## 2 Materials and Methods
### Process Model Description
#### 2.2.1 LNG Process Model Description  
In this section, an Air Products and Chemical Inc. (APCI) propane mixed refrigerant (C3 MR) liquefaction process was modelled. The C3 MR liquefaction process is favoured due to its suitability for small- to large-scale onshore and offshore natural gas liquefaction. In addition, this chosen liquefaction method is relatively easy, requires less energy, is a mature technology, and has positive economic advantages [21]. The complete facility for the LNG plant was not considered here, but rather the liquefaction only. Figure 4 displays the APCI's C3 MR liquefaction process. The propane stream (upward stream) is divided into two streams by a splitter mechanism (TEE-100): one for natural gas pre-cooling (middle stream) and one for MR pre-cooling (downward stream). The triple heat exchangers (E-106, E-107, and E-108) use one of the separated propane streams (C3-NG) to pre-cool the supply gas to minus 35 \({}^{\circ}\)C. Each heat exchanger's exit stream is directed to a two-phase separator, whose liquid output is transferred to a mixer for process recycling and whose gaseous output is transmitted via a valve and utilised in a corresponding heat exchanger device. Valves control the pressure of input streams operating in a specified pressure range across the phase. Half of the divided propane stream is used before entering the main cryogenic heat exchanger (MCHE) to cool the MR to minus 35 \({}^{\circ}\)C. This stream follows the preceding stream, entering another triple heat exchanger successfully, but the vapour is separated from the liquid and used for heat exchange. The compressors (K-103, K-105, and K-105) are used between the MIX-100 and MIX-102 mixers to decompress the propane stream and the cooler (E-109) to reach input conditions (stream C3-1, 30 \({}^{\circ}\)C, 1100 kPa). Two wound coil heat exchangers (LNG-1, LNG-2) cool feed gas to meet final product specifications. Each LNG exchanger can use two heat transmission streams. LNG-1 has three inlet streams:  
Figure 3: Average gas produced and flared in Field X per month. Source: Authors’ construction based on data collected from DPR in Nigeria.  
the vapour and liquid mixed refrigerant (MR) streams, and the natural gas pre-cooled stream. LNG-2 is supplied by gaseous MR and feed gas from LNG-1. The liquid MR flow travels through a valve to emerge as gas and then through a mixer (MIX-104) to combine with the LNG-2 exit vapour MR stream. The exit stream is heated by a heater (E-109) and rejoins LNG-1. The outlet stream is recycled through three coolers (E-100, E-101, and E-102) and three compressors (K-100, K-101, and K-102) for re-compression and cooling to inlet conditions (MR-1 stream, 30 \({}^{\circ}\)C, 4800 kPa). The vapour MR stream leaving the device is depressurised by a valve and returned to the heat exchanger, signifying the recycling of the streams at the second LNG exchanger. After the final gas flow through the valve, the LNG is brought to atmospheric pressure. LNG output is minus 162 \({}^{\circ}\)C. Table 1 provides the mixed refrigerant composition.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Component** & **Methane** & **Ethane** & **Propane** & **Nitrogen** \\ \hline \hline \multicolumn{1}{c}{Mole Fraction} & 0.45 & 0.45 & 0.02 & 0.08 \\ \hline \end{tabular}
\end{table}
Table 1: Mixed refrigerant composition for LNG process [22; 23].  
Figure 4: ASPEN HYSYS process flow diagram of the C3 MR process.