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Measurement of latent heat of tetra-n-butylammonium bromide (TBAB) hydrate

Tatsunori Asaoka*, Hiroyuki Kumano, Maki Serita

Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara-shi, Kanagawa 252-525, Japan

a r t i c l e i n f o

Article history:

Received 6 November 2012

Received in revised form

10 December 2012

Accepted 12 December 2012

Available online 19 December 2012

Keywords:

Thermal storage

Latent heat

Hydrate

Ice slurry

* Corresponding author. Tel.: þ81 42 759 621E-mail address: asaoka@me.aoyama.ac.jp

0140-7007/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.ijrefrig.2012.12.011

a b s t r a c t

Tetra-n-butylammonium bromide (TBAB) hydrate has favorable properties as a thermal

storage material. For example, the phase change temperature, which is from 0 to 12 �C, is

suitable for use in air conditioning, and the latent heat is relatively large. Thus, the ther-

mophysical properties of TBAB hydrate, such as latent heat, are of considerable interest. In

this study, a large size of pure TBAB hydrate crystal was formed and the latent heat of

TBAB hydrate was measured by melting the crystal. The latent heat of TBAB hydrate type 1

was estimated to be 210 kJ kg�1, while that for type 2 hydrate was 224 kJ kg�1. These values

agree with literature values estimated from the variation of specific enthalpy of TBAB

hydrate slurry, but are 10% larger than the other literature values measured using DSC.

ª 2013 Elsevier Ltd and IIR. All rights reserved.

Mesure de la chaleur latente de l’hydrate du bromure de tetra-n-butylammonium

Mots cles : accumulation thermique ; chaleur latente ; hydrate ; coulis de glace

1. Introduction

Tetra-n-butylammonium bromide (TBAB) hydrate has known

advantages as a thermal storage material for air-conditioning

applications. For example, the phase change temperature,

which is from 0 to 12 �C, is suitable for use in air conditioning,

and the latent heat is relatively large. Moreover, TBAB can be

utilized as a secondary refrigerant, because it is easy to

produce the hydrate slurry, which has high fluidity. Darbouret

et al. (2005) investigated the flow characteristics of TBAB

1; fax: þ81 42 759 6212.(T. Asaoka).

ier Ltd and IIR. All rights

hydrate slurry. Wenji et al. (2009) and Ma et al. (2010) reported

the heat transfer behavior of TBAB hydrate slurry. One of the

authors of the current study also performed further investi-

gation on the flow and heat transfer characteristics of TBAB

hydrate slurry (Kumano et al., 2011a, 2011b).

Relating to those studies, the thermophysical properties of

TBAB hydrate such as latent heat and hydration number are

needed. Oyama et al. (2005) measured the latent heat of TBAB

hydrate by using differential scanning calorimetry (DSC).

However, it is difficult to measure the latent heat of impure

reserved.

0 10 20 300

5

10

Concentration of solution, wt%

Tem

pera

ture

, o C

Type 2

Type 1

Fig. 1 e Relationship between hydrate slurry solution

concentration and temperature (from Kumano et al., 2006).

Nomenclature

C heat capacity, J K�1

c specific heat, J kg�1 K�1

L latent heat, J kg�1

T temperature, �Cm mass, kg

Subscripts

1 initial temperature of TBAB solution

2 final temperature of TBAB solution

cu copper block

e melting temperature

l TBAB solution

s TBAB hydrate crystal

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 9 9 2e9 9 7 993

materials, such as TBAB solution (hydrate), with high accu-

racy by using DSC, because it is difficult to produce pure

crystal due to the formation of concentration distribution in

the solution during the solidification. The concentration

distribution causes remaining of condensed solution and

incorporation of solute in the hydrate crystal. Ogoshi and

Takao (2004) have reported the thermophysical properties of

TBAB including latent heat. The values of latent heat reported

in these two reports agree well, but the method of measure-

ment in the latter report was not described. One of the authors

of the present study estimated the latent heat from variation

of specific enthalpy of TBAB hydrate slurry (Kumano et al.,

2006). The estimated value was 7e10% larger than those pre-

sented in other reports.

The authors of the present study have reported the varia-

tion of apparent latent heat of ice in ice slurry in previous

studies (Asaoka et al., 2011; Kumano et al., 2007). In those

studies, 30e100 g of ice was melted in aqueous solution, and

the latent heat was estimated from the temperature variation

of the solution. Given that the reliability of this method has

been confirmed, we have used the same method to measure

the latent heat of TBAB hydrate in the current study. A large

size of pure TBAB hydrate crystal was formed and the latent

heat of TBAB hydratewasmeasured using the crystal. Because

pure hydrate is used in this method, we believe that more

reliable results can be obtained than the other methods.

By cooling TBAB solution, TBAB hydrate is formed in the

solution. Oyama et al. (2005) presented the photograph of the

hydrate crystal in their report. Generally, crystals of TBAB

hydrate are crushed into tiny particles due to agitation and

the mixture becomes a slurry. It is quite difficult to separate

the hydrate crystal from the mixture, and a method is

required that can generate a large crystal before any

measurement can be performed. Hirata et al. (2003) reported

that the method of forming ice stalagmites is effective in

generating pure ice and excludes additives in it. In the

method, droplets of ethylene glycol solution are allowed to

fall onto a horizontal plate in a cooled room, and pure ice

forms on the plate. The remaining of condensed solution and

incorporation of solute in the crystal can be prevented in this

method. In this study, we generated pure TBAB hydrate

crystal using this method.

Fig. 1 shows the relationship between hydrate slurry

solution concentration and temperature (Kumano et al.,

2006). In addition, the phase diagram for TBAB hydrate with

wider range of concentration reported by Oyama et al. (2005)

must be also useful. TBAB solution forms two types of

hydrate, known as type 1 and type 2. Type 1 hydrate is

formed in the solution at high concentration, while type 2

forms at lower concentration. In this study, the crystals of

these two types of hydrate were generated and the latent

heats of them were measured.

2. Experiment

2.1. Procedure and apparatus for generating TBABhydrate crystal

Fig. 2 shows the apparatus used to generate TBAB hydrate

crystal. A brass plate was placed in a constant temperature

room and allowed to equilibrate to the room temperature.

Initially, a small TBAB hydrate crystal, which was produced

preliminarily, was placed on the plate. The nucleation of

hydrate was enhanced by this crystal, which we referred to as

the nucleus crystal. Droplets of TBAB solution were then

allowed to fall onto the plate at a mean mass flow rate of less

than 0.3 g min�1. TBAB hydrate crystal was then generated on

the plate. The temperature of the supplied TBAB solution was

controlled by using a heat exchanger, which was placed near

the outlet of the solution supply pipe. The temperature of the

supplied solutionwas controlled by adjusting the temperature

and mass flow rate of the water flow in the heat exchanger.

Various concentrations of TBAB solution were used for the

droplet supply solution. The temperature under solideliquid

equilibrium conditions depends on the hydrate type and the

concentration of the solution, as shown in Fig. 1. We refer to

these temperature and concentration conditions as equilib-

rium temperature and equilibrium concentration, respec-

tively. The temperature of the supplied solution at the outlet of

the supply pipe was maintained at the equilibrium tempera-

ture of the solution. Solidification of the supplied solution

never occurred due to supercooling under these conditions.

The room temperature was kept 2.5 �C below the equilibrium

Fig. 3 e Apparatus for measuring specific heat of TBAB

solution (a) and latent heat of TBAB hydrate (b). 1, copper

block; 2, TBAB solution; 3, Dewar vessel; 4, insulator; 5,

platinum resistance thermometer; 6, stirrer; 7, TBAB

hydrate crystal.

Fig. 2 e Apparatus for generating TBAB hydrate crystal.

1, TBAB solution; 2, insulator; 3, water flow for temperature

control; 4, TBAB hydrate crystal; 5, brass plate; 6, residual

TBAB solution; 7, constant temperature room.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 9 9 2e9 9 7994

temperature. The solution dropped onto the hydrate crystal

partially solidified and remaining solution became slightly

diluted. The remaining solution flowed out from the hydrate

crystal after its concentration decreases to the equilibrium

concentration at the applied room temperature.

The temperatures of the supplied solution, the brass plate,

and the air in the room were measured by using T-type ther-

mocouples. Each of these temperatures remained almost

constant during the generation of the hydrate crystal.

The concentration of TBAB solution can be conveniently

measured using a refractometer. The accuracy of the

measurement is �0.3% of the measured value and when the

measured value is 40 wt%, it includes a measurement error of

about �1 wt%.

2.2. Procedure and apparatus for measuring specificheat of TBAB solution and latent heat of TBAB hydrate

Fig. 3a shows the apparatus for measuring the specific heat of

TBAB solution. About 500 g of TBAB solution was placed in

a Dewar vessel. Four different concentrations of the solution

were used; 8, 25, 27, and 34 wt%. The solution was stirred

during the experiment to keep its temperature uniform. The

temperature of the solution was measured by platinum

resistance thermometer, and, initially, the temperature of the

solution was the same as the ambient temperature; 25 � 5 �C.The copper block was kept at constant temperature in hot

water, the temperature of which was measured by platinum

resistance thermometer. The temperature of the hot water

was about 70 �C. However, because the copper block was

covered with a plastic sheet, it did not contact the hot water

directly. The mass of the copper block was 1227.5 g. The

resolving power of the thermometer was 0.01 �C, and the error

of reading was �0.02 �C at most. With such an error in the

temperaturemeasurement, the error in the calculated specific

heat, cl, was less than 1%. Then the copper block was put into

the solution and the specific heat, cl, was calculated from the

temperature variation of the solution, using the energy

balance equation shown in Eq. (1).

mlclðT2 � T1Þ þmcuccuðT2 � TcuÞ þ CðT2 � T1Þ ¼ 0 (1)

where C represents the heat capacity of the apparatus and

ccu is the heat capacity of copper (Green and Perry, 2007).

The temperature variation of the solution was 5e10 �C.

Fig. 3b shows the apparatus for measuring the latent heat

of TBAB hydrate. About 500 g of TBAB solution was added to

the Dewar vessel. The concentration of the solution was equal

to the congruent concentration, which is defined as the

concentration at which the concentration of solution does not

change due to solidification. Generally, the solideliquid

equilibrium temperature at the congruent concentration is

known as the congruent melting point. The congruent

concentration depends on the type of hydrate. The solution

was stirred during the experiment, and the temperature was

measured by a platinum resistance thermometer. With an

Fig. 4 e TBAB hydrate crystal (type 1).

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error in the temperature measurement of about 0.02 �C, theerror in the calculated latent heat, L, is about 3%. Initially, the

temperature of the solution was the same as the ambient

temperature. Then the hydrate crystal is put into the solution,

and allowed to melt completely. The latent heat was calcu-

lated from the temperature variation of the solution, using the

energy balance equation shown in Eq. (2).

msLþmscsðTe �TsÞþmsclðT2 �TeÞþmlclðT2 �T1ÞþCðT2 �T1Þ ¼ 0

(2)

The temperature variation of the solution was 1e4 �C.

Tomeasure the heat capacity of the apparatus, C, the same

measurement was performed using tap water and ice

produced from tap water. Applying the specific heat of water

and ice (4.2 and 2.0 kJ kg�1 K�1) to Eq. (1), the heat capacity was

calculated. The measurement was performed four times, and

the mean value was 90 J K�1. Although the value had a varia-

tion of about �20 J K�1 in each measurement, the effect of the

variation on the calculated specific heat, cl, and on the latent

heat, L, was less than 1%. Themean valuewas used as C in this

investigation.

For the specific heat of hydrate, cs, the value reported by

Oyama et al. (2005) was used. The specific heat of type 1

hydrate at 5 �C is about 2.5 kJ kg�1 K�1, and that of type 2

hydrate at �0.2 �C (highest temperature in the reported value)

is about 2.5 kJ kg�1 K�1. Literature values were used for the

specific heat of hydrate, because the effect of error in it is not

significant. When the error in the specific heat of hydrate is

0.1 kJ kg�1 K�1, the error in the calculated latent heat, L, is

about 0.2%. Contrarily, the error in the specific heat of solution

is considerable. Thus the specific heat of solutionmeasured in

the present work was used as cl. When the error in the specific

heat of solution is 0.1 kJ kg�1 K�1, the error in the calculated

latent heat, L, is about 3%.

The resolving power of the electric balance used in these

measurements was 0.1 g. The effect of error in a mass of 0.1 g,

on the calculated specific heat, cl, and on the latent heat, L, is

less than 1%.

0 20 40

20

30

40

Concentration of supplied solution, wt%

Con

cent

ratio

n of

m

elte

d hy

drat

e, w

t%

Type of nucleus crystalType 1Type 2

Concentration of supplied solution is 8wt%

Fig. 5 e Effect of concentration of supplied solution on

concentration of melted hydrate. The congruent

concentration of type 1 and type 2 hydrate are shown as

solid line and broken line, respectively.

3. Results and discussion

3.1. Congruent concentration and hydration numberof TBAB hydrate

Fig. 4 shows the appearance of the TBAB hydrate crystal

generated in the present work. The generated crystal was

conical in shape, with the top of the cone being the point to

which the droplet of TBAB solution was supplied. White

powder near the right end of the crystal was identified as pure

TBAB solid, rather than the hydrate. On the surface of the

crystal, the hydrate was decomposed, because of the low

humidity in the constant temperature room. However, given

the relatively small mass of the solid TBAB powder, it was

considered to have no significant effect on themeasurements.

There was no difference in the appearance of type 1 and

type 2 hydrate crystals. As reported by Oyama et al. (2005), the

two types of hydrate appears different shape in micro scale

observation. However, the difference is not significant in large

crystals.

To measure the congruent concentration of TBAB hydrate,

the hydrate crystal produced in the experiment was melted

and the concentration of it was measured. The concentration

of melted hydrate is shown in Fig. 5. The experiments were

performed three times, using 8 wt% and 25 wt% solutions,

while one time under other conditions. The mean values are

shown on the figure, and the error bars represent the

maximum/minimum values from the three results.

When the concentration of supplied solution was low, the

concentration of melted hydrate is about 27 wt%. This

concentration is equal to the congruent concentration of type

2 hydrate, because type 2 hydrate forms under the condition,

as shown in the phase diagram (Fig. 1). When the concentra-

tion of supplied solution was high, the concentration of mel-

ted hydrate is about 34 wt%. This concentration is equal to the

congruent concentration of type 1 hydrate, because type 1

hydrate forms under the condition. When the concentration

of supplied solution was about 20 wt%, the concentration of

melted hydrate was not stable. This means that the generated

crystal contains both types of hydrates. When type 2 hydrate

was used as the nucleus crystal, the concentration of melted

Table 2 e Hydration number of TBAB hydrate.

Presentstudy

Kumanoet al. (2006)

Oyamaet al. (2005)

Ogoshi andTakao (2004)

Type 1 35 29 26 26

Type 2 47 44 38 36

4

4.5

t, kJ

kg–1

K–1

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 9 9 2e9 9 7996

hydrate was close to the congruent concentration of type 2

hydrate.When type 1 hydrate was used as the nucleus crystal,

the concentration became higher. This means that the

formation of the hydrate, which is the same type with nucleus

hydrate, tends to be facilitated. However, it was confirmed

that only type 2 hydrate is generated when the concentration

of supplied solution is 8wt%, even if the nucleus crystal is type

1. This is because type 1 hydrate cannot formunder such a low

concentration condition. When the concentration of supplied

solution was above 34 wt%, the incorporation of solute into

the hydrate crystal occurred. As shown in Fig. 5, the concen-

tration of melted hydrate increases over 34 wt% (congruent

concentration of type 1 hydrate) due to incorporation of the

solute. This means that pure hydrate cannot be obtained

under that condition.

The concentration of supplied solution was set at 25 wt%

for type 1 hydrate and 8 wt% for type 2 hydrate, when the

hydrate crystals used for themeasurement of latent heat were

generated. As shown in Fig. 5, a certain type of hydrate crystal

can be generated intentionally under these conditions.

The congruent concentrations of each type of hydrate are

shown in Table 1. The congruent concentrations were deter-

mined as the mean values of the concentration of melted

hydrates, which were generated from 25wt% solution for type

1, and 8 wt% solution for type 2. The reference values reported

by Oyama et al. (2005) are also listed. The values of hydration

number calculated from the congruent concentration are

shown in Table 2, and are compared with literature values

(Kumano et al., 2006; Oyama et al., 2005; Ogoshi and Takao,

2004). The hydration numbers presented in this work are

larger than those reported in the references.

In the report of Oyama et al. (2005), the congruent

concentration was estimated from the phase diagram. They

determined the congruent melting temperature as the peak of

the curve of solideliquid equilibrium. However, it is quite

difficult to determine the concentration at the peak

(congruent concentration) with high accuracy, since the

curvature is very small around the peak.

In the report of Kumano et al. (2006), the hydrate crystal

was separated from TBAB hydrate slurry, and the melted

concentration was measured. As shown in the report, the

measured value of hydration number varies 29e33 in type 1

hydrate, and 43e47 in type 2 hydrate. Considering the varia-

tion, it can be said that this result agrees with ours.

3.2. Specific heat of TBAB solution

The results for the measurement of specific heat of TBAB

solutions are shown in Fig. 6. The measurements using 8, 25,

27, and 34 wt% solutions were performed five, three, five, and

five times, respectively. The results of themeasurement using

Table 1 e Congruent concentration of TBAB hydrate(wt%).

Present study Oyama et al. (2005)

Type 1 34 40

Type 2 27 32

pure water are shown as 0 wt% solution. The data points

shown are the mean values, and the error bars represent the

maximum/minimum values of the results.

Comparing the specific heat of water measured in this

experimentwith the reference value (Green and Perry, 2007), it

is confirmed that reliable values can be obtained in this

method. In the measurement using 8 wt% solution, the

observed variation of �0.12 kJ kg�1 K�1 in the result for the

specific heat was the largest for our study. However, this

accuracy is sufficient for the calculation of latent heat,

because the error in the calculated latent heat, L, is about 3%

with the error in the specific heat. In addition, it should be

noted that only the results of 34 and 27 wt%, which are the

congruent concentration of type 1 and type 2 hydrate, were

used for the calculation of latent heat. Results of 8 and 25 wt%

were not used.

3.3. Latent heat of TBAB hydrate

Results of themeasurement of latent heat of TBAB hydrate are

shown in Table 3. The measurements were performed nine

times for each type of hydrate. The results are shown asmean

values (�maximum/minimum values), and reference values

(Kumano et al., 2006; Oyama et al., 2005; Ogoshi and Takao,

2004) are also presented for comparison.

The mean values of calculated latent heat were 210 kJ kg�1

in type 1 hydrate and 224 kJ kg�1 in type 2 hydrate. The vari-

ations of the results are less than 10% in the measurements.

The latent heat of TBAB hydrate presented in this work agrees

with the value reported by Kumano et al. (2006), which was

estimated from the variation of specific enthalpy of TBAB

hydrate slurry. However, our result is 10% larger than the

value reported by Oyama et al. (2005), which wasmeasured by

DSC. Because pure hydrate is used in the measurement of

present study, we believe that more reliable results can be

obtained than the other methods.

0 20 403.5

Concentration of TBAB solution, wt%

Spe

cifi

c he

a

Present studyGreen 2007

Fig. 6 e Specific heat of TBAB solution.

Table 3 e Latent heat of TBAB hydrate (kJ kgL1).

Presentstudy

Kumanoet al. (2006)

Oyamaet al. (2005)

Ogoshi andTakao (2004)

Type 1 210 � 10 215 193.18 193

Type 2 224 � 15 215 199.59 205

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 9 9 2e9 9 7 997

4. Conclusion

The latent heat of TBAB hydrate was measured by melting

a large size of pure crystal. The latent heat of type 1 TBAB

hydrate was estimated as 210 kJ kg�1 and that of type 2 was

estimated as 224 kJ kg�1. The values presented in this work

agree with the reference value estimated from the variation

of specific enthalpy of TBAB hydrate slurry, although it is

10% larger than the other reference value measured using

DSC.

r e f e r e n c e s

Asaoka, T., Kumano, H., Okada, M., Kose, H., 2011. Effect oftemperature on the effective latent heat of fusion of ice inaqueous solutions. Int. J. Refrigeration 33, 1533e1539.

Darbouret, M., Cournil, M., Herri, J.M., 2005. Rheological study ofTBAB hydrate slurries as secondary two-phase refrigerants.Int. J. Refrigeration 28, 663e671.

Green, D.W., Perry, R.H., 2007. Perry’s Chemical Engineers’Handbook, eighth ed. McGraw-Hill, New York, pp. 2e151,pp. 2e413.

Hirata, T., Inoue, T., Ishikawa, M., 2003. Ice formation phenomenaof water droplet fallen of a plate in cold room. Trans. JSRAE 20,517e522 (in Japanese).

Kumano, H., Saito, A., Okawa, S., Goto, Y., 2006. Study onfundamental characteristics of TBAB hydrate slurry. Trans.Jpn. Soc. Mech. Eng. 72 (724), 3089e3095.

Kumano, H., Asaoka, T., Saito, A., Okawa, S., 2007. Study on latentheat of fusion of ice in aqueous solutions. Int. J. Refrigeration30, 267e273.

Kumano, H., Hirata, T., Kudoh, T., 2011a. Experimental study onthe flow and heat transfer characteristics of a tetra-n-butylammonium bromide hydrate slurry (first report: flowcharacteristics). Int. J. Refrigeration 34, 1953e1962.

Kumano, H., Hirata, T., Kudoh, T., 2011b. Experimental study onthe flow and heat transfer characteristics of a tetra-n-butylammonium bromide hydrate slurry (second report: heattransfer characteristics). Int. J. Refrigeration 34, 1963e1971.

Ma, Z.W., Zhang, P., Wang, R.Z., Furui, S., Xi, G.N., 2010. Forcedflow and convective melting heat transfer of clathrate hydrateslurry in tubes. Int. J. Heat Mass. Trans. 53, 3745e3757.

Ogoshi, H., Takao, S., 2004. Air-conditioning system usingclathrate hydrate slurry. JFE Tech. Rev. 3, 1e5 (in Japanese).

Oyama, H., Shimada, W., Ebinuma, T., Kamata, Y., Takeya, S.,Uchida, T., Nagao, J., Narita, H., 2005. Phase diagram, latentheat, and specific heat of TBAB semiclathrate hydrate crystals.Fluid Phase Equilib. 234, 131e135.

Wenji, S., Rui, X., Chong, H., Shihui, H., Kaijun, D., Ziping, F., 2009.Experimental investigation on TBAB clathrate hydrate slurryflows in a horizontal tube: forced convective heat transferbehaviors. Int. J. Refrigeration 32, 1801e1807.