Michael J. Morgan
Phoenix Park Gas Processors Ltd.
Point Lisas, Trinidad
Mark R. Cousino
Conoco Inc.
San Angelo, Tex.
One of the largest gas processing plants in Latin America began operations June 15 in Trinidad. Startup came only 19 months after ground-breaking.
The $100 million, 650 MMcfd plant is located in the Point Lisas Industrial Estate in the western part of the island. It is a state-of-the-art, high-recovery cryogenic gas plant capable of removing 98% of the propane and all heavier components. Present and design material balances are shown in Table 1.
PERIODIC UPSETS
The National Gas Co. of Trinidad & Tobago Ltd. (NGC) receives unprocessed natural gas from oil and gas producing fields off the southeast coast of Trinidad (Fig. 1).
Gas flows through a dual pipeline system to supply a variety of industrial consumers (Table 2) primarily on the west side of the island. The gas is used for both fuel and feedstocks by the customers.
The composition of the unprocessed gas led these industrial customers to experience periodic liquid condensation and slightly varying heating values in their plants. These, in turn, were causing them short catalyst life and generally unsteady operations.
Additionally, the NGC recognized that no LPGs were being recovered for sale as a separate commodity.
After receiving a commissioned study, the NGC solicited proposals for the construction and operation of a gas-processing plant.
Responding to this request, Conoco Inc. and Pan West Engineers & Constructors, Houston, submitted a plan in February 1988 for a joint-venture company with NGC, and for financing the project and constructing a gas processing plant.
The NGC-Conoco-Pan West joint venture's proposal was selected and approved by the government in July 1988.
Negotiations culminated in the signing of a general agreement in January 1989. Between January and November 1989 numerous other agreements were negotiated, limited recourse financing was arranged through Citibank N.A., and all required government approvals were obtained.
Concurrently, Pan West and Conoco began engineering and design for the plant. Major design changes were required when the project scope was expanded to include product fractionation, refrigerated storage, and loading of ocean-going tankers over a to-be-constructed dock.
The plant site was subsequently relocated closer to the proposed dock location to reduce the costs associated with insulated product pipelines.
By November 1989, all agreements and government approvals were in place, the plant design was nearing completion, and Pan West mobilized to the job site to begin site preparation.
STRUCTURING, FINANCING
Phoenix Park Gas Processors Ltd. is an independent Trinidad and Tobago company owned by the National Gas Co. of Trinidad & Tobago Ltd. (49%), Conoco Inc. (41%), and Pan West Engineers & Constructors (10%).
During the design and construction, Pan West served as the prime contractor. Conoco provided commercial and project management expertise and operations management.
The project is funded with a combination of equity and debt financing. The final debt financing selected was through an industrial revenue bond issue utilizing Caribbean Basin Initiative "936" funds.
Propane and mixed butanes are sold to a variety of LPG suppliers who serve primarily the Caribbean basin and northern South America. These customers receive their product delivered at the Phoenix Park Gas Processors Ltd. dock onto ocean going tankers. (The dock and its operations will be discussed later.)
The plant's natural gasoline production is pipelined directly to state-owned Trintoc refinery for ultimate use in motor gasoline.
In order to meet the joint-venture commitments, numerous project-management challenges had to be overcome.
The construction schedule was constrained by the Trinidad rainy season, which lasts from June to December. Virtually all plant equipment, materials, and heavy-construction equipment had to be brought in by ocean-going freighters, making the timing of equipment from the vendors important.
The plant site is near the coast. This, coupled wit local earthquake activity, made foundation design critical.
After clearing 30 acres of undeveloped land, more than 100,000 cu yd of topsoil had to be removed. A semipermeable membrane was installed, and nearly 200,000 cu yd of sand were brought in and compacted.
More than 1,500 piles were driven into the ground to support equipment foundations, piperacks, and product-storage tanks.
Transporting the bulk of the project's equipment and material to Trinidad required 30 ocean shipments. A total of 3,200 tons of material was shipped by sea (Fig. 2), with another 50 by air freight.
Nearly all field construction was done by 16 Trinidad and Tobago subcontractors under the supervision of Pan West. Additionally, CBI was subcontracted to design and construct the two refrigerated product-storage tanks.
At the height of construction activities, 650 Trinidad and Tobago construction personnel were on the job site.
One of the highest priorities of the joint venture was to construct and ultimately operate the plant with best possible safety standards. The owners, prime contractor, and subcontractors were successful in constructing the plant without a single losttime accident: 1.25 million man-hr were worked without a single disabling injury.
PIPELINES, PROCESS
The NGC pipelines converge at the Phoenix Park valve station near the Point Lisas Industrial Estate. Pipelines were installed between this valve station and the plant to receive all of NGC's inlet gas and to return residue gas to NGC.
Because the entire supply of gas to Trinidad flows through this valve station, special precautions were taken in the plant design to ensure that there would never be an interruption of gas flow to NGC's customers.
Among these are parallel pressure-control valves at the valve station and at the plant site that are capable of bypassing the full 650 MMcfd of inlet gas to NGC's customers in the event of a total plant shutdown.
The plant is designed to receive as at 565 psia and 850 F. The gas is then dehydrated to a dew point of -150 F. utilizing a 3A molecular sieve specially selected to reduce conversion of H2S to COS (Fig. 3).
Four desiccant beds, each containing 79,000 lb of sieve, operate on 18.5-hr cycles with three beds on-line and one in regeneration. Regeneration of the beds is accomplished by heating 33 MMcfd of residue gas through a 20 MMBTU/hr gas-fired, induced-draft heater.
In the event of an electrical power outage or cryogenic plant shutdown, the plant can automatically continue dehydration and maintain hydrocarbon dew point conditioning of the gas in the "dry gas by-pass" mode of operation.
In this mode of operation, cooling from the isenthalpic expansion of the gas down to residue pressure and the subsequent exchange with dehydrated inlet gas produces 1,000 b/d of essentially C5+ liquids.
The resulting hydrocarbon dew point of the gas redelivered to NGC is 55-60 F. The condensed liquids are stabilized to atmospheric pressure by a 1,600 b/d stabilization system.
The stabilized condensate can be sent to storage or blended with natural gasoline, depending on product specifications.
RECOVERY, FRACTIONATION
The liquid-recovery section of the plant features a Pan West proprietary design for high propane recovery.
The scheme utilizes two towers: the propane-recovery tower which partially demethanizes the liquids in the cooling section and the deethanizer which deethanizes the liquid product at a higher temperature and pressure (Fig. 4).
This design results in 98% propane recovery with only a 200 psi pressure drop across the plant. Even with this low pressure drop, the design allows the plant to achieve a high level of recovery without inlet compression and without any residue-gas compression except for the compressor side of the turboexpander.
The propane-recovery tower is a 3.5% nickel-alloy column with 34 stainless steel trays.
Both bottom and side reboiler heat is provided by inlet-gas exchange, giving the column a temperature profile of - 120F. to 20F. from top to bottom.
Inlet-gas chilling by propane refrigeration accounts for nearly all of the energy removed in the cold plant. The remaining energy is removed in a Mafi-Trench Frame 5 turboexpander, one of the largest expanders Mafi-Trench has ever constructed for a gas-processing application.
With the equipment operating at 7,000 rpm, the resulting expansion to 390 psia generates approximately 2,500 hp which is subsequently used for residue-gas recompression.
The propane-recovery tower is not pressure controlled but "rides" residue-gas pressure by the manipulation of the turboexpander's inlet guide vanes.
Although the deethanizer's main purpose is to remove ethane and lighter components from the liquids recovered in the propane recovery tower, it also is designed to remove H2S to ensure the propane product meets specification requirements.
The tower has the capacity to handle 13,500 b/d of propane-butane condensate. The 52-tray column operates at 500 psia with a bottoms product temperature of 249 F. and an overhead temperature of 45 F.
Side reboiler heat is obtained from exchange with the warmer bottoms product while heating medium provides duty to the bottoms reboiler. The tower overhead is partially condensed with high-level propane refrigeration to produce reflux. The depropanizer is a 29-tray tower operating at 315 psia and a bottoms temperature of 280 F. The debutanizer is a 28-tray tower which operates at 120 psia and a bottoms temperature of 2770 F.
Both towers have aircooled condensers that also subcool the overheads stream by 15 F., resulting in a 120 F. overhead product. Heat-transfer fluid at 475 F. provides heat to both tower's reboilers.
With high-level and low-level propane refrigeration, the propane product is ultimately cooled to -33 F., when it is then routed to propane storage. The butane product is cooled with high level propane refrigeration to 25 F. and then flows to butane storage. Both towers have the ability to divert to rerun in the event off-specification product is produced.
PRODUCT STORAGE, LOADING
Propane and butane are stored in individual, fully refrigerated storage tanks (Fig. 5).
The butane tank is a 100,000 bbl, 100-ft diameter tank; the propane tank has a capacity of 150,000 bbl and measures 120 ft in diameter. Tank capacity corresponds to approximately 1 month of production at design throughput.
The tanks are of single-wall construction and have a design pressure of 2 psig. Both tanks operate at essentially atmospheric pressure at the product's normal boiling point (-44 F. for propane and 25 F. for butane).
Three 200-hp, rotary-vane compressors handle flash vapor and product boil off from the refrigerated storage tanks, thereby controlling the operating pressure of the tanks. The compressed vapors are condensed by propane refrigerant and returned to the storage tanks.
Tank pressure control is further maintained by way of relief to a separate LPG flare. As a last resort, four bellows-type relief valves can vent the tank contents.
Two product evaporators provide product vapor in the event that tank pressures fall below the operating limit. Should the limits of these evaporators be exceeded, fuel gas and the tank's own vacuum-relief devices prevent the tanks from being subjected to a vacuum.
The plant has the capability to deliver either fully refrigerated, semirefrigerated, or pressurized ambient product to ships, depending on the needs of the customer.
Three 200 hp, nine-stage, deep-well pumps can deliver product at rates of up to 5,000 bbl/hr. Ambient (50 F.) deliveries of propane are limited to the capacity of the loading heater, which is 1,500 bbl/hr. Propane-butane mixed product deliveries are also possible.
A 1 mile, 10-in. diameter insulated product line connects the plant with the LPG-loading terminal. The dock was constructed within an existing harbor in the Gulf of Paria.
The harbor, owned and operated by a neighboring ammonia producer, has a turning-basin radius of 1,000 ft. The dock has the flexibility of handling ocean-going LPG tankers as large as 70 x 154 m and drawing up to 7.7 m.
Product transfer is made through an 8 in., fully automatic, hydraulic loading arm. The arm also contains a "piggybacked" vapor return line.
Vapors unsuitable for liquid recovery can be directed to the plant's flare, while recoverable hydrocarbon vapors are routed to one of the three vapor-recovery compressors, depending on product content and purity.
An LPG-interface tank is located at the dock to collect liquids drained from the loading arm as well as the contaminated product interface that is created when products are changed. Interface detection can be made by observation of the temperature of the product at the dock and the interface being diverted accordingly.
These liquids are then pumped back to the plant's rerun system. Natural gasoline, upon production, is transported by pipeline directly to the state-owned Trintoc refinery for motor gasoline blending.
UTILITIES, MEASUREMENT
Approximately 2,100 kw of electrical power are required to operate the plant and tanker-loading operations.
The plant's electrical supply is purchased from the local utility and transformed down into a 480-y system. A 750-kw, diesel-driven standby generator starts automatically to provide emergency power to critical systems if purchased power is lost.
The entire dry-gas bypass mode of operation can be run off the emergency generator, ensuring a continuous supply of conditioned gas to NGC's customers should purchased power be lost.
The majority of the gas-cooling duty is provided by a mechanical refrigeration system. This system circulates 79 MMscfd of refrigerant propane through a conventional two-stage system.
Two Solar Centaur H model gas-fired turbines provide the power to run York centrifugal compressors for this service, each delivering 4,500 hp.
The two turbine-compressor sets are each controlled by their individual Solar control panels, featuring multi-color, programmable-logic controller (PLC) displays of all critical functions.
Reboil heat for the distillation towers is provided by a circulating heating-medium system delivering 1,900 gpm of 475 F. oil to the users.
A 57.7 MMBTU/hr gasfired heater provides most of the heat to the heating-medium stream. This is supplemented by 33.4 MMBTU/hr of additional heat recovered from the hot exhaust of the two refrigeration compressor turbine drivers.
As mentioned earlier, there is a separate gas-fired heater for the mol sieve-regeneration gas. There is no cooling water or steam used in the plant. All atmospheric level cooling is accomplished by air-cooled heat exchangers.
A distributed control system (DCS) monitors and controls all plant functions.
The DCS is a Texas Instruments Model 40 Tistar PLCbased system featuring three multi-color monitors for use by plant operators. The DCS not only processes analog control functions but also handles discrete functions such as shutdown logic and dehydration switching valve logic. An uninterruptible power supply provides a conditioned source of power to the DCS and to all field electronic instrumentation. Battery backup ensures that process control will not be interrupted by power outages.
Not only are plant operations controlled from the DCS, but all measurement calculations are performed as well. One PLC is dedicated to receiving a variety of gas and liquid flow-measurement data, including chromatograph data, and to performing all AGA3, AGA8, and mass-flow calculations.
Inlet and residue gas streams are each measured in four parallel senior orifice meters. All required parameters such as differential pressure, static pressure, and temperature are measured by electronic transmitters, whose signals the DCS sends to the measurement PLC for processing.
To ensure backup of these gas flows, a solar-powered measurement package records all data and calculates flows at the field locations.
Raw mix is measured with a turbine meter-based mass flow system.
As with the gas streams, all data, including analyses, are sent to the measurement PLC for the calculation of mass flow.
Process chromatographs update the status of all major gas and liquid streams in the plant every 15-30 min, depending on the individual stream.
A separate computer receives all chromatograph data and calculates physical properties as required. All heating values are calculated in accordance with GPA-2172.
Refrigerated tank inventories are measured by an Enraf tank-gauging system which is precise to 0.0625 in. Temperature compensation for volume is calculated by an Enraf-supplied software package.
For tanker loading, a separate mass-flow metering package is installed at the dock.
This package features a turbine meter, automatic proportional sampler, and an on site meter prover.
SAFETY, ENVIRONMENT
The plant was designed to conform with U.S. standards for safety and environmental conservation.
Among the features included to comply with these standards are the following:
- Containment dikes surrounding all liquid-storage tanks
- Dual flare system for high and low-pressure relief headers
- Tandem pump seals
- Gas and fire detectors throughout the plant and dock
- Extensive fireproofing of structural and equipment supports.
The plant has its own firefighting system based on an underground firewater loop, a 2,000 gpm electric-driven firewater pump, a 2,000 gpm diesel-driven firewater pump, and a 540,000 gal firewater-storage tank. Ail monitors and hydrants in the plant have the ability to quick-couple to the equipment used by the local municipal fire department, should they be called.
To ensure the quality of any liquid effluent leaving the plant, there is a sanitary sewage-treatment package and an oily water separator. Separate closed and open drain systems ensure the proper handling of any nonperiodic process waste streams.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
