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    Methyl Diethanolamine (MDEA)
 
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  • Comparison of Amines
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    Advantage of MDEA in gas treating
  • MDEA is a tertiary amine, is less basic and can be used in significantly higher concentrations. Table I show that for identical flows, MDEA has a greater capacity to react with acid gas because it can be used in higher concentrations. This advantage is enhanced by the fact that it is also selective, reacting with all of the hydrogen sulfide [H2S], and only part of the carbon di- oxide [CO2]. Both MEA and DEA react with all of the CO2 present in the acid gas.
  • This and other beneficial characteristics of MDEA result in promises which include increased capacity for existing units, decreased capital cost for new units, lower energy costs and higher selectivity than primary or secondary amines. Table II summarizes data from actual MDEA operations.
  • Conversions to MDEA have been made from MEA, DEA, DGA, DIPA, and so on. Solvent Circulation rates between 10 to 1600 gpm have proven successful.
  • In primary treating MDEA rich loading have averaged 0.5 moles of acid gas per mole of MDEA. Reboiler Steam requirements have ranged from 0.67 to 0.85 lbs per gpm of solvent in circulation.
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  • CO2 selectivity of 50% under 200 psig and higher at lower pressures have also been achieved. In general the industry is over-stripping the lean solvent to very low or trace loading (0.01 for primary, 0.001 for tail gas treating units (TGTU)) partly to ensure that gas specifications are met and partly because low-pressure steam is virtually free in many plants. Similarly with TGTUs the rich loading have been between 0.06 and 0.08. The steam requirements between 0.67 to 0.85 and CO2 selectivity in excess of 80% slip. (Slip is percentage of the CO2 passing through the column.)
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  • Solvent concentration between 35 to 50 percent has, been proved successful. Viscosity negatively affect the mass transfer for concentration above 50%. Lean solvent temperatures have been successful between 90 to 110 O F for TGTUs and as high as 130 to 160 O F for primary and liquid contactors as long as the hydrocarbons do not vaporize.
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  • Typical concentration between 35 to 50% and pickup rates as high as 0.45 or 0.50 moles acid gas per mole of MDEA significantly increase capacity of existing units and allow equipment to be considerably smaller for new units. Higher concentration and higher pickup rates correspond to lower solvent circulation rates for equivalent capacities.
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  • MDEA also delivers energy savings from reduced reboiler duties (reflux ratio of 0.5 to 1.0) and lower overhead condenser duties. It has proved to be highly selective for absorption of H2S when compared to CO2 resulting in even lower circulation rates and higher quality acid gas for recycle to sulfur recovery unit.
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  • Among MEA, DEA, and MDEA, MEA has worst reputation for corrosion related problems. It is well documented in literature, that MEA and DEA form degradation products when reacted with CO2 whereas MDEA does not. Operating MEA, DEA and MDEA plants have demonstrated that corrosion can be minimized under proper operating conditions. However based on plant experience and laboratory data, relative corrosivity of amines are ranked as follows: MEA >> DEA >> MDEA. Table VI Summarizes corrosion data for various amine-based solutions.
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  • To sum up, advantages of MDEA include, highest selectivity for hydrogen sulfide over carbon di- oxide, higher energy efficiency, greater acid gas removal capacity, higher resistance to degradation, smaller equipment size for the new plants and above all much less corrosivity as compared to primary and secondary amines.
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  • MDEA is tertiary amine and therefore carbamate formation with CO2 does not take place in MDEA based system. MEA and DEA form Carbamates with CO2. Therefore operation with MDEA is far more stable with no spurious shutdowns over longer periods.
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