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  Poster PO-13 PO-13.1 COST EFFECTIVE VAPOR HANDLING SYSTEMS IN AN LNG RECEIVING TERMINAL Joseph H. Cho Technology Advisor Heinz Kotzot Chief Technology Professional Leader David Coyle Technology Manager - LNG/Gas Processing/Alt Fuels Charles Durr Energy Technology Kellogg Brown & Root, Inc. Houston, Texas 77002  ABSTRACT The transfer of liquefied natural gas between the LNG storage tanks and the LNG ship during unloading will always result in significant vapor generation. The vapor quantity is subject to energy input from varies sources and changes as the unloading  proceeds. The vapor system in an import terminal needs to be designed for maximum vapor handling, but also needs to avoid over-design, considering capital investment and operation costs. The dynamic interaction between the storage tanks and the ship tanks during unloading have been investigated and analyzed based on actual operation data. This paper describes the relationships of the many parameters that result in vapor generation and  provides process design guidelines for the vapor handling system. The process of optimizing technology of the vapor handling system is also discussed in this paper.  Poster PO-13 PO-13.2 INTRODUCTION Boil-off Gas (BOG) generated in the receiving terminal consists mainly of methane and nitrogen. This BOG is a result of heat leaking through the tank walls, equipment, and associate piping system. The LNG pumping energy also contributes to BOG generation. However, the major portion of BOG is the displaced vapor in the onshore tank when filling with liquid. There is a big difference in vapor generation between the unloading mode and the holding mode (non-unloading operation). The BOG handling system is generally designed to meet the BOG generated during unloading operation. The important parameters that affect BOG generation volume are liquid transfer rate, operating pressure differences between two liquid containment systems and heat leak rate [1]. LNG loading/unloading causes interaction between two containment systems, resulting in changes of operating pressure and temperature of the containment systems (onshore storage tank and ship tank) as loading/unloading proceeds. During LNG unloading operation at an import terminal, the onshore tank operating pressure increases while LNG unloading from the LNG ship. A considerable amount of vapor generated by displacement of loaded LNG in the onshore tanks is sent back to the LNG ship to maintain a positive pressure. The remainder of the BOG is compressed and recondensed into send-out LNG. Consideration of this interaction between two containment systems of onshore tanks and a ship tanks is required to calculate the BOG during normal operation. If the estimated BOG rate is too high a large capital investment would be underutilized. Conversely, if the estimated BOG is too low, it may be inevitable to flare/vent the excess BOG during unloading operation. This paper analyzes the interaction between two LNG containment systems during unloading operation based on actual operation data. This LNG unloading interaction data can be used to better predict BOG generation during unloading and improve engineering practices.   FACILITY DESCRIPTION The Tong-young receiving terminal is a base load facility, which is the third terminal owned and operated by Korea Gas Corporation. LNG import capacity is about 10 million tones per annum (MTPA). Two types of regasifiers have been installed: Open Rack Type Vaporizer (ORV) and Submerged Combustion Vaporizer (SCV). The main facilities are: ã ORV: 6 units each for 200 MMScfd (equivalent to 180 t/h) ã SCV: 3 units each for 100 MMScfd (equivalent to 90 t/h) ã LNG Storage capacity: 10 tanks, each for 140,000 m 3   ã Unloading Jetty: for one ship unloading, equipped with three 16” liquid arms and one 16” vapor arm. ã Boil off Gas Compressors: 12,000 Nm 3 /h Technical data of the LNG Ship, from which operation data was obtained. ã Tank Type: membrane ã Capacity: 145,000 m 3   ã Number of cargo tanks: 4 with 2 pumps each ã Pump capacity: 1,450 m 3 /h x 145 mlc The designed unloading rate of the terminal is 11,000 m 3 /h. This is based on 12 hours unloading from a 135,000 m 3 /h ship. Figure 1 illustrates the simplified process flow diagram of an LNG Receiving Terminal.  Poster PO-13 PO-13.3  Figure 1 – LNG Receiving Terminal Simplified Process Flow Diagram UNLOADING CONDITIONS  A ship vapor saturation pressure of 174 mbarg was calculated based on the arrival LNG compositions and its temperature. N 2  concentration reduced from 0.67 mol% to 0.38 mol% during the 14-day voyage from Qatar to Tong-young terminal, Korea. The LNG Compositions at data collection point are tabulated in Table 1. Table 1 – LNG Compositions Compositions Loaded onto ship (mol %)  Arriving at Tong-young (mol %) N2 C1 C2 C3 i-C4 n-C4 0.67 92.95 6.03 0.33 0.01 0.01 0.38 92.98 6.24 0.36 0.02 0.02 Weather conditions are: ã  Ambient temperature: 29 – 34 ºC ã Wind: 3 – 5 m/s DATA COLLECTION Normally the vapor flow returning to the ship is not measured at the terminal, therefore the vapor balance was calculated from ship’s operation data, which were logged in 10 to 30 minutes intervals over a period that started during preparation for unloading until completion of ramp down. In order to investigate interaction between ship cargo tanks and onshore storage tanks, all main operating data in the receiving terminal were logged. The terminal operating data were obtained in 10 minutes intervals. This data collection started two hours prior to ship arrival at the dock and lasted until six hours after ramp down. Ship data were then compared against the terminal operating data and synchronized to be usable for data analysis. LNG Carrier    LNG Unloading   Arm   Vapor Return LineBoil-Off GasCompressor LNG Storage Tanks1 st  StageLNG PumpsRecondenser 2 nd   StageLNG PumpsFuel Gas   Vaporizer    s To Pipeline  Poster PO-13 PO-13.4 OPERATION DESCRIPTION During holding mode (no offloading operation from a ship), the BOG is pressurized by two or three compressors and re-liquefied by the two recondensers, which are operating in parallel. For unloading operation, two more compressors are started 6 – 8 hours prior to LNG ship arrival at the terminal. These extra compressors reduce the tank operating pressure from 230 mbarg to about 200 mbarg to provide some pressure cushion during the initial unloading operation.  After the ship completes the preparation for unloading of LNG, the terminal operator closes LNG recirculation valve as the ship ramps up LNG unloading. The LNG remaining in the recirculation lines, approximately 2,000 m 3,  are returned back to LNG storage tanks. This LNG in the recirculation lines is warmer than LNG in the send-out line. During the initial ramp-up period, this confined LNG is pushed back to storage tanks by the unloading LNG, generating a large amount of initial flash vapor. It might require starting additional BOG compressors, which will be determined by the plant operator depending on tank operating pressure. After this initial flash vapor generation, the BOG rate caused by displacement of unloaded LNG normalizes. The ship tank pressure gradually increases in the beginning of unloading due to heat leak. This operating pressure then decreases as LNG unloading is ramped up. The vapor return from the onshore tanks to the ship starts when the return vapor control valve located on board of the ship is opened. However, the vapor return valve does not open until the tank pressure drops below a threshold pressure. The valve opening of the vapor return line is manually controlled to maintain the ship tank pressure fairly constant during the unloading operation. The onshore tank operating pressure is sufficient to transport return vapor to the ship, so no gas blowers are needed for the 2,200m vapor return line. The balance is pressurized by BOG compressors and routed to the recondensers, where it is recondensed by heat exchange with send-out LNG. DATA ANALYSIS LNG UNLOADING  After preparation for unloading and line hookup is completed, it takes about two hours for the stripping pump to cool the LNG manifold and the unloading arms. After cooldown a cargo pump is started with total spill back into the ship tank by opening its kickback valve. Gradually, the unloading valve is opened and the recycle valve closed. The same procedure is applied with the other cargo pumps. Pump out flow is adjusted by monitoring pump motor ampere meter. Figure 2 illustrates unloading flow rate. During the initial stage of unloading, the flow rate is slightly higher than in the later stage. Ramp-up to full unloading rate and ramp-down takes about 1 hour each. Actual unloading rate is slightly higher than the design rate. The unloading rate gradually decreases when a pump in a small cargo shuts down with a low level of LNG in the ship tank. The ship tank vapor pressure is about 175 mbar when the ship arrives. During preparation of LNG unloading, the vapor pressure increases gradually. At the start of unloading, the vapor pressure decreases and is maintained around 120 – 130 mbarg (Fig. 3). The vapor pressure is directly affected by the return vapor flow rate. This is especially illustrated at point A in Fig. 3 and Fig. 4; when the vapor return flow and the vapor pressure fluctuate simultaneously. The vapor temperature shown in Fig 4 is measured in the ship tank. The vapor temperature measured at the jetty head is about 10 ºC lower. This temperature rise is due to heat leak in the vapor return arm and piping on ship deck. The vapor temperature decreases as unloading operation progresses which raise return vapor mass flow at constant volumetric flow. The peak vapor return flow is observed approximately at half time of the unloading phase.
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