Solvent circulation times and estimation of real-time solvent working capacity

To effectively understand the impact of dynamic scenarios on solvent CO₂ loading at the absorber inlet and outlet, time-shifting is utilised in order to estimate the time at which each “packet” of lean or rich solvent taken for analysis will reach the absorber packed bed inlet or outlet, respectively. The continuous online solvent measurement is also time-shifted using the same method. This is illustrated in Fig. 8, Fig. 10, Fig. 12, Fig. 14. It is possible to estimate when each discrete packet of solvent sampled at the lean inlet reaches the absorber, by estimating the total solvent inventory between the lean sampling port and the absorber inlet and dividing by the flow rate. It is also possible to determine when each rich solvent sample reaches the base of the packed bed by estimating the time required to flow through the absorber sump. The forward time-shifting of the lean loading measurements does not account for potential mixing effects in the main solvent tank, nor does the backwards time-shifting of rich loading measurements account for mixing effects in the absorber sump, as both rely on the assumption of plug flow. This time-shifting method has been applied to lean and rich titration measurements and continuous lean loading measurement values in Fig. 8, Fig. 10, Fig. 12, Fig. 14, in order to more effectively illustrate how the CO₂ capture rate responds to changes in solvent lean loading. The driving force for CO₂ absorption by the working solvent is dependent on its inlet CO₂ loading, and shifting the lean loading measurements to the absorber inlet shows more clearly how the capture rate and lean loading are related. However, the time-shifting method is unable to account for potential mixing effects in the solvent tank and pipework.

Since the times that the rich and lean samples taken for bench measurements generally do not match, the real-time solvent working capacity (defined as the difference in solvent loading between the absorber inlet and absorber outlet at any given point in time) is estimated using the capture rate and flow rates of CO₂ and MEA. This can be done using two methods:

1.

Each lean loading bench measurement is time-shifted forward to the time where it enters the absorber inlet, rich loading at the base of the packing is then estimated using the average capture rate and flow rates of CO₂ and MEA over the following minute

2.

Each rich loading bench measurement is time-shifted backwards to the time where it reaches the base of the absorber packing, lean loading at the absorber inlet is then estimated using the average capture rates and flow rates of CO₂ and MEA over the previous minute.