- Email: [email protected]
Desalting water costs using waste heat from power systems combined with the VTFE and vapor compression
Desalination, 44 (1983)
12S-135
129
Elsevier Science Publishers B.V.. Amsterdam --'Printed inTheNetherlands
DESALTING WATER COSTS USING WASTE HEAT FROM POWER SYSTEMS COMBINED WITH THE VTFE AND VAPOR COMPRESSION SAMUEL J. SENATORE
EbascoServicesInc., 2 World Trade Center,New York, NY 10048,U.S.A.
ABSTRACT An updated desalted water cost comparison Will be presented for the Reverse Osmosis, Multistage Flash Evaporator and the high efficiency performance Vapor Compression Vertical Tube Foaming Flow Evaporator (VTFE). The capital cost of the VCVTFE plant is based on a realistic conceptual design for a 2.5 mgd Single Purpose plant. There are several interesting desalting design concepts that will be presented which can only be realized through the use of the low
AT
vertical tube foaming flow evaporator.
Together with the
vapor compression cycle, high performance ratios of about 35 lbs. product per 1000 Btus can be readily achieved. These high efficiency concepts and use of waste heat are economical with current day high fuel costs. Conceptual designs will also be discussed of several distillation options using waste heat from a 20 MWe gas turbine.
A design
of a plant using the waste heat from the bottom end of a power plant for fresh water production from seawater will be presented, comparisons of these low energy consuming with conventional MSF and RO systems.
cost
concepts will be made
The turbine exhaust steam
(waste heat) normally rejected through the seawater cooled condensers can be Used to produce distilled water with an efficient vertical tube foaming flow single effect evaporator using enhanced fluttea tubes. A discussion will be presented on the VCVTFE process including
design
the options and selection of the critical hardware for the
efficient vapor compression plant design. A summary table will be presented which will include plant capital Costs and water costs
for several values of energy costs.
0011~9164/83/$03.00 0 1983EhevierScience Publishers B.V.
130
L'exposg presente une comparafson du co8t de l'eau dessalee pour les syst&ues: osmose inverse, Bvaporateur instantan B multiples &ages et 6vaporateur B compression de vapeur, tube vertical et flux moussant (VTFE). 11 traite &galement les conceptions de diffkentes options de distillateurs utilisant la chaleur perdue d'une turbine B gaz. Les frais de ces conceptions a basse consommetlon en 6nergie sont cornpar& riceux des systSmes conventionnels d'Bvaporation instantanee a Stages multiples et d'osmose inverse. La vapeur sortant de la turbine (chaleur perdue) peut gtre mise 2 profit pour produire de l'eau dfstillge. Un tableau presente le coat d'investissementde l'installationet le coat de l'eau par rapport 1 plusieurs valeurs de cogt de l'krergie. INTRODUCTION The costs of desalting seawater is made up of two major categories: namely, capital and annual operating costs. The capital costs consist of all the direct costs plus the indirect costs which can have many interpretations. For a fair and accurate comparison, the basis of this cost analysis is the timely and significant ORBL 1982 Report TM-8191. The capital costs of three processes, namely, Reverse Osmosis (RO), Multistage Flash Distillation (MSF) and the Vapor Compression Vertical Tube Evaporator (VCVTFE) were normalized for consistency for a 2.37 MGD conceptual design. The projected bottom line water costs results show that the VCVTFE process is more economical than the RO which is cheaper than the YSF desalting costs at today's high energy costs of about $32/bbl for crude oil or the equivalent of $5,5/106 Btu. The VCVTFE water cost is $5.74 per 1,000 gallons vs. $8.22 for the RO and $9.13 for the MSF process. The RO cost (without energy recovery) for desalting seawater is more than 40% higher than the VCVTFE costs. More importantly, as the fuel costs continue to escalate the VCVTFE process continues to become more attractive economically, since this process consumes less energy than either the RO or the MSF. The hybrid VCVTFE process herein described provides the method of reducing the energy cost of the distillation process by a factor of 3 to 4 in a single purpose desalination plant. The energy conversion efficiency for generating shaft power is about 35 to 40 percent, Hence, desalination processes using mechanical or electrical energy are penalized. The diesel driven VCVTFE overcomes this disadvantage. The hybrid VCVTFE plant discussed in this paper has a performance ratio of 31 lbs. product per 1,000 Btu of fuel input to the diesel drive. Generally, the performance ratio of distillation plants is based on the Btu in the steam supplied to the process. Hence, the boiler conversion efficiency will lower the overall plant P.R. A power plant waste heat VTFE design is also included in this discussion. This process uses only 10 Kwhr per 1,000 gallons of product water produced from seawater.
131 VCVTFE Plant Design The VCVTFE process, shown in Figure 1, for the 2.37 MGD plant consist of a diesel engine powering a vapor compressor that operates across a four-effect vertical tube evaporator. A heat recovery boiler is employed that recovers the heat from the diesel engine exhaust and cooling water jacket. Steam is generated in the heat recovery boiler at approximately 260'F saturated. This steam is used as the supplemental heat input to the first effect of the vertical tube foam evaporator. The vapor compressor receives the low-pressure steam from the fourth VTFE effect at 233'F with 8'F superheat (by recycling some discharge steam) and increases its temperature to 335OF superheated steam, which is then desuperheated to 250°F. The boiler steam in the first effect is held separate from the compressor discharged process steam. The system operates with a gross Delta-T for the four effects of only 25'F. When the boiling point elevation and intereffect pressure drop losses are deducted, the process results in a net Delta-T of about 5'F per effect. The VTFE test bundles have successfully operated with a Delta-T as low as 4.5'F. The energy consumption of the vapor compressor is only 3,400 HP and these 4 vertical tube effects result in a net water production of 1.95 MGD.
The 17
effect low temperature vertical tube foaming flow evaporator will produce about 0.42 MGD of additional product.
Waste Heat VTFE Design Power plant turbine exhaust steam (waste heat) normally rejected to the ocean through seawater-cooledcondensers, can be used to produce distilled water with an efficient Vertical Tube Foaming Flow (VTFE) evaporative condenser (1). Such a system was recently demonstrated by Dr. Sephton on a realistic scale in conjunctive operation with an inland power plant. Based on this demonstration, the commercial plant reported here was designed and bid for the production of distilled water for boiler feed from seawater utilizing turbine exhaust steam to drive the process. Feasibility of utilizing waste heat for distillation prior to its rejection to the ocean is provided by the energy-efficientvertical.tube foam evaporation (VTFE) process. In this method of evaporation, the liquid is induced into a foamy-layer flow over the heat transfer surface for thin-layer, wiped film evaporation that enhances the brineside coefficient of heat transfer substantially. This waste heat VTFE plant was designed to provide I.50m3 (40,000 gal.) per day of distillate having about 5 parts per million (ppm) of total dissolved salts (TDS), for use of boiler feed at a 37 MWe coastal power plant at Key West, Florida. Turbine exhaust steam was available at 46'C (115'F) and power
132
PIGURR
1
2.37 MGD DIESEL DRIVEN-VAPOR COMPRESSOR DESALTING FLOW DIAGRAM
FUEL IN, 9 - 26 X lo8 BTU/M
133
FIGURE
POWER
PLANT
WASTE-HEAT
*_-
2
DESALTING
TURBINE -__-
EXHAUST
STEAM
- - --mm 300000LBS,HR
POWER PLANT CONDENSER
-ORIFICE
,
EVAPORATOR
SURFACTANT ADDITION 10 PPM
PLATE
ZOO.15 LONG. 3” DIA. FLUTEDTUBES
%
SLOWDOWN
- KEY WEST
SEA WATER
134 plant coolant at 3.5 KPa (5 psi) and 24OC (75OF); coolant effluent (95'F) was available quirements
included
for use as evaporator the removal
the usual decarbonation The flow diagram through
warmed
reject
The deaerated scavenger,
of hydrogen
(and in parallel
feed is dosed with a selected
to
feed makeup.
the pre-existing
pressure
and is recycled
and vacuum
deaeration.
agent and an oxygen
as distillate
exhaust
hotwell,
power
Part of this
The vapor produced
flow.
The turbine
coolant passes
is con-
for use as
steam condensate
where
is
it mixes with the power
as power plant boiler
feed.
Cost Comparisons
The capital processes. generates
costs of desalting
The VCVTFE plant its own required
degraded
plants have been calculated
is a single purpose water
power by a diesel
The capital start-up,
intake and outfall
vary widely
The major hardware
The capital capacity
costs
includes
design.
on a common
The overall
cost
plant
site.
the site related work
cost (about
(about 40%).
cost estimate were interpolated
of 2.37 MGD.
$6.61/GPD.
cost estimate
plant
These costs were
and included
for any specific
erection,
costs amount to about 50% of the total project
The remaining
10%) and the indirect
on site
The plant and systems
in ORNL TM-8191.
to the VCVTFE and conceptual
costs can obviously
estimate.
plant equipment,
systems.
for a given southern 1J.S. location
basis and compared
is generated
fired with oil.
for the RO and MSF are furnished
normalized
For the MSF, extraction
engine.
costs include all necessary
and seawater
for the three
only plant which
electricity
steam is used as the heat source,
using steam from boilers
costs,
Seawater
to the ocean.
foaming
and enters the VTFE feed recycle
to the power plant condenser
Capital
2.
is used as VTFE feed after pretreatment
plant condensate,
re-
(10 ppm) in addition
flow through
rejected
densed and is pumped out to atmospherie
returned
sulfide
is shown in Figure
and is subsequently
power plant boiler
at 35OC
VTFE feed pretreatment
deaeration. utilized
the condenser
plant condenser)
feed.
The unit cost of water
This cost on the same basis
for an RO plant with a
capacity
from this RO plant is
as the VCVTFE was $5.76/GPD
and for
the MSF was $8.03/GPD. The capital hardware million
cost for the 2.37 MGD VCVTFE plant was estimated
proposals dollars
compressor,
from manufacturers.
The total hardware
and is made up of the major
diesel, waste heat boiler,
etc. for this plant. about 13.7 million
equipment
vertical
The total plant capital
dollars.
from actual
cost is about 6.45
including
tube evaporator,
cost including
the vapor piping,
drives,
the indirects
is
135 Operating Costs The energy cost of the VCVTFE process is only 51% of the energy cost of the RO process. The reason is that the RO process requires electric motor drives and the inherent electrical conversion efficiencies (about 40%) are much lower than the direct diesel drives for the VCVTFE process since 75% of the fuel.Btu's are used in this process. Based on the fuel input to the power plant the RO performance ratio calculates to be about 25 lbs. product per 1,000 Btu, whereas the VCVTFE has a performance ratio of 31. An interesting energy comparison shows that the Btu energy equivalent cost of power is about 4 times higher than the energy equivalent cost of the Btu in the fuel. Table 1 is a sununarycost comparison for the three processes for a 2.37 MGD capacity. In this Table, a water cost sensitivity analysis is estimated based on energy costs of $3.0/106 Btu (4.09c/kwhr),and $5.5/106 Btu (7.5c/kwhr).
TABLE 1
2.37 MGD DESALTING SEAWATER COST COMPARISON SUMMARY MSF CAPITAL
(THOUSANDS)
$/GPD
$t9,036
RO $15,675
8.03
6.61
VCVTFE $13,655 5.76
ANNUAL WATER COSTS (THOUSANDS) POWER @ $5.5/106 (7.5a/KWHR)
BTU
FIX CHARGES @I16% OTHER OPERATING
S/KG&L
2,611
2,122
1.066
3,426
2,622
2,456
475 -_ $ 6,712
$ 6,040
9.13
6.22
5.74
7.39
8.66
5.07
1,966
674 $4.220
@ S3Jl66 BTU (4.06dKWHRI $/KGAL
REFERENCES
(1)
"Desalination of Seawater With Waste Heat", H.H. Sephton of EnvirotechSephton Development Center and S.J. Senatore of Ebasco, presented at the WSIA 10th Annual Conference held in Honolulu, Hawaii, on July 25-29, 1982.
12S-135
129
Elsevier Science Publishers B.V.. Amsterdam --'Printed inTheNetherlands
DESALTING WATER COSTS USING WASTE HEAT FROM POWER SYSTEMS COMBINED WITH THE VTFE AND VAPOR COMPRESSION SAMUEL J. SENATORE
EbascoServicesInc., 2 World Trade Center,New York, NY 10048,U.S.A.
ABSTRACT An updated desalted water cost comparison Will be presented for the Reverse Osmosis, Multistage Flash Evaporator and the high efficiency performance Vapor Compression Vertical Tube Foaming Flow Evaporator (VTFE). The capital cost of the VCVTFE plant is based on a realistic conceptual design for a 2.5 mgd Single Purpose plant. There are several interesting desalting design concepts that will be presented which can only be realized through the use of the low
AT
vertical tube foaming flow evaporator.
Together with the
vapor compression cycle, high performance ratios of about 35 lbs. product per 1000 Btus can be readily achieved. These high efficiency concepts and use of waste heat are economical with current day high fuel costs. Conceptual designs will also be discussed of several distillation options using waste heat from a 20 MWe gas turbine.
A design
of a plant using the waste heat from the bottom end of a power plant for fresh water production from seawater will be presented, comparisons of these low energy consuming with conventional MSF and RO systems.
cost
concepts will be made
The turbine exhaust steam
(waste heat) normally rejected through the seawater cooled condensers can be Used to produce distilled water with an efficient vertical tube foaming flow single effect evaporator using enhanced fluttea tubes. A discussion will be presented on the VCVTFE process including
design
the options and selection of the critical hardware for the
efficient vapor compression plant design. A summary table will be presented which will include plant capital Costs and water costs
for several values of energy costs.
0011~9164/83/$03.00 0 1983EhevierScience Publishers B.V.
130
L'exposg presente une comparafson du co8t de l'eau dessalee pour les syst&ues: osmose inverse, Bvaporateur instantan B multiples &ages et 6vaporateur B compression de vapeur, tube vertical et flux moussant (VTFE). 11 traite &galement les conceptions de diffkentes options de distillateurs utilisant la chaleur perdue d'une turbine B gaz. Les frais de ces conceptions a basse consommetlon en 6nergie sont cornpar& riceux des systSmes conventionnels d'Bvaporation instantanee a Stages multiples et d'osmose inverse. La vapeur sortant de la turbine (chaleur perdue) peut gtre mise 2 profit pour produire de l'eau dfstillge. Un tableau presente le coat d'investissementde l'installationet le coat de l'eau par rapport 1 plusieurs valeurs de cogt de l'krergie. INTRODUCTION The costs of desalting seawater is made up of two major categories: namely, capital and annual operating costs. The capital costs consist of all the direct costs plus the indirect costs which can have many interpretations. For a fair and accurate comparison, the basis of this cost analysis is the timely and significant ORBL 1982 Report TM-8191. The capital costs of three processes, namely, Reverse Osmosis (RO), Multistage Flash Distillation (MSF) and the Vapor Compression Vertical Tube Evaporator (VCVTFE) were normalized for consistency for a 2.37 MGD conceptual design. The projected bottom line water costs results show that the VCVTFE process is more economical than the RO which is cheaper than the YSF desalting costs at today's high energy costs of about $32/bbl for crude oil or the equivalent of $5,5/106 Btu. The VCVTFE water cost is $5.74 per 1,000 gallons vs. $8.22 for the RO and $9.13 for the MSF process. The RO cost (without energy recovery) for desalting seawater is more than 40% higher than the VCVTFE costs. More importantly, as the fuel costs continue to escalate the VCVTFE process continues to become more attractive economically, since this process consumes less energy than either the RO or the MSF. The hybrid VCVTFE process herein described provides the method of reducing the energy cost of the distillation process by a factor of 3 to 4 in a single purpose desalination plant. The energy conversion efficiency for generating shaft power is about 35 to 40 percent, Hence, desalination processes using mechanical or electrical energy are penalized. The diesel driven VCVTFE overcomes this disadvantage. The hybrid VCVTFE plant discussed in this paper has a performance ratio of 31 lbs. product per 1,000 Btu of fuel input to the diesel drive. Generally, the performance ratio of distillation plants is based on the Btu in the steam supplied to the process. Hence, the boiler conversion efficiency will lower the overall plant P.R. A power plant waste heat VTFE design is also included in this discussion. This process uses only 10 Kwhr per 1,000 gallons of product water produced from seawater.
131 VCVTFE Plant Design The VCVTFE process, shown in Figure 1, for the 2.37 MGD plant consist of a diesel engine powering a vapor compressor that operates across a four-effect vertical tube evaporator. A heat recovery boiler is employed that recovers the heat from the diesel engine exhaust and cooling water jacket. Steam is generated in the heat recovery boiler at approximately 260'F saturated. This steam is used as the supplemental heat input to the first effect of the vertical tube foam evaporator. The vapor compressor receives the low-pressure steam from the fourth VTFE effect at 233'F with 8'F superheat (by recycling some discharge steam) and increases its temperature to 335OF superheated steam, which is then desuperheated to 250°F. The boiler steam in the first effect is held separate from the compressor discharged process steam. The system operates with a gross Delta-T for the four effects of only 25'F. When the boiling point elevation and intereffect pressure drop losses are deducted, the process results in a net Delta-T of about 5'F per effect. The VTFE test bundles have successfully operated with a Delta-T as low as 4.5'F. The energy consumption of the vapor compressor is only 3,400 HP and these 4 vertical tube effects result in a net water production of 1.95 MGD.
The 17
effect low temperature vertical tube foaming flow evaporator will produce about 0.42 MGD of additional product.
Waste Heat VTFE Design Power plant turbine exhaust steam (waste heat) normally rejected to the ocean through seawater-cooledcondensers, can be used to produce distilled water with an efficient Vertical Tube Foaming Flow (VTFE) evaporative condenser (1). Such a system was recently demonstrated by Dr. Sephton on a realistic scale in conjunctive operation with an inland power plant. Based on this demonstration, the commercial plant reported here was designed and bid for the production of distilled water for boiler feed from seawater utilizing turbine exhaust steam to drive the process. Feasibility of utilizing waste heat for distillation prior to its rejection to the ocean is provided by the energy-efficientvertical.tube foam evaporation (VTFE) process. In this method of evaporation, the liquid is induced into a foamy-layer flow over the heat transfer surface for thin-layer, wiped film evaporation that enhances the brineside coefficient of heat transfer substantially. This waste heat VTFE plant was designed to provide I.50m3 (40,000 gal.) per day of distillate having about 5 parts per million (ppm) of total dissolved salts (TDS), for use of boiler feed at a 37 MWe coastal power plant at Key West, Florida. Turbine exhaust steam was available at 46'C (115'F) and power
132
PIGURR
1
2.37 MGD DIESEL DRIVEN-VAPOR COMPRESSOR DESALTING FLOW DIAGRAM
FUEL IN, 9 - 26 X lo8 BTU/M
133
FIGURE
POWER
PLANT
WASTE-HEAT
*_-
2
DESALTING
TURBINE -__-
EXHAUST
STEAM
- - --mm 300000LBS,HR
POWER PLANT CONDENSER
-ORIFICE
,
EVAPORATOR
SURFACTANT ADDITION 10 PPM
PLATE
ZOO.15 LONG. 3” DIA. FLUTEDTUBES
%
SLOWDOWN
- KEY WEST
SEA WATER
134 plant coolant at 3.5 KPa (5 psi) and 24OC (75OF); coolant effluent (95'F) was available quirements
included
for use as evaporator the removal
the usual decarbonation The flow diagram through
warmed
reject
The deaerated scavenger,
of hydrogen
(and in parallel
feed is dosed with a selected
to
feed makeup.
the pre-existing
pressure
and is recycled
and vacuum
deaeration.
agent and an oxygen
as distillate
exhaust
hotwell,
power
Part of this
The vapor produced
flow.
The turbine
coolant passes
is con-
for use as
steam condensate
where
is
it mixes with the power
as power plant boiler
feed.
Cost Comparisons
The capital processes. generates
costs of desalting
The VCVTFE plant its own required
degraded
plants have been calculated
is a single purpose water
power by a diesel
The capital start-up,
intake and outfall
vary widely
The major hardware
The capital capacity
costs
includes
design.
on a common
The overall
cost
plant
site.
the site related work
cost (about
(about 40%).
cost estimate were interpolated
of 2.37 MGD.
$6.61/GPD.
cost estimate
plant
These costs were
and included
for any specific
erection,
costs amount to about 50% of the total project
The remaining
10%) and the indirect
on site
The plant and systems
in ORNL TM-8191.
to the VCVTFE and conceptual
costs can obviously
estimate.
plant equipment,
systems.
for a given southern 1J.S. location
basis and compared
is generated
fired with oil.
for the RO and MSF are furnished
normalized
For the MSF, extraction
engine.
costs include all necessary
and seawater
for the three
only plant which
electricity
steam is used as the heat source,
using steam from boilers
costs,
Seawater
to the ocean.
foaming
and enters the VTFE feed recycle
to the power plant condenser
Capital
2.
is used as VTFE feed after pretreatment
plant condensate,
re-
(10 ppm) in addition
flow through
rejected
densed and is pumped out to atmospherie
returned
sulfide
is shown in Figure
and is subsequently
power plant boiler
at 35OC
VTFE feed pretreatment
deaeration. utilized
the condenser
plant condenser)
feed.
The unit cost of water
This cost on the same basis
for an RO plant with a
capacity
from this RO plant is
as the VCVTFE was $5.76/GPD
and for
the MSF was $8.03/GPD. The capital hardware million
cost for the 2.37 MGD VCVTFE plant was estimated
proposals dollars
compressor,
from manufacturers.
The total hardware
and is made up of the major
diesel, waste heat boiler,
etc. for this plant. about 13.7 million
equipment
vertical
The total plant capital
dollars.
from actual
cost is about 6.45
including
tube evaporator,
cost including
the vapor piping,
drives,
the indirects
is
135 Operating Costs The energy cost of the VCVTFE process is only 51% of the energy cost of the RO process. The reason is that the RO process requires electric motor drives and the inherent electrical conversion efficiencies (about 40%) are much lower than the direct diesel drives for the VCVTFE process since 75% of the fuel.Btu's are used in this process. Based on the fuel input to the power plant the RO performance ratio calculates to be about 25 lbs. product per 1,000 Btu, whereas the VCVTFE has a performance ratio of 31. An interesting energy comparison shows that the Btu energy equivalent cost of power is about 4 times higher than the energy equivalent cost of the Btu in the fuel. Table 1 is a sununarycost comparison for the three processes for a 2.37 MGD capacity. In this Table, a water cost sensitivity analysis is estimated based on energy costs of $3.0/106 Btu (4.09c/kwhr),and $5.5/106 Btu (7.5c/kwhr).
TABLE 1
2.37 MGD DESALTING SEAWATER COST COMPARISON SUMMARY MSF CAPITAL
(THOUSANDS)
$/GPD
$t9,036
RO $15,675
8.03
6.61
VCVTFE $13,655 5.76
ANNUAL WATER COSTS (THOUSANDS) POWER @ $5.5/106 (7.5a/KWHR)
BTU
FIX CHARGES @I16% OTHER OPERATING
S/KG&L
2,611
2,122
1.066
3,426
2,622
2,456
475 -_ $ 6,712
$ 6,040
9.13
6.22
5.74
7.39
8.66
5.07
1,966
674 $4.220
@ S3Jl66 BTU (4.06dKWHRI $/KGAL
REFERENCES
(1)
"Desalination of Seawater With Waste Heat", H.H. Sephton of EnvirotechSephton Development Center and S.J. Senatore of Ebasco, presented at the WSIA 10th Annual Conference held in Honolulu, Hawaii, on July 25-29, 1982.