Introduction to LNG sampling at low pressure
A typical LNG mixture start to boil at -260°F (-162°C) at atmospheric pressure. For an application like LNG sampling at low pressure with Ship-to-Ship (StS) transfers, the typical pressures in the cross over manifolds are below 1 barg. This means that the LNG nearly start to reach its boiling point prior entering the sample probe.
ISO 8943: 2007 stated;
6.2.1 Sample probes shall be located at points in the pipeline where the LNG is in a sub-cooled condition.
The degree of sub-cooling at a sampling point shall be ascertained by observation of the temperature and pressure of the LNG at that point and comparing the temperature with the boiling point of the LNG at the same pressure as calculated from the composition of the LNG (see Annex A). In the case of multiple transfer lines, the sample probe shall be located downstream of the manifold, if one exists. Otherwise, each line shall be provided with a sampling point. Where multiple lines are provided with individual sampling points and the flow rates in the respective lines differ, the flow rate in each line shall be measured and the sample flows made proportional to these rates.
6.2.2 Sample probes shall be located at a point where the degree of sub-cooling is high.”
But what if the LNG will is near its bubble point and could never reach a sub cooled or a minimal sub-cooled condition?
How is it possible to sample representatively and vaporize to a homogeneous gas mixture for sampling and analysis purpose?
The solution is known and possible by sub-cooling the LNG probe and transfer line to its vaporizer by using the LNG’s latent heat or enthalpy. This paper will describe how sub cooling can be done by a combined Probe/Vaporizer.
Theoretical background of LNG sampling at low pressure
Based on a composition of natural gas and its Equation of State (EoS) a phase diagram can be generated as shown below. Based on the Peng-Robinson EoS, the LNG temperature must be below its bubble point (light blue line) prior to the LNG entering the vaporizer. This is what is called the sub-cooled region. The degree of sub-cooling is a function of the LNG composition, temperature and pressure.
So the lower the LNG pressure, the closer the LNG will be to its bubble point.
During the 5th Biennial NGSTech Conference New Orleans, USA. January 20 & 21, 2016, we presented a paper;
EXTREME SAMPLING: LIQUEFIED NATURAL GAS (LNG), by H-P Visser, Analytical Solutions and Products (ASaP) and Christiaan Mul, Analytical Solutions and Products (ASaP).
In this paper and presentation we described the financial impact (losses) and typical root causes of wrong LNG vaporization and LNG sampling at low pressure. All vaporization errors were due to the fact that the LNG passes it bubble point prior the analytical vaporizer itself!
Combined LNG Probe Vaporizer
In order to sample and vaporize LNG properly with the lowest uncertainty it is essential to keep the distance between the tip of the probe till the vaporizer as short as possible. As a matter of fact the vaporizer should be close-coupled with the probe without having thermal flow from the heater to the probe.
The photo and figures below illustrate suits an close couple probe vaporizer.
The combined Probe/Vaporizer (CPV) consists of three main sections as given in the illustration below.
The main sections and their functions are;
- Cold section: extract a sample, created sub-cooled conditions, control flow, transport sample to vaporizer.
- Thermal expansion section: eliminated material stresses between the cold- and heater sections, increases flow in order to eliminate back mixing.
- Heater section: flash evaporate LNG and mix the natural gas to a homogeneous mixture.
During the transfer from the tip of the probe to the vaporizer entrance must maintain a 100% bubble free liquid (LNG) state. The only way to achieve this to transport the sample under sub-cooled conditions. This apply for all cases but especially in the case of LNG sampling at low pressure while it reaches its boiling point.
In order to create sub-cooling, a small proportion of LNG is used. In the illustration below the dark blue line is representing the LNG needed for analysis. A slipstream of the LNG is filling a container around the LNG for analysis, represented by the light blue area. The light blue area is exposed to atmospheric pressure or the Boil Off Gas (BOG) header pressure.
By lowering the pressure of the LNG used for sub-cooling, the LNG will start to boil. In order to start to boil the LNG needs energy. Since it is an adiabatic system, the energy needs to come from its surroundings and its core, which is the LNG transfer line for analysis.
In the enthalpy diagram above the maximum degree of sub cooling is given and expressed as -5000 J/kg.
Test at a truck loading facility of Rolande LNG
In order to prove the theory and visualize the power of sub cooling, a test was performed at a truck loading facility of Rolande LNG in The Netherlands.
Two identical Phazers (commercial name of the combined Probe/Vaporizer) where installed at the filling manifold. Both Phazers were sampling the exact same LNG at the same time. The vaporized LNG of both Phazer were analyzed by the same analyzer (switching between the two Phazers).
LNG sampling at low pressure
By one of the Phazers the sub-cooling was shut-off (Phazer 1) while the other Phazer was in operation with sub-cooling (Phazer 2).
The results of the ethane concentration reading in the graph below show clearly that Phazer 1 start have negative influence of pre-vaporization while Phazer 2 has a perfect reading due to sub-cooling of the sample.
Results during actual StS transfers
In the period from march till end October nearly every week a Ship-to-Ship transfer took place on the FSRU moored in Dubai. The FSRU is shown on the photo below on the right hand side.
Below the trend of the Gross Calorific Value is given during the offloading of the LNG. It is important to mention that the LNG transfer pressure was as low as 0,8 barG
The buying joined venture party applies a so-called Minimum Performance Criteria, in order to judge every LNG transferred cargo.
Based on the analytical data gathered from the Phazers following statistics were generated.
The results were good and meet the minimum performance criteria, they also point to another interesting phenomenon; the enriching of the LNG, sometimes also called aging or weathering.
During the offloading of the LNG Carrier (LNGC) the level of LNG in all tanks is maximal which means that the vapor headspace above the LNG is minimal. The LNG and vapor headspace are in equilibrium with each other.
By transferring the LNG from the tanks of the LNGC, the vapor headspace volume becomes larger. This means that the equilibrium between the LNG and the vapor headspace is adjusted at every level. Consequently, the large the vapor headspace the more light-end molecules like methane and nitrogen will be in the vapor space. Actually the remaining LNG becomes richer.
This is exactly shown in the GCV trend line of figure 10.
So the whole data set is not suitable for statistics while it is not constant. In order to run proper statistics the data set must be divided into smaller intervals which are considered as constant.
It has been the first time that the customer observe such a trend which prove what they were always expecting, enriching of the LNG over time.
The data set was divided into small intervals of 7 measurements in order to compare to the minimum performance criteria. The statistical results are given below and it may be obvious that the results are exceeding the minimum performance criteria by far.
LNG sampling at low pressure conclusions
By the test and the actual ship to ship transferred gathered data it is proven that sub cooling is extremely beneficial and essential while vaporizing and LNG sampling at low pressure. It means that for LNG applications with higher pressures the same result will apply as well.
The phenomena of enriching the LNG during offloading a LNGC is also occurring at LNG storage tanks and LNG fuel tanks, in those cases it is often called weathering or aging of the LNG. With the performance of the Phazer aging can be measured and trended with makes it beneficial for motor management control for LNG fueled engines.
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