Thermodynamic and kinetic effect of biodegradable polymers on carbondioxide hydrates

Journal of Industrial and Engineering Chemistry 79 (2019) 131–145

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Thermodynamic and kinetic effect of biodegradable polymers on carbondioxide hydrates Sana Yaquba,b , Bhajan lala,b,* , Lau Kok Keonga,b a b

Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar 32610, Perak, Malaysia CO2 Research Centre (CO2RES), Universiti Teknologi PETRONAS, Bandar Seri Iskandar 32610, Perak, Malaysia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 March 2019 Received in revised form 11 May 2019 Accepted 8 June 2019 Available online 26 July 2019

In this work, the effect of biodegradable polymers, i.e. pectin (PC), sodium-carboxymethyl cellulose (NaCMC), tapioca starch (TS) and dextran (DX) on thermodynamics and kinetics of CO2 hydrates are evaluated on sapphire hydrate reactor. The CO2 hydrate liquid vapour equilibrium (HLwVE) data is evaluated in the presence of biopolymers (1.5 wt%) using isochoric T-cycle method at temperature and pressure ranging from 278.7 to 283.0 K and 2.3 to 4.3 MPa respectively. The effect of biopolymers on HLwVE curve is reported by measuring average increment temperature (DÜ). The constant cooling method is used to evaluate the kinetics of CO2 hydrates at 4.3 MPa in the presence of biopolymers (0.12– 1.5 wt%) at 274.15 K and 277.15 K. The inhibition effect of biopolymers on the kinetics of CO2 hydrate is reported by measuring induction time, hydrate formation rate and amount of gas consumed. The kinetic inhibition strength of biopolymers is compared with poly-N-vinylpyrrolidone (PVP) and with two noncommercial inhibitors, i.e. glycine and tetra-methyl ammonium chloride (TMACl) through relative inhibition strength (RIS). Results reveal that DX shows maximum increment temperature of 0.36 K. While PC and Na-CMC delayed CO2 hydrate nucleation for 423 and 181 min respectively. Additionally, biodegradation study on biopolymers indicates that, compared to PVP, biopolymers are easily biodegradable and show potential for gas hydrate offshore applications. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Keywords: Thermodynamic CO2 hydrate promoters Kinetic CO2 hydrate inhibitors Biopolymers Biodegradability Flow assurance

Introduction The non-stoichiometric crystalline compounds formed by the interaction of hydrogen-bonded water molecules and certain gas molecules (CH4, CO2, C2H6, etc.) are called “gas hydrates” [1,2]. The water molecules in a hydrate cage are stabilized into three different structures, i.e. sI, sII and sH. Small size gases such as CH4 (4.36 Å) and CO2 (5.12 Å) generally form sI hydrate structures [3]. CO2 is a primary greenhouse gas, either present as an impurity in natural gas or produced by combustion of fossil fuel in power plants. This greenhouse gas is transported from the point of capture to the suitable geological location for either sequestration or enhanced hydrocarbon recovery through a pipeline. The CO2 transport via pipelines encounters various complications including flow assurance, economic losses, safety and environmental hazards. In oil and gas transportation pipelines, favorable temperature and pressure conditions are often available that

* Corresponding author at: Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar 32610, Perak, Malaysia. E-mail address: [email protected] (B. lal).

build up gas hydrate, resulting in pipeline blockage [4]. The unnecessary pipeline shut down due to hydrate blockage results in significant financial losses for oil and gas companies. Additionally, the potential of gas hydrates for huge explosions in case of sudden depressurization is a major safety concern throughout the world. However, in the world energy industry, flow assurance for natural gas transportation and CO2 sequestration becomes one of the most challenging areas [5]. In this scenario, the injection of chemicals that could mitigate hydrate formation during natural gas transportation and CO2 sequestration is of vital importance. Thermodynamic hydrate inhibitors (THIs) are well-known chemicals that prevent hydrate formation by increasing the hydrate stability region [6–8]. Currently, methanol and ethylene glycol are commercially used which effectively inhibit hydrate formation by shifting hydrate phase boundary toward low temperature and high pressure. However, these THIs are using in large concentrations ranging from 10 to 60 wt% and their storage in offshore plants is a big problem [6,9,10]. Therefore, inhibitors that would use in small concentrations <1 wt% are seeking attention [6,10]. Such inhibitors are known as low dosage hydrate inhibitor (LDHIs), further classified as kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs).

https://doi.org/10.1016/j.jiec.2019.06.017 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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Nomenclature

pectin sodium-carboxymethyl cellulose tapioca starch dextran xanthan gum natrium-carboxy-methyl cellulose thermodynamic hydrate inhibitors low dosage hydrate inhibitors anti-agglomerates kinetic hydrate inhibitor polyvinylpyrrolidone polyvinylcaprolactam 1-ethyl-3-methylimidazolium tetrafluoroborate BMIM-BF4 1-butyl-3-methylimidazolium tetrafluoroborate AFPs antifreeze proteins CH4 methane CO2 carbondioxide C2H6 ethane HLwVE hydrate liquid vapor equilibrium SD standard deviation %RIP percentage relative inhibition power TMACl tetra methyl ammonium chloride TBAOH tetra butyl ammonium hydroxide Poly (VP/VC) poly vinyl pyrrolione-co-vinylcarpolactam ILs ionic liquids BOD5 biological oxygen demand COD chemical oxygen demand DM degree of methylation M.W. molecular weight WQI Water Quality Index PC Na-CMC TS DX XG LV-CMC THIs LDHIs AAs KHIs PVP PVCap EMIM-BF4

Extensively, KHIs delay hydrate formation, reduce nucleation rate and growth while AAs prevent lump formation of small hydrate crystals. In gas pipelines, the use of AAs is absurd since they require a liquid hydrocarbon phase; therefore KHIs are essential there [11,12]. KHIs have been utilized in the upstream oil industry for around 25 years to prevent blockage of streamlines with gas hydrates [13]. KHIs are mostly amphipathic in nature and watersoluble polymers. KHIs being low in poisonous severity and bioaccumulation, do not exhibit great biodegradability. Nonbiodegradability of KHIs is a major concern in the case of partially degraded products that could trigger chronic toxicity. KHIs such as polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) are polymeric in nature and delay hydrate formation either by increasing induction time (time lapse from the start of the experiment to the time where the detectable size of hydrate crystal appears) or by retarding hydrate growth [14,15]. Zhang et al. [16–18] studied the impact of PVP and silver nano particles (AgNPs) on co-composting of sewage sludge and agricultural waste. Even though PVP and PVCap show great kinetic performance, efforts are continuously made to replace these non-biodegradable polymeric KHIs [3,11] with environment-friendly biodegradable KHIs. The well-known biodegradable KHIs also recognized as natural green inhibitors include chitosan [19], starch [20–23], amino acids [24–29] and antifreeze proteins (AFPs) [30–33]. Studies show that methane hydrate formation is effectively inhibited in the presence of chitosan and Raisamyl starch [20,21]. At 273.7 K and 3.5 MPa, 0.5 mass% Raisamyl starch can delay methane hydrate nucleation for 94.7 min and reduced methane consumption better than

non-KHI system [21]. Chitosan (C3, M.W. = 2.2 * 106 g mol
1) can delay methane hydrate nucleation for 126.7 min at 274.3 K and 4.5 MPa [19]. While for the prevention of mix gas (methane + propane) hydrate tapioca starch can inhibit hydrate formation for 192 min at 279 K and 3.0 MPa [21]. Talaghat et al. [22] reported that at 280 K and 7 MPa, modified starch could delay methane and CO2 hydrate nucleation for 180.4 min and 80.3 min respectively. However, further studies revealed that few other polysaccharides such as xanthan gum and CMC effectively delay natural gas hydrate nucleation and improve rheological properties of drilling mud [34]. Kelland et al. [35] found that the addition of xanthan gum (XG) along with bentonite in deep-water drilling situations increased natural gas induction time more than 1200 min. Al-Adel et al. [36] evaluated the effect of type I AFPs on the methane hydrate formation in comparison to vinylpyrrolidone-co-vinylcaprolactam, poly (VP/VC) and found that AFPs decrease methane hydrate formation rate while their performance is similar to poly (VP/VC). However, due to solubility limitations, the THI effect of most natural green inhibitors have not been investigated in the literature. Xiaolan et al. [37] measured the shift in methane hydrate phase boundary, in terms of extra sub-cooling gained (i.e. DT) due to the addition of natrium carboxymethyl cellulose (LVCMC). It is noticed that 5 wt% LV-CMC show small thermodynamic inhibition effect with DT = 1.02 K. However, if an extra DT gained is higher than 0.5 K the chemical can be considered as a hydrate inhibitor [37]. Recently, Gupta et al. [38] evaluated the phase equilibrium of methane hydrate in aqueous solutions of xanthan gum and guar gum. Addition of 0.01–0.05 wt% xanthan gum showed average temperature depression ranging from 0.51 K to 0.88 K. However, different M.W. (6.4 * 105 g mol
1 XG-1 and 2.4 * 105 g mol
1 XG-2) and concentration of xanthan gum shown the effect on hydrate inhibition tendency. It should be noted that mostly KHIs are used to inhibit hydrate formation in the systems comprising methane gas or a mixture of hydrocarbons, but for the prevention of CO2 hydrates, only a few KHIs have been tested. The most important KHIs that have been tested for CO2 hydrate prevention are amino acids and ionic liquids (ILs). For example, Saad et al. [39] reported that tetramethylammonium chloride (TMACl) works as a potential KHI for CO2 hydrate prevention. It is found that compared to imidazolium-based ILs, TMACl is a much better KHIs and delayed CO2 hydrate formation for approximately 45 min [39]. In another study, it is stated that tetramethylammonium hydroxide (TMAOH), tetraethylammonium hydroxide (TEAOH) and tetrapropyl ammonium hydroxide (TPrAOH) are performed as KHI for mix gas (50 mol% CO2–50 mol% CH4) [40]. Del et al. [41] investigated kinetic inhibition performance of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) along with the 28 day biodegradation test. Their result reveals weak kinetic inhibition and poor biodegradability of imidazolium-based ILs. However, the non-biodegradability and toxicity of ILs is still a big problem for the petroleum industry [42]. Sa et al. [24] investigated the effects of five hydrophobic amino acids on CO2 hydrate formation and found that shorter alkyl chain amino acids such as glycine and L-leucine are better KHIs and reduced CO2 uptake 30% better than other amino acids. Naeiji et al. [27,43] evaluated the effect of glycine and L-leucine on THF hydrate formation and stated that glycine is more effective KHI. Furthermore, for preventing CO2 hydrate formation Roosta et al. [5] investigated four new structures of amino acids and compare the inhibition effects of these four amino acids with glycine, Lthreonine and PVP. It is found that 1 wt% glycine and histidine reduced CO2 uptake better than other amino acids, but their performance is not better than PVP. However, among all studied amino acids glycine has more potential to inhibit CO2 hydrates. Although natural green inhibitors included amino acids are biodegradable, and numerous

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studies are presented to understand how these materials can be used as an alternative KHIs [5,31–33,44,45]. However, there performance for CO2 hydrate mitigation along with biodegradability data is still unmapped. To find out whether biopolymers are potentially applicable for flow assurance, the study is undertaken into four objectives such as: (i) to investigate the kinetic inhibition performance of biopolymers (PC, Na-CMC, TS and DX) by measuring induction time, hydrate formation rate and moles of gas consumed at various concentrations of biopolymers (0.12, 0.20, 0.25, 0.50, 1.0 and 1.5 wt %) and at two sub-cooling temperatures (274.15 K and 277.15 K), (ii) to compare the kinetic inhibition performance of biopolymers with PVP and two best non-commercial KHIs i.e. glycine and TMACl through percentage relative inhibition strength (%RIS), (iii) to investigate the thermodynamic influence of biopolymers (1.5 wt%) on CO2 hydrate phase equilibria (HLwVE), (iv) to find out the biodegradable extent of biopolymers by performing biodegradability study. Experimental Materials The chemicals used in this study are summarized in Table 1. The structural details of biopolymers are presented in previous work [46]. To maintain the physicochemical properties of biopolymers, they are purchased commercially and used without further purification. An analytical balance of precision 0.01 mg is used to weight the powdered biopolymers. Apparatus The experimental setup used for gas hydrate measurements along with schematic diagram is explained and represented in [25,28,46,47]. However, Wheaton BOD incubation bottles (300 ml) are used for biological oxygen demand (BOD5) biodegradability test. All bottles are manufactured of Type I borosilicate glass. YSI 5000 dissolved oxygen meter is used to measure the amount of dissolved oxygen before and after incubation. DBR-200, HACH branded chemical oxygen demand (COD) reactor is used for the digestion of samples contain a blend of reagents and additives. DR3900, HACH spectrophotometer is used to measure the chemical oxygen demand of studied chemicals. The electrical conductivity, pH and turbidity are measured using Myron L EP-10 Multi-Range Analog conductivity meter, portable ECO Testr pH 2 meter and portable 2020we turbidity meter respectively. Thermodynamic measurement method The hydrate–aqueous liquid–vapour equilibrium (HLwVE) of H2O–CO2 and H2O–CO2–biopolymers system is measured using the isochoric T-cycle method. Before starting the experiment,

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sapphire hydrate reactor is thoroughly cleaned and washed with distilled water. To remover air or any other contaminants, the cell is then vacuumed, followed by the injection of liquid phase samples with or without biopolymers. Then sapphire hydrate reactor is immersed in a liquid bath where after reaching the initial temperature (2–3 K above the equilibrium temperature of CO2) the CO2 gas is injected in the cell up to desired experimental pressure. In these experiments, the 4.3–2.3 MPa pressure range is used to achieve phase boundary of H2O–CO2 and H2O–CO2– biopolymers hydrates. Once the stabilized conditions are attained (after 30 min), the adequate mixing is provided by the mechanical stirrer at 300 rpms which are enough to break the interface boundary of the liquid water. In the T-cycle method, initially fast cooling is used to reduce the temperature of the system and to facilitate the hydrate formation. After the experimental temperature is accomplished, the reactor is kept for an extended period of 3–4 h. The sudden pressure drop shows on data acquisition and by visual observation indicates hydrate formation. When no further pressure drop is observed, and hydrate formation is completed, then the reactor is heated to dissociate hydrate. Compare to hydrate formation; hydrate dissociation is a slow process. For hydrate dissociation initially, the system is heated fast, but near equilibrium point (predicted by CSMGem software) the system temperature is raised stepwise with a heating rate of 0.5 K/h. To determine the hydrate equilibrium point accurately at each step the system is managed to stay for 2–3 h. Therefore, completion of each THI experiment required approximately 48 h. For each run of the experiment, a pressure–temperature (P–T) diagram is obtained (Fig. 1). In Fig. 1, the point where heating and cooling lines meet each other is considered as the equilibrium point or hydrate dissociation point (shown with a red circle). The promotional thermodynamic effect of biopolymers is quantified by measuring average increment temperature (DÜ), calculated using Eq. (1) [4]:

DÜ ¼

1 Xm DT i¼1 m

ð1Þ

where m is the number of data points, and DT is the difference between measured hydrate dissociation temperature in the presence of biopolymers and pure water. Kinetic measurement method To perform kinetic measurements of H2O–CO2 and H2O–CO2– biopolymers hydrates a constant cooling method is used. Before performing kinetic experiments, the reactor is thoroughly clean, dried and vacuumed. In constant cooling method, first the reactor is loaded with 18 ml liquid sample (pure water or aqueous solutions of biopolymers: 0.12, 0.2, 0.25, 0.5, 1 and 1.5 wt% concentration). To keep sub-cooling temperatures constant, in all experiments the initial temperature is adjusted 2–3 K above the equilibrium temperature of CO2 at 4.3 MPa. Once the initial temperature is maintained the cell is pressurized with CO2 up to

Table 1 Materials for this study.

1 2 3 4 5 6 7 8 9

Chemicals

Symbol

Molecular weight (g mol
1)

Monomer

Deionized water Carbondioxide Pectin Sodium carboxymethyl cellulose Tapioca starch Dextran PVP Glycine Tetramethyl ammonium chloride

H2O CO2 PC Na-CMC TS DX PVP – TMACl

18 44.0 194.1 263.2 692.7 504.4 111.1 75.07 109.6

– – C6H10O7 C8H16NaO8 C27H48O20 C18H32O16 C6H9NO – –

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Fig. 1. Pressure–temperature diagram obtained by the isochoric T-cycle method, showing hydrate dissociation point of CO2–H2O–Na–CMC system.

experimental pressure and leftover 30 min for stabilization. After this, the system temperature is constantly decreased from 284.15 K to experimental temperature (274.15 K and 277.15 K) at the rate of 4 K/h. Simultaneously adequate stirring is provided to break the gas–liquid interface. Changes in the system temperature and pressure are recorded by the data acquisition in every 10 s. As soon as the temperature and pressure in the reactor become constant (after 4–6 h), the experiment is considered complete. To confirm the repeatability, all experiments are repeated three times, and the average readings are used to evaluate the kinetic performance of biopolymers. For every new experimental run fresh sample is prepared. The standard pressure versus time plot for CO2–H2O hydrate at 274 K is shown in Fig. 2. KHIs performance is investigated by measuring: induction time, amount of CO2 consumed, and hydrate formation rate. The time at which detectable nuclei are formed in a reactor is termed as induction time [3,25]. The induction time is determined using

Eq. (2) [48,49]: ti ¼ ts
th

ð2Þ

where ti, ts and th respectively denotes as induction time, time at the start of the experiment and the time where detectable hydrate is formed. The initial hydrate formation rate is calculated using Eq. (3) [50]: dn ¼ kðn0
ns Þ dt

ð3Þ

where k, n0, and ns respectively denote to initial hydrate formation rate, initial moles of CO2 and initial moles of CO2 consumed in the hydrate phase. The total amount of CO2 consumed is calculated by using Eq. (4):     PV PV Dn ¼
ð4Þ ZRT 0 ZRT t

Fig. 2. A standard pressure versus time plot for CO2–H2O hydrate, measured by constant cooling method at 274.15 K.

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where P, V, T, R and Z respectively refers to system pressure, gas volume, system gas temperature, universal constant and compressibility factor. The compressibility factor (Z) is calculated using Peng–Robinson's equation of state. The subscript ‘00 and ‘t’ represents the number (n) of moles of CO2 at time zero and time, t of complete hydrate formation. The performance of various KHIs is further compared by calculating percentage relative inhibition strength (%RIS) using Eq. (5) [51]:   t
t ð5Þ %RIS ¼ i  i ti where ti and ti respectively represents the induction time with KHI and without KHI. Biodegradability measurement method The biodegradability of biopolymers is evaluated by determining the BOD5/COD ratio [52]. The amount of dissolved oxygen required to biological organisms for the decay of organic matter, existing in the liquid sample is termed as biological oxygen demand (BOD) [53]. While chemical oxygen demand (COD) is the measure of the amount of oxygen required to oxidize soluble and particulate organic matter in water [54]. It generally quantifies the number of oxidizable pollutants in a liquid sample and measures organic matter by using chemical oxidizing agents. The standard procedure, 8043 is used to determine BOD5 [55–57]. In this method, 5 days are given for the incubation of liquid samples (aerated distilled water + BOD nutrient buffer + biopolymers) at 293.15 K in a dark place. Firstly, the incubation bottles are filled with liquid samples. Then the amount of dissolved oxygen in the liquid samples are measured using dissolved oxygen meter. After this, the incubation bottles are sealed with aluminum foil and placed in an incubator for 5 days. After the completion of the incubation period, the bottles are taken out, and the amount of dissolved oxygen (DO) is measured immediately. The following equation is finally used to get BOD5 values of various additives [56]: BOD5 ¼

Di
D0  300 S

ð6Þ

where Di and D0 respectively denotes initial dissolved oxygen, and final dissolved oxygen and S is the sample size, take 5 ml in this work. The standard reactor digestion method is used for COD measurement using HACH reagents [56]. Sulfuric acid and a strong oxidizing agent such as potassium dichromate are used as a reagent. Potassium dichromate is a hexavalent chromium salt that is bright orange in color, and when organic matter is oxidized by dichromate, it converts into a trivalent form of chromium which is dull green in color. In COD measurement initially, the liquid samples (reagent + biopolymers) are pretreated and heated in the digester for 2 h at 423.15 K. After digestion, the liquid samples are cooled at room temperature. Then the quantity of excess oxidants in a liquid sample is determined by the colorimetric method. In the colorimetric method, the consumption of dichromate is observed by the change in the absorbance of the liquid samples. The samples absorb at specific wavelengths due to the color of hexavalent and trivalent chromium. After cooling the amount of trivalent chromium in a sample is measured by the absorbance of samples at a wavelength of 600 nm in a spectrophotometer. It is important to mention here that in this work the ratio of BOD5/COD is used for investigating the biodegradability of biopolymers and other additives. BOD5/COD ratio is a well-known way to measure the biodegradability of wastewater [58]. The wastewater is considered easily biodegradable if the BOD5/COD

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ratio > 0.45. Based on the BOD5/COD ratio following is the standard range to distinguish between easily and hardly biodegradable materials [45].    

Easily biodegradable: BOD5/COD > 0.45 Biodegradable: BOD5/COD > 0.3 Hardly biodegradable: BOD5/COD < 0.3 Non-biodegradable: BOD5/COD < 0.2

Results and discussion Thermodynamic measurements Hydrate phase equilibria (HLwVE curve) of biopolymers To study the effect of biopolymers on the CO2 hydrate phase equilibrium the thermodynamic experiments are conducted in the pressure and temperature ranging from 4.3 MPa to 2.3 MPa and 278.5 K to 283.4 K respectively. The hydrate equilibrium curve of H2O–CO2 system is determined experimentally and predicted using commercial hydrate prediction software, CSMGem. The obtained results reflect that the measured HLwVE curve of H2O– CO2 is in great agreement with predicted equilibrium data which further verifies the accuracy of equipment and reliability of the methodology. The HLwVE curve of 1.5 wt% biopolymers is represented in Fig. 3, and equilibrium pressures and temperatures are mentioned in Table 2. Fig. 3 shows that biopolymers are somehow able to shift the thermodynamic phase boundary toward high temperature and low pressure, indicating a CO2 hydrate promotional effect. The promotional thermodynamic effect of biopolymers is further quantified by measuring the average increment temperature (DÜ). The calculated values of DÜ are represented in Fig. 4 reveals the increasing order of promotion strength as DX > Na-CMC > TS > PC. DX shows the highest promotional effect with an average increment temperature of 0.36 K. Firstly, the highest promotional impact of DX is probably due to its longer alkyl chain or parent chain among other studied biopolymers. The similar CO2 hydrate promotional behavior is observed by Saad et al. [4,40,59] in the presence of longer alkyl chain, tetrabutylammonium hydroxide (TBAOH, Ü = 1.15 K) and revealed that shorter alkyl chain ammonium based ionic liquids behave as hydrate inhibitors. Secondly, the highest promotional impact of DX is possibly due to more CO2 solubility in the presence of DX (data not shared). As more CO2 is soluble in the system having DX, the hydrate cages are more stable and required a higher temperature to dissociate [60,61]. Indeed, the promotional effect of biopolymers is insignificant, but it can affect the kinetic performance of DX for inhibiting CO2 hydrate formation due to additional sub-cooling temperature. Kinetic measurements Induction time The induction time or hydrate nucleation time is the principle kinetic parameter, generally used to evaluate KHI performance. In this work, induction time is considered as the time at which a detectable hydrate nucleus is formed inside a sapphire hydrate reactor. Fig. 5a and b, represents the deviation in induction time of CO2 hydrate formation in the presence of 0.5 wt% biopolymers at different sub-cooling temperatures (a) 274.15 K and (b) 277.15 K. The change in induction time by the addition of biopolymers is discussed with reference to pure water or the system without any KHI. Results revealed that in comparison to pure water the addition of biopolymers could enhance induction time by hindering the nucleation process. It is found that at 274.15 K in the non-KHI system the CO2 hydrates are formed in 41.2 min while for the

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Fig. 3. CO2 hydrate phase boundary in the presence of water and 1.5 wt% biopolymers.

Table 2 Equilibrium temperature and pressure of CO2 hydrate for pure water and 1.5 wt% biopolymers. H2O–CO2–Na-CMC

H2O–CO2–TS

T (K)

P (MPa)

H2O–CO2–PC T (K)

P (MPa)

T (K)

P (MPa)

T (K)

P (MPa)

T (K)

P (MPa)

278.75 280.85 282.4

2.32 3.21 4.1

278.7 281.3 283.1

2.41 3.32 4.28

279.05 281.3 283

2.39 3.24 4.31

279.35 282 282.5

2.56 3.46 3.81

279.35 281.5 282.45

2.47 3.37 3.83

H2O–CO2

H2O–CO2–DX

Fig. 4. Average increment temperature (DÜ) in the presence of 1.5 wt% biopolymers.

system containing 0.5 wt% DX the induction time is increased to 73 min (Fig. 5a). The addition of TS and Na-CMC further delayed hydrate nucleation for 93 min and 129 min respectively. Whereas, high methoxyl (DM: the degree of methylation > 50%) PC prolonged CO2 hydrate nucleation for 149 min and the induction time is increased almost four folds than pure water. However, by reducing the driving force to 277.15 K the induction time is slightly enhanced to 52 min for the non-KHI system. Correspondingly, for

the systems contain PC and Na-CMC the hydrate nucleation is delayed much longer at 277.15 K then at 274.15 K. At 277.15 K in the presence of Na-CMC induction time is enhanced almost 4 times than pure water. Whereas, PC prolonged CO2 hydrate nucleation for 423 min which is 8 times more than pure water. Results revealed that at 277.15 K induction time for PC is improved three times than its performance at high driving force, which leads to speedy nucleation. The side chain carboxylic acid group in PC might

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Fig. 5. Induction time of CO2 hydrate in the presence of water (solid black line) and 0.5 wt% biopolymers at different temperatures (a) 274.15 K and (b) 277.15 K and 4.3 MPa.

because hydrogen bonding with water molecules and causes a delay in hydrate nucleation [62,63]. In terms of percentage the inhibition performance of PC and Na-CMC at 277.15 K is increased 713% and 156% respectively while at 274.15 K it is increased to 262%, and 162% respectively. However, in the case of TS and DX, no noticeable increase in induction time is observed at 277.15 K. The availability of more non-freezing water in DX–CO2–H2O system as compared to other additives caused more DX–DX and DX–CO2 interactions than DX–H2O interactions hence induce hydrate nucleation [64]. A smaller amount of non-freezing water in PC, Na-CMC and TS boost biopolymer–H2O interactions than biopolymer–biopolymer or H2O–CO2 interactions. It leads, to delay in hydrate nucleation than observed in the presence of DX. Inclusively, all considered biopolymers observed to be well effective in delaying CO2 hydrate nucleation at both temperatures. In summary, the nucleation mechanism of biopolymers is based on the effective side chain, containing hydrophilic and hydrophobic functional groups. The hydrophilic functional groups of the KHIs disrupt the water structure, hence increasing the barrier to nucleation whereas adsorption of hydrophobic functional groups on the hydrate surface hindered the hydrate growth [65,66]. The concentration phenomenon is imperative in gas hydrate studies [67] since relying on concentration a similar substance can

act in various ways (inhibitor or promoter). Therefore, it is important to consider this impact while investigating the effect of biopolymers on CO2 hydrate formation kinetics at six concentrations ranging from 0.12 to 1.5 wt%. Fig. 6 shows that induction time increases by increasing additives concentrations from 0.12 wt % to 0.5 wt% and follow general KHI inhibition trend with concentration [21]. However, at 1 wt% except for TS, the induction time of all biopolymers is decreased (Fig. 6a). Further increase in the concentration of biopolymers to 1.5 wt%, insignificantly lingered the CO2 hydrates formation. Though, at 1.5 wt% kinetic promotional effect is observed in the presence of DX. At 277.15 K a similar trend of decreasing onset time with increasing biopolymer concentration is observed up to 0.5 wt%. However, the induction time is enhanced more at 277.15 K than 274.15 K. Qin et al. [68] observed the similar behavior of decreasing onset time with increasing triglycol concentration from 0 to 5 wt%. The induction time of triglycol is increased with increasing concentration from 0 to 2 wt% and started to decrease as concentration further increases. The possible reason for this peculiar behavior of biopolymers with increasing concentration might be associated with their thermodynamic promotional effect which causes additional sub-cooling at 1.5 wt% (section “Hydrate phase equilibria (HLwVE curve) of biopolymers”). A similar trend of decreasing induction time with

Fig. 6. The relationship of various biopolymers concentration and induction time of CO2 hydrate at (a) 274.15 K and (b) 277.15 K.

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increasing concentration (0.05–3 wt%) is observed by Shiva et al. [29] for ethane hydrate prevention in the presence of amino acids. Their study showed that the optimum concentration of glycine and leucine is 0.5 wt%, where ethane hydrate nucleation can delay for 112 and 75 min respectively. However, with increasing concentration from 0.5 wt% to 3 wt% the induction time for glycine decreased to 45 min. The justification of this behavior of amino acid is further clarified by measuring the sub-cooling temperature in the presence of 0.04. 0.41 and 4.04 wt% glycine by Sa et al. [24]. They found that the sub-cooling temperature in the concentration of 0.41 wt% is less than those observed in other concentrations. The sub-cooling temperature is proportionated with the driving force for crystal formation. Thus the inhibition potential of hydration is further at lower sub-cooling temperature (concentration about to 0.5% of glycine). Gas consumption The stochastic nature of hydrate nucleation complicates the evaluation criteria of KHIs. Therefore, the total moles of CO2 consumed during hydrate formation is further used to evaluate KHI performance. Fig. 7a and b shows that 0.5 wt% biopolymers reduce the amount of CO2 consumed at both experimental conditions. At

274.15 K for the system comprising PC less amount of gas is consumed for hydrate formation than pure water (Fig. 7a). While the addition of Na-CMC and TS reduced the usage of gas from 0.026 mol to 0.025 mol and 0.0249 mol respectively. In terms of percentage, the observed gas reduction is 4% and 4.4% respectively. While the presence of DX further reduced the gas consumption from 0.026 mol to 0.0238 mol. Fig. 7b reveals that at 277.15 K, DX and TS consumed 18% and 23% less gas than at 274.15 K, while in the presence of PC and Na-CMC the amount of gas consumed is reduced up to 20% at 274.15 K. The overall trend of gas uptake indicates that gas consumed by PC > Na-CMC > TS  DX. The larger monomer of Na-CMC (M. W. = 263.2 g mol
1) introduced more hydrophobic groups on the surface of hydrate crystal and hindered the gas to trapped inside the hydrate cages, hence reduced gas consumption than PC (M. W. = 194.14 g mol
1). Similarly, the larger monomer of TS (M.W. = 692.6 g mol
1) than DX (M.W. = 504.4 g mol
1) enables it to cover enormous hydrate surface and forbid gas molecules to trapped inside the hydrate cages by blocking the hydrate cavities, resulted in less or almost equal gas uptake. The relation between the molecular weight of monomers and gas uptake showed in Fig. 8 further supported the statement. It can

Fig. 7. Total moles of CO2 consumed in the presence of water (black straight line) and 0.5 wt% biopolymers at (a) 274.15 K and (b) 277.15 K.

Fig. 8. Relationship of molecular weight of the monomer and CO2 gas uptake.

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be seen that due to the introduction of more hydrophobic groups the high molecular weight biopolymers take less CO2 gas than low molecular weight KHIs. However, it is opposite to nucleation time where more mobility of smaller size KHIs perturb water structure and delayed hydrate nucleation and retard hydrate growth. Similar behavior of prolonged hydrate nucleation for low molecular weight PVCap is reported by Seo et al. [69]. Fig. 9a and b shows the effect of varying concentrations of PC and Na-CMC on CO2 gas uptake. Addition of various concentrations of these biopolymers effectively reduced gas consumption than pure water. At both temperatures, the decline in gas consumption with increasing concentration of PC and Na-CMC is uniform till 0.25 wt%. Conversely, at 0.5 wt% the amount of gas consumed during hydrate formation is increased someway but reduced with increasing concentration. Relative to Na-CMC the reduction in gas consumption with increasing concentration is more regular in the system having PC as KHI. In the system having PC the maximum CO2 gas consumed at 274.15 K and 277.15 K is 0.0147 mol and 0.0166 mol respectively. While in the system having Na-CMC the maximum CO2 consumption at 274.15 K and 277.15 K is 0.0223 mol and 0.0201 mol respectively which is almost twice than PC. However, both biopolymers reduced CO2 consumption greater than pure water.

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In Fig. 10a and b, it can be observed that the addition of TS and DX drastically reduced CO2 consumption than pure water at both temperatures. In the system containing DX, the CO2 consumption is reduced much more than TS at 274.15 K. The larger molecular weight of DX enables it to occupy enormous hydrate surface and lastly consumed less CO2 gas. The gas consumption is decreased with increasing concentration of DX and TS at both conditions. However, the trend is uneven for 0.12 wt% to 0.25 wt% concentrations of TS and DX where more CO2 is consumed. Though, the observed CO2 consumption is still lower than pure water (Fig. 10a). While at 0.5 wt% the gas consumption in the presence of DX is decreased (which is increased further with increasing concentration of DX) and increased by the addition of TS. The trend of gas uptake with increasing concentration of biopolymers is more uniform at 277.15 K. At 277.15 K the gas uptake decreased with increasing concentration of TS till 1 wt% while in the presence of DX only up to 0.25 wt% the gas uptake is decreased with increasing concentration. In the system having TS, the maximum CO2 gas consumed at 274.15 K and 277.15 K is 0.0248 mol and 0.0217 mol respectively, which is more than PC and Na-CMC. While in the system having DX the maximum CO2 consumption at 274.15 K and 277.15 K is 0.0243 mol and 0.0217 mol respectively. Overall, the addition of biopolymers reduced the amount of CO2 consumed

Fig. 9. Total moles of CO2 consumed during hydrate formation in the presence of water (black straight line) and various concentrations of PC and Na-CMC at (a) 274.15 K and (b) 277.15 K.

Fig. 10. Total moles of CO2 consumed during hydrate formation in the presence of water (black straight line) and various concentrations of TS and DX at (a) 274.15 K and (b) 277.15 K.

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Fig. 11. Initial formation rate of CO2 hydrate in the presence of water (blank) and 0.5 wt% biopolymers at (a) 274.15 K and (b) 277.15 K.

during hydrate formation, but the trend of gas uptake with concentration is not too much obvious. Initial hydrate formation rate The effect of biopolymers on the initial hydrate formation rate is determined and illustrated in Fig. 11. The addition of biopolymers reduced the initial hydrate formation rate better than pure water. At 274.15 K the presence of 0.5 wt% PC, Na-CMC and DX effectively reduced the hydrate formation rate while a small percentage of it is reduced by TS. PC and DX reduced the hydrate formation rate about 56% more than pure water. While the presence of Na-CMC and TS respectively reduced the hydrate formation rate about 45% and 27% more than pure water. However, at 277.15 K the hydrate formation rate is further reduced both in non-KHI and KHI systems. In nonKHI system, the hydrate formation rate is reduced to 4% than at 274.15 K. While at low driving force the addition of PC, DX and TS respectively reduced the formation rate up to 69%, 40% and 44% than pure water. However, in the system contain Na-CMC and PC the formation of the thin film at the interface limits the hydrate formation rate (as also observed visually during the experiment). In terms of percentage at 277.15 K, PC reduced the formation rate of about 13% than at 274.15 K while Na-CMC reduced it to about 32%. In general, the addition of all biopolymers efficiently reduced the

hydrate formation rate at both sub-cooling temperatures. Fig. 12a and b shows the relation of biopolymers concentration and initial formation rate at both sub-cooling temperatures. Likely, to induction time and CO2 gas uptake the hydrate formation rate is also concentration dependent and reduced linearly with increasing concentration of biopolymers. The smaller concentrations (0.12– 0.25 wt%) of PC and Na-CMC effectively reduce hydrate formation rate. For example, 0.25 wt% PC and Na-CMC reduce formation rate 40–54% better than pure water. A similar trend of decreasing CO2 growth rate with increasing concentration (0.5–1 wt%) in the presence of amino acids is observed by Roosta et al. [5]. They also concluded that smaller concentrations are more effective for reducing growth rate, while at high concentrations (1.5 wt%, 2 wt%) their study revealed a constant inhibitory effect. Moreover, at higher sub-cooling temperature, the rate is reduced linearly with increasing concentration of biopolymers. Therefore, 1.5 wt% PC reduced the hydrate formation rate of 281% while a similar concentration of Na-CMC reduced formation rate to 48% than pure water (Fig. 12a). While at lower sub-cooling temperature presence of 1.5 wt% Na-CMC and PC reduce formation rate from 0.000258 min
1 to 0.000134 min
1 (92%) and 0.000078 min
1 (230%) respectively. Hence, at both sub-cooling temperatures, PC reduced formation rate more than Na-CMC. The smaller molecules like PC being more mobile in solution, interact

Fig. 12. Initial formation rate of CO2 hydrate in the presence of water (black straight line) and different concentrations of PC and Na-CMC at (a) 274.15 K and (b) 277.15 K.

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Fig. 13. Initial formation rate of CO2 hydrate in the presence of water (black straight line) and different concentrations of TS and DX at (a) 274.15 K and (b) 277.15 K.

easily with former gas and reduce formation rate [51], additionally, compared to Na-CMC the low hydrophobicity of PC reduced the formation rate. Shiva et al. [29] reported a similar trend of decrease in ethane hydrate growth with the hydrophobicity of amino acids, observed reduced hydrate growth in the presence of low hydrophobic glycine than L-leucine. At both sub-cooling temperatures, the addition of TS and DX drastically reduce the hydrate formation rate as observed in Fig. 13a and b. Although, at higher driving force in the system containing DX the rate is reduced much more than TS. The reduction in the formation rate with increasing concentration of DX and TS is also observed at both conditions. But, at 0.2 wt% TS the trend is uneven, and the rate is enhanced. Though, hydrate formation rate is still lower than pure water (Fig. 13a). While at 0.2 wt% DX the rate is decreased which is increased latter with increasing concentration of DX. The trend of hydrate formation rate with increasing concentration of biopolymers is more uniform at lower driving force. At 277.15 K, the formation rate is decreased with increasing concentration of TS till 0.25 wt% while in the presence of DX the formation rate continuously decreased with increasing concentration except at 0.5 wt%. At 0.5 wt% the rate is respectively enhanced by 17% and 20% more than the rate at

0.25 wt% and 1 wt%. In the system having 1.5 wt% DX and TS, the maximum reduction in the formation rate at 277.15 K is 65% and 43% respectively which is less than PC and Na-CMC. Generally, the addition of biopolymers significantly reduces the hydrate formation rate at all the studied concentrations. Comparative study The conventional KHIs lose their inhibition efficiency at higher subcooling temperatures or at high driving force [12,45,70]. Therefore, the inhibition performance of 0.5 wt% biopolymers is compared with 0.5 wt % PVP. There are two reasons for opting this concentration such as: (i) in this study it is found that 0.5 wt% of biopolymers is the optimum concentration as maximum induction time, reduced growth rate and least gas uptake is observed at this concentration and (ii) in the presence of different concentrations of PVP (0.1, 0.5 and 1 wt%) for a methane/ propane gas mixture. Daraboina et al. [71] proposed that 0.5 wt% PVP showed higher induction time than other concentration. Consequently, it was declared that in this concentration (0.5 wt%) PVP inhibits hydration effectively. In Fig. 14, results show that 0.5 wt% PC and Na-CMC can enhance induction time more than PVP at both sub-cooling temperatures.

Fig. 14. Induction time of 0.5 wt% biopolymers in comparison with PVP (commercial KHI) at different sub-cooling temperatures.

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Fig. 15. CO2 uptake of 0.5 wt% biopolymers in comparison with PVP (commercial KHI) at different sub-cooling temperatures.

At higher sub-cooling temperature, the induction time of PC is enhanced 33% while the presence of Na-CMC increased the induction time to 15% than PVP. The reason for enhanced induction time of PC and Na-CMC than PVP perhaps lies in the fact of large molecular weight. As mentioned in Table 1 the molecular weight of PC and Na-CMC monomer is greater than PVP monomer. The large molecular weight of these biopolymers introduces more hydrophobic functional group on the surface of hydrate crystal and sterically block the gas molecules from entering in hydrate cavities. While the hydrophilic pendant groups (PC: COOH, Na-CMC: OH
, COO
) of the polymer binds with the hydrate surface in the early stages of nucleation, therefore, preventing the particles from reaching the critical size. These active side chain groups in PC and Na-CMC forms hydrogen bond with water molecules more prominently than PVP or any other biopolymer. Additionally, Fig. 14 shows that at lower sub-cooling temperature (277.15 K) the inhibition instigated by Na-CMC (181 min) is comparable to PVP (253 min) while, PC prolonged the hydrate nucleation 67% more than PVP. At lower sub-cooling temperature because of the low driving force, the induction time of PC is increased from 149 min to 423 min, reflecting that high driving force attributed to accelerated nucleation. However, the kinetic performance of DX and TS is not comparable to PVP at both sub-cooling temperatures. These type of KHIs can only be a good choice where hydrate inhibition together with high biodegradability is desired. Though, the significance of sub-cooling temperature or driving force in hydrate formation is observed by taking examples of PC and Na-CMC. Fig. 15 represents the comparison of CO2 uptake of biopolymers with PVP at different sub-cooling temperatures. It can be seen that at lower sub-cooling temperature presence of TS and DX reduced the CO2 uptake comparable to PVP, while the CO2 uptake by other biopolymers is not comparable with PVP. However, at higher subcooling temperature, compared to other biopolymers, the CO2 uptake of the system having DX is comparable to PVP. While other biopolymers reduce their efficiency at higher sub-cooling temperature. The CO2 uptake by DX is comparable to PVP at both subcooling temperatures. Fig. 16 shows the initial formation rate of CO2 hydrate for 0.5 wt % biopolymers in comparison with PVP at different sub-cooling temperatures. At 277.15 K, as compared to PVP biopolymers insignificantly reduce the hydrate formation rate. While at higher driving force the formation rate of PC and DX is comparable to PVP.

The formation rate is decreasing with decreasing sub-cooling temperature, which shows that at higher sub-cooling temperature even the formation rate is enhanced. In summary, at higher sub-cooling temperature biopolymers specifically, PC and Na-CMC are better KHIs than PVP. Moreover, in terms of gas consumption and hydrate formation rate the performance of PVP is much better than biopolymers, indicating that biopolymers are good in delaying hydrate nucleation, but once hydrate formed their inhibition efficiency (in terms of gas consumption and formation rate) is reduced. Kinetic measurements are typically depending on equipment, stirring speed and experimental method, which make it difficult to compare different KHIs, reported in the literature. However, using the percentage relative inhibition strength (%RIS) the kinetic performance of different KHIs can be easily compared. Glycine and tetramethylammonium chloride (TMACl) are the best introduced KHIs in the literature. Therefore, the experiments are also conducted with them. At 274.15 K the kinetic performance of these academically robust KHIs is compared with biopolymers in Fig. 17. The %RIS increases in the order of water < DX < TS < PVP < Na-CMC < glycine < PC < TMACl. The trend indicates the effective inhibition performance of Na-CMC and PC than PVP and glycine, while %RIS of TMACl is 23% more than PC. Addition of biopolymers increased the RIS from 80% to 260%. The %RIS data reveals that TMACl is much better KHI than biopolymers, but non-biodegradability of TMACl limits its application. Biodegradability In large scale offshore, industrial application the robust KHIs must meet the environmental and biodegradability requirements. So that any spillage from the pipeline could not cause a hazardous effect on aquatic life. The studied biopolymers are from natural sources and break through the poor biodegradability of commercial KHI. The biodegradability of biopolymers is evaluated by determining the BOD5/COD ratio. Other factors such as pH, turbidity and electrical conductivity also play vital role in the biodegradability of any material. The BOD5/COD ratio is a well know way to measure the biodegradability of wastewater and high value of it reflects a high degree of biodegradability. The wastewater is considered too hard to biodegrade if it's BOD5/ COD <0.2. The BOD5/COD ratio of all studied biopolymers along

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Fig. 16. Initial formation rate of CO2 hydrate for 0.5 wt% biopolymers in comparison with PVP (commercial KHI) at different sub-cooling temperatures.

Fig. 17. Percentage relative inhibition strength (%RIS) of water in comparison with 0.5 wt% biopolymers at 274.15 K. %RIS of glycine and TMACl is measured by performing experiments.

with glycine, TMACl and PVP is mentioned in Table 3 and represented in Fig. 18. It is found that among studied biopolymers the BOD5/COD ratio of PC is highest reflecting its more biodegradability which is further followed by TS, Na-CMC and DX. However, while comparing the biodegradability of biopolymers with academically robust KHIs, i.e. glycine and TMACl, it is seen that glycine is easy to biodegrade while TMACl is hard to biodegrade. The commercial KHI, i.e. PVP showed poor biodegradability than all biopolymers. The pH values obtained for biopolymers are in the range of 4.6– 7 shows that in terms of pH, different biopolymers have a different tendency toward biodegradability. The pH value of PC is 4.6 which

is quite acidic, and according to Malaysia wastewater standard, it lies in Class V (pH < 5) [72]. The pH values of TS, DX and Na-CMC are 6, 6.1 and 7.1 respectively and classified under class II (pH 6–7) [72]. However, the results are within the standard range of Malaysian Water Quality Index (WQI). The pH value for TS, DX and Na-CMC is much higher than PVP, glycine and TMACl. The photosynthetic algae activities that consume dissolved CO2 generally increase pH value [73]. Overall, pH ranging from 6.5 to 9 is considered appropriate for aquatic life because high and low pH can be destructive [74]. The dissolved oxygen (DO) of the biopolymer range from 6.19 to 8.61 mg L
1. Na-CMC shows the lowest DO value and classified under class II, while other biopolymers show the

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Table 3 Comparative summary of parameters used to evaluate the biodegradability of different KHIs.

a

Kinetic hydrate inhibitors

BOD5 (mg L
1)

COD (mg L
1)

BOD5/COD

DO (mg L
1)

Turbidity (NTU)

pH

Electrical conductivity (ms cm
1)

Biodegradable extenta

PC TS Glycine DX Na-CMC PVP TMACl

506.4 517.0 515.4 483.0 371.8 83.8 77.0

1196 1566 1123 1559 1150 1492 3000

0.423 0.330 0.459 0.310 0.323 0.056 0.026

8.44 8.61 8.59 8.05 6.19 1.39 1.28

250 300 1.27 2.23 6.19 18 0.71

4.6 6 5.7 6.1 7.1 5.5 5

900 42 13 10 200 70 >5000

Easy Easy Easy Easy Easy Hard Hard

Biodegradable extent is based on BOD5/COD ratio.

Fig. 18. Distribution of values for BOD5/COD of different KHIs.

highest DO value and classified under class I [72]. It is found that these results are within the standard acceptable levels of WQI for Malaysian river, which is more than 3 mg L
1. While PVP and TMACl show less amount of DO. The DO levels found in the presence of biopolymers are adequate both for the survival of planktons as well as to perform various physiological activities [74,75]. Overall, oxygen generally dissolved in surface water because of diffusion from the atmosphere and aquatic-plant photosynthesis. In general, dissolved oxygen is consumed by the degradation of organic matter in water [76]. Turbidity values of biopolymers vary between 2.23 and 300 NTU. TS, and PC shows the high turbidity of 300 and 250 NTU respectively, while Na-CMC and DX show the lowest turbidity of 6.19 and 2.23 NTU respectively. Also, these concentrations are within the standard permissible limits of WQI for Malaysian rivers and categorized as class I (turbidity < 10) [72]. The high value of turbidity in the presence of PC and TS is probably due to suspended particles. The large turbidity is generally related to possible microbiological contamination and the presence of suspended particles [77]. The suspended particles can be in the form of silt, plankton, mud, organic matter, and other microscopic or decomposed organisms. The turbidity value of PVP is much higher than Na-CMC and DX, but its turbidity is much lower than the PC and TS. However, the addition of TMACl as compared to biopolymers enhanced the clarity of wastewater. In the presence of biopolymers the conductivity varies from 10 to 900 ms cm
1, and the smallest value of conductivity is observed in the presence of DX, while the largest is in the presence of PC (Table 3). So the conductivity was found to be within the recommended level by WQI, Malaysia, and fell into the class I

[72,77]. Generally, most of the freshwater's conductivity is ranging from 10 to 1000 ms cm
1 though, it can exceed about 1000 ms cm
1 in the polluted water [78]. The conductivity in the presence of TMACl is even > 5000 ms cm
1 while the conductivity of PVP is in acceptable range. In summary, the biodegradability of biopolymers is evaluated by the BOD5/COD ratio and the increasing biodegradability trend follows as: glycine > PC > TS > CMC > DX > PVP > TMACl (Fig. 18). Conclusion In this work, the thermodynamic and kinetic effect of biodegradable polymers (pectin, Na-CMC, tapioca starch, dextran and xanthan gum) on CO2 hydrate formation is studied in sapphire hydrate reactor. The THI evaluation revealed that the presence of biopolymers increases CO2 hydrate phase boundary causing additional sub-cooling temperature. DX shows the highest thermodynamic promotional effect with an average increment temperature of 0.36 K. The KHI evaluation revealed that the presence of biopolymers enhances the induction time of CO2 hydrate. At lower driving force, PC and Na-CMC commendably delayed CO2 hydrate nucleation for 423 and 181 min respectively. Additionally, PC reduced the formation rate of about 13% while, Na-CMC reduced it to about 32%. However, the mole consumption of CO2 is also decreased with the increasing quantity of biopolymers and found that larger size biopolymers consume less CO2 gas. Comparative study reveals more induction time of PC than PVP at both sub-cooling temperatures. However, the rate and mole consumption of PVP is much lower than the biopolymers. The relative inhibition strength reveals that % RIP of water < DX < TS < PVP < NaCMC < glycine < PC < TMACl. While, biodegradability studies depicted

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