Brine substitute liquids for soil freezing at very low temperatures

Brine substitute liquids for soil freezing at very low temperatures

Engineering Geology, 18 (1981) 203--210 203 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands BRINE SUBSTITUTE LIQUID...

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Engineering Geology, 18 (1981) 203--210

203

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

BRINE SUBSTITUTE LIQUIDS FOR SOIL FREEZING AT VERY LOW TEMPERATURES

P. de PORCELLINIS and J.L. ROJO

Cimentaciones Especiales SA Rodio, Paseo de la Castellana, 130 4a, Madrid 16 (Spain) (Accepted for publication February 4, 1981)

ABSTRACT De Porcellinis, P. and Rojo, J.L., 1981. Brine substitute liquids for soil freezing at very low temperatures. Eng. Geol., 18: 203--210. The strength of a frozen soil increases with decreasing temperature. Furthermore, the speed it takes to form a frozen wall increases on lowering the temperature of the freezing liquid. With the traditional freezing systems using brine it is difficult to work with temperatures below --30°C. To go lower than this limit, it is necessary to substitute the brine by using freezing liquids that maintain good hydraulic and thermal characteristics at much lower temperatures. Different organic liquids have been tested and good results have been obtained with some aromatic hydrocarbon mixes from the terpene family. As a result of the research~ for practical purposes a by-product of distilling citrus fruit skins has been selected. This liquid solidifies at --100°C approx, and maintains a low viscosity rate below --30°C. The present paper describes the thermal and hydraulic properties of this product as a function of the temperature concerned and compares them to the same properties of classic brines of CaCI2. INTRODUC~ON

The technology of cold as applied to soil freezing, especially in so,ailed "indirect" processes, is a "borrowed" technique, so much so that it would not be untrue to say that the machinery and the methods used for producing and transferring cold do not differ greatly from those used in a standard ice factory. The most obvious limitations facing us are: --The minimum temperature that can be achieved (~ --30°C) does not allow the low-temperature soil properties to be fully exploited. --There are difficulties involved in transporting cold from the point of production over great distances, a problem that does not usually arise in standard industrial processes. --The need to resort to other methods (cryogenic gases) in emergencies because o f the impossibility of increasing the flow of frigories at specific local points. 0013--7952/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

204

--The intrinsic dangers present in brine solutions in the case of leaks from the freezing probe-tubes since these can produce a localized reduction of the freezing point of the soil which is frequently n o t detectable and the root of problems difficult to solve. The fact is that the thermal properties of the brines are precisely the cause o f many limitations. The considerable rate of viscosity that these solutions achieve at low temperatures, furthermore, makes it necessary to use great pumping power in order to move the brines through the circuits with the consequent loss of energy occurring in the final balance. And, in fact if, on the one hand, it is possible, using the right refrigerants to arrive at temperatures of --70°C in the evaporator of the freezing plant, on the other we have to limit these temperatures because of the increases in viscosity and the risk of solidification that the brines present. The increase in viscosity is also a limitation that drastically reduces the possibility of obtaining turbulent flows in the probe-tubes and the exchangers in general, a fact that limits the flow of heat in the liquid--wall exchange. REVIEW OF POSSIBLE H E A T - T R A N S F E R LIQUIDS

As a consequence of the foregoing, we searched for a new heat-transfer liquid that would combine all of the following properties: low melting point; good thermal properties at low temperatures; low viscosity; low vapour pressure at site temperatures; non-toxic; non-corrosive. If we discard the inorganic brines (NaC1 and CaC12 ) and the organic ones (alcohol or glycol solutions) that have the limitations we have mentioned, the usable liquids we are left with, are halocarbons and halogenated hydrocarbons. These products have in c o m m o n the property of possessing a low melting point (around --100°C), b u t all of them, with the exception of trichloroethylene, have high vapour pressures at site temperatures. Some of them, such as methylchloride, attack metals; all of them have high specific gravity. The comparative table (Table I) will give an idea of the main products available in this family. TABLE I Some properties of heat-transfer organic liquids (according to literature) Name

Specific gravity

Melting Boiling Specific Observations point (°C) point heat

(°C) Methylchloride

1.336

--96.7

40.0

0.288

Trichloroethylene

1.466

--73.0

87.0

0.233

Tri¢hlorofluoromethane

1.490

--111.0

23.7

0.208

corrodes aluminum; the gas is lethal same limitations; less vapour pressure non-toxic; suffocation risk; high vapour pressure

205

It can be seen from Table I, and basically for reasons of health hazards, corrosion and high vapour pressure, that these liquids in principle would not seem suitable for the applications required. In the classic literature on freezing processes no other possibilities appear; however, specific experiments have been carried out in the United States in the field of food preservation where organic aromatic liquids have been used. A rapid selection led us to study, among others, one product that is reasonably priced and readily available -- a by-product from the manufacture of natural essences, a terpene mix. A general picture of its physical properties is given in the following: 0.840 --specific gravity --96.9°C --melting point +177°C --boiling point ~1 cp --viscosity at 20°C ~0.445 kcal/kg °C --specific heat at 20°C --vapour pressure at 40°C 5 mm Hg none --toxicity --odour strong and pleasant --miscibility in water 0% From these properties we can deduce that the material a priori presents the following advantages: (1) Lower specific gravity than water, signifying that: (a) should any leaks occur, since it is neither soluble nor miscible in water, it will settle above the freatic level where freezing is not required; (b) it requires only a small amount of energy per pumped unit of volume. (2) Its vapour pressure at site temperatures is low, implying that few precautions are necessary in the installations. (3) It does not corrode metals; it only causes decomposition of natural rubber and some synthetic ones. COMPARATIVE

STUDY ON BRINE AND TERPENE

Fig.1 and Table II show the comparison between a CaC12 solution at 30% and this heat-transfer liquid where the thermal and hydraulic properties a r e concerned, giving the variations in these properties as a function of the temperature involved. There follows a theoretic study of the hydraulic and thermal properties of the two liquids being compared for a specific geometric layout and two different flow rates; average conditions obtained in the course of our practical freezing operations. Initial data: --internal diameter of the outer tube of the probe: 79 mm --external diameter of the inner tube of the probe: 25 mm --equivalent diameter de (De --Di): 0.054 m The experiment has been carried out for the following flow rates:

206 481~, /~

[~

I"E

~/ I TERPENE

I[

1130°/,,CclCt2 BRINE

|~

f • THEORETICAL CURVE I 7 l (THOMAS) ~÷ EXPERIMENTAL DATA

o

'

I

I

/

/~

30

/!' *

-

/

---Z]

-

'

~'

/

~

/

z'/

/

20

'

l

ml

./

i+ I

/

-10

-20

-30

-40

-50

-60

-70

oC -80

Fig.1. D y n a m i c viscosity plotted versus t e m p e r a t u r e for brine and terpene.

F, = 50 l/min = 0.83" 10 -3 ma/sec; F2 = 10 l/min = 0.167" 10 -3 m3/sec The Nusselt and Reynolds numbers corresponding to the liquids and flow rates under consideration were taken at temperatures of 0°C, --20°C, --40°C and --60°C. From these figures the heat transfer coefficients and pressure
c: (kcal/kg °C)

Specific heat

(Pr)

*B ffi CaCI2 brine at 30%; T = terpene.

Prandtl number

10-4

: \ ---~ m see °-'-C/

.(kgf .sec~ 7. \ - " ~ T ' - - / "

Viscosity

Thermal conductivity

~: (kgf/m 3)

23.9

0.291

1.26 30.46

0.442

1.604

840

T

0.650

6.02

1300

B

Type of fluid :*

Specific gravity

0°C

Temperature:

Comparison between physical properties

TABLE II

44.61

0.288

1.20 83.82

0~438

2.99

840

T

0~640

16.02

1300

B

--20°C

92.85

0.285

1.13 201.15

0.435 0.631

6.23

241.2

0.282

0.428

16.2

840 1300 840

1300 36.72

T

B

T

--60°C

B

--40°C

t~

208

TABLE

III

Comparison

of hydraulic

and thermal

O°C Brine: F,

(Re) 4312 h = 1416

F2

(Re) 868 h = 85.2

Terpene: F,

F2

properties

--20°C

(Nu) 60.7

(Nu) 3.65

(Re) 1620 h = 81.1

(Re) 326 h = 81.1

--40°C

(Nu) 3.65

(Nu) 3.65

(Re) 707 h = 79.1

(Re) 142 h = 79.1

--60°C

(Nu) 3.65

(gu) 3.65

(Re)

(Nu)

-h = --

--

(Re)

(Nu)

-h = --

--

(Re)

(Nu)

(Re)

(Nu)

(Re)

(Nu)

(Re)

(Nu)

10458 h = 612

113.6

5910 h = 477

88.57

2693 h = 317

60.21

1035 h = 19.1

3.65

(Re)

(Nu)

(Re)

(Nu)

(Re)

(Nu)

(Re)

(Nu)

208 h = 19.1

3.65

2104 h = 19.7

3.65

1129 h = 19.5

3.65

542 h = 19.3

3.65

F l o w r a t e : F , = 0 . 8 3 • 1 0 -3 m 3 / s e c ; F : = 0 . 1 6 7 • 1 0 -3 m 3 / s e c . E q u i v a l e n t d i a m e t e r d e = 0 . 0 5 4 m . h ( h e a t t r a n s f e r c o e f f i c i e n t ) ( k c a l / m 2 °C s e c ) • 1 0 4 . T u r b u l e n t flow /> 2 5 0 0 .

= Re

The heat flow rate will also depend upon the average temperature of the fluid that it is possible to reach with the existing freezing plant. From the theoretic details obtained and for the temperature intervals of --20°C and --60°C, the following can be deduced. (a) Turbulent flows with brine cannot be achieved for either of the flow rates under consideration, as a result the heat transfer coefficient cannot be greater than 81 • 10 -4 (kcal/m 2 sec °C) at temperatures below --20°C. (b) The pressure drop per linear metre at a temperature of --20°C is t o o high (from a practical point of view) for the brine flowing at rates of or over 50 1/min and this reduces the possibilities of heat transfer since the circulating mass remains limited. (c) Since, for practical purposes, a pressure drop in the region o f 2 kg/cm 2 is admissible for 100 linear metres o f probe-tubes in series, the values shown in Table IV indicate that where brine is concerned, for temperatures as low as --30°C, the maximum projected flow rates should not exceed 15 1/min, thus setting up a ceiling value for the liquid-wall thermal drop; for lower temperatures the pressure drop increases very rapidly, and the flow would ultimately need to be reduced. From these considerations, taken together with the preceding data, the result is that for CaC12 brine at 30% and temperatures lower than --30°C, the maximum heat flow obtainable is approximately 30 kcal per hour and linear metre of probe-tube and per degree centigrade (°C) of thermal drop between wall and fluid.

209

TABLE IV Pressure drop in the probe-tubes (per metre) (kgf/m 2) ~C

--20°C

--4~C

Brine: FI F2

1.57 0.12

6.37 1.28

14.62 2.94

Terpene: F1 F~

0.81 0.05

1.04 0.24

1.14 0.5

--6~C

m

6.45 1.29

Where terpene is concerned, we observe that for temperatures of--20°C and a stream of 50 l/rain, the value for (Re)d~ of 5,910 can be achieved, implying a flow rate that tends to be turbulent, maintaining a lower pressure drop than that obtained with brine at a flow rate of 10 1/min; thus, from a practical point of view, this turbulent flow can easily be achieved. If a turbulent flow is in fact produced, the h-value obtained is 4 7 7 - 10 -4 (kcal/m 2 sec °C); since the flow is a transitional one, close to turbulence, we can take a value in the region o f 3 0 0 . 1 0 -4 as being a realistic one. These values can be improved by increasing the outflow, something that is technically possible in view of the limited pressure drops. This h-value allows us to obtain (per linear metre of probe-tube and °C of thermal drop between fluid and wall) a heat flow of 108 kcal/h. For terpene at --60°C, the pressure drops in the circuit are similar to those for brine at --20°C for the same flow rates, demonstrating h o w viable it is to have the heat-transfer liquid circulate at --60°C which implies the possibility of achieving greater strength properties in frozen soils. FIELD TEST

In order to check the theoretic statements made, at least at a first approach, we devised a comparative test between brine and terpene, utilising equipment intended for other purposes (Fig.2). In the test the geometric parameters of the circuit were kept constant as also the pumping rate for the circulation and the freezing equipment. The idea was simply to measure the heat absorbed b y approximately one linear metre of probe-tube in a rudimentary calorimeter with CaC12 brine and terpene. In addition, in order to ascertain the qualitative effect of different circulation flow rates, the test was repeated using t w o types o f probes with different inner tube diameters. For each case the results obtained were as summarized in Table V. The comparative study of the results o f the field test confirms the theoretic statements made.

210

F i g . 2 . F i e l d t e s t e q u i p m e n t l a y o u t . S = f r e e z i n g p r o b e ( t y p e A a n d B); D = c a l o r i m e t e r ; B = circulating pump; C = external circuit; E = freezing plant evaporator; Q = flowmeter; PD = differential pressure gauge; T,--T 5 = thermometers. TABLE V Summary of the field test results Type A probe (d e = 0.054)

Liquid inflow temp. in the probe (°C) Circulating liquid flow rate (l/rain) Pressure drop inside the probe (cm H20) Heat in calorimeter (kcal) Test duration (h) Heat flow (kcal/h)

Type B probe (d e = 0.006)

brine

terpene

brine

terpene

--20> t~--23 14 50~H~100 2404 6.75 356.15

--20~ t~--42 47 ~0 2974 4.00 743.5

--20> t~--25 12.5 240~H<~290 2748 6.83 402.3

--20~ t~--42 48 260~H~280 3072 4.00 768.0

CONCLUSIONS

Utilizing a heat-transfer liquid with terpene properties as studied, and with conventional installations and technology, turbulent flow rates can be achieved with higher heat-transfer performance. Lower p o w e r losses can also result in the circulation of heat-ixansfer liquid used. The possibility opens up of achieving lower temperatures than --60°C, while still using conventional installations, with far better mechanical properties being attained in the frozen soils. REFERENCES Hiitte A c a d e m y , B e r l i n , 1 9 6 4 . D e s I n g e n i e u r s T a s c h e n b u c h ( 2 8 t h e d . , S p a n i s h t r a n s l a t i o n ) . Perry, R. and Chilton, C.H., 1973. Chemical Engineers' Handbook (5th ed.)McGraw-Hill, New York, N.Y. Rosenow, W.M. and Hartnett, J.P., 1973. Handbook for Heat Transfer. McGraw-Hill, New York, N.Y.