Conditioning natural gas samples often seems more art than science. This is partly due to the fact that a number of people involved in the design of these systems fail to apply the laws of nature to sample conditioning. A number of physical and chemical concepts should be taken into consideration; a thorough understanding of the physical relationship between liquids, gases and the surfaces that contain them is a plus for anyone involved in the design of conditioning systems.
The liquid in the line is dynamic and constantly changing shape.
The liquid present in the line is dynamic and constantly changing form: a liquid film is present along the pipe wall; aerosols constantly come into contact with the liquid film and become embedded in it; the liquid film accumulates and forms puddles; the gas flow tears the aerosols away from the liquid and the film; these aerosols are drawn back into the gas flow...
Because of these constant changes, it's impossible to extract a representative sample. Even if you could measure a sample with liquids, you would only sample the entrained aerosols, and only at that point of extraction. This would not represent the total amount of liquid present in different forms along the line, as the relationship between the liquids extracted and the total liquid load cannot be known. Finally, liquids must be separated, as there is no technology available to extract a gas sample containing a representative quantity of liquid entrained in the gas stream.
Liquid-vapor balance
Liquid-vapor equilibrium is a state of dynamic equilibrium where the number of vapor molecules condensing is statistically equal to the number of liquid molecules vaporizing.
The diagram opposite shows a container holding a pure substance (single component), with a certain number of molecules of the substance in the liquid phase and the rest in the gas phase. These molecules are dynamic (constantly in motion).
At constant temperature and pressure inside the pipe, the number of gas molecules condensing into liquid is equal to the number of liquid molecules evaporating in the gas phase. This system is said to be in liquid-vapor equilibrium. When this happens, the gas phase is saturated with liquid vapor AND at its dew point temperature.
Bear in mind that the amount of liquid is of no consequence; THE PRESENCE OF LIQUID is the important point! The container could be almost completely full of liquid with just a tiny bubble of steam, or there could be just a drop of liquid, and the rest steam. They always arrive at equilibrium.
We can upset this balance by changing temperature or pressure.
Liquid-vapor equilibrium of a pure substance
An increase in temperature or a decrease in pressure will result in additional vaporization of liquid, thus increasing the number of molecules in the gas phase. If the temperature is reduced or the pressure increased, some of the vapor will condense and return to the liquid phase, reducing the number of molecules in the gas phase. The equilibrium is momentarily upset, but is restored almost instantaneously.
The relative number of molecules in each phase depends on the pressure and temperature of the system. Suppose we increase the temperature inside the container. This will add energy to the molecules, and more molecules will tend to move from the liquid phase to the vapor state. A new liquid-vapor equilibrium will be reached, with more molecules in the vapor phase, and more molecules moving from one phase to the other. Once this dynamic equilibrium has been reached, the vapour phase will reach a new, higher vapour pressure. This is because the vapor pressure of a substance is proportional to the relative number of molecules of that substance in the vapor phase, a principle known as Raoult's Law.
Liquid-vapor balance of a mixture
We've talked about a pure substance. What if you have more than one component, such as alcohol and water?
Each individual component has its own unique vapor pressure, which varies with temperature. So what happens when they are mixed at the same temperature and pressure? Changes in temperature or pressure will change the composition of the gas phase, as each component vaporizes or condenses at a different rate. Let's take a look at a mixture containing several components, such as natural gas. At constant temperature and pressure, we'll again see liquid-vapor equilibrium as the number of molecules evaporating equals the number of molecules condensing.
Vapor phase pressure is related to the number of molecules in the vapor phase; however, the proportionality between the number of molecules in the liquid phase and the vapor phase will not be the same for each component. Even with mixing at a uniform temperature, one component may have 50% of its molecules in the vapor phase, while another component may have only 10% of its molecules in the vapor phase. If there are equal amounts of each component in the liquid phase, there will almost never be equal amounts of each component in the gas phase. Since the gas phase has different amounts of each component contributing to the overall vapor pressure, we say that each component has a different partial pressure. In natural gas mixtures, the lighter components (methane, ethane, etc.) will have higher vapor pressures than the heavier components (nonane, decane, etc.).
An increase in temperature or decrease in pressure will cause molecules to vaporize, but not all components will vaporize the same amount, resulting in a change in the composition of the gas phase. If this happens during sampling, we will mistakenly enrich the sample.
A decrease in temperature or an increase in pressure will cause molecules to condense, but not all components condense in proportion to their concentration in the gas phase, resulting in a change in the composition of the gas phase. The "heavier" components will condense first.
When liquid is entrained in the source gas, changes in temperature or pressure alter the composition of the gas phase. The natural gas industry uses composition analysis to obtain information on the physical constants of the gas (specific gravity/density, compressibility) in order to calculate the volume of gas flowing through the pipe (total flow rate) and to calculate the BTU value. And if the total flow rate or the BTU value are wrong, this will obviously distort the monetary value of the gas at the time of transfer of ownership.
Dew point and heating requirements
To prevent heavier hydrocarbons from condensing in the sampling system, API 14.1 makes the following recommendation: "Since hydrocarbon dew point measurements and calculations are sometimes uncertain, it is recommended that the sampled gas be maintained at least 17°C above the estimated hydrocarbon dew point throughout the sampling system." This means that all sampling system components (sampling probes, regulators, tubes, filters, sampling cylinders, etc.) must be maintained at least 17°C above the estimated hydrocarbon dew point. Note that the notion of 17°C above its dew point applies to components outside the pipeline. Internal components which are at the temperature of the gas flow do not require this safety feature.
The two main reasons for eliminating liquids are:
- analyzer protection
- sample integrity
When liquids are present, the gas is at its dew point and saturated. It is impossible to perform temperature or pressure changes on a gas sample if it contains liquids.
So what can we do?
- Separation of liquid from gas phase at line pressure and temperature conditions
- Use a probe with a membrane at the end inserted into the sample source to accomplish this task.
- Take steps to "de-saturate" the sample
- Lower the pressure
- Increase temperature
The solution to extracting a representative sample in the presence of entrained liquids is to use a membrane probe to separate the liquids under conditions of pressure and temperature. The membrane is a phase separation membrane.
Phase separation membranes
The most familiar type of membrane in hydrocarbon analysis is a chemically selective membrane used to dry gases. GENIE membranes are not chemically selective. They separate the liquid and gas phases. They provide an anti-liquid barrier while allowing the gas to flow without changing its composition.
The membrane separates liquids from gas samples, based on the property of surface tension. Liquid molecules cling tightly together as a "cohesive group"; gas and vapor molecules do not. Since liquid molecules cling together, they cannot flow through the membrane's small passages.
Gases and vapor molecules circulate freely. Liquid molecules cling together and, due to surface tension, cannot flow freely through. Remember .... chemically inert, not chemically selective.
Natural gas phase diagrams
The phase digram is a decision-making tool for the design and operation of sample conditioning systems. It helps determine the hydrocarbon dew point, the presence of liquids in the source and in the sample, and any reheating requirements.
A phase diagram is a graph showing the relationship between gas and liquid phases at different pressure and temperature conditions. The diagram opposite shows a typical natural gas composition.
De-saturation
When liquids are present, vapor and liquid are in equilibrium. The gas phase is saturated, at its dew point. What if we could wave a magic wand to separate gas from liquid without changing pressure or temperature, so that the gas contained no liquid and the liquid no gas?
In this hypothesis, if we lower the temperature or raise the pressure, this creates condensation and changes the composition of the sample. But if we raise the temperature or lower the pressure, nothing happens. In this way, we de-saturate the sample, moving it away from its dew point so that it doesn't condense during transport to the analyzer. In conclusion, if we can separate the liquids at pipe temperature and pressure, we won't change the composition, and we can then de-saturate the sample to prevent any change in composition. However, it is absolutely essential to separate the liquids BEFORE de-saturating (expanding and heating) the sample.
Adsorption and desorption
Adsorption is a surface phenomenon by which gas or liquid molecules attach themselves to solid surfaces through chemical or physical processes. The opposite phenomenon, the release of gas or liquid molecules from the solid surface, is desorption. Adsorption should not be confused with absorption, which is the penetration of a gas or liquid into another body, like water into the pores of a sponge.
Decreasing temperature and increasing pressure tend to increase adsorption, while increasing temperature and decreasing pressure tend to reduce adsorption. De-saturating the sample can also reduce adsorption.
The fact that the adsorption rate changes if temperature and pressure change means that the composition of the sample entering the analyzer may be different from the composition of the sample entering the sampling system if adsorption is not at equilibrium. When the ambient temperature is stable and the pressure, temperature and composition of the source sample are stable, adsorption in the sampling system will have reached equilibrium and the composition of the sample entering the analyzer will be the same as the composition of the sample entering the sampling system.
If the ambient temperature drops, adsorption will increase. Some of what enters the sampling system is then adsorbed before it reaches the analyzer. Until a new adsorption equilibrium is established, the composition entering the sampling system is not the same as that reaching the analyzer. The time required to establish the new adsorption equilibrium leads to a time lag.
Similarly, if the ambient temperature rises, the composition of the sample entering the analyzer will contain molecules that desorb from the inner surfaces of the sampling system, and will be different from the composition entering the sampling system, until the adsorption equilibrium is re-established at the new temperature.
Thomson-Joule effect and latent heat of vaporization
TheJoule-Thomson effect is a phenomenon in which the temperature of a gas decreases as it expands.
When a gas is expanded in a closed system, it expands and cools. This cooling sometimes lowers the temperature of the gas below its dew point, producing condensation. So a change in pressure can also change temperature. However, the Thomson-Joule effect is often not the cause of condensation, because generally speaking, lowering pressure also lowers the dew point temperature. Condensation can occur when the mixture is at a pressure above the cricondentherm (maximum temperature at which the 2 liquid and gaseous phases can coexist) on the dew point curve near the supercritical state.
The latent heat of vaporization is the heat required to change a liquid into a gas or vapor, without any change in temperature. When a material changes phase from liquid to gas, a certain amount of energy is involved. This amount of energy is called the enthalpy of vaporization, also known as the (latent) heat of vaporization or the heat of evaporation. The frost that appears on valves and expansion valves is due more to the latent heat of vaporization than to the Thomson-Joule effect. In any case, frost indicates the presence of liquid.
Liquid is present in sampling systems far more often than most people realize. It can have several origins, entrained in the gas flow or created by condensation. We don't necessarily see it. Signs such as erroneous analyses, icing on valves or regulators, on-line, spot or composite analyzers that don't agree with each other, can be the cause of the presence of liquid.
