Necessary Conditions and Goals of Processing :
The types of treatments carried out on gas streams usually are:
– liquid hydrocarbon recovery;
Treatment needs and objectives:
The first gas treatments are temporary. Their aim is to prevent the hydrates formation and they’re classified as follows:
– separation of free water;
– increase in the temperature of gas above that of hydrate formation;
– inhibitors injection to prevent hydrate formation.
Elimination of free water
The elimination of free water in the gas is carried out through separators that are installed at the well-head and at the entrance of the treatment stations.
This treatment removes the liquid phase from the two-phases coming out of the well head. The gaseous phase is saturated before the treatment and is still saturated (at pressure and temperature conditions) leaving the
Gas undergoes a temperature decrease flowing within the pipe from well head to the treatment station and expands (decreasing in pressure, caused by head losses along the pipe and across valves and fittings). The expansion causes a temperature loss (Joule-Thompson effect), since another condensation of moisture takes place, so that the liquid phase can be present at the inlet of the treatment station once again.
In the diagram representing the hydrates formation of a 0,7 specific gravity gas, the evidenced area defines the condition under which the hydrates can form.
From the same diagram it is evident that we have the gas phase increasing the temperature.
The treatment is temporary because heating the gas at constant pressure increases the capacity to hold vapour, however content is still the same.
So, when the temperature is lower than the previous values, gas returns into the hydrates formation area.
read more about Hydrate and Hydrate Prevention
Inhibitors are liquid substances that prevent and also eliminate the formation of hydrates. The inhibitors enter a solution with water, lowering its freezing point thus shifting the equilibrium of hydrates towards lower
temperature values. The inhibitors which are generally used are:
– ethyl Alcohol;
– diethylene Glycol.
Other chemical inhibitors are used in order to hinder, in case of sour gas, the corrosion of the machinery located at the wellhead.
Both treatments are temporary, since they do not eliminate the problem, but simply hand it over to the treatment stations, either for the elimination of water vapour or the elimination of corrosive agents such as CO2 and/or H2S.
After temporary treatments, gas is delivered to the treatment plant and then to users, according to the requested specifications.
The final treatments eliminating harmful elements from natural gas consists in:
1. reducing water (dehydration);
2. reducing the content of superior hydrocarbons (adsorption);
3. reducing the content of hydrogen sulphide and carbon dioxide
The above-mentioned treatments are carried out by appropriate types of plants according to the composition of natural gas. Moreover, the plants are managed to meet the user’s specifications.
The maximum concentration allowed for the various undesirable components are indicated as follows:
CHARACTERISTICS OF NATURAL GAS ACCORDING TO USERS REQUESTS
(These specifications can vary depending on the customer)
WATER Content usually expressed as dew point temperature
e.g. < 5 °C at 50 bar
HYDROGEN SULPHIDE Content = 2 ppm/Vol Max
CARBON DIOXIDE Content = 1,3 Mol% Max
NITROGEN Content = 6 Mol% Max
GASOLINE Content usually expressed as dew point temperature
e.g. < -10 °C at 50 bar