Gas turbine waste heat utilization for distillation

Desalination, 52 (1985) 105-122 Elsevier Science Publishers B, V ., Amsterdam - Printed in The Netherlands

105

GAS TURBINE WASTE HEAT UTILIZATION FOR DISTILLATION*

R . RAUTENBACH and B . ARZT Institut fur Verfahrenstechnih Professor Dr .-Ing. R. Rautenbach, Rheinisch-Westfalische Technische Hochschule Aachen, Turmstrasse 46, 5100 Aachen (F .R .G.) Tel . (0241)805428 ; Telex 08 32704 THAachen

SUMMARY

At present many gas turbines are in operation in the Near East and along the North African coast which are perfectly suited for a combination with a desalination unit . Several possible combinations are discussed in this paper, demonstrating the superiority of the MES process compared to the standard MSF process . The discussion is limited to one case of practical importance - an already existing gas turbine of a nominal capacity of 10 MW, located near the seashore . Furthermore the cost of heating steam from a waste heat boiler is compared with the cost of steam from other sources such as solar energy and fuel-fired boilers .

SYMBOLS

A CF Cp f GR MhE I k m

N p 4 r

- heat transfer area, m 2 - concentration factor - specific heat capacity, kJ/kg K - utility factor - gain ratio - latent heat of evaporation, kJ/kg - investment costs, DM - overall heat transfer coefficient, W/m 2 K - mass flow rate, kg/s - shaft power, kW - pressure, bar - heat consumption, kW - depreciation period, 1/a

*Presented at the Symposium on Economics of Water Desalination Processes prepared by the Working Party on Fresh Water from the Sea of the European Federation of Chemical Engineering and Dechema, Bad Soden, 8-10 October 1984 . 0011-9164185/$03 .30

0 1985 Elsevier Science Publishers B .V .

106

R . RAUTENBACH AND B . ARZT

T

- temperature, ° K gross temperature difference, ° K boiling point elevation, °K flash temperature difference, ° K temperature difference for heat exchange, ° K temperature difference, preheater, ° K terminal temperature difference, ° K boiler efficiency - overall efficiency of a condensation power plant - turbine efficiency

ATo OTBE

ATFL LITHE ATpg OTTTD nB

npp nT

Indices B C D DP el F ME MSF PE s S sy WHU

- brine - collector - distillate - dual purpose - electrical - feed - multiple effect - multi-stage flash - primary energy - steam - solar - system - waste heat utilization

1 . INTRODUCTION

At present more than 90% of the total amount of fresh water from the sea is produced by MSF evaporation plants . The specific water costs of such plants are determined to a very high degree by their prime energy consumption, even if the plants are designed as dual purpose plants . This is at least valid as long as the cost analysis is based on world market prices for the prime energy . In principle the specific costs of fresh water are determined by capital costs as well as variable costs and savings in either one of both will result in lower production costs . According to Fig . 1 even for an energy price well below the world market price it is much more promising to concentrate the efforts on reducing the specific energy consumption rather than on reducing the investment costs. More precisely stated : research and development must concentrate on reducing the specific energy consumption without increasing the specific investment costs at the same rate .



GAS TURBINE WASTE HEAT

107

6 5 8

MSF-Desalination plant : N=39 OR =10.5 TB .MAX =105 °C

5

TB .MIN = 40 ° C m D = 10 000 t/d IMSF - 11 .5' 10 6 DM

3,29 0 3 210 DM

Single purpose 300 DM

DM 600

r = 0,1625 f=90%

Dual purpose 300M

6D0

DM'

Fig . 1 . Cost relation between dual and single purpose plant . Investment costs only those for complete MSF evaporator including pumps .

This paper will discuss several alternatives of thermal energy supply for evaporation processes . Furthermore a multiple effect process will be discussed which combines the inherent simplicity of the MSF process with the, in practice thermodynamically superior, ME principle . 2 . COST OF HEATING STEAM

The cost of heating steam for an evaporation plant is determined by the investment costs of the "energy converter" and by the cost of fuel . Naturally the cost will depend on the steam conditions, i .e. its pressure and temperature . Four options for the generation of low pressure steam (p = 1 .7 bar and T = 115'C) will be considered - solar steam generation - single purpose steam generation - dual purpose steam generation - waste heat utilization The steam conditions will not be varied - they correspond to proven and safe top-brine temperatures of the evaporation process . Table I summarizes the assumptions on which the calculation of the steam costs is based . It should be noticed that in the case of waste heat utilization as well as in the case of solar steam generation the monetary value of the energy input is set equal to zero . This seems justified since contrary to the case of a dual



108

R . RAUTENBACH AND B . ARZT

purpose plant, by additional and in most cases later installed equipment, energy is utilized which would be lost otherwise . Fig. 2 shows the influence of fuel price on steam generation costs . It can be concluded from Fig . 2 that focused (or flat!) solar collectors demand high performance desalination processes - even if the specific capital costs of the evaporator are high. On the other hand, desalination units employing waste heat should be low in capital cost . A high performance is only justified if it can be achieved at reasonable capital costs of the evaporator . 125 Focused solar collector DM is 100 Steam temperature :115°C Steam pressure :1,7bar C

a `a c a a, E 0 a

75

50 0 0 0a

U

25

prp"' Dual purpos Waste heat boiler 200 400 5 10 Spec primary energy costs

600 DMI 15DMIGJ

Fig . 2 . Steam generation costs vs . primary energy costs .

3 . MULTIPLE EFFECT (ME) AND MULTI STAGE FLASH (MSF) EVAPORATION

MSF units are currently the most reliable source for the production of fresh water from the sea on a large scale and therefore, all thermal plants in the Near East or in North Africa are designed according to this principle . The



GAS TURBINE WASTE HEAT

109

TABLEI CALCULATION OF STEAM GENERATION COSTS

SOLAR

SINGLE PURPOSE

DUAL PURPOSE

gPE

RPE C PE A, 56E

-

PL C

PL

N.,

B

C

.

-,DI 1

1000 I

•

O

5

t

7 .9R •m

a k°

1050

- 0 .1627

8R £ -.

IC

C

SP 5 = r 5P _

QS'

- Ill DM/t

•

I

pE R

S

-

/kg

p0 l

, 11u

t0 . 7

90 t

fDP - 90 1

20 DS

I

0 .1627

t 5P I "s E SP 6 .80 DM/t 5

lkg/Pl 1

IDN . 10 6 1

T

-

tDY

-

IUP

PE

0.

1440

Imp

C

I PP

66

As •

I

GASTUREINE

-

DM/kkel

IT I RHU

0 .6'n

fWU

90 %

70 1

=

0,1627

rhHO .

°0P'N

I

I

20 0 .1627

r WU NNU •R s

ST D

2 .60 DM/t

RHD

g

- 2 .06 DM/t

1

t 0 .7

5

-c

unit capacity is about 2 ,0 0 t/d distil ate, the maximum brine temperature is either 90°C or about 1 0 ° C and the number of stages about 25 . The energy consumption of such plants is about 95 kWh/t distil ate in form of low-pressure heating-steam (GOR ~ 7) and about 4-5 kWh/t distillate in form of power for the pump-drives . There is the pos ibility of improving the standard MSF proces towards a higher GOR, i .e . to a lower specific energy consumption without increasing the top brine temperature . A significant increase in GOR, however, can only be achiev d by the ME principle (Fig . 3) . Multiple effect evaporation means that the heat of condensation of the vapor produced in the first ef ect is employed to produce an ap roximately equal amount of vapor in the second effect . This condenses in the third effect and so on . The ap roximate specif c heat consumption of ME is given by Eq . 2 in Table II where rF/rD is the ratio of feed flow to distillate flow and LSTBE+ATpythe temperature rise of the feed in the last preheater . The additional specific power consumption for pump drives amounts to about 2 kWh/t 0 . A comparison of the specific heat consumption of the MSF process and of the ME process (Table II) reveals that N, the number of effects in ME, is much more important for specific heat consumption than the number of



R . RAUTENBACH AND B . ARZT

110

W

I

r-



•

L .J

1

I 1

L .J

I ^~ L-~

Q . . N Ah E

- Steam

MD

I II mo

moz

mol

I

m.CP-ATPH

-K)

--- Distillate --- Brine

K

c,, AT Ah E

cp ATs

K-001 . . . a02

MEN

Fig . 3 . Flow diagram of the ME process. TABLE H APPROXIMATE CALCULATION OF MSF AND ME DESALINATION PROCESS

MSF :

QMSF

OhE 1

:

QME

OhE

mF

+ MD

TBmax

NMSF

MD

ME

1 + OTBE + ATTTD + OTNE

rv NME

- TB,m n

(1)

/

CP,F (ATPB + ATBE )

MD

A T,B ;- +ATBE =ME

CF

TT - TB min + CF TT - TB,min

1CP,F(ATFB+OTBE)

(2)

(3)

Note : Nmax,ME< ATBE

stages in MSF : A ME plant ne ds few r ef ects than a thermodynamical y equal MSF plant . In conventional plants, however, a single effect is much more expensive than a stage of a MSF plant . In Fig . 4 the number of effects is plotted against the specific heat consumption . According to Fig . 4, the specific heat consumption of a ME proces is distinctively lower than the consumption of a MSF proces with an equal number of stages .



GAS TURBINE WASTE HEAT

111

60 AT

0

ATBE ATNE AT

FL

0, 0

N 0

MSF

ATHE CF AT0

0 E

t

MSF 15K 0.9K 0.6K 1,5K 53 80K

ME 0.7K 5K 2A 36 80K

0

20

60 Specific heat consumption

60

kWh/t

Q/m o

Fig . 4 . Specific heat consumption vs . number of stages .

The superiority of the ME proces with respect to specific heat consumption becomes even more evident if reasonable minimum temperature differences for both processes are considered . 4 . SPECIFIC HEAT TRANSFER SURFACE AREA

In principle an economic comparison of both processes must be based on turnkey investment costs and on al variable costs . Nevertheless this paper will be confined to a comparison of - evaporator costs - costs related to heat consumption . Costs related to the consumption of power and chemicals, to labor and maintenance wil not be considered since they are of minor influence and - even more important - since the cost differences between both processes for most items are neglig bly smal . Cost of civil engineering, plant erection etc . cannot be considered since theydpendverymuchontheplantlocation .Iftheyare qualtobthproces es, which se ms to be a fair estimate, it is justified to exclude these costs . In Table III the simplified equations for the calculation of the specific heat transfer surface area are derived for both processes . It should be noticed, that the ME process incorporates two kinds of heat transfersurfaceinevrystage : an evaporator and a preheater section . According to Fig . 5 the ME process needs less specific heat transfer area than the MSF process in cases of a low number of stages . The difference is even more pronounced if processes with equal gain ratios are compared .



R . RAUTENBACH AND B . ARZT

112 TABLE III

SPECIFIC HEAT TRANSFER SURFACE AREA FOR ME AND MSF

L ~eraf,

AT

°TD

T

\

ATa

GME - kME AME ATHE" m0 Ah

0

E

=kw AAT,, = rim e Ch E 0PH = kPHAPH'ATPH .in = mF Ca AT0 .ATME mF _ CE rho ATME CE - 1 In11 . ) ATrro A AHE . AP.

ATa /N

AT, /N AT,"- In ATTT,-AT,/N

ATE

In11t 1 ATo l N ATrro

ATT70 AT0

A _ A! ma

kMSF

N

m0

In(I .1 NATTTO 1 AT,

m0

Mn _

kME ATHE

4

E a

12

M2

kgl

t/d

30

GF cF-1 `P

Ah

•

k PM

ATa ATMF

ATME In I1` ATTTO

1

I GR=194 .

3

MSF

0

m

GR=12,5

0

GR~9 20

ME

MSF

k ME (NSF] 3500W/m7 K 2500W/m2 K kFH 2500W/m'K

GR=9 10

ATTTD ATP_E

ATME

AT,

80K

C F 0

10

2,SK

0.7K

80K

2 .0

-

I

I

20

30

40

Number of stages

Fig . 5 . Specific heat transfer surface area vs . number of stages . For a higher number of stages (in Fig . 5 for higher numbers than 18), the ME process needs more specific heat transfer surface area than the MSF process . It must be emphasized, however, that the influence of the number of : while the gain ratio ef ects on the gain ratio is very dif erent in this region of the ME proces is markedly improved, the MSF proces is not much affected by increasing the number of stages!



GAS TURBINE WASTE HEAT

113

Despite the thermodynamic advantages of the ME proces , al major plants are built ac ording to the MSF principle, mainly because of the simplicity and reliabil ty of this concept . According to our opinion, the ME principle can compete with the inherent simplicity of the MSF proces only if the ME units are designed as "stacked" plants, i .e . the ef ects placed vertical y on top of each other . In this case, gravity and pres ure dif erence betwe n the ef ects can be utilized for the transport of the brine from effect to effect - only e major pump is neces ary for pumping the fe d through the fe d heat exchanger to the top of the stack (Fig . 6) . It should be noted that, according to Fig . 6, a ME plant inevitably wil include the MSF principle to a certain ext nt!

-IT

v

Fig. 6 . The MES process .

There are two important variations in MES design : LTV (long tube ver. In any case, the advantages of MES are : 1 . Even for higher concentration factors, the once-through principle can beralizd . As a consequence, concentrated brine wil be in contact only with the effects operating at lower temperatures . 2 . A simple mode of operation . Betw en ominal cap city and zero production, the distil ate production is total y control ed by the amount of live steam flowing into the unit. 3 . Almost any combination with vapor compres ion is pos ible . tical) and HT (horizontal tube)

114

R . RAUTENBACH AND B . ARZT

4 . Very high overal heat ransfer coef ic ents can be realized [3], permitting small temperature differences between the effects at economical rates of heat transfer area. This is especial y important for low-temperature waste h at u il zation . Fig . 7 is a design study for a HT MES unit with a cap city of 50 t/d . A special feature of the design is the ar angement of the preheaters at the exit side of the evaporator section . By this configuration the final condensation of the vapor in every ef ect is confined to the preheater section and as a consequence, positive vapor velocities are assured for the total length of the tubes in the evaporation section . This is important with respect to the venting of the tubes since the evaporator section with its possible high overall heat transfer coefficients is much

Fig . 7 . Design study of the HT MES concept

.

115

GAS TURBINE WASTE HEAT

more sensitive to inert gases, even in low concentrations, than the preheater section . The advantages of the MES concept and the result of the experiments on the hydraulics of the sieve plates for brine distribution and on overall heat transfer coefficients have been discussed in detail at the Florence conference in 1983 [5] . Based on these results which have been obtained from a HT MES unit with a capacity of about 10 t/d per ef ect, the capital . cost of medium sized MES units can be stimated with ac eptable ac uracy A very promising alternative to the tube-bundle evaporators discus ed be. 8) . fores m tobeth recntlyintroduce Bavex ® hybridevaport (Fig It consists of plates which are pressed and arranged in such a pattern that short tubes are formed for the evaporating brine and sinusoidal channels for . The wall thickthe condensing vapor . The flow pat ern is trictly cros -flow ness can be as low as 0 .2 mm in case of titanium-palladium and 0 .4 mm in case of Cu-Ni 10 Fe .

Fig . S . Evaporation side of the Bavex

® heat xchanger

.

The major advantages of this heat exchanger are - the very high packing density of about 240 m' heat ransfer surface area per ma . - low manufacturing costs of about 228 DM/m' in case of titanium and ca . 178 DM/m' in case of CuNi 10 Fe . - exc l ent overal heat ransfer coef ic ents

116

R. RAUTENBACH AND B . ARZT

This heat exchanger al ows the design of an extrem ly short evaporation ef ect and, consequently, the design of MES units comprised of a very high number of effects (Fig . 9) . An exp rimental plant with a nominal cap city of 50 t distil ate per day is operating since October 1984 in the United Emirates (Fig . 10) . The vaporator consi ts of 5 ef ects and util zes olar en rgy during the day . At night, the plant is operated with a boiler . The very high gain ratio f the evaporator is the logical consequence of the high specific cost of the solar collector field . It is understo d that in other cases, like the utilization of waste heat of gas turbines, a lower number of effects is optimal .

Fig . 9 . Ar angem nt of the Bavex

® heat exchanger in the MES unit

.



GAS TURBINE WASTE HEAT

117

Fig . 10 . 500 t/d MES plant at Al Ain . 5 . COMPARISON OF MSF AND MES IN CASE OF WASTE HEAT UTILIZATION OF GAS TURBINES

Since the waste heat of gas turbines is on a fairly high temperature level (ca . 450° C), it has to be decided first whether the waste heat utilization should include the production of power by a back pres ure steam turbine or not . A low investment cost solution is shown in Fig . 11 . Here the waste heat boiler is designed for the production of 10 bar live steam . This steam is fed into a steam injector coupled with a MES unit com. Among others, one advantage of this concept pris ng about 10 ef ects only



118

R . RAUTENBACH AND B . ARZT

is, that the desalination plant can operate at very low, i .e . "safe" top-brine temperatures (ca. 80 ° C), but stil with an excel ent gain ratio . The alternative is shown in Fig . 12 . In this case the waste heat boiler is designed for the production of superheated steam of about 40 bar and 40 °C which is utilized for desalination after pas ing a back pres ure turbine .

10°kW (-100RW TO DESALINATION PLANT)

0

BR 190-C

201/h CONOENSATE

Fig. 11 .WastehatuilzatonbyMES-TC

.

I

10000 k W 18 /h

Obor40°C Steomturbine with generator 2000 kW

20 t/h 120-C °C

Fig . 12 .Wastehatuilzationbygenrato andMES

10°C

Desalination plant

.



GAS TURBINE WASTE HEAT

119

At present many gas turbines are in operation in the Near East and along the North African coast which are perfectly suited for a combination with a desalination unit. These gas turbines operate in places where fresh water is ne de and they are located near the seashore . The discus ion of waste heat utilization shal be limited to such a case an lready xistng asturbineofanomialcpaityofIV, =10MW . Five alternatives for fresh water production will be discussed : - a standard MSF unit (21 + 3 stages and a top-brine temperature of 110 ° C) - a HT MES unit, operating at the same conditions and with identical investment costs - a HT MES unit, operating at the same conditions and with identical gain ratio - a HT MES-TC configuration with a low top-brine temperature . According to Fig. 2, the cost of steam produced from the waste heat of gas turbines wil be relatively ow and consequently a low investment costlow performance desalination unit should be optimal in most cases . Despite this, a MES configuration for maximum distil ate production (25 ef ect Bavex) wil be discus ed as the 5th alternative . In al cases, the evaporator costs have be n obtained from industry for CuNi 10 Fe plated shells and endplates and CuNi tubes resp .CuNiBavex plates, iev traysetc . But the general results of the process comparison will be valid for other construction materials as well . Again it should be noticed that the investment cost listed in Table IV, Fig . 13 and Table V are the costs of the evaporator including pumps, venting 9 10 6 DM

I

SALINITY 35g/kg CONCENTRATION FACTOR 1,8 SEAWATER INLET TEMP 25-C OPINE DISCHARGE TEMP
°C B. AX

90 ° G

6 I

N O V

--

O 0 U

10°C

I =3,510 6 DM -/-

3 6

I= Z2410 DM HT I

MES

I

N=10 N=16 5

0

3

10 6

15 Number of gtfects 9

20

12 Gain Ratio GR 15

Fig . 13 . MES investment costs vs . number of stages .

25



120

R . RAUTENBACH AND B . ARZT

TABLE IV MAIN DATA OF THE COMPARED PROCESSES

Type

MSF

HT MES HT MRS HT MES-TC

Number of stages Max . brine temperature Seawater inlet temperature Live steam temperature Concentration factor Heat transfer coefficient rec./evap . rej ./peh . Heat transfer area rec./evap . rej ./preh . Gain ratio Distillate production Power consumption Power generation Desalination plant Waste heat boiler Spec, investment costs

21+3 110 26 120 2 .0

?

2800 2400

? 7

4000 2500

7

4000 2000

7420 965 8 .0 3840 720 2000 3 .6 2 .5 1562

?

2744 2260 8 .0 3840 320 2000 7 2 .5 ?

?

24200 9800 18 8800 700 2000 7 2 .6 7

110 25 120 1 .8

? 7 ? 2000 3 .6 2 .5 ?

10 110 25 120 1 .8

7 80 25 180 1 .6

? ? 7 7 2 .0 7

MES-avex

25 110 25 120 2 .0

° C °C ° C W/m'K

t/d kW kW 106 -DM 10'-DM DM/t/d

TABLE V MAIN DATA OF THE COMPARED PROCESSES

Type

MSF

Number of stages Max . brine temperature Seawater inlet temperature Live steam temperature Concentrator factor Heat transfer coefficient rec./evap . rej,/preh . Heat transfer area rec./evap . rej . preh . Gain ratio Distillate production Power consumption Power generation Desalination plant Waste beat boiler Spec. investment costs

21+3 110 25 120 2 .0

HT MES HT MES HT MES-TC

14 110 25 120 1 .8

10 110 25 120 1 .8

10 80 25 180 1 .6

2800 2400

4000 2500

4000 2600

3300 2450

7420 965 8 .0 3840 720 2000 3 .5 2 .5 1562

5240 3715 10 .5 5040 400 2000 8 .5 2 .6 1200

2744 2260 8 .0 3840 320 2000 2.24 2.5 1234

8328 2671 10 .6 5040 420 8 .75 2 .0 1150

MES-Saver

25 110 25 120 2.0 4000 2000 24200 9800 18 8800 700 2000 11 .0 2 .5 1634

°C °C °C W/m'K

ms t/d kW kW '10'-DM 10 1 -DM DM/t/d



GAS TURBINE WASTE HEAT

121

sy tem, brine heater esp . final condensor, etc .,butin owayturnkeyplant costs . The main design data of the MSF and the MES process are listed in Table . The missing IV . The gain ratio of the MSF proces has be n fixed to GR = 8 data in Table IV, for example the gain ratio and the number of ef ects of the HT MES process for given evaporator costs can be deducted from Fig . 13 . At low top-brine temperatures the combination of MES and thermocompression is advantageous since the possible number of effects is limited in this case and since the effectiveness of a thermocompressor, i .e . the relative increase in gain ratio, is much more pronounced for a low number of effects than for a high number . In our case, i .e . in the case of 10 effects, the gain . ratio is increased by 31% by ad ing a thermoc mpres or According to Fig . 14 a number of effects of about 10 is optimal in case of tube-bundle evaporators and for waste heat utilization where the investment costs of the waste heat boiler are the only major cost item . In Table V, all data for the 5 alternatives considered in this paper are listed .

DM t 16

I

=80°C

Tewaz

ms

=10 kg Is

5DMlt 1,2 N

Ul 0 U W

0 10 3DM1

2DMlt

1 DMA

0,6 5

10 15 Number of stages

Fig. 14 . Specific distillate costs for MES-TC .



122

R . RAUTENBACH AND B . ARZT

6 . DISCUSSION OF THE RESULTS

It should be noticed that the cost related to the additional power generation, i.e . costs of the instal ed steam turbine, are excluded in Table V . Table V clearly indicates the superiority of the MES concept . A MES unit will produce much more distillate for comparable investment costs or, vice versa, the investment costs wil be considerably lower in case of equal distillate production . The an lysi of case 4 indicates that MES is especial y suited for low top-brine temperatures . Even for such a small operating range of 40 ° K a 10 ef ect unit can be safely operated . Without a steam ejector, the gain ratio will be 8 .4, i.e . a distillate production of 4040 t/d can be achieved, the investment costs will be 3 .3X106 DM . By the ad ition of the thermocompres or, the investment costs wil increase to the above listed 3 .75 X 10 6 DM, the corresponding distillate production will be 5040 t/d . In the case of maximum distil ate production, the capital costs of the evaporator are by far the highest . Nevertheless such an investment can be attractive since the ratio of total investment costs, i .e . cost of boiler and ful y equip ed evaporator to daily distil ate production is stil reasonable .

REFERENCES 1 R . Rautenbach and B. Arzt, As es ment of Present and Future Desalination Proces es with Respect o their Primary Energy Demand, Arabian J . Sci . Eng ., 8 (1983) 3 . 2 R . Rautenbach and B. Arzt, Large scale diesel driven vapor compres ion units, Desalination, 38 (1981) 75-84 . 3 R . Rautenbach and B . Arzt, Desalination in the 90th - A future for multiple-effectstack plants!, Pure Water, December 1982 . 4 L . Cavalieri and G . Nas er, Seaw ter desalination with very low en rgy consumption, Desalination, 44 (1983) 143-151 . 5 R . Rautenbach and B. Ant, Waste heat util zation of large dies l engines by thermocompression and low temperature multiple effect evaporation, Desalination, 44 (1983) 121-128 .