Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)12

World's First Large Full Triple-Pass RO Seawater
Desalination Plant

KENJI TANAKAKATSUNORI MATSUITAKAYOSHI HORI

HIDEO IWAHASHIKAZUHISA TAKEUCHIYOSHIAKI ITO|1. Introduction
The independent water, steam, and power (IWSP) project that was completed on January 1,
2009 in the Rabigh District of Saudi Arabia and has commenced commercial operations is intended
to supply water, steam, and electricity to a large petrochemical complex (Petro Rabigh) co-invested
equally by Sumitomo Chemical Company and Saudi Arabian Oil Company (Saudi ARAMCO), thenational petroleum company of Saudi Arabia. An order was placed Mitsubishi Heavy Industries,
Ltd. (MHI) on a full turnkey basis from supplying the equipment up to execution of site
work/commissioning, including civil work. In recent years, customers require high pure water andhigh efficiency for desalination facilities. In response to this trend, remarkable developments havedone actively in new processes and new membranes for the reverse osmosis (RO) method ofseawater desalination, by the engineering, procurement, and construction (EPC) contractors andRO membrane manufacturers. Therefore, we would like to introduce our own technology, called afull triple-pass RO process, which we developed to meet these requirements.

|2. Development and significance of full triple-pass RO process
The Rabigh IWSP Project is one of the largest seawater RO desalination plants in the world,with a water production capacity of 168,000 m3/day during normal operations and 192,000 m3/day
at maximum capacity. As the product water is used mainly for cooling water make up, high purewater, the same water quality as that produced by the evaporation method (TDS < 10 mg/l, Cl– < 5mg/l) was required. Even though it was really difficult for the conventional RO process to produce
such high pure water, MHI enabled to apply the RO method to this project by using a full
triple-pass RO method which was uniquely developed by ourselves.
In addition, the total number of trains could be reduced by scaling up the RO train capacity,which eliminated restrictions on the application of RO to large scale seawater desalination plants.
The RO method has been deemed to be superior to the evaporation method in terms of the delivery
period and its competitive price, as less high-alloy metals, such as nickel, chromium, and copper
are used for RO method which are less affected by steep increases in metal prices. The
development of this large-scale full triple-pass RO process means that it will assume a dominantposition in the high pure desalinated water market, which has been monopolized by the evaporation
method. The RO technology will be expected to have a big impact that will change the power
balance between the multistage flush evaporation method, the multi-effect desalination method andthe RO method in the seawater desalination market.

|3. Features of the full triple-pass RO
The features of the full triple-pass RO process (patent applications are pending in Japan andoverseas) are described below:(1) The triple pass RO process combines high-pressure RO membranes for seawater service with
low-pressure RO membranes for brackish water. Different types of high-pressure RO
membrane for seawater can be used, regardless of whether they are the hollow fiber type or
spiral wound type.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)13(2) Smooth start-up/shut-down is enabled by eliminating the intermediate tank at the outlet ofeach RO pass. Consequently, it is necessary to install a three-way valve or exclusive-use valve
to dump permeate outside the system, and then to avoid bio-fouling in the intermediate tank.
(3) By installing a three-way valve or exclusive-use valve to dump brine, brine from each RO
pass can also be recovered to the upstream RO pass.
(4) MHI has developed a unique operation system that uses its own hardware and software
products for a distributed control system (DCS), enabling automatic start-up/shut-down of acomplex RO system.
(5) When required, a bypass operation can also be selected considering the dispersion of RO
membrane performance by adding a bypass line of permeate to each RO pass.
These features enable the long-term commercial operation of the system with almost noburden on the operators. MHI and the RO membrane manufacturers guarantee RO performance for
8 years, allowing long-term operation and maintenance service if customers request it, and theseservices support the diffusion of this full triple-pass RO seawater desalination plant.

|4. Example of the first large-scale commercial operationin the world
As stated in the introduction, we completed the first full triple-pass RO seawater desalination
plant in the world which was constructed in Rabigh on schedule, and obtained excellent results
during commissioning and demonstrated the excellence and reliability of this unique MHI
technology. The customers and end users highly appreciate this reliable system, and we anticipate
that it will be adopted for similar new projects.
The actual commercial operation in Rabigh can be described as follows. Seawater is filteredthrough a dual-media filter and fed to the RO unit, which consists of three passes in series. This
plant uses high-pressure membranes manufactured by Toyobo Co. Ltd., a company that has a lot ofstable operation experience during long years around the Red Sea area for the first pass, andlow-pressure RO membranes manufactured by Nitto Denko Co., which possess high performancefor salt rejection for the second and third passes.
Figures 1 and 2 show the RO units for each pass installed at the site. The water flow rate foreach RO unit is 504 m3/h and 16 units are available (14 duty plus 2 standby). The recovery ratio(Rc) is 43% for the first pass RO and 90% for the second and third pass ROs. The brine seawaterfrom the first pass RO is discharged outside the system, while the brine from the second and thirdpass ROs is circulated obtaining an overall Rc of 40%.

Figure 1 The first-pass RO modules

These employ cellulose triacetate hollow fiber-type
RO membranes with a good reference for long,
stable operation in the Red Sea area.

Figure 2 Low-pressure RO modules
for the second and third passes
These employ polyamide spiral wound-type RO
membranes with high salt rejection performance.

Figure 3 shows an example of the results of a performance test. The electrical conductivityof the permeate in the first, second, and third pass ROs are 280, 26, and 11 μS/cm, respectively; theequivalent value of total dissolved solids (TDS) are 140, 13, and 5.5 mg/l respectively.
Consequently, excellent high pure water was achieved even though the plant is still in the initial
operating stage. Although seawater with a higher salinity than the original design salinity wassupplied continuously, a flow rate of 508 m3/h was actually achieved, which exceeds theguaranteed water flow rate of 504 m3/h with a sufficient margin.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)
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Figure 3 An example of the water quality and water flow rate during a performance test

Despite being conducted during the initial stage of operations, we achieved excellent water quality, and the flowrate of 508 m3/h exceeded the guaranteed water flow rate of 504 m3/h.

Figure 4 shows the product water quality of 16 RO units during commissioning.
The water quality from each RO unit was excellent, achieving the guaranteed value foreach unit. These results confirmed that our full triple pass RO plant will ensure thesupply of utilities to the petrochemical complex with high availability.

Figure 4 The water quality of 16 units during commissioning
The water quality of each unit satisfied the guaranteed value. The dispersion of the values arose because theperformance of each RO unit was under adjustment.

|5. Conclusion
In recent years, customers have required high pure water, and larger size, and higher
efficiency from RO plants. Consequently, MHI developed our own full triple-pass RO technology.
When this plant started operation, it was the second largest RO plant of many large RO seawater
desalination plants. We are proud that this has prompted the adoption of the RO method for thelarge high-purity desalination market, which had been monopolized by the evaporation method.
To meet the diverse future needs of customers, we will continue to develop seawater desalination
systems using the RO method, which is environmentally friendly and easy for operation andmaintenance.

Authors
Kenji TanakaManager,
Nagasaki Project Department,
Nagasaki Shipyard & Machinery
Works

Katsunori Matsui
Nagasaki Plant Engineering
Department,
Nagasaki Shipyard & Machinery
Works

Takayoshi Hori
Nagasaki Plant Engineering
Department,
Nagasaki Shipyard & Machinery
Works

Hideo Iwahashi
Manager,
Nagasaki Plant Engineering
Department,
Nagasaki Shipyard & Machinery
Works

Kazuhisa Takeuchi
Manager,
Chemical Laboratory,
Nagasaki Research &Development Center, TechnicalHeadquarters

Yoshiaki Ito
Chemical Laboratory,
Nagasaki Research & Development
Center, Technical Headquarters