Reprinted from MAR/APR 2013 LNGINDUSTRY

Analysing the weakest linkShane Hale, Emerson Process Management, Rosemount Analytical, USA, addresses the challenge of LNG analysis with gas chromatography technology.

T he use of gas chromatography as an analysis tool in the measurement and reporting of

LNG composition is both an industry standard and a unique challenge. The extremely low temperature of LNG and the problems with keeping

it in liquid form make sample handling complex, and the batch handling nature of the operation complicates reporting. The characteristics of the sample handling system and the chromatography capabilities should be designed specifically for LNG operations. The operator needs to be informed and

LNGINDUSTRY Reprinted from MAR/APR 2013

make careful decisions to ensure accuracy and long-term operational efficiency, while also avoiding costly disputes between the various stakeholders in the LNG fiscal transactions.

Sample handling – the weakest linkThe accuracy and reliability of LNG measurement is critical since the ship loading and unloading operations are highly time-sensitive and there are no second chances. The cost of keeping a ship in port is astronomical and delays due to measurement issues are unacceptable. Failures in a poorly designed or maintained sample handling system can result in inaccurate analysis and even damage to the measurement technology. An easy way to determine if the sample system is operating correctly is to look at the stability of the measurement.

The composition of the LNG at the ship-loading arms is usually very stable and the analysis results should show this. In loading operations, the composition from a single tank should be stable and large changes in composition should only occur if the source of the LNG changes. In offloading operations, the LNG in the ship’s tanks should be consistent and the composition change will be slight over the unloading operation as the pressure in the ship tanks decrease and the lower ends in the LNG boil off. However, if the composition reported by the gas chromatograph (GC) changes significantly from one analysis cycle to the next, it indicates that the vaporisation of the sample is not consistent and the analysis will not match the actual flowing LNG composition.

In order for the sample handling system to deliver an appropriate sample to the GC, the LNG must remain liquid up to the point of vaporisation and must then be vaporised uniformly into a single-phase vapour state. To reach this single-phase state, the sample must first go through a two-phase region in the sample handling system (Figures 1 and 2). If the sample is transported while in the two-phase region, the different velocities of the liquid and gas phases will cause the entire sample to change composition once it reaches the single-phase vapour state. If the LNG starts to vaporise in the sample lines, the nitrogen and methane will boil off first and produce pockets of gas in the liquid stream that reach the vaporiser at different times, resulting in varying compositions. The LNG sample reaching the vaporiser will consist of an unrepresentative liquid sample rich in the heavier components, as well as slugs of methane-rich vapour. The vapour sample leaving the vaporiser will be inconsistent, with dramatic variations in composition. This Figure 1. LNG phase diagram.

Figure 2. Example of LNG sample handling system.

faulty sample then goes into the GC and sample cylinders, and produces an incorrect analysis.

Using an accumulator to reconstitute the sample immediately after the vaporiser can help address some of these challenges, but it can only correct for small fluctuations. If there is substantial vaporisation in the sample lines, then large slugs of gas can actually insulate the vaporiser from the LNG, once again resulting in the sampled gas being unrepresentative of the actual flowing stream. Most modern installations use vacuum-jacketed tubing from the sample probe to the vaporiser, which is located within 7 ft (2 m) of the sample point to ensure the sample remains in the liquid phase right up to the vaporiser.

Once the sample enters the vaporiser, there are additional challenges to overcome. When the LNG sample enters the vaporiser as a pure liquid, sufficient heat must be added to the liquid sample to allow for over 600-to-1 expansion from the liquid phase to the gas phase, without causing sample fractionation. Recent advances in vaporiser design help address these issues. Traditional ‘water bath’ vaporisers are losing favour since they exacerbate the gas pocket formation and are relatively high maintenance. Vaporising regulators are common in the process industry and attempt to perform both the sample vaporising and pressure regulation functions. However, they lack both the heating capacity and volume expansion allowances required to do either job well for LNG, resulting in inconsistent vaporisation and selective fractionation of the sample. The more effective approach is to separate the vaporisation process from the gas pressure regulation function. The best performing vaporisers are designed specifically for LNG and flash a small liquid sample off quickly, providing very little restriction and allowing the sample to expand over 600 times in volume. The outlet temperature of the vaporiser should be monitored to prevent liquid carryover in order to ensure proper operation. The sample enters the accumulator after it is vaporised, and is then pressure-controlled and sent for analysis to the GC.

GCs designed for LNGWhen it comes to the measurement of LNG, the LNG operator needs to be aware of specific GC capabilities that will enhance accuracy and reduce operating costs. LNG composition is similar to natural gas, but has unique properties as a result of the process requirements involved in chilling the feed gas to the low temperatures necessary for LNG production. In pipeline gas, the CO2 levels are often controlled to just under 2% by the producers as this is the common tariff limit. However, the CO2 content in LNG is typically less than 50 ppm in LNG streams and must be kept this low to prevent solids from forming during liquefaction. The GC must be capable of accurately measuring the CO2 at these low levels and should have a lower detectable limit (LDL) of better than 25 ppm.

All design characteristics of a GC used for LNG measurement should be ruggedised for a marine environment as the GC is usually located on the dock close to the loading arms. In this environment, it is critical

that the analyser can hold up to the demanding salt-laden environment while taking up as little space on the dock as possible.

GCs used for LNG analysis should have an analysis repeatability of at least +/- 0.25 BTU per 1000 BTU (0.025% of energy value) to ensure accurate fiscal accounting. To reach this level of performance, some GCs are required to be housed in a temperature-controlled analyser shelter; however, this significantly increases costs, utility requirements and installation footprint. Alternatively, some GCs can operate at this level of performance across an extended temperature range (typically 0 ˚F – 130 ˚F/-17 ˚C – 55 ˚C) requiring very little protection from the elements (such as a sun-shield or three-sided shelter), which results in a much smaller installation footprint and lower utility and installation costs. As a result of the critical nature of the LNG custody transfer measurement, the end-user should require the manufacturer to demonstrate the repeatability of the unit purchased throughout the extended temperature range of the instrument.

The software criteriaAnalysis, reporting and communication software for LNG gas chromatography systems must be specifically designed for the application. GC software for laboratory applications is too complex for most non-expert GC users found in the field. The software for the online LNG GC should be simple and intuitive for the plant technicians and engineers to use effectively.

Unlike pipeline or process gas chromatographs that run 24/7, GCs used for LNG ship loading and unloading will only be run while the ship is in the dock. On a pipeline, an operator can calibrate and start the system locally since it will run continuously for long periods of time. For LNG custody transfer applications, the GC should be calibrated immediately prior to the ship loading, the analysis cycle started at the beginning of the loading/unloading operation and stopped at the end of the load, and the calibration validated after the load. To improve efficiency and save operator time, it is important that the GC can be remotely calibrated, started, stopped and validated from the control room. This means that the LNG GC must be able to communicate and provide the control mechanisms to host devices such as flow computers, SCADA systems and distributive control systems (DCS). The Modbus serial communication protocol is most commonly used over either serial or Ethernet communication links, because of the high number of values GCs report. A recent development is the use of Foundation Fieldbus for connection to the DCS for better interoperability with plant-wide diagnostic monitoring software with virtually no customisation. Whatever the protocol for host system communication, additional provision should be made to allow remote diagnostics for the GC-specific diagnostic software. Such capability allows highly trained personnel to analyse, diagnose and maintain multiple devices from a central location, often off-site. Reporting software must also be very specific to LNG requirements and provide the batch load averaging

LNGINDUSTRY Reprinted from MAR/APR 2013

functions and LNG specific calculations required by the custody transfer contracts.

Analysing the analysersThe measurement and reporting of the composition and energy values has a significant impact on the fiscal performance of LNG operations. Therefore, no operator can afford to take short-cuts or use incorrectly designed

systems for the analysis of the LNG. While often considerably more expensive than pipeline natural gas custody transfer systems, the up-front investment in the specialised LNG sample vaporisers and the LNG GC reporting systems will be quickly returned in the form of accurate and trouble-free analysis and reporting that avoids disputes over the value or quality of the LNG transferred.

Table 1. Typical compositions of LNG

Example A Example B Example C Example D Example E Example F

Methane 94.73% 92.3% 86.53% 89.94% 88.33% 91.8%

Ethane 3.8% 7.5% 12% 6% 6% 6%

Propane 1.17% 0.2% 1.33% 3% 4.3% 1.4%

iso-Butane 0.3% 0.06% 0.53% 0.5% 0.4%

n-Butane 0.08% 0.53% 0.5% 0.4%

iso-Pentane 0.37%

BTU 1063 1070 1124 1125 1154 1095

Wobbe index 1387 1391 1420 1420 1436 1404