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Effectiveness of Biosurfactant and Quaternary Ammonium Salt as Hydrate Anti-Agglomerants: Effect of Model Oil Concentration, Methanol as Co-Surfactant, and Salinity

J. Dalton York and Abbas Firoozabadi. Chemical Engineering, Yale University, New Haven, CT 06520

Low dosage hydrate inhibitors (LDHI) are desirable because of potential at low concentration. Of the two types of LDHI, anti-agglomerants have a clear advantage over kinetic inhibitors from effectiveness at high subcooling often encountered in offshore oil and gas production. The disadvantage in employing the still commonly-used organic thermodynamic inhibitors in these extreme environments is that massive amounts are required for hydrate inhibition. This means inhibitor costs are also very large but that water separated from such mixtures downstream must be purged of organic content before disposal. In this work, we present results from tetrahydrofuran (THF) hydrate anti-agglomeration with isooctane as model oil. A ratio of 1/1 parts (by weight) water/THF has been used throughout the work, but variations in amount of model oil and surfactant are crucial to the study. For mixtures where anti-agglomeration does not occur, small amounts of methanol as co-surfactant are employed. We also examine the effect of salinity.

A multiple-sample rocking apparatus is employed with temperature bath and thermocouple data acquisition to determine crystallization and dissociation characteristics. Visual inspection is made of the agglomeration state of the fluid mixtures at their lowest temperatures; observations show a clear distinction between what is deemed as hydrate plug or proper anti-agglomeration. Temperatures employed are as low as -20 oC. An assessment of the stability of emulsions is also made.

A rhamnolipid biosurfactant as well as a quat—i.e., dicetyl dimethyl ammonium chloride—are the anti-agglomerants of focus. Biosurfactants are becoming increasingly attractive, largely due to lower toxicity and increased biodegradability compared to chemical surfactants. Physicochemical properties are very attractive also. Only one recent study has shown that rhamnolipid may be an effective anti-agglomerant, but the data are limited.

Results show rhamnolipid and quat are effective down to 0.05 and 0.01 wt. %, respectively. These values are roughly an order of magnitude below what the current literature limit if anti-agglomerant effectiveness is. This changes when the amount of model oil is reduced, but small amounts of methanol can be of benefit; concentration here is important since it is widely known that organic thermodynamic inhibitors may encourage precipitation of dissolved salts. To our knowledge, this is the first report of any alcohol co-surfactant being used in anti-agglomeration. Similarly, limits of behavior when brine is included—via dissolved NaCl or MgCl2—will also be discussed. Our results show—in certain cases—some non-monotonic behavior in the agglomeration state patterns. It is also found that highly stable emulsions are not crucial to anti-agglomeration.