1 Jul, 2018
Methyl Diethanolamine (MDEA) Vaporization Loss in Gas Sweetening Process
In natural gas treating, there are several processes available for removing acid gases. Aqueous solutions of alkanolamines are the most widely used . The alkanolamines process is characterized as “mass transfer enhanced by chemical reactions” in which acid gases react directly or react through an acid-base buffer mechanism with alkanolamines to form nonvolatile ionic species. For further detail of sour gas treating refer to references [1-6].
According to Seagraves et al. , amine vaporization and degradation losses constitute a small portion of the overall solvent losses which can be by mechanical means, entrainment due to foaming and solubility, and vaporization and degradation. “The vaporization and degradation account for as little as 3% of the overall solution losses” . However, it can be significant at lower pressures.
In this Tip of The Month (TOTM), the effect of pressure and temperature on the MDEA vaporization loss from the contactor top, regenerator top and flash gas is investigated. Specifically, this study focuses on the variation of MDEA vaporization losses with the feed sour gas pressure in the range of 5.52 MPa to 8.28 MPa (800 psia to 1200 psia). For each pressure, temperature varied from 21.1 °C to 48.9 °C (70 °F to 120 °F).
By performing the rigorous computer simulations of an MDEA sweetening process, several charts for demonstrating the impact of pressure and temperature on the MDEA vaporization loss and other operating parameters like the lean MDEA solution circulation rate are presented.
For the purpose of illustration, this tip considers sweetening of 2.84 x 106 Sm3/d (100.2 MMSCFD) of a sour natural gas using MDEA. Table 1 presents its composition and flow rate. The feed sour gas pressure was varied from 5.52 MPa to 8.28 MPa with an increment of 0.690 MPa (800 psia to 1200 psia with an increment of 100 psia). For each pressure, the temperature was varied from 21.1 °C to 48.9 °C with an increment of 5.5 °C (70 °F to 120 °F with an increment of 10 °F). This tip uses ProMax  simulation software with “Amine Sweetening – PR” property package to perform all of the simulations.
Table 1. Feed composition and flow rate
Figure 1  presents a typical sweetening process flow diagram for the case study. Note this diagram has a trim cooler to control the top temperature of the absorber and a reflux condenser that minimizes the water and MDEA losses via the acid gas stream.
Figure 1. Simplified process flow diagram for an amine sweetening unit 
The following specifications/assumptions for the case study are considered:
►Feed sour gas is saturated with water
►Number of theoretical stages = 7
►Pressure drop = 20 kPa (3 psi)
►Lean amine solution temperature = Sour gas feed temperature. Typically lean solution should be 5.5 °C (10 °F) higher than feed gas, but there is no concern about hydrocarbon dewpoint for this feed.
►Number of theoretical stages = 10 (excluding condenser and reboiler)
►Feed rich solution pressure = 414 kPa (60 psia); typically stream 6 have a letdown valve to reduce pressure to the stripper column pressure.
►Feed rich solution temperature = 98.9 °C (210 °F ); this is conservative and could be 107 °C at 414 kPa (225 °F at 60 psia)
►Condenser temperature = 48.9 °C (120 °F ); this reflects warm climate with aerial cooler
►Pressure drop = 21 kPa (3 psi)
►Bottom pressure = 214 kPa (31 psia), about 126 °C (259 °F)
►Steam rate = 132 kg of steam/m3 of amine solution (1.1 lbm/gallon) times amine circulation rate
►Saturate steam pressure = 348 kPag (50 psig) at 147.7 °C (297.7 °F)
►Lean amine cooler pressure drop = 35 kPa (5 psi)
►Rich side pressure = 35 kPa (5 psi)
►Lean side pressure = 35 kPa (5 psi)
►Discharge Pressure = Feed sour gas pressure + 35 kPa (5 psi)
►Efficiency = 65 %
►Discharge Pressure = 350 kPa (50 psi)
►Efficiency = 65 %
Lean Amine Concentration and Circulation Rate
►MDEA concentration in lean amine = 50 weight %
►Lean amine circulation rate was adjusted (by solver tool) to reduce the H2S concentration in sweet gas to 4 ppmv (The calculated rates resulted in a total acid gas loading in rich solution in the range of ~0.28 to 0.54 mole acid gases/mole of MDEA)
►Total acid gas loadings in lean solution in the range of ~0.002 to 0.004 mole acid gases/mole of MDEA
Rich Solution Expansion Valve
►Flash tank pressure = 448 kPa (65 psia)
RESULTS AND DISCUSSIONS
For the above specifications, ProMax  is used to simulate the process flow diagram in Figure 1. The objective was to produce a sweet gas with 4 ppmv H2S and less than 3 mole % CO2. In order to meet these specifications, the required lean MDEA solution volumetric rate was determined by the solver tool and then the calculated operation parameters were recorded. The following properties are reported here and the rest are presented in the Appendix.
1. MDEA circulation rate and total rate of MDEA vaporization losses in
►Flash gas from the amine flash tank
►Acid gas from regenerator/stripper
2. H2S, CO2, and total acid gas loadings (mole acid gas/mole MDEA) in
3. H2S and CO2 concentration in the sweet gas.
4. Heat duties
►Regenerator/Stripper condenser and reboiler
►Lean amine (trim) cooler
5. Pump power requirements
Five feed gas pressures and for each pressure 6 temperatures were simulated. For clarity and to avoid crowded curves on the diagrams, only the results for the lowest, the average and the highest pressures are presented.
The variation of operational parameters as a function of pressure and temperature are presented in Figures 2 through 6 for the lean MDEA solution rate, total MDEA vaporization loss, ratio of the MDEA loss in sweet gas to loss in flash gas, mole % of CO2 in the sweet gas, and the total acid gas loadings in the lean and rich MDEA solutions, respectively. Even the lean amine temperature was set equal to feed gas temperature, the top tray and sweet gas temperatures from overhead were 5.5 °C to 11 °C (10 °F to 20 °F) higher than the feed gas temperatures. In all cases, the sweet gas pressure was 21 kPa (3 psi) lower than the feed gas pressure.
Figure 2 presents the variation of the lean MDEA solution rate, in the standard cubic meter per hour, Sm3/h (standard gallon per minute, sgpm), as a function of feed sour gas pressure and temperature. Figure 2 indicates that as the sour gas temperature increases the required lean MDEA circulation rate increases. However, as the sour gas pressure increases the required lean MDEA solution decreases. As expected the absorption process works better at a lower temperature and higher pressure.
Figure 2. Variation of lean MDEA volumetric rate with pressure and temperature
MDEA vaporization losses can occur with the sweet gas, flash gas and acid gas streams which are replaced by MDEA in the makeup stream. Figure 3 presents the variation of the rate of total MDEA vaporization losses as a function of the feed sour gas pressure and the sweet gas temperature. Figure 3 indicates that as the feed gas pressure and temperature increases the rate of total MDEA vaporization losses increases. For a given circulation rate, the rate of vaporization loss typically increases with increasing temperature but decreases with increasing pressure. However, as presented in Figure 2, the lean MDEA circulation rate increases with temperature.
A combination of the increasing effect of temperature and circulation rate overcomes the decreasing effect of higher pressure on the vaporization rate. In other words, at higher pressure the circulation rate is a bit lower, which means for the same gas inlet temperature, the sweet gas temperature is a bit higher and then MDEA losses are a bit higher. The low rate of MDEA vaporization loss is in agreement with the values in Fig 1 reported by Teletzke and Madhyani for 50 weight % MDEA at high pressure of 6.21 MPa (900 psia) . The MDEA vaporization losses from the top of the stripper/regenerator column were practically zero.
Figure 3. Variation of total MDEA vaporization loss with pressure and temperature
Figure 4 presents the variation of the ratio of MDEA vaporization loss in sweet gas to the loss in flash gas (mass basis) with pressure and temperature. Figure 4 indicates the losses with sweet gas is about 150 to 4600 times higher than the losses with flash gas. This figure verifies that major vaporization losses occur at the absorber/contractor overhead.
Figure 4. Variation of the ratio of MDEA vaporization loss in sweet gas to loss in flash gas (mass basis) with pressure and temperature
Figure 5 presents the variation of CO2 concentration in the sweet gas as a function of the feed sour gas pressure and temperature. The calculated CO2 concentrations in the sweet gas were from 1.2 to 2.6 % which are less than the specified value of 3 mole % for all pressures and temperatures considered. Figure 5 indicates that as the feed sour gas temperature increases the CO2 concentration in the sweet gas decreases. However, the feed sour gas pressure has a small effect on CO2 mole % at low temperature but at higher temperature CO2 mole % decreases as pressure increases. At higher temperature the MDEA circulation rate increases, which reduces the CO2 concentration in the sweet gas.
Figure 5. Variation of CO2 concentration in sweet gas with pressure and temperature
Figure 6 presents the variation of the total acid gas loadings in the lean and rich solutions as a function of the feed sour gas pressure and temperature. Figure 6 indicates that the lean solution acid gas loading is practically independent of the feed sour gas pressure but decreases with the temperature increase. At higher temperature, more acid gas comes out in the flash, and less acid gas goes to the regenerator. The fixed steam rate then does a better job at stripping. Some systems have a low pressure contactor on the flash gas, which would recapture the acid gases and perhaps change this result. Figure 6 also indicates that as the feed sour gas temperature increases the rich solution total acid gas loadings decrease but increases with increasing pressure. At higher temperature the circulation rate increases and lowers the total acid gas loadings. The acid gas pick-up increases with higher temperature, as the CO2 content of the sweet gas decreases. Rich loadings are lower due to increased circulation rate. Circulation rate increases faster than the increase in CO2 pickup.
Figure 6. Variation of lean and rich MDEA solution loadings with pressure and temperature
Based on the results obtained for the considered case study, this TOTM presents the following conclusions:
1. As the feed gas temperature to the contactor column increases, the lean MDEA solution rate increases whereas pressure has an opposite effect (Fig 2).
2. As the feed gas temperature to the contactor column increases, the total MDEA vaporization losses increase (Fig 3).
3. As the feed gas pressure to the contactor column increases, the total MDEA vaporization losses increase (Fig 3); however, vaporization losses can be significant in systems operating at very low pressure or with high contactor overhead temperatures.
4. The MDEA vaporization loss from the contactor top is about 150 to 4600 times higher than the loss with flash gas (Fig 4). The MDEA vaporization loss from the top of still/regenerator column is practically zero.
5. The CO2 mole % in sweet gas decreases with increasing temperature due to a higher circulation rate and better kinetics for CO2 absorption (Fig 5).
6. The lean and rich total acid gas loadings decrease with the feed sour gas temperature increase due to the higher circulation rate (Fig 6), whereas pressure has a smaller effect in the opposite direction.
7. Even though not studied in this TOTM, mechanical and entrainment losses from the contactor top and regenerator top, as well as losses due to filter change, are also sources of loss much higher than the vaporization losses presented here.
To learn more about similar cases and how to minimize operational troubles, we suggest attending our G6 (Gas Treating and Sulfur Recovery), G4 (Gas Conditioning and Processing), G5 (Advanced Applications in Gas Processing), PF4 (Oil Production and Processing Facilities) and PF49 (Troubleshooting Oil and Gas Processing Facilities) courses.
Written By: Dr. Mahmood Moshfeghian
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Figure 7. Variation of sour gas temperature with feed gas pressure and temperature
Figure 8. Variation of pumps power with feed gas pressure and temperature
Figure 9. Variation of reboiler and condenser duties with feed pressure and temperature
Figure 10. Variation of Lean-Rich HEX and cooler duties with feed gas pressure and temperature