TOPNIR LNG ONLINE ANALYSIS ALLOWS CONTROLLING CUSTODY TRANSFER AND ENABLES ADVANCED PROCESS CONTROL

1. Introduction

LNG quality is measured and classified by High Heating value (HHV), which is a characteristic of energy content, and by Wobbe Index (WI), which is used for Gas interchangeability in burning processes. Requirement in WI is different in the Far-East with WI generally greater than 52 MJ/m3 and in the UK and USA with a lower WI.

LNG is one of the energy sources showing continuous growth in parallel with global demand. Analysts predict the production capacity to double by 2012 and demand to grow about 8% per annum until 2015. In addition, new markets are opening rapidly in Europe and Asia as gas price is attractive by comparison with the price of crude oil. The main LNG reserves are located in the Middle-East, Australasia and Africa.

The stages involved in the LNG chain are exploration, liquefaction, shipping, storage and regasification. LNG is found in gas state during the exploration stage. It is then processed and liquefied in a LNG train. Liquid LNG will be stored at the LNG terminal then loaded onto dedicated vessels for transportation. LNG will be unloaded at the reception terminal and stored. LNG will finally be regasified so that it can be transported by pipe to the end users.

As LNG is transported and stored in liquid state at temperatures below -150°C, this imposes constraints on the metering systems used to measure accurately the quantities produced at the offshore plant, as well as the quantities received or exported at a jetty. The same physical constraints will apply on the analyzer systems used to measure the quality of the product on line.

Topnir has designed an online system that can be used at the jetty during custody import/export transfer. The system can also be used at the production plant for online monitoring and control of the LNG quality in conjunction with Advanced Process Control.

2. The Topnir online system for LNG

The sample probe is a device inserted into a LNG pipe used to sample LNG on a continuous basis and the vaporizer is used for the liquid to gas total transformation of sample feeding the Sampling System and Gas Chromatograph.

The sampling system allows storing samples in containers for the purpose of analysing LNG in the laboratory. At any stage during this entire online measurement process, LNG product shall not be fractionated into a liquid and a gas phase. The Opta-Periph solution is to control the sample liquid to gas transformation in supercritical state such that the vaporization process does not cause fractionation. This is a key condition to obtain the performance of 0.01 mJ/m3 repeatability in accordance to the ISO 6976 standard.

2.1 The sample probe

Figure 2 1: Picture of the Opta-Periph sample probe

The LNG product flowing in the process line is subcooled such that no vaporization is occurring in the pipes. The amount of energy that the LNG product will receive in the sample take-off to vaporizer inlet shall be minimized such that the sample will stay in liquid phase. In order to achieve this target, it is essential to determine the degree of subcooling of the sample and compare the result with the amount of energy received in the sampling probe up to vaporization point. Two calculations are provided hereafter, one reproduced from the ISO 8943 standard, one related to the Opta-Periph probe installed on Topnir application.

Table 2 1: LNG Data

In addition, the rise in enthalpy of the LNG sample, due to heat absorbed in the sample line, is calculated using the following equation:

Where:

enthalpy rise, in J/kg

absorbed heat, in W

flow rate, in kg/h

The Degree of Subcooling R is determined from Figure 2 2 displayed hereafter. For the ISO 8943 case, R is found to be 27,000 J/kg, and 51,000 J/kg for the Topnir case. Indications displayed on top of Figure 2 2 are related to the ISO 8943 case.

Figure 2 2: Enthalpy of the saturated liquid

For the two cases, the following results can be obtained:

Table 2 2: Enthalpy results

2.2 The vaporizer

Figure 2 3 below displays the pressure/temperature diagram for a LNG containing approximately 90% methane, where:

B Cricondenbar, in bar

T Cricondentherm, in °C

C Critical point, in bar

Figure 2 3: Pressure/Temperature Diagram

The objective is then to transform LNG from liquid to gas state in supercritical conditions at very high pressure (here above 75 bars), such that the LNG sample goes directly into the desired gas state. Such transformation is represented on Figure 2 3 by the dotted line.

Within the Opta-Periph sample probe and vaporizer, the following operations are taking place (see Figure 2 4):

" The temperature rises from -160°C to -150°C in the capillary tube, at LNG process pressure (minus some pressure drops in the sample line)

" A supercritical cell featuring pressure reductor/check valve integrated at the inlet of vaporizer coil has been designed for flashing at that point the liquid sample without back pressure effect to process line

" The vaporization in this cell (indicated as "critical point in the figure 2-4) of approximately 0.5 cc increases the temperature and the pressure, and leads to conditions of approximately 80 bars and -100°C

" The temperature is controlled at +65°C in the vaporizer, in order to obtain approximately +55°C at the outlet of the vaporizer

Figure 2-4: Topnir system application

Similarly to the sampling probe, no fractioned vaporization is taking place in the Opta-Periph vaporizer, ensuring that the required sample quality and stability is maintained carefully before being sent to the Gas Chromatograph.

2.3 The Gas Chromatograph

Figure 2 5: Gas Chromatograph

Note: The definition of the Wobbe Index is:

Where:

WI Wobbe Index, in MJ/m3

HHV High Heating Value in, MJ/kg

Rd relative density of the gas to air, in kg/m3

2.4 The aggregate autosampler

Figure 2 6: Aggregate autosampler

The Topnir online system for LNG can be located at an import/export jetty as well as part of an onshore production plant. Monitoring the quality of LNG during custody transfer at a jetty is critical to ensure that the transaction is in line with the terms of the commercial contract as the prediction of sample aging process during the cargo transportation cannot be accurate due to all modelling parameters to be considered. Monitoring the quality of LNG at a liquefaction plant is enabling the control of the LNG quality in real-time either through operator manual control, or through automated Advanced Process Control.

3. Applying APC to LNG plants

By definition, APC is a technology with multivariable, closed-loop model predictive control with constraint handling and some linear optimization. APC is multivariable as the controller accounts for the dependencies of several control loops in the process. APC is model predictive as it is aware of all the necessary dynamic relationships between independent and dependant process variables. APC is a closed-loop system capable of sending setpoints every minute to the regulatory control layer and correct itself in feed-back. Lastly, APC embeds a linear optimizer (LP or Linear Program) capable of defining the optimum operating point of the plant while honouring constraints.

3.1 The LNG process

Figure 3 1: LNG unit block

The inlet facilities section is designed to receive feedstock from the 2-phase feed pipelines produced by the offshore production facilities, and to provide gas/condensate separation. In order to protect aluminium in the main cryogenic heat exchanger (MCHE), mercury has to be removed from LNG in the gas treatment section before sending the gas to the liquefaction units. The Acid Gas Removal section is designed to remove hydrogen sulphide, carbon dioxide and other sulphur compounds by means of chemical/physical absorption from gas coming from the treated gas. The dehydration section is designed to dry the gas leaving the acid gas removal section, which is saturated with water, using molecular sieve driers. The objective of the gas chilling and liquefaction section and Refrigeration section is to produce Liquefied Natural Gas (LNG) from the treated natural gas. Refrigeration section consists in a propane circuit and a mixed refrigerant (MR) circuit. Liquefaction is achieved in the main cryogenic heat exchanger (MCHE). The purpose of the Refrigerant Preparation section is to produce ethane and propane suitable for refrigerant make-up, to separate heavy hydrocarbon (C5+) to avoid sending them to the MCHE and to produce a stabilised plant condensate. The function of the Nitrogen Rejection section is to flash the nitrogen from the LNG product and deliver the nitrogen rich gas to the fuel gas system. Finally, the Sulphur Recovery Unit is designed to recover sulphur (mainly under H2S form) contained in the acid gas feed from the Acid Gas Removal section, and to convert it into liquid sulphur.

3.2 APC objectives

* Absorb disturbances from offshore operations in the Slug Catcher

* Stabilize condensate stripper and ensure good stripping

* Maximize valuable products by running at high vapor pressure

* Minimize overall energy usage

Figure 3 2: LNG Inlet Facilities

The general operating objectives for the LNG liquefaction controller are typically:

* Maximize LNG production when running in maximum production mode

* Maximize valuable products when running at fixed feed rate according to relative prices of LNG, plant condensate and field condensate

* Run the temperature at the top of the Main Cryogenic Heat Exchanger against a target when so desired

* Avoid sending heavy hydrocarbons (C5+) or water in the MCHE

* Maintain the product qualities of LNG (HHV, WI) on specification

* Minimize overall energy usage

Figure 3 3: LNG liquefaction

3.3 APC design around MCHE

* Control the pressure in the Scrub column and LNG inlet to cold bundle

* Control the content of C5+ in the scrub column overhead

* Ensure Mixed-Refrigerant (MR) circuit is providing the appropriate duty to the MCHE

* Control the temperature profile along the MCHE

* Honor MR compressor constraints

* Honor Propane compressor constraints

* Control the quality of LNG at the Nitrogen Rejection section with balancing the re-injection of LPG

* Maximize LNG production, subject to exit LNG temperature target

The APC controller generally manipulates the 2 Joule-Thompson valves and the MR compressor speed in order to set the appropriate circulation rate of MR. This strategy will allow achieving the desired temperature at the top of the MCHE, while maintaining the temperature profile along the MCHE (warm, middle and cold bundles). Additional constraints might give a limitation to the MR circulation rate, such as maximum delta P limit in the cold bundle, compression ratio range and MR separator overhead maximum flow. The APC controller will also ensure that the MR compressor constraints - typically maximum exhaust temperature, distance from surge at each stage, suction pressure - are kept within limits. In order to achieve this, the APC generally manipulates the speed of the MR compressor and the kickback valves. For quality control purpose, the APC controller typically manipulates the suction pressure of the fuel gas compressor, as well as the flow of LPG re-injection from the Refrigeration Preparation section. The fuel gas compressor can be amperage constrained. If the MCHE exit temperature or the plant throughput starts to rise, then the load on the fuel gas compressor will start to increase. A typical case is that the fuel gas compressor reaches its amperes limit, which means that the throughput should not be further increased. During LNG maximization mode, the APC maximizes gas flow to the plant, while controlling MCHE top temperature, LNG HHV, WI and nitrogen content, as well as MR and fuel gas compressor constraints.

3.4 Benefits from APC

The Figure 3 4 hereafter illustrates a real-life application of APC on a LNG train. In the top part, the graph is showing a decrease in throughput decided by the plant supervisor due to ship loading logistical constraints. The MCHE temperature is kept very accurately at the desired target - here -147°C - as the APC adjusts automatically the 2 J-T valves (middle graph) as well as the MR compressor speed (bottom graph). All other process constraints and products qualities, including LNG WI and HHV, are kept safely within operating limits.

Figure 3 4: APC in action

4. Conclusion

The Topnir online system allows accurate measurement of WI in the liquefaction LNG trains, and therefore enables the successful implementation of Advanced Process Control. For the first time in this plant's history the MR speed loop and two JT valves were controlled automatically to achieve a desired LNG flow rate, temperature and WI quality. After a post-audit of the APC application, the benefits were quantified accurately and exhibited a project pay-back of 6 months.