https://doi.org/10.1016/j.ijggc.2013.12.005
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4.4.1. Justification for case study
Temperature bulge in conventional absorber was reported by Freguia and Rochelle (2003), Kvamsdal and Rochelle (2008), Kvamsdal et al. (2009). It limits the overall performance of the absorber. It is necessary to investigate temperature profile in RPB absorbers to determine if it has similar problem.
4.4.2. Setup of the case study
To implement this case study, lean MEA flow rate of 0.66 kg/s, rotor speed of 1000 rpm were selected. For 56 wt% lean MEA concentration process conditions refer to Case 1 Run 1 of Table 1 and for 75 wt% lean MEA concentration refer to Case 3 Run 1 of Table 2, in both input conditions the rotor speed is replaced with 1000 rpm. The flue gas temperature was maintained at 47 °C during the study. The temperature profile study was done over two lean MEA temperatures of 25 °C and 50 °C.
4.4.3. Results and discussion
As stated in Kvamsdal and Rochelle (2008) that magnitude and location of temperature bulge are given in term of liquid temperature profile, this is because gas and liquid temperature profiles are similar in shape but the gas temperature profile will be lagged due to the difference in heat capacities of the two phases and the L/G ratio.
Fig. 8, Fig. 9 shows liquid temperature profile in RPB, outer radius where flue gas enters RPB is taken as 0 m. At 55 wt% MEA concentration, temperature profile has a steady gradient for the two temperatures under study. On the other hand, steeper gradient is noticed close to the outer radius. Both results show there is no temperature bulge in RPB. This is likely due to higher solvent to gas ratio (L/G) which is 30 kg/kg. Kvamsdal and Rochelle (2008) stated for conventional absorber in case where no temperature bulge, the enthalpy of reaction must leave with the gas and liquid. At high liquid rates, the enthalpy will leave with the liquid, while at high gas rates it will leave with the gas. In Fig. 8, Fig. 9 it can be observed that the temperature of lean MEA increases from the inner diameter to the outer diameter. This is because of the gain in the enthalpy of reaction since we have greater liquid rate than the gas rate. Also it can be observed in Fig. 8, Fig. 9 that exit temperature for the solvent at 0 m is higher for 75 wt% MEA concentration than for 55 wt% this is because of greater enthalpy of reaction at higher concentration.
Fig. 8. Liquid temperature profile in RPB absorber at 25 °C lean MEA temperature.
Fig. 9. Liquid temperature profile in RPB absorber at 50 °C lean MEA temperature.
Another factor that contributes to having no temperature bulge in RPB absorber is high mixing capability, which enhances heat transfer and significantly reduces residence time. This is also because there are no liquid build-up since high gravity in RPB stimulates droplet flow and little film flow (Burn’s et al., 2000).
From the above findings we can see that RPB absorber does not need inter-cooling provided it is operated at the conditions being studied. From this, we can see that the cost of energy for inter-cooling is saved if we are using RPB absorber. The temperature profile shows that a better column performance could be found in intensified absorber using RPB.
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