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The Use of Amines in the Gas Sweetening Process
Aqueous mixtures of alkanolamines and / or alkyl-alkanolamines have been used in a regenerable process to remove acidic contaminants, usually hydrogen sulphide (H2S) and carbon dioxide (CO2) from natural gas streams for over 60 years.
This activity is driven by product specification necessary for sale, i.e. natural gas shall typically contain less than 4 ppm (v) H2S.
Amines used for this purpose include MEA (monoethanolamine), DGA (diglycolamine), DEA (diethanolamine), MDEA (methyl-diethanolamine), the latter being the most commonly used in recent years.
Acid gas is removed from the gas stream by counter-current absorption with an acid gas lean aqueous amine solution (lean amine) in a multi-stage absorber tower.
Since hydrocarbons and other non-reactive gases also dissolve in the lean amine solution, they are normally recovered by pressure reduction in a flash tank.
Amine solution chemistry permits acid gas removal from the rich amine solution at elevated temperatures. The process is facilitated by counter-current steam stripping in a multistage regenerator tower, operated at low pressure.
From the reboiler tank of the regenerator tower, lean amine is boostered, through a heat recovery exchanger and a cooler, to a surge tank, wherefrom it is recirculated to the gas absorber.
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Side-Stream Treatment of Amines Solution by Unidro: a Tailor Made Design
There are many contaminants present in gas treating amine solutions: most of them are degradation products derived from irreversible degradation of the base amine molecule itself (examples are ethylenediamine derivatives), but pollutants also include solids / particulates, hydrocarbons and process chemicals.
Particulates formed in the plant as well as those transported into the plant can be very bothersome; the potential for under deposit corrosion increases when the amount of circulated solids increases.
In general, all these contaminants can affect unit operation and cause significant corrosion, for this reason their concentration must be kept under control. Accumulation within the circulating amine is prevented by filtering a slip stream of the lean amine solution.
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The filter section typically includes a mechanical precoat type filter, to remove solid particles from the solution, followed by activated carbon filters, which remove chemical contaminants such as surfactants, large hydrocarbon molecules and degradation products; the last stage is filtration on cartridge guard filters. Unidro can provide complete lean amine filtration packages, tailor-made to Client’s requirements and Project specifications; in particular, two different types of mechanical precoat filters can be supplied depending on client’s specific requirements.
Unidro’s lean amine filtration packages fully satisfy the stringent safety criteria which normally apply when harmful process fluids such as amines are involved. The package is designed by Unidro according to the project requirements.
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The equipment is fabricated and assembled at shop, duly inspected and tested, then shipped to site and erected.
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The Technology: Precoat Filters
The concept of precoat filtration is that, in order to prevent filter elements from clogging and to secure fine filtration efficiency, filter elements are to be protected by depositing a coat of precoat material on their surface, prior to filtration.
For this purpose, precoat liquid is introduced into the filter vessel and filtered, then it is returned to the precoat tank.
Precoat solution is prepared by mixing uniformly part of the inlet lean amine solution with the precoat material, which is normally cellulose, diatomaceous earth, or similar others.
The proper selection of the precoat media is a key factor for successful operation and performances achievement of the precoat filtration unit.
During precoat solution circulation, the filter aid is deposited on the surface of filter elements, and forms a precoat layer.
At the end of the filtering cycle, when a pre-set differential pressure across the precoat filter is reached, the filtration is stopped, and the filtering elements are cleaned.
Depending on the selected precoat filter technology, the cleaning operation generates a slurry or a dry waste.
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The Equipment: Leaves Precoat Filters or Candles Precoat Filters
Leaves Precoat Filters consist of a horizontal cylindrical vessel with conical bottom, equipped with circular filtering leaves installed on a central internal collecting header and a washing device.
The filtering elements are vertical round leaves, which can filter through each side. The filter leaves are hold by a central shaft, which also acts as filtered liquid collector.
At the end of the filtering cycle, when a pre-set differential pressure is reached, the filtration is stopped, the filtering leaves are rotated by the shaft and the washing device is started.
The washing device consists of a horizontal pipe located on the top of the tank.
The pipe bears special spray-jets suitable to send a strong blade jet on the rotating filtering leaves and clean them effectively. After the sluicing the leaves are perfectly clean and ready for a new filtration cycle.
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The candles precoat filters consist of a vertical cylindrical vessel with conical bottom, with filtering candles fixed at their top on horizontal collecting manifolds.
Once the filtrate has passed through the filter candle, it flows into the peripheral tubes through the slots and then down to the bottom of the candle, before rising again in the central tube toward the registers.
When the filtration cycle is completed, cake discharge is brought about by counter-current gas pressure in the interior of the candle.
The quick-pulse gas blow-back into the filter elements instantly releases the cake, causing it to fall.
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The Equipment: Activated Carbon Filters and Cartridge Filters
Activated carbon filters are used to remove liquid contaminants such as corrosion inhibitors, amines degradation products, oils etc., thus reducing foaming problems, inhibiting corrosion, and maintaining the operating efficiency. All of this results in a reduction of operational and maintenance costs.
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In addition to the optimisation of the conventional process parameters (contact time, filtering velocity, etc.), the following critical points are carefully considered in Unidro’s design of Activated Carbon filters for amine service:
Proper selection of charcoal media: A high volume of pores is necessary, and a vast spread in pore size distribution is advisable for this application, in view of the variety of molecules to be adsorbed. Hardness and density are also important parameters.
Careful evaluation of pressure drops: Amine is a much more viscous fluid than water: special care must be taken when selecting the charcoal layer height and the filtering velocity.
Identification of backwash conditions: Although backwash is not a frequent operation (feed lean amine to the charcoal filters is pre-filtered), it is sometimes necessary to remove the fines which are inevitably generated within the charcoal layer.
Hydraulic conditions: Backwash velocity, fluid temperature, charcoal layer expansion. All these parameters require special care, also in view of the high fluid viscosity.
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At the outlet of the charcoal filters a battery of Cartridge guard filters is normally installed, to prevent fines possibly released by the charcoal media to enter the amine circuit.
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