Absorption is the intake of gases or vapors from the gas or gas-vapor mixtures by liquid absorbers.
The possibility of the absorption process is based on the gases solubility in liquids. The absorption process is selective and reversible, and it can be applied for hydrocarbon gas conditioning combined with the removal of water (selective dehydration process), hydrogen sulfide and mercaptan, carbon dioxide, and other components.
Absorbents are special liquids or solutions of an active component. Anyway, there is a number of requirements for absorbers, the most important of which are: high absorptivity, selectivity, low vapor pressure, chemical inertness to commonly used structural materials, non-toxicity, fire and explosion safety, availability, and low cost.
Due to the combination of absorption and desorption, the absorbent is suited for a multiple use, almost without losses, in closed-circuit apparatus: absorber-desorber-absorber (closed cycle), extracting the absorbed component in its pure form.
The hydrocarbon gas (natural, associated petroleum), containing water, hydrogen sulfide, or some other recoverable component, is lead to the absorber where it contacts with the absorbent. The treated gas is removed, and the absorbent solution, containing the absorbed component, goes to the heat exchanger where it is heated. The heated solution is pumped over to the desorber where the absorbed component is extracted (desorbed) by evaporation due to heating the absorbent with steam. Lean absorbent is supplied to the heat exchanger where it transfers heat to the wet absorbent during its countercurrent flow into the desorber; afterwards, it is lead into the refrigerator and, upon passing through the refrigerator, the absorbent is lead into the absorber again.
The cycle is closed. This is the mode of operation of the unit for the absorption and desorption recovery of certain vapors and gases from a gas mixture.
The process of dehydration by absorption is typically applied at medium to high gas flow rates and practically any pressure of the medium being purified, and medium dehydrated gas dew point values (up to -40…-50°С). Also, the gas can be treated to remove hydrogen sulfide and carbon dioxide in accordance with Gazprom’s industrial standard requirements.
Gas adsorption dehydration and purification units
Adsorption dehydration is the process of selective removal of water and other components molecules from the gas by the pores of a solid desiccant followed by the removal of contaminants from the pores applying the exposure to an external action.
Selecting the desiccants for the adsorption dehydration units, it necessary to take into account the gas composition, the presence of certain components in the gas that can affect the process and the final result.
One of the major benefits of adsorption is the fact that no preliminary drying of gas is required, because solid desiccants very well adsorb water, along with liquid hydrocarbons. As an adsorbent, solid porous substances with a large specific surface area are used. Some of these desiccants are: activated carbon, silica gels – dried silicic acid gel products, zeolites — hydrated sodium and calcium aluminosilicate minerals and synthetic zeolites.
The dynamic activity of an adsorbent characterizes the percentage of weight of adsorbed liquid to the weight of adsorbent. Typically, it is 4-7%. Industrial adsorbents have a sufficiently high activity, adsorption reversibility and simple regeneration, low resistance to the gas flow, and a high mechanical strength.
Adsorption dehydration units are used, as a rule, for a deep dehydration of gas with low gas dew points. The process runs in vertical adsorption units with loose desiccant beds.
Membrane units for natural gas dehydration and purification
The membrane separation of gas mixtures is based on the use of a selective gas components permeable membrane, i.e. feed mixture components permeate the membrane at different velocities. During gas conditioning in a membrane module, the wet feed stream is divided into two streams: permeate low pressure stream and residual stream. The driving force of the process is the difference in gas components partial pressures on the low and high pressure sides.
The high pressure stream is refined with the low permeability components (methane, ethane), and this is, as a rule, the objective of gas treatment.
The low pressure stream containing water, hydrogen sulfide, carbon dioxide, and heavy hydrocarbons can be utilized as a fuel or additionally processed.
Membranes are fabricated of glass, metals, and polymer materials and configured as plates, tubes, hollow fibers, and capillaries. Along with it, a very important feature of membrane modules is the so-called density of membrane installation, i.e. the membrane surface per unit of the apparatus volume.
Membrane units have a compact design and can be installed in highly prefabricated modules.