2.1.1. CO2 Capture Technologies

There are several routes available for CO2 capture technologies from the gas stream. The technologies are generally based on different physical and chemical processes comprising of absorption, cryogenic distillation, and membrane. The selection of technology route mainly relies on the type of plant, gas type, and CO2 separation level. In this study, to meet the Tangga Barat gas field sales gas specifications, suitable CO2 capture technologies that have been widely been used for CO2 separation from natural gas were evaluated. Four technologies have been selected for this techno-economic analysis study, which are:


Membrane is a type of semipermeable barrier that has the ability to separate components using various separation mechanisms such as sorption/diffusion, adsorption/diffusion, ion-conducting, and molecular sieving, with selection of organic material (polymeric) and inorganic material (metallic, zeolite, ceramic). Figure 3a shows the basic method for membrane separation for CO2 capture. Among all the available mechanisms, polymeric membrane with solution-diffusion has been commercially applied in the upstream business for natural gas separation at offshore CO2 containing gas fields. Natural gas separation utilizing polymeric hollow fiber cellulose triacetate membrane has been installed at an offshore platform in Thailand [36]. In this solution-diffusion process, it starts with gas molecules from the feed side being absorbed by the membrane, which is then diffused across the membrane matrix and finally desorbed through to the other permeate side. The membrane selectivity is dependent on the polymer molecular structure that allows preferential passing of selected gas based on their molecule sizes, while the membrane permeability is highly dependent on the gas solubility [37–39].

Chemical absorption technology for CO2 capture has been commonly applied in the petroleum and natural gas industry. CO2 is an acidic gas; therefore, chemical absorption of CO2 is a method that utilizes necessary solvents for acid-based neutralization reactions in gaseous streams/flue gas. In this process, CO2 reacts with chemical solvents to produce an intermediate compound that is weakly bonded, which is further broken down using heating, then regenerating back into the original solvent to produce pure CO2 stream. For more than 60 years, chemical absorption process using amine chemical solution, such as monoethanolamine (MEA), has been commercially applied in the natural gas industry and is considered as one of the most well-known chemical absorption technologies to absorb CO2 from natural gas [27]. In this process, in order to capture CO2, in a packed absorption column, the flue gas is bubbled through the solvent during which preferentially separates the CO2 from the flue gas stream [40,41]. Next, the solvent flows through a regenerator unit, whereby the absorbed CO2 is separated from the solvent by counterflowing the steam at temperature between 100–200 ◦C. This results in condensation of water vapor that leaves a high concentration of more than 99% of CO2 stream, which would then be compressed and used for storage or commercial. Finally, the lean solvent is cooled down to the temperature of 40–65 ◦C and is recycled back into the absorption column. In fundamental, the chemical reaction is shown as follows:

$$\rm C\_2H\_4OH\\NH\_2 \text{ (MEA)} + H\_2O + CO\_2 \leftrightarrow C\_2H\_4OH\\NH\_3^+ + HCO\_3^- \tag{1}$$

During the absorption process using MEA, the chemical reaction starts from left to right while during regeneration, the reaction is from the opposite direction, right to left [40,42].

**3b.** Separation with chemical/physical absorption.

Physical absorption is also a commercially available CO2 capture method in the petroleum and natural gas industry. In this physical solvent process, it employs organic solvents to absorb gas components physically rather than reacting chemically. The solubility of CO2 of solvents determines the separation rate of CO2 by physical absorption processes, in which the solubility hinge on the temperature and the partial pressure and of the feed gas. Low temperature and high CO2 partial pressure will increase the CO2 in the solvent solubility. Selexol (dimethylether of polyethylene glycol) is a liquid glycol-based solvent and has since been used to process natural gas especially for bulk CO2 removal. In the Selexol process, the flue gas is first dehydrated. After that, in an absorption column at a pressure about 450 psi with a low temperature of 0–5 ◦C, the flue gas is contacted with the solvent to produce a loaded CO2 solvent. Then, flash desorption or separation of the CO2 loaded solvent will regenerate the original solvent. Finally, the produced CO2 gas stream is compressed and stored while

the regenerated solvent is recycled back to the column [25,40,43]. Figure 3b shows the basic method for CO2 capture through absorption.

In the cryogenic distillation CO2 separation method, CO2 is purified by separating the gas mixtures using distillation at low temperature and fractional condensation, leveraging on their de-sublimation properties and different condensation level as shown in Figure 3c. The low-temperature distillation is a mature process used to liquefy and purify CO2 from relatively high purity (>90%) sources. The process starts with the cooling of the gas to a very low temperature of <−73.3 ◦C, to freeze out/liquefied and separate the CO2. During which, cryogenic air separation unit (ASU) was used to supply high purity oxygen to a boiler. This step condenses most of the moisture and removes any carried over particles. The produced high purity oxygen is then blended with the recycled flue gas before combusted in the boiler to maintain combustion conditions comparable to air fired configuration. This is crucial as the presently existing materials for construction could not survive the high temperatures resulting from combustion in pure oxygen [44]. Table 1 shows the advantages and disadvantages of the selected CO2 capture technologies.


**Table 1.** Advantages and disadvantages of the selected CO2 capture technologies.
