https://doi.org/10.1016/j.petlm.2016.11.002
“Due to the continuous and cyclic nature of a conventional MEA stripping process, the aim is to operate at a high working capacity over a narrow loading range. Moreover, desorption should take place quickly to permit fast cycle times, and hence reduce capital costs [7]. A target of 50% CO2 recovery is often sought (i.e. 50% of the CO2 content of the rich amine stream is recovered), however industrial systems typically operate at capture efficiencies as low as 15–30% [22]. The feasibility and steady-state performance of the microwave regeneration process was evaluated by completing multiple (six) back-to-back absorption-desorption cycles. The amine solution was loaded with an inlet gas stream of 20% CO2 balanced by N2 without heating. The CO2-rich solution was regenerated by microwave irradiation at either 70 °C or 90 °C to produce a CO2-lean solution. The breakthrough curves and solution temperatures of the multiple absorption-desorption cycles are outlined in Fig. 5. A blank run was also performed to allow the absorbed CO2 quantity to be determined. Apparent in Fig. 5, a large difference between the blank and first absorption runs indicates an initial loading of the solution with CO2. A portion of the bound CO2 is then recovered during the first desorption step. This partial recovery results in a lean solution in which some CO2 remains. The next absorption step then increases the total loading and subsequent regeneration produces more CO2 relative to the first desorption step. This is reflected in the outlet CO2 flow rates in Fig. 5(i). A higher lean loading means less CO2 is absorbed from the inlet gas stream and so the outlet CO2 concentration moves towards that of the blank. This trend continues until the solution reaches a steady-state limit, generally between one and two cycles.
Fig. 5. Breakthrough curves (solid lines) and temperatures (dashed lines) for six back-to-back absorption-desorption cycles using microwave regeneration. (a) Regeneration at 70 °C. (b) Regeneration at 90 °C. (i) Absorption step. (ii) Desorption step. Cycle number: 1 (black), 2 (blue), 3 (red), 4 (green), 5 (magenta), 6 (dark yellow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
As the lean loading increases, the heat of absorption decreases, and so aside from the first step, heat generated during absorption does not vary significantly over consecutive cycles. Rapid microwave heating to the target regeneration temperature occurs, consistent with the direct and volumetric nature of microwave heating. This very fast temperature increase quickly generates an appreciable amount of CO2, as seen from the desorption curves, which peak at early times and decline as more CO2 is removed. The breakthrough curves suggest an overall stable cyclic working capacity, where regeneration can take place at temperatures considerably lower than the industrial benchmark [5], [9]. Comparison of Figs. 5(a) and 5(b) clearly demonstrates an enhanced CO2 recovery at 90 °C compared to 70 °C. This is evidenced in both the absorption and desorption curves, where much more CO2 is released at 90 °C, allowing a larger re-uptake in the absorption step to reach steady-state. The CO2 desorption profile reaches a higher peak at 90 °C and declines more rapidly than for 70 °C, indicating a faster desorption rate at higher temperature. In the same respect, the absorption profile following 70 °C regeneration produces a higher CO2 outlet concentration sooner than after 90 °C regeneration, indicating a lower CO2 re-uptake due to a higher lean loading.
Integration of the CO2 curves from Fig. 5, normalized by the number of moles of MEA, permits the rich and lean loadings to be calculated. These are displayed in Fig. 6 for 70 °C and 90 °C regeneration along with the percentage CO2 recovery for each cycle. Fig. 6 supports the key findings drawn from Fig. 5. The first absorption-desorption loadings are lower than for subsequent cycles, reflecting the initial loading phase of the system as it approaches steady-state. Both temperature sets display stable cyclic CO2 capacities over finite loading ranges, with average rich loadings of around 0.5 mol CO2/mol MEA, indicating high capture efficiencies over consecutive cycles. Regeneration at 70 °C produces an average lean loading of 0.39 mol/mol with around 23% CO2 recovery each time, which lies within the acceptable working range [7]. Regeneration at 90 °C doubles the CO2 recovery to around 50%, providing lean loadings of 0.25 mol/mol. This is the optimum target for MEA scrubbing and has been achieved at temperatures 30–50 °C lower than with conventional methods [18]. Increasing the regeneration temperature even further could provide a higher percentage CO2 recovery, however this is not necessary, as optimal cyclic loadings have already been achieved at the lower regeneration temperatures used.
Fig. 6. Rich (grey bars) and lean (red bars) CO2 loadings for (a) 70 °C regeneration cycles and (b) 90 °C regeneration cycles. The percentage CO2 recovered between the absorption and desorption steps are shown as blue points. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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