Thermal analysis of the mixture of laboratory and commercial grades hexadecane and tetradecane

INT. COMM. HEAT MASS TRANSFER VoL 19, pp. 1-15, 1992 Printed in the USA

0735-1933/92 $5.00 + .00 Copyrighte1992 Pergamon Press plc

THERMAL ANALYSIS OF THE MIXTURE OF LABORATORY AND COMMERCIAL GRADES HEXADECANE AND TETRADECANE

Eunsoo Choi, Young I. Cho, and Harold G. Lorsch Department of Mechanical Engineering and Mechanics Drexel University, Philadelphia, PA 19104

(Communicated by J.P. Hartnett and W.J. Minkowycz)

ABSTRACT Tests were performed to characterize the thermal behavior of hexadecane and tetradeeane as a potential phase change material (PCM) to enhance the convective heat transfer performance of district cooling system. Commercial grade as well as laboratory grade of the PCMs were tested to compare their thermal characteristics. In order to vary the melting temperature of the mixture, hexadecane and te~adecane were mixed in different fractions, and their thermal behavior was experimentally evaluated. Test results for melting temperature and fusion energy for laboratory grade substances showed good agreement with data in the literature. However, values for commercial (i.e., technical) grade substances were found to be considerably lower than for laboratory grade materials. In the ranges from 0% to approximately 10% hexadecane in tetradeeane (or vice versa), the fusion energy of the mixtures was comparable to that of the pure substances. The melting temperature was found to decrease as the fractions of the two substances became more equal. This could be expressed by a simple equation.

The large amount of latent heat contained in a material during phase change has many applications in various heat transfer systems. It can be used as a high density energy storage material in a solar energy system [1]. It can also be used as a cooling material in electronic cooling technology or in a high power lithium battery [2]. Recently, a technique using microeneapsulated phase change material (PCM) partieles was introduced to enhance the convective heat transfer behavior in a circular robe flow [3]. One of the most important factors for the proper selection of a PCM is the phase transition temperature. For n-alkanes, these temperatures are in the range from 5.8oc to 65.4oc depending on the length of the CH2 chain in the alkane molecule, increasing with increasing chain length. The molecular structures of telradecane, pentadeeane, and hexadecane are shown in Fig. 1. The

2

E. Choi, Y.I. Cho and H.G. Lorsch

CH3 - CH2- CH2 '-

Vol. 19, No. 1

CH2-CH 3

Tetradecane

CH 2,i

Pentadecane

12

- CH2~-

13

CH3- CH2~-

CH 3

CH2-CH 3

Hexadecane

14

FIG 1 Molecular structure of n-alkanes melting temperatures of the laboratory grades of these substances are 5.8, 9.9, and 18.1oc, respectively [4]. Kauranen et al. [5] mixed two carboxylic acids, which have different melting temperatures, and measured the melting temperature of the mixture. They used a eutectic point as a proper mixing fraction to obtain isothermal melting. They obtained ten different melting temperatures from five different carboxylic acids by mixing two of them in various fractions. As a part of the development of advanced low-temperature heat transfer fluids to be used in district cooling systems, present tests were performed to characterize the thermal behavior of the mixture of hexadecane and tetradecane with the specific goal of using PCM particles suspended in water at temperatures used in district cooling loops. The temperature range of the working fluid of district cooling systems is between 3oc and 13oc. PCM mixtures must be selected to have transition temperatures compatible with the design temperature chosen for a particular primary cooling loop. This requires testing of various mixtures. Since pentadecane costs twice as much as hexadecane or tetradecane, and the prices of the laboratory grade n-alkanes are approximately four times those of commercial grades, commercial grade hexadecane and tetradecane were chosen for the present test program. Exnerimental Svstem and Calibration A duPont 1090 Differential Thermal Analyzer (DTA) was used in connection with a duPont 910 Differential Thermal Calorimeter (DSC) [6] in order to measure melting temperatures and heats of fusion of PCM mixtures. Because the average mass of the samples required by DSC system is only 2 to 8 rag, a Mettler AE166 scale with an accuracy of 0.01mg was used in order to obtain results with three significant figures. A typical output curve of the DSC system is shown in Fig. 2. At first, the DSC cell containing a PCM sample is cooled down to a much lower temperature than the expected melting temperature of the sample. As the heating block is heated at a constant rate, the temperature of the

Vol. 19, No. 1

ANALYSIS OF HEXADECANE AND TETRADECANE

3

reference sample pan also increases at a constant ram. If there is no phase change in the PCM sample pan, the temperature difference between the PCM sample pan and the reference sample pan will produce an almost-horizontal straight line, as r¢pmsented by the straight dashed line in Fig. 2. If there is any phase change in the PCM sample pan, the temperature difference between the two sample pans will result in a curve that deviates from the straight line. The area between the straight dashed line and that deviated curve represents the enorgy consumed for phase change, which is integrated numerically by a program built into the DSC. Two phase changes are shown in Fig. 2, which is a typical output for a mixture, in this case, of 40% hexadecane and 60% tetradecane. The first phase change represents a solid-solid phase change, whereas the second phase change represents a solid-liquid phase change or melting. The phase change temperature is obtained by extending a tangent line to the deviation curve and intersecting it with the original linear portion of the curve, as is illustrated in the figure. In order to calibrate the DSC system for the heat of fusion measurements, at least two sets of calibration data for two known substances are required. One set was represented by these

T

2nd Phase Change 1st Phase C h an g e

E o

o

o -r

Phase I ~ Fusion Energy Change I Temperature II ,L

Temperature (°C) FIG 2 Typical DSC output diagram

r

4

E. Choi, Y.I. Cho and H.G. Lorsch

Vol. 19, No. 1

measurements for the water-ice transition, the other set consisted of data for indium provided by the manufacturer. The cell constants for any specific melting temperature were then obtained by a linear interpolation with respect to temperature between the cell constants for water and indium. In order to determine the accuracy of the DSC system, nine samples with different amounts of distilled water were prepared and tested. The melting temperatures of the water samples are shown in Fig. 3. Although the samples were of different sizes, the measured values of melting temperature were consistently found to be 0°C; their standard deviation was 0.54oc. Figure 4 shows the measured fusion energies for the water samples. The solid horizontal line represents the fusion energy of water given in a handbook [4], which is equal to 333.67 J/g. The standard deviation of the measured values of fusion energy was 15.3 J/g, which is 4.6% of the mean value. In the range of 2 to 10 mg, there was no effect of sample size on the measured values of either melting temperature or heat of fusion. Results and Analyses

Laboratory grade (99% pure) hexadecane and tetradecane were used to prepare nineteen PCM samples with various fractions of hexadecane and tetradecane. The test results shown in Fig. 5 illustrate the effect of varying the proportions of the mixture. Pure hexadecane exhibits a single phase change as shown in Fig. 5a. In Fig. 5b, when 4% of tetradecane is added, a single phase change still occurs, but the onset temperature, i.e., the melting temperature, is changed 10

I

I

I

I

{J o

5 L_ ,4,=¢

a~ L..

Q.

0-6----0

0

E ¢)

~

-'3---0- 0

I-= e=

.m

-5

m

a) "10

,

0

i

2

,

I

,

4

i

6

,

I

8

,

10

Mass (mg) FIG3

Measured melting temperatures for water/ice samples of different size

Vol. 19, No. 1

ANALYSIS OF HEXADECANE AND TETRADECANE

5

400

O0

0

O

300 >, O~ Im

rUJ tO

200

IL

100

333.67 J/g Present Data

0

0

i

0

I

2

i

I

4

I

I

m

6

I

i

8

Mass (rng) FIG 4 Measured heats of fusion for water/ice samples of different size significantly. When larger amounts of tetradecane are added (i.e., 8% and 14.5% in Fig. 5c and 5d), the shape of the curves changes, indicating that a molecular rearrangement in the solid phase or a solid-solid phase change occurs at the temperature lower than the melting temperature. This solid-solid phase change increases as the fraction of tetradecane increases, resulting in a lowering of the primary melting point and the heat of fusion of the mixture. A similar effect is shown in Fig. 5j ~ 5f. The pure tetradecane in Fig. 5j shows a single phase change temperature (melting temperature). When small amounts of hexadecane are added, both the melting temperature and the heat of fusion are decreased. In the range of 94% to 60% mixtures, multiple phase changes take place. In the range of 96% to 100% mixtures, the curves suggest that the mixture can be treated as a homogeneous solution. The melting temperature of the mixture is significantly lowered as the amount of hexadecane increases. The phase change temperatures of all the tested samples are plotted in Fig. 6. The horizontal coordinate indicates the concentration of tetradecane by weight. Thus, 0% (left margin) indicates pure hexadecane, and 100% (right margin) indicates pure tetradecane. The measured data for the pure tetradecane and hexadecane show good agreement with published data [4]. The first phase change represents the solid-liquid phase change of the mixture, the second phase change represents the solid-solid phase change in a hexadecane-rich mixture, and the third phase change represents the solid-solid phase change in a tetradecane-rich mixture. In the range of 8% to 94% mixtures, a solid-solid phase change always occurs. The total phase-change

6

E. Choi, Y.I. Cho and H.G. Lorsch

(a) 0 %

(f) 60 %

(b) 4 %

(g) 80 %

(c) 8 %

(h) 94 %

(d) 14.5 %

(i) 96 %

(e) 20 %

(j) 100 %

Vol. 19, No

FIG 5 DSC output diagrams for different mixtures of laboratory grade hexadecane and tetradecane.

Percentages indicatethe weight fractionof tetradccane.The abscissarepresentstemperature, the ordinate represents heat flow rate.

Vol. 19, No. 1

ANALYSIS OF HEXADECANE AND TETRADECANE

F

Io

"~

0

7

t~ L-

Q.

E

-10

o-D--•

I.-. o) r-

-20

P

1st phase change 2rid phase change 3rd phase change Ref. 4 j ~

I~

/ -

D m u

=S

-30

4 0

,

0

I

,

20

I

40

,

I

,

60

I

80

,

100

Percentage of Tetradecane FIG 6 Measured melting temperatures for mixtures of laboratory grade hexadecane and tetradecane energy of the mixtures as a function of the mixture is shown in Fig. 7. It indicates that the energy for PCM mixtures is smaller than for pure substances. In the range of 8% to 94% mixtures, the solid-liquid phase change energy is decreased by the amount of the solid-solid phase change energy. In two extreme ranges (i.e., 0% to 10% and 90% to 100%), the actual fusion energies of those mixtures are larger than those in the range of 10% to 90%. The melting temperature in these two ranges decreases as a linear function of molarity and can be correlated by a simple equation. Kauranen et al. [5] used the following well-known relation [7] to represent the melting temperature of a mixture:

AHoi ..I._.1) Ln x i = - - ~

('~0,i Ti

where xi

= mole fraction of the main component i of the mixture

AH0,i = latent heat of fusion for pure component i (J/mol) = melting temperature of pure component i (K) To,i

(1)

8

E. Choi, Y.I. Cho and H.G. Lorsch

Vol. 19, No. 1

300

250 .-j >,

200

q)

rILl

¢0 u)

150

100

U.

50

o-•

O 0

i

I

I

20

I

I

40

Present data Ref. 4 I

60

=

I

=

80

100

Percentage of Tetradecane FIG 7 Measured heats of fusion for mixtures of laboratory grade hexadecane and tetradecane Ti

= melting temperature of a mixture containing component i (K)

R

= universal gas constant (8.315 J/K mol)

From the above relation, the decrease of the melting temperature can be represented by the following equation: T0,i - T i = T0. i [ 1-

1 ] 1- R T0,i Ln (1-xj) AH

(2)

where xj

= 1-xi = mole fraction of impurity

If xj is a very small value, the following simple approximate relation can be obtained: T0, i - T i = C R ( T0,i )2xj AH = Kf,i xj

(3)

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ANALYSIS OF HEXADECANE AND TETRADECANE

9

where C

= arbitrary constant

Kf,i

= melting temperature depression constant of main component i in the mixture

This means that, when a small amount of a component, the solute, is added to a large amount of the main component, the solvent, the melting temperature decrease is proportional to the mole fraction of the added solute. The melting temperature depression constant, Kf, of hexadecane was calculated from the data in Fig. 6 by linear regression, and the result is shown in Fig. 8. The molality is the number of the molecules of a solute in 1 kg of the solution. In this case hexadecane is the solvent and tetradecane the solute. Kf = -8.74°C/mol

(for laboratory grade hexadecane)

The same method was used to calculate the melting temperature depression constant, Kf, of telradecane, and the result is shown in Fig. 9. In this case, tetradecane is the solvent and hexadecane the solute.

0

i

i

i

lO

E

y = 17.14 - 8.74x

G)

i-

O)

e.

5

,i

q)

:S 0 0.0

i

I

0.2

i

I

0.4

,

I

0.6

,

0.8

Molality of Tetradecane FIG 8 Linear regression for the melting t~nperaturc depression constant of laboratory grade hexadecane (solvent) and laboratory grade tetradecane (solute). Kf = - 8.74oC/mole

10

E. Choi, Y.I. Cho and H.G. Lorsch

Vol. 19, No. 1

10

O i._

-I t~

en

E I-

0 e--~

y = 5.27 - 15.33x

=E "5

0.0

i

I

0.1

i

I

i

0.2

I

0.3

I

0.4

Molality of Hexadecane FIG 9 Linear regression for the melting temperature depression constant of laboratory grade tetradecane (solvent) and laboratory grade hexadecane (solute). Kf = - 15.33°C/mole Kf = -15.33°C/mol

(for laboratory grade tetradecane)

In the range of small concentrations of either component, the mixtures behave like pure substances, which is especially pertinent for PCM slurry applications where PCM particles are very fine. In that range, the melting temperature change can be predicted by a simple relation. If hexadecane is used as the solvent, the melting temperature can vary from about 13oc to 17.2oc. If tetradecane is used as the solvent, the melting temperature can vary from about l o c to 5.4oc. The melting temperature of mixtures between other combinations of n-alkanes can be obtained by this method. For the advanced heat transfer fluids being developed for district cooling applications, commercial grade PCMs must be used instead of laboratory grade because of the high cost of laboratory grade materials. Therefore, the above characterization tests were also performed with commercial grade PCMs. The trends of DSC curve changes are similar to those given in Fig. 5. The melting temperatures of commercial grade hexadecane and tetradecane are much lower than those of the laboratory grade substances. Test results of melting temperatures for various mixtures of the laboratory grade and commercial grade hexadecane and tetradecane are shown in Fig. 10. As

Vol. 19, No. 1

ANALYSIS OF HEXADECANE AND TETRADECANE

30

i

i

13•

o

O v

4) !._

I

Laboratory Grade Commercial Grade Ref. 4

o--A

11

20

d.-I

t~ Lo. E q)

10

ho~ en

4)

=S

"10

,

0

I

20

i

I

40

,

I

i

60

I

80

,

00

Percentage of Tetradecane FIG 10 Melting temperatures of mixtures of laboratory grade and commercial grade hexadecane and tetradecane shown, the melting temperature of the commercial grade hexadecane is 5. I°C lower than that of the laboratory grade, and the melting temperature of the commercial grade tetradecane is 3.7°C lower than that of the laboratory grade, suggesting that there are significant amounts of unknown impurities in the commercial grade substances. The fusion energies of the mixtures of commercial grade PCMs are also a little lower than those of the laboratory grade materials as shown in Fig. 11. The melting temperatures and fusion energies of both laboratory grade and commercial grade PCMs are summarized in Table 1. For small fractions of either component of commercial grade, the melting temperature decreases linearly like the laboratory grade. The melting temperature depression constants of the commercial grade hexadecane and tetradecane were obtained by the linear regression method. They are compared with those of the laboratory grade materials in Figs. 12 and 13. Kf = -6.57oC/mole Kf = -13.08°C/mole

(for commercial grade hexadecane) (forcommercial grade tetradecane)

12

E. Choi, Y.I. Cho and H.G. Lorsch

Vol. 19, No. I

300

250 O~ .-j

200

>,

Co Im rIJJ e-0 "~

150

100

u.

o--- Laboratory Grade EP-- Commercial Grade • Ref. 4

50

0

,

0

I

,

20

I

i

40

Percentage

I

60

,

I

,

80

100

of T e t r a d e c a n e

FIG 11 Heats of fusion of mixtures of laboratory grade and commercial grade hexadecane and tetradecane TABLE 1 Melting temperature and fusion energy for laboratory grade and commercial grade tetradecane and hexadecane n-Alkane

Melting temperature (°C)

Heat of Fusion (J/g)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ref.[4]

Measured lab. com.

Ref. [4]

Measured lab. com.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tetradecane Hexadeeane

5.8 18.1

5.4 17.2

1.7 12.1

227 236

234 224

206 185

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* lab. : laboratory grade * com. : commercial grade If the commercial grade hexadecane is used as a solvent, the melting temperature can vary from 9 o c to 12.1°C. If the commercial grade tetradecane is used as a solvent, the melting temperature can vary from -5°C to 1.7°C. The melting temperature depression constants and available melting temperature ranges of both laboratory and commercial grades are summarized in Table 2.

Vol. 19, No. 1

ANALYSIS OF HEXADECANE AND TETRADECANE

20

E

15

I

I

~

13

I

°'"

"'~..n

E

I-

ol I=

y=17.14-8.74x y=12.43-6.57x Laboratory Grade Commercial Grade

II i

........ O []

G)

:S 0

0.0

*

i

I

0.2

I

i

0.4

I

,

0.6

0.8

Molality of Tetradecane FIG 12 Linear regression of the melting temperature depression constants

TABLE 2 Melting temperature depression constants and available melting temperature range for laboratory grade and commercial grade tetradecane and hexadecane n-Alkane

Melting Temperature Depression (°C/mole)

Available range (oC)

...............................................................................................

lab.

com.

lab.

com.

...............................................................................................

Tetradecane Hexadecane

-15.33 -8.74

-13.08 -6.57

1-5.4 13-17.2

-5-1.7 9-12.1

...............................................................................................

* lab. : laboratory grade * com. : commercial grade

Experimental results for the melting temperature and fusion energy of laboratory grade hexadeeane and tetradecane show good agreement with handbook data. Mixtures of these two substances in the ranges of about 10% to 90% by weight clearly exhibit multiple phase changes, including solid-liquid phase change and solid-solid phase change. The solid-liquid phase change

14

E. Choi, Y,I. Cho and H.G. Lorsch

10

I

I

Vol. 19, No. 1

I

y=5.27-15.33x

........

¢.)

y=1.92-13.08x Laboratory Grade Commercial Grade

O

O v

[]

G) L_ -I

5q

t~ 3,. 4) D.

o

E . . . . "El. . . . "O,.,

Ic

.m 4,.,* D

4)

:S

-5 0.0

0.1

0.2

0.3

0.4

Molality of H e x a d e c a n e FIG 13 Linear regression of the melting temperature depression constants energies of these mixtures are much smaller than those of the pure substances because of the solidsolid phase change. In the ranges from 0% to 10% and from 90% to 100%, a single phase change, i.e., melting, is dominant. The fusion energies in these ranges are fairly high even though they are slightly lower than those of the pure substances. The melting temperature changes are significant and predictable by simple relations. The melting temperature depression constants are -8.74oC/mol for laboratory grade hexadecane, and -15.33 oC/mol for laboratory grade tetradecane. The results for commercial grade hexadecane and tetradecane show similar trends in the shapes of the phase change curves. However, their melting temperatures are significantly lower than those of the laboratory grade materials. In the ranges from 0% to 10% and from 90% to 100%, the melting temperature decreases linearly with fraction, as do laboratory grade substances. The temperature ranges for linear behavior are -5-1.7oc for commercial grade tetradecane, 1-5.4oc for laboratory grade tetradecane, 9~12.1°C for commercial grade hexadecane, and 13~17.2oc for laboratory grade hexadecane. The melting temperature depression constants are 6.57oC/mol for commercial grade hexadecane, and -13.08oC/mol for commercial grade tetradecane.

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ANALYSIS OF HEXADECANE AND TETRADECANE

15

Acknowled~emenl~ The authors acknowledge the financial support of the U.S. Department of Energy, Office of Buildings and Community Systems, under Grant No. DE-FG01-90CE26602.

References Lane, G., ed., Solar Heat Storage: Latent Heat Materials, Vol. I: Background and Scientific Principles, CRC Press, 1986. .

Cho, Y., and Chee, D., "Thermal Analysis of Prirnary Cylindrical Lithium Cells", Journal of the Electrochemical Society, Vol. 138, No.4, pp.927-930.

.

Charunyakorn, P., Sengurta, S., and Roy, S.K., "Forced convection heat transfer in microencapsulated phase change material slurries : flow in circular ducts", Int.J. Heat Mass Transfer, Voi.34, No.3. pp.819-833, 1991.

.

Lane, G., ed., Solar Heat Storage: Latent Heat Materials, Vol II: Technology, CRC Press, 1986.

.

Kauranen, P., Peippo, K., and Lund, P.D., "An Organic PCM Storage System with Adjustable Melting Temperature", Solar Energy, Vol.46, No.5, pp.275-278, 1991.

.

"Instruction Manual, 910 Differential Scanning Calorimeter (DSC) System," duPont Company, 1980.

7.

Castellan, G.W., Physical chemistry, 2 ed., Addison-Wesley, Reading, MA, 1971.