Foam and Antifoam Behavior of PDMS in MDEA-PZ Solution in the Presence of Different Degradation Products for CO₂ Absorption Process

Abstract

Absorption is one of the most established techniques to capture CO₂ from natural gas and post-combustion processes. Nevertheless, the absorption process frequently suffers from various operational issues, including foaming. The main objective of the current work is to elucidate the effect of degradation product on the foaming behavior in methyldiethanolamine (MDEA) and piperazine (PZ) solution and evaluate the antifoaming performance of polydimethylsiloxane (PDMS) antifoam. The foaming behavior was investigated based on types of degradation product, temperature, and gas flow rate. The presence of glycine, heptanoic acid, hexadecane, and bicine in MDEA-PZ solution cause significant foaming. The presence of hexadecane produced the highest amount of foam, followed by heptanoic acid, glycine and lastly bicine. It was found that increasing the gas flow rate increases foaming tendency and foam stability. Furthermore, increasing temperature increases foaming tendency, but reduces foam stability. Moreover, PDMS antifoam was able to reduce foam formation in the presence of different degradation products and at various temperatures and gas flow rates. It was found that PDMS antifoam works best in the presence of hexadecane with the highest average foam height reduction of 19%. Hence, this work will demonstrate the cause of foaming and the importance of antifoam in reducing its effect.

1. Introduction

The removal of acid gas from natural gas processing is an important process in which acid gases such as carbon dioxide (CO₂), hydrogen sulfide (H₂S), mercaptans (R-SH), and carbonyl sulfide (COS) are removed [1,2]. It is generally used in petrochemical plants and refineries. Acid gas removal or gas sweetening is one of the main components in carbon capture technology which is the first key step in the carbon capture, utilization, and storage (CCUS) industry. CCUS technology is important whereby it aims to reduce the emission of CO₂ as it is released to the environment and could cause severe aftereffects such as a rise in global temperature, climate change, and negative health effects. Furthermore, the presence of CO₂ in industrial processing may cause corrosion to downstream processing equipment and pipelines, and may decrease the economic value of natural gas. The alkanolamines-based absorption process is the most common method for CO₂ and H₂S removal. Some common types of alkanolamines are monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and piperazine (PZ). Among these amines, MDEA is widely regarded as a cost-effective solvent with excellent CO₂ absorption capabilities, low corrosion rate, and low heat regeneration [3]. However, the use of amine-based solvents has its disadvantage in which degradation of amine into contaminants that can cause foaming in alkanolamine solution [4].

The formation of foam in an absorption process is one of the main problems that commonly occur in gas-treating plants. Foams are generated by the presence of various impurities that are introduced into the absorption system [5]. Some of the main impurities in the acid gas sweetening process are the presence of contaminant and degradation products such as condensed liquid hydrocarbon, suspended solids, organic acids, and heat-stable salts [6]. The presence of degradation products reduces the surface tension of the amine solution. Gas bubbles rise to the surface of the liquid solvent by buoyancy force. A thin layer of elastic film around the gas bubbles stabilizes them as the gas bubbles rise to the top of the absorption column. This process results in foaming in the absorption process [7,8]. The formation of foam is an undesirable phenomenon, as it causes flooding in the absorption column, process performance reduction, carryover of solvent to downstream facilities, incomplete regeneration of solvent, change of pressure in the column, and decrease in absorption efficiency [6,9]. Therefore, antifoam is added to the absorption system to reduce the effect of foaming. The addition of antifoam serves as a quick and an effective way to reduce foam formation as well as destroying existing foam formation [10]. Unlike mechanical methods of foam control that are complicated to use and consume a lot of electrical power, the use of antifoaming agents is simple, economical, and does not affect the continual absorption process [11]. There are three classifications of antifoams that are commonly used in acid gas removal system. These three types are non-polar antifoam, polar antifoam, and solid-based antifoams.

Table 1. Typical Types of Antifoam.

Classification	Types of Antifoams	Ref.
Non-polar Antifoam	Mineral oils Silicone oils	[8]
Polar Antifoam	Fatty alcohol Alky amides Alky amines	[8]
	Polypropylene glycol (PPG) Polyalkylene glycol (PAG)	[12] [10]
Solid Antifoam	Fumed silica	[13]
	Treated silica Aluminum oxide	[8]

describes the types of antifoams in acid gas removal system. They are typically pre-added into a foaming system as either oil, hydrophobic solid compound, or a mixture of both compounds.

1.1. Foaming Tendency in Amine-Based Solution

Recently, many studies on the foaming behavior in acid gas removal systems have been studied. These works are crucial to show that foaming is an issue that needs to be addressed. This will also emphasize the importance of the foam prevention method, especially with the use of chemical antifoaming agents. Some studies on foaming behavior in acid gas removal are described in

Table 2. Experimental Works on Foaming Behavior in Acid Gas Removal Process in the Presence of Impurities.

Purpose	Details	Ref.
To investigate the tendency of foaming in aqueous MDEA with the presence of BHCL and HCB corrosion inhibitors.	Foaming increases with increasing gas flow rate As the temperature increase, foam formation increases as well. Foam volume increases as a larger pore-size diffuser is used.	[14]
To investigate the behavior of foam in MDEA in the presence of THEED, HEED, bHEP, bicine, acetic acid and octanoic acid as a degradation product.	Foam volume increased with the presence of THEED and HEED. bHEP and bicine show no effect on the foam volume. Acetic acid does not affect foam formation while octanoic acid increases foam formation.	[4]
To evaluate the foaming behavior of MDEA when subjected to various degradation products and contaminants.	Short-chain carboxylic acids (C3 and C4) do not show a visible effect on foam formation while long-chain carboxylic acids (C5 to C7) create higher foam formation. Adding formaldehyde increases the tendency of foaming. The presence of FeSO4 increases the foaming tendency. Liquid hydrocarbon of pentane and hexane decrease the tendency of foaming and stability.	[5]
To study the effect of impurities in the natural gas stream on the characteristic of foam behavior in MDEA and PZ solution.	As MDEA concentration increases, the foam height decreases. As the concentration of hydrocarbon increases, the foam formation in MDEA solution. The increase in MDEA concentration with constant concentration of hydrocarbon as contaminant decreases the foam height. The presence of iron sulfide and sodium chloride increases foam formation.	[15]
To investigate the effect of dissolved and undissolved organics in MDEA solution.	A longer chain of hydrocarbon increases foam formation. Excess hexane and n-heptane decrease foam formation. Lighter organic acids (C3) do not affect foam volume, while heavier organic acids (C5-C7) increase foam height. Toulene shows no effect on foam formation. Xylene increases foam formation, but adding it in excess removed the foam completely.	[16]
To investigate foaming behavior in MEA solution in the presence of various degradation product.	Increased N2 flow rate increase foaminess coefficient. Increasing solution volume increases the foam coefficient. The foaminess coefficient initially increases with MEA concentration and then declines An increase in CO2 loading increases the foaminess coefficient Solution temperature decreases the foaminess coefficient Presence of degradation product increase foam formation	[17]
To investigate the foaming behavior on the removal of CO2 in DEA solution.	n-hexane increases the foam stability and foam tendency of the amine solution. The increase in n-hexane concentration in DEA solution increase foam intensity and stability.	[18]

.

Based on Table 2, it is proven that the presence of impurities is an issue in the acid gas removal system as it increases foam formation. Thus, foam control is an important action that needs to be employed in order to reduce foam formation. From the above review in Table 2, it is evident that the work on the foaming behavior of organic acids, heat stable salts, and heavy hydrocarbon in MDEA+PZ solution is still limited. In addition, the effect of temperature and gas flow rate on the foaming behavior in MDEA and PZ solution is fairly minimal.

1.2. Antifoaming Mechanism

The antifoaming mechanism can be described in two processes which are entering (E) spreading (S) coefficient [19]. These coefficients are defined as below:

$$E={\sigma }_F+{\sigma }_{FD}-{\sigma }_D$$

(1)

$$S={\sigma }_F-{\sigma }_{FD}-{\sigma }_D$$

(2)

where:

${\sigma }_F$: surface tension of foaming medium

${\sigma }_{FD}$: interfacial tension of foaming medium/antifoam

${\sigma }_D$: surface tension of antifoam.

A positive entering coefficient (E) will allow the antifoam droplets to enter foam film of the bubbles. Once the antifoam droplet enters the foam film, spreading (S) of the droplet will follow suit in which it will force liquid away from the antifoam droplet. The drainage of liquid will lower the surface tension of the foam film and cause it to eventually rupture [20].

There are basically three important requirements for an antifoam: insolubility in foaming solution, ability to disperse into small droplets, and low surface tension to have a positive entry and spreading coefficient [21]. Most defoamers have a low surface tension of about 20–30 mNm⁻¹ which enables them to disperse into tiny droplets and enter the foam film of the foam. The difference in surface tension caused by spreading of tiny droplets across the foaming medium will cause the foam film to destabilize and rupture [22].

1.3. Antifoaming Behavior Using Silicone-Based Antifoam

The addition of antifoam is a chemical method used to prevent or reduce foam formation. Among the available antifoam agents, silicone-based antifoam is proven to be the most effective in preventing foam formation in the acid gas sweetening process. This is because silicone-based antifoams are versatile, compatible with both aqueous and non-aqueous solutions, easy to use, and economical. Polydimethylsiloxane (PDMS) is an efficient defoamer as it possesses several antifoaming properties such as low surface tension of 21 mN/m, good capability of spreading, good thermal stability, chemically inert, and insoluble in water [23]. In recent years, several studies have been conducted to investigate the use of antifoams in acid gas sweetening systems as described in

Table 3. Studies Using Silicone-Based Antifoam.

Silicone Antifoam	Solvent Used	Details	Ref.
Dow Corning Q2-3183 (Mixture of silicone and polyether glycol)	PZ	The antifoaming effect was carried out in 8 m PZ solution in the presence of Fe2+ and formaldehyde at 40 °C The addition of 1 ppm of antifoam was able to reduce foaminess by 15 to 20 times	[24]
SAG 7133 Composition: PDMS and 10% silica SAG 220 Composition: PDMS and 20% silica	MDEA, MEA	A 20% concentration of antifoam in 35–45 wt % of amine solution at 40–60 °C is used in this study. It was found that SAG 7133 is a better antifoam effect as compared to SAG 220 in MDEA solution. These antifoams are suitable when foaming is severe. However, silica particles may separate from PDMS and cause deposition.	[25,26]
SAG 7133 Composition: PDMS and 10% silica VP 5371 (Organic silicone) Composition: organic silicone and water	MDEA, MEA	Solubility of CO2 in the 45% MDEA solution decreased by 2% with SAG 7133 antifoam Solubility of CO2 in the 45% MDEA solution decreased by 1% with VP 5371 antifoam	[26]
Polydimethylsiloxane (100 cst and 300 sct)	DIPA, DEA	The antifoaming effect was carried out 30% DIPA or DEA solution containing H2S, H2, CH4 and C2-C4 hydrocarbon More than 800 ppm of antifoam is needed to reduce foam height	[27]

.

Although there is various silicone antifoam suitable for amine-based solvents, their antifoaming performance in MDEA+PZ solution has not yet been investigated thus far. Based on Table 3, the antifoaming performance of the silicone-based antifoam is evaluated based on its foam height and defoaming time. However, the physical factors that may affect the antifoaming ability such as temperature and gas flow rate have not been considered. Furthermore, the study of the antifoaming performance in MDEA+PZ solution in the presence of a degradation product is still limited.

To date, the study on the foaming behavior of MDEA and PZ solution in the presence of organic acid and heavy hydrocarbon for acid gas removal is still inadequate. Moreover, the research on antifoaming performance for the absorption process using MDEA and PZ is even more limited. Therefore, the objective of this study is to investigate the effect of degradation products on the foaming behavior in 33% MDEA and 5% PZ by weight solution and to evaluate the antifoaming performance of PDMS antifoam under different operating temperatures and gas flow rates. The concentration of 5% PZ is commonly used and has been shown to have the highest CO₂ absorption in MDEA solution [28,29,30]. Hence, 5% of PZ is used in this current study.

2. Materials and Method

2.1. Analysis of Foaming and Antifoaming Behavior in the MDEA+PZ System

In this present study, the effect of organic degradation products on foam behavior and formation in MDEA+PZ system has been examined. This will then help to emphasize the importance of antifoaming agents in reducing foam formation by degradation products. Four (4) degradation products—namely glycine, bicine, heptanoic acid, and hexadecane—were used as foaming agents found in the acid gas removal system. The presence of these degradation products is hypothesized to be able to increase the foaming behavior in the absorption unit and form stable foam [16,31,32].

The foaming behavior was evaluated using physicochemical characteristics of MDEA and PZ solution in the presence of different degradation products. Two factors that are used to evaluate the foaming behavior are the foaming tendency and foam stability. The foaming tendency is the ability of the solution to produce foam. The foaming tendency was measured in terms of foam height (mL). Foam stability is defined as the resistance of the foam to burst into liquid phase. The foam stability was measured by taking the time required for all the foam to break after the gas flow was stopped. In other words, longer foam break time signifies higher foam stability [14]. In addition, the effect of the degradation product on the solution’s physical properties (density and surface tension) was also studied as it affects the foaming behavior at different temperatures and gas flow rates.

After the foaming tendency and foam stability of MDEA+PZ solution in the presence of the degradation product were measured, PDMS antifoam was added, and the antifoaming performance was measured. The antifoaming ability was evaluated by calculating the % reduction in foam height using Equation (3). In addition, the foam stability of the MDEA+PZ solution before and after added of PDMS antifoam was also compared.

$$\% Reduction of Foam Height=\frac{x_2-x_1}{x_1}\times 100\%$$

(3)

x₂: Foam Height After Addition of Antifoam

x₁: Foam Height Before Addition of Antifoam

2.2. Materials

Fresh MDEA with a purity of >99% was obtained from Revlogi Materials (Puchong, Malaysia). Glycine, heptanoic acid, hexadecane, bicine, and piperazine (PZ) were obtained from Merck (Rahway, NJ, USA). Distilled water was prepared in the lab using a distillation unit. Pure nitrogen (N₂) gas was used to bubble into the test solution to produce foam.

2.3. Preparation of Test Solution

The amine solution (test sample) was prepared by mixing MDEA (with a purity of >99%), PZ (with a purity of >99%) and distilled water. The amine solvent has a total volume of 300 mL with a composition of 33% MDEA 5% PZ by weight where they were diluted in distilled water by constant stirring until homogenous. A total of 3.5 kmol/m³ amine concentration was prepared.

2.4. Experimental Setup

This experimental setup was designed based on the pneumatic method modified from the standard ASTM D892 [4,17,33]. Based on Figure 1, the experiment setup consists of a 1000 mL graduated cylinder test cell, silicone oil test bath, stirrer, immersed heater, temperature probe, stainless steel diffuser with a pore size of 80 μm, hollow glass rod, inlet and outlet air tubes, rotameter to control the gas flow, and N₂ gas supply.

The 1000 mL cylindrical test cell was immersed in the silicone oil bath. The top of the test cell was mounted tightly with a rubber stopper fitted with the hollow glass rod through the middle, one hole off-center to the left for the air outlet tube and one hole off-center to the right for the dosing point. The stainless-steel diffuser was fitted into the end of the hollow glass rod and adjusted so that the gas diffuser touched the center bottom of the graduated cylinder. The air inlet tube was fitted into the top of the hollow gas rod, which was wrapped around with a rubber hose. The air inlet tube directed the N₂ into the diffuser, which flowed into the test solution in the test cell. The air outlet tube would allow the gas to be emitted into the air vent. A stirrer was immersed into the silicone oil bath to ensure that heat was distributed evenly throughout the silicone oil [17].

2.5. Experimental Procedure

300 mL of the test solution and desired degradation product were placed into the test cell. The test cell was submerged into the silicone oil bath and will then be heated to the desired temperature for 40 min to ensure it reached thermal equilibrium. The N₂ gas was then turned on and sparged through the diffuser into the solution for about one minute or until the stable foam was formed to obtain the final foam height. Then, the N₂ gas supply was turned off and the foam break time was measured. The experiment was repeated with different degradation products and with PDMS antifoam at different temperatures and gas flow rates.

The density of the test solution was measured using Anton Paar DMA 4500M (Graz, Austria). The test solution surface tension was measured using Rame-Hart Advanced Goniometer (Succasuna, NJ, USA). The density and surface tension of the test solution was measured at different temperatures ranging from 55 °C to 85 °C.

2.6. Test Parameters

33% MDEA and 5% PZ were used to produce a 300 mL aqueous amine solution. The solution volume was heated to the desired temperature ranging from 55 °C to 85 °C. The solution volume used was 300 mL to maximize the contact time. N₂ gas which was bubbled into MDEA+PZ solution from a gas flow rate of 2 standard cubic feet per hour (SCFH) to 10SCFH. N₂ gas was used to prevent the degradation of amine solvent which can cause inconsistency in the experimental readings [16,24,31]. 20 g of glycine and bicine and 20 mL of heptanoic acid and hexadecane were added into the amine solution, which was approximately 6.5% of the amine solution. The amount of PDMS antifoam added to the test solution was fixed at 1 mL.

3. Foaming and Antifoaming Behavior in MDEA+PZ Solution

3.1. Foaming Behavior of MDEA+PZ Solution in the Presence of Glycine

Figure 2 shows the foam height (foaming tendency) of the MDEA+PZ solution in the presence of glycine. Glycine is an organic acid found in an acid gas removal system that is formed for the degradation of aqueous MDEA [32,34]. It was found that the presence of glycine was able to increase foam formation where the highest foam height produced is at 80 mL as the temperature and gas flow rate increased. An increase in gas flow rate increases the turbulence of the gas bubbles, thus increasing its foam height.

As shown in Figure 2, the increase in temperature of the solution from 55 °C to 65 °C increases the foam height and remains constant after 65 °C. Hence, a maximum foam height is reached at each respective temperature. The increase in foam formation is influenced by the density and surface tension of the solution. As shown in

Table 4. Physical Properties of MDEA+PZ Solution in the Presence of Glycine as Degradation Product.

Temperature (°C)	Density (g/cm3)	Surface Tension (mN/m)
55	1.039	47.95
65	1.033	46.57
75	1.026	44.94
85	1.019	43.96

, the increase in temperature decreases the density and surface tension of the solution. This in turn is able to improve the surface elasticity of the amine solvent in order to promote foam formation [16]. However, after 65 °C, the density and surface tension of the solution do not influence the foam height any longer. The reason for this is because the foam produced at higher temperatures is less stable (low foam stability) which is a result of the high turbulence flow of molecules at a higher temperature. This causes the foam produced to break before new foam can be produced [17]. Hence, preventing the foam height from increasing at higher temperatures.

Based on Figure 3, the foam break time increases as the gas flow rate increase at each operating temperature. The reason for this is that a higher gas flowrate induces more foam volume at a given amount of time. Hence, the foam stability increases as a larger foam volume is produced which increases the foam break time. However, the foam break time decreases as temperature increases. This is because foam produced at higher temperatures is less stable due to an increase in kinetic energy of the liquid molecule and foam system which decreases foam stability, hence shorter foam break time [35]. Furthermore, the decrease in foam break time with increased temperature is also influenced by the decrease in surface tension as liquid drainage from the foam film is faster with lower surface tension [20].

3.2. Antifoaming Performance of PDMS Antifoam in MDEA+PZ Solution in the Presence of Glycine

The antifoaming performance of PDMS antifoam in MDEA+PZ solution in the presence of glycine as a degradation product was evaluated. The antifoaming performance was evaluated based on the reduction in foam height from before and after the addition of PDMS. Based on Figure 4, the PDMS antifoam works best at 65 °C where it is able to reduce the foam height at every gas flow rate. The highest foam reduction was at 2SCFH where it was able to reduce 50% of the foam produced. In addition, it was also observed that PDMS antifoam does not perform well at 85 °C where it is unable to reduce the foam height at 4SCFH to 8SCFH. Similarly, the addition of PDMS at 6 and 8 SCFH under 75 °C show no antifoaming effect. This may be caused by a non-uniform gas flow distribution from the diffuser. Air bubbles may be trapped between the pores of the diffuser. This produces smaller foam bubbles that have higher surface tension and foam stability. Hence, no changes in foam height can be observed. However, as the gas flow rate increases to 10SCFH, the gas bubbles have enough pressure to force out the air trapped in between the pores of the diffusers, hence, a reduction in foam height is observed at higher gas flow rate.

Figure 5 shows the foam stability of MDEA+ PZ solution in the presence of glycine after the addition of PDMS antifoam. It was found that the foam break time decreases as temperature increases as foam formed at higher temperature are less stable. Furthermore, it was also observed that the foam break time increases as the gas flow rate increase, which is similar to the trend of the foam break time in Figure 3. However, the foam stability after the addition of PDMS is generally higher as compared to before the addition of PDMS antifoam. This is because the addition of PDMS antifoam may have increased the viscosity of the solution, thus making the foam more stable.

3.3. Foaming Behavior of MDEA+PZ Solution in the Presence of Heptanoic Acid

Heptanoic acid represents the organic acids found in amine degradation products, which is a major part of heat stable salts (HSS) in amine solvent. The presence of HSS is known to cause foams in MDEA+PZ solvent which disrupts the absorption process of CO₂ from natural gas [16]. As shown in Figure 6, the foam height of the solution has little change as the temperature increases in which the foam produced either decreases or remains the same—except for 65 °C and 8SCFH, 85 °C and 4SCFH, and 85 °C and 2SCFH where the foam height increased slightly. Other than that, the increase in gas flow rate creates higher foam volume as a larger volume of gas rises to the top of the liquid, thereby creating a higher foam height [17].

Based on

Table 5. Physical Properties of MDEA+PZ Solution in the Presence of Heptanoic Acid as Degradation Product.

Temperature (°C)	Density (g/cm3)	Surface Tension (mN/m)
55	1.012	38.68
65	1.006	37.83
75	0.997	37.35
85	0.987	36.54

, as the temperature increases, the density and surface tension of the solution decreases. However, the reduction in density and surface tension of the solution as the temperature increases has little effect on the foaming tendency of the solution. This is because foam produced at high temperatures has poor foam stability caused by the rapid movement of molecules as temperature increases. This hinders the foam height from increasing as the foam produced breaks easily so much to cause a decrease in foam height as shown in Figure 6 [17]. The slight increase in foam height at 65 °C and 8SCFH, 85 °C and 4SCFH, and 85 °C and 2SCFH may be caused by an uneven distribution of gas bubbles whereby smaller size gas bubbles are formed where they are harder to burst, thus increasing the foam height.

Figure 7 shows the foam break time of heptanoic acid decreases as temperature increases. It was found that as the temperature increases, the stability of the foam decreases due to the decrease in surface tension. The decrease in surface tension reduces the wettability of the foam film; thus, less time is required for the liquid to drain and burst the foam film [16]. This in turn prevents the foam height from increasing further as shown in Figure 6. However, the increase in gas flow rate increases the foam breaking time as a higher volume of gas is produced, hence requiring more time for the foam to rupture. The slight increase in the foam break time at 65 °C and at 8SCFH occur may be due to the increase in foam height from 80 mL at 55 °C to 90 mL at 65 °C.

3.4. Antifoaming Performance of PDMS Antifoam in MDEA+PZ Solution in the Presence of Heptanoic Acid

Figure 8 illustrates the antifoaming performance of PDMS antifoam in MDEA+PZ solution in the presence of heptanoic acid as a degradation product. It was observed that the antifoaming performance is at its most efficient at 85°C. At 2SCFH, it was able to reduce the foam height by 50% of the original foam height. However, at lower temperatures of 55 °C and 65 °C, the antifoaming performance is not as efficient. There was a slight increase in foam height at 2SCFH and 8SCFH at 55 °C, and 4SCFH at 65 °C. This may be due to the slight increase in viscosity at 55 °C after the addition of PDMS into a foaming solution at a lower temperature, thus causing more foam to be produced [36,37].

Based on Figure 9, the foam break time decreases as the temperature increases as the foam produced at higher temperature is less stable and breaks faster [38]. On the other hand, the foam break time increases as the gas flow rate increases as a higher volume of foam is produced. Comparing the foam stability before and after the addition of PDMS, the foam takes a longer time to break after the addition of PDMS, indicating that the foam is more stable. This behavior may be caused by the distribution of bubbles produced that are smaller and closer to each other which in turn makes it more stable.

3.5. Foaming Behavior of MDEA+PZ Solution in the Presence of Hexadecane

The foaming behavior of MDEA+PZ solution in the presence of hexadecane is evaluated in this section. The presence of hexadecane represents liquid heavy hydrocarbon, which is a surface active agent known to increase foaming tendency in acid gas removal systems [39]. Based on Figure 10, it was found that the presence of heavy hydrocarbon increases foam formation in MDEA+PZ solution at elevated temperatures and various gas flow rates. The increase in foam height with gas flow rate at each temperature is due to the fact that higher volumes of gas bubbles are produced at a higher rate per unit of time, thereby producing higher foam.

Besides this, the increase in the formation of foam as temperature rises occurs as the density and surface tension of the amine solution decrease due to oil spreading in the presence of heavy hydrocarbon as shown in

Table 6. Physical Properties of MDEA+PZ Solution in the Presence of Hexadecane as Degradation Product.

Temperature (°C)	Density (g/cm3)	Surface Tension (mN/m)
55	1.016	44.95
65	1.009	43.10
75	1.002	42.16
85	0.994	40.69

. This in turn helps to increase the elasticity of the surface of the solution during foaming and increases the susceptibility to bubble formation. The amount of foam produced at higher temperatures is much higher than the foam formed at a lower temperature [39,40].

Figure 11 shows that the foam stability increases as the temperature and gas flow rate increase. The reason for this is that a higher gas flowrate creates greater volume of foam as the number of gas bubbles formed increases. At the same time, the increase in the temperature of the amine solution decreases its surface tension, which in turn helps to produce more stable foam with longer foam break time. In addition to that, as heavy hydrocarbon is insoluble in MDEA+PZ solution, an oil layer is formed on the surface of the test solution. This significantly affects the foam stability of the amine solution. In this case, the foam stability (foam break time) increases with temperature.

3.6. Antifoaming Performance of PDMS Antifoam in MDEA+PZ Solution in the Presence of Hexadecane

Figure 12 shows the antifoaming performance of PDMS in MDEA+PZ solution in the presence of heavy hydrocarbon of hexadecane. It was found that PDMS antifoam works best at 65 °C and 75 °C. This indicates that PDMS antifoam is more efficient at higher temperatures rather than lower temperatures. In addition to that, it is also observed that it was able to reduce the foam height up to 33% of the initial foam height before the addition of PDMS antifoam.

Based on Figure 13, the foam stability decreases as the temperature increases due to rapid movement of liquid molecules at higher temperature, causing the liquid in the gas bubbles to drain faster. Furthermore, the foam break time increases as gas flow rate increases as more foam is produced. When comparing the foam stability before and after the addition of PDMS antifoam, the foam stability after the addition of PDMS is lower than that before the addition of PDMS antifoam. This behavior is different from the foam stability seen in the presence of glycine and heptanoic acid. This is because hexadecane is insoluble in MDEA+PZ solution, hence, the properties of the foam produced differ. The PDMS antifoam is able to enter the liquid film and reduce the surface tension of the local gas bubble film faster in the presence of hexadecane, which in turn causes faster drainage of liquid and lower foam stability.

3.7. Foaming Behavior of MDEA+PZ Solution in the Presence of Bicine

The foaming behavior of MDEA+PZ solution in the presence of bicine is investigated in this section. Bicine is an organic acid commonly found in the feed stream or is formed through amine degradation through oxidation. It is also known as corrosion-active contamination of amine solvent that can cause foaming and disrupt the absorption process [34,41]. Based on Figure 14, it was observed that the presence of bicine increases the foam formation as the gas flow rate increases. This is due to the fact that a higher gas flow rate induces more gas bubbles that rise to the top due to buoyancy force. The higher the gas flow rate, the higher the foam height will be.

As shown in

Table 7. Physical Properties of MDEA+PZ Solution in the Presence of Bicine as a Degradation Product.

Temperature (°C)	Density (g/cm3)	Surface Tension (mN/m)
55	1.034	46.83
65	1.026	45.59
75	1.019	43.69
85	1.009	42.7

, as the temperature increases, the density and surface tension of the solution decreases. The decrease in surface tension enhances the elasticity of the solution surface, causing higher formability of the solution. However, this is only true up to 65 °C in which the foam height beyond 65 °C does not increase as much. This is because the kinetic energy of the liquid molecule and foam bubble increases as temperature increases. This will then cause higher liquid drainage which in turn causes the foam to burst before the new foam is produced and thus stabilizes at a certain height [35,38].

Based on Figure 15, the foam break time increases as the gas flow rate increases. This is because a higher gas flowrate produces more stable foam as more and larger gas bubbles are produced. In addition to that, while the foam height increases as the temperature increases, the foam stability decreases. The reason for this is because the decrease in surface tension decreases the amount of liquid in the foam film. Hence, a lesser amount of time is needed to break the foam formation [16]. The decrease in foam break time as temperature increases is due to the turbulent flow of liquid molecules at high temperatures, causing rapid drainage of liquid from the gas bubble to the liquid phase [17].

3.8. Antifoaming Performance of PDMS Antifoam in MDEA+PZ Solution in the Presence of Bicine

Figure 16 below shows the antifoaming performance of PDMS in MDEA+PZ solution in the presence of bicine. It was found that the PDMS antifoam is the most efficient at 65 °C where it was able to reduce foam formation at each gas flow rate from 2SCFH to 10SCFH. The highest foam reduction that it was able to make is 33% of the foam height at 2SCFH. It was also found that PDMS antifoam was the least efficient at 55 °C where it was not able to reduce the foam height produced before the addition of antifoam, except at 6SCFH. An increase of 17% in foam height at 85 °C and 10SCFH may be caused by an uneven distribution of foam which would cause a slight increase in foam height.

Figure 17 describes the foam break time of MDEA+PZ solution in the presence of bicine after the addition of PDMS antifoam. It was observed that the foam stability reduces as temperature rises. This is because the foam formed at higher temperature is less stable and breaks faster. Additionally, it was found that the foam break time increases as the gas flow rate increases from 2 SCFH to 10 SCFH, which is in line with the trend of foam stability in Figure 15. However, the foam stability after the addition of PDMS is higher compared to before the addition of PDMS antifoam. This is because the addition of PDMS antifoam may have increased the viscosity of the solution, which in turn produces foam that is more stable [36,37].

3.9. Comparing Foaming Tendency and Antifoaming Performance

In this study, four (4) different types of degradation products were used to study their effect on the foaming tendency in MDEA+PZ solution. Figure 18 shows the average foam height under each temperature and gas flow rate of each degradation product in the MDEA+PZ solution. It was observed that the presence of hexadecane in MDEA+PZ solution produce the highest average foam height of 75 mL, followed by heptanoic acid at 64 mL, glycine at 56 mL, and lastly bicine at 43 mL. In other words, the foaming tendency of MDEA+PZ solution is primarily caused by heavy hydrocarbon, followed by HSS and other degradation products.

Figure 19 describes the average foam reduction under each temperature and gas flow rate in MDEA+PZ solution in the presence of different degradation products. It was found PDMS antifoam was able to reduce foam height that is formed in MDEA+PZ solution, even in the presence of different degradation products. Based on Figure 19, the PDMS antifoam performed more efficiently in the presence of hexadecane and less effectively in the presence of heptanoic acid, glycine, and bicine. The PDMS antifoam works by dispersing small droplets into the surface of the foam. Having a low surface tension of 21 mN/m, it is able to reduce the surface tension of the gas film and eventually cause the foam to destabilize and rupture [21,42].

4. Conclusions

In this paper, the foaming behavior of MDEA+PZ solution with the presence of different degradation products was studied. Four (4) types of degradation products were used to represent the contaminant or degradation product found in the acid gas removal system. From this experiment, it can be said that glycine, heptanoic acid, hexadecane, and bicine cause foam formation at various temperatures and gas flow rates. It is also observed that the presence of glycine, heptanoic acid, and bicine results in lower foaming tendency as compared to hexadecane. In general, foaming tendency increases with temperature and gas flow rate; foam stability decreases with temperature and increases with gas flow rate. In addition to that, the physical property of the test solution, particularly density and surface tension plays an important role in the foaming behavior of the solution in terms of foaming tendency and foam stability. As surface tension and density decrease, the foam tendency increases while the foam stability decreases.

Antifoams play an important role in gas-treating plants where they serve as a quick way to reduce foam formation. In this paper, the antifoaming performance of PDMS antifoam in MDEA+PZ solution in the presence of different degradation products was investigated. It was also found that PDMS antifoam is not effective at a lower temperature of 55 °C where it is unable to reduce foam formation at a selected gas flow rate in the presence of glycine, heptanoic acid, and bicine. On the other hand, the PDMS antifoam is the most efficient in the presence of heavy hydrocarbon where it was able to reduce foam formation at all temperatures and most of the gas flow rates with the highest average foam height reduction of 19%. It was observed that at 65 °C and 75 °C, the PDMS antifoam was the most efficient where it was able to reduce a maximum of 33% of its original foam height. Hence, it can be concluded that the presence of degradation product causes foaming issues in amine solution. From this study, it is proven that PDMS antifoam was able to reduce foam formation in MDEA+PZ solution in the presence of different degradation products.