Article pubs.acs.org/EF
Poly(vinylalkanamide)s as Kinetic Hydrate Inhibitors: Comparison of Poly(N‑vinylisobutyramide) with Poly(N‑isopropylacrylamide) Pei Cheng Chua,*,† Malcolm A. Kelland,† Hiroharu Ajiro,‡ Fumika Sugihara,‡ and Mitsuru Akashi‡ †
Department of Mathematics and Natural Sciences, Faculty of Science and Technology, University of Stavanger, N-4036 Stavanger, Norway ‡ Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ABSTRACT: Kinetic hydrate inhibitors (KHIs) are water-soluble polymers that have been used for almost 20 years as a method to prevent the formation of gas hydrate plugs. Most classes of KHI polymers contain amide groups. We have now found a simple route to synthesize poly(N-vinylalkanamide)s (PVamides) from polyvinylamine. In this paper, we present results on the KHI performance of PVamides with varying alkyl chain lengths in the side group. This has been performed in tests with a structure-IIforming synthetic natural gas in high-pressure rocker cells and on tetrahydrofuran (THF) structure II hydrate crystal growth. The KHI performance of poly(N-isopropylacrylamide)s (PNIPAMs) and poly(N-vinylisobutyramide)s (PNVIBAs) at varying molecular weights was also compared, because these two polymers differ only in the orientation of the amide group.
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INTRODUCTION In pipelines at thermodynamic conditions of elevated pressure and low temperature, water molecules tend to form gas hydrates by trapping low-molecular-weight natural gas molecules. Typical guest molecules include small hydrocarbons, such as methane, ethane, and propane, as well as carbon dioxide.1 Natural gas hydrate plugging is a costly and challenging problem for the oil and gas industry, especially for deepwater fields. One of the methods of prevention of gas hydrate formation is the use of low-dosage hydrate inhibitors (LDHIs). Kinetic hydrate inhibitors (KHIs) are a class of LDHIs that have been used in oil and gas field operations for almost 20 years.2,3 KHIs inhibit hydrate formation by perturbing the water structure to delay the hydrate particles from growing to the critical nuclear size and/or by absorbing onto the hydrate particle crystal surfaces and deforming and inhibiting further growth on that surface. The latter KHI mechanism has been verified for some polymeric KHIs.4 Most classes of KHI polymers contain amide groups. Previous work has shown that N-substituted (meth)acrylamide polymers show good hydrate inhibition performance, especially when the pendant groups are isopropyl.5,6 A range of poly(N-alkyl(meth)acrylamide)s have been developed and patented as KHIs.7−9 Studies on the performance of polyvinylamides, such as poly(N-alkyl-N-vinylalkanamide)s, as KHIs have also been reported.10,11 In this study, we have developed a simple synthetic route for poly(N-vinylalkanamide)s (PVamides) from polyvinylamine. In contrast to poly(N-alkyl-N-vinylamide)s, there is no N-alkyl group on the nitrogen atom in this class of polymers (Figure 1). The PVamides have been investigated for their ability to prevent structure II gas hydrate formation in high-pressure rocker cells and on their ability to prevent tetrahydrofuran (THF) structure II hydrate crystal growth. The KHI performance of poly(N-vinylisobutyramide)s (PNVIBAs) with varying molecular weights was compared to poly(N-isopropylacrylamide)s (PNIPAMs) of similar molecular weights to determine the influence of the orientation of the © 2012 American Chemical Society
Figure 1. Structure of poly(N-alkyl-N-vinylalkanamide)s (R and R′ = alkyl) and poly(N-vinylalkanamide)s (PVamine−R′CO, with R = H and R′ = methyl, ethyl, n-propyl, isopropyl, or isobutyl).
amide group in these isomeric structures (Figure 2). PNVIBA is known to have a lower critical solution temperature (LCST) of about 39 °C, whereas PNIPAM has a LCST of about 33 °C.12
Figure 2. Structures of (left) PNIPAM and (right) PNVIBA.
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EXPERIMENTAL METHODS: KHI PERFORMANCE TESTS
A commercial KHI product was used for comparison to the PNIPAMs and PNVIBAs. This is a low-molecular-weight (of about 2−3000 Da) Received: October 19, 2012 Revised: December 10, 2012 Published: December 12, 2012 183
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1:1 N-vinylcaprolactam (VCap)/N-vinylpyrrolidone (VP) co-polymer in water (supplied as Luvicap 55W by BASF, Germany). Polyvinylamine was obtained from BASF, Germany, as a 15 wt % partially neutralized solution in water with a molecular weight (Mw only available) of about 8−10 000 Da. This polyvinylamine is made by hydrolysis of poly(N-vinylformamide) and still contains a small percentage of unhydrolyzed formamide groups. Poly(N-vinylacetamide) (PNVA) was made from N-vinylacetamide in 2-propanol solvent using an azobisisobutyronitrile (AIBN) initiator. The molecular weight (Mn) of the PNVA is 14 000 Da [polydispersity index (PDI) = 2.11]. All other chemicals were obtained from VWR International or Sigma-Aldrich Chemical Co. A typical PVamide synthesis from polyvinylamine (Mw = 8−10 000 Da) is given here. To 10 g, a 15 wt % aqueous solution of polyvinylamine (1.5 g, 36.6 mmol) was added NaOH (1.46 g, 36.6 mmol) and dichloromethane (30 mL). The solution was stirred at 5−10 °C, and isobutyryl chloride was added dropwise. The mixture was stirred for a further 4 h, and the dichloromethane layer was separated. The aqueous phase was extracted again with more dichloromethane, and the extracts were combined, washed with 3.5 wt % NaCl solution, and dried over anhydrous Na2SO4. The solution was filtered, and the volatiles were removed in vacuo to leave a solid. The yield of polymer PVamine− iPrCO is 3.21 g (79%). This is almost pure PNVIBA and has a theoretical Mw of approximately 15 200−19 000 Da. The LCST of a 1 wt % solution in water was determined to be 41 °C. Other PVamine−R′CO polymers (Figure 1) were made similarly. PVamine−nPrCO (Mw = 15 200−19 000 Da) had a LCST of 44 °C; PVamine−EtCO (Mw = 13 800−17 200 Da) had a LCST of 83 °C; and PVamine−MeCO (Mw = 12 300−15 400 Da) had no LCST. PNIPAMs and PNVIBAs were synthesized using typical radical polymerization procedures as described in our previous works.8,9 The synthesized PNIPAMs and PNVIBAs are summarized in Table 1.
The constant cooling test procedure was as follows: (1) The 5000 ppm test solutions are prepared by dissolving 0.5 wt % polymer in 100 mL of deionized water. (2) A total of 20 mL of the test solution was used for each cell. (3) To maintain the composition of the gas phase, air in the cells was removed using a vacuum pump and purging with ca. 3 bar of SNG. This procedure is repeated twice. (4) The initial pressure in the cell is 76 bar at 20.5 °C. The cell was cooled from 20.5 to 2 °C over 18.5 h under rocking. The rocking rate is set to 20 rocks/min and a 40° rocking angle. (5) A connected computer logs the pressure and temperature of each cell during the experiment. A pressure drop is observed because of the temperature decrement of the cell, which is a closed system. From the pressure drop curve, it is possible to determine the start of hydrate formation and the fast, catastrophic hydrate formation. Figure 3 shows the pressure and temperature logged from all five cells during a standard constant cooling test using the higher molecular weight PNVIBAs and PNIPAMs in the rocker rig. Cells 1 and 2 are filled with 20 mL of PNIVIBA 21k, whereas cells 3, 4, and 5 are filled with 20 mL of PNIPAM 17k. Figure 4 is an example of pressure and temperature curves versus time during an 18.5 h standard constant cooling test using a single experiment with 5000 ppm of PNIPAM 6k. The first detectable hydrate formation occurred after 699 min (to) at the onset temperature To = 9.1 °C. The temperature for catastrophic hydrate formation Ta = 6.2 °C indicates fast hydrate formation after 870 min (ta). Usually 10 standard constant cooling tests were repeated for each polymer, except where clear statistical distinction in performance could be obtained with 5 experiments. The results show that the temperatures are homogeneous for all cells in the water bath and none of the cells contains any systematic errors that lead to better or worse results consistently. THF Hydrate Crystal Growth Experimental. The study on the inhibition of hydrate crystal growth can be carried out using THF, which forms a structure II hydrate at about 4.4 °C under atmospheric pressure in THF/water mixtures. This method used has been reported previously.16−20 A total of 900 mL of THF/water test solution (equilibrium temperature of about 3.2 °C) is prepared by mixing 26.28 g of NaCl and 170 g of THF in distilled water. The test polymer was dissolved in the aqueous THF/NaCl solution to the desired concentration. The beaker with the test solution is then placed in a stirred cooling bath preset to −0.5 °C (±0.05 °C), which represents about 3.8 °C subcooling. THF hydrate crystals were allowed to grow at the end of the glass tube for 60 min. After this time, the glass tube was removed and the weight and morphology of the crystals were recorded.
Table 1. PNIPAM and PNVIBA Samples Synthesized sample ID
polymer
molecular weight (Mn)
molecular weight (Mw)
PDI
PNIPAM 6k PNIPAM 17k PNVIBA 9k PNVIBA 21k
PNIPAM PNIPAM PNVIBA PNVIBA
6000 17000 9500 21400
15000 51000 18560 112000
2.50 3.00 1.95 5.23
High-Pressure Rocker Rig Tests with Synthetic Natural Gas (SNG). KHI performance tests were carried out in high-pressure steel rocking cells supplied by PSL Systemtechnikk, Germany. The rocker rig consists of five steel cells with a volume of 40 mL. Each cell contains a steel ball, which is used for agitation of the test liquid. The test procedure was a constant cooling test method similar to that used in our previous research.13 In all of the experiments, the cell was charged with a SNG mixture and distilled water as an aqueous phase, which will theoretically form structure II hydrates as the most thermodynamically stable hydrate. Table 2 shows the composition of the SNG. For the KHI experiments,
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RESULTS AND DISCUSSION Comparison of PVamides and Polymerization Methods. High-pressure natural gas hydrate constant cooling experiments were carried out using PVamides with different alkyl chain lengths on the side group. Table 3 summarizes the average onset temperatures (To) and fast hydrate formation temperatures (Ta). The To value is considered the most important of the two temperature parameters because this refers to the first detection of hydrate formation, after which crystal growth is more likely to lead to hydrate plugging. The percentage deviation (% dev) based on the average values is also given in the table to present the degree of stochastic nature of the KHI experiments. Almost all of the standard constant cooling tests gave 1−8% deviations on either side of the average. The % dev of PVamine−iPrCO and PVamine−nPrCO was about 12−30%. The scattering in the To and Ta values was expected because of the stochastic nature of hydrate formation in a small cell. The KHI performances of all polymers are significantly better than without an additive. PVamine−MeCO is actually the same polymer as PNVA, although PNVA is a purer homopolymer. Both polymers gave the poorest performances of any of the amide polymers (average To = 17.0 and 15.7 °C, respectively), underlining the fact that small hydrophobic methyl groups are
Table 2. Composition of SNG component
mol %
methane ethane propane isobutane n-butane N2 CO2
80.67 10.20 4.90 1.53 0.76 0.10 1.84
the initial pressure in the cell is 76 bar. The equilibrium temperature for our SNG−water system at this pressure was calculated using Calsep’s PVTSim software to be 20.5 °C. Laboratory experiments were used to determine the equilibrium temperature by standard slow hydrate dissociation.14,15 Experiments conducted previously with five cells gave 20.2 ± 0.05 °C for the SNG−water system at 76 bar. 184
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Figure 3. Pressure and temperature logged from all five cells during a standard constant cooling test. Cells 1 and 2 contain a different KHI than cells 3−5 (see the Results and Discussion).
Figure 4. Pressure and temperature curves versus time during a standard constant cooling test using 5000 ppm of PNIPAM 6k.
insufficiently large for good KHI performance. Moreover, a trend can be seen showing that an increase in the alkyl chain length also increases the KHI performance significantly. Thus, increasing the chain length from ethyl to n-propyl to increase the hydrophobicity should also improve the KHI performance of the PVamide. This is indeed what was observed. PVamine−nPrCO gave statistically better performance than PVamine−EtCO, dropping the average To from 12.7 to 8.5 °C. This was the best PVamide tested, but the performance was still statistically significantly worse than that of the commercial 1:1 VCap/VP co-polymer. However, the molecular weight of this co-polymer has been optimized for the best KHI performance, and it is probable that
a lower molecular weight PVamine−nPrCO would perform better. Further, the comparison of PVamine−EtCO and PVamine− iPrCO with a maximum carbon chain length of 2 carbons shows that the branched chain in the more hydrophobic iso-alkyl group gives significantly better inhibition performance. This fits to our expectation because the branching leads to a greater van der Waals interaction with water molecules either in the free water or on the hydrate particle surfaces. PVamine−iBuCO is however not water-soluble and could not be tested. However, a copolymer with 66% isopropyl/33% isobutyl groups gave a good performance with an average To value of 8.6 °C. 185
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Table 3. PVamide KHI Gas Hydrate Results (Average of 10 Rocking Cell Tests Using 5000 ppm of Polymer unless Otherwise Stated) polymer
To(av) (°C)
To(% dev)
Ta(av) (°C)
Ta(% dev)
To(av) − Ta(av) (°C)
no polymer 1:1 VCap/VP polymer (2500 ppm) 1:1 VCap/VP polymer PVamine−MeCO PNVA PVamine−EtCO PVamine−nPrCO PVamine−iPrCO PNVIBA 9k PVamine−iPrCO/iBuCO PVamine−iBuCO
18.6 8.1 6.3 17.0 15.7 12.7 8.5 9.3 9.4 8.6 not soluble
7.0−8.1 2.0−4.2 3.2−11.1 3.4−4.2 5.9−11.9 1.4−1.7 12.1−29.8 12.4−15.7 5.4−6.3 1.6−2.9
18.6 5.9 3.6 16.5 15.7 10.9 6.8 7.2 7.1 7.8
7.0−8.1 1.4−2.0 8.3−14.0 3.1−3.5 5.9−11.9 1.8−2.8 3.2−8.1 16.7−27.8 4.6−5.2 1.3−4.3
0 2.2 2.7 0.5 0 1.8 1.7 2.1 2.3 0.8
values between 8.8 and 9.9 °C and Ta values between 6.8 and 7.5 °C. For PNIPAM 17k, the To and Ta varied from 9.0 to 10.7 °C and from 7.1 to 8.0 °C, respectively. The To value of PNVIBA 21k varied between 10.4 and 12.0 °C, and the Ta value of PNVIBA 21k varied between 8.5 and 9.8 °C. Similar deviations in To and Ta were observed previously in our laboratories.24 Scatterings of results produced from small cells were expected, because of the stochasticity of hydrate formation. Independent sample t tests with equal variances assumed are used to show the differences in the To and Ta values between the polymers statistically. The result is said to be statistically significant when the p value is less than 0.05.25 The test results show that the best polymer tested was the commercial 1:1 VCap/VP polymer. The differences comparing the PNIPAMs and PNVIBAs by comparing the To and Ta values are statistically significant, judging from the p values. PNIPAM or PNVIBA polymers of lower molecular weights and closer to that of the VCap/VP co-polymer may have given higher performance, but these were unavailable. The results using no polymer gave the highest onset and rapid formation temperatures with a clear statistical difference to all of the polymers tested. The KHI performance of the polymers ranked using the To values is
It was also observed that the performance of PNVIBAs of comparable molecular weights but synthesized from two different methods (PVamine−iPrCO and PNVIBA) gave similar performance as KHIs. In summary, the KHI performance of the PVamides was best when the alkyl group was n-propyl, with good performance also obtained for the polymers with isopropyl groups or the co-polymer with isopropyl and isobutyl groups. We carried out THF hydrate crystal growth inhibition experiments with the PVamides synthesized using both methods. The growth rate and crystal morphology are presented in Table 4. For Table 4. Average THF Hydrate Crystal Growth Rate Results with 4000 ppm of PVamides chemical PVamine−MeCO PNVA PVamine−EtCO PVamine−nPrCO PVamine−iPrCO PVamine−iBuCO
growth rate (g/h) 0.80 0.82
crystal morphology fairly thin plates deformed at edges TIP TIP slushy lump TIP not measurable, slushy crystals
PVamides with a side group larger than the methyl group, we were not able to determine the THF crystal growth rate quantitatively. Thin plates were growing until the edge of the beakers. Therefore, it was not possible to weigh the amount of THF crystals inside the beakers. Such observation is noted as TIP in Table 4. Tests carried out with PVamine−nPrCO and PVamine− iBuCO formed slushy wet lumps that grew around the tip of the glass tube. These results show that PVamides affect the growth of THF hydrate crystals, especially with alkyl groups of 2−3 carbon atoms, as tests with no additive give octahedral crystals with sharp pyramidal shapes, as reported previously.21−23 Comparison of PNVIBA to PNIPAM. High-pressure KHI constant cooling experiments were carried out with the two different molecular weights of PNIPAMs and PNVIBAs. The onset temperatures (To) and fast hydrate formation temperatures (Ta) are presented in Figure 5. The average values are summarized in Table 5. These results are compared to results from distilled water without polymer addition. To determine the KHI performance of these PNIPAMs and PNVIBAs, the results with a commercial polymer (1:1 VCap/VP polymer) are also reported here. The standard constant cooling tests gave 1−10% deviations on either side of the average. For PNIPAM 6k, the To value varied between 8.9 and 10.1 °C and the Ta value varied between 6.2 and 6.7 °C. Similarly, the lower molecular weight PNVIBA gave To
1:1 VCap/VP polymer > PNIPAM 6k = PNVIBA 9k > PNIPAM 17k > PNVIBA 21k > no additive
A significant difference in polymer ranking based on the Ta values was also observed using the standard constant cooling test method. t tests of all of the samples gave p values of less than 0.05 (1:1 VCap/VP polymer/PNIPAM 6k, 2.32 × 10−5; PNIPAM 6k/PNVIBA 9k, 2.77 × 10−6; PNVIBA 9k/PNIPAM 17k, 0.0017; PNIPAM 17k/PNVIBA 21k, 8.39 × 10−9; and PNVIBA 21k/no additive, 1.10 × 10−17). This gives the following ranking: 1:1 VCap/VP polymer > PNIPAM 6k > PNVIBA 9k > PNIPAM 17k > PNVIBA 21k > no additive
To values represent the first detectable hydrate formation, whereas the difference between To and Ta indicates the slow hydrate growth period before rapid formation occurs. The lower molecular weight polymers gave lower average To values and longer delay until rapid formation than their higher molecular weight counterparts. Previous studies show that the performance of some KHIs in the same polymer class varies according to their molecular weight.3,26 Our results correspond with this expectation, as shown in the Table 5. With the increase of the molecular weight, a KHI inhibition performance decrease is observed for both PNIPAM and PNVIBA. 186
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Figure 5. To and Ta values from the standard constant cooling tests using 5000 ppm of polymers.
Table 5. Average Values of Onset (To) and Fast (Ta) Hydrate Formation Temperatures from 10 Rocking Cell Tests with 5000 ppm of PNIPAMs and PNVIBAs polymer
To(av) (°C)
To(% dev)
Ta(av) (°C)
Ta(% dev)
To(av) − Ta(av) (°C)
no polymer 1:1 VCap/VP polymer (2500 ppm) PNVIBA 9k PNVIBA 21k PNIPAM 6k PNIPAM 17k
18.6 8.1 9.4 11.0 9.4 9.8
7.0−8.1 2.0−4.2 5.4−6.3 5.3−9.3 5.0−7.8 6.3−9.0
18.6 5.9 7.1 9.4 6.4 7.6
7.0−8.1 1.4−2.0 4.6−5.2 3.9−9.9 3.4−4.4 5.7−6.2
0 2.2 2.3 1.6 3.0 2.2
weights from both synthesis routes gave a very similar KHI performance. We also observed that PVamides affect the growth of THF hydrate crystal morphology, giving plates rather than the normal octahedral crystals. In the high-pressure natural gas hydrate constant cooling experiments using PNIPAMs and PNVIBAs, we observed that the lower molecular weight polymers inhibit hydrate nucleation and crystal growth more than their higher molecular weight counterparts. At lower molecular weight, the performance of PNIPAM and PNVIBA in inhibiting hydrate nucleation is very similar but PNIPAM performed slightly better as a crystal growth inhibitor. Thus, at low molecular weight (which gives the best KHI performance), the orientation of the amide group (−CO− NH− or −NH−CO) does not seem to have a significant effect on the KHI performance. However, the KHI performance of PNIPAM is significantly better than PNVIBA at higher molecular weights.
The performance between the two lower molecular weight polymers by comparing the To values is not statistically significant, judging from the p values. This shows that PNIPAM 6k performed as well as PNVIBA 9k in inhibiting hydrate nucleation. On the other hand, PNIPAM 6k gave a longer delay (i.e., lower average Ta values with the same average To values) before fast hydrate formation and, consequently, plugging of the cells, which is an indicator that PNIPAM 6k is a better crystal growth inhibitor than PNVIBA 9k. For the higher molecular weight PNIPAM and PNVIBA, performance differences are observed in both the To and Ta values. PNIPAM 17k gave lower average To and Ta values than PNVIBA 21k, with clearly significant difference p values in independent sample t tests. When the differences between To and Ta values are compared, PNIPAM gave a longer delay from observable hydrate nucleation until rapid formation. The results suggest that, at higher molecular weights, PNIPAM performs better than PNVIBA as a KHI.
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CONCLUSION In this study, we have synthesized PVamides with different alkyl chain lengths on the side group. The synthesis was from either the vinyl monomers or polyvinylamine. High-pressure natural gas hydrate constant cooling experiments show that an increase in the alkyl chain length and branched chain to the iso-alkyl group increases the KHI performance significantly as long as water solubility is maintained. PVamine−iBuCO of similar molecular
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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NOMENCLATURE AIBN = azobisisobutyronitrile dx.doi.org/10.1021/ef301703w | Energy Fuels 2013, 27, 183−188
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av = average LCST = lower critical solution temperature Mn = number average molecular weight Mw = weight average molecular weight PDI = polydispersity index PVamide = poly(N-vinylalkanamide) Ta = catastrophic hydrate formation temperature (°C) ta = rapid hydrate formation time (min) to = induction time (min) To = onset temperature (°C) VCap = N-vinylcaprolactam VP = N-vinylpyrrolidone % dev = percentage deviation
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REFERENCES
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