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Methyl Diethanolamine
(MDEA) |
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APL is totally geared
up to meet customers Acid Gas Treating requirements so do call on us and
it would be our pleasure to work in close association with your company
to satisfactorily cater to your gas treating requirements : |
| 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.
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| 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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