Boronic and Organic Acids as Synergists for a Poly(N-vinylcaprolactam) Kinetic Hydrate Inhibitor

Malcolm A Kelland,Erik G Dirdal

doi:10.1021/acs.energyfuels.1c03556

Malcolm A Kelland, Erik G Dirdal

Open Access

https://doi.org/10.1021/acs.energyfuels.1c03556

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Abstract

A range of boronic acids have been investigated as synergists for the kinetic hydrate inhibitor (KHI) polymer, poly(N-vinylcaprolactam) (PVCap, Mw ≈ 10,000 g/mol) using high pressure rocking cells, a natural gas mixture, and a slow constant cooling (1 °C/h) test method from 76 bar. Surprisingly, unlike other classes of synergists such as alcohols and quaternary ammonium salts, the boronic acids that gave the best synergy had an alkyl or cycloalkyl tail with a maximum of a 3 carbon atom distance from the boron atom. The tail-branched iso-butylboronic acid was the best of these, yet it showed a negligible KHI effect when tested alone. However, consistent with the other classes of synergists, tail branching or use of a cyclic alkyl group was beneficial. Interestingly, boronic acids with chains of 5 to 6 carbon atoms, i.e., n-pentyl- and n-hexylboronic acids, were antagonistic to the PVCap KHI performance. For comparison, several organic acids were also investigated as synergists with PVCap. The same trend as for the boronic acids regarding the size and branching of the acid was seen. 3-Methylbutanoic acid gave the best synergy although worse than that of iso-butylboronic acid. The synergistic performance of sodium salts of some organic acids differed markedly to that of the free organic acids. Sodium 3,3-dimethylbutanoate gave the best synergy with PVCap.

Highlights

Kinetic hydrate inhibitors (KHIs) have been used since the mid 1990s to prevent plugging of flow lines with gas hydrates.[1−6] kinetic hydrate inhibitor (KHI) are a class of low-dosage hydrate inhibitors (LDHIs) and as the name suggests are added to well streams at low concentrations, usually between 0.3 and 3.0 wt % as the finished formulation at a pumpable viscosity
The Ta value can be useful as some KHI polymers are capable of arresting the growth rate considerably
If hydrate formation is somehow detected in a flow line, a slow growth rate can give enough time for remedial treatment before complete plugging of the line occurs

Summary

Introduction

Kinetic hydrate inhibitors (KHIs) have been used since the mid 1990s to prevent plugging of flow lines with gas hydrates.[1−6] KHIs are a class of low-dosage hydrate inhibitors (LDHIs) and as the name suggests are added to well streams at low concentrations, usually between 0.3 and 3.0 wt % as the finished formulation at a pumpable viscosity. Many other classes of polymers, both amides and non-amide-based functional groups, have been investigated.[2,4,6−12] In addition, a wide range of synergists or performance enhancers have been studied.[2−4,13−16] Synergism occurs when the interaction or cooperation of two or more substances produces a combined effect greater than the sum of their separate effects Based on this definition, many chemicals currently called synergists are only performance enhancers as these chemicals by themselves have poor ability to prevent gas hydrate formation. The word “synergist” is being used to cover both terms as this is the general industry practice

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