Many studies and reviews of process modification have already been published in open literatures (Oyenekan and Rochelle 2007; Le Moullec and Kanniche 2011a; Cousins et al. 2011; Ahn et al. 2013; Le Moullec et al. 2014), which contain a variety of amine based capture process modifications for the purpose of energy consumption reduction. However, most studies only evaluate process configurations for MEA solvent and the interaction between solvent and process is ignored (Le Moullec et al. 2014). Therefore, it is worth investigating the energy consumption of different amine solvents in different processes. In this work, ten different process configurations are simulated using MEA and DEA solvent to compare their thermal performances and investigate the energy saving by different process configurations. To make a more valuable and comprehensive evaluation on energy consumption reduction, the performance is presented in terms of reboiler duty as well as the total equivalent work. The reboiler duty is calculated in process simulation with PROII based on given process parameters. And the calculation method for the total equivalent work will be described in the following Sect. 4.1. It is worth mentioning that all simulations are restrained to maintain the temperature of amine solution below 120 °C, which is to avoid the degradation of MEA and DEA. The lean solution temperature is 40 °C.
Intercooled absorber (ICA)
Intercooled absorber is a widely studied and used modification (Aroonwilas and Veawab 2007; Karimi et al. 2011). Absorption of CO2 is an exothermic process which will lead to the temperature rise in the absorber. This has a negative effect on thermodynamic driving force for absorption and it results to lower the solvent absorption capacity. Figure 2 illustrates that this modification is to remove a part or all of the liquid flow from the absorber at one of its stages, cooling it and then injecting it back at the same part. Intercooled absorber is efficient in control of the temperature in the absorber column which can increase the carrying capacity of the solvent and hence reduce the required amount of recycling solvent as well as the size of equipment. In simulation work, 5th stage temperature is cooled down to 45 °C for MEA and DEA. As a result, the rich CO2 loading reaches 0.492 mol CO2/mol MEA, which is 0.465 mol CO2/mol MEA in conventional process. For DEA, 0.468 mol CO2/mol DEA is obtained as only 0.447 mol CO2/mol DEA before. It is found that the cycled lean amine solvent is reduced by 11.5% for MEA and 4.7% for DEA. Thus, 7.1% of reboiler duty is saved by MEA and DEA gains 2.8%. ICA is more efficient for MEA than DEA due to the heat of reaction with CO2 is higher for MEA. In such favourable process in thermodynamics, MEA gains more benefits by cooling in absorber.
Flue gas pre-cooling (FGP)
Flue gas pre-cooling is a simple modification discussed in the work of Tobiesen et al. (2007) and Le Moullec and Kanniche (2011a). As Fig. 3 shows, flue gas is cooled to a lower temperature before introduced to absorber. The principle of flue gas pre-cooled is similar with the intercooled absorber to some extent, which also lowers the temperature of vapour-liquid mixture in absorber and enhances CO2 absorption in thermodynamic aspect. Thus, higher rich loading solvent and less reboiler duty are foreseeable. Flue gas is cooled to 30 °C in simulation, and around 5% reduction in reboiler duty is achieved with MEA, compared with a 2% saving with DEA.
Rich solvent split (RSS)
This process modification is suggested as long ago as Eisenberg and Johnson (1979). In Fig. 4, it splits the cold rich loaded solvent into two flows, and the split one remains unheated when it enters the top of the stripper, while the other one is heated in the lean/rich heat exchanger and it is injected at lower stage. With the rich split modification, the heated rich solvent can reach a higher temperature at which CO2 can desorb more easily. Meanwhile, the vapour released from the rich solvent meets with the cold solvent injected above, which is able to strip a little CO2 from it. Thus, there is a reduction in reboiler duty. 10% of the rich solvent unheated is split to the top of stripper in simulation. There is a saving in reboiler duty of 7.7% by MEA and 7% by DEA. RSS has neutral effect on rich loading and solvent required as the absorption process remains the same. Different split rates larger than 10% are necessary to be used for further simulation.
Rich solvent pre-heating (RSP)
As Herrin (1989) proposed, the cold rich solvent can be heated by the hot vapour exiting the stripper as Fig. 5 shows, which can make use of the latent heat and reduce the cooling water required in stripper condenser. It seems to be efficient because the rich solvent can be heated twice, however, due to the temperature of the hot vapour is exactly similar with the rich solvent temperature after heated by hot lean solvent, even a little lower, the heat transfer cannot exist if all rich solvent is heated. No energy reduction obtained in the simulation of MEA or DEA. But obvious benefits will be gained if combining rich solvent pre-heating with rich solvent split because heat can be transferred to the split cold rich solvent (Ahn et al. 2013). Then the wasted heat can be used and other principles of energy saving are the same with rich solvent split, no more tautology or simulation here.
Solvent split flow (SSF)
The modification of split flow is first proposed by Shoeld (1934), which contains a partial regeneration cycle of lean solvent. A flow of semi-lean solvent is taken from the middle of the stripper, having heat exchange with the cold rich solvent and then injected to the middle of absorber. Among all the variant of split flow modifications, the most common one is described by Leites et al. (2003) and Aroonwilas and Veawab (2007) as Fig. 6. It is a combination of two modifications, simple split flow and rich solvent split. Furthermore, as the semi-lean solvent is cooled down before entering absorber, it also takes a little bit advantage of inter-cooling. Many parameters needs to be taken into account to reach a minimal energy consumption, for example, the stages to draw off semi-lean solvent from stripper and inject into absorber, the split fraction of cold rich solvent and semi-lean solvent, and the introduced stage of hot rich solvent. In principle and simulation, the semi-lean stream is drawing off from the middle of stripper to provide the cold rich solvent split with more heat. Since less rich solvent contacts with the hot lean solvent leaving stripper, hot inlet stream reaches higher temperature, then if it is injected at lower part of stripper, energy saving is further allowed. Optimal energy savings are found in simulation when taking all these factors into account. As a result, simulation shows that SSF can lead to a 7.6% of reduction on reboiler duty in MEA case, correspondingly with 7.8% in DEA.
It is worth mentioning that the required amount of circulating solvent becomes larger in the solvent split flow modification than in the conventional process because the average solvent working capacity is lowered. Bigger equipments such as columns and pumps are required to match with the flow rate.
Rich solvent flashing (RSF)
The principle of the modification of rich solvent flashing is to flash the inlet stream of stripper before entering, as Fig. 7 illustrates. By flashing the hot rich solvent, a little more CO2 is gained whereas vaporization lowers the temperature of liquid phase. In fact, this flashing process is same with conducting separation process at an ideal stage. The phenomenon happens everywhere in stripper. As a result, this modification does not obviously reduce energy consumption except providing one more stripping stage. The simulation result in this work is the same with what Le Moullec and Kanniche (2011a) claimed.
Stripper condensate bypass (SCB)
In the modification of stripper condensate bypass, the condensate liquid is not fed back to the top of stripper. Instead, this stream is directly injected to the absorber. This modification is used in the work of Oexmann and Kaher (2009) as Fig. 8. The simulation of this work provide a 0.6% reboiler duty saving with MEA and 0.4% with DEA, that is, stripper condensate bypass almost makes no difference in limiting energy consumption. Because of the small flow rate of condensate, the duty saving for heating it in stripper is restricted.
Stripper condensate heating (SCH)
The modification of stripper condensate heating is proposed and studied in Aroonwilas and Veawab (2007) and Ahn et al. (2013) as Fig. 9. As the vapour temperature is high at the top of stripper, stripper condensate heating is to make use of this to heat the stripper condensate, and then feeding the hot condensate back to the bottom of stripper to provide a little heat recovery. Nevertheless, it has been proved by theoretical analysis and simulation in this work that there is insignificant gain in energy consumption. Only 1% of reboiler duty is reduced both for MEA and DEA.
Lean vapour compression (LVC)
Lean vapour compression is one of the most widely suggested modifications in a variety of literatures and patents, such as Batteux and Godard (1983), Reddy et al. (2007), Woodhouse and Rushfeldt (2008). As Fig. 10 shows, the principle is to flash the hot lean solvent at a lower pressure, then compressing the hot vapour generated and re-injecting it into the bottom of stripper. As the vapour benefits from the sensible heat of hot lean solvent as well as recompression, it can reach a very high pressure and temperature, which can provide additional steam and heat in the column for stripping. In the simulation, the hot lean solvent is flashed to the atmospheric pressure and the adiabatic efficiency of the compressor is 80%, and this modification shows significant savings in reboiler duty. With MEA, a 12.8% of reduction is obtained, and as for DEA, LVC allows a gain of 11.9% of reduction in reboiler duty. However, it should be noted that as a compressor introduced here, it leads to the additional electricity consumption that cannot be neglected, and the performance of total energy saving compared with the conventional process will be detailed discussed in the following part.