Multiple effect flash (MEF) evaporator

Drsolinahn

- Elsevier Publithing

MULTIPLE

Compariy,

Amsterdam

- Printed in The Netherlands

EFFECT FL,ASH (MEF) EVAPORATOR

SUMMARY

The Muitiple Effect FIash* Evaporation System represents an improved vertical multiple effect system arranged to give a flow scheme resembling the multi-stage flash system. Performance characteristics are superior to other systems in showing reduced tube area requirements because of improved heat transfer. Feed-ffow distribution and interstage control problems have been solved by Vortex Level Control and Flow i)istributors*, small plastic devices located in the entrance of each brirre tube. Dropwise condensation has been achieved although further work needs to be done to insure reliabiiity. Economic projections for this process for sea water conversion appear very promising. SYMBOLS

A

-

4 C

-

=, d D G h

-

[Jr, k

-

N NIX,

-

brine tube area, outside, ft2 boiling number = (q/C Jz& solute concentration, Ib moles[ft2 heat capacity at constant pressure, Btuffb “F i-d. tube, ft molecular diffusivity, ft’/hr mass flux, ib/hr ft’ IocaI heat transfer coefficient, Btu/hr ft2 “F enthalpy diff. between sat. vap. and sat. iiq., BtufIb

rn~s-t~nsfer or

c~~cient,

(Ib moles~hr ft’)/(lb moles/ft3)

thermal conductivity of liquid, Rtu/hr ft2 “F mass transfer flux at phase boundary, lb moIes/hr ft’ Nussett(f.5) number for boiling or saturated liq. = @D/X-), dimensionIt3

P

-

absoiute pressure, lb/fP

* Patents applied for and assigned to the Regents of the University of California. Multiple Effit Rash

Evaporator,

U.S. Patent N0.3&?7,873,

issued Jan. 6, 1970. Desa~~~at~~n~7 (1970) 343-391

maintyp2-s ofevaporators in opp;ariun today, whirh make slqgewise to attain remmable rhcrmal econ~rny* are the muftipie eff&t and the multi-stage flash, The former is cxcmplified in the Long-Tube-Vertical CLTV) pIant of the Office of Saline Water(i). Tote I mgd O_S.W_ pfant. originally kcasctd in San Diego, and subseqwntJy mo*ed to Cuba, was of the multi-stage &sh t,ype. fn reomt years, the ‘Aash’ phnt has gained in popularity because of the simplicity of its design., and also because no boiling occurs inside the tubes, The latter presumably reduces the chances for scak f”ormation or foulinp, Active work on the MEF concept. which attempts to combine as many as possible of the desirable features ofeach process, was begun iti the stststmer of 1963, although many of rhc ideas predate this. An -attempt was also made to solve some of the probkms common to borh types of evaporators, such as corrusion and improved beat transfer. it has been shown by IkxigW) that, per stage, the multipie effect. process is more efficient than the multi-stage ffash. Thus* ir appeared desirabfc to retain the muitipk eff&z process while attempting to arrange the e&c& into a simpMied flow scheme that wotlld approximate the m&i&age fh system. Baiting was still &owed to occur in the tubes, as it is known that by pH controi, temperature The two

reuse

of heat

-CONDENSATE TO BOlLaA

PRODUCT 2

PRODUCT BRINE

-

CONCENTRATRD

INEWS TO

VACUUM gJ7.J ---,COOLANT PRODUCT Fig. 1. Multiple et&t flashevaporator(schcmntic).

pp. 3w48

b

3. MU@k CtTaY Ilash evaporator sysm (schcmaic). Rcvkd April 1%7.

MULTIPLE

EFFECT

FLASH

(MEF)

EVAPORATOR

349

limitation, and good feed distribution(3), scale can be controlled(4;. Further, any tube leakage would result only in some vapor loss rather than product contamination. One might consider the MEF concept as an improved LTV evaporator. Other improved versions of the LTV have been proposed by ICays( Standiford and Bjork(6), Detmanfl), The Dow Chemical Company(f), Oak Ridge Nationat Laboratory(l), Struthers Energy Systems(,‘a), and others. THE MEF SYSTEM

The MEF system differs from most of these other improved versions by employing short brine-evaporation tubes, having rehtively small diameters, in effects located directly above each other. The pressure drop between effects is taken at the entrance of the brine tubes. Tubes for preheating the feed are arranged as an integral part of the effects. Three alternate arrangements are presented in Figs. 1.2, and 3. it will be noted that. in all three. the effects are arranged to form a tall conicat.tower. Horizontal and inclined tube versions were also considered. but in general appeared to have certian disadvantages. mainly due to the effect of gravity on feed distribution and suppression of boiiing in Iow-pressure effects. Since the ‘Froduct Hash Alternate’ no-product-tube version (Fig. 2) is probably the best, its operation wi!l be described now in more detail. Pretreated feed sea water enters feed tubes near the bottom of the evaporator. From here it flows upward from effect to effect. being successively heated to higher and higher temperatures until it reaches the top of the evaporator, where it is further heated by indirect contact with steam. The hot feed now flows downward through flow distributing restrictions where fiashing occurs, and on into the top set of brine tubes where boiling occurs. The vapor and brine are separated. The vapor is condensed part& on the feed tubes and the rest on the brine tubes which open inta the effect beiow. The brine passes downward through Vortex Level Control and Ftow Distributors (see p”375) into these brine tubes. After flashing, the brine boils and exits as a mixture af brine and vapor. The condensed vapor is the product which is flashed from effect to effect for heat recovery. Vents are provided for the removal of nan-condensible gases from each effect. Concentrated brine and cooted product are removed from the Iowest (coldest) effect. It will be noted that no inter-effect pumps or controls are present. Flashing tends to accelerate the brine into the brine tubes for better heat transfer. Feed preheating is accomplished as if the evaporator were simply a multi-stage flash evaporator_ The vertical orientation gives a compact arrangement in which no brine contacts the structural outside waii. No recycIe of brine is anticipated to be desirable although, if it were added and the brine tubes removed, the evaporator would be simply a vertical multi-stage flash evaporator.

LEROY A. BROMLEY AND STANLEY M. READ

350 C5~1~5r~s5~

of~ro~~~~-tube

and

n5-~roi~t~~~-~~~~ wdorts

configurations of the MEF evaporator have been studied.

The producttube version (Fig. 1) has tubes that conduct the product from individual effects through the brine on the lower effkcts to the lowest, or condenser, effect. These tubes provide for cooling of the product by causing evaporation of some brine. This heat exchange takes ptace in a brine reservoir arranged so that the flow of brine is upwards around the outside of the product tubes. fn this Producr-tube version non-condensible gases are removed from the effects by the product tubes. The tubes are sized such that they do not run full and, therefore, periodically gulp small amounts of steam and other gases. In the no-product-tube or ‘Product Flash Alternate’ version (Fig. 2) the Two

product is allowed to flash from efkt to effect through an opening (orifice, valve, pipe, etc.) in the floor of the effect. The vaporized product must be recondensed and

tube area provided for this purpose, or, as in the case of our existing experimental evaporator. a lower production is expected. Non-condensible gases are removed by separate vent lines or lines with openings on each effect. in the experimental evaporator the prod&% lines, pr~v~o~~~y instakd in the pr~~~~-~~~ version, were converted to vent lines from each effect. Optimization cost studies have been run on both versions and less than 0_5$jtOOO gallons difference was found in the cost of water. The product-tube version is found to be cheaper. It also appears to be more efficient per efkct since heat given up in cooling the product is used to produce more product by direct evaporation from the brine. Except for these advantages of the product-tube

version, other considerations strongly favor the no-product-tube version. They are as follows: I. Product-tube heat transfer appears to be poor. The non-use of the product tubes, which represent IS% (9% effective) of the total tube area, only reduced production about 6 % and steam economy (lb. product/lb. steam) 4%. 2. EIlmination of the product tubes wou~dcons~de~b~~s~mp~ifycunstruct~o~

of a large ptant. 3. The product tubes were found to scale in areas insufficiently wetted by brine. 4. Cleaning and drop promoting of tubes is considerably more effketive if the product (which carries some promoter molecules) is flashed from effect to effect. A saving in treatment chemicals results. Large vortex level control and flow distributors (see p_ 375) would be a convenient way to allow the product to flash from effect to effect without appreciable loss of steam between effkfzts. Fig. 3, showing the MEF in&ding flash stages, is a schematic which may prove advantageous in the event that heat-transfer coefkients on the brine tubes in the low temperature eRkcts are found to be too low. In essence. the brine tubes are simply removed from those effects and a recirculating brine system added to give

’

hNtLTtPLE EFFECT FLAW

(hfEF) EVAPORATOR

351

a multi-stage flash system. The schematic (Fig. 3) shows four flash stages in the lower part of the MEF evaporator. Calculations indicate that this arrangement would only prove advantageous for the lowest temperature effects, and it appears that the economic advantage (if any) over the MEF ‘Product Flash Alternate’ (Fig. 2) is marginal, Rather than converting the low temperature effects to fiash stages. it may be better to use lluted, extended-surface, or other special brine tubes to improve heat transfer.

The feed pretreatment. used before the evaporator, is similar to that employed in most Iargeoperatingsea waterconversion plants today. Fig. 4 is a schematic of this. Sea water, or other feed material, is first freed of particulate matter. in the small MEF unit tested, this was accomplished by filtration through diatomaceous earth. in general, the MEF process wil; require the removal of smaller particulate matter than do the more conventional processes. This is because the fluid must pass through flow nozzles with throats as small as 0.08” diameter. After screening or filtration, acid is added to the sea water feed to prevent later deposition of carbonate or hydroxide scafc. The feed may or may not have been preheated before acid addition. The acidified sulution is sprayed into air to remove carbon dioxide. In the experimental equipment used. previous tests(8j showed that, with unheated feed, 1%2Qp! of the COZ remains after this treatment. The next step involves removal of gases, mainly CL, N,, and COz. by treating the feed in a vacuum degasser. One proposed degasser which should be particularly

effective in reducing oxygen to neariy zero concentration, is shown schematicsllly in Fig. 5. It will be noted that the bulk mass flow of the gases md water vapor is always away from the liquid being degassed, Finally, to insure complete removal of the last traces of oxygen, it is suggested that some type of scavenger IX added, such as metallic magnesium, iron. or perhaps a sulfite or other oxygen-getter. This pretreatment should minimize trouble due to scat& corrosion, and non-condensible gases. The deaerator used in the experimental test equipment consisted simply of a vacuum chamber into which was sprayed the acidified decarbonated sea water. This treatment was immediately followed by vacuum treatment in a string column (9). Previous tests(8, 10, II) on this vacuum degasser or deaerator showed that, depending on pH, anly from 2 to 6% of the original CO2 remains after this treatment (with cold feed) and that the oxygen level is reduced to about 30 ppb. An iron scavenger was used to reduce the oxygen below 5 ppb in a few tests, but ws not extensiveiy used as it was observed that copper tubing was quickly covered with a brown layer which may have been ferric hydroxide. It is not known whether this Iayer interferes with heat transfer or is advantageous in corrosion control although there is some evidence flla) to indicate that corrosion resistance may be improved. Desahafion,

7 (1970) 343-391

352

LEROY

A. BROMLk&’

AND

STANY_l%

M. READ

Another interzzsting possibility has been suggested by Roy and Yahalom(Zlb) This is the direct treatment of raw sea water by sulfur dioxide such as from a sulfur burner. This would liberate carbon dioxide just as does the present acid treatment. Sulfite would be Ieft in solution even after vacuum degassing to remove nitrogen, carbon dioxide and some SO,. This residual sulfite should remove ah oxygen and insure its continued absence fromthe feed and brine. The su~~te-bisulfiteeq~ilib~um would act as a buffer to help stabilize the pW of the brine. Experimental evaporafor

Fig 6 is a ‘-stage schematic representation of the experimental MEF evaporator. It was designed to enclose ail of the individual heat-transfer surfaces in glass pipe so that they could be viewed completely at any time. Because of the size limitation of glass pipe. it was neeessae to place the feed tubes. brine tubes, etc.. in separate glass pipes and to interconnect the pipes, as indicated in the schematic. As originally constructed. a product tube was run from each effect through a brine reservoir. This product tube also functioned as a vent tube. It was later modified to function only as a vent tube to simulate the ‘no-product-tube’ version (Fig. 2). For this purpose. external piping was added from near the bottom of the feed tubes and brine tubes on each effect through a vapor trap and a throttle valve to the next effect. The old product-tube lines were retained. with risers added at the bottom of the brine tubes. Thisenabled these lines to function as inerts removal lines since these entrances were slightly above the normal product level. The operation of the no-product-+& evaporator was much the same as the product-t&e version with the exception of the inter-effect product throttie valves which had to be adjusted with varying performance. These vaks were set so that there was no product build-up on an effect and no passage of steam to the next effect. In a large plant it is anticipated that this control could be accomplished by one or more large vortex ievel contro!lers (see page 375). Fig. 7 is a photograph of the evaporator in the support tower. Fig. 8 is a typical section. It will be noted that all copper tubing is contained in glass pipe, where\cr possible, for maximum visibility. Durin g operation it was possible to inspect the condensing surfaces, entrance and exits to brine tubes, liquid levels, etc., simpIy by removing sections of insulation. All heat transfer tubing was originally of commercially pure copper (99.9% copper, deoxidized high-phosphorous, about 0.007 % P, and about 0.002 % 0). Later, the copper condenser tubing was replaced by a~u~num brass. as the untreated sea water used for coo&g cawed excessiire corrosion of the copperSome aluminum brass tubes were also substituted for the copper brine tubes on the next-to-the-top effect. The final tubing artangement was such that the effects could be operated either with or without an individual product tube per effect. Venting of inert gases Desditmfion. 7 (1970) 343-39 1

Raw Seawa ---* I

g

I

Evaporator

Boiler

Coolant Out

NT TUBE

FEED TUBE’

I

-..

FEED

IBRINETUBE

p,p. 353.3:

$J

Evaporotor /

Ejector

H

i

Inert5

VAPO

FEED TUBE’

NT TUEE

‘BRINETUBE

VAPO '-ACID

FEEQ TUBE'

IFEED BRINE PRODUCT

Boiler

kaerator

Bciler

I

il

I

1

Inerts

VAPO

FEED TUBE’

NT TUBE __

‘BRINE TUflE

&FJ

FEED

m m

PRODUCT WATERVAPOR BRINE plus WATERVAPOR

BRINE

hwL+cPLE

EFFECT

FLASH

(MEF)

359

EVA~RATOR

Fig. 7. Multiple effect flash evaporator. U.C.S.D. Campus.

from the effects was accomplished by means of these individual ‘product’ tubes, whether or not they were carrying product. Venting proved to be inadequate only on the lowest (coldest) effect when the l/4” tube was also carrying product. Desalination,

7 (1970) 343-391

LEROY A. BROMLEY

Fig. 8.

AND STANLEY

M. READ

Multiple effect flash evaporator, typical sta@.

One vent (product)

tube was provided

per effect. All were f/4” o.d., 0.035”

wall-thickness. Feed tubes were continuous 3/8” o-d., 0.035” wail-thickness copper tubes from a coil. A simple tube straightener (Fig. 9) was used to allow the tubing to pass through hofes, about ten mils oversize, in the In-thick brass flanges between effects. A simple grommet-type seal was provided which was later loosened to allow more product ieakage between effects. This leakage was found to result in excessive corrosioa of the feed tubes and, hence, the grommets were again tightened. Desalination,

7 (1970)

343-391

MULTIPLE

=cr

FLUSH (MEF) WA~~RATOR

361

Fig. 9. Simple tube-straightener for MEF evaporator.

Dedinotion.

7 (1970) 343-391

LEROY .A. BROMLEY

362 I-ABLE

M. READ

I

EXPERIMEST.4L

__._--._

AND Sl-AKLEY

EVAPORATOk

.._..._-

DEI-AILS

-..___- --_.--.-

_..-- -.---.

No. of brine rttbes

Nominal brim

Vortex he1

rttbes

rttbe diumerer
canlrvf rftrorrr dianterer (in.)

9.00 9.00

5i8

0.077 0.083 0.083 0.092 0.103 0.107 0.130 0.138

Stage I?O.

11 11 9 8 6 5 4 4

.---.-l-__

Am2 of 6rimi [fr ’ )

8.83 9.16 7.65

8.18 7.85 7.85

C_.-.-.-

--

5/s 314 7/S 1 14 I? 1% -Condenser--

Brine tube length: 5 feet Two feed tubes, 3/S inch o.d. by 85 fixt tong Feed tube arua: 2 ft’ per effect Vortex tevei control throat length: 3 inch Effect height: 10 feet Condenser tube area: 30 ft’ AI1 heat transfer tubing: commcrcia! pure copper except condenser and a few brine tubes which were replaced with aluminum brass. One f-inch diameter vent tube per effect’ Pyrex pipe used to contain tubing ____I____________________..._...- __._._____

-- .._. ..__--..

..-__. .._ ___.__._ ..._

* In product-tube version of evaporator the product was also passed through these +-inch tubes where it was heat-exchanged with the brine.

It may be noted that in the no-product-tube version (Fig. 2). ali product is simply flashed from effect to effect, and. hence, product leakage is not objectionable as long as vapor leakage is minimized and feed-tube corrosion is not encountered. Certain of the experimental evaporator details are summarized in Table 1. Auxiiiar_s equipnrerrt

Sea water is pumped from the end of Scripps Pier (about 1000’ long), the intake being about six to ten feet above the sandy bottom and a similar distance below the surface. The water is conveyed to a sand fitter. &lost of the filtered sea water is used for the Scripps Aquarium and for other marine uses. For sea water conversion work a stream of this sea water, about eight gailons per minute, was filtered through diatomaceous earth, after which it was sent to a holding tank. The filtered sea water from the holding tank was acidified with concentrated (about 36N) HzSOJ to pH 4.5 and sprayed into air to remove CO,. Much of this equipment has been previously described(l0, II). After the air spray, the sea water Desalinarion. 7 (1970) 343-391

MULTIPLE

EFFECT

FL.ASH (MEF)

EVAPORATOR

363

was degassed in a vacuum deaerator (Fig. 10) from where it was pumped evaporator_

Fig.

10.

Vacuum

dcaentor.

The equipment was controlled central control panel (Fig, 11). Upefiziirtg

to the

from, and instrumentation

was located on. a

procedures

Once the auxiliary equipment was operating smoothly and pH vaks were steady. the start-up and control of the f&effect evaporator was very simple: 1. Evacuate the evaporator to within 0.05” Hg of the vapor pressure of any residual water in the system. Desalination,

7 ( 1970) X3-391

364

LEROY A. BROMLEY

Fig.

AND STANLEY

N. READ

11. Central control panel.

2. .Turn on feed flow to low value; also, turn on brine and product removal pumps. 3. Turn on steam to desired pressure. 4. Increase feed flow to desired value. The entire start-up of the evaporator was accomplished and steady-stage operating conditions were reached in about one hour. No instabilities have been observed. Shut-down witi;>ut product contamination was accomp!ished by turning off steam and reducing feed flow until product rate dropped to near zero. Feed flow was then stopped. Shut-down was easily accomplished in one-half hour. Only three operating precautions appear to be necessary to prevent scale: I. Maintain the pH of the fwd sea water to the evaporator on the acid side of neutral. It may be noted that the proposed commercial type of MEF will not have brine in contact with the supporting shelI, which might be of steel or concrete. 2. Limit the maximum temperature for sea water feed to about 275”F(Z3). This was not tested in this equipment. 3. Maintain the feed rate high enough to prevent over-evaporation. i.e. do not concentrate brine over about 3.5 times as concentrated as normal sea water. We have operated to 3.95 times the concentration of sea water, scale free. An essential correlation to this is that the feed be well distributed betw’een the tubes on the individual effects. This was accomplished quite satisfactorily by the vortex level control and flow distributors (plastic devices) one of which is set in the entrance to each brine tube. These simple plastic devices also provide, automatically,

al1 internal

control

necessary

for operation

of the evaporator.

It k

possible that a multiple effect evaporator operating at both the maximum brine temperature (about 275°F) and the maximum brine concentration may have some scaling tendencies at intermediate temperatures, as the equilibrium solubility of anhydrous calcium sulfate would probably be exceeded. Again, this possibility has not been studied in this evaporator. Desalination, 7 (I 970) 343-39 1

hlULl-IPLE

EFFECT

FLASH (MEF)

Perf~r~tt~n~e ehui~~~er~~~i~~ Typical data on the operation

recorded

365

EVAPORATOR

of the Seffket

MEF evaporator

has been

(33).

t6OC

REV-2570 l&Sf?iR

\ 1

0

1

2ro

It---

PREDKTEO

I

I

220 STEAM

Fig. 12. Prdudion

lFltMWlSE

1

I

230

CONDENSATIONS

I

I

1

240

TEMPERATURE;F

VS.steam temperature.

The effect of steam temperature on the production for the no-product-tubs MEF evaporator is shown in Fig. 12. It will be noted that production appears to increase linearly with steam temperature and is much higher with dropwise GOIIdensation than with filmwise. The fact that production is well above predicted vaiues indicates that actual Seat transfer is better than that calculated by existing correfations. The effect of feed flow and the manner of condensation is even more graphicatry shown in Fig. 13. Production uniformly increases with Feed flow. This is again for the no-product-tube version. The resulting final brine concentration is shown as a parameter on these curves. The dramatic effect ofchanging fromfilmwisecondens~esu~~~u~ion, 7 ( 1970) 343-391

366

LEROY

BRINE

BRINE

A. BROMLEY

AND

STANLEY

M. READ

CfflCEMTRATlON-DRO?HISE

CONCENTilATION-FILWISE

FEED, LB/HR Fig. 13. Production

vs. feed flow.

ation to dropwise is particularly evident. At a feed flow of about 2000 Ib./hr. changing from a filmwise condensation to dropwise condensation will result in a change in brine concentration from about two- to four-times that of normal sea

water. The range of variabtes investigated is summarized in Table 11. It wit1 be noted that the evaporator has functioned satisfactorily over a rather wide range of flow rates and temperatures_ Heat-transfer coefficients on the feed tubes appear to be about those that would be predicted from existing correlations. The Dittus-Boelter equation is used for inside coefficients. and the modified Nusselt equation for outside coefhcients as recommended by McAdams(l4). Brine tube coefficients art much larger than had been calculated by the methods of Wright et a/.(15). particularly at temperatures above about 150°F. Heat-transfer coefficients with dropwise condensation were even more impressive.

It was observed that on al1 the copper tubes, whether clean with a film of oxide {reddish-brown or black), the condensation was The foliowing cleaning methods have produced 100°? fi!mwise HNO, about 40% concentrated, NH,OH-(NH&CO, saturated, Desahariorr,

or coated only 100% filmwise. condensation: SO, 4- Cl, f 7 ( 1970) 343-39 I

MULTIPLE

EFFECT

FLASH (hfEF)

367

EVAPORATOR

TABLE 1L RAtiCiE

OF VARIABLES

_.__.__ _--

---.

IFiViZSl”lGA~O

.

-.-. _.._ _. .__.__._. _.-

AND

RESULTS ______-_

.I_(

I...

______---__-

-.-.-.----.-..--

_

--_1.-._.-._---.

Feed* salinity Feed fiow Feed temperatureto evaporator Feed pfi r~twsea water Feed pH after HzSOJ addition Fred pki to evapxator Feed oxygen to fvaporatOr Condenser now Condenser feed temperature Condenser temperaturerise

Condense&pressure Dcaerator prr?ssurs Steam temperature steam flow Product ftOW Product saiinify Product pH Product temperature Brine flow Brine concentration (without Brine pH Brine temperature Product/Steam GAMMA Brine tube heat flux

-

.___

_--.-_

.._..

3.34-3.38 lwi-2705 68-80 7.9-8.1

3.3-4-g 3.3-6.4 --m-40

*r 5,5oO-fO.6w 58-73 16-26

_

.._.

-____

_-.~

wt. percetrtsalt Ibjhr “F Normaf 4.5 Normal 6.0 parts per biliion lbjhr

“F “F

l.I-2.0

in. f+g

OS-O.8

in. Hg ‘F rbjhr lbfhr

213-246 108-34t @H-t953

3.539.0** M-6.1 72-M

307-I 177 scaling)

-.-___

1.88-3.95 X1-8.0 7g-106 4.59-6.48 266-1693 8020-33,430

parts per milfion “F Ib/hr times se3 water Normal 7 to 8 “F Ib/tb

Ib/hr ft Btufhr ft”

-,

. .___. _ _ .. - _.__. __-_--._.. __“_._ ,.._... __.____~.,.___ -____--Feed material is sea water at La JoHa, CaIifQmia

l

** A few product salinities

aver 39 were caused by a pinhofe feak in feed tube fsec Fig. 2%

steam. hot sea water, mechanized cleaning as by sanding or wire brushing. Oils, waxes, etc. must be avoided. Observed overall heat-transfer coefficients for the brine tubes appear to be mainly a function of effect vapor temperature, and are shown in Figs. 14 and 15. The best straight line from the data on clean tubes is Lf, = - 316.2 -i- 6.62t and fur oxidized tubes U, = - 64.37 + 4.625t.

(21

where U, is in Btufhr ft’ “F and t is vapor temperature in “F_ These equations are valid between about 110” and 220°F. The steam chest paints were not used because mixed condensation was observed to occur there. Desaiimztion, 7 (I

970) 343-39 1

$

FiUN NO.

1

i

600 i

000

1000 1

1200

1600

t 6.2 x 9.t 0lO.l

A 8.1

. 7.2

MOO 0 7.1

t

z il.1 Y II.2 0113 t II.4 a 12.1 0 12.2 l 12.3

/Rf

VAPOR

TEMPERATURE,

u

@F

L,

‘IS,

! a

X c

II

c 4. O&

.

230

ORDER LEAST SOUARES CURVE THROUGH ALL POINTS EXCEPT STEAM CHEST

STEAM CHEST POINTS MIXEO OROPWISE AND F~~~~lS~CONDE~S&T~ON /

r----------

240

250

i

if0

I80

190 VAPORT~~PERATUR~~OF

200

210

220

230

240

Id ORDERLEASTSQUARES CURVE THROUG~~ POINTS EXCEPT STEAM&EST

STEAM CHESTPOINTS MIXEOOROPWlSE AND FILMWISE CONOENSATION

250

LEROY A. BROhlLEY AND

370

STANLEY

M. READ

Calculated values for the feed tubes. with a sea water velocity of 7 ft/sec, are about 500 Btu/hr ft’ “F at 80°F and 900 at 220°F. Within experimental error. measured feed-tube coefhcients did not conflict with those calculated. It should be pointed out that the heat-transfer coefficients for the brine tubes are higher than those for the feed tubes at etevated temperatures. whereas the reverse is true at iow temperatures: the heat fluxes are_ however, larger in all cases for the brine tubes. This is mainty because the effective temperature driving force in the case of the brine tubes is higher than that for the feed tubes. It is further observed that the heat-transfer coetficients increased somewhat with brine flow rate. Since this is caused by an increase in the inside heat-transfer coefficient. it is best shown by an examination of the data taken with dropwise condensation (Fig. 16).

OO

I

t

,

500

1000 GAMMA.L8S/HA

Fig. $6. Heat transfer coefficient VS. brine wise conilensztion.

1500

2000

FT

flow per unit periphery of brine tube for drop-

It is wet1 known that dropwise condensation results in much superior heat transfer and, hence, considerable effort was made to achieve this in the EI-effect experimental MEF evaporator. Desalination,

7 ( 1970) 343-391

LWLTIPLE

EFFECT

FLASH

Dropwise

condensutiotr

371

(‘MEF) EVAPORATOR

The resuks of many of the preliminary tests on dropwise condensation have been p;.rbii&ed(i6). For operation in the Multiple Effect Flash evaporator, the following geiieral procedure was used: The tubes were cleaned. using SO, (in steam) with occasional small amounts of Cl,_ A small amount of octanoic acid also proved to be hefpful. The high-temperature efG_-ts, up to three of them, were easily promoted by injecting 25 to 100 cc of octar;oic ncid, containing I to 2 4% (ClaH3,S).,Si, directly into the feed. The lower effects were best promoted by injecting 10 to 30 cc of promoter solution directly i*tto the alternate effects. numbers 3. 5, and 7. Spots which had started to show filmwise condensation were usually returned to dropwise condition simply by reinjecting a small amount of SO2 with the steam. This seemed to be particularly effective if the filmwise condensation was due to ttie presence of a small amount of oxide. A major exception to this was areas which appeared to be coated with sulfide. This occurred mainly on effect number I (the first effect condensing vapor from boiling sea water). This black deposit was most difticult to remove. Even steam + SO? + Cl, was only partially effective in causing the deposit to sluff off. It appears that considerable effort should be made to prevent the entrance of any H2S into the evaporator. Overall-heat-transfer coefficients on the brine tubes. with dropwise condensation. are shown in Fig. 16. It will be noted that. particularly at higher temperatures, these are higher than any normally found in tubes having swirl promoters(ff). fluted tubes(18), or even finned tubes(l9); and are approaching those found with mechanically-assisted coeficients. as in rotating evaporators(If. 20) or the wipedfilm evaporator(2f). Although dropwise condensation appears to be the major factor in improving heat-transfer coefficients, it appears that the pfastic flow nozzle controllers. set in the entrance to each brir.e tube. have contributed substantiaily to the observed high inside coefficients. The experimentaliy observed overall heat transfer coefficients were fitted by least squares as a function of the vapor temperature, t, to give the following third order equation: uo = 5,186

-

90.82t

-i 0.55662

- 9.159 x 10-Y

(3)

with U0 in Btu/hr ft ’ “F and I in “F. This equation appears to give a reasonable extrapolation to higher temperatures. At about 300°F the equation gives a maximum of about 3,3QO. The equation should be used with caution for the following reasons. It has been tested only for 5-foot-long brine tubes with the vortex level control and flow distributors operating. Temperature drops between effects were limited and dependent on the &effect MEF evaporator used. De_wdinurion,7 (I 970) 343-39 I

LEROY

312

A. BROMLEX

AND

STANLEY

M. READ

It is suggested that. for small departures from these operating cor?ditions, the correlation Nub = const. (Re~)“~7”(X,)~o~3~5(B~)o~~9*(~~~)o~*

(41

of Wright et ai. be used as a basis for the change oicoetkients inside the brine tubes. even though the experime:rrtal coefficients at elevated temperatures are over a fatter of two larger than those calculated by the correlation. and the temperature dependence is obviously in error.

I

RUN’NO.

0 14.1 . 15.1 WOO-

At% + 19.1

+ 21.5 x 21.6 o 21.7 v 2l.B

x 19.2

a 21.9

0 19.3 v 19.4

2

n 19.5 *

2 20.1 Y 20.2 0 20.3

z

05 E g 2m ; z g Y s

-

.

’

f o * .

.

3rd ORDER

LEASTSQUARES CURVE

-22-i 22.2 22.3 22.4 22.5

* 20.4 . 21.1 0 21.2 .2L3 A2t.4

..

r A

a

0

A

+

A

A

IMAOEOUATE. THESE POINTS NOT USED IN CURVE DETEl?MINATfON.

0.

I 140

I

I I60 VAPOR

Fig. 17.

Overall

heat

dropwisecondensation.

trznsfer

coefkient

I

t

,

I

I80 TEMPERATURE, vs.

200

I

I

220

I

I 240

‘f

vapor temperatureoutside of brine tube,

Desalination. 7 (1970) 343-391

MULTIPLE

373

EFFECT FLASH (MEF) EVAPORATOR

The effect of brine flow rate on the brine-tube heat-transfer coefficients with dropwise condensation is shown in Fig. 16. It will be noted that coefficients increase with brine ffow rate. The abscissa chosen is the brine fiow per unit periphery. Gamma, and is proportional to the Reynolds No.. 4r/p. A similar plot made on log-log coordinates indicated that the power exponent on Gamma varied from about 0.25 at IOO’F to O-65 at 250°F whereas the correlation of Wright et af_iIf) would give about 0.52 independent of temperature_ Data scatter were such that either result may, however. be correct. For a true falling-film evaporator one would expect the heat-transfer coefficients to decrease as the brine flow increased if the film is in viscous flow or to increase only at about the 0.4 power for turbulent flow. Both the magnitude of coefficients and their rather rapid increase with flow rate, even at values of Reynolds No. which might be considered to lie in the viscous range. indicate that only ‘passibly for the largest and coldest brine tubes could the brine flow be called failing film. Forced two-phase flow is a better description of the brine flow and corresponds better to the ra?her turbulent mixture which is observed to exit from these tubes.

It had been found that, in the operation of a single-tube condenser(l6). good dropwise condensation continued for about two weeks without repromotion, provided the copper tube had been well cleaned before promoting with the octanoic acid-parafinic

BOO

$

thio-silane

’

i

5

I

1

IO

t

I

I5

I

20

I

UAY 1969

APRIL

Fig. 18. Effect

solution.

of

time on product rate after drop promotion. Rsaknatiun,

7 ( 1970) 343-39 I

LEROY A. BROWLEY AND STANLEY

374

ht. READ

A

results are presented in Fig. 18. It will be noted that the production fell rather rapidly at a rate of 3 to 4% per day for the first several days and then leveled off. The exact causes for this rather rapid deterioration are not known but, based on observations of the condensing surfaces. the following explanations are offered: I _ The copper and aluminum brass tubes on effect number 1, the first after the steam chest. darkened considerably and more rapidly became fiimwise than the tubes on the other effects. The darkening appeared to be due to sulfide formation, as it was more difficult to remove than oxide. The source of the sulfide may be from H,S produced by sulfatereducing bacteria or perhaps some mercaptan source resulting from decomposition of organic matter in the feed sea water. It was thought that these bacteria might be growing in the deaerator; however, microscopic examination( f2) showed the compie:e absence of suifhte-reducing bacteria (which would give off H,S) in the residues from it. The deaerator was evacuated to. and maintained below in Hg absolute about 95 “//,of the time during the last year. 2. On effect number 3 it was observed that the lower third of all the tubes had become filmwise even though the top two-thirds were still good?dropwise. The copper tubes were ail pink and clean. It appeared that non-condensible gases had accumulated on the lower third of the effect, resulting in promoter removal and filmwise condensation, even though the concentration was not high enough to cause apparent darkening due to oxide formation. 3. Whenever product water splashed or otherwise impinged on the tubes, the result was rapid change from dropwise to filmwise condensation. Non-contlensible gases

fn addition to non-condensible gases having an adverse effect on dropwise condensation it is well known that a diffusion resistance will be i~ttroduced at the vapor-liquid interface which may seriously interfere with heat transfer. Recent studies by Sparrow et af.(22). Tanner et uL(23). and others. have shown that the heat-transfer resistance (or increased temperature potential acquired) due to noncondensibie gases is more severe at low pressures than at high pressures. This may be seen from the following qualitative argument. Consider two condenser tubes operating at the same heat flux, but with one at one atmosphere (212”F), and the other at about 0.01 atmosphere (about 45°F). Assume the percent of non-condensible gases is the same in both cases. As a first approximation. the mass of water vapor condensed will be the same in both cases.

This is of course only possible for very low concentrations (-0.01% or less) of inert gases. One may estimate that the rate of removal of these non~ondensible gases from the liquid-vapor interface would be approximately proportional to the partial pressure of the inerts at the interface, It is interesting to note that, whereas at the lower pressure the velocity of approach of the water vapor would be about two orders of magnitudes higiter, the diffusion coefficient would also be higher by Desalinution, 7

(1970) 343-391

hKJLTlPLE EFFECT FLASH

(MIS)

EVAPORATOR

375

about the same amount. In any case, one sees that the partial pressure of the noncondensible gases at the interface would be about the same in both cases. The resu!ting temperature drop may be calculated by use of the Clapeyron equation: _dP dT

2 u .“____ Pi, = ._-T-ho IU2

or Ietting dP = AP = pressure non-condensibles,

(5) of inert gas which is pfoport~ona~

to the percent

(6) and hence we see that At. the effective roughly

inversely

proportional

temperature

loss across the gas film, is and proportional to has shown that as little as 0.01 % by

to the absolute

total pressure

the percent non-condensibles. Slegers(?&) volume can seriously reduce heat transfer. Since, at the lower pressure. the mo!e fraction of non-condensibles is apt to be even higher than at one atmosphere_ the deleterious effect of non-condensibles is even more pronounced. except possibly for the stratifying of the inert gas which may be more pronounced at the higher pressure. It is thus imperative that (I) Air-in leakage be kept to nearly zero; (2) Non-condensible gases should IIO~be bled from effect to effect unless they are kept out of the main vapor stream; (3) The feed must be degassed as completely as possible; and (4) Separate inert gas vents should he provided to each effect and to each critical part of that et%ct to insure adequate removal of these non-condenslble gases. As a criterion of the vacuum-tight system, it has been found that. if &he pressure in the closed evacuated 8-effect evaporator does not rise more than a few tenths of an inch of mercury overnight, the system is considered tight and air leakage appears acceptable. It is estimated that under this condition the maximum air concentration in the vapor stream in the evaporator while operating is about 20 ppm (by volume). It is recommended that the maximum be kept below 10 ppm for the lowest temperature effects. Yorrex levci control and flow distributors An essential part of the MEF design is the flow restrictors at the entrance of the brine tubes. Fig. 19 is a cross-sectional view of a typical ‘Vortex Level Control and Ftow Distributor’_ Fig_ 20 is a typical performance curve. The purpose of the plastic devices is to regulate the brine Row entering the tubes so that the foliowing conditions will prevail: I. Minimum brine hold-up in the MEF tower; 2. Adequate feed rate to all brine tubes;

LEROY

Sectton

-

Fig.

19. Vortes

A. BROMLEY

AND

STAPiLEY

M. READ

A-A 8 B-B

0.075.

level conrrol

and

tlow distributor.

3. Satisfactory operation, even when tube inlets are out of level by as much l/2-inch, avoiding costly leveling; 4. Pressure drop between effects to be used to maximum advantage by acceterating the brine into the tube to improve heat transfer; 5. Brine flow to be distributed evenly over the interior of the brine tubes; 6. Ability to operate over a wide range of overall feed rates: and 7. MI of the above to be accomplished at minimum cost. For most satisfactory performance. it has been found that these ‘nozzles’ should be smooth and wettable. Thus polysulfone, polypheneiene oxide (PPO), diallyi phthalate, or polycarbonate plastic is superior to polypropylene. Polycarbonate tends to craze and decompose at high temperatures in an aqueous enDesaharion,

7 (1970) 343-391

MULTIPLEEFFE~~

FLASH

(MEF)EVAPORATOR

377

vironment. Brass has been found to be satisfactory; porcelain or glass should also be satisfactory. The maximum flow is higher with the wettable materials, and the flow distribution is superior. When made in large quantities for a commercial scale evaporator the vortex level control could be injection molded.

sea Wokr sow. a? 4.1 @SC)

Exhauslinqto I otm.

Bc-

Throat Oia = 0.075’ Tube I 0 =0560’

0

I

I I

1

I

2

1

I

I

3

HEAD, inches lobowe bottom set holes)

Fig. 20. Typical performance inlet holes.

vortex level control and flow distributor

with two sets of

For the typical device shown in Fig. 19, it may be noted (Figure 20) that, as a ver_vcrunt approximation, the flow is linear with head for about two inches. At high liquid levels (Fig. 21) the flow is a maximum. Almost no swirl is imparted

to the fluid, and vaporization

only occurs af:er the throat of the device.

At lower levels. the flow enters the tangent inlet holes and distinct vortex is developed.

The flow is reduced markedly when the tip of this vortex penetrates the throat. Since the pressure in the throat is well below the pressure of the effect from which the saturated liquid is leaving, the liquid is superheated and rapid vaporization occurs into the vortex tube of vapor. Unless the liquid level is quite low, this may cause the flow to oscillate between a high and low level. This in no way hinders operation_ At low liquid levels, a somewhat larger continuous tube of vapor extends downward through the throat of the nozzle. This results in a rather low flow, but appears to give good gow distribution within the tube. The amount of vapor passed between effects appears to be completely negligible from a heat-balance viewpoint. The gradual curvature of the outlet carries nearly all of the fluid to the wall, and insures complete wetting of the wall. Desalination,

7 (1970) 343-39 I

375

LEROY

A. BRDMLEY

AND

STANLEY

hf. READ

LOWLloulO l.Evn vw?TEx F-lmyEo

--

---rr--> --- FL -.

y=

-_--

--_ _=

-F--Z - --_ -_. E

--z--. -,- .-=2.-Z --+

z -. .

SE&N

A-A

Fig. 21. Vortex loci control aad flow distributor.

Sizing vortex level cvntrols

Design calculations showed that about 85% of the pressure drop between effects should be taken across the vortex level control and Bow distributors. Since no vortex exists when the Sow is a maximum. and the nozzle functions as a simple single-phase nozzle, with almost no pressure recovery beyond the throat. it is possible to size the nozzIe for maximum flow conditions. The maximum desired brine Row per tube is estimated based on the maximum production possible and the minimum final brine concentration to be ahowed. Single-phase liquid flow is assumed, and the caIculations made in the usual way for a simple nozzle. In construction. every effort should be made to insure a smooth and wettable entrance and throat of these vortex level controis to prevent premature flashing. It is, however, possible that this may occasionally occur on some defect. and, therefore, it is Desalination. 7 ( 1970)343-39I

~~ULTIPL.F EFFECT FLASH (MEF) EVAIJ~RAT~R

recommended outlined

that

the throats

be sIightly

379 larger

than

given

by the ca!culation

above.

Over 4300 hours a,f i~te~mi~ten~ operation have produced a negi~gib~eamoun~ of state. This was checked by examining aI1 brine tubes with a borescope. Unty a few

small thin deposits were found. and most of them were IarseIy dried NaCl formed in the evacuated evaporator upon shutdown. Perftlrmance characteristics at the end of this period approximated those at the beginning. During most of this extended period the foiiowing conditions were maintained for the no-product-tube modef: 214 to 220’F Steam temperature Feed flow 1800 to 1940 Ib/hr Feed pH after acid addition 4.5 Under these conditions with a steam temperature of 214°F. production has ranged from 870 lbjhr with all filmwise condensation to 1220 15/hr with all excettent dropwise condensation. Product salinities have ranged from 6 ppm to 25 ppm. The higher valrte was obtained during periods of high pruductiort, indicating a small amount of carry-over of brine. The production ranged mainly from 5 to 6 Ibjlb of steam consumed for the %effect evaporator_ The variation is a function of insulation, weather conditions, etc. The exact upper temperature limit for long-term scale-free operation with pW control is not known. but, based on reported performance of others(24, 25) and some uon-evaporation tests of the author’s group in other equipment at 300°F. it wollld appear to be definitely above 275°F yet not as high as 300°F. On the other

hand. Fabuss and Lu(2.5~) indicate that anhydrous calciumsulfate might precipitate from sea water as tow as XO’F ant! even perhaps lower. Mulford(26) reported in tests on the San Diego multi-stage flash piant that no scare was observed at 250°F with a brine which was about f .8 times as concentrated as sea water. Simpson and

Nutchinson(i3? raters, together

have summarized the experience gained from commercial cvapowith some experiments of their own using acid-treated feed. and that it is possibie to operate above 275°F on once-through sea water

conclude feed and to concentrate

sea water to about a factor of 4 without

brine-reject temperature is near tindings of the author*% pwtq.

Optimirarion

cr;Ic&,:!tons

100°F.

This

latter

would

indicate

forming

observation

that

the

scale if the

agrees

highest

with the

temperature

possible. which will not chase scale. should be used. The benefit in going above 275°F is not fnrge in a dust-purpose pIant but does resuh in considerable cost

savings in a single-purpose

water p&W.

it had been previously reported sea water was essentially non-corrosive

that, in the complete absence of oxygen, to copper. Although this view still appears Desalilrnlion, 7 (1970) 343-39 t

LEROY A. BROMLEY

380

AND STANLEY

M. READ

to be correct, it must be somewhat modified, as some unexpected corrosion has been encountered from other causes. The oxygen content of the sea water entering the evaporator was not zero even though the latter was well deaerated. No regular analyses were made for oxygen although previous tcsts(Z1) had shown that the oxygen content of sea water coming from the deaerator was consistently between 20 and 40 ppb. A modified Winkler technique(Z6~) was used for ihose analyses. A eontinuot‘s oxygen nnalyzer(J6&, used for boiler feed water, was used for a short time but with only limited success. One can calculate the maximum possible riite of corrosion of a copper pipe, containing flowing deaerated sea water, if one assumes that the rate is diffusioncontrolled. It is further assumed that every molecule of oxygen that reaches the surface reacts according to the equation 4Hf

+ Oz f 4Cu --, 4Cu+

The mass-transfer

+ 2H,O

rate is calculaied

(7)

by the correlation(Z7)

found to hold for

soluble wall columns:

kd =‘b .

073

_

Reoos3 Sc’i3

(8)

D

The flux in moles of oxygen arriving

at the surface would be

IV = kc

(9)

where c is the bulk concentration of oxygen. At 25°C (77”F), for a f” id. copper tube containing deacrated sea water with a bulk oxygen concentration of 30 ppb and at a flow tate of 6 ftjsec, one may calculate(33) that the corrosion rate would be 0.005” per year. At 150°C (302°F) the maximum possible rate would be about 8 times as large. Even with only 1 ppb oxygen at 125°C (257-F) the calcuiated maximum rate is about 0.001” per year. It is thus evident that, if surface reaction kinetics are not limiting, which is probably true at high temperatures, the bulk oxygen concentration of flowing sea water should be kept be!ow about 1 ppb. This would require use of a scavenger in addition to deaeration. Cormsion in es-perimenral evaporaror All heat-transfer tubing of the evaporator was originally of pure copper. The condenser and a few brine tubes on effect no. 1 were later replaced by aluminum brass. Representative tubes were sectioned and examined under a IO-70 power zoom stereo microscope. Several evidences of severe corrosion were observed. Most of the tubing had been used for over 4O!Mlhours. Desalination,7 (1970)

343-391

MULTIPLE

Feed

EFFECT

FLASH

(MEF)

381

EVAPORATOR

tubes

These were originally continuous tubes of copper extending through ali eight effects. The deaerated sea water was heated in these by water vapor from each effect condensing on the tubes. A grommet seal was made around the tubes at each tube sheet. This seal was later loosened to allow product and some noncondensible gas to flow downward at high velocity between the tube and tube sheet.

‘_ ____- _...

,. ._c _. ” a_,..;-: - c..- ,<. ..a. .... _‘_.,.:-_ _ :. _

.

.. _

,-.

._-

::. _., ,

Fig. 22. Feed tube corrosion caused by flow of product and non-condensable the outside of the copper tubes.

gases over

This resulted in very severe corrosion oi the feed tube (see Fig. 22). A tight seal should be maintained where the feed tubes pass through the tube sheets, or other protective measures taken. The inside of the feed tubes showed evidence of a very serious corrosion situation. indeed, one small hole developed in one feed tube on effect no. 3 at about I8G”F by corroding through from the inside (see Fig. 23). It was apparent that the large pit was due to an occlusion in the copper. Careful examination indicated several other small pits and a general roughening of inside surface (see Fig. 24). Oesalinalion,

7 (I 970) 343-391

382

A. BROMLEY AND STANLEY M.

READ

Brine rubes Some has been near the and outside the brine where non-condensiblc were poorly This was mainly to of chlorine, was used cleaning and of drop promoter from these tubes. Oxygen would probably have caused oxidation in time

Fig. 23. Corrosion pit in copper feed tube. This was caused by an occlusion Deaerafed sezt water flowing at about 10 ftjsec temp. 180’F. Magnifkation x 40.

in the metal.

in any case. Other spots on the outside of several of the tubes, that showed appreciable corrosion, occurred where they touched the glass surrounding them. There appeared to be copper removal around these spots and some evidence of deposition of copper in the centers. This same phenomena occurred with an aluminum brass tube (see Fig. 25). The etching on the outside of the tubes was, in general, rather mild. Except for one pit about 6 mils deep, all others were less than I mil deep. There was no evidence of corrosion at the bottom of the brine tubes on the outside where a shallow layer of product contacted them. The interiors of all the brine tubes were examined with a borescope. A few small thin salt deposits were observed, but no signs of corrosion were evident. Tubes from each stage were sectioned and examined_ Although at about 70x magnification all appeared to be strongly etched inside, the deepest pit located was Desalinurion,

7 ( 1970) 343-391

MULTtPLE

EFFECT

FLASH

(MEF)

3x3

EVAPORATOR

about t mif deep and there appeared to be no evidence of any serious corrosion. A typical surface is shown in Fig. 26.

of

Fig. 24. Typical corrosion of interior copper feed tube with deaeratrd se& water Ilowing at ten ft/sec temp. 180°F. Magniftczxtion x 40.

The condenser tubes were cooled with high velocity (about 10 ft,‘sec) raw sea water in the tubes. It was expected that corrosion might be severe, and it was. Pitting occurred from the inside near the warm end of the tubes. AIuminum brass tubing. substituted later, also showed evidence of serious pitting corrosion due to high-velocity raw sea water.

A Dutch-weave. 12 x 64 mesh. Monel screen (equivalent to 60-80 mesh) was located just before the evaporator in the deaerated feed sea water stream. Except that it had ruptured at tbe wefd seam, the Monet itself appeared to be in excellent condition with no evidence of corrosion. Fine 2OCmesh Monet screens, located at the inlet of the degasser, were completely corroded. Desafinaricm, 7 (8 970)

343-391

384

LEROY

A. BROMLEY

AND

ST_\SLEY

hf. READ

fig. 25. Corrosion of outside of aluminum brass brine tube where it contact& the +SS wall. Magnification 2: 10.

Alf P.V.C. pipe used at room temperature remained in excellent condition. In seven years the Dacron cloth backing up the diatumaceous earth in the filter was never replaced. Nylon strings in the deaerator were still mder tension. and in excellent condition after about nine years of ilse in deaerated sea water. Polycarbonate plastic deteriorated badly even at about 100°F in brine. Potysulfone appeared to hold up weti, but ~11s only tested over a few months at about 220°F in deaerated sea water. Polyphenekne oxide fP.P_O.) appeared to surface-etch rather rapidly fthree months), b&t otherwise remained in good condition. Teflon gaskets and O-rings have not shown any deterioration except coid ffow.

Drsahtation,

7 ( 1970)

343-391

MULTIPLE

EFFECT FLASH (MEF) EVAPORATOR

Fig. 26. Typical surfan: of inside of copper brine tube showing etching but no major pits. Magnification x 40.

Bronze pump parts. valves, etc. exposed to deaerated sea water remained in excelient condition for years although those exposed to normal sea water, and especially acidified sea water, corroded rather rapidly.

Ethylene-propyfene rubber appears to be rather resistant to hot sea water. O-rings of this material have been used in other equipment in contact with sea water up to 200°C for short periods (several hours). Although some permanent set resulted, they were not softened. nor did they tend to crack. They were superior to silicone or Viton and much better than neoprene.

DesaIinatiorr. 7 ( 1970)

343-39 I

LEROY A. BRONLEY AND STANLEY M. READ

386

Reconuttendatium on corrosiott control Based on our observations and the experience of others. it is recommended that: I. Sea water not only be degassed but aho scavenged of oxygen or oxidizing agents before entering the evaporator. 2. All tubes carrying sea water under pressure within the evaporator be of 95-r5 copper-nickef (even though we have not tested this ahoy) or other corrosionresistant affoy. Careful metaaflurgicaf controt should be maintained as random samples of new 90-10 Cu-Ni tubing had ;x large number of serious surface impcrfections and pits up to 5 mils deep. 3. Continuous tubes, passing between stages or effects, must be sealed or otherwise protected from corrosion caused by leakage along the tube at the tube sheet.

4. Tubes carrying

evaporating brine may be of copper or perhaps admiraity not touch a solid surface fexccpt at ends). specifications and vacuum tests should be appfied to aii

(not tested). Tubes should

5. Very stringent stages or eRkcts,,expected to be below one atmosphere. to insureabsence ofair feaks-

A number of computer optimization eaiculations have been made for this Multiple Effect Ffash system. Aff indicate that. for a large system. this process shoufd have distinct economic advantages over the multi-stage flash system. One typical example for a 155-mgd dual-purpose plant is given in Table 111.This program, which contains the elements needed in the design of a MEF evaporation plant, wifl be provided by the author upon request. The reader is cautioned that the costs are relative an3 were made from cost data avaihbte in 1963. cottcitcsiotts

Advantages of the MEF forced-downRow process and comparisons which may be made with other processes. including fatling-film. multipte efl’ect. and multi-stage fiash. on the basis of its observed performance. are: 1. Thermodynamic Efficiency - Sirto,- the MEF process is in reality an improved multiple effect process. the effect efhciency is at feast that of the usuaI multiple effect which, per effect (stage)_ is superior to the multi-stage gash. 2. Volume of Equipment - Because ofcompact arrangement, relatively smafi tubing. higher beat-transfer coefficients. and large. effective thermal driving force for heat transfer. the total equipment volume of a commercial plant shoufd be Ies: for a given production than for competitive processes. 3. Versatifity - The MEF evaporator is capabfe of being operated cfEcientIy over a wide range of conditions without equipment modification. 4. Flow Scheme - This resembles the conventionai flash process in that all evaporation and condensation takes place within the major piece of equipment. Dcsdindion,

7 ( 1970) 343-391

AWLTIPLE

TABLE

EFFECT

(MEF)

EVAPORATOR

387

111

CALCULATED

Fitoht

FLASH

iSO_~tco

EFFECT

OF

FlLRtWtSE

DUAL-puwwsE

____._.. _. -__ ____

_____._-.+.

VS. DROPWISE

*tui_~tp~E ..,___

COVDEXSATION

EFFECT

n.631

_

. ._._ _-

_.__

(~wi

0s

COST OF CONVERftD

mmm

-.-__-.

SEA

WxrER

(~~-PRoN~c~-wBE~~

. .c-. -_- _._.__~.

._-

.ez&rMmoNf

Item

ti,ti’r 27SS”F 65°F 10.7~“!I000 Ibs O.K/kWh 7_-t~/&ar 4.0yQfear 3 30 days: year ~~i;ttninttrn bnss* tutbe!+ in stcvI shell 7 itJ%ec Apprortimateky twice brine tube iength

Steam templx-ature Sea water temperature Steam cost Electricity cost Fixed charge %&Year lift int. charge Operittion ltlWrii&i Veto&it_v in feed tubes Feed tube length

‘Irpe

of Cundeftsr7riun

Fi%nw&e No. of

cY;tporatOrS

No.

eects

of

3 15

Height Diameter botrom Wuneter top Tube area per evaporator Lbs. prod..&s. steam Feed tube o.d. Brim! tube 0.d. Brine tube kngth Brine concentration

243 t&t fro feel 30 feet 2.n4 ? $06 ft’ 10.9 t3.50’ I!.?” to r 4” 7.5 feet 3.5 :,’ sea water

8.4 t.7 1.8 6.5 I.8 1.6 0.29 21 g*+

-__ ._.._._ __,__.-_--..-

--.---..-_-.-__.I_..-_.._.,--~

.

i”?roj7lcisp 3 16 245 feet 47 Fit 24 feet 1.91 2. Wft”

12.6 0.50’ 3/d’ to I!’ 7.0 feet 3.5 Y sea water

7.3 1.6 1.8 5.1 1.4 t% l&6* --e-m

-

-

Calculated cost of product wafer for copper tubes was 0.2~ cheaper. Aluminum bmssandcopper were assumed to cost the same per tube of equal six. Costa are as of Dee; 1963. l * These calculated costs should only be considered to be reprexntntive. l

Dexrlirtntion.

7 if9761 343-393

LEROY A. BROMLEY AND STAFLEY

388 OTIIER

OFTIMIZED

M. READ

RESULTS

The temperature at the cold end.

difference between effects at the hot end should b: about 40:4, of that

About SSg< of the pressure drop bcrween cffccts should be taken through the vortex Ie~_4 control and flow distributors. As d very crude approximation. the brine tube area per efEct should be about the same for all effec?s and from 3 to 5 times the feed-tube area per effect, depending on whether condenslltion is expected to be dropwise or filmwise, respectivctq-. Condenser-tuti area (incIudin8 feed-t;& area on fourst cffcctj should be 3 to 5 times the fecxW.&e area per efTect. All tube sizes cou!d je shifted appreciably from optimum values without s;gnificantiy airecting the cos s. The optimum feed \clocity cf 7 ft,‘sec could be shifted from 5 tQ 9 ft/see without adversely affecting costs.

5. Brine Concentration in Hot Effects - Because of no recirculation, this is minimum. as with the typical multiple effect. 6. Brine discharge concentration - Limited only by possible scaie formation in the coldest effect. 7. Outside structural wa!l and main tube sheets are not contacted by brine. The pH of the product which does contact these members may need to be raised to minimize corrosion. 8. Tube leakage - About 80 7; of the tubing area in the MEF is that of brine tubes; perforation ofthese results only in some vapor short-circuiting between effects rather than in product contamination. This should allow somewhat cheaper and Iess corrosion-resistant material to be used for these tubes. 9. Vapor piping - This is held to a minimum, as in the multi-stage gash procesi. 10. Controls - Control of rhe entire MEF evaporator is accomplished by regulating the steam temperature (or pressure) and the gross feed flow rate to give the desired product rate and brine concentration. Ah internal control is accomplished automatically by the vortex level control and flow distributors. 1 I. Heat transfer coefhcients - With dropwise condensation at elevated temperatures, these are measured to be higher than for any other known existing process using plain tubing without mechanical assistance. It is believed that. per unit cost, these coefficients are the highest known. With filmwise condensation, the coefficients may still be considered to be high. 12. Heat fluxes - Measured heat fluxes have been much higher (by severaf hundred percent) than those obtained for other processes. Not only are the heattransfer coefficients high, but the temperature driving forces are the same as for the multiple effect sIrstem, which are higher than for the usual multi-stage flash system. 13. Pressure drop between effects - This is fully utilized by the vortex level control and flow distributors to assist heat transfer. a

Desalinat;bn,

7 (1970) 343-39 I

MULTIPLE

EFFECT

FLASH

(LlEF)

EVAPORATOR

389

13. Pumps - Fewer pumps are required than in either the typical multiple effect or multi-stage flash. or in other propo.;ed modifications of them. 15 Pumping power - Evven though a rather large vertical lift is required. estimated power requirements are Iess than for other processes. This is because of the absence of brine recirculation and of the smali vertical lift per effect. 16. Esternal heat exchangers - Nose required. 17. Maximum stenm temperature - This should be as high or higher than for other processes. There is no brine recircutation. Fcrd is easily distr‘butrd evenly among the tubes on the hottest stage. 18. Piping - As with the multi-stage flash. piping is held to a minimum. 19. Feed pretreatment - Except for a finer screening of the feed. pretreatment should be the same as with other processes. 20. Feed heating - This is the same as for the multistage flash system. 2t. Flash evaporation efhciency - Since liquids are more or less in free fat1 between effects. no boiling-point &ppression due to hydrostatic head isencountered in contrast to horizontal flash equipment. 22. Land requirements - This equipment occupies the smalfest Iand area of any operating or proposed equipment. 23. Maintenance - The rather large height of this equipment may make maintenance somewhat more costly. However. the brine does not contact the support shell. and scale formation should be zero (or at a minimum). 1-l. Product salinity - It is possible to control this with baffles and demisters. 25. Projected water costs - Both initial equipment cost and annual water cost appear to be lower than for other processes invoiving large-state equipment.

ACKKOWLEDGESIENT

Funds for the support of this research were furnished by the State of California and allocated to the investigations by the Water Resources Center of the University of California. The authors wish especially to acknowledge the assistance of Anthony E. Diamond. who was a co-inventor of the MEF process and was responsible for some of the design and operation of the equipment. Others who have contributed significantly to this project include John W. Hughes, James Q_ Smith, Wayne W. White. and James S. Wright_ Most of this work was performed at the Sea Water Test Facihty, Scripps rnstitution of Oceanography, University of California. San Diego. The cooperation of the personnel at that location was appreciated.

Dt~x~lirrutio,r, 7 (I 970) 343-391

LEROY A. BROMLEY

390

AND STANLEY

M. READ

REFERENCES 1. OFFICE OF SALINE WATER. U.S. Department of the Interior. Annrrat Saline Wafer Conversion Reporrs. 1959- 1967. Processes for Recovery of Fresh Water from Saline 2. B. F. Dovotz. Review of Distiiiation Waters, A&wxes in Chemistry Series No. 38. Am- Cbem. Sot., Washington. D.C.. 1963, p.1. 3. D. D. KAYS. Desalting Sea Water in the Multiple Effect-Long Tube Vertical Evaporator. Proc. Firsr Intera Symp. on Ffafer Oesahmtion, IVashi~,tgrom, D-C.. Ocr. 3-9, 196.5.3 (1967) 23. Ofice o/ S&ire Wurer, RCS. 4. S. F. h’lULFDiiD. Scale Control in Sea Water Evaporators. Develop. Frogr. Rep!. No. I-13. 1965. Prw. 4 R’esfcm 5. D. D. K~\Ys, Advances in Vertical Tube Piants for Sea Water Distillation. Burrr atin Power Synrp.. Los Angles. Cnlifi. April I!&Y. 6. F. C. STAS~XFORD AND H. F. BJORK, Large Plants for Salt Water Conversion, Chem. &ug. Prqr.. 63 (1967) 70. Combination Process in Large Desalting Plants, C/rent. Eug. Progr.. 63 7. R. F- Dtrrww, ( I9671 80. 73. STRUTHER~ ENERGY Sy.st~~s [SC.. Design and Economic Studs of a Gas Turbine Powered Vapor Compmssion Plant for Eiapomtion of Sea Water, @@ice of Saline Warer. Res Develop. Prcgr. Rep?. No. 377. 1969. UNIV. of Chef,. BERRELEY. Sea Wafer Cotwersion ilrliururnry Reporr No. 64-t (1963

Progress

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L- A- BROMLEY- S. M. R-D ABD S. S. BUPARA. Falfing Liquid Sheets. tmt. eng. Ctzem., 52 (1960) 311. L. A. BROMLEY, Multiple Effrct Rotating Evaporator. Desatiffariorr.I (1966) 367. L. A. BROMLEY. Multiple Effect Rotating Evaporator, Univ. of CutiJ Warer Resources Cenrer Contribution No. t00. April 1965. I !a. ARTHUR, D. LII-TLE, INC.. Suivey of Condenser Tube Life in Salt Water Service. O&e uf Saline Water, Res. Develop. Progr. Repr. No. 278, 1967. I !b.A. ROY AND J. YAH~LOM. Protection of Sea Water De-Salting Eauioment by Owaen Scavenging. lncl Intern. Congress on Marine Corrosion and Forttiig. kherrs. Grek, &S. 12. MICHA~X. CAHS, private communication to the author, 1968. 13. H. C. Stwso~ AND M. HUKHINSON. Desatinarion. 2 (1967) 308. New York, NY., 1954. p-331. 14. W. H. MCADAMS, Heat Transmission, 3 cd., McGraw-Hill. 15. R. M. WRIGHT. G. F. SOS~ERVILLE. R. L. SAlr;t AND L. A. BROS~LEY, Downftow Boiling of Water and a-Buttiol in Uniformly Heated Tub, Ctrerx &g_ Progr. S_smp. Set. No. 57, 61 (I%!+) 220.

Condensation 16. L. A. BROM~EY,J. W. PORTER ASV S- M. READ. Promotion of Drop-by-Drop of Steam from Sea Water on a Venicat Copper Tube. Ant_ 1sz.s. Chenr. Engrs. f., 14 (1968) 245. 17. OAK RIDGE NATIONAL LABORATORY, Wuter Desatinurion i~rformafiun MeeGtg, Mas 10-t I, 19&5,OKNL-3%2.

18.

GENERAL

ELECTRIC Co.,

Pilot Plant. Ofice

Operation

and Maintenance

of 3i.000

gpd Thin-Film

Distillation

DfSatine Water, Res. Develop. Progr. t?ept- No. 181, 1966.

of Film Condensation on Vertical Tubes by Longitudinal 19. DAVID G. THOMM, Enhancement Fins, An?. Inst. Chem. Engrs. J., 14 (1968) 644. of Hickman 20. BATTELLE MEMORIAL ISST~TUIE, Summary Report on a Study and Development Sea Water Still, Office of Satine Water, Res. Devetop. Progr. Rept. No. 43, 1960. 21. GENERAL ELECCR~CCo., Economic and Technical Evaluation of the Wiped-Film E\iaporator, 22. 23.

Ofice uf Saline Water. Rex DeveIop. Progr. Rep?. No. i05, 1964. E. M. SPARROW, W. J- Mmzow~cs ASD M. SADDY, fn~enz. J. Ifed Mass Trwsfer, 10 (1967) 1829. D. W. TAXPSBR,D. POPE, C_ J. POTIXR AND D. WBT, Intern. J. Hear Mass Tratzsfer. 11 (1968) 181.

23a. L. St_~~ctts, private communication

to the author

(1968).

Desalination, 7 ( 1970) 343-39 1

~LTIPLE

EFFECT FLASH

(MEF)

391

EVAPORATOR

23.

Fourth Annual Report Saline Water Conversion Denomstration Plant No. I, Freeport, Texas. O&X of Saline Water. Res. Develop. Progr. Rept. XI. 171, 1966. 25. 8~LDwts-Lt~-H~altLTO~ CORP., Scale Control for Saline Water Conversion Distillation Plants, Ofice of Suline Water, Res. Develop. Progr. Rept. No. 186, 1966. 2%~ 8. M. Fneuss AND C. H. Lu. Precipitation of C’alcium Sulfate in Desalination Units, Preprint of Paper from S_vmp.on &tine Wafer Conversion. rim. Clrcnt. SM.. ISS Nuff. nfeefing Sati Framisco, Paper No. 5, March-April 1968. p.31. 26. S. F. MULFORD, private communication to the author. 1%8. 26a. AMLRICAS Socrmr FOR TESTWG MATERIAIS. Committee D-19 on Industrial Water (ASTM Special Teehnirzi Publication No. 1+8-D). Dissolved Oxygen in industrial Water, 1960. 26b.C~~m~mo~ ISSTRUXNT COYPAXY, Ix;c., Dissolved Oxygen Analyzer Recorder. 27. R. E. THEYRAL, hfass Trunsfe#r Operations. 2 ed., McGraw-Hill. New York, N.Y.. 1968, pp.56. 62. 28. JOIST MET. WATER Dm. OF S. CAL., OmcE OFSALISE WATER, Aroosw ENERGY Cohtntss~os, Engineering und Economic SIII& Phases I and II for a Conrbinatim Desulring Plant. A.E.C. Div. of Tttth. Info. TID-22330 (Vol. I).

Nuclear Power and

29. K. A_ KRAUS. R. P. HAMMOSD.and several other authors from the Oak Ridge National laboratory. Taken from Abstmcts of Papers, Water and Desalination Information Meeting, May 7-8. 1968. 30. -4. FRANKEL. Flash Evaporators from the Distillation of Sea Water. Pruc. Inst. hfech. Engrs... 173 (1960) 317. 31. BECHTELCORP., Cost Studies of Large Multi-stage Flash Safine Water Conversion Plants, O.@ieeof SaIitre Water. Res_ Deveiop. Progr. Repr. LVF. 116. 19s. 32. R. W. LOCKHART ANO R. C. MARTISELLL Chcnr. Engr. Progr.. 45 (1949) 39. 33_ J._.A. BROMLEY A?ct) S. M. REAP. Multiple Effect Flash (MEF) Evaporator. Univ. of Calif. Seu Water Conversion L.ubmator_v Report No. 684. Berkeley. Calif., Dec. 1968.

Desalination. 7 (1970) 343-391