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pplied to Qatar, the much clichéd ‘country of extremes’ is no exaggeration. A small outcrop on the Arabian Peninsula, the country's interior is a sand covered barren plain on which summer temperatures

can approach 50˚C (122˚F). In its coastal capital city, Doha, futuristic super towers soar above the Arabian Gulf shores from which its legendary pearl divers once embarked in search of undersea riches. Qatar is also home to one third of the world's natural gas reserves. In just a few decades, the country's energy resources have transformed it to one of the richest; Qatar's citizens today have the planet's highest per capita income.

Qatar's growing population and sparse rainfall, exacerbated by the world's changing climate patterns, have also made it one of the countries most likely to face ‘exceptional water related challenges for the foreseeable future,’ according to the

World Resources Institute, a non-profit research organisation. Natural freshwater resources are scarce and much of the water used in the country's homes and businesses comes from the sea and must pass through energy intensive desalination plants. In response to looming water challenges, the Qatari government has undertaken measures to reduce the country's water consumption, which, at 460 l/d per person, is one of the world's highest.

Oasis of liquid energyThis focus on conserving water provides the backdrop for one of the world's largest, most complex and challenging energy projects, the Pearl gas to liquids (GTL) plant at Ras Laffan Industrial City, 85 km north of Doha. An integrated offshore and onshore development, the plant relies on the Fischer-Tropsch process in which methane is converted into liquid products

Jeroen Boom and Joe Rehbein, Veolia Water Technologies, explain how a closed circuit zero liquid discharge system is used to recover, treat and reuse industrial process water at the Pearl GTL facility in Qatar.

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through an exothermic reaction. The process generates considerable amounts of water but also heat, which, in turn, creates a requirement for large amounts of cooling water.

A joint project of Qatar Petroleum and Shell, the Pearl GTL facility today converts up to 1.6 billion ft3/d of wellhead gas into liquid fuels. Products include cleaner burning diesel and aviation fuel, kerosene, base oils for advanced lubricants, the plastics feedstock naphtha, and paraffin, which is used to produce detergents. Pearl GTL has the capacity to produce 140 000 boe/d of liquid products and 120 000 boe/d of condensates, liquefied petroleum gas and ethane for industrial uses. At the peak of construction, 52 000 workers from 65 countries were deployed

on the site, and overall it took the team 500 million man hours to design and build.

Now in its fourth year of operation, Pearl is still the world's largest GTL plant and has helped make Qatar the GTL capital of the world. It can make enough diesel to fill over 160 000 cars a day and enough synthetic oil each year to make lubricants for more than 225 million cars.

Conserving every drop of waterAs it prepared to build its state of the art GTL plant, Shell needed to meet the government's stringent water conservation requirements. Of the many challenges faced by Middle Eastern countries in order to achieve sustainable development, the most fundamental is water management. No matter how abundant the region's energy resources, without proper water management, countries face the problem of being either unable to exploit the resource or destroying their environment in doing so. The industrial capacity to manage water efficiently was paramount for the Pearl GTL project.

An integrated water management design that would enable the reuse of treated process water to feed boilers and cooling systems, eliminate fresh water intake and result in zero liquid discharge (ZLD) was needed. The water treatment unit's reliability was critical. An additional process complication was making the plant water neutral. This required creating an efficient plant from scratch in a land with no available fresh water. Veolia, a global specialist in water and wastewater treatment, was hired to build a sustainable water treatment facility that could recycle the effluent generated from the GTL process.

For the Pearl facility, the objective was to treat the considerable volume of water generated by the complex GTL process of converting gas into liquid hydrocarbons, and ancillary processes, and to make it available for reuse in the various processes of the complex.

The Veolia designed system uses microorganisms to remove chemical impurities from the water, which then undergoes a process to remove salts. Most of this treated water is then used to absorb the heat generated in the GTL process. Part of the water is converted into steam to drive the turbines generating the plant's electrical power.

Unique process, zero dischargeThe closed circuit zero liquid discharge system was chosen, despite its significant complexity. More than 10 separate treatment steps are necessary, along with intermediate storage tanks with an overall capacity of more than 40 000 m3/d. Numerous parameters must be monitored to meet such industrial standards, including hydrogen potential, nitrate concentration and conductivity.

Within the plant, water management is no simpler in the face of three problem areas: scaling, the deposits on structures which damage or raise energy consumption; corrosion, causing damage and premature aging of facilities; and disposal.

Shell has a ZLD facility, not only for its effluent water from its GTL processes, but also to treat all other water streams inside the fence. In total, 12 different water streams were to be treated entering the effluent treatment plant (ETP) at eight different locations, requiring five different recycled water qualities. Each effluent stream presents its own unique challenge when designing an entire water treatment system to

Figure 1. Controlled discharge facility (CDF) for collection of various liquid waste streams.

Figure 3. View of the biotreater basins and clarifiers for removal of dissolved organics and settling of biomass.

Figure 2. Top view of the flotation basin on the left and the coagulation-flocculation chambers on the right.

Reprinted from Janaury 2016HYDROCARBON ENGINEERING

efficiently and effectively minimise waste or eliminate the discharge of wastewater.

The solution The solution, which is based on a long experience in treating and managing diverse streams, includes specification and complete system design utilising complementary water treatment expertise in reverse osmosis, softening, and clarification.

The effluent treatment plant, completed by Veolia, uses technologies including ultra-filtration and reverse osmosis to treat wastewater to enable their complete reuse within the factory process. Reverse osmosis brine treatment is carried out by evaporation and crystallisation, a technology achieving zero liquid discharge where only salt crystals are produced.

The design build of the Pearl GTL complex effluent treatment plant, one of the world's largest ZLD facilities, was conducted by a consortium formed from a 50/50 joint venture between Veolia and Saipem, and a local construction company, Al Jabar.

In engineering, procuring and delivering the ZLD system, Veolia provided the following technologies and solutions for volume reduction and zero liquid discharge, based on its proven HPD® Brine Evaporator and Brine Crystallizer technologies:

n Major and ancillary process equipment. n Sweet and sour CPI separators. n Wax removal pits. n Oil and water tankage. n A controlled discharge facility. n Heat exchangers. n Flocculation and flotation units. n Aerobic biological treatment (conventional) with circular

settlers (conventional). n Submerged ultra-filtration. n 2-pass reverse osmosis with dedicated extra brine staging to

minimise feed flow to the evaporator. n Irrigation water treatment by UV. n Evaporation and crystallisation of concentrated waste

streams (ZLD system). n Cooling water blowdown treatment (submerged

ultra-filtration and reverse osmosis). n Mineral sludge dewatering with centrifuge. n Activated sludge dewatering with centrifuge.

Veolia offers several options in designing the optimal system for achieving zero discharge or waste reduction objectives, water treatment efficiency and energy consumption. HPD evaporation system capacities can range from around 12 m3/hr (50 gpm) to greater than 270 m3/hr (1200 gpm) per unit.

Brine concentrators based on falling film technology used as wastewater evaporators are an excellent technology for energy efficiency with water recovery of greater than 90% as high purity distillate. Recovered water in a brine concentrator can be recycled to a variety of plant processes, such as cooling tower or boiler makeup. These systems can be driven by mechanical vapour recompression (MVR) or with live steam, if allowed by plant economies.

Further concentration of effluents is achieved using brine crystalliser systems. When used in conjunction with brine concentrators, they comprise a true zero liquid discharge system.

Step by step toward zeroDuring pretreatment, water from a variety of sources is processed in different ways. Waxes and oils are removed and the water is cooled. It is then joined by process water from the GTL reactors. In the main treatment area, the flow is split into two identical lines. Each starts with a buffer tank, which stores water in order to control flow and variations in composition into the rest of the line.

During the next stage, called flocculation-flotation, air is passed through the water to remove suspended solids and dispersed organics. The water then enters a biotreater where biological treatment is used to remove most of the dissolved organic compounds. Two clarifiers perform a final decantation treatment before the water undergoes ultra-filtration. Ultra-filtration uses submerged membranes to remove any solids still suspended in the water. Dissolved solids are removed through reverse osmosis membranes and clean water is sent back to the GTL complex for use in boilers, cooling towers and other applications.

Water from cooling towers and boilers is treated separately in a specific ultra-filtration and reverse osmosis unit before entering the final stage that makes this plant unique. Brine, a byproduct of the reverse osmosis, is then processed through evaporation and crystallisation so that the water is separated from the salt and returned to the complex for reuse. Every ion present or added in the process is converted into salt, while more than 99% of the water that flows into the treatment plant is recycled and reused, achieving zero liquid discharge for the whole site.

ConclusionWhile additional application of ZLD technology at energy production facilities has been slowed in part by the changing economics of shifting global energy markets, the successful demonstration at Pearl GTL may take on increasing importance as impending water shortages around the globe drive local authorities to impose new conservation measures. The capability may prove particularly valuable in countries in the Middle East and elsewhere in the world wherein the ability to develop economically is closely linked to effective water management strategies.

The Pearl GTL facility demonstrates the feasibility of implementing ZLD technology on an extremely large scale. Indeed, it is the world's largest industrial water processing plant for recovery, treatment and reuse of industrial process water. Each day, the water treatment plant processes an average of 40 000 m3, or more than half a tonne of water per second, without discharging a drop of liquid to the environment. Now that is an impressive scale of processes.

Figure 4. Three evaporator-crystalliser trains are applied to recover the last drops of water.