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Vapour compression distillation using waste heat absorption systems
Desalination, 68 (1988) 57-68 ElsevierSciencePublishers B.V., Amsterdam-- Printed in The Netherlands
57
Vapour Compression Distillation using Waste Heat Absorption Systems S.E. ALY Mechanical Engineering Department, King Abdulaziz University, P.O. Box 9027, Jeddah 21413 (Saudi Arabia)
(ReceivedApril 12, 1987)
SUMMARY
A novel combined vapour compression/multi-effect boiling waste heat recovery system suitable for variable temperature energy sources is presented. The multi-effect boiling system comprises 14 vertical tube evaporator effects with an extended evaporation range from 114°C to 36°C and operates in the vapour compression mode. The vapour is compressed using a LiBr-water absorption machine fired by the exhaust of a gas turbine. The absorber receives the vapour produced in the last effect of the multi-effect boiling system while the top effect receives the steam produced in the generator of the absorption machine. Analysis of the combined system shows its operational flexibility compared to other waste heat recovery systems. Using the exhaust of a GE-32670 kW gas turbine, the proposed combination would produce up to 3.44 mgd of fresh water at a performance ratio of 10 and a product rate of 4.3 gal/kWh. This is 44% higher than that of existing waste heat recovery distillation systems, with no expense for extra firing.
SYMBOLS
BBT BTT C h L m M MEB MVC
- - brine bottom temperature, °C brine top temperature, ° C - - specific heat, kJ/kg K enthalpy, kJ/kg - - latent heat of evaporation, kJ/kg mass flow rate, kg/s total flow rate, kg/s multi-effect boiling - - mechanical vapour compression
0011-9164/88/$03.50
© 1988 Elsevier Science Publishers B.V.
58 MTLE P R
Q T TVC W H R S
X Y
multi-temperature level evaporators - - performance ratio - heat load, M W - temperature, °C - - thermovapour compression - - waste heat recovery system concentration - flashing ratio - -
Subscripts a b d f g m n s v
----------
absorber brine distillate feed generator m-th effect n-th effect supply steam vapour
INTRODUCTION Vapour compression (VC) distillation plants are known to be compact and efficient systems which require energy to perform compression. Vapour compression is usually associated with multi-effect boiling ( M E B ) with either vertical tube evaporators ( V T E ) or horizontal tube evaporators ( H T E ) . One of the main advantages of VC is the reuse of the vapour generated in the last effect, after elevating its pressure and temperature by compression, in the M E B top effect. The vapour is usually compressed by either mechanical ( M V C ) or thermal ( T V C ) means. In MVC, mechanically driven compressors are used while in TVC, steam jet boosters are used [ 1,2 ]. The MVC system raises the vapour temperature to a level higher than that of the saturation conditions in the top effect. The difference in temperature is essential for the evaporation process in this effect. Capacities and possible pressure ratios of the available vapour compressors play major roles in the limitations imposed on the MVC systems. Hence a small number of effects with small intereffect temperature differences are applied to minimize the mechanical energy input required for driving the compressor and usually compression ratios of about 1.58 are recommended [ 3 ]. Compressor maintenance for smooth operation presents a big problem for the operator. Carryover can cause difficulties and would affect the unit performance. This could be reduced by using demisters, b u t the pressure drop across the compressor would
59 increase, giving a higher compression ratio. Moreover operating at low temperature levels would increase the handled volume considerably and the compressor power would increase accordingly. Thus it is a common practice to use MVC for a limited number of effects at temperatures close to the atmospheric pressure conditions. In TVC, a steamjet compressor is operated by external motive steam of higher pressure and temperature, supplied by a boiler for instance. The motive steam sucks the vapour produced in the last effect by expansion in an ejector to a pressure slightly lower than that of this effect. The mixture of vapours is then compressed in a diffuser to a pressure that meets the requirement in the top effect. An amount equivalent to the withdrawn vapour proceeds down into the MEB system while the rest returns to the boiler loop. The efficiency of the steamjet ejector is quite low, 25-30% [ 4 ], and it drops rapidly whenever design conditions are altered. Moreover the ejector can operate only across a limited number of effects otherwise the amount of motive steam required would increase significantly. Indeed MVC requires expensive items such as the compressor with all its limitations and drawbacks while TVC requires a steamboiler. However, for both systems practical limitations are imposed on the capacity and number of effects. The capacity of vapour compression distillation is rarely above 600 m3/day and in some designes it reaches 1500 m3/day [ 14]. However, a VC system could play a bigger role in the desalination industry if larger plants were built and/or higher performance ratios were attained. In the present proposed arrangement, a VC distillation system is operated using a LiBr-water absorption machine. The analysis shows that such a combination provides the VC system with a number of attractive features. These include the system's ability to adapt to different sources of energy with differing temperatures, for example geothermal energy or gas turbine exhaust.
Combined absorption-distillation system The proposed system comprises two subsystems, namely a LiBr-water absorption system and a MEB system of the VTE type. A V T E / M E B system is selected rather than a multi-stage flash (MSF) system due to the nature of the present arrangement. In a recycled MSF system, for example the operating conditions in the heat rejection section, and thus the evaporation range, are controlled and limited by the temperature of the seawater. However, in the MEB system the operating temperature in the n-th effect is controlled by the saturation temperature in the ( n - 1 )-th and ( n + 1)-th effects. Hence, the condensation of the vapour produced in any effect is determined by the brine temperature of the succeeding effect rather than by the seawater temperature. Thus, the temperature of the MEB last effect can be set at any level provided that the vapour produced is efficiently withdrawn out of the system. Indeed this chracteristic makes the MEB system suitable to operate in conjunction with VC distillation systems.
60
Sat steom I
I
Co!
.... --~
I
nomizer Throttle vo~e
Vopour Absorber
Tll
Evaporator
I 5pill over
I
/0.
qa Fig. I. LiBr-water absorption machine.
Details of the combined absorption-distillation system are given in this paper along with case study calculations to demonstrate the advantages of the proposed arrangement.
LiBr- Water absorption system Basically this absorption machine is employed for refrigeration purposes. It operates in a heat-driven cycle where the LiBr (absorbent) is used to absorb the water vapour (refrigerant) produced in the evaporator as shown in Fig. 1. The low pressure water vapour released in the evaporator is absorbed by the strong LiBr solution and is converted to liquid under the evaporator conditions, with the heat released during the absorption process being carried away by the cooling water. From the absorber, the weak LiBr solution is heated in an economizer (liquid/liquid heat exchanger) before being introduced to the generator where it is heated further by an external heat supply. Since LiBr is non-volatile, pure water vapour is released under the generator conditions. It passes to the condenser while the strong solution leaves the generator via the economizer and passes through a pressure reducing valve before being admitted to the absorber. From the condenser, saturated water is throttled down to a prespecified pressure, after which it is introduced to the evaporator where it evaporates by heat received from the surroundings. The vapour released at the evaporator low pressure is absorbed in the absorber and the cycle is repeated. Commercial LiBr-water absorption refrigeration units are available with a
61 capacity of up to 3000 TR [ 5 ]. The evaporators in these machines usually work at temperatures above 0 ° C in order to avoid the LiBr crystallization problem. Thus the evaporators work at temperatures around 5 ° C with a corresponding pressure of 0.87 kPa [ 6 ]. One of the attractive features of the absorption machine is the possibility of operation over a range of boiling temperatures. This is achieved through the manipulation and adjustment of the solution concentration in various parts of the cycle. This feature is important for the proposed arrangement since an efficient waste heat recovery system should be able to match the nature of such a heat source. These sources, which may be geothermal energy or gas turbine exhaust for example, have varying temperatures. In Jeddah, within a period of less than 8 y, 27 gas turbines were commissioned to establish one of the largest gas turbine power plants in the world. With 11 units of 44.4 MW each from General Electric (GE), and 16 units of 51.7 MW each from Brown Bovari Co. (BBC), the plant has a total installed capacity of 1315 MW. At a specific fuel consumption of 0.36 1/kWh, each GE turbine for example, would produce about 170 kg/s of exhaust gases at 536 ° C. If these gasses are cooled down to 130 °C before being discarded to the atmosphere, then about 72 MW thermal energy can be extracted from each GE unit. For comparison, the present analysis is based on a GE-5000 gas turbine with a gross power output of 32,670 kW. This turbine exhausts 124 kg/s of hot gases at 430 ° C. If this is brought down to 130 °C in the absorption machine, 38,688 kW would be available to supply the generator with its heat requirement to produce saturated steam in the generator conditions. Instead of passing the steam to a condenser, as in conventional absorption machines, it is fed to the steam chest of the MEB top effect, as shown in Fig. 2. Similarly, the MEB last effect replaces the evaporator in a conventional absorption machine and thus, theoretically speaking, it can operate at 5 ° C (0.87 kPa). However, in the present arrangement the top effect would receive steam from the generator at 120 °C (200 kPa) whereas the last effect operates at 36 °C (6 kPa). As shown in Fig. 3, the equilibrium temperature in the generator is 140°C with a LiBr concentration of 42%. The absorber operates at 42 °C with a LiBr concentration of 30% and is cooled by seawater at 30 ° C. From ASHRAE charts [ 7 ], the enthalpy of the strong solution at the generator exit is h4 = 327 kJ/kg whereas the enthalpy of the weak solution at the absorber exit is hi = 105 kJ/kg LiBr mass flow balance for the generator yields m3 X3 = m4 X4 + m7 X7 The total mass flow balance in the generator gives F n 3 __~/n 4 - ] - / n 7
62
/
1 I
GE- 5000 Gasturbine /
I L;b::;iLO Y~I °ur
I
I I I I I I I I
VTE / MEB
Fig. 2. Wasteheat recoveryvapourcompressionblockdiagram. generator 43o'c
I
•
~-
Fee~:iHeaters
(~) . 17
~oc
I
Ms
'
~2o'c
i
_ -
"I '!
:',
®
(~)/'~'---- t" (~} tEm~gency
1 Mb
I-Sea tel-I
b
"
T To seo
Fig. 3. Combinedsystemflowdiagram. where X is the LiBr concentration and the subscripts correspond to points in Fig. 3. For a unit mass flow rate of saturated steam leaving the generator, the strong solution leaves the generator with m4 of 2.5 kg/s while the weak solution leaves the absorber at ms of 3.5 kg/s. Heat balance for the economizer (Fig. 4a) gives m4 ( h 4 - h s ) = m 3 ( h 3 - h 2 ) At t3 of 88°C, then t5 would have a level of 68°C at a LiBr concentration of 42%. Under these operating conditions, there will be no risk of LiBr crystallization of the strong solution at the exit of the liquid/liquid heat exchanger. A heat balance across the generator (Fig. 4b ) gives the generator load (Qg) per unit mass flow rate of saturated steam produced
63 (a)
(b)
.~,
430 "C
~o'c
~30"c - ' / /' .. 88"C j t ~ B~ Sdut~on
ss~
- ~0 "c
68~ ~ < ~ ~2 "¢
Fig. 4. Heat recovery system - - Temperature distribution. Economizer temperature profile; Generator temperature profile.
Qg
=
2705
kJ/kg
Assume that 10% of the available energy in the gas turbine exhaust is lost to the surroundings. Therefore the steam produced in the generator (roT) would be m7
__m 12.8
kg/s
This saturated steam ( M s = m 7 ) is fed to the M E B top effect at 120°C. The absorber heat balance yields the total absorbed load (Q~) as 1s~ etfect 13th FH
2nd effect 121h FH ~
11 th FH
11 th effect
12 th effect
2rid FH
13t;h effect
14th effect
1st FH Mf (36 "C)
~l-j~ .... ~
1
"-
i'
120 C
!
I ',°o::;=;;.. Fig. 5. VC/MEB arrangement.
•
c
64 Qa =
me he -
Ms hlo - ml hi =
33 MW
With cooling seawater available at 30 °C with a terminal temperature difference of 6 ° C, the absorber would require a seawater flowrate of 1.3 m3/s. This could be delivered using a single-stage centrifugal pump. The pressure of the weak solution in the absorber is elevated to the generator pressure using a pump. With a 70% efficiency, the pumping power required would amount to 12.5 kW. Seawater that leaves the absorber at 36 ° C is divided into two streams; one stream feeds the MEB distillator (Mr) while the second returns to the sea. It is interesting to estimate the equivalent cooling load for the present LiBr-water absorption machine. Following the standard method for cooling calculations, the machine would have an equivalent load of 7580 TR (ton refrigeration) which is possible by doubling the size of existing machines of 3000 TR. The coefficient of performance (COP) of the equivalent cooling machine would be about 0.76.
VC/MEB-distiUation system Indeed a combined VC/MEB distillator is by no means a new system [ 11-13 ]. However, the present combination is a unique system consisting of 14 effects of the VTE type operating in the vapour compression mode. The top effect is connected to the generator of the LiBr absorption machine while the last effect is connected to the absorber of the machine. Therefore, theoretically speaking, it is possible to extend the MEB evaporation range down to the temperature level encountered in conventional absorbers which is lower than the seawater temperature. However, in the present arrangement,the top effect receives supply steam Ms from the generator at 120°C while the last effect operates at 36 ° C. This shown in Fig. 5. Assume an equal intereffect temperature difference of 6 ° C. Then the proposed VC/MEB system would have an evaporation range of 78 °C across 14 effects with the top effect operating at 114 ° C. In a conventional MEB system, with a seawater-cooled bottom condenser, the last effect temperature is set at around 48 ° C. Thus, for an evaporation range of 78 °C the top effect temperature has to be 126°C compared with the present level of 114°C. With nonconcentrated seawater at the top effect, this temperature reduction would significantly reduce the level of mineral scale deposition at the hot end of the plant. From the top effect down to the 12th effect (48°C), a VTE system of the climbing film type can be used in a similar fashion to those of Scheveshenko [8] while a VTE system of the falling film type can be used for effects with lower temperatures. Indeed it is advantageous to apply the falling film VTE system for all the effects since it minimizes the maintenance and spare parts required.
65 The feed stream Mf to the MEB system is taken from the hot seawater effluent from the absorber of the LiBr machine at 36 ° C. Through a train of 13 regenerative feed heaters (FH) the feed is heated until it matches the top effect temperature ( T1 ). Feed is introduced to the first FH which is associated with the 12th effect (48°C) and proceeds until it enters the last FH which is heated by the supply steam from the generator. Steam supply from the generator (Ms) would produce vapour by boiling in the top effect as well as heating the feed stream in the last FH. The condensate from the supply steam may or may not join the distillate stream, depending on the generator design and the purity of the supplied steam. From the first (top) down to the 12th effect, three streams would leave the n-th effect namely My,n, Md.n and Mb,,. It is assumed that the vapour would generate by boiling an equivalent amount in the succeeding effect while distillate and brine would generate vapour by flashing. These streams are calculated using the following expressions
Mv,n = M s - M f y ( l - y ) n-1 Md,n =nMs - M r ( l - y ) n-~ Mb,n = M r [ 2 - ( l - y ) n] - n M s These are derived on the basis of constant values for the following variables: Specific heat, C; latent heat of evaporation, L; intereffect temperature difference, AT; interfeed heater temperature difference, At=AT; flashing ratio,
y(=CAT/L). From the 12th effect, three streams emerge, Mv,~2, Md,~2 and Mb,~2. These streams or some of them could be then entirely or partially introduced to the succeeding bottom effects (13th onward ), depending on the design parameters of the plant. As mentioned before, evaporation in these effects can extend down to 5 ° C, but a level of 36 ° C is satisfactory to demonstrate the advantages of the present arrangement. Let My, Md and Ms be the vapour, distillate and brine introduced to the 13th effect. T h e n the corresponding streams leaving the mth effect ( m > 12) are estimated using the followign expressions
M,,m=Mv+(Mb+Md) [ 1 - ( l - y ) z] Md,m = zMv + Md [(1 --y)Z -- 1 + ( z + 1 ) y ] / y + M b [(1 --y) ~ -- 1 +zy]/y
Mb,m =Mb[1--zy-- ( 1 - y ) ~ + l ] / y - z M v - M d [ ( 1 - y ) z+~ - 1 + (z+ l )y]/y where z= ( m - 12 ). In the bottom effects, the vapour generated in any effect is condensed by the brine of the next effect. This continues until the last effect is reached where the vapour released is directed towards the absorber of the LiBr machine. This should compensate for the steam Ms fed to the top effect, thus completing the cycle. Hence for Ms of 12.8 kg/s and m equal to 14 the distillation system would require a seawater feed Mf of 232 kg/s. The fresh water produced (Md) would
66 TABLE I Comparison of waste heat recovery systems
WHRS type
Product, mgd gal/kWh BTT, °C BBT, oC PR No.effects Heat requriement per unitdistillate, kJ/kg
RO [9]
MTLEfMEB [9]
VC/MEB
Rankine/RO
5-VTE
LiBr m/c
3.2 4 130 46 9.2 15
3.4 4.3 114 36 10 14
248
231
2.8 3.5 8.2 284
amount to 3.44 mgd and the blowdown would be discarded at a concentration factor of 2.8. In conjunction with the gas turbine used, the proposed arrangement would maintain a product ratio of 4.3 gal/kWh.
Combined system performance The V C / M E B system characteristics are demonstrated through the realized performance ratio ( P R ) . This represents the ratio of liters of product fresh water (distillate) per 2330 k J thermal energy supplied, defined by the formula P R = Md × 2330/heat input ( k W ) Accordingly, the proposed waste heat recovery of a V C / M E B system can maintain a P R slightly above 10. This arrangement is compared with other waste heat recovery systems ( W H R S ) [ 9 ] and is shown in Table I. This table is based on utilizing the exhaust of a GE-5000, 32,670 k W gas turbine. It demonstrates the various advantages of combining the L i B r - w a t e r absorption machine with the V C / M E B system. Heat requirement per unti distillate produced is 231 k J / k g compared with 248 k J / k g for the multi temperature level evaporators ( M T L E ) and 284 k J / k g for the R a n k i n e / R O bottoming cycle [9 ]. The proposed arrangement offers many operating advantages as well as a number of unique features. It offers the possibility of extending the evaporation range beyond the seawater temperature. Moreover, it offers the operator the choice of either maintaining the level of scale deposition at the extended evaporation range or maintaining the evaporation range at a reduced level of mineral deposition. Another feature of the proposed arrangement is the oper-
67 ating flexibility regarding the temperature level of the waste heat source available, e.g. gas turbine at full and part loads. It is possible to compare the present system with existing combined gas turbine/distillation arrangements based on the product ratio. Consider the plant in Qatar for instance [ 10 ]. It employs auxiliary boilers in conjunction with gas turbines exhaust and produces fresh water at the rate of 3 g a l / k W h while the rate of the system proposed here is 4.33 g a l / k W h at no extra firing cost. This means that combining the LiBr absorption machine with the V C / M E B system in the waste heat recovery system resulted in 44 % more fresh water than was obtained using auxiliary boilers with the distillation unit. CONCLUSIONS
This article presents a novel combined system for more efficient utilization of gas turbine exhaust in producing fresh water from the sea. The proposed waste heat recovery system comprises a 14 effect V T E / M E B system operating in the vapour compression mode. The compression is applied by connecting the M E B system to a L i B r - w a t e r absorption machine. This is fired by the hot exhaust gases of a gas turbine. Steam produced in the machine generator is supplied to the M E B top effect and the vapour generated in the last effect passes to the absorber of the machine. T h u s the V T E / M E B system replaces the condenser and the evaporator in a conventional LiBr absorption machine. The proposed arrangement has been shown and analyzed in this paper, with various advantages of the system being demonstrated. Using the exhaust of a GE-5000, 32,670 k W gas turbine, the present system, at no extra firing cost, produces up to 3.44 mgd fresh water at a product rate of 4.3 gal/kWh. The system evaporation range extends from 114°C to 36°C and the system performance ratio is 10.
REFERENCES 1 S.E. Aly, Energy savings in distillation plants by using vapor thermo-compression, Desalination, 49 (1984) 37. 2
S.E. Aly, Efficient energy utilization in single purpose desalination plants, Desalination, 58
(1986) 99. 3 S.Senatore, Vapour compression distillation. In: A. Delyannis and E. Delyannis (Eds.), Proc. 7th. Int. Syrup. Fresh Water from the Sea, Amsterdom, 1 (1980) 351. 4 J. Elliot, Vapourcompressiontheory, short course on desalination technology,Jeddah, March 18-30, 1980. 5 W. Stoecker, Refrigeration and Air Conditioning, McGraw-Hill, New York, Tara McGrawHill edn., 1974. 6 W. Stoecker and J. Jones, Refrigeration and air conditioning, McGraw-Hill, New York, International Student edn., 1982. 7 ASHRAE, Fundamentals Handbook, 17, 1981.
68 8
A.Egorov, V.G. Shatsillo,B.M. Borissov,I.P.Lazarev and I.G.Vakhin, Questions of improvement in thermal desalinationtechnology at the operating plants in the town of Schevshenko. In: A. Delyannis and E. Delyannis (Eds.), Proc. 7th. Int. Syrup. Fresh Water from the sea, Amsterdam, 1 (1980) 205. 9 S.E.AIy,Gas turbien waste heat recovery distillationsystem, J. Heat Recovery Systems, 7 (4) (1987) 375. 10 N. Wade, Water and power in desalination--Choice of desalinationand power plants,Middle East Water & Sewage, (Sept/Oct. 1985) 3. 11 D. Hoffman, Low Temperature evaporation plants,C.E.P., Oct. 1981, pp. 59-62. 12 U. Fisher,One year of operation of IDE's M E D in U S Virgin Islands,Desalination,44 (1983) 73-84. 13 U. Fisher, A. Aviram and A. Gendel, Ashdod M E low temperature desalination.One year of operation,Desalination,55 (1985) 13-32. 14 M. Lucas and F. Murat, Desalination by mechanical vapour compression operationalresults after one year operation of the Flamanville unit. Comparison with other processes by evaporation,Desalination,55 (1985) 33-42.
57
Vapour Compression Distillation using Waste Heat Absorption Systems S.E. ALY Mechanical Engineering Department, King Abdulaziz University, P.O. Box 9027, Jeddah 21413 (Saudi Arabia)
(ReceivedApril 12, 1987)
SUMMARY
A novel combined vapour compression/multi-effect boiling waste heat recovery system suitable for variable temperature energy sources is presented. The multi-effect boiling system comprises 14 vertical tube evaporator effects with an extended evaporation range from 114°C to 36°C and operates in the vapour compression mode. The vapour is compressed using a LiBr-water absorption machine fired by the exhaust of a gas turbine. The absorber receives the vapour produced in the last effect of the multi-effect boiling system while the top effect receives the steam produced in the generator of the absorption machine. Analysis of the combined system shows its operational flexibility compared to other waste heat recovery systems. Using the exhaust of a GE-32670 kW gas turbine, the proposed combination would produce up to 3.44 mgd of fresh water at a performance ratio of 10 and a product rate of 4.3 gal/kWh. This is 44% higher than that of existing waste heat recovery distillation systems, with no expense for extra firing.
SYMBOLS
BBT BTT C h L m M MEB MVC
- - brine bottom temperature, °C brine top temperature, ° C - - specific heat, kJ/kg K enthalpy, kJ/kg - - latent heat of evaporation, kJ/kg mass flow rate, kg/s total flow rate, kg/s multi-effect boiling - - mechanical vapour compression
0011-9164/88/$03.50
© 1988 Elsevier Science Publishers B.V.
58 MTLE P R
Q T TVC W H R S
X Y
multi-temperature level evaporators - - performance ratio - heat load, M W - temperature, °C - - thermovapour compression - - waste heat recovery system concentration - flashing ratio - -
Subscripts a b d f g m n s v
----------
absorber brine distillate feed generator m-th effect n-th effect supply steam vapour
INTRODUCTION Vapour compression (VC) distillation plants are known to be compact and efficient systems which require energy to perform compression. Vapour compression is usually associated with multi-effect boiling ( M E B ) with either vertical tube evaporators ( V T E ) or horizontal tube evaporators ( H T E ) . One of the main advantages of VC is the reuse of the vapour generated in the last effect, after elevating its pressure and temperature by compression, in the M E B top effect. The vapour is usually compressed by either mechanical ( M V C ) or thermal ( T V C ) means. In MVC, mechanically driven compressors are used while in TVC, steam jet boosters are used [ 1,2 ]. The MVC system raises the vapour temperature to a level higher than that of the saturation conditions in the top effect. The difference in temperature is essential for the evaporation process in this effect. Capacities and possible pressure ratios of the available vapour compressors play major roles in the limitations imposed on the MVC systems. Hence a small number of effects with small intereffect temperature differences are applied to minimize the mechanical energy input required for driving the compressor and usually compression ratios of about 1.58 are recommended [ 3 ]. Compressor maintenance for smooth operation presents a big problem for the operator. Carryover can cause difficulties and would affect the unit performance. This could be reduced by using demisters, b u t the pressure drop across the compressor would
59 increase, giving a higher compression ratio. Moreover operating at low temperature levels would increase the handled volume considerably and the compressor power would increase accordingly. Thus it is a common practice to use MVC for a limited number of effects at temperatures close to the atmospheric pressure conditions. In TVC, a steamjet compressor is operated by external motive steam of higher pressure and temperature, supplied by a boiler for instance. The motive steam sucks the vapour produced in the last effect by expansion in an ejector to a pressure slightly lower than that of this effect. The mixture of vapours is then compressed in a diffuser to a pressure that meets the requirement in the top effect. An amount equivalent to the withdrawn vapour proceeds down into the MEB system while the rest returns to the boiler loop. The efficiency of the steamjet ejector is quite low, 25-30% [ 4 ], and it drops rapidly whenever design conditions are altered. Moreover the ejector can operate only across a limited number of effects otherwise the amount of motive steam required would increase significantly. Indeed MVC requires expensive items such as the compressor with all its limitations and drawbacks while TVC requires a steamboiler. However, for both systems practical limitations are imposed on the capacity and number of effects. The capacity of vapour compression distillation is rarely above 600 m3/day and in some designes it reaches 1500 m3/day [ 14]. However, a VC system could play a bigger role in the desalination industry if larger plants were built and/or higher performance ratios were attained. In the present proposed arrangement, a VC distillation system is operated using a LiBr-water absorption machine. The analysis shows that such a combination provides the VC system with a number of attractive features. These include the system's ability to adapt to different sources of energy with differing temperatures, for example geothermal energy or gas turbine exhaust.
Combined absorption-distillation system The proposed system comprises two subsystems, namely a LiBr-water absorption system and a MEB system of the VTE type. A V T E / M E B system is selected rather than a multi-stage flash (MSF) system due to the nature of the present arrangement. In a recycled MSF system, for example the operating conditions in the heat rejection section, and thus the evaporation range, are controlled and limited by the temperature of the seawater. However, in the MEB system the operating temperature in the n-th effect is controlled by the saturation temperature in the ( n - 1 )-th and ( n + 1)-th effects. Hence, the condensation of the vapour produced in any effect is determined by the brine temperature of the succeeding effect rather than by the seawater temperature. Thus, the temperature of the MEB last effect can be set at any level provided that the vapour produced is efficiently withdrawn out of the system. Indeed this chracteristic makes the MEB system suitable to operate in conjunction with VC distillation systems.
60
Sat steom I
I
Co!
.... --~
I
nomizer Throttle vo~e
Vopour Absorber
Tll
Evaporator
I 5pill over
I
/0.
qa Fig. I. LiBr-water absorption machine.
Details of the combined absorption-distillation system are given in this paper along with case study calculations to demonstrate the advantages of the proposed arrangement.
LiBr- Water absorption system Basically this absorption machine is employed for refrigeration purposes. It operates in a heat-driven cycle where the LiBr (absorbent) is used to absorb the water vapour (refrigerant) produced in the evaporator as shown in Fig. 1. The low pressure water vapour released in the evaporator is absorbed by the strong LiBr solution and is converted to liquid under the evaporator conditions, with the heat released during the absorption process being carried away by the cooling water. From the absorber, the weak LiBr solution is heated in an economizer (liquid/liquid heat exchanger) before being introduced to the generator where it is heated further by an external heat supply. Since LiBr is non-volatile, pure water vapour is released under the generator conditions. It passes to the condenser while the strong solution leaves the generator via the economizer and passes through a pressure reducing valve before being admitted to the absorber. From the condenser, saturated water is throttled down to a prespecified pressure, after which it is introduced to the evaporator where it evaporates by heat received from the surroundings. The vapour released at the evaporator low pressure is absorbed in the absorber and the cycle is repeated. Commercial LiBr-water absorption refrigeration units are available with a
61 capacity of up to 3000 TR [ 5 ]. The evaporators in these machines usually work at temperatures above 0 ° C in order to avoid the LiBr crystallization problem. Thus the evaporators work at temperatures around 5 ° C with a corresponding pressure of 0.87 kPa [ 6 ]. One of the attractive features of the absorption machine is the possibility of operation over a range of boiling temperatures. This is achieved through the manipulation and adjustment of the solution concentration in various parts of the cycle. This feature is important for the proposed arrangement since an efficient waste heat recovery system should be able to match the nature of such a heat source. These sources, which may be geothermal energy or gas turbine exhaust for example, have varying temperatures. In Jeddah, within a period of less than 8 y, 27 gas turbines were commissioned to establish one of the largest gas turbine power plants in the world. With 11 units of 44.4 MW each from General Electric (GE), and 16 units of 51.7 MW each from Brown Bovari Co. (BBC), the plant has a total installed capacity of 1315 MW. At a specific fuel consumption of 0.36 1/kWh, each GE turbine for example, would produce about 170 kg/s of exhaust gases at 536 ° C. If these gasses are cooled down to 130 °C before being discarded to the atmosphere, then about 72 MW thermal energy can be extracted from each GE unit. For comparison, the present analysis is based on a GE-5000 gas turbine with a gross power output of 32,670 kW. This turbine exhausts 124 kg/s of hot gases at 430 ° C. If this is brought down to 130 °C in the absorption machine, 38,688 kW would be available to supply the generator with its heat requirement to produce saturated steam in the generator conditions. Instead of passing the steam to a condenser, as in conventional absorption machines, it is fed to the steam chest of the MEB top effect, as shown in Fig. 2. Similarly, the MEB last effect replaces the evaporator in a conventional absorption machine and thus, theoretically speaking, it can operate at 5 ° C (0.87 kPa). However, in the present arrangement the top effect would receive steam from the generator at 120 °C (200 kPa) whereas the last effect operates at 36 °C (6 kPa). As shown in Fig. 3, the equilibrium temperature in the generator is 140°C with a LiBr concentration of 42%. The absorber operates at 42 °C with a LiBr concentration of 30% and is cooled by seawater at 30 ° C. From ASHRAE charts [ 7 ], the enthalpy of the strong solution at the generator exit is h4 = 327 kJ/kg whereas the enthalpy of the weak solution at the absorber exit is hi = 105 kJ/kg LiBr mass flow balance for the generator yields m3 X3 = m4 X4 + m7 X7 The total mass flow balance in the generator gives F n 3 __~/n 4 - ] - / n 7
62
/
1 I
GE- 5000 Gasturbine /
I L;b::;iLO Y~I °ur
I
I I I I I I I I
VTE / MEB
Fig. 2. Wasteheat recoveryvapourcompressionblockdiagram. generator 43o'c
I
•
~-
Fee~:iHeaters
(~) . 17
~oc
I
Ms
'
~2o'c
i
_ -
"I '!
:',
®
(~)/'~'---- t" (~} tEm~gency
1 Mb
I-Sea tel-I
b
"
T To seo
Fig. 3. Combinedsystemflowdiagram. where X is the LiBr concentration and the subscripts correspond to points in Fig. 3. For a unit mass flow rate of saturated steam leaving the generator, the strong solution leaves the generator with m4 of 2.5 kg/s while the weak solution leaves the absorber at ms of 3.5 kg/s. Heat balance for the economizer (Fig. 4a) gives m4 ( h 4 - h s ) = m 3 ( h 3 - h 2 ) At t3 of 88°C, then t5 would have a level of 68°C at a LiBr concentration of 42%. Under these operating conditions, there will be no risk of LiBr crystallization of the strong solution at the exit of the liquid/liquid heat exchanger. A heat balance across the generator (Fig. 4b ) gives the generator load (Qg) per unit mass flow rate of saturated steam produced
63 (a)
(b)
.~,
430 "C
~o'c
~30"c - ' / /' .. 88"C j t ~ B~ Sdut~on
ss~
- ~0 "c
68~ ~ < ~ ~2 "¢
Fig. 4. Heat recovery system - - Temperature distribution. Economizer temperature profile; Generator temperature profile.
Qg
=
2705
kJ/kg
Assume that 10% of the available energy in the gas turbine exhaust is lost to the surroundings. Therefore the steam produced in the generator (roT) would be m7
__m 12.8
kg/s
This saturated steam ( M s = m 7 ) is fed to the M E B top effect at 120°C. The absorber heat balance yields the total absorbed load (Q~) as 1s~ etfect 13th FH
2nd effect 121h FH ~
11 th FH
11 th effect
12 th effect
2rid FH
13t;h effect
14th effect
1st FH Mf (36 "C)
~l-j~ .... ~
1
"-
i'
120 C
!
I ',°o::;=;;.. Fig. 5. VC/MEB arrangement.
•
c
64 Qa =
me he -
Ms hlo - ml hi =
33 MW
With cooling seawater available at 30 °C with a terminal temperature difference of 6 ° C, the absorber would require a seawater flowrate of 1.3 m3/s. This could be delivered using a single-stage centrifugal pump. The pressure of the weak solution in the absorber is elevated to the generator pressure using a pump. With a 70% efficiency, the pumping power required would amount to 12.5 kW. Seawater that leaves the absorber at 36 ° C is divided into two streams; one stream feeds the MEB distillator (Mr) while the second returns to the sea. It is interesting to estimate the equivalent cooling load for the present LiBr-water absorption machine. Following the standard method for cooling calculations, the machine would have an equivalent load of 7580 TR (ton refrigeration) which is possible by doubling the size of existing machines of 3000 TR. The coefficient of performance (COP) of the equivalent cooling machine would be about 0.76.
VC/MEB-distiUation system Indeed a combined VC/MEB distillator is by no means a new system [ 11-13 ]. However, the present combination is a unique system consisting of 14 effects of the VTE type operating in the vapour compression mode. The top effect is connected to the generator of the LiBr absorption machine while the last effect is connected to the absorber of the machine. Therefore, theoretically speaking, it is possible to extend the MEB evaporation range down to the temperature level encountered in conventional absorbers which is lower than the seawater temperature. However, in the present arrangement,the top effect receives supply steam Ms from the generator at 120°C while the last effect operates at 36 ° C. This shown in Fig. 5. Assume an equal intereffect temperature difference of 6 ° C. Then the proposed VC/MEB system would have an evaporation range of 78 °C across 14 effects with the top effect operating at 114 ° C. In a conventional MEB system, with a seawater-cooled bottom condenser, the last effect temperature is set at around 48 ° C. Thus, for an evaporation range of 78 °C the top effect temperature has to be 126°C compared with the present level of 114°C. With nonconcentrated seawater at the top effect, this temperature reduction would significantly reduce the level of mineral scale deposition at the hot end of the plant. From the top effect down to the 12th effect (48°C), a VTE system of the climbing film type can be used in a similar fashion to those of Scheveshenko [8] while a VTE system of the falling film type can be used for effects with lower temperatures. Indeed it is advantageous to apply the falling film VTE system for all the effects since it minimizes the maintenance and spare parts required.
65 The feed stream Mf to the MEB system is taken from the hot seawater effluent from the absorber of the LiBr machine at 36 ° C. Through a train of 13 regenerative feed heaters (FH) the feed is heated until it matches the top effect temperature ( T1 ). Feed is introduced to the first FH which is associated with the 12th effect (48°C) and proceeds until it enters the last FH which is heated by the supply steam from the generator. Steam supply from the generator (Ms) would produce vapour by boiling in the top effect as well as heating the feed stream in the last FH. The condensate from the supply steam may or may not join the distillate stream, depending on the generator design and the purity of the supplied steam. From the first (top) down to the 12th effect, three streams would leave the n-th effect namely My,n, Md.n and Mb,,. It is assumed that the vapour would generate by boiling an equivalent amount in the succeeding effect while distillate and brine would generate vapour by flashing. These streams are calculated using the following expressions
Mv,n = M s - M f y ( l - y ) n-1 Md,n =nMs - M r ( l - y ) n-~ Mb,n = M r [ 2 - ( l - y ) n] - n M s These are derived on the basis of constant values for the following variables: Specific heat, C; latent heat of evaporation, L; intereffect temperature difference, AT; interfeed heater temperature difference, At=AT; flashing ratio,
y(=CAT/L). From the 12th effect, three streams emerge, Mv,~2, Md,~2 and Mb,~2. These streams or some of them could be then entirely or partially introduced to the succeeding bottom effects (13th onward ), depending on the design parameters of the plant. As mentioned before, evaporation in these effects can extend down to 5 ° C, but a level of 36 ° C is satisfactory to demonstrate the advantages of the present arrangement. Let My, Md and Ms be the vapour, distillate and brine introduced to the 13th effect. T h e n the corresponding streams leaving the mth effect ( m > 12) are estimated using the followign expressions
M,,m=Mv+(Mb+Md) [ 1 - ( l - y ) z] Md,m = zMv + Md [(1 --y)Z -- 1 + ( z + 1 ) y ] / y + M b [(1 --y) ~ -- 1 +zy]/y
Mb,m =Mb[1--zy-- ( 1 - y ) ~ + l ] / y - z M v - M d [ ( 1 - y ) z+~ - 1 + (z+ l )y]/y where z= ( m - 12 ). In the bottom effects, the vapour generated in any effect is condensed by the brine of the next effect. This continues until the last effect is reached where the vapour released is directed towards the absorber of the LiBr machine. This should compensate for the steam Ms fed to the top effect, thus completing the cycle. Hence for Ms of 12.8 kg/s and m equal to 14 the distillation system would require a seawater feed Mf of 232 kg/s. The fresh water produced (Md) would
66 TABLE I Comparison of waste heat recovery systems
WHRS type
Product, mgd gal/kWh BTT, °C BBT, oC PR No.effects Heat requriement per unitdistillate, kJ/kg
RO [9]
MTLEfMEB [9]
VC/MEB
Rankine/RO
5-VTE
LiBr m/c
3.2 4 130 46 9.2 15
3.4 4.3 114 36 10 14
248
231
2.8 3.5 8.2 284
amount to 3.44 mgd and the blowdown would be discarded at a concentration factor of 2.8. In conjunction with the gas turbine used, the proposed arrangement would maintain a product ratio of 4.3 gal/kWh.
Combined system performance The V C / M E B system characteristics are demonstrated through the realized performance ratio ( P R ) . This represents the ratio of liters of product fresh water (distillate) per 2330 k J thermal energy supplied, defined by the formula P R = Md × 2330/heat input ( k W ) Accordingly, the proposed waste heat recovery of a V C / M E B system can maintain a P R slightly above 10. This arrangement is compared with other waste heat recovery systems ( W H R S ) [ 9 ] and is shown in Table I. This table is based on utilizing the exhaust of a GE-5000, 32,670 k W gas turbine. It demonstrates the various advantages of combining the L i B r - w a t e r absorption machine with the V C / M E B system. Heat requirement per unti distillate produced is 231 k J / k g compared with 248 k J / k g for the multi temperature level evaporators ( M T L E ) and 284 k J / k g for the R a n k i n e / R O bottoming cycle [9 ]. The proposed arrangement offers many operating advantages as well as a number of unique features. It offers the possibility of extending the evaporation range beyond the seawater temperature. Moreover, it offers the operator the choice of either maintaining the level of scale deposition at the extended evaporation range or maintaining the evaporation range at a reduced level of mineral deposition. Another feature of the proposed arrangement is the oper-
67 ating flexibility regarding the temperature level of the waste heat source available, e.g. gas turbine at full and part loads. It is possible to compare the present system with existing combined gas turbine/distillation arrangements based on the product ratio. Consider the plant in Qatar for instance [ 10 ]. It employs auxiliary boilers in conjunction with gas turbines exhaust and produces fresh water at the rate of 3 g a l / k W h while the rate of the system proposed here is 4.33 g a l / k W h at no extra firing cost. This means that combining the LiBr absorption machine with the V C / M E B system in the waste heat recovery system resulted in 44 % more fresh water than was obtained using auxiliary boilers with the distillation unit. CONCLUSIONS
This article presents a novel combined system for more efficient utilization of gas turbine exhaust in producing fresh water from the sea. The proposed waste heat recovery system comprises a 14 effect V T E / M E B system operating in the vapour compression mode. The compression is applied by connecting the M E B system to a L i B r - w a t e r absorption machine. This is fired by the hot exhaust gases of a gas turbine. Steam produced in the machine generator is supplied to the M E B top effect and the vapour generated in the last effect passes to the absorber of the machine. T h u s the V T E / M E B system replaces the condenser and the evaporator in a conventional LiBr absorption machine. The proposed arrangement has been shown and analyzed in this paper, with various advantages of the system being demonstrated. Using the exhaust of a GE-5000, 32,670 k W gas turbine, the present system, at no extra firing cost, produces up to 3.44 mgd fresh water at a product rate of 4.3 gal/kWh. The system evaporation range extends from 114°C to 36°C and the system performance ratio is 10.
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S.E. Aly, Efficient energy utilization in single purpose desalination plants, Desalination, 58
(1986) 99. 3 S.Senatore, Vapour compression distillation. In: A. Delyannis and E. Delyannis (Eds.), Proc. 7th. Int. Syrup. Fresh Water from the Sea, Amsterdom, 1 (1980) 351. 4 J. Elliot, Vapourcompressiontheory, short course on desalination technology,Jeddah, March 18-30, 1980. 5 W. Stoecker, Refrigeration and Air Conditioning, McGraw-Hill, New York, Tara McGrawHill edn., 1974. 6 W. Stoecker and J. Jones, Refrigeration and air conditioning, McGraw-Hill, New York, International Student edn., 1982. 7 ASHRAE, Fundamentals Handbook, 17, 1981.
68 8
A.Egorov, V.G. Shatsillo,B.M. Borissov,I.P.Lazarev and I.G.Vakhin, Questions of improvement in thermal desalinationtechnology at the operating plants in the town of Schevshenko. In: A. Delyannis and E. Delyannis (Eds.), Proc. 7th. Int. Syrup. Fresh Water from the sea, Amsterdam, 1 (1980) 205. 9 S.E.AIy,Gas turbien waste heat recovery distillationsystem, J. Heat Recovery Systems, 7 (4) (1987) 375. 10 N. Wade, Water and power in desalination--Choice of desalinationand power plants,Middle East Water & Sewage, (Sept/Oct. 1985) 3. 11 D. Hoffman, Low Temperature evaporation plants,C.E.P., Oct. 1981, pp. 59-62. 12 U. Fisher,One year of operation of IDE's M E D in U S Virgin Islands,Desalination,44 (1983) 73-84. 13 U. Fisher, A. Aviram and A. Gendel, Ashdod M E low temperature desalination.One year of operation,Desalination,55 (1985) 13-32. 14 M. Lucas and F. Murat, Desalination by mechanical vapour compression operationalresults after one year operation of the Flamanville unit. Comparison with other processes by evaporation,Desalination,55 (1985) 33-42.