LNG FRACTIONATION
Ortloff has developed a number of processes for efficient recovery of hydrocarbon liquids from LNG streams. These LFP designs are able to recover extremely high levels of hydrocarbon liquids (either NGL or LPG) with no external compression and with flexible and reliable operation.
One of the main advantages of integrating the liquids recovery into the vaporization step is that there is more than enough refrigeration available in the LNG stream to supply the requirements for liquids recovery. While a typical natural gas plant must generate refrigeration via expansion and recompression to provide the duty required to make the component separation, the integrated LFP designs require no additional refrigeration beyond that already present in the LNG. Although turboexpanders are used in some of these designs, their purpose is simply power recovery rather than generating additional refrigeration. Application of a turboexpander/compressor can be especially useful in a straddle-plant-type application where the LNG is delivered to the liquids recovery unit at high pressure.
Separation of heavier hydrocarbons from a predominately methane stream must be carried out at relatively low pressure due to phase behavior characteristics, and the residual methane stream must leave the overhead of the distillation column as a vapor to avoid excessive refrigeration requirements. As a result, a significant portion of the inlet LNG stream must be vaporized either upstream of the distillation column or within the column itself. It is important to note that the overall heat duty of the vaporization process is essentially the same whether the liquids recovery unit is in service or if the LNG stream is being vaporized directly into the pipeline. Thus, the system can be integrated so that the heat transfer fluid can be directed to provide heat to either the liquids recovery unit or the vaporizer units, depending on the mode of plant operation. This type of process is very suitable for an air vaporization facility since the majority of the heat is required at low temperatures and can be supplied by atmospheric heat. The only heat duty in the liquids recovery unit that must be supplied at higher temperature is that of the distillation column reboiler, a small percentage of the overall heat duty required. An LFP design is presently being integrated in this fashion into a U.S. plant which uses a combination of air vaporization and SCVs for process heat and final LNG vaporization.
In the LFP designs, the residue gas (primarily methane) from the demethanizer (or deethanizer) is condensed and subcooled so that it can be pumped to pipeline pressure and then re-vaporized. This is accomplished by cross exchange of the residue gas stream with inlet LNG to condense and subcool the residue gas stream, and also to preheat the inlet LNG stream before it feeds the distillation column. To achieve this, the operating conditions of the inlet LNG and the residue gas must be such that the cooling curve characteristics are appropriate for the cross heat exchange step. In some instances, it can be advantageous to work expand the high pressure vaporized inlet to the distillation column and apply the power recovered to the residue gas prior to condensing and subcooling. This serves to ensure that total condensation of the residue stream can be accomplished, and minimizes the amount of power required to increase the stream to the required pipeline pressure by using pumping rather than compression.
A Mollier diagram illustrating the theoretical steps of an LFP design for ethane recovery is shown in Figure 4 on the following page. In the traditional re-gasification process, the cold LNG in the storage tank (A) is pumped to pipeline pressure (B) and then heated to vaporize it before it flows to the pipeline (C). When the LFP plant is integrated into the facility, a portion of the high pressure LNG is heated to an intermediate temperature (D) and supplied to the distillation column as a liquid reflux stream. The remainder of the LNG is heated to higher temperature (E), then work expanded in a turboexpander to the column operating pressure (F) to generate power per the enthalpy change along path E-F. After fractionation, the residue gas leaving the top of the column (G) is partially recompressed (H) in the booster compressor connected to the turboexpander, cooled (I) as it heats the cold LNG, pumped to pipeline pressure (J), and then flows to the vaporizers to be heated before flowing to the gas transmission pipeline (C).
For ethane recovery designs, compressing the column overhead to intermediate pressure before cooling it can substantially reduce the duty required to make the fluid suitable for pumping. Contrast the enthalpy change for path H-I to what would be required if the column overhead is cooled instead (path G-I'). In addition, the pumping power (path I-J) is now quite small, making the power requirements of LFP very low. Note that the power required to compress the column overhead to this intermediate pressure (path G-H) is small, so only a portion of the incoming LNG is required to power the turboexpander. This allows optimizing the power recovery more or less independently of the liquids recovery level.
Krunal Yuvaraj Bhosale
Chemical Engineer
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