Natural Gas Refining
Some natural gas wells furnish gas of very high purity, that is, almost pure methane. However, most hydrocarbon streams are complex mixtures of hundreds of different compounds. A typical wellstream is a high velocity, turbulent, constantly expanding mixture of gases and hydrocarbons intimately mixed with water vapor, free water, solids, and other contaminants.
Contaminant removal processes can be divided into two groups: dehydration and purification. The principle reasons for the importance of natural gas dehydration include the following:
1. Liquid water and natural gas can form solid ice-like hydrates which can plug valves, piping, etc.
2. Natural gas containing liquid water is corrosive, particularly if it contains CO2 or H2S.
3. Water vapor in natural gas pipelines may condense, causing sluggish flow conditions.
4. Water vapor increases the volume and decreases the heating value of natural gas thus leading to reduced line capacity.
5. Dehydration of natural gas prior to cryogenic processing is a must to prevent ice formation on low temperature heat exchangers.
Of these, the most common reason for dehydration is the prevention of hydrate formation in gas pipelines. Natural gas hydrates are solid crystalline compounds formed by the chemical combination of natural gas and water under pressure at temperatures considerably above the freezing point of water. In the presence of free water, hydrates will form when the temperature is below a certain point called the hydrate temperature.
Hydrate formation is often confused with condensation and the distinction between the two must be clearly understood.
Condensation of water from natural gas under pressure occurs when the temperature is at or below the dewpoint at that pressure. Free water obtained under such conditions is essential to formation of hydrates which will occur at or below the hydrate temperature at the same pressure.
During the flow of natural gas, it is necessary to avoid conditions that promote the formation of hydrates. This is essential since hydrates may choke the flow stream, surface lines, and other equipment. Hydrate formation in the flow stream results in a lower value for measured wellhead pressures. In a flowrate measuring device, hydrate formation results in lower apparent flow rate. Excessive hydrate formation may also completely block flowlines and service equipment.
Thus, the need for prevention of hydrate formation is obvious, and the easiest way to eliminate hydrates is to substantially remove the water from the natural gas stream. The most efficient method for removing the water present in a natural gas stream is by adsorption with a solid desiccant such as molecular sieve or activated alumina.
Another important application for desiccant drying is the liquefaction of natural gas. Methane is converted into a liquid in a cryogenic process at -285°F (-176°C) and atmospheric pressure. There is a 600 to 1 reduction in volume. As a liquid, large volumes of methane can be easily transported and/or stored. Natural gas companies liquefy and store gas (1 to 20 MMSCFD) during low demand periods and use the stored liquid during high periods of demand. Natural gas found in remote areas can be liquefied and transported to places of demand. Because of the low dewpoints needed for the cryogenic production of LNG, dryers are used.