Pilot Test and Simulation of an Advanced Amine Process for CO 2 Capture

Transcription

1 Pilot Test and Simulation of an Advanced Amine Process for CO 2 Capture Xi Chen, Barath Baburao, Frederic Vitse * Alstom Power, 1409 Centerpoint Blvd, Knoxville, TN Summary An Advanced Amine Process (AAP) was extensively tested in the Alstom Power pilot plant facility located at Växjö, Sweden. The performance of a proprietary solvent was evaluated under varied flow schemes. The test campaign was comprised of several series of tests, including energy consumption at varied ratio of liquid to gas flow rate in absorber with 90% CO 2 removal, effect of absorber packing height, and energy performance under advanced flow schemes as compared to conventional ones. With the advanced flow schemes, the energy demand for CO 2 separation excluding CO 2 compression was reduced by 13% - 21% compared to the conventional flow scheme, at 90% CO 2 capture. The predictive capability of a process simulator was examined against the pilot plant data for absorber and regenerator. The correlations for mass transfer and interfacial area were developed and implemented for the rate-based modeling of the absorber. The comparison between the simulation results and the pilot plant data shows that the model is capable of satisfactorily predicting the performance of AAP under different process conditions and configurations. Comparison of the simulation to the experimental results in terms of deviation for key parameters is summarized in the following table. a: Table 1. Deviations between simulation and experimental results for AAP performance. Predicted Variable Average Absolute Deviation (AAD a ) BIAS b Absorber CO 2 removal percentage (wrt. Liquid) 4.7% -0.8% Absorber rich CO 2 concentration 2.7% -0.4% Absorber outlet solvent temperature ( C) 1.8 c -0.4 c Absorber outlet gas temperature ( C) 2.2 c -0.3 c Regenerator lean Loading 2.8% -0.2% Regenerator outlet gas flow rate 14.8% -6.5% Regenerator outlet gas temperature ( C ) 3.4 c -1.3 c Regenerator outlet solvent temperature ( C ) 0.4 c 0.3 c 1 AAD n 1 n y i, calc y y i 1 i, exp n y y i,exp 100% i, calc i,exp BIAS 100% n i 1 yi, exp b: n number of data points c: For temperature, the arithmetic mean value of deviations is used. * Corresponding author. Tel.: ; fax: address: frederic.vitse@power.alstom.com.

2 1. Introduction Amine-based post combustion CO 2 capture is a promising solution for mitigation of CO 2 emission from power plants. Alstom has been actively developing Advanced Amine Process (AAP) as one of the variety of CO 2 capture technologies in its portfolio. Alstom has obtained extensive operational experience from multiple AAP pilot facilities, one of which is a 1 tonne CO 2 /day pilot plant located at Växjö, Sweden (Figure 1). a)

3 Figure 1. b) VäxjöAdvanced Amine pilot plant, Alstom Power. a) closed, b) open. This facility, in operation between May 2011 and March 2012, achieved 90% CO 2 capture efficiencies using a proprietary solvent. The campaign was comprised of four batches of tests: Energy consumption at varied liquid-to-gas ratio for conventional flow scheme Effect of packing height in a conventional flow scheme set-up Effect of absorber intercooling and recycling in energy consumption with conventional regeneration scheme Performance of Advanced Flow Scheme (AFS) The use of intercooling and recycling as well as AFS have been discussed in two recent patents (Baburao et al. 2013, Leister et al. 2012) respectively. Process optimization has led to progressive reduction in energy consumption between 13% - 21% for AAP. (Baburao et al. 2012) This paper presents some of the results from this pilot campaign as well as the validation of a process simulation tool using steady-state plant data. Pilot plant operational results and model

4 development efforts are added to Alstom s know-how of designing and operating larger demonstration plants. 2. Process Description The schematic process flow diagram for AAP in conventional flow scheme is shown in Figure 2. Inlet gas was cooled and saturated with water in Direct Contact Heater (DCC) before being sent to absorber. For the absorption block, tests of different packing heights, intercooling positions and recycling configurations were performed to find the optimal configuration. The outlet gas from absorber goes through a water wash section and a condenser to remove amine and water vapor before being emitted. Flue Gas Stripped Oxygen Water Rich Solution Lean Solution CO 2 Clean Flue Gas To Stack CO 2 Cooler CO 2 Product Water Wash Cooler Water Wash Pump CO 2 Absorber Lean Cooler Rich Solution Pump Lean/Rich X Exchanger Regenerator Column Flue Gas From DCC Reboiler CO 2 Absorption Lean Solution Pump CO 2 Regeneration Figure 2. Conventional Flow Scheme for AAP As opposed to conventional flow scheme, in which only one exchanger is used between lean and rich amine streams, in AFS the cool rich amine solvent from absorbers is split, and the heat exchange duty is accomplished by multiple heat exchangers. The hot rich streams are then sent to different heights of the regenerator column. The regenerator column is equipped with three sections of packing. 3. Process Simulation Absorber Several flowsheet diagrams were created to simulate different absorber configurations. Rigorous rate-based separation calculation is implemented for the simulation of the absorber. Packing characterization was conducted in a separate study. The surface area model and mass transfer correlations developed therein were implemented in the simulations. Heat loss is assumed to be negligible.

5 Calculated Absorber CO 2 Removal Rate (%) Regenerator The inputs in regenerator simulation included temperature, pressure, flow rate and compositions of inlet streams, the column overhead pressure and reboiler duty. The temperature, composition and flow rate of the outlet amine solvent and the outlet gas are the main outputs to be examined. The regenerator is modeled with rate-based method. To make column convergence faster and easier, equilibrium-based simulation was run first to provide initial values for rate-based simulation. 4. Simulation Results and Discussion Absorber A parity plot for the comparison of CO 2 removal rate between simulation and measurements is shown in Figure 3. The tests with different numbers of packing sections or configurations have been differentiated with different colors and symbols. The values used for experimental CO 2 removal rate is the average of that calculated from gas phase composition change and that from liquid composition change. The latter two values are represented by the two end points of each error bar respectively. As can be seen from the figure, most of the simulation results agree with the measurement within ±5% if the extension of the error bars is considered. Simulation agrees well with pilot plant data, although for certain cases the deviation is somewhat significant. 100% 95% 90% 85% +5% 80% 75% 70% 65% -5% 3 PS 4 PS 5 PS 6 PS 7 PS 7 PS w/ IC&RC Figure 3. 60% 60% 65% 70% 75% 80% 85% 90% 95% 100% Experimental Absorber CO 2 Removal Rate (%) Comparison of predicted CO 2 removal efficiency to experimental measurements for absorber. (PS = Packing Sections, IC = Intercooling, RC = Recycling) The rich CO 2 concentration predicted by the simulations is compared to experimental data in Figure 4. The values calculated by simulation matches the measured ones with the deviation less than 5%. Especially, good agreement between experiments and simulation is obtained for the richer rich

6 Calculated Absorber CO 2 Conc. (wt%) of Outlet Amine loading regime. The deviation between simulation and data on the very rich end could be possibly attributed to the error associated with rich CO 2 sample collection and loading measurements % 5.0-5% Abs 4 Abs 5 Abs Abs 7 Abs 7 Abs w/ IC&RC Experimental Absorber CO 2 Conc. (wt%) of Outlet Amine Figure 4. Comparison of predicted rich CO 2 concentration to the calculated value based on experimental measurements for absorber. In Figure 5, the predictions for outlet amine solvent temperature agree well with measurements within ±1 C. The good agreement indicates that heat of absorption and heat of capacity are well modeled by the simulator.

7 Calculated Absorber Outlet Amine Temp. ( C) C 50-1 C PS 4 PS 5 PS 6 PS 7 PS 7 PS w/ IC&RC Figure Experimental Absorber Outlet Amine Temp. ( C) Comparison of predicted outlet amine solvent temperature to experimental measurements for absorber. Figure 6 shows that outlet gas temperature for absorbers are well predicted by simulation. For some cases with no intercooling, the outlet gas temperature is over predicted by up to 3 C. A 2 C in the predicted outlet gas temperature is a conservative uncertainty.

8 Calculated Absorber Outlet Gas Temp. ( C) C -2 C PS 4 PS 5 PS 6 PS 7 PS 7 PS w/ IC&RC Experimental Absorber Outlet Gas Temp. ( C) Figure 6. Comparison of predicted outlet gas temperature to experimental measurements for absorber. Regenerator Figure 7 compares the predicted outlet gas flow rates to the experimental values. They agree with each other within ±20%. The disagreement could be attributed to the potential experimental errors in the flow measurement.

9 Calculated Outlet Gas Flowrate (kg/hr) % 20-20% Experimental Outlet Gas Flowrate (kg/hr) Figure 7. Comparison of predicted outlet gas flow rates with measured values for the total condenser. As shown in Figure 8, the predicted CO 2 concentration and temperature of the amine solvent stream at the outlet of the reboiler is in good agreement with experimental values for all AFS tests. The deviations in the CO 2 concentration and the temperature are less than 5% and 1 C respectively.

10 Cal. outlet amine Temp. ( C) Calculated CO 2 Conc. in outlet amine (wt%) % -5% Experimental CO 2 Conc. in outlet amine (wt%) a) C C 115 Figure Exp. outlet amine Temp. ( C) b) Comparison of predicted CO 2 concentration (a) and temperature (b) of the outlet amine solvent to measured values for the regenerator in AFS. The temperature at the top of the regenerator column, which is also the temperature for the inlet stream to the partial condenser, is compared between simulation and experiment in Figure 9. The predictions for the vapor stream at the top of the packing are in good agreement with those experimental

11 Cal. column top Temp. ( C) values with only a few exceptions. It is found that the mass transfer correlation selected is of significant impact on the outcome of the simulations. This is believed to be due to the change in gas mass transfer coefficient and the rate of water condensation from gas to liquid. The error on the prediction of the vapor stream temperature coming out of the packing top is about ± 3 C C -3 C AFS Conventional Exp. column top Temp. ( C) Figure 9. Comparison of predicted temperature at the top of the regenerator column (inlet temperature for the partial condenser) to the experimental values. 5. Conclusions An Advanced Amine Process with varied absorber configurations and flow schemes was tested in Alstom s facility at Växjö, Sweden. Simulation satisfactorily predicts the performance of absorber and regenerator under various process conditions. The knowledge and experience gained from the operation and simulation of VäxjöAAP will be used, together with other AAP pilot tests conducted by Alstom, to further improve amine-based CO 2 removal process. References Baburao, Barath and Schubert, Craig. Advanced intercooling and recycling in CO 2 absorption. U.S. Patent No. 8,460,436. Jun 11, Baburao, Barath; Pontbriand, Michael et al. Advanced Amine Process Technology Pilot Plant at Le Havre: First Operations and Results. The 11 th International Conference on Greenhouse Gas Technology, Kyoto, Japan, November 18-22, 2012.

12 Leister, Jonathan W.; Baburao, Barath; Vitse, Frederic. Method and System for Removal of Gaseous Contaminants. U.S. Patent 2012/ A1, July 12, 2012.