Fused chemical reactions to remediate paraffin plugging in sub-sea pipelines.

Abstract

In this work, a timed release scheme of the citric acid-catalyst was used to fuse the highly exothermic reaction between ammonium chloride and sodium nitrite in a fused chemical reaction (FCR) system. The timed release was obtained by encapsulating the acid-catalyst in gelatin capsules then coating the capsules with a water-soluble polymer. The highly exothermic FCR system is demonstrated to be a feasible solution for the billion-dollar problem of paraffin deposition during crude oil production and transportation operations in sub-sea pipelines. Studies of the exothermic reaction in an isothermal reactor showed that hydrogen ions catalyze the reaction by changing the concentration of the reacting species, not by changing the reaction pathways. The rate-limiting step involves the S_N2 reaction between aqueous molecular ammonia and nitrogen trioxide to form nitrosamine. The activation energy of the reaction was found to be about 65.7 kJ/mol experimentally which compares very favorably (within 11%) with the value found from molecular modeling. The facilitated diffusion of hydrogen ions away from the polymer interface is the principal process that determines the rate limiting step as well as the overall rate of the polymer dissolution. The facilitated diffusion effect increases significantly with an initial increase in the carrier concentration, then approaches a limit at high carrier concentration. However, there are optimum values of the carrier's <italic>pK_a</italic> and of the solution pH which give a maximum facilitation effect. A homogenous chemico-diffusion model can predict concentration profiles of all species across the diffusion boundary layer and polymer dissolution rates which agree well with experimental results. Good agreement between simulation and experimental results for the FCR system in both batch and flow conditions was achieved. Batch experiments showed that the heat release is controlled by the thickness of the polymeric coating and can be delayed as long as 20 hours. Flow experiments demonstrated that the FCR system could be controlled to provide a substantial amount of effective heat in-situ. This large amount of effective heat is sufficient to soften and melt the wax deposit. The model for the FCR system in flow conditions was also extended to apply in a typical sub-sea pipeline.