1 Gas Sweetening Processes By Amines Figure 15 Schematic of amine gas-sweetening process flow diagram. 4.4.3.3 Design Considerations This lecture describes some design aspects for the major equipment used in both MEA and DEA systems. 4.4.3.3.1 Amine Absorber Amine absorbers use countercurrent flow through a trayed or packed tower to provide intimate contact between the amine solvent and the sour gas so that the H2S and CO2 molecules can transfer from the gas phase to the solvent liquid phase. In tray columns, a liquid level is maintained on each tray by a weir usually 2 or 3 inches high. The gas passes up from underneath the trays through openings in the trays such as perforations, bubble caps, or valves and disperses into bubbles through the liquid, forming a froth. The gas disengages from the froth, travels through a vapor space, providing time for entrained amine solution to fall back down to the liquid on the tray, and passes through the next tray above.
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1
Gas Sweetening Processes
By Amines
Figure 15 Schematic of amine gas-sweetening process flow diagram.
4.4.3.3 Design Considerations
This lecture describes some design aspects for the major equipment
used in both MEA and DEA systems.
4.4.3.3.1 Amine Absorber
Amine absorbers use countercurrent flow through a trayed or packed tower to provide
intimate contact between the amine solvent and the sour gas so that the H2S and CO2
molecules can transfer from the gas phase to the solvent liquid phase.
In tray columns, a liquid level is maintained on each tray by a weir usually 2 or 3 inches
high. The gas passes up from underneath the trays through openings in the trays such as
perforations, bubble caps, or valves and disperses into bubbles through the liquid, forming
a froth. The gas disengages from the froth, travels through a vapor space, providing time
for entrained amine solution to fall back down to the liquid on the tray, and passes through
the next tray above.
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Schematic of Bubble Cap Trays
Schematic of Valve Trays
In packed columns the liquid solvent is dispersed in the gas stream by forming a film over
the packing, providing a large surface area for CO2 and H2S transfer from the gas to the
liquid solvent.
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Schematic of an Absorption/Desorption Cycle using Packed Column
As Absorber
The degree of sweetening achieved is largely dependent on
the number of trays or
the height of packing available in the absorber.
Twenty valve-type trays (spaced 24 inches apart) or
the equivalent height in packing column
are common and are often a standard design.
Typically, small-diameter towers use packing, whereas
larger towers use stainless steel trays.
The cross-sectional area of the contactor (absorber)
is sized for the gas and amine flow rates,
where the maximum gas superficial velocity is obtained from the Souders and Brown (1932)
equation as:
VSG = 0.25 [([([([(ρamine−ρgas)/ρgas]0.5
, ft/sec 4.14
It may be necessary to reduce the gas velocity by 25 to 35% to avoid jet flooding and
by 15% to allow for foaming.
Use an amine velocity of 0.25 ft/sec in the downcomer.
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In most cases a mist eliminator pad is installed near the gas outlet of the absorber
(the distance between the top tay and the mist pad is 3 to 4 feet)
to trap entrained solvent, and
an outlet knockout drum, similar to the inlet separator for the gas feed, is provided to
collect solvent carryover.
Some contactors have a water wash consisting of two to five trays at the top of the absorber
to minimize vaporization losses of amine,
which is often found in low-pressure MEA (monoethanolamine) systems.
Absorbers will usually have multiple feed points,
allowing the option of introducing the lean amine lower in the column or at multiple trays.
If carbon dioxide absorption is desired, all of the lean amine should, in general, be fed on
the top tray, thus utilizing all available stages.
4.4.3.3.2 Amine Pumps
There are several different amine pumps in each of the processes.
The amine booster and reflux pumps are centrifugal,
preferably in-line or horizontal.
Selection of the circulation pump depends on the contactor pressure and the amine
circulation rate.
Normally, reciprocating pumps are preferred only if the absorber pressure is very high.
However, centrifugal pumps are used for low pressures (e.g., 100 psig) and
multistage horizontal centrifugal pumps for high pressures (e.g., 700 psig) or high
circulation rates (e.g., 300 gal/min).
In sizing and rating pumps
use a low positive suction pressure of 3 to 10 psig.
The circulation flow rates for amine systems can be determined from
the acid gas flow rates by selecting a solution concentration and an acid gas loading.
For this purpose, the following equation can be used:
Q = [k(QG)(MF)] / [ρ(WF)(AG)] 4.15
where
Q is circulation rate for amine systems, gal/min;
K is constant (112 for MEA system and 192 for DEA system);
QG is gas flow rate, MMscfd;
MF is total acid–gas fraction in inlet gas, moles acid gas/mole inlet gas;
WF is amine weight fraction, lb amine/lb solution;
ρ is solution density, lb/gal at 60◦F; and
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AG is acid gas loading, mole acid gas/mole amine.
The rich solution acid gas loading depends on
the acid gas partial pressure and corrosiveness of solution.
The normal range of this parameter is 0.45 to 0.52 lbmole acid-gas/lbmole amine for MEA
and 0.43 to 0.73 lbmole acid-gas/lbmole amine for DEA systems.
For design,
the following solution strengths and loading are recommended to provide an effective
system without an excess of corrosion:
MEA system:
WF = 20 wt.%
AG = 0.33 mole acid gas/mole MEA
DEA system:
WF = 35 wt.%
AG = 0.5 mole acid gas/mole DEA
For the recommended concentrations,
the densities at 60◦F are
20% MEA = 8.41 lb/gal = 0.028 mole MEA/gal
35% DEA = 8.71 lb/gal = 0.029 mole DEA/gal
Using these design limits,
Equation (4-15) can be simplified to the following equation (Arnold and Stewart, 1999):
Q = k, (QG)(MF) 4.16
where
k, is constant
(201 for MEA system and 126 for DEA system).
The circulation rate determined with these equations should be increased by 10–15% to
supply an excess of amine.
Since the reboiler duty is almost always tied directly to the circulation rate, lower
circulation rates reduce the overall energy requirements. Lower circulation rates also tend
to increase the CO2 slip and can improve the quality of the feed to the sulfur recovery unit.
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4.4.3.3.3 Flash Tank
The rich amine solution from the absorber enters a flah tank,
allowing the lightest of the hydrocarbons to flash.
A small percentage of acid gases will also flash when the pressure is reduced.
The heavier hydrocarbons remain as a liquid, but separate from the aqueous amine,
forming a separate liquid layer. Because the hydrocarbons have a lower density than the
aqueous amine, they form the upper liquid layer and can be skimmed off the top.
Therefore, a provision should be made to remove these liquid hydrocarbons.
Typically the flash tanks are designed for 2 to 3 minutes of retention time
for the amine solution while operating half-full.
4.4.3.3.4 Amine Reboiler
The amine reboiler provides the heat input to an amine stripper, which
reverses the chemical reactions and drives off the acid gases.
Two different reboiler designs are used in amine plants:
thermosiphon and kettle.
Thermosiphon reboilers return the heated amine solution and steam to the regenerator
tower by the same pipe.
Schematic of a Thermosyphon Reboiler
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Kettle reboilers return the heated amine solution and steam to the regenerator tower in
different pipes.
Schematic of a Kettle Reboiler
The reboiler heat duty includes:
(1) sensible heat required to raise the temperatures of the rich amine feed, the reflux, and
the makeup water to the temperature of the reboiler,
(2) heat of reaction to break chemical bonds between the acid gas molecules and the amine,
(3) heat of vaporization of water to produce a stripping vapor of steam.
The heat duty and transfer area of the amine reboiler can be determined as follows (Jones
and Perry, 1973):
HR = 432000 × Q 4.17
A = 11.30 × Q 4.18
where
Q is amine circulation flow rate, gal/min;
HR is heat duty of amine reboiler, Btu/min; and
A is heat transfer area of reboiler, ft2.
The reboiler duty should be maintained as low as possible, but must be adequate to
regenerate the amine solution sufficiently to meet the sweet gas requirements and
to ensure that the CO2 loadings in the reboiler do not cause excessive corrosion.
Higher reboiler duties do not reduce circulation rates to any degree and just consume
energy.
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Reboiler temperature is dependent on
solution concentration,
flare/vent line back pressure, and/or
residual CO2 content required.
It is good practice to operate the reboiler at as low a temperature as possible.
The normal operating range for reboiler temperature is 225 to 260◦F for MEA and
230 to 250◦F for DEA systems.
4.4.3.3.5 Amine Stripper
Amine strippers use heat and steam to reverse the chemical reactions with CO2 and H2S.
The steam acts as a stripping gas to remove the CO2 and H2S from the liquid solution and
to carry these gases to the overhead.
Like the absorber, the stripper is either a tray or a packed column with approximately
20 trays or the equivalent height in packing.
To minimize amine vaporization loss,
there may be a water wash section at the top of the column with an additional four to six
trays.
The rich amine feed is introduced on the third or fourth tray from the top.
The lean amine is removed at the bottom of the stripper and
acid gases are removed from the top.
Liquid flow rates are greatest near the bottom tray of the tower where the liquid from the
bottom tray must provide the lean amine flow rate from the tower plus enough water to
provide the steam generated by the reboiler.
The lean amine circulation rate is known, and from the reboiler duty, pressure, and
temperature, the amount of steam generated and thus the amount of water can be
calculated.
The vapor flow rate within the tower must be studied at both ends of the stripper.
The higher of these vapor rates should be used to size the tower for vapor.
At the bottom of the tower the vapor rate equals the amount of steam generated in the
reboiler.
Near the top of the tower, the vapor rate equals
the steam rate overhead plus the acid gas rate.
The steam overhead can be calculated from the steam generated in the reboiler by
subtracting the amount of steam condensed by raising the lean amine from its inlet
temperature to the reboiler temperature and the amount of steam condensed by vaporizing
the acid gases.
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4.4.3.3.6 Amine Condensers
Amine-stripper condensers are typically overhead air-cooled and fin-fan exchangers.