Download Simulation of large capacity MSF brine circulation plants

Desalination 204 (2007) 501–514
Simulation of large capacity MSF brine circulation plants
Nabil M. Abdel-Jabbara,b,*, Hazim Mohameed Qiblaweya, Farouq S. Mjallic,
Hisham Ettouneyd
a
Department of Chemical Engineering, Jordan University of Science and Technology, Jordan
Tel: (+971) 65152907; Fax: +(971) 65152979; email: [email protected]
b
Chemical Engineering Department, American University of Sharjah, United Arab Emirates
c
Department of Chemical Engineering, University of Qatar, Qatar
d
Department of Chemical Engineering, Kuwait University, Kuwait
Received 22 January 2006; accepted 15 February 2006
Abstract
New multistage flashing desalination units (MSF) are being constructed with large capacity that may vary
from 50,000 to 75,000 m3/d. This is almost 2–3 times the conventional unit capacity of 27,000–32,000 m3/d, which
were common for the units installed in the 1980’s. Most of the Gulf States and several countries across the world
are acquiring the large units to take the merits of reduced product cost caused by the large unit capacity.
Literature studies and field reports indicate that the unit product in these units is almost identical to seawater
reverse osmosis and the multiple effect evaporation, with an average of $0.5/m3. This paper focuses on modeling
and simulation of the performance characteristics of these large units. Mathematical modeling of the MSF process
is well established in the literature. However, several of the design parameters have to be adjusted in order to take
into considerations the characteristics of the large unit capacity. The analysis focuses on evaluation of the weir
loading, the dimensions of the condenser tube bundle, demister dimensions, stage dimensions, and temperature
and flow rate profiles. The model predictions are validated against field data for a number of existing units.
Keywords: Multistage flash desalination; modeling; design; costing
1. Introduction
One of the striking features of the MSF is
its reliability. Field experience shows that a
large number of existing MSF plants has
exceeded the intended life time [1–2]. Several
of these plants are going through
*Corresponding author.
Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation
between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European
Desalination Society and the University of Montpellier II, Montpellier, France, 21-25 May 2006.
0011-9164/07/$– See front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.desal.2006.02.047
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N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
rehabilitation. More efficient construction
materials are used in the rehabilitation and
newly designed and streamlined components.
Such replacements are taking place in all
areas of the plant, which may include venting
system, demisters, tubing, partitions, and
pumping units. Maintaining and updating of
existing MSF plants is motivated by the
excellent process reliability and the ability to
continually operate for durations of more
than 2 years. Success of the MSF process
has resulted in drastic increase in the unit
production capacity to large values of
50,000–75,000 m3/d. In the 1960’s the first
MSF units had unit capacity of 500 m3/d.
The unit capacity then increased to the well
known capacity of 27,000–32,000 m3/d,
which were introduced in the late 1970’s
[3–4]. Recent economic evaluation of large
desalination systems show that the competitive
unit product cost of MSF, MED, and RO.
Although, RO consumes a close to 25% of
the total energy of MSF and MED, but, its
unit product cost is almost similar, when
membrane replacement cost is considered. In
addition, competitive tendering, pricing, and
less restrictive system design allowed for massive and economical increase in the unit capacity of the MSF and MED plants [5].
Modeling of the MSF process is well
established in the literature [6–20]. Mathematical models include short cut techniques
[6–8], which are very useful to provide quick
estimates of main process characteristics, i.e.,
performance ratio (defined as the amount of
distillate product per unit mass of heat
steam), condenser heat transfer area, and
flow rates of various streams. More detailed
steady state models [9–14] are thought to
evaluate additional design characteristics of
the system, i.e., stage variations in the
amount of flashed off vapor from the distillate stream, thermodynamic losses, heat
transfer coefficient. MSF models include
also system dynamics [15–18], which are
used to study system transient behavior and
to optimize system controllers. In addition,
thermo-economic models are thought to optimize the unit product cost. [19–20].
This study is motivated by the need to
present to the literature an integrated model
on design of large scale MSF units. Such
systems are becoming the industry standard.
The analysis focuses on evaluation of various
system design characteristics, which includes
stage dimensions, weir loading, demister
length, as well as flow rates and temperature
profiles, and performance ratio. The next
sections include system description, model,
system analysis, and comparison against
field data.
2. Process description
Fig. 1 shows schematic of the MSF flashing
stage. As is shown, the flashing stage includes
the condenser tubes, demister, venting line,
distillate tray, inlet/outlet brine orifices, and
air baffle. The processes that take place inside
the flashing stage are given below:
The inlet brine flashes off as it enters the
flashing stage. This is because the saturation
Tube Bundle
Air Baffle
Vent line
Demister
Demister
Distillate Tray
Outlet Brine
Flashed off
Vapor
Inlet Brine
Brine Pool
Fig. 1. MSF flashing stage.
Submerged
Orifice
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
temperature/pressure of the brine is higher
than the stage temperature/pressure. Flashing implies that the formed vapor extracts
its latent heat from the brine stream. Therefore, the brine temperature decreases as
the brine flows across the stages.
The brine orifice is designed to control the
brine flow. Also, it maintains sufficient
brine head within the stages to prevent
vapor escape between stages.
The stage width and length should be
large enough to maintain the vapor velocity below 6 m/s. This is necessary to limit
entrainment of brine droplets in the vapor
stream and to allow for settling of the
brine droplets.
The flashed off brine flows through the
demister, where most of the entrained
brine droplets are captured by the demister
wires. Continued removal of the droplets
would result in the increase in the
captured droplet size. This would result
in detachment of the large droplets and
settling back to the brine pool.
As the vapor flows to the condenser tubes,
where it releases its latent heat and condenses. The condensed vapor accumulates
in the distillate tray and flows to the next
stage. The inlet distillate stream has a
higher temperature and it flashes off as it
flows between stages. This flashed off
vapor also condenses on the outside
surface of the condenser tubes.
Vapor condensation results in the increase
of the temperature of the brine stream
flowing inside the tubes. This is a very
important design feature, where most of
the thermal energy provided by the heating steam is recovered by the brine stream
before entering the brine heater.
Non-condensable gases are also released
during the evaporation process. The noncondensable gases include oxygen, nitrogen, and carbon dioxide. Carbon dioxide
503
might be generated due to the decomposition of calcium carbonate at high temperature. The gases have very low
thermal conductivity and it must be
removed from the system. This is achieved
by the use of vent lines, which are placed
between the flashing stages. The vent
lines, especially at the high temperature
end, are directly connected to the vacuum
steam jet ejector. This is because of the
high release rate of the non-condensable
gases. After the first few stages, the vent
lines allow the non-condensable gases to
flow across the stages until it reaches the
last stage, where the vent line is attached
to the vacuum steam jet ejector.
The non-condensable gases act as an insulating blanket around the condenser
tubes. Also, if it is left to accumulate
within the stages, it would reduce the
vapor partial pressure and its temperature. As a result, the brine flowing inside
the tubes will not reach the desired design
temperature. Another shortcoming of the
non-condensable gases, especially oxygen
and carbon dioxide, is the fact that it
promotes corrosion reactions within the
system.
The air baffle shown within the stage is
placed to control vapor flow through the
vent line and to prevent condensate
splashing in the vent line. Approximately,
2.5% of the vapor formed in each stage is
lost through the vent line; however, close
to 90% of this vapor is recovered in the
steam ejector condenser.
The MSF process with brine circulation is
shown in Fig. 2. As is shown, the flashing
stages are divided into two sections, which
include the heat recovery and rejection sections. The heat recovery stages involve heating of the brine recycle stream inside the
condenser tubes from the temperature of the
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Cooling Seawater
Vacuum Steam Ejector
Motive Steam
Non-Condensable Gases
Condenser
Condensate
Outlet Cooling Seawater
Distillate
Trays
Condenser
Tubes
Cooling
Seawater
Intake
Seawater
Heating
Steam
Distillate
Product
Condensate
Demister
Brine
Pool
Feed
Brine
Brine Blowdown
Brine
Recycle
Chemicals
Fig. 2. Multistage flash desalination with brine recycle.
brine blow down to a higher temperature
close to the top brine temperature. Therefore,
the amount of heating steam used in the brine
heater is minimized. The heat rejection section usually contains three stages. The heat
rejection section functions to control the temperature of the intake seawater and reject the
excess heat added in the brine heater. As is
shown the flow rate of the intake seawater is
equal to the sum of the flow rates of the feed
seawater and the cooling seawater. During
winter operation part of the rejected cooling
seawater is recycled and mixed with the
intake seawater. This is to control the intake
seawater temperature. This practice is common during winter operation. Control of the
feed seawater temperature prevents reduction
of the last stage system temperature, which
would result in the increase of the specific
volume of the flashed off vapor and subsequent increase in the vapor velocity as well as
the amount of entrained brine. This would
result in the increase of the product salinity.
Therefore, the product stream might not be
suitable for use as makeup boiler water. This
problem is circumvented by using part of the
distillate product in the first stages.
It should be noted that the temperatures
of the cooling and feed seawater steam leaving the condenser tubes from the heat rejection section is identical to the temperature
of the brine blow down leaving the last
flashing stage. This is necessary to prevent
thermal shock upon mixing of the feed
seawater in the brine pool of the last stage.
If the temperature of the feed stream and
the brine blow down are different then
the calcium bicarbonate would decompose
and result in precipitation of calcium
carbonate inside the condenser tubes of the
heat recovery section.
Approximately, there are nine pumps used
in the MSF system. The pumps are used for
the intake seawater, rejected cooling seawater, recycled cooling seawater, feed seawater, brine blow down, distillate product,
steam condensate, chemicals, and ejector condensate. The largest of these pumps is the
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
brine recycle pump. For example, a plant
with a production rate of 30,000 m3/d,
would require a brine recycle pump with a
capacity of 300,000 m3/d. The feed seawater
pump would be limited to 75,000 m3/d and
the brine blow down pump would have a
capacity of 45,000 m3/d. Other pumping
units will have a much smaller capacity;
where the heating steam condensate pump
will have a capacity of 3000 m3/d and the
ejector condensate pump will be limited to
1000 m3/d. The rejected cooling seawater
pump will have a capacity of 150,000 m3/d
and recycled cooling seawater pump will
have a capacity of 90,000 m3/d.
3. Mathematical model
The assumptions used to develop the
mathematical model include the following:
Stead state operation, which is the industry
standard. Although, system operation
may experience seasonal temperature
variations of the intake seawater, but, such
variations are slow and the system parameters are adjusted accordingly. Another
factor that may change the system characteristics is the tube fouling, which
results in the increase of the thermal resistance for heat transfer. This problem is
encountered through the use of on-line
ball cleaning system or tube acid cleaning,
which restores conditions to near clean
operation.
Heat losses to the surroundings are negligible. This assumption is valid, since, the
surface to volume ratio of the MSF plants
is very small. Also, the temperature of the
low temperature stage is very close to the
ambient temperature, which reduces the
rate of heat transfer to the surroundings.
Equal heat transfer area in each flashing
stage in the heat recovery section.
505
Equal heat transfer area in each flashing
stage in the heat rejection section.
The heat capacities for feed seawater,
brine, and distillate product depend on
temperature and composition.
The overall heat transfer coefficients in
the evaporators depends on the following
parameters:
&
Flow rate of the condensing vapor.
&
Flow rate of the brine inside the condenser tubes.
&
Temperatures of the condensing vapor
and the brine.
&
Physical properties of the condensing
vapor and the brine, which includes
thermal conductivity, viscosity, density, and specific heat.
&
The tube material, diameter, and wall
thickness.
&
The fouling resistance.
&
The percentage of the non-condensable
gases.
The overall heat transfer coefficient is the
sum of the thermal resistances expressed in
terms of the inside and outside heat transfer
coefficient, the fouling resistance, and the
thermal resistance of the condenser tube.
The latent heat of formed/condensed
vapor depends on temperature.
Thermodynamic losses include the boiling
point elevation (BPE), the non-equilibrium allowance (NEA), and demister
losses (Tp).
The distillate product is salt free.
Schematics of the MSF variables in the
flashing stage and the brine heater are
shown in Figs. 3 and 4.
The model equations are written for the
heat recovery and the heat rejection sections.
It should be noted that the balance are similar for the two sections; this is except for the
balance of the first stage and the condenser
balance equations in the heat rejection
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Inlet Seawater
MR, XR, Tfj+1
Condenser Tubes
Outlet Seawater
MR, XR, Tfj
Flashing
Vapor
Dj, Tcj
Inlet Distillate
j–1
∑ Dk , Tcj – 1
Outlet Distillate
j
∑ Dk, Tcj
k =1
k=1
Brine Orifice
Demister
Inlet Brine
Bj – 1, Xbj – 1, Tbj – 1
Outlet Brine
Bj, Xbj, Tbj
Flashing Vapor
Dj, Tvj
Brine Pool
Design/Operation
Parameters:
Tube diameter
Tube wall thickness.
Number of Tubes.
Stage Length, Width,
Height.
Tube Length
Heat Transfer Area
Distillate Flow Rate
Brine Flow Rate
Brine Recycle Flow
Rate
Temperature Rise of
the Brine Recycle.
Brine Temperature
Drop.
Brine Orifice
Dimensions
Demister width,
Length, Thickness
Fig. 3. Flashing stage and process variables.
Design/Operation Parameters:
Heating Steam
Ms, Ts
Brine Recycle
MR, Tf1, XR
Condensate
Ms, Ts
Brine Recycle
MR, Tbo, XR
Tube diameter
Tube wall thickness.
Number of Tubes.
Shell diameter.
Tube Length
Heat Transfer Area
Flow Rate of Heating Steam
Heating Steam
Temperature
Temperature Rise of the
Brine Recycle.
Brine Recycle Flow Rate.
Fig. 4. Brine heater process variables.
section. The model equations include mass
balance of stage j, which is given by
Bj1 þ
j1
X
Dk ¼ Bj þ
k¼1
j
X
Dk
ð1Þ
For the first stage Bj1 is equal to MR and
Xbj1 is equal to XR. Therefore, the salt balance in the first stage is given by
XR MR ¼ Xb1 B1
ð4Þ
k¼1
It should be noted for the first stageP
the term
Bj1 is equal to MR and the term is j1
k¼1 Dk
equal to zero. Therefore, the mass balance for
the first stage is reduced to the following
MR ¼ D1 þ B1
ð2Þ
The stage salt balance is given by
Xbj Bj ¼ Xbj1 Bj1
ð3Þ
The energy balance for the flashing brine
is given by
Dj lvj ¼ Bj1 Cpb ðTbj1 Tbj Þ
ð5Þ
For the first stage, the term Bj1 is equal
to MR and Tbj1 is equal to the top brine
temperature Tbo . Therefore, the brine energy
balance in the first stage is given by
D1 v1 ¼ MR Cpb ðTbo Tb1 Þ
ð6Þ
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
The energy balance for the condenser
tubes is given by
Dj cj þ Cpd ðTcj1 Tcj Þ
j1
X
MR CpR ðTfj Tfjþ1 Þ ¼ Urj Ar ðLMTDÞrj ð10Þ
k¼1
ð7Þ
It should be noted that in the last stage in
the heat recovery section, the temperature
Tfjþ1 isPequal Tbn . Also, for the first stage the
term j1
k¼1 Dk is equal to zero; therefore, the
condenser energy balance in the first stage is
reduced to the following form
Dj cj ¼ MR Cpf ðTfj Tfjþ1 Þ
ð8Þ
In the heat rejection section, the condenser
tubes energy balance is given by
Dj cj þ Cpd ðTcj1 Tcj Þ
j1
X
transfer equation for the condenser tubes in
the heat recovery section is given by
Dk
¼ MR Cpf ðTfj Tfjþ1 Þ
where ðLMTDÞrj is given by
ðLMTDÞrj ¼ ðTfj Tfjþ1 Þ=ln½ðTcj Tfjþ1 Þ=ðTcj Tfj Þ
ð11Þ
In the heat rejection section, the heat
transfer equation for the condenser is given
by
ðMf þ Mcw ÞCpf ðTfj Tfjþ1 Þ ¼ Ucj Ac ðLMTDÞcj
ð12Þ
The flow of the brine recycle and its salinity are obtained by performing material balance on the mixer/splitter shown in Fig. 5.
This gives the following relations
MR ¼ Bn þ Mf Mb
XR ¼ ðBn Xbn þ Mf Xf Mb Xbn Þ=MR
Dk
k¼1
¼ ðMf þ Mcw ÞCpf ðTfj Tfjþ1 Þ
507
ð9Þ
In the last stage of the heat rejection section, the temperature Tfj is equal to Tbn, and
in the last stage of the heat rejection, the
temperature Tfjþ1 is equal to Tcw. The heat
The term Mb is obtained from performing
overall material and salt balance on the plant,
where
Mf ¼ Md þ Mb
Cooling Seawater
Removes Excess Heat
Feed Seawater
Heating
Steam
Feed and cooling
Seawater
MSF Plant
ð13Þ
ð14Þ
Distillate Product
Flashing
vapor
Energy Recovery
Brine Blow Down
Mixer/splitter
Brine Recycle
Fig. 5. Heat and mass transfer in MSF brine circulation plant.
ð15Þ
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N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
Xf Mf ¼ Xbn Mb
ð16Þ
The heat transfer equation for the brine
heater is given by
Ms ls ¼ Uh Ah ðLMTDÞh
ð17Þ
ntj ¼ 4ðMcw þ Mf Þ=ðVcw dj 2 Þ
The vapor temperature below the demister
is less than the brine boiling temperature by
the boiling point elevation and the non-equilibrium allowance. This relation is given by
Tvj ¼ Tbj BPEj NEAj
where
ðLMTDÞh ¼ ðTbo Tf1 Þ=ln½ðTs Tf1 Þ=ðTs Tbo Þ
ð18Þ
The energy balance equation for the brine
heater
Ms s ¼ MR Cph ðTbo Tf1 Þ
ð19Þ
The width of all stages is set equal the
width of the first stage, which has the highest
brine load. This relation is given by
Wst ¼ MR =wb
ð20Þ
Demister length is given
Lp ¼ ðDvv Þ=ðVv Wst Þ
ð21Þ
The vapor velocity through the demister
varies between 2 m/s in the first stage and 12 m/s
in the last stage. The stage length is given
Lst ¼ Lp þ Ltb
ð22Þ
The tube bundle length is obtained as a
function of number of tubes, tube diameter,
and tube spacing, where
Ltb ¼ nt 1=2 dSt
ð23Þ
The number of condenser tubes in the heat
recovery and the heat sections are obtained as
a function of the stream flow rate and velocity. These equations are given by
ntr ¼ 4MR =ðVR dr 2 Þ
ð24Þ
ð25Þ
ð26Þ
The vapor temperature above the demister
is given by
Tcj ¼ Tvj Tpj
ð27Þ
4. Degree of freedom analysis and solution
method
Each flashing stage contains five balance
Equas (1, 3, 5, 7–8) and five unknowns
ðDj ; Bj ; Xbj ; Tbj ; and Tfj Þ. Also, there are two
constraints that apply to the system; the first
is the equality of the system production capacity and the summation of the distillate produced in each flashing stage. The second
constraint is the equality of the outlet seawater temperature from the condenser tubes
in the first stage in the heat rejection section
and the brine blow down temperature. Therefore, the number of system unknowns is equal
to the heat transfer areas per stage in the heat
recovery and heat rejection stages as well as
the 5 unknowns in each flashing stage.
Solution of the system equations would
define the temperature, flow rate, and salinity
profiles across the flashing stages. The flow
rate and salinity of the outlet streams are then
used to calculate the brine recycle flow rate
and salinity. Also, the brine heater energy
balance and the heat transfer equation are
used to calculate the heating steam flow rate
and the brine heater heat transfer area. The
model Eqs (1–24) are highly non-linear. This
is because the equations are interlinked and
contains non-linear terms. Therefore, an
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
iterative procedure is needed to solve the
equation system. Newton’s method is used
to solve the model equations. The solution
scheme is shown in Fig. 6.
As is shown, the solution starts with definition of the system design parameters, which
includes capacity (Md), cooling seawater flow
rate (Mcw), number of flashing stages (n),
509
vapor velocity in the last stage (Vv), brine
velocity inside the tubes (Vb), heating steam
temperature (Ts), top brine temperature Tbo ,
intake seawater salinity (Xf), outer diameter
of condenser tube (do), wall thickness of the
condenser tubes (dw), thermal conductivity of
the condenser tubes (kw), discharge coefficient
of the brine weirs (Cd), brine loading per unit
Define System Parameters:
Md, Mcw, n, Vv, Vb, Ts, Tf, Tbo, Xf, do, dw, kw,
Cd, ε, wb, Lp, ρp, dp, Rfh, Rfc
Calculate the Flow Rates of the Brine Blow
Down and Feed (Eqs. 15-16) Mf, Mb
Calculate Initial Guess for:
MR, XR, Ac, Ar, Tfj, Tbj, Xbj, Dj, Bj
For Each Stage Calculate:
Tvj, Tcj, Hj, HGj, NEAj, BPEj, Cpf, Cpb, Cpd, μb, kv,
hij, hoj, Ucj vvj, ΔTpj, ΔPpj
Evaluate the Residuals of the Balance Equations in
Each Stage: Eqs. 1-12
Solve the Residual Equations Using Newton’s Method
Check Convergence:
|error difference in the calculated variables in two iterations| ≤ ε
Yes
Calculate the Heating Steam Flow Rate and the Brine
Heater Heat Transfer Area Ms, Ah, Uh
Print Solution
Fig. 6. Solution scheme for the MSF mathematical model.
No
510
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
length of the interstage weirs (wb), demister
thickness (Lp), demister bulk density (rp),
demister wire diameter (dp), condenser fouling resistance Rfc , brine heater fouling resistance Rfh , and error tolerance for the solution
method (e).
The flow rates of the feed and brine blow
down (Mf and Mb) are calculated by simultaneous solution of Eqs. (15–16). This is followed
by constructing an initial guess for the condenser
areas in heat recovery and heat rejection stages
(Ar and Ac), the temperature profiles of the
flashing brine (Tbj) and the feed seawater flowing inside the condenser tubes (Tfj), and the
profiles of the brine salinity (Xbj), brine flow
rate (Bj), and distillate product (Dj). Also, the
brine recycle flow rate and salinity (MR and
XR) are obtained by solution of Eqs. (13–14).
This guess is used to calculate various parameters in each stage, which include the physical properties (Cpf, Cpb, Cpd, b, kv) the heat
transfer coefficients (hbj, hvj), the overall heat
transfer coefficient (ucj), the vapor and condensation temperatures, and the thermodynamic
losses (BPE, NEA), and the drop in temperature and pressure drop across the demister
(Pp, Tp). The residuals of Eqs. (1–12) are
then evaluated and new profiles are established
for the system temperatures, flow rates, and
salinity. The solution error, which is the summation for the difference between the calculated variables in two successive iterations, is
checked against the error tolerance (e). If the
solution error is less than the tolerance the
iterations are terminated; if not then a new
iteration is performed. Newton’s method is
known to give quadratic convergence as the
exact solution of the residual equations is
approached. This criterion is found to hold
upon solution of the model equations for
wide range system parameters, which includes
number of stages, capacity, feed seawater temperature and salinity, top brine temperature,
and heating steam temperature.
5. Results and discussion
Results and analysis of the MSF system
includes calculations of the stage dimensions,
tube bundle length, the demister length, and
the flow rate and temperature profiles. Other
design characteristics include the system performance ratio, specific heat transfer area,
and specific flow rates of various streams.
The analysis is made for the conditions
shown in Table 1.
Variations in the specific heat transfer area
as a function of the system production capacity and top brine temperature are shown in
Fig. 7. As is shown, the specific heat transfer
area decreases with the increase in the top
brine temperature. This is because of the
increase in the flashing range and the temperature drop per stage, which increase the driving
force for heat transfer. Also, the specific heat
transfer area increases with the increase in the
production capacity. This is because of the
increase in the system thermal load.
Variations in the stage length as a function
of the top brine temperature and the production capacity are shown in Fig. 8. As is
shown, the top brine temperature has small
effect on the stage length. There is small
decrease in the stage length is caused by the
reduction in the dimension of the tube bundle
upon the increase in the top brine
Table 1
Design parameters
Variable
Value
24
43,200–77,760 m3/d
36,000 ppm
30 C
105–115 C
Heating steam temperature (Ts) (Tbo þ 10) C
70,000
Salinity of brine reject Xbn
Stage width
20 m
Performance ratio
9
Number of stages (n)
Distillate flow rate (Md)
Intake seawater salinity (Xcw)
Intake seawater temperature ( C)
Top brine temperature Tbo
Specific Heat Transfer Area (m2/(kg/s))
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
210
Product Flow Rate (m3/d)
205
900
700
500
200
195
190
185
180
175
104
106
108
110
112
Top Brine Temperature (°C)
114
116
Fig. 7. Variations in the specific heat transfer area as
a function of the top brine temperature and product
flow rate.
9
Product Flow Rate (m3/d)
900
700
500
Stage Length (m)
8
7
6
5
4
3
104
106
108
110
112
Top Brine Temperature (°C)
114
116
Fig. 8. Variations in the stage length as a function of
the top brine temperature and product flow rate.
temperature. On the other hand, the increase
in the production capacity requires larger
heat transfer area, which causes increase in
the tube bundle dimensions and the stage
length. A similar behavior is found for the
change in stage height, where it vary between
5.7 and 7 m upon the increase in the production capacity and decrease of the top brine
temperature.
Upon the increase in the top brine temperature from 105 to 110 C, the specific flow
511
rate for cooling water varies over a range of
7.4–8.5 and the specific flow rate of brine
recycle varies over a range of 7.42–8.54. On
the other hand, increasing the production
capacity has no effect on the specific flow
rate of cooling water or the specific flow
rate of brine recycle. This is because the
system performance ratio is kept constant at
a value of 9. This implies that the amount of
input heat per unit product is also kept constant, which implies use of a constant amount
of cooling seawater and brine recycle per unit
product.
6. Comparison against field data
Comparison of model predictions against
field data is made for a number of existing
MSF units in the Gulf States. The comparison results are shown in Table 2. The input
design data include the number of flashing
stages, the system capacity, the top brine temperature, and the stage width. The data were
obtained from web site of the manufacturing
company Italimpianti (http://www.italimpianti.it). The calculated variables include
the performance ratio, the stage length, the
stage height, the demister length, and the
specific flow rates of the cooling water and
brine recycle.
The results shown in the table are for
small, medium, and large production capacity
systems, where it varies from 15,000 to
75670 m3/d. The stage width for these systems varies over a range of 8–23.8 m, as the
system capacity increases. As is shown, the
MSF performance ratio varies between 8
and 9.5. This is except for the unit with a
capacity of 15,000 m3/d, which has a performance ratio of 6. Also, it should be noted
that specific flow rate of the cooling seawater
and brine recycle varies over a range of 8–9.
Similarly, the stage length and height varies
over a range of 3.5–7 m.
8.6
8.85
8
8
9.27
8.6
9.3
8.45
8.6
8
12.82
8.88
8.6
8.57
8
7.9
8
7.4
198
141
184
189
194
197
199
199
205
2.1
2.3
2
2
2.5
2
2.2
2.3
2
260
193
226
237
282
242
270
311
288
5.6
4.7
5.4
5.8
6.4
5.5
5.9
6.6
6.2
4.66
3.64
4.25
4.7
5.7
4.4
5
6
5.2
9
6
8.8
9
8
8.9
8.6
9
9.5
107–112
105–112
115
105
105
110
105
111
110
37000
15000
34080
45480
60530
34000
45400
75670
58000
14.2
8
14
17.8
23
14
18
23.8
20
9.53
174
2.3
293
6.2
5.58
8
19
112
57600
The results shown in Table 2 indicate that
stage dimensions of the MSF system are
strongly dependent on the stage width. The
stage width is set to maintain the weir loading
within a range of 200–300 kg/(m s). Lower
weir loading would imply high residence
time for the brine stream within the stage,
which would result in high brine levels. In
this case, the minimum distance between the
top of the brine pool and the demister will
not be achieved. As a result, brine entrainment in the distillate vapor will increase and
would result in higher product salinity.
Higher weir loading is also not desirable
because it may result in increased vibrations
within the system and subsequent damage to
the weir components.
Although, no actual data were available
for the stage length, stage height, demister
length, the specific heat transfer area, and
the specific flow rates of the brine recycle
and cooling seawater, but, the predicted
values are well within known field practice,
For example stage height and length are
known to vary to vary between 4 and 6 m.
Also, the specific flow rate of the brine
recycle is known to vary over a range of 9–
10. Similarly, the specific flow rate of the heat
transfer area is reported in several studies to
vary between 200 and 300 m2/(kg/s)
21
15
21
21
19
21
21
21
24
20
7. Conclusions
Al Taweelah ‘‘B’’ (Abu
Dhabi–UAE)
Al Hidd (Bahrain)
Ruwais (UAE)
Jebel Ali ‘‘G’’ (Dubai UAE)
Jebel Ali ‘‘K’’ (Dubai UAE)
Jebel Ali ‘‘K’’ 2 (Dubai UAE)
Mirfa (Abu Dhabi–UAE)
Ras Laffan (Qatar)
Shuweihat (Abu Dhabi–UAE)
Subyia (Kuwait)
Plant
8.36
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
Top brine Stage PR Stage Stage Weir
Demister Specific heat
Number Capacity temperature width
length height loading length
transfer area
of stages (m3/d)
sMCW sMR
( C)
(m)
(m)
(m)
(m)
(m)
[m2/(kg/s)]
Table 2
Comparison of model predictions against field data
512
This paper presented a mathematical
model and analysis for large capacity MSF
process. Analysis is presented for variations
in main design parameters, which includes
stage dimensions, performance ratio, specific
heat transfer area, and specific flow rates of
cooling seawater and brine recycle. The analysis is made as a function of system capacity,
top brine temperature, and number of stages.
The main outcome of this analysis is that the
system design is strongly dependent on
N. M. Abdel-Jabbar et al. / Desalination 204 (2007) 501–514
capacity and stage width. The width is set to
achieve weir loading of 200–300. Also, the
width is limited to a value of 20–25 m,
which is set by the available tube length for
the cross flow configuration. As a result, the
remaining stage dimensions vary over a narrow range of 3.5–7 m for the stage height and
length. Also, the demister length vary
between 1 and 2 m. Similarly, maintaining
the system performance ratio between 8 and
10 result in limited variations in the specific
flow rate of the cooling seawater and the
brine recycle.
8. Acknowledgement
The authors wish to acknowledge the
financial support of the Middle East Desalination Research Center (MEDRC), Oman,
project # 04-AS-001.
Nomenclature
Heat transfer area, m2
Brine flow from each flashing stage,
kg/s
BPE
Boiling point elevation, C
Cp
Specific heat at constant pressure,
kJ/kg K
D
Distillate formed in each flashing
stage, kg/s
L
Length, m
LMTD Logarithmic mean temperature
difference, C
M
Mass flow rate, kg/s
Number of tubes
nt
NEA
Non-equilibrium allowance, C
Tube spacing, m
St
T
Temperature, C
U
Overall heat transfer coefficient,
kW/(m2 C)
v
Specific volume, m3/kg
V
Velocity, m/s
Brine loading per unit width of the
wb
flashing stage, kg/(m s)
A
B
Wst
X
513
Stage width, m
Salinity, ppm
Greek Symbols
Latent heat, kJ/kg
Density, kg/m3
Subscripts
1
b
c
cw
d
f
h
j
n
p
r
R
s
st
t
tb
v
First stage
Brine
Condensation temperature
Cooling seawater
Distillate product
Feed stream
Brine heater
Heat rejection section
Last stage
Demister
Heat recovery section
Brine recycle
Heating steam
Stage
Tube
Tube bundle
Vapor
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