Hydrate Growth Inhibition by Poly(vinyl caprolactam) Released from Microcarriers under Turbulent Mixing Conditions

Abstract

Core-shell microcarriers were microfluidically prepared by using water-in-oil-in-water double-emulsion drops as a template. The aqueous core contained a kinetic hydrate inhibitor (KHI) of poly(vinyl caprolactam) (PVCap), and the shell was made of a cross-linked polymer. To make the microcarriers selectively release PVCap at temperatures where hydrate formation occurs under a constant shear flow, the relative shell thickness to the radius of the microcarrier was set to 0.11. The hydrate inhibition performance of PVCap released from the microcarriers was investigated using continuous cooling and constant subcooling in a high-pressure autoclave. Longer hydrate onset times were observed for the PVCap microcarriers compared to bulk water, suggesting that hydrate nucleation was inhibited by PVCap released from the microcarriers. The obtained subcooling temperature for the PVCap microcarriers was 11.3 degrees C, which was close to that of the PVCap bulk solution at 10.8 degrees C. The hydrate growth was faster for the PVCap microcarriers than for bulk water, but the effective growth period was shorter, resulting in a lower hydrate fraction in the liquid phase. Although the PVCap microcarriers performed successfully under continuous cooling, limited performance was observed with constant subcooling. Successful hydrate inhibition was sometimes observed, but fast hydrate formation was also observed over repeated experiments. This result is because microcarriers are designed to rupture under a constant shear flow. Thus, more studies are required to improve the design of microcarriers to release the inner KHI solution, even in cold-restart operations. Nevertheless, microcarriers provide a flexible way to inject KHI into subsea flowlines, as many different types of KHIs can be simultaneously delivered at a proper dose using a set of distinct microcarriers.