Efficient and cost-effective amine purification process
In refineries and gas processing complexes, H₂S and CO₂ present in the sour gases are removed using an aqueous solution of alkanolamine.
Venkata Srinu Gollapalli, Saptarshi Paul, Balaji Lakavath and V Kamesh Jayanti
Engineers India Limited
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Different types of amines, such as mono-ethanolamine (MEA), di-ethanolamine (DEA), and methyl di-ethanolamine (MDEA), are used in an amine-based gas sweetening plant. Strong acid anions like formates, acetates, thiocyanates, and chlorides can react with amine molecules to form heat stable salts (HSS). These salts are formed from thermal and chemical degradation of amine molecules. High temperature in the amine regeneration reboiler and the reaction of amine with COS and/or O₂ results in the formation of HSS.
The presence of HSS in an amine circulation loop can result in corrosion, leading to stress corrosion cracking, high maintenance cost, frequent filter replacement, foaming, plugging in the amine absorption column, decrease in amine absorption efficiency, heat exchanger and reboiler tube fouling, as well as overall unit malfunctioning. Reduction of HSS in alkanolamine solution ensures stable and uniform operation of amine-treating plants.
It is essential to remove HSS from amine for smooth and trouble-free operation. Different processes are used in the removal of HSS from alkanolamine solutions. Each process has its merits and demerits. The distillation process is used where HSS are concentrated by leaving the amine in the vapour phase. This process is not energy efficient, and a vacuum system is adopted to reduce the temperature in order to avoid thermal degradation of the alkanolamine.
Another process is electrodialysis, which uses an ion-selective membrane to separate HSS by maintaining potential differences across the membranes. The efficiency of the process depends on the membranes used, but the cost of the electrodialysis system is relatively high. Based on a literature survey and patent search, the ion exchange process is one of the most economical processes to remove HSS from amine solution.
EIL R&D has developed an ion exchange-based amine purification process utilising a proprietary resin. The process takes a slip stream (~10% of lean amine flow to the gas sweetening plant) and passes it through a packed bed adsorber containing ion exchange resin. The process brings down the total HSS to less than 0.5 wt%, which is acceptable for trouble-free operation of any gas sweetening plant. The resin bed is then regenerated, ready for reuse in the next cycle.
The ion exchange-based process follows a few main steps in sequence. They are activation of resin by caustic solution, water wash to remove traces of caustic, amine circulation for HSS adsorption on resin bed (adsorption cycle), water wash to remove traces of amine, and regeneration of resin bed with a caustic solution (regeneration cycle). A brief process schematic diagram is illustrated in Figure 1.
Salient Features of Amine Purification Process
• Ensures significant removal of HSS (i.e. <0.5 wt% in treated amine), thereby preserving amine quality and reducing make-up amine expenditures
• Uses dedicated resin beds, which are highly efficient and easily regenerable
• Uses 5-10 wt% caustic solution and demineralised water for resin bed regeneration, which are easily available in the refinery. No new chemicals are needed
• Is designed for batch operation of adsorption/regeneration; hence, providing continuous amine purification at all times
• Operates at ambient temperatures. No heating/cooling utilities are required
Implementation of this process needs minimum investment and minimum shutdown period.
This indigenous technology, ‘Ion exchange resin process for removal of heat stable salts (HSS) from alkanolamine solution’, has been patented (Indian Patent No: 356304). High HSS removal efficiency, easy regeneration, and low Capex and Opex are some of the novel benefits of the process. Currently, this technology is under commercialisation at an Indian refinery.
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