process for the transference of heat,at' relatively high temperatures possess practically complete non-inflammability and extreme stability at very high temperatures, temperatures considerably in excess of their boiling points.

The use of certain aromatic hydrocarbons and derivatives of aromatic hydrocarbons as heat transfer media has been previously suggested.

These compositions usually contain the aromatic objections. For example, in the mixture di- 85 phenyl, diphenyl oxide and naphthalene, the naphthalene is said to be present primarily for the purpose of avoiding undue decomposition of the heat transfer compound at the elevated temperature to which it is subjected in use. Diphenyl and mixtures of diphenyl and diphenyl oxide are' subject to this decomposition, Diphenyl oxide is admixed with diphenyl primarily for the purpose of lowering the freezing point and rendering the mixture liquid at room temperature.

Various other heat transfer agents for transferring heat at relatively high temperatures have also been suggested, chief among which are chlorinated benzene, chlorinated toluene, and mixtures comprising both chlorinated benzene and chlorinated toluene. Unfortunately, it has been necessary to prepare chlorinated benzene and chlorinated toluene of relatively high chlorine content in order to secure a product having a sufliciently low degree of infiammability'to permit of its use at relatively elevated temperatures.

Thus, it has been necessary to utilize products prepared by chlorinating benzene and toluene until there have been introduced at least three chlorine atoms per mole of starting material if a product is ,to besecured which will not have a 5 flash point below 1. about C. Chlorinated benzene and chlorinated toluene containing sufficient chlorine to form the trichloride are unduly'corrosive to metal surfaces at elevated temperatures. This has been especially noticeable 10 when these products are utilized as heat transferagents at temperatures closely approximating their boiling points such as temperatures of 210 C. or above, although it is noticeable to a substantial extent at any temperature in excess of 100 0.. The extreme corrosion of chlorinated benzene and chlorinated toluene of sufficiently high chlorine content to render them reasonably non-inflammable has greatly restricted their use for purposes of heat transference. Neither chlorinated benzene nor chlorinated toluene, nor any mixture comprising one or both of these constituents, has received any commercial acceptance as a medium for use in transferring heat at relatively elevated temperatures where metallic' surfaces come into contact 'with the heat transfer medium.

The heat transfer agents with which this inventionisvconcerned and which are utilized in our improved process forv the transference of 30 heat at temperatures above 100 C. are readily prepared from relatively inexpensive starting materials. As heat transferiagents these compounds Y are open to none of the objections which have seriously restricted the commercial use of the 35 a heat transfer compositions now known to the art. For example, the various mixtures of diphenyl and similar agents such as diphenyl oxide and naphthalene are considered too inflammable at temperatures in excess of 100 C. to permit of 40 their use .in heat transfer processes except in relatively isolated instances. Commercial mixtures involving these ingredients which are sold for heat transfer purposes under the trade-mark names Dowtherm A and Dowtherm C have 45 been found to have flash points of 102 C. and C., respectively. The'flre points of these compositions are also very low, that of 'Dowtherm A being 120 C. while Dowtherm C" will readily catch fire at a temperature but slightly 50 above its flash point, one of about C. As distinguished from these materials th' improved ,heat transfer agents" which we utilize in our process of transferring heat at temperatures in excess of about 100 C. have no fire points below. 65

1 their boiling points and flash points but slightly below their boiling points.

While chlorinated toluene and chlorinated benzene, or mixtures comprising one or both of these constituents, are not objectionable because of relatively low flash or fire points, they are extremely objectionable when used at temperatures in excess of 100 C. in that-they are definitely corrosive to metallic equipment. As a matter of fact at temperatures in excess of about 210 0. they are so corrosive as to constitute a serious hazard that the metallic equipment will fail if used continuously for heat transfer purposes for even relatively short periods of time. a

For the transference of heat at temperatures in excess of 100 C. and, more especially, for the transference of heat at temperatures in excess of about 210 C. the prior art has had no satisfactory heat transfer agent. The various mixtures containing diphenyl, diphenyl oxide or naphthalene, while not corrosive to metallic equipment are so relatively inflammable as to constitute a serious danger when used at temperatures much in excess of 100 C. in the event some leak should occur in the heat transfer apparatus. While chlorinated benzene and chlorinated toluene are not subject to this high degree of inflammability. when they are used for heat transfer purposes at relatively high temperatures it has not been practransfer agents which we utilize in our improved process of heat transference at relatively high temperatures, in addition to being substantially non-inflammable and having relatively high flash and fire points, possess a heat capacity which is superior to that of the various commercial mixtures now sold for heat transfer purposes. Their density is greater'tha'n the commerial heat transference agents now utilized which also results in certain further advantages and greater versatility in the process of heat transference. But the outstanding characteristic of our heat transfer agents which render them strikingly superior and of outstanding importance when utilized for transferring heat at temperatures in excess of 100 C. is their remarkable stability. This stability, which is indeed surprising and is observed whether there be moisture or. air present in the heat transfer agent or not, means that the metallic equipment utilized to hold the agent in subjected to practically no corrosion. It will be appreciated that the elimination of corrosion difiiculties is an essential factor in the commercial acceptance of any heat transfer medium. Its surprisingly slight corrosive action to metals, as compared with the relatively high corrosive action of its chlorinated homologs toluene and benzene, is the outstanding factor permitting the use of these compounds in our improved heat transfer process.

The compositions which we propose to use as heat transfer media and a novel and improved process for preparing them are described and claimed in our co-pending application Serial No. 68,919, filed March 14, 1936. The use of certain products prepared from these compositionsand of the compositions themselves, as dielectric media and as ingredients in dielectric mixtures for various electrical purposes, is described and claimed in our co-pending application, Serial Number 53,099, filed December 5, 1935, issued December 13, 1938 as U. S. P. 2,139,964. 5

It is therefore one of the objects of this invention to provide a composition of matter that will be useful for high temperature heat transfer purposes. Another object of this invention is to provide a heat transfer medium which can be 10 utilized in our novel process for transferring heat at temperatures in excess of 210 C. Still other objects of our invention are to utilize in a heat transfer process at elevated temperatures a heat transfer agent which will be substantially non 15 inflammable under all ordinary conditions of use.

Among other and ancillary objects of our invention are the preparation of these improved heat transfer agents for use in our novel process of transferring heat at elevated temperatures 20 from relatively inexpensive starting materials.

It may be further mentioned that our process utilizes heat exchange media of superior heat exchange properties, due essentially to their satisfactory heat capacity, extreme stability, greater :5 density, low freezing point, relatively high boiling points, and liquid characteristics under all ordinary conditions of use. The selection of specific heat transfer agents answering these requirements and their use in our improved process con- 30 stitutes still another object of our invention.

There are certain definite advantages in utilizing a heat transfer medium which is a liquid under ordinary conditions of use. Water, which is now widely used in power plants and miscel- 35 laneous heat transfer applications both as a liquid and for the generation of steam, is now utilized in certain types of apparatus and it is desirable that any new heat transfer medium be similar to water, at least in being a liquid so that it may 3' replace this usual heat transfer agent with a tions wherein it is essential that it be brought into thermal contact but kept out of physical 55 contact with a fluid which is to be heated.

Our improved heat transfer media are pre pared by the chlorination of ethyl benzene. Ethylbenzene, an aromatic compound having the formula CsH5.C2H5, may be readily prepared in180 any one of anumber of ways. As described and claimed in our co-pending applicationpreviously referred to, Serial No. 68,919, ethylbenzene is preferably chlorinated with gaseous chlorine in the presence of a catalytic material such .as me-' tallic iron. A relatively low temperature is employed (below C.) and light is excluded from the reaction vessel. Under these conditions the chlorinereplaces the .nuclear hydrogen atoms, and chlorination of the ethyl side chain does not' occur. I That chlorination of the ethyl side chain has not occurred is readily shown by refluxing the product withalcoholic potassium hydroxide solution. No change in the normality of such u a solution is observed, thus indicating the absence of chlorine in the side chain.

When ethylbenzene is chlorinated in this manner products which have certain definite charpurities and certain isomers of dichlorethylbenzene may be present in relatively small amount.

By fractional distillation certain fractions can be separated from the products prepared by acteristics result. However the chracteristics of chlorinating ethyl benzene, which fractions are 5 the product depend on the number of chlorine themselves useful as heat transfer media. The .atoms introduced per mole of ethylbenzene. following table gives certain data regarding the Until about 4.6 atoms of chlorine per mole of percentages and boiling points of various fracethylbenzen'e have been introduced, the reaction tions which may be separated by fractional disproducts are liquid at room temperature. When tillation. Nine specific compositions prepared 4.6 or more chlorine atoms are introduced per by chlorinating ethylbenzene until there are mole of ethylbenzene, the product approaches a present specific amounts of chlorine from 3.0 solid in characteristics until about 5.0 atoms of to 5.0 atoms of chlorine per mole of ethylbenzene chlorine have been introduced, when the product and described in this table. Any other combecomes a crystalline solid consisting essentially positions prepared by chlorinating ethylbenzene of pentachlorethylbenzene. until there are present per mole of ethylbenzene Our invention comprises the process of utilizing any other number of chlorine atoms can, of for high temperature heat transfer the heat course, be readily prepared and their use as heat transfer media prepared by the chlorination of transfer media is contemplated as part of our ethylbenzene until there are present from 3.0 up invention.

' Table I Number of chlorine atoms introduced per molecule of ethyl benzene Fraction U' Character oi product at room temperature Liquid Liquid Liquid Liquid Liquid Liquid Liquid Semi-solid Solid Percent Percent Percent Percent Percent Percent Percent I. Boiling pt. to 235 8. 9 II. Boiling pt. 235250 76.8 III. Boiling pt. 250-260 9.4 IV. Boiling pt. 260-270 5.4 V. Boiling pt. 270-285 VI. Boiling pt. 285295 vn. Boilin pt. 295305 VIVII. Resi ue Nil to and including 5.0 atoms of chlorine per mole of ethylbenzene. While the products prepared by chlorinating ethylbenzene until the chlorine content ranges from 3.0 to 4.0 atoms of chlorine per mole of ethylbenzene, are substantially nonfiammable, i. e., will not flash below 80 0., these products are not regarded as substantially noninfiammable. These products will burn if suincient oxygen is present and they are heated to a relatively elevated temperature. Our preferred compositions, therefore, comprise the products prepared by chlorinating ethylbenzene until from 4.0 to 5.0 atoms of chlorine per mole of ethylbenzene are present, which products are entirely non-inflammable, although products prepared by chlorinating ethylbenzene until the chlorine con tent is from 3.0 to 4.0 atoms of chlorine per mole of ethylbenzene can be employed for heat exchange purposes in some installations.

As will be apparent from our previously mentioned co-pending application. Serial No. 68,919, we are not entirely certain as to just what prod ucts are present in our compositions, prepared by chlorinating ethylbenzene with gaseous chlorine or equivalent chlorinating agent under conditions such that chlorine substitution in the ethyl side chain is substantially completely avoided and is not observable by the usual test. Under certain conditions these compositions contain mixtures of various isomers of trichlorethylbenzene and various isomers of tetrachlorethylbenzene. Under other conditions the mixtures consist essentially of various isomers .of tetrachlorethylbenzene and pentachlorethylbenzene. Again the product may contain mixtures of various isomers of trichlorethylbenzene, various isomers of tetrachlorethylbenzene and pentachlorethylbenzene. Under other conditions various additional products such as im- It is possible to regulate the temperature at which the heat transfer liquid will boil, which is customarily the temperature to which the material to be heated is subjected, by selecting any particular fraction having a boiling point between 235 and 305 C. Thus if a boiling point within the range 285-295 C. is desired, fraction VI of the above table is selected. This avoids the necessity of applying pressure to elevate the boiling point, or operating under reduced pressure 'so as to have a lower boiling point which is now sometimes necessary with the heat exchange materials now known to the art.

It is apparent that our improved heat transfer compositions may be employed for any purpose where heat transfer media are now used for the transference of heat at temperatures in excess of 100 C. or, more particularly, at temperatures in excess of 210 C. Thus they may be used in boilers for high pressure steam generators, as anti-freeze fluids in automobile radiators, as oil baths for laboratory and plant manufacturing processes, and to replace the liquids now generally used in heat transfer apparatus in factories. Other uses will occur to those skilled in the art. For example, they may be used as high temperature heat-transfer fluids in power generation to replace water or they may be employed in the inter-stage re-heating of steam. In general, the heat transfer compositions described may be used for heat transference at elevated temperatures anywhere in the oil, chemical, manufacturing or other industries where noninflammable, extremely stable, heat transfer media which are liquids of high boiling point, are necessary.

The fact that our improved heat transfer agents have "no action upon the common metallic materials used in engineering construction when employed at temperatures of 100 C. in the transference of heat is apparent from the following table. In explanation it may be stated that various fractions of chlorinated ethylbenzene having boiling points within the range 270 to 310 C. were heated to a temperature of 100 C. in contact with metallic test strips for periods of '72 hours. At the conclusion of these 72-hour test periods the change in weight of each test strip was determined, as well as the color of the chlorinated thylbenzene heat transfer agents. The results a e summarized below.

vated temperature, but neither is copper or aluminum, other frequently encountered materials of construction employed in installations where heat Table H is to be transferred. The following table illustrates the results of various tests carried out at T Time temperatures wherein the chlorinated ethylbenemper' metal in Color of heat I Metal! contact store at contact Loss in transfer zene is used for transferring heat at a temperawithheatedheat g gg g gggg gglgfi g agent t ture of 270 C. or at temperatures in excess of transfer agent being t his; strip gag? that temperature.

9 ea c 1'8 8! agent Table IV C'. Hours Metal with 100 72 0.0002 g Water-white. which heat 'lem era- Time of L953 100 72. No lam.-. Do. ggggg g g zgggg transfer tureo heat #2:; 2 3 2 $8 3% 11:13:21: 38: 332.35. n am Both copper and 100 72 0.0003 g. (Cu Do. iron teststrip. strip) l 0.0004g.( e .Do. (7. Hours Gram strip) Ethyl chlorobenzene Iron 270 6 0.

(B. P. 270-285 0.). Ethyl chlorobenzene ..do 270 6 0.0050 It is thus evident that at 100 C. the chlorinated ggz fig fifig do 310 6 0 0m ethylbenzene heat transfer agents which we util- Y R 305410. ize in our improved process of heat transference enzene 0opper 210 6 0. 002

. ubst ti 1] (B.P.270-305 0.). at elevated temperature have 3 an y no Ethyl chlorobenzene Aluminum 270 6 0.0011 corrosive action on the various metals used for REW- 05 0.).

structural purposes in engineering.

In order to illustrate just how much less corrosive to metals the chlorinated ethylbenzene heat tests were carried out. The temperature selected for heat transference was 210 0., which is substantiallythe boiling point of trichlorobenzene. In this case the test periods were only six hours as at the end of this time the test samples present in the trichloroben'zene and chlorinated toluene heat transfer agents, and in mixtures containing these agents, were so badly corroded as to make immediately evident the corrosive nature of these compositions at elevated temperatures.

It will be noted that the loss in weight of the iron test strip in contact with the ethyl chlorobenzene heat transfer agent was practically negligible. The losses in weight of the test strips in contact with the chlorinated benzene and chlorinated toluene heat transfer agents were so marked as to make it immediately evident that these products would so seriously corrode iron materials of construction as to render it impossible to use this metal for commercial construc- It is thus evident that even at these exceedingly I It may also be pointed out that our improved heat transfer agents comprising chlorinated ethylbenzene are highly resistant to oxidation. These agents are practically not attacked by acidiiied potassium permanganate solution, an extremely strong oxidizing agent, even when refiuxed with this oxidizing agent for 24 hours.

Attempts to analyze these heat transfer agents for chlorine loss by the usual method in a Parr bomb were also unsuccessful. In this method a sample of the material to be tested, together with sodium peroxide and some combustible organic material such as sugar, are placed in a closed bomb. The mixture is ignited anda hi h temperature is developed-within the closed system.

This temperature, which is the combustion temperature of sugar in the presence of substantially pure oxygen, is very high. Tests at the end of .such an experiment gave no determinable value 1 for the chlorine released from our improved heat transfer agents, thus indicating that even under bined hydrolysis and oxidation the chlorinated aisaeae ethylbenzenes are extremely stable. This is illustrated by an experiment wherein 50 cc. of our novel heat transfer agents were refluxed with ,50 cc. of water and air continuously blown through for at least a period of six' hours. This resulted in only 0.0006% decomposition based on the chlorine involved. When a mixture of 92% trichlorobenzene and 8% trichlorotoluene was subjected to a similar test the percentage decomposed was 0.013%. Chlorinated toluenes decomposed to the extent of 0.048% under similar conditions. The resistance of our improved heat transfer agents to combined hydrolysis and oxidation isillustrated in a remarkable way by these comparative tests.

In our improved process of heat transfer the heat transfer agent is brought into indirect contact with the material to be heated. Where heat transfer agents having boiling points up to 310 C. are utilized for heat transference at temperatures below 310 C. but above 100 C., the agent may be utilized in an ordinary system open to the atmosphere. In view of the low toxicity and very high flash and the points of our heat transfer agents there is absolutely no danger under these conditions.

Our heat transfer agents can also be utilized at temperatures above their-boiling points taking care to insure the maintenance of a pressure system. Thus, a product such as that described in Table IV and having a boiling point within the range 270 to 285? C. may be used at temperatures up to 310 C. by simply completely enclos: ing it in a jacket surrounding the material to which heat is to be transferred. Under these circumstances the heat transfer agent remains liquid and pressure is developed within the heating jacket. The pressures developed are in no way excessive since even if a material having a boiling point of 270 C. were utilized for transferring heat in a pressure system at 310 C., the pressure developed would be moderate. when such apparatus is shut down after use and the heat transfer agent is at room temperature in all cases it will be a liquid. This is an added distinct advantage of our improved heat transfer agents.

In view of their low melting point, the melting point of most fractions comprising our improved heat transfer media being below about 10 C., the heat transfer agent remains a liquid when the apparatus is cooling down after use and does not solidify as when various other heat transfer agents now known to the art are employed.

Other modes of applying the principles of our invention may be devised in place of those herein explained, and various changes may be made in the process of transferring heat at elevated temperatures without departing from the scope of our invention. For example, various equivalents of the compositions herein described may be utilized for heat transference at temperatures above 100 C. and, more particularly, for heat transference at temperatures above 210 C., which will nevertheless come withi'fithe purview of our invention. The scope of our invention is not to be restricted to the details, numerical amounts, and

conditions, previously given as illustrative of our preferred embodiment, but isto be ascertained in accordance with the appended claims.

We claim: I I 1. The method of transferring heat which com-' prises heating a medium comprising nuclear chlorinated ethylbenzene containing at least 3 atoms of chlorine per molecule of ethylbenzene to a temperature within the range 100 C. to 310 C.

' and passing said medium in indirect heat transfer relationship with a body of material to be heated. 1

2. The method of transferring heat which comprises heating a medium-comprising nuclear chlorinated ethylbenzene containing 3 to 5 atoms of chlorine per molecule of ethylbenzene to a temperature within the range 100 C. to 310 C. and

passing said medium in indirect heat transfer relationship with a body of material to be heated.

3. The method of transferring heat which comprises heating a medium comprising nuclear chlorinated ethylbenzene containing 4 to 5 atoms of chlorine per molecule of ethylbenzene to a temperature within the range 100 C. to 310 C. and passing said medium in indirect heat transfer relationship with a body of material to be heated.

4. The method of transferring heat which comprises heating amedium comprising nuclear chlorinated ethylbenzene containing approximately 4 atoms of chlorine per molecule of ethylbenzene to a temperature within the range 100 C. to 310 C. and passing said medium in indirect heat transfer relationship with a body of material to be heated.

5. The method of transferring heat which comprises heating a medium comprising nuclear chlorinated ethylbenzene containing at least 3 atoms of chlorine per molecule of ethylbenzene to a temperature within the range 210 C. to 310 C. and passing said medium in indirect heat transfer relationship with a' body of material to be heated.

6. The method of transferring heat which comperature within the range 210 C. to 310 C. and

passing said medium in indirect heat transfer I relationship with a body of material to be heated.

7. The method of transferring heat which comprises heating a medium comprising nuclearchlorinated ethylbenzene'containing 4 to 5 atoms'of chlorine per molecule of ethylbenzene-to a temperature within the range 210 C. to 310 C. and v passing said medium in indirect heat transfer relationship with a body of material to be heated.

8. The method of transferring heat which comprises heating a medium comprising nuclear chlorinated ethylbenzene containing approximately 4 atoms of chlorine per molecule of ethylbenzene to a temperature within the range 210 C. to 310 C. and passing said medium in indirect heat transfer relationship with a body of material to be heated.

ARTHUR A. LEVINE. OLIVER W. CASS.