Abu Dhabi plant on track for major expansion

A combined-cycle boiler has been installed as part of the OGD project.

The Gas Processing & Pipelines Division/Processing Directorate (GP&PD) of the Abu Dhabi National Oil Co. (Adnoc) has undertaken the expansion as part of the Onshore Gas Development project. The expansion was covered in the October 1996 edition of OPEC Bulletin in an article prepared by Adnoc.

Fuel for power

Gas is the main fuel used in the Emirate of Abu Dhabi to generate most of its water and electricity; consumption almost doubled between 1985 and 1995, said Adnoc.

The Abu Dhabi water and electricity department has always been a major customer for gas, accounting for more than 80% of the total gas consumption.

Gas is also used as an industrial fuel in the two refineries at Ruwais and Umm Al Nar, as well as in the cement factory at Al Ain. And methane is the feedstock for production of fertilizer at the Ruwais plant, operated by Fertil.

The GP&PD of Adnoc has sole responsibility for processing and supplying gas to consumers. Currently, the department supplies about 700-900 MMscfd of gas to industrial consumers and about the same quantity of high-pressure gas to wells for injection into various reservoirs.

The first modern gas-processing plant at Habshan was built in 1983 to process only Thammama C gas and is therefore still referred to as the "Thammama C Plant," said Adnoc.

This plant is actually located, however, on the Bab oil and gas field and also exploits gas from the Thammama B and F reservoirs. Thammama B is primarily an oil reservoir; Thammama C and F, on the other hand, are basically gas reservoirs.

The plant was initially built with a processing capacity of 450 MMscfd of feed gas and later debottlenecked to 540 MMscfd. It has two identical trains, each with gas separation, condensate stabilization, sweetening, dehydration, dewpointing, and sulfur recovery. And there is a common unit for NGL-recovery treatment.

The old plant also has its own utilities and offsite units, such as steam boilers, power generators, water treatment, nitrogen, and instrument and plant air systems. In addition to the process and utility units, there are two gas turbine-driven compressors, each of 140 MMscfd for reinjection of excess gas to the oil reservoir (Fig. 1 [31588 bytes]).

This plant uses diethanol amine (DEA) for sweetening, triethylene glycol (TEG) for dehydration, turboexpander-recompressor for dewpointing, and a three-bed Claus process for sulfur recovery. The treated gas ("sales gas") produced from this plant contains less than 4 ppm of H2S. The other products are condensate, NGL, and sulfur.

Besides the gas plant, GP&PD operates an extensive pipeline network of more than 1,000 km which connects all the major plants and other industrial sites in the Emirate, said Adnoc.

The lean gas available by processing associated gas at Abu Dhabi Gas Industries Ltd.'s (Gasco) NGL plants at Bu Hasa, Asab, and Bab is also compressed at the respective lean-gas compressor stations then fed into the GP&PD network.

Expansion

Gas consumption in Abu Dhabi Emirate has nearly doubled during the last decade, the annual average in 1995 being about 565 MMscfd. This growth will continue in the next decade, said Adnoc, making it necessary to augment gas-production capacity of the existing gas plant at Habshan.

This led in 1991 to the launching of a major expansion, the Onshore Gas Development (OGD) project. Phase 1 of the project has just been completed and most of the major units are operating.

This project was initiated to achieve four major objectives:

1. Meet increased consumer demand for gas

2. Increase revenue by production of condensate, NGL, and sulfur

3. Process excess associated gas produced at the Bab oil separation plant

4. Maintain oil reservoir pressure by gas injection.

Advanced features

The OGD Project consists of the installation of new processing facilities fully integrated with the existing Thammama C plant. It is one of the most modern, highly efficient gas processing plants in the world, said Adnoc, embodying advanced instrumentation, automation, and controls, and safety systems.

The project aims mainly at processing feed gas from Thammama C and F gas reservoirs, besides processing excess Thammama B associated gas available from neighboring oil production facilities at Bab (about 5 km from the gas plant). Bab is operated by the Abu Dhabi Co. for Onshore Operations.

Thammama F reservoir is richer in condensate content (150 bbl/1 MMscf) and is also sweeter (0.5% H₂S) compared to the Thammama C reservoir, which typically contains only about 35 bbl/MMscf of condensate and more than 3% H₂S. Thammama B associated gas also contains about 3-4% H₂S.

Begun in 1991, the OGD project has since gone through various engineering stages including front-end engineering design and detail design, followed by construction and commissioning.

As of the end of 1996, with the plant mostly complete and operating, created gas-production capacity had more than tripled and output of other products was also up many times (Table 1 [10526 bytes]).

OGD plant

Adnoc said the new plant consists of three independent trains:

• Trains 1 and 2, each with 350 MMscfd feed capacity, process a mixture of Thammama B and C gas.

• Train 3, on the other hand, has 600 MMscfd capacity and processes exclusively Thammama F gas.

In Trains 1 and 2, the amount of B gas may vary from 40 to 60%. Each of the three trains consists of a gas-sweetening unit using MDEA, followed by molecular-sieve drying, deep NGL recovery, and sulfur-recovery units.

Treated Thammama F gas (residue gas) from Train 3 is reinjected to F reservoir using two new gas-turbine-driven injection compressors each with 400 MMscfd capacity discharging at 400-bar pressure.

Thammama B/C residue gas from Trains 1 and 2 is sent directly to the pipeline network under its own pressure of about 42 bar for supply to consumers. A portion of B/C residue gas from Train 1 and 2 provides makeup for the shrinkage in Thammama F reservoir fluid and is reinjected to the reservoir along with treated F gas from Train 3.

The additional units installed under each section and a schematic flow diagram of the OGD units appear in Table 2 [9456 bytes] and Fig. 2 [44601 bytes], respectively.

Condensate treatment

The additional raw condensate produced from the Thammama C and F gas separators is treated separately in two independent stabilization units, said Adnoc.

The stabilized condensate produced from the existing two units, together with the condensate from the two new OGD units, is stored in two floating-roof storage tanks of 195,000 bbl each, where it is mixed with the pentane-plus streams from the NGL-recovery units.

The pressure of the mixed condensate thus produced is maintained at about 8 psi Rvp. The total condensate produced from all streams, including existing units, is about 130,000 b/d, with the condensate produced from F stream alone accounting for 70-80% of this total.

The condensate from these tanks is pumped through a new 1-in. pipeline to new OGD condensate storage and handling at Ruwais. This facility has six floating-roof tanks with a total capacity of 8,000 cu m.

At Ruwais, the condensate may either be loaded directly into ships or spiked into crude oil. In the future, it may also be routed to the Ruwais refinery for fractionation.

As mentioned, Adnoc said the NGL-recovery section uses a deep NGL-recovery process in which gas is chilled with propane as a refrigerant, in addition to a turboexpander-recompressor unit. The gas cools to as low as -70 to -80° C. which results in a propane recovery of a minimum of 90%.

The total NGL produced from the three OGD units and the two existing units is about 5,000 metric tons/day. This is stored in three new Horton spheres, each of 2,335 cu m capacity.

NGL is then pumped 5 km through a 10-in. line to Bab and injected into Gasco's NGL pipeline going to a fractionation plant at Ruwais.

The acid gases produced from the three OGD gas-sweetening units are mixed together and treated in three new identical processing trains of a sulfur-recovery unit, with the cold bed adsorption (CBA) process.

Each has an output capacity of 600 metric tons/day of molten liquid sulfur, giving a total sulfur output of about 1,800 metric tons/day.

The molten sulfur is degassed in the respective sumps to a specification of less than 10 ppm H₂S by use of Comprimo's degassing process, which adds a very small dosage of quinoline as catalyst in the sump and agitates thoroughly.

The degassed liquid sulfur is then stored in three steam-heated tanks and dispatched about 110 km in insulated road tankers to central granulation plant at Ruwais.

Processes

• Selective sweetening. In OGD, sour gas is sweetened with the selective MDEA process licensed from SNPA, France. This removes mainly H₂S, which is the most toxic and corrosive component, down to the desired low level, in this case 10 ppm.

  More than 60% of the CO₂ is retained in the gas, giving a higher volumetric yield of treated gas. This is desirable in this case because a major part of the gas is reinjected into the reservoir.

  In addition, the size of the plant and the equipment in the amine regeneration section are much reduced because CO₂ is not removed from the sour gas.

• Deep NGL recovery. A high efficiency, deep NGL-recovery process is used in OGD to recover more than 90% of C3+ components contained in the sweetened gas fed into the NGL-recovery units.

  These units are designed to operate over a wide range of variation in feed-gas composition and turn-down ratio. Propane chilling combined with turboexpander-recompressor and heat exchangers are used optimally for chilling of the feedgas in order to achieve maximum liquid NGL recovery.

  The unit is designed and engineered by Technip of France with a process guarantee.

• CBA sulfur recovery. Cold bed adsorption (CBA) is used for recovery of sulfur from the acid gas produced from the MDEA sweetening units of OGD.

  The units are designed and engineered by Pritchard Corp., U.S., and guaranteed to give a minimum recovery efficiency of 98.5%. The units, said Adnoc, are highly efficient in heat recovery and reliable in operation.

Control philosophy

All the process and utility units of the existing plant, as well as those of the OGD plant, said Adnoc, are controlled from a main control room, through a computerized, distributed control system (DCS).

The injection compressors, Thammama B feed-gas compressors and the pipeline network, including the Bab lean gas compressor station (5 km away), are all controlled from another central control room through the same DCS, both control rooms being interfaced for necessary data transfer and controls.

The DCS uses Bailey's Infi 90 system. All signals between the field instruments and the control rooms are transmitted through a dual infinite-data highway, via the local instrument shelters. All hardware in the DCS is dual-redundant, thus making it highly reliable.

The plant is provided with PLC-based emergency shutdown systems (ESD), fire and gas detection system, machine condition monitoring systems for major equipment, all interfaced with the central DCS.

All the feed-gas and injection wells can be controlled and monitored by a supervisory control and data acquisition (scada) system, also connected with the DCS.

The pipeline network and the compressor stations are remotely monitored/controlled by a supervisory control system, which is also interfaced with the DCS.

Safety; the environment

Adnoc said that safety issues are of the highest importance and precede all other concerns in the gas plant. Both the old plant and the new OGD plant have been designed with several modern safety systems and features, such as ESD systems, fire and gas detection system, and remote plant surveillance system.

An elaborate and extensive fire-water network runs throughout the plant with adequate numbers of hydrants, monitors, and deluge systems. Environment-friendly Inergen and CO₂ systems are used in appropriate places like in the control rooms, instrument shelters, and electrical substations for automatic fire fighting in these areas.

Adnoc said that the greatest care has been taken in planning and designing the OGD plant so as to have a minimum impact on the environment. The release of solid, liquid, and gaseous effluents has been minimized through proper selection of process, plant, and equipment.

Flaring of hydrocarbon and acid gases is extremely restricted and adopted only as the last resort in emergency and upset conditions, which are rare. Several H₂S sensors are installed around the plant in vulnerable locations to detect any leakage of H₂S.

Flares are also continuously monitored through video cameras to keep a check on any undesirable release of gases to flare and the resulting smoke pollution. The ambient air quality is also monitored in the surrounding inhabited areas continuously to detect the daily average SO₂ concentration in the air.

From the record of operation of the gas plant since 1990 (when the monitoring system was installed), the daily average SO₂ concentration in the atmosphere has always been well below the limit prescribed by the World Health Organization for ambient air quality for the continuous exposure of human beings.

Liquid sulfur produced from the sulfur-recovery units, Adnoc said, is also degassed to reduce dissolved H₂S to a safe limit of a maximum of 10 ppm. If this were not done, most of the H₂S would be likely to be liberated into the atmosphere during handling and transportation.

Special attention has been paid to effluent sour water streams, which are all treated in sour-water stripper units to remove H₂S, followed by oil recovery and bacteria control operation, before finally being injected into wells for safe disposal in specified underground strata.

And since the late 1980s, the entire sewage outflow from the plant and its residential complex has been treated, stabilized, and bacteria-controlled in a separate unit to an acceptable hygienic standard. The clarified water is reused locally for irrigation.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.