It is well known to use thermally regenerable liquid solvents for extracting the acid compounds contained in a gas, in particular in a natural gas. Examples of the most commonly used solvents are aqueous amine solutions and some physical solvents such as sulfolane, methanol, N-formyl morpholine, acetyl morpholine, propylene carbonate.
These methods generally involve a stage of extraction of the CO2 contained in the gas to be treated by contacting this gas with the regenerated solvent in an absorber operating at the pressure of the gas to be treated, followed by a thermal regeneration stage, generally at a pressure slightly above atmospheric pressure, generally between 1 and 5 bar, preferably between 1.5 and 3 bar. This thermal regeneration is generally carried out in a column equipped at the bottom with a reboiler and at the top with a condenser allowing to cool the acid compounds released by the regeneration and to recycle the condensates to the top of the regenerator as reflux.
When the pressure of the gas to be treated is notably higher than the atmospheric pressure, for example in the case of a natural gas that has to be treated at a pressure of the order of 70 bar, the acid gas-rich solvent obtained at the absorber bottom can contain significant amounts of dissolved hydrocarbons. It is then common practice to carry out a stage of release of these dissolved hydrocarbons vaporized by simple expansion of the acid gas-rich solvent. This expansion is carried out at an intermediate pressure between that of the raw gas to be treated and that of the thermal regeneration stage, typically of the order of 5 to 15 bar. A gas containing the lighter dissolved hydrocarbons, predominant in proportion by volume, which can be used as combustion gas, is thus separated from the CO2-rich solvent. This gas is sometimes washed by a stream of regenerated solvent coming from the thermal stage so as to re-absorb the acid compounds released upon expansion. This washing of the gas released by expansion is generally performed in a column placed directly on the separator drum between the gas and the expanded liquid. The solvent thus laden with acid compounds is directly mixed with the expanded solvent and sent to the thermal regeneration stage.
In order to reduce the heat consumptions of these methods, a stage of thermal exchange between the rich solvent, after expansion, and the regenerated solvent obtained hot at the bottom of the regeneration column is generally carried out.
Regeneration of these solvents produces a gaseous effluent rich in acid compounds. When the raw gas contains significant amounts of heavy hydrocarbons (several hundred ppmv for example), these gases are found in significant proportions in the acid gas at the regenerator top. Indeed, although the stage of expansion of the acid gas-rich solvent obtained at the regenerator bottom allows to release the major part of the light hydrocarbons (methane, ethane, etc.) dissolved in the solvent at the absorber bottom, it does not allow to extract the major part of the heavier compounds. An acid gas that can contain several hundred ppmv hydrocarbons is thus commonly obtained at the regenerator top. The significant proportion of these compounds in the acid gas induces an increase in the VOC (Volatile Organic Compound) content and can lead to failure to meet the VOC specifications. In a vent, the VOCs are made up of the following hydrocarbon compounds: linear alkanes (methane is sometimes excluded), cyclo-alkanes, aromatics (benzene, toluene, ethyl benzene and xylenes). It is then necessary to provide an acid gas post-treatment stage such as incineration, which may involve costly equipment and high energy consumption (combustion gas consumption).
Acid gas is sometimes reinjected into the well in order to facilitate the extraction stage (EOR), notably in the case of decarbonation. It is therefore necessary to compress the water vapour-saturated acid gas. This compression requiring several stages generates water condensates. The hydrocarbons present in the acid gas are then found in these aqueous condensates, which significantly increases the cost of the associated condensate treatment required to remove the polluting hydrocarbons (notably aromatics).
To overcome these drawbacks, it is possible to adsorb the hydrocarbons present in the acid gas on a suitable material (activated charcoal for example). This method requires an additional processing unit that may be expensive as regards investment (case of a regenerable adsorbent) or operating costs (case of non-regenerable adsorbents).
The present invention provides a simple and inexpensive method that requires only a small number of additional equipments for separating, upon regeneration, the major part of the hydrocarbons co-absorbed by the solvent from the major part of the acid gases absorbed by the solvent. The method achieves this goal using a LP (Low Pressure) flash system allowing the hydrocarbon content at the regenerator top to be controlled.
The method thus allows the hydrocarbon content at the regenerator top to be controlled, which affords the following advantages:
by optimizing the pressure and temperature conditions of the LP flash system, it is possible to obtain an acid gas that can be released to the atmosphere without a post-treatment stage that is usually required to meet the environmental standards in terms of VOC content, notably in the presence of aromatic compounds,
it is then possible to do without the incinerator dedicated to the destruction of the hydrocarbons present in the acid gas. Of course, an incinerator is necessary to purify the vents of the LP flash drums, but it is smaller because the flow of gas to be incinerated is much lower (5 to 15% of the acid gas flow rate),
the flow of gas to be incinerated being lower, the incineration-dedicated fuel gas consumption is also significantly reduced,
the flow of amine to be regenerated being slightly lower with the present invention, a gain is also achieved as regards the size of the regenerator and the feed of the reboiler,
for decarbonation applications, the acid gas obtained with the present invention contains water and CO2, and much less hydrocarbons. If the acid gas is to be reinjected for enhanced oil recovery purposes (EOR), the treatments usually necessary for purifying the condensates (water here) extracted from the various compression stages are greatly reduced, or even suppressed. Due to its purity, the water can in some cases be directly recycled to the amine unit with a reduced makeup water consumption,
in order to meet environmental standards, the energy consumption of the method is substantially reduced due to the low combustion gas consumption according to the scheme of the present invention.