Mass transfer coefficient of conventional amines

https://doi.org/10.1039/C7RA01352C

“Tontiwachwuthikul, et al.⁶⁷ have reported the mass transfer performance of CO₂ absorption into MEA and AMP by measuring the temperature and concentration along the height of the absorption packed column. They performed several experiments in a laboratory-scale absorber packed column with 12.7 mm ceramic Berl saddles packing type. The column was made of six packed bed sections with a total packing height of 6.55 m and a 0.1 m diameter. The profiles along the height of the packed column were obtained for different liquid to gas ratios, inlet CO₂ concentration in feed of flue gas, and amine concentrations. They did not determine the K_GCO2a_V values directly; rather, they modeled the packed column for CO₂ absorption with an MEA solution by applying a rate-based model. Aroonwilas and Tontiwachwuthikul⁶⁸ have studied experimentally K_GCO2a_V for CO₂ absorption into AMP solution. They performed their experiments on a laboratory-scale absorption column of 1.1 m packing height and a 0.019 m diameter, packed with EX-type structured packing with a specific surface area of approximately 1700 m² m⁻³. Their experimental results showed that the values of K_GCO2a_V at fixed operating parameters were unaffected by the CO₂ partial pressure over a range of 3–15 kPa. They also compared the values of K_GCO2a_V for two cases—columns packed with ceramic Berl saddles packing and EX-type structured packing at the same operating conditions. They found that the value of K_GCO2a_V for EX-type structured packing was six times greater than for the ceramic Berl saddles packing. Afterwards, Aroonwilas and Tontiwachwuthikul⁶³ have studied experimentally the K_GCO2a_V for CO₂ absorption into AMP solution under operating conditions different from their previous work.⁶⁸ They found that the effect of the CO₂ partial pressure on the values of K_GCO2a_V changed slightly at pressures above 6 kPa and the values of K_GCO2a_V reduced from 1 to 6 kPa. For flow rates in the absorber column, the K_GCO2a_V was unaffected by the gas flow rate but the liquid flow rate had a pronounced effect on K_GCO2a_V, which increased the values of K_GCO2a_V by increasing the liquid flow rate in the range of 6.1–14.8 m³ m⁻² h⁻¹. In addition, by increasing the CO₂ loading of the AMP solution, the K_GCO2a_V values decreased. Demontigny, et al.⁵⁸ have reported experimental data of CO₂ absorption into ultra-highly concentrated MEA solutions (up to 9 kmol m⁻³) and investigated the effects of process parameters on K_GCO2a_V in three pilot-scale absorption columns packed with random (16 mm Pall Ring and IMTP-15) and structured packing (Gempak 4A). The diameter and height of the absorption packed columns were 0.1 and 2.4 m, respectively. Their results showed that the values of K_GCO2a_V increased with increasing liquid flow rates and were unaffected by the gas flow rate. By increasing the CO₂ partial pressure and CO₂ loading, the values of K_GCO2a_V decreased. In relation to the MEA concentration, which was one of their important works, by increasing the MEA concentration up to 4 kmol m⁻³, the values of K_GCO2a_V decreased with a mild slope but increased in the range of 4–9 kmol m⁻³. They also studied the effect of the packing type on the K_GCO2a_V values, and found that structured packing (Gempak 4A) had a better performance compared with random packing (16 mm Pall Ring and IMTP-15). When comparing 16 mm Pall Ring packing with IMTP-15 packing, the IMTP-15 had greater K_GCO2a_V values. Aroonwilas, et al.⁶⁹ have performed experiments on the performance of three types of structured packing (laboratory-scale (EX), pilot-scale (Gempak 4A), and industrial-scale (SulzerBX)) in terms of the K_GCO2a_V coefficient. The experimental data was reported for CO₂ absorption into sodium hydroxide (NaOH), MEA and AMP solutions. The laboratory-scale absorption packed column was packed with 20 packing elements of EX and had a total packing height of 1.1 m and a 0.019 m diameter. The pilot-scale absorption packed column was packed with Gempak 4A stainless steel and the packing height varied between 0.98 and 2.21 m, and the absorber had a 0.1 m diameter. The third case was an industrial-scale absorption–desorption unit in which an absorber column was packed with six elements of Sulzer BX gauze structured packing, and the column had a total packing height of 1.02 m and a 0.25 m. Their results indicated that the values of K_GCO2a_V increased with an increasing liquid flow rate and liquid concentration and were unaffected by the gas flow rate. The values of K_GCO2a_V decreased with increasing CO₂ concentrations up to 15%. The values of K_GCO2a_V increased with solvent temperature from 20 °C to 37 °C and decreased with temperatures from 40 °C to 65 °C. When comparing structured packing (Gempak 4A) and IMTP-25 packing, the Gempak 4A provided two times greater K_GCO2a_V values. Aroonwilas and Veawab⁶⁵ have comprehensively investigated the performance of conventional amines such as MEA, DEA, DIPA (diisopropanolamine), MDEA, and AMP; in addition, they have investigated blends including MEA–MDEA, DEA–MDEA, MEA–AMP, and DEA–AMP. They performed the experiments in a laboratory-scale absorption column with a 2 m packing height and a 0.02 m diameter with 36 DX-type elements of structured packing. Their results were presented based on the CO₂ removal efficiency, absorber height requirement, effective mass-transfer area, and K_GCO2a_V, under identical conditions for the liquid flow rate and CO₂ loading. Their result showed that the CO₂ removal efficiency in a CO₂ loading of zero was in the order MEA > DEA > AMP > DIPA > MDEA. The value of 100% of CO₂ removal efficiency was obtained for MEA, DEA, and AMP, requiring a 0.75, 1.75, and 2.0 m height of the packed column, respectively. Therefore, MEA showed a better performance in comparison with other studied amines. For blended amines, the value of 100% of CO₂ removal efficiency was obtained for MEA–AMP, DEA–AMP, MEA–MDEA, and DEA–MDEA, requiring a 1.2, 2.3, 3.3, and 5.4 m height of the packed column, respectively. The authors also assessed the performance in terms of the effective mass-transfer area under identical processing parameters and found that MEA provided the highest mass-transfer area among the tested amines, including DEA, DIPA, and MDEA. They also showed that the values of K_GCO2a_V at different CO₂ loadings for MEA were higher compared with other tested amines such as DEA, AMP, DIPA, and MDEA. Setameteekul, et al.⁷⁰ studied the mass transfer performance for CO₂ absorption in a MEA and MDEA blended amine. The experiments were performed based on the factorial experimental design method (a statistical method), and conducted more than 106 tests with three replications in an absorption column packed with a DX-type packing. The packing height varied between 0.165 and 0.825 m, and the absorber had a 0.02 m diameter. The results of the work by Setameteekul, et al.⁷⁰ indicated that the solvent temperature and solvent concentration have the largest effects on the K_GCO2a_V values and the other process parameters have smaller effects. Dey and Aroonwilas⁶⁶ used the blended MEA–AMP amine to determine K_GCO2a_V by using only two data sampling points of CO₂ concentrations at the bottom and top of an absorber column. In fact, they obtained the average values of K_GCO2a_V for an absorber column packed with a DX-type structured packing. Their results showed that the K_GCO2a_V values increased with an increasing liquid flow rate, temperature, and total amine concentrations, and decreased with increasing CO₂ partial pressure of the feed gas and CO₂ loading of the amines. The addition of higher concentrations of MEA in the mixed MEA–AMP amine led to an increase in the K_GCO2a_V values, except at high CO₂ loadings. This was because of a lower reaction rate of CO₂ with AMP compared with MEA.

Jeon, et al.⁷¹ have studied the mass transfer performance and effect of adding ammonia (NH₃) to AMP and MDEA. They determined the K_GCO2a_V values in an absorption packed column with a 1.5 m packing height and a 0.05 m diameter by testing two packing types including 6 mm ceramic Raschig rings, and a wire gauze laboratory-structured packing. They showed that the K_GCO2a_V values at a CO₂ partial pressure of 15 kPa increased for both mentioned systems by using structured packing, and increased even more by adding NH₃ from 1 wt% to 3 wt%. They also showed that the K_GCO2a_V values increased at lower CO₂ partial pressure and higher liquid-to-gas ratios. The overall conclusion of their work was that adding NH₃ to AMP and MDEA and using structured packing produced higher K_GCO2a_V values. Li, et al.⁷² have performed experiments for CO₂ absorption using an NH₃ solution to determine the K_GCO2a_V values in an absorber column packed with a novel structured packing with diversion windows type. The height of the absorber column and column diameter were 2.4 m (packing height: 2 m) and 0.15 m, respectively. Their results showed that K_GCO2a_V was enhanced by increasing the liquid flow rate and its concentration. However, the K_GCO2a_V values decreased when the CO₂ partial pressure increased to 8 kPa, and were unaffected by the gas flow rate. Kang, et al.⁷³ have tested various packing types including ceramic Raschig rings, Berl saddles, a structured gauze packing and a hybrid of Raschig rings and a structured packing in different ratios to investigate the mass transfer performance of a CO₂–MEA–AMP system. Their results showed that CO₂ removal efficiencies of Raschig rings, Berl saddles, and the structured packing materials provided higher values for the MEA than the AMP solution, and that the structured packing had a greater efficiency than the random packing. They improved the performance of single random and structured packing materials by mixing them in ratios of 1 : 1, 2 : 1, and 1 : 2. The optimal performance was obtained for the 2 : 1 ratio (structured packing/Raschig rings). The K_GCO2a_V parameter decreased in the order 2 : 1 hybrid packing > structured packing > Raschig rings > Berl saddles.

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