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LNG Vaporizers

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  Open Rack Vaporizer An Open Rack Vaporizer (ORV) is a heat exchanger that uses seawater as the source of heat. The preferred seawater temperature for ORV operation is above 5°C. ORV units are generally constructed of aluminum alloy for mechanical strength suitable to operate at the cryogenic temperature. The material has high thermal conductivity which is effective for heat transfer equipment. The tubes are arranged in panels, connected through the LNG inlet and the regasified product outlet piping manifolds and hung from a rack (Figure 1). The panels are coated externally with zinc alloy, providing corrosion protection against seawater. Process Open rack vaporizers (ORV) use seawater as the thermal energy source in a direct heat system to vaporize the LNG. To control algae growth within the system, sodium hypochlorite (chlorine) is injected on the intake side of the system. The treated seawater is then pumped to the top of the water box and travels down along the outer surface of the tube heat exchanger panels, while LNG flows upward through these tubes and is vaporized.4 The cooled seawater collects in a basin under the open rack vaporizer and is discharged through the water outfall, while the vaporized natural gas is removed from the top header of the system. Because this technology relies on seawater as the primary heat source, it is only effective where seawater temperatures exceed approximately 63 degrees Fahrenheit.  Drawbacks ã   Open rack vaporizer technology requires large volumes of water, which could adversely affect marine life. ã   The cooled and treated seawater that is returned to the ocean could potentially affect marine life and water quality. ã   Although the ORVs do not directly produce air pollution emissions, powering the seawater pumps For large regasification terminals where significant amounts of water are required, in-depth evaluation and assessment of the seawater system must be performed. the key issues and design  parameter must be established early in the project, such as: Is the seawater quality suitable for operating an ORV system? ã Does the seawater containing significant amounts of heavy metal ions? These ions will attack the zinc aluminum alloy coating and will shorten its life. ã Does the seawater contain significant amount of sand and suspended solids? Excessive sediment will cause jamming of the water trough and the tube panel. Proper seawater intake filtration system must be designed to prevent silts, sands and sea life from reaching the seawater  pumps and exchangers. ã The design must consider the environmental impacts of the seawater intake and outfall system, and minimize the destruction of marine life during the construction period and normal plant operation. ã Chlorination of the seawater is necessary to slow down marine growth. However, residual chlorine in the seawater effluent can impact the marine life and the usage must be minimized. ã Seawater discharge temperature must comply with local regulation. The temperature drop of seawater is typically limited to 5°C in most locations. ã Location of the seawater intake and outfall must be studied to avoid cold seawater recirculation. ã If site is located in a cold climate region, supplementary heating may be necessary to maintain the outlet gas temperature. Boiloff gas from LNG storage tanks can be used as fuel to these heaters. ã Is a backup vaporization system provided? This may be necessary during partial shutdown of the seawater system or during peaking demand operation.  ã Is the regasification facility located close to a waste heat source, such as a power plant? Heat integration using waste heat can reduce regasification duty and would minimize the environmental impacts. Submerged Combustion Vaporizer Process A typical SCV system is shown in Figure 2. 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. To minimize the NOx emissions, low NOx burners can be used to meet the 40 ppm NOx limit. The NOx level can be further reduced by using a Selective Catalytic Reduction (SCR) system to meet the 5 ppm specification if more stringent emission requirements are needed, at a significant cost impact. Advantages ã   Since the thermal capacity of the water bath is high, itis possible to maintain a stable operation even for suddenstart-ups/shutdowns and rapid load fluctuations. ã   SCV is very reliable and have very good safety records.  ã   Leakage of gas can be quickly detected by hydrocarbon detectors which will result in a  plant shutdown. ã   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. ã   SCVs are compact and do not require much plot area when compared to the other vaporizer options. Drawbacks ã   The submerged combustion vaporizer system produces large quantities of air emissions from the flue gas. This can be reduced through exhaust gas control technology, but adds significant operating costs to the SCV system .   ã   The controls for the submerged combustion vaporizers are more complex when compared to the open rack vaporizers (ORV). ã   During operation, SCVs consume anywhere from 1.5 to 2.0 percent of the LNG cargo to fuel the combustion burner, which is a significant operating cost. Ambient Air Vaporizers The approaches can be categorized into two basic methods for recovering low level heat from ambient air. They are indirect contact and direct contact. 1)Direct Contact Ambient Air Technologies   Process Ambient air vaporization (AAV) technology uses ambient air as the thermal energy source to vaporize theliquefied natural gas. The LNG is distributed througha series of surface heat exchangerswarm ambient air enters from the top of the exchanger, exchanges heat directly to the cold LNG flowing in the specialized finned tubes tovaporize LNG. The air is cooled and flows downward along the outside of the finned tubes and leaves from the bottom of the exchanger. The air flowis controlled on the outside of the exchanger throughnatural  buoyancy of the cooled, dense air, or by installingforced-draft air fans. Frost forming on the vaporizer is an issue because the LNG is vaporized directly against the air (direct heat system) and it cools air reaches its dew point and starts to condense water s. Frost build-up reduces performanceand heat transfer. Deicing or Defrosting is necessaryto avoid dense ice buildup on the surface of the heat exchanger tubes.In order to maintain the overall availability of the LNG vaporizer capacity, additional units are installed to overcome vaporizer regeneration activityThe use of force draft fans can reduce the defrosting time but would require additional fan horsepower. The performance of ambient air vaporizers depends on the LNG inlet and outlet conditions and more importantly the site conditions and environment factors, such
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