The choice of a vaporization system is an important first step in the development of a LNG import terminal, since it impacts capital expenditure, expenditure, operating costs, operating flexibility and reliability, emissions as well as public perception and regulatory compliance. Historically, LNG import terminals have generally used either Open Rack Vaporizers (ORV) or Submerged Combustion Vaporizers (SCV) for LNG regasification purposes. ORVs are widely used in Asia and Europe, and are well proven in baseload LNG regasification service. SCVs have been used in the four existing import terminals in the USA. When compared to other vaporization technologies, the higher emissions from ORV’s and SCV’s have prompted requirements to evaluate alternative vaporization systems. Recent developments in alternative vaporizer technologies include ambient air vaporizers and shell and tubevaporizers with or without intermediate fluid and/or combinations of each and there now exists proven design and operating experience. This study examined the applicability and suitability of several different types of LNG vaporization v aporization technologies technologies that are currently available. Based on this experience, the WorleyParsons- SOFREGAZ alliance developed an 800 million standard ft3/d IFV vapourisation system, in which glycol or propane is used as intermediate fluid and seawater as primary heating medium. Both systems (glycol or propane) can be used for newbuild FSRUs or converted LNGC to FSRU facilities. The system makes use of offshore oil and gas standards to provide the reliability and maintainability required for greater than 20 year lifetime on station. In addition, the system is highly flexible, allowing for the fluctuation of the power plant loads and also the ability to increase the regasification capacity over the life of the facility as demand increases, i.e. “a fit for but not with” philosophy was adopted, to minimise the initial capital expenditure and to keep the operational cost as low as possible, by avoiding the need to maintain equipment that is not required in the early life of the facility. The IFV system was chosen for its compactness, the ability to marinise the system, the track record of comparable systems in the industry and in the floating production industry. At the core of the system is a shell and tube vapouriser through which the high pressure LNG is vapourised into natural gas using glycol or propane as a heating medium. This type of system was also chosen because it is considerably more environmentally friendly than closed loop systems which require gas to be burned to heat the LNG. Depending on the power generation the system provides a 75% CO2 emission reduction compared to a system that uses submerged combustion vapourisers (SCV) of similar capacity. One variable that may allow for reduced system cost is altering the working fluid used in the system. Use of a phase change material like propane will reduce the required heat exchanger size by utilizing latent heat transfer within the tubes, thereby reducing the necessary heat transfer surface area of the unit. Other working fluids will have varying degrees of specific heat capacity, with an associated reduction or increase in required heat transfer surface area and pumping power. Regardless of the freezing point of the working fluid, operating constraints are in place to minimize freezing on the outside of the heat exchanger tubes. One variable that may allow for reduced system cost is altering the primary working fluid used in the system. Use of a phase change material like propane will reduce the required plate and frame and shell and tube heat exchanger size by utilizing latent heat transfer on the primary fluid side of these exchangers, thereby reducing the necessary heat transfer surface area of the unit. Other working fluids will have varying degrees of specific heat capacity,
with an associated reduction or increase in required heat transfer surface area and pumping power. Other technologies to evaluate are cross-flow heating towers vs. the counterflow design in this study. Cross flow towers have a wider footprint yet lower height, which could provide a reduction in total installed cost, yet no change in operating costs.
Propane Cascade regasification system In this system, LNG is heated by propane in a closed loop and the propane is heated by seawater. In situations where the seawater is too cold to supply all the required heating energy, additional heat can be introduced. The cascade concept is recommended instead of directly heat exchanging with seawater. LNG heat exchanged directly with seawater increases the risk of freezing the seawater in the heat exchanger. Propane as a secondary medium is suggested because of its thermodynamically stable fluid with a low freezing point and commercially available at low cost. The introduction of flammable and explosive fluid, propane requires careful evaluation for siting equipment and cost increases A HAZID analysis has been carried out with DNV to identify risks related to the design and operation of the regasification system. For example, it was concluded that the cascade system is a safer system than the pure seawater based. LNG enters a cryogenic pump capable of producing the required send-out pressure (e.g. up to 130 bar has been studied). LNG at the required discharge pressure is heated in two stages. In the first stage LNG is heated from 160°C to -10°C in a compact printed circuit heat exchanger with propane as a heating medium. In the second stage, LNG can be heated further using seawater as a heating medium in a shell and tube heat exchanger. In the LNG/Propane heat exchanger, heat is exchanged against propane circulating in a closed loop. The propane enters the heat exchanger at approx. 0°C at 4.7 bar as gas. In the heat exchanging process propane is condensed, and leaves the exchanger in a liquid state at approximately -5°C. The propane in the closed loop is then pumped by the circulating pump and heated against seawater in titanium semi welded plate heat exchangers. In these heat exchangers, the propane is evaporated and heated to 0°C before Artist impression of an LNG FSRU with a Hamworthy regasification plant returning as gas to the printed circuit heat exchanger. Although propane and refrigerant have low flash points that are ideal for heat transfer, the operational risks are much higher when handling these types of fluids, and these fluids are very costly.
If an intermediate fluid vaporizer system operates with propane or refrigerant as the intermediate fluid, then these fluids add a potentially hazardous material to the facility operations.
Glycol Hamworthy has also developed a steam heated system for areas where seawater is not allowed to be used to heat the LNG. Such systems are contracted to Hoegh LNG SRVs for serving the Neptune offshore terminal at Massachusetts. Each unit has 2 x 50% booster pumps installed in canisters, designed for a send out pressure of 130 bar. The LNG is then pumped through a shell & tube type heat exchanger heated by a glycol water mixture which is looped through a compact heat exchanger with steam from the SRVs steam system. The natural gas is then passed to the export manifold. The water/glycol mixture has a high flash point, requiring a larger heat transfer area, which results in a larger system than the propane or refrigerant systems. However, the water/glycol fluid system is more cost effective and the associated operational risks are r elatively low. An IFV system that uses the water/glycol mixture is considered a safer way to operate. Lastly, depending on the combustion process used to heat the intermediate fluid, air emissions are also an environmental concern, unless the system uses waste heat r ecovery. The two basic choices are a closed-loop system and an open-loop system. In the closed loop system, boilers are used to produce steam; the steam heats a fluid in a closed loop that is circulated through heat exchangers to warm the LNG. In the open-loop system, seawater would be circulated through heat exchangers to warm the LNG. The closed-loop system would be required for proper operation in the northeast portion of the United States because of the low water temperatures during winter months. In addition, the closed-loop is less harmful to the marine environment. The Neptune vaporization system would be a closed loop and would use an intermediate media (water/glycol) to heat the LNG in the regasification units. Steam produced by the marine boilers is used as the heat source for the water/glycol. The water/glycol enters the LNG shell and tube heat exchanger at approximately 90 degrees Celsius (°C) and leaves at approximately 20°C after evaporating and warming the LNG. It is then pumped via a circulation pump and re-heated by steam produced in the marine boilers before it is returned to the shell and tube heat exchanger. A printed circuit heat exchanger (PCHE) is used to transfer heat from the marine boilers to the water/glycol fluid. LNG at –160°C and approximately 5 bars in the SRV’s tanks is pressurized in multi stage centrifugal pump(s) to a pressure up to 120 bars prior to entering the shell and tube heat exchanger. The LNG is then evaporated and heated to approximately 0°C. In the heat exchanger, LNG passes through tubes where it is evaporated/heated in the pipes by the water/glycol surrounding the pipes.
