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\u00a0psia\u00a0to 1200\u00a0psia). For each pressure, temperature varied from 21.1 \u00b0C to 48.9 \u00b0C (70 \u00b0F to 120 \u00b0F).<\/p>\n
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.<\/p>\n
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CASE STUDY:<\/strong><\/p>\n For the purpose of illustration, this tip considers sweetening of 2.84 x 106<\/sup>\u00a0Sm3<\/sup>\/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\u00a0increment\u00a0of 0.690 MPa (800\u00a0psia\u00a0to 1200\u00a0psia\u00a0with an increment of 100\u00a0psia). For each pressure, the temperature was varied from 21.1 \u00b0C to 48.9 \u00b0C with an increment of 5.5 \u00b0C (70 \u00b0F to 120 \u00b0F with an increment of 10 \u00b0F). This tip uses ProMax [1] simulation software with \u201cAmine Sweetening \u2013 PR\u201d property package to perform all of the simulations.<\/p>\n <\/p>\n Table 1<\/strong>. Feed composition and flow rate<\/p>\n <\/p>\n Figure 1 [7] presents a typical\u00a0sweetening\u00a0process flow diagram for the case study. Note this diagram has a trim cooler to control the top temperature of the\u00a0absorber\u00a0and a reflux condenser that minimizes the water and MDEA losses via the acid gas stream.<\/p>\n <\/p>\n Figure 1<\/strong>.\u00a0Simplified process flow diagram for an amine sweetening unit [7]<\/em><\/p>\n <\/p>\n The following specifications\/assumptions for the case study are considered:<\/p>\n Absorber\/Contactor Column<\/strong><\/p>\n \u25baFeed sour gas is saturated with water<\/p>\n \u25baNumber of theoretical stages = 7<\/p>\n \u25baPressure drop = 20 kPa (3 psi)<\/p>\n \u25baLean amine solution temperature\u00a0 = Sour gas feed temperature. Typically lean solution should be 5.5 \u00b0C (10 \u00b0F) higher than feed gas, but there is no concern about hydrocarbon dewpoint for this feed.<\/p>\n <\/p>\n Regenerator\/Stripper Column<\/strong><\/p>\n \u25baNumber of theoretical stages = 10 (excluding condenser and reboiler)<\/p>\n \u25baFeed rich solution pressure = 414 kPa (60 psia); typically stream 6 have a letdown valve to reduce pressure to the stripper column pressure.<\/p>\n \u25baFeed rich solution temperature = 98.9 \u00b0C (210 \u00b0F ); this is conservative and could be 107 \u00b0C at 414 kPa (225 \u00b0F at 60 psia)<\/p>\n \u25baCondenser temperature = 48.9 \u00b0C\u00a0(120 \u00b0F ); this reflects warm climate with aerial cooler<\/p>\n \u25baPressure drop = 21 kPa (3 psi)<\/p>\n \u25baBottom pressure and temperature\u00a0= 214 kPa (31 psia), about 126 \u00b0C (259 \u00b0F)<\/p>\n <\/p>\n Reboiler Duty<\/strong><\/p>\n \u25baSteam rate = 132 kg of steam\/m3<\/sup>\u00a0of amine solution (1.1 lbm<\/sub>\/gallon) times amine circulation rate<\/p>\n \u25baSaturated steam pressure = 348 kPag (50 psig) at 147.7 \u00b0C (297.7 \u00b0F)<\/p>\n <\/p>\n Heat Exchangers<\/strong><\/p>\n \u25baLean amine cooler pressure drop\u00a0 = 35 kPa\u00a0 (5 psi)<\/p>\n \u25baRich side pressure\u00a0 = 35 kPa (5 psi)<\/p>\n \u25baLean side pressure\u00a0 = 35 kPa (5 psi)<\/p>\n <\/p>\n Main Pump<\/strong><\/p>\n \u25baDischarge Pressure = Feed sour gas pressure + 35 kPa (5 psi)<\/p>\n \u25baEfficiency = 65 %<\/p>\n <\/p>\n Reflux Pump<\/strong><\/p>\n \u25baDischarge Pressure = 350 kPa (50 psi)<\/p>\n \u25baEfficiency = 65 %<\/p>\n <\/p>\n Lean Amine Concentration and Circulation Rate<\/strong><\/p>\n \u25baMDEA concentration in lean amine = 50 weight %<\/p>\n \u25baLean amine circulation rate was adjusted (by solver tool) to reduce the H2<\/sub>S concentration in sweet\u00a0gas to 4 ppmv (The calculated rates resulted in a total acid gas loading in rich solution in the range of ~0.28 to0.54 mole\u00a0acid gases\/mole of MDEA)<\/p>\n \u25baTotal acid gas loadings in lean solution in the range of ~0.002 to 0.004 mole\u00a0acid gases\/mole of MDEA<\/p>\n <\/p>\n Rich Solution Expansion Valve<\/strong><\/p>\n \u25baFlash tank pressure = 448 kPa (65 psia)<\/p>\n <\/p>\n <\/p>\n RESULTS AND DISCUSSIONS<\/b><\/span><\/p>\n For the above specifications, ProMax [7] is used to simulate the process flow diagram in Figure 1. The objective was to produce a sweet gas with 4 ppmv H2<\/sub>S and less than 3 mole % CO2<\/sub>. 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\u00a0recorded. The following properties are reported here and the rest are presented in the Appendix.<\/p>\n <\/p>\n 1. MDEA circulation rate and total rate of MDEA vaporization losses in<\/p>\n \u25baSweet gas<\/p>\n \u25baFlash gas from the amine flash tank<\/p>\n \u25baAcid gas from regenerator\/stripper<\/p>\n 2. H2<\/sub>S, CO2<\/sub>, and total acid gas loadings (mole acid gas\/mole MDEA) in<\/p>\n \u25baLean amine<\/p>\n \u25baRich amine<\/p>\n 3. H2<\/sub>S and CO2<\/sub>\u00a0concentration in the sweet gas.<\/p>\n 4. Heat duties<\/p>\n \u25baRegenerator\/Stripper condenser and reboiler<\/p>\n \u25baLean-Rich exchanger<\/p>\n \u25baLean amine (trim) cooler<\/p>\n 5. Pump power requirements<\/p>\n \u25baReflux pump<\/p>\n \u25baMain pump<\/p>\n <\/p>\n 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\u00a0and the highest pressures are presented.<\/p>\n 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<\/sub>\u00a0in 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 \u00b0C to 11 \u00b0C (10 \u00b0F to 20 \u00b0F) higher than the feed gas temperatures. In all\u00a0cases, the sweet gas pressure was 21 kPa (3 psi) lower than the feed gas pressure.<\/p>\n Figure 2 presents the variation of the lean MDEA solution rate, in the standard cubic meter per hour, Sm3<\/sup>\/h (standard gallon per minute,\u00a0sgpm), 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\u00a0a lower\u00a0temperature and higher pressure.<\/p>\n <\/p>\n Figure 2.<\/strong>\u00a0Variation of lean MDEA volumetric rate with pressure and temperature<\/em><\/p>\n <\/p>\n 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.<\/p>\n 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\u00a0higher\u00a0pressure 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 %\u00a0 MDEA at high pressure of 6.21 MPa (900\u00a0psia) [8]. The MDEA vaporization losses from the top of the stripper\/regenerator column\u00a0were\u00a0practically zero.<\/p>\n <\/p>\n Figure 3<\/strong>. Variation of total MDEA vaporization loss with pressure and\u00a0temperature<\/em><\/p>\n <\/p>\n Figure 4 presents the variation of the ratio of MDEA vaporization loss in sweet gas to\u00a0the loss\u00a0in flash gas (mass basis) with pressure\u00a0and\u00a0temperature. 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.<\/p>\n <\/p>\n Figure 4<\/strong>. Variation of the ratio of MDEA vaporization loss in sweet gas to\u00a0<\/em>loss<\/em>\u00a0in flash gas (mass basis) with pressure and temperature<\/em><\/p>\n <\/p>\n Figure 5 presents the variation of CO2<\/sub>\u00a0concentration in the sweet gas as a function of the feed sour gas pressure and temperature. The calculated CO2<\/sub>\u00a0concentrations 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<\/sub>\u00a0concentration in the sweet gas decreases. However, the feed sour gas pressure has a small effect on CO2<\/sub>\u00a0mole % at low temperature but at\u00a0higher temperature CO2<\/sub>\u00a0mole % decreases as pressure increases. At higher temperature the MDEA circulation rate increases, which reduces the CO2<\/sub>\u00a0concentration in the sweet gas.<\/p>\n <\/p>\n Figure 5<\/strong>. Variation of CO2<\/sub>\u00a0concentration in sweet gas with pressure and\u00a0temperature<\/em><\/p>\n <\/p>\n 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\u00a0low pressure\u00a0contactor 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<\/sub>\u00a0content of the sweet gas decreases. Rich loadings are lower due to increased circulation rate.\u00a0 Circulation rate increases faster than\u00a0the increase\u00a0in\u00a0CO2<\/sub>\u00a0pickup.<\/p>\n <\/p>\n Figure 6.<\/strong>\u00a0Variation of lean and rich MDEA solution loadings with pressure and\u00a0 temperature<\/em><\/p>\n <\/p>\n <\/p>\n CONCLUSIONS:<\/strong><\/p>\n Based on the results obtained for the considered case study, this TOTM presents the following conclusions:<\/p>\n 1.<\/strong>\u00a0As the feed gas temperature to the contactor column increases, the lean MDEA solution rate increases whereas pressure has an opposite effect (Fig 2).<\/p>\n 2.<\/strong>\u00a0As the feed gas temperature to the contactor column increases, the total MDEA vaporization losses increase (Fig 3).<\/p>\n 3.<\/strong>\u00a0As 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.<\/p>\n 4.<\/strong>\u00a0The 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.<\/p>\n 5.<\/strong>\u00a0The CO2<\/sub>\u00a0mole % in sweet gas decreases with increasing temperature due to a higher circulation rate and better kinetics for CO2<\/sub>\u00a0absorption (Fig 5).<\/p>\n 6.<\/strong>\u00a0The 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.<\/p>\n 7.<\/strong>\u00a0Even 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\u00a0much higher than the vaporization losses presented here.<\/p>\n To learn more about similar cases and how to minimize operational troubles, we suggest attending our\u00a0G6<\/strong><\/a>(Gas Treating and Sulfur Recovery<\/a>)<\/a>,<\/strong><\/u>\u00a0G4 (<\/strong>Gas Conditioning and Processing)<\/strong><\/a>,<\/strong>\u00a0G5 (<\/strong>Advanced Applications in Gas Processing)<\/strong><\/a>,<\/strong>\u00a0PF4<\/strong>\u00a0(Oil Production and Processing Facilities)<\/a>\u00a0and\u00a0PF49<\/strong>\u00a0(Troubleshooting Oil and Gas Processing Facilities)<\/a>\u00a0courses.<\/p>\n Written By: Dr. Mahmood Moshfeghian<\/em><\/p>\n <\/p>\n <\/p>\n Sign up to receive Tip of the Month email updates!<\/em><\/p>\n![](\"https:\/\/i0.wp.com\/www.petroskills.com\/images\/jul18-fac\/chart.png\")
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