# Optimal Energy Integration and Off-Design Analysis of an Amine-Based Natural Gas Sweetening Unit
## 6 Conclusions  
This study investigates the use of waste heat as a means to increase the energy efficiency of sweetening units in the context of natural gas upstream processing. Carbon capture in natural gas processing plants could lead to significant emission reductions in the case that the captured CO\({}_{2}\) is stored or re-used, but its impact on the energy balance of the process is significant. A heat integration scheme consisting of two steps was proposed to address this issue. Firstly, the direct integration of high-temperature waste heat is proposed, and secondly, the use of ORC is used as a bottoming cycle technology. The following conclusions were drawn:  
Direct heat integration of inlet natural gas compression units with the amine sweetening unit leads to considerable energy savings ranging from 8 to 20 MW, which is equivalent to 20 to 100% of the thermal requirements of the amine unit. The highest integration potential was met at low CO\({}_{2}\) content, where the energy demand of the process is low compared to the available amount of waste heat. Such integration results in a net improvement of energy and environmental performances of the background process. Moreover, if performed in the early design stages, it contributes to cutting CAPEX down by reducing the size of the external heating equipment.  
ORC integration for secondary waste heat was feasible in most of the investigated domains. However, the net energy output was found to be highly sensitive to process conditions. Its value ranges from 100 up to 2600 kW. The results show that the most favorable conditions for ORC integration are a CO\({}_{2}\) content lower than 5% and a compression ratio larger than 2. The results also showed that for a CO\({}_{2}\) content lower than 4% and compression ratios smaller than 1.6, the waste heat quality was not high enough to ensure the evaporation of the selected fluids. From a net energy output point of view, R290 was found to outperform R600a and R601a in most scenarios. Yet, it requires a much higher  
Figure 14: Distribution of energy output, (**a**) Energy savings from direct heat integration, (**b**) Net energy output (R290), (**c**) Net energy output (R600a), and (**d**) Net energy output (R601a).  
evaporator pressure and fluid flow rate along with engineering challenges associated with its wet expansion. A net power output of 2.6 MW could be achieved in some conditions.  
Finally, the off-design analysis offered a better potential assessment of the proposed solution through quantification of the impact of temperature fluctuations of waste heat sources and sinks. A deviation of \(\pm\)68 to \(\pm\)103% around the nominal energy savings was found for the direct heat integration, and deviations from the nominal net ORC power output were sensitive to the working fluid. They ranged from 60 to 132% for R290, from 47 to 142% for R600a, and between 52 and 135% for R601a.  
This work reveals that ORC can play an important role as a waste heat recovery technology. In future work, we will extend the concept developed in this study to include high-temperature heat pumps, which are emerging as a promising technology for industrial decarbonization.  
Conceptualization, A.B.; Methodology, A.B.; Validation, S.B. and G.L.; Data curation, A.B. and M.G.; Writing--original draft, A.B.; Writing--review & editing, M.G. and G.L.; Supervision, G.L. All authors have read and agreed to the published version of the manuscript.  
The authors are grateful to the Belgian Federal Public Service, who partially supported [PERSON] through the Energy Transition Fund and the Procura project.  
The industrial data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.