# Optimal Energy Integration and Off-Design Analysis of an Amine-Based Natural Gas Sweetening Unit
## 5 Off-Design Performance Analysis  
In a natural gas processing plant, the variation of inlet conditions and the simultaneous actions of several controllers induce fluctuations in the heat sources used to power the heat recovery system proposed in this study, which lead to instabilities that affect its efficiency and result in a deviation from the expected nominal performance. For a more realistic assessment of the behavior of the proposed heat integration solution, an off-design analysis was conducted, taking into account real-time temperature and flow rate fluctuations of all the waste heat sources as well as the ambient temperature.  
In this regard, real-time data from a natural gas processing plant operated by Sonatrach were extracted at a 50 min interval over 31 days of stable operations. The signals were used to create a perturbation around the conditions of a steady-state scenario. The scenario with 7% CO\({}_{2}\) and 2.66 compression ratio was selected, as it was the closest to the conditions of the real plant. Table 5 summarizes the characteristics of the fluctuating input signals. For the three working fluids, the optimized design parameters of the heat integration scheme were maintained and the model was then evaluated at each point.  
The off-design analysis was conducted under the following assumptions:  
* The heat transfer area (UA) of each heat exchanger obtained using design conditions was maintained in the off-design analysis.
* The pump was assumed to deliver the required flow rate and outlet pressure at the design efficiency--an acceptable assumption given that the ORC fluid flow rate was found to range within \(\pm\)10% of the design value.
* The turbine was also assumed capable of delivering the expansion ratios despite the fluctuating conditions. However, due to the large deviation in expander energy output  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Unit** & **R290** & **R600a** & **R601a** \\ \hline
**Evaporator pressure** & kPa & 2506 \(\pm\) 32\% & 1007 \(\pm\) 53\% & 400 \(\pm\) 145\% \\
**Net power** & kW & 872 \(\pm\) 64\% & 839 \(\pm\) 75\% & 743 \(\pm\) 71\% \\
**Efficiency** & \% & 4.55 \(\pm\) 36\% & 4.56 \(\pm\) 43\% & 4.55 \(\pm\) 58\% \\
**Flow rate** & m\({}^{3}\)/h & 389 \(\pm\) 40\% & 305 \(\pm\) 43\% & 231 \(\pm\) 40\% \\
**Pumping power** & kW & 376 \(\pm\) 52\% & 232 \(\pm\) 47\% & 177 \(\pm\) 35\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Averaged ORC results of all scenarios at optimal conditions.  
Figure 11: Mapping of ORC net power output all across the investigated domains.  
their exclusive dependence on the waste heat from the turbine exhaust. On another hand, the profile of ORC net power output of all fluids presents a wider bi-modal distribution. This behavior is due to its dependence on several waste heat sources as well as the ambient temperature. The results also show that each fluid performs at a different efficiency and demonstrates a different sensitivity to the same changes in inlet conditions. R290 showed the highest efficiency and the narrowest variation around the nominal point. Efficiency results are depicted in Figure 13.