Mixing amines can increase capacity or improve efficiency in an amine sweetening plant.1
Changing the amine type from monoethanolamine (MEA) or diethanolamine (DEA) to MDEA is done to overcome specific plant problems that can be categorized into three main groups:
- The amine-treating section of the plant is a bottleneck for plant-capacity increase or feed-gas composition change.
- Sour-gas quality delivered to a Claus unit is such that it requires improvements; that is, an H2S concentration increase by introduction of CO2 slippage in treated gas.
- Corrosion.
In some cases, generic MDEA works, but product gas specifications are not normally achieved.2 In other cases, it does not work,3 and tailoring a mixed amine is needed to fulfill the plant requirements.
This proved the case in the 102 million standard cu m/d (MMscmd) Fajr gas plant in southwest Iran, 30 km west of Jam. It receives sour gas from gas-condensate reservoirs of Nar and Kangan.
The plant has eight similar trains, each with a capacity of 12.75 MMscmd. The process in each train is typical DEA sweetening for H2S and CO2 removal followed by a Merox catalyst unit for mercaptan removal and a typical propane refrigeration system for dew point control.
There are also corresponding utilities available in the plant.
Advantages, disadvantages
MDEA is a tertiary amine and cannot react directly with CO2 to produce carbamate ion. This gives very interesting capabilities regarding gas-sweetening processes.
Generally, advantages of MDEA could be summarized as follows:
- Higher solution concentration.
- Higher loading with fewer corrosion problems.
- Lower heat of reaction, leading to better stripping.
- Higher resistance to degradation.
- Lower vapor pressure.
- Selectivity toward H2S in presence of CO2.
The clear disadvantage of MDEA becomes evident when CO2 slippage is not desired. In such cases, combining the advantages of various amines is one solution, in which the optimum solution concentration should be carefully investigated and depends on:
- Absorber pressure.
- Acid gas concentration in the feed.
- Contactor operating temperature.
- Sweet gas specification.
- Absorber number of trays.
Steam consumption
In an amine unit, the stripper reboiler's job is to:
- Increase the temperature of rich solvent to the temperature of the reboiler.
- Increase the temperature of the reflux to the temperature of the reboiler.
- Maintain the heat requirement to reverse the reaction of the acid gases and amine.
- Vaporize water for acid gas carrying through the stripper tower up to the reflux drum.
Therefore, MDEA steam consumption is less than for other amines. This is due, in part, to the fact that MDEA is less corrosive. As such, it can be used in a high concentration that, in turn, will result in a lower circulation rate.
In addition, the amount of thermal energy and level of acid-gas reactions are less throughout this process.
The processes for DEA and a blend of MDEA/DEA are similar2 in that they eliminate the need for new equipment, process variables, or antifoaming agent, just to name a few.
Case study
Feed gas for the Fajr gas-treating plant contains 1.7 mol % CO2, 380 mg/std. cu m H2S, and 30 mg/std. cu m mercaptan.
The plant receives feed gas at 76 bar (approximately 1,100 psi) and a temperature range of 40-55° C.
Feed gas entering each train is subjected to a typical sweetening plant in which H2S and CO2 content is absorbed by DEA then forwarded to a tower where mercaptans are reacted with caustic solution activated by Merox catalyst.
Finally, a propane system is used for gas conditioning and dew point control.
In such a process, the treated gas leaving the absorber should not contain CO2 greater than 30 ppm (vol); otherwise deactivation of the Merox catalyst may result.
The plant underwent a revamping study for capacity increase up to 13.75 MMscmd for each train in 1998. One of the more serious bottlenecks was caused by the steam requirements in stripper reboilers. Although the reboiler duty was performing acceptably, there was insufficient steam.
There were two alternatives for the extra steam demand: installing an additional boiler or replacing the existing solvent with MDEA, which consumes less steam.
A simulation for MDEA using PROII simulation software declared the adaptability of MDEA to the process variables except CO2 concentration in the gas leaving the amine absorber that was much higher than the value required, 30 ppm (vol).
Simulation results using a blend of DEA and MDEA declared the capability of blend solvent to fulfill all process specification requirements
Test run
A test run in one train has been in progress since Nov. 27, 1999. Table 1 shows plant operations of the test for slightly more than the first month of the run.
As PROII predicted, MDEA alone could not reduce the concentration of CO2 in treated gas less than 30 ppm (vol). At present, an optimized blend of MDEA-DEA based on experimental data is used. The result is that a feed gas flow rate of 14 MMscmd is achieved in the system.
To obtain the maximum advantage of MDEA, the concentration of DEA in the mixture is maintained as low as possible to achieve a satisfactory concentration of CO2 in the system.
The following constraints5 6 have been adopted to control the corrosion:
- Total amine concentration: 45 wt %.
- Max rich loading: 0.4 mole, acid gas/mole total amine.
- Reflux ratio: 1 lb/mole rich total amine.
The unit train has been operated under the new condition (MDEA-DEA blend) since November 1999 without problem. The concentration of CO2 in the sweet gas was less than 30 ppm (vol).
Comparing data with other trains operating with DEA at the same condition is very revealing:
- Reduction in circulation rate: 25%.
- Reduction in steam consumption: 30%.
- Reduction in amine loss: 30% (based on 4-month operation).
- Total Fe concentration in amine: 0.67 ppm (wt), based on lab test dated Apr. 2, 2000.
As far as corrosion is concerned, based on published data2 and the aforementioned preventive methodology, it is expected that no severe corrosion will be faced. It will, however, be investigated by routine tests for Fe concentration in amine solution and visual inspection after 10 months of operation.
Acknowledgment
The authors acknowledge Messrs. G. Barvayeh, H. Farhang, H. Shamshiri, A. Ahmadi, and R. Afrang of the Fajr gas plant for their participation in test-run and data collection.
References
- Spears, M.L., et al., "Converting to DEA/MDEA mix ups sweetening capacity," OGJ, Aug. 12 1996, p. 63.
- Law, D., "New MDEA design in gas plant improves sweetening, reduces CO2," OGJ, Aug. 29, 1994, p. 83.
- Thomas, C.D., "Specialty amines cope with varying loads," Chemical Engineering, January 1993, p.145.
- Dupart, M.S., et al., "Understanding corrosion in alkanolamine gas treating plants," Hydrocarbon Processing, April 1993, p. 75 (Part 1); and May 1993, p. 89 (Part 2).
- Mak, H.Y., "Gas plant converts amine unit to MDEA-based solvent," Hydrocarbon Processing, October 1992, p. 91.
- Dupart, M.S., et al., "Comparing laboratory and plant data for MDEA/DEA blends," Hydrocarbon Processing, April 1999. p. 81.
The authors
Homayoun S. Saremi is a senior process specialist of operation management technical services in National Iranian Gas Co. (NIGC), Tehran. He holds a BS in chemical engineering from Petroleum University of Iran.
Norouzali Abdi is a senior process engineer in the technical and engineering department at the Fajr gas plant, Jam, Iran. He holds a BS in chemical engineering from Polytechnique University of Iran.
Mohammad A. Kashani is a shift supervisor at the Fajr gas plant. He holds a BS in chemical engineering from Petroleum University of Iran.
