- Email: [email protected]
Solutions for coupling a mechanical vapour compression distiller with a multi-stage-flash evaporator
Desalination, 45 (1983)143-152 Elsevier Science PublishersB.V.,Amsterdam--Printed inTheNetherlands
143
SOLUTIONS FOR COUPLING A MECHANICAL VAPOUR COMPRESSION DISTILLER WITH A MULTI-STAGE-FLASH EVAPORATOR K.
Genthner' and M.M. El-Allawy2
'Universit%t Bremen FB4, P.O.Box 330440, D-2800 Bremen 33 (WestGermany) 2Buckau-Walther AG, P.O.Box 210120, D-2800 Bremen 21 (West-Germany)
ABSTRACT
The paper discusses various familiar and new process combinations of multi-stage-flash (MSF) and mechanical vapour compression (MVC) distillation. Coupling of an MVC distiller to an MSF unit considerably reduces the consumption of energy and chemicals as well as feed water requirements. The plant is extremely flexible with respect to partial load operation and adaption to varying availabilities of thermal and electric&l energy. An existing MSF distiller with a capacity of up to 2000 m3/d can be extended by coupling it with a simplified MVC module. Investment and operation costs are notably lower than with the installation of additional MSF units. Ce rapport di?crit des combinaisons connues et nouvelles du pro&d& de distillation flash (MSF) et de distillation par compression m& canique des vapeurs (MVC). En raccordant une unit& de MVC avec une unit& de MSF on peut reduire dans une forte mesure la consommation en gnergie et en produits chimiques ainsi que les besoins en eau brute. En ce qui concerne le service 2 charge partielle et l'adaptation aux sources diverses d'6nergie thermique et Glectrique, cette unit& est t&s flexible. Le dgbit d'une installation MSF ayant une capacit6 jusqu'au 2000 m3/j peut Btre augment6 en raccordant cette installation avec un module de MVC simplifi8. Les frais d'investissement et d'exploitation sont pour ce raccordement notamment plus bas que d'installer des installations MSF suppl6mentaires. Es werden bekannte und neue Koppelungen eines Entspannungsverdampfers (MSF) mit einem durch mechanische Briidenkompression betriebenen Verdampfer (MVC) untersucht. Durch die Ankoppelung eines MVC-Verdampfers an eine MSF-Anlage werden der Energiebedarf, der Verbrauch an Chemikalien und der Rohwasserbedarf erheblich reduziert. Die Anlage ist in ihrem Teillastverhalten und mit der Mtiglichkeit, sich wechselnden Verfiigbarkeiten von thermischer und elektrischer Energie anzupassen, sehr flexibel. Eine bestehende MSF-Anlage mit einer Tagesleistung bis 2000 m3/d kann auf einfache Weise durch Ankoppelung eines MVC-Moduls erweitert werden. Investitionsund Betriebskosten sind nennenswert geringer als bei Installation zu.sZtzlicher MSF-Anlagen.
OOll-9164/83/$03.00 0 1983ElsevierSciencePublishersB.V.
144 INTRODUCTION Various suggestions of coupling a mechanical vapour compression (MVC) distiller in series with a multi-stage flash (MSF) evaporator have been investigated in the past. The present paper additionally presents solutions for a parallel link between the two processes. No MSF-MVC combination has yet - to our knowledge
been suc-
cessfully realized. In the years of low fuel costs the goal of reducing energy consumption was superceded by higher installation costs, lack of confidence in the lifetime of compressors and by the complexity of the solutions. Vapour compression technology has now certainly reached a state of high reliability. The other aspects still apply for the majority of new desalination projects. However, there do exist a number of cases where a combination of the features of both MSF and MVC distillation bears considerable economic attraction. In cases where high or medium pressure steam is available for desalination, the exploitation of steam expansion energy in the turbine of a MVC module and subsequent utilization of condensation enthalpy in an MSF distiller presents a low energy solution. In a dual purpose plant, an MSF-MVC combination can make water production more independent of varying power delivery. Many small desalination plants in remote locations consist of an arrangement of MSF distillers and Diesel generators. Often the generators are overrated to satisfy extreme municipal demands, whereas water production very soon requires extension. The installation of additional MSF evaporators usually calls for costly extensions of the seawater intake, seawater pretreatment and other ancillaries. These investments can be saved if it is possible to link the feed water, blow-down and distillate streams of an MVC module to an existing MSFevaporator. At the same time, assuming a duplication of water production, specific fuel consumption is reduced by approximately 30 %. Usually the capacity of the existing generators can satisfy the power requirements of an MVC extension. CHARACTERISTICS OF THE BASIC PROCESSES MSF AND MVC Fig. 1 shows the flow diagram of a conventional MSF process in once-through design. Fig. 2 shows the flow diagram of a single effect, horizontal tube MVC process. Both flow sheets represent standard evaporators in the desalination program of Buckau-Walther. The following discussion of process combinations relates to these sheets for reference.
145
I
Fig.
1:
Flow diagram tor.
of a multi-stage-flash
once-through
evapora-
DISTILLATE
F'ig.
2:
Flow diagram compression
of a single-effect (MVC) evaporator
mechanical
vapour
146 Table 1 lists the characteristics data of both reference processes. Only the more customary range of parameters in practice is stated. The columns marked BW refer to the data of standard BuckauWalther package units with capacities between 250 and 2000 m3/d. In cases where numerical data are required in the following, reference is made to these units. MSF
__-_-
_ _---
IrIVC
- ._._. _ -
Thermal energy co"sum#io"
(kJ/ky)
250 - 400
Electrical energy input (kJ/kg) Raw water requirement (kg seawater / kg distillate)
17-18
13
(9-12) ref.12
Number of stages / effects
12-
Anti-scale chemical consumption assnming high-tan erature additives (y of them. per m 9 of distillate)
110
28
lfi- 22 (once throug 10 - 26 (recycle) 2.2 - 3.6 (log mean)
Total heat transfer area (m2 per m3 daily output)
4 5 - 85
20
-
required
N
890
lot
290
14-
Max. brine temperature (%I
Ileat transfer temperature diff. (K)
BW
BW
2-
.60.5
3
2.8
(90-10s8) 45- 75 ref.90
(50-651 ref.60
18
l-
1
ref.18
6
2.7
3-
7
1 .4-
3
2 - 3.2
2
6
4-5
Table 1: Customary range of process data of MSF distillers (excluding acid dosed plants) and MVC distillers. The columns marked BW refer to standard Bockau-Walther package "nits adopted for refere"ce in this study.
PROCESS COMBINATIONS AND THEIR PECULIARITIES MVC AND MSF DISTILLERS COUPLED IN SERIES. Fig. 3.1 and 3.2 show process combinations that have been similarly suggested in the past. The MVC module is coupled to the hot end of an MSF unit. In Fig. 3.1 the MSF unit serves as preheater for the MVC distiller. Before entering the MSF brine heater, a partial stream of preheated seawater SW2 is diverted to the MVC distiller. Discharge brine BD2 is returned at slightly higher temperature to the MSF cycle upstream of the brine heater. Brine flux through the flash chambers of the MSF distiller is reduced by the amount of distillate D2 produced in the MVC module. This distillate flashed from stage to stage of the MSF module, thus compensating for reduced heat recovery from the brine. The serial arrangement in Fig. 3.2 is a modification of the first. The MVC module serves as brine heater for the MSF unit, thus
147 requiring additional heat transfer areas and steam from an external source. For more flexibility in practice, a separate brine heater to bypass the MVC module would be installed. If the compressor is driven by a back pressure turbine, an economical solution would be to dimension the MSF unit for a heat input that is covered approximately by the exhaust steam.
Fig. 3.1: Serial Link of an MVC evaporator to an MSF evaporator. i&SF unit serves as preheater for MVC unit.
Fig. 3.2: Serial Combination of MSF and MVC evaporators. Steam turbine driven compressor. MVC unit serves as brine heater for MSF unit. PARALLEL LINK OF AN MVC MODULE TO A MSF EVAPORATOR The Buckau-Walther MVC standard package distiller has an evaporation temperature near 60°C. This was found to be a good compromise for reliable scale control, low energy consumption, low seawater demand, plant size and preheater performance. Most other MVC seawater distillers also operate at temperatures below 70°C. In order to couple a low temperature MVC distiller with an MSF unit without abandoning
the full range of MSF operation, the two pro-
cesses have to be coupled in parallel. Feed water for the MVC module can either be diverted from the preheater / condenser stream
148 or from a flash chamber of the MSF unit. In Fig. 4 feed water for the MVC module is diverted from the water box of an MSF condenser with appropriate temperature. The diverted stream can vary between minimum MVC feed and the total MSF seawater flow. Effluent brine BD2 from the MVC evaporator is returned to the MSF heat recovery condensers. The MVC distillate D2 is added to the distillate stream of the MSF unit either in a stage with appropriate temperature or - to simplify the design into the distillate hot-well of the last stage. In Fig. 5.1 and 5.2 MVC feed water SW2 is diverted from the sump of an MSF flash chamber. The effluent brine RD2 can either be returned to the same flash chamber (Fig. 5.1) or it can be led to flash into a subsequent stage (Fig. 5.2). The distillate D2 is either added to the adjacent upstream stage or into the distillate hot-well of the last stage. The interaction between the two evaporators can be stabilized by connecting the vapour space at the end of the MVC evaporator tubes to the vapour space of an appropriate MSF chamber. ENERGY BALANCE In any of the above process combinations the enthalpy usually discharged by an MVC distiller becomes available to the MSF distiller. In the case of the reference distiller in Fig. 2 and Table 1 this amounts specifically to 55 kJ per kg of distillate. The benefit to be gained from this energy depends on the MS?? stage to which the MVC module is linked and the relative capacity of the two evaporators. Assuming equal capacities, a computer simulation of the configuration shown in Fig. 5.1 verified a reduction of brine heater energy consumption from 290 kJ/kg to 270 kJ/kg, i.e. a rise in performance ration from 8 to 8.6
149
Fig. 4: Parallel link of an MVC-evaporator to an MSF unit. Seawater is extracted from one or more MSF unit water boxes to the MVC-cycle and is fed back into the MSF-preheater/condenser system.
Fig. 5.1: Parallel link of an MIX-evaporator to an MSF unit. Brine is extracted from a flash chamber of the MSF unit. Discharge brine from the MVC-evaporator is returned to the same chamber.
150
Fig. 5.2: Parallel link of an MVC-evaporator to an MSF unit. Brine for the MVC-evaporator is extracted from one stage of the MS&unit
and is discharged back into the next
down-stream stage. It can be shown that a surplus energy of 55 kJ/kg, introduced to some intermediate stage of an MSF evaporator will not produce instabilities in the energy balance and operation of the MSF unit. Part of this energy can be understood to compensate for roughly 25 kJ/kg of thermal losses by convention, radiation and venting. The temperature profile through the MSF evaporator is only slightly distorted to higher temperatures. The preheater recovery temperature and the brine discharge temperature are very slightly raised. In -Table 2 the specific thermal and electrical energy consumption of an MSF-MVC combination according to Fig. 5.1 is compared with that of two equal MSF distillers. Assuming that electrical power is produced by Diesel generators, the total specific fuel consumption of the combined process is 6.5 g/kg, that of the MSF units 9.5 g/kg.
151
.--Total seawater demand (kg seawater / kg distillate) Thermal energy (kJ/kg) Electrical energy LkJ/Kg) Total fuel consumption (g/kg) Anti-scale chemic 1s (g of them. ljer"I' d:st.)
2 x MSF 12
MSF-""C -6
290
135
17
30
9.5 18
ECO”Omy --
50
%
6.6
31 %
9
50 %
Table 2: Comparison of specific consumption data. TWO MSF distillers and a MSF-WC
combination for the case that a MSP plant capacity Is
to be doubled. Reference units Fig. 1, Table
1 and
Fig.
5.1.
SCALE CONTROL CHEMICALS In a parallel combination ofa low temperature MVC distiller and an MSF distiller, the required dosage of anti-scale chemicals is determined by the MSF unit. Assuming equal capacity of both modules, the specific chemical consumption is accordingly half that of an MSF distiller, cf. Table 2. In a serial combination the MVC distiller operates at top brine temperature of the MSF unit and with higher concentration than in the reference MSF distiller. This bears a risk of scaling in an MVC evaporator and the brine heater if not carefully taken into account. PARTIAL LOAD OPERATION Production rates of MSF distillers can be lowered with stable operation to approximately 50 %. The output of an MVC distiller can be reduced to 20 8 - 40 8, depending on the seawater temperature. In a combination of the two processes various schemes for partial load operation can be devised. Reductions down to IO % are feasible. In the very low range of production either one of the modules is taken out of operation, the choice depending on the energy - thermal or electrical - available. INSTALLATION COST CONSIDERATIONS Compared with the MVC distiller shown as flow sheet in Fig. 2, the IWC module in combination with an MSF distiller can dispense various components. The MSF unit acquires the function of the preheaters the evacuation and non-condensibles extractions system the chemical dosing station the deaerator certain pumps, depending on the coupling configuration.
162
Part of the costs saved here are certainly consumed by essential coupling arrangements. If an MSF plant is to be extended, the installation of additional MSF distillers also demands extension of at least part of the following systems: - the seawater intake and seawater transfer to distillers - seawater pretreatment: filtration and chlorination - brine reject system - stores for chemicals and fuel. If site arrangement permits the link of an MVC module to the existing MSF evaporators, considerable costs for these ancillary installations can be saved. CONCLUSION The link of an MVC distiller module to an MSF evaporator can be an economic solution if an existing MSF plant is to be extended. Due to its flexibility with respect to partical load operation and varying energy availabilities, the MSF-MVC combination is worth considering in certain cases. Compared with the conventional MSF distiller, the specific seawater demand and the consumption of chemicals are reduced to the order of 50 8, specific fuel consumption is reduced to 65 % - 75 %.
143
SOLUTIONS FOR COUPLING A MECHANICAL VAPOUR COMPRESSION DISTILLER WITH A MULTI-STAGE-FLASH EVAPORATOR K.
Genthner' and M.M. El-Allawy2
'Universit%t Bremen FB4, P.O.Box 330440, D-2800 Bremen 33 (WestGermany) 2Buckau-Walther AG, P.O.Box 210120, D-2800 Bremen 21 (West-Germany)
ABSTRACT
The paper discusses various familiar and new process combinations of multi-stage-flash (MSF) and mechanical vapour compression (MVC) distillation. Coupling of an MVC distiller to an MSF unit considerably reduces the consumption of energy and chemicals as well as feed water requirements. The plant is extremely flexible with respect to partial load operation and adaption to varying availabilities of thermal and electric&l energy. An existing MSF distiller with a capacity of up to 2000 m3/d can be extended by coupling it with a simplified MVC module. Investment and operation costs are notably lower than with the installation of additional MSF units. Ce rapport di?crit des combinaisons connues et nouvelles du pro&d& de distillation flash (MSF) et de distillation par compression m& canique des vapeurs (MVC). En raccordant une unit& de MVC avec une unit& de MSF on peut reduire dans une forte mesure la consommation en gnergie et en produits chimiques ainsi que les besoins en eau brute. En ce qui concerne le service 2 charge partielle et l'adaptation aux sources diverses d'6nergie thermique et Glectrique, cette unit& est t&s flexible. Le dgbit d'une installation MSF ayant une capacit6 jusqu'au 2000 m3/j peut Btre augment6 en raccordant cette installation avec un module de MVC simplifi8. Les frais d'investissement et d'exploitation sont pour ce raccordement notamment plus bas que d'installer des installations MSF suppl6mentaires. Es werden bekannte und neue Koppelungen eines Entspannungsverdampfers (MSF) mit einem durch mechanische Briidenkompression betriebenen Verdampfer (MVC) untersucht. Durch die Ankoppelung eines MVC-Verdampfers an eine MSF-Anlage werden der Energiebedarf, der Verbrauch an Chemikalien und der Rohwasserbedarf erheblich reduziert. Die Anlage ist in ihrem Teillastverhalten und mit der Mtiglichkeit, sich wechselnden Verfiigbarkeiten von thermischer und elektrischer Energie anzupassen, sehr flexibel. Eine bestehende MSF-Anlage mit einer Tagesleistung bis 2000 m3/d kann auf einfache Weise durch Ankoppelung eines MVC-Moduls erweitert werden. Investitionsund Betriebskosten sind nennenswert geringer als bei Installation zu.sZtzlicher MSF-Anlagen.
OOll-9164/83/$03.00 0 1983ElsevierSciencePublishersB.V.
144 INTRODUCTION Various suggestions of coupling a mechanical vapour compression (MVC) distiller in series with a multi-stage flash (MSF) evaporator have been investigated in the past. The present paper additionally presents solutions for a parallel link between the two processes. No MSF-MVC combination has yet - to our knowledge
been suc-
cessfully realized. In the years of low fuel costs the goal of reducing energy consumption was superceded by higher installation costs, lack of confidence in the lifetime of compressors and by the complexity of the solutions. Vapour compression technology has now certainly reached a state of high reliability. The other aspects still apply for the majority of new desalination projects. However, there do exist a number of cases where a combination of the features of both MSF and MVC distillation bears considerable economic attraction. In cases where high or medium pressure steam is available for desalination, the exploitation of steam expansion energy in the turbine of a MVC module and subsequent utilization of condensation enthalpy in an MSF distiller presents a low energy solution. In a dual purpose plant, an MSF-MVC combination can make water production more independent of varying power delivery. Many small desalination plants in remote locations consist of an arrangement of MSF distillers and Diesel generators. Often the generators are overrated to satisfy extreme municipal demands, whereas water production very soon requires extension. The installation of additional MSF evaporators usually calls for costly extensions of the seawater intake, seawater pretreatment and other ancillaries. These investments can be saved if it is possible to link the feed water, blow-down and distillate streams of an MVC module to an existing MSFevaporator. At the same time, assuming a duplication of water production, specific fuel consumption is reduced by approximately 30 %. Usually the capacity of the existing generators can satisfy the power requirements of an MVC extension. CHARACTERISTICS OF THE BASIC PROCESSES MSF AND MVC Fig. 1 shows the flow diagram of a conventional MSF process in once-through design. Fig. 2 shows the flow diagram of a single effect, horizontal tube MVC process. Both flow sheets represent standard evaporators in the desalination program of Buckau-Walther. The following discussion of process combinations relates to these sheets for reference.
145
I
Fig.
1:
Flow diagram tor.
of a multi-stage-flash
once-through
evapora-
DISTILLATE
F'ig.
2:
Flow diagram compression
of a single-effect (MVC) evaporator
mechanical
vapour
146 Table 1 lists the characteristics data of both reference processes. Only the more customary range of parameters in practice is stated. The columns marked BW refer to the data of standard BuckauWalther package units with capacities between 250 and 2000 m3/d. In cases where numerical data are required in the following, reference is made to these units. MSF
__-_-
_ _---
IrIVC
- ._._. _ -
Thermal energy co"sum#io"
(kJ/ky)
250 - 400
Electrical energy input (kJ/kg) Raw water requirement (kg seawater / kg distillate)
17-18
13
(9-12) ref.12
Number of stages / effects
12-
Anti-scale chemical consumption assnming high-tan erature additives (y of them. per m 9 of distillate)
110
28
lfi- 22 (once throug 10 - 26 (recycle) 2.2 - 3.6 (log mean)
Total heat transfer area (m2 per m3 daily output)
4 5 - 85
20
-
required
N
890
lot
290
14-
Max. brine temperature (%I
Ileat transfer temperature diff. (K)
BW
BW
2-
.60.5
3
2.8
(90-10s8) 45- 75 ref.90
(50-651 ref.60
18
l-
1
ref.18
6
2.7
3-
7
1 .4-
3
2 - 3.2
2
6
4-5
Table 1: Customary range of process data of MSF distillers (excluding acid dosed plants) and MVC distillers. The columns marked BW refer to standard Bockau-Walther package "nits adopted for refere"ce in this study.
PROCESS COMBINATIONS AND THEIR PECULIARITIES MVC AND MSF DISTILLERS COUPLED IN SERIES. Fig. 3.1 and 3.2 show process combinations that have been similarly suggested in the past. The MVC module is coupled to the hot end of an MSF unit. In Fig. 3.1 the MSF unit serves as preheater for the MVC distiller. Before entering the MSF brine heater, a partial stream of preheated seawater SW2 is diverted to the MVC distiller. Discharge brine BD2 is returned at slightly higher temperature to the MSF cycle upstream of the brine heater. Brine flux through the flash chambers of the MSF distiller is reduced by the amount of distillate D2 produced in the MVC module. This distillate flashed from stage to stage of the MSF module, thus compensating for reduced heat recovery from the brine. The serial arrangement in Fig. 3.2 is a modification of the first. The MVC module serves as brine heater for the MSF unit, thus
147 requiring additional heat transfer areas and steam from an external source. For more flexibility in practice, a separate brine heater to bypass the MVC module would be installed. If the compressor is driven by a back pressure turbine, an economical solution would be to dimension the MSF unit for a heat input that is covered approximately by the exhaust steam.
Fig. 3.1: Serial Link of an MVC evaporator to an MSF evaporator. i&SF unit serves as preheater for MVC unit.
Fig. 3.2: Serial Combination of MSF and MVC evaporators. Steam turbine driven compressor. MVC unit serves as brine heater for MSF unit. PARALLEL LINK OF AN MVC MODULE TO A MSF EVAPORATOR The Buckau-Walther MVC standard package distiller has an evaporation temperature near 60°C. This was found to be a good compromise for reliable scale control, low energy consumption, low seawater demand, plant size and preheater performance. Most other MVC seawater distillers also operate at temperatures below 70°C. In order to couple a low temperature MVC distiller with an MSF unit without abandoning
the full range of MSF operation, the two pro-
cesses have to be coupled in parallel. Feed water for the MVC module can either be diverted from the preheater / condenser stream
148 or from a flash chamber of the MSF unit. In Fig. 4 feed water for the MVC module is diverted from the water box of an MSF condenser with appropriate temperature. The diverted stream can vary between minimum MVC feed and the total MSF seawater flow. Effluent brine BD2 from the MVC evaporator is returned to the MSF heat recovery condensers. The MVC distillate D2 is added to the distillate stream of the MSF unit either in a stage with appropriate temperature or - to simplify the design into the distillate hot-well of the last stage. In Fig. 5.1 and 5.2 MVC feed water SW2 is diverted from the sump of an MSF flash chamber. The effluent brine RD2 can either be returned to the same flash chamber (Fig. 5.1) or it can be led to flash into a subsequent stage (Fig. 5.2). The distillate D2 is either added to the adjacent upstream stage or into the distillate hot-well of the last stage. The interaction between the two evaporators can be stabilized by connecting the vapour space at the end of the MVC evaporator tubes to the vapour space of an appropriate MSF chamber. ENERGY BALANCE In any of the above process combinations the enthalpy usually discharged by an MVC distiller becomes available to the MSF distiller. In the case of the reference distiller in Fig. 2 and Table 1 this amounts specifically to 55 kJ per kg of distillate. The benefit to be gained from this energy depends on the MS?? stage to which the MVC module is linked and the relative capacity of the two evaporators. Assuming equal capacities, a computer simulation of the configuration shown in Fig. 5.1 verified a reduction of brine heater energy consumption from 290 kJ/kg to 270 kJ/kg, i.e. a rise in performance ration from 8 to 8.6
149
Fig. 4: Parallel link of an MVC-evaporator to an MSF unit. Seawater is extracted from one or more MSF unit water boxes to the MVC-cycle and is fed back into the MSF-preheater/condenser system.
Fig. 5.1: Parallel link of an MIX-evaporator to an MSF unit. Brine is extracted from a flash chamber of the MSF unit. Discharge brine from the MVC-evaporator is returned to the same chamber.
150
Fig. 5.2: Parallel link of an MVC-evaporator to an MSF unit. Brine for the MVC-evaporator is extracted from one stage of the MS&unit
and is discharged back into the next
down-stream stage. It can be shown that a surplus energy of 55 kJ/kg, introduced to some intermediate stage of an MSF evaporator will not produce instabilities in the energy balance and operation of the MSF unit. Part of this energy can be understood to compensate for roughly 25 kJ/kg of thermal losses by convention, radiation and venting. The temperature profile through the MSF evaporator is only slightly distorted to higher temperatures. The preheater recovery temperature and the brine discharge temperature are very slightly raised. In -Table 2 the specific thermal and electrical energy consumption of an MSF-MVC combination according to Fig. 5.1 is compared with that of two equal MSF distillers. Assuming that electrical power is produced by Diesel generators, the total specific fuel consumption of the combined process is 6.5 g/kg, that of the MSF units 9.5 g/kg.
151
.--Total seawater demand (kg seawater / kg distillate) Thermal energy (kJ/kg) Electrical energy LkJ/Kg) Total fuel consumption (g/kg) Anti-scale chemic 1s (g of them. ljer"I' d:st.)
2 x MSF 12
MSF-""C -6
290
135
17
30
9.5 18
ECO”Omy --
50
%
6.6
31 %
9
50 %
Table 2: Comparison of specific consumption data. TWO MSF distillers and a MSF-WC
combination for the case that a MSP plant capacity Is
to be doubled. Reference units Fig. 1, Table
1 and
Fig.
5.1.
SCALE CONTROL CHEMICALS In a parallel combination ofa low temperature MVC distiller and an MSF distiller, the required dosage of anti-scale chemicals is determined by the MSF unit. Assuming equal capacity of both modules, the specific chemical consumption is accordingly half that of an MSF distiller, cf. Table 2. In a serial combination the MVC distiller operates at top brine temperature of the MSF unit and with higher concentration than in the reference MSF distiller. This bears a risk of scaling in an MVC evaporator and the brine heater if not carefully taken into account. PARTIAL LOAD OPERATION Production rates of MSF distillers can be lowered with stable operation to approximately 50 %. The output of an MVC distiller can be reduced to 20 8 - 40 8, depending on the seawater temperature. In a combination of the two processes various schemes for partial load operation can be devised. Reductions down to IO % are feasible. In the very low range of production either one of the modules is taken out of operation, the choice depending on the energy - thermal or electrical - available. INSTALLATION COST CONSIDERATIONS Compared with the MVC distiller shown as flow sheet in Fig. 2, the IWC module in combination with an MSF distiller can dispense various components. The MSF unit acquires the function of the preheaters the evacuation and non-condensibles extractions system the chemical dosing station the deaerator certain pumps, depending on the coupling configuration.
162
Part of the costs saved here are certainly consumed by essential coupling arrangements. If an MSF plant is to be extended, the installation of additional MSF distillers also demands extension of at least part of the following systems: - the seawater intake and seawater transfer to distillers - seawater pretreatment: filtration and chlorination - brine reject system - stores for chemicals and fuel. If site arrangement permits the link of an MVC module to the existing MSF evaporators, considerable costs for these ancillary installations can be saved. CONCLUSION The link of an MVC distiller module to an MSF evaporator can be an economic solution if an existing MSF plant is to be extended. Due to its flexibility with respect to partical load operation and varying energy availabilities, the MSF-MVC combination is worth considering in certain cases. Compared with the conventional MSF distiller, the specific seawater demand and the consumption of chemicals are reduced to the order of 50 8, specific fuel consumption is reduced to 65 % - 75 %.