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BRINE CHEMISTRY/COMBINED HEAT AND MASS TRANSFER

EPRI Research Pro jec t 653-1

D. W. Shannon D. W. F a l e t t i D. L. Lessor J . R. Morrey

Bat te l le , Pac i f i c Northwest Laboratories

INTRODUCTION

o r concerns i n geothermal development today i s the extent o f the resource base. W i l l a "geothermal f ue l supply" l a s t the 20 t o 30 years needed t o obta in a sa t is fac to ry re tu rn on the investment of an e l e c t r i c generating p lant? I f the p l a n t investment i s to- be successful, we must a lso assure r e l i a b l e p l a n t operation, a h igh p l a n t factor, and the disposal o f the spent geothermal f l u i d s i n an environmentally acceptable manner.

There have been a number o f studies t r y i n g t o optimize a geothermal power p lan t cyc le (such as mult istage f l ash versus binary f l u i d ) f o r various geothermal reservoirs. So fa r these studies have no t taken i n t o consideration t h a t the power cyc le chosen, and the engineering detai 1 s o f temperatures, pressures, and f 1 ows can profoundly a l t e r the extent o f sca l ing and corrosion, and the po ten t ia l f o r plugging the waste i n j e c t i o n wells. Scal in i s no t an inherent charac ter is t i c of

This program has as i t s objectives:

geothermal f l u id ; E resu l t s from t + e process used t o ex t rac t the energy.

0 develop a data base on the chemical fac to rs a f fec t i ng scal ing

0 develop computer models t o estimate scal ing rates i n p l a n t components

develop compute p l a n t e l e c t r i c output and maintenance requirements

0 odels of the impact of sca l ing on long-term

We recognize t h a t such computer models have l i m i t a t i o n s and can only give back what has been programed. Obviously, a t the present l eve l o f the s ta te o f the ar t , our understanding i s s t i 11 incomplete; therefore, our model s w i 11 be incomplete. We must accept t h a t these are f i r s t generation models.

However, computer models can be useful:

0 The massive memory allows consideration o f many fac ts simultaneously. An important f a c t i s l ess l i k e l y t o be over1 ooked.

2-1 5

I

e The models w i l l be useful educational t oo l s t o s c i e n t i s t s and engineers new t o geothermal techno1 ogy .

e The models provide a framework w i t h i n which t o plan f i e l d work and def ine t e s t data t o be gathered.

e The model s w i l l permit comparison o f one reservo i r w i t h another and w i t h various power cycle concepts.

Our program centers around the development o f four computer codes:

EQUILIB - an equi l ibr ium chemistry code t h a t takes a b r ine model and cal- culates what minerals would become insolub le and how much would p r e c i p i t a t e w i th changed temperatures, pressure and volumes i n a power cycle

components FLOSCAL - a code t o estimate the bui ldup r a t e o f scale on pipes and

PLANT - an extensive thermohydraulics code t h a t optimizes a t yp i ca l mult istage f l a s h p lan t o r binary cycle p l a n t f o r a reservo i r and then calculates p lan t degradation due t o scale bui ldup

GEOSCALE - a time-dependent code t o combine the above codes t o assess when and how the performance o f a geothermal power p l a n t w i l l degrade w i t h time as a r e s u l t o f scale bui ldup

EyUILIB

Why Does Scale Form?

When a geothermal b r i ne flashes, the gases fract ionate t o the steam phase (Figure 1). This process causes pH changes t h a t a f f e c t c a l c i t e s o l u b i l i t y (Figure 2 ) and s u l f i d e s o l u b i l i t y . As temperatures drop, the s o l u b i l i t y o f quartz, c r i s t o b a l i t e , o r amorphous s i l i c a can be exceeded, and one o r more forms o f s i l i c a can be prec ip i ta ted (Figure 3 ) . Other factors are shown i n Table 1.

What Process Parameters A f fec t Scaling?

When we examine the factors a f fec t i ng scal ing we see t h a t there are many (Table 2). P rec ip i t a t i on o f a mineral does no t necessari ly lead t o scale formation, Because mass transport must take place towards a pipe wal l and s t i c k i n g must occur i n order f o r a scale growth t o form. Thus i t i s the i n te rp lay o f chemical and thermohydraul i c factors t h a t cont ro ls scale growth.

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Table 1

hd WHY DOES SCALE FORM?

TYPE CAUSES

S i 1 i ca and s i 1 i cates Temperature drop decreases so lub i l i ty Steam loss concentrates br ine pH changes a f f e c t k i n e t i c s

Steam loss concentrates br ine

Temperature drop decreases sol ubi 1 i ty C02 loss increases pH

Fe+2 i o n prec ip i ta tes on surfaces and i n other scale deposi t s

Ca lc i t e C02 loss increases pH

Sul f i des

I r o n deposits from corrosion

Carry - o v e r Incomplete steam separation r e s u l t s i n aerosol carry-over o f s a l t s

Sul fates Temperature o r pressure changes decrease sol ubi 1 i ty Mixing d i f f e r e n t f l u i d s - barium i n one stream and su l fa te i n another = BaS04 scale

Table 2

IMPORTANT FACTORS AFFECTING GEOTHERMAL SCALING

0

0

0

0

0

0

0

0

0

0

0

0

Brine composition Gases present and pH - C02, H2S, NH3, HC1, H2, O2

Temperature i n reservoi r F l u i d produced s ing le phase o r 2 phase Degree o f f lash ing and steam f r a c t i o n D i s t r i b u t i o n of gases between 1 i q u i d and vapor

Oxidation-reduction potent i a1 Br ine concentrat ion from steam loss Nucl eation-growth phenomena Deposit ion surface

T and P

I d s number and other

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L,

I

Figure 1 . The Concentration o f Gases in the Water Phase Remaining a f t e r the Equil ibrium Separation o f Steam Source: A.J. Ellis (1-4)

20 60 100 140 180 m 260 m 340 3en TEMPERATURE. OC

F'igure 2. The Solubility o f Calcite Figure 3 . Silica Solubility in in Water up to 300" at Water Source: Barnes Various Partial Pressures and Rimstidt o f Carbon Dioxide Source: A. J. Ellis (1-2.)

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The code EQUILIB only looks a t p a r t o f the problem - the chemical d r i v ing forces af fect ing prec ip i ta t ion . This i s very important t o assess what minerals are %hennodynamically possible. The concept o f EQUILIB i s i l l u s t r a t e d i n Figure 4. Some o f the important chemical fac to rs are considered i n Figure 5.

C c U a P l f f IWlLll C c U a P l f f IWlLll

Figure 4. Concept o f EUU!!.IB

TEMPERATURE

0 CONCENTRATIONS Q ALL BRINE CATIONS AND ANIONS

PH PARTIAL PRESSURES OF GASE

ACTIVIlY COEFFICIENTS 0 IONIC STRENGTH WHICH A

C(MPONENTS THAT CONTROL OXIDATION POTENTIAL

SOLUBILIlES OF SOLID MINERALS THAT COULD FORM 0 AQUEOUS PHASE ECUILIBRIA THAT DlSTRIBUlECOMPONMTS AMONG

0 ~ . T O T M MASS BALANCE BETWEEN AOUEOUS AND GAS PHASES

MANY SPECIES AND CWPLEXfS

Figure 5. Factors t o Be Included i n Equi l ibr ium Chemistry Model

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/- u The code EYUILIB i s presently operational using a data base o r i g i n a l l y developed by H. C. Helgeson for another purpose. We have used the code t o v e r i f y a mineral s t a b i l i t y diagram (Figure 6 ) where the code co r rec t l y i d e n t i f i e d the s t a b i l i t y f i e l d s of s ider i te , p y r i t e and hematite. We used EQUILIB t o ca lcu late what cor- ros ion products would form on carbon steel and compared the r e s u l t w i th actual experimental r e s u l t s (Table 2A). The code has a1 so been used t o compare scal ing i n a HEBER heat exchange tube where su l f ides and s i l i c a were predicted t o deposit. When the same br ine was flashed i n a code ca lcu lat ion, ca l c i t e , s i l i c a , and i r o n s i l i c a t e were predicted t o deposit. We are presently expanding the data base t o include more species o f i n t e r e s t t o geothermal power plants.

.LO. . . ~

F igure 6. EUUILIB Calculations Compared t o Published Stabi 1 i ty F ie lds o f Fe Phases a t 25°C Eh-pH Diagram from

Garrel s and Chr i s t (1-3)

Table 2 A

EQUILIB CODE PREDICTIONS OF CORROSION PRODUCTS ON CARBON STEEL

1% NaCl, pH 7.5 T “C EQUILIB Experimental

50 FeC03 None detected

150 Fe3O4 Fe3O4

250 Fe3O4 Fe3O4

1% NaCl , pH 4.8 1% NaCl, pH 4.8 + H S EQUILIB Experimental EQUILIB Experimencal

Fe++ 85% Fe FeS2 FeS

10% FeC03

FeC03 FeC03 FeC03 80% FeC03

FeS2 10% FeS

5% FeS2

Fe3O4 70% Fe3O4 Fe3O4 Not run + 30% FeC03

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1

I

I

, I I

I &,

I

I

FLOSCAL

FLOSCAL is a code i n its beginning stages of development. I t will take the output of EQUILIB (which predicts what minerals w i l l precipitate) and estimate how f a s t scale will bui ld up on walls. The computational approach for FLOSCAL is given i n Figures 7 and 8. The FLOSCAL data base is presently the most inadequate because we need equations t o describe scaling kinetics mathematically. We are concen- t ra t ing on the species i n Table 3, recognizing more specimens must be added a t some future time (sulfides, s i l i ca tes , sulfates, etc.).

I I I I 1 I I I I ___. - I ANo0As ' EOUILIE

PRESSURES FROM 1 I o w o s r n m ~ ~ i ~

I I I I I

OEmSRlONIATE I AMOUNT

AMOUNT I OECOSREO

OEPOLITEO I

I EIYILARTO I mow smE*U I

I I I 1 I I I I I I

I I

I I

I I 1 I I I I I I I I I

U O Y E N T I

I

I I I I !

Figure 7. Computational Approach

INPUT

FLOWSTREAM SECTION GEOMETRY -:LOW AREAS - LENQTHS

0 FLOW DETERMININE PARAMETERS - FRICTION FACTORS - ELEVATION CHANGES

0 FLUID CHARACTERIZATION AT UPSTREAM END - TEMPERATURE - PRESSURE - QUALITY - MASS FLOW RATE - SOLUTE CONCENTRATIONS - GAS PARTIAL PRESSURES - PH

NSTREAM FLOW

T. X. V DOWNSTREAM SCALE FORMATION RATES

0 DOWNSTREAM CONCENTRATIONS

F i gure 8. FLOSCAL Characteri s t i cs

2-21

Table 3

DEPOSITION FORMULATIONS

CALCIUM CARBONATE

0 Attenuation length model 0 Corre la t ion f o r attenuation length t o be developed

QUARTZ

0 Reactor r a t e model (H. L. Barnes)

AMORPHOUS SILICA

0 Deposition only i f amorphous s i l i c a s o l u b i l i t y exceeded

0 Attenuation length o r mixed k i n e t i c s model t o be devel oped f o r scale cor re la t ions

We have a por t ion o f our e f f o r t devoted t o laboratory experiments on the k ine t i cs of c a l c i t e and s i l i c a i n two-phase f low (Tables 4, 5) and analysis o f real scale deposits from the f i e l d (Figure 9, Tables 6, 7) .

Table 4

KINETIC EXPERIMENTS

TEST OBJECTIVE

0 Define the in te rac t ions o f temperature, s a l i n i t y , chemistry, and hydraul ics on scal ing rates during f lash ing i n two-phase f low

TEST PARAMETERS

0 Reservoir Temp Flash Temp % Steam

290C (554F) 171C 28 235C (455F) 143C 19 180C (356F) 116C 13

0 S a l i n i t y 0.58% and 5.8%

0 Chemistry Saturated w i th CaCO Si02 a t reservo i r temperature and 1 a% COz overpressure

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1 i

i i ! i I t i i

i

Table 5 W INITIAL OBSERVATIONS FROM SCALING KINETIC TESTS

0 CaC03 deposits i n both c a l c i t e and aragonite forms

0 CaC03 deposits very rap id l y a i n chemical equ i l ib r ium a few cm downstream from f l ash p o i n t

0 CaC03 scal i ng increases as reservo i r temperature drops

0 CaC03 scal ing decreases as s a l i n i t y increases

0 Si02 scal ing i s much slower - 20-30 hours t o equ i l ib r ium

0 Si02 sca l ing occurred only i n 290OC t e s t

0 Si02 sca l ing increases as s a l i n i t y increases

Table 6

ATOMIC PERCENTAGE OF MAJOR CONSTITUENTS OF THE SCALE DEPOSITS IN HEBER TUBES’

E2-OUT EZ-OUT E%-OUT E4-OUT (66OC) (56°C) ELEMENT (173OC) (116OC) (84°C) - - E l - I N

31.7 42.7 46.4 67 .O S 19.8

Sb 7.0

Fe 44.3

s i 5.4

As 8.9

Zn 5.1

Ca 4.6

Pb 0.1

T1

3.8 27.5 7.7 23.8 ,

27.8 2.1 17.7 0.4

10.5 9.0 23.1 4.4

1.9 3.2 0.6 1.2

9.3 0.7 0.9 0.2

8.8 0.7 0.2 0.3

‘0.8 2.2 0.2 0.3

- - 0.4 1.3

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66% d f I E4 dl I * I s0c i i i

~4 n

Figure 9. Sampies Taken from Heber Heat Exchanger Tubes

Table 7

PHASE IDENTIFICATION BY X-RAY DIFFRACTION FOR SCALE DEPOSITS I N HEBER TUBES

D i f f r a c t i on E1-IN El-OUT E2-OUT E3-OUT ECOUT Method (173OC (116OC) (84OC) (66OC) (56OC)

As-received deposits

- A-Phase* - A- Ph a se D i ffractometer - Guinier pliotograph A-Phase A-Phase A-Phase A-Phase A-Phase

( FeSl ( FeSbS) (FeSbS)

De posi t s heat-treated a t 35OOC f o r A-Phase A-Phase Sb2S3 A-Phase Sb2S3 12 hours and (FeS) (FeS, Sb) (Sb)

(FeSbs)

*A-Phase: Amorphous phase o r f i n e grain c rys ta l l i ne phase w i th gra in s ize below 50 fi

( 1: Small quanti ty of c rys ta l l i ne phase

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PLANT

A computer model has been developed t o simulate two types o f geothermal power plants, the f lashed steam p l a n t and the binary cycle plant. This computer model n o t only establ ishes a baseline descr ipt ion o f the power plants, bu t a lso simulates the performance o f these power p lants as scale bui ldup occurs.

The two t yp i ca l power p l a n t flow diagrams are given i n Figure 10 and Figure 11. Up t o four stages o f f lash ing can be accommodated. The inputs t o the code, general process informat ion provided the user on the code, output, and components modeled are given i n Tables 8 through 13. I n Figures 12-16 some o f the component model s are diagrammed.

We have the code PLANT running w i th manual i npu t o f scale thicknesses. Two such cases are given i n Table 14 and Figures 17 and 18, where the-impact o f scale bui ldup on power output i s i l l u s t r a t e d .

bd

Figure 10. Flash Steam Plant Figure 11. Subc r i t i ca l Binary F lu id Cvcle Power Plant

Table 8

INPUT TO PLANT CODE

RESERVOIR PROPERTIES 0 Thermodynamic 0 Composition 0 Well f lowrates

0 Binary o r f l a s h steam 0 Size 0 P lan t component options

PLANT PARAMETERS

METEOROLOGICAL CONDITIONS

4 d 2-25

Table 9

GENERAL PROCESS INFORMATION

Geometry o f Speci f ic P1 ant Components 0 Diameter 0 Length a Cross-sectional area 0 Descript ion o f i n te rna l conf igurat ion

Degradation of component e f f i c i enc ies due t o scal ing

Pressure and heat losses i n system

Heat t rans fer coe f f i c i en ts where appropriate

Scaling condit ions a t key p lan t locat ions

Descript ions of f low streams other than br ine

A1 te ra t ions i n p l a n t base1 ine operating condit ions

In ternal p lan t e l e c t r i c a l consumption

Power output

due t o deposit ion

Br ine condit ions a t over 90 locat ions i n p lan t

Temperature Veloci ty Pres sure Thermodynamic phase Enthalpy F1 ow r a t e Density V i scosi ty Ut% o f dissolved Reynolds number

so l i ds (and species)

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Table 10

COMPONENTS WITH STATE POINTS

Well and Transmission System

We1 1 s El bows Valves Tees Pumps P i pes

Flashed Steam Plant

F1 ashers Valves Separators Turbine Steam scrubber Condenser

Binary F l u i d Plant

Heat exchangers . Valves

i

ble 11

,INFORMATION GIVEN AT EACH STATE POINT

Temperature ( "F Pressure (psia) Enthalpy ( B t u / l b ) Density (lbm/cu f t ) Weight percent o f NaCl Weight percent of 7 other species Flow velocity ( f t /sec) Phase or steam fraction Instantaneous scaling rate Instantaneous corrosion rate Scale thickness (mills) Percent reduction i n component

efficiency due t scale (-%I

Corrosion 1 imi t Design diameter (f t) ~

Mass flow rate (10 s lbm/hr)

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Table 12

LIST OF MODELS FOR FLASHED STEAM SYSTEM COMPONENTS THAT ARE I N CONTACT WITH GEOTHERMAL FLUID

Production wel ls (reservoir no t included) Brine pump Transmi ss i on 1 i nes F1 asher separator Steam scrubber Turbine Condenser Gas e jector

Table 13

LIST OF COMPONENT MODELS FOR BINARY SYSTEM WHICH ARE I N CONTACT WITH THE GEOTHERMAL FLUID

0 0 Brine pumps e Transmission system 0 Geothermal/working f l u i d heat exchanger(s1

Production we1 1 s ( reservoir not included)

tT-i3 L SCRUO WATER

LINE

STEAM FLOW SCRUBBERCAP

SCRUB WATER OUTLET

l -

Figure 12. Ben Ho l t Steam Scrubber Figure 13. Di rect Contact Condenser

2-28

CONOENSERStZEL

. * O D U D U Y r n R WELLNOOEL

C R I Y R E O U I I E Y E I R .

0 MANIFORLO NODES C DIRECTION OF FLOW

ylxtwauE*

OLODE VALVE AT WELLHEAD b €UOWS AT PERIMETER WELLS e TEES AT INTERNAL WELLS d OATE VALVE AT YANIFOLO

ELBOW AT PERIMETER MANIFOU)

f TEE AT INTERNAL MANIFOLO 0 EXPANSION LOOP WITH ELBOWS

.LRORYANCE DUE TO SCALINO

b’

L ULCTOR WEIIDLSION . U C E 8 8 UII D U E 0 ~ I O C O I I T I O N N ,-I TO muma AREA REOUCWN

F igure 14. Steam J e t Eject ion Figure 15. Transmission System Matr ix

2-29 L,

Table 14

EFFECT OF SCALE ON POWER OUTPUT

INPUT CONDITIONS:

Flashed Steam Plant

0 200°C brine, compressed l i q u i d a t wellhead, w i t h 7% dissolved sol i d s

0 Double-flash system w i th f lash ing a t the p l a n t

0 44 MWe gross power output

Binary Cycle Plant

0 205OC brine, compressed l i q u i d a t wellhead, w i th 7% dissolved sol i d s

0 Subcr i t i ca l cyc le using isobutane as the working f l u i d

0 55 MWe gross power output

GROSS POWER W P U T . NET POWER OUTPUT

45.m - . rclm

m l u ) 6 o o m I m SCALE THICKNESS. imills)

Figure 17. P lant Power Output Flashed Steam Plant

2- 30

W 55.m -

s0.W - MODE E 5 .s 45.m -

40.W -

5.W - MODE SL

SCALE THICKNESS (mills1

Figure 18. Power O u t p u t Binary Cycle Plant

GEOSCALE

A l l of the above material will be integrated i n t o a large code called GEOSCALE, which will permit assessing the time-dependent performance of a geothermal power p lan t . The general flow logic of GEOSCALE is i l lustrated i n Figure 19.

CONCLUSIONS

A t the present time we are about 50% complete w i t h t h i s program and expect t o have the codes available for use by August 1978.

Figure 19. GEOSCALE Cal cul a t i on F1 ow

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