The LNG and BOG enter the packed section. This type of BOG recondenser works by adjusting the amount of packed-section area available for condensing, by means of partial flooding of the packed bed. To achieve partial flooding while ensuring a constant level and net positive section head (NPSH) for the high-pressure (HP) pumps, a physical separation between the two packed sections and the annular space is required.
The level and pressure in the annular section are kept constant. The level and pressure in the packed section can be distinct from those in the annular section. Both the pressure and level in the packed section self-adjust in response to varying BOG/LNG ratios.
At high BOG/LNG ratios, the liquid level in the packed-bed section decreases automatically, through pushing of liquid to the annular space, as the pressure in this section increases. This process exposes more of the packed bed, increasing the heat transfer/condensation area to create equilibrium conditions-commonly referred to as the "auto-regulating" effect.
Likewise, a lower BOG/LNG ratio tends to reduce pressure in the packed section, which then results in a level increase in the packed section. In other words, this type of recondenser tends to adjust itself. Both the level and pressure in the two sections are different.
Pressure control. For stable operation of the BOG recondenser, the BOG compressor discharge pressure and annulus pressure must be controlled. Also, the annulus level and the BOG recondenser outlet temperature must be guarded to ensure sufficient NPSH for the HP pumps. The following examples of pressure control have been proven in the industry.
Level and total volume control. To ensure constant HP pump conditions, the LNG level in the annular space of the BOG recondenser is controlled via manipulating the LNG inlet valve to the packed-bed section of the BOG recondenser. The level controller must be tuned so that moderate fluctuations in the BOG recondenser level are allowed to prevent unacceptable disturbances to the LP pump operation. The control valve should have a high turndown ratio and high resolution.
As discussed previously, the level in the packed-bed section is not directly regulated; it will vary as a result of the BOG/LNG ratio. The higher the BOG/LNG injection ratio, the less contacting area is required for recondensation, and the higher the level will reach. When there is less contacting area, the pressure will rise and the level will decrease due to the increased pressure.
Conversely, when the BOG/LNG ratio is high, the pressure in the packed section will rise and the level will drop. This results in an increase of the available area for recondensation; the pressure will fall, and the level will rise again.
A practical result of this floating packed-section level is the following inverse-response phenomenon. If the BOG/LNG ratio increases, then the level in the packed bed will decrease, and the level in the annulus will initially increase because the pressure in the central section increases (pushing LNG from the packed section into the annular space), which forces the level controller (LC) to close the LNG inlet valve. This reduces the level in the packed-bed section even further.
To avoid this inverse response of the level control system, an LNG total volume control, based on both the level in the packed section and the level in the annulus, has been developed and implemented. This response, which is potentially caused by uncontrolled introduction of padding gas, may displace liquid from the packed-bed section to the annulus and vice versa, making the simple annulus level an unstable parameter for controlling the flow of LNG into the recondenser.
For total volume control, liquid levels in both the BOG recondenser annulus and the BOG recondenser packed bed will be measured and used for calculation of total LNG volume in the BOG recondenser. The calculated total LNG volume, rather than the level in the annular space, is controlled by regulating the flow of LNG to the packed bed. By using total volume control, any transient conditions are ignored and the inverse response is eliminated.
The total volume of LNG in the BOG recondenser is calculated as shown in Eq. 1:
LNG volume = Vf × π × Rb2 × Lb + (π Rv2 − πRb2) × La(1)
Vf=Volume of fractional voids
Lb=Core section level