VAPORIZERS Types of Vaporizers:Typical types of vaporizers that have been used worldwide for LNG regasification are:
Open Rack Vaporizers (ORV)
Submerged Combustion Vaporizers (SCV)
Ambient Air Vaporizers (AAV)
Intermediate Fluid Vaporizers (IFV)
Shell and tube vaporizers
Fired Heater Vaporization System
Selection of Vaporizers :Traditionally, base load regasification terminals have used following types of vaporizers: 70% uses the Open rack Vaporizer (ORV), 25% uses the Submerged Combustion Vaporizer (SCV) and the remaining 5% uses the Intermediate Fluid vaporizer (IFV). In addition to these vaporizers, other types of vaporizers such as the Ambient Air Vaporizers (AAV), Shell and tube vaporizers, Fired Heater Vaporization System have been used in smaller regasification plants and peak shaving facilities.
Important factors that should be considered in the LNG vaporizer selection process are:• Site conditions and plant location • Availability and reliability of the heat source • Customer demand fluctuation • Emission permit limits • Regulatory restrictions with respect to the use of seawater • Vaporizer capacity and operating parameters
• Safety in design • Operating flexibility and reliability • Capital and the operating cost
Selection of vaporizer in terms of the Location of terminal:The countries where regasification terminals are located can be broadly classified into two regions. First, there is the equatorial countries where the site ambient temperatures are fairly constant and do not fall below 18°C during winter. Second, there is the sub-equatorial region where the site ambient temperatures can fall below 18 °C during winter months. The following countries fall under the equatorial region definition: • Asian Countries (Southern India, Indonesia, Thailand, Malaysia, Singapore, Philippines) • North American Countries (Mexico) • South American Countries (Brazil)
Whereas following countries may fall under the subequatorial definition: • Asian Countries (China, Japan, Mid-West and Mid-East of India) • South American Countries (Chile, Argentina) • European Countries (Spain, UK, France)
Open rack vaporizers (ORV) and submerged combustion vaporizers (SCV) are the most common vaporization methods in existing regasification terminals, which have generally been located in the subequatorial region. Recent LNG receiving terminal activities have been shifting to the equatorial region where the weather is warmer, and the use of intermediate fluid vaporizers (IFV) is found to be attractive.
Regasification Facilities:• Open Rack Vaporizer (ORV) :An Open Rack Vaporizer (ORV) is a heat exchanger that uses seawater as the source of heat. But sea water generally contains lots of heavy metal ions, solid particles i.e. salts and also suitable
to algae growth. So, as a precaution desalination of sea water and injection of sodium hypochlorite must be done before using sea water as source of heat. The treated sea water is pumped to the top of the trough / water box and travelled down contacting with the outer surface of the tube. LNG flows upward into the tube entering from bottom and as successively moving upward into the tubes vaporization taking place. The cooled sea water is collected on lower header of vaporizer and discharged. The vaporized natural gas is collected from the top through upper header.
Advantages:•
The construction and operation is very simple.
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No direct air emission into environment
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Easy to operate and maintain and lower operating cost
Disadvantage:•
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ORV requires large volume of sea water (25000-65000 m of water per hour) which greatly affects marine life
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Higher capital cost
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Sea water is discharged back at 5-8 C cooler than intake that creates great potential
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environment impact
• Submerged Combustion Vaporizers (SCV):Natural gas produced by the terminal is burnt and used as heating medium in SCV. LNG vaporization using fuel gas for heating typically consumes approximately 1.5 % of the vaporized LNG as fuel. LNG flows through a stainless steel tube coil that is submerged in a water bath which is heated by direct contact with hot flue gases from a submerged gas burner. Flue gases are sparged into the water using a distributor located under the heat transfer tubes. The sparging action promotes turbulence resulting in a high heat transfer rate and a high thermal efficiency (over 98%). The turbulence also reduces deposits or scales that can build up on the heat transfer surface. The bath water is acidic as the combustion gas products (CO2) are condensed in the water. Caustic chemical such as sodium carbonate and sodium bicarbonate can be added to the bath water to control the pH value and to protect the tubes against corrosion. The excess combustion water must be neutralized before being discharged to the open water.
Advantages:•
High thermal efficiency over 95 %
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Smaller in size so required less space
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There is no danger of explosion, due to the fact that the temperature of the water bath always stays below the ignition point of natural gas.
Disadvantages:•
LNG vaporization using fuel gas for heating typically consumes approximately 1.5 % of the vaporized LNG as fuel, which reduces the plant output and the revenue of the terminal
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SCV creates air emission by producing NOX around 30-80 ppm
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Bath water will required treatment before disposal
• Ambient Air Vaporizers (AAV) :Air is another source of "free" heat and would avoid the use of fuel gas and the generation of greenhouse gas from SCVs. AAV consists of direct contact, long, vertical heat exchange tubes that facilitate downward air draft. This is due to the warmer less dense air at the top being lighter than the cold denser air at the bottom. Ambient air vaporizers utilize air in a natural or forced draft vertical arrangement. To avoid dense ice buildup on the surface of the heat exchanger tubes, deicing or defrosting with a 4-8 hour cycle is typically required. Long operating cycles lead to dense ice on the exchanger tubes, requiring longer defrosting time. Defrosting requires the exchanger to be placed on a standby mode, and can be completed by natural draft convection or force draft air fans.
Advantages:•
In the environmental sensitive parts of the world, the use of sea water may not be allowed and could also be difficult to permit. In this case, the use of ambient air heat is the next best choice.
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Ambient air heater is advantageous in hot climate equatorial regions where ambient temperature is high all year round.
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Water condensation and melting ice can also be collected and used as a source of service/potable water.
Disadvantage:•
Fog around the vaporizer areas can pose a visibility problem, which is generated by condensation of the moist air outside.
• Intermediate Fluid Heating:The LNG vaporizing via intermediate fluid utilizes Heat Transfer Fluid (HTF) in a closed loop to transfer heat to vaporize LNG. Two types of Heat Transfer Fluids are typically utilized for LNG vaporization: • Glycol-Water • Hydrocarbon Based HTF (Propane, Butane or Mixed Refrigerant)
Glycol-water Intermediate Fluid Vaporizer (IFV):This system typically uses glycol-water as an intermediate heat transfer fluid. Ethylene glycol or propylene glycol or other low freezing heat transfer fluids are suitable for this application. Heat transfer for LNG vaporization occurs in a shell and tube exchanger. Warm glycol-water flows through the intermediate fluid vaporizers where it rejects heat to vaporize LNG. The IFV is a conventional shell and tube exchanger which is also known as Shell and Tube Vaporizer (STV). There are several options to warm the glycol-water solution prior to recycling it back into the shell and tube LNG vaporizers, such as: • Air heater • Reverse cooling tower • Seawater heater • Waste heat recovery system or fired heater
Intermediate Fluid (Hydrocarbon) in Rankine Cycle:This system uses propane, butane or other hydrocarbon refrigerant as an intermediate heat transfer fluid (HTF). The use of a hydrocarbon avoids the potential freezing problems encountered with seawater. This vaporizer arrangement allows the use of cold seawater as low as 1°C to minimize fuel consumption in the downstream trim heater. LNG heating is achieved using two heat exchangers operating in series: a first evaporator exchanger that uses the latent heat of propane condensation to partially heat LNG, and a
second heat exchanger using seawater to further heat the LNG to the final temperature. The second exchanger is also used to vaporize propane that is recycled to the first exchanger. Since the heating by seawater only occurs in the second exchanger, it avoids direct contact with cryogenic LNG, and hence freezing of seawater can be avoided. For this reason, seawater close to freezing can be used in this configuration.
Disadvantages:•
As IFV is using sea water as heat transfer agent, environmental issues arises during operation.
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Increased cost from use of intermediate fluid
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As the processed sea water is discharged back to sea, the sea water become entrained & possesses a risk to the marine life.
