Improving LNG density and calorific value calculations
The aim of this work package is to reduce uncertainties in the calculation of LNG density. This will be achieved by validating density calculation methods and by assessing the impact of non-ideal mixing in ship tanks on the calculation of transferred energy. A primary LNG densitometer will be developed in task 4.1. with an order of magnitude better uncertainty down to 0.02% compared with 0.2-0.3% for systems that have been used in the past decades to produce significantly improved density reference data in task 4.2. The validity of these results will be checked with additional reference data that is produced with an alternative cryogenic pycnometer method in task 4.3. The thus obtained consistency checked set of new reference data will be used to validate or improve the many existing LNG density equations of state in task 4.4. The many possible implementations of density calculation methods, the non-homogeneous conditions in LNG tankers and variable reference conditions have a large impact on uncertainty and financial risks. This will be quantified by conducting a thorough study in tasks 4.5 and 4.6.
The density calculation from known composition can be calculated using many different equations which results in uncertainties between 0.3% and 1% depending on the composition
Description of Work
Task 4.1: Development of a primary LNG densitometer (RUB, E.ON Ruhrgas, VSL, PTB)
The aim of this task is to develop a primary LNG densitometer based on preceding work that resulted in a conceptual design of an accurate LNG densitometer. Detailed feasibility studies have shown that this new densitometer enables measurements in the whole liquid phase (saturated and homogeneous liquid) of LNG and LNG-like mixtures with excellent accuracy (+/-0.02% in density) and without phase equilibrium related decomposition of the liquid in the measuring cell. For details on the concept and related references see the included REG application.
Based on this conceptual design and other preceding work, the detail engineering including the detailed thermal layout and the control and measurement concepts will be developed. A report illustrating the design of the new densitometer in detail will be delivered [D4.1.1]. Some essential parts with long delivery times will be ordered in advance (in parallel to the detailed design) [D4.1.2]. The new densitometer will be completed within 12 months, including test measurements on methane. Operability of the main components of the densitometer will be reported by month 20 including a validation by VSL of the liquefaction of natural gas reference material [D4.1.3]. Traceability to SI will be established at the highest level by very accurate calibration of the sinkers at the beginning of the measurements and end of the project [D4.1.4]. A report on the complete validation of the new densitometer will be delivered by the end of month 24 of the JRP (month 18 of the REG) [D4.1.5.]
This task will be active from month 6 until month 24.
Task 4.2: Produce experimental reference data with LNG densitometer (RUB, E.ON Ruhrgas)
The aim of this task is to use the new LNG densitometer to measure reference densities for at least four LNG-like mixtures [D4.2.1 and D4.2.2], gravimetrically prepared with the highest achievable accuracy and best traceability by VSL. This way, a set of traceable reference data for the density of LNG will become available. The uncertainty of the new data will be at least one order of magnitude smaller (down to 0.02%) than the uncertainty of the existing data (0.2-0.3%.) For the first time, densities of the homogeneous liquid will be described consistently to the density of the saturated liquid. To date only density data for the saturated liquid are available in the literature. For typical LNG compositions the new data set will form a whole new basis for the evaluation and development of density calculations and for the calibration of secondary density measurement devices.
This task will be active from month 25 until month 36.
Task 4.3: Produce experimental reference data with LNG pycnometer system (INRIM, VSL, RUB)
The aim of this task is to extend the pycnometer density standard at INRIM (currently operating between -15°C and 150°C) for operation at LNG temperature (-163°C) with a target uncertainty of between 0.1% - 0.01%. INRIM will design and set up an apparatus in which a sample cell of known volume (pycnometer) is connected to a reservoir of LNG by means of a valve [D4.3.1]. The whole circuit will be housed in a temperature-controlled cryostat. Before assembly of the circuit, the cell volume will be measured exactly at the operating temperature and its weight including the valve will be determined [D4.3.2]. After assembly, a low vacuum is drawn inside the cell and in the line that connects it to the reservoir. The cryostat is set at the working temperature and the reservoir is gently filled to ensure that the fluid remains in liquid phase. The process of liquefaction of natural gas reference material will be validated by VSL. After the filling of the measuring cell, the cell and the valve will be weighed again. Experimental reference data will be produced for at least four LNG-like mixtures similar to the ones used for the LNG densitometer [D4.3.3]. The pycnometer reference data will be compared with the densitometer reference data [D4.3.4].
This task will be active from month 13 until month 36.
Task 4.4: Validating and comparing Equations of State used for density calculation (RUB, VSL, TUV NEL, Elengy, E.ON Ruhrgas, INRIM)
The aim of this task is to use the results from task 4.2 and 4.3 to validate and improve the methods for calculating the LNG density based on composition [D4.4.1]. The results of the validation will be used as input for the development of guidelines and standards (WP5)
An overview of all relevant equations and methods for density calculation will be produced by Elengy. The financial implications of different methods being applied to a cargo will be evaluated. By analysing the information compiled, the most accurate method will be recommended for custody transfer. This will be based on agreement with available experimental measurements from task 4.2 and 4.3. [D4.4.2].
This task will first be active from month 3 until month 36.
Task 4.5: Calculation of LNG Calorific Value at different reference conditions and the impact on total energy transferred (TUV NEL, PTB)
The aim of this task is to compare calculations of LNG Calorific Value at various reference conditions (Temperatures: 0, 15, 20 and 25°C) used by available standards and clearly demonstrate the consequence of using one reference condition against the other on the calculated energy transfer. Real examples of LNG cargoes will be used for these calculations [D4.5.1]. An accurate calculation of the energy content of LNG in the real fluid state is not described in any standard or guideline. The correction related to the enthalpy difference between the liquid state and standard conditions is significant, contrary to the case of pipeline gas. To calculate this correction accurate information on the composition of the LNG and on enthalpy differences between the actual state of the liquid and the gaseous reference state of the pure component calorific values is needed. Based on the work in task 3.1 and task 4.4 the combined uncertainty budget for calorific values and enthalpies of formation under the real LNG pipeline conditions will be developed and made available to WP5 [D4.5.2].
This task will be active from month 6 until month 26.
Task 4.6: Assessing impact of temperature, composition and density gradients in tanks on the total measurement uncertainty (TUV NEL, Elengy, ENAGAS)
The following real data will be collected from the field [D4.6.1]:
The two data sets will be used to perform an analysis of impact of temperature and composition gradients in tankers on the density and gross heating value calculations. The corresponding contribution to the overall uncertainty of LNG energy transfer will be estimated [D4.6.2].
This task will be active from month 10 until month 22.
Summary of Deliverables for WP4
|Deliverable number||Deliverable description||Lead Participant||Other Participants||Delivery date|
|D4.1.1||A report illustrating the design of the new densitometer in detail is completed.||RUB||PTB, E.ON Ruhrgas||APRIL 2011|
|D4.1.2||Essential parts with long delivery times are ordered.||RUB||DECEMBER 2010|
|D4.1.3||Densitometer test measurements on methane completed and passed. Operability of the main components of the densitometer reported including a validation of the method of liquefaction of natural gas reference material.||RUB||VSL, PTB||DECEMBER 2011|
|D4.1.4||Traceability to SI established at the highest level by very accurate calibration of the sinkers.||PTB||DECEMBER 2011|
|D4.1.5||A report on the complete validation of the new densitometer is completed.||RUB||E.ON Ruhrgas||APRIL 2012|
|D4.2.1||Experimental reference data measured for first two LNG-like mixtures.||RUB||E.ON Ruhrgas||OCTOBER 2012|
|D4.2.2||Experimental reference data measured for two other LNG-like mixtures.||RUB||E.ON Ruhrgas||APRIL 2013|
|D4.3.1||Cryogenic pycnometer is completed and functionality tests have passed.||INRIM||OCTOBER 2011|
|D4.3.2||Pycnometer volume accurately measured at the operating temperature and its weight (incl. The valve) is determined.||INRIM||APRIL 2012|
|D4.3.3||Experimental reference date measured for at least four LNG-like mixtures.||INRIM||VSL||OCTOBER 2012|
|D4.3.4||Degree of equivalence established between densitometer and pycnometer LNG reference data.||INRIM||RUB||APRIL 2013|
|D4.4.1||Equations of State validated. (the selected EoS have been adjusted to better match reference density data).||RUB||E.on Ruhrgas, VSL||APRIL 2013|
|D4.4.2||Comparison of multiple density calculation methods with experimental reference data.||TUV NEL||Elengy, INRIM||JUNE 2011|
|D4.5.1||Report with calculations of differences between LNG calorific value at various applied reference conditions.||TUV NEL||JUNE 2011|
|D4.5.2||Report with uncertainty budget for calorific values and enthalpies of formation under real liquid conditions.||PTB||JANUARY 2012|
|D4.6.1||Data collected on temperature & composition under real circumstances.||Elengy||TUV NEL, ENAGAS||JUNE 2011|
|D4.6.2||Uncertainty estimation completed for LNG energy transferred based on data from 4.6.1.||TUV NEL||Elengy, ENAGAS||FEBRUARY 2012|