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Optimization of Large-Scale High Temperature MED-TVC
Systems: Numerical & Experimental Verification
SeungWon Ihm, Karim Bourouni, Charbel Siklawi, Muhammed Adnan Saroosh, Ahmed Nafey, JaeSeung
Choi and JongKyou Park
DOOSAN Water R&D Center, Monarch Office Tower, Level 29, Sheikh Zayed Road, Dubai, UAE
(E-mail: [email protected])
Abstract
Thermal desalination of seawater has been an answer for the shortage of drinkable water especially
in the Middle East and North Africa. Many developing countries have serious water and electricity
shortage, which renders water and power supply a matter of national security that requires cost
effective and reliable processes. Improving efficiency of thermal desalination technology will be a
good solution for a stable growth of these countries. MED process with TVC has recently drawn
much attention because of its reliability and lower energy consumption. During the development
of a new efficient large-scale MED-TVC process, Doosan verified the performance of the
developed process by theoretical study, pilot plant testing, two-phase computational fluid
dynamics study, etc. In this paper, efficiency enhancement by increasing TBT up to 85°C and the
recent design optimization of large-scale (more than 5 MIGD/unit) with high temperature (higher
than 70°C in TBT) MED-TVC system are discussed and presented with verifications.
Keywords
Seawater Desalination; Multi Effect Distillation (MED); Thermo Vapor Compressor (TVC); high
temperature
1. INTRODUCTION
With global warming and climate change issues, water shortage is becoming a more serious threat
to many countries. Thermal desalination technologies, usually combined with power plant, provide
high quality drinkable water. Nowadays MED (Multi Effect Distillation) with TVC (Thermo Vapor
Compressor) is getting attraction due to its lower energy consumption. Though its high efficiency in
heat transfer from falling film evaporation, one of the obstacles for MED in the giant-size market
has been its limited unit capacity for last several decades. However, recently the unit capacity of
MED has become comparable to that of MSF (Multi Stage Flash). For example, presently the
largest unit capacity of MED is 15 MIGD (Million Imperial Gallon per Day, 1MIGD=4,546m3/day
water production) which is under construction by Doosan Heavy Industries and Construction Co.
Ltd. (Doosan here after) with SWCC (Saline Water Conversion Corporation) in Yanbu, Saudi
Arabia, comparable capacity to the largest MSF unit of 20 MIGD for the world largest Ras AlKhair Phase 1 project, being installed in the same country also by Doosan.
Another limit in MED is its low TBT (Top Brine Temperature), usually less than 70°C due to the
risk of scale deposition, while the TBT of MSF is nowadays higher than 110°C. An increase in TBT
allows to use a wider temperature range in MED system, thus a design with higher number of
Effects (or Stages) which consequently improves GOR (Gain Output Ratio, GOR = product water
[kg] / steam consumption [kg]). Also, a larger temperature difference between Effects helps to
design more economic and compact evaporator, as the required heat transfer area is less for the
same heat transfer rate. Therefore, several investigations were carried on the increase of TBT in
MED-TVC by using different techniques. Ophir and Gendel (2009) suggested an extended thermal
operation range by increasing TBT from 70 to 85°C. Hamed et al. (2009) presented a tri-hybrid
desalination system integrating NF (Nano-Filtration) with both RO (Reverse Osmosis) and MEDTVC to increase TBT up to 125°C.
In order to investigate the practical possibility of increasing the operating temperature, Doosan has
conducted high TBT MED pilot tests and design optimization efforts, through its MED technology
development plan. In the following sections, the main experimental results from a small and precise
pilot and from a huge, more practical size pilot are discussed, followed by design optimization
study based on them.
2. ADVANCED HT-MED PILOT EXPERIMENT
Doosan Advanced HT(High Temperature)-MED pilot is a containerized experimental unit with
varying capacity from 70 to 150 kg/hr of distillate production. It consists of one falling film
horizontal tube evaporator (single Effect), condenser, steam boiler, vacuum pump, pre-heater,
chemical dosing system, control system, and auxiliaries as illustrated in Figure 2-1. This pilot was
built in 2008 and experiments for heat transfer and scale formation at higher TBT have been
conducted since this date.
Because most of commercial MED or MED-TVC plants have been designed with TBT lower than
70°C, this pilot aims to explore design parameters higher than 70°C, and reaching up to 120°C.
Until 2010 experiments had been conducted to verify and derive a proprietary Doosan heat transfer
correlation and scale index at higher temperature. Different TBT, anti-scalant dosage rate, feed rate
(liquid loading for tube wettability), and test duration ranging from 24 hours up to 18 days were
employed as test variables. The feed brine salinity is 64,000ppm or higher with brine recirculation
mode which simulates the salinity at the bottom of tube bundle, considering tough seawater
condition in the Middle East region.
Figure 2-1. Operation window of Advanced HT-MED pilot
The main result obtained from this investigation is related to the influence of the TBT on the overall
heat transfer coefficient, U. Figure 2-2 shows the increase in U with increased Effect temperature. A
comparison between experimental results and Doosan heat transfer correlation shows a good
agreement, thus confirming Doosan’s higher TBT MED design will be reliable in terms of adequate
heat transfer area. (Refer Rezazadeh et al. (2010) for more details on the experiment, and Bourouni
et al. (2011) for Doosan scale model development.)
Figure 2-2. Overall heat transfer coefficient trend on temperature
After analyzing experimental results from Advanced HT-MED pilot, high TBT MED-TVC
demonstration in a larger pilot was planned for further verification with commercial size tube
bundle and TVC.
3. 0.1MIGD HT MED-TVC PILOT EXPERIMENT
For large capacity MED study, Doosan installed a huge 0.1MIGD MED-TVC pilot plant in
Changwon, Korea, in 2004 (Figure 3-1). It has five Effects, TVC, condenser, three pre-heaters, twostage ejector condensers, and plate-type heat exchangers for winter use. A fire-and-flue-tube boiler
produces 3 ton/hr steam at 10 bara (bar absolute) for this pilot, and real time data are being
monitored and logged in a control room. It produces 450 ton/hr distillate at 63°C of TBT with GOR
8. A lot of studies have been conducted with this pilot to understand practical performance and
design variables of MED evaporator and TVC. Though the pilot capacity is small compared to a
commercial evaporator, the bundle height is designed to be about 2m to have close-to-real number
of tube rows which is one critical design point in MED to study tube bundle wettability with
different feed system, resultant scale formation risk, thermal losses, etc.
Figure 3-1. Doosan 0.1MIGD MED-TVC pilot system in Changwon
In 2010, the pilot was redesigned and refurbished to study practical heat transfer coefficients, TVC
performance, and optimal operating conditions at higher TBT. The new TBT is equal to 84.5°C
while BBT is controlled at 67°C. Note that the BBT is much higher than conventional design, as the
interested temperature range to be explored is above 70°C. With the controlled partial brine
recirculation, the feed water salinity becomes 45,000 to 53,000ppm when Korean seawater
condition in Masan bay has a seasonal variation of 27,000 to 32,000ppm. Three different TVCs
with 5 to 6 bara motive steam have been tested. Figure 3-2 shows the schematic diagram of the
refurbished HT MED-TVC pilot.
Figure 3-2. Revised HT MED-TVC pilot schematic diagram with brine recirculation
(Ej-Cond: ejector condensers, PH: pre-heater, BBDP: brine blow-down pump,
DP: distillate pump, TP: temperature pump)
The following items have been studied experimentally during last two years:
- TVC performance test at 100% and partial load conditions
- Feed flow rate optimization test to study required liquid loading rate for each Effect
- Anti-scalant dosage rate optimization test
- Long term operational demonstration
Again, the practical heat transfer coefficients at higher temperature region have been validated as
well.
With the experimental validations, main design parameters such as heat transfer coefficients, TVC
E/R (Entrainment Ratio = suction steam flow rate / motive steam flow rate), liquid loading
conditions are used for the design optimization study in the following chapter.
4. DESIGN OPTIMIZATION STUDY FOR LARGE-SCALE HT MED-TVC
Compared to the conventional MED-TVC systems with TBT 66°C, new designs with TBT 85°C are
numerically studied with two different motive steam pressures, 4.5 and 15 bara, at saturated
condition. Typical Middle East seawater conditions are considered, which are 45,000ppm salinity
and 35°C temperature in summer. In design stage, 5MIGD capacity is considered for heat and mass
balance calculation with single TVC, from which 15 or 20MIGD large-scale MED-TVC can be
designed in a parallel manner with multiple TVC and tube bundle.
Varying GOR, eight configurations are studied for TBT 66°C cases, summarized in Table 4.1, and
six configurations are studied for TBT 85°C cases, summarized in Table 4.2, respectively.
With increased TBT of 85°C from 66°C, the useable temperature range becomes almost double,
which allows higher GOR with reasonable heat transfer area requirement. Also, higher GOR means
less steam requirement for the same distillate production capacity, resulting in required seawater
intake size reduction due to lowered heat reject from the system.
Table 4-1. Case study for design optimization with TBT 66°C
Motive steam
Case study
GOR
PR
Pressure (bara)
#1
4.5
8
7.61
# of Effect
5
#2
4.5
10
9.50
7
#3
4.5
12
11.37
8
#4
4.5
14
13.25
9
#5
15
8
7.28
5
#6
15
10
9.32
6
#7
15
12
11.18
8
#8
15
14
13.02
9
(PR: Performance Ratio = product water [kg] x 2,326 [kJ/kg] / energy input by steam [kJ])
Table 4-2. Case study for design optimization with TBT 85°C
Motive steam
Case study
GOR
PR
Pressure (bara)
#9
4.5
12
11.76
# of Effect
10
#10
4.5
14
13.71
11
#11
4.5
16
15.66
13
#12
15
12
11.55
9
#13
15
14
13.46
11
#14
15
16
15.37
12
Comparisons are made for major requirements of each design cases, normalized to the value of case
#7 as a reference case.
Figure 4-1. Motive steam flow rate requirement
Figure 4-2. Seawater flow rate requirement
Because the cases are based on GOR rather than PR which considers motive steam quality as well,
the motive steam requirement is exactly inversely proportional to each design GOR value. However,
the seawater flow rate is determined to reject the heat supplied by motive steam in the evaporator
system in which steam quality also needs to be counted. Therefore the required seawater flow rate is
a little higher when higher quality (higher pressure and temperature) motive steam is considered.
Figure 4-3. Heat transfer area requirement for evaporator tubes
Figure 4-4. Heat transfer area requirement for pre-heaters and condenser
When the evaporator cost is estimated, the first and most important step is to calculate the required
heat transfer area. Once the heat transfer area is obtained, the required cost for other components,
e.g. shell, tube support, pipe, etc. could be estimated. Figure 4-4 clearly shows how efficiently the
evaporator can be designed when TBT is increased by about 20°C. Higher pressure motive steam
will be more attractive at higher TBT and/or higher GOR design, because better TVC performance
in terms of E/R is preferred to get higher GOR (even for higher compression of suction steam.) The
required heat transfer area for pre-heaters and condenser is normalized to the same reference value
of evaporator tubes. Usually more corrosion-resistant (thus expensive) material is used for PH and
condenser, but the required heat transfer area for these parts is found to be similar in spite of TBT
and/or GOR increase.
5. RESULTS AND DISCUSSION
The effects of increasing TBT on MED heat transfer coefficient, scale formation, and TVC
performance have been studied experimentally using two high temperature MED pilots. The overall
heat transfer coefficient gradually increases with temperature, and several key design parameters are
studied and verified. More practical studies conducted with 0.1MIGD HT MED-TVC pilot confirm
the viability of Doosan HT MED-TVC configuration.
With confidence from experimental studies, numerical design optimization study is conducted to
see the benefits of higher TBT MED-TVC design from 66°C to 85°C. Compared to the reference
case #7, if we consider case #14 as a higher TBT MED-TVC configuration, we can save steam and
seawater demand by about 25% while maintaining the same required heat transfer area. If we
consider the case #13, we can still save steam and seawater demand by about 14% with about 17%
less total heat transfer area.
Usually GOR for thermal desalination has ranged from 8 to 10 for oil-producing countries in the
Middle East, though higher GOR has been preferred by other countries to save steam and power
requirement but with additional cost for heat transfer area. As energy price is getting higher and
higher nowadays, higher GOR design is becoming more attractive for oil-producing Middle Eastern
countries as well. As analyzed in this paper, higher TBT MED-TVC could be an answer for saving
energy while keeping or even saving material cost, though increasing TBT in MED shall be
achieved after conducting enough studies, including close-to-real size test, because once scale is
deposited on the outer surface of the tubes, there is no physical way to remove but to shut down for
acid-cleaning.
REFERENCES
Bourouni, K., Ihm, S.-W., Rezazadeh, F., Nafey, A., Park, J.-K., Nied, S., Schuermann, G. 2011
Analysis of Scale in High-Temperature MED Distillers, IDA World Congress, Perth, Australia, Sep.
4-9.
Hamed, O.A., Al-Ghannam, M. N., Al-Shail, K. A., Farooque, A. M., Al-Rasheed, R., Al-Fozan, S.,
Al-Arifi, A. R., Hirai, M., Taniguchi, Y., Araki, S., Harada, K., Maekawa, K. 2009 Successful
Operation of MED/TVC Desalination Process at TBT of 125’C without Scaling, IDA World
Congress, Dubai, UAE, Nov. 7-12.
Ophir, A., Gendel, A. 2009 Latest Developments in MED and MVC Thermal Desalination
Processes, IDA World Congress, Dubai, UAE, Nov. 7-12.
Rezazadeh, F., Ihm, S.-W., Bourouni, K., Choi, H.-S., Yousuf, M., Park, J.-K. 2010 Advanced
MED-TVC Design, 2010 Asia-Pacific Conference on Desalination and Water Reclamation,
Qingdao, China, June 22-25.