DOI: 10.1002/ente.201500004 Influence of Low-Dosage Hydrate Inhibitors on Methane Clathrate Hydrate Formation and Dissociation Kinetics Asheesh Kumar, Tushar Sakpal, and Rajnish Kumar* [a] Introduction Gas hydrates are nonstoichiometric, crystalline, inclusion compounds formed by association of a suitable gas (also smaller hydrocarbon liquids) with water under high-pressure and low-temperature conditions. These conditions are fre- quently encountered in oil and gas transport lines, which leads to the undesirable formation of gas hydrates. If this growth of the hydrates is left unattended, their formation may plug the pipelines, which would result in a fatal acci- dent. [1] Thus, flow assurance in these pipelines involves con- siderable effort and expenses to ensure that this blockage does not happen. Thermodynamic hydrate inhibitors such as methanol and glycols are used to prevent the formation of gas hydrates and to avoid plugging of the transportation and process pipelines for safety and economic reasons. [2–6] How- ever, the use of water-soluble thermodynamic inhibitors is not a cost-effective method, because high concentrations (20–40 wt %) of these inhibitors are required to prevent hy- drate formation. [7] In the last two decades, economic and en- vironmental factors have motivated research and develop- ment to identify alternative low-dosage (1 wt %) hydrate inhibitors (LDHIs). [8, 9] These LDHIs can be further classified as kinetic hydrate inhibitors (KHIs) and antiagglomerates (AAs). Whereas KHIs delay the onset of hydrate nucleation and crystal growth, AAs prevent the agglomeration and dep- osition of the formed hydrate crystals. [10] These hydrate in- hibitors allow one to have enough time to transport the fluid even under conditions that are favorable for hydrate forma- tion and growth. In a recent study, Daraboina et al. observed the effect of different KHIs and confirmed that certain KHIs perform both as nucleation inhibitors and as growth inhibi- tors. [9] Commercial KHIs are based on specific classes of water-soluble polymers, [7, 11–18] and polyvinylpyrrolidone (PVP), polyvinylcaprolactam, and dimethyl amino ethyl methacrylates [19] are common examples. The mechanism for kinetic hydrate inhibition (nucleation inhibition) by water-soluble polymers is still not fully under- stood, or at least it is not available to the public domain. Generally, researchers relate growth inhibition to adsorption of the polymer onto hydrate embryos or crystal struc- tures, [8, 20] although some work now suggests that water per- turbation phenomena are also critical for hydrate inhibi- tion. [21–23] Polymers having hydrophobic groups interact with the surfaces of the hydrates through van der Waals interac- tions, whereas the amide hydrogen atom (in PVP) bonds with water molecules on the surfaces of the hydrates. The combined effect disrupts further growth of the hydrate parti- cles. [8] Naturally derived additives such as antifreeze proteins (AFPs) from animal and plant sources have also been exten- sively studied as green hydrate inhibitors. [9, 16–18, 24–30] Natural polymers such as starches can also be termed green inhibi- tors that are nontoxic and biodegradable. [31] Starch is a poly- saccharide consisting of anhydroglucose, amylase, and amylo- pectin units. Amylose is a linear chain spiral polymer that can bind to a hydrophobic guest molecule, whereas the highly branched amylopectin component of starch prevents agglomeration of hydrate nuclei. In the present work, two different polysaccharides were chosen as gas hydrate inhibi- tors, and their hydrate inhibition tendency was compared to that of two other polymeric inhibitors [i.e., PVP and poly- acrylic acid (PAA)]. The polysaccharides used in this study This work investigates the effect of low-dosage hydrate in- hibitors (LDHIs) on methane hydrate formation and dissoci- ation. The hydrate inhibitors used in this study were the sodium salt of polyacrylic acid, a polysaccharide chitosan, and the linear sulfated polysaccharide i-carrageenan; the in- hibiting behavior of these additives were compared with that of the commonly used hydrate inhibitor polyvinylpyrrolidone for methane hydrate formation. A LDHI concentration of 1 wt % was found to increase the induction time relative to that at a LDHI concentration of 0.1 wt %. Chitosan was found to be better than the others in reducing nucleation and the growth rate of the hydrate at a concentration of 1 wt %. At a lower concentration of 0.1 wt %, nucleation in- hibition was minimal, however, growth inhibition was signifi- cant. The effect of these inhibitors on the decomposition rate of the hydrate was also studied, and the decomposition kinet- ics at a constant driving force in excess of three-phase equi- librium is reported. [a] A. Kumar, T. Sakpal, Dr. R. Kumar Chemical Engineering and Process Development Division National Chemical Laboratory Pune-411008 (India) E-mail: [email protected]Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/ente.201500004. Energy Technol. 2000, 00, 1 – 10 # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1& These are not the final page numbers! ÞÞ
Transcript
DOI: 10.1002/ente.201500004
Influence of Low-Dosage Hydrate Inhibitors on MethaneClathrate Hydrate Formation and Dissociation KineticsAsheesh Kumar, Tushar Sakpal, and Rajnish Kumar*[a]
Introduction
Gas hydrates are nonstoichiometric, crystalline, inclusioncompounds formed by association of a suitable gas (alsosmaller hydrocarbon liquids) with water under high-pressureand low-temperature conditions. These conditions are fre-quently encountered in oil and gas transport lines, whichleads to the undesirable formation of gas hydrates. If thisgrowth of the hydrates is left unattended, their formationmay plug the pipelines, which would result in a fatal acci-dent.[1] Thus, flow assurance in these pipelines involves con-siderable effort and expenses to ensure that this blockagedoes not happen. Thermodynamic hydrate inhibitors such asmethanol and glycols are used to prevent the formation ofgas hydrates and to avoid plugging of the transportation andprocess pipelines for safety and economic reasons.[2–6] How-ever, the use of water-soluble thermodynamic inhibitors isnot a cost-effective method, because high concentrations(20–40 wt %) of these inhibitors are required to prevent hy-drate formation.[7] In the last two decades, economic and en-vironmental factors have motivated research and develop-ment to identify alternative low-dosage (�1 wt %) hydrateinhibitors (LDHIs).[8,9] These LDHIs can be further classifiedas kinetic hydrate inhibitors (KHIs) and antiagglomerates(AAs). Whereas KHIs delay the onset of hydrate nucleationand crystal growth, AAs prevent the agglomeration and dep-osition of the formed hydrate crystals.[10] These hydrate in-hibitors allow one to have enough time to transport the fluideven under conditions that are favorable for hydrate forma-tion and growth. In a recent study, Daraboina et al. observedthe effect of different KHIs and confirmed that certain KHIsperform both as nucleation inhibitors and as growth inhibi-tors.[9] Commercial KHIs are based on specific classes ofwater-soluble polymers,[7,11–18] and polyvinylpyrrolidone(PVP), polyvinylcaprolactam, and dimethyl amino ethylmethacrylates[19] are common examples.
The mechanism for kinetic hydrate inhibition (nucleationinhibition) by water-soluble polymers is still not fully under-stood, or at least it is not available to the public domain.Generally, researchers relate growth inhibition to adsorptionof the polymer onto hydrate embryos or crystal struc-tures,[8,20] although some work now suggests that water per-turbation phenomena are also critical for hydrate inhibi-tion.[21–23] Polymers having hydrophobic groups interact withthe surfaces of the hydrates through van der Waals interac-tions, whereas the amide hydrogen atom (in PVP) bondswith water molecules on the surfaces of the hydrates. Thecombined effect disrupts further growth of the hydrate parti-cles.[8] Naturally derived additives such as antifreeze proteins(AFPs) from animal and plant sources have also been exten-sively studied as green hydrate inhibitors.[9,16–18,24–30] Naturalpolymers such as starches can also be termed green inhibi-tors that are nontoxic and biodegradable.[31] Starch is a poly-saccharide consisting of anhydroglucose, amylase, and amylo-pectin units. Amylose is a linear chain spiral polymer thatcan bind to a hydrophobic guest molecule, whereas thehighly branched amylopectin component of starch preventsagglomeration of hydrate nuclei. In the present work, twodifferent polysaccharides were chosen as gas hydrate inhibi-tors, and their hydrate inhibition tendency was compared tothat of two other polymeric inhibitors [i.e. , PVP and poly-acrylic acid (PAA)]. The polysaccharides used in this study
This work investigates the effect of low-dosage hydrate in-hibitors (LDHIs) on methane hydrate formation and dissoci-ation. The hydrate inhibitors used in this study were thesodium salt of polyacrylic acid, a polysaccharide chitosan,and the linear sulfated polysaccharide i-carrageenan; the in-hibiting behavior of these additives were compared with thatof the commonly used hydrate inhibitor polyvinylpyrrolidonefor methane hydrate formation. A LDHI concentration of1 wt % was found to increase the induction time relative to
that at a LDHI concentration of 0.1 wt %. Chitosan wasfound to be better than the others in reducing nucleationand the growth rate of the hydrate at a concentration of1 wt %. At a lower concentration of 0.1 wt %, nucleation in-hibition was minimal, however, growth inhibition was signifi-cant. The effect of these inhibitors on the decomposition rateof the hydrate was also studied, and the decomposition kinet-ics at a constant driving force in excess of three-phase equi-librium is reported.
[a] A. Kumar, T. Sakpal, Dr. R. KumarChemical Engineering and Process Development DivisionNational Chemical LaboratoryPune-411008 (India)E-mail: [email protected]
Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/ente.201500004.
were chitosan (CS) and the linear sulfated polysaccharide i-carrageenan (CAR).
The synthetic polymer PVP is a commonly used kinetic hy-drate inhibitor that consists of five-membered lactam ringsattached to a carbon backbone. The lactam rings are charac-terized by an amide group (�N�C=O) attached to the poly-mer backbone. This class of polymeric hydrate inhibitors hasbeen patented.[32,33] PAA is a polymer of acrylic acid. Inwater at neutral pH, PAA acts as an anionic polymer; itloses its protons and acquires a negative charge. This makesPAA a polyelectrolyte with the ability to absorb and retainwater and swell to many times its original volume. Recently,Su et al. used it as a support to improve the kinetics and re-cyclability of H2 enclathration within clathrate hydrates.[34,35]
Electrolytes such as sodium chloride, potassium chloride, andcalcium chloride are also known to be hydrate inhibitors,[36, 37]
and thus, PAA seems to be an ideal candidate to inhibit thenucleation and growth of hydrate crystals. Al-Adel has alsoused the sodium salt of PAA (average molecular weight:2100 and 6000 g mol�1) as a kinetic inhibitor of methane hy-drate. Experiments were conducted at a constant tempera-ture of 4 8C and a pressure of 6.5 MPa. The polymer wastested for its hydrate inhibition activity at concentrations of0.1 and 0.5 wt %.[38] Chitosan is a linear polysaccharide com-posed of randomly distributed b-(1-4)-linked d-glucosamine(deacetylated unit) and N-acetyl-d-glucosamine (acetylat-ed unit) moieties. Chitosan iswater insoluble, but it doesshow the tendency to partici-pate in hydrogen bonding inthe presence of water[39] andit can be dispersed in waterat high rotation speeds(�200 rpm). Owing to the pres-ence of multiple hydroxylgroups and C�O�C linkages, itwas speculated that chitosanmay prevent the formation ofhydrates. Density functionaltheory suggests that the pres-ence of chitosan reduces thestrength of hydrogen-bonded512 water cages and thus may in-hibit the formation of hy-drates.[40]
Carrageenans or carrageeninsare a family of linear sulfatedpolysaccharides that can be ex-tracted from red algae. Thebackbone structure of carra-geenans is made up of repeat-ing (1-4)-b-d-galactopyranosyl-(1-3)-a-d-galactopyranosyl dis-accharide units with residues of3,6-anhydrogalactose. However,carrageenans differ in their pat-
tern of linkage and degree of sulfation and, thus, show differ-ent physiochemical properties.[41] They are a known cryopro-tectant and are widely used as stabilizing agents in icecreams.[42] Chitosan as a hydrate inhibitor[39] has shown posi-tive results for gas hydrate inhibition; however, carrageenansas kinetic inhibitors for gas hydrate formation have not beenexplored properly.[43,44] Figure 1 illustrates the chemical struc-ture of i-carrageenan, chitosan, polyacrylic acid and PVP.Zeng et al.[16] have observed that PVP, as a kinetic inhibitor,inhibits the formation of hydrates as a function of concentra-tion. Similar results were found by Kumar et al.[5] for Struc-ture II (sII) hydrate formation of methane and propane. De-tailed morphological studies performed by Kumar et al.showed that PVP increases the induction time (delayed nu-cleation of hydrate crystal) with increasing concentrations;however, catastrophic hydrate growth was observed if the hy-drate was formed with a 1 wt % PVP solution in water (cata-strophic growth was not found for 0.1 wt % PVP water solu-tions).[5] On the basis of this information, in the present workinhibitor concentrations as high as 1 wt % and as low as0.1 wt % were used. The degree of subcooling (DT), which isessential for faster hydrate growth, was kept at 280 K. Sub-cooling is the difference between the hydrate equilibriumtemperature (281.5 K) and the operating temperature(274.5 K) at a given pressure (6 MPa).[8]
Figure 1. Chemical structures of i-caragenan, chitosan, polyacrylic acid, and polyvinylpyrrolidone.
Influence of LDHIs on hydrate nucleation and growth
Table 1 summarizes the experimental conditions used for hy-drate formation, the water to hydrate conversion, moles ofmethane consumed during hydrate formation, and the induc-tion time for the experiments performed with 1 wt % LDHIs.As seen in the table, hydrate-formation experiments per-formed in the presence of the high concentration of LDHIsshow significant delay (higher induction time) in hydrate nu-cleation. The induction times of these three LDHIs, that is,PVP, CS, and PAA, are significantly higher than the time re-quired for hydrate formation in pure water, and this exhibitsthe nucleation inhibition characteristics of these LDHIs. Theinduction time generally decreases over repeated runs; how-ever, in the presence of CAR, the induction time is inde-pendent of memory and is higher in repeated measurements.As seen in Table 1, for every repeat experiment, the induc-tion time decreases, and this is generally explained by thememory effect. It is suggested that there are residual hydratestructures and/or dissolved gas in the memory solution thatare responsible for the shorter nucleation time.[45–47] Howev-er, in the case of 1 wt % CAR, the memory effect is not ob-served, and the induction time for experiments with CAR in-creases with every memory run. A similar observation wasmade by Lee et al.[31] in the formation of hydrates in naturalgas in the presence of different starches in water. Zenget al.[29] also observed that the addition of biologically de-
rived inhibitors eliminated the memory effect completely(memory suppression).
Table 2 summarizes the experimental conditions used forhydrate formation, the water to hydrate conversion, moles ofmethane consumed during hydrate formation, and the induc-tion time for the experiments performed with 0.1 wt %LDHIs. In the presence of a low concentration of LDHIs, noinhibition in hydrate nucleation was observed and the induc-tion time was similar to that obtained in the pure watersystem. However, according to the hydrate conversions re-ported in Tables 1 and 2, the lower concentration of theLDHIs was equally effective at inhibiting hydrate growth.Also through visual observation, it was concluded that thehydrates formed at the lower concentration of LDHIs havehigher flowability (for better flow assurance) than the hy-drates synthesized at the higher LDHI concentration.
Figure 2 a shows a typical gas hydrate formation curve inthe presence of different inhibitors (pressure drop due to hy-drate formation) during a fresh run. The gas-uptake profilealso shows the induction time during each of these runs. Forthe fresh runs, PAA and CS are significantly better in inhibit-ing hydrate nucleation than PVP and CAR. However, if theinduction time is averaged over multiple runs (including hy-drate formation from the fresh solution) it becomes clearthat all the LDHIs (1 wt %) perform equally well at inhibit-ing hydrate nucleation except CAR (Table 1). All of the ex-periments in the presence of the LDHIs were repeated manytimes for a set of fresh and memory gas-uptake profiles. Acomplete set of one such run is shown as a pressure drop due
Table 1. Summary of experiments conducted for methane hydrate formation at 274.5 K and 6 MPa along with duration of the experiments, induction times,gas consumption, and water to hydrate conversions by using 1.0 wt % LDHIs.
System Exp. no. Sample Induction Mean induction End of Methane consumed Final water-to-hydratestate time [h] time[a] [h] exp. [h] [molgas molwater
to hydrate formation in Figure 2 b–d; the relevant informa-tion is also shown in Table 1.
Figure 3 shows the influence of 1 wt % LDHI on methanehydrate growth. Time zero in the graph corresponds to the
nucleation point. As seen inFigure 3a, the initial rate of hy-drate growth for PVP/waterand CS/water solutions isslower than that for PAA/waterand CAR/water solutions. Dar-aboina and Linga[48] reportedsimilar results of slow methane/propane (90.5:9.5 mol %) hy-drate growth in the presence of1 wt % PVP. The gas-uptakeprofile shows that, overall, thewater to hydrate conversion inthe presence of the LDHIs(PAA is an exception) is lowerthan hydrate formation in purewater. Interestingly, once thehydrate nucleates, PAA fails toinhibit hydrate growth. Thus, at1 wt % concentration, PAA(LDHI) works as a hydrategrowth promoter, which notonly enhances the initial rate ofhydrate formation but also ach-ieves maximum water to hy-drate conversion in the shortest
Table 2. Summary of experiments conducted for methane hydrate formation at 274.5 K and 6 MPa along with duration of experiments, induction times, gasconsumption, and water to hydrate conversions by using 0.1 wt % LDHIs.
System Exp. no. Sample Induction Mean induction End of Methane consumed Final water-to-hydratestate time [h] time[a] [h] exp. [h] [molgas molwater
[a] For fresh runs; values in parentheses correspond to standard deviation.
Figure 2. (a) Pressure drop due to methane hydrate formation at 1 wt % LDHI (fresh experiments). Pressure dropfor fresh and repeated (memory) experiments in the presence of (b) 1 wt% PAA, (c) 1 wt % CAR, and (d) 1 wt %CS.
time. Al-Adel[38] reported similar results for methane hydrategrowth in the presence of PAA; in that study, a lower molec-ular weight (2100 Da) PAA was used at a concentration of0.5 wt %. Out of the four additives used for this study, therate of hydrate growth is the slowest for CS; this is reflectedby the gas-uptake profile, which shows the lowest gas con-sumption, even after 10 h. Hydrate formation from CS/watersolution (Figures 2 d and 3 d) shows good inhibition of hy-drate nucleation as well as growth. Weak hydrophilic interac-tions of the CS molecules with water molecules interferewith hydrogen bonding (required for hydrate nucleation),which thus inhibits hydrate formation. The low hydrategrowth rate in the presence of CS suggests that this long-chain polymeric molecule hinders hydrate growth by actingas an anti-agglomerant.
Englezos and co-workers argued that the lower hydrategrowth in the presence of LDHIs is expected, because theyadsorb to the hydrate surfaces, which thereby reduces the ef-fective water to gas contact area and the growth rate.[2,5] Dar-aboina et al.[9] observed similar behavior in the presence ofPVP and AFPs. It was suggested that LDHIs, once close tothe hydrate surface, have a high probability of being “anch-ored” through a chemical group that fills an empty cage,which not only results in the inhibition of further crystalgrowth but also partially explains the reduction in gas
uptake. However, molecular level spectroscopy measure-ments are required to prove this hypothesis.
Figure 4 shows the influence of 0.1 wt % LDHIs on meth-ane hydrate growth (pressure drop). Data from repeated ex-periments with standard deviations are presented in Fig-ure S1 (Supporting Information). Time zero in the graph cor-responds to the nucleation point. It is clear from the figure
Figure 3. (a) Effect of kinetic inhibitors on methane hydrate growth for experiments conducted with 1 wt% LDHI. Time zero in the graph corresponds to the nu-cleation point for the experiment. Methane gas uptake for hydrate growth for fresh and repeated (memory) experiments in the presence of (b) 1 wt % PAA,(c) 1 wt % CAR, and (d) 1 wt % CS.
Figure 4. Effect of kinetic inhibitors (0.1 wt %) on methane hydrate growth(average pressure drop). Time zero in the graph corresponds to the nuclea-tion point for the experiment.
that PAA is not an effective inhibitor of hydrate growth.This result is not much different from that obtained at higherPAA concentrations. However, at a low concentration of0.1 wt %, CAR significantly reduces the rate of hydrategrowth relative to the pure water system, and it is importantto note that the initial rate of hydrate growth at a higherconcentration of CAR (1 wt %) is significantly higher thanthat with no inhibitor system. Figure 4 reveals that both CSand PVP reduce hydrate growth at the higher and lower con-centrations; however, CS is a better hydrate inhibitor thanPVP at both of the concentrations studied in this work.
Influence of LDHIs on hydrate dissociation
It is equally important to understand the decomposition pro-file of the solid hydrate phase, as these hydrates form in thepresence of LDHIs and thus may show different decomposi-tion kinetics than pure methane/water hydrates. To evaluatethe effect of LDHIs on methane hydrate dissociation kinet-ics, the formed hydrates were decomposed at temperaturesof 283, 288, and 293 K. This approach resembles a typicalthermal stimulation approach followed in industry to get ridof hydrate blockages in oil and gas pipelines.
Tables S1 and S2 summarize all the results obtained in thepresence/absence of 1 and 0.1 wt % inhibitors for the dissoci-ation of methane hydrate. Figure 5 shows the gas release
data during one such hydrate decomposition at 293 K. Con-trol experiments (no hydrate experiment, NHE) were per-formed with water and methane; the change in the reactorpressure was recorded, which resulted from an increase intemperature alone. An increase in pressure as a result of thedecomposition of the solid hydrate phase is shown as themethane hydrate experiment (MHE) gas-release profile. Re-lease of methane gas resulting from hydrate dissociationalone was calculated by subtracting these two gas-releaseprofiles, which is shown as the MHE–NHE gas-release pro-
file. Figure S3 represents the calculated methane recoveryalong with the temperature profile (dissociation at 293 K). Asignificant drop in the reactor temperature at approximately282–283 K (owing to the endothermic nature of hydrate dis-sociation) coupled with a sharp rise in the methane releaserate are indicative of enhanced hydrate decomposition. Theequilibrium temperature at the experimental pressure is ex-pected to be approximately 281.5 K.[36] Figure S4 shows a sim-ilar graph for hydrate dissociation in the presence of PVP at283 K (just above equilibrium). As expected, hydrate decom-position is slow at lower temperatures, and the time requiredto reach a similar amount of methane recovery is longer thanthat required at 293 K. However, the rates of hydrate decom-position in the presence of different LDHIs are very differ-ent, even at similar dissociation temperatures.
Figure 6 compares the kinetics for the initial rate of hy-drate decomposition at 293 K for hydrates formed at high
concentrations of the inhibitors (1 wt %); it is observed thatat 293 K, which is far from equilibrium, the LDHIs enhancethe rate of dissociation. Daraboina et al.[9] reported similarresults with KHIs (AFPs and PVP) although on a muchsmaller scale. As seen in the figure, the initial rate of dissoci-ation at 293 K is much faster for hydrates synthesized in thepresence of 1 wt % CS, CAR, and PAA than for the systemin which no inhibitor is present. For the CAR system, ap-proximately 30 % recovery is achieved in the first 15 min,and for the CS system, 80 % of the hydrate decomposed inthe first 20 min, which is twice as fast as the system with noinhibitor. The complete data set is included in Figure S5.
Figure 7 compares the rate of methane recovery during thehydrate dissociation process at 293 K at low concentrationsof the inhibitors (0.1 wt %). Data from repeated experimentswith standard deviations are presented in Figure S6. Again, itcan be seen that the initial rate of hydrate dissociation forCAR and CS is faster than for other systems. However, theaverage methane recovery at the end of the experiment issimilar in all systems (Figure S6). Figure 8 compares the rate
Figure 6. Comparison of methane recovery from methane gas hydrate; meth-ane was released by thermal stimulation at 293 K in the presence/absence of1 wt % LDHI.
Figure 5. Moles of gas released during thermal stimulation (dissociation at293 K) for the control experiment (NHE), methane hydrate experiment(MHE, hydrate dissociation and thermal expansion), and the calculated gasrecovery curve due to decomposition (MHE–NHE).
of methane recovery during the hydrate dissociation processat 283 K at 1 wt % LDHIs. The presence of the kinetic inhib-itors slows down the decomposition process at 283 K. Finalmethane recovery is similar in all systems; however, in thepresence of CS, the rate of methane release is much slowerand it takes almost 5 h to recover 86 % of methane from thehydrate phase (Table S1).[9,48] Comparison of the dissociationbehavior of methane hydrate (in terms of methane recovery)in the presence/absence of the LDHIs at different tempera-tures (283, 288, and 293 K) is given in Figure S7. The resultsof the hydrate decomposition kinetics, as shown in this workin the presence of LDHIs, are very important for efficienthydrate plug removal in oil and gas pipelines.
Conclusions
Hydrate formation and dissociation experiments were per-formed in a stirred tank reactor in the presence of low-dosage hydrate inhibitors (LDHIs) at two different concen-trations (1 and 0.1 wt %). All the LDHIs studied in this work
significantly delayed methane hydrate nucleation at the highconcentration but showed no significant inhibition at the lowconcentration. Chitosan (CS) and polyvinylpyrrolidone(PVP) showed hydrate growth inhibition at the low and highconcentrations, but CS was a more effective inhibitor thanPVP. It was also observed that 1 wt % polyacrylic acid and i-carrageenan (CAR) promoted hydrate growth rate. Howev-er, at the low concentration CAR reduced the rate of hy-drate growth, similar to CS and PVP. Thus, CS not only in-hibits hydrate nucleation but also reduces the rate of hydrategrowth significantly. One of the important finding of thiswork is that the hydrate decomposition kinetics in the pres-ence of the LDHIs are temperature dependent and show anenhanced rate of hydrate decomposition relative to that ofthe pure water hydrate at 293 K.
Experimental Section
Materials
Methane gas with a certified purity of more than 99.9% was sup-plied by De-luxe Industrial Gases, India. The kinetic inhibitors,polyvinylpyrrolidone (PVP average molecular weight 40 000 Da),polyacrylic acid sodium salt (PAA, average molecular weight5100), chitosan (CS, low molecular weight, �50 000–190000 Da),and i-carrageenan (CAR, high molecular weight �200000–800000 Da), were purchased from Sigma–Aldrich, India.
Experimental apparatus
The apparatus and procedure are described as follows. In a typi-cal experiment, a hydrate-forming gas and inhibitor-water solu-tion were brought in contact at a suitable temperature and pres-sure to form a gas hydrate. The objective was to measure the in-duction time for hydrate formation (crystallization) and the rateof hydrate growth by measuring gas uptake in a stirred tank re-actor. Figure 9 shows the schematic of the experimental setup. Itconsisted of a 400 mL SS-316 high-pressure hydrate crystallizer(CR) equipped with a magnetically coupled internal stirrer. Thecomplete setup consisted of thermocouples, a vent and safetyvalve, a gas sample inlet and an extraction valve, a liquid sampleextraction valve, and a pressure transducer. Resistance tempera-ture detectors (Pt-100) were used to measure the temperature ofthe reactor contents. All pressure measurements were made withpressure transducers (Wika) within the 0–25 MPa range anda 0.075% accuracy of the span. The crystallizer was immersed ina temperature-controlled water bath containing a 40:60 wt % eth-ylene glycol/water mixture, which allowed the temperature in thehydrate crystallizer to remain constant.
Experimental procedure for hydrate formation
Hydrate-formation experiments in a stirred tank reactor wereperformed in batch mode with a fixed amount of water (120 mL)at a constant temperature of approximately 274.5 K; a drop inpressure was measured to calculate gas consumption (details ofthe calculation are given below). In all the kinetic inhibition ex-periments, 0.1 and 1 wt % LDHIs in water solution were used formethane hydrate formation. Circulating ethylene glycol/watermixture from the water bath was employed to cool the reactor atthe experimental temperature of 274.5 K. Once the desired tem-
Figure 7. Comparison of methane recovery from methane gas hydrate; meth-ane was released by thermal stimulation at 293 K in the presence/absence of0.1 wt% LDHI (average of all experiments).
Figure 8. Comparison of methane recovery from methane gas hydrate; meth-ane was released by thermal stimulation at 283 K in the presence/absence of1 wt % LDHI.
perature (inside the crystallizer) was reached, the crystallizer wasflushed (3 �) by repeating pressurization and depressurizationcycles with methane gas. The crystallizer was then pressurizedwith methane gas to the experimental pressure of 6 MPa. Theequilibrium methane hydrate formation temperature at 6 MPawas 281.5 K calculated with CSMHYD,[36] and thus experimentswere performed at a subcooling of 7 K. Once the crystallizer waspressurized and the desired temperature was recorded, hydrateformation was initiated by switching on the stirrer at 200 rpm.The temperature and pressure of the reactor were recordedevery 5 s; the induction time coincided with a sudden increase inthe temperature, because hydrate formation is an exothermicprocess. Once the hydrate had nucleated, the pressure in the re-actor started to drop, which was used to measure the gas uptakerate. The memory experiments were conducted by using thesame procedure; the memory experiments were started 3 h aftercomplete decomposition of the hydrates formed in fresh solu-tions.[49]
Calculation for the amount of gas consumed
As the gas in the crystallizer was consumed for hydrate forma-tion, the pressure in the reactor dropped. The total number ofmoles of gas that was consumed for hydrate formation was calcu-lated by Equation (1):
ðDnH;#Þt ¼ VCRP
zRT
� �0�VCR
PzRT
� �t
ð1Þ
in which z is the compressibility factor calculated by Pitzer�s cor-relation,[50] and it is calculated from the gas uptake data, whichrecords the temperature and pressure profile of the reactor with
respect to time. VCR is the volume of the gas phase of the crystal-lizer; P and T are the pressure and temperature of the crystalli-zer at any given time, respectively, and R is the gas constant.
Water to hydrate conversion calculation
Conversion of water to hydrate was determined by using Equa-tion (2):
Conversionwater-to-hydrate ¼DnH;# � hydration number
nH2O� 100 ð2Þ
in which DnH,fl is the number of moles of gas consumed for hy-drate formation at the end of the experiment, determined fromthe gas uptake, and nH2O is the total number of moles of water inthe system.[51,52]
For simple hydrates, the hydra-tion number is related to the frac-tional occupancy of the large andsmall cavities, qL, and qS, respec-tively, for structure I [Eq. (3)]:
Hydration number ðnÞ ¼ 466qL þ 2qS
ð3Þ
The hydration number is nothingbut the number of water mole-cules per guest molecule ina stable hydrate. A hydrationnumber of 6.1 for structure Imethane hydrate is used from theliterature.[53]
Hydrate decomposition proce-dure
At the end of the hydrate forma-tion experiments, the effects ofthe hydrate inhibitors on the hy-drate decomposition kineticswere studied at temperatures of283, 288, and 293 K. During thehydrate decomposition process,
the pressure inside the reactor increased as the gas was releasedfrom the solid hydrate phase. The increase in the pressure in thereactor was recorded every 5 s and this was used to measure thegas-release kinetics. An increase in pressure owing to an increasein the temperature alone was calculated and deducted from thegas-release profile to calculate the exact number of moles of gasreleased during the hydrate decomposition step. A control ex-periment with water (120 mL) and a suitable methane gas pres-sure (no hydrate) was also performed to cross check the increasein the pressure due to increasing the temperature alone. Hydratedissociation by thermal stimulation has routinely been performedto study the rate of methane decomposition.[9,54] Methane recov-ery percentages from the above hydrate decomposition experi-ment were calculated by using Equation (4):[55, 56]
Figure 9. Schematic of the experimental apparatus.
in which (DnH,fl)tendis the number of moles consumed for hydrate
formation at the end of a typical hydrate formation experimentand (DnH,›)t is the number of moles of gas released from the hy-drate during hydrate decomposition at any given time.
Acknowledgements
The authors gratefully acknowledge financial support for thiswork from the Council of Scientific and Industrial Research(CSIR) (Tap Coal: CSC 0102). A.K. wishes to acknowledgethe CSIR for providing a senior research fellowship (SRF).We thank Prof. Praveen Linga for technical discussionsduring his visits to NCL.
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Received: January 8, 2015Revised: January 30, 2015Published online on && &&, 0000
Influence of Low-Dosage HydrateInhibitors on Methane ClathrateHydrate Formation and DissociationKinetics
It�s in the pipeline: High-pressure andlow-temperature conditions are fre-quently encountered in oil and gasproduction and in transportation lines;this can potentially lead to a plug inthe pipelines through hydrate forma-tion. The present work is the study oflow-dosage hydrate inhibitors (LDHIs)that inhibit the formation of hydratesunder such conditions and ensure flowby either delaying the nucleation ofhydrate crystals or by acting as antiag-glomerates.
