Properties of the Hydrates of Fluorocarbons 142b and 12B1

14 Properties of the Hydrates of Fluorocarbons 142b and 12B1 Comparison of Six Agents for Use in the Hydrate Process FREDERICK A. BRIGGS

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1

a n d ALLEN J. BARDUHN

Department of Chemical Engineering, Syracuse University, Syracuse, N. Y.

T h e r m o d y n a m i c d a t a on the systems F-142B (CH CCIF )-H O-NaCl and F-12B1 (CCIF Br)-H O-NaCI include p r e s s u r e - t e m p e r a t u r e phase d i a g r a m s , heats of f o r m a t i o n , h y d r a t e compositions, 3

and

2

2

2

the t e m p e r a t u r e depressions

d u e to

2

NaCl.

Both h y d r a t i n g agents h a v e p r o p e r t i e s w h i c h a p p e a r useful in the h y d r a t e process f o r d e s a l t i n g sea w a t e r .

Six h y d r a t i n g agents a r e c o m p a r e d b y

calculating

the

energy

requirements

and

total

heat e x c h a n g e r surface r e q u i r e d f o r e a c h process when

producing

fresh w a t e r .

1,000,000

gallons

per d a y

of

The agents c o n s i d e r e d a r e p r o p a n e ,

F-21(CHCI F),

F-12B1,

F-31(CH CIF)

in o r d e r of increasing critical f o r m a -

2

2

tion t e m p e r a t u r e .

F-142b, High

methyl bromide, and

temperature

hydrating

agents h a v e a distinct a d v a n t a g e w i t h respect to e n e r g y r e q u i r e m e n t s a n d capital costs of heat e x changers.

Energy

requirements low er than

20

kw.-hr. p e r 1 0 0 0 gallons a p p e a r e a s i l y possible.

J he hydrate process for demineralizing sea water has been described and discussed in several publications (2, 3, 6). The development of the process is in its beginning stages, the first such work having been started at Syracuse University and at the Koppers Co. in 1959. Among the problems needing solution is that of choosing the most economical hydrating agent. To make such a decision requires a rather large amount of information on the properties of each possible agent and its hydrate. A preliminary screening can be made, however, on the basis of the hydrate critical decomposition temperature and pressure only, since a lower temperature limit of about 5 ° C . can be set, so that the process will have a significant advantage over freezing and an upper pressure limit of around 5 to 6 atm. can be set to keep the process equipment cost at a reasonable value. These are not firm limits, but they exclude many substances. 1

Present address, E . I. d u P o n t de N e m o u r s & C o . , Inc., W a y n e s b o r o , V a . 190

Saline Water Conversion—II Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

14. BRIGGS

AND

BARDUHN

Fluorocarbons

in the Hydrate

Process

191

O n the basis of the pressure a n d temperature limits thus specified the number of hydrating agents w h i c h have any reasonable chance of becoming i m portant can be reduced to about 12, shown i n Table I. Further work has been reported on F-31, methyl bromide, and F - 2 1 (2) and on propane ( 6 ) ; and this paper presents thermodynamic data on F-142b and F-12B1. Except for chlorine, similar data can probably be predicted for the remaining six agents w i t h sufficient accuracy for economic evaluation studies ( I ) .


T a b l e I. Hydrating Agent Chlorine M e t h y l chloride F-31(CH C1F) F«152a(CH CHF ) M e t h y l bromide F-142b(CH CClF ) F-12(CC1 F ) F-12Bl(CClBrF ) F-22Bl(CHBrF ) F-21(CHC1 F) F-11(CC1 F) Propane 2

3

2

3

2

2

2

2

2

2

3

Promising Hydrating Agents for Desalination Temperature ° C. ° F. 28.7 21 17.9 15.3 14.7 13.1 12.1 9.9 9.9 8.7 8.0 5.7

84 70 64 60 58 56 54 50 50 48 46 42

Pressure Mm.

P.s.i.g.

4570

74 (60) est. 27 51 8 19 52 10 24 0 -7 65

2147 3382 1151 1743 3435 1272 2012 760 419 4141

It is not impossible, but it seems improbable, that any new hydrating agents w i l l be discovered that are superior to those listed i n Table I, since a rather systematic search has been made among the hydrocarbons a n d their halogenated derivatives, and no other class of compounds seems as likely to form hydrates a n d simultaneously to have the properties useful i n desalting ( I , 3 ) . M a n y properties other than those presented w i l l determine the eventual worth of an agent i n a desalting process. A m o n g these are the stability i n water, the solubility i n water and salt solutions, the rates of nucleation a n d growth a n d the nature of the hydrate crystals, and the cost of the agent.

TEMPERATURE ° C .

Figure 1.

Experimental phase diagram for F-142b-water-NaCl system



192

A D V A N C E S IN CHEMISTRY

SERIES

TEMPERATURE C . P

Figure 2.

Calculated phase diagram for F-142b-water-NaCl system

Hydrates of F-142b and F-12B1 The pressure-temperature phase diagrams for these two substances w i t h H 0 and N a C l are presented i n Figures 1 to 4 and the data are tabulated i n Tables II to V . F r o m these data have been calculated heats of formation, hydrate compositions, and the depression of the formation temperature due to N a C l . T h e i m portant measured and derived quantities for both systems are summarized i n Table V I . 2

-10

-8

- 6

-4

- 2

0

+2

4

TEMPERATURE

Figure 3.

6

8

10

12

14

16

18

°C.

Experimental phase diagram for F-12Bl^waterNaCl system


14. BRIGGS AND T a b l e II.

BARDUHN

Fluorocarbons

in the Hydrate

Process

193

E x p e r i m e n t a l P h a s e Equilibrium D a t a f o r the System F - l 4 2 b - W a t e r

CHzCClFt (Gas), Water Hydrate P, mm. l°, c.

CH CCIF t, °C. 3

2

(Gas + Liq.), Hydrate P, mm. Below t


c

0.19 1.09 2.12 3.07 4.00 5.26 6.00 7.00 8.03 9.03 10.09 11.06 12.51

110.5 135.4 167.7 203 252.8 329.6 397.8 485.5 599.4 746.3 911.2 1168 1588

-2 0 1 1 3 3 4 6 8 10 11 12 12

80 63 80 83 06 82 38 00 10 10 71 50 95

13 13 13 14 15 15 15 16 16 17 18 18

10 60 87 17 15 18 62 25 59 10 11 17

1000.5 1148.0 1181.5 1184.5 1234 1267 1290 1367 1459.5 1569.5 1679 1720 1736 Above t

c

CH CCIF 3

2

(Gas ), Ice, Hydrate

-7.78 -7.24 -5.73 -4.85 -2.98 -2.56 -1.96 -1.04

1 -10

69.2 71.2 76.6 82.5 89.6 91.1 93.1 101.5

1 -8

I -6

Figure 4.

I -4

TEMPERATURE 1 0 0 0 / T ° K. I I I I I I -2 0 +2 4 6 8 TEMPERATURE °C.

Calculated phase diagram for system

I 10

1748 1779 1816 1830 1904 1891 1906 1967 2004 2031.5 2105 2101

I 12

I 14

I 16

L— 18

F-12Bl-water-NaCl

Figures 1 and 3 show the original experimental data points and Figures 2 and 4 are calculated from the data on heats of formation and hydrate compositions and are so constructed as to be more useful in engineering design. The experimental apparatus and techniques have been described (4, 5, 7), as have the methods of calculation (2).


A D V A N C E S IN

194 T a b l e III.


Point (Fig. 1)

E x p e r i m e n t a l Phase Equilibrium D a t a f o r the System F-142b-Water-Sodium Chloride

W, Wt.% NaCl 6.12 6.05 8.13 8.11 8.01 7.98 7.86 3.99 4.11 6.19 6.25 5.93

a b c d e f g h i j k I

CHEMISTRY SERIES

P

/, ° C .

Pi

4.66 5.69 4.24 3.34 1.22 5.58 6.90 2.95 4.15 2.98 4.91 1.88

559.4 706.9 635.4 515.5 306.0 835.9 1082.0 300.9 398.1 396.3 587.2 286.1

a

b

2

n

(No Salt)

c

293.0 366.6 267.4 219.4 137.1 357.9 475.8 201.4 262.1 202.7 309.5 158.8

17.58 18.01 17.14 17.03 16.29 17.06 16.72 17.32 17.38 17.96 16.99 16.64 A v . 17.18 ± 0 . 4 0 (av. dev.)

° P i . E x p e r i m e n t a l e q u i l i b r i u m pressure for given temp, a n d % N a C l i n m m . P. C a l c u l a t e d pressure for given temp, but w i t h no salt present i n m m . See 5a, Table V I . n. C a l c u l a t e d moles of water per mole of agent i n hydrate using P i a n d P2 w i t h M i l l e r and Strong equations (2). b

2

c

T a b l e IV.

E x p e r i m e n t a l Phase Equilibrium D a t a f o r the System F - 1 2 B 1 - W a t e r

CClBrF (Gas), 2

t, ° c.

Water, Hydrate P, mm.

CClBrF (Gas + Liq.), Hydrate t,°C. P, mm. 2

153.3 186.1 229.9 278.8 354.9 431.8 541.2 611.7 667.3 955.2 1267

0.08 1.02 2.02 2.95 4.06 4.89 6.05 6.50 7.09 8.49 9.73

Table V.

Below t

c

-1.51 0.13 2.04 3.60 5.03 6.77 8.03 9.59

834.3 890.4 947.3 1033.0 1067.5 1134.0 1185.5 1251.0 Above t

c

10.60 11.27 12.40 13.30 14.93 17.74

1295.5 1334.0 1387.0 1434.5 1513.5 1663.0

E x p e r i m e n t a l Phase Equilibrium D a t a f o r the System F-12B1-Water-Sodium Chloride

Point (Fig. 3)

W, Wt.% NaCl

a b

8.14 8.12 8.09 8.02 4.15 4.11 4.09 4.05 4.01

c

d e f g h i

t,°C. -2.04 0.28 2.55 3.55 -1.44 1.00 3.53 5.65 7.61

Pi

P (No Salt)

n

214.1 355.9 594.8 742.2 158.3 271.3 475.3 762.8 1152

92.4 156.6 260.0 324.2 106.0 184.1 322.9 513.0 782.2

16.82 16.44 16.49 16.63 16.72 16.22 16.45 17.03 16.32

2

A v . 16.57 =b 0.21 (av. dev.)


14. BRIGGS Table VI.

AND

BARDUHN

Fluorocarbons

in the

Hydrate

Process

195

Critical Decomposition Constants, H e a t of Reaction, Compositions,

a n d T e m p e r a t u r e Depressions Due to Salts f o r F-142b a n d F-12B1 ( M = agent, W = water, H = hydrate) H y d r a t e c o m p n . , moles H 0 / m o l e agent, n C r i t i c a l d e c o m p n . temp. ° C. ° F. C r i t i c a l decompn. pressure Mm. Hg P.s.i.a. Invariant point w i t h ice Temp., ° C . Pressure, m m . H g Constants i n equation logP = A-B/(t + 273.16) a. M ( g ) + n W ( l ) = H (Figs. 1 a n d 3, AB) A B b. M ( g ) + H = M ( l ) + H (Figs. 1 a n d 3, AC) A B c. M ( g ) + n W ( i c e ) = H ( F i g . 1, BE) A B Compressibility factor of M ( g ) at crit. temp, and press., Z Heats of reaction ( - A H = 4.574BZ) a. M ( g ) + b W ( 1 ) = H ( k c a l . / m o l e ) b. M ( g ) + H = M ( l ) + H ( k c a l . / m o l e ) c. M ( g ) + wW(ice) = H ( k c a l . / m o l e ) d. M ( l ) + bW(1) = H(B.t.u./lb. H 0 ) Depression of critical decompn. temp, by N a C l , ° C . 2 % (by weight) 4% 6% 8% 10% Depression of critical decompn. temp, by 6 % sea salt solution, ° F . H y d r a t e reactor operating conditions allowing 3 ° F . for d r i v i n g force i n 6 % sea salt solution Temp., ° F. Pressure, p.s.i.g.


2

4.

F-12B1

F-742b 17.18

16.57

13.09 55.56

9.9 49.8

1743 33.7

1272 24.5

-0.04 103.4

0 . 0 (assurr 147

28.78541 7311.8

28.95234 7316.6

7.33942 1173.0

7.48253 1239.3

8.00995 1637.5 0.930

0*952

m m

2

10.

31.11 4.99 7.492 152

31.86 5.40

1.10 2.27 3.49 4.77 6.22

1.04 2.15 3.32 4.54 5.92

5.82

5.53

46.74 13.9

41.3 6.1

159 '.7

Calculated Energy and Heat Exchanger Requirements for Hydrate Processes Adequate data are n o w available to estimate the energy requirements for converting sea water to fresh water b y the hydrate process, using the hydrating agents F - 2 1 , F - 1 2 B 1 , 142b, methyl bromide, and F - 3 1 . In addition, calculations have been made for comparison purposes on the propane hydrate process and on the direct contact freezing process, using n-butane as the refrigerant. Energy requirements are calculated for the primary and secondary compressors and estimated for the feed, recycle, and cooling water pumps for a plant producing 1,000,000 gallons of fresh water per day. T h e areas of the feed-product exchangers and secondary refrigerant condensers are also calculated. The general flowsheet used is shown i n F i g u r e 5. A n y work or heat required for stripping of products or deaerating of feed is presumed small and is not considered. Power for lighting, various small pumps, instruments, and other miscellaneous uses is ignored also. Energy requirements for agitation i n the reactor beyond that supplied b y the brine recirculation w o u l d also increase the given figures. The assumptions used to make the calculations are listed i n Table V I I . Saline Water Conversion—II Advances in Chemistry; American Chemical Society: Washington, DC, 1963.


196

A D V A N C E S IN

Figure 5.

CHEMISTRY SERIES

Flowsheet of hydrate process for desalting sea water

The results of the calculations are summarized i n Table V I I I , i n w h i c h the hydrating agents are arranged i n order of increasing critical decomposition temperatures. The following items i n this summary are worthy of comment. 1. T h e energy requirement for the primary compressor is practically independent of the hydrating agent used and averages 700 h p . , w i t h a maximum variation of 3%. This is due partly to the identical nature of a l l reversible heat p u m p i n g processes and partly to the fact that the driving forces i n the reactor and melter have been assumed at 3 ° and 2 ° F . for all agents. T h e 3 ° F . driving force i n the reactor is based on Koppers' work on propane, and assuming the same value for the other hydrates, may be overly liberal or overly conservative. Without more kinetic data, however, the accuracy of this assumption cannot be judged. The 2 ° F . driving force i n the condenser-melter is based on the work of Carrier and of Blaw-Knox on ice and is probably applicable indiscriminately to hydrates, since the process is physical and not chemical i n nature. 2. T h e work required of the secondary compressor decreases regularly w i t h increasing operating temperature of the reactor. It w o u l d be reduced to zero for a hydrate such as chlorine hydrate, for w h i c h the reactor w o u l d operate above the ambient temperature assumed. If the sea water feed were at about 5 5 ° F . no secondary compressor w o u l d be needed for F - 3 1 , since the excess vapor could be condensed w i t h sea water w i t h no compression. The amount of heat to be rejected to the atmosphere is fairly constant. It is the work required to pump it to the rejection temperature that decreases with increased operating temperature levels. T h u s the secondary refrigerant condenser has a fairly fixed area also. The work required i n this cycle is due mainly to incomplete heat exchange, but a small fraction is due to the work input of the compressors and p u m p . Heat leakage is negligible, although it has been included. 3. T h e feed-product heat exchanger areas decrease regularly w i t h increased operating temperature and they may be eliminated completely for F - 3 1 . This w o u l d represent a very substantial saving i n initial plant cost, since these exSaline Water Conversion—II Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

14. BRIGGS AND T a b l e VII.

BARDUHN

Fluorocarbons

In the Hydrate

Process

197

Assumptions Used f o r Estimating O p e r a t i n g Conditions in H y d r a t e Process

Sea water feed

3 . 5 % total salts

Temp. 70° F .


Reactor conditions Brine out = 6 % salts S l u r r y out = 5 % hydrate solids T e m p . = crit. d e c o m p n . temp. — (depress, due to 6 % sea salts - f 3 ° F . for d r i v i n g force) Press. = vapor press, of agent at operating temp. P r i m a r y compressor Suction press. = reactor press. Discharge press. = vapor press, of agent at (crit. d e c o m p n . temp, -f- 2 ° F . ) A d i a b a t i c eff. = 8 0 % D r i v e r eff. = 9 0 % Secondary compressor Suction press. = critical decompn. press, of hydrate Discharge press. = agent vapor press, at 8 0 ° F . A d i a b a t i c eff. = 8 0 % D r i v e r eff. = 9 0 % H e a t exchangers Feed vs. products T e m p , of both products out = 6 2 . 5 ° F . ( 7 . 5 ° F . approach) O v e r - a l l heat transfer coefficient = 200 B.t.u./hr. sq. ft.° F . Secondary refrig. condenser O v e r - a l l heat transfer coefficient = 125 B.t.u./hr. sq. ft.° F . Sea water i n at 7 0 ° F . a n d out at 7 4 ° F . Secondary refrig. load H e a t added to system due to Incomplete feed-product heat exchange A l l work input to both compressors W o r k input of recycle brine pumps H e a t leakage = 10,000 (75 - *

crit

. ) , B.t.u./hr. for 1,000,000 gal./day product

W a s h water W a t e r for ice washing = 5 % of net fresh water product Pumps A l l pumps A l l motors Recycle pumps C o o l i n g water pumps Feed pumps

8 9 % efficiency 9 0 % efficiency O p e r a t i n g press, difference = 10 p.s.i. 20 h p . per m g d . product O p e r a t i n g press, difference = reactor gage press, - f 15 psi. N o allowance made for energy recovery from product streams

changers alone represent 10 to 15% of the initial plant investment for the lower temperature processes. In the case of F-31, two possibilities for heat exchange between feed and products have been considered.

In case I no such exchange is used and in case

II heat is exchanged between the feed and brine out only.

The fresh water

product temperature is too near the feed temperature to exchange heat when a 7.5° F. approach is assumed. The addition of 1600 sq. feet of feed-brine exchangers reduces the energy required by 7% and the secondary refrigerant condenser area by 29%, for a net decrease in total required exchanger area of 15%. 4. The inlet volumes to the compressors depend mainly on the reactor operating pressures, the high pressure agents giving the lower volumes, since the moles of gas going to each compressor is substantially constant, with a variation of only about 10%. 5. Since the main contribution to the secondary refrigeration cycle load is the incomplete heat exchange between the feed and products, the energy required for this cycle may be varied at the expense of feed-product heat exchanger area.


198

ADVANCES

T a b l e VIII.

SERIES

C a l c u l a t e d O p e r a t i n g Conditions f o r the H y d r a t e a n d Freezing Process Using V a r i o u s A g e n t s Basis.

1,000,000 gallons of fresh water product per day

n-Butane + Ice Critical decompn. Temp., °F. Press., p.s.i.g.


IN CHEMISTRY

32 0.3

Propane

F-21

F-12B1

F-U2b

F-31 Case I, Case II, no feedfeedbrine Methyl prod. exchg. Bromide exchg. only

42.3 65.3

47.6 0.0

49.8 9.9

55.6 19.3

58.5 7.6

64.2 26.8

64.2 26.8

33.2 55.0

39.0 -2.5

41.4 6.1

46.8 13.9

49.4 4.0

56.1 21.0

56.1 21.0

Reactor conditions Temp., °F. Press., p.s.i.g.

23 -2.3

Feed-product exchanger areas, sq. ft. Brine Water Total

10,750 7,100 17,850

8,200 4,950 13,150

6,700 3,580 10,280

5,910 3,100 9,010

4,400 1,675 6,075

3,790 910 4,700

0 0 0

1,600 0 1,600

Primary compressor Horsepower (80%eff.) 677 Inlet volume, cu.ft./min. 43,000

715

708

725

720

710

704

680

10,000

45,600

29,300

21,600

33,000

18,600

17,900

363

284

263

223

183

166

118

1,345

5,200

3,430

2,580

3,500

2,430

1,730

Secondary compressor Horsepower (80%eff.) 448 Inlet volume, cu.ft./min. 5,710 Secondary refrig. cond. heat load, M B.t.u./hr. 2

Area, sq.ft. Major pumps, total hp. Compressor and pump, energy totals Horsepower Kw.-hr./lOOO gal. (incl. 90% driver eff.)

9.2

9.1

8.85

8.80

8.65

9.1

10.7

7.6

9,350

9,300

9,000

9,000

8,800

9,300

10,900

7,700

Ill

184

140

169

158

176

177

176

1,236

1,262

1,132

1,157

1,101

1,069

1,047

974

21.6

25.1

22.5

23.0

21.9

21.2

20.8

19.4

A 7 . 5 ° F. approach temperature was assumed for these calculations. A 5 ° F. approach would increase the exchanger area and reduce the secondary compressor load, and a 1 0 ° F. approach would do the opposite. An economic balance will eventually determine how best to apportion these variables. With direct contact heat exchangers in which a recirculating immiscible fluid is alternately exchanged against a hot stream and a cold stream, it appears that temperature differences may be reduced to about 3 ° F. economically. In the hydrate process the obvious immiscible agent to use is the hydrating agent. J*the exchange between the feed water and cold agent the feed will be cooled and simultaneously saturated with agent. This may reduce the required reactor volume somewhat. In the exchange between cold products and warm agent, little agent will dissolve, since the products will be saturated on entering these exchangers. Such systems have been studied by Woodward ( 9 ) .



14. BRIGGS

AND

BARDUHN

Fluorocarbons

in the Hydrate

Process

199

6. The total energy requirement for pumps and compressors decreases fairly regularly from 25.1 kw.-hr. per 1000 gallons for propane hydrate to 19.4 kw.-hr. per 1000 gallons for F-31 hydrate, a reduction of 23%. The advantage of the F-31 hydrate process over the freezing process is 21% in this regard. By reducing the approach temperatures in the heat exchangers and/or reducing the feed water temperature below 7 0 ° F. the energy requirement for F-31 hydrate process can be reduced to 17 kw.-hr. per 1000 gallons. The energy requirements may be expressed in units other than kilowatt-hours per 1000 gallons and it is interesting to convert these figures to foot-pounds of energy per pound of water, which is numerically equal to the number of feet that water can be elevated vertically with 100% efficiency. The conversion factor is 1 kw.-hr. per 1000 gallons = 319 foot elevation. Thus a requirement of 20 kw.-hr. for making 1000 gallons of fresh water from the sea is equivalent to raising the water at least 6380 feet vertically. This gives a visual comparison of the energy required to desalt sea water with that required to pump it uphill, and tells us that from the energy standpoint it is equally disadvantageous to be without fresh water at sea level and to have fresh water available several thousand feet below you. 7. The total heat exchange surface decreases regularly from 22,450 sq. feet for propane to 9300 for F-31 hydrate, a reduction of 58%. The advantage of F-31 over freezing is a 66% reduction of this quantity. These calculations point out clearly the two main advantages of a high temperature hydrate process over the freezing process or a low temperature hydrate process—namely, the substantial reductions possible in both heat exchanger investment and energy requirements. The energy advantage has been commented on by Wiegandt (8). Acknowledgment This research was sponsored b y the Office of Saline Water, to which appreciation is expressed. Theodore Kulpa and Richard Collette made most of the calculations for Table VIII. Literature Cited

(1) Barduhn, A. J., H u ,Y.C.,Briggs, F. A., Ann. Rept. 2 to Office of Saline Water (December 1961). (2) Barduhn, A. J. Towlson, H. E., Hu,Y.C.,A.I.Ch.E. J. 8, 176-83 (1962). (3) Barduhn, A. J., Towlson, H. E., H u ,Y.C.,Research and Dev. Progr. Rept. 44 to Office of Saline Water (September 1960). (4) Briggs, F. A., "Gas Hydrates of F-142b and F-12B1," thesis, Syracuse University, 1962. (5) Hu, Y. C., "Determination of Gas Hydrate Compositions by Thermodynamic Methods," thesis, Syracuse University, 1961. (6) Knox, W. G., Hess. M., Jones, G. E., Smith, H. B., Chem. Engr. Progr.57,66-71 (1961). (7) Towlson, H. E., "Gas Hydrates," thesis, Syracuse University 1960. (8) Wiegandt, H. F., Division of Water and Waste Chemistry, 139th Meeting, ACS, St. Louis,Mo.,March 1961. (9) Woodward, T., Chem. Engr. Progr.57,52 (1961). R E C E I V E D June 21, 1962.


Properties of the Hydrates of Fluorocarbons 142b and 12B1

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