CA1037065A - Stabilization of trichlorodifluoro benzenes - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Trichlorodifluorobenzene is thermally stabilized, particularly in the presence of engineering metals by contact-ing it with solid alkaline earth carbonates or with alkali or alkaline earth borates and phosphates in a sufficient amount.
The method is particularly useful in Rankine-cycle engines using trichlorodifluorobenzenes as the working fluid, when the stabilizer is contained in the boiler.

Description

~3~7~65 ~ h~s invention relateæ to a me~hod o~ therm~lly stabllizing trlchlorodi~luorobenzene, particularly in ~he presence o~ engineering metals.
The use o~ trichlorodifluorobenzenses as a power fluid in a rot~ry Rankine-cycle engine has been disclosed in French Patent No. 72.29277 gxanted Maxch 5, 1973. ~hile the trlchloro-di~luorobenzenes ha~e excellent thermal ~tability in their own right, the~e i5 need ~or ~urther improvement to prolong their use ln this and other heat tra~s~er applications a~d to preven~
the *orma~ion o~ undesirable biphenyl t~pe products on decompo-sition, and also o* gaseous by~product.
Accordin~ to -the pres~nt invention a method is provided ~or thermally stabilizing a trichloro~i~luorobenzene heat trans~er fluid as the ~luid is cycled through a temperature gxadlent including temperatures abo~e about 260C while in contact with hea~ trans~er surfaces, particularly o~ steel, aluminum or ~ nickel~base allo~, wherein said ~luid is contacted wlth a thermally stabillzing amount o~ an acid accepting solid :~
selected from alkaline eaxth ~arbonates and alkali and alk~line earth bora~es and phosphate~. Pre*erably the acid accepting solid is addad in amounts ~rom 1 to 5 percen~ by weight o~ the heat transfer ~luid.
It is contemplated that the heat trans~er met~ods of thi~ invention will be particulaxly use~ul in dense ~apor tur-bines where the thermodyn~c advantages o~ trichlorodi~luoro~
ben~ene can be extended to higher temperatures and longer oper~
ating times becau~e o~ the better thermal stability and lower corrosivity a~forded b~ the stabiliæers. The use~ul temperature range ~or this application is generally between the normal :
boilin~ point and the critical temperature of the fluid. An ~,~ . . . .

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~3~65 especially useful embodiment of the method utilizes liquid trichlorodifluoxobenzene at temperatures from about 300C to about 400C in a rotary engine such as described in the U.S.
Patent 3 613 368 lssued to William A. Doerned and co~nonly assigned~ It will also be understood that trichlorodifluoro-benYene u~ed in the ma~ner taught by this invention can be advantageously applied in other heat exch~nge processes not in~olving vapor impul~ion, and generally operating at lower temperatures. It wlll be appreciated that problems of thermal stability and corrosivity to metals ~re more pronounced at higher terhperatures. Wh~le the advanta~es o~ this method become more quickl~ apparent at higher temperatures, the alternative economic advantage of extended operability at lower temperatures can be readily in~erred and is none the less i~portant, While the individual isomers of trichlorodi~luoro-benzene are use~ul rluids in the practice o~ this invention economic advantages will generall~ recommend the use of isomer mixtures. Among the six positions isomers of C6XC13F2 there is little di~erence in boiling pointO about 203C, and critical ~emperature around 430C. The smallest liquid range (mp 50) is shown by 2,3,4-trlchloro-1,5-dl~luorobenzene; the largest by 1,2,5-trichloro-3,4-di~luorobenzene (mp -25C) which isomer also shows especially good thermal stability. The p~rticular mix of isomers usual~y available will be determ~ned by the synthesis procedure used. A convenient method o~ preparation is by heating pentachlorobenzene with potassium ~luoride in sul~olane (tetramethylene sulfone) as described in the aforementioned French Patent 72.29277. This process typically yields a mlxture com-prising

3~- 12.2% 1,3,5-trichloro-2,4-difluorobenzene 58~2% 1,2~-trichloro-3g5-di~luorobenzene - , , , , , , ;~

~;~;3711~6~i 6.3~ 1~2,3-trichloro-~,5-dl~luorobenzene 15.6~ 1,2,5-trichloro-3,4-di~luorobenzene 6.8% 2,3 9 4--trichloro-lg5-di~luorobenzene O.9% 1,3,4-~richloro-2~5-di~luorobenzene This mixture has a melting point o.f about -40C, a boiling point of about 203C a~d a critical.temperature o~
~27~C. Mixtures o~ other isomeric proportions will also bene~i~
from the stabilizing methods disclosed herein, as will mi~tures o~ trichlorodi~luorobenæene with other halobenzene fluidsq The solid stabilizers ~or use with trichlorodiMuoro-benzene are generally recognlzed as acid acceptors a~d may :. .
~unc~ion by a mech~ni~m rel~ted to the removal o~ catal~stic a~id : :
species which are intermediate~ in the thermal decomposition or in the corrosi~e attack on metals. Generally both the thern~ : ~
decomposition o~ the ~luid to ~orm the ~desirable polychlori- . -nated biphenyl~ and it8 corrosivity to metals are simult~neously reduced by the action of th~ stabillzer~. However, not all alkali~e substances are e~ective. Cextain ~trongly.alkaline substances such as Na CD3 and CaO may actually promote the decom- :~
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posi-tio~ of the chlorofluoroben~ene~
By alkall metal is meant all the metals o~ perlodic group IA o:E a~omic n~unber 3 to 55. B;r alkaline earkh metal is .
meant magnesium as well aB calcium, strontium~ a~d barium but ~ .
the term as u~ed herein does not include the metal ber~llium. .
The term "phosphate" includes sondensed phosphates .~
and polyphospha~e~ as well as the si~pl~ phosphates9 Na3PO~9 -:
Na2HP0~ and NaH~P04.
The solid ~nor~anic 5tabilizers of this in~ention may be b~ought ln co~tact with the ~apor phase but prefer~b~y ~h y ...
30 are brou~ht in c~ntact with the liq~id phase o~ the he~t trans- :
~er ~luld to be most e~e~tive. It i~ believed that thermal . ,~ - -. . - . .
:. , ~i5 decomposition and corroslve attack upon metals under heat transrer condltions occurs prlmarily in the llquid phase or at the llquid-vapor interrace rather than in the saturated vapor.
The stabllizers may be used as powders or pellets dlstributed throughout the liquld volume or they may be conrined to a sepa-rate region through which ~he rluid rlOws. AlternatiYely the stablllzing compositions ~ay be present as a coatlng on the metal surface. Such a coating provides intimate contact at an ef~ec-tive site, but it may also reduce the erriciency of heat transfer.
Still another method of stabilizer disposition is to rorm the solid, with binders if necessary, into a shaped article such as an open grid which can then be fired to form a sel~-supporting shape approprlate to the configuration o~ the heat transfer system. The optlmum distribution o~ the stabillzer can readily be determined ~or any of the wlde range o~ heat transfer con~ig-urations in which such fluids are used.
The amount Or stabilizer needed to pro~ide adequate protectlon against thermal decomposltlon can be qulte small and wlll ln general depend upon its distrlbution and on the u~e conditions. A significant improvement in stabllity can be measured whenever the bulk temperature Or the rluid i~ at least about 260C. Amounts of the order Or flve percent by weight o~
~the heat trancrer rluid ln the system are usually adequate even at temperatures as hlgh as 400C. For less severe~use conditions i amounts as ~mall as one percent or less are uæe~ul ~or lmproving ~ ~ th*rmal stabillty.
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, The ~tabillzation is e~fective when the liquid ls contained in most materials Or conskruction appllcable ~or heat exchange surraces. Steels, including high-temperature, high-strength alloys such as high chromlum steels and nickel-base } alloys can be used, as well as the more ~conomlcal plain carbon . .
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steel whlch is servlceable at lower temperatures and ~tre~ses.
Stabilization is also e~fected in the presence of aluminum.
~ven in inert vessels ~uch as glass, the high temperature utility of trichlorodi~luorobenzene iæ increased by a reduction in the rate o~ formation o~ polychlorinated biphenyls ~hich constitute a toxlc hazard o~ long persistence.
r~hls inv~ntion is ~urther illustrated by the following example~, which should not, however, be construed as ~ully dilineating the seope o~ this discove~y~
B _ ~n the following exa~ples the thermal stabili~y o~
trichloroaifluoro~en2ene was determined by sealing th~
liquld i~omeric mixture obtained from pentachlorobenzene-EF ~ynthe~is in about one gram quantltieæ into small PYREX*
glass tubes with small amounts (usu~lly 0.05 g) of the ~-stabilizing soll~ A small metal co~pon usually about 2 3 cm2 completely lmmersed in th~ liquid was used to test corrosive attack. me sealed tubes along with controls containing un~tabillzed liquid were kept ~or the indicated time at a temperature o~ 350 ~r 400C~ A~ter cooling and opening the tube~ the weight lo~s by corrosion was mea~ed a~ter washing, lightly rubbing with a paper towel to remove any ~uperficial coating and drying. The extent o~ thermal decompo~ition o~ the liquid was indlcated b~ ~ ~
measuring;the amoun~ of blphenyls in the liquid by standard .:
~apor phase chromatographic techniques.
~ , ' " -, A 1.0 g ~ample o~ a mixture o~ C6HC13F~ iæomer3 was kept in contact with a coupon o~ type 430 stalnles~ :
~teel at 350C ~or 100 days. At the end o~ that time de~ote~ trade mark - , 1~ ,'' . ' ~ ' "

-1~1370~5 corrosion o~ the stainle~s steel amounted to 3.3 mg met~l lost per cm2 o~ metal surface. Biphenyl ~ormation amount~d to 1.3%. Th~ ~ame experiment was carried out but with 0.05 g Na2BL~07 present. Now corrosion was measured a~
0.00 mg per cm and biphen~l rormation amounted to 0.13%.
E~ample 2 The conditions of e~ample 1 were ~epeated (350 ~or 100 days), but the test metal wa~ type P-ll steel (1.3%
Cr, 0.5% Mo). Thi~ showed a corro~lon o~ 5.8 mg per cm2 and biphenyl formatlon of' 1.3%~ Repeating the experiment ~ith 0.05 g Na2B40~ preæent ~ave a corrosion of 0.83 mg per c~2 ~nd biphenyl ~ormation of 0.~7%. :
Ex m ~3 :, ~
A loO g æample o~ a mixture o~ C6HC13F2 isomeræ wa~
placed in contact with a coupon o~ nlckel and kept at 400 ~or 7 da~. The m~tal showed a weight loss corresponding :
to 1.0 ~g per cm2 and biphenyl ~ormation of 0.54%. The ~ -experiment was re~e&ted except that 0.05 g CaC03 ~a~ added.
No~ the weight lo~æ wa~ 0.00 mg per cm2 and biphenyl ~orma-tion 0~39~.
Exa~le 4 Under the condition~ o~ Example 3, a sa~ple of INCONEL* X_750 (70% Nl9 16% Cr, 2.5% Ti, 11.5~ Fe) had a wel~ht loss o~ 1.0 mg per cm2 a~d biphenyl ~ormatlon o~
3.~% ln the ab~ence o~ additive. With O.05 g o~ GaC03 the weight loss was o.o6 mg per cm2 and biphenyl ~ormation WB~ 2.4%. .
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Under tha co~ditlons o~ E~ample 3, type 1018 steel had a weight loss o~ 6.2 mg per cm2 and biphenyl * denotes trade ~rk ~:
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~3 ~ormation o~ 6~5~o With 0.05g of CaC03 the welght los~
~as 4.4 mg per cm2 and biphen~l formation 3.4%.
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Under the conditions o~ Example 3, a sample o~
type P-ll steel had a weight loss o~ 6.7mg per c~2 and biphenyl ~ormation wa 4.L~% without additiYe. ~ith 0.05g Na5P3010 the wei.~ht loss was 5.8mg per cm2 and the biphenyl formation was 2.7~. In the presence of 0~01 g Na2B407 the weigh~ loss wa~ 4.8mg per cm2 and bipher~l ~or~ation was 3.7%. ~ith 0.05~ Na2B407 comparable re~ults were 2~8mg per cm2 and 3.8% biphenyl.
Example 7 ~.
A l~Og sample o~ C6HC13~2 isomers was placed in con~act with type P-ll teel at 400 ~or 28 days. We~ht los~ amounted to 11.8mg per cm2 and biphenyl ~ormation ~as 22~. In the presence o~ 0.05g Na2B~07 the weight loss was 11.8m~ per cm2 ~nd biphenyl rormation wa~ 8.3%. In the presence o~ OaO5g CaC03 ~eight loss was 9.5mg per cm2 and biphenyl ~ormation was 7~9%.
When the tests were completed a~ter 28 days the ~ample tube~ were opened within a sealed system so tha~ any gas developed durlng the te~t could be mea~ured. ;-~he unstabilized liquid as a result o~ decomposition ~ -generated 7.6 cc (STP) o~ hydrogen gas, while no measurable g&~ was found in the samæle tubes stabilized with Na2B~ 7 :~
or CaC03. Reduced hydrogen e~olution ls particularly important ~or ~luids used in a Ranklne cycle engine since the non-condensable gas otherwsie tends to bl~nket the tube area a~d reduce condenser capaci~y.
Example 8 A l.Og ~ample of C6HC13P2 isom~ir mlxture in a :~

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7~65 sealed gla~s tube at 400~ ~or 7 d~y~ ~n the absence of metal was 2.0% co~erted to biphen~l In the pre~ence o~ O.05 g of Na5P3010 the conver~ion to biphenyl was O.51%.
In the presencc o~ Na2B4p7 biphenyl ~ormatlon wa~ 0.79%;
with ~a2P~013 0.72%, with I,i3PO~ and with CaC03 0.052%.

Portions of a C6HC13F2 isomer mixture 0.3 cc each were sealed i~to each of th~ee P~EX gla~s tube~
contalning aluminum couponæ weighing about 0.2 g. One tube contained 0.05 g CaO; another contained 0.05 g CaC03;
the other a control, contained no additional solid. The tub~s were ~ealed ~nd kept at 400C. B~ the 14th day the Al coupon was coated in the control tube. The aluminum ln the tube with CaC03 was ~till very clean~ me tube containing cao waæ completely black with no liquid any longer ~isible. After 28 da~s the control tube was quite black with a heav~ coating on the couponl while the tube containing CaC03 ~ppeared to be unchanged. I~hen the tube~
were open0d and the liqulds analysed the control tube contained 0.42~ biphenyls while the tube containin~ CaC03 had only 0011% biphenyl~
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A 3 g sample of C6HC13F2 was placed in a tube of 80 cc ~olum~ together with two coupon~ of 26~1 sta~nle~ 6teel ha~ing -~
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a to~al surface 6.8 cm2 and ~eight o~ 309605 g. me air wa~ ~
, evacuated ~rom the tube and ~ealedO Another 80 cc tube wa6 loaded with 3 g o~ C6HC13F2, two coupons of 26~1 stainless steel (6~8 c~2, 307~06 g), a~d 0.1 g o~ ~a2B~07, e~acuated and ~ealed.
Both o~ the tubes were heated at 432C ~or 7 days. Under the~e ~ -conditions, the C6HC13F2 i~ complet~ly in the gas phase at 10 _ g _ , , , ~ , , . ;~

~0;317~1165 atmospheres pressure. At the end o~ this tlme, the metal coupons were wa~hed in water and acetone. me coupons that were in the presence o~ ~a~B407 showed a weight gain o~ Ool~g whereas the coupons that had no add~tive showed a weight loss o~ 5.1mg. The biphen~l ~nalysls showed a decomposition o~
0022% ~or the sample wlth Na2B1~07 while the control showed 0.41% decomposition.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of heat transfer wherein trichloro-difluorobenzene heat transfer fluid is cycled through a thermal gradient including temperatures above about 260°C., while being circulated over heat transfer surfaces, particularly of steel, aluminum or nickel-base alloys, the improvement which comprises contacting the trichlorodifluorobenzene heat transfer fluid with a thermally stabilizing amount of an acid accepting solid selected from alkali metal borates and phosphates and alkaline earth metal carbonates, borates and phosphates, said alkaline earth metal being selected from the group consisting of magnesium, calcium, strontium and barium.

2. Method according to Claim 1 wherein 1-5% by weight of the acid accepting solid is present.

3. The method of Claim 2 wherein said acid acceptor is sodium borate.

4. The method of Claim 2 wherein said acid acceptor is calcium carbonate.

5. The method of Claim 2 wherein said acid acceptor is Na5P3010.

6. The method of Claim 2 wherein said acid acceptor is Na6P6013.